166 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
Coordinated AMBER and MIDI observations of the Mira variable RR Aql
We have used near- and mid-infrared interferometry to investigate the
pulsating atmosphere and the circumstellar environment of the Mira variable RR
Aql. Observations were taken with the VLTI/AMBER (near infrared) and the
VLTI/MIDI (mid infrared) instruments. We have obtained a total of 15 MIDI
epochs between Apr 9, 2004 and Jul 28, 2007 covering 4 pulsation cycles and one
AMBER epoch on Sep 9, 2006 at phase 2.82. This work is also part of an ongoing
project of joint VLTI and VLBA observations to study the connection between
stellar pulsation and the mass loss process. Here we present a comparison of
the AMBER visibility data to a simple uniform disk model as well as to
predictions by recent self-excited dynamic model atmospheres. The best fitting
photospheric angular diameter of the model atmosphere at phase 2.82 is 9.9 +/-
2.4 mas.Comment: 4 pages, 3 figures, to appear in Proc. of Cool Stars 1
Washington Correlator
This report summarizes the activities of the Washington Correlator for 2012. The Washington Correlator provides up to 80 hours of attended processing per week plus up to 40 hours of unattended operation, primarily supporting Earth Orientation and astrometric observations. In 2012, the major programs supported include the IVS-R4, IVS-INT, APSG, and CRF observing sessions
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
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
SiO masers in TX Cam: Simultaneous VLBA observations of two 43 GHz masers at four epochs
We present the results of simultaneous high resolution observations of v=1
and v=2, J=1-0 SiO masers toward TX Cam at four epochs covering a stellar
cycle. Near maser maximum (Epochs III and IV), the individual components of
both masers are distributed in ring-like structures but the ring is severely
disrupted near stellar maser minimum (Epochs I and II). In Epochs III and IV
there is a large overlap between the radii at which the two maser transitions
occur. However in both epochs the average radius of the v=2 maser ring is
smaller than for the v=1 maser ring, the difference being larger for Epoch IV.
The observed relative ring radii in the two transitions, and the trends on the
ring thickness, are close to those predicted by the model of Humphreys et al.
(\cite{humphreys02}). In many individual features there is an almost exact
overlap in space and velocity of emission from the two transitions, arguing
against pure radiative pumping. At both Epochs III and IV in many spectral
features only 50% of the flux density is recovered in our images, implying
significant smooth maser structure. For both transitions we find that red- and
blue-shifted masers occur in all parts of the rings, with relatively few masers
at the systemic velocity. Thus there is no evidence for rotation, although the
blue-shifted masers are somewhat more prominent to the west. At all four epochs
red-shifted components are generally brighter than blue-shifted ones. At Epochs
III and IV, we see many filamentary or spoke-like features in both v=1 and v=2
masers, especially in the red-shifted gas. These spokes show systematic
velocity gradients consistent with a decelerating outward flow with increasing
radius. We outline a possible model to explain why, given the presence of these
spokes, there is a deficit of maser features at the systemic velocity.Comment: 15 pages, 10 figs, accepted to A&A, Abstract is reduced (see the
paper for full length
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