321 research outputs found
Understanding AGB evolution in Galactic bulge stars from high-resolution infrared spectroscopy
An analysis of high-resolution near-infrared spectra of a sample of 45
asymptotic giant branch (AGB) stars towards the Galactic bulge is presented.
The sample consists of two subsamples, a larger one in the inner and
intermediate bulge, and a smaller one in the outer bulge. The data are analysed
with the help of hydrostatic model atmospheres and spectral synthesis. We
derive the radial velocity of all stars, and the atmospheric chemical mix
([Fe/H], C/O, C/C, Al, Si, Ti, and Y) where possible. Our ability
to model the spectra is mainly limited by the (in)completeness of atomic and
molecular line lists, at least for temperatures down to K. We find that the subsample in the inner and intermediate
bulge is quite homogeneous, with a slightly sub-solar mean metallicity and only
few stars with super-solar metallicity, in agreement with previous studies of
non-variable M-type giants in the bulge. All sample stars are oxygen-rich,
C/O1.0. The C/O and carbon isotopic ratios suggest that third dredge-up
(3DUP) is absent among the sample stars, except for two stars in the outer
bulge that are known to contain technetium. These stars are also more
metal-poor than the stars in the intermediate or inner bulge. Current stellar
masses are determined from linear pulsation models. The masses, metallicities
and 3DUP behaviour are compared to AGB evolutionary models. We conclude that
these models are partly in conflict with our observations. Furthermore, we
conclude that the stars in the inner and intermediate bulge belong to a more
metal-rich population that follows bar-like kinematics, whereas the stars in
the outer bulge belong to the metal-poor, spheroidal bulge population.Comment: 21 pages, 13 figures, 6 tables (incl. appendix), years of work,
published in MNRA
Synthetic photometry for M and K giants and stellar evolution: hydrostatic dust-free model atmospheres and chemical abundances
Based on a grid of hydrostatic spherical COMARCS models for cool stars, we have calculated observable properties of these objects, which will be mainly used in combination with stellar evolution tracks and population synthesis tools. The high-resolution opacity sampling and low-resolution convolved spectra as well as bolometric corrections for a large number of filter systems are made electronically available. We exploit those data to study the effect of mass, C/O ratio and nitrogen abundance on the photometry of K and M giants. Depending on effective temperature, surface gravity and the chosen wavelength ranges, variations of the investigated parameters cause very weak to moderate and, in the case of C/O values close to 1, even strong shifts of the colours. For the usage with stellar evolution calculations, they will be treated as correction factors applied to the results of an interpolation in the main quantities. When we compare the synthetic photometry to observed relations and to data from the Galactic bulge, we find in general a good agreement. Deviations appear for the coolest giants showing pulsations, mass-loss and dust shells, which cannot be described by hydrostatic models
A Dramatic Decrease in Carbon Star Formation in M31
We analyze resolved stellar near-infrared photometry of 21 HST fields in M31
to constrain the impact of metallicity on the formation of carbon stars.
Observations of nearby galaxies show that the carbon stars are increasingly
rare at higher metallicity. Models indicate that carbon star formation
efficiency drops due to the decrease in dredge-up efficiency in metal-rich
thermally-pulsing Asymptotic Giant Branch (TP-AGB) stars, coupled to a higher
initial abundance of oxygen. However, while models predict a metallicity
ceiling above which carbon stars cannot form, previous observations have not
yet pinpointed this limit. Our new observations reliably separate carbon stars
from M-type TP-AGB stars across 2.6-13.7 kpc of M31's metal-rich disk using HST
WFC3/IR medium-band filters. We find that the ratio of C to M stars (C/M)
decreases more rapidly than extrapolations of observations in more metal-poor
galaxies, resulting in a C/M that is too low by more than a factor of 10 in the
innermost fields and indicating a dramatic decline in C star formation
efficiency at metallicities higher than [M/H] -0.1 dex. The
metallicity ceiling remains undetected, but must occur at metallicities higher
than what is measured in M31's inner disk ([M/H] +0.06 dex).Comment: 16 pages, 13 Figures; text clarifications in response to the referee.
Results are unchanged; accepted for publication in Ap
AESOPUS 2.0: Low-Temperature Opacities with Solid Grains
In this study we compute the equation of state and Rosseland mean opacity
from temperatures of T~30000 K down to T~400 K, pushing the capabilities of the
AESOPUS code (Marigo et al., 2022; Marigo & Aringer, 2009) into the regime
where solid grains can form. The GGchem code (Woitke et al. 2018) is used to
solve the chemistry for temperatures less than ~3000 K. Atoms, molecules, and
dust grains in thermodynamic equilibrium are all included in the equation of
state. To incorporate monochromatic atomic and molecular cross sections, an
optimized opacity sampling technique is used. The Mie theory is employed to
calculate the opacity of 43 grain species. Tables of Rosseland mean opacities
for scaled-solar compositions are provided. Based on our computing resources,
opacities for other chemical patterns, as well as various grain sizes,
porosity, and shapes, can be easily computed upon user request to the
corresponding author.Comment: 14 pages, 8 figures, resubmitted to ApJ following moderate revisio
Elemental abundances in AGB stars and the formation of the Galactic bulge
We obtained high-resolution near-IR spectra of 45 AGB stars located in the
Galactic bulge. The aim of the project is to determine key elemental abundances
in these stars to help constrain the formation history of the bulge. A further
aim is to link the photospheric abundances to the dust species found in the
winds of the stars. Here we present a progress report of the analysis of the
spectra.Comment: 2 pages, 1 figure. To appear in the proceedings of the conference
"Assembling the Puzzle of the Milky Way", Le Grand-Bornand, France, 17-22
April 2011, European Physical Journal, editors C. Reyl\'e, A. Robin and M.
Schulthei
The mass-loss, expansion velocities, and dust production rates of carbon stars in the Magellanic Clouds
The properties of carbon stars in the Magellanic Clouds (MCs) and their total dust production rates are predicted by fitting their spectral energy distributions (SED) over pre-computed grids of spectra reprocessed by dust. The grids are calculated as a function of the
stellar parameters by consistently following the growth for several dust species in their circumstellar envelopes, coupled with a stationary wind. Dust radiative transfer is computed taking as input the results of the dust growth calculations. The optical constants for amorphous carbon are selected in order to reproduce different observations in the
infrared and optical bands of Gaia Data Release 2. We find a tail of extreme mass-losing carbon stars in the Large Magellanic Cloud (LMC) with low gas-to-dust ratios that is not present in the Small Magellanic Cloud (SMC). Typical gas-to-dust ratios are around 700 for the extreme stars, but they can be down to similar to 160-200 and similar to 100 for
a few sources in the SMC and in the LMC, respectively. The total dust production rate for the carbon star population is similar to 1.77 +/- 0.45 x 10(-5) M-circle dot yr(-1), for the LMC, and similar to 2.52 +/- 0.96 x 10(-6) M-circle dot yr(-1), for the SMC. The extreme carbon stars
observed with the Atacama Large Millimeter Array and their wind speed are studied in detail. For the most dust-obscured star in this sample the estimated mass-loss rate is similar to 6.3 x 10(-5) M-circle dot yr(-1). The grids of spectra are available at:(1) and included in the
SED-fitting python package for fitting evolved stars.(2
Near-IR spectroscopy of OH/IR stars in the Galactic Centre
Context. Galactic Centre (GC) OH/IR stars can be, based on the expansion
velocities of their circumstellar shells, divided into two groups which are
kinematically different and therefore are believed to have evolved from
different stellar populations. Aims. To study the metallicity distribution of
the OH/IR stars population in the GC on basis of a theoretical relation between
EW(Na), EW(Ca) and EW(CO) and the metallicity. Methods. For 70 OH/IR stars in
the GC, we obtained near-IR spectra. The equivalent line widths of NaI, CaI,
12CO(2,0) and the curvature of the spectrum around 1.6 micron due to water
absorption are determined. Results. The near-IR spectrum of OH/IR stars is
influenced by several physical processes. OH/IR stars are variable stars
suffering high mass-loss rates. The dust that is formed around the stars
strongly influences the near-IR spectra and reduces the equivalent line widths
of NaI, CaI. A similar effect is caused by the water content in the outer
atmosphere of the OH/IR star. Because of these effects, it is not possible with
our low resolution near-infrared spectroscopy to determine the metallicities of
these stars.Comment: 20 pages, 21 figures, accepted for publication in A&A on 18/04/200
AMBER/VLTI observations of 5 giant stars
While the search for exoplanets around main sequence stars more massive than
the Sun have found relatively few such objects, surveys performed around giant
stars have led to the discovery of more than 30 new exoplanets. The interest in
studying planet hosting giant stars resides in the possibility of investigating
planet formation around stars more massive than the Sun. Masses of isolated
giant stars up to now were only estimated from evolutionary tracks, which led
to different results depending on the physics considered. To calibrate the
theory, it is therefore important to measure a large number of giant star
diameters and masses as much as possible independent of physical models. We aim
in the determination of diameters and effective temperatures of 5 giant stars,
one of which is known to host a planet. AMBER/VLTI observations with the ATs
were executed in low resolution mode on 5 giant stars. In order to measure high
accurate calibrated squared visibilities, a calibrator-star-calibrator
observational sequence was performed. We measured the uniform disk and
limb-darkened angular diameters of 4 giant stars. The effective temperatures
were also derived by combining the bolometric luminosities and the
interferometric diameters. Lower effective temperatures were found when
compared to spectroscopic measurements. The giant star HD12438 was found to
have an unknown companion star at an angular separation of ~ 12 mas. Radial
velocity measurements present in the literature confirm the presence of a
companion with a very long orbital period (P ~ 11.4 years).}Comment: accepted for publication in A&
Is there a metallicity ceiling to form carbon stars? - A novel technique reveals a scarcity of C stars in the inner M31 disk
We use medium-band near-infrared (NIR) Hubble Space Telescope WFC3 photometry with model NIR spectra of asymptotic giant branch (AGB) stars to develop a new tool for efficiently distinguishing carbon-rich (C-type) AGB stars from oxygen-rich (M-type) AGB stars in galaxies at the edge of and outside the Local Group. We present the results of a test of this method on a region of the inner disk of M31, where we find a surprising lack of C stars, contrary to the findings of previous C star searches in other regions of M31. We find only one candidate C star (plus up to six additional, less certain C star candidates), resulting in an extremely low ratio of C to M stars (C/M = (3.3^(+20)_(-0.1)) x 10^(-4)) that is one to two orders of magnitude lower than other C/M estimates in M31. The low C/M ratio is likely due to the high metallicity in this region which impedes stars from achieving C/O > 1 in their atmospheres. These observations provide stringent constraints to evolutionary models of metal-rich AGB stars and suggest that there is a metallicity threshold above which M stars are unable to make the transition to C stars, dramatically affecting AGB mass loss and dust production and, consequently, the observed global properties of metal-rich galaxies
Three-micron spectra of AGB stars and supergiants in nearby galaxies
The dependence of stellar molecular bands on the metallicity is studied using
infrared L-band spectra of AGB stars (both carbon-rich and oxygen-rich) and
M-type supergiants in the Large and Small Magellanic Clouds (LMC and SMC) and
in the Sagittarius Dwarf Spheroidal Galaxy. The spectra cover SiO bands for
oxygen-rich stars, and acetylene (C2H2), CH and HCN bands for carbon-rich AGB
stars. The equivalent width of acetylene is found to be high even at low
metallicity. The high C2H2 abundance can be explained with a high
carbon-to-oxygen (C/O) ratio for lower metallicity carbon stars. In contrast,
the HCN equivalent width is low: fewer than half of the extra-galactic carbon
stars show the 3.5micron HCN band, and only a few LMC stars show high HCN
equivalent width. HCN abundances are limited by both nitrogen and carbon
elemental abundances. The amount of synthesized nitrogen depends on the initial
mass, and stars with high luminosity (i.e. high initial mass) could have a high
HCN abundance. CH bands are found in both the extra-galactic and Galactic
carbon stars. None of the oxygen-rich LMC stars show SiO bands, except one
possible detection in a low quality spectrum. The limits on the equivalent
widths of the SiO bands are below the expectation of up to 30angstrom for LMC
metallicity. Several possible explanations are discussed. The observations
imply that LMC and SMC carbon stars could reach mass-loss rates as high as
their Galactic counterparts, because there are more carbon atoms available and
more carbonaceous dust can be formed. On the other hand, the lack of SiO
suggests less dust and lower mass-loss rates in low-metallicity oxygen-rich
stars. The effect on the ISM dust enrichment is discussed.Comment: accepted for A&
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