201 research outputs found
Thermal Alteration of Labile Elements in Carbonaceous Chondrites
Carbonaceous chondrite meteorites are some of the oldest Solar System
planetary materials available for study. The CI group has bulk abundances of
elements similar to those of the solar photosphere. Of particular interest in
carbonaceous chondrite compositions are labile elements, which vaporize and
mobilize efficiently during post-accretionary parent-body heating events. Thus,
they can record low-temperature alteration events throughout asteroid
evolution. However, the precise nature of labile-element mobilization in
planetary materials is unknown. Here we characterize the thermally induced
movements of the labile elements S, As, Se, Te, Cd, Sb, and Hg in carbonaceous
chondrites by conducting experimental simulations of volatile-element
mobilization during thermal metamorphism. This process results in appreciable
loss of some elements at temperatures as low as 500 K. This work builds on
previous laboratory heating experiments on primitive meteorites and shows the
sensitivity of chondrite compositions to excursions in temperature. Elements
such as S and Hg have the most active response to temperature across different
meteorite groups. Labile element mobilization in primitive meteorites is
essential for quantifying elemental fractionation that occurred on asteroids
early in Solar System history. This work is relevant to maintaining a pristine
sample from asteroid (101955) Bennu from the OSIRIS-REx mission and
constraining the past orbital history of Bennu. Additionally, we discuss
thermal effects on surface processes of near-Earth asteroids, including the
thermal history of "rock comets" such as (3200) Phaethon. This work is also
critical for constraining the concentrations of contaminants in vaporized water
extracted from asteroid regolith as part of future in situ resource utilization
for sustained robotic and human space exploration.Comment: 12 pages of text, 3 tables, 7 figures, accepted by Icaru
Adaptive State Estimation for Nonminimum-Phase Systems with Uncertain Harmonic Inputs
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90727/1/AIAA-2011-6315-484.pd
On multicomponent effects in stellar winds of stars at extremely low metallicity
We calculate multicomponent line-driven wind models of stars at extremely low metallicity suitable for massive first generation stars. For most of the models we find that the multicomponent wind nature is not important for either wind dynamics or for wind temperature stratification. However, for stars with the lowest metallicities we find that multicomponent effects influence the wind structure. These effects range from pure heating to possible fallback of the nonabsorbing wind component. We present a simple formula for the calculation of metallicity for which the multicomponent effects become important. We show that the importance of the multicomponent nature of winds of low metallicity stars is characterised not only by the low density of driving ions, but also by lower mass-loss rate
Direct observation of Levy flight of holes in bulk n-InP
We study the photoluminescence spectra excited at an edge side of n-InP slabs
and observed from the broadside. In a moderately doped sample the intensity
drops off as a power-law function of the distance from the excitation - up to
several millimeters - with no change in the spectral shape.The hole
distribution is described by a stationary Levy-flight process over more than
two orders of magnitude in both the distance and hole concentration. For
heavily-doped samples, the power law is truncated by free-carrier absorption.
Our experiments are near-perfectly described by the Biberman-Holstein transport
equation with parameters found from independent optical experiments.Comment: 4 pages, 3 figure
Central Stars of Planetary Nebulae in the Large Magellanic Cloud: A Far-UV Spectroscopic Analysis
We observed seven central stars of planetary nebulae (CSPN) in the Large
Magellanic Cloud (LMC) with the Far Ultraviolet Spectroscopic Explorer (FUSE),
and performed a model-based analysis of these spectra in conjunction with
Hubble Space Telescope (HST) spectra in the UV and optical range to determine
the stellar and nebular parameters. Most of the objects show wind features, and
they have effective temperatures ranging from 38 to 60 kK with mass-loss rates
of ~= 5x10^-8 Msun/yr. Five of the objects have typical LMC abundances. One
object (SMP LMC 61) is a [WC4] star, and we fit its spectra with He/C/O-rich
abundances typical of the [WC] class, and find its atmosphere to be
iron-deficient. Most objects have very hot (T ~> 2000 K) molecular hydrogen in
their nebulae, which may indicate a shocked environment. One of these (SMP LMC
62) also displays OVI 1032-38 nebular emission lines, rarely observed in PN.Comment: 53 pages, 15 figures (11 color). Accepted for publication in Ap
Far-UV Spectroscopic Analyses of Four Central Stars of Planetary Nebulae
We analyze the Far-UV/UV spectra of four central stars of planetary nebulae
with strong wind features -- NGC 2371, Abell 78, IC 4776 and NGC 1535, and
derive their photospheric and wind parameters by modeling high-resolution FUSE
(Far-Ultraviolet Spectroscopic Explorer) data in the Far-UV and HST-STIS and
IUE data in the UV with spherical non-LTE line-blanketed model atmospheres.
Abell 78 is a hydrogen-deficient transitional [WR]-PG 1159 object, and we find
NGC 2371 to be in the same stage, both migrating from the constant-luminosity
phase to the white dwarf cooling sequence with Teff ~= 120 kK, Mdot ~= 5x10^-8
Msun/yr. NGC 1535 is a ``hydrogen-rich'' O(H) CSPN, and the exact nature of IC
4776 is ambiguous, although it appears to be helium burning. Both objects lie
on the constant-luminosity branch of post-AGB evolution and have Teff ~= 65 kK,
Mdot ~= 1x10^-8 Msun/yr. Thus, both the H-rich and H-deficient channels of PN
evolution are represented in our sample. We also investigate the effects of
including higher ionization stages of iron (up to FeX) in the model atmosphere
calculations of these hot objects (usually neglected in previous analyses), and
find iron to be a useful diagnostic of the stellar parameters in some cases.
The Far-UV spectra of all four objects show evidence of hot (T ~ 300 K)
molecular hydrogen in their circumstellar environments.Comment: 38 pages, 8 figures (6 color). Accepted for publication in Ap
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
Predictions for mass-loss rates and terminal wind velocities of massive O-type stars
Mass loss forms an important aspect of the evolution of massive stars, as
well as for the enrichment of the surrounding ISM. Our goal is to predict
accurate mass-loss rates and terminal wind velocities. These quantities can be
compared to empirical values, thereby testing radiation-driven wind models. One
specific issue is that of the "weak-wind problem", where empirically derived
mass-loss rates fall orders of magnitude short of predicted values. We employ
an established Monte Carlo model and a recently suggested new line acceleration
formalism to solve the wind dynamics consistently. We provide a new grid of
mass-loss rates and terminal wind velocities of O stars, and compare the values
to empirical results. Our models fail to provide mass-loss rates for
main-sequence stars below a luminosity of log(L/Lsun) = 5.2, where we run into
a fundamental limit. At luminosities below this critical value there is
insufficient momentum transferred in the region below the sonic point to
kick-start the acceleration. This problem occurs at the location of the onset
of the weak-wind problem. For O dwarfs, the boundary between being able to
start a wind, and failing to do so, is at spectral type O6/O6.5. The direct
cause of this failure is a combination of the lower luminosity and a lack of Fe
V lines at the wind base. This might indicate that another mechanism is
required to provide the necessary driving to initiate the wind. For stars more
luminous than log(L/Lsun) = 5.2, our new mass-loss rates are in excellent
agreement with the mass-loss prescription by Vink et al. 2000. This implies
that the main assumption entering the method of the Vink et al. prescriptions -
i.e. that the momentum equation is not explicitly solved for - does not
compromise the reliability of the Vink et al. results for this part of
parameter space (Abridged).Comment: 10 pages, 10 figures, Astronomy & Astrophysics (in press
Chandra spectroscopy of the hot star beta Crucis and the discovery of a pre-main-sequence companion
In order to test the O star wind-shock scenario for X-ray production in less
luminous stars with weaker winds, we made a pointed 74 ks observation of the
nearby early B giant, beta Cru (B0.5 III), with the Chandra HETGS. We find that
the X-ray spectrum is quite soft, with a dominant thermal component near 3
million K, and that the emission lines are resolved but quite narrow, with
half-widths of 150 km/s. The forbidden-to-intercombination line ratios of Ne IX
and Mg XI indicate that the hot plasma is distributed in the wind, rather than
confined near the photosphere. It is difficult to understand the X-ray data in
the context of the standard wind-shock paradigm for OB stars, primarily because
of the narrow lines, but also because of the high X-ray production efficiency.
A scenario in which the bulk of the outer wind is shock heated is broadly
consistent with the data, but not very well motivated theoretically. It is
possible that magnetic channeling could explain the X-ray properties, although
no field has been detected on beta Cru. We detected periodic variability in the
hard (hnu > 1 keV) X-rays, modulated on the known optical period of 4.58 hours,
which is the period of the primary beta Cep pulsation mode for this star. We
also have detected, for the first time, an apparent companion to beta Cru at a
projected separation of 4 arcsec. This companion was likely never seen in
optical images because of the presumed very high contrast between it and beta
Cru in the optical. However, the brightness contrast in the X-ray is only 3:1,
which is consistent with the companion being an X-ray active low-mass
pre-main-sequence star. The companion's X-ray spectrum is relatively hard and
variable, as would be expected from a post T Tauri star.Comment: Accepted for publication in MNRAS; 19 pages, 15 figures, some in
color; version with higher-resolution figures available at
http://astro.swarthmore.edu/~cohen/papers/bcru_mnras2008.pd
Mass-loss rates of Very Massive Stars
We discuss the basic physics of hot-star winds and we provide mass-loss rates
for (very) massive stars. Whilst the emphasis is on theoretical concepts and
line-force modelling, we also discuss the current state of observations and
empirical modelling, and address the issue of wind clumping.Comment: 36 pages, 15 figures, Book Chapter in "Very Massive Stars in the
Local Universe", Springer, Ed. Jorick S. Vin
- …