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Eggenberger, P., S. Deheuvels, A. Miglio, S. Ekström, C. Georgy, G. Meynet, N. Lagarde, et al. "Asteroseismology of evolved stars to constrain the internal transport of angular momentum." Astronomy & Astrophysics 621 (January 2019): A66. http://dx.doi.org/10.1051/0004-6361/201833447.
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Context. The observations of solar-like oscillations in evolved stars have brought important constraints on their internal rotation rates. To correctly reproduce these data, an efficient transport mechanism is needed in addition to the transport of angular momentum by meridional circulation and shear instability. The efficiency of this undetermined process is found to increase both with the mass and the evolutionary stage during the red giant phase. Aims. We study the efficiency of the transport of angular momentum during the subgiant phase. Methods. The efficiency of the unknown transport mechanism is determined during the subgiant phase by comparing rotating models computed with an additional corresponding viscosity to the asteroseismic measurements of both core and surface-rotation rates for six subgiants observed by the Kepler spacecraft. We then investigate the change in the efficiency of this transport of angular momentum with stellar mass and evolution during the subgiant phase. Results. The precise asteroseismic measurements of both core and surface-rotation rates available for the six Kepler targets enable a precise determination of the efficiency of the transport of angular momentum needed for each of these subgiants. These results are found to be insensitive to all the uncertainties related to the modelling of rotational effects before the post-main sequence (poMS) phase. An interesting exception in this context is the case of young subgiants (typical values of log(g) close to 4), because their rotational properties are sensitive to the degree of radial differential rotation on the main sequence (MS). These young subgiants constitute therefore perfect targets to constrain the transport of angular momentum on the MS from asteroseismic observations of evolved stars. As for red giants, we find that the efficiency of the additional transport process increases with the mass of the star during the subgiant phase. However, the efficiency of this undetermined mechanism decreases with evolution during the subgiant phase, contrary to what is found for red giants. Consequently, a transport process with an efficiency that increases with the degree of radial differential rotation cannot account for the core-rotation rates of subgiants, while it correctly reproduces the rotation rates of red giant stars. This suggests that the physical nature of the additional mechanism needed for the internal transport of angular momentum may be different in subgiant and red giant stars.
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Eggenberger, P., J. W. den Hartogh, G. Buldgen, G. Meynet, S. J. A. J. Salmon, and S. Deheuvels. "Asteroseismology of evolved stars to constrain the internal transport of angular momentum." Astronomy & Astrophysics 631 (November 2019): L6. http://dx.doi.org/10.1051/0004-6361/201936348.
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Context. Asteroseismic observations enable the characterisation of the internal rotation of evolved stars. These measurements reveal that an unknown efficient angular momentum (AM) transport mechanism is needed for subgiant and red giant stars in addition to hydrodynamic transport processes. A revised prescription for AM transport by the magnetic Tayler instability has been recently proposed as a possible candidate for such a missing mechanism. Aims. We compare the rotational properties predicted by this magnetic AM transport to asteroseismic constraints obtained for evolved stars with a particular focus on the subgiant phase. Methods. We computed models accounting for the recent prescription for AM transport by the Tayler instability with the Geneva stellar evolution code for subgiant and red giant stars, for which an asteroseismic determination of both core and surface rotation rates is available. Results. The revised prescription for the transport by the Tayler instability leads to low core rotation rates after the main sequence that are in better global agreement with asteroseismic measurements than those predicted by models with purely hydrodynamic processes or with the original Tayler-Spruit dynamo. A detailed comparison with asteroseismic data shows that the rotational properties of at most two of the six subgiants can be correctly reproduced by models accounting for this revised magnetic transport process. This result is obtained independently of the value adopted for the calibration parameter in this prescription. We also find that this transport by the Tayler instability faces difficulties in simultaneously reproducing asteroseismic measurements available for subgiant and red giant stars. The low values of the calibration parameter needed to correctly reproduce the rotational properties of two of the six subgiants lead to core rotation rates during the red giant phase that are too high. Inversely, the higher values of this parameter needed to reproduce the core rotation rates of red giants lead to a very low degree of radial differential rotation before the red giant phase, which is in contradiction with the internal rotation of subgiant stars. Conclusions. In its present form, the revised prescription for the transport by the Tayler instability does not provide a complete solution to the missing AM transport revealed by asteroseismology of evolved stars.
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Vauclair, S. "Turbulence and the Lithium Abundance in Giants and Main-Sequence Disk and Halo Stars." Symposium - International Astronomical Union 145 (1991): 327–40. http://dx.doi.org/10.1017/s0074180900227472.
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The present situation concerning the observations of lithium in subgiant and giant stars is reviewed. These observations, compared to the determinations of lithium in main-sequence stars, can be used to obtain more stringents constraints on the dynamics of the stellar gas in the main-sequence as well as in the subgiant and giant stages. The macroscopic motions expected in stars are discussed from a theoretical point of view and compared to the observations, with special focusing on the theories of rotation-induced turbulence.
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Ruiz–Lapuente, Pilar, Jonay González Hernández, Hugo Tabernero, David Montes, Ramon Canal, Javier Mendez, and Luigi Bedin. "Survey for the Binary Progenitor in SN1006 and Update on SN1572." Proceedings of the International Astronomical Union 7, S281 (July 2011): 322–25. http://dx.doi.org/10.1017/s1743921312015293.
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AbstractWe have completed a survey down to R = 15 mag of the stars within a circle of 4 arcmin radius around the nominal center of the remnant of SN 1006, one of the three historical Type Ia supernovae (the other two being SN 1572 and SN 1604), in search of a possible surviving binary companion of the white dwarf whose explosion gave rise to the supernova. The stellar parameters (effective temperature, surface gravity, and metallicity), as well as the radial velocities of all the stars, have been measured from spectra obtained with the UVES spectrograph at the VLT, and from the former and the available photometry, distances have been determined. Chemical abundances of the Fe-peak elements Cr, Mn, Co, and Ni have also been measured to check for possible contamination of the stellar surface by the supernova ejecta. The limiting magnitude of the survey would allow us to find stellar companions of the red-giant type, subgiant stars, and main–sequence stars down to F5–6. Unlike in SN 1572, where a subgiant of type G0–1 has been proposed as the companion of SN 1572, for SN 1006 we can discard the possibility that SN 1006 had a red giant or subgiant companion.
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Pinçon, C., K. Belkacem, M. J. Goupil, and J. P. Marques. "Can plume-induced internal gravity waves regulate the core rotation of subgiant stars?" Astronomy & Astrophysics 605 (September 2017): A31. http://dx.doi.org/10.1051/0004-6361/201730998.
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Context. The seismic data provided by the space-borne missions CoRoT and Kepler enabled us to probe the internal rotation of thousands of evolved low-mass stars. Subsequently, several studies showed that current stellar evolution codes are unable to reproduce the low core rotation rates observed in these stars. These results indicate that an additional angular momentum transport process is necessary to counteract the spin up due to the core contraction during the post-main sequence evolution. For several candidates, the transport induced by internal gravity waves (IGW) could play a non-negligible role. Aims. We aim to investigate the effect of IGW generated by penetrative convection on the internal rotation of low-mass stars from the subgiant branch to the beginning of the red giant branch. Methods. A semi-analytical excitation model was used to estimate the angular momentum wave flux. The characteristic timescale associated with the angular momentum transport by IGW was computed and compared to the contraction timescale throughout the radiative region of stellar models at different evolutionary stages. Results. We show that IGW can efficiently counteract the contraction-driven spin up of the core of subgiant stars if the amplitude of the radial-differential rotation (between the center of the star and the top of the radiative zone) is higher than a threshold value. This threshold depends on the evolutionary stage and is comparable to the differential rotation rates inferred for a sample of subgiant stars observed by the satellite Kepler. Such an agreement can therefore be interpreted as the consequence of a regulation mechanism driven by IGW. This result is obtained under the assumption of a smooth rotation profile in the radiative region and holds true even if a wide range of values is considered for the parameters of the generation model. In contrast, on the red giant branch, we find that IGW remain insufficient, on their own, to explain the observations because of an excessive radiative damping. Conclusions. IGW generated by penetrative convection are able to efficiently extract angular momentum from the core of stars on the subgiant branch and accordingly have to be taken into account. Moreover, agreements with the observations reinforce the idea that their effect is essential to regulate the amplitude of the radial-differential rotation in subgiant stars. On the red giant branch, another transport mechanism must likely be invoked.
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Hekker, S., and A. Mazumdar. "Solar-like oscillations in subgiant and red-giant stars: mixed modes." Proceedings of the International Astronomical Union 9, S301 (August 2013): 325–31. http://dx.doi.org/10.1017/s1743921313014531.
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AbstractThanks to significant improvements in high-resolution spectrographs and the launch of dedicated space missions MOST, CoRoT and Kepler, the number of subgiants and red-giant stars with detected oscillations has increased significantly over the last decade. The amount of detail that can now be resolved in the oscillation patterns does allow for in-depth investigations of the internal structures of these stars. One phenomenon that plays an important role in such studies are mixed modes. These are modes that carry information of the inner radiative region as well as from the convective outer part of the star allowing to probe different depths of the stars.Here, we describe mixed modes and highlight some recent results obtained using mixed modes observed in subgiants and red-giant stars.
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Pinçon, C., K. Belkacem, and M. J. Goupil. "Excitation of internal gravity waves by penetrative convection." EAS Publications Series 82 (2019): 247–51. http://dx.doi.org/10.1051/eas/1982024.
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We investigate the ability of internal gravity waves that are generated by penetrative convection to redistribute angular momentum in the internal radiative zone of evolved low-mass stars. To do so, we use the semianalytical excitation model recently proposed by Pinçon et al. 2016. We briefly report the preliminary results of the study focusing on the subgiant and red giant branch stars.
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Hon, Marc, Dennis Stello, Rafael A. García, Savita Mathur, Sanjib Sharma, Isabel L. Colman, and Lisa Bugnet. "A search for red giant solar-like oscillations in all Kepler data." Monthly Notices of the Royal Astronomical Society 485, no. 4 (March 4, 2019): 5616–30. http://dx.doi.org/10.1093/mnras/stz622.
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ABSTRACT The recently published Kepler mission Data Release 25 (DR25) reported on ∼197 000 targets observed during the mission. Despite this, no wide search for red giants showing solar-like oscillations have been made across all stars observed in Kepler’s long-cadence mode. In this work, we perform this task using custom apertures on the Kepler pixel files and detect oscillations in 21 914 stars, representing the largest sample of solar-like oscillating stars to date. We measure their frequency at maximum power, νmax, down to $\nu _{\mathrm{max}}\simeq 4\, \mu$Hz and obtain log (g) estimates with a typical uncertainty below 0.05 dex, which is superior to typical measurements from spectroscopy. Additionally, the νmax distribution of our detections show good agreement with results from a simulated model of the Milky Way, with a ratio of observed to predicted stars of 0.992 for stars with $10 \lt \nu _{\mathrm{max}}\lt 270\, \mu$Hz. Among our red giant detections, we find 909 to be dwarf/subgiant stars whose flux signal is polluted by a neighbouring giant as a result of using larger photometric apertures than those used by the NASA Kepler science processing pipeline. We further find that only 293 of the polluting giants are known Kepler targets. The remainder comprises over 600 newly identified oscillating red giants, with many expected to belong to the Galactic halo, serendipitously falling within the Kepler pixel files of targeted stars.
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Simon, Theodore, and Stephen A. Drake. "The evolution of chromospheric activity of cool giant and subgiant stars." Astrophysical Journal 346 (November 1989): 303. http://dx.doi.org/10.1086/168012.
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Corsaro, E., J. M. McKeever, and J. S. Kuszlewicz. "Fast and Automated Peak Bagging with DIAMONDS (FAMED)." Astronomy & Astrophysics 640 (August 2020): A130. http://dx.doi.org/10.1051/0004-6361/202037930.
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Stars of low and intermediate mass that exhibit oscillations may show tens of detectable oscillation modes each. Oscillation modes are a powerful tool to constrain the internal structure and rotational dynamics of the star, hence allowing one to obtain an accurate stellar age. The tens of thousands of solar-like oscillators that have been discovered thus far are representative of the large diversity of fundamental stellar properties and evolutionary stages available. Because of the wide range of oscillation features that can be recognized in such stars, it is particularly challenging to properly characterize the oscillation modes in detail, especially in light of large stellar samples. Overcoming this issue requires an automated approach, which has to be fast, reliable, and flexible at the same time. In addition, this approach should not only be capable of extracting the oscillation mode properties of frequency, linewidth, and amplitude from stars in different evolutionary stages, but also able to assign a correct mode identification for each of the modes extracted. Here we present the new freely available pipeline FAMED (Fast and AutoMated pEak bagging with DIAMONDS), which is capable of performing an automated and detailed asteroseismic analysis in stars ranging from the main sequence up to the core-helium-burning phase of stellar evolution. This, therefore, includes subgiant stars, stars evolving along the red giant branch (RGB), and stars likely evolving toward the early asymptotic giant branch. In this paper, we additionally show how FAMED can detect rotation from dipolar oscillation modes in main sequence, subgiant, low-luminosity RGB, and core-helium-burning stars.
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da Silva, Ronaldo, André de C. Milone, and Helio J. Rocha-Pinto. "Homogeneous abundance analysis of FGK dwarf, subgiant, and giant stars with and without giant planets." Astronomy & Astrophysics 580 (July 23, 2015): A24. http://dx.doi.org/10.1051/0004-6361/201525770.
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CANTO MARTINS, B. L., J. D. DO NASCIMENTO, J. R. DE MEDEIROS, A. LÈBRE, O. RICHARD, P. DE LAVERNY, and C. MELO. "LITHIUM IN THE STELLAR OPEN CLUSTER M67." International Journal of Modern Physics D 16, no. 02n03 (February 2007): 391–96. http://dx.doi.org/10.1142/s021827180701016x.
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In this work we present atmospheric parameters and Li abundances, obtained from a detailed spectroscopic analysis, for a large sample of stars with different evolutionary stages (turn-off, subgiant and giant stars) in the open cluster M67. The observations were carried out with high resolution (R ~ 47,000) and high S/N using the UVES+FLAMES at VLT/UT2. From available photometry and computed synthetic spectra for the region around the lithium line at 6707.75 Å, we derive atmospheric parameters (T eff , log g, [ Fe/H ], v sin i) and Li abundances for each star, in order to better understand the level of mixing and convective dilution of evolved stars in M67.
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Deheuvels, S., J. Ballot, P. Eggenberger, F. Spada, A. Noll, and J. W. den Hartogh. "Seismic evidence for near solid-body rotation in two Kepler subgiants and implications for angular momentum transport." Astronomy & Astrophysics 641 (September 2020): A117. http://dx.doi.org/10.1051/0004-6361/202038578.
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Context. Asteroseismic measurements of the internal rotation of subgiants and red giants all show the need for invoking a more efficient transport of angular momentum than theoretically predicted. Constraints on the core rotation rate are available starting from the base of the red giant branch (RGB) and we are still lacking information on the internal rotation of less evolved subgiants. Aims. We identify two young Kepler subgiants, KIC 8524425 and KIC 5955122, whose mixed modes are clearly split by rotation. We aim to probe their internal rotation profile and assess the efficiency of the angular momentum transport during this phase of the evolution. Methods. Using the full Kepler data set, we extracted the mode frequencies and rotational splittings for the two stars using a Bayesian approach. We then performed a detailed seismic modeling of both targets and used the rotational kernels to invert their internal rotation profiles using the MOLA inversion method. We thus obtained estimates of the average rotation rates in the g-mode cavity (⟨Ω⟩g) and in the p-mode cavity (⟨Ω⟩p). Results. We found that both stars are rotating nearly as solid bodies, with core-envelope contrasts of ⟨Ω⟩g/⟨Ω⟩p = 0.68 ± 0.47 for KIC 8524425 and ⟨Ω⟩g/⟨Ω⟩p = 0.72 ± 0.37 for KIC 5955122. This result shows that the internal transport of angular momentum has to occur faster than the timescale at which differential rotation is forced in these stars (between 300 Myr and 600 Myr). By modeling the additional transport of angular momentum as a diffusive process with a constant viscosity νadd, we found that values of νadd > 5 × 104 cm2 s−1 are required to account for the internal rotation of KIC 8524425, and νadd > 1.5 × 105 cm2 s−1 for KIC 5955122. These values are lower than or comparable to the efficiency of the core-envelope coupling during the main sequence, as given by the surface rotation of stars in open clusters. On the other hand, they are higher than the viscosity needed to reproduce the rotation of subgiants near the base of the RGB. Conclusions. Our results yield further evidence that the efficiency of the internal redistribution of angular momentum decreases during the subgiant phase. We thus bring new constraints that will need to be accounted for by mechanisms that are proposed as candidates for angular momentum transport in subgiants and red giants.
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Primas, Francesca, Karin Lind, Corinne Charbonnel, Frank Grundahl, and Martin Asplund. "The lithium history of NGC 6397." Proceedings of the International Astronomical Union 5, S266 (August 2009): 143–48. http://dx.doi.org/10.1017/s1743921309990986.
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AbstractThe primordial lithium abundance inferred from WMAP and standard Big Bang nucleosysnthesis is approximately three times higher than the plateau value measured in old metal-poor Population II stars, suggesting that these stars have undergone atmospheric Li depletion. To constrain the physics responsible for such depletion, we conducted a homogeneous analysis of a large sample of stars in the metal-poor globular cluster NGC 6397, covering all evolutionary phases from below the main-sequence turnoff to high up the red-giant branch (RGB). The dwarf, turnoff, and early subgiant stars form a thin abundance plateau, with a sharpe edge in the middle of the subgiant branch, where Li dilution caused by the inward extension of the convective envelope starts (the beginning of the so-called first dredge up). A second steep abundance drop is seen at the RGB bump, again highlighting the need for the onset of nonstandard mixing in this evolutionary phase. Moreover, by also measuring the sodium abundances of the targets, we have gained insight into the degree of pollution by early cluster self-enrichement, and may separate highly polluted, Li-poor and Na-rich stars from stars formed from pristine material. Our observational findings strictly limit both the extent of lithium surface depletion, which in turn constrains the efficiency of mixing below the outer convection zone, and the resulting spread in lithium abundance in metal-poor turn-off stars.
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Kitamura, Masatoshi, and Yasuhisa Nakamura. "Empirical Determination of the Gravity-Darkening Exponent for the Secondary Components Filling the Roche Lobe in Semi-Detached Close Binary Systems." International Astronomical Union Colloquium 108 (1988): 217–18. http://dx.doi.org/10.1017/s0252921100093866.
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The ordinary semi-detached close binary system consists of a main-sequence primary and subgiant (or giant) secondary component where the latter fills the Roche lobe. From a quantitative analysis of the observed ellipticity effect, Kitamura and Nakamura (1986) have deduced empirical values of the exponent of gravity-darkening for distorted main-sequence stars in detached systems and found that the empirical values of the exponent for these stars with early-type spectra are close to the unity, indicating that the subsurface layers of early-main sequence stars in close binaries are actually in radiative equilibrium. The exponent of gravity-darkening can be defined by H ∝ gα with H as the bolonetric surface brightness and g as the local gravity on the stellar surface.
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Grieves, Nolan, Louise D. Nielsen, Jose I. Vines, Edward M. Bryant, Samuel Gill, François Bouchy, Monika Lendl, et al. "NGTS-13b: a hot 4.8 Jupiter-mass planet transiting a subgiant star." Astronomy & Astrophysics 647 (March 2021): A180. http://dx.doi.org/10.1051/0004-6361/202039586.
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We report the discovery of the massive hot Jupiter NGTS-13b by the Next Generation Transit Survey (NGTS). The V = 12.7 host star is likely in the subgiant evolutionary phase with logg* = 4.04 ± 0.05, Teff = 5819 ± 73 K, M* = 1.30−0.18+0.11 M⊙, and R* = 1.79 ± 0.06 R⊙. The NGTS detected a transiting planet with a period of P = 4.12 days around the star, which was later validated with the Transiting Exoplanet Survey Satellite (TESS; TIC 454069765). We confirm the planet using radial velocities from the CORALIE spectrograph. Using NGTS and TESS full-frame image photometry combined with CORALIE radial velocities, we determine NGTS-13b to have a radius of RP = 1.142 ± 0.046 RJup, a mass of MP = 4.84 ± 0.44 MJup, and an eccentricity of e = 0.086 ± 0.034. Previous studies have suggested that ~4 MJup may be the border separating two formation scenarios (e.g., core accretion and disk instability) and that massive giant planets share similar formation mechanisms as lower-mass brown dwarfs. NGTS-13b is just above 4 MJup, making it an important addition to the statistical sample needed to understand the differences between various classes of substellar companions. The high metallicity of NGTS-13, [Fe/H] = 0.25 ± 0.17, does not support previous suggestions that massive giants are found preferentially around lower metallicity host stars, but NGTS-13b does support findings that more massive and evolved hosts may have a higher occurrence of close-in massive planets than lower-mass unevolved stars.
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Walker, A. R., C. E. Martínez-Vázquez, M. Monelli, A. K. Vivas, G. Bono, C. Gallart, S. Cassisi, et al. "A DECam view of the diffuse dwarf galaxy Crater II: the colour–magnitude diagram." Monthly Notices of the Royal Astronomical Society 490, no. 3 (October 10, 2019): 4121–32. http://dx.doi.org/10.1093/mnras/stz2826.
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ABSTRACT We present a deep Blanco/DECam colour–magnitude diagram (CMD) for the large but very diffuse Milky Way satellite dwarf galaxy Crater II. The CMD shows only old stars with a clearly bifurcated subgiant branch (SGB) that feeds a narrow red giant branch. The horizontal branch (HB) shows many RR Lyrae and red HB stars. Comparing the CMD with [Fe/H] = −2.0 and [α/Fe] = + 0.3 alpha-enhanced BaSTI isochrones indicates a mean age of 12.5 Gyr for the main event and a mean age of 10.5 Gyr for the brighter SGB. With such multiple star formation events Crater II shows similarity to more massive dwarfs that have intermediate age populations, however for Crater II there was early quenching of the star formation and no intermediate age or younger stars are present. The spatial distribution of Crater II stars overall is elliptical in the plane of the sky, the detailed distribution shows a lack of strong central concentration, and some inhomogeneities. The 10.5 Gyr subgiant and upper main-sequence stars show a slightly higher central concentration when compared to the 12.5 Gyr population. Matching to Gaia DR2 we find the proper motion of Crater II: μαcos δ = −0.14 ± 0.07, μδ = −0.10 ± 0.04 mas yr−1, approximately perpendicular to the semimajor axis of Crater II. Our results provide constraints on the star formation and chemical enrichment history of Crater II, but cannot definitively determine whether or not substantial mass has been lost over its lifetime.
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Nielsen, L. D., F. Bouchy, O. Turner, H. Giles, A. Suárez Mascareño, C. Lovis, M. Marmier, et al. "A Jovian planet in an eccentric 11.5 day orbit around HD 1397 discovered by TESS." Astronomy & Astrophysics 623 (March 2019): A100. http://dx.doi.org/10.1051/0004-6361/201834577.
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The Transiting Exoplanet Survey Satellite TESS has begun a new age of exoplanet discoveries around bright host stars. We present the discovery of HD 1397b (TOI-120.01), a giant planet in an 11.54-day eccentric orbit around a bright (V= 7.9) G-type subgiant. We estimate both host star and planetary parameters consistently usingEXOFASTv2based on TESS time-series photometry of transits and radial velocity measurements with CORALIE and MINERVA-Australis. We also present high angular resolution imaging with NaCo to rule out any nearby eclipsing binaries. We find that HD 1397b is a Jovian planet, with a mass of 0.415 ± 0.020MJand a radius of 1.026 ± 0.026RJ. Characterising giant planets in short-period eccentric orbits, such as HD 1397b, is important for understanding and testing theories for the formation and migration of giant planets as well as planet-star interactions.
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Bonsor, Amy, Grant M. Kennedy, Justin R. Crepp, John A. Johnson, Mark C. Wyatt, Bruce Sibthorpe, and Kate Y. L. Su. "The Population of Debris Discs Orbiting Subgiants." Proceedings of the International Astronomical Union 8, S299 (June 2013): 328–29. http://dx.doi.org/10.1017/s174392131300879x.
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AbstractWhilst debris discs orbiting main-sequence stars are well studied, very little is known regarding their fate when the star evolves onto the giant branch. For intermediate mass (A-type) stars, giants provide a unique opportunity to detect planets using the radial velocity technique, otherwise prohibited by high jitter levels and rotationally broadened lines in main-sequence intermediate mass (A-type) stars. Such stars can provide key insights into the structure of planetary systems around intermediate mass stars. In our Herschel OT1 program (PI Bonsor) we searched for the presence of debris discs orbiting a sample of 36 subgiants, half of which have RV detected companions. Our best detection is the resolved debris disc orbiting κ CrB.
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Martins, B. L. Canto, S. Vieira, C. A. O. Torres, G. R. Quast, L. da Silva, R. de La Reza, C. H. F. de Melo, and J. R. de Medeiros. "Lithium survey in evolved stars observed in the sacy project." Proceedings of the International Astronomical Union 5, S266 (August 2009): 368. http://dx.doi.org/10.1017/s1743921309991335.
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AbstractThe primary goal of the sacy project (Search for Associations Containing Young Stars) was to identify possible associations of stars younger than the Pleiades association among optical counterparts of ROSAT X-ray-bright sources. The study of the chemical abundance in stars located in regions of stellar formation is extremely important to understand stellar nucleo-synthesis, the physical mechanisms controlling mixing in stellar interiors, and chemical enrichment in the Galaxy. The present work highights the first results of a chemical-abundance study of evolved stars identified in the sacy survey. For this, we performed a detailed spectroscopic analysis for the determination of atmospheric parameters and Li abundance for a sample of giant and subgiant stars. The observations were carried out with high resolution using the FEROS (R = 48 000) échelle spectrograph. We measured the stellar parameters (Teff, log g, vmic, [Fe/H]) from LTE analysis in the complete range of 420-1100 nm. Li abundance was derived from the region around the lithium line at 6707.78 Å for the entire sample of stars.
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Smith, V. V., and D. L. Lambert. "Lithium in late-type giants. IV - The subgiant CH stars." Astrophysical Journal 303 (April 1986): 226. http://dx.doi.org/10.1086/164068.
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Dambis, A. K. "Adjusting the distance scales of old and intermediate-age tracers via statistical parallaxes." Proceedings of the International Astronomical Union 8, S289 (August 2012): 44–47. http://dx.doi.org/10.1017/s1743921312021084.
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AbstractThe method of statistical-parallax determination is used to adjust the photometric distance-scale zero points and test the selection of several samples of old and intermediate-age tracer objects in the Milky Way, including RR Lyrae- and Mira-type variables, blue and red horizontal-branch stars, and red giants. We obtain reasonable and stable absolute-magnitude (and, hence, photometric distance-scale) corrections for all samples except for the red horizontal-branch stars, which appear to be significantly contaminated by main-sequence turn-off and subgiant stars.
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Allen, D. M., B. V. Castilho, L. Pasquini, B. Barbuy, and P. Molaro. "Li Abundance in Evolved Stars of NGC 6397." Symposium - International Astronomical Union 198 (2000): 354–55. http://dx.doi.org/10.1017/s0074180900166823.
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Five giants and 11 subgiants of the metal-poor globular cluster NGC 6397 are analysed. In this Poster we present the lithium abundances derived. The present Li abundances and those of turnoff stars by Pasquini & Molaro (1996) are complementary in terms of stellar evolution stage, and show the Li abundances decreasing off the main sequence along the red giant branch.
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McClure, R. D. "The Role of Binarity in the Evolution of Peculiar Red Giants." International Astronomical Union Colloquium 106 (1989): 196–204. http://dx.doi.org/10.1017/s0252921100062904.
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There are several types of Peculiar Red Giants (PRG's) which have enhanced s-process elements and/or carbon, but which cannot be explained by mixing during helium shell-flashing in the late stages of Asymptotic Giant Branch (AGB) evolution. These are the Ball, CH, sgCH, and the hotter R-type carbon stars. All these PRG's have absolute magnitudes which range down to zero and fainter. The Ball stars (Bidelman & Keenan 1951) are G-K giants whose spectra have strong CN and CH bands, and in extreme cases bands of C2 become noticeable. In addition they have strong lines of s-process elements such as Ball and SrII. The CH stars (Keenan 1942) are Population II equivalents of the Ball stars, and show similar spectral features, but with weaker metal lines, and usually stronger carbon bands. Bond (1974) introduced a new class of late F and G stars named the CH subgiants, which he suggests are fainter than the classical CH and Ball stars. They appear to be of mixed population (Luck & Bond 1982), and they are probably subgiants which will eventually evolve up the giant branch to become classical CH and Ball stars. The R-type carbon stars, at least the hotter (R0-R4) ones, differ from many of the other peculiar red giants in that they do not show enhanced s-process element abundances. The R stars have strong carbon bands, but relative to the N-type carbon stars they have less blue and ultraviolet absorption in their spectra. They range in absolute magnitude from near zero, similar to the Ball stars, up to several magnitudes brighter than this.
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Reddy, Arumalla B. S. "Omega Centauri: weak MgH band in red giants directly traces the helium content." Monthly Notices of the Royal Astronomical Society 495, no. 1 (May 14, 2020): 383–401. http://dx.doi.org/10.1093/mnras/staa1081.
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ABSTRACT High spectral resolution and high signal-to-noise ratio optical spectra of red giants in the globular cluster Omega Centauri are analysed for stellar parameters and chemical abundances of 15 elements including helium by either line equivalent widths or synthetic spectrum analyses. The simultaneous abundance analysis of MgH and Mg lines adopting theoretical photospheres and a combination of He/H ratios proved to be the only powerful probe to evaluate helium abundances of red giants cooler than 4400 K, wherein otherwise helium line transitions (He i 10830 and 5876 Å) present for a direct spectral line analysis. The impact of helium-enhanced model photospheres on the resulting abundance ratios is smaller than 0.15 dex, in agreement with past studies. The first indirect spectroscopic helium abundances measured in this paper for the most metal-rich cluster members reveal the discovery of seven He-enhanced giants ($\Delta Y=+0.15 \pm 0.04$), the largest such sample found spectroscopically to date. The average metallicity of −0.79 ± 0.06 dex and abundances for O, Na, Al, Si, Ca, Ti, Ni, Ba, and La are consistent with values found for the red giant branch (RGB-a) and subgiant branch (SGB-a) populations of Omega Centauri, suggesting an evolutionary connection among samples. The He enhancement in giants is associated with larger s-process elemental abundances, which correlate with Al and anticorrelate with O. These results support the formation of He-enhanced, metal-rich population of Omega Centauri out of the interstellar medium enriched with the ejecta of fast rotating massive stars, binaries exploding as supernovae, and asymptotic giant branch (AGB) stars.
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Souto, Diogo, C. Allende Prieto, Katia Cunha, Marc Pinsonneault, Verne V. Smith, R. Garcia-Dias, Jo Bovy, et al. "Chemical Abundances of Main-sequence, Turnoff, Subgiant, and Red Giant Stars from APOGEE Spectra. II. Atomic Diffusion in M67 Stars." Astrophysical Journal 874, no. 1 (March 27, 2019): 97. http://dx.doi.org/10.3847/1538-4357/ab0b43.
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Husti, Laura, Roberto Gallino, Sara Bisterzo, Oscar Straniero, and Sergio Cristallo. "Barium Stars: Theoretical Interpretation." Publications of the Astronomical Society of Australia 26, no. 3 (2009): 176–83. http://dx.doi.org/10.1071/as08065.
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AbstractBarium stars are extrinsic Asymptotic Giant Branch (AGB) stars. They present the s-enhancement characteristic for AGB and post-AGB stars, but are in an earlier evolutionary stage (main sequence dwarfs, subgiants, red giants). They are believed to form in binary systems, where a more massive companion evolved faster, produced the s-elements during its AGB phase, polluted the present barium star through stellar winds and became a white dwarf. The samples of barium stars of Allen & Barbuy (2006) and of Smiljanic et al. (2007) are analysed here. Spectra of both samples were obtained at high-resolution and high S/N. We compare these observations with AGB nucleosynthesis models using different initial masses and a spread of 13C-pocket efficiencies. Once a consistent solution is found for the whole elemental distribution of abundances, a proper dilution factor is applied. This dilution is explained by the fact that the s-rich material transferred from the AGB to the nowadays observed stars is mixed with the envelope of the accretor. We also analyse the mass transfer process, and obtain the wind velocity for giants and subgiants with known orbital period. We find evidence that thermohaline mixing is acting inside main sequence dwarfs and we present a method for estimating its depth.
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Siarkowski, M. "Imaging stellar coronae from eclipsing binary X-ray light curves." Symposium - International Astronomical Union 176 (1996): 469–76. http://dx.doi.org/10.1017/s0074180900083480.
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The Sun is the only star whose X-ray emitting, strongly inhomogenous corona can be spatially resolved via direct observations. For other late type-stars it is known that coronae do exist, but the spatial distribution of their emission is largely unknown. However in the case of eclipsing binaries this spatial structure can be potentially deduced from the orbital modulation of the observed X-ray light curve. The best candidates for this kind of analysis are RS CVn binaries, the most active and luminous late-type X-ray coronal sources. These are detached binaries with periods typically between 0.5 and 20 days, in which one or both stars have evolved into subgiant or giant of spectral type G or K. For short orbital periods (< 14 days) the tidal forces lead to synchronization of the orbital and rotational periods, so these systems rotate rigidly.
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Petit, P., J. F. Donati, G. A. Wade, J. D. Landstreet, T. A. A. Sigut, S. L. S. Shorlin, S. Bagnulo, et al. "Surface Differential Rotation of Evolved Fast Rotators." Symposium - International Astronomical Union 215 (2004): 294–99. http://dx.doi.org/10.1017/s0074180900195762.
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Zeeman-Doppler Imaging enables one to estimate the short term temporal evolution of surface brightness and magnetic structures, under the effect of differential rotation. We present here spectropolarimetric observations secured between 1998 and 2002 for two evolved active stars: the K1 subgiant of the RS CVn system HR 1099 and the single FK Com giant HD 199178. Differential rotation is detected both from brightness and magnetic images, indicating that the rotational shear, roughly solar in magnitude for the single star, is significantly weaker in the binary system. This result suggests that tidal forces, rather than stellar evolution, could be responsible for the lower rotational shear and thus the longer spot lifetime reported for binary systems.
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Escorza, A., D. Karinkuzhi, A. Jorissen, L. Siess, H. Van Winckel, D. Pourbaix, C. Johnston, et al. "Barium and related stars, and their white-dwarf companions." Astronomy & Astrophysics 626 (June 2019): A128. http://dx.doi.org/10.1051/0004-6361/201935390.
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Barium (Ba) dwarfs and CH subgiants are the less evolved analogues of Ba and CH giants. They are F- to G-type main-sequence stars polluted with heavy elements by their binary companions when the companion was on the asymptotic giant branch (AGB). This companion is now a white dwarf that in most cases cannot be directly detected. We present a large systematic study of 60 objects classified as Ba dwarfs or CH subgiants. Combining radial-velocity measurements from HERMES and SALT high-resolution spectra with radial-velocity data from CORAVEL and CORALIE, we determine the orbital parameters of 27 systems. We also derive their masses by comparing their location in the Hertzsprung–Russell diagram with evolutionary models. We confirm that Ba dwarfs and CH subgiants are not at different evolutionary stages, and that they have similar metallicities, despite their different names. Additionally, Ba giants appear significantly more massive than their main-sequence analogues. This is likely due to observational biases against the detection of hotter main-sequence post-mass-transfer objects. Combining our spectroscopic orbits with the HIPPARCOS astrometric data, we derive the orbital inclination and the mass of the WD companion for four systems. Since this cannot be done for all systems in our sample yet (but should be possible with upcoming Gaia data releases), we also analyse the mass-function distribution of our binaries. We can model this distribution with very narrow mass distributions for the two components and random orbital orientations on the sky. Finally, based on BINSTAR evolutionary models, we suggest that the orbital evolution of low-mass Ba systems can be affected by a second phase of interactions along the red giant branch of the Ba star, which impact the eccentricities and periods of the giants.
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Caffau, E., L. Monaco, P. Bonifacio, L. Sbordone, M. Haywood, M. Spite, P. Di Matteo, et al. "High-speed stars: Galactic hitchhikers." Astronomy & Astrophysics 638 (June 2020): A122. http://dx.doi.org/10.1051/0004-6361/202038057.
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Context. The search for stars born in the very early stages of the Milky Way star formation history is of paramount importance in the study of the early Universe since their chemistry carries irreplaceable information on the conditions in which early star formation and galaxy buildup took place. The search for these objects has generally taken the form of expensive surveys for faint extremely metal-poor stars, the most obvious but not the only candidates to a very early formation. Aims. Thanks to Gaia DR2 radial velocities and proper motions, we identified 72 bright cool stars displaying heliocentric transverse velocities in excess of 500 km s−1. These objects are most likely members of extreme outer-halo populations, either formed in the early Milky Way build-up or accreted from since-destroyed self-gravitating stellar systems. Methods. We analysed low-resolution FORS spectra of the 72 stars in the sample and derived the abundances of a few elements. Despite the large uncertainties on the radial velocity determination, we derived reliable orbital parameters for these objects. Results. The stars analysed are mainly slightly metal poor, with a few very metal-poor stars. Their chemical composition is much more homogeneous than expected. All the stars have very eccentric halo orbits, some extending well beyond the expected dimension of the Milky Way. Conclusions. These stars can be the result of a disrupted small galaxy or they could have been globular cluster members. Age estimates suggest that some of them are evolved blue stragglers, now on the subgiant or asymptotic giant branches.
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Löbling, L., H. M. J. Boffin, and D. Jones. "Spectral analysis of the barium central star of the planetary nebula Hen 2−39." Astronomy & Astrophysics 624 (April 2019): A1. http://dx.doi.org/10.1051/0004-6361/201834466.
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Context. Barium stars are peculiar red giants characterized by an overabundance of the elements synthesized in the slow neutron-capture nucleosynthesis (s-process elements) along with an enrichment in carbon. These stars are discovered in binaries with white dwarf companions. The more recently formed of these stars are still surrounded by a planetary nebula. Aims. Precise abundance determinations of the various s-process elements, of further key elements that act as indicators for effectiveness of nucleosynthesis on the asymptotic giant branch and, especially, of the lightest, short-lived radionuclide technetium will establish constraints for the formation of s-process elements in asymptotic giant branch stars as well as mass transfer through, for example, stellar wind, Roche-lobe overflow, and common-envelope evolution. Methods. We performed a detailed spectral analysis of the K-type subgiant central star of the planetary nebula Hen 2−39 based on high-resolution optical spectra obtained with the Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope using local thermodynamic equilibrium model atmospheres. Results. We confirm the effective temperature of Teff = (4350 ± 150) K for the central star of the planetary nebula Hen 2−39. It has a photospheric carbon enrichment of [C∕H] = 0.36 ± 0.08 and a barium overabundance of [Ba∕Fe] = 1.8 ± 0.5. We find a deficiency for most of the iron-group elements (calcium to iron) and establish an upper abundance limit for technetium (log ɛTc < 2.5). Conclusions. The quality of the available optical spectra is not sufficient to measure abundances of all s-process elements accurately. Despite large uncertainties on the abundances as well as on the model yields, the derived abundances are most consistent with a progenitor mass in the range 1.75–3.00 M⊙ and a metallicity of [Fe∕H] = −0.3 ± 1.0. This result leads to the conclusion that the formation of such systems requires a relatively large mass transfer that is most easily obtained via wind-Roche lobe overflow.
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Drake, Natalia A., and Claudio B. Pereira. "CNO and Li abundances in Barium-enriched stars." Proceedings of the International Astronomical Union 2, S239 (August 2006): 304–6. http://dx.doi.org/10.1017/s1743921307000610.
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AbstractCNO and Li abundances and 12C/13C isotopic ratios have been derived for two metal-poor Ba giants, HD 104340 and HD 206983, and two CH subgiants, HD 50264 and HD 87080. High resolution spectra obtained with the 1.52 m telescope and the FEROS echelle spectrograph at ESO, La Silla, Chile, were used in this study. CNO and Li abundances so as 12C/13C isotopic ratios were determined by applying the synthetic spectrum method to the lines of C2, CH, CN, [O I’ and Li I. Our analysis showed that the giant stars studied here have quite different natures: HD 206983 is a metal-poor barium star while HD 104340, although showing enhancement of s-process elements, should not be considered as a classical barium star: its barium syndrome can be explained by internal nucleosynthesis. The low metallicity giant HD 104340 can experience deeper convective mixing and, consequently, a larger dredge-up of CNO-cycle products compared to normal red giants. Light element abundance pattern of HD 104340 resembles anomalies resulting from the appearance on the stellar surface of material enriched in triple-α and CNO cycling.
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de la Reza, Ramiro, and Carolina Chavero. "On Lifetimes of Dusty Debris Disks around A-type Stars." Proceedings of the International Astronomical Union 8, S299 (June 2013): 336–37. http://dx.doi.org/10.1017/s1743921313008831.
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AbstractUsing a collection of 40 debris disks (DD) around dwarf A-type stars (from which 17 are spatially resolved) having well determined stellar ages (Vican 2012), we found that for a collisional self-stirring model of flux distributions (Kenyon & Bromley, 2008, 2010) dusty DD occupy for a large part the terminal age positions corresponding the end of the MS phase. This situation does not necessarily mean that the dust generation in these DD is finished at this stage. Dusty DD can continue their lives in the next Post-MS stages, as is the case of the recent first resolved dusty DD with planets around the K-type subgiant star κ CrB. This star is considered a “retired” A star (Bonsor et al. 2013). In order to explain the apparent lack of dusty DD in the giant phase, we propose here that this could be due, in part, to a dilution of any planetesimal belt existing around these stars, with the corresponding diminishing of collisions and subsequent dust generation. This dilution is provoked by the dynamical expansion of this belt produced by the important stellar mass loss during the RGB phase. This scenario is being studied by means of numerical simulations by de la Reza, Roig & Chavero 2013 (in preparation).
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Hirano, Teruyuki, Bun'ei Sato, Kento Masuda, Othman Michel Benomar, Yoichi Takeda, Masashi Omiya, and Hiroki Harakawa. "Search for Close-in Planets around Evolved Stars with Phase-curve variations and Radial Velocity Measurements." Proceedings of the International Astronomical Union 11, A29A (August 2015): 63–64. http://dx.doi.org/10.1017/s1743921316002404.
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AbstractTidal interactions are a key process to understand the evolution history of close-in exoplanets. But tidals still have a large uncertainty in their prediction for the damping timescales of stellar obliquity and semi-major axis. We have worked on a search for transiting giant planets around evolved stars, for which few close-in planets were discovered. It has been reported that evolved stars lack close-in planets, which is often attributed to the tidal evolution and/or engulfment of close-in planets by the hosts. Meanwhile, Kepler has detected a certain fraction of transiting planet candidates around evolved stars. Confirming the planetary nature for these candidates is especially important since the comparison between the occurrence rates of close-in planets around main sequence stars and evolved stars provides a unique opportunity to discuss the final stage of close-in planets. With the aim of confirming KOI planet candidates around evolved stars, we measured precision radial velocities (RVs) for evolved stars with transiting planet candidates using Subaru/HDS. We also developed a new code which simultaneously models and fits the observed RVs and phase-curve variations in the Kepler data (e.g., transits, stellar ellipsoidal variations, and planet emission/reflected light). As a result of applying the global fit to KOI giants/subgiants, we confirmed two giant planets around evolved stars (Kepler-91 and KOI-1894), as well as revealed that KOI-977 is more likely a false positive.
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Bragaglia, Angela. "The quite complex “Simple Stellar Populations” of globular clusters." Proceedings of the International Astronomical Union 5, S268 (November 2009): 119–28. http://dx.doi.org/10.1017/s1743921310003959.
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AbstractThere is compelling observational evidence that globular clusters (GCs) are quite complex objects. A growing body of photometric results indicate that the evolutionary sequences are not simply isochrones in the observational plane -as believed until a few years ago- from the main sequence, to the subgiant, giant, and horizontal branches. The strongest indication of complexity comes however from the chemistry, from internal dispersion in iron abundance in a few cases, and in light elements (C, N, O, Na, Mg, Al, etc.) in all GCs. This universality means that the complexity is intrinsic to the GCs and is most probably related to their formation mechanisms. The extent of the variations in light elements abundances is dependent on the GC mass, but mass is not the only modulating factor; metallicity, age, and possibly orbit can play a role. Finally, one of the many consequences of this new way of looking at GCs is that their stars may show different He contents.
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Villanova, S., G. Piotto, A. F. Marino, A. P. Milone, A. Bellini, L. R. Bedin, Y. Momany, and A. Renzini. "The multipopulation phenomenon in Galactic globular clusters: M4 and M22." Proceedings of the International Astronomical Union 5, S266 (August 2009): 326–32. http://dx.doi.org/10.1017/s1743921309991219.
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AbstractWe present an abundance analysis based on high-resolution spectra of red-giant-branch (RGB) stars in the Galactic globular clusters NGC 6121 (M4) and NGC 6656 (M22). Our aim was to study their stellar population in the context of the multipopulation phenomenon recently discovered to affect some globular clusters. Analysis was performed for the following elements: O, Na, Mg, Al, Ca, Fe, Y, and Ba. Spectroscopic data were completed by high-precision wide-field U BV IC ground-based photometry and HST/ACS observations. For M4, we find a well-defined Na–O anticorrelation composed of two distinct groups of stars with significantly different Na and O content. The two groups of Na-rich and Na-poor stars populate two different regions along the RGB. As regards M22, Na and O follow the well-known anticorrelation found in many other GCs. However, at odds with M4, it appears to be continuous without any hint of clumpiness. On the other hand, we identified two clearly separated groups of stars with significantly different abundances of the s-process elements Y, Zr and Ba. The relative numbers of the members of both groups are very similar to the ratio of the stars in the two subgiant branches of M22 recently found by Piotto (2009). The s-element-rich stars are also richer in iron and have higher Ca abundances. This makes M22 the second cluster after ω Centauri where an intrinsic spread in Fe was found. Both spectroscopic and photometric results imply the presence of two stellar populations in M4 and M22, even if both clusters have completely different characteristics.
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Kučinskas, A., J. Klevas, H. G. Ludwig, P. Bonifacio, M. Steffen, and E. Caffau. "Using the CIFIST grid of CO5BOLD 3D model atmospheres to study the effects of stellar granulation on photometric colours." Astronomy & Astrophysics 613 (May 2018): A24. http://dx.doi.org/10.1051/0004-6361/201732447.
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Aims. We studied the influence of convection on the spectral energy distributions (SEDs), photometric magnitudes, and colour indices of different types of stars across the H–R diagram. Methods. The 3D hydrodynamical CO5BOLD, averaged ⟨3D⟩, and 1D hydrostatic LHD model atmospheres were used to compute SEDs of stars on the main sequence (MS), main sequence turn-off (TO), subgiant branch (SGB), and red giant branch (RGB), in each case at two different effective temperatures and two metallicities, [M∕H] = 0.0 and − 2.0. Using the obtained SEDs, we calculated photometric magnitudes and colour indices in the broad-band Johnson-Cousins UBVRI and 2MASS JHKs, and the medium-band Strömgren uvby photometric systems. Results. The 3D–1D differences in photometric magnitudes and colour indices are small in both photometric systems and typically do not exceed ± 0.03 mag. Only in the case of the coolest giants located on the upper RGB are the differences in the U and u bands able reach ≈−0.2 mag at [M∕H] = 0.0 and ≈−0.1 mag at [M∕H] = −2.0. Generally, the 3D–1D differences are largest in the blue-UV part of the spectrum and decrease towards longer wavelengths. They are also sensitive to the effective temperature and are significantly smaller in hotter stars. Metallicity also plays a role and leads to slightly larger 3D–1D differences at [M∕H] = 0.0. All these patterns are caused by a complex interplay between the radiation field, opacities, and horizontal temperature fluctuations that occur due to convective motions in stellar atmospheres. Although small, the 3D–1D differences in the magnitudes and colour indices are nevertheless comparable to or larger than typical photometric uncertainties and may therefore cause non-negligible systematic differences in the estimated effective temperatures.
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Barker, A. J., C. A. Jones, and S. M. Tobias. "Angular momentum transport, layering, and zonal jet formation by the GSF instability: non-linear simulations at a general latitude." Monthly Notices of the Royal Astronomical Society 495, no. 1 (May 14, 2020): 1468–90. http://dx.doi.org/10.1093/mnras/staa1327.
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ABSTRACT We continue our investigation into the non-linear evolution of the Goldreich–Schubert–Fricke (GSF) instability in differentially rotating radiation zones. This instability may be a key player in transporting angular momentum in stars and giant planets, but its non-linear evolution remains mostly unexplored. In a previous paper we considered the equatorial instability, whereas here we simulate the instability at a general latitude for the first time. We adopt a local Cartesian Boussinesq model in a modified shearing box for most of our simulations, but we also perform some simulations with stress-free, impenetrable, radial boundaries. We first revisit the linear instability and derive some new results, before studying its non-linear evolution. The instability is found to behave very differently compared with its behaviour at the equator. In particular, here we observe the development of strong zonal jets (‘layering’ in the angular momentum), which can considerably enhance angular momentum transport, particularly in axisymmetric simulations. The jets are, in general, tilted with respect to the local gravity by an angle that corresponds initially with that of the linear modes, but which evolves with time and depends on the strength of the flow. The instability transports angular momentum much more efficiently (by several orders of magnitude) than it does at the equator, and we estimate that the GSF instability could contribute to the missing angular momentum transport required in both red giant and subgiant stars. It could also play a role in the long-term evolution of the solar tachocline and the atmospheric dynamics of hot Jupiters.
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Gratton, R., S. Lucatello, E. Carretta, and A. Bragaglia. "Spectroscopic evidence of multiple populations in globular clusters." Proceedings of the International Astronomical Union 10, H16 (August 2012): 230–31. http://dx.doi.org/10.1017/s1743921314005547.
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AbstractWe review spectroscopic evidence of multiple stellar populations in globular clusters. First, we lay down the basic data: the C-N, Na-O, Mg-Al anti-correlations among red giants and main sequence stars, and discuss how they appear to be general properties of globular clusters, in spite of cluster-to-cluster differences. We will then describe what is currently known about He from spectroscopy. We will then present the implications and current observations for the interpretation of the horizontal branches, showing that the multiple population phenomenon is strongly related to the distribution of stars along them. We will briefly mention the spectroscopic evidence related to some less understood cases, like the clusters with multiple subgiant branches. Finally, we summarize the relation between multiple populations and general properties for globular clusters, and their implications for the formation scenario.
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Mosser, B., C. Pinçon, K. Belkacem, M. Takata, and M. Vrard. "Period spacings in red giants." Astronomy & Astrophysics 600 (March 17, 2017): A1. http://dx.doi.org/10.1051/0004-6361/201630053.
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Context. The power of asteroseismology relies on the capability of global oscillations to infer the stellar structure. For evolved stars, we benefit from unique information directly carried out by mixed modes that probe their radiative cores. This third article of the series devoted to mixed modes in red giants focuses on their coupling factors, which have remained largely unexploited up to now. Aims. With the measurement of coupling factors, we intend to give physical constraints on the regions surrounding the radiative core and the hydrogen-burning shell of subgiants and red giants. Methods. A new method for measuring the coupling factor of mixed modes was implemented, which was derived from the method recently implemented for measuring period spacings. This new method was automated so that it could be applied to a large sample of stars. Results. Coupling factors of mixed modes were measured for thousands of red giants. They show specific variation with mass and evolutionary stage. Weak coupling is observed for the most evolved stars on the red giant branch only; large coupling factors are measured at the transition between subgiants and red giants as well as in the red clump. Conclusions. The measurement of coupling factors in dipole mixed modes provides a new insight into the inner interior structure of evolved stars. While the large frequency separation and the asymptotic period spacings probe the envelope and core, respectively, the coupling factor is directly sensitive to the intermediate region in between and helps determine its extent. Observationally, the determination of the coupling factor is a prior to precise fits of the mixed-mode pattern and can now be used to address further properties of the mixed-mode pattern, such as the signature of buoyancy glitches and core rotation.
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Cayrel, Roger. "The Effective Temperature Determination." Highlights of Astronomy 10 (1995): 395–98. http://dx.doi.org/10.1017/s1539299600011527.
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The content of my oral contribution has been reduced here to a guide to the recent litterature on the subject.The effective temperature of a star can be obtained directly by measuring the total flux received on earth from it, and its angular diameter. In principle interferometrie methods could determine both the limb-darkening law of the stellar disk and the angular diameter of the disk. In practice, in order to obtain a decent accuracy on the angular diameter, interferometrie methods have so far borrowed the limb-darkening law from stellar atmosphere theory, in order to concentrate all the observational information on a single parameter. The total number of stellar diameters and effective temperatures obtained by this basic method remains very small (about 30 stars) and concerns exclusively giant or subgiant stars. The reader is invited to look at the following references and those therein (Di Benedetto 1993, Mozurkewich et al. 1991, Alonso et al. 1994). Smalley and Dworetsky (A&A in press) have reinvestigated the old Code et al. (1976) calibration with more recent spatial UV data, for B, A, and F stars, and have found no significant changes. The accuracy of the basic method is at the best of the order of 1.5 %, and there is a deep need for more data, and for doubling the current accuracy. Actually there is a strong effort for developping interferometry both from the ground (ESO VLTI, USNO astrometric interferometer) andfrom space (OSI project, Colavita et al. 1993; GAIA project at ESA). So there is hope for a not to far future.
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Árnadóttir, A. S., S. Feltzing, and I. Lundström. "The ability of intermediate-band Strömgren photometry to correctly identify dwarf, subgiant, and giant stars and provide stellar metallicities and surface gravities." Astronomy and Astrophysics 521 (October 2010): A40. http://dx.doi.org/10.1051/0004-6361/200913544.
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Kraft, Robert P. "Abundance Ratios in Metal-Poor Globular Clusters: Deep Mixing and its Effect on Stellar Populations of the Galactic Halo." Highlights of Astronomy 11, no. 1 (1998): 53–57. http://dx.doi.org/10.1017/s1539299600019961.
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Only a bit more than 25 years ago, it seemed possible to assume that all Galactic globular clusters were chemically homogeneous. There were indications that star-to-star Fe abundance variations existed in ω Cen, but this massive cluster appeared to be unique. Following Osborn’s (1971) initial discovery, Zinn’s (1973) observation that M92 asymptotic giant branch (AGB) stars had weaker G-bands than subgiants with equivalent temperatures provided the first extensive evidence that there might be variations in the abundances of the light elements in an otherwise “normal” cluster. Since then star-to-star variations in the abundances of C, N, O, Na, Mg and Al have been observed in all cases in which sample sizes have exceeded 5-10 stars, e.g., in clusters such as M92, M15, M13, M3, ω Cen, MIO and M5. Among giants in these clusters one finds large surface O abundance differences, and these are intimately related to differences of other light element abundances, not only of C and N, but also of Na, Mg and Al (cf. reviews by Suntzeff 1993, Briley et al 1994, and Kraft 1994). The abundances of Na and O, as well as Al and Mg, are anticorrelated. Prime examples are found among giants in M15 (Sneden et al 1997), M13 (Pilachowski et al 1996; Shetrone 1996a,b; and Kraft et al 1997) and ω Cen (Norris & Da Costa 1995a,b).These observed anticorrelations almost certainly result from proton- capture chains that convert C to N, 0 to N, Ne to Na and Mg to Al in or near the hydrogen fusion layers of evolved cluster stars. But which stars? An appealing idea is that during the giant branch lifetimes of the low-mass stars that we now observe, substantial portions of the stellar envelopes have been cycled through regions near the H-burning shell where proton-capture nucleosynthesis can occur. This so-called “evolutionary” scenario involving deep envelope mixing in first ascent red giant branch (RGB) stars has been studied by Denissenkov & Denissenkova (1990), Langer & Hoffman (1995), Cavallo et al (1996, 1997) and Langer et al (1997). The mixing mechanism that brings proton-capture products to the surface is poorly understood (Denissenkov & Weiss 1996, Denissenkov et al 1997, Langer et al 1997), but deep mixing driven by angular momentum has been suggested (Sweigart & Mengel 1979, Kraft 1994, Langer & Hoffman 1995, Sweigart 1997).
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Cole, Andrew A., Aaron J. Grocholski, Doug Geisler, Ata Sarajedini, Verne V. Smith, and Eline Tolstoy. "Breaking the age–metallicity degeneracy: The metallicity distribution and star formation history of the Large Magellanic Cloud." Proceedings of the International Astronomical Union 4, S256 (July 2008): 263–68. http://dx.doi.org/10.1017/s174392130802855x.
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AbstractWe have obtained metallicities from near-infrared calcium triplet spectroscopy for nearly a thousand red giants in 28 fields spanning a range of radial distances from the center of the bar to near the tidal radius. We have used these data to investigate the radius-metallicity and age-metallicity relations. A powerful application of these data is in conjunction with the analysis of deep HST color–magnitude diagrams (CMDs). Most of the power in determining a robust star-formation history from a CMD comes from the main-sequence turnoff and subgiant branches. The age-metallicity degeneracy that results is largely broken by the red giant branch color, but theoretical model RGB colors remain uncertain. By incorporating the observed metallicity distribution function into the modelling process, a star-formation history with massively increased precision and accuracy can be derived. We incorporate the observed metallicity distribution of the LMC bar into a maximum-likelihood analysis of the bar CMD, and present a new star formation history and age–metallicity relation for the bar. The bar is certainly younger than the disk as a whole, and the most reliable estimates of its age are in the 5–6 Gyr range, when the mean gas abundance of the LMC had already increased to [Fe/H] ≳ −0.6. There is no obvious metallicity gradient among the old stars in the LMC disk out to a distance of 8–10 kpc, but the bar is more metal-rich than the disk by ≈0.1–0.2 dex. This is likely to be the result of the bar's younger average age. In both disk and bar, 95% of the red giants are more metal-rich than [Fe/H] = −1.2.
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Souto, Diogo, Katia Cunha, Verne V. Smith, C. Allende Prieto, D. A. García-Hernández, Marc Pinsonneault, Parker Holzer, et al. "Chemical Abundances of Main-sequence, Turnoff, Subgiant, and Red Giant Stars from APOGEE Spectra. I. Signatures of Diffusion in the Open Cluster M67." Astrophysical Journal 857, no. 1 (April 9, 2018): 14. http://dx.doi.org/10.3847/1538-4357/aab612.
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Stonkutė, R., M. Čeponis, A. Leščinskaitė, R. Naujalis, and V. Vansevičius. "Dwarf irregular galaxy Leo A extends even farther, according to HST WFC3 photometry." Astronomy & Astrophysics 614 (June 2018): A144. http://dx.doi.org/10.1051/0004-6361/201731840.
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Context. Leo A is an isolated gas-rich dwarf irregular galaxy of low stellar mass and metallicity residing at the outskirts of the Local Group. Ages of the stellar populations in Leo A range from ~10 Myr to ~10 Gyr. A large stellar halo (up to the deprojected distance of ~1.7 kpc) was discovered based on Subaru wide-field photometry of red giant branch stars. Aims. We aim to study stellar populations at the very outer region of the Leo A galaxy. Methods. We analysed HST WFC3 archive observation data in the field that is partly located beyond the previously known limits of the Leo A galaxy. We performed photometry of star-like objects in the F475W and F814W passbands and studied the spatial distribution of the Leo A stars below the horizontal branch. Results. We report a discovery of stellar halo populations (subgiants and faint red giants) in the Leo A galaxy extending up to ~2.3 kpc deprojected distance from the galaxy centre. Analysis of the colour-magnitude diagram suggests old (>5 Gyr) stellar populations of very low metallicity (Z ~ 0.0001).
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Shaw, A. W., C. O. Heinke, T. J. Maccarone, G. R. Sivakoff, J. Strader, A. Bahramian, N. Degenaar, et al. "The Swift Bulge Survey: optical and near-IR follow-up featuring a likely symbiotic X-ray binary and a focused wind CV." Monthly Notices of the Royal Astronomical Society 492, no. 3 (January 14, 2020): 4344–60. http://dx.doi.org/10.1093/mnras/staa105.
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ABSTRACT The nature of very faint X-ray transients (VFXTs) – transient X-ray sources that peak at luminosities $L_X\lesssim 10^{36} {\rm \, erg \, s^{-1}}$ – is poorly understood. The faint and often short-lived outbursts make characterizing VFXTs and their multiwavelength counterparts difficult. In 2017 April we initiated the Swift Bulge Survey, a shallow X-ray survey of ∼16 square degrees around the Galactic centre with the Neil Gehrels Swift Observatory. The survey has been designed to detect new and known VFXTs, with follow-up programmes arranged to study their multiwavelength counterparts. Here we detail the optical and near-infrared follow-up of four sources detected in the first year of the Swift Bulge Survey. The known neutron star binary IGR J17445-2747 has a K4III donor, indicating a potential symbiotic X-ray binary nature and the first such source to show X-ray bursts. We also find one nearby M-dwarf (1SXPS J174215.0-291453) and one system without a clear near-IR counterpart (Swift J175233.9-290952). Finally, 3XMM J174417.2-293944 has a subgiant donor, an 8.7 d orbital period, and a likely white dwarf accretor; we argue that this is the first detection of a white dwarf accreting from a gravitationally focused wind. A key finding of our follow-up campaign is that binaries containing (sub)giant stars may make a substantial contribution to the VFXT population.
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Mashonkina, L., T. Sitnova, S. A. Yakovleva, and A. K. Belyaev. "Influence of inelastic collisions with hydrogen atoms on the non-local thermodynamic equilibrium line formation for Fe I and Fe II in the 1D model atmospheres of late-type stars." Astronomy & Astrophysics 631 (October 17, 2019): A43. http://dx.doi.org/10.1051/0004-6361/201935753.
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Context. Iron plays a crucial role in studies of late-type stars. In their atmospheres, neutral iron is the minority species, and lines of Fe I are subject to the departures from local thermodynamic equilibrium (LTE). In contrast, one believes that LTE is a realistic approximation for Fe II lines. The main source of the uncertainties in the non-LTE (NLTE) calculations for cool atmospheres is a treatment of inelastic collisions with hydrogen atoms. Aims. Our aim is to investigate the effect of Fe I + H I and Fe II + H I collisions and their different treatments on the Fe I/Fe II ionisation equilibrium and iron abundance determinations for three Galactic halo benchmark stars (HD 84937, HD 122563, and HD 140283) and a sample of 38 very metal-poor giants in the dwarf galaxies with well known distances. Methods. We performed the NLTE calculations for Fe I–Fe II by applying quantum-mechanical rate coefficients for collisions with H I from recent papers. Results. We find that collisions with H I serve as efficient thermalisation processes for Fe II, to an extent that the NLTE abundance corrections for Fe II lines do not exceed 0.02 dex, in absolute value, for [Fe/H] ≳−3, and reach +0.06 dex at [Fe/H] ~−4. For a given star, different treatments of Fe I + H I collisions lead to similar average NLTE abundances from the Fe I lines, although discrepancies in the NLTE abundance corrections exist for individual lines. By using quantum-mechanical collisional data and the Gaia-based surface gravity, we obtain consistent abundances from the two ionisation stages, Fe I and Fe II, for red giant HD 122563. For turn-off star HD 84937, and subgiant HD 140283, we analyse the iron lines in the visible and the ultra-violet (UV, 1968–2990 Å) ranges. For either Fe I or Fe II, abundances from the visible and UV lines are found to be consistent in each star. The NLTE abundances from the two ionisation stages agree within 0.10 dex and 0.13 dex for two different treatments of Fe I + H I collisions. The Fe I/Fe II ionisation equilibrium is achieved for each star of our stellar sample in the dwarf galaxies, with the exception of stars at [Fe/H] ≲−3.7.
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Niedzielski, A., E. Villaver, M. Adamów, K. Kowalik, A. Wolszczan, and G. Maciejewski. "Tracking Advanced Planetary Systems (TAPAS) with HARPS-N." Astronomy & Astrophysics 648 (April 2021): A58. http://dx.doi.org/10.1051/0004-6361/202037892.
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Context. We present the current status of and new results from our search for exoplanets in a sample of solar-mass evolved stars observed with the HARPS-N and the 3.6 m Telescopio Nazionale Galileo (TNG), and the High-Resolution Spectrograph (HRS) and the 9.2 m Hobby-Eberly Telescope (HET). Aims. The aim of this project is to detect and characterize planetary-mass companions to solar-mass stars in a sample of 122 targets at various stages of evolution from the main sequence to the red giant branch, mostly subgiants and giants, selected from the Pennsylvania-Toruń Planet Search sample, and to use this sample to study relations between stellar properties, such as metallicity, luminosity, and the planet occurrence rate. Methods. This work is based on precise radial velocity (RV) measurements. We have observed the program stars for up to 14 yr with the HET/HRS and the TNG/HARPS-N. Results. We present the analysis of RV measurements with the HET/HRS and the TNG/HARPS-N of four solar-mass stars, HD 4760, HD 96992, BD+02 3313, and TYC 0434-04538-1. We found that HD 4760 hosts a companion with a minimum mass of 13.9 MJ (a = 1.14 au, e = 0.23); HD 96992 is a host to a m sin i = 1.14 MJ companion on an a = 1.24 au and e = 0.41 orbit, and TYC 0434-04538-1 hosts an m sin i = 6.1 MJ companion on an a = 0.66 au and e = 0.08 orbit. In the case of BD+02 3313 we found a correlation between the measured RVs and one of the stellar activity indicators, suggesting that the observed RV variations may either originate in stellar activity or be caused by the presence of an unresolved companion. We also discuss the current status of the project and a statistical analysis of the RV variations in our sample of target stars. Conclusions. In our sample of 122 solar-mass stars, 49 ± 5% of them appear to be single and 16 ± 3% spectroscopic binaries. The three giants hosting low-mass companions presented in this paper join the six previously identified giants in the sample.