766 research outputs found
HyRec: A fast and highly accurate primordial hydrogen and helium recombination code
We present a state-of-the-art primordial recombination code, HyRec, including
all the physical effects that have been shown to significantly affect
recombination. The computation of helium recombination includes simple analytic
treatments of hydrogen continuum opacity in the He I 2 1P - 1 1S line, the He
I] 2 3P - 1 1S line, and treats feedback between these lines within the
on-the-spot approximation. Hydrogen recombination is computed using the
effective multilevel atom method, virtually accounting for an infinite number
of excited states. We account for two-photon transitions from 2s and higher
levels as well as frequency diffusion in Lyman-alpha with a full radiative
transfer calculation. We present a new method to evolve the radiation field
simultaneously with the level populations and the free electron fraction. These
computations are sped up by taking advantage of the particular sparseness
pattern of the equations describing the radiative transfer. The computation
time for a full recombination history is ~2 seconds. This makes our code well
suited for inclusion in Monte Carlo Markov chains for cosmological parameter
estimation from upcoming high-precision cosmic microwave background anisotropy
measurements.Comment: Version accepted by PRD. Numerical integration switches adapted to be
well behaved for a wide range of cosmologies (Sec. V E). HyRec is available
at http://www.tapir.caltech.edu/~yacine/hyrec/hyrec.htm
Environment assisted electron capture
Electron capture by {\it isolated} atoms and ions proceeds by
photorecombination. In this process a species captures a free electron by
emitting a photon which carries away the excess energy. It is shown here that
in the presence of an {\it environment} a competing non-radiative electron
capture process can take place due to long range electron correlation. In this
interatomic (intermolecular) process the excess energy is transferred to
neighboring species. The asymptotic expression for the cross section of this
process is derived. We demonstrate by explicit examples that under realizable
conditions the cross section of this interatomic process can clearly dominate
that of photorecombination
Ionized Gas Kinematics and Morphology in Sgr B2 Main on 1000 AU Scales
We have imaged the Sgr B2 Main region with the Very Large Array in the BnA
configuration ( = 0\farcs13) in both the H52 (45.453
GHz) radio recombination line (RRL) and 7 mm continuum emission. At a distance
of 8500 pc, this spatial resolution corresponds to a physical scale of 0.005 pc
(1100 AU). The current observations detect H52 emission in 12
individual ultracompact (UC) and hypercompact (HC) HII regions. Two of the
sources with detected H52 emission have broad
(V50 \kms) recombination lines, and two of the sources
show lines with peaks at more than one velocity. We use line parameters from
the H52 lines and our previous H66 line observations to
determine the relative contribution of thermal, pressure and kinematic
broadening, and electron density. These new observations suggest that pressure
broadening can account for the broad lines in some of the sources, but that gas
motions (e.g. turbulence, accretion or outflow) contribute significantly to the
broad lines in at least one of the sources (Sgr B2 F3).Comment: 10 pages, 2 figure
PPAK Integral Field Spectroscopy survey of the Orion Nebula: Data Release
We present a low-resolution spectroscopic survey of the Orion nebula which
data we release for public use. In this article, we intend to show the possible
applications of this dataset analyzing some of the main properties of the
nebula. We perform an integral field spectroscopy mosaic of an area of ~5' X 6'
centered on the Trapezium region of the nebula, including the ionization front
to the south-east. The analysis of the line fluxes and line ratios of both the
individual and integrated spectra allowed us to determine the main
characteristics of the ionization throughtout the nebula.The final dataset
comprises 8182 individual spectra, which sample each one a circular area of
\~2.7" diameter. The data can be downloaded as a single row-stacked spectra
fits file plus a position table or as an interpolated datacube with a final
sampling of 1.5"/pixel. The integrated spectrum across the field-of-view was
used to obtain the main integrated properties of the nebula, including the
electron density and temperature, the dust extinction, the Halpha integrated
flux (after correcting for dust reddening), and the main diagnostic line
ratios. The individual spectra were used to obtain line intensity maps of the
different detected lines. These maps were used to study the distribution of the
ionized hydrogen, the dust extinction, the electron density and temperature,
and the helium and oxygen abundance...Comment: 13 pages, 8 figures, accepted for publishing in Astronomy &
Astrophysic
Post common envelope binaries from SDSS. II : identification of 9 close binaries with VLT/FORS2
Context. Post common envelope binaries (PCEBs) consisting of a white dwarf and a main sequence star are ideal systems to use to calibrate current theories of angular momentum loss in close compact binary stars. The potential held by PCEBs for further development of close binary evolution could so far not be exploited due to the small number of known systems and the inhomogeneity of the sample. The Sloan Digital Sky Survey is changing this scene dramatically, as it is very efficient in identifying white dwarf/main sequence (WDMS) binaries, including both wide systems whose stellar components evolve like single stars and − more interesting in the context of close binary evolution − PCEBs.
Aims. We pursue a large-scale follow-up survey to identify and characterise the PCEBs among the WDMS binaries that have been found with SDSS. We use a two-step strategy with the identification of PCEBs among WDMS binaries in the first phase and orbital period determinations in the second phase. Here we present first results of our ESO-VLT/FORS2 pilot study that targets the identification of the PCEBs among the fainter (g >∼18.5) SDSSWDMS binaries.
Methods. From published SDSS catalogues we selected 26 WDMS binaries to be observed with ESO-VLT/FORS2 in service mode.
The design of the observations was to get two spectra per object separated by at least one night.We used the Na I λλ 8183.27, 8194.81 doublet to measure radial velocity variations of our targets and a spectral decomposition/fitting technique to determine the white dwarf
effective temperatures and surface gravities, masses, and secondary star spectral types for all WDMS binaries in our sample.
Results. Among the 26 observed WDMS binaries, we find 9 strong PCEB candidates showing clear (≥3σ) radial velocity variations, and we estimate the fraction of PCEBs among SDSS WDMS binaries to be ∼35 ± 12%. We find indications of a dependence of the relative number of PCEBs among SDSSWDMS binaries on the spectral type of the secondary star. These results are subject to small number statistics and need to be confirmed by additional observations. Using Magellan-Clay/LDSS3, we measured the orbital periods of two PCEB candidates, SDSS J1047+0523 and SDSS J1414–0132, to be 9.17 h and 17.48 h, respectively.
Conclusions. This pilot study demonstrates that our survey is highly efficient in identifying PCEBs among the SDSSWDMS binaries, and it will indeed provide the observational parameters that are needed to constrain the theoretical models of close binary evolution
The line parameters and ratios as the physical probe of the line emitting regions in AGN
Here we discuss the physical conditions in the emission line regions (ELR) of
active galactic nuclei (AGN), with the special emphasize on the unresolved
problems, e.g. the stratification of the Broad Line Region (BLR) or the failure
of the photoionization to explain the strong observed optical Fe II emission.
We use here different line fluxes in order to probe the properties of the ELR,
such as the hydrogen Balmer lines (Ha to He), the helium lines from two
subsequent ionization levels (He II 4686 and He I 5876) and the strongest Fe II
lines in the wavelength interval 4400-5400 \AA. We found that the hydrogen
Balmer and helium lines can be used for the estimates of the physical
parameters of the BLR, and we show that the Fe II emission is mostly emitted
from an intermediate line region (ILR), that is located further away from the
central continuum source than the BLR.Comment: 8 pages, 9 figures, 2 tables, New Astronomy Reviews (Proceeding of
7th SCSLSA), in pres
Cosmic Reionisation by Stellar Sources: Population II Stars
We study the reionisation of the Universe by stellar sources using a
numerical approach that combines fast 3D radiative transfer calculations with
high resolution hydrodynamical simulations. Ionising fluxes for the sources are
derived from intrinsic star formation rates computed in the underlying
hydrodynamical simulations. Our mass resolution limit for sources is M~ 4.0 x
10^7 h^-1 M_sol, which is roughly an order of magnitude smaller than in
previous studies of this kind. Our calculations reveal that the reionisation
process is sensitive to the inclusion of dim sources with masses below ~10^9
h^-1 M_sol. We present the results of our reionisation simulation assuming a
range of escape fractions for ionising photons and make statistical comparisons
with observational constraints on the neutral fraction of hydrogen at z~6
derived from the z=6.28 SDSS quasar of Becker and coworkers. Our best fitting
model has an escape fraction of ~20% and causes reionisation to occur by z~8,
although the IGM remains fairly opaque until z~6. In order to simultaneously
match the observations from the z=6.28 SDSS quasar and the optical depth
measurement from WMAP with the sources modeled here, we require an evolving
escape fraction that rises from f_esc=0.20 near z~6 to f_esc>~10 at z~18.Comment: 42 pages, 13 figure
Comparing simulations of ionisation triggered star formation and observations in RCW 120
Massive clumps within the swept-up shells of bubbles, like that surrounding
the galactic HII region RCW 120, have been interpreted in terms of the Collect
and Collapse (C&C) mechanism for triggered star formation. The cold, dusty
clumps surrounding RCW 120 are arranged in an almost spherical shell and
harbour many young stellar objects. By performing high-resolution,
three-dimensional SPH simulations of HII regions expanding into fractal
molecular clouds, we investigate whether the formation of massive clumps in
dense, swept-up shells necessarily requires the C&C mechanism. In a second
step, we use RADMC-3D to compute the synthetic dust continuum emission from our
simulations, in order to compare them with observations of RCW 120 made with
APEX-LABOCA at 870 micron. We show that a distribution of clumps similar to the
one seen in RCW 120 can readily be explained by a non-uniform initial molecular
cloud structure. Hence, a shell-like configuration of massive clumps does not
imply that the C&C mechanism is at work. Rather, we find a hybrid form of
triggering, which combines elements of C&C and Radiatively Driven Implosion
(RDI). In addition, we investigate the reliability of deriving clump masses
from their 870 micron emission. We find that for clumps with more than 100
M_sun the observational estimates are accurate to within a factor of two and
that, even at these long wavelengths, it is important to account for the
radiative heating from triggered, embedded protostars.Comment: 10 pages, 7 figures, submitted to MNRA
Spatially resolved spectroscopy of Coma cluster early-type galaxies - II:the minor axis dataset
We present minor axis, off set major axis and one diagonal long slit spectra for 10 E and S0 galaxies of the Coma cluster drawn from a magnitude-limited sample studied before. We derive rotation curves, velocity dispersion profiles and the H-3 and H-4 coefficients of the Hermite decomposition of the line of sight velocity distribution. Moreover, we derive the line index profiles of Mg, Fe and Hbeta line indices and assess their errors. The data will be used to construct dynamical models of the galaxies and study their stellar populations
HI ``Tails'' from Cometary Globules in IC1396
IC 1396 is a relatively nearby (750 pc), large (>2 deg), HII region ionized
by a single O6.5V star and containing bright-rimmed cometary globules. We have
made the first arcmin resolution images of atomic hydrogen toward IC 1396, and
have found remarkable ``tail''-like structures associated with some of the
globules and extending up to 6.5 pc radially away from the central ionizing
star. These HI ``tails'' may be material which has been ablated from the
globule through ionization and/or photodissociation and then accelerated away
from the globule by the stellar wind, but which has since drifted into the
``shadow'' of the globules.
This report presents the first results of the Galactic Plane Survey Project
recently begun by the Dominion Radio Astrophysical Observatory.Comment: 11 pages, 5 postscript figures, uses aaspp4.sty macros, submitted in
uuencoded gzipped tar format, accepted for publication in Astrophysical
Journal Letters, colour figures available at
http://www.drao.nrc.ca/~schieven/news_sep95/ic1396.htm
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