331 research outputs found
NGC 2419, M92, and the Age Gradient in the Galactic Halo
The WFPC2 camera on HST has been used to obtain deep main sequence photometry
of the low-metallicity ([Fe/H]=-2.14), outer-halo globular cluster NGC 2419. A
differential fit of the NGC 2419 CMD to that of the similarly metal-poor \
standard cluster M92 shows that they have virtually identical principal
sequences and thus the same age to well within 1 Gyr. Since other
low-metallicity clusters throughout the Milky Way halo have this same age to
within the 1-Gyr precision of the differential age technique, we conclude that
the earliest star (or globular cluster) formation began at essentially the same
time everywhere in the Galactic halo throughout a region now almost 200 kpc in
diameter. Thus for the metal-poorest clusters in the halo there is no
detectable age gradient with Galactocentric distance. To estimate the absolute
age of NGC 2419 and M92, we fit newly computed isochrones transformed through
model-atmosphere calculations to the (M_V,V-I) plane, with assumed distance
scales that represent the range currently debated in the literature.
Unconstrained isochrone fits give M_V(RR) = 0.55 \pm 0.06 and a resulting age
of 14 to 15 Gyr. Incorporating the full effects of helium diffusion would
further reduce this estimate by about 1 Gyr. A distance scale as bright as
M_V(RR) = 0.15 for [Fe/H] = -2, as has recently been reported, would leave
several serious problems which have no obvious solution in the context of
current stellar models.Comment: 32 pages, aastex, 9 postscript figures; accepted for publication in
AJ, September 1997. Also available by e-mail from [email protected]
White Dwarfs in Globular Clusters: HST Observations of M4
Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258
white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial
distances from the cluster center were observed and sizeable WD populations
were found in all three. The location of these WDs in the color-magnitude
diagram, their mean mass of 0.51()M, and their luminosity
function confirm basic tenets of stellar evolution theory and support the
results from current WD cooling theory. The WDs are used to extend the cluster
main-sequence mass function upward to stars that have already completed their
nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and
found to be at most a few percent of all the main-sequence stars. The most
ancient WDs found are about 9 Gyr old, a level which is set solely by the
photometric limits of our data. Even though this is less than the age of M4, we
discuss how these cooling WDs can eventually be used to check the turnoff ages
of globular clusters and hence constrain the age of the Universe.Comment: 46 pages, latex, no figures included, figures available at
ftp://ftp.astro.ubc.ca/pub/richer/wdfig.uu size 2.7Mb. To be published in the
Astrophysical Journa
Ontogenetic changes in skeletal muscle fiber type, fiber diameter and myoglobin concentration in the Northern elephant seal (Mirounga angustirostris)
© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Frontiers in Physiology 5 (2014): 217, doi:10.3389/fphys.2014.00217.Northern elephant seals (Mirounga angustirostris) (NES) are known to be deep, long-duration divers and to sustain long-repeated patterns of breath-hold, or apnea. Some phocid dives remain within the bounds of aerobic metabolism, accompanied by physiological responses inducing lung compression, bradycardia, and peripheral vasoconstriction. Current data suggest an absence of type IIb fibers in pinniped locomotory musculature. To date, no fiber type data exist for NES, a consummate deep diver. In this study, NES were biopsied in the wild. Ontogenetic changes in skeletal muscle were revealed through succinate dehydrogenase (SDH) based fiber typing. Results indicated a predominance of uniformly shaped, large type I fibers and elevated myoglobin (Mb) concentrations in the longissimus dorsi (LD) muscle of adults. No type II muscle fibers were detected in any adult sampled. This was in contrast to the juvenile animals that demonstrated type II myosin in Western Blot analysis, indicative of an ontogenetic change in skeletal muscle with maturation. These data support previous hypotheses that the absence of type II fibers indicates reliance on aerobic metabolism during dives, as well as a depressed metabolic rate and low energy locomotion. We also suggest that the lack of type IIb fibers (adults) may provide a protection against ischemia reperfusion (IR) injury in vasoconstricted peripheral skeletal muscle.Funding was provided by the Baylor University Faculty
Research Investment Program (StephenJ.Trumble)
Magnetar-like X-ray Bursts from an Anomalous X-ray Pulsar
Anomalous X-ray Pulsars (AXPs) are a class of rare X-ray pulsars whose energy
source has been perplexing for some 20 years. Unlike other, better understood
X-ray pulsars, AXPs cannot be powered by rotation or by accretion from a binary
companion, hence the designation ``anomalous.'' AXP rotational and radiative
properties are strikingly similar to those of another class of exotic objects,
the Soft Gamma Repeaters (SGRs). However, the defining property of SGRs, namely
their low-energy gamma-ray and X-ray bursts, have heretofore not been seen in
AXPs. SGRs are thought to be ``magnetars,'' young neutron stars powered by the
decay of an ultra-high magnetic field. The suggestion that AXPs are magnetars
has been controversial. Here we report the discovery, from the direction of AXP
1E 1048-5937, of two X-ray bursts that have many properties similar to those of
SGR bursts. These events imply a close relationship between AXPs and SGRs, with
both being magnetars.Comment: 14 pages, 2 figures, accepted for publication in Nature. Note: The
content of this paper is embargoed until 1900 hrs London time / 1400 US
Eastern Time on Sept 1
Bubbles in live-stranded dolphins
© The Author(s), 2011. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Proceedings of the Royal Society B : Biological Sciences 279 (2012): 1396-1404, doi:10.1098/rspb.2011.1754.Bubbles in supersaturated tissues and blood occur in beaked whales stranded near sonar exercises, and post-mortem in dolphins bycaught at depth and then hauled to the surface. To evaluate live dolphins for bubbles, liver, kidneys, eyes and blubberâmuscle interface of live-stranded and capture-release dolphins were scanned with B-mode ultrasound. Gas was identified in kidneys of 21 of 22 live-stranded dolphins and in the hepatic portal vasculature of 2 of 22. Nine then died or were euthanized and bubble presence corroborated by computer tomography and necropsy, 13 were released of which all but two did not re-strand. Bubbles were not detected in 20 live wild dolphins examined during health assessments in shallow water. Off-gassing of supersaturated blood and tissues was the most probable origin for the gas bubbles. In contrast to marine mammals repeatedly diving in the wild, stranded animals are unable to recompress by diving, and thus may retain bubbles. Since the majority of beached dolphins released did not re-strand it also suggests that minor bubble formation is tolerated and will not lead to clinically significant decompression sickness.Funding for this work was provided by the US Office of
Naval Research Award no. N000140811220 and the
International Fund for Animal Welfare
Photometric redshifts for the CFHTLS T0004 Deep and Wide fields
We compute photometric redshifts based on the template-fitting method in the
fourth public release of the Canada-France-Hawaii Telescope Legacy Survey. This
unique multi-colour catalogue comprises u*,g',r',i',z' photometry in four deep
fields of 1 deg2 each and 35 deg2 distributed over three Wide fields. Our
photometric redshifts are calibrated with and compared to 16,983 high-quality
spectroscopic redshifts from several surveys. We find a dispersion of 0.028 and
an outlier rate of 3.5% in the Deep field at i'AB < 24 and a dispersion of
0.036 and an outlier rate of 2.8% in the Wide field at i'AB < 22.5. Beyond i'AB
= 22.5 in the Wide field the number of outliers rises from 5% to 10% at i'AB<23
and i'AB<24 respectively. For the Wide sample, we find the systematic redshift
bias keeps below 1% to i'AB < 22.5, whereas we find no significant bias in the
Deep field. We investigated the effect of tile-to-tile photometric variations
and demonstrate that the accuracy of our photometric redshifts is reduced by at
most 21%. We separate stars from galaxies using both the size and colour
information, reducing the contamination by stars in our catalogues from 50% to
8% at i'AB < 22.5 in fields with the highest stellar density while keeping a
complete galaxy sample. Our CFHTLS T0004 photometric redshifts are distributed
to the community. Our release include 592,891 (i'AB < 22.5) and 244,701 (i'AB <
24) reliable galaxy photometric redshifts in the Wide and Deep fields,
respectively.Comment: 18 pages, 17 figure
Hubble Space Telescope Observations of White Dwarfs in the Globular Cluster M4
With the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope,
we have discovered in M4 (NGC 6121, C 1620-264) the first extensive sequence of
cooling white dwarfs seen in a globular cluster. Adopting a distance modulus of
(m-M)_V = 12.65 and a reddening of E(B-V) = 0.37, we show that the sequence,
which extends over 9 < M_U < 13, is comprised of white dwarfs of mass \sim 0.5
M_{\odot}. The total mass loss from the present turnoff to the white dwarf
sequence is 0.31 M_{\odot} and the intrinsic dispersion in the mean mass
appears to be < 0.05 M_{\odot}. Both the location of the white dwarf cooling
sequence in the cluster color-magnitude diagram and the cumulative luminosity
function attest to the basic correctness and completeness of the physics in
theoretical models for the upper three magnitudes of the observed white dwarf
cooling sequence. To test the theory in globular clusters at cooling ages
beyond \sim 3 \times 10^8 years will require deeper and more complete data.Comment: To appear in the Astrophysical Journal Letters, 451, September 20,
1995. The text is also available as a postscript file from
http://www.astro.ubc.ca/people/richer/m4wd/m4wd.ps with postscript files of
the figures available in http://www.astro.ubc.ca/people/richer/m4wd/fig1.ps
http://www.astro.ubc.ca/people/richer/m4wd/fig2.ps
http://www.astro.ubc.ca/people/richer/m4wd/fig3.p
Cosmic Shear Analysis with CFHTLS Deep data
We present the first cosmic shear measurements obtained from the T0001
release of the Canada-France-Hawaii Telescope Legacy Survey. The data set
covers three uncorrelated patches (D1, D3 and D4) of one square degree each
observed in u*, g', r', i' and z' bands, out to i'=25.5. The depth and the
multicolored observations done in deep fields enable several data quality
controls. The lensing signal is detected in both r' and i' bands and shows
similar amplitude and slope in both filters. B-modes are found to be
statistically zero at all scales. Using multi-color information, we derived a
photometric redshift for each galaxy and separate the sample into medium and
high-z galaxies. A stronger shear signal is detected from the high-z subsample
than from the low-z subsample, as expected from weak lensing tomography. While
further work is needed to model the effects of errors in the photometric
redshifts, this results suggests that it will be possible to obtain constraints
on the growth of dark matter fluctuations with lensing wide field surveys. The
various quality tests and analysis discussed in this work demonstrate that
MegaPrime/Megacam instrument produces excellent quality data. The combined Deep
and Wide surveys give sigma_8= 0.89 pm 0.06 assuming the Peacock & Dodds
non-linear scheme and sigma_8=0.86 pm 0.05 for the halo fitting model and
Omega_m=0.3. We assumed a Cold Dark Matter model with flat geometry.
Systematics, Hubble constant and redshift uncertainties have been marginalized
over. Using only data from the Deep survey, the 1 sigma upper bound for w_0,
the constant equation of state parameter is w_0 < -0.8.Comment: 14 pages, 16 figures, accepted A&
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