449 research outputs found
Modeling the Near-Infrared Luminosity Functions of Young Stellar Clusters
We present the results of numerical experiments designed to evaluate the
usefulness of near-infrared luminosity functions for constraining the Initial
Mass Function (IMF) of young stellar populations. From this numerical modeling,
we find that the luminosity function of a young stellar population is
considerably more sensitive to variations in the underlying initial mass
function than to either variations in the star forming history or assumed
pre-main-sequence (PMS) mass-to-luminosity relation. To illustrate the
potential effectiveness of using the KLF of a young cluster to constrain its
IMF, we model the observed K band luminosity function of the nearby Trapezium
cluster. Our derived mass function for the Trapezium spans two orders of
magnitude in stellar mass (5 Msun to 0.02 Msun), has a peak near the hydrogen
burning limit, and has an IMF for Brown Dwarfs which steadily decreases with
decreasing mass.Comment: To appear in ApJ (1 April 2000). 37 pages including 11 figures, AAS:
ver 5.
X-ray view of IC348 in the light of an updated cluster census
We study the properties of the coronae of the low-mass stars in the young
(~2-3Myr), nearby (~310pc) open cluster IC348 combining X-ray and
optical/infrared data. The four existing Chandra observations of IC348 are
merged, thus providing a deeper and spatially more complete X-ray view than
previous X-ray studies of the cluster. We have compiled a comprehensive catalog
of IC348 members taking into account recent updates to the cluster census. Our
data collection comprises fundamental stellar parameters, infrared excess
indicating the presence of disks, Halpha emission as a tracer of chromospheric
emission or accretion and mass accretion rates. We have detected 290 X-ray
sources in four merged Chandra exposures, of which 187 are associated with
known cluster members. Only four of the X-ray sources are brown dwarfs
(spectral type M6 and later). The detection rate is highest for diskless Class
III stars and increases with stellar mass. This may be explained with higher
X-ray luminosities for higher mass and later evolutionary stage that is evident
in the X-ray luminosity functions. In particular, we find that for the lowest
examined masses (0.1-0.25 Msun) there is a difference between the X-ray
luminosity functions of accreting and non-accreting stars (classified on the
basis of their Halpha emission strength) as well as those of disk-bearing and
diskless stars (classified on the basis of the slope of the spectral energy
distribution). These differences disappear for higher masses. This is related
to our finding that the L_x/L_bol ratio is non-constant across the
mass/luminosity sequence of IC348 with a decrease towards lower luminosity
stars. Our analysis of an analogous stellar sample in the Orion Nebula Cluster
suggests that the decline of L_x/L_ bol for young stars at the low-mass end of
the stellar sequence is likely universal.Comment: Accepted for publication in Astronomy & Astrophysic
The substellar mass function in the central region of the open cluster Praesepe from deep LBT observations
Studies of the mass function (MF) of open clusters of different ages allow us
to probe the efficiency with which brown dwarfs (BDs) are evaporated from
clusters to populate the field. Surveys in old clusters (age > 100 Myr) do not
suffer so severely from several problems encountered in young clusters, such as
intra-cluster extinction and large uncertainties in BD models. Here we present
the results of a deep photometric survey to study the MF of the old open
cluster Praesepe (age 590 Myr and distance 190 pc), down to a 5 sigma detection
limit at i~25.6 mag (~40M_Jup). We identify 62 cluster member candidates, of
which 40 are substellar, from comparison with predictions from a dusty
atmosphere model. The MF rises from the substellar boundary until ~60M_Jup and
then declines. This is quite different from the form inferred for other open
clusters older than 50 Myr, but seems to be similar to those found in very
young open cluster, whose MFs peak at ~10M_Jup. Either Praesepe really does
have a different MF from other clusters or they had similar initial MFs but
have differed in their dynamical evolution. We further have identified six
foreground T dwarf candidates towards Praesepe, which require follow-up
spectroscopy to confirm their nature.Comment: 8 pages, 5 figures, to appear in the online proceedings of the Cool
Stars 16 conferenc
The Structure of the Star-forming Cluster RCW 38
We present a study of the structure of the high mass star-forming region
RCW~38 and the spatial distribution of its young stellar population. Spitzer
IRAC photometry 3-8um are combined with 2MASS near-IR data to identify young
stellar objects by IR-excess emission from their circumstellar material.
Chandra X-ray data are used to identify class III pre-main sequence stars
lacking circumstellar material. We identify 624 YSOs: 23 class 0/I and 90 flat
spectrum protostars, 437 Class II stars, and 74 Class III stars. We also
identify 29 (27 new) O star candidates over the IRAC field. Seventy-two stars
exhibit IR-variability, including seven class 0/I and 12 flat spectrum YSOs. A
further 177 tentative candidates are identified by their location in the IRAC
[3.6] vs. [3.6]-[5.8] cmd. We find strong evidence of subclustering in the
region. Three subclusters were identified surrounding the central cluster, with
massive and variable stars in each subcluster. The central region shows
evidence of distinct spatial distributions of the protostars and pre-main
sequence stars. A previously detected IR cluster, DB2001_Obj36, has been
established as a subcluster of RCW 38. This suggests that star formation in RCW
38 occurs over a more extended area than previously thought. The gas to dust
ratio is examined using the X-ray derived hydrogen column density, N_H and the
K-band extinction, and found to be consistent with the diffuse ISM, in contrast
with Serpens & NGC1333. We posit that the high photoionising flux of massive
stars in RCW 38 affects the agglomeration of the dust grains.Comment: 98 pages, 15 figure
Why are the K dwarfs in the Pleiades so Blue?
The K dwarfs in the Pleiades fall nearly one half magnitude below a main
sequence isochrone when plotted in a color-magnitude diagram utilizing V
magnitude as the luminosity index and B-V as the color index. This peculiarity
has been known for forty years but has gone unexplained and mostly ignored.
When compared to Praesepe members, the Pleiades K dwarfs again are subluminous
(or blue) in a color-magnitude diagram using B-V as the color index. However,
using V-I as the color index, stars in the two clusters are coincident to M_V ~
10; using V-K as the color index, Pleiades late K and M stars fall above the
main sequence locus defined by Praesepe members. We believe that the anomalous
spectral energy distributions for the Pleiades K dwarfs, as compared to older
clusters, are a consequence of rapid stellar rotation and may be primarily due
to spottedness. If so, the required areal filling factor for the cool component
has to be very large (=> 50%). Weak-lined T Tauri stars have similar color
anomalies, and we suspect this is a common feature of all very young K dwarfs
(sp. type > K3). The peculiar spectral energy distribution needs to be
considered in deriving accurate pre-main sequence isochrone-fitting ages for
clusters like the Pleiades since the age derived will depend on the temperature
index used.Comment: 41 pages, 15 figures, AASTeX5.0. Accepted 05 May 2003; Scheduled for
publication in the Astronomical Journal (August 2003
Brown dwarfs and very low mass stars in the Praesepe open cluster: a dynamically unevolved mass function?
[Abridged] In this paper, we present the results of a photometric survey to
identify low mass and brown dwarf members of the old open cluster Praesepe (age
of 590[+150][-120]Myr and distance of 190[+6.0][-5.8]pc) and use this to infer
its mass function which we compare with that of other clusters. We have
performed an optical (Ic-band) and near-infrared (J and Ks-band) photometric
survey of Praesepe with a spatial coverage of 3.1deg^2. With 5sigma detection
limits of Ic=23.4 and J=20.0, our survey is sensitive to objects with masses
from about 0.6 to 0.05Msol. The mass function of Praesepe rises from 0.6Msol
down to 0.1Msol and then turns-over at ~0.1Msol. The rise observed is in
agreement with the mass function derived by previous studies, including a
survey based on proper motion and photometry. Comparing our mass function with
that for another open cluster with a similar age, the Hyades (age ~ 600Myr), we
see a significant difference. Possible reasons are that dynamical evaporation
has not influenced the Hyades and Praesepe in the same way, or that the
clusters did not have the same initial mass function, or that dynamical
interactions have modified the evolution of one or both clusters. Although a
difference in the binary fractions of the clusters could cause the observed
(i.e. system) mass functions to differ, measurements in the literature give no
evidence for a significant difference in the binary fractions of the two
clusters. Of our cluster candidates, six have masses predicted to be equal to
or below the stellar/substellar boundary at 0.072Msol.Comment: 11 pages, 11 figures, accepted for publication in A&A. Higher
resolution of Figures 2-3-4-5 in A&A published version. Revised version
corrected for Englis
Fifty Years of IMF Variation: The Intermediate-Mass Stars
I track the history of star count estimates of the Milky Way field star and
open cluster IMFs, concentrating on the neglected mass range from 1 to 15
M. The prevalent belief in a universal IMF appears to be without
basis for this mass range. Two recent estimates of the field star IMF using
different methods and samples give values of the average logarithmic slope
between -1.7 and -2.1 in the mass range 1.1 to 4 M. Two
older estimates between 2 and 15 M disagree severely; the field IMF
in this range is essentially unknown from star counts. Variations in
among open cluster IMFs in this mass range have not decreased despite numerous
detailed studies, even for studies using homogeneous data and reduction
procedures and including only clusters with a significant mass range. These
cluster variations \textit{might} be due to the combined effects of sampling,
systematic errors, stellar evolution uncertainties, dynamical evolution, and
unresolved binaries. If so, then the cluster data are consistent with a
universal IMF, but are also consistent with sizeable variations. The cluster
data do not allow an estimate of an average IMF or because the average
depends on the choice of weighting procedure and other effects. If the spread
in cluster IMFs is in excess of the effects listed above, real IMF variations
must occur that do not depend much on physical conditions explored so far. The
complexity of the star formation process seen in observations and simulations
suggests that large realization-to-realization differences might be expected,
in which case an individual cluster IMF would be in part the product of
evolutionary contingency in star formation, and the function of interest is the
probability distribution of IMF parameters.Comment: 18 pages, including 4 figures: invited talk presented at the
conference on "IMF@50: The Stellar Initial Mass Function Fifty Years Later"
held at Abbazia di Spineto, Siena, Italy, May 2004; to be published by Kluwer
Academic Publishers, edited by E. Corbelli, F. Palla, and H. Zinnecke
A multi-wavelength census of stellar contents in the young cluster NGC 1624
We present a comprehensive multi-wavelength analysis of the young cluster NGC
1624 associated with the H II region Sh2-212 using optical UBVRI photometry,
optical spectroscopy and GMRT radio continuum mapping along with the
near-infrared (NIR) JHK archival data. Reddening E(B-V) and distance to the
cluster are estimated to be 0.76 - 1.00 mag and 6.0 +/- 0.8 kpc, respectively.
Present analysis yields a spectral class of O6.5V for the main ionizing source
of the region. The distribution of YSOs in (J-H)/ (H-K) NIR colour-colour
diagram shows that a majority of them have A_V 4 mag. Based on the NIR
excess characteristics, we identified 120 probable candidate YSOs in this
region which yield a disk frequency of ~ 20%. These YSOs are found to have an
age spread of ~ 5 Myr with a median age of ~ 2-3 Myr and a mass range of ~ 0.1
- 3.0 . A significant number of YSOs are located close to the cluster
centre and we detect an enhanced density of reddened YSOs located/projected
close to the molecular clumps at the periphery of NGC 1624. This indicates that
the YSOs located within the cluster core are relatively older in comparison to
those located/projected near the clumps. From the radio continuum flux,
spectral class of the ionizing source of the ultra-compact H II region at the
periphery of Sh2-212 is estimated to be ~ B0.5V. From optical data, slope of
the mass function (MF) , in the mass range can
be represented by a single power law with a slope -1.18 +/- 0.10, whereas the
NIR data in the mass range yields = -1.31
+/- 0.15. The slope of the K-band luminosity function (KLF) for the cluster is
found to be 0.30 +/- 0.06 which is in agreement with the values obtained for
other young clusters.Comment: Accepted for publication in MNRA
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