39 research outputs found
An X-Ray Survey of the Open Cluster NGC 6475 (M7) with ROSAT
A ROSAT x-ray survey, with complimentary optical photometry, of the open cluster NGC 6475 has enabled the detection of approx. 50 late-F to K0 and approx. 70 K/M dwarf new candidate members, providing the first reliable detection of low-mass stars in this low. galactic latitude, 220 Myr old cluster. The x-ray observations reported here have a typical limiting sensitivity of L(sub x) approx. equal to 10(exp 29) erg/s. The detection frequency of early type cluster members is consistent with the hypothesis that the x-ray emitting early type stars are binary systems with an unseen, low-mass secondary producing the x rays. The ratio between x-ray and bolometric luminosity among NGC 6475 members saturates at a spectral-type/color which is intermediate between that in much younger and in much older clusters, consistent with rotational spindown of solar-type stars upon their arrival on the ZAMS. The upper envelope of x-ray luminosity as a function of spectral type is comparable to that of the Pleiades, with the observed spread in x-ray luminosity among low-mass members being likely due to the presence of binaries and relatively rapid rotators. However, the list of x-ray selected candidate members is likely biased against low-mass, slowly rotating single stars. While some preliminary spectroscopic information is given in an appendix, further spectroscopic observations of the new candidate members will aid in interpreting the coronal activity among solar-type NGC 6475 members and their relation to similar stars in older and younger open clusters
Simultaneous X-ray spectroscopy of YY Gem with Chandra and XMM-Newton
We report on a detailed study of the X-ray spectrum of the nearby eclipsing
spectroscopic binary YY Gem. Observations were obtained simultaneously with
both large X-ray observatories, XMM-Newton and Chandra. We compare the
high-resolution spectra acquired with the Reflection Grating Spectrometer
onboard XMM-Newton and with the Low Energy Transmission Grating Spectrometer
onboard Chandra, and evidence in direct comparison the good performance of both
instruments in terms of wavelength and flux calibration. The strongest lines in
the X-ray spectrum of YY Gem are from oxygen. Oxygen line ratios indicate the
presence of a low-temperature component (1-4 MK) with density n_e < 2 10^{10}
cm^-3. The X-ray lightcurve reveals two flares and a dip corresponding to the
secondary eclipse. An increase of the density during phases of high activity is
suggested from time-resolved spectroscopy. Time-resolved global fitting of the
European Photon Imaging Camera CCD spectrum traces the evolution of temperature
and emission measure during the flares. These medium-resolution spectra show
that temperatures > 10^7 K are relevant in the corona of YY Gem although not as
dominant as the lower temperatures represented by the strongest lines in the
high-resolution spectrum. Magnetic loops with length on the order of 10^9 cm,
i.e., about 5 % of the radius of each star, are inferred from a comparison with
a one-dimensional hydrodynamic model. This suggests that the flares did not
erupt in the (presumably more extended) inter-binary magnetosphere but are
related to one of the components of the binary.Comment: 15 pages, accepted for publication in A&
The Human Ecology and Geography of Burning in an Unstable Savanna Environment
According to new ecological theories, many savannas are inherently in disequilibrium and can flip from tree-dominated to grass-dominated landscapes depending upon the disturbance regime. In particular, a shift in a fire regime to a more frequent and intensive one can radically alter the tree-to-grass ratio in a given savanna. Drawing upon the ecological buffering model we argue that savanna persistence requires a relatively stable fire regime. We hypothesize that anthropogenic burning practices perform this function by producing a regular annual spatiotemporal pattern of fire that is linked to vegetation type. We test this hypothesis using a study of two areas, one in Mali and the other Burkina Faso. We use two sources of satellite data to produce an 11-year time series of the spatiotemporal pattern of fires and an example of the annual burned area pattern these fires produce. We combine the analysis of satellite imagery with interviews of rural inhabitants who set fires to understand the logic underlying the patterns of fire. Analysis of a time series of imagery reveals a strikingly regular annual spatiotemporal pattern of burning for both study areas, which cannot be explained by the regional climatic pattern alone. We conclude that the regularity of the annual fire regime in West Africa is a human-ecological phenomenon closely linked to vegetation type and controlled by people\u27s burning practices. We argue that the anthropogenic burning regime serves to buffer the savanna and maintain its ecological stability
Ages of A-type Vega-like stars from uvby Photometry
We have estimated the ages of a sample of A-type Vega-like stars by using
Str\"{o}mgren \emph{uvby$\beta} photometric data and theoretical evolutionary
tracks. We find that 13 percent of these A stars have been reported as
Vega-like stars in the literature and that the ages of this subset run the
gamut from very young (50~Myr) to old (1~Gyr), with no obvious age difference
compared to those of field A stars. We clearly show that the fractional IR
luminosity decreases with the ages of Vega-like stars.Comment: 4pages text, 3 tables, 3 figures, Accepted in Ap
FUSE observations of molecular hydrogen on the line of sight towards HD141569A
We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE)
spectrum of HD141569A, a transitional object known to possess a circumstellar
disk. We observe two components of gas at widely different temperatures along
the line of sight. We detect cold H2, which is thermalized up to J=2 at a
kinetic temperature of 51K. Such low temperatures are typical of the diffuse
interstellar medium. Since the line of sight to HD141569A does not pass through
its disk, it appears that we are observing the cold H2 in a low extinction
envelope associated with the high Galactic latitude dark cloud complex L134N,
which is in the same direction and at nearly the same distance as HD141569A.
The column densities of the higher J-levels of H2 suggest the presence of warm
gas along the line of sight. The excitation conditions do not seem to be
consistent with what is generally observed in diffuse interstellar clouds. The
observed radial velocity of the gas implies that the UV spectral lines we
observe are likely interstellar in origin rather than circumstellar, although
our absorption line study does not definitely rule out the possibility that the
warm gas is close to the star. The discovery of such warm gas along the line of
sight may provide evidence for turbulent phenomena in the dark cloud L134N.Comment: accepted for publication in A&
Automated in vivo approaches for characterizing HAB in coastal waters: the Jerico-Next European Research Infrastructure
A "hair-raising" history of alopecia areata
YesA 3500âyearâold papyrus from ancient Egypt provides a list of treatments for many diseases including âbite hair loss,â most likely alopecia areata (AA). The treatment of AA remained largely unchanged for over 1500 years. In 30 CE, Celsus described AA presenting as scalp alopecia in spots or the âwindings of a snakeâ and suggested treatment with caustic compounds and scarification. The first âmodernâ description of AA came in 1813, though treatment still largely employed caustic agents. From the midâ19th century onwards, various hypotheses of AA development were put forward including infectious microbes (1843), nerve defects (1858), physical trauma and psychological stress (1881), focal inflammation (1891), diseased teeth (1902), toxins (1912) and endocrine disorders (1913). The 1950s brought new treatment developments with the first use of corticosteroid compounds (1952), and the first suggestion that AA was an autoimmune disease (1958). Research progressively shifted towards identifying hair follicleâspecific autoantibodies (1995). The potential role of lymphocytes in AA was made implicit with immunohistological studies (1980s). However, studies confirming their functional role were not published until the development of rodent models (1990s). Genetic studies, particularly genomeâwide association studies, have now come to the forefront and open up a new era of AA investigation (2000s). Today, AA research is actively focused on genetics, the microbiome, dietary modulators, the role of atopy, immune cell types in AA pathogenesis, primary antigenic targets, mechanisms by which immune cells influence hair growth, and of course the development of new treatments based on these discoveries.Alopecia UK