510 research outputs found
Are there brown dwarfs in globular clusters?
We present an analytical method for constraining the substellar initial mass
function in globular clusters, based on the observed frequency of transit
events. Globular clusters typically have very high stellar densities where
close encounters are relatively common, and thus tidal capture can occur to
form close binary systems. Encounters between main sequence stars and
lower-mass objects can result in tidal capture if the mass ratio is > 0.01. If
brown dwarfs exist in significant numbers, they too will be found in close
binaries, and some fraction of their number should be revealed as they transit
their stellar companions. We calculate the rate of tidal capture of brown
dwarfs in both segregated and unsegregated clusters, and find that the tidal
capture is more likely to occur over an initial relaxation time before
equipartition occurs. The lack of any such transits in recent HST monitoring of
47 Tuc implies an upper limit on the frequency of brown dwarfs (< 15 % relative
to stars) which is significantly below that measured in the galactic field and
young clusters.Comment: MNRAS in pres
Spectroscopic classification of red high proper motion objects in the Southern Sky
We present the results of spectroscopic follow-up observations for a sample
of 71 red objects with high proper motions in the range 0.08-1.14 arcsec/yr as
detected using APM and SSS measurements of multi-epoch photographic Schmidt
plates. Red objects were selected by combining the photographic BjRI magnitudes
with 2MASS near-infrared JHKs magnitudes. Some 50 of the 71 spectroscopically
classified objects turn out to be late-type (>M6) dwarfs and in more detail,
the sample includes 35 ultracool dwarfs with spectral types between M8 and L2,
some previously reported, as well as five M-type subdwarfs, including a cool
esdM6 object, SSSPM J0500-5406. Distance estimates based on the spectral types
and 2MASS J magnitudes place almost all of the late-type (>M6) dwarfs within 50
pc, with 25 objects located inside the 25 pc limit of the catalogue of nearby
stars. Most of the early-type M dwarfs are located at larger distances of
100-200 pc, suggesting halo kinematics for some of them. All objects with
Halpha equivalent widths larger than 10 Angstroms have relatively small
tangential velocities (<50 km/s). Finally, some late-type but blue objects are
candidate binaries.Comment: accepted on 06 June 2005 for publication in A&A, 22 pages, 14
figures, 7 table
NGC 3603 - a Local Template for Massive Young Clusters
We present a study of the star cluster associated with the massive Galactic
HII region NGC3603 based on near-IR broad-- and narrowband observations taken
with ISAAC/VLT under excellent seeing conditions (<0.4''). We discuss
color-color diagrams and address the impact of the high UV flux on the disk
evolution of the low-mass stars.Comment: 3 pages, 3 figures. To appear in the Proceedings of IAU Symposium 207
"Extragalactic Star Clusters", eds. E. Grebel, D. Geisler and D. Minitt
The low-mass Initial Mass Function in the 30 Doradus starburst cluster
We present deep Hubble Space Telescope (HST) NICMOS 2 F160W band observations
of the central 56*57" (14pc*14.25pc) region around R136 in the starburst
cluster 30 Dor (NGC 2070) located in the Large Magellanic Cloud. Our aim is to
derive the stellar Initial Mass Function (IMF) down to ~1 Msun in order to test
whether the IMF in a massive metal-poor cluster is similar to that observed in
nearby young clusters and the field in our Galaxy. We estimate the mean age of
the cluster to be 3 Myr by combining our F160W photometry with previously
obtained HST WFPC2 optical F555W and F814W band photometry and comparing the
stellar locus in the color-magnitude diagram with main sequence and pre-main
sequence isochrones. The color-magnitude diagrams show the presence of
differential extinction and possibly an age spread of a few megayears. We
convert the magnitudes into masses adopting both a single mean age of 3 Myr
isochrone and a constant star formation history from 2 to 4 Myr. We derive the
IMF after correcting for incompleteness due to crowding. The faintest stars
detected have a mass of 0.5 Msun and the data are more than 50% complete
outside a radius of 5 pc down to a mass limit of 1.1 Msun for 3 Myr old
objects. We find an IMF of dN/dlog(M) M^(-1.20+-0.2) over the mass range
1.1--20 Msun only slightly shallower than a Salpeter IMF. In particular, we
find no strong evidence for a flattening of the IMF down to 1.1 Msun at a
distance of 5 pc from the center, in contrast to a flattening at 2 Msun at a
radius of 2 pc, reported in a previous optical HST study. We examine several
possible reasons for the different results. If the IMF determined here applies
to the whole cluster, the cluster would be massive enough to remain bound and
evolve into a relatively low-mass globular cluster.Comment: Accepted in ApJ. Abstract abridge
OVRO N2H+ Observations of Class 0 Protostars: Constraints on the Formation of Binary Stars
We present the results of an interferometric study of the N2H+(1--0) emission
from nine nearby, isolated, low-mass protostellar cores, using the OVRO
millimeter array. The main goal of this study is the kinematic characterization
of the cores in terms of rotation, turbulence, and fragmentation. Eight of the
nine objects have compact N2H+ cores with FWHM radii of 1200 -- 3500 AU,
spatially coinciding with the thermal dust continuum emission. The only more
evolved (Class I) object in the sample (CB 188) shows only faint and extended
N2H+ emission. The mean N2H+ line width was found to be 0.37 km/s. Estimated
virial masses range from 0.3 to 1.2 M_sun. We find that thermal and turbulent
energy support are about equally important in these cores, while rotational
support is negligible. The measured velocity gradients across the cores range
from 6 to 24 km/s/pc. Assuming these gradients are produced by bulk rotation,
we find that the specific angular momenta of the observed Class 0 protostellar
cores are intermediate between those of dense (prestellar) molecular cloud
cores and the orbital angular momenta of wide PMS binary systems. There appears
to be no evolution (decrease) of angular momentum from the smallest prestellar
cores via protostellar cores to wide PMS binary systems. In the context that
most protostellar cores are assumed to fragment and form binary stars, this
means that most of the angular momentum contained in the collapse region is
transformed into orbital angular momentum of the resulting stellar binary
systems.Comment: 35 pages, 9 figures (one in color), 6 tables. Accepted by ApJ (to
appear in Nov. 2007
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