31 research outputs found
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
Race/Ethnicity, Gender, and Trajectories of Depressive Symptoms Across Early- and Mid-Life Among the Add Health Cohort
While disparities in depressive symptoms by race/ethnicity and gender have been documented, left unclear is how such status characteristics intersect to influence mental health, particularly across early life and among a diverse set of population subgroups. This study investigates how intra- and inter-individual trends in depressive symptoms unfold across a 30-year span (ages 12–42) and are structured by the intersection of race/ethnicity and gender among White, Black, Hispanic, and Asian American young adults (N = 18,566). Analyses use data from the National Longitudinal Study of Adolescent to Adult Health, a nationally representative study of adolescents who have been followed through their fourth decade of life. We draw on Waves I–IV and a representative subsample of the brand new Wave V data. Growth curve models indicated depressive symptoms decreased across adolescence and young adulthood before increasing in the early 30s. Racial/ethnic minorities reported more depressive symptoms than Whites. Women reported more depressive symptoms than men and experienced especially steep increases in their late 30s. Racial/ethnic-gender disparities remained stable with age, except for Hispanic-White disparities among women and Asian American-White disparities among men, which narrowed with age. Overall, findings demonstrate dynamic inequalities across a longer period of the life span than was previously known, as well as heterogeneity in trajectories of poor mental health within and between racial/ethnic-gender groups. Results also suggest that Black and Asian American women experience the highest mental health risks and that interventions for reducing disparities in depressive symptoms should focus on adults in their late 20s/early 30s, particularly women of color
The depths of despair among US adults entering midlife
Objectives. To test whether indicators of despair are rising among US adults as they age toward midlife and whether this rise is concentrated among low-educated Whites and in rural areas. Methods. We used data from the National Longitudinal Study of Adolescent to Adult Health, a nationally representative study of US adolescents in 1994. Our sample was restricted to individuals who participated in 1 or more of 5 waves (1994-2017) and self-identified as non-Hispanic White, non-Hispanic Black, or Hispanic (n = 18 446).We examined change in indicators of despair from adolescence to adulthood using multilevel regression analysis, testing for differences by race/ethnicity, education, and rurality. Results. We found evidence of rising despair among this cohort over the past decade. This increase was not restricted to low-educated Whites or to rural areas. Conclusions. Results suggest that generally rising despair among the young adult cohort now reaching midlife that cuts across racial/ethnic, educational, and geographic groups may presage rising midlife mortality for these subgroups in the next decade
A New Approach to Systematic Uncertainties and Self-Consistency in Helium Abundance Determinations
Tests of big bang nucleosynthesis and early universe cosmology require
precision measurements for helium abundance determinations. However, efforts to
determine the primordial helium abundance via observations of metal poor H II
regions have been limited by significant uncertainties. This work builds upon
previous work by providing an updated and extended program in evaluating these
uncertainties. Procedural consistency is achieved by integrating the hydrogen
based reddening correction with the helium based abundance calculation, i.e.,
all physical parameters are solved for simultaneously. We include new atomic
data for helium recombination and collisional emission based upon recent work
by Porter et al. and wavelength dependent corrections to underlying absorption
are investigated. The set of physical parameters has been expanded here to
include the effects of neutral hydrogen collisional emission. Because of a
degeneracy between the solutions for density and temperature, the precision of
the helium abundance determinations is limited. Also, at lower temperatures (T
\lesssim 13,000 K) the neutral hydrogen fraction is poorly constrained
resulting in a larger uncertainty in the helium abundances. Thus the derived
errors on the helium abundances for individual objects are larger than those
typical of previous studies. The updated emissivities and neutral hydrogen
correction generally raise the abundance. From a regression to zero
metallicity, we find Y_p as 0.2561 \pm 0.0108, in broad agreement with the WMAP
result. Tests with synthetic data show a potential for distinct improvement,
via removal of underlying absorption, using higher resolution spectra. A small
bias in the abundance determination can be reduced significantly and the
calculated helium abundance error can be reduced by \sim 25%.Comment: 51 pages, 13 figure
New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation
(abridged) The heating mechanism at high densities during M dwarf flares is
poorly understood. Spectra of M dwarf flares in the optical and
near-ultraviolet wavelength regimes have revealed three continuum components
during the impulsive phase: 1) an energetically dominant blackbody component
with a color temperature of T 10,000 K in the blue-optical, 2) a smaller
amount of Balmer continuum emission in the near-ultraviolet at lambda 3646
Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer
lines. These properties are not reproduced by models that employ a typical
"solar-type" flare heating level in nonthermal electrons, and therefore our
understanding of these spectra is limited to a phenomenological interpretation.
We present a new 1D radiative-hydrodynamic model of an M dwarf flare from
precipitating nonthermal electrons with a large energy flux of erg
cm s. The simulation produces bright continuum emission from a
dense, hot chromospheric condensation. For the first time, the observed color
temperature and Balmer jump ratio are produced self-consistently in a
radiative-hydrodynamic flare model. We find that a T 10,000 K
blackbody-like continuum component and a small Balmer jump ratio result from
optically thick Balmer and Paschen recombination radiation, and thus the
properties of the flux spectrum are caused by blue light escaping over a larger
physical depth range compared to red and near-ultraviolet light. To model the
near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer
lines, we include the extra Balmer continuum opacity from Landau-Zener
transitions that result from merged, high order energy levels of hydrogen in a
dense, partially ionized atmosphere. This reveals a new diagnostic of ambient
charge density in the densest regions of the atmosphere that are heated during
dMe and solar flares.Comment: 50 pages, 2 tables, 13 figures. Accepted for publication in the Solar
Physics Topical Issue, "Solar and Stellar Flares". Version 2 (June 22, 2015):
updated to include comments by Guest Editor. The final publication is
available at Springer via http://dx.doi.org/10.1007/s11207-015-0708-
Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling
This review paper outlines background information and covers recent advances
made via the analysis of spectra and images of prominence plasma and the
increased sophistication of non-LTE (ie when there is a departure from Local
Thermodynamic Equilibrium) radiative transfer models. We first describe the
spectral inversion techniques that have been used to infer the plasma
parameters important for the general properties of the prominence plasma in
both its cool core and the hotter prominence-corona transition region. We also
review studies devoted to the observation of bulk motions of the prominence
plasma and to the determination of prominence mass. However, a simple inversion
of spectroscopic data usually fails when the lines become optically thick at
certain wavelengths. Therefore, complex non-LTE models become necessary. We
thus present the basics of non-LTE radiative transfer theory and the associated
multi-level radiative transfer problems. The main results of one- and
two-dimensional models of the prominences and their fine-structures are
presented. We then discuss the energy balance in various prominence models.
Finally, we outline the outstanding observational and theoretical questions,
and the directions for future progress in our understanding of solar
prominences.Comment: 96 pages, 37 figures, Space Science Reviews. Some figures may have a
better resolution in the published version. New version reflects minor
changes brought after proof editin