238 research outputs found
The imprints of the Galactic Bar on the Thick Disk with RAVE
We study the kinematics of a local sample of stars, located within a cylinder of 500 pc radius centered on the Sun, in the RAVE data set. We find clear asymmetries in the v R v∞ velocity distributions of thin and thick disk stars: there are more stars moving radially outward for low azimuthal velocities and more radially inward for high azimuthal velocities. Such asymmetries have been previously reported for the thin disk as being due to the Galactic bar, but this is the first time that the same type of structures are seen in the thick disk. Our findings imply that the velocities of thick-disk stars should no longer be described by Schwarzschilds, multivariate Gaussian or purely axisymmetric distributions. Furthermore, the nature of previously reported substructures in the thick disk needs to be revisited as these could be associated with dynamical resonances rather than to accretion events. It is clear that dynamical models of the Galaxy must fit the 3D velocity distributions of the disks, rather than the projected 1D, if we are to understand the Galaxy fully
Cross-Identification of Stars with Unknown Proper Motions
The cross-identification of sources in separate catalogs is one of the most
basic tasks in observational astronomy. It is, however, surprisingly difficult
and generally ill-defined. Recently Budav\'ari & Szalay (2008) formulated the
problem in the realm of probability theory, and laid down the statistical
foundations of an extensible methodology. In this paper, we apply their
Bayesian approach to stars that, we know, can move measurably on the sky, with
detectable proper motion, and show how to associate their observations. We
study models on a sample of stars in the Sloan Digital Sky Survey, which allow
for an unknown proper motion per object, and demonstrate the improvements over
the analytic static model. Our models and conclusions are directly applicable
to upcoming surveys such as PanSTARRS, the Dark Energy Survey, Sky Mapper, and
the LSST, whose data sets will contain hundreds of millions of stars observed
multiple times over several years.Comment: 10 pages, 5 figure
M2000 : an astrometric catalog in the Bordeaux Carte du Ciel zone +11 degrees < {delta} < +18 degrees
During four years, systematic observations have been conducted in drift scan
mode with the Bordeaux automated meridian circle in the declination band [+11 ;
+18]. The resulting astrometric catalog includes about 2 300 000 stars down to
the magnitude limit V_M=16.3. Nearly all stars (96%) have been observed at
least 6 times, the catalog being complete down to V_M=15.4. The median internal
standard error in position is about 35 mas in the V_M magnitude range [11 ;
15], which degrades to about 50 mas when the faintest stars are considered.
M2000 provides also one band photometry with a median internal standard error
of 0.04 mag. Comparisons with the Hipparcos and bright part of Tycho-2 catalogs
have enabled to estimate external errors in position to be lower than 40 mas.
In this zone and at epoch 1998, the faint part of Tycho-2 is found to have an
accuracy of 116 mas in alpha instead of 82 mas deduced from the model-based
standard errors given in the catalog.Comment: The catalogue can be fetched directly from:
ftp://cdsarc.u-strasbg.fr/cats/I/272 or queried from:
http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=I/272 More information at :
http://www.observ.u-bordeaux.fr/~soubiran/m2000.ht
Constraining the Galaxy's dark halo with RAVE stars
We use the kinematics of giant stars that lie within kpc of the plane to measure the vertical profile of mass density near the
Sun. We find that the dark mass contained within the isodensity surface of the
dark halo that passes through the Sun
(), and the surface density within
kpc of the plane () are almost
independent of the (oblate) halo's axis ratio . If the halo is spherical, 46
per cent of the radial force on the Sun is provided by baryons, and only 4.3
per cent of the Galaxy's mass is baryonic. If the halo is flattened, the
baryons contribute even less strongly to the local radial force and to the
Galaxy's mass. The dark-matter density at the location of the Sun is
.
When combined with other literature results we find hints for a mildly oblate
dark halo with . Our value for the dark mass within the solar
radius is larger than that predicted by cosmological dark-matter-only
simulations but in good agreement with simulations once the effects of baryonic
infall are taken into account. Our mass models consist of three
double-exponential discs, an oblate bulge and a Navarro-Frenk-White dark-matter
halo, and we model the dynamics of the RAVE stars in the corresponding
gravitational fields by finding distribution functions that
depend on three action integrals. Statistical errors are completely swamped by
systematic uncertainties, the most important of which are the distance to the
stars in the photometric and spectroscopic samples and the solar distance to
the Galactic centre. Systematics other than the flattening of the dark halo
yield overall uncertainties per cent.Comment: 20 pages, 17 figures, accepted for publication in MNRA
Estimation of the Tilt of the Stellar Velocity Ellipsoid from RAVE and Implications for Mass Models
We present a measure of the inclination of the velocity ellipsoid at 1 kpc
below the Galactic plane using a sample of red clump giants from the RAVE DR2
release. We find that the velocity ellipsoid is tilted towards the Galactic
plane with an inclination of 7.3 +/-1.8 degree. We compare this value to
computed inclinations for two mass models of the Milky Way. We find that our
measurement is consistent with a short scale length of the stellar disc (Rd ~2
kpc) if the dark halo is oblate or with a long scale length (Rd~3 kpc) if the
dark halo is prolate. Once combined with independent constraints on the
flattening of the halo, our measurement suggests that the scale length is
approximately halfway between these two extreme values, with a preferred range
[2.5-2.7] kpc for a nearly spherical halo. Nevertheless, no model can be
clearly ruled out. With the continuation of the RAVE survey, it will be
possible to provide a strong constraint on the mass distribution of the Milky
Way using refined measurements of the orientation of the velocity ellipsoid.Comment: Accepted for publication in MNRAS, 10 pages, 9 figures, 2 table
The asymmetric drift, the local standard of rest, and implications from RAVE data
Context. The determination of the local standard of rest (LSR), which corresponds to the measurement of the peculiar motion of the Sun based on the derivation of the asymmetric drift of stellar populations, is still a matter of debate. The classical value of the tangential peculiar motion of the Sun with respect to the LSR was challenged in recent years, claiming a significantly larger value.
Aims. We present an improved Jeans analysis, which allows a better interpretation of the measured kinematics of stellar populations in the Milky Way disc. We show that the Radial Velocity Experiment (RAVE) sample of dwarf stars is an excellent data set to derive tighter boundary conditions to chemodynamical evolution models of the extended solar neighbourhood.
Methods. We propose an improved version of the Stromberg relation with the radial scalelengths as the only unknown. We redetermine the asymmetric drift and the LSR for dwarf stars based on RAVE data. Additionally, we discuss the impact of adopting a different LSR value on the individual scalelengths of the subpopulations.
Results. Binning RAVE stars in metallicity reveals a bigger asymmetric drift (corresponding to a smaller radial scalelength) for more metal-rich populations. With the standard assumption of velocity-dispersion independent radial scalelengths in each metallicity bin, we redetermine the LSR. The new Stromberg equation yields a joint LSR value of V-circle dot = 3.06 +/- 0.68 km s(-1), which is even smaller than the classical value based on Hipparcos data. The corresponding radial scalelength increases from 1.6 kpc for the metal-rich bin to 2.9 kpc for the metal-poor bin, with a trend of an even larger scalelength for young metal-poor stars. When adopting the recent Schonrich value of V-circle dot = 12.24 km s(-1) for the LSR, the new Stromberg equation yields much larger individual radial scalelengths of the RAVE subpopulations, which seem unphysical in part.
Conclusions. The new Stromberg equation allows a cleaner interpretation of the kinematic data of disc stars in terms of radial scalelengths. Lifting the LSR value by a few km s(-1) compared to the classical value results in strongly increased radial scalelengths with a trend of smaller values for larger velocity dispersions
On the local birth place of Geminga
Using estimates of the distance and proper motion of Geminga and the
constraints on its radial velocity posed by the shape of its bow shock, we
investigate its birth place by tracing its space motion backwards in time. Our
results exclude the lambda Ori association as the origin site because of the
large distance between both objects at any time. Our simulations place the
birth region at approximately 90-240 pc from the Sun, between 197 degrees and
199 degrees in Galactic longitude and -16 degrees and -8 degrees in latitude,
most probably inside the Cas-Tau OB association or the Ori OB1a association. We
discard the possibility of the progenitor being a massive field star. The
association of Geminga with either stellar association implies an upper limit
of M = 15 Msun for the mass of its progenitor. We also propose new members for
the Cas-Tau and Ori OB1 associations.Comment: 6 pages, 5 figures. Accepted for publication in Astronomy &
Astrophysic
Chromospherically Active Stars in the RAVE Survey. I. The Catalogue
RAVE, the unbiased magnitude limited survey of the southern sky stars,
contained 456,676 medium-resolution spectra at the time of our analysis.
Spectra cover the CaII IRT range which is a known indicator of chromospheric
activity. Our previous work (Matijevi\v{c} et al. 2012) classified all spectra
using locally linear embedding. It identified 53,347 cases with a suggested
emission component in calcium lines. Here we use a spectral subtraction
technique to measure the properties of this emission. Synthetic templates are
replaced by the observed spectra of non-active stars to bypass the difficult
computations of non-LTE profiles of the line cores and stellar parameter
dependence. We derive both the equivalent width of the excess emission for each
calcium line on a 5\AA\ wide interval and their sum EW_IRT for ~44,000
candidate active dwarf stars with S/N>20 and with no respect to the source of
their emission flux. From these ~14,000 show a detectable chromospheric flux
with at least 2\sigma\ confidence level. Our set of active stars vastly
enlarges previously known samples. Atmospheric parameters and in some cases
radial velocities of active stars derived from automatic pipeline suffer from
systematic shifts due to their shallower calcium lines. We re-estimate the
effective temperature, metallicity and radial velocities for candidate active
stars. The overall distribution of activity levels shows a bimodal shape, with
the first peak coinciding with non-active stars and the second with the pre
main-sequence cases. The catalogue will be publicly available with the next
RAVE public data releases.Comment: 13 pages, 9 figure
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