89 research outputs found
A Near-Infrared (JHK) Survey of the Vicinity of the HII region NGC 7538: Evidence for a Young Embedded Cluster
We describe the results of two near infrared (K-band) imaging surveys and a
three color (JHK) survey of the vicinity of NGC 7538. The limiting magnitudes
are K ~ 16.5 and K ~ 17.5 mag for the K-band surveys and K ~ 15 mag for the JHK
survey. We identify more than 2000 and 9000 near-infrared (NIR) sources on the
images of the two K-band surveys and 786 NIR sources in the JHK survey. From
color-color diagrams, we derive a reddening law for background stars and
identify 238 stars with NIR excesses. Contour maps indicate a high density peak
coincident with a concentration of stars with NIR excesses. We identify this
peak as a young, embedded cluster and confirm this result with the K-band
luminosity function, color histograms, and color-magnitude diagrams. The center
of the cluster is at RA = 23:13:39.34, DEC = 61:29:18.9. The cluster radius is
3' ~ 2.5 pc for an adopted distance, d ~ 2.8 kpc. For d = 2.8 kpc, and
reddening, E_{J-K} = 0.55 mag, the slope of the logarithmic K-band luminosity
function (KLF) of the cluster, s ~ 0.32 +- 0.03, agrees well with previous
results for L1630 (s = 0.34) and M17 (s = 0.26).Comment: 26 pages with 11 figures. Accepted by Astronomical Journa
The Milky Way Project: A statistical study of massive star formation associated with infrared bubbles
The Milky Way Project citizen science initiative recently increased the
number of known infrared bubbles in the inner Galactic plane by an order of
magnitude compared to previous studies. We present a detailed statistical
analysis of this dataset with the Red MSX Source catalog of massive young
stellar sources to investigate the association of these bubbles with massive
star formation. We particularly address the question of massive triggered star
formation near infrared bubbles. We find a strong positional correlation of
massive young stellar objects (MYSOs) and H II regions with Milky Way Project
bubbles at separations of < 2 bubble radii. As bubble sizes increase, a
statistically significant overdensity of massive young sources emerges in the
region of the bubble rims, possibly indicating the occurrence of triggered star
formation. Based on numbers of bubble-associated RMS sources we find that
67+/-3% of MYSOs and (ultra)compact H II regions appear associated with a
bubble. We estimate that approximately 22+/-2% of massive young stars may have
formed as a result of feedback from expanding H II regions. Using MYSO-bubble
correlations, we serendipitously recovered the location of the recently
discovered massive cluster Mercer 81, suggesting the potential of such analyses
for discovery of heavily extincted distant clusters.Comment: 16 pages, 17 figures. Accepted for publication in ApJ, comments
welcome. Milky Way Project public data release available at
http://www.milkywayproject.org/dat
The X-ray emission from Young Stellar Objects in the rho Ophiuchi cloud core as seen by XMM-Newton
We observed the main core F of the rho Ophiuchi cloud, an active star-forming
region located at ~140 pc, using XMM-Newton with an exposure of 33 ks. We
detect 87 X-ray sources within the 30' diameter field-of-view of the it EPIC
imaging detector array. We cross-correlate the positions of XMM-Newton X-ray
sources with previous X-ray and infrared (IR) catalogs: 25 previously unknown
X-ray sources are found from our observation; 43 X-ray sources are detected by
both XMM-Newton and Chandra; 68 XMM-Newton X-ray sources have 2MASS near-IR
counterparts. We show that XMM-Newton and Chandra have comparable sensitivity
for point source detection when the exposure time is set to ~30 ks for both. We
detect X-ray emission from 7 Class I sources, 26 Class II sources, and 17 Class
III sources. The X-ray detection rate of Class I sources is very high (64 %),
which is consistent with previous Chandra observations in this area. We propose
that 15 X-ray sources are new class III candidates, which doubles the number of
known Class III sources, and helps to complete the census of YSOs in this area.
We also detect X-ray emission from two young bona fide brown dwarfs, GY310 and
GY141, out of three known in the field of view. GY141 appears brighter by
nearly two orders of magnitude than in the Chandra observation. We extract
X-ray light curves and spectra from these YSOs, and find some of them showed
weak X-ray flares. We observed an X-ray flare from the bona fide brown dwarf
GY310. We find as in the previous Chandra observation of this region that Class
I sources tend to have higher temperatures and heavier X-ray absorptions than
Class II and III sources.Comment: 17 pages, 13 figures, 4 tables, accepted by A&
Near-Infrared Imaging Polarimetry of Young Stellar Objects in rho-Ophiuchi
The results of a near-infrared (J H K LP) imaging linear polarimetry survey
of 20 young stellar objects (YSOs) in rho Ophiuchi are presented. The majority
of the sources are unresolved, with K-band polarizations, P_K < 6 per cent.
Several objects are associated with extended reflection nebulae. These objects
have centrosymmetric vector patterns with polarization discs over their cores;
maximum polarizations of P_K > 20 per cent are seen over their envelopes.
Correlations are observed between the degree of core polarization and the
evolutionary status inferred from the spectral energy distribution. K-band core
polarizations >6 per cent are only observed in Class I YSOs. A 3D Monte Carlo
model with oblate grains aligned with a magnetic field is used to investigate
the flux distributions and polarization structures of three of the rho Oph YSOs
with extended nebulae. A rho proportional to r^(-1.5) power law for the density
is applied throughout the envelopes. The large-scale centrosymmetric
polarization structures are due to scattering. However, the polarization
structure in the bright core of the nebula appears to require dichroic
extinction by aligned non-spherical dust grains. The position angle indicates a
toroidal magnetic field in the inner part of the envelope. Since the measured
polarizations attributed to dichroic extinction are usually <10 per cent, the
grains must either be nearly spherical or very weakly aligned. The higher
polarizations observed in the outer parts of the reflection nebulae require
that the dust grains responsible for scattering have maximum grain sizes <=1.05
microns.Comment: 26 pages. Accepted by MNRAS. Available as online early versio
Star Formation in Massive Protoclusters in the Monoceros OB1 Dark Cloud
We present far-infrared, submillimetre, and millimetre observations of bright
IRAS sources and outflows that are associated with massive CS clumps in the
Monoceros OB1 Dark Cloud. Individual star-forming cores are identified within
each clump. We show that combining submillimetre maps, obtained with SCUBA on
the JCMT, with HIRES-processed and modelled IRAS data is a powerful technique
that can be used to place better limits on individual source contributions to
the far-infrared flux in clustered regions. Three previously categorized "Class
I objects" are shown to consist of multiple sources in different evolutionary
stages. In each case, the IRAS point source dominates the flux at 12 and 25
microns. In two cases, the IRAS point source is not evident at submillimetre
wavelengths. The submillimetre sources contribute significantly to the 60 and
100 micron fluxes, dominating the flux in the 100 micron waveband. Using fluxes
derived from our technique, we present the spectral energy distribution and
physical parameters for an intermediate-mass Class 0 object in one of the
regions. Our new CO J=2-1 outflow maps of the three regions studied indicate
complex morphology suggestive of multiple driving sources. We discuss the
possible implications of our results for published correlations between outflow
momentum deposition rates and "source" luminosities, and for using these
derived properties to estimate the ratio of mass ejection rates to mass
accretion rates onto protostars.Comment: 12 pages, 11 gzipped gif figures, LaTex file and MNRAS style files,
accepted by MNRAS, v2: reference typos and author affiliation have been
correcte
Triggering Collapse of the Presolar Dense Cloud Core and Injecting Short-Lived Radioisotopes with a Shock Wave. I. Varied Shock Speeds
The discovery of decay products of a short-lived radioisotope (SLRI) in the
Allende meteorite led to the hypothesis that a supernova shock wave transported
freshly synthesized SLRI to the presolar dense cloud core, triggered its
self-gravitational collapse, and injected the SLRI into the core. Previous
multidimensional numerical calculations of the shock-cloud collision process
showed that this hypothesis is plausible when the shock wave and dense cloud
core are assumed to remain isothermal at ~10 K, but not when compressional
heating to ~1000 K is assumed. Our two-dimensional models (Boss et al. 2008)
with the FLASH2.5 adaptive mesh refinement (AMR) hydrodynamics code have shown
that a 20 km/sec shock front can simultaneously trigger collapse of a 1 solar
mass core and inject shock wave material, provided that cooling by molecular
species such as H2O, CO, and H2 is included. Here we present the results for
similar calculations with shock speeds ranging from 1 km/sec to 100 km/sec. We
find that shock speeds in the range from 5 km/sec to 70 km/sec are able to
trigger the collapse of a 2.2 solar mass cloud while simultaneously injecting
shock wave material: lower speed shocks do not achieve injection, while higher
speed shocks do not trigger sustained collapse. The calculations continue to
support the shock-wave trigger hypothesis for the formation of the solar
system, though the injection efficiencies in the present models are lower than
desired.Comment: 39 pages, 14 figures. in press, Ap
An Infrared Multiplicity Survey of Class I/Flat-Spectrum Systems in the Rho Ophiuchi and Serpens Molecular Clouds
We present new near- and mid-infrared observations of 19 Class
I/flat-spectrum young stellar objects in the nearby Rho Oph (d=125pc) and
Serpens (d=310pc) dark clouds. These observations are part of a larger
systematic infrared multiplicity survey of Class I/flat-spectrum objects in the
nearest dark clouds. We find 7/19 (37% +/- 14%) of the sources surveyed to be
multiple systems over a separation range of ~150 - 1800 AU. This is consistent
with the fraction of multiple systems found among older pre-main-sequence stars
in each of the Taurus, Rho Oph, Chamaeleon, Lupus, and Corona Australis
star-forming regions over a similar separation range. However, solar-type
main-sequence stars in the solar neighborhood have a fraction approximately
one-third that of our Class I/flat- spectrum sample (11% +/- 3%). This may be
attributed to evolutionary effects or environmental differences. An examination
of the spectral energy distributions of the SVS 20 and WL 1 binaries reveals
that the individual components of each source exhibit the same SED
classifications, similar to what one typically finds for binary T Tauri star
(TTS) systems, where the companion of a classical TTS also tends to be of the
same SED type.Comment: 30 pages, 10 figures, 4 tables, accepted for publication in the A
Results from DROXO. III. Observation, source list and X-ray properties of sources detected in the "Deep Rho Ophiuchi XMM-Newton Observation"
X-rays from very young stars are powerful probes to investigate the
mechanisms at work in the very first stages of the star formation and the
origin of X-ray emission in very young stars. We present results from a 500 ks
long observation of the Rho Ophiuchi cloud with a XMM-Newton large program
named DROXO, aiming at studying the X-ray emission of deeply embedded Young
Stellar Objects (YSOs). The data acquired during the DROXO program were reduced
with SAS software, and filtered in time and energy to improve the signal to
noise of detected sources; light curves and spectra were obtained. We detected
111 sources, 61 of them associated with rho Ophiuchi YSOs as identified from
infrared observations with ISOCAM. Specifically, we detected 9 out of 11 Class
I, 31 out of 48 Class II and 15 out 16 Class III objects. Six objects out of 21
classified Class III candidates are also detected. At the same time we suggest
that 15 Class III candidates that remain undetected at log Lx < 28.3 are not
related to the cloud population. The global detection rate is ~64%. We have
achieved a flux sensitivity of ~5 x 10^{-15} erg s^{-1} cm^{-2}. The Lx to
L_bol ratio shows saturation at a value of ~ -3.5 for stars with T_eff <= 5000
K or 0.7 M_sun as observed in the Orion Nebula. The plasma temperatures and the
spectrum absorption show a decline with YSO class, with Class I YSOs being
hotter and more absorbed than Class II and III YSOs. In one star (GY 266) with
infrared counterpart in 2MASS and Spitzer catalogs we have detected a soft
excess in the X-ray spectrum which is best fitted by a cold thermal component
less absorbed than the main thermal component of the plasma. Such a soft
component hints to the presence of plasma heated by shocks due to jets outside
the dense circumstellar material.Comment: Accepted for publication on Astronomy & Astrophysics journa
A new scheme of radiation transfer in H II regions including transient heating of grains
A new scheme of radiation transfer for understanding infrared spectra of H II
regions, has been developed. This scheme considers non-equilibrium processes
(e. g. transient heating of the very small grains, VSG; and the polycyclic
aromatic hydrocarbon, PAH) also, in addition to the equilibrium thermal
emission from normal dust grains (BG). The spherically symmetric interstellar
dust cloud is segmented into a large number of "onion skin" shells in order to
implement the non-equilibrium processes. The scheme attempts to fit the
observed SED originating from the dust component, by exploring the following
parameters : (i) geometrical details of the dust cloud, (ii) PAH size and
abundance, (iii) composition of normal grains (BG), (iv) radial distribution of
all dust (BG, VSG & PAH).
The scheme has been applied to a set of five compact H II regions (IRAS
18116- 1646, 18162-2048, 19442+2427, 22308+5812 & 18434-0242) whose spectra are
available with adequate spectral resolution. The best fit models and inferences
about the parameters for these sources are presented.Comment: 16 pages total including 3 tables and 2 figure
- …