231 research outputs found
Quiescent H2 Emission From Pre-Main Sequence Stars in Chamaeleon I
We report the discovery of quiescent emission from molecular hydrogen gas
located in the circumstellar disks of six pre-main sequence stars, including
two weak-line T Tauri stars (TTS), and one Herbig AeBe star, in the Chamaeleon
I star forming region. For two of these stars, we also place upper limits on
the 2->1 S(1)/1->0 S(1) line ratios of 0.4 and 0.5. Of the 11 pre-main sequence
sources now known to be sources of quiescent near-infrared hydrogen emission,
four possess transitional disks, which suggests that detectable levels of H
emission and the presence of inner disk holes are correlated. These H
detections demonstrate that these inner holes are not completely devoid of gas,
in agreement with the presence of observable accretion signatures for all four
of these stars and the recent detections of [Ne II] emission from three of
them. The overlap in [Ne II] and H detections hints at a possible
correlation between these two features and suggests a shared excitation
mechanism of high energy photons. Our models, combined with the kinematic
information from the H lines, locate the bulk of the emitting gas at a few
tens of AU from the stars. We also find a correlation between H detections
and those targets which possess the largest H equivalent widths,
suggesting a link between accretion activity and quiescent H emission. We
conclude that quiescent H emission from relatively hot gas within the disks
of TTS is most likely related to on-going accretion activity, the production of
UV photons and/or X-rays, and the evolutionary status of the dust grain
populations in the inner disks.Comment: 12 pages, emulateapj, Accepted by Ap
Infrared Space Observatory Polarimetric Imaging of the Egg Nebula (RAFGL 2688)
We present polarimetric imaging of the protoplanetary nebula RAFGL 2688
obtained at 4.5 microns with the Infrared Space Observatory (ISO). We have
deconvolved the images to remove the signature of the point spread function of
the ISO telescope, to the extent possible. The deconvolved 4.5 micron image and
polarimetric map reveal a bright point source with faint, surrounding
reflection nebulosity. The reflection nebula is brightest to the
north-northeast, in agreement with previous ground- and space-based infrared
imaging. Comparison with previous near-infrared polarimetric imaging suggests
that the polarization of starlight induced by the dust grains in RAFGL 2688 is
more or less independent of wavelength between 2 microns and 4.5 microns. This,
in turn, indicates that scattering dominates over thermal emission at
wavelengths as long as ~5 microns, and that the dust grains have characteristic
radii < 1 micron.Comment: 27 pages, 9 figures; to appear in the Astronomical Journal, May 2002
issu
ROSAT X-ray Spectral Properties of Nearby Young Associations: TW Hydrae, Tucana-Horologium, and the beta Pic Moving Group
We present archival ROSAT data for three recently identified, nearby (D<70
pc), young (~10-40 Myr) stellar associations: the TW Hydrae Association, the
Tucana-Horologium Association, and the beta Pic Moving Group. The distributions
of ROSAT X-ray hardness ratios (HR1, HR2) for these three groups, whose
membership is dominated by low-mass, weak-lined T Tauri stars, are tightly
clustered and very similar to one another. The value of HR1 for TW Hya itself
-- the only bona fide classical T Tauri star in any of the nearby groups -- is
clearly anomalous among these nearby young stars. We compare the hardness ratio
distributions of stars in the three nearby groups with those of T Tauri stars,
the Hyades, and main sequence dwarfs in the field. This comparison demonstrates
that the X-ray spectra of F through M stars soften with age, and that F and G
stars evolve more rapidly in X-ray spectral hardness than do K and M stars. It
is as yet unclear whether this trend can be attributed to age-dependent changes
in the intrinsic X-ray spectra of stars of type F and later, to a decrease in
the column density of circumstellar gas (e.g., in residual protoplanetary
disks), or to the diminishing contributions of star-disk interactions to X-ray
emission. Regardless, these results demonstrate that analysis of archival ROSAT
X-ray spectral data can help both to identify nearby, young associations and to
ascertain the X-ray emission properties of members of known associations.Comment: 17 pages, 4 figures; accepted by the Astrophysical Journa
Structural insights into RNA processing by the human RISC-loading complex.
Targeted gene silencing by RNA interference (RNAi) requires loading of a short guide RNA (small interfering RNA (siRNA) or microRNA (miRNA)) onto an Argonaute protein to form the functional center of an RNA-induced silencing complex (RISC). In humans, Argonaute2 (AGO2) assembles with the guide RNA-generating enzyme Dicer and the RNA-binding protein TRBP to form a RISC-loading complex (RLC), which is necessary for efficient transfer of nascent siRNAs and miRNAs from Dicer to AGO2. Here, using single-particle EM analysis, we show that human Dicer has an L-shaped structure. The RLC Dicer's N-terminal DExH/D domain, located in a short 'base branch', interacts with TRBP, whereas its C-terminal catalytic domains in the main body are proximal to AGO2. A model generated by docking the available atomic structures of Dicer and Argonaute homologs into the RLC reconstruction suggests a mechanism for siRNA transfer from Dicer to AGO2
Gas and Dust Associated with the Strange, Isolated, Star BP Piscium
We have carried out a multiwavelength observational campaign demonstrating
some of the remarkable properties of the infrared-bright variable star BP Psc.
Surrounded by a compact dusty, gaseous disk, this little-studied late-G (or
early-K) type star emits about 75% of its detected energy flux at infrared
wavelengths. Evidence for accretion of gas in conjunction with narrow bi-polar
jets and Herbig-Haro objects is apparently consistent with classification of BP
Psc as a pre-main sequence star, as postulated in most previous studies. If
young, then BP Psc would be one of the nearest and oldest known classical T
Tauri stars. However, such an evolutionary classification encounters various
problems that are absent or much less severe if BP Psc is instead a luminosity
class III post-main sequence star. In this case, it would be the first known
example of a first ascent giant surrounded by a massive molecular disk with
accompanying rapid gas accretion and prominent jets and HH objects. In this
model, the genesis of the massive dusty gaseous disk could be a consequence of
the envelopment of a low mass companion star. Properties in the disk may be
conducive to the current formation of planets, a gigayear or more after the
formation of BP Psc itself.Comment: Accepted for Astrophysical Journal New version with minor changes:
includes fixing a typo on the 3rd line of the paragraph that follows Equa 4
and adding a new reference (Nordhaus and Blackman 2006
The varied roles of nuclear receptors during vertebrate embryonic development
Nuclear receptors comprise a superfamily of sequence-specific transcription factors whose members have diverse roles during development. This review will summarize the developmental roles of selected members of the nuclear receptor superfamily
Simultaneous consumption of pentose and hexose sugars: an optimal microbial phenotype for efficient fermentation of lignocellulosic biomass
Lignocellulosic biomass is an attractive carbon source for bio-based fuel and chemical production; however, its compositional heterogeneity hinders its commercial use. Since most microbes possess carbon catabolite repression (CCR), mixed sugars derived from the lignocellulose are consumed sequentially, reducing the efficacy of the overall process. To overcome this barrier, microbes that exhibit the simultaneous consumption of mixed sugars have been isolated and/or developed and evaluated for the lignocellulosic biomass utilization. Specific strains of Escherichia coli, Saccharomyces cerevisiae, and Zymomonas mobilis have been engineered for simultaneous glucose and xylose utilization via mutagenesis or introduction of a xylose metabolic pathway. Other microbes, such as Lactobacillus brevis, Lactobacillus buchneri, and Candida shehatae possess a relaxed CCR mechanism, showing simultaneous consumption of glucose and xylose. By exploiting CCR-negative phenotypes, various integrated processes have been developed that incorporate both enzyme hydrolysis of lignocellulosic material and mixed sugar fermentation, thereby enabling greater productivity and fermentation efficacy
Performance of the CMS Cathode Strip Chambers with Cosmic Rays
The Cathode Strip Chambers (CSCs) constitute the primary muon tracking device
in the CMS endcaps. Their performance has been evaluated using data taken
during a cosmic ray run in fall 2008. Measured noise levels are low, with the
number of noisy channels well below 1%. Coordinate resolution was measured for
all types of chambers, and fall in the range 47 microns to 243 microns. The
efficiencies for local charged track triggers, for hit and for segments
reconstruction were measured, and are above 99%. The timing resolution per
layer is approximately 5 ns
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