306 research outputs found
Looking for Distributed Star Formation in L1630: A Near-infrared (J, H, K) Survey
We have carried out a simultaneous, multi-band (J, H, K) survey over an area
of 1320 arcmin^2 in the L1630 region, concentrating on the region away from the
dense molecular cores and with modest visual extinctions (\leq 10 mag).
Previous studies found that star formation in L1630 occurs mainly in four
localized clusters, which in turn are associated with the four most massive
molecular cores (Lada et al. 1991; Lada 1992). The goal of this study is to
look for a distributed population of pre-main-sequence stars in the outlying
areas outside the known star-forming cores. More than 60% of the
pre-main-sequence stars in the active star forming regions of NGC 2024 and NGC
2023 show a near-infrared excess in the color-color diagram. In the outlying
areas of L1630, excluding the known star forming regions, we found that among
510 infrared sources with the near-infrared colors ((J-H) and (H-K)) determined
and photometric uncertainty at K better than 0.10 mag, the fraction of the
sources with a near-infrared excess is 3%--8%; the surface density of the
sources with a near-infrared excess is less than half of that found in the
distributed population in L1641, and 1/20 of that in the young cluster NGC
2023. This extremely low fraction and low surface density of sources with a
near-infrared excess strongly indicates that recent star formation activity has
been very low in the outlying region of L1630. The sources without a
near-infrared excess could be either background/foreground field stars, or
associated with the cloud, but formed a long time ago (more than 2 Myrs). Our
results are consistent with McKee's model of photoionization-regulated star
formation.Comment: 30 pages, 10 figures To appear in ApJ Oct 1997, Vol 48
Ammonia Imaging of the Disks in the NGC 1333 IRAS 4A Protobinary System
The NGC 1333 IRAS 4A protobinary was observed in the ammonia (2, 2) and (3,
3) lines and in the 1.3 cm continuum with a high resolution (about 1.0 arcsec).
The ammonia maps show two compact sources, one for each protostar, and they are
probably protostellar accretion disks. The disk associated with IRAS 4A2 is
seen nearly edge-on and shows an indication of rotation. The A2 disk is
brighter in the ammonia lines but dimmer in the dust continuum than its sibling
disk, with the ammonia-to-dust flux ratios different by about an order of
magnitude. This difference suggests that the twin disks have surprisingly
dissimilar characters, one gas-rich and the other dusty. The A2 disk may be
unusually active or hot, as indicated by its association with water vapor
masers. The existence of two very dissimilar disks in a binary system suggests
that the formation process of multiple systems has a controlling agent lacking
in the isolated star formation process and that stars belonging to a multiple
system do not necessarily evolve in phase with each other
V1647 Orionis: Reinvigorated Accretion and the Re-Appearance of McNeil's Nebula
In late 2003, the young eruptive variable star V1647 Orionis optically
brightened by over 5 magnitudes, stayed bright for around 26 months, and then
decline to its pre-outburst level. In August 2008 the star was reported to have
unexpectedly brightened yet again and we herein present the first detailed
observations of this new outburst. Photometrically, the star is now as bright
as it ever was following the 2003 eruption. Spectroscopically, a pronounced P
Cygni profile is again seen in Halpha with an absorption trough extending to
-700 km/s. In the near-infrared, the spectrum now possesses very weak CO
overtone bandhead absorption in contrast to the strong bandhead emission seen
soon after the 2003 event. Water vapor absorption is also much stronger than
previously seen. We discuss the current outburst below and relate it to the
earlier event.Comment: 6 pages, 3 figure
Interferometric Mapping of Magnetic fields: NGC2071IR
We present polarization maps of NGC2071IR from thermal dust emission at 1.3
mm and from CO J= line emission. The observations were obtained using
the Berkeley-Illinois-Maryland Association array in the period 2002-2004. We
detected dust and line polarized emission from NGC2071IR that we used to
constrain the morphology of the magnetic field. From CO J= polarized
emission we found evidence for a magnetic field in the powerful bipolar outflow
present in this region. We calculated a visual extinction mag from our dust observations. This result, when compared with early
single dish work, seems to show that dust grains emit polarized radiation
efficiently at higher densities than previously thought. Mechanical alignment
by the outflow is proposed to explain the polarization pattern observed in
NGC2071IR, which is consistent with the observed flattening in this source.Comment: 17 pages, 4 Figures, Accepted for publication in Ap
A Catalog of Young Stellar Groups and Clusters Within 1kpc of the Sun
We present a catalog of near-infrared surveys of young (<= a few 10^6 yr)
stellar groups and clusters within 1 kpc from the Sun, based on an extensive
search of the literature from the past ten years. We find 143 surveys from 69
published articles, covering 73 different regions. The number distribution of
stars in a region has a median of 28 and a mean of 100. About 80% of the stars
are in clusters with at least 100 members. By a rough classification of the
groups and clusters based on the number of their associated stars, we show that
most of the stars form in large clusters. The spatial distribution of cataloged
regions in the Galactic plane shows a relative lack of observed stellar groups
and clusters in the range 270 deg < l < 60 deg of Galactic longitude,
reflecting our location between the Local and Sagittarius arms. This
compilation is intended as a useful resource for future studies of nearby young
regions of multiple star formation.Comment: 16 pages, 2 tables, 2 figures. To appear in Astronomical Journa
The 3-Dimensional Structure of HH 32 from GMOS IFU Spetroscopy
We present new high resolution spectroscopic observations of the Herbig-Haro
object HH 32 from System Verification observations made with the GMOS IFU at
Gemini North Observatory. The 3D spectral data covers a 8''.7 x 5''.85 spatial
field and 4820 - 7040 Angstrom spectral region centered on the HH~32 A knot
complex. We show the position-dependent line profiles and radial velocity
channel maps of the Halpha line, as well as line ratio velocity channel maps of
[OIII]5007/Halpha, [OI]6300/Halpha, [NII]6583/Halpha, [SII](6716+6730)/Halpha
and [SII]6716/6730. We find that the line emission and the line ratios vary
significantly on spatial scales of ~1'' and over velocities of ~50 km/s. A
``3/2-D'' bow shock model is qualitatively successful at reproducing the
general features of the radial velocity channel maps, but it does not show the
same complexity as the data and it fails to reproduce the line ratios in our
high spatial resolution maps. The observations of HH 32 A show two or three
superimposed bow shocks with separations of ~3'', which we interpret as
evidence of a line of sight superposition of two or three working surfaces
located along the redshifted body of the HH 32 outflow.Comment: Accepted for Publication in the Astronomical Journal (January 2004
A FLAMINGOS Deep Near Infrared Imaging Survey of the Rosette Complex I: Identification and Distribution of the Embedded Population
We present the results of a deep near-infrared imaging survey of the Rosette
Complex. We studied the distribution of young embedded sources using a
variation of the Nearest Neighbor Method applied to a carefully selected sample
of near-infrared excess (NIRX) stars which trace the latest episode of star
formation in the complex. Our analysis confirmed the existence of seven
clusters previously detected in the molecular cloud, and identified four more
clusters across the complex. We determined that 60% of the young stars in the
complex and 86% of the stars within the molecular cloud are contained in
clusters, implying that the majority of stars in the Rosette formed in embedded
clusters. We compare the sizes, infrared excess fractions and average
extinction towards individual clusters to investigate their early evolution and
expansion. We found that the average infrared excess fraction of clusters
increases as a function of distance from NGC 2244, implying a temporal sequence
of star formation across the complex. This sequence appears to be primordial,
possibly resulting from the formation and evolution of the molecular cloud and
not from the interaction with the HII region.Comment: Accepted by Astrophysical Journa
Variability of the NGC 1333 IRAS 4A Outflow: Molecular Hydrogen and Silicon Monoxide Images
The NGC 1333 region was observed in the H2 1-0 S(1) line. The H2 images cover
a 5' x 7' region around IRAS 4. Numerous H2 emission features were detected.
The northeast-southwest bipolar outflow driven by IRAS 4A was studied by
combining the H2 images with SiO maps published previously. The SiO-H2 outflows
are continuous on the southwestern side but show a gap on the northeastern
side. The southwestern outflow lobe curves smoothly, and the position angle
increases with the distance from the driving source. The base and the outer tip
of the northeastern outflow lobe are located at positions opposite to the
corresponding parts of the southwestern lobe. This point-symmetry suggests that
the outflow axis may be drifting or precessing clockwise in the plane of the
sky and that the cause of the axis drift may be intrinsic to the outflow
engine. The axis drift model is supported by the asymmetric lateral intensity
profile of the SiO outflow. The axis drift rate is about 0.011 deg yr-1. The
middle part of the northeastern outflow does not exactly follow the point
symmetry because of the superposition of two different kinds of directional
variability: the axis drift of the driving source and the deflection by a dense
core. The axis drift model provides a good explanation for the large deflection
angle of the northeastern outflow. Other H2 emission features around the IRAS 4
region are discussed briefly. Some of them are newly found outflows, and some
are associated with outflows already known before
Circumstellar Disks in the IC 348 Cluster
We report the results of the first sensitive L-band (3.4 micron) imaging
survey of the young IC 348 cluster in Perseus. In conjunction with previously
acquired JHK (1.25, 1.65, 2.2 micron) observations, we use L-band data to
obtain a census of the circumstellar disk population to m_K=m_L<=12.0 in the
central 110 square arcmin region of the cluster. An analysis of the JHKL colors
of 107 sources indicates that 65% +/- 8% of the cluster membership possesses
(inner) disks. This fraction is lower than those (86% +/- 8% and 80% +/- 7%)
obtained from similar JHKL surveys of the younger NGC 2024 and Trapezium
clusters, suggesting that the disk fraction in clusters decreases with cluster
age. Sources with circumstellar disks in IC 348 have a median age of 0.9 Myr,
while the diskless sources have a median age of 1.4 Myr, for a cluster distance
of 320 pc. Although the difference in the median ages between the two
populations is only marginally significant, our results suggest that over a
timescale of 2 - 3 Myr, more than a third of the disks in the IC 348 cluster
disappear. Moreover, we find that at a very high confidence level, the disk
fraction is a function of spectral type. All stars earlier than G appear
diskless, while stars with spectral types G and later have a disk fraction
ranging between 50% - 67%, with the latest type stars having the higher disk
fraction. This suggests that the disks around stars with spectral types G and
earlier have evolved more rapidly than those with later spectral types. The
L-band disk fraction for sources with similar ages in both IC 348 and Taurus is
the same, within the errors, suggesting that, at least in clusters with no O
stars, the disk lifetime is independent of environment.Comment: 25 pages, 4 figures. Paper to appear in April A
The COMPLETE Nature of the Warm Dust Ring in Perseus
The Perseus molecular cloud complex is a ~30pc long chain of molecular clouds
most well-known for the two star-forming clusters NGC1333 and IC348 and the
well-studied outflow source in B5. However, when studied at mid- to
far-infrared wavelengths the region is dominated by a ~10pc diameter shell of
warm dust, likely generated by an HII region caused by the early B-star
HD278942. Using a revised calibration technique the COMPLETE team has produced
high-sensitivity temperature and column-density maps of the Perseus region from
IRAS Sky Survey Atlas (ISSA) 60 and 100um data. In this paper, we combine the
ISSA based dust-emission maps with other observations collected as part of the
COMPLETE Survey, along with archival H-alpha and MSX observations. Molecular
line observations from FCRAO and extinction maps constructed by applying the
NICER method to the 2MASS catalog provide independent estimates of the ``true''
column-density of the shell. H-alpha emission in the region of the shell
confirms that it is most likely an HII region located behind the cloud complex,
and 8um data from MSX indicates that the shell may be interacting with the
cloud. Finally, the two polarisation components previously seen towards
background stars in the region can be explained by the association of the
stronger component with the shell. If confirmed, this would be the first
observation of a parsec-scale swept-up magnetic field.Comment: Accepted by ApJ. Figures have been compressed - full resolution
version available at http://cfa-www.harvard.edu/COMPLETE/results.htm
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