946 research outputs found
The Galactic bulge as seen in optical surveys
The bulge is a region of the Galaxy of tremendous interest for understanding
galaxy formation. However measuring photometry and kinematics in it raises
several inherent issues, such as severe crowding and high extinction in the
visible. Using the Besancon Galaxy model and a 3D extinction map, we estimate
the stellar density as a function of longitude, latitude and apparent magnitude
and we deduce the possibility of reaching and measuring bulge stars with Gaia.
We also present an ongoing analysis of the bulge using the Canada-France-Hawaii
Telescope.Comment: In SF2A-2008: Proceedings of the Annual meeting of the French Society
of Astronomy and Astrophysic
Stellar sources in the ISOGAL intermediate bulge fields
We present a study of ISOGAL sources in the "intermediate" galactic bulge
( 2, 1--4), observed by
ISOCAM at 7 and 15 . In combination with near-infrared (I, J, K) data of DENIS survey, complemented by 2MASS data, we discuss the nature of
the ISOGAL sources, their luminosities, the interstellar extinction and the
mass-loss rates. A large fraction of the 1464 detected sources at 15
are AGB stars above the RGB tip, a number of them show an excess in
([7]-[15]) and (K-[15]) colours, characteristic
of mass-loss. The latter, especially (K-[15]), provide
estimates of the mass-loss rates and show their distribution in the range
10 to 10 M/yr.Comment: 16 pages, accepted for publication in Astronomy and Astrophysic
When the Milky Way turned off the lights: APOGEE provides evidence of star formation quenching in our Galaxy
Quenching, the cessation of star formation, is one of the most significant
events in the life cycle of galaxies. We show here the first evidence that the
Milky Way experienced a generalised quenching of its star formation at the end
of its thick disk formation 9 Gyr ago. Elemental abundances of stars
studied as part of the APOGEE survey reveal indeed that in less than 2
Gyr the star formation rate in our Galaxy dropped by an order-of-magnitude.
Because of the tight correlation between age and alpha abundance, this event
reflects in the dearth of stars along the inner disk sequence in the
[Fe/H]-[/Fe] plane. Before this phase, which lasted about 1.5 Gyr, the
Milky Way was actively forming stars. Afterwards, the star formation resumed at
a much lower level to form the thin disk. These events are very well matched by
the latest observation of MW-type progenitors at high redshifts. In late type
galaxies, quenching is believed to be related to a long and secular exhaustion
of gas. In our Galaxy, it occurred on a much shorter time scale, while the
chemical continuity before and after the quenching indicates that it was not
due to the exhaustion of the gas. While quenching is generally associated with
spheroids, our results show that it also occurs in galaxies like the Milky Way,
possibly when they are undergoing a morphological transition from thick to thin
disks. Given the demographics of late type galaxies in the local universe, in
which classical bulges are rare, we suggest further that this may hold true
generally in galaxies with mass lower than or approximately , where
quenching could be directly a consequence of thick disk formation. We emphasize
that the quenching phase in the Milky Way could be contemporaneous with, and
related to, the formation of the bar. We sketch a scenario on how a strong bar
may inhibit star formation.Comment: 17 pages, 8 figures. Published versio
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
Recent star formation in the inner Galactic Bulge seen by ISOGAL. I - Classification of bright mid-IR sources in a test field
Context: The stellar populations in the central region of the Galaxy are
poorly known because of the high visual extinction and very great source
density in this direction.
Aims: To use recent infrared surveys for studying the dusty stellar objects
in this region.
Methods: We analyse the content of a 20x20 arcmin^2 field centred at
(l,b)=(-0.27,-0.06) observed at 7 and 15 microns as part of the ISOGAL survey.
These ISO observations are more than an order of magnitude better in
sensitivity and spatial resolution than the IRAS observations. The sources are
cross-associated with other catalogues to identify various types of objects. We
then derive criteria to distinguish young objects from post-main sequence
stars.
Results: We find that a sample of about 50 young stellar objects and
ultra-compact HII regions emerges, out of a population of evolved AGB stars. We
demonstrate that the sources colours and spatial extents, as they appear in the
ISOGAL catalogue, possibly complemented with MSX photometry at 21 microns, can
be used to determine whether the ISOGAL sources brighter than 300 mJy at 15
microns (or [15] < 4.5 mag) are young objects or late-type evolved stars.Comment: 15 pages, 12 figures. Accepted for publication in Astronomy and
Astrophysic
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
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
Near-IR spectra of ISOGAL sources in the Inner Galactic Bulge
In this work we present near-IR spectra (HK-band) of a sample of 107 sources
with mid-IR excesses at 7 and 15 m detected during the ISOGAL survey.
Making use of the DENIS interstellar extinction map from Schultheis et al.
(1999) we derive luminosities and find that the vs.~
and diagrams are powerful tools for identifying
supergiants, AGB stars, giants and young stellar objects. The majority of our
sample are AGB stars (~ 80%) while we find four good supergiant candidates,
nine young stellar objects and 12 RGB candidates. We have used the most recent
relation by Jeong et al. (2002) based on recent theoretical
modeling of dust formation of AGB stars to determine mass-loss rates. However,
the uncertainties in the mass-loss rates are rather large. The mass-loss rates
of the supergiants are comparable with those in the solar neighbourhood while
the long-period Variables cover a mass-loss range from . The red giant candidateslie at the lower end of the
mass-loss rate range between . We used the
equivalent width of the CO bandhead at 2.3 , the NaI doublet and the
CaI triplet to estimate metallicities using the relation by Ram\'{\i}rez et al.
(\cite{Ramirez2000}). The metallicity distribution of the ISOGAL objects shows
a mean [Fe/H] -0.25 dex with a dispersion of which is
in agreement with the values of Ram\'{i}rez et al. (\cite{Ramirez2000}) for
Galactic Bulge fields between and . A
comparison with the solar neighbourhood sample of Lan\c{c}on & Wood (LW) shows
that our sample is ~ 0.5 dex more metal-rich on average.Comment: 23 pages, 9 figures, 2 appendix with IR spectra. accepted for A&
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&
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