632 research outputs found
Detection of Molecular Hydrogen Orbiting a "Naked" T Tauri Star
Astronomers have established that for a few million years newborn stars
possess disks of orbiting gas and dust. Such disks, which are likely sites of
planet formation, appear to disappear once these stars reach ages of 5-10 times
10^6 yr; yet, >= 10^7 yr is thought necessary for giant planet formation. If
disks dissipate in less time than is needed for giant planet formation, such
planets may be rare and those known around nearby stars would be anomalies.
Herein, we report the discovery of H_2 gas orbiting a weak-lined T Tauri star
heretofore presumed nearly devoid of circumstellar material. We estimate that a
significant amount of H_2 persists in the gas phase, but only a tiny fraction
of this mass emits in the near-infrared. We propose that this star possesses an
evolved disk that has escaped detection thus far because much of the dust has
coagulated into planetesimals. This discovery suggests that the theory that
disks are largely absent around such stars should be reconsidered. The
widespread presence of such disks would indicate that planetesimals can form
quickly and giant planet formation can proceed to completion before the gas in
circumstellar disks disperses.Comment: latex 12 pages, including 1 figur
Pentaquark searches at FOCUS
We find no evidence for high-energy photoproduction of pentaquarks at 1540
MeV/, 1862 MeV/, or 3099 MeV/ using decay modes ,
, and , respectively.Comment: Proceedings from talk at 2004 DPF Meeting at University of
California, Riversid
On the SigmaN cusp in the pp -> pK+Lambda reaction
Measurements of the reaction at = 2.28 GeV have
been carried out at COSY-TOF. In addition to the FSI and
resonance excitation effects a pronounced narrow structure is observed in the
Dalitz plot and in its projection on the -invariant mass. The
structure appears at the N threshold and is interpreted as
N cusp effect. The observed width of 20 MeV/ is substantially
broader than anticipated from previous inclusive measurements. Angular
distributions of this cusp structure are shown to be dissimilar to those in the
residual channel, but similar to those observed in the
channel
Analysis of diversity genetic of Moroccan net blotch populations using amplified fragment length polymorphism (AFLP) markers
Net blotch caused by Pyrenophora teres f. teres is the most harmful foliar disease in barley generating significant economic losses in Morocco. Populations of P. teres f. teres were collected from different regions of Morocco. Thirty five (35) P. teres f. teres isolates, single conidial, were isolated and were subjected to molecular study using amplified fragment length polymorphism (AFLP) technique. Out of the fourteen primers combinations tested, four primers combinations were selected to disclose the polymorphism between the different P. teres f. teres isolates. The molecular characterization of these isolates showed high degree of polymorphism reaching 95% and identifying 25 specific genotypes. The genetic variability of the different isolates of P. teres f. teres within and between Moroccan regions was highlighted, disclosing no linkage between the isolates and their geographical origins. This result might be due to informal material flow between regions.Key words: Barley, net blotch, Pyrenophora teres f. teres, amplified fragment length polymorphism (AFLP), genetic diversity
Physical Conditions of Accreting Gas in T Tauri Star Systems
We present results from a low resolution (R~300) near-infrared spectroscopic
variability survey of actively accreting T Tauri stars (TTS) in the
Taurus-Auriga star forming region. Paschen and Brackett series H I
recombination lines were detected in 73 spectra of 15 classical T Tauri
systems. The values of the Pan/PaB, Brn/BrG, and BrG/Pan H I line ratios for
all observations exhibit a scatter of < 20% about the weighted mean, not only
from source to source, but also for epoch-to-epoch variations in the same
source. A representative or `global' value was determined for each ratio in
both the Paschen and Brackett series as well as the BrG/Pan line ratios. A
comparison of observed line ratio values was made to those predicted by the
temperature and electron density dependent models of Case B hydrogen
recombination line theory. The measured line ratios are statistically well-fit
by a tightly constrained range of temperatures (T < 2000 K) and electron
densities 1e9 < n_e < 1e10 cm^-3. A comparison of the observed line ratio
values to the values predicted by the optically thick and thin local
thermodynamic equilibrium cases rules out these conditions for the emitting H I
gas. Therefore, the emission is consistent with having an origin in a non-LTE
recombining gas. While the range of electron densities is consistent with the
gas densities predicted by existing magnetospheric accretion models, the
temperature range constrained by the Case B comparison is considerably lower
than that expected for accreting gas. The cooler gas temperatures will require
a non-thermal excitation process (e.g., coronal/accretion-related X-rays and UV
photons) to power the observed line emission.Comment: 12 pages, emulateapj format, Accepted for publication in Ap
The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000A (II): Measurement of Foreground Zodiacal Light
We present a measurement of the absolute surface brightness of the zodiacal
light (3900-5100A) toward a fixed extragalactic target at high ecliptic
latitude based on moderate resolution (~1.3A per pixel) spectrophotometry
obtained with the du Pont 2.5m telescope at Las Campanas Observatory in Chile.
This measurement and contemporaneous Hubble Space Telescope data from WFPC2 and
FOS comprise a coordinated program to measure the mean flux of the diffuse
extragalactic background light (EBL). The zodiacal light at optical wavelengths
results from scattering by interplanetary dust, so that the zodiacal light flux
toward any extragalactic target varies seasonally with the position of the
Earth. This measurement of zodiacal light is therefore relevant to the specific
observations (date and target field) under discussion. To obtain this result,
we have developed a technique that uses the strength of the zodiacal Fraunhofer
lines to identify the absolute flux of the zodiacal light in the
multiple-component night sky spectrum. Statistical uncertainties in the result
are 0.6% (1 sigma). However, the dominant source of uncertainty is systematic
errors, which we estimate to be 1.1% (1 sigma). We discuss the contributions
included in this estimate explicitly. The systematic errors in this result
contribute 25% in quadrature to the final error in our coordinated EBL
measurement, which is presented in the first paper of this series.Comment: Accepted for publication in ApJ, 22 pages using emulateapj.sty,
version with higher resolution figures available at
http://www.astro.lsa.umich.edu/~rab/publications.html or at
http://nedwww.ipac.caltech.edu/level5/Sep01/Bernstein2/frames.htm
Production of Lambda and Sigma^0 hyperons in proton-proton collisions
This paper reports results on simultaneous measurements of the reaction
channels pp -> pK+\Lambda and pp -> pK+\Sigma^0 at excess energies of 204, 239,
and 284 MeV (\Lambda) and 127, 162, and 207 MeV (\Sigma^0). Total and
differential cross sections are given for both reactions. It is concluded from
the measured total cross sections that the high energy limit of the cross
section ratio is almost reached at an excess energy of only about 200 MeV. From
the differential distributions observed in the overall CMS as well as in the
Jackson and helicity frames, a significant contribution of interfering nucleon
resonances to the \Lambda production mechanism is concluded while resonant
\Sigma^0-production seems to be of lesser importance and takes place only
through specific partial waves of the entrance channel. The data also indicate
that kaon exchange plays a minor role in the case of \Lambda- but an important
role for \Sigma^0-production. Thus the peculiar energy dependence of the
\Lambda-to-\Sigma^0 cross section ratio appears in a new light as its
explanation requires more than mere differences between the p\Lambda and the
p\Sigma^0 final state interaction. The data provide a benchmark for theoretical
models already available or yet to come.Comment: 18 pages, 10 figures; accepted by The European Physical Journal A
(EPJ A
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
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