Physical processes working in the stellar interiors as well as the evolution
of stars depend on some fundamental stellar properties, such as mass, radius,
luminosity, and chemical abundances. A ...classical way to test stellar interior
models is to compare the predicted and observed location of a star on
theoretical evolutionary tracks in a H-R diagram. This requires the best
possible determinations of stellar mass, radius, luminosity and abundances. To
derive its fundamental parameters, we observed the well-known rapidly
oscillating Ap star, $\gamma$ Equ, using the visible spectro-interferometer
VEGA installed on the optical CHARA array. We computed the calibrated squared
visibility and derived the limb-darkened diameter. We used the whole energy
flux distribution, the parallax and this angular diameter to determine the
luminosity and the effective temperature of the star. We obtained a
limb-darkened angular diameter of 0.564~$\pm$~0.017~mas and deduced a radius of
$R$~=~2.20~$\pm$~0.12~${\rm R_{\odot}}$. Without considering the multiple
nature of the system, we derived a bolometric flux of $(3.12\pm 0.21)\times
10^{-7}$ erg~cm$^{-2}$~s$^{-1}$ and an effective temperature of
7364~$\pm$~235~K, which is below the effective temperature that has been
previously determined. Under the same conditions we found a luminosity of
$L$~=~12.8~$\pm$~1.4~${\rm L_{\odot}}$. When the contribution of the closest
companion to the bolometric flux is considered, we found that the effective
temperature and luminosity of the primary star can be, respectively, up to
$\sim$~100~K and up to $\sim$~0.8~L$_\odot$ smaller than the values mentioned
above.These new values of the radius and effective temperature should bring
further constraints on the asteroseismic modelling of the star.
A crucial issue in star formation is to understand the physical mechanism by
which mass is accreted onto and ejected by a young star. The visible
spectrometer VEGA on the CHARA array can be an ...efficient means of probing the
structure and the kinematics of the hot circumstellar gas at sub-AU. For the
first time, we observed the Herbig Ae star AB Aur in the H$\alpha$ emission
line, using the VEGA low spectral resolution on two baselines of the array. We
computed and calibrated the spectral visibilities between 610 nm and 700 nm. To
simultaneously reproduce the line profile and the visibility, we used a 1-D
radiative transfer code that calculates level populations for hydrogen atoms in
a spherical geometry and synthetic spectro-interferometric observables. We
clearly resolved AB Aur in the H$\alpha$ line and in a part of the continuum,
even at the smallest baseline of 34 m. The small P-Cygni absorption feature is
indicative of an outflow but could not be explained by a spherical stellar wind
model. Instead, it favors a magneto-centrifugal X-disk or disk-wind geometry.
The fit of the spectral visibilities could not be accounted for by a wind
alone, so we considered a brightness asymmetry possibly caused by large-scale
nebulosity or by the known spiral structures, inducing a visibility modulation
around H$\alpha$. Thanks to the unique capabilities of VEGA, we managed to
simultaneously record for the first time a spectrum at a resolution of 1700 and
spectral visibilities in the visible range on a target as faint as $m_{V}$ =
7.1. It was possible to rule out a spherical geometry for the wind of AB Aur
and provide realistic solutions to account for the H$\alpha$ emission
compatible with magneto-centrifugal acceleration. The study illustrates the
advantages of optical interferometry and motivates observations of other bright
young stars to shed light on the accretion/ejection processes.
The Solar System Odyssey mission uses modern-day high-precision experimental techniques to test the laws of fundamental physics which determine dynamics in the solar system. It could lead to major ...discoveries by using demonstrated technologies. The mission proposes to perform a set of precision gravitation experiments from the vicinity of Earth to the outer Solar System. Its scientific objectives can be summarized as follows: i) test of the gravity force law in the Solar System up to and beyond the orbit of Saturn; ii) precise investigation of navigation anomalies at the fly-bys; iii) measurement of Eddington's parameter at occultations; iv) mapping of gravity field in the outer solar system and study of the Kuiper belt. To this aim, the Odyssey mission is built up on a main spacecraft, designed to fly up to 13 AU, with the following components: a) a high-precision accelerometer, with bias-rejection system, measuring the deviation of the trajectory from the geodesics; b) Ka-band transponders, as for Cassini, for a precise range and Doppler measurement up to 13 AU, with additional VLBI equipment; c) optional laser equipment, which would allow one to improve the range and Doppler measurement. In this baseline concept, the main spacecraft is designed to operate beyond the Saturn orbit, up to 13 AU. It experiences multiple planetary fly-bys at Earth, Mars or Venus, and Jupiter. The cruise and fly-by phases allow the mission to achieve its baseline scientific objectives (i) to iii) in the above list). In addition to this baseline concept, the Odyssey mission proposes the release of the Enigma radio-beacon at Saturn, allowing one to extend the deep space gravity test up to at least 50 AU, while achieving the scientific objective of a mapping of gravity field in the outer Solar System.
Physical processes working in the stellar interiors as well as the evolution of stars depend on some fundamental stellar properties, such as mass, radius, luminosity, and chemical abundances. A ...classical way to test stellar interior models is to compare the predicted and observed location of a star on theoretical evolutionary tracks in a H-R diagram. This requires the best possible determinations of stellar mass, radius, luminosity and abundances. To derive its fundamental parameters, we observed the well-known rapidly oscillating Ap star, \(\gamma\) Equ, using the visible spectro-interferometer VEGA installed on the optical CHARA array. We computed the calibrated squared visibility and derived the limb-darkened diameter. We used the whole energy flux distribution, the parallax and this angular diameter to determine the luminosity and the effective temperature of the star. We obtained a limb-darkened angular diameter of 0.564~\(\pm\)~0.017~mas and deduced a radius of \(R\)~=~2.20~\(\pm\)~0.12~\({\rm R_{\odot}}\). Without considering the multiple nature of the system, we derived a bolometric flux of \((3.12\pm 0.21)\times 10^{-7}\) erg~cm\(^{-2}\)~s\(^{-1}\) and an effective temperature of 7364~\(\pm\)~235~K, which is below the effective temperature that has been previously determined. Under the same conditions we found a luminosity of \(L\)~=~12.8~\(\pm\)~1.4~\({\rm L_{\odot}}\). When the contribution of the closest companion to the bolometric flux is considered, we found that the effective temperature and luminosity of the primary star can be, respectively, up to \(\sim\)~100~K and up to \(\sim\)~0.8~L\(_\odot\) smaller than the values mentioned above.These new values of the radius and effective temperature should bring further constraints on the asteroseismic modelling of the star.
A crucial issue in star formation is to understand the physical mechanism by which mass is accreted onto and ejected by a young star. The visible spectrometer VEGA on the CHARA array can be an ...efficient means of probing the structure and the kinematics of the hot circumstellar gas at sub-AU. For the first time, we observed the Herbig Ae star AB Aur in the H\(\alpha\) emission line, using the VEGA low spectral resolution on two baselines of the array. We computed and calibrated the spectral visibilities between 610 nm and 700 nm. To simultaneously reproduce the line profile and the visibility, we used a 1-D radiative transfer code that calculates level populations for hydrogen atoms in a spherical geometry and synthetic spectro-interferometric observables. We clearly resolved AB Aur in the H\(\alpha\) line and in a part of the continuum, even at the smallest baseline of 34 m. The small P-Cygni absorption feature is indicative of an outflow but could not be explained by a spherical stellar wind model. Instead, it favors a magneto-centrifugal X-disk or disk-wind geometry. The fit of the spectral visibilities could not be accounted for by a wind alone, so we considered a brightness asymmetry possibly caused by large-scale nebulosity or by the known spiral structures, inducing a visibility modulation around H\(\alpha\). Thanks to the unique capabilities of VEGA, we managed to simultaneously record for the first time a spectrum at a resolution of 1700 and spectral visibilities in the visible range on a target as faint as \(m_{V}\) = 7.1. It was possible to rule out a spherical geometry for the wind of AB Aur and provide realistic solutions to account for the H\(\alpha\) emission compatible with magneto-centrifugal acceleration. The study illustrates the advantages of optical interferometry and motivates observations of other bright young stars to shed light on the accretion/ejection processes.
