12,891 research outputs found
Echelle Spectroscopy of gamma-ray binary 1FGL J1018.6-5856
We observed the Fermi-discovered gamma-ray binary 1FGL J1018.6-5856 at 20
epochs over 50 days using the CHIRON spectrograph, obtaining spectra at
R~25,000 covering 4090-8908A. The average spectrum confirms an O6 V((f))
spectral type and extinction E(B-V) = 1.35+/-0.04. Variable absorption line
equivalent widths suggest substantial contamination by wind line features. The
limited S/N ratio hindered accurate continuum definition and prevented
measurement of a high quality radial velocity curve. Nevertheless, the best
data indicate a radial velocity amplitude <40 km/s for the He II lines and
substantially lower for H I. We argue that this indicates a most likely compact
object mass <2.2Msun. While black hole solutions are not excluded, a neutron
star source of the gamma-ray emission seems preferred.Comment: 5 figures. To appear in the Astrophysical Journa
Slowing the stork : better health for women through family planning
Each year 500,000 women die from causes related to pregnancy - 99 percent of them in developing countries. While many of those pregnancies are unwanted and could have been prevented by family planning, only a minority of developing country couples use effective contraceptive methods. For some women, pregnancy represents a major health risk. Others, of lower risk, do not want any more children. This paper discusses the factors which determine women's use of contraceptives, and how family planning programs reach the large numbers of women at risk from further pregnancies. The most successful family planning policies offer women a variety of contraceptive methods tailored to specific age groups and educational levels. Much program experience suggests that family planning is one of, if not the most cost-effective means of averting maternal deaths. The savings generated by family planning services could be invested in saving the lives and health of women who do want to have more children.Health Monitoring&Evaluation,Adolescent Health,Reproductive Health,Early Child and Children's Health,Gender and Health
Wind-Interaction Models for the Early Afterglows of Gamma-Ray Bursts: The Case of GRB 021004
Wind-interaction models for gamma-ray burst afterglows predict that the
optical emission from the reverse shock drops below that from the forward shock
within 100s of seconds of the burst. The typical frequency of the
synchrotron emission from the forward shock passes through the optical band
typically on a timescale of minutes to hours. Before the passage of ,
the optical flux evolves as and after the passage, the decay
steepens to , where is the exponent for the assumed
power-law energy distribution of nonthermal electrons and is typically . The steepening in the slope of temporal decay should be readily
identifiable in the early afterglow light curves. We propose that such a
steepening was observed in the R-band light curve of GRB 021004 around day 0.1.
Available data at several radio frequencies are consistent with this
interpretation, as are the X-ray observations around day~1. The early evolution
of GRB 021004 contrasts with that of GRB 990123, which can be described by
emission from interaction with a constant density medium.Comment: 16 pages, 1 figure, submitted to ApJ
The most massive galaxies in clusters are already fully grown at
By constructing scaling relations for galaxies in the massive cluster
MACSJ0717.5 at and comparing with those of Coma, we model the
luminosity evolution of the stellar populations and the structural evolution of
the galaxies. We calculate magnitudes, surface brightnesses and effective radii
using HST/ACS images and velocity dispersions using Gemini/GMOS spectra, and
present a catalogue of our measurements for 17 galaxies. We also generate
photometric catalogues for galaxies from the HST imaging. With
these, we construct the colour-magnitude relation, the fundamental plane, the
mass-to-light versus mass relation, the mass-size relation and the
mass-velocity dispersion relation for both clusters. We present a new, coherent
way of modelling these scaling relations simultaneously using a simple physical
model in order to infer the evolution in luminosity, size and velocity
dispersion as a function of redshift, and show that the data can be fully
accounted for with this model. We find that (a) the evolution in size and
velocity dispersion undergone by these galaxies between and is mild, with and , and (b) the stellar populations are old, Gyr,
with a Gyr dispersion in age, and are consistent with evolving purely
passively since with . The implication is that these galaxies formed their stars early and
subsequently grew dissipationlessly so as to have their mass already in place
by , and suggests a dominant role for dry mergers, which may have
accelerated the growth in these high-density cluster environments.Comment: 20 pages; accepted for publication in MNRA
A Multi-wavelength Study of the Host Environment of SMBHB 4C+37.11
4C+37.11, at z=0.055 shows two compact radio nuclei, imaged by VLBI at 7mas
separation, making it the closest known resolved super-massive black hole
binary (SMBHB). An important question is whether this unique object is young,
caught on the way to a gravitational in-spiral and merger, or has `stalled' at
7pc. We describe new radio/optical/X-ray observations of the massive host and
its surrounding X-ray halo. These data reveal X-ray/optical channels following
the radio outflow and large scale edges in the X-ray halo. These structures are
promising targets for further study which should elucidate their relationship
to the unique SMBHB core.Comment: To appear in the Astrophysical Journa
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Near-axis subsidence rates, hydrothermal circulation, and thermal structure of mid-ocean ridge crests
We systematically investigated near-axis subsidence on the ridge flanks of intermediate and fast spreading mid-ocean ridges using bathymetric data from well-surveyed portions of the Southeast Indian Ridge (spreading at 72-76 mm/yr), the northern East Pacific Rise (91-96 mm/yr), and the southern East Pacific Rise (144 mm/yr). In all three regions, the mean subsidence rate of young (< 1 - 1.4 Ma) seafloor is less than 220 m/m.y1/2. The distribution of individual estimates shows a distinct peak at 180 - 220 m/m.y.1/2 with few profiles having subsidence rates greater than 275 m/m.y.1/2. The observed subsidence rates are significantly lower than both the worldwide average (~350 m/m.y.1/2) and subsidence rates observed for older lithosphere at the same ridge segments. Intense hydrothermal circulation at the ridge axis can result in low subsidence rates on the adjacent ridge flanks provided the vigorous flow is confined to the immediate vicinity (< ~5 km) of the axis. According to our model, the extremely vigorous hydrothermal circulation ceases off-axis, and conductive heat flow becomes a primary mechanism of vertical heat transport on the ridge flanks. The very low geothermal gradient within the cooled portion of the uppermost lithosphere retards conductive cooling, and the cooled area needs to be heated from below before a geothermal gradient can be established which permits significant heat to be conducted out of the lithosphere. As a result, subsidence in very young (~0.1 - 1 Ma) lithosphere is suppressed. A simple one-dimensional thermal model with a Nusselt number parameterization was used to estimate the effect of hydrothermal circulation. An upper layer with a high Nusselt number and the half-space that it overlies are initially at a temperature of 1300°C and the surface is maintained at 0°C. After 0.1 m.y. of cooling (about 3.5 to 7 km from the spreading axis), the Nusselt number of the top layer is set to 1 so that normal conduction is simulated in the cooled layer. We used an explicit finite difference method to solve for the temporal changes in temperature with depth. This model produces subsidence rates in the range that we observe for Nu in the range of 15-30. Isotherms resulting from the modeling imply rapid lithospheric thickening very near the axis, which is incompatible with most current models for the formation of the bathymetric axial high observed at fast spreading mid-ocean ridges
Limnological Features of Some Northwestern Iowa Lakes
Quantitative information on the morphology, watershed characteristics, water transparency, water chemistry and algal crops of six Iowa lakes is summarized. Lake West Okoboji had less oxygen present in the hypolimnion in 1950-1973 than in 1919-1928, indicating an increase in eutrophication. On the basis of increasing plant-nutrient concentrations, increasing summer algal standing crops and decreasing water transparency, the lakes can be ranked thus: Lake West Okoboji, Big Spirit Lake, Lake East Okoboji (including Upper Gar and Minnewashta) and Lower Gar Lake. These differences among lakes are related to the ratio of watershed area to lake volume, which controls the impact of annual nutrient inputs from the watersheds
A Survey of Water Transparency in Iowa Lakes
Measurements of Secchi disk transparency were made in 50 Iowa lakes and reservoirs in the summer of 1975. Averages of July and August readings for individual lakes ranged from 0.1 to 2.7 m. The man-made lakes in the southern part of the state generally had greater transparencies than the natural lakes in the north. Reduced transparency was related more to algal density than to suspended inorganic matter
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