822 research outputs found
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&
The Optical Gravitational Lensing Experiment. Is Interstellar Extinction Toward the Galactic Center Anomalous?
Photometry of the Galactic bulge, collected during the OGLE-II microlensing
search, indicates high and non-uniform interstellar extinction toward the
observed fields. We use the mean I-band magnitude and V-I color of red clump
stars as a tracer of interstellar extinction toward four small regions of the
Galactic bulge with highly variable reddening. Similar test is performed for
the most reddened region observed in the LMC.
We find that the slope of the location of red clump stars in the
color-magnitude diagrams (CMDs) in the Galactic bulge is significantly smaller
than the slope of the reddening line following the standard extinction law
(R_V=3.1) for approximations of the extinction curve by both Cardelli, Clayton
and Mathis (1989, CCM89) and Fitzpatrick (1999, F99). The differences are much
larger for the CCM89 approximation which, on the other hand, indicates the same
slopes for the control field in the LMC, contrary to the F99 approximation. We
discuss possible systematic effects that could cause the observed discrepancy.
Anomalous extinction toward the Galactic bulge seems to be the most natural
explanation. Our data indicate that, generally, the ratio of the total to
selective absorption, R_VI, is much smaller toward the Galactic bulge than the
value corresponding to the standard extinction curve (R_V=3.1). However, R_VI
varies from one line-of-sight to another.
Our results explain why the red clump and RR Lyr stars in the Baade's window
dereddened with standard value of R_VI are redder compared to those of the
local population.Comment: 16 pages. Accepted for publication in ApJ. Major changes include:
comparison of the OGLE-II photometry with other data, additional comparison
of the observed reddening line with that resulting from approximation of the
standard extinction curve by Fitzpatrick (1999
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
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
Mapping the Milky Way bulge at high resolution: the 3D dust extinction, CO, and X factor maps
Three dimensional interstellar extinction maps provide a powerful tool for
stellar population analysis. We use data from the VISTA Variables in the Via
Lactea survey together with the Besan\c{c}on stellar population synthesis model
of the Galaxy to determine interstellar extinction as a function of distance in
the Galactic bulge covering and . We adopted a
recently developed method to calculate the colour excess. First we constructed
the H-Ks vs. Ks and J-Ks vs. Ks colour-magnitude diagrams based on the VVV
catalogues that matched 2MASS. Then, based on the temperature-colour relation
for M giants and the distance-colour relations, we derived the extinction as a
function of distance. The observed colours were shifted to match the intrinsic
colours in the Besan\c{c}on model as a function of distance iteratively. This
created an extinction map with three dimensions: two spatial and one distance
dimension along each line of sight towards the bulge. We present a 3D
extinction map that covers the whole VVV area with a resolution of 6' x 6',
using distance bins of 0.5 kpc. The high resolution and depth of the photometry
allows us to derive extinction maps for a range of distances up to 10 kpc and
up to 30 magnitudes of extinction in . Integrated maps show the same
dust features and consistent values as other 2D maps. We discuss the spatial
distribution of dust features in the line of sight, which suggests that there
is much material in front of the Galactic bar, specifically between 5-7 kpc. We
compare our dust extinction map with high-resolution maps towards
the Galactic bulge, where we find a good correlation between and
. We determine the X factor by combining the CO map and our dust
extinction map. Our derived average value is consistent with the canonical
value of the Milky Way.Comment: 11 pages, 18 figures, accepted for publication in
Astronomy&Astrophysic
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
A three dimensional extinction map of the Galactic Anticentre from multi-band photometry
We present a three dimensional extinction map in band. The map has a
spatial angular resolution, depending on latitude, between 3 -- 9\,arcmin and
covers the entire XSTPS-GAC survey area of over 6,000\, for Galactic
longitude and latitude . By cross-matching the photometric catalog of the Xuyi Schmidt
Telescope Photometric Survey of the Galactic Anticentre (XSTPS-GAC) with those
of 2MASS and WISE, we have built a multi-band photometric stellar sample of
about 30 million stars and applied spectral energy distribution (SED) fitting
to the sample. By combining photometric data from the optical to the
near-infrared, we are able to break the degeneracy between the intrinsic
stellar colours and the amounts of extinction by dust grains for stars with
high photometric accuracy, and trace the extinction as a function of distance
for low Galactic latitude and thus highly extincted regions. This has allowed
us to derive the best-fit extinction and distance information of more than 13
million stars, which are used to construct the three dimensional extinction
map. We have also applied a Rayleigh-Jeans colour excess (RJCE) method to the
data using the 2MASS and WISE colour . The resulting RJCE extinction
map is consistent with the integrated two dimensional map deduced using the
best-fit SED algorithm. However for individual stars, the amounts of extinction
yielded by the RJCE method suffer from larger errors than those given by the
best-fit SED algorithm.Comment: 20 pages, 18 figures, accepted in MNRA
Abundances of disk and Bulge giants from high-resolution optical spectra III. Sc, V, Cr, Mn, Co, Ni
Context. Recent observations of the Bulge, e.g., its X-shape, cylindrical
stellar motions, and a potential fraction of young stars propose that it formed
through secular evolution of the disk and not through gas dissipation and/or
mergers, as thought previously. Aims. We measure abundances of six iron-peak
elements (Sc, V, Cr, Mn, Co and Ni) in the local thin and thick disks as well
as the Bulge to provide additional observational constraints for Galaxy
formation and chemical evolution models. Methods. We use high-resolution
optical spectra of 291 K giants in the local disk mostly obtained by the FIES
at NOT (signal-to-noise (S/N) ratio of 80-100) and 45 K giants in the Bulge
obtained by the UVES/FLAMES at VLT (S/N ratio of 10-80). We measure abundances
in SME and apply NLTE corrections to the [Mn/Fe] and [Co/Fe] ratios. To
discriminate between the thin and thick, we use stellar metallicity,
[Ti/Fe]-ratios, and kinematics from Gaia DR2 (proper motions and the radial
velocities). Results. The observed disk trend of V is more enhanced in the
thick disk, while the Co disk trend shows a minor enhancement in the thick
disk. The Bulge trends of V and Co appear even more enhanced w.r.t. the thick
disk, but within the uncertainties. The [Ni/Fe] ratio seems slightly
overabundant in the thick disk and the Bulge w.r.t. the thin disk, although the
difference is minor. The disk and Bulge trends of Sc, Cr and Mn overlap
strongly. Conclusions. The somewhat enhanced [(V,Co)/Fe] ratios observed in the
Bulge suggest that the local thick disk and the Bulge might have experienced
different chemical enrichment and evolutionary paths. However, we are unable to
predict the exact evolutionary path of the Bulge solely based on these
observations. Galactic chemical evolution models could, on the other hand,
provide that using these results.Comment: Accepted for publication in A&A; 25 pages, 15 figure
Reddening and metallicity maps of the Milky Way bulge from VVV and 2MASS II. The complete high resolution extinction map and implications for Bulge studies
We use the Vista Variables in the Via Lactea (VVV) ESO public survey data to
measure extinction values in the complete area of the Galactic bulge covered by
the survey at high resolution. We derive reddening values using the method
described in Paper I. This is based on measuring the mean (J-Ks) color of red
clump giants in small subfields of 2' to 6' in the following bulge area:
-10.3<b<+5.1 and -10<l<+10.4. To determine the reddening values E(J-Ks) for
each region, we measure the RC color and compare it to the (J-Ks) color of RC
stars measured in Baade's window, for which we adopt E(B-V)=0.55. This allows
us to construct a reddening map sensitive to small scale variations minimizing
the problems arising from differential extinction. The significant reddening
variations are clearly observed on spatial scales as small as 2'. We find a
good agreement between our extinction measurements and Schlegel maps in the
outer bulge, but, as already stated in the literature the Schlegel maps are not
reliable for regions within |b| < 6. In the inner regions we compare our
results with maps derived from DENIS and Spitzer surveys. While we find good
agreement with other studies in the corresponding overlapping regions, our
extinction map has better quality due to both higher resolution and a more
complete spatial coverage in the Bulge. We investigate the importance of
differential reddening and demonstrate the need for high resolution extinction
maps for detailed studies of Bulge stellar populations and structure. The
extinction variations on scales of up to 2'-6', must be taken into account when
analysing the stellar populations of the Bulge.Comment: Accepted for publication in A&
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
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