515 research outputs found
Athena: A New Code for Astrophysical MHD
A new code for astrophysical magnetohydrodynamics (MHD) is described. The
code has been designed to be easily extensible for use with static and adaptive
mesh refinement. It combines higher-order Godunov methods with the constrained
transport (CT) technique to enforce the divergence-free constraint on the
magnetic field. Discretization is based on cell-centered volume-averages for
mass, momentum, and energy, and face-centered area-averages for the magnetic
field. Novel features of the algorithm include (1) a consistent framework for
computing the time- and edge-averaged electric fields used by CT to evolve the
magnetic field from the time- and area-averaged Godunov fluxes, (2) the
extension to MHD of spatial reconstruction schemes that involve a
dimensionally-split time advance, and (3) the extension to MHD of two different
dimensionally-unsplit integration methods. Implementation of the algorithm in
both C and Fortran95 is detailed, including strategies for parallelization
using domain decomposition. Results from a test suite which includes problems
in one-, two-, and three-dimensions for both hydrodynamics and MHD are given,
not only to demonstrate the fidelity of the algorithms, but also to enable
comparisons to other methods. The source code is freely available for download
on the web.Comment: 61 pages, 36 figures. accepted by ApJ
Underascertainment of radiotherapy receipt in Surveillance, Epidemiology, and End Results registry data
BACKGROUND: Surveillance, Epidemiology, and End Results (SEER) registry data have been used to suggest underuse and disparities in receipt of radiotherapy. Prior studies have cautioned that SEER may underascertain radiotherapy but lacked adequate representation to assess whether underascertainment varies by geography or patient sociodemographic characteristics. The authors sought to determine rates and correlates of underascertainment of radiotherapy in recent SEER data. METHODS: The authors evaluated data from 2290 survey respondents with nonmetastatic breast cancer, aged 20 to 79 years, diagnosed from June of 2005 to February 2007 in Detroit and Los Angeles and reported to SEER registries (73% response rate). Survey responses regarding treatment and sociodemographic factors were merged with SEER data. The authors compared radiotherapy receipt as reported by patients versus SEER records. The authors then assessed correlates of radiotherapy underascertainment in SEER. RESULTS: Of 1292 patients who reported receiving radiotherapy, 273 were coded as not receiving radiotherapy in SEER (underascertained). Underascertainment was more common in Los Angeles than in Detroit (32.0% vs 11.25%, P < .001). On multivariate analysis, radiotherapy underascertainment was significantly associated in each registry (Los Angeles, Detroit) with stage ( P = .008, P = .026), income ( P < .001, P = .050), mastectomy receipt ( P < .001, P < .001), chemotherapy receipt ( P < .001, P = .045), and diagnosis at a hospital that was not accredited by the American College of Surgeons ( P < .001, P < .001). In Los Angeles, additional significant variables included younger age ( P < .001), nonprivate insurance ( P < .001), and delayed receipt of radiotherapy ( P < .001). CONCLUSIONS: SEER registry data as currently collected may not be an appropriate source for documentation of rates of radiotherapy receipt or investigation of geographic variation in the radiation treatment of breast cancer. Cancer 2011;. © 2011 American Cancer Society. This study found that the Los Angeles Cancer Surveillance Program, among the largest Surveillance, Epidemiology, and End Results (SEER) registries, coded that radiation was not received in nearly a third of cases in which breast cancer patients themselves reported radiation receipt, whereas ascertainment of radiation receipt was much more complete in another large SEER registry, that of the Metropolitan Detroit Cancer Surveillance System. SEER registry data as currently collected may not be an appropriate source for documentation of rates of radiotherapy receipt or geographic disparities.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90319/1/26295_ftp.pd
Super-Keplerian Frequencies in Accretion Disks. Implications for Mass and Spin Measurements of Compact Objects from X-ray Variability Studies
The detection of fast quasi-periodic variability from accreting black holes
and neutron stars has been used to constrain their masses, radii, and spins. If
the observed oscillations are linear modes in the accretion disks, then bounds
can be placed on the properties of the central objects by assuming that these
modes are locally sub-Keplerian. If, on the other hand, the observed
oscillations correspond to non-linear resonances between disk modes, then the
properties of the central objects can be measured by assuming that the resonant
modes are excited at the same radial annulus in the disk. In this paper, we use
numerical simulations of vertically integrated, axisymmetric hydrodynamic
accretion disks to provide examples of situations in which the assumptions
implicit in both methods are not satisfied. We then discuss our results for the
robustness of the mass and spin measurements of compact objects from
variability studies.Comment: 12 pages, 4 figures, submitted to Ap
Exploring the Local Milky Way: M Dwarfs as Tracers of Galactic Populations
We have assembled a spectroscopic sample of low-mass dwarfs observed as part
of the Sloan Digital Sky Survey along one Galactic sightline, designed to
investigate the observable properties of the thin and thick disks. This sample
of ~7400 K and M stars also has measured ugriz photometry, proper motions, and
radial velocities. We have computed UVW space motion distributions, and
investigate their structure with respect to vertical distance from the Galactic
Plane. We place constraints on the velocity dispersions of the thin and thick
disks, using two-component Gaussian fits. We also compare these kinematic
distributions to a leading Galactic model. Finally, we investigate other
possible observable differences between the thin and thick disks, such as
color, active fraction and metallicity.Comment: 11 pages, 12 figures, Accepted by A
Dynamic proteome profiling of individual proteins in human skeletal muscle after a high-fat diet and resistance exercise.
It is generally accepted that muscle adaptation to resistance exercise (REX) training is underpinned by contraction-induced, increased rates of protein synthesis and dietary protein availability. By using dynamic proteome profiling (DPP), we investigated the contribution of both synthesis and breakdown to changes in abundance on a protein-by-protein basis in human skeletal muscle. Age-matched, overweight males consumed 9 d of a high-fat, low-carbohydrate diet during which time they either undertook 3 sessions of REX or performed no exercise. Precursor enrichment and the rate of incorporation of deuterium oxide into newly synthesized muscle proteins were determined by mass spectrometry. Ninety proteins were included in the DPP, with 28 proteins exhibiting significant responses to REX. The most common pattern of response was an increase in turnover, followed by an increase in abundance with no detectable increase in protein synthesis. Here, we provide novel evidence that demonstrates that the contribution of synthesis and breakdown to changes in protein abundance induced by REX differ on a protein-by-protein basis. We also highlight the importance of the degradation of individual muscle proteins after exercise in human skeletal muscle.-Camera, D. M., Burniston, J. G., Pogson, M. A., Smiles, W. J., Hawley, J. A. Dynamic proteome profiling of individual proteins in human skeletal muscle after a high-fat diet and resistance exercise
Magnetically Driven Accretion Flows in the Kerr Metric II: Structure of the Magnetic Field
We present a detailed analysis of the magnetic field structure found in
general relativistic 3D MHD simulations of accreting tori in the Kerr metric
with different black hole spins. Among the properties analyzed are the field
strength as a function of position and black hole spin, the shapes of field
lines, the degree to which they connect different regions, and their degree of
tangling. We investigate prior speculations about the structure of the magnetic
fields and discuss how frequently certain configurations are seen in the
simulations. We also analyze the distribution of current density, with a view
toward identifying possible locations for magnetic energy dissipation.Comment: Submitted to ApJ. PDF and PostScript files with high-resolution
figures are available at http://www.pha.jhu.edu/~shirose/GRMHD/PaperII
Cataclysmic Variables from SDSS II. The Second Year
The first full year of operation following the commissioning year of the
Sloan Digital Sky Survey has revealed a wide variety of newly discovered
cataclysmic variables. We show the SDSS spectra of forty-two cataclysmic
variables observed in 2002, of which thirty-five are new classifications, four
are known dwarf novae (CT Hya, RZ Leo, T Leo and BZ UMa), one is a known CV
identified from a previous quasar survey (Aqr1) and two are known ROSAT or
FIRST discovered CVs (RX J09445+0357, FIRST J102347.6+003841). The SDSS
positions, colors and spectra of all forty-two systems are presented. In
addition, the results of follow-up studies of several of these objects identify
the orbital periods, velocity curves and polarization that provide the system
geometry and accretion properties. While most of the SDSS discovered systems
are faint (>18th mag) with low accretion rates (as implied from their spectral
characteristics), there are also a few bright objects which may have escaped
previous surveys due to changes in the mass transfer rate.Comment: Accepted for publication in The Astronomical Journal, Vol. 126, Sep.
2003, 44 pages, 25 figures (now with adjacent captions), AASTeX v5.
The magnetic nature of disk accretion onto black holes
Although disk accretion onto compact objects - white dwarfs, neutron stars,
and black holes - is central to much of high energy astrophysics, the
mechanisms which enable this process have remained observationally elusive.
Accretion disks must transfer angular momentum for matter to travel radially
inward onto the compact object. Internal viscosity from magnetic processes and
disk winds can in principle both transfer angular momentum, but hitherto we
lacked evidence that either occurs. Here we report that an X-ray-absorbing wind
discovered in an observation of the stellar-mass black hole binary GRO J1655-40
must be powered by a magnetic process that can also drive accretion through the
disk. Detailed spectral analysis and modeling of the wind shows that it can
only be powered by pressure generated by magnetic viscosity internal to the
disk or magnetocentrifugal forces. This result demonstrates that disk accretion
onto black holes is a fundamentally magnetic process.Comment: 15 pages, 2 color figures, accepted for publication in Nature.
Supplemental materials may be obtained by clicking
http://www.astro.lsa.umich.edu/~jonmm/nature1655.p
AMPK is essential for energy homeostasis regulation and glucose sensing by POMC and AgRP neurons
Hypothalamic AMP-activated protein kinase (AMPK) has been suggested to act as a key sensing mechanism, responding to hormones and nutrients in the regulation of energy homeostasis. However, the precise neuronal populations and cellular mechanisms involved are unclear. The effects of long-term manipulation of hypothalamic AMPK on energy balance are also unknown. To directly address such issues, we generated POMC alpha 2KO and AgRP alpha 2KO mice lacking AMPK alpha 2 in proopiomelanocortin- (POMC-) and agouti-related protein-expressing (AgRP-expressing) neurons, key regulators of energy homeostasis. POMC alpha 2KO mice developed obesity due to reduced energy expenditure and dysregulated food intake but remained sensitive to leptin. in contrast, AgRPa2KO mice developed an age-dependent lean phenotype with increased sensitivity to a melanocortin agonist. Electrophysiological studies in AMPK alpha 2-deficient POMC or AgRP neurons revealed normal leptin or insulin action but absent responses to alterations in extracellular glucose levels, showing that glucose-sensing signaling mechanisms in these neurons are distinct from those pathways utilized by leptin or insulin. Taken together with the divergent phenotypes of POMC alpha 2KO and AgRP alpha 2KO mice, our findings suggest that while AMPK plays a key role in hypothalamic function, it does not act as a general sensor and integrator of energy homeostasis in the mediobasal hypothalamus
Chandra Observation of V426 Oph: Weighing the Evidence for a Magnetic White Dwarf
We report the results of a 45 ks Chandra observation of the cataclysmic
variable V426 Ophiuchus. The high resolution spectrum from the high-energy
transmission grating spectrometer is most consistent with a cooling flow model,
placing V426 Oph among the group of CVs including U Gem and EX Hya. An
uninterrupted lightcurve was also constructed, in which we detect a significant
4.2 hr modulation together with its first harmonic at 2.1 hrs. Reanalysis of
archival Ginga, and ROSAT X-ray lightcurves also reveals modulations at periods
consistent with 4.2 and/or 2.1 hrs. Furthermore, optical photometry in V,
simultaneous with the Chandra observation, indicates a modulation
anti-correlated with the X-ray, and later more extensive R band photometry
finds a signal at ~2.1 hrs. The earlier reported X-ray periods at ~0.5 and 1
hrs appear to be only transient and quasi-periodic in nature. In contrast, the
4.2 hr period or its harmonic are stable and persistent in X-ray/optical data
from 1988 to 2003. This periodicity is clearly distinct from the 6.85 hr orbit,
and could be due to the spin of the white dwarf. If this is the case, V426 Oph
would be the first long period intermediate polar with a ratio P_spin/P_orb of
0.6. However, this interpretation requires unreasonable values of magnetic
field strength and mass accretion rate.Comment: 11 pages, 7 figures, 5 tables. Accepted for publication in the
Astrophysical Journal (to appear August 2004
- …