1,295 research outputs found
Large-scale Breit-Pauli R-matrix calculations for transition probabilities of Fe V
Ab initio theoretical calculations are reported for the electric (E1) dipole
allowed and intercombination fine structure transitions in Fe V using the
Breit-Pauli R-matrix (BPRM) method. We obtain 3865 bound fine structure levels
of Fe V and oscillator strengths, Einstein A-coefficients and
line strengths. In addition to the relativistic effects, the intermediate
coupling calculations include extensive electron correlation effects that
represent the complex configuration interaction (CI). Fe V bound levels are
obtained with angular and spin symmetries and of the (e + Fe VI)
system such that = 5,3,1, 10, . The bound levels are
obtained as solutions of the Breit-Pauli (e + ion) Hamiltonian for each ,
and are designated according to the `collision' channel quantum numbers. A
major task has been the identification of these large number of bound fine
structure levels in terms of standard spectroscopic designations. A new scheme,
based on the analysis of quantum defects and channel wavefunctions, has been
developed. The identification scheme aims particularly to determine the
completeness of the results in terms of all possible bound levels for
applications to analysis of experimental measurements and plasma modeling. An
uncertainty of 10-20% for most transitions is estimated.Comment: 31 pages, 1 figure, Physica Scripta (in press
Off-Critical Logarithmic Minimal Models
We consider the integrable minimal models , corresponding
to the perturbation off-criticality, in the {\it logarithmic
limit\,} , where are coprime and the
limit is taken through coprime values of . We view these off-critical
minimal models as the continuum scaling limit of the
Forrester-Baxter Restricted Solid-On-Solid (RSOS) models on the square lattice.
Applying Corner Transfer Matrices to the Forrester-Baxter RSOS models in Regime
III, we argue that taking first the thermodynamic limit and second the {\it
logarithmic limit\,} yields off-critical logarithmic minimal models corresponding to the perturbation of the critical
logarithmic minimal models . Specifically, in accord with the
Kyoto correspondence principle, we show that the logarithmic limit of the
one-dimensional configurational sums yields finitized quasi-rational characters
of the Kac representations of the critical logarithmic minimal models . We also calculate the logarithmic limit of certain off-critical
observables related to One Point Functions and show that the
associated critical exponents
produce all conformal dimensions in the infinitely extended Kac table. The corresponding Kac labels
satisfy . The exponent is obtained from the logarithmic limit of the free energy giving the
conformal dimension for the perturbing field . As befits a non-unitary
theory, some observables diverge at criticality.Comment: 18 pages, 5 figures; version 3 contains amplifications and minor
typographical correction
Age, Metallicity and Star Formation History of Cluster Galaxies at z~0.3 F
We investigate the color-magnitude distribution in the rich cluster AC 118 at
z=0.31. The sample is selected by the photometric redshift technique, allowing
to study a wide range of properties of stellar populations, and is complete in
the K-band, allowing to study these properties up to a given galaxy mass. We
use galaxy templates based on population synthesis models to translate the
physical properties of the stellar populations - formation epoch, time-scale of
star formation, and metallicity - into observed magnitudes and colors. In this
way we show that a sharp luminosity-metallicity relation is inferred without
any assumption on the galaxy formation scenario (either monolithic or
hierarchical). Our data exclude significant differences in star formation
histories along the color-magnitude relation, and therefore confirm a pure
metallicity interpretation for its origin, with an early (z~5) formation epoch
for the bulk of stellar populations. The dispersion in the color-magnitude
diagram implies that fainter galaxies in our sample (K~18) ceased to form stars
as late as z~0.5, in agreement with the picture that these galaxies were
recently accreted into the cluster environment. The trend with redshift of the
total stellar mass shows that half of the luminous mass in AC 118 was already
formed at $z~2, but also that 20% of the stars formed at z<1.Comment: 16 pages, 10 figures. ApJ in pres
Ising tricriticality and the dilute A model
Some universal amplitude ratios appropriate to the peturbation
of the c=7/10 minimal field theory, the subleading magnetic perturbation of the
tricritical Ising model, are explicitly demonstrated in the dilute A model,
in regime 1.Comment: 8 pages, LaTeX using iop macro
On the Nature of Andromeda IV
Lying at a projected distance of 40' or 9 kpc from the centre of M31,
Andromeda IV is an enigmatic object first discovered during van den Bergh's
search for dwarf spheroidal companions to M31. Being bluer, more compact and
higher surface brightness than other known dwarf spheroidals, it has been
suggested that And IV is either a relatively old `star cloud' in the outer disk
of M31 or a background dwarf galaxy. We present deep HST WFPC2 observations of
And IV and the surrounding field which, along with ground-based long-slit
spectroscopy and Halpha imagery, are used to decipher the true nature of this
puzzling object. We find compelling evidence that And IV is a background galaxy
seen through the disk of M31. The moderate surface brightness (SB(V)~24), very
blue colour (V-I<~0.6), low current star formation rate (~0.001 solar mass/yr)
and low metallicity (~10% solar) reported here are consistent with And IV being
a small dwarf irregular galaxy, perhaps similar to Local Group dwarfs such as
IC 1613 and Sextans A. Although the distance to And IV is not tightly
constrained with the current dataset, various arguments suggest it lies in the
range 5<~D<~8 Mpc, placing it well outside the confines of the Local Group. It
may be associated with a loose group of galaxies, containing major members UGC
64, IC 1727 and NGC 784. We report an updated position and radial velocity for
And IV.Comment: 26 pages, LaTex with 9 figures (including 6 jpg plates). Accepted for
publication in A
Star formation in disk galaxies driven by primordial H_2
We show that gaseous \HI disks of primordial composition irradiated by an
external radiation field can develop a multiphase medium with temperatures
between 10^2 and 10^4 K due to the formation of molecular hydrogen. For a given
\HI column density there is a critical value of the radiation field below which
only the cold \HI phase can exist. Due to a time decreasing quasar background,
the gas starts cooling slowly after recombination until the lowest stable
temperature in the warm phase is reached at a critical redshift .
Below this redshift the formation of molecular hydrogen promotes a rapid
transition towards the cold \HI phase. We find that disks of protogalaxies with
10^{20}\simlt N_{HI}\simlt 10^{21} cm^{-2} are gravitationally stable at
K and can start their star formation history only at z \simlt
z_{cr}\sim 2, after the gas in the central portion of the disk has cooled to
temperatures T\simlt 300 K. Such a delayed starbust phase in galaxies of low
gas surface density and low dynamical mass can disrupt the disks and cause them
to fade away. These objects could contribute significantly to the faint blue
galaxy population.Comment: 16 pages (LaTeX), 2 Figures to be published in Astrophysical Journal
Letter
Cepheid Mass-loss and the Pulsation -- Evolutionary Mass Discrepancy
I investigate the discrepancy between the evolution and pulsation masses for
Cepheid variables. A number of recent works have proposed that non-canonical
mass-loss can account for the mass discrepancy. This mass-loss would be such
that a 5Mo star loses approximately 20% of its mass by arriving at the Cepheid
instability strip; a 14Mo star, none. Such findings would pose a serious
challenge to our understanding of mass-loss. I revisit these results in light
of the Padova stellar evolutionary models and find evolutionary masses are
()% greater than pulsation masses for Cepheids between 5<M/Mo<14. I
find that mild internal mixing in the main-sequence progenitor of the Cepheid
are able to account for this mass discrepancy.Comment: 15 pages, 3 figures, ApJ accepte
High Carbon in I Zwicky 18: New Results from Hubble Space Telescope Spectroscopy
We present new measurements of the gas-phase C/O abundance ratio in both the
NW and SE components of the extremely metal-poor dwarf irregular galaxy I Zw
18, based on ultraviolet spectroscopy of the two H II regions using the Faint
Object Spectrograph on the Hubble Space Telescope. We determine values of log
C/O = -0.63 +/- 0.10 for the NW component and log C/O = -0.56 +/- 0.09 for the
SE component. In comparison, log C/O = -0.37 in the sun, while log C/O = -0.85
+/- 0.07 in the three most metal-poor irregular galaxies measured by Garnett et
al. (1995a). Our measurements show that C/O in I Zw 18 is significantly higher
than in other comparably metal-poor irregular galaxies, and above predictions
for the expected C/O from massive star nucleosynthesis. These results suggest
that carbon in I Zw 18 has been enhanced by an earlier population of lower-mass
carbon producing stars; this idea is supported by stellar photometry of I Zw 18
and its companion, which demonstrate that the current bursts of massive stars
were not the first. Despite its very low metallicity, it is likely that I Zw 18
is not a ``primeval'' galaxy.Comment: 14 pages including 4 figures; uses aaspp4.sty. Accepted for
publication in ApJ. Postscript version also available by e-mail request to
author at [email protected]
Detection of the Central Star of the Planetary Nebula NGC 6302
NGC 6302 is one of the highest ionization planetary nebulae known and shows
emission from species with ionization potential >300eV. The temperature of the
central star must be >200,000K to photoionize the nebula, and has been
suggested to be up to ~ 400,000K. On account of the dense dust and molecular
disc, the central star has not convincingly been directly imaged until now. NGC
6302 was imaged in six narrow band filters by Wide Field Camera 3 on HST as
part of the Servicing Mission 4 Early Release Observations. The central star is
directly detected for the first time, and is situated at the nebula centre on
the foreground side of the tilted equatorial disc. The magnitudes of the
central star have been reliably measured in two filters(F469N and F673N).
Assuming a hot black body, the reddening has been measured from the
(4688-6766\AA) colour and a value of c=3.1, A_v=6.6 mag determined. A G-K main
sequence binary companion can be excluded. The position of the star on the HR
diagram suggests a fairly massive PN central star of about 0.64,M_sun close to
the white dwarf cooling track. A fit to the evolutionary tracks for
(T,L,t)=(200,000K, 2000L_sun, 2200yr), where t is the nebular age, is obtained;
however the luminosity and temperature remain uncertain. The model tracks
predict that the star is rapidly evolving, and fading at a rate of almost 1 %
per year. Future observations could test this prediction.Comment: 13 pages, 5 figures, submitted to ApJ Letters on 25.09.2009 accepted
on 19.10.200
Theoretical He I Emissivities in the Case B Approximation
We calculate the He I case B recombination cascade spectrum using improved
radiative and collisional data. We present new emissivities over a range of
electron temperatures and densities. The differences between our results and
the current standard are large enough to have a significant effect not only on
the interpretation of observed spectra of a wide variety of objects but also on
determinations of the primordial helium abundance.Comment: Accepted to ApJ
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