102 research outputs found
The fundamental parameters of the roAp star Equulei
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, 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~~0.017~mas and deduced a radius of
~=~2.20~~0.12~. Without considering the multiple
nature of the system, we derived a bolometric flux of erg~cm~s and an effective temperature of
7364~~235~K, which is below the effective temperature that has been
previously determined. Under the same conditions we found a luminosity of
~=~12.8~~1.4~. 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
~100~K and up to ~0.8~L 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.Comment: Accepted by A&
Time, spatial, and spectral resolution of the Halpha line-formation region of Deneb and Rigel with the VEGA/CHARA interferometer
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%
Tests with a Carlina-type diluted telescope; Primary coherencing
Studies are under way to propose a new generation of post-VLTI
interferometers. The Carlina concept studied at the Haute- Provence Observatory
is one of the proposed solutions. It consists in an optical interferometer
configured like a diluted version of the Arecibo radio telescope: above the
diluted primary mirror made of fixed cospherical segments, a helium balloon (or
cables suspended between two mountains), carries a gondola containing the focal
optics. Since 2003, we have been building a technical demonstrator of this
diluted telescope. First fringes were obtained in May 2004 with two
closely-spaced primary segments and a CCD on the focal gondola. We have been
testing the whole optical train with three primary mirrors. The main aim of
this article is to describe the metrology that we have conceived, and tested
under the helium balloon to align the primary mirrors separate by 5-10 m on the
ground with an accuracy of a few microns. The servo loop stabilizes the mirror
of metrology under the helium balloon with an accuracy better than 5 mm while
it moves horizontally by 30 cm in open loop by 10-20 km/h of wind. We have
obtained the white fringes of metrology; i.e., the three mirrors are aligned
(cospherized) with an accuracy of {\approx} 1 micron. We show data proving the
stability of fringes over 15 minutes, therefore providing evidence that the
mechanical parts are stabilized within a few microns. This is an important step
that demonstrates the feasibility of building a diluted telescope using cables
strained between cliffs or under a balloon. Carlina, like the MMT or LBT, could
be one of the first members of a new class of telescopes named diluted
telescopes.Comment: 18 pages, 17 figures, A&A, accepte
An investigation of the close environment of beta Cep with the VEGA/CHARA interferometer
High-precision interferometric measurements of pulsating stars help to
characterize their close environment. In 1974, a close companion was discovered
around the pulsating star beta Cep using the speckle interferometry technique
and features at the limit of resolution (20 milli-arcsecond or mas) of the
instrument were mentioned that may be due to circumstellar material. Beta Cep
has a magnetic field that might be responsible for a spherical shell or
ring-like structure around the star as described by the MHD models. Using the
visible recombiner VEGA installed on the CHARA long-baseline interferometer at
Mt. Wilson, we aim to determine the angular diameter of beta Cep and resolve
its close environment with a spatial resolution up to 1 mas level. Medium
spectral resolution (R=6000) observations of beta Cep were secured with the
VEGA instrument over the years 2008 and 2009. These observations were performed
with the S1S2 (30m) and W1W2 (100m) baselines of the array. We investigated
several models to reproduce our observations. A large-scale structure of a few
mas is clearly detected around the star with a typical flux relative
contribution of 0.23 +- 0.02. Our best model is a co-rotational geometrical
thin ring around the star as predicted by magnetically-confined wind shock
models. The ring inner diameter is 8.2 +- 0.8 mas and the width is 0.6 +- 0.7
mas. The orientation of the rotation axis on the plane of the sky is PA = 60 +-
1 deg, while the best fit of the mean angular diameter of beta Cep gives UD[V]
= 0.22 +- 0.05 mas. Our data are compatible with the predicted position of the
close companion of beta Cep. These results bring additional constraints on the
fundamental parameters and on the future MHD and asteroseismological models of
the star.Comment: Paper accepted for publication in A&A (in press
Chromosphere of K giant stars Geometrical extent and spatial structure detection
We aim to constrain the geometrical extent of the chromosphere of non-binary
K giant stars and detect any spatial structures in the chromosphere. We
performed observations with the CHARA interferometer and the VEGA beam combiner
at optical wavelengths. We observed seven non-binary K giant stars. We measured
the ratio of the radii of the photosphere to the chromosphere using the
interferometric measurements in the Halpha and the Ca II infrared triplet line
cores. For beta Ceti, spectro-interferometric observations are compared to an
non-local thermal equilibrium (NLTE) semi-empirical model atmosphere including
a chromosphere. The NLTE computations provide line intensities and contribution
functions that indicate the relative locations where the line cores are formed
and can constrain the size of the limb-darkened disk of the stars with
chromospheres. We measured the angular diameter of seven K giant stars and
deduced their fundamental parameters: effective temperatures, radii,
luminosities, and masses. We determined the geometrical extent of the
chromosphere for four giant stars. The chromosphere extents obtained range
between 16% to 47% of the stellar radius. The NLTE computations confirm that
the Ca II/849 nm line core is deeper in the chromosphere of ? Cet than either
of the Ca II/854 nm and Ca II/866 nm line cores. We present a modified version
of a semi-empirical model atmosphere derived by fitting the Ca II triplet line
cores of this star. In four of our targets, we also detect the signature of a
differential signal showing the presence of asymmetries in the chromospheres.
Conclusions. It is the first time that geometrical extents and structure in the
chromospheres of non-binary K giant stars are determined by interferometry.
These observations provide strong constrains on stellar atmosphere models.Comment: 10 pages, 12 figure
Autómata de Lattice Boltzmann para modelar la difusión óptica en materiales translúcidos
La interrogación de objetos traslúcidos mediante luz láser en el rango infrarrojo cercano es una técnica para recabar información tomográfica que está siendo usada cada vez más en diagnóstico médico y en inspecciones industriales. En este trabajo se presenta una estrategia para la simulación de la difusión de luz visible en materiales translúcidos basada en el método de Lattice Bolzmann (LBM). LBM es un autómata celular que simula fenómenos de transporte a nivel macroscópico mediante una representación mesoscópica, muy fácil de implementar y altamente paralelizable. En nuestro caso el transporte de fotones en la materia se modela mediante una matriz de colisión y absorción definida en cada celda del dominio espacial simulado. La grilla de soporte es tridimensional y los resultados son visualizados superponiendo los elementos de una malla triangular. El modelo fue validado con datos experimentales medidos en un fantoma de laboratorio. Se presentan también las posibles aplicaciones del autómata en un motor de visualizaciónSociedad Argentina de Informática e Investigación Operativ
The H line forming region of AB Aur spatially resolved at sub-AU with the VEGA/CHARA spectro-interferometer
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 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 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. 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 =
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 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
First AMBER/VLTI observations of hot massive stars
AMBER is the first near infrared focal instrument of the VLTI. It combines
three telescopes and produces spectrally resolved interferometric measures.
This paper discusses some preliminary results of the first scientific
observations of AMBER with three Unit Telescopes at medium (1500) and high
(12000) spectral resolution. We derive a first set of constraints on the
structure of the circumstellar material around the Wolf Rayet Gamma2 Velorum
and the LBV Eta Carinae
Spectrally resolved interferometric observations of α Cephei and physical modeling of fast rotating stars
This is the final version of the article. Available from the publisher via the DOI in this record.Context. When a given observational quantity depends on several stellar physical parameters, it is generally very difficult to obtain observational constraints for each of them individually. Therefore, we studied under which conditions constraints for some individual parameters can be achieved for fast rotators, knowing that their geometry is modified by the rapid rotation which causes a non-uniform surface brightness distribution.
Aims. We aim to study the sensitivity of interferometric observables on the position angle of the rotation axis (PA) of a rapidly rotating star, and whether other physical parameters can influence the determination of PA, and also the influence of the surface differential rotation on the determination of the β exponent in the gravity darkening law that enters the interpretation of interferometric observations, using α Cep as a test star.
Methods. We used differential phases obtained from observations carried out in the Hα absorption line of α Cep with the VEGA/CHARA interferometer at high spectral resolution, R = 30 000 to study the kinematics in the atmosphere of the star.
Results. We studied the influence of the gravity darkening effect (GDE) on the determination of the PA of the rotation axis of α Cep and determined its value, PA = −157-10°+17°. We conclude that the GDE has a weak influence on the dispersed phases. We showed that the surface differential rotation can have a rather strong influence on the determination of the gravity darkening exponent. A new method of determining the inclination angle of the stellar rotational axis is suggested. We conclude that differential phases obtained with spectro-interferometry carried out on the Hα line can in principle lead to an estimate of the stellar inclination angle i. However, to determine both i and the differential rotation parameter α, lines free from the Stark effect and that have collision-dominated source functions are to be preferred.VEGA is a collaboration between CHARA and
OCA/LAOG/CRAL/LESIA that has been supported by the French programs
PNPS and ASHRA, by INSU and by the Région PACA. The project has
obviously benefitted from the strong support of the OCA and CHARA technical
teams. The CHARA Array is operated with support from the National Science
Foundation through grant AST-0908253, the W. M. Keck Foundation, the
NASA Exoplanet Science Institute, and from Georgia State University. This
work has made use of the BeSS database, operated at GEPI, Observatoire de
Meudon, France: http://basebe.obspm.fr, use of the Jean-Marie Mariotti
Center SearchCal service1 co-developed by FIZEAU and LAOG, and of
CDS Astronomical Databases SIMBAD and VIZIER2. We are grateful to an
anonymous referee for her/his valuable suggestions that helped to improve the
presentation of our results
Direct constraint on the distance of y2 Velorum from AMBER/VLTI observations
In this work, we present the first AMBER observations, of the Wolf-Rayet and
O (WR+O) star binary system y2 Velorum. The AMBER instrument was used with the
telescopes UT2, UT3, and UT4 on baselines ranging from 46m to 85m. It delivered
spectrally dispersed visibilities, as well as differential and closure phases,
with a resolution R = 1500 in the spectral band 1.95-2.17 micron. We interpret
these data in the context of a binary system with unresolved components,
neglecting in a first approximation the wind-wind collision zone flux
contribution. We show that the AMBER observables result primarily from the
contribution of the individual components of the WR+O binary system. We discuss
several interpretations of the residuals, and speculate on the detection of an
additional continuum component, originating from the free-free emission
associated with the wind-wind collision zone (WWCZ), and contributing at most
to the observed K-band flux at the 5% level. The expected absolute separation
and position angle at the time of observations were 5.1±0.9mas and
66±15° respectively. However, we infer a separation of
3.62+0.11-0.30 mas and a position angle of 73+9-11°. Our analysis thus
implies that the binary system lies at a distance of 368+38-13 pc, in agreement
with recent spectrophotometric estimates, but significantly larger than the
Hipparcos value of 258+41-31 pc
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