38 research outputs found
The WIYN Open Cluster Study: A New Color-Magnitude Diagram for M35 - A Twin of the Pleiades
M35 (NGC 2168) is rich, young, and in some regards a betÂter laboratory for stellar cluster research at 100 million years than the Pleiades. Yet, while M35 has been the subject of intriguing photometric, astrometric, and theoretical studies, to the best of our knowledge no pubÂlished CCD-based photometry exists for this cluster. We have obtained relatively wide-field (23\u27 x23\u27) precise UBVRJ CCD photometry for M35 as part of the WIYN Open Cluster Study (WOCS) at the 0.9m telescope at Kitt Peak National Observatory. The photometry extends from the main sequence turn off to beyond V = 19. We find that M35 is ~ 100 Myrs old, with (m-M)v = 10.25, and E(B-V)=0.3 based on fitting Yale isochrones. We also perform a differential comparison between M35 and the Pleiades, and we explore the cluster dynamical state
WIYN Open Cluster Study. II. UBVRI CCD Photometry of the Open Cluster NGC 188
We present high-precision UBVRI CCD photometry of the old open cluster NGC 188. Our color-magnitude diagram extends from near the red giant branch tip to as faint as ~5 mag below the main-sequence turnoff. From an analysis of these data along with published photometry for M67, we draw the following conclusions: (1) From the UBV two-color diagram, we find a reddening of E(B ̶ V) = 0.04 ± 0.02 for M67 and E(B ̶ V) = 0.09 ± 0.02 for NGC 188. (2) Based on main-sequence fitting to solar abundance isochrones, the distance moduli turn out to be (m - M)v = 9.69 ± 0.11 for M67 and (m - M)v = 11.44 ± 0.08 for NGC 188. (3) The comparison of the CMDs to theoretical isochrones indicates that an amount of core convective overshoot equivalent to 0.10 of a pressure scale height is appropriate for M67, while no overshoot is required to fit the CMD of NGC 188. These isochrones suggest that NGC 188 is 3.0 ± 0.7 Gyr older than M67. (4) There is a clear indication of mass segregation in both M67 and NGC 188, with the most massive stars (M/Mâ \u3e 1.1) being more centrally concentrated than those that are the least massive (0.8 â„ M/Mâ \u3e 0.65)
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
A New Proper Motion Determination of Leo I
We measure the absolute proper motion of Leo I using a WFPC2/HST data set
that spans up to 10 years, to date the longest time baseline utilized for this
satellite. The measurement relies on ~ 2300 Leo I stars located near the center
of light of the galaxy; the correction to absolute proper motion is based on
174 Gaia EDR3 stars and 10 galaxies. Having generated highly-precise, relative
proper motions for all Gaia EDR3 stars in our WFPC2 field of study, our
correction to the absolute EDR3 system does not rely on these Gaia stars being
Leo I members. This new determination also benefits from a recently improved
astrometric calibration of WFPC2. The resulting proper-motion value, (mu_alpha,
mu_delta) = (-0.007 +- 0.035, -0.119 +-0.026) mas/yr is in agreement with
recent, large-area, Gaia EDR3-based determinations. We discuss all the recent
measurements of Leo I's proper motion and adopt a combined, multi-study average
of (mu_alpha_3meas, mu_delta_3meas) = (-0.036 +- 0.016, -0.130 +- 0.010)
mas/yr. This value of absolute proper motion for Leo I indicates its orbital
pole is well aligned with that of the Vast Polar Structure, defined by the
majority of the brightest dwarf-spheroidal satellites of the Milky Way.Comment: accepted for publication in The 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
The Group Evolution Multiwavelength Study (GEMS): bimodal luminosity functions in galaxy groups
We present B and R-band luminosity functions (LF) for a sample of 25 nearby
groups of galaxies. We find that the LFs of the groups with low X-ray
luminosity (L_X < 10^{41.7} erg/s) are significantly different from those of
the X-ray brighter groups, showing a prominent dip around M_b = -18. While both
categories show lack of late-type galaxies in their central regions, X-ray dim
groups also show a more marked concentration of optical luminosity towards the
centre. A toy simulation shows that in the low velocity dispersion environment,
as in the X-ray dim group, dynamical friction would facilitate more rapid
merging, thus depleting intermediate-luminosity galaxies to form a few giant
central galaxies, resulting in the prominent dip seen in our LFs. We suggest
that X-ray dim (or low velocity dispersion) groups are the present sites of
rapid dynamical evolution rather than their X-ray bright counterparts, and may
be the modern precursors of fossil groups. We predict that these groups of low
velocity dispersion would harbour younger stellar populations than groups or
clusters with higher velocity dispersion.Comment: 9 pages, 12 figures, accepted for publication in MNRA
Hubble Space Telescope 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 2454 objects in the magnitude range of 14.2 < m_(F775W) < 24.7, thus probing the stellar masses of the ONC from ~0.4 Mâ down to ~0.02 Mâ over an area of ~550 arcminÂČ. We provide a number of internal velocity dispersion estimates for the ONC that indicate a weak dependence on stellar location and mass. There is good agreement with the published velocity dispersion estimates, although nearly all of them (including ours at Ï_(v,x) = 0.94 and Ï_(v,y) = 1.25 mas yrâ»Âč) might be biased by the overlapping young stellar populations of Orion A. We identified four new ONC candidate runaways based on HST and the Gaia DR 2 data, all with masses less than ~1 Mâ. The total census of known candidate runaway sources is 10âone of the largest samples ever found in any Milky Way open star cluster. Surprisingly, none of them have tangential velocities exceeding 20 km sâ»Âč. If most of them indeed originated in the ONC, it may compel the 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
Population Parameters of Intermediate-Age Star Clusters in the Large Magellanic Cloud. I. NGC 1846 and its Wide Main Sequence Turnoff
The Advanced Camera for Surveys on board the Hubble Space Telescope has been
used to obtain deep, high-resolution images of the intermediate-age star
cluster NGC 1846 in the Large Magellanic Cloud. We present new color-magnitude
diagrams (CMDs) based on F435W, F555W, and F814W imaging. We test the
previously observed broad main sequence turnoff region for "contamination" by
field stars and (evolved) binary star systems. We find that while these impact
the number of objects in this region, none can fully account for the large
color spread. Our results therefore solidify the recent finding that stars in
the main sequence turnoff region of this cluster have a large spread in color
which is unrelated to measurement errors or contamination by field stars, and
likely due to a ~300 Myr range in the ages of cluster stars. An unbiased
estimate of the stellar density distribution across the main sequence turnoff
region shows that the spread is fairly continuous rather than strongly bimodal
as suggested previously. We fit the CMDs with several different sets of
theoretical isochrones, and determine systematic uncertainties for population
parameters when derived using any one set of isochrones. We note a degeneracy
between age and [alpha/Fe], which can be lifted by matching the shape
(curvature) of the full red giant branch in the CMD. We find that stars in the
upper part of the main sequence turnoff region are more centrally concentrated
than those in any other region of the CMD, including more massive red giant
branch and asymptotic giant branch stars. We consider several possible
formation scenarios which account for the unusual features observed in the CMD
of NGC 1846.Comment: 17 pages, 17 figures, in emulateapj format. Figures 1 and 2 have been
downgraded in resolution in this version. Accepted for publication in The
Astronomical Journa