10,069 research outputs found
The New Eclipsing Cataclysmic Variable SDSS 154453+2553
The cataclysmic variable SDSS154453+2553 was recently identified in the Sloan
Digital Sky Survey. We obtained spectra and photometry at the MDM Observatory,
which revealed an eclipse with a 6.03 hour period. The H{\alpha} emission line
exhibits a strong rotational disturbance during eclipse, indicating that it
arises in an accretion disk. A contribution from an M-type companion is also
observed. Time-series photometry during eclipse gives an ephemeris of
2454878.0062(15) + 0.251282(2)E. We present spectroscopy through the orbit and
eclipse photometry. Our analysis of the secondary star indicates a distance of
800 {\pm} 180 pc.Comment: 6 pages, 3 figures, Accepted for publication in PAS
Spatial structures in a simple model of population dynamics for parasite-host interactions
Spatial patterning can be crucially important for understanding the behavior
of interacting populations. Here we investigate a simple model of parasite and
host populations in which parasites are random walkers that must come into
contact with a host in order to reproduce. We focus on the spatial arrangement
of parasites around a single host, and we derive using analytics and numerical
simulations the necessary conditions placed on the parasite fecundity and
lifetime for the populations long-term survival. We also show that the parasite
population can be pushed to extinction by a large drift velocity, but,
counterintuitively, a small drift velocity generally increases the parasite
population.Comment: 6 pages, 6 figure
X-rays from T Tau: A test case for accreting T Tauri stars
We test models for the generation of X-rays in accreting T Tauri stars (TTS),
using X-ray data from the classical TTS T Tau. High-resolution spectroscopy
from the Reflection Grating Spectrometers on XMM-Newton is used to infer
electron densities, element abundances and the thermal structure of the X-ray
source. We also discuss the ultraviolet light curve obtained by the Optical
Monitor, and complementary ground-based photometry. A high-resolution image
from Chandra constrains contributions from the two companions of T Tau N. The
X-ray grating spectrum is rich in emission lines, but shows an unusual mixture
of features from very hot (~30 MK) and very cool (1-3 MK) plasma, both emitted
by similar amounts of emission measure. The cool plasma confirms the picture of
a soft excess in the form of an enhanced OVII/OVIII Lya flux ratio, similar to
that previously reported for other accreting TTS. Diagnostics from lines formed
by this plasma indicate low electron densities (<~ 1E10 cm-3). The Ne/Fe
abundance ratio is consistent with a trend in pre-main sequence stars in which
this ratio depends on spectral type, but not on accretion. On the basis of line
density diagnostics, we conclude that the density of the cool ``soft-excess''
plasma is orders of magnitude below that predicted for an accretion shock,
assuming previously determined accretion rates of (3-6)E-8 M_sun/y. We argue
that loading of magnetic field lines with infalling material suppresses the
heating process in a part of the corona. We thus suggest that the X-ray
production of T Tau is influenced by the accretion process although the X-rays
may not form in the bulk of the accretion footpoints.Comment: 12 pages, 7 figures, A&A style. Accepted by A&A, to appear in a
special section/issue dedicated to the XMM-Newton Extended Survey of the
Taurus Molecular Cloud (XEST). See also
http://www.issibern.ch/teams/Taurus/papers.htm
Species measurements in a hypersonic, hydrogen-air, combustion wake
A continuously sampling, time-of-flight mass spectrometer has been used to measure relative species concentrations in a two-dimensional, hydrogen-air combustion wake at mainstream Mach numbers exceeding 5. The experiments, which were conducted in a free piston shock tunnel, yielded distributions of hydrogen, oxygen, nitrogen, water and nitric oxide at stagnation enthalpies ranging from 5.6 MJ kg(exp -1) to 1.2 MJ kg(exp -1) and at a distance of approximately 100 times the thickness of the initial hydrogen jet. The amount of hydrogen that was mixed in stoichiometric proportions was approximately independent of the stagnation enthalpy, in spite of the fact that the proportion of hydrogen in the wake increased with stagnation enthalpy. Roughly 50 percent of the mixed hydrogen underwent combustion at the highest enthalpy. The proportion of hydrogen reacting to water could be approximately predicted using reaction rates based on mainstream temperatures
Mass spectrometer measurements of test gas composition in a shock tunnel
Shock tunnels afford a means of generating hypersonic flow at high stagnation enthalpies, but they have the disadvantage that thermochemical effects make the composition of the test flow different to that of ambient air. The composition can be predicted by numerical calculations of the nozzle flow expansion, using simplified thermochemical models and, in the absence of experimental measurements, it has been necessary to accept the results given by these calculations. This note reports measurements of test gas composition, at stagnation enthalpies up to 12.5 MJ.kg(exp -1), taken with a time-of-flight mass spectrometer. Limited results have been obtained in previous measurements. These were taken at higher stagnation enthalpies, and used a quadruple mass spectrometer. The time-of-flight method was preferred here because it enabled a number of complete mass spectra to be obtained in each test, and because it gives good mass resolution over the range of interest with air (up to 50 a.m.a.)
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