104 research outputs found
New insights into the dust formation of oxygen-rich AGB stars
We observed the AGB stars S Ori, GX Mon and R Cnc with the MIDI instrument at
the VLTI. We compared the data to radiative transfer models of the dust shells,
where the central stellar intensity profiles were described by dust-free
dynamic model atmospheres. We used Al2O3 and warm silicate grains. Our S Ori
and R Cnc data could be well described by an Al2O3 dust shell alone, and our GX
Mon data by a mix of an Al2O3 and a silicate shell. The best-fit parameters for
S Ori and R Cnc included photospheric angular diameters Theta(Phot) of
9.7+/-1.0mas and 12.3+/-1.0mas, optical depths tau(V)(Al2O3) of 1.5+/-0.5 and
1.35+/-0.2, and inner radii R(in) of 1.9+/-0.3R(Phot) and 2.2+/-0.3R(Phot),
respectively. Best-fit parameters for GX Mon were Theta(Phot)=8.7+/-1.3mas,
tau(V)(Al2O3)=1.9+/-0.6, R(in)(Al2O3)=2.1+/-0.3R(Phot),
tau(V)(silicate)=3.2+/-0.5, and R(in)(silicate)=4.6+/-0.2R(Phot). Our model
fits constrain the chemical composition and the inner boundary radii of the
dust shells, as well as the photospheric angular diameters. Our interferometric
results are consistent with Al2O3 grains condensing close to the stellar
surface at about 2 stellar radii, co-located with the extended atmosphere and
SiO maser emission, and warm silicate grains at larger distances of about 4--5
stellar radii. We verified that the number densities of aluminum can match that
of the best-fit Al2O3 dust shell near the inner dust radius in sufficiently
extended atmospheres, confirming that Al2O3 grains can be seed particles for
the further dust condensation. Together with literature data of the mass-loss
rates, our sample is consistent with a hypothesis that stars with low mass-loss
rates form primarily dust that preserves the spectral properties of Al2O3, and
stars with higher mass-loss rate form dust with properties of warm silicates.Comment: 20 pages, 10 figure
OH 12.8-0.9: A New Water-Fountain Source
We present observational evidence that the OH/IR star OH 12.8-0.9 is the
fourth in a class of objects previously dubbed "water-fountain" sources. Using
the Very Long Baseline Array, we produced the first images of the water maser
emission associated with OH 12.8-0.9. We find that the masers are located in
two compact regions with an angular separation of ~109 mas on the sky. The axis
of separation between the two maser regions is at a position angle of 1.5 deg.
East of North with the blue-shifted (-80.5 to -85.5 km/s) masers located to the
North and the red-shifted (-32.0 to -35.5 km/s) masers to the South. In
addition, we find that the blue- and red-shifted masers are distributed along
arc-like structures ~10-12 mas across oriented roughly perpendicular to the
separation axis. The morphology exhibited by the water masers is suggestive of
an axisymmetric wind with the masers tracing bow shocks formed as the wind
impacts the ambient medium. This bipolar jet-like structure is typical of the
three other confirmed water-fountain sources. When combined with the previously
observed spectral characteristics of OH 12.8-0.9, the observed spatio-kinematic
structure of the water masers provides strong evidence that OH 12.8-0.9 is
indeed a member of the water-fountain class.Comment: 12 pages, 2 figures (1 color), accepted for publication in the Ap J
Letter
Structure and shaping processes within the extended atmospheres of AGB stars
We present recent studies using the near-infrared instrument AMBER of the VLT
Interferometer (VLTI) to investigate the structure and shaping processes within
the extended atmosphere of AGB stars. Spectrally resolved near-infrared AMBER
observations of the Mira variable S Ori have revealed wavelength-dependent
apparent angular sizes. These data were successfully compared to dynamic model
atmospheres, which predict wavelength-dependent radii because of geometrically
extended molecular layers. Most recently, AMBER closure phase measurements of
several AGB stars have also revealed wavelength-dependent deviations from 0/180
deg., indicating deviations from point symmetry. The variation of closure phase
with wavelength indicates a complex non-spherical stratification of the
extended atmosphere, and may reveal whether observed asymmetries are located
near the photosphere or in the outer molecular layers. Concurrent observations
of SiO masers located within the extended molecular layers provide us with
additional information on the morphology, conditions, and kinematics of this
shell. These observations promise to provide us with new important insights
into the shaping processes at work during the AGB phase. With improved imaging
capabilities at the VLTI, we expect to extend the successful story of imaging
studies of planetary nebulae to the photosphere and extended outer atmosphere
of AGB stars.Comment: 6 pages, Proc. of "Asymmetric Planetary Nebulae V", A.A. Zijlstra, F.
Lykou, I. McDonald, and E. Lagadec (eds.), Jodrell Bank Centre for
Astrophysics, Manchester, UK, 201
The Mira variable S Ori: Relationships between the photosphere, molecular layer, dust shell, and SiO maser shell at 4 epochs
We present the first multi-epoch study that includes concurrent mid-infrared
and radio interferometry of an oxygen-rich Mira star. We obtained mid-infrared
interferometry of S Ori with VLTI/MIDI at four epochs between December 2004 and
December 2005. We concurrently observed v=1, J=1-0 (43.1 GHz), and v=2, J=1-0
(42.8 GHz) SiO maser emission toward S Ori with the VLBA at three epochs. The
MIDI data are analyzed using self-excited dynamic model atmospheres including
molecular layers, complemented by a radiative transfer model of the
circumstellar dust shell. The VLBA data are reduced to the spatial structure
and kinematics of the maser spots. The modeling of our MIDI data results in
phase-dependent continuum photospheric angular diameters between about 7.9 mas
(Phase 0.55) and 9.7 mas (Phase 1.16). The dust shell can best be modeled with
Al2O3 grains using phase-dependent inner boundary radii between 1.8 and 2.4
photospheric radii. The dust shell appears to be more compact with greater
optical depth near visual minimum, and more extended with lower optical depth
after visual maximum. The ratios of the SiO maser ring radii to the
photospheric radii are between about 1.9 and 2.4. The maser spots mark the
region of the molecular atmospheric layers just beyond the steepest decrease in
the mid-infrared model intensity profile. Their velocity structure indicates a
radial gas expansion. Al2O3 dust grains and SiO maser spots form at relatively
small radii of 1.8-2.4 photospheric radii. Our results suggest increased mass
loss and dust formation close to the surface near the minimum visual phase,
when Al2O3 dust grains are co-located with the molecular gas and the SiO maser
shells, and a more expanded dust shell after visual maximum. Silicon does not
appear to be bound in dust, as our data show no sign of silicate grains.Comment: Accepted for publication in A&A. See ESO press release 25/07 at
http://www.eso.org/public/outreach/press-rel/pr-2007/pr-25-07.htm
Ground-State SiO Maser Emission Toward Evolved Stars
We have made the first unambiguous detection of vibrational ground-state
maser emission from SiO toward six evolved stars. Using the Very Large Array,
we simultaneously observed the v=0, J=1-0, 43.4-GHz, ground-state and the v=1,
J=1-0, 43.1-GHz, first excited-state transitions of SiO toward the oxygen-rich
evolved stars IRC+10011, o Ceti, W Hya, RX Boo, NML Cyg, and R Cas and the
S-type star chi Cyg. We detected at least one v=0 SiO maser feature from six of
the seven stars observed, with peak maser brightness temperatures ranging from
10,000 K to 108,800 K. In fact, four of the seven v=0 spectra show multiple
maser peaks, a phenomenon which has not been previously observed. Ground-state
thermal emission was detected for one of the stars, RX Boo, with a peak
brightness temperature of 200 K. Comparing the v=0 and the v=1 transitions, we
find that the ground-state masers are much weaker with spectral characteristics
different from those of the first excited-state masers. For four of the seven
stars the velocity dispersion is smaller for the v=0 emission than for the v=1
emission, for one star the dispersions are roughly equivalent, and for two
stars (one of which is RX Boo) the velocity spread of the v=0 emission is
larger. In most cases, the peak flux density in the v=0 emission spectrum does
not coincide with the v=1 maser peak. Although the angular resolution of these
VLA observations were insufficient to completely resolve the spatial structure
of the SiO emission, the SiO spot maps produced from the interferometric image
cubes suggest that the v=0 masers are more extended than their v=1
counterparts
Axial Symmetry and Rotation in the SiO Maser Shell of IK Tauri
We observed v=1, J=1-0 43-GHz SiO maser emission toward the Mira variable IK
Tauri (IK Tau) using the Very Long Baseline Array (VLBA). The images resulting
from these observations show that SiO masers form a highly elliptical ring of
emission approximately 58 x 32 mas with an axial ratio of 1.8:1. The major axis
of this elliptical distribution is oriented at position angle of ~59 deg. The
line-of-sight velocity structure of the SiO masers has an apparent axis of
symmetry consistent with the elongation axis of the maser distribution.
Relative to the assumed stellar velocity of 35 km/s, the blue- and red-shifted
masers were found to lie to the northwest and southeast of this symmetry axis
respectively. This velocity structure suggests a NW-SE rotation of the SiO
maser shell with an equatorial velocity, which we determine to be ~3.6 km/s.
Such a NW-SE rotation is in agreement with a circumstellar envelope geometry
invoked to explain previous water and OH maser observations. In this geometry,
water and OH masers are preferentially created in a region of enhanced density
along the NE-SW equator orthogonal to the rotation/polar axis suggested by the
SiO maser velocities.Comment: 17 Pages, 4 figures (2 color); accepted for publication in Ap
Mid-infrared interferometric monitoring of evolved stars - The dust shell around the Mira variable RR Aql at 13 epochs
We obtained 13 epochs of mid-infrared interferometry with the MIDI instrument
at the VLTI between April 2004 and July 2007, covering pulsation phases
0.45-0.85 within four cycles. The data are modeled with a radiative transfer
model of the dust shell where the central stellar intensity profile is
described by a series of dust-free dynamic model atmospheres based on
self-excited pulsation models. We examined two dust species, silicate and Al2O3
grains. We performed model simulations using variations in model phase and dust
shell parameters to investigate the expected variability of our photometric and
interferometric data. The observed visibility spectra do not show any
indication of variations as a function of pulsation phase and cycle. The
observed photometry spectra may indicate intracycle and cycle-to-cycle
variations at the level of 1-2 standard deviations. The best-fitting model for
our average pulsation phase of 0.64+/-0.15 includes the dynamic model
atmosphere M21n (T_model=2550 K) with a photospheric angular diameter of
7.6+/-0.6 mas, and a silicate dust shell with an optical depth of 2.8+/-0.8, an
inner radius of 4.1+/-0.7 R_Phot, and a power-law index of the density
distribution of 2.6+/-0.3. The addition of an Al2O3 dust shell did not improve
the model fit. The photospheric angular diameter corresponds to a radius of
520^+230_-140 R_sun and an effective temperature of ~ 2420+/-200 K. Our
modeling simulations confirm that significant visibility variations are not
expected for RR Aql at mid-infrared wavelengths within our uncertainties.
We conclude that our RR Aql data can be described by a pulsating atmosphere
surrounded by a silicate dust shell. The effects of the pulsation on the
mid-infrared flux and visibility values are expected to be less than about 25%
and 20%, respectively, and are too low to be detected within our measurement
uncertainties.Comment: 16 pages, 14 figures. Accepted for publication in A&
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