1,410 research outputs found
Spectral analysis of the sdO K 648, the exciting star of the planetary nebula Ps 1 in the globular cluster M 15 (NGC 7078)
We present a spectral analysis of the sdO central star K 648 based on
medium-resolution optical and high-resolution UV spectra. The photospheric
parameters are determined by means of state-of-the-art NLTE model atmosphere
techniques.
We found Teff = 39 +/- 2 kK and log g = 3.9 +/- 0.2. The helium (He/H=0.08)
and oxygen (O/H=0.001) abundances are about solar while carbon is enriched by a
factor of 2.5 (C/H=0.001). Nitrogen (N/H = 10**(-6), [N/H] = -2.0) appears at a
sub-solar value. However, these metal abundances are much higher than the
cluster's metallicity M 15: [Fe/H] = -2.25).
The surface composition appears to be a mixture of the original hydrogen-rich
material and products of helium burning (3 alpha process) which have been mixed
up to the surface. The abundances of He, C, and N are consistent with the
nebular abundance, while O is considerably more abundant in the photosphere
than in the nebula.
From a comparison of its position in the log Teff - log g plane with
evolutionary calculations a mass of 0.57 (+0.02, -0.01) Msun and a luminosity
of 3810 +/- 1200 Lsun are deduced.
Our spectroscopic distance d = 11.1 (+2.4, -2.9) kpc is in agreement with the
distance of M 15 as determined by Alves et al. (2000).
From the GHRS spectra we measure a radial velocity of vrad = -130 km/sec.Comment: 8 pages, 13 figure
Two candidate brown dwarf companions around core helium-burning stars
Hot subdwarf stars of spectral type B (sdBs) are evolved, core helium-burning
objects. The formation of those objects is puzzling, because the progenitor
star has to lose almost its entire hydrogen envelope in the red-giant phase.
Binary interactions have been invoked, but single sdBs exist as well. We report
the discovery of two close hot subdwarf binaries with small radial velocity
amplitudes. Follow-up photometry revealed reflection effects originating from
cool irradiated companions, but no eclipses. The lower mass limits for the
companions of CPD-64481 () and PHL\,457
() are significantly below the stellar mass limit. Hence
they could be brown dwarfs unless the inclination is unfavourable. Two very
similar systems have already been reported. The probability that none of them
is a brown dwarf is very small, 0.02%. Hence we provide further evidence that
substellar companions with masses that low are able to eject a common envelope
and form an sdB star. Furthermore, we find that the properties of the observed
sample of hot subdwarfs in reflection effect binaries is consistent with a
scenario where single sdBs can still be formed via common envelope events, but
their low-mass substellar companions do not survive.Comment: accepted to A&
A criterion to discriminate between solar and cosmic ray forcing of the terrestrial climate
International audienceThere is increasing evidence that there exist interstellar-terrestrial relations and that the heliosphere's effectivity to serve as a protecting shield for the Earth, specifically against cosmic rays, is varying in time. Nonetheless, a debate is going on whether, amongst other drivers, the Sun or the cosmic rays are influencing the terrestrial climate, particularly on periods of hundred years and shorter. As the modelling of the transport of cosmic rays in the heliosphere has evolved from pure test particle simulations to far more consistent treatments, one can explain various correlations within the framework of physical models and one can make quantitative predictions regarding terrestrial indicators of interstellar-terrestrial relations. This level of understanding and modelling allows to identify a criterion with which one can discriminate between solar and cosmic ray forcing on a period of several decades. We define such a criterion and discuss related existing observations
Improving Data Quality for a Dairy Pollutant Emissions Study
The National Air Emissions Monitoring Study (NAEMS) was sanctioned by the EPA to determine the characteristics of airborne pollutant emissions from confined broiler, egg, pork, and dairy housing. Fifteen representative monitoring sites were selected around the U.S., at which influent and effluent pollutant concentrations were measured in conjunction with airflow and climatic data. Due to the monumental nature of this study and the potential ramifications of its findings, it is of vital importance that the data collected by the researchers and utilized by the EPA be as complete and accurate as possible. To improve the validity of the data collected at a dairy facility in New York, it was necessary to review the work of previous data analysts while studying the field notes that were logged by scientists onsite during data collection. This allowed for the correction of perceived errors in the handling of the data. When sensor data were deemed invalid or missing, redundant data were substituted. Any unnecessarily flagged out data were restored. The use of these strategies led to a significant improvement in data quality. For example, data completeness for ambient temperature and relative humidity were increased by over 6%, while atmospheric pressure data saw an improvement of more than 18% after substituting data from the nearest NWS weather station. These and other improvements to this data set will allow EPA to develop more accurate dairy facility emissions models that will have substantial, wide-ranging effects for both producers and consumers in the U.S. dairy industry
The Deuterium, Oxygen, and Nitrogen Abundance Toward LSE 44
We present measurements of the column densities of interstellar DI, OI, NI,
and H2 made with FUSE, and of HI made with IUE toward the sdO star LSE 44, at a
distance of 554+/-66 pc. This target is among the seven most distant Galactic
sight lines for which these abundance ratios have been measured. The column
densities were estimated by profile fitting and curve of growth analyses. We
find D/H = (2.24 +1.39 -1.32)E-5, D/O = (1.99 +1.30 -0.67)E-2, D/N = (2.75
+1.19 -0.89)E-1, and O/H = (1.13 +0.96 -0.71)E-3 (2 sigma). Of the most distant
Galactic sight lines for which the deuterium abundance has been measured LSE 44
is one of the few with D/H higher than the Local Bubble value, but D/O toward
all these targets is below the Local Bubble value and more uniform than the D/H
distribution. (Abstract abridged.)Comment: 20 pages, including 9 figures. Accepted for publication in Ap
- âŠ