1,186 research outputs found
Testing Binary Population Synthesis Models with Hot Subdwarfs
Models of binary star interactions have been successful in explaining the
origin of field hot subdwarf (sdB) stars in short period systems, but
longer-period systems that formed via Roche-lobe overflow (RLOF) mass transfer
from the present sdB to its companion have received less attention. We map sets
of initial binaries into present-day binaries that include sdBs and
main-sequence stars, distinguishing "observable" sdBs from "hidden" ones. We
aim to find out whether (1) the existing catalogues of sdBs are sufficiently
fair samples of all the kinds of sdB binaries that theory predicts; or instead
whether (2) large predicted hidden populations mandate the construction of new
catalogues, perhaps using wide-field imaging surveys such as 2MASS, SDSS, and
Galex. We also report on a pilot study to identify hidden subdwarfs, using
2MASS and GALEX data.Comment: 3 pages with 2 figures. Uses AIP style files. To appear in Future
Directions in Ultraviolet Astronomy, ed. Michael E. VanSteenberg (AIP Conf
Proc
A dynamical magnetosphere model for periodic Halpha emission from the slowly rotating magnetic O star HD191612
The magnetic O-star HD191612 exhibits strongly variable, cyclic Balmer line
emission on a 538-day period. We show here that its variable Halpha emission
can be well reproduced by the rotational phase variation of synthetic spectra
computed directly from full radiation magneto-hydrodynamical simulations of a
magnetically confined wind. In slow rotators such as HD191612, wind material on
closed magnetic field loops falls back to the star, but the transient
suspension of material within the loops leads to a statistically overdense, low
velocity region around the magnetic equator, causing the spectral variations.
We contrast such "dynamical magnetospheres" (DMs) with the more steady-state
"centrifugal magnetospheres" of stars with rapid rotation, and discuss the
prospects of using this DM paradigm to explain periodic line emission from also
other non-rapidly rotating magnetic massive stars.Comment: 5 pages, 5 figures, accepted for publication in MNRAS letter
Bisphenol A Data in NHANES Suggest Longer than Expected Half-Life, Substantial Nonfood Exposure, or Both
Reproduced with permission from Environmental Health Perspectives. doi:10.1289/ehp.0800376It is commonly stated in the literature on human exposure to bisphenol A (BPA) that food is the predominant BPA exposure source, and that BPA is rapidly and completely cleared from the body. If this is correct, BPA levels in fasting individuals should decrease with increased fasting time. We set out to investigate the relationship between urine BPA concentration and fastÂŹing time in a population-based sample. Overall, BPA levels did not decline rapidly with fasting time in this sample. This suggests substantial nonfood exposure, accumulation in body tissues such as fat, or both. Explaining these findings may require experimental pharmacokinetic studies of chronic BPA exposure, further examination of BPA levels and effects in fat, and a search for important nonfood sources.This work was supported by National Institute of Environmental Health Sciences Training Grant ES07026 and University of Rochester Environmental Health Sciences Center Grant ES01247
Biases in Expansion Distances of Novae Arising from the Prolate Geometry of Nova Shells
(abridged) Expansion distances (or expansion parallaxes) for classical novae
are based on comparing a measurement of the shell expansion velocity,
multiplied by the time since outburst, with some measure of the angular size of
the shell. We review and formalize this method in the case of prolate
spheroidal shells. We present expressions for the maximum line-of-sight
velocity from a complete, expanding shell and for its projected major and minor
axes, in terms of the intrinsic axis ratio and the inclination of the polar
axis to the line of sight. For six distinct definitions of ``angular size'', we
tabulate the error in distance that is introduced under the assumption of
spherical symmetry (i.e., without correcting for inclination and axis ratio).
The errors can be significant and systematic, affecting studies of novae
whether considered individually or statistically. Each of the six estimators
overpredicts the distance when the polar axis is close to the line of sight,
and most underpredict the distance when the polar axis is close to the plane of
the sky. The straight mean of the projected semimajor and semiminor axes gives
the least distance bias for an ensemble of randomly oriented prolate shells.
The best individual expansion distances, however, result from a full
spatio-kinematic modeling of the nova shell. We discuss several practical
complications that affect expansion distance measurements of real nova shells.
Nova shell expansion distances be based on velocity and angular size
measurements made contemporaneously if possible, and the same ions and
transitions should be used for the imaging and velocity measurements. We
emphasize the need for complete and explicit reporting of measurement
procedures and results, regardless of the specific method used.Comment: 21 pages, LaTeX, uses aasms4.sty, to be published in Publ. Astron.
Soc. of the Pacific, May 200
A Spectroscopic Reconnaissance of UV-Bright Stars
We have carried out spectroscopic observations and made preliminary
classifications of 62 UV-bright stars identified by Lanning on plates taken by
A. Sandage. The goal was to search for "interesting" objects, such as
emission-line stars, hot sub-dwarfs, and high-gravity stars. Our targets were
grouped into two samples, a bright, B < 13, sample of 35 stars observed with
the Kitt Peak 2.1m telescope and a faint, 13< B < 16, sample of 27 stars
observed with the Hobby-Eberly Telescope. We find 39% fairly normal O-mid B
stars, 15% late ~B-late A stars and 32% F-G stars, with 13% of the stars being
high gravity objects, composite, or otherwise peculiar. Included are four
emission-line stars, three composite systems. Thus one out of every ten Lanning
stars is "interesting" and may deserve individual study. Stars in the bright
sample are often found to be late F or early G stars, although this sample does
include interesting stars as well. No such large contamination occurs among the
fainter stars, however, owing to "deselection" of these stars by interstellar
reddening in the low-latitude fields of the survey.Comment: 9 pages in total, to appear in February 2002 issue of P.A.S.
Ciliate microzooplankton from the Northeastern Gulf of Mexico
Microzooplankton mediate a critical juncture of autotrophic and heterotrophic microbial production in the water column. Taxonomic and ecological work on this group has been substantial, yet few reports exist for the offshore waters of the Gulf of Mexico (GOM). This report focuses on protists in the phylum Ciliophora collected at stations spanning the continental shelf in the northeastern GOM. We hypothesized that patterns of spatial distribution across the region would be westâeast along the coast, rather than northâsouth coastal to offshore, reflecting major freshwater sources. Samples were obtained by 10â”m plankton net for microscopy and by filtration of seawater for DNA extraction and ciliate-specific clone sequencing. Microscopy and molecular analysis recovered 46 and 156 taxa, respectively. Some visually identified taxa were missing from the sequence analysis and sequences from unknown species dominated molecular results. Differences were apparent with both dominant and rare taxa between February and July sampling and across a trophic gradient from coastal influenced stations to those more representative of the offshore environment. This report provides new data on ciliate microzooplankton richness and distribution in the GOM and adds to our understanding of microzooplankton diversity in the ocean
The Post-Common Envelope and Pre-Cataclysmic Binary PG 1224+309
We have made extensive spectroscopic and photometric observations of PG
1224+309, a close binary containing a DA white dwarf primary and an M4+
secondary. The H alpha line is in emission due to irradiation of the M-star by
the hot white dwarf and is seen to vary around the orbit. From the radial
velocities of the H alpha line we derive a period of P = 0.258689 +/- 0.000004
days and a semi-amplitude of K_Halpha = 160 +/- 8 km/s. We estimate a
correction Delta_K = 21 +/- 2 km/s, where K_M = K_Halpha + Delta_K. Radial
velocity variations of the white dwarf reveal a semi-amplitude of K_WD = 112
+/- 14 km/s. The blue spectrum of the white dwarf is well fit by a synthetic
spectrum having T_eff = 29,300 K and log(g) = 7.38. The white dwarf contributes
97% of the light at 4500 Angstroms and virtually all of the light blueward of
3800 Angstroms. No eclipses are observed. The mass inferred for the white dwarf
depends on the assumed mass of the thin residual hydrogen envelope: 0.40 < M_WD
< 0.45 solar masses for hydrogen envelope masses of 0 < M_H < 4.0E-4 solar
masses. We argue that the mass of the white dwarf is closer to 0.45 solar
masses, hence it appears that the white dwarf has a relatively large residual
hydrogen envelope. The mass of the M-star is then M_M = 0.28 +/- 0.05 solar
masses, and the inclination is i = 77 +/- 7 degrees. We discuss briefly how PG
1224+309 may be used to constrain theories of close binary star evolution, and
the past and future histories of PG 1224+309 itself. The star is both a
``post-common envelope'' star and a ``pre-cataclysmic binary'' star. Mass
transfer by Roche-lobe overflow should commence in about 10 Gyr.Comment: 17 pages, 8 figures, AAS LaTeX, to appear in AJ, March 199
A Radial Velocity Study of Composite-Spectra Hot Subdwarf Stars with the Hobby-Eberly Telescope
Many hot subdwarf stars show composite spectral energy distributions
indicative of cool main sequence companions. Binary population synthesis (BPS)
models demonstrate such systems can be formed via Roche lobe overflow or common
envelope evolution but disagree on whether the resulting orbital periods will
be long (years) or short (days). Few studies have been carried out to assess
the orbital parameters of these spectroscopic composite binaries; current
observations suggest the periods are long. To help address this problem, we
selected fifteen moderately-bright (V~13) hot subdwarfs with F-K dwarf
companions and monitored their radial velocities (RVs) from January 2005 to
July 2008 using the bench-mounted Medium Resolution Spectrograph on the
Hobby-Eberly Telescope (HET). Here we describe the details of our observing,
reduction, and analysis techniques and present preliminary results for all
targets. By combining the HET data with recent observations from the Mercator
telescope, we are able to calculate precise orbital solutions for three systems
using more than 6 years of observations. We also present an up-to-date period
histogram for all known hot subdwarf binaries, which suggests those with F-K
main sequence companions tend to have orbital periods on the order of several
years. Such long periods challenge the predictions of conventional BPS models,
although a larger sample is needed for a thorough assessment of the models'
predictive success. Lastly, one of our targets has an eccentric orbit, implying
some composite-spectrum systems might have formerly been hierarchical triple
systems, in which the inner binary merged to create the hot subdwarf.Comment: Published in The Astrophysical Journal, Volume 758, Issue 1, article
id. 58 (2012). References updated and Equation (5) corrected. 12 pages, 5
figures, 5 table
Automorphisms of graphs of cyclic splittings of free groups
We prove that any isometry of the graph of cyclic splittings of a finitely
generated free group of rank is induced by an outer automorphism
of . The same statement also applies to the graphs of maximally-cyclic
splittings, and of very small splittings.Comment: 22 pages, 5 figures. Small modifications. To appear in Geometriae
Dedicat
Variation of all-cause and cause-specific mortality with body mass index in one million Swedish parent-son pairs:An instrumental variable analysis
BackgroundHigh body mass index (BMI) is associated with mortality, but the pervasive problem of confounding and reverse causality in observational studies limits inference about the direction and magnitude of causal effects. We aimed to obtain estimates of the causal association of BMI with all-cause and cause-specific mortality.Methods and findingsIn a record-linked, intergenerational prospective study from the general population of Sweden, we used two-sample instrumental variable (IV) analysis with data from 996,898 fathers (282,407 deaths) and 1,013,083 mothers (153,043 deaths) and their sons followed up from January 1, 1961, until December 31, 2004. Sons' BMI was used as the instrument for parents' BMI to compute hazard ratios (HRs) for risk of mortality per standard deviation (SD) higher parents' BMI. Using offspring exposure as an instrument for parents' exposure is unlikely to be affected by reverse causality (an important source of bias in this context) and reduces confounding. IV analyses supported causal associations between higher BMI and greater risk of all-cause mortality (HR [95% confidence interval (CI)] per SD higher fathers' BMI: 1.29 [1.26-1.31] and mothers' BMI: 1.39 [1.35-1.42]) and overall cancer mortality (HR per SD higher fathers' BMI: 1.20 [1.16-1.24] and mothers' BMI: 1.29 [1.24-1.34]), including 9 site-specific cancers in men (bladder, colorectum, gallbladder, kidney, liver, lung, lymphatic system, pancreas, and stomach) and 11 site-specific cancers in women (gallbladder, kidney, liver, lung, lymphatic system, ovaries, pancreas, stomach, uterus, cervix, and endometrium). There was evidence supporting causal associations between higher BMI in mothers and greater risk of mortality from kidney disease (HR: 2.17 [1.68-2.81]) and lower risk of mortality from suicide (HR: 0.77 [0.65-0.90]). In both sexes, there was evidence supporting causal associations between higher BMI and mortality from cardiovascular diseases (CVDs), stroke, diabetes, and respiratory diseases. We were unable to test the association between sons' and mothers' BMIs (as mothers' data were unavailable) or whether the instrument was independent of unmeasured or residual confounding; however, the associations between parents' mortality and sons' BMI were negligibly influenced by adjustment for available confounders.ConclusionsConsistent with previous large-scale meta-analyses and reviews, results supported the causal role of higher BMI in increasing the risk of several common causes of death, including cancers with increasing global incidence. We also found positive effects of BMI on mortality from respiratory disease, prostate cancer, and lung cancer, which has been inconsistently reported in the literature, suggesting that the causal role of higher BMI in mortality from these diseases may be underestimated. Furthermore, we expect different patterns of bias in the current observational and IV analyses; therefore, the similarities between our findings from both methods increases confidence in the results. These findings support efforts to understand the mechanisms underpinning these effects to inform targeted interventions and develop population-based strategies to reduce rising obesity levels for disease prevention
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