125 research outputs found
New analysis in the field of open cluster Collinder 223
The present study of the open cluster Collinder 223 (Cr 223) has been mainly
depended on the photoelectric data of Claria & Lapasset (1991; hereafter CL91).
This data of CL91 has been used with the cluster's image of AAO-DSS in order to
re-investigate and improve the main parameters of Cr 223. Stellar count has
been achieved to determine the stellar density, the cluster's center and the
cluster's diameter. In addition, the luminosity function, mass function, and
the total mass of the cluster have been estimated.Comment: 12 pages, 8 figure
Galactic Extinction from Colors and Counts of Field Galaxies in WFPC2 Frames: An Application to GRB 970228
We develop the ``simulated extinction method'' to measure average foreground
Galactic extinction from field galaxy number-counts and colors. The method
comprises simulating extinction in suitable reference fields by changing the
isophotal detection limit. This procedure takes into account selection effects,
in particular, the change in isophotal detection limit (and hence in isophotal
magnitude completeness limit) with extinction, and the galaxy color--magnitude
relation.
We present a first application of the method to the HST WFPC2 images of the
gamma-ray burster GRB 970228. Four different WFPC2 high-latitude fields,
including the HDF, are used as reference to measure the average extinction
towards the GRB in the F606W passband. From the counts, we derive an average
extinction of A_V = 0.5 mag, but the dispersion of 0.4 mag between the
estimates from the different reference fields is significantly larger than can
be accounted by Poisson plus clustering uncertainties. Although the counts
differ, the average colors of the field galaxies agree well. The extinction
implied by the average color difference between the GRB field and the reference
galaxies is A_V = 0.6 mag, with a dispersion in the estimated extinction from
the four reference fields of only 0.1 mag. All our estimates are in good
agreement with the value of 0.81\pm0.27 mag obtained by Burstein & Heiles, and
with the extinction of 0.78\pm0.12 measured by Schlegel et al. from maps of
dust IR emission. However, the discrepancy between the widely varying counts
and the very stable colors in these high-latitude fields is worth
investigating.Comment: 14 pages, 2 figures; submitted to the Astrophysical Journa
Trumpler 20 - an old and rich open cluster
We show that the open cluster Trumpler 20, contrary to the earlier findings,
is actually an old Galactic open cluster. New CCD photometry and
high-resolution spectroscopy are used to derive the main parameters of this
cluster. At [Fe/H]=-0.11 for a single red giant star, the metallicity is
slightly subsolar. The best fit to the color-magnitude diagrams is achieved
using a 1.3 Gyr isochrone with convective overshoot. The cluster appears to
have a significant reddening at E(B-V)=0.46 (for B0 spectral type), although
for red giants this high reddening yields the color temperature exceeding the
spectroscopic T_eff by about 200 K. Trumpler 20 is a very rich open cluster,
containing at least 700 members brighter than M_V=+4. It may extend over the
field-of-view available in our study at 20'x20'.Comment: 7 pages, 5 figures; accepted for publication in MNRA
The Globular Cluster Population of NGC 7457: Clues to the Evolution of Field S0 Galaxies
In this paper we present the results of a wide-field imaging study of the
globular cluster (GC) system of the field S0 galaxy NGC 7457. To derive the
global properties of the GC system, we obtained deep BVR images with the WIYN
3.5 m telescope and Minimosaic Imager and studied the GC population of NGC 7457
to a projected radius of approximately 30 kpc. Our ground-based data were
combined with archival and published Hubble Space Telescope data to probe the
properties of the GC system close to the galaxy center and reduce contamination
in the GC candidate sample from foreground stars and background galaxies. We
performed surface photometry of NGC 7457 and compared the galaxy's surface
brightness profile with the surface density profile of the GC system. The
profiles have similar shapes in the inner 1 arcminute (3.9 kpc), but the GC
system profile appears to flatten relative to the galaxy light at larger radii.
The GC system of NGC 7457 is noticeably elliptical in our images; we measure an
ellipticity of 0.66 +/- 0.14 for the GC distribution, which is consistent with
our measured ellipticity of the galaxy light. We integrated the radial surface
density profile of the GC system to derive a total number of GCs N_GC = 210 +/-
30. The GC specific frequency normalized by the galaxy luminosity and mass are
S_N = 3.1 +/- 0.7 and T = 4.8 +/- 1.1, respectively. Comparing the derived GC
system properties and other empirical data for NGC 7457 to S0 formation
scenarios suggests that this field S0 galaxy may have formed in an unequal-mass
merger.Comment: 40 pages, 10 figures, accepted for publication in The Astrophysical
Journa
Lifting the Dusty Veil With Near- and Mid-Infrared Photometry: III. Two-Dimensional Extinction Maps of the Galactic Midplane Using the Rayleigh-Jeans Color Excess Method
We provide new, high-resolution A(Ks) extinction maps of the heavily reddened
Galactic midplane based on the Rayleigh-Jeans Color Excess ("RJCE") method.
RJCE determines star-by-star reddening based on a combination of near- and
mid-infrared photometry. The new RJCE-generated maps have 2 x 2 arcmin pixels
and span some of the most severely extinguished regions of the Galaxy -- those
covered with Spitzer+IRAC imaging by the GLIMPSE-I, -II, -3D, and Vela-Carina
surveys, from 256<l<65 deg and, in general, for |b| <= 1-1.5 deg (extending up
to |b|<=4 deg in the bulge). Using RJCE extinction measurements, we generate
dereddened color-magnitude diagrams and, in turn, create maps based on main
sequence, red clump, and red giant star tracers, each probing different
distances and thereby providing coarse three-dimensional information on the
relative placement of dust cloud structures. The maps generated from red giant
stars, which reach to ~18-20 kpc, probe beyond most of the Milky Way extinction
in most directions and provide close to a "total Galactic extinction" map -- at
minimum they provide high angular resolution maps of lower limits on A(Ks).
Because these maps are generated directly from measurements of reddening by the
very dust being mapped, rather than inferred on the basis of some less direct
means, they are likely the most accurate to date for charting in detail the
highly patchy differential extinction in the Galactic midplane. We provide
downloadable FITS files and an IDL tool for retrieving extinction values for
any line of sight within our mapped regions.Comment: 23 pages, 5 figures, accepted for publication in ApJ
Chandra X-ray Sources in M101
A deep (98.2 ks) Chandra Cycle-1 observation has revealed a wealth of
discrete X-ray sources as well as diffuse emission in the nearby face-on spiral
galaxy M101. From this rich dataset we have created a catalog of the 110
sources from the S3 chip detected with a significance of >3 sigma,
corresponding to a flux of ~1.0E-16 ergs/cm/cm/s and a luminosity of 1.0E36
ergs/s for a distance to M101 of 7.2 Mpc. The sources display a distinct
correlation with the spiral arms and include a variety of X-ray binaries,
supersoft sources, supernova remnants, and other objects of which only ~27 are
likely to be background sources. There are only a few sources in the interarm
regions, and most of these have X-ray colors consistent with that of background
AGNs. The derived log N-log S relation for the sources in M101 (background
subtracted) has a slope of -0.80+/-0.05 over the range of 1.0E36 - 1.0E38
ergs/s. The nucleus is resolved into 2 nearly identical X-ray sources, each
with a 0.5-2.0 keV flux of 4.0E37 ergs/s. One of these sources coincides with
the optical nucleus, and the other coincides with a cluster of stars 110 pc to
the south.Comment: 39 pages including 13 figures and 4 tables; ApJ, in pres
The Solar Neighborhood. XXVI. AP Col: The Closest (8.4 pc) Pre-Main-Sequence Star
We present the results of a multi-technique investigation of the M4.5Ve flare
star AP Col, which we discover to be the nearest pre-main-sequence star. These
include astrometric data from the CTIO 0.9m, from which we derive a proper
motion of 342.0+/-0.5 mas yr^-1, a trigonometric parallax of 119.21+/-0.98 mas
(8.39+/-0.07 pc), and photometry and photometric variability at optical
wavelengths. We also provide spectroscopic data, including radial velocity
(22.4+/-0.3 km s^-1), lithium Equivalent Width (EW) (0.28+/-0.02 A), H-alpha EW
(-6.0 to -35 A), {\it vsini} (11+/-1 km s^-1), and gravity indicators from the
Siding Spring 2.3-m WiFeS, Lick 3-m Hamilton echelle, and Keck-I HIRES echelle
spectrographs. The combined observations demonstrate that AP Col is the closer
of only two known systems within 10 pc of the Sun younger than 100 Myr. Given
its space motion and apparent age of 12-50 Myr, AP Col is likely a member of
the recently proposed ~40 Myr old Argus/IC 2391 association.Comment: 31 pages, 11 figure
The Mass and Structure of the Pleiades Star Cluster from 2MASS
We present the results of a large scale search for new members of the
Pleiades star cluster using 2MASS near-infrared photometry and proper motions
derived from POSS plates digitized by the USNO PMM program. The search extends
to a 10 degree radius around the cluster, well beyond the presumed tidal
radius, to a limiting magnitude of R ~ 20, corresponding to ~ 0.07 M_sun at the
distance and age of the Pleiades. Multi-object spectroscopy for 528 candidates
verifies that the search was extremely effective at detecting cluster stars in
the 1 - 0.1 M_sun mass range using the distribution of H_alpha emission
strengths as an estimate of sample contamination by field stars.
When combined with previously identified, higher mass stars, this search
provides a sensitive measurement of the stellar mass function and dynamical
structure of the Pleiades. The degree of tidal elongation of the halo agrees
well with current N body simulation results. Tidal truncation affects masses
below ~ 1 M_sun. The cluster contains a total mass ~ 800 M_sun. Evidence for a
flatter mass function in the core than in the halo indicates the depletion of
stars in the core with mass less than ~ 0.5 M_sun, relative to stars with mass
\~1 - 0.5 M_sun, and implies a preference for very low mass objects to populate
the halo or escape. The overall mass function is best fitted with a lognormal
form that becomes flat at ~ 0.1 M_sun. Whether sufficient dynamical evaporation
has occurred to detectably flatten the initial mass function, via preferential
escape of very low mass stars and brown dwarfs, is undetermined, pending better
membership information for stars at large radial distances.Comment: 19 pages, 14 figures, 2 tables, accepted by AJ, to appear April 200
Lifting the Dusty Veil With Near- and Mid-Infrared Photometry: I. Description and Applications of the Rayleigh-Jeans Color Excess Method
The Milky Way (MW) remains a primary laboratory for understanding the
structure and evolution of spiral galaxies, but typically we are denied clear
views of MW stellar populations at low Galactic latitudes because of extinction
by interstellar dust. However, the combination of 2MASS near-infrared (NIR) and
Spitzer-IRAC mid-infrared (MIR) photometry enables a powerful method for
determining the line of sight reddening to any star: the sampled wavelengths
lie in the Rayleigh-Jeans part of the spectral energy distribution of most
stars, where, to first order, all stars have essentially the same intrinsic
color. Thus, changes in stellar NIR-MIR colors due to interstellar reddening
are readily apparent, and (under an assumed extinction law) the observed colors
and magnitudes of stars can be easily and accurately restored to their
intrinsic values, greatly increasing their usefulness for Galactic structure
studies. In this paper we explore this "Rayleigh-Jeans Color Excess" (RJCE)
method and demonstrate that use of even a simple variant of the RJCE method
based on a single reference color, (H-[4.5um]), can rather accurately remove
dust effects from previously uninterpretable 2MASS color-magnitude diagrams of
stars in fields along the heavily reddened Galactic mid-plane, with results far
superior to those derived from application of other dereddening methods. We
also show that "total" Galactic midplane extinction looks rather different from
that predicted using 100um emission maps from the IRAS/ISSA and COBE/DIRBE
instruments as presented by Schlegel et al. Instead, the Galactic mid-plane
extinction strongly resembles the distribution of 13-CO (J=1->0) emission.
Future papers will focus on refining the RJCE method and applying the technique
to understand better not only dust and its distribution, but the distribution
of stars intermixed with the dust in the low-latitude Galaxy.Comment: Accepted to ApJ; 21 pages, 17 figure
The OSACA Database and a Kinematic Analysis of Stars in the Solar Neighborhood
We transformed radial velocities compiled from more than 1400 published
sources, including the Geneva--Copenhagen survey of the solar neighborhood
(CORAVEL-CfA), into a uniform system based on the radial velocities of 854
standard stars in our list. This enabled us to calculate the average weighted
radial velocities for more than 25~000 HIPPARCOS stars located in the local
Galactic spiral arm (Orion arm) with a median error of +-1 km/s. We use these
radial velocities together with the stars' coordinates, parallaxes, and proper
motions to determine their Galactic coordinates and space velocities. These
quantities, along with other parameters of the stars, are available from the
continuously updated Orion Spiral Arm CAtalogue (OSACA) and the associated
database. We perform a kinematic analysis of the stars by applying an
Ogorodnikov-Milne model to the OSACA data. The kinematics of the nearest single
and multiple main-sequence stars differ substantially. We used distant
(r\approx 0.2 kpc) stars of mixed spectral composition to estimate the angular
velocity of the Galactic rotation -25.7+-1.2 km/s/kpc, and the vertex
deviation,l=13+-2 degrees, and detect a negative K effect. This negative K
effect is most conspicuous in the motion of A0-A5 giants, and is equal to
K=-13.1+-2.0 km/s/kpc.Comment: 16 pages, 8 figure
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