865 research outputs found
Detailed Abundances for the Old Population near the Galactic Center: I. Metallicity distribution of the Nuclear Star Cluster
We report the first high spectral resolution study of 17 M giants
kinematically confirmed to lie within a few parsecs of the Galactic Center,
using R=24,000 spectroscopy from Keck/NIRSPEC and a new linelist for the
infrared K band. We consider their luminosities and kinematics, which classify
these stars as members of the older stellar population and the central cluster.
We find a median metallicity of =-0.16 and a large spread from
approximately -0.3 to +0.3 (quartiles). We find that the highest metallicities
are [Fe/H]<+0.6, with most of the stars being at or below the Solar iron
abundance. The abundances and the abundance distribution strongly resembles
that of the Galactic bulge rather than disk or halo; in our small sample we
find no statistical evidence for a dependence of velocity dispersion on
metallicity.Comment: 18 pages, 14 figures, accepted for publication in A
AGAPEROS: Searching for variable stars in the LMC Bar II. Temporal and near-IR analysis of Long-Period Variables
We analysed the light curves of a large sample of long period variables in
the LMC from the AGAPEROS catalogue. The (non)regularity of the light change is
discussed in detail showing that the majority of the light curves cannot be
described properly by a single period. We show that semiregular and small
amplitude variability do not necessarily correlate as has been assumed in
several previous studies. Using near-infrared data from the DENIS survey we
correlate the light change with colours and luminosities of the objects. These
results are used to compare long period variables in the LMC with LPVs in the
Galactic Bulge and in the solar neighborhood.Comment: 12 pages, 15 figures accepted for publication in A&
Evidence against anomalous compositions for giants in the Galactic Nuclear Star Cluster
Very strong Sc I lines have been found recently in cool M giants in the
Nuclear Star Cluster in the Galactic Center. Interpreting these as anomalously
high scandium abundances in the Galactic Center would imply a unique
enhancement signature and chemical evolution history for nuclear star clusters,
and a potential test for models of chemical enrichment in these objects. We
present high resolution K-band spectra (NIRSPEC/Keck II) of cool M giants
situated in the solar neighborhood and compare them with spectra of M giants in
the Nuclear Star Cluster. We clearly identify strong Sc I lines in our solar
neighborhood sample as well as in the Nuclear Star Cluster sample. The strong
Sc I lines in M giants are therefore not unique to stars in the Nuclear Star
Cluster and we argue that the strong lines are a property of the line formation
process that currently escapes accurate theoretical modeling. We further
conclude that for giant stars with effective temperatures below approximately
3800 K these Sc I lines should not be used for deriving the scandium abundances
in any astrophysical environment until we better understand how these lines are
formed. We also discuss the lines of vanadium, titanium, and yttrium identified
in the spectra, which demonstrate a similar striking increase in strength below
3500 K effective temperature.Comment: 11 pages, 6 figures, accepted for publication in Ap
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
Mid-IR period-magnitude relations for AGB stars
Asymptotic Giant Branch variables are found to obey period-luminosity
relations in the mid-IR similar to those seen at K_S (2.14 microns), even at 24
microns where emission from circumstellar dust is expected to be dominant.
Their loci in the M, logP diagrams are essentially the same for the LMC and for
NGC6522 in spite of different ages and metallicities. There is no systematic
trend of slope with wavelength. The offsets of the apparent magnitude vs. logP
relations imply a difference between the two fields of 3.8 in distance modulus.
The colours of the variables confirm that a principal period with log P > 1.75
is a necessary condition for detectable mass-loss. At the longest observed
wavelength, 24 microns, many semi-regular variables have dust shells comparable
in luminosity to those around Miras. There is a clear bifurcation in LMC
colour-magnitude diagrams involving 24 micron magnitudes.Comment: 5 pages, 4 figure
Interstellar Extinction and Long-Period Variables in the Galactic Center
We use the Spitzer IRAC catalogue of the Galactic Center (GC) point sources
(Ramirez et al. 2008) and combine it with new isochrones (Marigo et al. 2008)
to derive extinctions based on photometry of red giants and asymptotic giant
branch (AGB) stars. This new extinction map extends to much higher values of Av
than previoulsy available. Our new extinction map of the GC region covers 2.0 x
1.4 degree (280 x 200 pc at a distance of 8 kpc). We apply it to deredden the
LPVs found by Glass et al. (2001) near the GC. We make period-magnitude
diagrams and compare them to those from other regions of different metallicity.
The Glass-LPVs follow well-defined period-luminosity relations (PL) in the
IRAC filter bands at 3.6, 4.5, 5.8, and 8.0 micron. The period-luminosity
relations are similar to those in the Large Magellanic Cloud, suggesting that
the PL relation in the IRAC bands is universal. We use ISOGAL data to derive
mass-loss rates and find for the Glass-LPV sample some correlation between
mass-loss and pulsation period, as expected theoretically.The GC has an excess
of high luminosity and long period LPVs compared to the Bulge, which supports
previous suggestions that it contains a younger stellar population.Comment: 12 pages, 11 figures accepted for publication in Astronomy &
Astrophysic
Explanatory Supplement of the ISOGAL-DENIS Point Source Catalogue
We present version 1.0 of the ISOGAL-DENIS Point Source Catalogue (PSC),
containing more than 100,000 point sources detected at 7 and/or 15 micron in
the ISOGAL survey of the inner Galaxy with the ISOCAM instrument on board the
Infrared Space Observatory (ISO). These sources are cross-identified, wherever
possible, with near-infrared (0.8-2.2 micron) data from the DENIS survey. The
overall surface covered by the ISOGAL survey is about 16 square degrees, mostly
(95%) distributed near the Galactic plane (|b| < 1 deg), where the source
extraction can become confusion limited and perturbed by the high background
emission. Therefore, special care has been taken aimed at limiting the
photometric error to ~0.2 magnitude down to a sensitivity limit of typically 10
mJy. The present paper gives a complete description of the entries and the
information which can be found in this catalogue, as well as a detailed
discussion of the data processing and the quality checks which have been
completed. The catalogue is available via the VizieR Service at the Centre de
Donn\'ees Astronomiques de Strasbourg (CDS,
http://vizier.u-strasbg.fr/viz-bin/VizieR/) and also via the server at the
Institut d'Astrophysique de Paris (http://www-isogal.iap.fr/). A more complete
version of this paper, including a detailed description of the data processing,
is available in electronic form through the ADS service.Comment: 21 pages, 7 figures. A&A in press. Full length version with 32
figures and detailed description of the data processing is available here:
http://www-isogal.iap.fr/Publications/ExplSupplFull.ps.g
Effect of quantum confinement on exciton-phonon interactions
We investigate the homogeneous linewidth of localized type-I excitons in
type-II GaAs/AlAs superlattices. These localizing centers represent the
intermediate case between quasi-two-dimensional (Q2D) and
quasi-zero-dimensional localizations. The temperature dependence of the
homogeneous linewidth is obtained with high precision from
micro-photoluminescence spectra. We confirm the reduced interaction of the
excitons with their environment with decreasing dimensionality except for the
coupling to LO-phonons. The low-temperature limit for the linewidth of these
localized excitons is five times smaller than that of Q2D excitons. The
coefficient of exciton-acoustic-phonon interaction is 5 ~ 6 times smaller than
that of Q2D excitons. An enhancement of the average exciton-LO-phonon
interaction by localization is found in our sample. But this interaction is
very sensitive to the detailed structure of the localizing centers.Comment: 6 pages, 4 figure
Detection of the Milky Way spiral arms in dust from 3D mapping
Large stellar surveys are sensitive to interstellar dust through the effects
of reddening. Using extinctions measured from photometry and spectroscopy,
together with three-dimensional (3D) positions of individual stars, it is
possible to construct a three-dimensional dust map. We present the first
continuous map of the dust distribution in the Galactic disk out to 7 kpc
within 100 pc of the Galactic midplane, using red clump and giant stars from
SDSS APOGEE DR14. We use a non-parametric method based on Gaussian Processes to
map the dust density, which is the local property of the ISM rather than an
integrated quantity. This method models the dust correlation between points in
3D space and can capture arbitrary variations, unconstrained by a pre-specified
functional form. This produces a continuous map without line-of-sight
artefacts. Our resulting map traces some features of the local Galactic spiral
arms, even though the model contains no prior suggestion of spiral arms, nor
any underlying model for the Galactic structure. This is the first time that
such evident arm structures have been captured by a dust density map in the
Milky Way. Our resulting map also traces some of the known giant molecular
clouds in the Galaxy and puts some constraints on their distances, some of
which were hitherto relatively uncertain.Comment: Accepted for publication in A&A, 9 pages, 7 figure
Chemical trends in the Galactic halo from APOGEE data
Indexación: Web of Science; Scopus.The galaxy formation process in the A cold dark matter scenario can be constrained from the analysis of stars in the Milky Way's halo system. We examine the variation of chemical abundances in distant halo stars observed by the Apache Point Observatory Galactic Evolution Experiment ( APOGEE), as a function of distance from the Galactic Centre ( r) and iron abundance ([M/H]), in the range 5 less than or similar to r less than or similar to 30 kpc and - 2.5 15 kpc and [M/H] > - 1.1 (larger in the case of O, Mg, and S) with respect to the nearest halo stars. This result confirms previous claims for low-alpha stars found at larger distances. Chemical differences in elements with other nucleosynthetic origins (Ni, K, Na, and Al) are also detected. C and N do not provide reliable information about the interstellar medium from which stars formed because our sample comprises red giant branch and asymptotic giant branch stars and can experience mixing of material to their surfaces.https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stw286
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