1,044 research outputs found
A MERLIN Observation of PSR B1951+32 and its associated Plerion
In an investigative 16 hour L band observation using the MERLIN radio
interferometric array, we have resolved both the pulsar PSR B1951+32 and
structure within the flat spectral radio continuum region, believed to be the
synchrotron nebula associated with the interaction of the pulsar and its `host'
supernova remnant CTB 80. The extended structure we see, significant at
4.5 , is of dimensions 2.5" 0.75", and suggests a sharp bow
shaped arc of shocked emission, which is correlated with similar structure
observed in lower resolution radio maps and X-ray images. Using this MERLIN
data as a new astrometric reference for other multiwavelength data we can place
the pulsar at one edge of the HST reported optical synchrotron knot, ruling out
previous suggested optical counterparts, and allowing an elementary analysis of
the optical synchrotron emission which appears to trail the pulsar. The latter
is possibly a consequence of pulsar wind replenishment, and we suggest that the
knot is a result of magnetohydrodynamic (MHD) instabilities. These being so, it
suggests a dynamical nature to the optical knot, which will require high
resolution optical observations to confirm.Comment: 12 pages, 2 figures. Accepted for publication in ApJ
A simultaneous search for prompt radio emission associated with the short GRB 170112A using the all-sky imaging capability of the OVRO-LWA
We have conducted the most sensitive low frequency (below 100 MHz) search to
date for prompt, low-frequency radio emission associated with short-duration
gamma-ray bursts (GRBs), using the Owens Valley Radio Observatory Long
Wavelength Array (OVRO-LWA). The OVRO-LWA's nearly full-hemisphere
field-of-view (, square degrees) allows us to search for
low-frequency (sub- MHz) counterparts for a large sample of the subset of
GRB events for which prompt radio emission has been predicted. Following the
detection of short GRB 170112A by Swift, we used all-sky OVRO-LWA images
spanning one hour prior to and two hours following the GRB event to search for
a transient source coincident with the position of GRB 170112A. We detect no
transient source, with our most constraining flux density limit of
for frequencies spanning . We
place constraints on a number of models predicting prompt, low-frequency radio
emission accompanying short GRBs and their potential binary neutron star merger
progenitors, and place an upper limit of on the fraction of energy released in the prompt radio
emission. These observations serve as a pilot effort for a program targeting a
wider sample of both short and long GRBs with the OVRO-LWA, including bursts
with confirmed redshift measurements which are critical to placing the most
constraining limits on prompt radio emission models, as well as a program for
the follow-up of gravitational wave compact binary coalescence events detected
by advanced LIGO and Virgo.Comment: 14 pages, 5 figures, ApJ submitte
Periodic Bursts of Coherent Radio Emission from an Ultracool Dwarf
We report the detection of periodic (p = 1.96 hours) bursts of extremely
bright, 100% circularly polarized, coherent radio emission from the M9 dwarf
TVLM 513-46546. Simultaneous photometric monitoring observations have
established this periodicity to be the rotation period of the dwarf. These
bursts, which were not present in previous observations of this target, confirm
that ultracool dwarfs can generate persistent levels of broadband, coherent
radio emission, associated with the presence of kG magnetic fields in a
large-scale, stable configuration. Compact sources located at the magnetic
polar regions produce highly beamed emission generated by the electron
cyclotron maser instability, the same mechanism known to generate planetary
coherent radio emission in our solar system. The narrow beams of radiation pass
our line of sight as the dwarf rotates, producing the associated periodic
bursts. The resulting radio light curves are analogous to the periodic light
curves associated with pulsar radio emission highlighting TVLM 513-46546 as the
prototype of a new class of transient radio source.Comment: 12 pages, 3 figures, accepted for publication in ApJ Letter
Design, implementation, and testing of a cryogenic loading capability on an engineering neutron diffractometer
A novel capability was designed, implemented, and tested for in situ neutron diffraction measurements during loading at cryogenic temperatures on the spectrometer for materials research at temperature and stress at Los Alamos National Laboratory. This capability allowed for the application of dynamic compressive forces of up to 250 kN on standard samples controlled at temperatures between 300 and 90 K. The approach comprised of cooling thermally isolated compression platens that in turn conductively cooled the sample in an aluminum vacuum chamber which was nominally transparent to the incident and diffracted neutrons. The cooling/heat rate and final temperature were controlled by regulating the flow of liquid nitrogen in channels inside the platens that were connected through bellows to the mechanical actuator of the load frame and by heaters placed on the platens. Various performance parameters of this system are reported here. The system was used to investigate deformation in Ni-Ti-Fe shape memory alloys at cryogenic temperatures and preliminary results are presented
A Catalogue of Optically Selected Cores
We present a new catalogue of 406 dense cores optically selected by using the
STScI Digitized Sky Survey (DSS). In this catalogue 306 cores have neither an
Embedded YSO (EYSO) nor a Pre-Main-Sequence (PMS) star, 94 cores have EYSOs (1
core has both an EYSO and a PMS star), and 6 cores have PMS star only. Our
sample of dense cores in the catalogue is fairly complete within a category of
northern Lynds class 5, 6 clouds, and southern Hartley et al. (1986)'s class A
clouds, providing a database useful for the systematic study of dense cores.
Most of the cores listed in the catalogue have diameters between
pc with a mean of pc. The sizes ( pc in the mean) of
cores with EYSOs are found to be usually larger than the sizes ( pc
in the mean) of starless cores. The typical mean gas density of the cores is
. Most of the cores are more likely elongated than
spherical (mean aspect ratio: ). The ratio of the number of cores
with EYSOs to the number of starless cores for our sample is about 0.3,
suggesting that the typical lifetime of starless cores is Myr, about
3 times longer than the duration of the Class 0 and Class I phases. This
lifetime is shorter than expected from models of ambipolar diffusion, by
factors of 2-44.Comment: 22 pages, 8 figures, 3 tables, and to appear in ApJS.
Harvard-Smithsonian Center for Astrophysic
On the Identification of High Mass Star Forming Regions using IRAS: Contamination by Low-Mass Protostars
We present the results of a survey of a small sample (14) of low-mass
protostars (L_IR < 10^3 Lsun) for 6.7 GHz methanol maser emission performed
using the ATNF Parkes radio telescope. No new masers were discovered. We find
that the lower luminosity limit for maser emission is near 10^3 Lsun, by
comparison of the sources in our sample with previously detected methanol maser
sources. We examine the IRAS properties of our sample and compare them with
sources previously observed for methanol maser emission, almost all of which
satisfy the Wood & Churchwell criterion for selecting candidate UCHII regions.
We find that about half of our sample satisfy this criterion, and in addition
almost all of this subgroup have integrated fluxes between 25 and 60 microns
that are similar to sources with detectable methanol maser emission. By
identifying a number of low-mass protostars in this work and from the
literature that satisfy the Wood & Churchwell criterion for candidate UCHII
regions, we show conclusively for the first time that the fainter flux end of
their sample is contaminated by lower-mass non-ionizing sources, confirming the
suggestion by van der Walt and Ramesh & Sridharan.Comment: 8 pages with 2 figures. Accepted by Ap
How is rape a weapon of war?: feminist international relations, modes of critical explanation and the study of wartime sexual violence
Rape is a weapon of war. Establishing this now common claim has been an achievement of feminist scholarship and activism and reveals wartime sexual violence as a social act marked by gendered power. But the consensus that rape is a weapon of war obscures important, and frequently unacknowledged, differences in ways of understanding and explaining it. This article opens these differences to analysis. Drawing on recent debates regarding the philosophy of social science in IR and social theory, it interprets feminist accounts of wartime sexual violence in terms of modes of critical explanation – expansive styles of reasoning that foreground particular actors, mechanisms, reasons and stories in the formulation of research. The idea of a mode of critical explanation is expanded upon through a discussion of the role of three elements (analytical wagers, narrative scripts and normative orientations) which accomplish the theoretical work of modes. Substantive feminist accounts of wartime sexual violence are then differentiated in terms of three modes – of instrumentality, unreason and mythology – which implicitly structure different understandings of how rape might be a weapon of war. These modes shape political and ethical projects and so impact not only on questions of scholarly content but also on the ways in which we attempt to mitigate and abolish war rape. Thinking in terms of feminist modes of critical explanation consequently encourages further work in an unfolding research agenda. It clarifes the ways in which an apparently commonality of position can conceal meaningful disagreements about human action. Exposing these disagreements opens up new possibilities for the analysis of war rape
Abundances of Molecular Species in Barnard 68
Abundances for 5 molecules (C18O, CS, NH3, H2CO, and C3H2) and 1 molecular
ion (N2H+) and upper limits for the abundances of 1 molecule (13CO) and 1
molecular ion (HCO+) are derived for gas within the Bok globule Barnard 68
(B68). The abundances were determined using our own BIMA millimeter
interferometer data and single-dish data gathered from the literature, in
conjunction with a Monte Carlo radiative transfer model. Since B68 is the only
starless core to have its density structure strongly constrained via extinction
mapping, a major uncertainty has been removed from these determinations. All
abundances for B68 are lower than those derived for translucent and cold dense
clouds, but perhaps only significantly for N2H+, NH3, and C3H2. Depletion of CS
toward the extinction peak of B68 is hinted at by the large offset between the
extinction peak and the position of maximum CS line brightness. Abundances
derived here for C18O and N2H+ are consistent with other, recently determined
values at positions observed in common.Comment: 16 pages, 1 figure, accepted by AJ, typo corrected, reference removed
in Section 4.
The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. I. Chamaeleon II Observed with MIPS
We present maps of over 1.5 square degrees in Chamaeleon (Cha) II at 24, 70,
and 160 micron observed with the Spitzer Space Telescope Multiband Imaging
Photometer for Spitzer (MIPS) and a 1.2 square degree millimeter map from SIMBA
on the Swedish-ESO Submillimetre Telescope (SEST). The c2d Spitzer Legacy
Team's data reduction pipeline is described in detail. Over 1500 24 micron
sources and 41 70 micron sources were detected by MIPS with fluxes greater than
10-sigma. More than 40 potential YSOs are identified with a MIPS and 2MASS
color-color diagram and by their spectral indices, including two previously
unknown sources with 24 micron excesses. Our new SIMBA millimeter map of Cha II
shows that only a small fraction of the gas is in compact structures with high
column densities. The extended emission seen by MIPS is compared with previous
CO observations. Some selected interesting sources, including two detected at 1
mm, associated with Cha II are discussed in detail and their SEDs presented.
The classification of these sources using MIPS data is found to be consistent
with previous studies.Comment: 44 pages, 12 figures (1 color), to be published in Ap
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