Letteratura scientifica selezionata sul tema "Parallasse stellare annua"

In astrometry, construction of stellar catalogues favors solar barycentric coordinates. Classically, they are calculated by linearly removing stellar aberration and annual parallaxes from measured quantities yielded by Earth-bound instruments. A global approach is proposed to account for those two effects from a more general relativistic point of view. Implications for VLBI measurements and HIPPARCOS measurements of stellar positions are discussed.

AbstractWe have carried out multi-epoch VLBI observations of the H2O maser sources associated with young stellar objects (YSOs) in nearby molecular clouds with VERA (VLBI Exploration of Radio Astrometry), which is a newly constructed VLBI network in Japan (Kobayashiet al. 2003). The main goal of our study is to measure the absolute proper motions and distances to nearby molecular clouds within 1 kpc from the Sun, to reveal their 3-dimensional structures and dynamical properties. Using the VERA dual-beam receiving system (Honmaet al. 2003), we have carried out phase-referencing VLBI observations and measured annual parallaxes and absolute proper motions of the H2O maser features with respect to the extragalactic radio sources. We have successfully detected the annual parallax of one of the H2O maser features in Orion KL to be 2.29±0.10 mas, corresponding to the distance of 437±19 pc from the Sun (Hirotaet al. 2007). In addition, the annual parallax of SVS13 in NGC 1333 is also determined to be4.10±0.17 mas, corresponding to the distance of 244±10 pc from the Sun, although the life time of the maser features are only 6 months. The absolute proper motions of the H2O maser features associated with Orion KL and NGC 1333 are derived, possibly indicating the outflow motions from the YSOs as well as the systemic motions of the powering sources.

POINTS is a space-based optical astrometric interferometer, capable of measuring the angular separation of two stars about 90° apart with 5-microarcsec (μas) nominal accuracy. During the intended ten-year mission, a repeated survey of a few hundred targets over the whole sky, including a few bright quasars, will establish a “rigid” reference grid with 0.5 μas position uncertainties. At that level, the grid will be free of regional biases and tied to the extra-Galactic frame that is our present best candidate for an inertial frame. POINTS will also determine parallaxes and annual proper motions at about the same level. Further, the planetary ephemeris frame will be tied through stellar aberration to the grid at about 300 μas. Additional targets of interest, to a limiting magnitude of greater than 20, will be observed relative to the grid, yielding determinations with uncertainties depending on the observing schedule. Measurement at the microarcsec/year level of the apparent relative velocities of quasars that are widely separated on the sky will severely test the assumption of cosmological quasar distances and may also constrain models of the early Universe.

AbstractNano-JASMINE is a nano-size astrometry satellite that will carry out astrometry measurements of nearby bright stars for more than one year. This will enable us to detect annual parallaxes of stars within 300 pc from the Sun. We expect the satellite to be launched as a piggy-back system as early as in 2009 into a Sun synchronized orbit at the altitude between 500 and 800 km. Being equipped with a beam combiner, the satellite has a capability to observe two different fields simultaneously and will be able to carry out HIPPARCOS-type observations along great circles. A 5 cm all aluminum made reflecting telescope with a aluminum beam combiner is developed. Using the on-board CCD controller, experiments with a real star have been executed. A communication band width is insufficient to transfer all imaging data, hence, we developed an onboard data processing system that extracts stellar image data from vast amount of imaging data. A newly developed 2K × 1K fully-depleted CCD will be used for the mission. It will work in the time delayed integration(TDI) mode. The bus system has been designed with special consideration of the following two points. Those are the thermal stabilization of the telescope and the accuracy of the altitude control. The former is essential to achieve high astrometric accuracies, on the order of 1 mas. Therefore relative angle of the beam combiner must be stable within 1 mas. A 3-axes control of the satellite will be realized by using fiber gyro and triaxial reaction wheel system and careful treatment of various disturbing forces.

Abstract We report the results of astrometric VLBI observations toward the Mira variable star BX Cam using the VERA VLBI array. The observation was performed from 2012 February to 2014 November. The parallax obtained is 1.73 ± 0.03 mas corresponding to a distance of 0.58 ± 0.01 kpc. The parallax of this source was reported in Gaia DR2 as 4.13 ± 0.25 mas, and there is a 240% difference between these two measurements. Astrometric results from our VLBI observations show that we exactly traced the angular motions of the seven maser spots in BX Cam. We calculated the stellar luminosities using both parallaxes, and obtained luminosities of $L_{\ast }^{\mathrm{VERA}} = 4950\pm 170\, L_{\odot }$ and $L_{\ast }^{\mathrm{Gaia}} = 870\pm 110\, L_{\odot }$. These deduced luminosities also support the validity of the parallax that we determined with VERA. Evaluating the two parallaxes, we conclude that the parallax of 1.73 ± 0.03 mas from the VERA observations is correct for BX Cam. We obtained the systemic motion of BX Cam as (μαcos δsys, ${\mu }_{\delta }^{\mathrm{sys}}$) = (13.48 ± 0.14, −34.30 ± 0.18) mas yr−1. The total of 73 H2O maser spots detected from our VLBI observations shows a spatial distribution of 30 au × 80 au, with a strong elongation along the north–south direction. They show outflows with a three-dimensional velocity of 14.79 ± 1.40 km s−1. From a comparison between the time variations of the V-band magnitudes and the H2O masers, we found that the variation of the H2O masers is relevant to that seen in the V band even though the H2O masers do not recover their maximum flux in each cycle.

In astrofisica la determinazione delle distanze ha sempre assunto un ruolo molto importante, poiché essa consente una più accurata comprensione della struttura e dell’evoluzione dell’universo in cui ci troviamo. Ci sono molti metodi che possono essere seguiti, a seconda degli oggetti di studio e dello scopo della ricerca. I metodi diretti (geometrici), utilizzabili solo nei dintorni del Sistema Solare, permettono di calibrare quelli indiretti (indicatori primari e secondari), i quali sono utilizzabili a distanze molto maggiori. Ogni metodo consente di accurare il successivo, creando così un meccanismo di misura chiamato scala delle distanze cosmiche, la quale ha come fine ultimo la calibrazione della legge di Hubble, di fondamentale importanza in cosmologia.