2,060 research outputs found
The Life and Death of Dense Molecular Clumps in the Large Magellanic Cloud
We report the results of a high spatial (parsec) resolution HCO+ (J = 1-0)
and HCN (J = 1-0) emission survey toward the giant molecular clouds of the star
formation regions N105, N113, N159, and N44 in the Large Magellanic Cloud. The
HCO+ and HCN observations at 89.2 and 88.6 GHz, respectively, were conducted in
the compact configuration of the Australia Telescope Compact Array. The
emission is imaged into individual clumps with masses between 10^2 and 10^4
solar masses and radii of <1 pc to ~2 pc. Many of the clumps are coincident
with indicators of current massive star formation, indicating that many of the
clumps are associated with deeply-embedded forming stars and star clusters. We
find that massive YSO-bearing clumps tend to be larger (>1 pc), more massive (M
> 10^3 solar masses), and have higher surface densities (~1 g cm^-2), while
clumps without signs of star formation are smaller (<1 pc), less massive (M <
10^3 solar masses), and have lower surface densities (~0.1 g cm^-2). The dearth
of massive (M >10^3 solar masses) clumps not bearing massive YSOs suggests the
onset of star formation occurs rapidly once the clump has attained physical
properties favorable to massive star formation. Using a large sample of LMC
massive YSO mid-IR spectra, we estimate that ~2/3 of the massive YSOs for which
there are Spitzer mid-IR spectra are no longer located in molecular clumps; we
estimate that these young stars/clusters have destroyed their natal clumps on a
time scale of at least 3 x 10^{5}$ yrs.Comment: Accepted to ApJ 3-19-201
Supernova Remnants and Star Formation in the Large Magellanic Cloud
It has often been suggested that supernova remnants (SNRs) can trigger star
formation. To investigate the relationship between SNRs and star formation, we
have examined the known sample of 45 SNRs in the Large Magellanic Cloud to
search for associated young stellar objects (YSOs) and molecular clouds. We
find seven SNRs associated with both YSOs and molecular clouds, three SNRs
associated with YSOs but not molecular clouds, and eight SNRs near molecular
clouds but not associated with YSOs. Among the 10 SNRs associated with YSOs,
the association between the YSOs and SNRs can be either rejected or cannot be
convincingly established for eight cases. Only two SNRs have YSOs closely
aligned along their rims; however, the time elapsed since the SNR began to
interact with the YSOs' natal clouds is much shorter than the contraction
timescales of the YSOs, and thus we do not see any evidence of SNR-triggered
star formation in the LMC. The 15 SNRs that are near molecular clouds may
trigger star formation in the future when the SNR shocks have slowed down to
<45 km/s. We discuss how SNRs can alter the physical properties and abundances
of YSOs.Comment: 24 pages, 5 figures, 1 table, Accepted for publication in the August
2010 edition of the Astronomical Journa
Dynamics in Star-forming Cores (DiSCo): Project Overview and the First Look toward the B1 and NGC 1333 Regions in Perseus
The internal velocity structure within dense gaseous cores plays a crucial
role in providing the initial conditions for star formation in molecular
clouds. However, the kinematic properties of dense gas at core scales (~0.01 -
0.1 pc) has not been extensively characterized because of instrument
limitations until the unique capabilities of GBT-Argus became available. The
ongoing GBT-Argus Large Program, Dynamics in Star-forming Cores (DiSCo) thus
aims to investigate the origin and distribution of angular momenta of
star-forming cores. DiSCo will survey all starless cores and Class 0
protostellar cores in the Perseus molecular complex down to ~0.01 pc scales
with < 0.05 km/s velocity resolution using the dense gas tracer NH.
Here, we present the first datasets from DiSCo toward the B1 and NGC 1333
regions in Perseus. Our results suggest that a dense core's internal velocity
structure has little correlation with other core-scale properties, indicating
these gas motions may be originated externally from cloud-scale turbulence.
These first datasets also reaffirm the ability of GBT-Argus for studying dense
core velocity structure and provided an empirical basis for future studies that
address the angular momentum problem with a statistically broad sample.Comment: 17 pages, 12 figures, accepted by MNRA
Complex Structure in Class 0 Protostellar Envelopes
We use archived IRAC images from the Spitzer Space Telescope to show that
many Class 0 protostars exhibit complex, irregular, and non-axisymmetric
structure within their dusty envelopes. Our 8 m extinction maps probe some
of the densest regions in these protostellar envelopes. Many of the systems are
observed to have highly irregular and non-axisymmetric morphologies on scales
>= 1000 AU, with a quarter of the sample exhibiting filamentary or flattened
dense structures. Complex envelope structure is observed in regions spatially
distinct from outflow cavities, and the densest structures often show no
systematic alignment perpendicular to the cavities. These results indicate that
mass ejection is not responsible for much of the irregular morphologies we
detect; rather, we suggest that the observed envelope complexity is mostly the
result of collapse from protostellar cores with initially non-equilibrium
structures. The striking non-axisymmetry in many envelopes could provide
favorable conditions for the formation of binary systems. We also note that
protostars in the sample appear to be formed preferentially near the edges of
clouds or bends in filaments, suggesting formation by gravitational focusing.Comment: 20 pages, 11 figures, 3 Tables, accepted to ApJ. Paper with full
resolution figures available at
http://www.astro.lsa.umich.edu/~jjtobin/Envelopes.pd
Complex Structure in Class 0 Protostellar Envelopes II: Kinematic Structure from Single-Dish and Interferometric Molecular Line Mapping
We present a study of dense molecular gas kinematics in seventeen nearby
protostellar systems using single-dish and interferometric molecular line
observations. The non-axisymmetric envelopes around a sample of Class 0/I
protostars were mapped in the N2H+ (J=1-0) tracer with the IRAM 30m, CARMA and
PdBI as well as NH3 (1,1) with the VLA. The molecular line emission is used to
construct line-center velocity and linewidth maps for all sources to examine
the kinematic structure in the envelopes on spatial scales from 0.1 pc to ~1000
AU. The direction of the large-scale velocity gradients from single-dish
mapping is within 45 degrees of normal to the outflow axis in more than half
the sample. Furthermore, the velocity gradients are often quite substantial,
the average being ~2.3 km\s\pc. The interferometric data often reveal
small-scale velocity structure, departing from the more gradual large-scale
velocity gradients. In some cases, this likely indicates accelerating infall
and/or rotational spin-up in the inner envelope; the median velocity gradient
from the interferometric data is ~10.7 km/s/pc. In two systems, we detect
high-velocity HCO+ (J=1-0) emission inside the highest-velocity \nthp\
emission. This enables us to study the infall and rotation close to the disk
and estimate the central object masses. The velocity fields observed on large
and small-scales are more complex than would be expected from rotation alone,
suggesting that complex envelope structure enables other dynamical processes
(i.e. infall) to affect the velocity field.Comment: 85 Pages, 31 Figures, 11 Tables, Accepted to ApJ
Characterizing the Cool KOIs III. KOI-961: A Small Star with Large Proper Motion and Three Small Planets
We present the characterization of the star KOI 961, an M dwarf with transit
signals indicative of three short-period exoplanets, originally discovered by
the Kepler Mission. We proceed by comparing KOI 961 to Barnard's Star, a
nearby, well-characterized mid-M dwarf. By comparing colors, optical and
near-infrared spectra, we find remarkable agreement between the two, implying
similar effective temperatures and metallicities. Both are metal-poor compared
to the Solar neighborhood, have low projected rotational velocity, high
absolute radial velocity, large proper motion and no quiescent H-alpha
emission--all of which is consistent with being old M dwarfs. We combine
empirical measurements of Barnard's Star and expectations from evolutionary
isochrones to estimate KOI 961's mass (0.13 +/- 0.05 Msun), radius (0.17 +/-
0.04 Rsun) and luminosity (2.40 x 10^(-3.0 +/- 0.3) Lsun). We calculate KOI
961's distance (38.7 +/- 6.3 pc) and space motions, which, like Barnard's Star,
are consistent with a high scale-height population in the Milky Way. We perform
an independent multi-transit fit to the public Kepler light curve and
significantly revise the transit parameters for the three planets. We calculate
the false-positive probability for each planet-candidate, and find a less than
1% chance that any one of the transiting signals is due to a background or
hierarchical eclipsing binary, validating the planetary nature of the transits.
The best-fitting radii for all three planets are less than 1 Rearth, with KOI
961.03 being Mars-sized (Rp = 0.57 +/- 0.18 Rearth), and they represent some of
the smallest exoplanets detected to date.Comment: Accepted to Ap
The Magellanic Mopra Assessment (MAGMA). I. The Molecular Cloud Population of the Large Magellanic Cloud
We present the properties of an extensive sample of molecular clouds in the
Large Magellanic Cloud (LMC) mapped at 11 pc resolution in the CO(1-0) line. We
identify clouds as regions of connected CO emission, and find that the
distributions of cloud sizes, fluxes and masses are sensitive to the choice of
decomposition parameters. In all cases, however, the luminosity function of CO
clouds is steeper than dN/dL \propto L^{-2}, suggesting that a substantial
fraction of mass is in low-mass clouds. A correlation between size and
linewidth, while apparent for the largest emission structures, breaks down when
those structures are decomposed into smaller structures. We argue that the
correlation between virial mass and CO luminosity is the result of comparing
two covariant quantities, with the correlation appearing tighter on larger
scales where a size-linewidth relation holds. The virial parameter (the ratio
of a cloud's kinetic to self-gravitational energy) shows a wide range of values
and exhibits no clear trends with the CO luminosity or the likelihood of
hosting young stellar object (YSO) candidates, casting further doubt on the
assumption of virialization for molecular clouds in the LMC. Higher CO
luminosity increases the likelihood of a cloud harboring a YSO candidate, and
more luminous YSOs are more likely to be coincident with detectable CO
emission, confirming the close link between giant molecular clouds and massive
star formation.Comment: Accepted by ApJS; 22 pages in emulateapj format; full-resolution
version and data tables available at http://mmwave.astro.illinois.edu/magma
A Triple Protostar System Formed via Fragmentation of a Gravitationally Unstable Disk
Binary and multiple star systems are a frequent outcome of the star formation
process, and as a result, almost half of all sun-like stars have at least one
companion star. Theoretical studies indicate that there are two main pathways
that can operate concurrently to form binary/multiple star systems: large scale
fragmentation of turbulent gas cores and filaments or smaller scale
fragmentation of a massive protostellar disk due to gravitational instability.
Observational evidence for turbulent fragmentation on scales of 1000~AU has
recently emerged. Previous evidence for disk fragmentation was limited to
inferences based on the separations of more-evolved pre-main sequence and
protostellar multiple systems. The triple protostar system L1448 IRS3B is an
ideal candidate to search for evidence of disk fragmentation. L1448 IRS3B is in
an early phase of the star formation process, likely less than 150,000 years in
age, and all protostars in the system are separated by 200~AU. Here we
report observations of dust and molecular gas emission that reveal a disk with
spiral structure surrounding the three protostars. Two protostars near the
center of the disk are separated by 61 AU, and a tertiary protostar is
coincident with a spiral arm in the outer disk at a 183 AU separation. The
inferred mass of the central pair of protostellar objects is 1 M,
while the disk surrounding the three protostars has a total mass of 0.30
M_{\sun}. The tertiary protostar itself has a minimum mass of 0.085
M. We demonstrate that the disk around L1448 IRS3B appears susceptible
to disk fragmentation at radii between 150~AU and 320~AU, overlapping with the
location of the tertiary protostar. This is consistent with models for a
protostellar disk that has recently undergone gravitational instability,
spawning one or two companion stars.Comment: Published in Nature on Oct. 27th. 24 pages, 8 figure
HAWC+ Far-infrared Observations of the Magnetic Field Geometry in M51 and NGC 891
Abstract: Stratospheric Observatory for Infrared Astronomy High-resolution Airborne Wideband Camera Plus polarimetry at 154 μm is reported for the face-on galaxy M51 and the edge-on galaxy NGC 891. For M51, the polarization vectors generally follow the spiral pattern defined by the molecular gas distribution, the far-infrared (FIR) intensity contours, and other tracers of star formation. The fractional polarization is much lower in the FIR-bright central regions than in the outer regions, and we rule out loss of grain alignment and variations in magnetic field strength as causes. When compared with existing synchrotron observations, which sample different regions with different weighting, we find the net position angles are strongly correlated, the fractional polarizations are moderately correlated, but the polarized intensities are uncorrelated. We argue that the low fractional polarization in the central regions must be due to significant numbers of highly turbulent segments across the beam and along lines of sight in the beam in the central 3 kpc of M51. For NGC 891, the FIR polarization vectors within an intensity contour of 1500 are oriented very close to the plane of the galaxy. The FIR polarimetry is probably sampling the magnetic field geometry in NGC 891 much deeper into the disk than is possible with NIR polarimetry and radio synchrotron measurements. In some locations in NGC 891, the FIR polarization is very low, suggesting we are preferentially viewing the magnetic field mostly along the line of sight, down the length of embedded spiral arms. There is tentative evidence for a vertical field in the polarized emission off the plane of the disk
Search for New Physics with Jets and Missing Transverse Momentum in pp collisions at sqrt(s) = 7 TeV
A search for new physics is presented based on an event signature of at least
three jets accompanied by large missing transverse momentum, using a data
sample corresponding to an integrated luminosity of 36 inverse picobarns
collected in proton--proton collisions at sqrt(s)=7 TeV with the CMS detector
at the LHC. No excess of events is observed above the expected standard model
backgrounds, which are all estimated from the data. Exclusion limits are
presented for the constrained minimal supersymmetric extension of the standard
model. Cross section limits are also presented using simplified models with new
particles decaying to an undetected particle and one or two jets
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