1,893 research outputs found
A Systematic Search for Supersoft X-Ray Sources in the ROSAT All-Sky Survey
We have conducted a systematic search for supersoft X-ray sources using the
ROSAT all-sky survey data. With the optical identification of the selected
sources being almost complete, we discuss the statistics of the various source
classes and their observability. Besides supersoft close binary sources this
search also can be used to estimate the number of isolated neutron stars in the
Galaxy, such as those described by Stocke et al. 1995 and Walter et al. 1996.Comment: 4 pages postscript, Proc. of Workshop on Supersoft X-Ray Sources, to
appear in Lecture Notes in Physics vol. 472 (1996
A New Giant Stellar Structure in the Outer Halo of M31
The Sloan Digital Sky Survey has revealed an overdensity of luminous red
giant stars ~ 3 degrees (40 projected kpc) to the northeast of M31, which we
have called Andromeda NE. The line-of-sight distance to Andromeda NE is within
approximately 50 kpc of M31; Andromeda NE is not a physically unrelated
projection. Andromeda NE has a g-band absolute magnitude of ~ -11.6 and central
surface brightness of ~ 29 mag/sq.arcsec, making it nearly two orders of
magnitude more diffuse than any known Local Group dwarf galaxy at that
luminosity. Based on its distance and morphology, Andromeda NE is likely
undergoing tidal disruption. Andromeda NE's red giant branch color is unlike
that of M31's present-day outer disk or the stellar stream reported by Ibata et
al. (2001), arguing against a direct link between Andromeda NE and these
structures. However, Andromeda NE has a red giant branch color similar to that
of the G1 clump; it is possible that these structures are both material torn
off of M31's disk in the distant past, or that these are both part of one
ancient stellar stream.Comment: 11 pages, 3 figures; ApJ Letters accepted versio
Earth incorporated: centralization and variegation in the global company network
Over the past 20 years, a widening gulf has appeared between the increasingly internationalized financing arrangements of the world’s leading corporations, and the persistence of nationally compartmentalized approaches to the study of corporate control. In lieu of direct empirical evidence on corporate control at the global level, the most widespread assumption is that the globalization of ownership has taken the form of an expansion of arms-length, market-based arrangements traditionally prevailing in the Anglo-American economies. Here, however, we challenge this assumption, both empirically and conceptually. Empirically, we show that three quarters of the world’s 205 largest firms by sales are linked to a single global company network of concentrated (5%) ownership ties. This network has a hierarchically centralized organization, with a dominant “global network core” of US fund managers ringed by a more geographically diverse “state capitalist periphery.” Conceptually, we argue that the this architecture can be broadly explained through a Polanyian “variegated capitalist” model of contradictory market institutionalization, with the formation of the global company network actually a counterintuitive product of global financial marketization. In order to understand this process of network formation, however, it is necessary to extend Polanyi’s model of a double movement mediated through political interventions in the market, to incorporate Veblenian processes of evolutionary institutional change mediated through the market
The Birth of an Ultra-Luminous X-ray Source in M83
A previously undetected X-ray source (L_X<10**36 erg/s) in the strongly
star-forming galaxy M83 entered an ultraluminous state between August 2009 and
December 2010. It was first seen with Chandra on 23 December 2010 at L_X ~ 4
10**39 ergs/s, and has remained ultraluminous through our most recent
observations in December 2011, with typical flux variation of a factor of two.
The spectrum is well fitted by a combination of absorbed power-law and disk
black-body models. While the relative contributions of the models varies with
time, we have seen no evidence for a canonical state transition. The luminosity
and spectral properties are consistent with accretion powered by a black hole
with M_BH ~ 40-100 solar masses. In July 2011 we found a luminous, blue optical
counterpart which had not been seen in deep HST observations obtained in August
2009. These optical observations suggest that the donor star is a low-mass star
undergoing Roche-lobe overflow, and that the blue optical emission seen during
the outburst is coming from an irradiated accretion disk. This source shows
that ultraluminous X-ray sources (ULXs) with low-mass companions are an
important component of the ULX population in star-forming galaxies, and
provides further evidence that the blue optical counterparts of some ULXs need
not indicate a young, high-mass companion, but rather that they may indicate
X-ray reprocessing.Comment: 40 pages, 7 figures, accepted for publication in Ap
Accretion-powered chromospheres in classical T Tauri stars
(Abridged) Optical spectra of classical T Tauri stars (cTTS) are rich in
emission lines of low-excitation species that are composed of narrow and broad
components, related to two regions with different kinematics, densities, and
temperatures. The photospheric spectrum is often veiled by an excess continuous
emission. This veiling is usually attributed to radiation from a heated region
beneath the accretion shock. The aim of this research is to clarify the nature
of the veiling, and whether the narrow chromospheric lines of Fe I and other
metals represent a standard chromosphere of a late-type star, or are induced by
mass accretion. From high-resolution spectroscopy of DR Tauri we found that the
amount of veiling in this star varies from practically nothing to factors more
than 10 times the stellar continuum intensity, and that the veiling is caused
by both a non-photospheric continuum and chromospheric line emission filling in
the photospheric absorption lines. This effect can be shown to exist in several
other T Tauri stars. We conclude that enhanced chromospheric emission in cTTS
is linked not only to solar-like magnetic activity, but is powered to a greater
extent by the accreting gas. We suggest that the area of enhanced chromospheric
emission is induced by mass accretion, which modifies the local structure of
stellar atmosphere in an area that is more extended than the hot accretion
spot. The narrow emission lines from this extended area are responsible for the
extra component in the veiling through line-filling of photospheric absorption
lines.Comment: 11 pages, 13 figure
New insights on the Draco dwarf spheroidal galaxy from SDSS: a larger radius and no tidal tails
We have investigated the spatial extent and structure of the Draco dwarf
spheroidal galaxy using deep wide-field multicolor CCD photometry from the
Sloan Digital Sky Survey (SDSS). Our study covers an area of 27 square degrees
around the center of the Draco dwarf. We show that the spatial distribution of
Draco's red giants, red horizontal branch stars and subgiants down to i=21.7
mag does not provide evidence for the existence of tidally induced tails or a
halo of unbound stars. The radial profile can be fit by King models as well as
by a generalized exponential. The core radius and the limiting (or tidal)
radius along the major axis are 7.7' and 40.1', respectively, making Draco 40%
larger than previously measured. Down to our magnitude limit tidal effects can
only exist at a level of 10^-3 of the central surface density of Draco or
below. The regular structure of Draco found from the new data argues against it
being a portion of an unbound tidal stream and lends support to the assumption
of dynamical equilibrium. We estimate Draco's total mass to be 2.2 - 3.5 times
10^7 solar masses. We obtain an overall mass-to-light ratio of 146+-42 or
92+-28 solar masses depending on the details of the mass and luminosity
estimates. In summary, our results strengthen the case for a strongly dark
matter dominated, bound stellar system. (Abstract strongly abridged).Comment: 30 pages, 11 figures (in part with degraded resolution). Accepted for
publication in the Astronomical Journa
Maternal Malaria and Gravidity Interact to Modify Infant Susceptibility to Malaria
BACKGROUND: In endemic areas, placental malaria due to Plasmodium falciparum is most frequent and severe in first-time mothers, and increases the risk of infant mortality in their offspring. Placental malaria may increase the susceptibility of infants to malaria parasitemia, but evidence for this effect is inconclusive. METHODS AND FINDINGS: During 2002–2004, we monitored parasitemia in 453 infants, including 69 who were born to mothers with placental malaria, in a region of northeastern Tanzania where malaria transmission is intense. We used a Cox proportional hazards model to evaluate the time from birth to first parasitemia, and a generalized estimating equations logistic regression model to evaluate risk of any parasitemia throughout the first year of life. Compared with infants whose mothers did not have placental malaria at delivery (“PM-negative”), offspring of mothers with placental malaria at delivery (“PM-positive”) were 41% more likely to experience their first parasitemia at a younger age (adjusted hazard ratio [AHR] = 1.41, 95% confidence interval [CI] 1.01–1.99). The odds of parasitemia throughout infancy were strongly modified by the interaction between placental malaria and gravidity (p for interaction = 0.008, Type 3 likelihood ratio test). Offspring of PM-negative primigravidae had lower odds of parasitemia during infancy (adjusted odds ratio [AOR] = 0.67, 95% CI 0.50–0.91) than offspring of PM-negative multigravidae, and offspring of PM-positive primigravidae had the lowest odds (AOR = 0.21, 95% CI 0.09–0.47). In contrast, offspring of PM-positive multigravidae had significantly higher odds of parasitemia (AOR = 1.59, 95% CI 1.16–2.17). CONCLUSION: Although parasitemia is more frequent in primigravid than multigravid women, the converse is true in their offspring, especially in offspring of PM-positive women. While placental malaria is known to increase mortality risk for first-born infants, it surprisingly reduced their risk of parasitemia in this study. Placental malaria of multigravidae, on the other hand, is a strong risk factor for parasitemia during infancy, and therefore preventive antimalarial chemotherapy administered to multigravid women close to term may reduce the frequency of parasitemia in their offspring
Is type II diabetes mellitus (NIDDM) a surgical disease?
Since February 1, 1980, 515 morbidly obese patients have undergone
the Greenville gastric bypass (GGB) operation. Of these,
212 (41.2%) were euglycemic, 288 (55.9%) were either diabetic
or had glucose intolerance, and 15 (2.9%) were unable to complete
the evaluation. After the operation, only 30 (5.8%) patients remained
diabetic (and 20 of these improved), 457 (88.7%) became
and have remained euglycemic, and inadequate data prevented
classification of the other 28 (5.4%). The patients who failed to
return to normal glucose values were older and their diabetes
was of longer duration than those who did. The effect of the
GGB was not only limited to the correction of abnormal glucose
levels. The GGB also corrected the abnormal levels of fasting
insulin and glycosylated hemoglobin in a cohort of 52 consecutive
severely obese patients with non-insulin-dependent diabetes. The
GGB effectively controls weight. If morbid obesity is defined as
100 pounds over ideal body weight, 89% of the patients are no
longer "morbidly" obese within 2 years. In most patients, the
control of the weight has been well maintained during the 11
years of follow-up; most of the upward creep in weight of 20.8%
between 24 and 132 months was from the 49 (9.5%) patients
who had staple line breakdowns between the large and small
gastric pouches. Non-insulin-dependent diabetes, previously
considered a chronic unrelenting disease, can be controlled in
the severely obese by the gastric bypass. Whether the correction
of glucose metabolism affects the complications of diabetes is
unknown. Whether the gastric bypass should be considered for
patients with advanced non-insulin-dependent diabetes but who
are not severely obese deserves consideration. The GGB has an
unacceptably high rate of staple line failure. Accordingly, the
authors have recently changed their procedure to one that divides
the stomach rather than partitions it with staples. Originally published Annals of Surgery, Vol. 215, No. 6, June 199
Sloan Digital Sky Survey Imaging of Low Galactic Latitude Fields: Technical Summary and Data Release
The Sloan Digital Sky Survey (SDSS) mosaic camera and telescope have obtained
five-band optical-wavelength imaging near the Galactic plane outside of the
nominal survey boundaries. These additional data were obtained during
commissioning and subsequent testing of the SDSS observing system, and they
provide unique wide-area imaging data in regions of high obscuration and star
formation, including numerous young stellar objects, Herbig-Haro objects and
young star clusters. Because these data are outside the Survey regions in the
Galactic caps, they are not part of the standard SDSS data releases. This paper
presents imaging data for 832 square degrees of sky (including repeats), in the
star-forming regions of Orion, Taurus, and Cygnus. About 470 square degrees are
now released to the public, with the remainder to follow at the time of SDSS
Data Release 4. The public data in Orion include the star-forming region NGC
2068/NGC 2071/HH24 and a large part of Barnard's loop.Comment: 31 pages, 9 figures (3 missing to save space), accepted by AJ, in
press, see http://photo.astro.princeton.edu/oriondatarelease for data and
paper with all figure
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