568 research outputs found
Origin of nascent lineages and the mechanisms used to prime second-strand DNA synthesis in the R1 and R2 retrotransposons of Drosophila
Comparative analysis of 12 Drosophila genomes reveals insights into the evolution and mechanism of integration of R1 and R2 retrotransposons
The Three-Dimensional Structure of Cassiopeia A
We used the Spitzer Space Telescope's Infrared Spectrograph to map nearly the
entire extent of Cassiopeia A between 5-40 micron. Using infrared and Chandra
X-ray Doppler velocity measurements, along with the locations of optical ejecta
beyond the forward shock, we constructed a 3-D model of the remnant. The
structure of Cas A can be characterized into a spherical component, a tilted
thick disk, and multiple ejecta jets/pistons and optical fast-moving knots all
populating the thick disk plane. The Bright Ring in Cas A identifies the
intersection between the thick plane/pistons and a roughly spherical reverse
shock. The ejecta pistons indicate a radial velocity gradient in the explosion.
Some ejecta pistons are bipolar with oppositely-directed flows about the
expansion center while some ejecta pistons show no such symmetry. Some ejecta
pistons appear to maintain the integrity of the nuclear burning layers while
others appear to have punched through the outer layers. The ejecta pistons
indicate a radial velocity gradient in the explosion. In 3-D, the Fe jet in the
southeast occupies a "hole" in the Si-group emission and does not represent
"overturning", as previously thought. Although interaction with the
circumstellar medium affects the detailed appearance of the remnant and may
affect the visibility of the southeast Fe jet, the bulk of the symmetries and
asymmetries in Cas A are intrinsic to the explosion.Comment: Accepted to ApJ. 54 pages, 21 figures. For high resolution figures
and associated mpeg movie and 3D PDF files, see
http://homepages.spa.umn.edu/~tdelaney/pape
The X-ray Spectrum of the Vela Pulsar Resolved with Chandra
We report the results of the spectral analysis of two observations of the
Vela pulsar with the Chandra X-ray observatory. The spectrum of the pulsar does
not show statistically significant spectral lines in the observed 0.25-8.0 keV
band. Similar to middle-aged pulsars with detected thermal emission, the
spectrum consists of two distinct components. The softer component can be
modeled as a magnetic hydrogen atmosphere spectrum - for the pulsar magnetic
field G and neutron star mass and radius
km, we obtain \tef^\infty =0.68\pm 0.03 MK, erg s, pc (the
effective temperature, bolometric luminosity, and radius are as measured by a
distant observer). The effective temperature is lower than that predicted by
standard neutron star cooling models. A standard blackbody fit gives MK,
erg s ( is the distance in units of 250 pc); the blackbody
temperature corresponds to a radius, km, much
smaller than realistic neutron star radii. The harder component can be modeled
as a power-law spectrum, with parameters depending on the model adopted for the
soft component - , erg s and , erg s for the hydrogen atmosphere and blackbody soft
component, respectively ( is the photon index, is the luminosity
in the 0.2--8 keV band). The extrapolation of the power-law component of the
former fit towards lower energies matches the optical flux at --1.45.Comment: Submitted to ApJ, three figures; color figure 1 can be found at
http://www.xray.mpe.mpg.de/~zavlin/pub_list.htm
On the Expansion Rate, Age, and Distance of the Supernova Remnant G266.2-1.2 (Vela Jr.)
An analysis of Chandra ACIS data for two relatively bright and narrow
portions of the northwestern rim of G266.2-1.2 (a.k.a. RX J0852.0-4622 or Vela
Jr.) reveal evidence of a radial displacement of 2.40 +/- 0.56 arcsec between
2003 and 2008. The corresponding expansion rate (0.42 +/- 0.10 arcsec/yr or
13.6 +/- 4.2%/kyr) is about half the rate reported for an analysis of
XMM-Newton data from a similar, but not identical, portion of the rim over a
similar, but not identical, time interval (0.84 +/- 0.23 arcsec/yr, Katsuda et
al. 2008a). If the Chandra rate is representative of the remnant as a whole,
then the results of a hydrodynamic analysis suggest that G266.2-1.2 is between
2.4 and 5.1 kyr old if it is expanding into a uniform ambient medium (whether
or not it was produced by a Type Ia or Type II event). If the remnant is
expanding into the material shed by a steady stellar wind, then the age could
be as much as 50% higher. The Chandra expansion rate and a requirement that the
shock speed be greater than or equal to 1000 km/s yields a lower limit on the
distance of 0.5 kpc. An analysis of previously-published distance estimates and
constraints suggests G266.2-1.2 is no further than 1.0 kpc. This range of
distances is consistent with the distance to the nearer of two groups of
material in the Vela Molecular Ridge (0.7 +/- 0.2 kpc, Liseau et al. 1992) and
to the Vel OB1 association (0.8 kpc, Eggen 1982).Comment: 30 pages, 7 figure
The Arecibo Galaxy Environments Survey - Description and Early Results
The Arecibo Galaxy Environments Survey (AGES) is a 2000-hour neutral hydrogen
(HI) survey using the new Arecibo L-band Feed Array (ALFA) multibeam instrument
at Arecibo Observatory. It will cover 200 square degrees of sky, sampling a
range of environments from the Local Void through to the Virgo Cluster with
higher sensitivity, spatial resolution and velocity resolution than previous
neutral hydrogen surveys.Comment: IAU Symposium 235 - Galaxy Evolution across the Hubble Time. 3 pages
(only 1 st to appear in printed proceedings), 3 figures. Uses iaus.cls. Full
resolution PDF is available from
http://www2.naic.edu/alfa/ealfa/AGES-IAUS235-Minchin.pdf and a PDF of the
poster is available from http://www2.naic.edu/alfa/ealfa/AGES-IAU-Minchin.pd
The Arecibo Galaxy Environment Survey III: Observations Toward the Galaxy Pair NGC 7332/7339 and the Isolated Galaxy NGC 1156
Two 5 square degree regions around the NGC 7332/9 galaxy pair and the
isolated galaxy NGC 1156 have been mapped in the 21-cm line of neutral hydrogen
(HI) with the Arecibo L-band Feed Array out to a redshift of ~0.065
km/s) as part of the Arecibo Galaxy Environment Survey. One of the aims of this
survey is to investigate the environment of galaxies by identifying dwarf
companions and interaction remnants; both of these areas provide the potential
for such discoveries. The neutral hydrogen observations were complemented by
optical and radio follow-up observations with a number of telescopes. A total
of 87 galaxies were found, of which 39 (45 per cent) were previously cataloged
and 15 (17 per cent) have prior redshifts. Two dwarf galaxies have been
discovered in the NGC 7332 group and a single dwarf galaxy in the vicinity NGC
1156 . A parallel optical search of the area revealed one further possible
dwarf galaxy near NGC 7332.Comment: 18 pages, 17 figures, uses emulateap
- …