259 research outputs found
An HST/WFPC2 Survey for Brown Dwarf Binaries in the alpha Per and the Pleiades Open Clusters
We present the results of a high-resolution imaging survey for brown dwarf
(BD) binaries in two open clusters. The observations were carried out with
WFPC2 onboard HST. Our sample consists of 8 BD candidates in the alpha Per
cluster and 25 BD candidates in the Pleiades. We have resolved 4 binaries in
the Pleiades with separations in the range 0".094--0".058, corresponding to
projected separations between 11.7~AU and 7.2~AU. No binaries were found among
the alpha Per targets. Three of the binaries have proper motions consistent
with cluster membership in the Pleiades cluster, and for one of them we report
the detection of Halpha in emission and LiI absorption obtained from
Keck~II/ESI spectroscopy. One of the binaries does not have a proper motion
consistent with Pleiades membership. We estimate that BD binaries wider than
12~AU are less frequent than 9% in the alphaPer and Pleiades clusters. This is
consistent with an extension to substellar masses of a trend observed among
stellar binaries: the maximum semimajor axis of binary systems decreases with
decreasing primary mass. We find a binary frequency of 2 binaries over 13 BDs
with confirmed proper motion membership in the Pleiades, corresponding to a
binary fraction of 15%(1 sigma error bar +15%/-5%). These binaries are limited
to the separation range 7-12~AU and their mass ratios are larger than 0.7. The
relatively high binary frequency (>10%), the bias to separations smaller than
about 15 AU and the trend to high mass ratios (q>0.7) are fundamental
properties of BDs. Current theories of BD formation do not appear to provide a
good description of all these properties.Comment: Accepted by ApJ (scheduled publication in volume 594, September 1,
2003
Correlated errors in Hipparcos parallaxes towards the Pleiades and the Hyades
We show that the errors in the Hipparcos parallaxes towards the Pleiades and
the Hyades open clusters are spatially correlated over angular scales of 2 to 3
deg, with an amplitude of up to 2 mas. This correlation is stronger than
expected based on the analysis of the Hipparcos catalog. We predict the
parallaxes of individual cluster members, pi_pm, from their Hipparcos proper
motions, assuming that all cluster members have the same space velocity. We
compare pi_pm with their Hipparcos parallaxes, pi_Hip, and find that there are
significant spatial correlations in pi_Hip. We derive a distance modulus to the
Pleiades of 5.58 +- 0.18 mag using the radial-velocity gradient method. This
value, agrees very well with the distance modulus of 5.60 +- 0.04 mag
determined using the main-sequence fitting technique, compared with the value
of 5.33 +- 0.06 inferred from the average of the Hipparcos parallaxes of the
Pleiades members. We show that the difference between the main-sequence fitting
distance and the Hipparcos parallax distance can arise from spatially
correlated errors in the Hipparcos parallaxes of individual Pleiades members.
Although the Hipparcos parallax errors towards the Hyades are spatially
correlated in a manner similar to those of the Pleiades, the center of the
Hyades is located on a node of this spatial structure. Therefore, the parallax
errors cancel out when the average distance is estimated, leading to a mean
Hyades distance modulus that agrees with the pre-Hipparcos value. We speculate
that these spatial correlations are also responsible for the discrepant
distances that are inferred using the mean Hipparcos parallaxes to some open
clusters. Finally, we note that our conclusions are based on a purely geometric
method and do not rely on any models of stellar isochrones.Comment: 33 pages including 10 Figures, revised version accepted for
publication in Ap
CTQ 414: A New Gravitational Lens
We report the discovery and ground based observations of the new
gravitational lens CTQ 414. The source quasar lies at a redshift of z = 1.29
with a B magnitude of 17.6. Ground based optical imaging reveals two point
sources separated by 1.2 arcsec with a magnitude difference of roughly 1 mag.
Subtraction of two stellar point spread functions from images obtained in
subarcsecond seeing consistently leaves behind a faint, residual object. Fits
for two point sources plus an extended object places the fainter object
collinear with the two brighter components. Subsequent HST/NICMOS observations
have confirmed the identification of the fainter object as the lensing galaxy.
VLA observations at 8.46 GHz reveal that all components of the lensing system
are radio quiet down to the 0.2 mJy flux level.Comment: Latex, 18 pages including 2 ps figures; accepted for publication in
A
Bright Variable Stars in NGC 6819 - An Open Cluster in the Kepler Field
We describe a variability study of the moderately old open cluster NGC 6819.
We have detected 4 new detached eclipsing binaries near the cluster turnoff
(one of which may be in a triple system). Several of these systems should be
able to provide mass and radius information, and can therefore constrain the
age of the cluster. We have also newly detected one possible detached binary
member about 3.5 magnitudes below the turnoff. One EW-type binary (probably not
a cluster member) shows unusually strong night-to-night light curve variations
in sets of observations separated by 8 years. According to the best current
information, the three brightest variables we detected (2 of them new) are
cluster members, making them blue stragglers. One is a delta Scu pulsating
variable, one is a close but detached binary, and the third contains a detached
short period binary that shows total eclipses. In each case, however, there is
evidence hinting that the system may have been produced through the interaction
of more than two stars.Comment: 33 pages, 15 figures, accepted to A
Lithium in Blanco1: Implications for Stellar Mixing
We obtain lithium abundances for G and K stars in Blanco 1, an open cluster
with an age similar to, or slightly younger than, the Pleiades. We critically
examine previous spectroscopic abundance analyses of Blanco 1 and conclude that
while there were flaws in earlier work, it is likely that Blanco 1 is close in
overall metallicity to the older Hyades cluster and more metal-rich than the
Pleiades. However, we find Blanco 1 has Li abundances and rotation rates
similar to the Pleiades, contradicting predictions from standard stellar
evolution models, in which convective pre-main sequence (PMS) Li depletion
should increase rapidly with metallicity. If the high metallicity of Blanco 1
is subsequently confirmed, our observations imply (1) that a currently unknown
mechanism severely inhibits PMS Li depletion, (2) that additional non-standard
mixing modes, such as those driven by rotation and angular momentum loss, are
then responsible for main sequence Li depletion between the ages of Blanco 1
and the Hyades, and (3) that in clusters younger than the Hyades, metallicity
plays only a minor role in determining the amount of Li depletion among G and K
stars. These conclusions suggest that Li abundance remains a useful age
indicator among young (less than 700 Myr) stars even when metallicities are
unknown. If non-standard mixing is effective in Population I stars, the
primordial Li abundance could be significantly larger than present day
Population II Li abundances, due to prior Li depletion.Comment: 18 pages, 3 figs. To appear in ApJ Vol. 511 (Jan 20 1999
The Infrared Ca II triplet as metallicity indicator
From observations of almost 500 RGB stars in 29 Galactic open and globular
clusters, we have investigated the behaviour of the infrared Ca II triplet
(8498, 8542 and 8662 \AA) in the age range 13Age/Gyr0.25 and the
metallicity range [Fe/H] +0.47. These are the widest ranges of
ages and metallicities in which the behaviour of the Ca II triplet lines has
been investigated in a homogeneous way. We report the first empirical study of
the variation of the CaII triplet lines strength, for given metallicities, with
respect to luminosity. We find that the sequence defined by each cluster in the
Luminosity-Ca plane is not exactly linear. However, when only stars in
a small magnitude interval are observed, the sequences can be considered as
linear. We have studied the the Ca II triplet lines on three metallicities
scales. While a linear correlation between the reduced equivalent width (
or ) versus metallicity is found in the \citet{cg97} and \citet{ki03}
scales, a second order term needs to be added when the \citet{zw84} scale is
adopted. We investigate the role of age from the wide range of ages covered by
our sample. We find that age has a weak influence on the final relationship.
Finally, the relationship derived here is used to estimate the metallicities of
three poorly studied open clusters: Berkeley 39, Trumpler 5 and Collinder 110.
For the latter, the metallicity derived here is the first spectroscopic
estimate available.Comment: 52 pages, 16 figures, accepted for publication in Astronomical
Journa
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
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
An optical and near IR study of the old open cluster NGC 2141
We report on CCD optical (B and V passbands) and near IR (J and K bands)
observations in the region of the old open cluster NGC 2141. By combining the
two sets of photometry (500 stars in common) we derive new estimates of the
cluster fundamental parameters. We confirm that the cluster is 2.5 Gyrs old,
but, with respect to previous investigations, we obtain a slightly larger
reddening (E(B-V)=0.40), and a slightly shorter distance (3.8 kpc) from the
Sun. Finally we present Luminosity Function (LF) in the V band, which is
another age indicator. We provide a good fit for the age range inferred from
isochrones by assuming the Kroupa et al. (1993) IMF up to =5.0. We
interpret the disagreemt at fainter magnitudes as an evidence of mass
segregation.Comment: 8 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
WIYN Open Cluster Study. XXIV. Stellar Radial-Velocity Measurements in NGC 6819
We present the current results from our ongoing radial-velocity survey of the
intermediate-age (2.4 Gyr) open cluster NGC 6819. Using both newly observed and
other available photometry and astrometry we define a primary target sample of
1454 stars that includes main-sequence, subgiant, giant, and blue straggler
stars, spanning a magnitude range of 11<=V<=16.5 and an approximate mass range
of 1.1 to 1.6 Msun. Our sample covers a 23 arcminute (13 pc) square field of
view centered on the cluster. We have measured 6571 radial velocities for an
unbiased sample of 1207 stars in the direction of the open cluster NGC 6819,
with a single-measurement precision of 0.4 km/s for most narrow-lined stars. We
use our radial-velocity data to calculate membership probabilities for stars
with >= 3 measurements, providing the first comprehensive membership study of
the cluster core that includes stars from the giant branch through the upper
main sequence. We identify 480 cluster members. Additionally, we identify
velocity-variable systems, all of which are likely hard binaries that
dynamically power the cluster. Using our single cluster members, we find a
cluster average radial velocity of 2.34 +/- 0.05 km/s . We use our kinematic
cluster members to construct a cleaned color- magnitude diagram from which we
identify rich giant, subgiant, and blue straggler populations and a
well-defined red clump. The cluster displays a morphology near the cluster
turnoff clearly indicative of core convective overshoot. Finally, we discuss a
few stars of note, one of which is a short-period red-clump binary that we
suggest may be the product of a dynamical encounter.Comment: 11 pages, accepted for publication in A
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