545 research outputs found
Luminosity Functions of Spitzer Identified Protostars in Nine Nearby Molecular Clouds
We identify protostars in Spitzer surveys of nine star-forming molecular
clouds within 1 kpc: Serpens, Perseus, Ophiuchus, Chamaeleon, Lupus, Taurus,
Orion, Cep OB3, and Mon R2, which combined host over 700 protostar candidates.
Our diverse cloud sample allows us to compare protostar luminosity functions in
these varied environments. We combine photometry from 2MASS J, H, and Ks bands
and Spitzer IRAC and MIPS 24 micron bands to create 1 - 24 micron spectral
energy distributions (SEDs). Using protostars from the c2d survey with
well-determined bolometric luminosities (Lbol), we derive a relationship
between Lbol, L_MIR (integrated from 1 - 24 microns), and SED slope.
Estimations of Lbol for protostar candidates are combined to create luminosity
functions for each cloud. Contamination due to edge-on disks, reddened Class II
sources, and galaxies is estimated and removed from the luminosity functions.
We find that luminosity functions for high mass star forming clouds peak near 1
Lsun and show a tail extending toward luminosities above 100 Lsun. The
luminosity functions of the low mass star forming clouds do not exhibit a
common peak, however the combined luminosity function of these regions peaks
below 1 Lsun. Finally, we examine the luminosity functions as a function of the
local surface density of YSOs. In the Orion molecular cloud, we find a
significant difference between the luminosity functions of protostars in
regions of high and low stellar density, the former of which is biased toward
more luminous sources. This may be the result of primordial mass segregation,
although this interpretation is not unique. We compare our luminosity functions
to those predicted by models and find that our observed luminosity functions
are best matched by models which invoke competitive accretion, although we do
not find strong agreement of the high mass star forming clouds with any of the
models.Comment: 76 pages, 18 figures, 7 tables. Accepted for publication in the
Astronomical Journa
The Properties of X-ray Luminous Young Stellar Objects in the NGC 1333 and Serpens Embedded Clusters
We present Chandra X-ray data of the NGC 1333 embedded cluster, combining
these data with existing Chandra data, Sptizer photometry and ground based
spectroscopy of both the NGC 1333 & Serpens North clusters to perform a
detailed study of the X-ray properties of two of the nearest embedded clusters
to the Sun. In NGC 1333, a total of 95 cluster members are detected in X-rays,
of which 54 were previously identified with Spitzer. Of the Spitzer sources, we
detect 23% of the Class I protostars, 53% of the Flat Spectrum sources, 52% of
the Class II, and 50% of the Transition Disk YSOs. Forty-one Class III members
of the cluster are identified, bringing the total identified YSO population to
178. The X-ray Luminosity Functions (XLFs) of the NGC 1333 and Serpens clusters
are compared to each other and the Orion Nebula Cluster. Based on this
comparison, we obtain a new distance for the Serpens cluster of 360+22/-13 pc.
The X-ray luminosity was found to depend on the bolometric luminosity as in
previous studies of other clusters, and that Lx depends primarily on the
stellar surface area. In the NGC 1333 cluster, the Class III sources have a
somewhat higher X-ray luminosity for a given surface area. We also find
evidence in NGC 1333 for a jump in the X-ray luminosity between spectral types
of M0 and K7, we speculate that this may result from the presence of radiative
zones in the K-stars. The gas column density vs. extinction in the NGC 1333 was
found to be N_H = 0.89 +/- 0.13 x 10^22 A_K, this is lower than expected of the
standard ISM but similar to that found previously in the Serpens Cloud Core.Comment: 58 pages, 14 figures, accepted by A
IRAS 20050+2720: Anatomy of a young stellar cluster
IRAS 20050+2720 is young star forming region at a distance of 700 pc without
apparent high mass stars. We present results of our multiwavelength study of
IRAS 20050+2720 which includes observations by Chandra and Spitzer, and 2MASS
and UBVRI photometry. In total, about 300 YSOs in different evolutionary stages
are found. We characterize the distribution of young stellar objects (YSOs) in
this region using a minimum spanning tree (MST) analysis. We newly identify a
second cluster core, which consists mostly of class II objects, about 10 arcmin
from the center of the cloud. YSOs of earlier evolutionary stages are more
clustered than more evolved objects. The X-ray luminosity function (XLF) of
IRAS 20050+2720 is roughly lognormal, but steeper than the XLF of the more
massive Orion nebula complex. IRAS 20050+2720 shows a lower N_H/A_K ratio
compared with the diffuse ISM.Comment: 15 pages, 12 figures, accepted by A
SPITZER observations of the λ Orionis cluster. II. Disks around solar-type and low-mass stars
We present IRAC/MIPS Spitzer Space Telescope observations of the solar-type and the low-mass stellar population
of the young (~5Myr) λ Orionis cluster. Combining optical and Two Micron All Sky Survey photometry, we identify
436 stars as probable members of the cluster. Given the distance (450 pc) and the age of the cluster, our sample
ranges in mass from 2 M_⊙ to objects below the substellar limit. With the addition of the Spitzer mid-infrared data,
we have identified 49 stars bearing disks in the stellar cluster. Using spectral energy distribution slopes, we place
objects in several classes: non-excess stars (diskless), stars with optically thick disks, stars with “evolved disks”
(with smaller excesses than optically thick disk systems), and “transitional disk” candidates (in which the inner
disk is partially or fully cleared). The disk fraction depends on the stellar mass, ranging from ~6% for K-type stars (R_C − J 4). We confirm the dependence of disk
fraction on stellar mass in this age range found in other studies. Regarding clustering levels, the overall fraction of disks in the λ Orionis cluster is similar to those reported in other stellar groups with ages normally quoted as ~5Myr
A Survey of High Contrast Stellar Flares Observed by Chandra
The X-ray light curves of pre-main sequence stars can show variability in the
form of flares altering a baseline characteristic activity level; the largest
X-ray flares are characterized by a rapid rise to more than 10 times the
characteristic count rate, followed by a slower quasi-exponential decay.
Analysis of these high-contrast X-ray flares enables the study of the innermost
magnetic fields of pre-main sequence stars. We have scanned the ANCHORS
database of Chandra observations of star-forming regions to extend the study of
flare events on pre-main sequence stars both in sky coverage and in volume. We
developed a sample of 30 high-contrast flares out of the 14,000 stars of
various ages and masses available in ANCHORS at the start of our study.
Applying methods of time-resolved spectral analysis, we obtain the
temperatures, confining magnetic field strengths, and loop lengths of these
bright, energetic flares. The results of the flare analysis are compared to the
2MASS and Spitzer data available for the stars in our sample. We find that the
longest flare loop lengths (of order several stellar radii) are only seen on
stars whose IR data indicates the presence of disks. This suggests that the
longest flares may stretch all the way to the disk. Such long flares tend to be
more tenuous than the other large flares studied. A wide range of loop lengths
are observed, indicating that different types of flares may occur on disked
young stellar objects.Comment: 38 pages, 8 figures, 4 table
The Spatial Distribution of Star Formation in the Solar Neighbourhood: Do all stars form in clusters?
We present a global study of low mass, young stellar object (YSO) surface
densities in nearby (< 500 pc) star forming regions based on a comprehensive
collection of Spitzer Space Telescope surveys. We show that the distribution of
YSO surface densities in the solar neighbourhood is a smooth distribution,
being adequately described by a lognormal function from a few to 10^3 YSOs per
pc^2, with a peak at 22 stars/pc^2 and a dispersion of 0.85. We do not find
evidence for multiple discrete modes of star-formation (e.g. clustered and
distributed). Comparing the observed surface density distribution to previously
reported surface density threshold definitions of clusters, we find that the
fraction of stars in clusters is crucially dependent on the adopted
definitions, ranging from 40 to 90%. However, we find that only a low fraction
(< 26%) of stars are formed in dense environments where their
formation/evolution (along with their circumstellar disks and/or planets) may
be affected by the close proximity of their low-mass neighbours.Comment: 6 pages, 2 figures, MNRAS letters, accepte
Spitzer Imaging of the Nearby Rich Young Cluster, Cep OB3b
We map the full extent of a rich massive young cluster in the Cep OB3b
association with the IRAC and MIPS instruments aboard the {\it Spitzer} Space
Telescope and the ACIS instrument aboard the X-Ray Observatory.
At 700 pc, it is revealed to be the second nearest large ( member),
young ( Myr) cluster known. In contrast to the nearest large cluster, the
Orion Nebula Cluster, Cep OB3b is only lightly obscured and is mostly located
in a large cavity carved out of the surrounding molecular cloud. Our infrared
and X-ray datasets, as well as visible photometry from the literature, are used
to take a census of the young stars in Cep OB3b. We find that the young stars
within the cluster are concentrated in two sub-clusters; an eastern
sub-cluster, near the Cep B molecular clump, and a western sub-cluster, near
the Cep F molecular clump. Using our census of young stars, we examine the
fraction of young stars with infrared excesses indicative of circumstellar
disks. We create a map of the disk fraction throughout the cluster and find
that it is spatially variable. Due to these spatial variations, the two
sub-clusters exhibit substantially different average disk fractions from each
other: and . We discuss whether the discrepant disk
fractions are due to the photodestruction of disks by the high mass members of
the cluster or whether they result from differences in the ages of the
sub-clusters. We conclude that the discrepant disk fractions are most likely
due to differences in the ages.Comment: 48 Pages, 12 figures, 6 table
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
Abundant cyanopolyynes as a probe of infall in the Serpens South cluster-forming region
We have detected bright HC7N J = 21-20 emission toward multiple locations in
the Serpens South cluster-forming region using the K-Band Focal Plane Array at
the Robert C. Byrd Green Bank Telescope. HC7N is seen primarily toward cold
filamentary structures that have yet to form stars, largely avoiding the dense
gas associated with small protostellar groups and the main central cluster of
Serpens South. Where detected, the HC7N abundances are similar to those found
in other nearby star forming regions. Toward some HC7N `clumps', we find
consistent variations in the line centroids relative to NH3 (1,1) emission, as
well as systematic increases in the HC7N non-thermal line widths, which we
argue reveal infall motions onto dense filaments within Serpens South with
minimum mass accretion rates of M ~ 2-5 M_sun Myr^-1. The relative abundance of
NH3 to HC7N suggests that the HC7N is tracing gas that has been at densities n
~ 10^4 cm^-3, for timescales t < 1-2 x 10^5 yr. Since HC7N emission peaks are
rarely co-located with those of either NH3 or continuum, it is likely that
Serpens South is not particularly remarkable in its abundance of HC7N, but
instead the serendipitous mapping of HC7N simultaneously with NH3 has allowed
us to detect HC7N at low abundances in regions where it otherwise may not have
been looked for. This result extends the known star-forming regions containing
significant HC7N emission from typically quiescent regions, like the Taurus
molecular cloud, to more complex, active environments.Comment: 19 pages, 13 figures, accepted to MNRAS. Version with full resolution
figures available at http://www.dunlap.utoronto.ca/~friesen/Friesen_HC7N.pd
Pre-main sequence stars in the Cepheus flare region
We present results of optical spectroscopic and BVR_CI_C photometric
observations of 77 pre-main sequence (PMS) stars in the Cepheus flare region. A
total of 64 of these are newly confirmed PMS stars, originally selected from
various published candidate lists. We estimate effective temperatures and
luminosities for the PMS stars, and comparing the results with pre-main
sequence evolutionary models we estimate stellar masses of 0.2-2.4M_sun and
stellar ages of 0.1-15 Myr. Among the PMS stars, we identify 15 visual binaries
with separations of 2-10 arcsec. From archival IRAS, 2MASS, and Spitzer data,
we construct their spectral energy distributions and classify 5% of the stars
as Class I, 10% as Flat SED, 60% as Class II, and 3% as Class III young stellar
objects (YSOs). We identify 12 CTTS and 2 WTTS as members of NGC 7023, with
mean age of 1.6 Myr. The 13 PMS stars associated with L1228 belong to three
small aggregates: RNO 129, L1228A, and L1228S. The age distribution of the 17
PMS stars associated with L1251 suggests that star formation has propagated
with the expansion of the Cepheus flare shell. We detect sparse aggregates of
6-7 Myr old PMS stars around the dark clouds L1177 and L1219, at a distance of
400 pc. Three T Tauri stars appear to be associated with the Herbig Ae star SV
Cep at a distance of 600 pc. Our results confirm that the molecular complex in
the Cepheus flare region contains clouds of various distances and star forming
histories.Comment: 61 pages, 27 figures, 8 tables; accepted for publication by ApJ
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