414 research outputs found
Galactic structure studies with BATC star counts
We report the first results of star counts carried out with the National
Astronomical Observatories (NAOC) 60/90 cm Schmidt Telescope in 15
intermediate-band filters from 3000 to 10000 {\AA} in the BATC survey. We
analyze a sample of over 1400 main sequence stars (V), which lie
in the field with central coordinates R.A.= and
DEC=47 (J2000). The field of view is 0.95
deg, and the spatial scale was 1\arcsec.67. In our model, the
distribution of stars perpendicular to the plane of the Galaxy is given by two
exponential disks (thin disk plus thick disk) and a de Vaucouleurs halo. Based
on star counts, we derive the scale heights of the thin disk to be
pc and of the thick disk to be pc,
respectively, with a local density of of the thin disk. We find that
the observed counts support an axial ratio of for a de Vaucouleurs
law, implying a more flattened halo.
We also derive the stellar luminosity function (SLF) for the thin disk, and
it partly agrees with the Hipparcos luminosity function.Comment: 17pages,9 figure
Galactic structure from the Calar Alto Deep Imaging Survey (CADIS)
We used 1627 faint (15.5< R<23) stars in five fields of the Calar Alto Deep
Imaging Survey (CADIS) to estimate the structure parameters of the Galaxy. The
results were derived by applying two complementary methods: first by fitting
the density distribution function to the measured density of stars
perpendicular to the Galactic plane, and second by modelling the observed
colors and apparent magnitudes of the stars in the field, using Monte Carlo
simulations. The best-fitting model of the Galaxy is then determined by
minimising the C-statistic, a modified chisquared. Our model includes a double
exponential for the stellar disk with scaleheights h_1 and h_2 and a power law
halo with exponent alpha. 24480 different parameter combinations have been
simulated. Both methods yield consistent results: the best fitting parameter
combination is alpha=3.0 (or alpha=2.5, if we allow for a flattening of the
halo with an axial ratio of (c/a)=0.6), h_1=300 pc, h_2=900 pc, and the
contribution of thick disk stars to the disk stars in the solar neighbourhood
is found to be between 4 and 10%.Comment: 16 pages, 10 figures, submitted to Astronomy and Astrophysic
The Information Of The Milky Way From 2MASS Whole Sky Star Count: The Structure Parameters
The Ks band differential star count of the Two Micron All Sky Survey (2MASS)
is used to derive the global structure parameters of the smooth components of
the Milky Way. To avoid complication introduced by other fine structures and
significant extinction near and at the Galactic plane, we only consider
Galactic latitude |b| > 30 degree data. The star count data is fitted with a
threecomponent model: double exponential thin disk and thick disk, and a power
law decay oblate halo. Using maximum likelihood the best-fit local density of
thin disk is n0 = 0.030 +- 0.002 stars/pc^3. The best-fit scale-height and
length of the thin disk are Hz1 = 360+-10 pc and Hr1 = 3.7+-1.0 kpc, and those
of the thick disk are and Hz2 = 1020+-30 pc and Hr2 = 5.0+-1.0 kpc, the local
thick-to-thin disk density ratio is f2 = 7+-1%. The best-fit axis ratio, power
law index and local density ratio of the oblate halo are kappa = 0.55+-0.15, p
= 2.6+-0.6 and fh = 0.20+-0:10%, respectively. Moreover, we find some
degeneracy among the key parameters (e.g. n0,Hz1, f2 and Hz2). Any pair of
these parameters are anticorrelated to each other. The 2MASS data can be
well-fitted by several possible combinations of parameters. This is probably
the reason that there is a wide range of values for the structure parameters in
literature similar to this study. Since only medium and high Galactic latitude
data are analyzed, the fitting is very insensitive to the scale-lengths of the
disks.Comment: 25 pages, 4 figures, accepted by ApJ on July 15 201
A different approach for the estimation of Galactic model parameters
We estimated the Galactic model parameters by means of a new approach based
on the comparison of the observed space density functions per absolute
magnitude interval with a unique density law for each population individually,
and via the procedure in situ for the field SA 114 (,
; 4.239 square-degree; J2000). The separation of stars into
different populations has been carried out by their spatial distribution. The
new approach reveals that model parameters are absolute magnitude dependent.
The scale height for thin disk decreases monotonously from absolutely bright to
absolutely faint stars in a range 265-495 pc, but there is a discontunity at
the absolute magnitude where the squared secans hiperbolicus
density law replaces the exponential one. The range of the scale-height for
thick disk, dominant in the absolute magnitude interval , is
less: 805-970 pc. The local space density for thick disk relative to thin disk
decreases from 9.5% to 5.2% when one goes from the absolutely bright to faint
magnitudes. Halo is dominant in three absolute magnitude intervals and the
axial ratio for this component is almost the same for these intervals where
. The same holds for the local space density relative to the thin
disk with range (0.02-0.15)%. The model parameters estimated by comparison of
the observed space density functions combined for three populations per
absolute magnitude interval with the combined density laws agree with the cited
values in the literature. Also each parameter is equal to at least one of the
corresponding parameters estimated for different absolute magnitude intervals
by the new approach. We argue that the most appropriate Galactic model
parameters are those, that are magnitude dependent.Comment: 14 pages, including 16 figures and 16 tables, accepted for
publication in MNRA
Starcounts Redivivus. IV. Density Laws Through Photometric Parallaxes
In an effort to more precisely define the spatial distribution of Galactic
field stars, we present an analysis of the photometric parallaxes of 70,000
stars covering nearly 15 square degrees in seven Kapteyn Selected Areas. We
address the affects of Malmquist Bias, subgiant/giant contamination,
metallicity and binary stars upon the derived density laws. The affect of
binary stars is the most significant. We find that while the disk-like
populations of the Milky Way are easily constrained in a simultaneous analysis
of all seven fields, no good simultaneous solution for the halo is found. We
have applied halo density laws taken from other studies and find that the
Besancon flattened power law halo model (c/a=0.6, r^-2.75) produces the best
fit to our data. With this halo, the thick disk has a scale height of 750 pc
with an 8.5% normalization to the old disk. The old disk scale height is
280-300 pc. Corrected for a binary fraction of 50%, these scale heights are 940
pc and 350-375 pc, respectively. Even with this model, there are systematic
discrepancies between the observed and predicted density distributions. Our
model produces density overpredictions in the inner Galaxy and density
underpredictions in the outer Galaxy. A possible solution is modeling the
stellar halo as a two-component system in which the halo has a flattened inner
distribution and a roughly spherical, but substructured outer distribution.
Further reconciliation could be provided by a flared thick disk, a structure
consistent with a merger origin for that population. (Abridged)Comment: 66 pages, accepted to Astrophysical journal, some figures compresse
The Red Giant Branch Tip and Bump of the Leo II dwarf spheroidal galaxy
We present V and I photometry of a 9.4' X 9.4' field centered on the dwarf
spheroidal galaxy Leo II. The Tip of the Red Giant Branch is identified at
I^{TRGB}=17.83 +/- 0.03 and adopting = -1.53 +/- 0.2 from the
comparison of RGB stars with Galactic templates, we obtain a distance modulus
(m-M)_0=21.84 +/- 0.13, corresponding to a distance D=233 +/- 15 Kpc. Two
significant bumps have been detected in the Luminosity Function of the Red
Giant Branch. The fainter bump (B1, at V=21.76 +/- 0.05) is the RGB bump of the
dominant stellar population, while the actual nature of the brightest one (B2,
at V=21.35 +/- 0.05) cannot be firmly assessed on the basis of the available
data, it can be due to the Asymptotic Giant Branch Clump of the main population
or it may be a secondary RGB bump. The luminosity of the main RGB bump (B1)
suggests that the majority of RGB stars in Leo II belongs to a population that
is ~4 gyr younger than the classical Galactic globular clusters. The stars
belonging to the He-burning Red Clump are shown to be significantly more
centrally concentrated than RR Lyrae and Blue Horizontal Branch stars, probing
the existence of an age/metallicity radial gradient in this remote dwarf
spheroidal.Comment: Accepted for publication by MNRAS. Latex, 10 pages, 8 .ps figure
High Resolution HDS/SUBARU chemical abundances of the young stellar cluster Palomar 1
Context. Palomar\,1 is a peculiar globular cluster (GC). It is the youngest
Galactic GC and it has been tentatively associated to several of the
substructures recently discovered in the Milky Way (MW), including the Canis
Major (CMa) overdensity and the Galactic Anticenter Stellar Structure (GASS).
Aims. In order to provide further insights into its origin, we present the
first high resolution chemical abundance analysis for one red giant in Pal\,1.
Methods. We obtained high resolution (R=30000) spectra for one red giant star
in Pal\,1 using the High Dispersion Spectrograph (HDS) mounted at the SUBARU
telescope. We used ATLAS-9 model atmospheres coupled with the SYNTHE and WIDTH
calculation codes to derive chemical abundances from the measured line
equivalent widths of 18 among , Iron-peak, light and heavy elements.
Results. The Palomar~1 chemical pattern is broadly compatible to that of the MW
open clusters population and similar to disk stars. It is, instead, remarkably
different from that of the Sagittarius (Sgr) dwarf spheroidal galaxy.
Conclusions. If Pal\,1 association with either CMa or GASS will be confirmed,
this will imply that these systems had a chemical evolution similar to that of
the Galactic disk.Comment: 13 pages, 9 figures, 5 tables. Accepted for publication in Astronomy
and Astrophysics. Minor changes in the tex
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