3,069 research outputs found
Microstructure mapping: a new method for imaging deformation-induced microstructural features of ice on the grain scale
The rapid evolution of the exciting star of the Stingray Nebula
SAO244567, the exciting star of the Stingray nebula, is rapidly evolving.
Previous analyses suggested that it has heated up from an effective temperature
of about 21kK in 1971 to over 50kK in the 1990s. Canonical post-asymptotic
giant branch evolution suggests a relatively high mass while previous analyses
indicate a low-mass star. Fitting line profiles from static and expanding
non-LTE model atmospheres to the observed UV and optical spectra, taken during
1988-2013, allowed us to study the temporal change of effective temperature,
surface gravity, mass-loss rate, and terminal wind velocity. In addition, we
determined the chemical composition of the atmosphere. We find that the central
star has steadily increased its effective temperature from 38kK in 1988 to a
peak value of 60kK in 2002. During the same time, the star was contracting, as
concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a
drop in luminosity. Simultaneously, the mass-loss rate declined from log
(dM/dt/Msun/yr)=-9.0 to -11.6 and the terminal wind velocity increased from
1800km/s to 2800km/s. Since around 2002, the star stopped heating and has
cooled down again to 55kK by 2006. It has a largely solar surface composition
with the exception of slightly subsolar carbon, phosphorus, and sulfur. By
comparison with stellar-evolution calculations, we confirm that SAO244567 must
be a low-mass star (M < 0.55 Msun). However, the slow evolution of the
respective stellar evolutionary models is in strong contrast to the observed
fast evolution and the young planetary nebula with a kinematical age of only
about 1000 years. We speculate that the star could be a late He-shell flash
object. Alternatively, it could be the outcome of close-binary evolution. Then
SAO244567 would be a low-mass (0.354 Msun) helium prewhite dwarf after the
common-envelope phase, during which the planetary nebula was ejected.Comment: 16 pages, 13 figures, accepted for publication in A&
Conflicts and solutions related to marine turtle conservation initiatives in the Caribbean basin: identifying new challenges
Conflicts among and between local, national, regional and international stakeholders involved in marine turtle conservation are increasing. Often, they arise because of different socio-economic backgrounds of the people or groups involved. Here, we identified and assessed the conservation-based conflicts occurring in 24 of the 39 Caribbean countries, including their frequency, level of severity, number of stakeholders' groups involved, the degree to which they hinder conservation goals, and potential solutions. Using a cross-sectional social survey, we evaluated the presence and details of conservation conflicts provided by 72 respondents. The respondents included conservation-based project leaders, researchers, people involved in policy-based decision-making, conservation volunteers (community-based conservation groups), and species experts with experience working on marine turtle conservation programs in the Caribbean. The respondents identified 136 conflicts, and we grouped them into 16 different categories. The most commonly mentioned causes of conflicts were: 1) the ‘lack of enforcement by local authorities to support conservation-based legislation or programs’ (18%); 2) ‘legal consumption of turtles by one sector of community clashing the conservation aspirations of other sectors of community (14%); and 3) ’variable enforcement of legislation to limit/prohibit use across range states of the species (10%). From our data it is also apparent that illicit activities in the region are also likely to impact the future success of conservation or monitoring based projects and programs. Overall, an exhaustive review was carried out, and the potential solutions were gathered. Due to the level of severity (physical violence) that some conflicts have reached, achieving solutions will be challenging without mediation, mutual cooperation around shared values, and adaptive management arrangements. Achieving this will require combinations of bottom up and top down collaborative governance approaches
How Massive Single Stars End their Life
How massive stars die -- what sort of explosion and remnant each produces --
depends chiefly on the masses of their helium cores and hydrogen envelopes at
death. For single stars, stellar winds are the only means of mass loss, and
these are chiefly a function of the metallicity of the star. We discuss how
metallicity, and a simplified prescription for its effect on mass loss, affects
the evolution and final fate of massive stars. We map, as a function of mass
and metallicity, where black holes and neutron stars are likely to form and
where different types of supernovae are produced. Integrating over an initial
mass function, we derive the relative populations as a function of metallicity.
Provided single stars rotate rapidly enough at death, we speculate upon stellar
populations that might produce gamma-ray bursts and jet-driven supernovae.Comment: 24 pages, 9 figues, submitted to Ap
Observational properties of massive black hole binary progenitors
The first directly detected gravitational waves (GW 150914) were emitted by
two coalescing black holes (BHs) with masses of ~36Msun and ~29Msun. Several
scenarios have been proposed to put this detection into an astrophysical
context. The evolution of an isolated massive binary system is among commonly
considered models. Various groups have performed detailed binary-evolution
calculations that lead to BH merger events. However, the question remains open
as to whether binary systems with the predicted properties really exist. The
aim of this paper is to help observers to close this gap by providing spectral
characteristics of massive binary BH progenitors during a phase where at least
one of the companions is still non-degenerate. Stellar evolution models predict
fundamental stellar parameters. Using these as input for our stellar atmosphere
code (PoWR), we compute a set of models for selected evolutionary stages of
massive merging BH progenitors at different metallicities. The synthetic
spectra obtained from our atmosphere calculations reveal that progenitors of
massive BH merger events start their lives as O2-3V stars that evolve to
early-type blue supergiants before they undergo core-collapse during the
Wolf-Rayet phase. When the primary has collapsed, the remaining system will
appear as a wind-fed high-mass X-ray binary. We provide feedback parameters,
broad band magnitudes, and spectral templates that should help to identify such
binaries in the future. Comparisons of empirically determined mass-loss rates
with those assumed by evolution calculations reveal significant differences.
The consideration of the empirical mass-loss rates in evolution calculations
will possibly entail a shift of the maximum in the predicted binary-BH merger
rate to higher metallicities, that is, more candidates should be expected in
our cosmic neighborhood than previously assumed.Comment: 64 pages, 30 figures, accepted for publication in Astronomy &
Astrophysics, v2: typos correcte
Effects of the stellar wind on X-ray spectra of Cygnus X-3
We study X-ray spectra of Cyg X-3 from BeppoSAX, taking into account
absorption and emission in the strong stellar wind of its companion. We find
the intrinsic X-ray spectra are well modelled by disc blackbody emission, its
upscattering by hot electrons with a hybrid distribution, and by Compton
reflection. These spectra are strongly modified by absorption and reprocessing
in the stellar wind, which we model using the photoionization code cloudy. The
form of the observed spectra implies the wind is composed of two phases. A hot
tenuous plasma containing most of the wind mass is required to account for the
observed features of very strongly ionized Fe. Small dense cool clumps filling
<0.01 of the volume are required to absorb the soft X-ray excess, which is
emitted by the hot phase but not present in the data. The total mass-loss rate
is found to be (0.6--1.6) x 10^-5 solar masses per year. We also discuss the
feasibility of the continuum model dominated by Compton reflection, which we
find to best describe our data. The intrinsic luminosities of our models
suggest that the compact object is a black hole.Comment: MNRAS, in pres
Variational quantum Monte Carlo calculations for solid surfaces
Quantum Monte Carlo methods have proven to predict atomic and bulk properties
of light and non-light elements with high accuracy. Here we report on the first
variational quantum Monte Carlo (VMC) calculations for solid surfaces. Taking
the boundary condition for the simulation from a finite layer geometry, the
Hamiltonian, including a nonlocal pseudopotential, is cast in a layer resolved
form and evaluated with a two-dimensional Ewald summation technique. The exact
cancellation of all Jellium contributions to the Hamiltonian is ensured. The
many-body trial wave function consists of a Slater determinant with
parameterized localized orbitals and a Jastrow factor with a common two-body
term plus a new confinement term representing further variational freedom to
take into account the existence of the surface. We present results for the
ideal (110) surface of Galliumarsenide for different system sizes. With the
optimized trial wave function, we determine some properties related to a solid
surface to illustrate that VMC techniques provide standard results under full
inclusion of many-body effects at solid surfaces.Comment: 9 pages with 2 figures (eps) included, Latex 2.09, uses REVTEX style,
submitted to Phys. Rev.
Emission-Line Properties of z > 4 Quasars
We present results of a program of high signal-to-noise spectroscopy for 44
QSOs at redshifts > 4 using the MMT and Keck observatories. The quasar spectra
cover 1100 -- 1700 A in the rest frame for sources spanning a luminosity range
of approximately 2 orders of magnitude. Comparisons between these data and
spectra of lower redshift quasars reveal a high degree of similarity, although
differences are present in the profiles and the strengths of some emission
features. An examination of the luminosity dependence of the emission lines
reveals evidence for a weak or absent Baldwin effect among z > 4 QSOs. We
compare measurements for objects in our sample with results from other high
redshift surveys characterized by different selection techniques. Distributions
of equivalent widths for these different ensembles are consistent with a common
parent population, suggesting that our sample is not strongly biased, or in any
case, subject to selection effects that are not significantly different from
other surveys, including the Sloan Digital Sky Survey. Based on this
comparison, we tentatively conclude that the trends identified here are
representative of high z QSOs. In particular, the data bolster indications of
supersolar metallicities in these luminous, high-z sources, which support
scenarios that assume substantial star formation at epochs preceding or
concurrent with the QSO phenomena.Comment: 26 pages (incl. 9 figures), AASTeX v5.0, to appear in The
Astrophysical Journa
A Systematic Search for Corotating Interaction Regions in Apparently Single Galactic WR Stars. I. Characterizing the Variability
We present the results of a systematic search for large-scale spectroscopic
variability in apparently single Wolf-Rayet stars brighter than ~12.5. In this
first paper we characterize the various forms of variability detected and
distinguish several separate groups. For each star in our sample, we obtained
4-5 high-resolution spectra with a signal-to-noise ratio ~100. Our ultimate
goal is to identify new candidates presenting variability that potentially
comes from Co-rotating Interaction Regions (CIR).
Out of a sample of 25 stars, 10 were found to display large-scale changes of
which 4 are of CIR-type (WR1, WR115, WR120 and WR134). The star WR134 was
already known to show such changes from previous studies. Three WN8 stars
present a different type of large-scale variability and we believe deserve a
group of their own. Also, all three WC9d stars in our sample present
large-scale variability, but it remains to be checked if these are binaries, as
many dust-making WR stars are double. Finally, of the remaining stars, 10 were
found to show small-amplitude spectral changes which we attribute to normal
line-profile variability due to inhomogeneities in the wind, and 5 were found
to show no spectral variability, as far as can be concluded from the data in
hand.
Follow-up studies are required to identify potential periods for our
candidates showing CIR-type changes and eventually estimate a rotation rate for
these WR stars.Comment: 25 pages, 13 figure
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