545 research outputs found
[TiII] and [NiII] emission from the strontium filament of eta Carinae
We study the nature of the [TiII] and [NiII] emission from the so-called
strontium filament found in the ejecta of eta Carinae. To this purpose we
employ multilevel models of the TiII and NiII systems which are used to
investigate the physical condition of the filament and the excitation
mechanisms of the observed lines. For the TiII ion, for which no atomic data
was previously available, we carry out ab initio calculations of radiative
transition rates and electron impact excitation rate coefficients. It is found
that the observed spectrum is consistent with the lines being excited in a
mostly neutral region with an electron density of the order of cm
and a temperature around 6000 K. In analyzing three observations with different
slit orientations recorded between March~2000 and November~2001 we find line
ratios that change among various observations, in a way consistent with changes
of up to an order of magnitude in the strength of the continuum radiation
field. These changes result from different samplings of the extended filament,
due to the different slit orientations used for each observation, and yield
clues on the spatial extent and optical depth of the filament. The observed
emission indicates a large Ti/Ni abundance ratio relative to solar abundances.
It is suggested that the observed high Ti/Ni ratio in gas is caused by dust-gas
fractionation processes and does not reflect the absolute Ti/Ni ratio in the
ejecta of \etacar. We study the condensation chemistry of Ti, Ni and Fe within
the filament and suggest that the observed gas phase overabundance of TiComment: 14 paginas, 12 figure
Habitable Planet Formation in Binary-Planetary Systems
Recent radial velocity observations have indicated that Jovian-type planets
can exist in moderately close binary star systems. Numerical simulations of the
dynamical stability of terrestrial-class planets in such environments have
shown that, in addition to their giant planets, these systems can also harbor
Earth-like objects. In this paper, we study the late stage of terrestrial
planet formation in such binary-planetary systems, and present the results of
the simulations of the formation of Earth-like bodies in their habitable zones.
We consider a circumprimary disk of Moon- to Mars-sized objects and numerically
integrate the orbits of these bodies at the presence of the Jovian-type planet
of the system and for different values of the mass, semimajor axis, and orbital
eccentricity of the secondary star. Results indicate that, Earth-like objects,
with substantial amounts of water, can form in the habitable zone of the
primary star. Simulations also indicate that, by transferring angular momentum
from the secondary star to protoplanetary objects, the giant planet of the
system plays a key role in the radial mixing of these bodies and the water
contents of the final terrestrial planets. We will discuss the results of our
simulation and show that the formation of habitable planets in binary-planetary
systems is more probable in binaries with moderate to large perihelia.Comment: 27 pages, 11 figures, submitted for publicatio
Gas phase atomic metals in the circumstellar envelope of IRC+10216
We report the results of a search for gas phase atomic metals in the
circumstellar envelope of the AGB carbon star IRC+10216. The search was made
using high resolution (R=50000) optical absorption spectroscopy of a backgound
star that probes the envelope on a line of sight 35" from the center. The metal
species that we detect in the envelope include NaI, KI, CaI, CaII, CrI, and
FeI, with upper limits for AlI, MnI, TiI, TiII, and SrII. The observations are
used to determine the metal abundances in the gas phase and the condensation
onto grains. The metal depletions range from a factor of 5 for Na to 300 for
Ca, with some similarity to the depletion pattern in interstellar clouds. Our
results directly constrain the condensation efficiency of metals in a
carbon-rich circumstellar envelope and the mix of solid and gas phase metals
returned by the star to the ISM. The abundances of the uncondensed metal atoms
that we observe are typically larger than the abundances of the metal-bearing
molecules detected in the envelope. The metal atoms are therefore the major
metal species in the gas phase and likely play a key role in the metal
chemistry.Comment: 11 pages, 8 Figures. Accepted by Astronomy and Astrophysic
Tungsten isotopic compositions in stardust SiC grains from the Murchison meteorite: Constraints on the s-process in the Hf-Ta-W-Re-Os region
We report the first tungsten isotopic measurements in stardust silicon
carbide (SiC) grains recovered from the Murchison carbonaceous chondrite. The
isotopes 182W, 183W, 184W, 186W and 179Hf, 180Hf were measured on both an
aggregate (KJB fraction) and single stardust SiC grains (LS+LU fraction)
believed to have condensed in the outflows of low-mass carbon-rich asymptotic
giant branch (AGB) stars with close-to-solar metallicity. The SiC aggregate
shows small deviations from terrestrial (=solar) composition in the 182W/184W
and 183W/184W ratios, with deficits in 182W and 183W with respect to 184W. The
186W/184W ratio, however, shows no apparent deviation from the solar value.
Tungsten isotopic measurements in single mainstream stardust SiC grains
revealed lower than solar 182W/184W, 183W/184W, and 186W/184W ratios. We have
compared the SiC data with theoretical predictions of the evolution of W
isotopic ratios in the envelopes of AGB stars. These ratios are affected by the
slow neutron-capture process and match the SiC data regarding their 182W/184W,
183W/184W, and 179Hf/180Hf isotopic compositions, although a small adjustment
in the s-process production of 183W is needed in order to have a better
agreement between the SiC data and model predictions. The models cannot explain
the 186W/184W ratios observed in the SiC grains, even when the current 185W
neutron-capture cross section is increased by a factor of two. Further study is
required to better assess how model uncertainties (e.g., the formation of the
13C neutron source, the mass-loss law, the modelling of the third dredge-up,
and the efficiency of the 22Ne neutron source) may affect current s-process
predictions.Comment: Accepted for Publication on The Astrophysical Journal 43 pages, 2
tables, 7 figure
High-resolution spectroscopy of QY Sge -- An obscured RV Tauri variable?
The first high-resolution optical spectra of QY Sge are presented and
discussed. Menzies & Whitelock (1988) on the basis of photometry and
low-resolution spectra suggested that this G0I supergiant was obscured by dust
and seen only by scattered light from a circumstellar reflection nebula. The
new spectra confirm and extend this picture. Photospheric lines are unusually
broad indicating scattering of photons from dust in the stellar wind. Presence
of very broad Na D emission lines is confirmed. Sharp emission lines from low
levels of abundant neutral metal atoms are reported for the first time. An
abundance analysis of photospheric lines shows that the stellar atmosphere is
of approximately solar composition but with highly condensible (e.g., Sc and
Ti) elements depleted by factors of 5 to 10.Comment: 8 pages, 9 figures, accepted for publication in MNRA
On the oxygen isotopic composition of the Solar System
The 18O/17O ratio of the Solar System is 5.2 while that of the interstellar
medium (ISM) and young stellar objects is ~4. This difference cannot be
explained by pollution of the Sun's natal molecular cloud by 18O-rich supernova
ejecta because (1) the necessary B-star progenitors live longer than the
duration of star formation in molecular clouds; (2) the delivery of ejecta gas
is too inefficient and the amount of dust in supernova ejecta is too small
compared to the required pollution (2% of total mass or ~20% of oxygen); and
(3) the predicted amounts of concomitant short-lived radionuclides (SLRs)
conflicts with the abundances of 26Al and 41Ca in the early Solar System.
Proposals for the introduction of 18O-rich material must also be consistent
with any explanation for the origin of the observed slope-one relationship
between 17O/16O and 18O/16O in the high-temperature components of primitive
meteorites. The difference in 18O/17O ratios can be explained by enrichment of
the ISM by the 17O-rich winds of asymptotic giant branch (AGB) stars, the
sequestration of comparatively 18O-rich gas from star-forming regions into
long-lived, low-mass stars, and a monotonic decrease in the 18O/17O ratio of
interstellar gas. At plausible rates of star formation and gas infall, Galactic
chemical evolution does not follow a slope-one line in an three-isotope plot,
but instead moves along a steeper trajectory towards an 17O-rich state.
Evolution of the ISM and star-forming gas by AGB winds also explains the
difference in the carbon isotope ratios of the Solar System and ISM.Comment: accepted to ApJ Letter
Self-Consistent Model Atmospheres and the Cooling of the Solar System's Giant Planets
We compute grids of radiative-convective model atmospheres for Jupiter,
Saturn, Uranus, and Neptune over a range of intrinsic fluxes and surface
gravities. The atmosphere grids serve as an upper boundary condition for models
of the thermal evolution of the planets. Unlike previous work, we customize
these grids for the specific properties of each planet, including the
appropriate chemical abundances and incident fluxes as a function of solar
system age. Using these grids, we compute new models of the thermal evolution
of the major planets in an attempt to match their measured luminosities at
their known ages. Compared to previous work, we find longer cooling times,
predominantly due to higher atmospheric opacity at young ages. For all planets,
we employ simple "standard" cooling models that feature adiabatic temperature
gradients in the interior H/He and water layers, and an initially hot starting
point for the calculation of subsequent cooling. For Jupiter we find a model
cooling age 10% longer than previous work, a modest quantitative difference.
This may indicate that the hydrogen equation of state used here overestimates
the temperatures in the deep interior of the planet. For Saturn we find a model
cooling age 20% longer than previous work. However, an additional energy
source, such as that due to helium phase separation, is still clearly needed.
For Neptune, unlike in work from the 1980s and 1990s, we match the measured
Teff of the planet with a model that also matches the planet's current gravity
field. This is predominantly due to advances in the equation of state of water.
This may indicate that the planet possesses no barriers to efficient convection
in its deep interior. However, for Uranus, our models exacerbate the well-known
problem that Uranus is far cooler than calculations predict, which could imply
strong barriers to interior convective cooling.Comment: 45 pages, Accepted to Ap
A Time-Dependent Radiative Model of HD209458b
We present a time-dependent radiative model of the atmosphere of HD209458b
and investigate its thermal structure and chemical composition. In a first
step, the stellar heating profile and radiative timescales were calculated
under planet-averaged insolation conditions. We find that 99.99% of the
incoming stellar flux has been absorbed before reaching the 7 bar level.
Stellar photons cannot therefore penetrate deeply enough to explain the large
radius of the planet. We derive a radiative time constant which increases with
depth and reaches about 8 hr at 0.1 bar and 2.3 days at 1 bar. Time-dependent
temperature profiles were also calculated, in the limit of a zonal wind that is
independent on height (i.e. solid-body rotation) and constant absorption
coefficients. We predict day-night variations of the effective temperature of
\~600 K, for an equatorial rotation rate of 1 km/s, in good agreement with the
predictions by Showman &Guillot (2002). This rotation rate yields day-to-night
temperature variations in excess of 600 K above the 0.1-bar level. These
variations rapidly decrease with depth below the 1-bar level and become
negligible below the ~5--bar level for rotation rates of at least 0.5 km/s. At
high altitudes (mbar pressures or less), the night temperatures are low enough
to allow sodium to condense into Na2S. Synthetic transit spectra of the visible
Na doublet show a much weaker sodium absorption on the morning limb than on the
evening limb. The calculated dimming of the sodium feature during planetary
transites agrees with the value reported by Charbonneau et al. (2002).Comment: 9 pages, 8 figures, replaced with the revised versio
Aluminum-, Calcium- And Titanium-Rich Oxide Stardust In Ordinary Chondrite Meteorites
We report isotopic data for a total of 96 presolar oxide grains found in
residues of several unequilibrated ordinary chondrite meteorites. Identified
grain types include Al2O3, MgAl2O4, hibonite (CaAl12O19) and Ti oxide. This
work greatly increases the presolar hibonite database, and is the first report
of presolar Ti oxide. O-isotopic compositions of the grains span previously
observed ranges and indicate an origin in red giant and asymptotic giant branch
(AGB) stars of low mass (<2.5 MSun) for most grains. Cool bottom processing in
the parent AGB stars is required to explain isotopic compositions of many
grains. Potassium-41 enrichments in hibonite grains are attributable to in situ
decay of now-extinct 41Ca. Inferred initial 41Ca/40Ca ratios are in good
agreement with model predictions for low-mass AGB star envelopes, provided that
ionization suppresses 41Ca decay. Stable Mg and Ca isotopic ratios of most of
the hibonite grains reflect primarily the initial compositions of the parent
stars and are generally consistent with expectations for Galactic chemical
evolution, but require some local interstellar chemical inhomogeneity. Very
high 17O/16O or 25Mg/24Mg ratios suggest an origin for some grains in binary
star systems where mass transfer from an evolved companion has altered the
parent star compositions. A supernova origin for the hitherto enigmatic
18O-rich Group 4 grains is strongly supported by multi-element isotopic data
for two grains. The Group 4 data are consistent with an origin in a single
supernova in which variable amounts of material from the deep 16O-rich interior
mixed with a unique end-member mixture of the outer layers. The Ti oxide grains
primarily formed in low-mass AGB stars. They are smaller and rarer than
presolar Al2O3, reflecting the lower abundance of Ti than Al in AGB envelopes.Comment: Accepted for publication in ApJ; 47 pages, 13 figure
Possible thermochemical disequilibrium in the atmosphere of the exoplanet GJ 436b
The nearby extrasolar planet GJ 436b--which has been labelled as a 'hot
Neptune'--reveals itself by the dimming of light as it crosses in front of and
behind its parent star as seen from Earth. Respectively known as the primary
transit and secondary eclipse, the former constrains the planet's radius and
mass, and the latter constrains the planet's temperature and, with measurements
at multiple wavelengths, its atmospheric composition. Previous work using
transmission spectroscopy failed to detect the 1.4-\mu m water vapour band,
leaving the planet's atmospheric composition poorly constrained. Here we report
the detection of planetary thermal emission from the dayside of GJ 436b at
multiple infrared wavelengths during the secondary eclipse. The best-fit
compositional models contain a high CO abundance and a substantial methane
(CH4) deficiency relative to thermochemical equilibrium models for the
predicted hydrogen-dominated atmosphere. Moreover, we report the presence of
some H2O and traces of CO2. Because CH4 is expected to be the dominant
carbon-bearing species, disequilibrium processes such as vertical mixing and
polymerization of methane into substances such as ethylene may be required to
explain the hot Neptune's small CH4-to-CO ratio, which is at least 10^5 times
smaller than predicted
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