5,264 research outputs found
Automated search for galactic star clusters in large multiband surveys: I. Discovery of 15 new open clusters in the Galactic anticenter region
Aims: According to some estimations, there are as many as 100000 open
clusters in the Galaxy, but less than 2000 of them have been discovered,
measured, and cataloged. We plan to undertake data mining of multiwavelength
surveys to find new star clusters. Methods: We have developed a new method to
search automatically for star clusters in very large stellar catalogs, which is
based on convolution with density functions. We have applied this method to a
subset of the Two Micron All Sky Survey catalog toward the Galactic anticenter.
We also developed a method to verify whether detected stellar groups are real
star clusters, which tests whether the stars that form the spatial density peak
also fall onto a single isochrone in the color-magnitude diagram. By fitting an
isochrone to the data, we estimate at the same time the main physical
parameters of a cluster: age, distance, color excess. Results: For the present
paper, we carried out a detailed analysis of 88 overdensity peaks detected in a
field of degrees near the Galactic anticenter. From this analysis,
15 overdensities were confirmed to be new open clusters and the physical and
structural parameters were determined for 12 of them; 10 of them were
previously suspected to be open clusters by Kronberger (2006) and Froebrich
(2007). The properties were also determined for 13 yet-unstudied known open
clusters, thus almost tripling the sample of open clusters with studied
parameters in the anticenter. The parameters determined with this method showed
a good agreement with published data for a set of well-known clusters.Comment: accepted to A&
Extinction Maps Toward The Milky Way Bulge: Two-Dimensional And Three-Dimensional Tests With APOGEE
Galactic interstellar extinction maps are powerful and necessary tools for Milky Way structure and stellar population analyses, particularly toward the heavily reddened bulge and in the midplane. However, due to the difficulty of obtaining reliable extinction measures and distances for a large number of stars that are independent of these maps, tests of their accuracy and systematics have been limited. Our goal is to assess a variety of photometric stellar extinction estimates, including both two-dimensional and three-dimensional extinction maps, using independent extinction measures based on a large spectroscopic sample of stars toward the Milky Way bulge. We employ stellar atmospheric parameters derived from high-resolution H-band Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectra, combined with theoretical stellar isochrones, to calculate line-of-sight extinction and distances for a sample of more than 2400 giants toward the Milky Way bulge. We compare these extinction values to those predicted by individual near-IR and near+mid-IR stellar colors, two-dimensional bulge extinction maps, and three-dimensional extinction maps. The long baseline, near+mid-IR stellar colors are, on average, the most accurate predictors of the APOGEE extinction estimates, and the two-dimensional and three-dimensional extinction maps derived from different stellar populations along different sightlines show varying degrees of reliability. We present the results of all of the comparisons and discuss reasons for the observed discrepancies. We also demonstrate how the particular stellar atmospheric models adopted can have a strong impact on this type of analysis, and discuss related caveats.NSF Astronomy & Astrophysics Postdoctoral Fellowship AST-1203017Physics Frontier Center/Joint Institute for Nuclear Astrophysics (JINA) PHY 08-22648U.S. National Science FoundationAlfred P. Sloan FoundationParticipating InstitutionsU.S. Department of Energy Office of Science ANR-12-BS05-0015-01Astronom
Photometry Results for the Globular Clusters M10 and M12: Extinction Maps, Color-Magnitude Diagrams, and Variable Star Candidates
We report on photometry results of the equatorial globular clusters (GCs) M10
and M12. These two clusters are part of our sample of GCs which we are probing
for the existence of photometrically varying eclipsing binary stars. During the
search for binaries in M10 and M12, we discovered the signature of differential
reddening across the fields of the clusters. The effect is stronger for M10
than for M12. Using our previously described dereddening technique, we create
differential extinction maps for the clusters which dramatically improve the
appearance of the color-magnitude diagrams (CMDs). Comparison of our maps with
the dust emissivity maps of Schlegel, Finkbeiner, & Davis (SFD) shows good
agreement in terms of spatial extinction features. Several methods of adding an
E_{V-I} zero point to our differential maps are presented of which isochrone
fitting proved to be the most successful. Our E_{V-I} values fall within the
range of widely varying literature values. More specifically, our reddening
zero point estimate for M12 agrees well with the SFD estimate, whereas the one
for M10 falls below the SFD value. Our search for variable stars in the
clusters produced a total of five variables: three in M10 and two in M12. The
M10 variables include a binary system of the W Ursa Majoris (W UMa) type, a
background RR Lyrae star, and an SX Phoenicis pulsator, none of which is
physically associated with M10. M12's variables are two W UMa binaries, one of
which is most likely a member of the cluster. We present the phased photometry
lightcurves for the variable stars, estimate their distances, and show their
locations in the fields and the CMDs of the GCs.Comment: 22 pages, 21 figures, to be published in AJ October 2002. For a
higher-resolution version of this paper, please visit
http://www.astro.lsa.umich.edu/~kaspar/M10_M12_photometry.ps.gz (gzipped
postscript) or http://www.astro.lsa.umich.edu/~kaspar/M10_M12_photometry.pdf
(pdf file
HST survey of the Orion Nebula Cluster in the HO 1.4 m absorption band: I. A census of substellar and planetary mass objects
In order to obtain a complete census of the stellar and sub-stellar
population, down to a few M in the Myr old Orion Nebula
Cluster, we used the infrared channel of the Wide Field Camera 3 of the Hubble
Space Telescope with the F139M and F130N filters. These bandpasses correspond
to the m HO absorption feature and an adjacent line-free continuum
region. Out of detected sources, (about ) appear fainter
than m (Vega mag) in the F130N filter, a brightness corresponding to
the hydrogen-burning limit mass (M) at Myr. Of
these, however, only sources have a negative F130M-139N color index,
indicative of the presence of HO vapor in absorption, and can therefore be
classified as bona-fide M and L dwarfs, with effective temperatures T K at an assumed Myr cluster age. On our color-magnitude diagram, this
population of sources with HO absorption appears clearly distinct from the
larger background population of highly reddened stars and galaxies with
positive F130M-F139N color index, and can be traced down to the sensitivity
limit of our survey, m, corresponding to a Myr old
M, planetary mass object under about 2 magnitudes of visual
extinction. Theoretical models of the BT-Settl family predicting substellar
isochrones of and Myr (down to M) fail to reproduce
the observed HO color index at MM. We perform a
Bayesian analysis to determine extinction, mass and effective temperature of
each sub-stellar member of our sample, together with its membership
probability.Comment: Accepted for publication in the Astrophysical Journal. The resolution
of several figures has been downgraded to comply with the size limit of arXiv
submission
Approximating Stellar Orbits: Improving on Epicycle Theory
Already slightly eccentric orbits, such as those occupied by many old stars
in the Galactic disk, are not well approximated by Lindblad's epicycle theory.
Here, alternative approximations for flat orbits in axisymmetric stellar
systems are derived and compared to results from numeric integrations. All of
these approximations are more accurate than Lindblad's classical theory. I also
present approximate, but canonical, maps from ordinary phase-space coordinates
to a set of action-angle variables.
Unfortunately, the most accurate orbit approximation leads to non-analytical
R(t). However, from this approximation simple and yet very accurate estimates
can be derived for the peri- and apo-centers, frequencies, and actions
integrals of galactic orbits, even for high eccentricities. Moreover, further
approximating this approximation allows for an analytical R(t) and still an
accurate approximation to galactic orbits, even with high eccentricities.Comment: accepted for publication in AJ; 12 pages LaTeX, 9 figures (coloured
only here, not in AJ) uses aas2pp4.st
Ruprecht 147: The oldest nearby open cluster as a new benchmark for stellar astrophysics
Ruprecht 147 is a hitherto unappreciated open cluster that holds great
promise as a standard in fundamental stellar astrophysics. We have conducted a
radial velocity survey of astrometric candidates with Lick, Palomar, and MMT
observatories and have identified over 100 members, including 5 blue
stragglers, 11 red giants, and 5 double-lined spectroscopic binaries (SB2s). We
estimate the cluster metallicity from spectroscopic analysis, using
Spectroscopy Made Easy (SME), and find it to be [M/H] = +0.07 \pm 0.03. We have
obtained deep CFHT/MegaCam g'r'i' photometry and fit Padova isochrones to the
(g' - i') and 2MASS (J - K) CMDs using the \tau^2 maximum-likelihood procedure
of Naylor (2009), and an alternative method using 2D cross-correlations
developed in this work. We find best fits for isochrones at age t = 2.5 \pm
0.25 Gyr, m - M = 7.35 \pm 0.1, and A_V = 0.25 \pm 0.05, with additional
uncertainty from the unresolved binary population and possibility of
differential extinction across this large cluster. The inferred age is heavily
dependent by our choice of stellar evolution model: fitting Dartmouth and
PARSEC models yield age parameters of 3 Gyr and 3.25 Gyr respectively. At
approximately 300 pc and 3 Gyr, Ruprecht 147 is by far the oldest nearby star
cluster.Comment: 31 pages, 21 figures, 6 tables. Comments welcom
An Extinction Map and Color Magnitude Diagram for the Globular Cluster NGC 3201
Differential variations of up to mag on a scale of
arcminutes across NGC 3201 are presented in the form of an extinction map. This
map, created by calculating average values for stars in small
subregions of the field with respect to a fiducial region, greatly improves the
appearance of the CMD of the cluster. We describe how we implemented this
technique in detail with our data for NGC 3201. A comparison between our map
and that of the same region extracted from the COBE/DIRBE reddening maps
published by Schlegel, Finkbeiner, & Davis (1998) (SFD) displays larger-scale
similarities between the two maps as well as smaller-scale features which show
up in our map but not in the SFD map. Several methods of determining an
zeropoint to add to our differential extinction map are presented.
Isochrone fitting proved to be the most successful one, but it produces an
average for the cluster which is smaller than previously published
values by . Finally, our results seem to support the statement
by Arce & Goodman (1999) that the SFD maps overestimate the reddening in
regions of high extinction.Comment: 19 pages, 12 figures, 1 table, accepted for publication in AJ (March
2001). Full resolution version may be obtained at
http://www.astro.lsa.umich.edu/users/kaspar/html/ngc3201.pdf (PDF) and at
http://www.astro.lsa.umich.edu/users/kaspar/html/ngc3201.ps.gz (PS
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