BA-type supergiants are amongst the most optically-bright stars. They are observable in extragalactic environments, hence potential accurate distance indicators. Emission activity in the Halpha line ...of the BA supergiants Rigel (B8Ia) and Deneb (A2Ia) is indicative of presence of localized time-dependent mass ejections. Here, we employ optical interferometry to study the Halpha line-formation region in these stellar environments. High spatial- (0.001 arcsec) and spectral- (R=30 000) resolution observations of Halpha were obtained with the visible recombiner VEGA installed on the CHARA interferometer, using the S1S2 array-baseline (34m). Six independent observations were done on Deneb over the years 2008 and 2009, and two on Rigel in 2009. We analyze this dataset with the 1D non-LTE radiative-transfer code CMFGEN, and assess the impact of the wind on the visible and near-IR interferometric signatures, using both Balmer-line and continuum photons. We observe a visibility decrease in Halpha for both Rigel and Deneb, suggesting that the line-formation region is extended (1.5-1.75 R*). We observe a significant visibility decrease for Deneb in the SiII6371 line. We witness time variations in the differential phase for Deneb, implying an inhomogeneous and unsteady circumstellar environment, while no such variability is seen in differential visibilities. Radiative-transfer modeling of Deneb, with allowance for stellar-wind mass loss, accounts fairly well for the observed decrease in the Halpha visibility. Based on the observed differential visibilities, we estimate that the mass-loss rate of Deneb has changed by less than 5%.
Studying exoplanet host stars is of the utmost importance to establish the link between the presence of exoplanets around various types of stars and to understand the respective evolution of stars ...and exoplanets. Using the limb-darkened diameter (LDD) obtained from interferometric data, we determine the fundamental parameters of four exoplanet host stars. We are particularly interested in the F4 main-sequence star, {\theta} Cyg, for which Kepler has recently revealed solar-like oscillations that are unexpected for this type of star. Furthermore, recent photometric and spectroscopic measurements with SOPHIE and ELODIE (OHP) show evidence of a quasi-periodic radial velocity of \sim150 days. Models of this periodic change in radial velocity predict either a complex planetary system orbiting the star, or a new and unidentified stellar pulsation mode. We performed interferometric observations of {\theta} Cyg, 14 Andromedae, {\upsilon} Andromedae and 42 Draconis for two years with VEGA/CHARA (Mount Wilson, California) in several three-telescope configurations. We measured accurate limb darkened diameters and derived their radius, mass and temperature using empirical laws. We obtain new accurate fundamental parameters for stars 14 And, {\upsilon} And and 42 Dra. We also obtained limb darkened diameters with a minimum precision of \sim 1.3%, leading to minimum planet masses of Msini=5.33\pm 0.57, 0.62 \pm 0.09 and 3.79\pm0.29 MJup for 14 And b, {\upsilon} And b and 42 Dra b, respectively. The interferometric measurements of {\theta} Cyg show a significant diameter variability that remains unexplained up to now. We propose that the presence of these discrepancies in the interferometric data is caused by either an intrinsic variation of the star or an unknown close companion orbiting around it.
A good knowledge of the angular diameters of stars used to calibrate the observables in stellar interferometry is fundamental. As the available precision for giant stars is worse than the required ...per cent level, we aim to improve the knowledge of many diameters using MATISSE (Multiple AperTure mid-Infrared SpectroScopic Experiment) data in its different instrumental configurations. Using the squared visibility MATISSE observable, we compute the angular diameter value, which ensures the best-fitting curves, assuming an intensity distribution of a uniform disc. We take into account that the transfer function varies over the wavelength and is different from one instrumental configuration to another. The uncertainties on the diameters are estimated using the residual bootstrap method. Using the low spectral resolution mode in the L band, we observed a set of 35 potential calibrators selected in the Mid-infrared stellar Diameter and Flux Compilation Catalogue with diameters ranging from about 1 to 3 mas. We reach a precision on the diameter estimates in the range 0.6 per cent to 4.1 per cent. The study of the stability of the transfer function in visibility over two nights makes us confident in our results. In addition, we identify one star, 75 Vir initially present in the calibrator lists, for which our method does not converge, and prove to be a binary star. This leads us to the conclusion that our method is actually necessary to improve the quality of the astrophysical results obtained with MATISSE, and that it can be used as a useful tool for 'bad calibrator' detection.
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.
Context: FS Canis Majoris (FS CMa, HD 45677) is an unclassified Be star surrounded by an inclined dust disk. The evolutionary stage of FS CMa is still debated. Perpendicular to the circumstellar ...disk, a bipolar outflow was detected. Infrared aperture-synthesis imaging provides us with a unique opportunity to study the disk structure. Aims: Our aim is to study the intensity distribution of the disk of FS CMa in the mid-infrared L and N bands. Methods: We performed aperture-synthesis imaging of FS CMa with the MATISSE instrument (Multi AperTure mid-Infrared SpectroScopic Experiment) in the low spectral resolution mode to obtain images in the L and N bands. We computed radiative transfer models that reproduce the L- and N-band intensity distributions of the resolved disks. Results: We present L- and N-band aperture-synthesis images of FS CMa reconstructed in the wavelength bands of 3.4-3.8 and 8.6-9.0 micrometer. In the L-band image, the inner rim region of an inclined circumstellar disk and the central object can be seen with a spatial resolution of 2.7 milliarcsec (mas). An inner disk cavity with an angular diameter of 6x12mas is resolved. The L-band disk consists of a bright northwestern (NW) disk region and a much fainter southeastern (SE) region. The images suggest that we are looking at the bright inner wall of the NW disk rim, which is on the far side of the disk. In the N band, only the bright NW disk region is seen. In addition to deriving the inclination and the inner disk radius, fitting the reconstructed brightness distributions via radiative transfer modeling allows one to constrain the innermost disk structure, in particular the shape of the inner disk rim.
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.
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.
PDF
