290 research outputs found
Dust crystallinity in protoplanetary disks: the effect of diffusion/viscosity ratio
The process of turbulent radial mixing in protoplanetary disks has strong
relevance to the analysis of the spatial distribution of crystalline dust
species in disks around young stars and to studies of the composition of
meteorites and comets in our own solar system.
A debate has gone on in the recent literature on the ratio of the effective
viscosity coefficient (responsible for accretion) to the turbulent
diffusion coefficient (responsible for mixing). Numerical
magneto-hydrodynamic simulations have yielded values between
(Carballido, Stone & Pringle, 2005) and (Johansen & Klahr,
2005}). Here we present two analytic arguments for the ratio which
are based on elegant, though strongly simplified assumptions. We argue that
whichever of these numbers comes closest to reality may be determined {\em
observationally} by using spatially resolved mid-infrared measurements of
protoplanetary disks around Herbig stars. If meridional flows are present in
the disk, then we expect less abundance of crystalline dust in the surface
layers, a prediction which can likewise be observationally tested with
mid-infrared interferometers.Comment: 9 pages, 5 figures, accepted for publication in A&
The 44Ti-powered spectrum of SN 1987A
SN 1987A provides a unique opportunity to study the evolution of a supernova
from explosion into very late phases. Due to the rich chemical structure, the
multitude of physical process involved, and extensive radiative transfer
effects, detailed modeling is needed to interpret the emission from this and
other supernovae. In this paper, we analyze the late-time (~8 years) HST
spectrum of the SN 1987A ejecta, where 44Ti is the dominant power source. Based
on an explosion model for a 19 Msun progenitor, we compute a model spectrum by
calculating the degradation of positrons and gamma-rays from the radioactive
decays, solving the equations governing temperature, ionization balance and
NLTE level populations, and treating the radiative transfer with a Monte Carlo
technique. We obtain a UV/optical/NIR model spectrum which is found to
reproduce most of the lines in the observed spectrum to good accuracy. We find
non-local radiative transfer in atomic lines to be an important process also at
this late stage of the supernova, with ~30% of the emergent flux in the optical
and NIR coming from scattering/fluorescence. We investigate the question of
where the positrons deposit their energy, and favor the scenario where they are
locally trapped in the Fe/He clumps by a magnetic field. Energy deposition into
these largely neutral Fe/He clumps makes Fe I lines prominent in the emergent
spectrum. Using the best available estimates for the dust extinction, we
determine the amount of 44Ti produced in the explosion to 1.5\pm0.5 * 10^-4
Msun.Comment: 23 pages, 9 figures. 44Ti mass updated from 1.4E-4 to 1.5E-4 Msu
Galaxy Zoo: Are Bars Responsible for the Feeding of Active Galactic Nuclei at 0.2 < z < 1.0?
We present a new study investigating whether active galactic nuclei (AGN)
beyond the local universe are preferentially fed via large-scale bars. Our
investigation combines data from Chandra and Galaxy Zoo: Hubble (GZH) in the
AEGIS, COSMOS, and GOODS-S surveys to create samples of face-on, disc galaxies
at 0.2 < z < 1.0. We use a novel method to robustly compare a sample of 120 AGN
host galaxies, defined to have 10^42 erg/s < L_X < 10^44 erg/s, with inactive
control galaxies matched in stellar mass, rest-frame colour, size, Sersic
index, and redshift. Using the GZH bar classifications of each sample, we
demonstrate that AGN hosts show no statistically significant enhancement in bar
fraction or average bar likelihood compared to closely-matched inactive
galaxies. In detail, we find that the AGN bar fraction cannot be enhanced above
the control bar fraction by more than a factor of two, at 99.7% confidence. We
similarly find no significant difference in the AGN fraction among barred and
non-barred galaxies. Thus we find no compelling evidence that large-scale bars
directly fuel AGN at 0.2<z<1.0. This result, coupled with previous results at
z=0, implies that moderate-luminosity AGN have not been preferentially fed by
large-scale bars since z=1. Furthermore, given the low bar fractions at z>1,
our findings suggest that large-scale bars have likely never directly been a
dominant fueling mechanism for supermassive black hole growth.Comment: 13 pages, 5 figures, 2 tables, accepted by MNRA
Excitation of emission lines by fluorescence and recombination in IC 418
We predict intensities of lines of CII, NI, NII, OI and OII and compare them
with a deep spectroscopic survey of IC 418 to test the effect of excitation of
nebular emission lines by continuum fluorescence of starlight. Our calculations
use a nebular model and a synthetic spectrum of its central star to take into
account excitation of the lines by continuum fluorescence and recombination.
The NII spectrum is mostly produced by fluorescence due to the low excitation
conditions of the nebula, but many CII and OII lines have more excitation by
fluorescence than recombination. In the neutral envelope, the NI permitted
lines are excited by fluorescence, and almost all the OI lines are excited by
recombination. Electron excitation produces the forbidden optical lines of OI,
but continuum fluorescence excites most of the NI forbidden line intensities.
Lines excited by fluorescence of light below the Lyman limit thus suggest a new
diagnostic to explore the photodissociation region of a nebula.Comment: 2 pages, 4 figures, to appear in proceedings of the IAU Symposium
283: "Planetary Nebulae: An Eye to the Future", Eds.: A. Manchado, L.
Stanghellini and D. Schoenberne
The Shaping of T Cell Receptor Recognition by Self-Tolerance
SummaryDuring selection of the TÂ cell repertoire, the immune system navigates the subtle distinction between self-restriction and self-tolerance, yet how this is achieved is unclear. Here we describe how self-tolerance toward a trans-HLA (human leukocyte antigen) allotype shapes TÂ cell receptor (TCR) recognition of an Epstein-Barr virus (EBV) determinant (FLRGRAYGL). The recognition of HLA-B8-FLRGRAYGL by two archetypal TCRs was compared. One was a publicly selected TCR, LC13, that is alloreactive with HLA-B44; the other, CF34, lacks HLA-B44 reactivity because it arises when HLA-B44 is coinherited in trans with HLA-B8. Whereas the alloreactive LC13 TCR docked at the C terminus of HLA-B8-FLRGRAYGL, the CF34 TCR docked at the NÂ terminus of HLA-B8-FLRGRAYGL, which coincided with a polymorphic region between HLA-B8 and HLA-B44. The markedly contrasting footprints of the LC13 and CF34 TCRs provided a portrait of how self-tolerance shapes the specificity of TCRs selected into the immune repertoire
Simulating the formation and evolution of galaxies: Multi-phase description of the interstellar medium, star formation, and energy feedback
We present a multi-phase representation of the ISM in NB-TSPH simulations of
galaxy formation and evolution with particular attention to the case of
early-type galaxies. Cold gas clouds are described by the so-called sticky
particles algorithm. They can freely move throughout the hot ISM medium; stars
form within these clouds and the mass exchange among the three baryonic phases
(hot gas, cold clouds, stars) is governed by radiative and Compton cooling and
energy feedback by supernova (SN) explosions, stellar winds, and UV radiation.
We also consider thermal conduction, cloud-cloud collisions, and chemical
enrichment. Our model agrees with and improves upon previous studies on the
same subject. The results for the star formation rate are very promising and
agree with recent observational data on early-type galaxies. These models lend
further support to the revised monolithic scheme of galaxy formation, which has
recently been also strengthened by high redshift data leading to the so-called
downsizing and top-down scenarios.Comment: 17 pages, 17 figure
Continuum and line modeling of disks around young stars II. Line diagnostics for GASPS from the DENT grid
Aims. We want to understand the chemistry and physics of disks on the basis
of a large unbiased and statistically relevant grid of disk models. One of the
main goals is to explore the diagnostic power of various gas emission lines and
line ratios for deriving main disk parameters such as the gas mass. Methods. We
explore the results of the DENT grid (Disk Evolution with Neat Theory) that
consists of 300 000 disk models with 11 free parameters. Through a statistical
analysis, we search for correlations and trends in an effort to find tools for
disk diagnostic. Results. All calculated quantities like species masses,
temperatures, continuum and line fluxes differ by several orders of magnitude
across the entire parameter space. The broad distribution of these quantities
as a function of input parameters shows the limitation of using a prototype T
Tauri or Herbig Ae/Be disk model. The statistical analysis of the DENT grid
shows that CO gas is rarely the dominant carbon reservoir in disks. Models with
large inner radii (10 times the dust condensation radius) and/or shallow
surface density gradients lack massive gas phase water reservoirs. Also, 60% of
the disks have gas temperatures averaged over the oxygen mass in the range
between 15 and 70 K; the average gas temperatures for CO and O differ by less
than a factor two. Studying the observational diagnostics, the [CII] 158 \mum
fine structure line flux is very sensitive to the stellar UV flux and presence
of a UV excess and it traces the outer disk radius (Rout). In the submm, the CO
low J rotational lines also trace Rout. Low [OI] 63/145 line ratios (< a few)
can be explained with cool atomic O gas in the uppermost surface layers leading
to self-absorption in the 63 \mum line; this occurs mostly for massive
non-flaring, settled disk models without UV excess. ... abbreviatedComment: 15 pages, 25 figures, accepted for publication in A&
Radial Velocity Studies of Close Binary Stars. XI
Radial-velocity measurements and sine-curve fits to the orbital radial
velocity variations are presented for ten close binary systems: DU Boo, ET Boo,
TX Cnc, V1073 Cyg, HL Dra, AK Her, VW LMi, V566 Oph, TV UMi and AG Vir. By this
contribution, the DDO program has reached the point of 100 published radial
velocity orbits. The radial velocities have been determined using an improved
fitting technique which uses rotational profiles to approximate individual
peaks in broadening functions.
Three systems, ET Boo, VW LMi and TV UMi, were found to be quadruple while AG
Vir appears to be a spectroscopic triple. ET Boo, a member of a close visual
binary with years, was previously known to be a multiple
system, but we show that the second component is actually a close,
non-eclipsing binary. The new observations enabled us to determine the
spectroscopic orbits of the companion, non-eclipsing pairs in ET Boo and VW
LMi. The particularly interesting case is VW LMi, where the period of the
mutual revolution of the two spectroscopic binaries is only 355 days.
While most of the studied eclipsing pairs are contact binaries, ET Boo is
composed of two double-lined detached binaries and HL Dra is single-lined
detached or semi-detached system. Five systems of this group were observed
spectroscopically before: TX Cnc, V1073 Cyg, AK Her (as a single-lined binary),
V566 Oph, AG Vir, but our new data are of much higher quality than the previous
studies.Comment: Accepted by AJ, August 2006, 10 figures, 3 table
The effects of changes in the order of verbal labels and numerical values on children's scores on attitude and rating scales
Research with adults has shown that variations in verbal labels and numerical scale values on rating scales can affect the responses given. However, few studies have been conducted with children. The study aimed to examine potential differences in childrenâs responses to Likert-type rating scales according to their anchor points and scale direction, and to see whether or not such differences were stable over time. 130 British children, aged 9 to 11, completed six sets of Likert-type rating scales, presented in four different ways varying the position of positive labels and numerical values. The results showed, both initially and 8-12 weeks later, that presenting a positive label or a high score on the left of a scale led to significantly higher mean scores than did the other variations. These findings indicate that different arrangements of rating scales can produce different results which has clear implications for the administration of scales with children
Non-thermal emission processes in massive binaries
In this paper, I present a general discussion of several astrophysical
processes likely to play a role in the production of non-thermal emission in
massive stars, with emphasis on massive binaries. Even though the discussion
will start in the radio domain where the non-thermal emission was first
detected, the census of physical processes involved in the non-thermal emission
from massive stars shows that many spectral domains are concerned, from the
radio to the very high energies.
First, the theoretical aspects of the non-thermal emission from early-type
stars will be addressed. The main topics that will be discussed are
respectively the physics of individual stellar winds and their interaction in
binary systems, the acceleration of relativistic electrons, the magnetic field
of massive stars, and finally the non-thermal emission processes relevant to
the case of massive stars. Second, this general qualitative discussion will be
followed by a more quantitative one, devoted to the most probable scenario
where non-thermal radio emitters are massive binaries. I will show how several
stellar, wind and orbital parameters can be combined in order to make some
semi-quantitative predictions on the high-energy counterpart to the non-thermal
emission detected in the radio domain.
These theoretical considerations will be followed by a census of results
obtained so far, and related to this topic... (see paper for full abstract)Comment: 47 pages, 5 postscript figures, accepted for publication in Astronomy
and Astrophysics Review. Astronomy and Astrophysics Review, in pres
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