Issue |
A&A
Volume 511, February 2010
|
|
---|---|---|
Article Number | A51 | |
Number of page(s) | 8 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361/200913288 | |
Published online | 09 March 2010 |
VLTI/AMBER spectro-interferometric
imaging of VX Sagittarii's inhomogenous outer
atmosphere![[*]](/icons/foot_motif.png)
A. Chiavassa1,2 - S. Lacour3 - F. Millour4 - T. Driebe4 - M. Wittkowski5 - B. Plez2 - E. Thiébaut6 - E. Josselin2 - B. Freytag7,8 - M. Scholz9,10 - X. Haubois3
1 - Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1,
Postfach 1317, 85741 Garching b. München, Germany
2 - GRAAL, Université de Montpellier II - IPM, CNRS, Place Eugène
Bataillon 34095 Montpellier Cedex 05, France
3 - Observatoire de Paris, LESIA, CNRS/UMR 8109, 92190 Meudon, France
4 - Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121
Bonn, Germany
5 - ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
6 - AIRI/Observatoire de Lyon, France and Jean-Marie Mariotti Center,
France
7 - Centre de Recherche Astrophysique de Lyon, UMR 5574: CNRS,
Université de Lyon, École Normale Supérieure de Lyon, 46 allée
d'Italie, 69364 Lyon Cedex 07, France
8 - Department of Physics and Astronomy, Division of Astronomy and
Space Physics, Uppsala University, Box 515, 751 20 Uppsala,
Sweden
9
- Zentrum für Astronomie der Universität Heidelberg (ZAH), Institut für
Theoretische Astrophysik, Albert Ueberle-Str. 2, 69120 Heidelberg,
Germany
10 - Sydney Institute for Astronomy, School of Physics, University of
Sydney, Sydney, NSW 2006, Australia
Received 13 September 2009 / Accepted 23 November 2009
Abstract
Aims. We aim to explore the photosphere of the very
cool
late-type star VX Sgr and in particular the
characterization
of molecular layers above the continuum forming photosphere.
Methods. We obtained interferometric observations
with the
VLTI/AMBER interferometer using the fringe tracker FINITO in the
spectral domain 1.45-2.50 m with a spectral resolution of
and baselines ranging from 15 to 88 m. We performed
independent image reconstruction for different wavelength bins and fit
the interferometric data with a geometrical toy model. We also compared
the data to 1D dynamical models of Miras atmosphere and to 3D
hydrodynamical simulations of red supergiant (RSG) and asymptotic giant
branch (AGB) stars.
Results. Reconstructed images and visibilities show
a strong wavelength dependence. The H-band
images display two bright spots whose positions are confirmed by the
geometrical toy model. The inhomogeneities are qualitatively predicted
by 3D simulations. At
m and in the
region 2.35-2.50
m, the
photosphere appears extended and the radius is larger than in the H band.
In this spectral region, the geometrical toy model locates a
third
bright spot outside the photosphere that can be a feature of the
molecular layers. The wavelength dependence of the visibility can be
qualitatively explained by 1D dynamical models of Mira
atmospheres. The best-fitting photospheric models show a good match
with the observed visibilities and give a photospheric diameter of
0.50 mas. The H2O molecule
seems to be the dominant absorber in the molecular layers.
Conclusions. We show that the atmosphere of
VX Sgr seems to
resemble Mira/AGB star model atmospheres more closely than do
RSG model atmospheres. In particular, we see
molecular
(water) layers that are typical of Mira stars.
Key words: stars: AGB and post-AGB - stars: atmospheres - stars: individual: VX Sagittarii - techniques: interferometric - supergiants
1 Introduction
VX Sagittarii (HD 165674) is a cool semi-regular variable with a long
mean period of 732 days (Kholopov
et al. 1987). Lockwood
& Wing (1982) reports a spectral type varying from
M5.5 (near the time of visual maximum) to M9.8
(at minimum light). Lockwood
& Wing (1982) determined
that the effective temperature of VX Sgr ranges between 3300
and 2400 K (maximum to minimum light). García-Hernández
et al. (2007) find =
2900 K at the time of their high-resolution spectroscopic
observation, when the star was near minimum light (AAVSO
). Lockwood
& Wing
also find that VX Sgr exhibits stronger CN and
VO bands
than to Mira variables with similar temperature. Enhanced
CN absorption is an indicator of high luminosity in RSGs of an
earlier type and also, together with VO, of S stars. Speck
et al. (2000) categorized VX Sgr as an
oxygen-rich star and found a strong silicate feature at 10
m,
indicates a dusty circumstellar environment. Using aperture-masking and
IR/optical-telescope array interferometry at 2.16
m, Monnier
et al. (2004) reveals that VX Sgr exhibits
a dusty environment with a flux contribution of about 20
in the K band
and some evidence of departure from circular symmetry, even if
they could not place strong limits on possible asymmetries because of
calibration uncertainties. The dusty environment is confirmed by
HST images (Schuster
et al. 2006). VX Sgr's circumstellar
environment is the result of the heavy mass loss experienced by the
star (1.3
yr-1,
CO measurements; Knapp
et al. 1989). The mass-loss process appears to be
particularly asymmetric for the inner regions (Chapman
& Cohen 1986). Using AAVSO data, Kamohara
et al. (2005)
show that the optical light curve has a much smaller amplitude of about
2 mag in the years 1998-2003, much less than the
usual
6-7 mag. An examination of AAVSO data shows
that the
decrease to this smaller amplitude has happened several times in the
past 70 years
and that the star is probably currently in that state.
The classification of VX Sgr as a red supergiant or an AGB is
thus not firmly established. A further constraint can be
brought
by an estimate of its luminosity, in order to better ascertain its
position in the HR diagram. Humphreys
et al. (1972) placed VX Sgr in the vicinity
of the Sgr OB1 cluster at 1.7 kpc; Murakawa
et al. (2003) find 1.8
0.5 kpc measuring the GHz
H2O maser expansion; Chen
et al. (2007) reports a distance of 1.57
0.27 kpc using 43 GHz SiO maser proper
motions; finally, the trigonometric parallax of Hipparcos (van
Leeuwen 2007)
gives a distance of 0.262(+0.655/-0.109) kpc, which is
probably
unreliable because of the size and asymmetry of the stellar
photosphere.
Using AAVSO data, we find VX Sgr was at maximum luminosity
during our observations, and we assume a
of 3200 to 3400 K. With the 2MASS K magnitude
(Cutri
et al. 2003), assuming a distance d=1.7 kpc
and using data for Galactic red supergiants from Levesque
et al. (2005), we derive a luminosity
0.25 (
0.6).
The error bar accounts for uncertainties in the photometry and in the
assumed
at the time of our observation, affecting the bolometric correction
at K.
Circumstellar emission in IR may increase the luminosity by a few
tenths of magnitude. Putting the star at 1.3 kpc would
decrease
the estimated luminosity to
0.25. The radius is then about 1200
,
and
= 2
to 4. This is a too high luminosity for an
AGB star (e.g., Vassiliadis
& Wood 1993). Even compared to so-called super-AGB
stars, where the most recent models show a maximum of
with typical masses ranging between
(Siess 2006)
and
(Poelarends
et al. 2008), VX Sgr's luminosity is
extremely high. García-Hernández
et al. (2009) find AGB stars with similar
luminosities and masses of
6-7
showing Rb enhancement in the Magellanic Clouds, and they
argue that
these AGB stars may be more luminous due to a contribution
from hot bottom burning. However, García-Hernández
et al. (2007),
using synthetic spectra based on classical hydrostatic model
atmospheres for cool stars with extensive line lists, find
VX Sgr
to be the only Li-poor, long-period,
high OH expansion velocity star of their Galactic
AGB sample.
On the other hand, VX Sgr's low effective temperature and
large
V variability are quite atypical for RSGs, although Levesque
et al. (2007) find high-variability, low-
RSGs in the Magellanic Clouds. Heger
et al. (1997) studied the pulsations properties in
red supergiants from 10 to 20
with a high luminosity-to-mass ratio and show that very large pulsation
periods, amplitudes, and mass-loss rates may be expected to occur at
and beyond central helium ex-haustion over the time scale of the last
few 104 years.
This could lead to an overall dimming of the star after a period of
stronger oscillations subsequent to enhanced mass loss and ejection of
a dust shell that screens the stellar radiation. It appears
that
the evolutionary status of VX Sgr is still not well
established,
more investigations are needed. In particular its chemical
composition should be scrutinized.
We discuss here interferometric observations of VX Sgr made with the VLTI/AMBER instrument in the near IR. The aim of this paper is to study the continuum-forming photosphere, and the characterization of molecular layers of VX Sgr probing different wavelengths in the H and K bands.
2 Observations and data reduction
We obtained near-infrared interferometry data of VX Sgr with
the Very Large Telescope Interferometer (VLTI, Haguenauer
et al. 2008) using the near-infrared beam combiner
AMBER (Petrov
et al. 2007), which simultaneously covers the J,
H, and K bands with a
spectral resolution of
.
VX Sgr has been observed in less than 1.5 months
using the
AT configurations: A0-D0-H0, D0-H0-G1, and E0-G0-H0. The
fringe
tracker FINITO (Le Bouquin
et al. 2008)
was used for all the observations. In addition to the science
target, three calibrator stars were observed close in time and
interleaved with VX Sgr: HD 169916 (K0IV),
HD 146545
(K5III), and HD 166295 (K2III/IV). The calibrator diameters
were
retrieved from Richichi
et al. (2005), Bordé
et al. (2002), and Richichi
& Percheron (2002). The
diameter errors are about
.
Details are reported in Table 1 with the
projected baseline lengths (
), the position
angles (PA
),
the spectral interval, the calibrators used, the optical seeing, and
the coherence time.
Raw visibilities and closure phases were computed with the
latest version of the amdlib package
(release 2.2, Tatulli
et al. 2007) and the yorick
interface provided
by the Jean-Marie Mariotti Center. Individual frames were averaged
after selecting the best
frames based only on the fringe SNR and with a piston smaller
than
m. We
decided to discard J band fringes because
the data quality was significantly worse than for longer wavelengths.
In addition, we used the addons by Millour et al. (2008, release 1.53) to calibrate the datasets. The transfer function measurement for one night is shown in Fig. 1. Both science and calibrator targets have the same detector integration time of 0.05 ms, and after the calibration, VX Sgr data were averaged. The error bars on the calibrated visibilities include the statistical error of averaging the single frames, the errors of the calibration stars' angular diameters, and the scatter of the transfer function measurements. This scatter (top panel of Fig. 1) is much larger than the internal errors, computed by the reduction software. This basically means that the visibilities errors (between 0.05 and 0.1) we use in this paper reflect calibration issues simultaneously affecting a whole range of wavelengths, while the wavelength-to-wavelength error is much smaller (typically 0.01 to 0.05). While the visibility errors seem large in a single dataset, the wavelength-variation errors of the same dataset are small.
![]() |
Figure 1:
Transfer function for the night 2008-07-03. Top panel:
squared visibilities averaged over the region 2.1-2.2 |
Open with DEXTER |
The absolute wavelength correction was done using the telluric Kitt
Peak spectra, which we convolved to match the spectral resolution of
the AMBER data. In VX Sgr data, the band
gaps (i.e.,
between J and H and H
and K)
are visible, and we made a linear two-component adjustment of the
wavelength scale, which gave a systematic offset
of -0.21 m
with respect to the initial AMBER table and a
wavelength
stretch.
Figure 2
shows the
final UV-plane coverage of all observations that successfully passed
all steps of the data reduction and calibration quality control. The
north-west south-east direction
is not completely covered because of the actual AT geometry,
while
the east-west direction is particularly favored. The star exhibits high
wavelength-dependent visibilities and has a clear non-zero, non-180 closure
phase (bottom panel of Fig. 1), showing
asymmetries in the intensity distribution.
Table 1: Observations log for the AMBER observations of VX Sgr, carried out using FINITO and an integration time of 50 ms.
![]() |
Figure 2: UV-coverage for of the AMBER observations of VX Sgr. The radial extension of the uv tracks reflects the spectral coverage of our AMBER measurements, covering the H and the K bands. |
Open with DEXTER |
3 Image reconstruction
The first step in our analysis is a chromatic image
reconstruction
of our data to probe different layers in the photosphere and above. The
image reconstruction process is similar to the one performed for
T Lep in Le Bouquin
et al. (2009). For the reconstruction, we used the
MIRA software package (Le Besnerais et al. 2008; Thiébaut
2008; Cotton
et al. 2008). The image was sought by minimizing a
so-called cost function
which is the sum of a regularization term plus
data-related terms. The data terms enforce agreement of the model image
with the measured data (visibilities). The regularization term is
a minimization
between the reconstructed image and an expected
image. The expected image is issued from a preliminary image
reconstruction, strongly constrained by the assumption of circularity.
Each spectral bin has been processed independently (Fig. 3, left
column). The reconstructed images clearly highlight different behaviors
across the wavelength range: in the H band (
m),
the intensity distribution is inhomogeneous and a bifurcation
of
the image core into a few bright ``spots'' is visible. At
m and at the
upper K band edge (
m) the
radius appears extended and much larger than in the H band.
Artifacts may be introduced by the poor UV-plane coverage in one
direction (north-west south-east, see Fig. 2).
However, the detection of inhomogeneities is not in doubt because there
are clear signatures in the closure phases (Fig. 1).
![]() |
Figure 3:
Left column: reconstructed images of
VX Sgr for several AMBER spectral bins across the H
and K bands. The intensity, I,
is normalized to the range [0, 1] and plotted
as I0.33. Some
contour lines are indicated (0.251, 0.275, 0.314, 0.372, 0.588, 0.889,
0.929,
0.976, 0.983, 0.990, 0.997). The resolution of the interferometer is
illustrated in the bottom
left part of each image by the PSF of an 88 |
Open with DEXTER |
4 Geometrical toy models
The reconstructed images are not good enough to firmly establish
inhomogeneities on VX Sgr's photosphere because the different
sources of errors can smooth out the information. Thus,
to confirm
their presence and to constrain their flux relative to the total flux,
we used a model-fitting approach. We approximated the star with a
uniform disk and its extended shells by a Gaussian disk and
added
a series of point sources to model the spots. We performed a fit with
all the observed wavelengths together, but used five regularly
spaced reference wavelengths to effectively
compute the model (1.54, 1.78, 2.03, 2.28, and 2.52 m).
To match the data with the model, we interpolated the
parameters
for other wavelengths using cubic spline interpolation. The global
optimum of the fit was found using a set of simulated annealing
algorithms complemented by standard gradient descent algorithms (see Millour
et al. 2009,
for a first application of this approach). A model
with three
spots (i.e., performing a fit with a uniform disk, a Gaussian
disk, and three spots) significantly enhance the fit (
), especially for the
visibilities and closure phases up to spatial frequency of
cycles/arcsec.
Spatial frequencies above this value correspond to a single measurement
at the largest baseline length. The details about the fits are reported
in Table 2.
Our toy model probably does not perfectly describe the object
at the
highest angular resolution available, but it gives a
reasonable
fit to intermediate angular resolutions. We also note that two of the
bright spots are located at the position of the stellar disk, and one
has to be outside the stellar disk in order to
fit the observed closure phases. We estimate that the upper value of
the spots' flux is
of the total flux and that the flux of the spot located outside the
photosphere very likely has a flux equal to zero in the H band.
Figure 3
(right column) displays the appearance of the toy model at some
selected wavelengths. As a final remark, these images
resemble the reconstructed images in the left column of the figure. We
also note that the bright spot located outside the photosphere is
particularly visible in the lower right part of each panel of the
reconstructed images at lower wavelength and somehow less evident at
longer wavelength even if it is expected by the geometrical model.
Table
2:
Parameters of the best geometrical toy model (
).
What we can conclude from this model-fitting approach is the following:
- a model with three spots can explain the visibilities and
closure phases for spatial frequencies
cycles/arcsec. Higher spatial frequencies are probed in our dataset by only one observation and indicate a more complicated object shape at smaller scales;
- the best-fitting model locates two of the spots inside the supposed photosphere of the star, but the third spot, mandatory for explaining the closure phase deviations, is located outside the photosphere and it is brighter at longer wavelengths;
- the spots have individual fluxes that, at all wavelengths, do not exceed 10% of the total flux of the star, for all wavelengths.
5 Model fitting
The visibility data of VX Sgr show significant
wavelength-dependent
features. We fitted the visibility curves and the closure phases using
a Gaussian disk for each spectral channel, which gives more
robust
results than a uniform disk model. Figure 4
illustrates the change in apparent size with wavelength. This is
consistent with what is visible in the reconstructed images
(Fig. 3):
the radius gets larger between 1.8 and 2.1 m, reaching
local maximum at 2.0
m,
then gets smaller again to a minimum around 2.15 to
2.25
m,
and eventually becomes much larger toward 2.5
m. However,
Fig. 4
only gives a rough idea of the apparent diameter of VX Sgr
because the intensity profile is not a Gaussian disk.
![]() |
Figure 4: VX Sgr Gaussian FWHM values as a function of the wavelength compared to the model M18 predictions. The dashed line indicates the smallest values. The position of the H2O bands are also indicated (after Lançon & Wood 2000). |
Open with DEXTER |
To reproduce the wavelength dependence, we used the one-dimensional dynamic models of Ireland et al. (2004b,a) for oxygen-rich Mira stars that include the effect of molecular layers in the outer atmosphere. These are self-excited dynamic models whose gray atmospheric temperature stratification was re-computed on the basis of non-gray extinction coefficients that contain all relevant molecular absorbers (e.g., H2O, CO, TiO; solar abundances) but do not contain dust. The stellar parameters were assumed to be close to those of the M-type Mira prototypes o Cet and R Leo. This model series has been successfully used by Wittkowski et al. (2008,2007) to explain AMBER observation of the Mira star S Ori. Because the VX Sgr stellar parameters are poorly understood, there is no fully consistent model of this star, and the models we used can only show typical characteristics of such a star. Figure 5 shows the comparison for short (left and central panels) and long (right panel) projected baselines. The wavelength dependence of the visibility is similar for all the nights.
![]() |
Figure 5:
Measured VX Sgr visibility data (red crosses with error bars) compared
to the visibilities derived from (i) the 1D Mira atmosphere
model M18 (model with a phase |
Open with DEXTER |
For the fit, we followed the fitting procedure used by Wittkowski
et al. (2008)
and used only the short baseline data obtained with E0-G0-H0, because
we do not expect to match details of the intensity profile probed at
high spatial frequencies with the M series that has not been
designed to match a star like VX Sgr. The dynamic models
predict
large changes in the monochromatic radius caused by the molecular
layers above the continuum-forming region in the stellar photosphere.
The bandpasses around 1.9 and 2.5 m are
significantly affected by H2O molecules
with some contribution of CO in the H band
and in the long-wavelength part K band.
The molecular-band effects change significantly with phase as shown in Wittkowski
et al. (2008). The model M18
(see Table 3)
provides the best fit to the VX Sgr data among the
20 available phase and cycle combinations of the
M series.
Table 3: Parameters of the simulations, all with solar metallicity.
The M model qualitatively explains the overall wavelength dependence of
the apparent stellar radius (Fig. 5).
However, while the fit is very good at short baselines, the comparison
is less good at long baselines. Two reasons can explain this
discrepancy: (i) the stellar parameters of the
M models are
not suited to VX Sgr; (ii) there are some surface
inhomogeneities detected in the data, which are not in the
M model. The angular photospheric diameter corresponding to
the
reference
radius at 1.04 m
is
0.43 mas for short baselines where the fit is better. The
error on
the diameter includes systematic calibration and model uncertainties.
This result agrees with what has been found by Monnier
et al. (2004),
0.4 mas. However, our models do not include dust, while Monnier
et al. find a dusty environment around the star with
a flux contribution of about 20
in the K band.
We also fitted Gaussian FWHM values to the synthetic visibility values based on the M18 model intensity profiles for each spectral channel, using the same procedure that was used to fit the measured data. The comparison between the fit to the M18 model intensity profiles and the fit to the measured data is shown in Fig. 4.
The available Mira models show good agreement with VX Sgr's data. However, this comparison can just give a basic qualitative picture of VX Sgr's surface, and more detailed interpretations of these data must be addressed to next generation models with stellar parameters appropriate for VX Sgr, which are currently not available.
![]() |
Figure 6:
Reconstructed image at 1.6 |
Open with DEXTER |
6 Complementary comparison with three-dimensional simulations
In this section, we quantify whether the observed asymmetries are
consistent with three-dimensional simulations of surface convection in
RSG and AGB stars and whether their chaotic photospheric
structure
is adequate for explaining the different surface layers structures
observed. The simulations are carried out with CO5BOLD
(Freytag
et al. 2002; Freytag
& Höfner 2008). Parameters of the models are given in
Table 3.
The pulsations are not artificially added to the models
(e.g. by a
piston) but are self-excited. Excitation by unstationary sonic
convective motions are responsible for the pulsations. Molecular
opacities are taken into account, but radiation transport is treated in
a gray approximation, ignoring radiation pressure and dust opacities.
Dynamical pressure lets the density drop much more slowly than expected
for a hydrostatic atmosphere. In our 3D simulations, the
average
density drops exponentially, and there is no sign of a wind or an
extended shell with relatively high densities. Some more technical
information can be
found in Freytag
& Höfner (2008), the CO5BOLD Online
User Manual, and in a forthcoming
paper by Freytag (2010, in prep.).
![]() |
Figure 7:
Top left panel: synthetic image from the
RSG simulation at 1.6 |
Open with DEXTER |
![]() |
Figure 8: Same as in Fig. 7 for the AGB simulation. Top right panel: original AGB image, the range is [0; 130 000] erg cm-2 s-1 Å-1. |
Open with DEXTER |
We computed
synthetic images from these simulations at the same wavelengths as the
observations using our 3D radiative transfer code OPTIM3D (Chiavassa
et al. 2009). Then, we generated visibility curves
using the method described by Chiavassa
et al..
In the case of the RSG simulation, the synthetic
images do
not change strongly in diameter, shape, and
asymmetries across the wavelength range of the observations,
so that the observed wavelength dependence of the visibility
is
not explained by this approach (Fig. 5).
The synthetic images of the AGB simulation show a noticeable
wavelength dependence, and their agreement with the observed
visibilities is slightly better than the RSG model,
in particular at short baselines (left and central panel, blue
dots of Fig. 5).
Nevertheless,
the two 3D simulations do not provide a better fit to our
AMBER data than the Mira models described
in Sect. 5.
At last, to explain the detected spots on the surface of the
reconstructed images (Fig. 3),
we compare the synthetic images corresponding to the best-fitting
visibility curves of the 3D simulations
(see Fig. 5)
to the reconstructed image at 1.6 m (Fig. 6). This
wavelength corresponds roughly to the H-1 continuous
opacity minimum (i.e., the photosphere becomes more
transparent).
The reconstructed image shows two spots on the
surface. Figure 7
displays the synthetic images of the RSG star, which show
large convective cells (
mas), and on top of
that there are small-scale granules (
mas).
The observed visibility points do not show a good agreement especially
at long baselines where the expected signal is much lower than the
simulation's predictions.
The AGB star (Fig. 7) displays a pulsating stellar atmosphere with strong asymmetric structures caused by the shocks. The convective envelope is hardly visible because of the optical thickness of the atmosphere even without including the dust opacity tables (Freytag & Höfner 2008). The AGB's visibilities and closure phases already show large departures from symmetry in the first lobe, but even in this case, the long baseline data points cannot be explained.
That there are spots on the reconstructed image of VX Sgr can be qualitatively explained by the synthetic images, even if the distribution of the spots in our simulations has little chance of being exactly the one we observed with AMBER. Unfortunately, we cannot constrain the 3D simulations in terms of surface intensity contrast using these observations because we could not determine an accurate value for the spots' flux level (Table 2).
7 Conclusions
Our AMBER observations unveil, for the first time, the shape of
VX Sgr's surface. The individual wavelength image
reconstruction
was carried out using the MIRA software. VX Sgr displays
visibilities and images with strong wavelength dependence:
(i) surface asymmetries in the H band
(
m) and
(ii) an extended radius at
and 2.35-2.50
m.
We find that a geometrical toy model composed of a uniform disk plus a
Gaussian disk and three spots gives a reasonable fit for the
AMBER data. We claim that at least two spots are present on
the
photosphere of VX Sgr, and we show that this is qualitatively
predicted by a 3D hydrodynamical simulations of
stellar
convection. In addition, the toy model shows that one spot
resides
outside the photosphere, and it is brighter at longer wavelengths. This
spot's location outside the photosphere could be related to a complex
and irregular structure in the surrounding of VX Sgr as
already
detected by Kervella
et al. (2009)
for the RSG alpha Ori. Also the two bright spots that
appear
at the position on the photosphere may both originate in the depths of
the continuum photosphere or higher up in the molecular layers.
In fact, these spots appear at all wavelengths (Fig. 3),
also at the water bands, which hide a large part of the continuum
photosphere. This might be a hint that they originate in the molecular
layers far above the continuum photosphere.
We used 1D dynamical oxygen-rich Mira model predictions to explain the visibility data points, and we found that the fit is very good at short baselines. We conclude that H2O molecules strongly affect the visibility, making this molecule seem to be a dominant absorber in the molecular layers. The atmospheric structure of VX Sgr seems to qualitatively resemble Mira-star models, which show molecular layers above the continuum forming layer. In addition, its photosphere shows bright spots that can be related to giant-cell surface convection and possibly to the molecular layer. However, we must point out that the Mira model used here has stellar parameters that are not consistent with what we expect of VX Sgr, and there are only observations from a single epoch to do the interpretation. Because of various uncertainties, the classification of VX Sgr must be investigated further in the near future by first, combining multi-epoch photometric and spectroscopic observations to determine its stellar parameters better, and second, by observing with interferometers at higher spectral resolution to study the changes across the H and K bands in greater detail using new theoretical models with consistent stellar parameters.
AcknowledgementsWe would like to thank the entire VLTI team and in particular J.-B. Le Bouquin. Moreover, we acknowledge John Monnier, Aldo Serenelli, Josef Hron, and Claudia Paladini for the enlightening discussions. A.C., B.P., and E.J. acknowledge financial support from ANR (ANR-06-BLAN-0105). We would like also to thank the JMMC team. The CNRS is acknowledged for supporting us with the Guaranteed Time Observations with AMBER. We thank the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research. We also thank CINES, France, and UPPMAX, Sweden, for providing the computational resources for the 3D simulations.
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Footnotes
- ...
atmosphere
- Based on the observations made with VLTI-ESO Paranal, Chile under the programs IDs 081.D-0005(A, B, C, D, E, F, G, H).
- ... (AAVSO
- www.aavso.org
- ... Manual
- www.astro.uu.se/ bf/co5bold_main.html
All Tables
Table 1: Observations log for the AMBER observations of VX Sgr, carried out using FINITO and an integration time of 50 ms.
Table 2:
Parameters of the best geometrical toy model (
).
Table 3: Parameters of the simulations, all with solar metallicity.
All Figures
![]() |
Figure 1:
Transfer function for the night 2008-07-03. Top panel: squared visibilities averaged over the region 2.1-2.2 |
Open with DEXTER | |
In the text |
![]() |
Figure 2: UV-coverage for of the AMBER observations of VX Sgr. The radial extension of the uv tracks reflects the spectral coverage of our AMBER measurements, covering the H and the K bands. |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Left column: reconstructed images of VX Sgr for several AMBER spectral bins across the H and K bands. The intensity, I, is normalized to the range [0, 1] and plotted as I0.33. Some contour lines are indicated (0.251, 0.275, 0.314, 0.372, 0.588, 0.889, 0.929,
0.976, 0.983, 0.990, 0.997). The resolution of the interferometer is illustrated in the bottom
left part of each image by the PSF of an 88 |
Open with DEXTER | |
In the text |
![]() |
Figure 4: VX Sgr Gaussian FWHM values as a function of the wavelength compared to the model M18 predictions. The dashed line indicates the smallest values. The position of the H2O bands are also indicated (after Lançon & Wood 2000). |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Measured VX Sgr visibility data (red crosses with error bars) compared
to the visibilities derived from (i) the 1D Mira atmosphere
model M18 (model with a phase |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Reconstructed image at 1.6 |
Open with DEXTER | |
In the text |
![]() |
Figure 7:
Top left panel: synthetic image from the RSG simulation at 1.6 |
Open with DEXTER | |
In the text |
![]() |
Figure 8: Same as in Fig. 7 for the AGB simulation. Top right panel: original AGB image, the range is [0; 130 000] erg cm-2 s-1 Å-1. |
Open with DEXTER | |
In the text |
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