196 research outputs found
A dynamical magnetosphere model for periodic Halpha emission from the slowly rotating magnetic O star HD191612
The magnetic O-star HD191612 exhibits strongly variable, cyclic Balmer line
emission on a 538-day period. We show here that its variable Halpha emission
can be well reproduced by the rotational phase variation of synthetic spectra
computed directly from full radiation magneto-hydrodynamical simulations of a
magnetically confined wind. In slow rotators such as HD191612, wind material on
closed magnetic field loops falls back to the star, but the transient
suspension of material within the loops leads to a statistically overdense, low
velocity region around the magnetic equator, causing the spectral variations.
We contrast such "dynamical magnetospheres" (DMs) with the more steady-state
"centrifugal magnetospheres" of stars with rapid rotation, and discuss the
prospects of using this DM paradigm to explain periodic line emission from also
other non-rapidly rotating magnetic massive stars.Comment: 5 pages, 5 figures, accepted for publication in MNRAS letter
An Interstellar Conduction Front Within a Wolf-Rayet Ring Nebula Observed with the GHRS
With the High Resolution Spectrograph aboard the Hubble Space Telescope we
obtained high signal-to-noise (S/N > 200 - 600 per 17 km/s resolution element)
spectra of narrow absorption lines toward the Wolf-Rayet star HD 50896. The
ring nebula S308 that surrounds this star is thought to be caused by a
pressure-driven bubble bounded by circumstellar gas (most likely from a red
supergiant or luminous blue variable progenitor) pushed aside by a strong
stellar wind. Our observation has shown for the first time that blueshifted
(approximately 70 km/s relative to the star) absorption components of C IV and
N V arise in a conduction front between the hot interior of the bubble and the
cold shell of swept-up material. These lines set limits on models of the
conduction front. Nitrogen in the shell appears to be overabundant by a factor
~10. The P Cygni profiles of N V and C IV are variable, possibly due to a
suspected binary companion to HD 50896.Comment: 32 pages, Latex, to appear in the Astrophysical Journal, April, 199
The Low- and Intermediate-Mass Stellar Population in the Small Magellanic Cloud: The Central Stars of Planetary Nebulae
We present a study on the central stars (CSs) of Planetary Nebulae (PNe)
observed in the Small Magellanic Cloud (SMC) with the Space Telescope Imaging
Spectrograph instrument on-board the HST. The stellar magnitudes have been
measured using broad-band photometry, and Zanstra analysis of the nebulae
provided the stellar temperatures. From the location of the CSs on the HR
diagram, and by comparing the observed CSs with current models of stellar
evolution, we infer the CSs masses. We examine closely the possibility of light
contamination in the bandpass from an unrecognized stellar companion, and we
establish strong constraints on the existence and nature of any binary
companion. We find an average mass of 0.63 Msun, which is similar to the mass
obtained for a sample of CSs in the LMC (0.65 Msun). However, the SMC and LMC
CS mass distributions differ slightly, the SMC sample lacking an
intermediate-mass stellar population (0.65 to 0.75 Msun). We discuss the
significance and possible reasons for the difference between the two mass
distributions. In particular, we consider the differences in the star formation
history between the clouds and the mass-loss rate dependence on metallicity.Comment: 30 pages, 6 figures, 5 tables. To be published in ApJ (October 20
The reintroduction of large carnivores to the Eastern Cape, South Africa: an assessment
Recently, conservation estate in South Africa's Eastern Cape Province has increased 10-fold resulting in large predators being increasingly reintroduced to restore ecological integrity and maximize tourism. We describe the reintroductions of large carnivores (>10 kg) that have occurred in the Eastern Cape and use various criteria to assess their success. Lion Panthera leo reintroduction has been highly successful with a population of 56 currently extant in the region and problems of overpopulation arising. The African wild dog Lycaon pictus population has increased to 24 from a founder population of 11. Preliminary results for spotted hyaenas Crocuta crocuta also indicate success. Wild populations of leopards Panthera pardus exist on several reserves and have been supplemented by translocated individuals, although deaths of known individuals have occurred and no estimate of reproduction is available. Cheetah Acinonyx jubatus reintroduction has also been less successful with 36 individuals reintroduced and 23 cubs being born but only 41 individuals surviving in 2005. Criteria for assessing the success of reintroductions of species that naturally occur in low densities, such as top predators, generally have limited value. Carrying capacity for large predators is unknown and continued monitoring and intensive management will be necessary in enclosed, and possibly all, conservation areas in the Eastern Cape to ensure conservation success
Hopkins Ultraviolet Telescope Observations of Her X-1
We have obtained a far-ultraviolet spectrum of the X-ray binary Hercules
X-1/HZ Herculis using the Hopkins Ultraviolet Telescope aboard the Astro-1
space shuttle mission in 1990 December. This is the first spectrum of Her X-1
that extends down to the Lyman limit at 912 A. We observed emission lines of O
VI, N V, and C IV, and the far UV continuum extending to the Lyman limit. We
examine the conditions of the emitting gas through line strengths, line ratios,
and doublet ratios. The UV flux is lower by about a factor of 2 than expected
at the orbital phase of the observation. We model the UV continuum with a
simple power-law and with a detailed model of an X-ray-illuminated accretion
disk and companion star. The power-law provides a superior fit, as the detailed
model predicts too little flux below 1200 A. We note, however, that there are
uncertainties in the interstellar reddening, in the background airglow
spectrum, and in the long-term phase of the accretion disk. We have searched
the data for UV line and continuum pulsations near the neutron star spin period
but found none at a detectable level.Comment: 19 pages, 5 figures, accepted by the Astrophysical Journa
Space telescope Imaging Spectrograph ultraviolet spectra of LMC planetary nebulae. A study of carbon abundances and stellar evolution
We acquired spectra of 24 LMC PNe in the 1150--3000 \AA range in order to
determine carbon and other ionic abundances. The sample more than doubles the
number of LMC PNe with good quality UV spectra in this wavelength range, and
whose optical images are available in the {\it HST} archive. The {\it Space
Telescope Imaging Spectrograph} was used with a very large aperture to obtain
virtually slit-less spectra, thus the monochromatic images in the major nebulae
emission lines are also available. The analysis of the data shows extremely
good quality spectra. This paper presents the emission lines identified and
measured, and the calculation of the ionic abundances of the emitting carbon
and other ions, and total carbon abundance. P-Cygni profiles have been found in
a fraction of the nebulae, and the limiting velocities of the stellar winds
estimated. The total carbon abundance can be inferred reliably in most nebulae.
We found that the average carbon abundance in round and elliptical PNe is one
order of magnitude larger than that of the bipolar PNe, while elliptical and
round PNe with a bipolar core have a bimodal behavior. This results confirm
that bipolarity in LMC PNe is tightly correlated with high mass progenitors.
When compared to predicted yields, we found that the observed abundance ratio
show a shift toward higher carbon abundances, that may be due to initial
conditions assumed in the models not appropriate for LMC PNe.Comment: Ap. J., in pres
Models of X-ray Photoionization in LMC X-4: Slices of a Stellar Wind
We show that the orbital variation in the UV P Cygni lines of the X-ray
binary LMC X-4 results when X-rays photoionize nearly the entire region outside
of the X-ray shadow of the normal star. We fit models to HST GHRS observations
of N V and C IV P Cygni line profiles. Analytic methods assuming a spherically
symmetric wind show that the wind velocity law is well-fit by v~(1-1/r)^beta,
where beta is likely 1.4-1.6 and definitely <2.5. Escape probability models can
fit the observed P Cygni profiles, and provide measurements of the stellar wind
parameters. The fits determine Lx/Mdot=2.6+/-0.1 x10^43 erg/s/Msun yr, where Lx
is the X-ray luminosity and Mdot is the mass-loss rate of the star. Allowing an
inhomogeneous wind improves the fits. IUE spectra show greater P Cygni
absorption during the second half of the orbit than during the first. We
discuss possible causes of this effect.Comment: 56 pages, 12 figures, to be published in the Astrophysical Journa
Deliberative multiattribute valuation of ecosystem services across a range of regional land-use, socioeconomic, and climate scenarios for the upper Merrimack River watershed, New Hampshire, USA
We evaluate the relative desirability of alternative futures for the upper Merrimack River watershed in New Hampshire, USA based on the value of ecosystem services at the end of the 21st century as gauged by its present-day inhabitants. This evaluation is accomplished by integrating land-use and socioeconomic scenarios, downscaled climate projections, biogeophysical simulation models, and the results of a citizen-stakeholder deliberative multicriteria evaluation. We find that although there are some trade-offs between alternative plausible futures, for the most part, it can be expected that future inhabitants of the watershed will be most satisfied if land-use planning in the intervening years prioritizes water supply and flood protection as well as maintenance of existing farmland and forest cover. With respect to climate change, it is expected that future watershed inhabitants will be more negatively affected by the projected loss of snow cover than the anticipated increase in hot summer days. More important than the specific results for the upper Merrimack River watershed, this integrative assessment demonstrates the complex yet ultimately informative potential to link stakeholder engagement with scenario generation, ecosystem models, and multiattribute evaluation for informing regional-scale planning and decision making
A mass-loss rate determination for zeta Puppis from the quantitative analysis of X-ray emission line profiles
We fit every emission line in the high-resolution Chandra grating spectrum of
zeta Pup with an empirical line profile model that accounts for the effects of
Doppler broadening and attenuation by the bulk wind. For each of sixteen lines
or line complexes that can be reliably measured, we determine a best-fitting
fiducial optical depth, tau_* = kappa*Mdot/4{pi}R_{\ast}v_{\infty}, and place
confidence limits on this parameter. These sixteen lines include seven that
have not previously been reported on in the literature. The extended wavelength
range of these lines allows us to infer, for the first time, a clear increase
in tau_* with line wavelength, as expected from the wavelength increase of
bound-free absorption opacity. The small overall values of tau_*, reflected in
the rather modest asymmetry in the line profiles, can moreover all be fit
simultaneously by simply assuming a moderate mass-loss rate of 3.5 \pm 0.3
\times 10^{-6} Msun/yr, without any need to invoke porosity effects in the
wind. The quoted uncertainty is statistical, but the largest source of
uncertainty in the derived mass-loss rate is due to the uncertainty in the
elemental abundances of zeta Pup, which affects the continuum opacity of the
wind, and which we estimate to be a factor of two. Even so, the mass-loss rate
we find is significantly below the most recent smooth-wind H-alpha mass-loss
rate determinations for zeta Pup, but is in line with newer determinations that
account for small-scale wind clumping. If zeta Pup is representative of other
massive stars, these results will have important implications for stellar and
galactic evolution.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society. 17 pages, including 14 figures (7 color
Magmatic and geotectonic significance of Santa Elena Peninsula, Costa Rica
We present a new integrated interpretation of the geochemistry and geotectonic significance of the Santa Elena Peninsula, which is divided in three units: 1) an overthrust allocthonous unit of ultramafic and mafic rocks, the Santa Elena Nappe; 2) an autochthonous basaltic sedimentary suite, resting immediately below the overthrust, the Santa Rosa Accretionary Complex; and 3) Islas Murciélago pillow and massive basaltic flows. In the Santa Elena Nappe three petrological affinities have been recognized: 1) the ultramafic complex, that corresponds to depleted (MORB-like) mantle serpentinizated peridotites, with very low TiO2 and high Ni and Cr; 2) the pegmatitic gabbros, layered gabbros and plagiogranites and basaltic dikes with low TiO2 (0.89%). These mafic associations have geochemical signatures that suggest an island arc origin and petrographic evidences of low grade metamorphism and hydrothermal alteration. The Santa Rosa Accretionary Complex includes pelagic and volcanoclastic sediments, tuffs and alkaline magmatic rocks, originated by low degree melting of enrichment OIB mantle source, and probably related with seamount portions incorporated into the accretionary prism. Islas Murciélago pillow and massive basalts show no clear structural relationship with the rest of the units, but are geochemically similar to the dolerites of the Santa Elena Nappe. Sr, Nd, and Pb isotopic ratios of the Santa Elena Nappe and the Santa Elena Accretionary Complex samples do not correspond to the Galapagos Mantle array, and have different mantle reservoirs and geochemical characteristics than the Nicoya Complex
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