484 research outputs found
HST astrometry in the Orion Nebula Cluster: census of low-mass runaways
We present a catalog of high-precision proper motions in the Orion Nebula
Cluster (ONC), based on Treasury Program observations with the Hubble Space
Telescope's (HST) ACS/WFC camera. Our catalog contains 2,454 objects in the
magnitude range of , thus probing the stellar masses
of the ONC from 0.4 down to 0.02 over an area
of 550 arcmin. We provide a number of internal velocity dispersion
estimates for the ONC that indicate a weak dependence on the stellar location
and mass. There is good agreement with the published velocity dispersion
estimates, although nearly all of them (including ours at
and mas yr) might be biased by the overlapping young
stellar populations of Orion A. We identified 4 new ONC candidate runaways
based on HST and the Gaia DR2 data, all with masses less than 1
. The total census of known candidate runaway sources is 10 -- one of
the largest samples ever found in any Milky Way open star cluster.
Surprisingly, none of them has the tangential velocity exceeding 20 km
s. If most of them indeed originated in the ONC, it may compel
re-examination of dynamical processes in very young star clusters. It appears
that the mass function of the ONC is not significantly affected by the lost
runaways.Comment: 16 pages, 10 figures, 5 tables. Accepted for publication in A
Ground-based astrometry with wide field imagers. V. Application to near-infrared detectors: HAWK-I@VLT/ESO
High-precision astrometry requires accurate point-spread function modeling
and accurate geometric-distortion corrections. This paper demonstrates that it
is possible to achieve both requirements with data collected at the high acuity
wide-field K-band imager (HAWK-I), a wide-field imager installed at the Nasmyth
focus of UT4/VLT ESO 8m telescope. Our final astrometric precision reaches ~3
mas per coordinate for a well-exposed star in a single image with a systematic
error less than 0.1 mas. We constructed calibrated astro-photometric catalogs
and atlases of seven fields: the Baade's Window, NGC 6656, NGC 6121, NGC 6822,
NGC 6388, NGC 104, and the James Webb Space Telescope calibration field in the
Large Magellanic Cloud. We make these catalogs and images electronically
available to the community. Furthermore, as a demonstration of the efficacy of
our approach, we combined archival material taken with the optical wide-field
imager at the MPI/ESO 2.2m with HAWK-I observations. We showed that we are able
to achieve an excellent separation between cluster members and field objects
for NGC 6656 and NGC 6121 with a time base-line of about 8 years. Using both
HST and HAWK-I data, we also study the radial distribution of the SGB
populations in NGC 6656 and conclude that the radial trend is flat within our
uncertainty. We also provide membership probabilities for most of the stars in
NGC 6656 and NGC 6121 catalogs and estimate membership for the published
variable stars in these two fields.Comment: 36 pages (included appendix), 13 tables, 35 figures (26 in low
resolution), accepted for publication in Astronomy and Astrophysics. Online
materials will be soon available on CDS. Meanwhile, online materials can be
requested directly to the first autho
The Southern Proper Motion Program III. A Near-Complete Catalog to V=17.5
We present the third installment of the Yale/San Juan Southern Proper Motion
Catalog, SPM3. Absolute proper motions, positions, and photographic B,V
photometry are given for roughly 10.7 million objects, primarily stars, down to
a magnitude of V=17.5. The Catalog covers an irregular area of 3700 square
degrees, between the declinations of -20 and -45 degrees, excluding the
Galactic plane. The proper-motion precision, for well-measured stars, is
estimated to be 4.0 mas/yr. Unlike previous releases of the SPM Catalog, the
proper motions are on the International Celestial Reference System by way of
Hipparcos Catalog stars, and have an estimated systematic uncertainty of 0.4
mas/yr. The SPM3 Catalog is available via electronic
transfer,(http://www.astro.yale.edu/astrom/) As an example of the potential of
the SPM3 proper motions, we examine the Galactocentric velocities of a group of
metal-poor, main-sequence A stars. The majority of these exhibit thick-disk
kinematics, lending support to their interpretation as thick-disk blue
stragglers, as opposed to being an accreted component.Comment: 23 pages, 10 figures, accepted for publication in Astronomical
Journa
Lick Northern Proper Motion Program. III. Lick NPM2 Catalog
The Lick Northern Proper Motion (NPM) program, a two-epoch (1947-1988)
photographic survey of the northern two-thirds of the sky (Dec. > -23 deg), has
measured absolute proper motions, on an inertial system defined by distant
galaxies, for 380,000 stars from 8 <B < 18. The 1993 NPM1 Catalog contains
148,940 stars in 899 fields outside the Milky Way's zone of avoidance. The 2003
NPM2 Catalog contains 232,062 stars in the remaining 347 NPM fields near the
plane of the Milky Way. This paper describes the NPM2 star selection, plate
measurements, astrometric and photometric data reductions, and catalog
compilation. The NPM2 Catalog contains 120,000 faint (B > 14) anonymous stars
for astrometry and galactic studies, 92,000 bright (B < 14) positional
reference stars, and 35,000 special stars chosen for astrophysical interest.
The NPM2 proper motions are on the ICRS system, via Tycho-2 stars, to an
accuracy of 0.5 mas/yr in each field. RMS proper motion precision is 6 mas/yr.
Positional errors average 80 mas at the mean plate epoch 1968, and 200 mas at
the NPM2 catalog epoch 2000. NPM2 photographic photometry errors average 0.18
mag in B, and 0.20 mag in B-V. The NPM2 Catalog and the updated (to J2000) NPM1
Catalog are available at the CDS Strasbourg data center and on the NPM WWW site
(http://www.ucolick.org/~npm). The NPM2 Catalog completes the Lick Northern
Proper Motion program after a half-century of work by three generations of Lick
Observatory astronomers. The NPM Catalogs will serve as a database for research
in galactic structure, stellar kinematics, and astrometry.Comment: 44 pages, 6 figures, accepted for publication in September 2004
Astronomical Journa
A large local rotational speed for the Galaxy found from proper-motions: Implications for the mass of the Milky-Way
Predictions from a Galactic Structure and Kinematic model are compared to the
absolute proper-motions of about 30,000 randomly selected stars with derived from the Southern Proper-Motion Program (SPM) toward
the South Galactic Pole. The absolute nature of the SPM proper-motions allow us
to measure not only the relative motion of the Sun with respect to the local
disk, but also, and most importantly, the overall state of rotation of the
local disk with respect to galaxies. The SPM data are best fit by models having
a solar peculiar motion of +5 km~s in the V-component (pointing in the
direction of Galactic rotation), a large LSR speed of 270 km~s, and a
disk velocity ellipsoid that points towards the Galactic center. We stress,
however, that these results rest crucially on the assumptions of both
axisymmetry and equilibrium dynamics.
The absolute proper-motions in the U-component indicate a solar peculiar
motion of km~s, with no need for a local expansion or
contraction term.
The implications of the large LSR speed are discussed in terms of
gravitational mass of the Galaxy inferred from the most recent and accurate
determination for the proper-motion of the LMC. We find that our derived value
for the LSR is consistent both with the mass of the Galaxy inferred from the
motion of the Clouds ( to kpc), as well
as the timing argument, based on the binary motion of M31 and the Milky Way,
and Leo I and the Milky Way ( to
kpc).Comment: 7 pages (AAS Latex macro v4.0), 2 B&W postscript figures, accepted
for publication on ApJ, Letters sectio
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 effects of differential reddening and stellar rotation on the appearance of multiple populations in star clusters: the case of Trumpler 20
We present a detailed analysis of the upper main sequence of the 1.3 Gyr old
open cluster Trumpler 20. High accuracy BV photometry combined with the Very
Large Telescope/FLAMES medium-resolution spectroscopy of 954 stars is essential
to understanding the unusual appearance of the color-magnitude diagram (CMD),
initially suggesting multiple populations in Trumpler 20. We show that
differential reddening is a dominant contributor to the apparent
splitting/widening of the main-sequence turnoff region. At its extreme, the
excess differential reddening reaches Delta(B-V)=0.1 while the adopted minimum
reddening for the cluster is E(B-V)=0.36. A unique sample of measured projected
rotational velocities indicates that stellar rotation is high near the
main-sequence turnoff, reaching vsin i=180 km/s. By dividing the upper
main-sequence stars into equal groups of slow and fast rotators, we find that
fast rotators have a marginal blueshift of delta(V-I)=-0.01, corresponding to a
difference in the median vsin i of 60 km/s between these subsamples. We
conclude that stellar rotation has an insignificant effect on the morphology of
the upper main sequence of this intermediate-age open cluster. Trumpler 20
appears to contain a single coeval population of stars but there is evidence
that the red clump is extended.Comment: 16 pages, 5 figures, accepted for publication in ApJ
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