313 research outputs found
The investigation of ELAIS field by Vega photometry: Absolute - magnitude dependent on the Galactic model parameters
We estimate the density laws of the Galactic stellar populations as a
function of absolute magnitude in a near-polar Galactic field. The density laws
are determined by the direct fit to photometric parallaxes from Vega photometry
in the ELAIS (l=84.27, b=+44.90; 6.571 deg^2) field both independently for each
population and simultaneously for all stellar populations. Stars have been
separated into different populations based on their spatial location. The thick
disc and halo best fit by an exponential. However, the thin disc best fits by
using a sech^2 law for stars at faint absolute magnitudes, (10,11], (11,12] and
(12,13], whereas an exponential law for stars at relatively bright absolute
magnitudes, (5,6], (6,7], (7,8], (8,9] and (9,10]. The scaleheights for the
sech^2 density laws are the equivalent exponential scaleheights. Galactic model
parameters are absolute magnitude dependent: The scaleheight for thin disc
decreases monotonically from stars at bright absolute magnitudes to stars at
faint absolute magnitudes in the range 363-163 pc, except the minimum H=211 pc
at (9,10] where sech density law fits better. Its local density is flat at
bright absolute magnitudes but it increases at faint absolute magnitudes. For
thick disc, the scaleheight is flat within the uncertainties. The local space
density of thick disc relative to the local space density for the thin disc is
almost flat at absolute magnitude intervals (5,6] and (6,7], 7.59 and 7.41%
respectively, whereas it decreases down to 3.31% at absolute magnitude interval
(7,8]. The axial ratio for the halo is e=0.60, 0.73 and 0.78 for the absolute
magnitude intervals (4,5], (5,6] and (6,7] respectively, and its local space
density relative to the local space density for the thin disc is 0.06 and 0.04%
for the intervals (5,6], and (6,7] respectively.Comment: 15 pages, including 16 figures and 15 tables, accepted for
publication in MNRA
Local Stellar Kinematics from RAVE data - VII. Metallicity Gradients from Red Clump Stars
We investigate the Milky Way Galaxy's radial and vertical metallicity
gradients using a sample of 47,406 red clump stars from the RAVE DR4. This
sample is more than twice the size of the largest sample in the literature
investigating radial and vertical metallicity gradients. The absolute magnitude
of Groenewegen (2008) is used to determine distances to our sample stars. The
resulting distances agree with the RAVE DR4 distances Binney et al. (2014) of
the same stars. Our photometric method also provides distances to 6185 stars
that are not assigned a distance in RAVE DR4. The metallicity gradients are
calculated with their current orbital positions ( and ) and with
their orbital properties (mean Galactocentric distance, and ),
as a function of the distance to the Galactic plane:
d[Fe/H]/d- dex/kpc for kpc and
d[Fe/H]/d- dex/kpc for kpc. This
reaffirms the radial metallicity gradient in the thin disc but highlights that
gradients are sensitive to the selection effects caused by the difference
between and . The radial gradient is flat in the distance
interval 0.5-1 kpc from the plane and then becomes positive greater than 1 kpc
from the plane. The radial metallicity gradients are also eccentricity
dependent. We showed that d[Fe/H]/d-, -,
- and - dex/kpc for , ,
and sub-samples, respectively, in the distance
interval kpc. Similar trend is found for vertical
metallicity gradients. Both the radial and vertical metallicity gradients are
found to become shallower as the eccentricity of the sample increases. These
findings can be used to constrain different formation scenarios of the thick
and thin discs.Comment: 18 pages, including 16 figures and 6 tables, accepted for publication
in PAS
Local stellar kinematics from RAVE data: III. Radial and Vertical Metallicity Gradients based on Red Clump Stars
We investigate radial and vertical metallicity gradients for a sample of red
clump stars from the RAdial Velocity Experiment (RAVE) Data Release 3. We
select a total of 6781 stars, using a selection of colour, surface gravity and
uncertainty in the derived space motion, and calculate for each star a
probabilistic (kinematic) population assignment to a thin or thick disc using
space motion and additionally another (dynamical) assignment using stellar
vertical orbital eccentricity. We derive almost equal metallicity gradients as
a function of Galactocentric distance for the high probability thin disc stars
and for stars with vertical orbital eccentricities consistent with being
dynamically young, e_v<=0.07, i.e. d[M/H]/dR_m = -0.041(0.003) and d[M/H]/dR_m
= -0.041(0.007) dex/kpc. Metallicity gradients as a function of distance from
the Galactic plane for the same populations are steeper, i.e. d[M/H]/dz_{max} =
-0.109(0.008) and d[M/H]/dz_{max} = -0.260(0.031) dex/kpc, respectively. R_m
and z_{max} are the arithmetic mean of the perigalactic and apogalactic
distances, and the maximum distance to the Galactic plane, respectively.
Samples including more thick disc red clump giant stars show systematically
shallower abundance gradients. These findings can be used to distinguish
between different formation scenarios of the thick and thin discs.Comment: 27 pages, including 15 figures and 4 tables, accepted for publication
in MNRA
CCD BV and 2MASS photometric study of the open cluster NGC 1513
We present CCD BV and JHK 2MASS photometric data for the open cluster
NGC 1513. We observed 609 stars in the direction of the cluster up to a
limiting magnitude of mag. The star count method shows that the
centre of the cluster lies at ,
and its angular size is arcmin.
The optical and near-infrared two-colour diagrams reveal the colour excesses in
the direction of the cluster as , and
mag. These results are consistent with normal
interstellar extinction values. Optical and near-infrared Zero Age
Main-Sequences (ZAMS) provided an average distance modulus of
mag, which can be translated into a distance of
pc. Finally, using Padova isochrones we determined the metallicity
and age of the cluster as ( dex) and
, respectively.Comment: 15 pages, 12 figures and 4 tables, accepted for publication in
Astrophysics & Space Scienc
A CCD Study of High Latitude Galactic Structure: Testing the Model Parameters
We interpret published CCD UBVI data to deduce the stellar density
distribution and metallicity distribution function in the region from 2-8 kpc
from the Galactic Plane, and compare our results to several star count models.
A feature of extant star count models is degeneracy between the adopted scale
heights of the thin and thick disks, and their local normalisation. We
illustrate the utility of this small data set, and future larger sets (e.g.
SDSS), by explicitly considering consistency between the derived density laws,
and the implied solar neighbourhood luminosity function. Our data set, from
Hall et al.(1996) (l=52, b=-39) contains 566 stars, selected to be consistent
with stellar loci in colour-colour diagrams. Our analysis supports the
parameterisation of the recent SDSS galaxy model of Chen et al.(2001), except
in preferring the stellar halo axis ratio to be 0.84. Photometric
metal-abundances have been derived for 329 stars using a new calibration. The
vertical distance-dependent metallicity distribution function, if parameterised
by a single mean value, can be described by a metallicity gradient -0.2 dex/kpc for the inner halo.
The data are however better described as the sum of three discrete distribution
functions, each of which has a small or zero internal gradient. The changing
mix of thin disk, thick disk and halo populations with distance from the plane
generates an illusion of a smooth gradient.Comment: 14 pages, 43 gif figures, accepted for publication in MNRA
A Universal Vertical Stellar Density Distribution Law for the Galaxy
We reduced the observational logarithmic space densities in the vertical
direction up to 8 kpc from the galactic plane, for stars with absolute
magnitudes (5,6], (6,7] and [5,10] in the fields #0952+5245 and SA114, to a
single exponential density law. One of three parameters in the quadratic
expression of the density law corresponds to the local space density for stars
with absolute magnitudes in question. There is no need of any definition for
scaleheights or population types. We confirm with the arguments of non-discrete
thin and thick discs for our Galaxy and propose a single structure up to
several kiloparsecs from the galactic plane. The logarithmic space densities
evaluated by this law for the ELAIS field fit to the observational ones.
Whereas, there are considerable offsets for the logarithmic space densities
produced by two sets of classical galactic model parameters from the
observational ones, for the same field.Comment: 9 pages, 1 figure and 10 tables, accepted for publication in
Astrophysics & Space Scienc
A New Procedure for the Photometric Parallax Estimation
We present a new procedure for photometric parallax estimation. The data for
1236 stars provide calibrations between the absolute magnitude offset from the
Hyades main-sequence and the UV-excess for different (B-V)o colour-index
intervals, i.e.: (0.3 0.4), (0.4 0.5), (0.5 0.6), (0.6 0.7), (0.7 0.8), (0.8
0.9), (0.9 1.0), and (1.0 1.1). The mean difference between the original and
estimated absolute magnitudes and the corresponding standard deviation are
rather small, +0.0002 and +/-0.0613 mag. The procedure has been adapted to the
Sloan photometry by means of colour equations and applied to a set of
artificial stars with different metallicity. The comparison of the absolute
magnitudes estimated by the new procedure and the canonical one indicates that
a single colour-magnitude diagram does not supply reliable absolute magnitudes
for stars with large range of metallicity.Comment: 26 pages, including 12 tables and 6 figures, accepted for publication
in PAS
Local stellar kinematics from RAVE data - II. Radial metallicity gradient
We investigate radial metallicity gradients for a sample of dwarf stars from the RAdial Velocity Experiment (RAVE) Data Release 3 (DR3). We select a total of approximately 17 000 F-type and G-type dwarfs, using a selection of colour, log g and uncertainty in the derived space motion, and calculate for each star a probabilistic (kinematic) population assignment to a thick or thin disc using space motion and additionally another (dynamical) assignment using stellar vertical orbital eccentricity. We additionally subsample by colour, to provide samples biased toward young thin-disc and older thin-disc stars. We derive a metallicity gradient as a function of Galactocentric radial distance, i.e. d[M/H]/dRm=−0.051 ± 0.005 dex kpc−1, for the youngest sample, F-type stars with vertical orbital eccentricities ev≤ 0.04. Samples biased toward older thin-disc stars show systematically shallower abundance gradients
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