88 research outputs found
Direct diameter measurement of a star filling its Roche Lobe: The semi-detached binary SS Leporis spatially resolved with VINCI/VLTI
Stellar evolution in close binary systems is strongly influenced by mass
transfer from one star to the other when one component fills its zero-velocity
surface or Roche Lobe. SS Lep is a fairly nearby close binary showing the Algol
paradox and a shell spectrum, both indicative of (past) mass transfer. To study
the process of mass transfer and its evolutionary consequences, we aim at a
direct characterisation of the spatial dimensions of the different components
of SS Lep with IR interferometry. We use VINCI/VLTI interferometric
observations in the K band and photometric observations from the UV to the
far-IR. The visibilities are interpreted with simple geometrical models and the
Spectral Energy Distribution (SED) is decomposed into the three main
components: A star, M star and dust shell/disk. From the SED, we find that the
main emitters in the K band are the M star and the circumstellar environment.
Both are spatially resolved with the VINCI observations, showing the excess to
be circumbinary and showing the M star to have a size equal to its Roche Lobe.
We conclude that we have, for the first time, directly resolved a star filling
its Roche Lobe. The resulting mass transfer is probably the cause of (1) the
circumbinary dust disk of which we see the hot inner region spatially resolved
in our observations, (2) the unusually high luminosity of the A star and (3)
the shell spectrum seen in the UV and optical spectra.Comment: 4 pages, 2 figures, accepted for publication in A&A Letters on
26/05/200
Imaging the spotty surface of Betelgeuse in the H band
This paper reports on H-band interferometric observations of Betelgeuse made
at the three-telescope interferometer IOTA. We image Betelgeuse and its
asymmetries to understand the spatial variation of the photosphere, including
its diameter, limb darkening, effective temperature, surrounding brightness,
and bright (or dark) star spots. We used different theoretical simulations of
the photosphere and dusty environment to model the visibility data. We made
images with parametric modeling and two image reconstruction algorithms: MIRA
and WISARD. We measure an average limb-darkened diameter of 44.28 +/- 0.15 mas
with linear and quadratic models and a Rosseland diameter of 45.03 +/- 0.12 mas
with a MARCS model. These measurements lead us to derive an updated effective
temperature of 3600 +/- 66 K. We detect a fully-resolved environment to which
the silicate dust shell is likely to contribute. By using two imaging
reconstruction algorithms, we unveiled two bright spots on the surface of
Betelgeuse. One spot has a diameter of about 11 mas and accounts for about 8.5%
of the total flux. The second one is unresolved (diameter < 9 mas) with 4.5% of
the total flux. Resolved images of Betelgeuse in the H band are asymmetric at
the level of a few percent. The MOLsphere is not detected in this wavelength
range. The amount of measured limb-darkening is in good agreement with model
predictions. The two spots imaged at the surface of the star are potential
signatures of convective cells.Comment: 10 pages, 10 figures, accepted for publication in A&A, references
adde
Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles. II. CO line survey of evolved stars: derivation of mass-loss rate formulae
The VLTI/MIDI view on the inner mass loss of evolved stars from the Herschel MESS sample
The mass-loss process from evolved stars is a key ingredient for our
understanding of many fields of astrophysics, including stellar evolution and
the chemical enrichment of the interstellar medium via stellar yields. One the
main unsolved questions is the geometry of the mass-loss process. Taking
advantage of the results from the Herschel Mass loss of Evolved StarS (MESS)
programme, we initiated a coordinated effort to characterise the geometry of
mass loss from evolved red giants at various spatial scales. For this purpose
we used the MID-infrared interferometric Instrument (MIDI) to resolve the inner
envelope of 14 asymptotic giant branch stars (AGBs) in the MESS sample. In this
contribution we present an overview of the interferometric data collected
within the frame of our Large Programme, and we also add archive data for
completeness. We studied the geometry of the inner atmosphere by comparing the
observations with predictions from different geometric models. Asymmetries are
detected for five O-rich and S-type, suggesting that asymmetries in the N band
are more common among stars with such chemistry. We speculate that this fact is
related to the characteristics of the dust grains. Except for one star, no
interferometric variability is detected, i.e. the changes in size of the shells
of non-mira stars correspond to changes of the visibility of less than 10%. The
observed spectral variability confirms previous findings from the literature.
The detection of dust in our sample follows the location of the AGBs in the
IRAS colour-colour diagram: more dust is detected around oxygen-rich stars in
region II and in the carbon stars in region VII. The SiC dust feature does not
appear in the visibility spectrum of UAnt and SSct, which are two carbon stars
with detached shells. This finding has implications for the theory of SiC dust
formation.Comment: 43 pages, 31 figures; accepted for publication in Astronomy &
Astrophysics. Abstract shortened for compilation reasons. Metadata correcte
Pre-maximum spectro-imaging of the Mira star T Lep with AMBER/VLTI
Diffuse envelopes around Mira variables are among the most important sources
influencing the chemical evolution of galaxies. However they represent an
observational challenge because of their complex spectral features and their
rapid temporal variability. We constrained the exact brightness distribution of
the Mira star TLep with a model-independent analysis. We obtained single-epoch
interferometric observations with a dataset continuous in the spectral domain
(1.5-2.4mum) and in the spatial domain (baselines ranging from 11 to 96m). We
performed a model independent image reconstruction for each spectral bin using
the MIRA software. We completed the analysis by modeling the data with a simple
star+layer model inspired from the images. Reconstructed images confirm the
general picture of a central star partially obscured by the surrounding
molecular shell of changing opacity. At 1.7mum, the shell becomes optically
thin, with corresponding emission appearing as a ring circling the star. This
is the first direct evidence of the spherical morphology of the molecular
shell. Model fitting confirmed a spherical layer of constant size and changing
opacity over the wavelengths. Rough modeling points to a continuum opacity
within the shell, in addition to the CO and H2O features. Accordingly, it
appeared impossible to model the data by a photosphere alone in any of the
spectral bins.Comment: Accepted in A&
Periodic mass loss episodes due to an oscillation mode with variable amplitude in the hot supergiant HD50064
We aim to interpret the photometric and spectroscopic variability of the
luminous blue variable supergiant HD\,50064 ().CoRoT space photometry
and follow-up high-resolution spectroscopy, with a time base of 137\,d and
169\,d, respectively, was gathered, analysed and interpreted using standard
time series analysis and light curve modelling methods as well as spectral line
diagnostics.The space photometry reveals one period of 37\,d, which undergoes a
sudden amplitude change with a factor 1.6. The pulsation period is confirmed in
the spectroscopy, which additionally reveals metal line radial velocity values
differing by km\,s depending on the spectral line and on the
epoch. We estimate \teff13\,500\,K, \logg1.5 from the equivalent
width of Si lines. The Balmer lines reveal that the star undergoes episodes of
changing mass loss on a time scale similar to the changes in the photometric
and spectroscopic variability, with an average value of (in M\,yr). We tentatively interpret the 37\,d
period as due to a strange mode oscillation.Comment: 4 pages, accepted for publication in Astronomy & Astrophysics Letter
The 2003-4 multisite photometric campaign for the Beta Cephei and eclipsing star 16 (EN) Lacertae with an Appendix on 2 Andromedae, the variable comparison star
A multisite photometric campaign for the Beta Cephei and eclipsing variable
16 Lacertae is reported. 749 h of high-quality differential photoelectric
Stromgren, Johnson and Geneva time-series photometry were obtained with ten
telescopes during 185 nights. After removing the pulsation contribution, an
attempt was made to solve the resulting eclipse light curve by means of the
computer program EBOP. Although a unique solution was not obtained, the range
of solutions could be constrained by comparing computed positions of the
secondary component in the Hertzsprung-Russell diagram with evolutionary
tracks.
For three high-amplitude pulsation modes, the uvy and the Geneva UBG
amplitude ratios are derived and compared with the theoretical ones for
spherical-harmonic degrees l <= 4. The highest degree, l = 4, is shown to be
incompatible with the observations. One mode is found to be radial, one is l =
1, while in the remaining case l = 2 or 3.
The present multisite observations are combined with the archival photometry
in order to investigate the long-term variation of the amplitudes and phases of
the three high-amplitude pulsation modes. The radial mode shows a
non-sinusoidal variation on a time-scale of 73 yr. The l = 1 mode is a triplet
with unequal frequency spacing, giving rise to two beat-periods, 720.7 d and
29.1 yr. The amplitude and phase of the l = 2 or 3 mode vary on time-scales of
380.5 d and 43 yr.
The light variation of 2 And, one of the comparison stars, is discussed in
the Appendix.Comment: 18 pages, 19 figures, accepted for publication in MNRA
Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles - II. CO line survey of evolved stars: derivation of mass-loss rate formulae
We aim to (1) set up simple and general analytical expressions to estimate
mass-loss rates of evolved stars, and (2) from those calculate estimates for
the mass-loss rates of asymptotic giant branch (AGB), red supergiant (RSG), and
yellow hypergiant stars in our galactic sample. Rotationally excited lines of
CO are a very robust diagnostic in the study of circumstellar envelopes (CSEs).
When sampling different layers of the CSE, observations of these molecular
lines lead to detailed profiles of kinetic temperature, expansion velocity, and
density. A state-of-the-art, nonlocal thermal equilibrium, and co-moving frame
radiative transfer code that predicts CO line intensities in the CSEs of
late-type stars is used in deriving relations between stellar and
molecular-line parameters, on the one hand, and mass-loss rate, on the other.
We present analytical expressions for estimating the mass-loss rates of evolved
stellar objects for 8 rotational transitions of the CO molecule, apply them to
our extensive CO data set covering 47 stars, and compare our results to those
of previous studies. Our expressions account for line saturation and resolving
of the envelope, thereby allowing accurate determination of very high mass-loss
rates. We argue that, for estimates based on a single rotational line, the
CO(2-1) transition provides the most reliable mass-loss rate. The mass-loss
rates calculated for the AGB stars range from 4x10^-8 Msun/yr up to 8x10^-5
Msun/yr. For RSGs they reach values between 2x10^-7 Msun/yr and 3x10^-4
Msun/yr. The estimates for the set of CO transitions allow time variability to
be identified in the mass-loss rate. Possible mass-loss-rate variability is
traced for 7 of the sample stars. We find a clear relation between the
pulsation periods of the AGB stars and their derived mass-loss rates, with a
levelling off at approx. 3x10^-5 Msun/yr for periods exceeding 850 days.Comment: Accepted for publication by Astronomy and Astrophysics, 24 pages + 28
pages appendix, 20 figure
The mass-loss rates of red supergiants and the de Jager prescription
Mass loss of red supergiants (RSG) is important for the evolution of massive
stars, but is not fully explained. Several empirical prescriptions have been
proposed, trying to express the mass-loss rate (Mdot) as a function of
fundamental stellar parameters (mass, luminosity, effective temperature). Our
goal is to test whether the de Jager et al. (1988) prescription, used in some
stellar evolution models, is still valid in view of more recent mass-loss
determinations. By considering 40 Galactic RSGs presenting an infrared excess
and an IRAS 60-mu flux larger than 2 Jy, and assuming a gas-to-dust mass ratio
of 200, it is found that the de Jager rate agrees within a factor 4 with most
Mdot estimates based on the 60-mu signal. It is also in agreement with 6 of the
only 8 Galactic RSGs for which Mdot can be measured more directly through
observations of the circumstellar gas. The two objects that do not follow the
de Jager prescription (by an order of magnitude) are mu Cep and NML Cyg. We
have also considered the RSGs of the Magellanic Clouds. Thanks to the works of
Groenewegen et al. (2009) and Bonanos et al. (2010), we find that the RSGs of
the SMC have Mdots consistent with the de Jager rate scaled by
(Z/Zsun)**(alpha), where Z is the metallicity and alpha is 0.7. The situation
is less clear for the LMC RSGs. In particular, for luminosties larger than
1.6E+05 Lsun, one finds numerous RSGs (except WOH-G64) having Mdot
significantly smaller than the de Jager rate, and indicating that Mdot would no
longer increase with L. Before this odd situation is confirmed through further
analysis of LMC RSGs, we suggest to keep the de Jager prescription unchanged at
solar metallicity in the stellar evolutionary models and to apply a
(Z/Zsun)**0.7 dependence.Comment: 13 pages, 9 figures. Accepted by Astronomy and Astrophysic
A mid-infrared imaging survey of post-AGB stars
Post-AGB stars are key objects for the study of the dramatic morphological changes of low- to intermediate-mass stars on their evolution from the Asymptotic Giant Branch (AGB) towards the planetary nebula stage. There is growing evidences that binary interaction processes may very well have a determining role in the shaping process of many objects, but so far direct evidence is still weak. We aim at a systematic study of the dust distribution around a large sample of post-AGB stars as a probe of the symmetry breaking in the nebulae around these systems. We used imaging in the mid-infrared to study the inner part of these evolved stars to probe direct emission from dusty structures in the core of post-AGB stars in order to better understand their shaping mechanisms. We imaged a sample of 93 evolved stars and nebulae in the mid-infrared using VISIR/VLT, T-Recs/Gemini South and Michelle/Gemini North. We found that all the the proto-planetary nebulae we resolved show a clear departure from spherical symmetry. 59 out of the 93 observed targets appear to be non resolved. The resolved targets can be divided in two categories. The nebulae with a dense central core, that are either bipolar and multipolar. The nebulae with no central core have an elliptical morphology. The dense central torus observed likely host binary systems which triggered fast outflows that shaped the nebulae
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