81 research outputs found
The Spitzer Survey of Interstellar Clouds in the Gould Belt. III. A Multi-Wavelength View of Corona Australis
We present Spitzer Space Telescope IRAC and MIPS observations of a 0.85 deg^2
field including the Corona Australis (CrA) star-forming region. At a distance
of 130 pc, CrA is one of the closest regions known to be actively forming
stars, particularly within its embedded association, the Coronet. Using the
Spitzer data, we identify 51 young stellar objects (YSOs) in CrA which include
sources in the well-studied Coronet cluster as well as distributed throughout
the molecular cloud. Twelve of the YSOs discussed are new candidates, one of
which is located in the Coronet. Known YSOs retrieved from the literature are
also added to the list, and a total of 116 candidate YSOs in CrA are compiled.
Based on these YSO candidates, the star formation rate is computed to be 12 M_o
Myr^-1, similar to that of the Lupus clouds. A clustering analysis was also
performed, finding that the main cluster core, consisting of 68 members, is
elongated (having an aspect ratio of 2.36), with a circular radius of 0.59 pc
and mean surface density of 150 pc^-2.
In addition, we analyze outflows and jets in CrA by means of new CO and H_2
data. We present 1.3 mm interferometric continuum observations made with the
Submillimeter Array (SMA) covering R CrA, IRS 5, IRS 7, and IRAS 18595-3712
(IRAS 32). We also present multi-epoch H_2 maps and detect jets and outflows,
study their proper motions, and identify exciting sources. The Spitzer and
ISAAC/VLT observations of IRAS 32 show a bipolar precessing jet, which drives a
CO (2-1) outflow detected in the SMA observations. There is also clear evidence
for a parsec-scale precessing outflow, E-W oriented, and originating in the SMA
2 region, likely driven by SMA 2 or IRS 7A.Comment: Accepted for publication in ApJS. 112 pages, 42 figures (quality
reduced), 13 tables. Full resolution version can be found at
http://www.cfa.harvard.edu/~dpeterson/CrA/CrA_highres.pd
Variable Sodium Absorption in a Low-Extinction Type Ia Supernova
Recent observations have revealed that some Type Ia supernovae exhibit
narrow, time-variable Na I D absorption features. The origin of the absorbing
material is controversial, but it may suggest the presence of circumstellar gas
in the progenitor system prior to the explosion, with significant implications
for the nature of the supernova progenitors. We present the third detection of
such variable absorption, based on six epochs of high-resolution spectroscopy
of the Type Ia supernova SN 2007le from Keck and the HET. The data span ~3
months, from 5 days before maximum light to 90 days after maximum. We find that
one component of the Na D absorption lines strengthened significantly with
time, indicating a total column density increase of ~2.5 x 10^12 cm^-2. The
changes are most prominent after maximum light rather than at earlier times
when the UV flux from the SN peaks. As with SN 2006X, we detect no change in
the Ca II H&K lines over the same time period, rendering line-of-sight effects
improbable and suggesting a circumstellar origin for the absorbing material.
Unlike the previous two SNe exhibiting variable absorption, SN 2007le is not
highly reddened (E_B-V = 0.27 mag), also pointing toward circumstellar rather
than interstellar absorption. Photoionization models show that the data are
consistent with a dense (10^7 cm^-3) cloud or clouds of gas located ~0.1 pc
from the explosion. These results broadly support the single-degenerate
scenario previously proposed to explain the variable absorption, with mass loss
from a nondegenerate companion star responsible for providing the circumstellar
gas. We also present tentative evidence for narrow Halpha emission associated
with the SN, which will require followup observations at late times to confirm.
[abridged]Comment: 16 pages, 10 figures (8 in color), 5 tables. Accepted for publication
in Ap
PerR Confers Phagocytic Killing Resistance and Allows Pharyngeal Colonization by Group A Streptococcus
The peroxide response transcriptional regulator, PerR, is thought to contribute to virulence of group A Streptococcus (GAS); however, the specific mechanism through which it enhances adaptation for survival in the human host remains unknown. Here, we identify a critical role of PerR-regulated gene expression in GAS phagocytosis resistance and in virulence during pharyngeal infection. Deletion of perR in M-type 3 strain 003Sm was associated with reduced resistance to phagocytic killing in human blood and by murine macrophages in vitro. The increased phagocytic killing of the perR mutant was abrogated in the presence of the general oxidative burst inhibitor diphenyleneiodonium chloride (DPI), a result that suggests PerR-dependent gene expression counteracts the phagocyte oxidative burst. Moreover, an isogenic perR mutant was severely attenuated in a baboon model of GAS pharyngitis. In competitive infection experiments, the perR mutant was cleared from two animals at 24 h and from four of five animals by day 14, in sharp contrast to wild-type bacteria that persisted in the same five animals for 28 to 42 d. GAS genomic microarrays were used to compare wild-type and perR mutant transcriptomes in order to characterize the PerR regulon of GAS. These studies identified 42 PerR-dependent loci, the majority of which had not been previously recognized. Surprisingly, a large proportion of these loci are involved in sugar utilization and transport, in addition to oxidative stress adaptive responses and virulence. This finding suggests a novel role for PerR in mediating sugar uptake and utilization that, together with phagocytic killing resistance, may contribute to GAS fitness in the infected host. We conclude that PerR controls expression of a diverse regulon that enhances GAS resistance to phagocytic killing and allows adaptation for survival in the pharynx
- âŚ