930 research outputs found
A Gravitational Redshift Determination of the Mean Mass of White Dwarfs. DA Stars
We measure apparent velocities (v_app) of the Halpha and Hbeta Balmer line
cores for 449 non-binary thin disk normal DA white dwarfs (WDs) using optical
spectra taken for the ESO SN Ia Progenitor surveY (SPY; Napiwotzki et al.
2001). Assuming these WDs are nearby and co-moving, we correct our velocities
to the Local Standard of Rest so that the remaining stellar motions are random.
By averaging over the sample, we are left with the mean gravitational redshift,
: we find = = 32.57 +/- 1.17 km/s. Using the mass-radius
relation from evolutionary models, this translates to a mean mass of 0.647
+0.013 -0.014 Msun. We interpret this as the mean mass for all DAs. Our results
are in agreement with previous gravitational redshift studies but are
significantly higher than all previous spectroscopic determinations except the
recent findings of Tremblay & Bergeron (2009). Since the gravitational redshift
method is independent of surface gravity from atmosphere models, we investigate
the mean mass of DAs with spectroscopic Teff both above and below 12000 K; fits
to line profiles give a rapid increase in the mean mass with decreasing Teff.
Our results are consistent with no significant change in mean mass: ^hot =
0.640 +/- 0.014 Msun and ^cool = 0.686 +0.035 -0.039 Msun.Comment: Accepted for publication in ApJ, 14 pages, 12 figure
Hubble Space Telescope Spectroscopy of the Balmer lines in Sirius B
Sirius B is the nearest and brightest of all white dwarfs, but it is very
difficult to observe at visible wavelengths due to the overwhelming scattered
light contribution from Sirius A. However, from space we can take advantage of
the superb spatial resolution of the Hubble Space Telescope to resolve the A
and B components. Since the closest approach in 1993, the separation between
the two stars has become increasingly favourable and we have recently been able
to obtain a spectrum of the complete Balmer line series for Sirius B using
HST?s Space Telescope Imaging Spectrograph (STIS). The quality of the STIS
spectra greatly exceed that of previous ground-based spectra, and can be used
to provide an important determination of the stellar temperature (Teff =
25193K) and gravity (log g = 8.556). In addition we have obtained a new, more
accurate, gravitational red-shift of 80.42 +/- 4.83 km s-1 for Sirius B.
Combining these results with the photometric data and the Hipparcos parallax we
obtain new determinations of the stellar mass for comparison with the
theoretical mass-radius relation. However, there are some disparities between
the results obtained independently from log g and the gravitational redshift
which may arise from flux losses in the narrow 50x0.2arcsec slit. Combining our
measurements of Teff and log g with the Wood (1995) evolutionary mass-radius
relation we get a best estimate for the white dwarf mass of 0.978 M. Within the
overall uncertainties, this is in agreement with a mass of 1.02 M obtained by
matching our new gravitational red-shift to the theoretical M/R relation.Comment: 11 pages, 6 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
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Deubiquitination of Dishevelled by Usp14 is required for Wnt signaling
Dishevelled (Dvl) is a key regulator of Wnt signaling both in the canonical and non-canonical pathways. Here we report the identification of a regulatory domain of ubiquitination (RDU) in the C-terminus of Dvl. Mutations in the RDU resulted in accumulation of polyubiquitinated forms of Dvl, which were mainly K63 linked. Small interfering RNA-based screening identified Usp14 as a mediator of Dvl deubiquitination. Genetic and chemical suppression of Usp14 activity caused an increase in Dvl polyubiquitination and significantly impaired downstream Wnt signaling. These data suggest that Usp14 functions as a positive regulator of the Wnt signaling pathway. Consistently, tissue microarray analysis of colon cancer revealed a strong correlation between the levels of Usp14 and β-catenin, which suggests an oncogenic role for Usp14 via enhancement of Wnt/β-catenin signaling
Supernova Simulations and Strategies For the Dark Energy Survey
We present an analysis of supernova light curves simulated for the upcoming
Dark Energy Survey (DES) supernova search. The simulations employ a code suite
that generates and fits realistic light curves in order to obtain distance
modulus/redshift pairs that are passed to a cosmology fitter. We investigated
several different survey strategies including field selection, supernova
selection biases, and photometric redshift measurements. Using the results of
this study, we chose a 30 square degree search area in the griz filter set. We
forecast 1) that this survey will provide a homogeneous sample of up to 4000
Type Ia supernovae in the redshift range 0.05<z<1.2, and 2) that the increased
red efficiency of the DES camera will significantly improve high-redshift color
measurements. The redshift of each supernova with an identified host galaxy
will be obtained from spectroscopic observations of the host. A supernova
spectrum will be obtained for a subset of the sample, which will be utilized
for control studies. In addition, we have investigated the use of combined
photometric redshifts taking into account data from both the host and
supernova. We have investigated and estimated the likely contamination from
core-collapse supernovae based on photometric identification, and have found
that a Type Ia supernova sample purity of up to 98% is obtainable given
specific assumptions. Furthermore, we present systematic uncertainties due to
sample purity, photometric calibration, dust extinction priors, filter-centroid
shifts, and inter-calibration. We conclude by estimating the uncertainty on the
cosmological parameters that will be measured from the DES supernova data.Comment: 46 pages, 30 figures, resubmitted to ApJ as Revision 2 (final author
revision), which has subtle editorial differences compared to the published
paper (ApJ, 753, 152). Note that this posting includes PDF only due to a bug
in either the latex macros or the arXiv submission system. The source files
are available in the DES document database:
http://des-docdb.fnal.gov/cgi-bin/ShowDocument?docid=624
An upper limit on the electron-neutrino flux from the HiRes detector
Air-fluorescence detectors such as the High Resolution Fly's Eye (HiRes)
detector are very sensitive to upward-going, Earth-skimming ultrahigh energy
electron-neutrino-induced showers. This is due to the relatively large
interaction cross sections of these high-energy neutrinos and to the
Landau-Pomeranchuk-Migdal (LPM) effect. The LPM effect causes a significant
decrease in the cross sections for bremsstrahlung and pair production, allowing
charged-current electron-neutrino-induced showers occurring deep in the Earth's
crust to be detectable as they exit the Earth into the atmosphere. A search for
upward-going neutrino-induced showers in the HiRes-II monocular dataset has
yielded a null result. From an LPM calculation of the energy spectrum of
charged particles as a function of primary energy and depth for
electron-induced showers in rock, we calculate the shape of the resulting
profile of these showers in air. We describe a full detector Monte Carlo
simulation to determine the detector response to upward-going
electron-neutrino-induced cascades and present an upper limit on the flux of
electron-neutrinos.Comment: 13 pages, 3 figures. submitted to Astrophysical Journa
Interplane Transport and Superfluid Density in Layered Superconductors
We report on generic trends in the behavior of the interlayer penetration
depth of several different classes of quasi two-dimensional
superconductors including cuprates, SrRuO, transition metal
dichalcogenides and organic materials of the -series. Analysis
of these trends reveals two distinct patterns in the scaling between the values
of and the magnitude of the DC conductivity: one realized in the
systems with a Fermi liquid (FL) ground state and the other seen in systems
with a marked deviation from the FL response. The latter pattern is found
primarily in under-doped cuprates and indicates a dramatic enhancement (factor
) of the energy scale associated with the formation of
the condensate compared to the data for the FL materials. We discuss
implications of these results for the understanding of pairing in high-
cuprates.Comment: 4 pages, 2 figure
Search for Global Dipole Enhancements in the HiRes-I Monocular Data above 10^{18.5} eV
Several proposed source models for Ultra-High Energy Cosmic Rays (UHECRs)
consist of dipole distributions oriented towards major astrophysical landmarks
such as the galactic center, M87, or Centaurus A. We use a comparison between
real data and simulated data to show that the HiRes-I monocular data for
energies above 10^{18.5} eV is, in fact, consistent with an isotropic source
model. We then explore methods to quantify our sensitivity to dipole source
models oriented towards the Galactic Center, M87, and Centaurus A.Comment: 17 pages, 31 figure
Search for Correlations between HiRes Stereo Events and Active Galactic Nuclei
We have searched for correlations between the pointing directions of
ultrahigh energy cosmic rays observed by the High Resolution Fly's Eye
experiment and Active Galactic Nuclei (AGN) visible from its northern
hemisphere location. No correlations, other than random correlations, have been
found. We report our results using search parameters prescribed by the Pierre
Auger collaboration. Using these parameters, the Auger collaboration concludes
that a positive correlation exists for sources visible to their southern
hemisphere location. We also describe results using two methods for determining
the chance probability of correlations: one in which a hypothesis is formed
from scanning one half of the data and tested on the second half, and another
which involves a scan over the entire data set. The most significant
correlation found occurred with a chance probability of 24%.Comment: 13 pages, 1 table, 5 figure
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