340 research outputs found
Theoretical UBVRI colors of iron core white dwarfs
We explore photometric properties of hypothetical iron core white dwarfs and
compute their expected colors in UBVRI Johnson broadband system. Atmospheres of
iron core WDs in this paper consist of pure iron covered by a pure hydrogen
layer of an arbitrary column mass. LTE model atmospheres and theoretical
spectra are calculated on the basis of Los Alamos TOPS opacities and the
equation of state from the OPAL project, suitable for nonideal Fe and H gases.
We have also computed UBVRI colors of the models and determined an area on the
B-V vs. U-B and U-B vs. V-I planes, occupied by both pure Fe, and pure H model
atmospheres of WD stars. Finally, we search for iron core white dwarf
candidates in the available literature.Comment: 13 pages, 12 figures, Astronomy & Astrophysics (2003) in prin
Multiwavelength Observations of the Hot DB Star PG 0112+104
We present a comprehensive multiwavelength analysis of the hot DB white dwarf
PG 0112+104. Our analysis relies on newly-acquired FUSE observations, on
medium-resolution FOS and GHRS data, on archival high-resolution GHRS
observations, on optical spectrophotometry both in the blue and around Halpha,
as well as on time-resolved photometry. From the optical data, we derive a
self-consistent effective temperature of 31,300+-500 K, a surface gravity of
log g = 7.8 +- 0.1 (M=0.52 Msun), and a hydrogen abundance of log N(H)/N(He) <
-4.0. The FUSE spectra reveal the presence of CII and CIII lines that
complement the previous detection of CII transitions with the GHRS. The
improved carbon abundance in this hot object is log N(C)/N(He) = -6.15 +- 0.23.
No photospheric features associated with other heavy elements are detected. We
reconsider the role of PG 0112+104 in the definition of the blue edge of the
V777 Her instability strip in light of our high-speed photometry, and contrast
our results with those of previous observations carried out at the McDonald
Observatory.Comment: 10 pages in emulateapj, 9 figures, accepted for publication in Ap
AOD Distributions and Trends of Major Aerosol Species over a Selection of the World's Most Populated Cities Based on the 1st Version of NASA's MERRA Aerosol Reanalysis
NASA recently extended the Modern-Era Retrospective Analysis for Research and Application (MERRA) with an atmospheric aerosol reanalysis which includes five particulate species: sulfate, organic matter, black carbon, mineral dust and sea salt. The MERRA Aerosol Reanalysis (MERRAero) is an innovative tool to study air quality issues around the world for its global and constant coverage and its distinction of aerosol speciation expressed in the form of aerosol optical depth (AOD). The purpose of this manuscript is to apply MERRAero to the study of urban air pollution at the global scale by analyzing the AOD over a period of 13 years (2003-2015) and over a selection of 200 of the world's most populated cities in order to assess the impacts of urbanization, industrialization, air quality regulations and regional transport which affect urban aerosol load. Environmental regulations and the recent global economic recession have helped to decrease the AOD and sulfate aerosols in most cities in North America, Europe and Japan. Rapid industrialization in China over the last two decades resulted in Chinese cities having the highest AOD values in the world. China has nevertheless recently implemented emission control measures which are showing early signs of success in many cities of Southern China where AOD has decreased substantially over the last 13 years. The AOD over South American cities, which is dominated by carbonaceous aerosols, has also decreased over the last decade due to an increase in commodity prices which slowed deforestation activities in the Amazon rainforest. At the opposite, recent urbanization and industrialization in India and Bangladesh resulted in a strong increase of AOD, sulfate and carbonaceous aerosols in most cities of these two countries. The AOD over most cities in Northern Africa and Western Asia changed little over the last decade. Emissions of natural aerosols, which cities in these two regions tend to be mostly composed of, don't tend to fluctuate significantly on an annual basis
An Upper Bound to the Space Density of Interstellar Comets
Two well-studied white dwarfs with helium-dominated atmospheres (DBs) each
possess less hydrogen than carried by a single average-mass comet. Plausibly,
the wind rates from these stars are low enough that most accreted hydrogen
remains with the star. If so, and presuming their nominal effective
temperatures, then these DBs have been minimally impacted by interstellar
comets during their 50 Myr cooling age; interstellar iceballs with radii
between 10 m and 2 km contain less than 1% of all interstellar oxygen. This
analysis suggests that most stars do not produce comets at the rate predicted
by "optimistic" scenarios for the formation of the Oort cloud.Comment: Astronomical Journal, accepte
A new look at the pulsating DB white dwarf GD 358:Line-of-sight velocity measurements and constraints on model atmospheres
We report on our findings of the bright, pulsating, helium atmosphere white
dwarf GD 358, based on time-resolved optical spectrophotometry. We identify 5
real pulsation modes and at least 6 combination modes at frequencies consistent
with those found in previous observations. The measured Doppler shifts from our
spectra show variations with amplitudes of up to 5.5 km/s at the frequencies
inferred from the flux variations. We conclude that these are variations in the
line-of-sight velocities associated with the pulsational motion. We use the
observed flux and velocity amplitudes and phases to test theoretical
predictions within the convective driving framework, and compare these with
similar observations of the hydrogen atmosphere white dwarf pulsators (DAVs).
The wavelength dependence of the fractional pulsation amplitudes (chromatic
amplitudes) allows us to conclude that all five real modes share the same
spherical degree, most likely, l=1. This is consistent with previous
identifications based solely on photometry. We find that a high signal-to-noise
mean spectrum on its own is not enough to determine the atmospheric parameters
and that there are small but significant discrepancies between the observations
and model atmospheres. The source of these remains to be identified. While we
infer T_eff=24kK and log g~8.0 from the mean spectrum, the chromatic
amplitudes, which are a measure of the derivative of the flux with respect to
the temperature, unambiguously favour a higher effective temperature, 27kK,
which is more in line with independent determinations from ultra-violet
spectra.Comment: 14 pages, 11 figures; accepted for publication in A&
Technical Report Series on Global Modeling and Data Assimilation
NASA's Global Modeling and Assimilation Office has extended the Modern-Era Retrospective Analysis for Research and Application (MERRA) tool with five atmospheric aerosol species (sulfates, organic carbon, black carbon, mineral dust and sea salt). This inclusion of aerosol reanalysis data is now known as MERRAero. This study analyses a ten-year period (July 2002 - June 2012) MERRAero aerosol reanalysis applied to the study of aerosol optical depth (AOD) and its trends for the aforementioned aerosol species over the world's major cities (with a population of over 2 million inhabitants). We found that a proportion of various aerosol species in total AOD exhibited a geographical dependence. Cities in industrialized regions (North America, Europe, central and eastern Asia) are characterized by a strong proportion of sulfate aerosols. Organic carbon aerosols are dominant over cities which are located in regions where biomass burning frequently occurs (South America and southern Africa). Mineral dust dominates other aerosol species in cities located in proximity to the major deserts (northern Africa and western Asia). Sea salt aerosols are prominent in coastal cities but are dominant aerosol species in very few of them. AOD trends are declining over cities in North America, Europe and Japan, as a result of effective air quality regulation. By contrast, the economic boom in China and India has led to increasing AOD trends over most cities in these two highly-populated countries. Increasing AOD trends over cities in the Middle East are caused by increasing desert dust
Precursor flares in OJ 287
We have studied three most recent precursor flares in the light curve of the
blazar OJ 287 while invoking the presence of a precessing binary black hole in
the system to explain the nature of these flares. Precursor flare timings from
the historical light curves are compared with theoretical predictions from our
model that incorporate effects of an accretion disk and post-Newtonian
description for the binary black hole orbit. We find that the precursor flares
coincide with the secondary black hole descending towards the accretion disk of
the primary black hole from the observed side, with a mean z-component of
approximately z_c = 4000 AU. We use this model of precursor flares to predict
that precursor flare of similar nature should happen around 2020.96 before the
next major outburst in 2022.Comment: to appear in the Astrophysical Journa
Precise mass and radius values for the white dwarf and low mass M dwarf in the pre-cataclysmic binary NN Serpentis
We derive precise system parameters for the pre-cataclysmic binary, NN Ser.
From light curve fitting we find an orbital inclination of i = 89.6 +/- 0.2
deg. From the HeII absorption line we find K_{WD}= 62.3 +/- 1.9 km/s. The
irradiation-induced emission lines from the surface of the secondary star give
a range of observed radial velocities. The corrected values give a radial
velocity of K_{sec}= 301 +/- 3 km/s, with an error dominated by the systematic
effects of the model. This leads to a binary separation of a = 0.934 +/- 0.009
R_{sun}, radii of R_{WD} = 0.0211 +/- 0.0002 R_{sun} and R_{sec} = 0.149 +/-
0.002 R_{sun} and masses of M_{WD} = 0.535 +/- 0.012 M_{sun} and M_{sec} =
0.111 +/- 0.004 M_{sun}. The masses and radii of both components of NN Ser were
measured independently of any mass-radius relation. For the white dwarf, the
measured mass, radius and temperature show excellent agreement with a `thick'
hydrogen layer of fractional mass M_{H}/{M}_{WD} = 10^{-4}. The measured radius
of the secondary star is 10% larger than predicted by models, however,
correcting for irradiation accounts for most of this inconsistency, hence the
secondary star in NN Ser is one of the first precisely measured very low mass
objects to show good agreement with models. ULTRACAM r', i' and z' photometry
taken during the primary eclipse determines the colours of the secondary star
as (r'-i')_{sec}= 1.4 +/- 0.1 and (i'-z')_{sec} = 0.8 +/- 0.1 which corresponds
to a spectral type of M4 +/- 0.5. This is consistent with the derived mass,
demonstrating that there is no detectable heating of the unirradiated face,
despite intercepting radiative energy from the white dwarf which exceeds its
own luminosity by over a factor of 20.Comment: 20 pages, 17 figures, 8 tables, minor changes, accepted for
publication in MNRA
Pulsational Mapping of Calcium Across the Surface of a White Dwarf
We constrain the distribution of calcium across the surface of the white
dwarf star G29-38 by combining time series spectroscopy from Gemini-North with
global time series photometry from the Whole Earth Telescope. G29-38 is
actively accreting metals from a known debris disk. Since the metals sink
significantly faster than they mix across the surface, any inhomogeneity in the
accretion process will appear as an inhomogeneity of the metals on the surface
of the star. We measure the flux amplitudes and the calcium equivalent width
amplitudes for two large pulsations excited on G29-38 in 2008. The ratio of
these amplitudes best fits a model for polar accretion of calcium and rules out
equatorial accretion.Comment: Accepted to the Astrophysical Journal. 16 pages, 10 figures
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