5,344 research outputs found
A VLA Survey For Faint Compact Radio Sources in the Orion Nebula Cluster
We present Karl G. Janksy Very Large Array (VLA) 1.3 cm, 3.6 cm, and 6 cm
continuum maps of compact radio sources in the Orion Nebular Cluster. We
mosaicked 34 square arcminutes at 1.3 cm, 70 square arcminutes at 3.6 cm and
109 square arcminutes at 6 cm, containing 778 near-infrared detected YSOs and
190 HST-identified proplyds (with significant overlap between those
characterizations). We detected radio emission from 175 compact radio sources
in the ONC, including 26 sources that were detected for the first time at these
wavelengths. For each detected source we fit a simple free-free and dust
emission model to characterize the radio emission. We extrapolate the free-free
emission spectrum model for each source to ALMA bands to illustrate how these
measurements could be used to correctly measure protoplanetary disk dust masses
from sub-millimeter flux measurements. Finally, we compare the fluxes measured
in this survey with previously measured fluxes for our targets, as well as four
separate epochs of 1.3 cm data, to search for and quantify variability of our
sources.Comment: 13 pages, 6 figures, 4 tables, ApJ, in pres
Outflow 20--2000 AU from a High-Mass Protostar in W51-IRS2
We present the results of the first high angular resolution observations of
SiO maser emission towards the star forming region W51-IRS2 made with the Very
Large Array (VLA) and Very Long Baseline Array (VLBA). Our images of the water
maser emission in W51-IRS2 reveal two maser complexes bracketing the SiO maser
source. One of these water maser complexes appears to trace a bow shock whose
opening angle is consistent with the opening angle observed in the distribution
of SiO maser emission. A comparison of our water maser image with an image
constructed from data acquired 19 years earlier clearly shows the persistence
and motion of this bow shock. The proper motions correspond to an outflow
velocity of 80 km/s, which is consistent with the data of 19 years ago (that
spanned 2 years). We have discovered a two-armed linear structure in the SiO
maser emission on scales of ~25 AU, and we find a velocity gradient on the
order of 0.1 km/s/AU along the arms. We propose that the SiO maser source
traces the limbs of an accelerating bipolar outflow close to an obscured
protostar. We estimate that the outflow makes an angle of <20 degrees with
respect to the plane of the sky. Our measurement of the acceleration is
consistent with a reported drift in the line-of-sight velocity of the W51 SiO
maser source.Comment: 19 pages, 5 figures (including 3 color). Accepted for publication in
ApJ (April 1, 2001 issue
The Circumstellar Disk Mass Distribution in the Orion Trapezium Cluster
We present the results of a submillimeter interferometric survey of
circumstellar disks in the Trapezium Cluster of Orion. We observed the 880
micron continuum emission from 55 disks using the Submillimeter Array, and
detected 28 disks above 3sigma significance with fluxes between 6-70 mJy and
rms noise between 0.7-5.3 mJy. Dust masses and upper limits are derived from
the submillimeter excess above free-free emission extrapolated from longer
wavelength observations. Above our completeness limit of 0.0084 solar masses,
the disk mass distribution is similar to that of Class II disks in
Taurus-Auriga and rho Ophiuchus but is truncated at 0.04 solar masses. We show
that the disk mass and radius distributions are consistent with the formation
of the Trapezium Cluster disks ~1 Myr ago and subsequent photoevaporation by
the ultraviolet radiation field from Theta-1 Ori C. The fraction of disks which
contain a minimum mass solar nebula within 60 AU radius is estimated to be
11-13% in both Taurus and the Trapezium Cluster, which suggests the potential
for forming Solar Systems is not compromised in this massive star forming
region.Comment: Accepted for publication in ApJL (2009 Feb 3
On Phase Transition of -Type Crystals by Cluster Variation Method
The Cluster Variation Method (CVM) is applied to the Ishibashi model for
ammonium dihydrogen phosphate () of a typical hydrogen
bonded anti-ferroelectric crystal. The staggered and the uniform susceptibility
without hysteresis are calculated at equilibrium. On the other hand, by making
use of the natural iteration method (NIM) for the CVM, hysteresis phenomena of
uniform susceptibility versus temperature observed in experiments is well
explained on the basis of local minimum in Landau type variational free energy.
The polarization curves against the uniform field is also calculated.Comment: 14 pages, 10 figure
Evaluating the impact of flexible alcohol trading hours on violence: an interrupted time series analysis.
BACKGROUND: On November 24(th) 2005, the Government of England and Wales removed regulatory restrictions on the times at which licensed premises could sell alcohol. This study tests availability theory by treating the implementation of Licensing Act (2003) as a natural experiment in alcohol policy. METHODS: An interrupted time series design was employed to estimate the Act's immediate and delayed impact on violence in the City of Manchester (Population 464,200). We collected police recorded rates of violence, robbery, and total crime between the 1st of February 2004 and the 31st of December 2007. Events were aggregated by week, yielding a total of 204 observations (95 pre-, and 109 post-intervention). Secondary analysis examined changes in daily patterns of violence. Pre- and post-intervention events were separated into four three-hour segments 18∶00-20∶59, 21∶00-23.59, 00∶00-02∶59, 03∶00-05∶59. RESULTS: Analysis found no evidence that the Licensing Act (2003) affected the overall volume of violence. However, analyses of night-time violence found a gradual and permanent shift of weekend violence into later parts of the night. The results estimated an initial increase of 27.5% between 03∶00 to 06∶00 (ω = 0.2433, 95% CI = 0.06, 0.42), which increased to 36% by the end of the study period (δ = -0.897, 95% CI = -1.02, -0.77). CONCLUSIONS: This study found no evidence that a national policy increasing the physical availability of alcohol affected the overall volume of violence. There was, however, evidence suggesting that the policy may be associated with changes to patterns of violence in the early morning (3 a.m. to 6 a.m.)
Stellar and Molecular Radii of a Mira Star: First Observations with the Keck Interferometer Grism
Using a new grism at the Keck Interferometer, we obtained spectrally
dispersed (R ~ 230) interferometric measurements of the Mira star R Vir. These
data show that the measured radius of the emission varies substantially from
2.0-2.4 microns. Simple models can reproduce these wavelength-dependent
variations using extended molecular layers, which absorb stellar radiation and
re-emit it at longer wavelengths. Because we observe spectral regions with and
without substantial molecular opacity, we determine the stellar photospheric
radius, uncontaminated by molecular emission. We infer that most of the
molecular opacity arises at approximately twice the radius of the stellar
photosphere.Comment: 12 pages, including 3 figures. Accepted by ApJ
Protoplanetary Disk Masses in the Young NGC 2024 Cluster
We present the results from a Submillimeter Array survey of the 887 micron
continuum emission from the protoplanetary disks around 95 young stars in the
young cluster NGC 2024. Emission was detected from 22 infrared sources, with
flux densities from ~5 to 330 mJy; upper limits (at 3sigma) for the other 73
sources range from 3 to 24 mJy. For standard assumptions, the corresponding
disk masses range from ~0.003 to 0.2Msolar, with upper limits at
0.002--0.01Msolar. The NGC 2024 sample has a slightly more populated tail at
the high end of its disk mass distribution compared to other clusters, but
without more information on the nature of the sample hosts it remains unclear
if this difference is statistically significant or a superficial selection
effect. Unlike in the Orion Trapezium, there is no evidence for a disk mass
dependence on the (projected) separation from the massive star IRS2b in the NGC
2024 cluster. We suggest that this is due to either the cluster youth or a
comparatively weaker photoionizing radiation field.Comment: ApJ, in pres
ALMA Observations of the Largest Proto-Planetary Disk in the Orion Nebula, 114-426: A CO Silhouette
We present ALMA observations of the largest protoplanetary disk in the Orion
Nebula, 114-426. Detectable 345 GHz (856 micron) dust continuum is produced
only in the 350 AU central region of the ~1000 AU diameter silhouette seen
against the bright H-alpha background in HST images. Assuming optically thin
dust emission at 345 GHz, a gas-to-dust ratio of 100, and a grain temperature
of 20 K, the disk gas-mass is estimated to be 3.1 +/- 0.6 Jupiter masses. If
most solids and ices have have been incorporated into large grains, however,
this value is a lower limit. The disk is not detected in dense-gas tracers such
as HCO+ J=4-3, HCN J=4-3, or CS =7-6. These results may indicate that the
114-426 disk is evolved and depleted in some light organic compounds found in
molecular clouds. The CO J=3-2 line is seen in absorption against the bright 50
to 80 K background of the Orion A molecular cloud over the full spatial extent
and a little beyond the dust continuum emission. The CO absorption reaches a
depth of 27 K below the background CO emission at VLSR ~6.7 km/s about 0.52
arcseconds (210 AU) northeast and 12 K below the background CO emission at VLSR
~ 9.7 km/s about 0.34 arcseconds (140 AU) southwest of the suspected location
of the central star, implying that the embedded star has a mass less than 1
Solar mass .Comment: 20 pages, 4 figure
Effect of inoculants of different composition on the quality of rye silages harvested at different stages of maturity
Winter rye (Secale cereale L.), one of the small-grain winter annuals, can be used as a cover crop for protection against soil erosion for absorption of unused soil nitrogen, and for cattle feed by preserving as silage. The experiment was conducted with the objective to evaluate the potential of the blend of homofermentative and hetero- and homofermentative lactic acid bacteria (LAB) as a rye silage additive. Early-cut rye (at boot stage, wilted) and whole-crop rye (at milk and soft dough stages of grain) were ensiled in laboratory mini-silos with (1) a blend of homofermentative LAB strains containing Lactobacillus plantarum (DSM26571), Enterococcus faecium (DSM22502), and Lactococcus lactis (NCIMB30117), (2) a blend of hetero- and homofermentative LAB strains containing Lactobacillus plantarum (DSM26571), Enterococcus faecium (DSM22502), and Lactobacillus buchneri (DSM22501), or (3) a blend of hetero- and homofermentative LAB strains containing Lactobacillus buchneri (DSM22501) and Lactococcus lactis (DSM11037). They were compared to ensiling without additive. After 60 days of fermentation at room temperature, mini-silos were opened, sampled for proximate analysis, forage hygiene, fermentation profile, and subjected to an aerobic stability (AS) test. Although the addition of homofermentative LAB strains was effective in reducing fermentation losses, it impaired the aerobic stability of rye silages. The combination of hetero- and homofermentative LAB strains was effective in reducing the aerobic deterioration of the rye silages by supporting a low pH value and inhibiting the proliferation of yeast and moulds
A Submillimeter Array Survey of Protoplanetary Disks in the Orion Nebula Cluster
We present the full results of our 3-year long Submillimeter Array survey of
protoplanetary disks in the Orion Nebula Cluster. We imaged 23 fields at 880
microns and 2 fields at 1330 microns, covering an area of ~6.5 arcmin^2 and
containing 67 disks. We detected 42 disks with fluxes between 6-135 mJy and at
rms noise levels between 0.6 to 5.3 mJy/beam. Thermal dust emission above any
free-free component was measured in 40 of the 42 detections, and the inferred
disk masses range from 0.003-0.07 Msolar. We find that disks located within 0.3
pc of theta^1 Ori C have a truncated mass distribution, while disks located
beyond 0.3 pc have masses more comparable to those found in low-mass star
forming regions. The disk mass distribution in Orion has a distance dependence,
with a derived relationship max(M_(disk)) = 0.046Msolar(d/0.3pc)^0.33 for the
maximum disk masses. We found evidence of grain growth in disk 197-427, the
only disk detected at both 880 microns and 1330 microns with the SMA. Despite
the rapid erosion of the outer parts of the Orion disks by photoevaporation,
the potential for planet formation remains high in this massive star forming
region, with approximately 18% of the surveyed disks having masses greater than
or equal to 0.01 Msolar within 60 AU.Comment: Accepted for publication in ApJ, 36 pages, 10 figure
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