956 research outputs found
A Simple Model for the Absorption of Starlight by Dust in Galaxies
We present a new model to compute the effects of dust on the integrated
spectral properties of galaxies, based on an idealized prescription of the main
features of the interstellar medium (ISM). The model includes the ionization of
HII regions in the interiors of the dense clouds in which stars form and the
influence of the finite lifetime of these clouds on the absorption of
radiation. We compute the production of emission lines and the absorption of
continuum radiation in the HII regions and the subsequent transfer of line and
continuum radiation in the surrounding HI regions and the ambient ISM. This
enables us to interpret simultaneously all the observations of a homogeneous
sample of nearby UV-selected starburst galaxies, including the ratio of far-IR
to UV luminosities, the ratio of Halpha to Hbeta luminosities, the Halpha
equivalent width, and the UV spectral slope. We show that the finite lifetime
of stellar birth clouds is a key ingredient to resolve an apparent discrepancy
between the attenuation of line and continuum photons in starburst galaxies. In
addition, we find that an effective absorption curve proportional to
lambda^-0.7 reproduces the observed relation between the ratio of far-IR to UV
luminosities and the UV spectral slope. We interpret this relation most simply
as a sequence in the overall dust content of the galaxies. The shallow
wavelength dependence of the effective absorption curve is compatible with the
steepness of known extinction curves if the dust has a patchy distribution. In
particular, we find that a random distribution of discrete clouds with optical
depths similar to those in the Milky Way provides a consistent interpretation
of all the observations. Our model for absorption can be incorporated easily
into any population synthesis model. (abridged)Comment: To appear in the 2000 July 20 issue of the Astrophysical Journal; 19
pages with 13 embedded PS figures (emulateapj5.sty
Formation of Primordial Protostars
The evolution of collapsing metal free protostellar clouds is investigated
for various masses and initial conditions.
We perform hydrodynamical calculations for spherically symmetric clouds
taking account of radiative transfer of the molecular hydrogen lines and the
continuum, as well as of chemistry of the molecular hydrogen.
The collapse is found to proceed almost self-similarly like Larson-Penston
similarity solution.
In the course of the collapse, efficient three-body processes transform
atomic hydrogen in an inner region of \sim 1 M_{\sun} entirely into molecular
form.
However, hydrogen in the outer part remains totally atomic although there is
an intervening transitional layer of several solar masses, where hydrogen is in
partially molecular form.
No opaque transient core is formed although clouds become optically thick to
H collision-induced absorption continuum, since H dissociation
follows successively.
When the central part of the cloud reaches stellar densities (), a very small hydrostatic core (\sim
5 \times 10^{-3} M_{\sun}) is formed and subsequently grows in mass as the
ambient gas accretes onto it.
The mass accretion rate is estimated to be 3.7 \times 10^{-2} M_{\sun}
{\rm yr^{-1}} (M_{\ast}/M_{\sun})^{-0.37}, where is instantaneous
mass of the central core, by using a similarity solution which reproduces the
evolution of the cloud before the core formation.Comment: 20 pages, 5 Postscript figures, uses AAS LaTe
Radiative transfer effects in primordial hydrogen recombination
The calculation of a highly accurate cosmological recombination history has
been the object of particular attention recently, as it constitutes the major
theoretical uncertainty when predicting the angular power spectrum of Cosmic
Microwave Background anisotropies. Lyman transitions, in particular the
Lyman-alpha line, have long been recognized as one of the bottlenecks of
recombination, due to their very low escape probabilities. The Sobolev
approximation does not describe radiative transfer in the vicinity of Lyman
lines to a sufficient degree of accuracy, and several corrections have already
been computed in other works. In this paper, the impact of some previously
ignored radiative transfer effects is calculated. First, the effect of Thomson
scattering in the vicinity of the Lyman-alpha line is evaluated, using a full
redistribution kernel incorporated into a radiative transfer code. The effect
of feedback of distortions generated by the optically thick deuterium
Lyman-alpha line blueward of the hydrogen line is investigated with an analytic
approximation. It is shown that both effects are negligible during cosmological
hydrogen recombination. Secondly, the importance of high-lying, non overlapping
Lyman transitions is assessed. It is shown that escape from lines above
Ly-gamma and frequency diffusion in Ly-beta and higher lines can be neglected
without loss of accuracy. Thirdly, a formalism generalizing the Sobolev
approximation is developed to account for the overlap of the high-lying Lyman
lines, which is shown to lead to negligible changes to the recombination
history. Finally, the possibility of a cosmological hydrogen recombination
maser is investigated. It is shown that there is no such maser in the purely
radiative treatment presented here.Comment: 23 pages, 4 figures, to be submitted to PR
Importance of Compton scattering to radiation spectra of isolated neutron stars
Model atmospheres of isolated neutron stars with low magnetic field are
calculated with Compton scattering taking into account. Models with effective
temperatures 1, 3 and 5 MK, with two values of surface gravity log(g)g = 13.9
and 14.3), and different chemical compositions are calculated. Radiation
spectra computed with Compton scattering are softer than the computed with
Thomson scattering at high energies (E > 5 keV) for hot (T_eff > 1 MK)
atmospheres with hydrogen-helium composition. Compton scattering is more
significant to hydrogen models with low surface gravity. The emergent spectra
of the hottest (T_eff > 3 MK) model atmospheres can be described by diluted
blackbody spectra with hardness factors ~ 1.6 - 1.9. Compton scattering is less
important for models with solar abundance of heavy elements.Comment: Proceedings of the 363. WE-Heraeus Seminar on: Neutron Stars and
Pulsars (Posters and contributed talks) Physikzentrum Bad Honnef, Germany,
May.14-19, 2006, eds. W.Becker, H.H.Huang, MPE Report 291, pp.173-17
How exactly did the Universe become neutral?
We present a refined treatment of H, He I, and He II recombination in the
early Universe. The difference from previous calculations is that we use
multi-level atoms and evolve the population of each level with redshift by
including all bound-bound and bound-free transitions. In this framework we
follow several hundred atomic energy levels for H, He I, and He II combined.
The main improvements of this method over previous recombination calculations
are: (1) allowing excited atomic level populations to depart from an
equilibrium distribution; (2) replacing the total recombination coefficient
with recombination to and photoionization from each level directly at each
redshift step; and (3) correct treatment of the He I atom, including the
triplet and singlet states. We find that the ionization fraction x_e = n_e/n_H
is approximately 10% smaller at redshifts <~800 than in previous calculations,
due to the non-equilibrium of the excited states of H, which is caused by the
strong but cool radiation field at those redshifts. In addition we find that He
I recombination is delayed compared with previous calculations, and occurs only
just before H recombination. These changes in turn can affect the predicted
power spectrum of microwave anisotropies at the few percent level. Other
improvements such as including molecular and ionic species of H, including
complete heating and cooling terms for the evolution of the matter temperature,
including collisional rates, and including feedback of the secondary spectral
distortions on the radiation field, produce negligible change to x_e. The lower
x_e at low z found in this work affects the abundances of H molecular and ionic
species by 10-25%. However this difference is probably not larger than other
uncertainties in the reaction rates.Comment: 24 pages, including 18 figures, using emulateapj.sty, to appear in
ApJ, the code recfast can be obtained at
http://www.astro.ubc.ca/people/scott/recfast.html (in FORTRAN) and
http://cfa-www.harvard.edu/~sasselov/rec/ (in C
Photoemission of BiSe with Circularly Polarized Light: Probe of Spin Polarization or Means for Spin Manipulation?
Topological insulators are characterized by Dirac cone surface states with
electron spins aligned in the surface plane and perpendicular to their momenta.
Recent theoretical and experimental work implied that this specific spin
texture should enable control of photoelectron spins by circularly polarized
light. However, these reports questioned the so far accepted interpretation of
spin-resolved photoelectron spectroscopy. We solve this puzzle and show that
vacuum ultraviolet photons (50-70 eV) with linear or circular polarization
probe indeed the initial state spin texture of BiSe while circularly
polarized 6 eV low energy photons flip the electron spins out of plane and
reverse their spin polarization. Our photoemission calculations, considering
the interplay between the varying probing depth, dipole selection rules and
spin-dependent scattering effects involving initial and final states explain
these findings, and reveal proper conditions for light-induced spin
manipulation. This paves the way for future applications of topological
insulators in opto-spintronic devices.Comment: Submitted for publication (2013
Mapping the energy landscape of biomolecules using single molecule force correlation spectroscopy (FCS): Theory and applications
In the current AFM experiments the distribution of unfolding times, P(t), is
measured by applying a constant stretching force f_s from which the apparent
unfolding rate is obtained. To describe the complexity of the underlying energy
landscape requires additional probes that can incorporate the dynamics of
tension propagation and relaxation of the polypeptide chain upon force quench.
We introduce a theory of force correlation spectroscopy (FCS) to map the
parameters of the energy landscape of proteins. In the FCS the joint
distribution, P(T,t) of folding and unfolding times is constructed by repeated
application of cycles of stretching at constant fs, separated by release
periods T during which the force is quenched to f_q<f_s. During the release
period, the protein can collapse to a manifold of compact states or refold. We
show that P(T,t) can be used to resolve the kinetics of unfolding as well as
formation of native contacts and to extract the parameters of the energy
landscape using chain extension as the reaction coordinate and P(T,t). We
illustrate the utility of the proposed formalism by analyzing simulations of
unfolding-refolding trajectories of a coarse-grained protein S1 with beta-sheet
architecture for several values of f_s, T and f_q=0. The simulations of
stretch-relax trajectories are used to map many of the parameters that
characterize the energy landscape of S1.Comment: 23 pages, 9 figures; accepted to Biophysical Journa
Constraints on a Universal IMF from UV to Near-IR Galaxy Luminosity Densities
We obtain constraints on the slope of a universal stellar initial mass
function (IMF) over a range of cosmic star-formation histories (SFH) using
z=0.1 luminosity densities in the range from 0.2 to 2.2 microns. The age-IMF
degeneracy of integrated spectra of stellar populations can be broken for the
Universe as a whole by using direct measurements of (relative) cosmic SFH from
high-redshift observations. These have only marginal dependence on
uncertainties in the IMF, whereas, fitting to local luminosity densities
depends strongly on both cosmic SFH and the IMF. We fit to these measurements
using population synthesis and find the best-fit IMF power-law slope to be
Gamma=1.15+-0.2 (0.5 < M/M_solar < 120). This slope is in good agreement with
the Salpeter IMF slope (Gamma=1.35). A strong upper limit of Gamma<1.7 is
obtained which effectively rules out the Scalo IMF due to its too low fraction
of high-mass stars. This upper limit is at the 99.7% confidence level if we
assume a closed-box chemical evolution scenario and 95% if we assume constant
solar metallicity. Fitting to the H-alpha line luminosity density, we obtain a
best-fit IMF slope in good agreement with that derived from broadband
measurements. Marginalizing over cosmic SFH and IMF slope, we obtain (95% conf.
ranges, h=1): omega_stars = 1.1-2.0 E-3 for the stellar mass density; rho_sfr =
0.7-4.1 E-2 M_solar/yr/Mpc^3 for the star-formation rate density, and; rho_L =
1.2-1.7 E+35 W/Mpc^3 for the bolometric, attenuated, stellar, luminosity
density (0.09-5 microns). Comparing this total stellar emission with an
estimate of the total dust emission implies a relatively modest average
attenuation in the UV (<=1 magnitude at 0.2 microns).Comment: 16 pages, accepted by Ap
Feedback in the Antennae Galaxies (NGC 4038/9): I. High-Resolution Infrared Spectroscopy of Winds from Super Star Clusters
We present high-resolution (R ~ 24,600) near-IR spectroscopy of the youngest
super star clusters (SSCs) in the prototypical starburst merger, the Antennae
Galaxies. These SSCs are young (3-7 Myr old) and massive (10^5 - 10^7 M_sun for
a Kroupa IMF) and their spectra are characterized by broad, extended Br-gamma
emission, so we refer to them as emission-line clusters (ELCs) to distinguish
them from older SSCs. The Brgamma lines of most ELCs have supersonic widths
(60-110 km/s FWHM) and non-Gaussian wings whose velocities exceed the clusters'
escape velocities. This high-velocity unbound gas is flowing out in winds that
are powered by the clusters' massive O and W-R stars over the course of at
least several crossing times. The large sizes of some ELCs relative to those of
older SSCs may be due to expansion caused by these outflows; many of the ELCs
may not survive as bound stellar systems, but rather dissipate rapidly into the
field population. The observed tendency of older ELCs to be more compact than
young ones is consistent with the preferential survival of the most
concentrated clusters at a given age.Comment: Accepted to Ap
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