5,071 research outputs found
Thermodynamics of viscous dark energy in an RSII braneworld
We show that for an RSII braneworld filled with interacting viscous dark
energy and dark matter, one can always rewrite the Friedmann equation in the
form of the first law of thermodynamics, , at apparent horizon.
In addition, the generalized second law of thermodynamics can fulfilled in a
region enclosed by the apparent horizon on the brane for both constant and time
variable 5-dynamical Newton's constant . These results hold regardless of
the specific form of the dark energy. Our study further support that in an
accelerating universe with spatial curvature, the apparent horizon is a
physical boundary from the thermodynamical point of view.Comment: 11 page
Calculating three thermal coefficients from one data set
© 2019 Academic Publications. We study the problem of determining three thermal coefficients from one set data of a model problem rising in thermodynamics. This is an inverse problem, that is to coincide the solution of the differential equation with actual experimental results. The used method is based on minimizing the solution of the problem with the experimental data. Both the direct and inverse problems are described and numerical results are given
Spectral synthesis analysis and radial velocity study of the northern F-, G-, and K-type flare-stars
In this publication we present a study of the general physical, chemical
properties and radial velocity monitoring of young active stars. We derive
temperatures, log g, [Fe/H], v sin i, and R_{spec} values for eight stars. The
detailed analysis reveals that the stars are not as homogeneous in their
premier physical parameters as well as in the age distribution. In 4/5 we found
a periodic radial velocity signal which origins in surface features the fifth
is surprisingly inactive and shows little variation.Comment: 8 pages, 9 figures, accepted by MNRA
MOST photometry of the enigmatic PMS pulsator HD 142666
We present precise photometry of the pulsating Herbig Ae star HD 142666
obtained in two consecutive years with the MOST (Microvariability & Oscilations
of STars) satellite.
Previously, only a single pulsation period was known for HD 142666. The MOST
photometry reveals that HD 142666 is multi-periodic. However, the unique
identification of pulsation frequencies is complicated by the presence of
irregular variability caused by the star's circumstellar dust disk. The two
light curves obtained with MOST in 2006 and 2007 provided data of unprecedented
quality to study the pulsations in HD 142666 and also to monitor the
circumstellar variability.
We attribute 12 frequencies to pulsation. Model fits to the three frequencies
with the highest amplitudes lie well outside the uncertainty box for the star's
position in the HR diagram based on published values.
The models suggest that either (1) the published estimate of the luminosity
of HD 142666, based on a relation between circumstellar disk radius and stellar
luminosity, is too high and/or (2) additional physics such as mass accretion
may be needed in our models to accurately fit both the observed frequencies and
HD 142666's position in the HR diagram.Comment: 10 pages, 11 figures, accepted for publication by Astronomy and
Astrophysic
Global atmospheric budget of acetaldehyde: 3-D model analysis and constraints from in-situ and satellite observations
We construct a global atmospheric budget for acetaldehyde using a 3-D model of atmospheric chemistry (GEOS-Chem), and use an ensemble of observations to evaluate present understanding of its sources and sinks. Hydrocarbon oxidation provides the largest acetaldehyde source in the model (128 Tg a<sup>&minus;1</sup>, a factor of 4 greater than the previous estimate), with alkanes, alkenes, and ethanol the main precursors. There is also a minor source from isoprene oxidation. We use an updated chemical mechanism for GEOS-Chem, and photochemical acetaldehyde yields are consistent with the Master Chemical Mechanism. We present a new approach to quantifying the acetaldehyde air-sea flux based on the global distribution of light absorption due to colored dissolved organic matter (CDOM) derived from satellite ocean color observations. The resulting net ocean emission is 57 Tg a<sup>&minus;1</sup>, the second largest global source of acetaldehyde. A key uncertainty is the acetaldehyde turnover time in the ocean mixed layer, with quantitative model evaluation over the ocean complicated by known measurement artifacts in clean air. Simulated concentrations in surface air over the ocean generally agree well with aircraft measurements, though the model tends to overestimate the vertical gradient. PAN:NO<sub>x</sub> ratios are well-simulated in the marine boundary layer, providing some support for the modeled ocean source. We introduce the Model of Emissions of Gases and Aerosols from Nature (MEGANv2.1) for acetaldehyde and ethanol and use it to quantify their net flux from living terrestrial plants. Including emissions from decaying plants the total direct acetaldehyde source from the land biosphere is 23 Tg a<sup>&minus;1</sup>. Other terrestrial acetaldehyde sources include biomass burning (3 Tg a<sup>&minus;1</sup>) and anthropogenic emissions (2 Tg a<sup>&minus;1</sup>). Simulated concentrations in the continental boundary layer are generally unbiased and capture the spatial gradients seen in observations over North America, Europe, and tropical South America. However, the model underestimates acetaldehyde levels in urban outflow, suggesting a missing source in polluted air. Ubiquitous high measured concentrations in the free troposphere are not captured by the model, and based on present understanding are not consistent with concurrent measurements of PAN and NO<sub>x</sub>: we find no compelling evidence for a widespread missing acetaldehyde source in the free troposphere. We estimate the current US source of ethanol and acetaldehyde (primary + secondary) at 1.3 Tg a<sup>&minus;1</sup> and 7.8 Tg a<sup>&minus;1</sup>, approximately 60{%} and 480% of the corresponding increases expected for a national transition from gasoline to ethanol fuel
Development of the CNPP Prices Database
Data are available at: http://www.cnpp.usda.gov/USDAFoodPlansCostofFood.htmfood prices, USDA Food Plans, NHANES, CNPP Prices Database, Consumer/Household Economics, Demand and Price Analysis, Food Consumption/Nutrition/Food Safety,
YREC: The Yale Rotating Stellar Evolution Code
The stellar evolution code YREC is outlined with emphasis on its applications
to helio- and asteroseismology. The procedure for calculating calibrated solar
and stellar models is described. Other features of the code such as a non-local
treatment of convective core overshoot, and the implementation of a
parametrized description of turbulence in stellar models, are considered in
some detail. The code has been extensively used for other astrophysical
applications, some of which are briefly mentioned at the end of the paper.Comment: 10 pages, 2 figures, ApSS accepte
Confirmation of the Planet Hypothesis for the Long-period Radial Velocity Variations of Beta Geminorum
We present precise stellar radial velocity measurements for the K giant star
Beta Gem spanning over 25 years. These data show that the long period low
amplitude radial velocity variations found by Hatzes & Cochran (1993) are
long-lived and coherent. An examination of the Ca II K emission, spectral line
shapes from high resolution data (R = 210,000), and Hipparcos photometry show
no significant variations of these quantities with the RV period. These data
confirm the planetary companion hypothesis suggested by Hatzes & Cochran
(1993). An orbital solution assuming a stellar mass of 1.7 M_sun yields a
period, P = 589.6 days, a minimum mass of 2.3 M_Jupiter, and a semi-major axis,
and a = 1.6 AU. The orbit is nearly circular (e = 0.02). Beta Gem is the
seventh intermediate mass star shown to host a sub-stellar companion and
suggests that planet-formation around stars much more massive than the sun may
common.Comment: 10 pages, 9 figures, Astronomy and Astrophysics, in pres
The young, active binary star EK Draconis
EK Dra (HD 129333) is a young, active, nearby star that is orbited by a low
mass companion. By combining new speckle observations with old and new radial
velocity measurements we find that the orbit is highly eccentric with
e=0.82\pm0.03, and we derive the true masses of both components. The masses are
and , for the primary and
secondary, respectively. From high resolution spectra we derive a new of K, and a of , which is different to
previous estimates. However, the new spectroscopic distance differs by only
5.8% to the distance derived by parallax measurement of the Hipparcos satellite
and thus the stellar parameters are presumably more realistic than older
determinations. We derive a somewhat higher value for the metallicity of
. EK Dra turns out to be one of the few nearby young stars
which will evolve similar to the sun. The precise radial velocity measurements
taken in the course of this program also allows us to shed more light on to the
activity of this star. In 2001 and 2002 we find a periodic signal of the radial
velocity variations with a period of days which we interpret as
the rotation period. This signal vanishes in 2003. However the signal can be
recovered if only the spectra in which the photospheric lines are asymmetric
are used. On the other hand, we do not find a close correlation between the
asymmetry of photospheric lines and the radial velocity.Comment: 10 pages, 11 figures, accepted by A&
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