91 research outputs found
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
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
Radio and IR interferometry of SiO maser stars
Radio and infrared interferometry of SiO maser stars provide complementary
information on the atmosphere and circumstellar environment at comparable
spatial resolution. Here, we present the latest results on the atmospheric
structure and the dust condensation region of AGB stars based on our recent
infrared spectro-interferometric observations, which represent the environment
of SiO masers. We discuss, as an example, new results from simultaneous VLTI
and VLBA observations of the Mira variable AGB star R Cnc, including VLTI near-
and mid-infrared interferometry, as well as VLBA observations of the SiO maser
emission toward this source. We present preliminary results from a monitoring
campaign of high-frequency SiO maser emission toward evolved stars obtained
with the APEX telescope, which also serves as a precursor of ALMA images of the
SiO emitting region. We speculate that large-scale long-period chaotic motion
in the extended molecular atmosphere may be the physical reason for observed
deviations from point symmetry of atmospheric molecular layers, and for the
observed erratic variability of high-frequency SiO maser emissionComment: 8 pages, 4 figures, submitted to Proc. IAU Symp. 287 "Cosmic masers -
from OH to H_0", R.S. Booth, E.M.L. Humphreys, W.H.T. Vlemmings (eds.),
invited pape
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
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&
Fundamental stellar parameters of benchmark stars from CHARA interferometry. I. Metal-poor stars
Benchmark stars are crucial as validating standards for current as well as
future large stellar surveys of the Milky Way. However, the number of suitable
metal-poor benchmarks is currently limited. We aim to construct a new set of
metal-poor benchmarks, based on reliable interferometric effective temperature
() determinations and a homogeneous analysis with a desired
precision of in . We observed ten late-type metal-poor
dwarf and giants: HD2665, HD6755, HD6833, HD103095, HD122563, HD127243,
HD140283, HD175305, HD221170, and HD224930. Only three of the ten stars
(HD103095, HD122563, and HD140283) have previously been used as benchmarks. For
the observations, we used the high angular resolution optical interferometric
instrument PAVO at the CHARA array. We modelled angular diameters using 3D limb
darkening models and determined directly from the
Stefan-Boltzmann relation, with an iterative procedure to interpolate over
tables of bolometric corrections. Surface gravities () were estimated
from comparisons to Dartmouth stellar evolution model tracks. We collected
spectroscopic observations from the ELODIE and FIES spectrographs and estimated
metallicities () from a 1D non-LTE abundance analysis of
unblended lines of neutral and singly ionized iron. We inferred
to better than for five of the stars (HD103095, HD122563, HD127243,
HD140283, and HD224930). The of the other five stars are
reliable to between ; the higher uncertainty on the for
those stars is mainly due to their having a larger uncertainty in the
bolometric fluxes. We also determined and with
median uncertainties of and ,
respectively. These ten stars can, therefore, be adopted as a new, reliable set
of metal-poor benchmarks.Comment: 13 pages, 7 figures, 8 tables + 10 online tables, abstract shortened
to meet arXiv requirements, accepted in A&
Exploring wind-driving dust species in cool luminous giants I. Basic criteria and dynamical models of M-type AGB stars
This work is part of an ongoing effort aiming at identifying the actual
wind-drivers among the dust species observed in circumstellar envelopes. In
particular, we focus on the interplay between a strong stellar radiation field
and the dust formation process. To identify critical properties of potential
wind-driving dust species we use detailed radiation-hydrodynamical models which
include a parameterized dust description, complemented by simple analytical
estimates to help with the physical interpretation of the numerical results.
The adopted dust description is constructed to mimic different chemical and
optical dust properties in order to systematically study the effects of a
realistic radiation field on the second stage of the mass loss mechanism. We
see distinct trends in which combinations of optical and chemical dust
properties are needed to trigger an outflow. Dust species with a low
condensation temperature and a NIR absorption coefficient that decreases
strongly with wavelength will not condense close enough to the stellar surface
to be considered as potential wind-drivers. Our models confirm that metallic
iron and Fe-bearing silicates are not viable as wind-drivers due to their
near-infrared optical properties and resulting large condensation distances.
TiO2 is also excluded as a wind-driver due to the low abundance of Ti. Other
species, such a SiO2 and Al2O3, are less clear-cut cases due to uncertainties
in the optical and chemical data and further work is needed. A strong candidate
is Mg2SiO4 with grain sizes of 0.1-1 micron, where scattering contributes
significantly to the radiative acceleration, as suggested by earlier
theoretical work and supported by recent observations.Comment: 15 pages, 12 figure
The geometry of the close environment of SV Psc as probed by VLTI/MIDI
Context. SV Psc is an asymptotic giant branch (AGB) star surrounded by an
oxygen-rich dust envelope. The mm-CO line profile of the object's outflow shows
a clear double-component structure. Because of the high angular resolution,
mid-IR interferometry may give strong constraints on the origin of this
composite profile.
Aims. The aim of this work is to investigate the morphology of the
environment around SV Psc using high-angular resolution interferometry
observations in the mid-IR with the Very Large Telescope MID-infrared
Interferometric instrument (VLTI/MIDI).
Methods. Interferometric data in the N-band taken at different baseline
lengths (ranging from 32-64 m) and position angles (73- 142{\deg}) allow a
study of the morphology of the circumstellar environment close to the star. The
data are interpreted on the basis of 2-dimensional, chromatic geometrical
models using the fitting software tool GEM-FIND developed for this purpose.
Results. The results favor two scenarios: (i) the presence of a highly
inclined, optically thin, dusty disk surrounding the central star; (ii) the
presence of an unresolved binary companion at a separation of 13.7 AU and a
position angle of 121.8{\deg} NE. The derived orbital period of the binary is
38.1 yr. This detection is in good agreement with hydrodynamic simulations
showing that a close companion could be responsible for the entrainment of the
gas and dust into a circumbinary structure.Comment: 10 pages, 12 figure
The extended atmospheres of Mira variables probed by VLTI, VLBA, and APEX
We present an overview on our project to study the extended atmospheres and
dust formation zones of Mira stars using coordinated observations with the Very
Large Telescope Interferometer (VLTI), the Very Long Baseline Array (VLBA), and
the Atacama Pathfinder Experiment (APEX). The data are interpreted using an
approach of combining recent dynamic model atmospheres with a radiative
transfer model of the dust shell, and combining the resulting model structure
with a maser propagation model.Comment: 5 pages, to appear in Proc. of "Why Galaxies Care About AGB Stars
II", ASP Conf. Ser., Franz Kerschbaum, Thomas Lebzelter, and Bob Wing (eds.
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