49 research outputs found
Spectroscopic Identification of Faint White Dwarf Candidates in the Praesepe Open Star Cluster
We present spectroscopic observations of the remaining four candidate white
dwarfs in Praesepe. All four candidates are quasars with redshifts between 0.8
and 2.8. One quasar, LB 6072, is observed to have a strong metal-line
absorption system blueward of the quasar redshift. The lack of additional white
dwarfs in Praesepe leaves the total known white-dwarf population of the cluster
at five, well below the number expected from commonly-assumed initial mass
functions, though several undiscovered cluster WDs may lie in the outer regions
of the cluster. All known Praesepe member white dwarfs are concentrated within
0.6 deg of the cluster center, and the radial profile of cluster white dwarfs
is quite similar to the profile of massive cluster stars. This profile is
mildly inconsistent with that of ~1Mo cluster stars and suggests that the white
dwarfs did not receive a velocity kick during the progenitor star's mass loss
phases. If complete, the observed Praesepe white dwarf population is consistent
with a steeper high-end initial-mass function than commonly assumed, though the
calculated slopes are inconsistent with the present-day mass function of
Praesepe. Searches for white dwarfs outside the core of Praesepe and further
study of the white dwarf populations of additional open clusters is necessary
to constrain further the underlying cause of the white dwarf deficit.Comment: 6 pages, 5 figures, uses emulateapj.cls; accepted for publication in
the Astronomical Journa
The Effect of the Outer Lindblad Resonance of the Galactic Bar on the Local Stellar Velocity Distribution
Hydro-dynamical modeling of the inner Galaxy suggest that the radius of the
outer Lindblad resonance (OLR) of the Galactic bar lies in the vicinity of the
Sun. How does this resonance affect the distribution function in the outer
parts of a barred disk, and can we identify any effect of the resonance in the
velocity distribution f(v) actually observed in the solar neighborhood? To
answer these questions, detailed simulations of f(v) in the outer parts of an
exponential stellar disks with nearly flat rotation curves and a rotating
central bar have been performed. For a model resembling the old stellar disk,
the OLR causes a distinct feature in f(v) over a significant fraction of the
outer disk. For positions <2kpc outside the OLR radius and at bar angles of
\~10-70 degrees, f(v) inhibits a bi-modality between the low-velocity stars
moving like the local standard of rest (LSR) and a secondary mode of stars
predominantly moving outward and rotating more slowly than the LSR.
Such a bi-modality is indeed present in f(v) inferred from the Hipparcos data
for late-type stars in the solar neighborhood. If one interpretes this observed
bi-modality as induced by the OLR -- and there are hardly any viable
alternatives -- then one is forced to deduce that the OLR radius is slightly
smaller than Ro. Moreover, by a quantitative comparison of the observed with
the simulated distributions one finds that the pattern speed of the bar is
1.85+/-0.15 times the local circular frequency, where the error is dominated by
the uncertainty in bar angle and local circular speed.
Also other, less prominent but still significant, features in the observed
f(v) resemble properties of the simulated velocity distributions, in particular
a ripple caused by orbits trapped in the outer 1:1 resonance.Comment: 14 pages, 10 figures (Fig.2 in full resolution available upon
request), accepted for publication in A
The Homogeneity of Interstellar Krypton in the Galactic Disk
We present an analysis of high resolution HST Space Telescope Imaging
Spectrograph (STIS) observations of Kr I 1236 Angstrom absorption in seven
sight lines that probe a variety of interstellar environments. In combination
with krypton and hydrogen column densities derived from current and archival
STIS and Far-Ultraviolet Spectroscopic Explorer data, the number of sight lines
with reliable Kr/H ISM abundance ratios has been increased by 50% to
26--including paths that sample a range of nearly 5 orders of magnitude in
f(H_2), over 2 orders of magnitude in , and extending up to 4.8 kpc in
length. For sight lines contained entirely within the local spiral arm (the
Orion Spur), the spread of Kr/H ratios about the mean of log_10[N(Kr)/N(H)]_ISM
= -9.02+/-0.02 is remarkably tight (0.06 dex), less than the typical datapoint
uncertainty. Intriguingly, the only two sight lines that extend through
neighboring structures, in particular gas associated with the
Carina/Sagittarius Arm, exhibit relatively large, near-solar krypton abundances
(log_10[N(Kr)/N(H)]_combined = -8.75+0.09_-0.11). Although these deviations are
only measured at the 2 sigma level, they suggest the possibility that krypton
abundances beyond the Orion Spur may differ from the local value.Comment: 15 page, including 3 figures and 2 tables, accepted for publication
in Ap
The Distribution of Stellar Mass in the Pleiades
As part of an effort to understand the origin of open clusters, we present a
statistical analysis of the currently observed Pleiades. Starting with a
photometric catalog of the cluster, we employ a maximum likelihood technique to
determine the mass distribution of its members, including single stars and both
components of binary systems. We find that the overall binary fraction for
unresolved pairs is 68%. Extrapolating to include resolved systems, this
fraction climbs to about 76%, significantly higher than the accepted field-star
result. Both figures are sensitive to the cluster age, for which we have used
the currently favored value of 125 Myr. The primary and secondary masses within
binaries are correlated, in the sense that their ratios are closer to unity
than under the hypothesis of random pairing. We map out the spatial variation
of the cluster's projected and three-dimensional mass and number densities.
Finally, we revisit the issue of mass segregation in the Pleiades. We find
unambiguous evidence of segregation, and introduce a new method for quantifying
it.Comment: 41 pages, 14 figures To Be Published in The Astrophysical Journa
The Homogeneity of Interstellar Oxygen in the Galactic Disk
We present an analysis of high resolution HST Space Telescope Imaging
Spectrograph (STIS) observations of O I 1356 and H I Lyman-alpha absorption in
36 sight lines that probe a variety of Galactic disk environments and include
paths that range over nearly 4 orders of magnitude in f(H_2), over 2 orders of
magnitude in mean sight line density, and that extend up to 6.5 kpc in length.
Consequently, we have undertaken the study of gas-phase O/H abundance ratio
homogeneity using the current sample and previously published Goddard
High-Resolution Spectrograph (GHRS) results. Two distinct trends are identified
in the 56 sight line sample: an apparent decrease in gas-phase oxygen abundance
with increasing mean sight line density and a gap between the mean O/H ratio
for sight lines shorter and longer than about 800 pc. The first effect is a
smooth transition between two depletion levels associated with large mean
density intervals; it is centered near a density of 1.5 cm^-3 and is similar to
trends evident in gas-phase abundances of other elements. Paths less dense than
the central value exhibit a mean O/H ratio of log_10 (O/H) = -3.41+/-0.01 (or
390+/-10 ppm), which is consistent with averages determined for several long,
low-density paths observed by STIS (Andre et al. 2003) and short low-density
paths observed by FUSE (Moos et al. 2002). Sight lines of higher mean density
exhibit an average O/H value of log_10 (O/H) = -3.55+/-0.02 (284+/-12 ppm). The
datapoints for low-density paths are scattered more widely than those for
denser sight lines, due to O/H ratios for paths shorter than 800 pc that are
generally about 0.10 dex lower than the values for longer ones.Comment: 33 pages, including 8 figures and 4 tables; accepted for publication
in ApJ, tentatively in Oct 200
Hubble Space Telescope Photometry of Hodge 301: An "Old" Star Cluster in 30 Doradus
We present Hubble Space Telescope Planetary Camera UVI data for the
little-studied cluster Hodge 301 3' northwest of 30 Doradus' central ionizing
cluster R136. The average reddening of Hodge 301 is found to be =
(0.28+-0.05) mag from published infrared and ultraviolet photometry. Using two
different sets of evolutionary models, we derive an age of ~ 20-25 Myr for
Hodge 301, which makes it roughly 10 times as old as R136. Hodge 301 is the
most prominent representative of the oldest population in the 30 Dor starburst
region; a region that has undergone multiple star formation events. This range
of ages is an important consideration for the modelling of starburst regions.
Hodge 301 shows a widened upper main sequence largely caused by Be stars. We
present a list of Be star candidates. The slope of the initial mass function
for intermediate-mass main sequence stars ranging from 10 to 1.3 solar masses
is found to be -1.4+-0.1 in good agreement with a Salpeter law. There is no
indication for a truncation or change of slope of the IMF within this mass
range. In accordance with the age of Hodge 301 no obvious pre-main-sequence
stars are seen down to 1 solar mass. We estimate that up to 41+-7 stars with
more than 12 solar masses may have turned into supernovae since the formation
of the cluster. Multiple supernova explosions are the most likely origin of the
extremely violent gas motions and the diffuse X-ray emission observed in the
cluster surroundings.Comment: To appear in the Astronomical Journal (Feb 2000 issue). 16 pages in
two-column style. 9 separate figures, in part in significantly reduced
resolution for space reasons (bitmapped postscript or jpg
Galactic Kinematics Towards the South Galactic Pole. First Results from the Yale-San Juan Southern Proper-Motion Program
The predictions from a Galactic Structure and Kinematic model code are
compared to the color counts and absolute proper-motions derived from the
Southern Proper-Motion survey covering more than 700 toward the South
Galactic Pole in the range . The theoretical assumptions
and associated computational procedures, the geometry for the kinematic model,
and the adopted parameters are presented in detail and compared to other
Galactic Kinematic models of its kind. The data to which the model is compared
consists of more than 30,000 randomly selected stars, and it is best fit by
models with 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 always points towards the
Galactic center. 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 fainter absolute motions show an indication
that the thick-disk must exhibit a rather steep velocity gradient of about -36
km s kpc with respect to the LSR. We are not able to set
constraints on the overall rotation for the halo, nor on the thick-disk or halo
velocity dispersions. Some substructure in the U & V proper-motions could be
present in the brighter bins , and it might be indicative
of (disk) moving groups.Comment: 24 double-column pages, 12 tables, AAS Latex macros v4.0, 19 B&W
figures, 1 color figure. Accepted for publication on The Astronomical Journa
Near and Mid-IR Photometry of the Pleiades, and a New List of Substellar Candidate Members
We make use of new near and mid-IR photometry of the Pleiades cluster in
order to help identify proposed cluster members. We also use the new photometry
with previously published photometry to define the single-star main sequence
locus at the age of the Pleiades in a variety of color-magnitude planes.
The new near and mid-IR photometry extend effectively two magnitudes deeper
than the 2MASS All-Sky Point Source catalog, and hence allow us to select a new
set of candidate very low mass and sub-stellar mass members of the Pleiades in
the central square degree of the cluster. We identify 42 new candidate members
fainter than Ks =14 (corresponding to 0.1 Mo). These candidate members should
eventually allow a better estimate of the cluster mass function to be made down
to of order 0.04 solar masses.
We also use new IRAC data, in particular the images obtained at 8 um, in
order to comment briefly on interstellar dust in and near the Pleiades. We
confirm, as expected, that -- with one exception -- a sample of low mass stars
recently identified as having 24 um excesses due to debris disks do not have
significant excesses at IRAC wavelengths. However, evidence is also presented
that several of the Pleiades high mass stars are found to be impacting with
local condensations of the molecular cloud that is passing through the Pleiades
at the current epoch.Comment: Accepted to ApJS; data tables and embedded-figure version available
at http://spider.ipac.caltech.edu/staff/stauffer/pleiades07