4,279 research outputs found
IRAS 16293-2422: Evidence for Infall onto a Counter-Rotating Protostellar Accretion Disk
We report high spatial resolution VLA observations of the low-mass
star-forming region IRAS 16293-2422 using four molecular probes: ethyl cyanide
(CHCHCN), methyl formate (CHOCHO), formic acid (HCOOH), and the
ground vibrational state of silicon monoxide (SiO). Ethyl cyanide emiss ion has
a spatial scale of and encompasses binary cores A and B as
determined by continuum emission peaks. Surrounded by formic acid emission,
methyl formate emission has a spatial scale of and is confined to core
B. SiO emission shows two velocity components with spatial scales less than
2 that map northeast of the A and B symmetry axis. The redshifted
SiO is northwest of blueshifted SiO along a position angle of
which is approximately parallel to the A and B symmetry axis. We
interpret the spatial position offset in red and blueshifted SiO emission as
due to rotation of a protostellar accretion disk and we derive 1.4
M interior to the SiO emission. In the same vicinity, Mundy et al.
(1986) also concluded rotation of a nearly edge-on disk from OVRO observations
of much stronger and ubiquitous CO emission but the direction of
rotation is opposite to the SiO emission findings. Taken together, SiO and
CO data suggest evidence for a counter-rotating disk. Moreover, archival
BIMA array CO data show an inverse P Cygni profile with the strongest
absorption in close proximity to the SiO emission, indicating unambiguous
material infall toward the counter-rotating protostellar disk at a new source
location within the IRAS 16293-2422 complex. The details of these observations
and our interpretations are discussed.Comment: 18 pages, 5 figures, accepted for publication in the Astrophysical
Journa
Control valve: Hot gas fast response
The design of a warm gas control valve is reviewed outlining the problems posed by the requirement for extremely fast response combined with a severe environment and a hot, dirty, and corrosive operating fluid
Non-Thermal Continuum toward SGRB2(N-LMH)
An analysis of continuum antenna temperatures observed in the Green Bank
Telescope (GBT) spectrometer bandpasses is presented for observations toward
SgrB2(N-LMH). Since 2004, we have identified four new prebiotic molecules
toward this source by means of rotational transitions between low energy
levels; concurrently, we have observed significant continuum in the GBT
spectrometer bandpasses centered at 85 different frequencies in the range of 1
to 48 GHz. The continuum heavily influences the molecular spectral features
since we have observed far more absorption lines than emission lines for each
of these new molecular species. Hence, it is important to understand the
nature, distribution, and intensity of the underlying continuum in the GBT
bandpasses for the purposes of radiative transfer, i.e. the means by which
reliable molecular abundances are estimated. We find that the GBT spectrometer
bandpass continuum is consistent with optically-thin, non thermal (synchrotron)
emission with a flux density spectral index of -0.7 and a Gaussian source size
of ~143" at 1 GHz that decreases with increasing frequency as nu^(-0.52). Some
support for this model is provided by high frequency Very Large Array (VLA)
observations of SgrB2.Comment: Accepted for Publication in the Astrophysical Journal Letter
Lateral Shock of the R Aquarii Jet
The R Aqr jet was observed with the VLA B-configuration at two epochs separated by approximately 13.2 yr. Comparison of the resulting 6 cm continuum images show that the radio jet has undergone a lateral counterclockwise rotation of approximately 6 deg-12 deg on the plane of the sky. The model of jet parcels on independent trajectories is difficult to reconcile with these observations and leads us to consider a path-oriented jet (i.e., younger parcels follow the same path as older parcels). Comparison of the most recent radio image with a nearly contemporaneous HST/FOC ultraviolet image at approximately 2330 Angstroms suggests that the ultraviolet emission lies along the leading side of the rotating radio jet. In conjunction with a proper motion analysis of the jet material that yields empirical space-velocity and resulting acceleration-magnitude relationships as a function of distance from the central source, we evaluate the observational results in terms of a schematic model in which the jet emission consists of plane-parallel isothermal shocks along the leading edge of rotation. In such a radiating shock, the ultraviolet-emitting region is consistent with the adiabatic region in the form of a high-temperature, low-density sheath that surrounds the cooled postshock radio-emitting region. Within the context of the schematic model, we obtain the temperatures, densities, and pressures within the preshock, adiabatic, and postshock regions as a function of distance from the central source; the physical parameters so derived compare favorably to previously published estimates. We obtain a total jet mass of 3.1 x 10(exp -5) solar mass and an age of approximately 115 yr. We evaluate the model in the context of its density-boundary condition, its applicability to an episodic or quasi-continuous jet, and angular momentum considerations
Screening for autism in preterm children : diagnostic utility of the Social Communication Questionnaire
Objective Preterm survivors are at high risk for autism
spectrum disorders (ASD). The diagnostic utility of the
Social Communication Questionnaire (SCQ) in screening
for ASD was assessed in extremely preterm children at
11 years of age.
Design All babies born at <26 weeks gestation
in UK and Ireland from March through December
1995 were recruited to the EPICure Study. Of 307
survivors, 219 (71%) were assessed at 11 years.
Parents of 173 children completed the SCQ to screen
for autistic features and the Development and Well
Being Assessment (DAWBA) psychiatric interview.
A consensus diagnosis of ASD was assigned by two
child psychiatrists following review of the DAWBA
parental interview and corresponding DAWBA teacher
questionnaire.
Setting Community-based follow-up.
Results Using the established SCQ cut-off (scores
≥15), 28 (16%) extremely preterm children screened
positive for ASD. Eleven (6%) were assigned a
diagnosis of ASD. Using this cut-off, the SCQ had 82%
sensitivity and 88% specifi city for identifying ASD in this
population. Using a receiver operating characteristic
curve, SCQ scores ≥14 had optimal diagnostic utility
(area under curve: 0.94; sensitivity: 91%; specifi city:
86%). Positive predictive value was relatively low
(31%) resulting in numerous over-referrals. However,
children with false positive screens had signifi cantly
worse neuro-developmental, cognitive and behavioural
outcomes than those with true negative screens.
Conclusion The SCQ has good diagnostic utility for
identifying ASD in extremely preterm children and
is a useful screening tool in this population. Children
with false positive screens represent a high-risk group
in whom further diagnostic assessment would be
benefi cial
Lateral Shock of the R Aquarii Jet
The R Aqr jet was observed with the VLA B-configuration at two epochs separated by ~13.2 yr. Comparison of the resulting 6 cm continuum images show that the radio jet has undergone a lateral counterclockwise rotation of ~6-12 on the plane of the sky. The model of jet parcels on independent trajectories is difficult to reconcile with these observations and leads us to consider a path-oriented jet (i.e., younger parcels follow the same path as older parcels). Comparison of the most recent radio image with a nearly contemporaneous HST /FOC ultraviolet image at ~2330 A suggests that the ultraviolet emission lies along the leading side of the rotating radio jet. In conjunction with a proper motion analysis of the jet material that yields empirical space-velocity and resulting acceleration-magnitude relationships as a function of distance from the central source, we evaluate the observational results in terms of a schematic model in which the jet emission consists of plane-parallel isothermal shocks along the leading edge of rotation. In such a radiating shock, the ultraviolet-emitting region is consistent with the adiabatic region in the form of a high-temperature, low-density sheath that surrounds the cooled postshock radio-emitting region. Within the context of the schematic model, we obtain the temperatures, densities, and pressures within the preshock, adiabatic, and postshock regions as a function of distance from the central source; the physical parameters so derived compare favorably to previously published estimates. We obtain a total jet mass of 3.1x10^-5 M and an age of ~115 yr. We evaluate the model in the context of its density-boundary condition, its applicability to an episodic or quasi-continuous jet, and angular momentum considerations
High Spatial Resolution VLA Observations of the R Aquarii Jet
High spatial resolution observations (~1 ) of the jet feature associated with the symbiotic variable R Aquarii were obtained with the Very Large Array (VLA). The peak radio intensity of the jet lies at a 29~3 P.A. with respect to the radio emission from R Aquarii itself. If the line defined by the jet and star is extended ~196 , it intercepts a previously reported and heretofore unresolved radio source. In our high spatial resolution 6 cm map, this feature is resolved into a compact double radio source, whose peak intensity lies on an axis defined by the jet and star. The possible association of this feature with R Aquarii or with the extended filamentary nebula that surrounds the system cannot be determined from these radio morphology studies alone. If this feature is associated with R Aquarii, it may represent ejecta from the system which occurred previously. Moreover, a new unresolved radio feature has been detected - 2\u27\u27:7 from the central star at - 45° P.A. It may represent material recently ejected from the system, perhaps as the object precesses. Weak evidence for a counter-jet is suggested from radio contours centered on R Aquarii
Ultraviolet Variability and Mass Expulsion from R Aquarii
Ultraviolet spectra obtained in the 1200-3200 A range over the course of 4 yr with the International Ultraviolet Explorer (IUE) indicate that the extended nebular features which resemble a jet in the peculiar variable R Aquarii (M7e+pec) have increased in excitation in 1985. This is indicated by the appearance of He II Al640 and N v AA1238, 1240 in the jet features and is consistent with the detection of soft X-rays, as found with EXOSAT in 1985. We have analyzed the emission properties of the compact H II region that surrounds the unresolved binary, and those of the extended nebular jet, from the low-resolution IUE spectra which we obtained of these regions. In particular, the UV line intensities observed in the jet appear variable on a time scale of ~1.5 yr. A new accretion disk model is proposed that explains the kinematic and ionization properties of discrete components which comprise the jet emission nebulosity, the appearance of the jet in the 1980s, and morphology that uniquely characterizes the R Aquarii system at radio, optical, UV, and X-ray wavelengths
Evidence for Extended Radio Emission Surrounding RX Puppis
We report evidence for ~1 extended structure in 6 cm continuum emission emanating from the symbiotic star system RX Puppis. We did not detect hourly continuum flux changes as suggested in previous radio experiments by others. Our observations indicate that the predominant nature of the radio emission is thermal and consistent with an optically thick stellar wind emanating from the symbiotic star system. Our results are discussed with regard to other similar stellar binary systems
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