1,052 research outputs found
Four new HgMn stars: HD 18104, HD 30085, HD 32867, HD 53588
We have detected four new HgMn stars, while monitoring a sample of apparently
slowly rotating superficially normal bright late B and early A stars in the
northern hemisphere. Important classification lines of Hg II and Mn II are
found as conspicuous features in the high resolution SOPHIE spectra of these
stars (R = 75000). Several lines of Hg II, Mn II and Fe II have been
synthesized using model atmospheres and the spectrum synthesis code SYNSPEC48
including hyperfine structure of various isotopes when relevant. These
synthetic spectra have been compared to high resolution high signal-to-noise
observations of these stars in order to derive abundances of these key
elements. The four stars are found to have distinct enhancements of Hg and Mn
which show that these stars are not superficially normal B and A stars, but
actually are new HgMn stars and should reclassified as such.Comment: 4 pages, 3 figures, accepted in A&
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The role of natural variability in projections of climate change impacts on U.S. ozone pollution
Climate change can impact air quality by altering the atmospheric conditions that determine pollutant concentrations. Over large regions of the U.S., projected changes in climate are expected to favor formation of ground-level ozone and aggravate associated health effects. However, modeling studies exploring air quality-climate interactions have often overlooked the role of natural variability, a major source of uncertainty in projections. Here we use the largest ensemble simulation of climate-induced changes in air quality generated to date to assess its influence on estimates of climate change impacts on U.S. ozone. We find that natural variability can significantly alter the robustness of projections of the future climate's effect on ozone pollution. In this study, a 15 year simulation length minimum is required to identify a distinct anthropogenic-forced signal. Therefore, we suggest that studies assessing air quality impacts use multidecadal simulations or initial condition ensembles. With natural variability, impacts attributable to climate may be difficult to discern before midcentury or under stabilization scenarios
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Natural Variability in Projections of Climate Change Impacts on Fine Particulate Matter Pollution
Variations in meteorology associated with climate change can impact fine particulate matter (PM2.5) pollution by affecting natural emissions, atmospheric chemistry, and pollutant transport. However, substantial discrepancies exist among model-based projections of PM2.5 impacts driven by anthropogenic climate change. Natural variability can significantly contribute to the uncertainty in these estimates. Using a large ensemble of climate and atmospheric chemistry simulations, we evaluate the influence of natural variability on projections of climate change impacts on PM2.5 pollution in the United States. We find that natural variability in simulated PM2.5 can be comparable or larger than reported estimates of anthropogenic-induced climate impacts. Relative to mean concentrations, the variability in projected PM2.5 climate impacts can also exceed that of ozone impacts. Based on our projections, we recommend that analyses aiming to isolate the effect climate change on PM2.5 use 10 years or more of modeling to capture the internal variability in air quality and increase confidence that the anthropogenic-forced effect is differentiated from the noise introduced by natural variability. Projections at a regional scale or under greenhouse gas mitigation scenarios can require additional modeling to attribute impacts to climate change. Adequately considering natural variability can be an important step toward explaining the inconsistencies in estimates of climate-induced impacts on PM2.5. Improved treatment of natural variability through extended modeling lengths or initial condition ensembles can reduce uncertainty in air quality projections and improve assessments of climate policy risks and benefits
A new method for the inversion of atmospheric parameters of A/Am stars
We present an automated procedure that derives simultaneously the effective
temperature , the surface gravity logg, the metallicity [Fe/H], and
the equatorial projected rotational velocity vsini for "normal" A and Am stars.
The procedure is based on the principal component analysis inversion method of
Paletou et al. (2015a). A sample of 322 high resolution spectra of F0-B9 stars,
retrieved from the Polarbase, SOPHIE, and ELODIE databases, were used to test
this technique with real data. We have selected the spectral region from
4400-5000\AA\ as it contains many metallic lines and the Balmer H line.
Using 3 datasets at resolving powers of R=42000, 65000 and 76000, about
6.6x synthetic spectra were calculated to build a large learning
database. The Online Power Iteration algorithm was applied to these learning
datasets to estimate the principal components (PC). The projection of spectra
onto the few PCs offered an efficient comparison metric in a low dimensional
space. The spectra of the well known A0- and A1-type stars, Vega and Sirius A,
were used as control spectra in the three databases. Spectra of other well
known A-type stars were also employed in order to characterize the accuracy of
the inversion technique. All observational spectra were inverted and
atmospheric parameters derived. After removal of a few outliers, the
PCA-inversion method appears to be very efficient in determining ,
[Fe/H], and vsini for A/Am stars. The derived parameters agree very well with
previous determinations. Using a statistical approach, deviations of around 150
K, 0.35 dex, 0.15 dex, and 2 km/s were found for , logg, [Fe/H], and
vsini with respect to literature values for A-type stars. The PCA-inversion
proves to be a very fast, practical, and reliable tool for estimating stellar
parameters of FGK and A stars, and deriving effective temperatures of M stars.Comment: 16 pages, 9 figures. Accepted in A&
The Quasar-frame Velocity Distribution of Narrow CIV Absorbers
We report on a survey for narrow (FWHM < 600 km/s) CIV absorption lines in a
sample of bright quasars at redshifts in the Sloan Digital
Sky Survey. Our main goal is to understand the relationship of narrow CIV
absorbers to quasar outflows and, more generally, to quasar environments. We
determine velocity zero-points using the broad MgII emission line, and then
measure the absorbers' quasar-frame velocity distribution. We examine the
distribution of lines arising in quasar outflows by subtracting model fits to
the contributions from cosmologically intervening absorbers and absorption due
to the quasar host galaxy or cluster environment. We find a substantial number
( per cent) of absorbers with REW \AA in the velocity range
+750 km/s \la v \la +12000 km/s are intrinsic to the AGN outflow. This
`outflow fraction' peaks near km/s with a value of . At velocities below km/s the incidence
of outflowing systems drops, possibly due to geometric effects or to the
over-ionization of gas that is nearer the accretion disk. Furthermore, we find
that outflow-absorbers are on average broader and stronger than
cosmologically-intervening systems. Finally, we find that per cent of
the quasars in our sample exhibit narrow, outflowing CIV absorption with REW \AA, slightly larger than that for broad absorption line systems.Comment: 11 pages, 9 figures, accepted for publication in MNRA
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