71 research outputs found
Long period variables and mass loss in the globular clusters NGC 362 and NGC 2808
The pulsation periods of long period variables (LPVs) depend on their mass
and helium abundance as well as on their luminosity and metal abundance.
Comparison of the observed periods of LPVs in globular clusters with models is
capable of revealing the amount of mass lost on the giant branch and the helium
abundance.} {We aim to determine the amount of mass loss that has occurred on
the giant branches of the low metallicity globular clusters NGC 362 and NGC
2808. We also aim to see if the LPVs in NGC 2808 can tell us about helium
abundance variations in this cluster.} We have used optical monitoring of NGC
362 and NGC 2808 to determine periods for the LPVs in these clusters. We have
made linear pulsation models for the pulsating stars in these clusters taking
into account variations in mass and helium abundance. Reliable periods have
been determined for 11 LPVs in NGC 362 and 15 LPVs in NGC 2808. Comparison of
the observed variables with models in the logP - K diagram shows that mass loss
of ~0.15-0.2 Msun is required on the first giant branch in these clusters, in
agreement with estimates from other methods. In NGC 2808, there is evidence
that a high helium abundance of Y~0.4 is required to explain the periods of
several of the LPVs. It would be interesting to determine periods for LPVs in
other Galactic globular clusters where a helium abundance variation is
suspected to see if the completely independent test for a high helium abundance
provided by the LPVs can confirm the high helium abundance estimates.Comment: 13 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
Two barium stars in the Galactic bulge
Barium stars conserve important information on the s-process and the third
dredge-up in intermediate mass stars. Their discovery in various environments
is therefore of great help to test nucleosynthesis and mixing models. Our aim
is to analyse two stars with a very strong barium line detected in a large
survey of red giants in the Galactic bulge. Abundance analysis was done
comparing synthetic model spectra based on the COMARCS code with our medium
resolution spectra. Abundances of Ba, La, Y, and Fe were determined. Beside the
two main targets, the analysis was also applied to two comparison stars. We
confirm that both stars are barium stars. They are the first ones of this kind
identified in the Galactic bulge. Their barium excesses are among the largest
values found up to now. The elemental abundances are compared with current
nucleosynthesis and mixing models. Furthermore, we estimate a frequency of
barium stars in the Galactic bulge of about 1%, which is identical to the value
for disc stars.Comment: 4 pages, accepted for publication in A&
Oxygen isotopic ratios in intermediate-mass red giants
Context. The abundances of the three main isotopes of oxygen are altered in
the course of the CNO-cycle. When the first dredge-up mixes the burning
products to the surface, the nucleosynthesis processes can be probed by
measuring oxygen isotopic ratios. Aims. By measuring 16O/17O and 16O/18O in red
giants of known mass we compare the isotope ratios with predictions from
stellar and galactic evolution modelling. Methods. Oxygen isotopic ratios were
derived from the K-band spectra of six red giants. The sample red giants are
open cluster members with known masses of between 1.8 and 4.5 Msun . The
abundance determination employs synthetic spectra calculated with the COMARCS
code. The effect of uncertainties in the nuclear reaction rates, the mixing
length, and of a change in the initial abundance of the oxygen isotopes was
determined by a set of nucleosynthesis and mixing models using the FUNS code.
Results. The observed 16O/17O ratios are in good agreement with the model
results, even if the measured values do not present clear evidence of a
variation with the stellar mass. The observed 16O/18O ratios are clearly lower
than the predictions from our reference model. Variations in nuclear reaction
rates and mixing length parameter both have only a very weak effect on the
predicted values. The 12C/13C ratios of the K giants studied implies the
absence of extra-mixing in these objects. Conclusions. A comparison with
galactic chemical evolution models indicates that the 16O/18O abundance ratio
underwent a faster decrease than predicted. To explain the observed ratios, the
most likely scenario is a higher initial 18O abundance combined with a lower
initial 16 O abundance. Comparing the measured 18 O/17 O ratio with the
corresponding value for the ISM points towards an initial enhancement of 17O as
well. Limitations imposed by the observations prevent this from being a
conclusive result.Comment: 9 pages, accepted for publication in Astronomy & Astrophysic
Abundance analysis for long-period variables II. RGB and AGB stars in the globular cluster 47\,Tuc
Asymptotic giant branch (AGB) stars play a key role in the enrichment of
galaxies with heavy elements. Due to their large amplitude variability, the
measurement of elemental abundances is a highly challenging task that has not
been solved in a satisfactory way yet.
Following our previous work we use hydrostatic and dynamical model
atmospheres to simulate observed high-resolution near-infrared spectra of 12
variable and non-variable red giants in the globular cluster 47 Tuc. The 47 Tuc
red giants are independently well-characterized in important parameters (mass,
metallicity, luminosity). The principal aim was to compare synthetic spectra
based on the dynamical models with observational spectra of 47 Tuc variables.
Assuming that the abundances are unchanged on the upper giant branch in these
low-mass stars, our goal is to estimate the impact of atmospheric dynamics on
the abundance determination.
We present new measurements of the C/O and 12C/13C ratio for 5 non-variable
red giants in 47Tuc. The equivalent widths measured for our 7 variable stars
strongly differ from the non-variable stars and cannot be reproduced by either
hydrostatic or dynamical model atmospheres. Nevertheless, the dynamical models
fit the observed spectra of long-period variables much better than any
hydrostatic model. For some spectral features, the variations in the line
intensities predicted by dynamical models over a pulsation cycle give similar
values as a sequence of hydrostatic models with varying temperature and
constant surface gravity.Comment: 16 pages, 12 figures; accepted for publication in A&
CRIRES-POP: a library of high resolution spectra in the near-infrared. III. Line identification in the K-giant 10 Leo
Context: High-resolution spectra in the near-infrared (NIR) are an important
tool for the detailed study of stellar atmospheres. The accurate identification
of elements and molecules in these spectra can be used to determine chemical
abundances and physical conditions in the photosphere of the observed star.
Such identifications require precise line positions and strengths of both
atomic and molecular features. Aims: This work focusses on the full
identification of absorption lines in the NIR spectrum of the K-giant 10 Leo,
including previously unidentified lines. The large number and complexity of the
observed absorption lines require a deep search for potential spectral
signatures to enable an unambiguous assignment to specific elements or
molecular species. We aim to improve the published line lists of metals, some
of which are determined by model calculations only, and many of which presently
lack the completeness and accuracy of line parameters. Methods: The CRIRES-POP
project provided high-resolution, high signal-to-noise ratio (S/N) spectra of
several bright stars in the 1 to 5 m range. For the K-giant 10 Leo, a
spectrum corrected for telluric absorption and with precise wavelength
calibration is available. This has been analysed by comparison with model
spectra and up-to-date line lists. Results: We identified lines of 29 elements
and eight molecular species. While the positions of many known lines could be
confirmed, about 6% of all lines detected in 10 Leo could not be attributed to
any known feature. For CO and its isotopologues, molecular constants could be
derived and several additional lines identified. We report major
inconsistencies for some prominent lines. In addition, abundances for several
key elements in 10 Leo are provided.Comment: Accepted for publication in A&
Constraining the structure and formation of the Galactic bulge from a field in its outskirts. FLAMES-GIRAFFE spectra of about 400 red giants around (l,b)=(0{\deg},-10{\deg})
The presence of two stellar populations in the Milky Way bulge has been
reported recently. We aim at studying the abundances and kinematics of stars in
the outer bulge, thereby providing additional constraints on models of its
formation. Spectra of 401 red giant stars in a field at
(l,b)=(0{\deg},-10{\deg}) were obtained with FLAMES at the VLT. Stars of
luminosities down to below the two bulge red clumps (RCs) are included. From
these spectra we measure general metallicities, abundances of Fe and the
alpha-elements, and radial velocities (RV) of the stars. These measurements as
well as photometric data are compared to simulations with the Besancon and
TRILEGAL models of the Galaxy. We confirm the presence of two populations among
our sample stars: i) a metal-rich one at [M/H] ~+0.3, comprising about 30% of
the sample, with low RV dispersion and low alpha-abundance, and ii) a
metal-poor population at [M/H] ~-0.6 with high RV dispersion and high
alpha-abundance. The metal-rich population could be connected to the Galactic
bar. We identify this population as the carrier of the double RC feature. We do
not find a significant difference in metallicity or RV between the two RCs, a
small difference in metallicity being probably due to a selection effect. The
RV dispersion agrees well with predictions of the Besancon Galaxy model, but
the metallicity of the "thick bulge" model component should be shifted to lower
metallicity by 0.2 to 0.3dex to well reproduce the observations. We present
evidence that the metallicity distribution function depends on the evolutionary
state of the sample stars, suggesting that enhanced mass loss preferentially
removes metal-rich stars. We also confirm the decrease of \alpha-element
over-abundance with increasing metallicity.Comment: 19 pages (excluding on-line table), 21 figures, accepted for
publication in A&
Dust, pulsation, chromospheres and their role in driving mass loss from red giants in Galactic globular clusters
Context: Mass loss from red giants in old globular clusters affects the
horizontal branch (HB) morphology and post-HB stellar evolution including the
production of ultraviolet-bright stars, dredge up of nucleosynthesis products
and replenishment of the intra-cluster medium. Studies of mass loss in globular
clusters also allows one to investigate the metallicity dependence of the mass
loss from cool, low-mass stars down to very low metallicities.
Aims: We present an analysis of new VLT/UVES spectra of 47 red giants in the
Galactic globular clusters 47 Tuc (NGC 104), NGC 362, omega Cen (NGC 5139), NGC
6388, M54 (NGC 6715) and M15 (NGC 7078). The spectra cover the wavelength
region 6100-9900A at a resolving power of R = 110,000. Some of these stars are
known to exhibit mid-infrared excess emission indicative of circumstellar dust.
Our aim is to detect signatures of mass loss, identify the mechanism(s)
responsible for such outflows, and measure the mass-loss rates.
Methods: We determine for each star its effective temperature, luminosity,
radius and escape velocity. We analyse the H-alpha and near-infrared calcium
triplet lines for evidence of outflows, pulsation and chromospheric activity,
and present a simple model for estimating mass-loss rates from the H-alpha line
profile. We compare our results with a variety of other, independent methods.
Results: We argue that a chromosphere persists in Galactic globular cluster
giants and controls the mass-loss rate to late-K/early-M spectral types, where
pulsation becomes strong enough to drive shock waves at luminosities above the
RGB tip. This transition may be metallicity-dependent. We find mass-loss rates
of ~10^-7 to 10^-5 solar masses per year, largely independent of metallicity.Comment: 23 pages, 17 figures, accepted for publication in Astronomy and
Astrophysic
Modelling Long-Period Variables -- II. Fundamental mode pulsation in the nonlinear regime
Long-period variability in luminous red giants has several promising
applications, all of which require models able to accurately predict pulsation
periods. Linear pulsation models have proven successful in reproducing the
observed periods of overtone modes in evolved red giants, but they fail to
accurately predict their fundamental mode periods. Here, we use a 1D
hydrodynamic code to investigate the long-period variability of M-type
asymptotic giant branch stars in the nonlinear regime. We examine the period
and stability of low-order radial pulsation modes as a function of mass and
radius, and find overtone mode periods in complete agreement with predictions
from linear pulsation models. In contrast, nonlinear models predict an earlier
onset of dominant fundamental mode pulsation, and shorter periods at large
radii. Both features lead to a substantially better agreement with
observations, that we verify against OGLE and Gaia data for the Magellanic
Clouds. We provide simple analytic relations describing the nonlinear
fundamental mode period-mass-radius relation. Differences with respect to
linear predictions originate from the readjustment of the envelope structure
induced by large-amplitude pulsation. We investigate the impact of turbulent
viscosity on linear and nonlinear pulsation, and probe possible effects of
varying metallicity and carbon abundance.Comment: 18 pages, 17 figures; accepted for publication in MNRA
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