1,170 research outputs found
The time variation in infrared water-vapour bands in Mira variables
The time variation in the water-vapour bands in oxygen-rich Mira variables
has been investigated using multi-epoch ISO/SWS spectra of four Mira variables
in the 2.5-4.0 micron region. All four stars show H2O bands in absorption
around minimum in the visual light curve. At maximum, H2O emission features
appear in the ~3.5-4.0 micronm region, while the features at shorter
wavelengths remain in absorption. These H2O bands in the 2.5-4.0 micron region
originate from the extended atmosphere.
The analysis has been carried out with a disk shape, slab geometry model. The
observed H2O bands are reproduced by two layers; a `hot' layer with an
excitation temperature of 2000 K and a `cool' layer with an excitation
temperature of 1000-1400 K. The radii of the `hot' layer (R_hot) are ~1 R_* at
visual minimum and 2 R_* at maximum, where R_* is a radius of background source
of the model. The time variation of R_hot/R_* from 1 to 2 is attributed to the
actual variation in the radius of the H2O layer. A high H2O density shell
occurs near the surface of the star around minimum, and moves out with the
stellar pulsation. This shell gradually fades away after maximum, and a new
high H2O density shell is formed in the inner region again at the next minimum.
Due to large optical depth of H2O, the near-infrared variability is dominated
by the H2O layer, and the L'-band flux correlates with the area of the H2O
shell. The infrared molecular bands trace the structure of the extended
atmosphere and impose appreciable effects on near-infrared light curve of Mira
variables.Comment: 15 pages, 16 figures, accepted by A&
Probing the mass-loss history of the unusual Mira variable R Hydrae through its infrared CO wind
Context. The unusual Mira variable R Hya is well known for its declining period between AD 1770 and 1950, which is possibly attributed to a recent thermal pulse. Aims. The goal of this study is to probe the circumstellar envelope (CSE) around R Hya and to check for a correlation between the derived density structure and the declining period. Methods. We investigate the CSE around R Hya by performing an in-depth analysis of (1.) the photospheric light scattered by three vibration-rotation transitions in the fundamental band of CO at 4.6 m; and (2.) the pure rotational CO J = 1-0 through 6-5 emission lines excited in the CSE. The vibrational-rotational lines trace the inner CSE within 3.5´´, whereas the pure rotational CO lines are sensitive probes of the cooler gas further out in the CSE. Results. The combined analysis bear evidence of a change in mass-loss rate some 220 yr ago (at ~150 or ~1.9 arcsec from the star). While the mass-loss rate before AD 1770 is estimated to be ~ 2 imes 10^ /yr, the present day mass-loss rate is a factor of ~20 lower. The derived mass-loss history nicely agrees with the mass-loss rate estimates by Zijlstra et al. (2002) on the basis of the period decline. Moreover, the recent detection of an AGB-ISM bow shock around R Hya at 100 arcsec to the west by Wareing et al. (2006) shows that the detached shell seen in the 60 m IRAS images can be explained by a slowing-down of the stellar wind by surrounding matter and that no extra mass-loss modulation around 1-2 arcmin needs to be invoked. Conclusions. Our results give empirical evidence to the thermal-pulse model, which is capable of explaining both the period evolution and the mass-loss history of R Hya
The Frequency of Rapid Rotation Among K Giant Stars
We present the results of a search for unusually rapidly rotating giant stars
in a large sample of K giants (~1300 stars) that had been spectroscopically
monitored as potential targets for the Space Interferometry Mission's
Astrometric Grid. The stars in this catalog are much fainter and typically more
metal-poor than those of other catalogs of red giant star rotational
velocities, but the spectra generally only have signal-to-noise (S/N) of
~20-60, making the measurement of the widths of individual lines difficult. To
compensate for this, we have developed a cross-correlation method to derive
rotational velocities in moderate S/N echelle spectra to efficiently probe this
sample for rapid rotator candidates. We have discovered 28 new red giant rapid
rotators as well as one extreme rapid rotator with a vsini of 86.4 km/s. Rapid
rotators comprise 2.2% of our sample, which is consistent with other surveys of
brighter, more metal-rich K giant stars. Although we find that the temperature
distribution of rapid rotators is similar to that of the slow rotators, this
may not be the case with the distributions of surface gravity and metallicity.
The rapid rotators show a slight overabundance of low gravity stars and as a
group are significantly more metal-poor than the slow rotators, which may
indicate that the rotators are tidally-locked binaries.Comment: Accepted for publication in ApJ. 25 pages, 9 figures, 3 tables.
Tables 1 and 2 are provided in their full form as plain text ancillary file
A Model for the Formation of Large Circumbinary Disks Around Post AGB Stars
We propose that the large, radius of ~1000 AU, circumbinary rotating disks
observed around some post-asymptotic giant branch (post-AGB) binary stars are
formed from slow AGB wind material that is pushed back to the center of the
nebula by wide jets. We perform 2D-axisymmetrical numerical simulations of fast
and wide jets that interact with the previously ejected slow AGB wind. In each
system there are two oppositely launched jets, but we use the symmetry of the
problem and simulate only one jet. A large circularization-flow (vortex) is
formed to the side of the jet which together with the thermal pressure of the
shocked jet material accelerate cold slow-wind gas back to the center from
distances of ~1000-10000 AU. We find for the parameters we use that up to 0.001
Mo is back-flowing to the center. We conjecture that the orbital angular
momentum of the disk material results from the non-axisymmetric structure of
jets launched by an orbiting companion. This conjecture will have to be tested
with 3D numerical codes.Comment: New Astronomy, in pres
The puzzling dredge-up pattern in NGC 1978
Low-mass stars are element factories that efficiently release their products
in the final stages of their evolution by means of stellar winds. Since they
are large in number, they contribute significantly to the cosmic matter cycle.
To assess this contribution quantitatively, it is crucial to obtain a detailed
picture of the stellar interior, particularly with regard to nucleosynthesis
and mixing mechanisms. We seek to benchmark stellar evolutionary models of
low-mass stars. In particular, we measure the surface abundance of ^{12}C in
thermally pulsing AGB stars with well-known mass and metallicity, which can be
used to infer information about the onset and efficiency of the third
dredge-up. We recorded high-resolution near-infrared spectra of AGB stars in
the LMC cluster NGC 1978. The sample comprised both oxygen-rich and carbon-rich
stars, and is well-constrained in terms of the stellar mass, metallicity, and
age. We derived the C/O and ^{12}C/^{13}C ratio from the target spectra by a
comparison to synthetic spectra. Then, we compared the outcomes of stellar
evolutionary models with our measurements. The M stars in NGC 1978 show values
of C/O and ^{12}C/^{13}C that can best be explained with moderate extra-mixing
on the RGB coupled to a moderate oxygen enhancement in the chemical
composition. These oxygen-rich stars do not seem to have undergone third
dredge-up episodes (yet). The C stars show carbon-to-oxygen and carbon isotopic
ratios consistent with the occurrence of the third dredge-up. We did not find S
stars in this cluster. None of the theoretical schemes that we considered was
able to reproduce the observations appropriately. Instead, we discuss some
non-standard scenarios to explain the puzzling abundance pattern in NGC 1978.Comment: 16 pages, 9 figures, 4 tables, accepted for publication in A&A,
language revise
Chemical compositions of stars in two stellar streams from the Galactic thick disk
We present abundances for 20 elements for stars in two stellar streams
identified by Arifyanto & Fuchs (2006, A&A, 449, 533): 18 stars from the
Arcturus stream and 26 from a new stream, which we call AF06 stream, both from
the Galactic thick disk. Results show both streams are metal-poor and very old
(1014 Gyrs) with kinematics and abundances overlapping with the properties
of local field thick disk stars. Both streams exhibit a range in metallicity
but with relative elemental abundances that are identical to those of thick
disk stars of the same metallicity. These results show that neither stream can
result from dissolution of an open cluster. It is highly unlikely that either
stream represents tidal debris from an accreted satellite galaxy. Both streams
most probably owe their origin to dynamical perturbations within the Galaxy.Comment: 10 figures, Accepted for publication in MNRA
Kinematics and H_2 morphology of the multipolar Post-AGB star IRAS 16594-4656
context: The spectrum of IRAS 16594-4656 shows shock excited H_2 emission and
collisionally excited emission lines such as[O I],[C I],and [Fe II]. aim: The
goal is to determine the location of the H_2 and [Fe II] shock emission, to
determine the shock velocities,and constrain the physical properties in the
shock. methods: High resolution spectra of the H_2 1-0 S(1),H_2 2-1 S(1), [Fe
II], and Pa emission lines were obtained with the near infrared
spectrograph Phoenix on Gemini South. results: The position-velocity diagrams
of H_2 1-0 S(1), H_2 2-1 S(1), and [Fe II] are presented. The H_2 and [Fe II]
emission is spatially extended. The collisionally excited [O I] and [C I]
optical emission lines have a similar double peaked profile compared to the
extracted H_2 profile and appear to be produced in the same shock. They all
indicate an expansion velocity of ~8 km/s and the presence of a neutral, very
high density region with about 3 x 10^6 to 5 x10^7 cm. The
[Fe II] emission however is single peaked. It has a gaussian FWHM of 30 km/s
and a total width of 62 km/s at 1% of the peak. The Pa profile is even
wider with a gaussian FWHM of 48 km/s and a total width of 75 km/s at 1% of the
peak. conclusions: The H emission is excited in a slow 5 to 20 km/s shock
into dense material at the edge of the lobes, caused by the interaction of the
AGB ejecta and the post-AGB wind. The 3D representation of the H_2 data shows a
hollow structure with less H_2 emission in the equatorial region. The [Fe II]
emission is not present in the lobes, but originates close to the central star
in fast shocks in the post-AGB wind or in a disk. The Pa emission also
appears to originate close to the star.Comment: 11 pages and 8 figures; A&A in press; the paper includig high
resolution figures can be downloaded from
http://homepage.oma.be/gsteene/publications.htm
The origin of the split red clump in the Galactic bulge of the Milky Way
Near the minor axis of the Galactic bulge, at latitudes b < -5 degrees, the
red giant clump stars are split into two components along the line of sight. We
investigate this split using the three fields from the ARGOS survey that lie on
the minor axis at (l,b) = (0,-5), (0,-7.5), (0,-10) degrees. The separation is
evident for stars with [Fe/H] > -0.5 in the two higher-latitude fields, but not
in the field at b = -5 degrees. Stars with [Fe/H] < -0.5 do not show the split.
We compare the spatial distribution and kinematics of the clump stars with
predictions from an evolutionary N-body model of a bulge that grew from a disk
via bar-related instabilities. The density distribution of the peanut-shaped
model is depressed near its minor axis. This produces a bimodal distribution of
stars along the line of sight through the bulge near its minor axis, very much
as seen in our observations. The observed and modelled kinematics of the two
groups of stars are also similar. We conclude that the split red clump of the
bulge is probably a generic feature of boxy/peanut bulges that grew from disks,
and that the disk from which the bulge grew had relatively few stars with
[Fe/H] < -0.5Comment: 12 pages, 9 figures, accepted for publication in Ap
Red supergiants as potential Type IIn supernova progenitors: Spatially resolved 4.6 micron CO emission around VY CMa and Betelgeuse
We present high-resolution 4.6micron CO spectra of the circumstellar
environments of two RSGs that are potential SN progenitors: Betelgeuse and VY
CMa. Around Betelgeuse, 12CO emission within 3arcsec follows a mildly clumpy
but otherwise spherical shell, smaller than its 55arcsec shell in KI
lambda7699. In stark contrast, 4.6micron CO emission around VY CMa is
coincident with bright KI in its clumpy asymmetric reflection nebula, within
5arcsec of the star. Our CO data reveal redshifted features not seen in KI
spectra of VY CMa, indicating a more isotropic distribution of gas punctuated
by randomly distributed asymmetric clumps. The relative CO and KI distribution
in Betelgeuse arises from ionization effects within a steady wind, whereas in
VY CMa, KI is emitted from skins of CO cloudlets resulting from episodic mass
ejections 500--1000 yr ago. In both cases, CO and KI trace potential pre-SN
circumstellar matter: we conclude that an extreme RSG like VY CMa might produce
a Type IIn event like SN1988Z if it were to explode in its current state, but
Betelgeuse will not. VY CMa demonstrates that LBVs are not necessarily the only
progenitors of SNe IIn, but it underscores the requirement that SNe IIn suffer
enhanced episodic mass loss shortly before exploding.Comment: 16 pages, AJ accepte
Fundamental Parameters and Chemical Composition of Arcturus
We derive a self-consistent set of atmospheric parameters and abundances of
17 elements for the red giant star Arcturus: Teff = 4286+/-30 K, logg =
1.66+/-0.05, and [Fe/H] = -0.52+/-0.04. The effective temperature was
determined using model atmosphere fits to the observed spectral energy
distribution from the blue to the mid-infrared (0.44 to 10 um). The surface
gravity was calculated using the trigonometric parallax of the star and stellar
evolution models. A differential abundance analysis relative to the solar
spectrum allowed us to derive iron abundances from equivalent width
measurements of 37 FeI and 9 FeII lines, unblended in the spectra of both
Arcturus and the Sun; the [Fe/H] value adopted is derived from FeI lines. We
also determine the mass, radius, and age of Arcturus: M = 1.08+/-0.06 Msun, R =
25.4+/-0.2 Rsun, and t = 7.1(+1.5/-1.2) Gyr. Finally, abundances of the
following elements are measured from an equivalent width analysis of atomic
features: C, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, and Zn. We
find the chemical composition of Arcturus typical of that of a local thick-disk
star, consistent with its kinematics.Comment: ApJ, in pres
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