30 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
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&
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.
Inhomogeneities in molecular layers of Mira atmospheres
We obtained K-band spectro-interferometric observations of the Miras R Cnc, X
Hya, W Vel, and RW Vel with a spectral resolution of 1500 using the VLTI/AMBER
instrument. We obtained concurrent JHKL photometry using the the Mk II
instrument at the SAAO. Our sources have wavelength-dependent visibility values
that are consistent with earlier low-resolution AMBER observations of S Ori and
with the predictions of dynamic model atmosphere series based on self-excited
pulsation models. The wavelength-dependent UD diameters show a minimum near the
near-continuum bandpass at 2.25 um. They increase by up to 30% toward the H2O
band at 2.0 um and by up to 70% at the CO bandheads. The dynamic model
atmosphere series show a consistent wavelength-dependence, and their parameters
such as the visual phase, effective temperature, and distances are consistent
with independent estimates. The closure phases have significantly
wavelength-dependent non-zero values indicating deviations from point symmetry.
For example, the R Cnc closure phase is 110 degr in the 2.0 um H2O band,
corresponding for instance to an additional unresolved spot contributing 3% of
the total flux at a separation of ~4 mas. Our observations are consistent with
the predictions of the latest dynamic model atmosphere series based on
self-excited pulsation models. The wavelength-dependent radius variations are
interpreted as the effect of molecular layers. The wavelength-dependent closure
phase values are indicative of deviations from point symmetry at all
wavelengths, thus a complex non-spherical stratification of the extended
atmosphere. In particular, the significant deviation from point symmetry in the
H2O band is interpreted as a signature on large scales of inhomogeneities or
clumps in the water vapor layer. The observed inhomogeneities might be caused
by pulsation- and shock-induced chaotic motion in the extended atmosphere.Comment: 5 pages, 2 figures, accepted for publication as a Letter in Astronomy
and Astrophysic
Accurate effective temperatures of the metal-poor benchmark stars HD140283, HD122563, and HD103095 from CHARA interferometry
Large stellar surveys of the Milky Way require validation with reference to a
set of "benchmark" stars whose fundamental properties are well-determined. For
metal-poor benchmark stars, disagreement between spectroscopic and
interferometric effective temperatures has called the reliability of the
temperature scale into question. We present new interferometric measurements of
three metal-poor benchmark stars, HD 140283, HD 122563, and HD 103095, from
which we determine their effective temperatures. The angular sizes of all the
stars were determined from observations with the PAVO beam combiner at visible
wavelengths at the CHARA array, with additional observations of HD 103095 made
with the VEGA instrument, also at the CHARA array. Together with
photometrically derived bolometric fluxes, the angular diameters give a direct
measurement of the effective temperature. For HD 140283 we find {\theta}_LD =
0.324+/-0.005 mas, Teff = 5787+/-48 K; for HD 122563, {\theta}_LD =
0.926+/-0.011 mas, Teff = 4636+/-37 K; and for HD 103095 {\theta}_LD =
0.595+/-0.007 mas, Teff = 5140+/-49 K. Our temperatures for HD 140283 and HD
103095 are hotter than the previous interferometric measurements by 253 K and
322 K, respectively. We find good agreement between our temperatures and recent
spectroscopic and photometric estimates. We conclude some previous
interferometric measurements have been affected by systematic uncertainties
larger than their quoted errors
The GALAH Survey : Non-LTE departure coefficients for large spectroscopic surveys
19 pages, 25 figures, 2 tables, arXiv abstract abridged; accepted for publication in A&AMassive sets of stellar spectroscopic observations are rapidly becoming available and these can be used to determine the chemical composition and evolution of the Galaxy with unprecedented precision. One of the major challenges in this endeavour involves constructing realistic models of stellar spectra with which to reliably determine stellar abundances. At present, large stellar surveys commonly use simplified models that assume that the stellar atmospheres are approximately in local thermodynamic equilibrium (LTE). To test and ultimately relax this assumption, we have performed non-LTE calculations for different elements (H, Li, C, N, O, Na, Mg, Al, Si, K, Ca, Mn, and Ba), using recent model atoms that have physically-motivated descriptions for the inelastic collisions with neutral hydrogen, across a grid of 1D MARCS model atmospheres that spans , , and . We present the grids of departure coefficients that have been implemented into the GALAH DR3 analysis pipeline in order to complement the extant non-LTE grid for iron. We also present a detailed line-by-line re-analysis of stars from GALAH DR3. We found that relaxing LTE can change the abundances by between and for different lines and stars. Taking departures from LTE into account can reduce the dispersion in the versus plane by up to , and it can remove spurious differences between the dwarfs and giants by up to . The resulting abundance slopes can thus be qualitatively different in non-LTE, possibly with important implications for the chemical evolution of our Galaxy.Peer reviewe
First Results from the Hertzsprung SONG Telescope: Asteroseismology of the G5 Subgiant Star μ Herculis
We report the first asteroseismic results obtained with the Hertzsprung Stellar Observations Network Group Telescope from an extensive high-precision radial-velocity observing campaign of the subgiant μ Herculis. The data set was collected during 215 nights in 2014 and 2015. We detected a total of 49 oscillation modes with l values from zero to three, including some l = 1 mixed modes. Based on the rotational splitting observed in l = 1 modes, we determine a rotational period of 52 days and a stellar inclination angle of 63°. The parameters obtained through modeling of the observed oscillation frequencies agree very well with independent observations and imply a stellar mass between 1.11 and 1.15 and an age of Gyr. Furthermore, the high-quality data allowed us to determine the acoustic depths of the He ii ionization layer and the base of the convection zone