290 research outputs found
Testing a Predictive Theoretical Model for the Mass Loss Rates of Cool Stars
The basic mechanisms responsible for producing winds from cool, late-type
stars are still largely unknown. We take inspiration from recent progress in
understanding solar wind acceleration to develop a physically motivated model
of the time-steady mass loss rates of cool main-sequence stars and evolved
giants. This model follows the energy flux of magnetohydrodynamic turbulence
from a subsurface convection zone to its eventual dissipation and escape
through open magnetic flux tubes. We show how Alfven waves and turbulence can
produce winds in either a hot corona or a cool extended chromosphere, and we
specify the conditions that determine whether or not coronal heating occurs.
These models do not utilize arbitrary normalization factors, but instead
predict the mass loss rate directly from a star's fundamental properties. We
take account of stellar magnetic activity by extending standard
age-activity-rotation indicators to include the evolution of the filling factor
of strong photospheric magnetic fields. We compared the predicted mass loss
rates with observed values for 47 stars and found significantly better
agreement than was obtained from the popular scaling laws of Reimers,
Schroeder, and Cuntz. The algorithm used to compute cool-star mass loss rates
is provided as a self-contained and efficient computer code. We anticipate that
the results from this kind of model can be incorporated straightforwardly into
stellar evolution calculations and population synthesis techniques.Comment: 23 pages (emulateapj style), 14 figures, ApJ, in press. A brief IDL
subroutine that implements the model described in this paper will be
distributed as "online-only material," and this code is also available at
http://www.cfa.harvard.edu/~scranmer/cranmer_data.htm
Candidate X-ray-Emitting OB Stars in the Carina Nebula Identified Via Infrared Spectral Energy Distributions
We report the results of a new survey of massive, OB stars throughout the
Carina Nebula using the X-ray point source catalog provided by the Chandra
Carina Complex Project (CCCP) in conjunction with infrared (IR) photometry from
the Two Micron All-Sky Survey and the Spitzer Space Telescope Vela--Carina
survey. Mid-IR photometry is relatively unaffected by extinction, hence it
provides strong constraints on the luminosities of OB stars, assuming that
their association with the Carina Nebula, and hence their distance, is
confirmed. We fit model stellar atmospheres to the optical (UBV) and IR
spectral energy distributions (SEDs) of 182 OB stars with known spectral types
and measure the bolometric luminosity and extinction for each star. We find
that the extinction law measured toward the OB stars has two components:
Av=1--1.5 mag produced by foreground dust with a ratio of total-to-selective
absorption Rv=3.1 plus a contribution from local dust with Rv>4.0 in the Carina
molecular clouds that increases as Av increases. Using X-ray emission as a
strong indicator of association with Carina, we identify 94 candidate OB stars
with Lbol\geq10^4 Lsun by fitting their IR SEDs. If the candidate OB stars are
eventually confirmed by follow-up spectroscopic observations, the number of
cataloged OB stars in the Carina Nebula will increase by ~50%. Correcting for
incompleteness due to OB stars falling below the Lbol cutoff or the CCCP
detection limit, these results potentially double the size of the young massive
stellar population.Comment: 19 pages, 8 figures, accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers, including a version of this article with
high-quality figures, are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html (through 2011
at least
GRIPS - Gamma-Ray Imaging, Polarimetry and Spectroscopy
We propose to perform a continuously scanning all-sky survey from 200 keV to
80 MeV achieving a sensitivity which is better by a factor of 40 or more
compared to the previous missions in this energy range. The Gamma-Ray Imaging,
Polarimetry and Spectroscopy (GRIPS) mission addresses fundamental questions in
ESA's Cosmic Vision plan. Among the major themes of the strategic plan, GRIPS
has its focus on the evolving, violent Universe, exploring a unique energy
window. We propose to investigate -ray bursts and blazars, the
mechanisms behind supernova explosions, nucleosynthesis and spallation, the
enigmatic origin of positrons in our Galaxy, and the nature of radiation
processes and particle acceleration in extreme cosmic sources including pulsars
and magnetars. The natural energy scale for these non-thermal processes is of
the order of MeV. Although they can be partially and indirectly studied using
other methods, only the proposed GRIPS measurements will provide direct access
to their primary photons. GRIPS will be a driver for the study of transient
sources in the era of neutrino and gravitational wave observatories such as
IceCUBE and LISA, establishing a new type of diagnostics in relativistic and
nuclear astrophysics. This will support extrapolations to investigate star
formation, galaxy evolution, and black hole formation at high redshifts.Comment: to appear in Exp. Astron., special vol. on M3-Call of ESA's Cosmic
Vision 2010; 25 p., 25 figs; see also www.grips-mission.e
Turbulence-driven Polar Winds from T Tauri Stars Energized by Magnetospheric Accretion
Pre-main-sequence stars are observed to be surrounded by both accretion flows
and some kind of wind or jet-like outflow. Recent work by Matt and Pudritz has
suggested that if classical T Tauri stars exhibit stellar winds with mass loss
rates about 0.1 times their accretion rates, the wind can carry away enough
angular momentum to keep the stars from being spun up unrealistically by
accretion. This paper presents a preliminary set of theoretical models of
accretion-driven winds from the polar regions of T Tauri stars. These models
are based on recently published self-consistent simulations of the Sun's
coronal heating and wind acceleration. In addition to the convection-driven MHD
turbulence (which dominates in the solar case), we add another source of wave
energy at the photosphere that is driven by the impact of plasma in neighboring
flux tubes undergoing magnetospheric accretion. This added energy, determined
quantitatively from the far-field theory of MHD wave generation, is sufficient
to produce T Tauri-like mass loss rates of at least 0.01 times the accretion
rate. While still about an order of magnitude below the level required for
efficient angular momentum removal, these are the first self-consistent models
of T Tauri winds that agree reasonably well with a range of observational mass
loss constraints. The youngest modeled stellar winds are supported by Alfven
wave pressure, they have low temperatures ("extended chromospheres"), and they
are likely to be unstable to the formation of counterpropagating shocks and
clumps far from the star.Comment: 19 pages (emulateapj style), 13 figures, ApJ, in press (v. 689,
December 10, 2008
Genome-Wide Association Meta-analysis of Neuropathologic Features of Alzheimer's Disease and Related Dementias
Alzheimer's disease (AD) and related dementias are a major public health challenge and present a therapeutic imperative for which we need additional insight into molecular pathogenesis. We performed a genome-wide association study and analysis of known genetic risk loci for AD dementia using neuropathologic data from 4,914 brain autopsies. Neuropathologic data were used to define clinico-pathologic AD dementia or controls, assess core neuropathologic features of AD (neuritic plaques, NPs; neurofibrillary tangles, NFTs), and evaluate commonly co-morbid neuropathologic changes: cerebral amyloid angiopathy (CAA), Lewy body disease (LBD), hippocampal sclerosis of the elderly (HS), and vascular brain injury (VBI). Genome-wide significance was observed for clinico-pathologic AD dementia, NPs, NFTs, CAA, and LBD with a number of variants in and around the apolipoprotein E gene (APOE). GalNAc transferase 7 (GALNT7), ATP-Binding Cassette, Sub-Family G (WHITE), Member 1 (ABCG1), and an intergenic region on chromosome 9 were associated with NP score; and Potassium Large Conductance Calcium-Activated Channel, Subfamily M, Beta Member 2 (KCNMB2) was strongly associated with HS. Twelve of the 21 non-APOE genetic risk loci for clinically-defined AD dementia were confirmed in our clinico-pathologic sample: CR1, BIN1, CLU, MS4A6A, PICALM, ABCA7, CD33, PTK2B, SORL1, MEF2C, ZCWPW1, and CASS4 with 9 of these 12 loci showing larger odds ratio in the clinico-pathologic sample. Correlation of effect sizes for risk of AD dementia with effect size for NFTs or NPs showed positive correlation, while those for risk of VBI showed a moderate negative correlation. The other co-morbid neuropathologic features showed only nominal association with the known AD loci. Our results discovered new genetic associations with specific neuropathologic features and aligned known genetic risk for AD dementia with specific neuropathologic changes in the largest brain autopsy study of AD and related dementias
Feasibility study of computed tomography colonography using limited bowel preparation at normal and low-dose levels study
The purpose was to evaluate low-dose CT colonography without cathartic cleansing in terms of image quality, polyp visualization and patient acceptance. Sixty-one patients scheduled for colonoscopy started a low-fiber diet, lactulose and amidotrizoic-acid for fecal tagging 2 days prior to the CT scan (standard dose, 5.8–8.2 mSv). The original raw data of 51 patients were modified and reconstructed at simulated 2.3 and 0.7 mSv levels. Two observers evaluated the standard dose scan regarding image quality and polyps. A third evaluated the presence of polyps at all three mSv levels in a blinded prospective way. All observers were blinded to the reference standard: colonoscopy. At three times patients were given questionnaires relating to their experiences and preference. Image quality was sufficient in all patients, but significantly lower in the cecum, sigmoid and rectum. The two observers correctly identified respectively 10/15 (67%) and 9/15 (60%) polyps ≥10 mm, with 5 and 8 false-positive lesions (standard dose scan). Dose reduction down to 0.7 mSv was not associated with significant changes in diagnostic value (polyps ≥10 mm). Eighty percent of patients preferred CT colonography and 13% preferred colonoscopy (P<0.001). CT colonography without cleansing is preferred to colonoscopy and shows sufficient image quality and moderate sensitivity, without impaired diagnostic value at dose-levels as low as 0.7 mSv
A Statistical Study of Threshold Rotation Rates for the Formation of Disks around Be Stars
This paper presents a detailed statistical determination of the equatorial
rotation rates of classical Be stars. The rapid rotation of Be stars is likely
to be linked to the ejection of gas that forms dense circumstellar disks. The
physical origins of these disks are not understood, though it is generally
believed that the ability to spin up matter into a Keplerian disk depends on
how close the stellar rotation speed is to the critical speed at which the
centrifugal force cancels gravity. There has been recent disagreement between
the traditional idea that Be stars rotate between 50 and 80 percent of their
critical speeds and new ideas (inspired by the tendency for gravity darkening
to mask rapid rotation at the equator) that their rotation may be very nearly
critical. This paper utilizes Monte Carlo forward modeling to simulate
distributions of the projected rotation speed (v sin i), taking into account
gravity darkening, limb darkening, and observational uncertainties. A
chi-squared minimization procedure was used to find the distribution parameters
that best reproduce observed v sin i distributions from R. Yudin's database.
Early-type (O7e-B2e) Be stars were found to exhibit a roughly uniform spread of
intrinsic rotation speed that extends from 40 to 60 percent up to 100 percent
of critical. Late-type (B3e-A0e) Be stars exhibit progressively narrower ranges
of rotation speed as the effective temperature decreases; the lower limit rises
to reach critical rotation for the coolest Be stars. The derived lower limits
on equatorial rotation speed represent conservative threshold rotation rates
for the onset of the Be phenomenon. The significantly subcritical speeds found
for early-type Be stars represent strong constraints on physical models of
angular momentum deposition in Be star disks.Comment: 36 pages (AASTeX), 11 figures, Ap. J., in press (November 20, 2005
A candidate regulatory variant at the TREM gene cluster associates with decreased Alzheimer's disease risk and increased TREML1 and TREM2 brain gene expression
Introduction: We hypothesized that common Alzheimer's disease (AD)-associated variants within the triggering receptor expressed on myeloid (TREM) gene cluster influence disease through gene expression.
Methods: Expression microarrays on temporal cortex and cerebellum from ∼400 neuropathologically diagnosed subjects and two independent RNAseq replication cohorts were used for expression quantitative trait locus analysis.
Results: A variant within a DNase hypersensitive site 5′ of TREM2, rs9357347-C, associates with reduced AD risk and increased TREML1 and TREM2 levels (uncorrected P = 6.3 × 10−3 and 4.6 × 10−2, respectively). Meta-analysis on expression quantitative trait locus results from three independent data sets (n = 1006) confirmed these associations (uncorrected P = 3.4 × 10−2 and 3.5 × 10−3, Bonferroni-corrected P = 6.7 × 10−2 and 7.1 × 10−3, respectively).
Discussion: Our findings point to rs9357347 as a functional regulatory variant that contributes to a protective effect observed at the TREM locus in the International Genomics of Alzheimer's Project genome-wide association study meta-analysis and suggest concomitant increase in TREML1 and TREM2 brain levels as a potential mechanism for protection from AD
Excessive Biologic Response to IFNβ Is Associated with Poor Treatment Response in Patients with Multiple Sclerosis
Interferon-beta (IFNβ) is used to inhibit disease activity in multiple sclerosis (MS), but its mechanisms of action are incompletely understood, individual treatment response varies, and biological markers predicting response to treatment have yet to be identified.he relationship between the molecular response to IFNβ and treatment response was determined in 85 patients using a longitudinal design in which treatment effect was categorized by brain magnetic resonance imaging as good (n = 70) or poor response (n = 15). Molecular response was quantified using a customized cDNA macroarray assay for 166 IFN-regulated genes (IRGs).The molecular response to IFNβ differed significantly between patients in the pattern and number of regulated genes. The molecular response was strikingly stable for individuals for as long as 24 months, however, suggesting an individual ‘IFN response fingerprint’. Unexpectedly, patients with poor response showed an exaggerated molecular response. IRG induction ratios demonstrated an exaggerated molecular response at both the first and 6-month IFNβ injections.MS patients exhibit individually unique but temporally stable biological responses to IFNβ. Poor treatment response is not explained by the duration of biological effects or the specific genes induced. Rather, individuals with poor treatment response have a generally exaggerated biological response to type 1 IFN injections. We hypothesize that the molecular response to type I IFN identifies a pathogenetically distinct subset of MS patients whose disease is driven in part by innate immunity. The findings suggest a strategy for biologically based, rational use of IFNβ for individual MS patients
The complex genetics of gait speed:Genome-wide meta-analysis approach
Emerging evidence suggests that the basis for variation in late-life mobility is attributable, in part, to genetic factors, which may become increasingly important with age. Our objective was to systematically assess the contribution of genetic variation to gait speed in older individuals. We conducted a meta-analysis of gait speed GWASs in 31,478 older adults from 17 cohorts of the CHARGE consortium, and validated our results in 2,588 older adults from 4 independent studies. We followed our initial discoveries with network and eQTL analysis of candidate signals in tissues. The meta-analysis resulted in a list of 536 suggestive genome wide significant SNPs in or near 69 genes. Further interrogation with Pathway Analysis placed gait speed as a polygenic complex trait in five major networks. Subsequent eQTL analysis revealed several SNPs significantly associated with the expression of PRSS16, WDSUB1 and PTPRT, which in addition to the meta-analysis and pathway suggested that genetic effects on gait speed may occur through synaptic function and neuronal development pathways. No genome-wide significant signals for gait speed were identified from this moderately large sample of older adults, suggesting that more refined physical function phenotypes will be needed to identify the genetic basis of gait speed in aging
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