370 research outputs found
A new look at the kinematics of the bulge from an N-body model
(Abridged) By using an N-body simulation of a bulge that was formed via a bar
instability mechanism, we analyse the imprints of the initial (i.e. before bar
formation) location of stars on the bulge kinematics, in particular on the
heliocentric radial velocity distribution of bulge stars. Four different
latitudes were considered: , , , and
, along the bulge minor axis as well as outside it, at
and . The bulge X-shaped structure comprises
stars that formed in the disk at different locations. Stars formed in the outer
disk, beyond the end of the bar, which are part of the boxy peanut-bulge
structure may show peaks in the velocity distributions at positive and negative
heliocentric radial velocities with high absolute values that can be larger
than 100 , depending on the observed direction. In some
cases the structure of the velocity field is more complex and several peaks are
observed. Stars formed in the inner disk, the most numerous, contribute
predominantly to the X-shaped structure and present different kinematic
characteristics. Our results may enable us to interpret the cold high-velocity
peak observed in the APOGEE commissioning data, as well as the excess of
high-velocity stars in the near and far arms of the X-shaped structure at
= and =. When compared with real data, the kinematic
picture becomes more complex due to the possible presence in the observed
samples of classical bulge and/or thick disk stars. Overall, our results point
to the existence of complex patterns and structures in the bulge velocity
fields, which are generated by the bar. This suggests that caution should be
used when interpreting the bulge kinematics: the presence of substructures,
peaks and clumps in the velocity fields is not necessarily a sign of past
accretion events.Comment: 21 pages, 18 figures. Accepted for publication in A&
The Giraffe Inner Bulge Survey (GIBS) II. Metallicity distributions and alpha element abundances at fixed Galactic latitude
High resolution (R22,500) spectra for 400 red clump giants, in four
fields within and , were obtained within the GIRAFFE
Inner Bulge Survey (GIBS) project. To this sample we added another 400
stars in Baade's Window, observed with the identical instrumental
configuration. We constructed the metallicity distributions for the entire
sample, as well as for each field individually, in order to investigate the
presence of gradients or field-to-field variations in the shape of the
distributions. The metallicity distributions in the five fields are consistent
with being drawn from a single parent population, indicating the absence of a
gradient along the major axis of the Galactic bar. The global metallicity
distribution is well fitted by two Gaussians. The metal poor component is
rather broad, with a mean at dex and dex.
The metal-rich one is narrower, with mean and
dex. The [Mg/Fe] ratio follows a tight trend with [Fe/H], with enhancement with
respect to solar in the metal-poor regime, similar to the one observed for
giant stars in the local thick disc. [Ca/Fe] abundances follow a similar trend,
but with a considerably larger scatter than [Mg/Fe]. A decrease in [Mg/Fe] is
observed at dex. This \textit{knee} is in agreement with our
previous bulge study of K-giants along the minor axis, but is 0.1 dex lower in
metallicity than the one reported for the Bulge micro lensed dwarf and
sub-giant stars. We found no variation in -element abundance
distributions between different fields.Comment: Accepted for publication in A&
The GIRAFFE Inner Bulge Survey (GIBS). I. Survey Description and a kinematical map of the Milky Way bulge
The Galactic bulge is a massive, old component of the Milky Way. It is known
to host a bar, and it has recently been demonstrated to have a pronounced
boxy/peanut structure in its outer region. Several independent studies suggest
the presence of more than one stellar populations in the bulge, with different
origins and a relative fraction changing across the bulge area. This is the
first of a series of papers presenting the results of the Giraffe Inner Bulge
Survey, carried out at the ESO-VLT with the multifibre spectrograph FLAMES.
Spectra of ~5000 red clump giants in 24 bulge fields have been obtained at
resolution R=6500, in the infrared Calcium triplet wavelength region at 8500
{\AA}. They are used to derive radial velocities and metallicities, based on
new calibration specifically devised for this project. Radial velocities for
another ~1200 bulge red clump giants, obtained from similar archive data, have
been added to the sample. Higher resolution spectra have been obtained for 450
additional stars at latitude b=-3.5, with the aim of investigating chemical
abundance patterns variations with longitude, across the inner bulge. In total
we present here radial velocities for 6392 RC stars. We derive a radial
velocity, and velocity dispersion map of the Milky Way bulge, useful to be
compared with similar maps of external bulges, and to infer the expected
velocities and dispersion at any line of sight. The K-type giants kinematics is
consistent with the cylindrical rotation pattern of M-giants from the BRAVA
survey. Our sample enables to extend this result to latitude b=-2, closer to
the Galactic plane than probed by previous surveys. Finally, we find strong
evidence for a velocity dispersion peak at (0,-1) and (0,-2), possibly
indicative of a high density peak in the central 250 pc of the bulgeComment: A&A in pres
Alpha element abundances and gradients in the Milky Way bulge from FLAMES-GIRAFFE spectra of 650 K giants
We obtained FLAMES-GIRAFFE spectra (R=22,500) at the ESO Very Large Telescope
for 650 bulge red giant branch (RGB) stars and performed spectral synthesis to
measure Mg, Ca, Ti, and Si abundances. This sample is composed of 474 giant
stars observed in 3 fields along the minor axis of the Galactic bulge and at
latitudes b=-4, b=-6, b=-12. Another 176 stars belong to a field containing the
globular cluster NGC 6553, located at b=-3 and 5 degrees away from the other
three fields along the major axis. Our results confirm, with large number
statistics, the chemical similarity between the Galactic bulge and thick disk,
which are both enhanced in alpha elements when compared to the thin disk. In
the same context, we analyze [alpha/Fe] vs. [Fe/H] trends across different
bulge regions. The most metal rich stars, showing low [alpha/Fe] ratios at b=-4
disappear at higher Galactic latitudes in agreement with the observed
metallicity gradient in the bulge. Metal-poor stars ([Fe/H]<-0.2) show a
remarkable homogeneity at different bulge locations. We have obtained further
constrains for the formation scenario of the Galactic bulge. A metal-poor
component chemically indistinguishable from the thick disk hints for a fast and
early formation for both the bulge and the thick disk. Such a component shows
no variation, neither in abundances nor kinematics, among different bulge
regions. A metal-rich component showing low [alpha/Fe] similar to those of the
thin disk disappears at larger latitudes. This allows us to trace a component
formed through fast early mergers (classical bulge) and a disk/bar component
formed on a more extended timescale.Comment: 13 pages, 17 figures. Accepted for publication in Astronomy and
Astrophysic
3D kinematics through the X-shaped Milky Way bulge
Context. It has recently been discovered that the Galactic bulge is X-shaped, with the two southern arms of the X both crossing the lines of sight at l = 0 and | b| > 4, hence producing a double red clump in the bulge color magnitude diagram. Dynamical models predict the formation of X-shaped bulges as extreme cases of boxy-peanut bulges. However, since X-shaped bulges were known to be present only in external galaxies, models have never been compared to 3D kinematical data for individual stars.
Aims. We study the orbital motion of Galactic bulge stars in the two arms (overdensities) of the X in the southern hemisphere. The goal is to provide observational constraints to bulge formation models that predict the formation of X-shapes through bar dynamical instabilities.
Methods. Radial velocities have been obtained for a sample of 454 bulge giants, roughly equally distributed between the bright and the faint red clump, in a field at (l,b) = (0, −6). Proper motions were derived for all red clump stars in the same field by combining images from two epochs, which were obtained 11 years apart, with WFI at the 2.2 m at La Silla. The observed field contains the globular cluster NGC 6558, whose member stars were used to assess the accuracy of the proper motion measurement. At the same time, as a by-product, we provide the first proper motion measurement of NGC 6558. The proper motions for the spectroscopic subsample are analyzed for a subsample of 352 stars, taking into account the radial velocities and metallicities measured from near-infrared calcium triplet lines.
Results. The radial velocity distribution of stars in the bright red clump, which traces the closer overdensity of bulge stars, shows an excess of stars moving towards the Sun. Similarly, an excess of stars receding from the Sun is seen in the far overdensity, which is traced by faint red clump stars. This is explained by the presence of stars on elongated orbits, which are most likely streaming along the arms of the X-shaped bulge. Proper motions for these stars are consistent with qualitative predictions of dynamical models of peanut-shaped bulges. Surprisingly, stars on elongated orbits have preferentially metal-poor (subsolar) metallicities, while the metal rich ones, in both overdensities, are preferentially found in more axisymmetric orbits. The observed proper motion of NGC 6558 has been measured as (μlcos   (b),μb) = (0.30   ±   0.14, −0.43 ± 0.13), with a velocity dispersion of (σlcos(b),σb) = (1.8,1.7) mas/yr. This is the first proper motion measurement for this cluster
GYES, a multifibre spectrograph for the CFHT
We have chosen the name of GYES, one of the mythological giants with one
hundred arms, offspring of Gaia and Uranus, for our instrument study of a
multifibre spectrograph for the prime focus of the Canada-France-Hawaii
Telescope. Such an instrument could provide an excellent ground-based
complement for the Gaia mission and a northern complement to the HERMES project
on the AAT. The CFHT is well known for providing a stable prime focus
environment, with a large field of view, which has hosted several imaging
instruments, but has never hosted a multifibre spectrograph. Building upon the
experience gained at GEPI with FLAMES-Giraffe and X-Shooter, we are
investigating the feasibility of a high multiplex spectrograph (about 500
fibres) over a field of view 1 degree in diameter. We are investigating an
instrument with resolution in the range 15000 to 30000, which should provide
accurate chemical abundances for stars down to 16th magnitude and radial
velocities, accurate to 1 km/s for fainter stars. The study is led by
GEPI-Observatoire de Paris with a contribution from Oxford for the study of the
positioner. The financing for the study comes from INSU CSAA and Observatoire
de Paris. The conceptual study will be delivered to CFHT for review by October
1st 2010.Comment: Contributed talk at the Gaia ELSA conference 2010, S\`evres 7-11 June
2010, to be published on the EAS Series, Editors: C. Turon, F. Arenou & F.
Meynadie
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&
Reddening and metallicity maps of the Milky Way Bulge from VVV and 2MASS. I.The method and minor axis maps
We present a method to obtain reddening maps and to trace structure and
metallicity gradients of the bulge using data from the recently started ESO
public survey Vista Variables in the Via Lactea (VVV). We derive the mean J-Ks
color of the red clump (RC) giants in 1835 subfields in the Bulge region with
-8<b<-0.4 and 0.2<l<1.7, and compare it to the color of RC stars in Baade's
Window for which we adopt E(B-V)=0.55. This allows us to derive the reddening
map on a small enough scale to minimize the problems arising from differential
extinction. The dereddened magnitudes are then used to build the bulge
luminosity function in regions of 0.4 x 0.4 deg to obtain the mean RC
magnitudes. These are used as distance indicator in order to trace the bulge
structure. Finally, for each subfield we derive photometric metallicities
through interpolation of red giant branch colors on a set of empirical ridge
lines. The photometric metallicity distributions are compared to metallicity
distributions obtained from high resolution spectroscopy in the same regions.
The reddening determination is sensitive to small scale variations which are
clearly visible in our maps. The luminosity function clearly shows the double
RC recently discovered in 2MASS and OGLE III datasets, hence allowing to trace
the X-shape morphology of the bulge. Finally, the mean of the derived
photometric metallicity distributions are in remarkable agreement with those
obtained from spectroscopy. The VVV survey presents a unique tool to map the
bulge properties by means of the consistent method presented here. The
remarkable agreement between our results and those presented in literature for
the minor axis allows us to safely extend our method to the whole region
covered by the survey.Comment: Accepted for publication in A&
Astrometric orbits of SB9 stars
Hipparcos Intermediate Astrometric Data (IAD) have been used to derive
astrometric orbital elements for spectroscopic binaries from the newly released
Ninth Catalogue of Spectroscopic Binary Orbits (SB9). Among the 1374 binaries
from SB9 which have an HIP entry, 282 have detectable orbital astrometric
motion (at the 5% significance level). Among those, only 70 have astrometric
orbital elements that are reliably determined (according to specific
statistical tests discussed in the paper), and for the first time for 20
systems, representing a 10% increase relative to the 235 DMSA/O systems already
present in the Hipparcos Double and Multiple Systems Annex.
The detection of the astrometric orbital motion when the Hipparcos IAD are
supplemented by the spectroscopic orbital elements is close to 100% for
binaries with only one visible component, provided that the period is in the 50
- 1000 d range and the parallax is larger than 5 mas. This result is an
interesting testbed to guide the choice of algorithms and statistical tests to
be used in the search for astrometric binaries during the forthcoming ESA Gaia
mission.
Finally, orbital inclinations provided by the present analysis have been used
to derive several astrophysical quantities. For instance, 29 among the 70
systems with reliable astrometric orbital elements involve main sequence stars
for which the companion mass could be derived. Some interesting conclusions may
be drawn from this new set of stellar masses, like the enigmatic nature of the
companion to the Hyades F dwarf HIP 20935. This system has a mass ratio of 0.98
but the companion remains elusive.Comment: Astronomy & Astrophysics, in press (16 pages, 12 figures); also
available at http://www.astro.ulb.ac.be/Html/ps.html#Astrometr
The large scale dust lanes of the Galactic bar
(abridged) By comparing the distribution of dust and gas in the central
regions of the Galaxy, we aim to obtain new insights into the properties of the
offset dust lanes leading the bar's major axis in the Milky Way. On the one
hand, the molecular emission of the dust lanes is extracted from the observed
CO l-b-V distribution according to the interpretation of a dynamical model. On
the other hand, a three dimensional extinction map of the Galactic central
region constructed from near-infrared observations is used as a tracer of the
dust itself and clearly reveals dust lanes in its face-on projection.
Comparison of the position of both independent detections of the dust lanes is
performed in the (l, b) plane. These two completely independent methods are
used to provide a coherent picture of the dust lanes in the Milky Way bar. In
both the gas and dust distributions, the dust lanes are found to be out of the
Galactic plane, appearing at negative latitudes for l > 0 deg and at positive
latitudes for l < 0 deg. However, even though there is substantial overlap
between the two components, they are offset from one another with the dust
appearing to lie closer to the b = 0 deg plane. Two scenarios are proposed to
explain the observed offset. The first involves grain destruction by the bar
shock and reformation downstream. Due to the decrease in velocity caused by the
shock, this occurs at lower z. The second assumes that the gas and dust remain
on a common tilted plane, but that the molecular gas decouples from the Milky
Way's magnetic field, itself strong enough to resist the shear of the bar's
shock. The diffuse gas and dust remain coupled to the field and are carried
further downstream. This second scenario has recently been suggested in order
to explain observations of the barred galaxy NGC 1097.Comment: 4 pages, 5 figures, accepted for publication in Astronomy and
Astrophysics letter
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