2,940 research outputs found
Low energy electron scattering from DNA and RNA bases: shape resonances and radiation damage
Calculations are carried out to determine elastic scattering cross sections
and resonance energies for low energy electron impact on uracil and on each of
the DNA bases (thymine, cytosine, adenine, guanine), for isolated molecules in
their equilibrium geometry. Our calculations are compared with available theory
and experiment. We also attempt to correlate this information with experimental
dissociation patterns through an analysis of the temporary anion structures
that are formed by electron capture in shape resonances.Comment: 20 pages, 12 figures, submitted to J. Chem. Phy
Light-Level Geolocation Reveals The Migration Route And Non-Breeding Location Of An Antillean Nighthawk (Chordeiles Gundlachii)
The Antillean Nighthawk’s (Chordeiles gundlachii) migration routes and non-breeding location were previously unknown. We deployed a geolocator on a female Antillean Nighthawk found breeding on the Lesser Antilles island of Guadeloupe and tracked her annual movements between 2013 and 2014. Her journey included a 2-month stopover on Isla La Tortuga, Venezuela, during southward migration, and a non-breeding season in the remote forestlands of the state of Amazonas, Brazil, approximately 2,100 km south of her breeding grounds. Her migration route was geographically similar in both the fall and spring, following a north-south trajectory, but lacked a prolonged stopover in the spring
Red Supergiants in the Andromeda Galaxy (M31)
Red supergiants are a short-lived stage in the evolution of moderately
massive stars (10-25Mo), and as such their location in the H-R diagram provides
an exacting test of stellar evolutionary models. Since massive star evolution
is strongly affected by the amount of mass-loss a star suffers, and since the
mass-loss rates depend upon metallicity, it is highly desirable to study the
physical properties of these stars in galaxies of various metallicities. Here
we identify a sample of red supergiants in M31 (the most metal-rich of the
Local Group galaxies) and derive their physical properties by fitting MARCS
atmosphere models to moderate resolution optical spectroscopy, and from V-K
photometry.Comment: Accepted for publication in the Astrophysical Journa
The Physical Properties of the Red Supergiant WOH G64: The Largest Star Known?
WOH G64 is an unusual red supergiant (RSG) in the Large Magellanic Cloud
(LMC), with a number of properties that set it apart from the rest of the LMC
RSG population, including a thick circumstellar dust torus, an unusually late
spectral type, maser activity, and nebular emission lines. Its reported
physical properties are also extreme, including the largest radius for any star
known and an effective temperature that is much cooler than other RSGs in the
LMC, both of which are at variance with stellar evolutionary theory. We fit
moderate-resolution optical spectrophotometry of WOH G64 with the MARCS stellar
atmosphere models, determining an effective temperature of 3400 +/- 25 K. We
obtain a similar result from the star's broadband V - K colors. With this
effective temperature, and taking into account the flux contribution from the
aysmmetric circumstellar dust envelope, we calculate log(L/L_sun) = 5.45 +/-
0.05 for WOH G64, quite similar to the luminosity reported by Ohnaka and
collaborators based on their radiative transfer modeling of the star's dust
torus. We determine a radius of R/R_sun = 1540, bringing the size of WOH G64
and its position on the H-R diagram into agreement with the largest known
Galactic RSGs, although it is still extreme for the LMC. In addition, we use
the Ca II triplet absorption feature to determine a radial velocity of 294 +/-
2 km/s for the star; this is the same radial velocity as the rotating gas in
the LMC's disk, which confirms its membership in the LMC and precludes it from
being an unusual Galactic halo giant. Finally, we describe the star's unusual
nebula emission spectrum; the gas is nitrogen-rich and shock-heated, and
displays a radial velocity that is significantly more positive than the star
itself by 50 km/s.Comment: 25 pages, 5 figures; accepted for publication in The Astronomical
Journa
The impact of mass-loss on the evolution and pre-supernova properties of red supergiants
The post main-sequence evolution of massive stars is very sensitive to many
parameters of the stellar models. Key parameters are the mixing processes, the
metallicity, the mass-loss rate and the effect of a close companion. We study
how the red supergiant lifetimes, the tracks in the Hertzsprung-Russel diagram
(HRD), the positions in this diagram of the pre-supernova progenitor as well as
the structure of the stars at that time change for various mass-loss rates
during the red supergiant phase (RSG), and for two different initial rotation
velocities. The surface abundances of RSGs are much more sensitive to rotation
than to the mass-loss rates during that phase. A change of the RSG mass-loss
rate has a strong impact on the RSG lifetimes and therefore on the luminosity
function of RSGs. At solar metallicity, the enhanced mass-loss rate models do
produce significant changes on the populations of blue, yellow and red
supergiants. When extended blue loops or blue ward excursions are produced by
enhanced mass-loss, the models predict that a majority of blue (yellow)
supergiants are post RSG objects. These post RSG stars are predicted to show
much smaller surface rotational velocities than similar blue supergiants on
their first crossing of the HR gap. The position in the HRD of the end point of
the evolution depends on the mass of the hydrogen envelope. More precisely,
whenever, at the pre-supernova stage, the H-rich envelope contains more than
about 5\% of the initial mass, the star is a red supergiant, and whenever the
H-rich envelope contains less than 1\% of the total mass the star is a blue
supergiant. For intermediate situations, intermediate colors/effective
temperatures are obtained. Yellow progenitors for core collapse supernovae can
be explained by the enhanced mass-loss rate models, while the red progenitors
are better fitted by the standard mass-loss rate models.Comment: 19 pages, 11 figures, 6 tables, accepted for publication in Astronomy
and Astrophysic
Molecular Density Functional Theory of Water describing Hydrophobicity at Short and Long Length Scales
We present an extension of our recently introduced molecular density
functional theory of water [G. Jeanmairet et al., J. Phys. Chem. Lett. 4, 619,
2013] to the solvation of hydrophobic solutes of various sizes, going from
angstroms to nanometers. The theory is based on the quadratic expansion of the
excess free energy in terms of two classical density fields, the particle
density and the multipolar polarization density. Its implementation requires as
input a molecular model of water and three measurable bulk properties, namely
the structure factor and the k-dependent longitudinal and transverse dielectric
susceptibilities. The fine three-dimensional water structure around small
hydrophobic molecules is found to be well reproduced. In contrast the computed
solvation free-energies appear overestimated and do not exhibit the correct
qualitative behavior when the hydrophobic solute is grown in size. These
shortcomings are corrected, in the spirit of the Lum-Chandler-Weeks theory, by
complementing the functional with a truncated hard-sphere functional acting
beyond quadratic order in density. It makes the resulting functional compatible
with the Van-der-Waals theory of liquid-vapor coexistence at long range.
Compared to available molecular simulations, the approach yields reasonable
solvation structure and free energy of hard or soft spheres of increasing size,
with a correct qualitative transition from a volume-driven to a surface-driven
regime at the nanometer scale.Comment: 24 pages, 8 figure
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