7,296 research outputs found
To a Star : A I\u27Etoile
https://digitalcommons.library.umaine.edu/mmb-ps/1953/thumbnail.jp
Chemical abundances and kinematics of barium stars
In this paper we present an homogeneous analysis of photospheric abundances
based on high-resolution spectroscopy of a sample of 182 barium stars and
candidates. We determined atmospheric parameters, spectroscopic distances,
stellar masses, ages, luminosities and scale height, radial velocities,
abundances of the Na, Al, -elements, iron-peak elements, and s-process
elements Y, Zr, La, Ce, and Nd. We employed the local-thermodynamic-equilibrium
model atmospheres of Kurucz and the spectral analysis code {\sc moog}. We found
that the metallicities, the temperatures and the surface gravities for barium
stars can not be represented by a single gaussian distribution. The abundances
of -elements and iron peak elements are similar to those of field giants
with the same metallicity. Sodium presents some degree of enrichment in more
evolved stars that could be attributed to the NeNa cycle. As expected, the
barium stars show overabundance of the elements created by the s-process. By
measuring the mean heavy-element abundance pattern as given by the ratio
[s/Fe], we found that the barium stars present several degrees of enrichment.
We also obtained the [hs/ls] ratio by measuring the photospheric abundances of
the Ba-peak and the Zr-peak elements. Our results indicated that the [s/Fe] and
the [hs/ls] ratios are strongly anti-correlated with the metallicity. Our
kinematical analysis showed that 90% of the barium stars belong to the thin
disk population. Based on their luminosities, none of the barium stars are
luminous enough to be an AGB star, nor to become self-enriched in the s-process
elements. Finally, we determined that the barium stars also follow an
age-metallicity relation.Comment: 30 pages, 26 figures, 18 tables, accepted for publication in MNRA
Indicators of implicit and explicit social anxiety influence threat-related interpretive bias as a function of working memory capacity
Interpretive biases play a crucial role in anxiety disorders. The aim of the current study was to examine factors that determine the relative strength of threat-related interpretive biases that are characteristic of individuals high in social anxiety. Different (dual process) models argue that both implicit and explicit processes determine information processing biases and behavior, and that their impact is moderated by the availability of executive resources such as working memory capacity (WMC). Based on these models, we expected indicators of implicit social anxiety to predict threat-related interpretive bias in individuals low, but not high in WMC. Indicators of explicit social anxiety should predict threat-related interpretive bias in individuals high, but not low in WMC. As expected, WMC moderated the impact of implicit social anxiety on threat-related interpretive bias, although the simple slope for individuals low in WMC was not statistically significant. The hypotheses regarding explicit social anxiety (with fear of negative evaluation used as an indicator) were fully supported. The clinical implications of these findings are discussed
Exact relativistic stellar models with liquid surface. I. Generalizing Buchdahl's polytrope
A family of exact relativistic stellar models is described. The family
generalizes Buchdahl's n=1 polytropic solution. The matter content is a perfect
fluid and, excluding Buchdahl's original model, it behaves as a liquid at low
pressures in the sense that the energy density is non-zero in the zero pressure
limit. The equation of state has two free parameters, a scaling and a stiffness
parameter. Depending on the value of the stiffness parameter the fluid
behaviour can be divided in four different types. Physical quantities such as
masses, radii and surface redshifts as well as density and pressure profiles
are calculated and displayed graphically. Leaving the details to a later
publication, it is noted that one of the equation of state types can quite
accurately approximate the equation of state of real cold matter in the outer
regions of neutron stars. Finally, it is observed that the given equation of
state does not admit models with a conical singularity at the center.Comment: 19 pages, 12 figures (16 eps files), LaTeX2e with the standard
packages amssymb, amsmath, graphicx, subfigure, psfra
The evolutionary state of short-period magnetic white dwarf binaries
We present phase-resolved spectroscopy of two new short-period low accretion rate magnetic binaries, SDSS J125044.42+154957.3 (Porb= 86 min) and SDSS J151415.65+074446.5 (Porb= 89 min). Both systems were previously identified as magnetic white dwarfs from the Zeeman splitting of the Balmer absorption lines in their optical spectra. Their spectral energy distributions exhibit a large near-infrared excess, which we interpret as a combination of cyclotron emission and possibly a late-type companion star. No absorption features from the companion are seen in our optical spectra. We derive the orbital periods from a narrow, variable Hα emission line which we show to originate on the companion star. The high radial velocity amplitude measured in both systems suggests a high orbital inclination, but we find no evidence for eclipses in our data. The two new systems resemble the polar EF Eri in its prolonged low state and also SDSS J121209.31+013627.7, a known magnetic white dwarf plus possible brown dwarf binary, which was also recovered by our method
Stably accessing octave-spanning microresonator frequency combs in the soliton regime
Microresonator frequency combs can be an enabling technology for optical
frequency synthesis and timekeeping in low size, weight, and power
architectures. Such systems require comb operation in low-noise, phase-coherent
states such as solitons, with broad spectral bandwidths (e.g., octave-spanning)
for self-referencing to detect the carrier-envelope offset frequency. However,
stably accessing such states is complicated by thermo-optic dispersion. For
example, in the Si3N4 platform, precisely dispersion-engineered structures can
support broadband operation, but microsecond thermal time constants have
necessitated fast pump power or frequency control to stabilize the solitons. In
contrast, here we consider how broadband soliton states can be accessed with
simple pump laser frequency tuning, at a rate much slower than the thermal
dynamics. We demonstrate octave-spanning soliton frequency combs in Si3N4
microresonators, including the generation of a multi-soliton state with a pump
power near 40 mW and a single-soliton state with a pump power near 120 mW. We
also develop a simplified two-step analysis to explain how these states are
accessed in a thermally stable way without fast control of the pump laser, and
outline the required thermal properties for such operation. Our model agrees
with experimental results as well as numerical simulations based on a
Lugiato-Lefever equation that incorporates thermo-optic dispersion. Moreover,
it also explains an experimental observation that a member of an adjacent mode
family on the red-detuned side of the pump mode can mitigate the thermal
requirements for accessing soliton states
Empires and Percolation: Stochastic Merging of Adjacent Regions
We introduce a stochastic model in which adjacent planar regions merge
stochastically at some rate , and observe analogies with the
well-studied topics of mean-field coagulation and of bond percolation. Do
infinite regions appear in finite time? We give a simple condition on
for this {\em hegemony} property to hold, and another simple condition for it
to not hold, but there is a large gap between these conditions, which includes
the case . For this case, a non-rigorous analytic
argument and simulations suggest hegemony.Comment: 13 page
- …