188 research outputs found
The Discovery of the Most Metal-Rich White Dwarf: Composition of a Tidally Disrupted Extrasolar Dwarf Planet
Cool white dwarf stars are usually found to have an outer atmosphere that is
practically pure in hydrogen or helium. However, a small fraction have traces
of heavy elements that must originate from the accretion of extrinsic material,
most probably circumstellar matter. Upon examining thousands of Sloan Digital
Sky Survey spectra, we discovered that the helium-atmosphere white dwarf SDSS
J073842.56+183509.6 shows the most severe metal pollution ever seen in the
outermost layers of such stars. We present here a quantitative analysis of this
exciting star by combining high S/N follow-up spectroscopic and photometric
observations with model atmospheres and evolutionary models. We determine the
global structural properties of our target star, as well as the abundances of
the most significant pollutants in its atmosphere, i.e., H, O, Na, Mg, Si, Ca,
and Fe. The relative abundances of these elements imply that the source of the
accreted material has a composition similar to that of Bulk Earth. We also
report the signature of a circumstellar disk revealed through a large infrared
excess in JHK photometry. Combined with our inferred estimate of the mass of
the accreted material, this strongly suggests that we are witnessing the
remains of a tidally disrupted extrasolar body that was as large as Ceres.Comment: 7 pages in emulateapj, 5 figures, accepted for publication in Ap
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
Chandra grating spectroscopy of three hot white dwarfs
High-resolution soft X-ray spectroscopic observations of single hot white
dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have
observed two white dwarfs, one is of spectral type DA (LB 1919) and the other
is a non-DA of spectral type PG1159 (PG 1520+525). The spectra of both stars
are analyzed, together with an archival Chandra spectrum of another DA white
dwarf (GD 246). The soft X-ray spectra of the two DA white dwarfs are
investigated in order to study the effect of gravitational settling and
radiative levitation of metals in their photospheres. LB 1919 is of interest
because it has a significantly lower metallicity than DAs with otherwise
similar atmospheric parameters. GD 246 is the only white dwarf known that shows
identifiable individual iron lines in the soft X-ray range. For the PG1159
star, a precise effective temperature determination is performed in order to
confine the position of the blue edge of the GW Vir instability region in the
HRD. (abridged)Comment: A&A, in pres
Ecosystem transpiration and evaporation: Insights from three water flux partitioning methods across FLUXNET sites
We apply and compare three widely applicable methods for estimating ecosystem transpiration (T) from eddy covariance (EC) data across 251 FLUXNET sites globally. All three methods are based on the coupled water and carbon relationship, but they differ in assumptions and parameterizations. Intercomparison of the three daily T estimates shows high correlation among methods (R between .89 and .94), but a spread in magnitudes of T/ET (evapotranspiration) from 45% to 77%. When compared at six sites with concurrent EC and sap flow measurements, all three ECâbased T estimates show higher correlation to sap flowâbased T than ECâbased ET. The partitioning methods show expected tendencies of T/ET increasing with dryness (vapor pressure deficit and days since rain) and with leaf area index (LAI). Analysis of 140 sites with highâquality estimates for at least two continuous years shows that T/ET variability was 1.6 times higher across sites than across years. Spatial variability of T/ET was primarily driven by vegetation and soil characteristics (e.g., crop or grass designation, minimum annual LAI, soil coarse fragment volume) rather than climatic variables such as mean/standard deviation of temperature or precipitation. Overall, T and T/ET patterns are plausible and qualitatively consistent among the different water flux partitioning methods implying a significant advance made for estimating and understanding T globally, while the magnitudes remain uncertain. Our results represent the first extensive EC dataâbased estimates of ecosystem T permitting a dataâdriven perspective on the role of plantsâ water use for global water and carbon cycling in a changing climate.We acknowledge insightful discussions with Dario Papale and apologize for having a cappuccino after lunch. We further acknowledge Ulrich Weber for preparing the cappuccino. M.G. acknowledges funding by Swiss National Science Foundation project ICOSâCH Phase 2 20FI20_173691. L.Ć . was supported by the Ministry of Education, Youth and Sports of the Czech Republic within the CzeCOS program, grant number LM2015061, and by SustESâAdaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions (CZ.02.1.01/0.0/0.0/16_019/0000797). G.W. acknowledges support by the Austrian National Science Fund (FWF, project I03859) and the Province of South Tyrol (âCycling of carbon and water in mountain ecosystems under changing climate and land useâ). R.P. was supported by grants CGL2014â55883âJIN, RTI2018â095297âJâI00 (Spain), and by a Humboldt Research Fellowship for Experienced Researchers (Germany). This work used eddy covariance data acquired and shared by the FLUXNET community, including these networks: AmeriâFlux, AfriFlux, AsiaFlux, CarboAfrica, CarboEuropeIP, CarboItaly, CarboMont, ChinaFlux, FluxnetâCanada, GreenGrass, ICOS, KoFlux, LBA, NECC, OzFluxâTERN, TCOSâSiberia, and USCCC. The ERAâInterim reanalysis data are provided by ECMWF and processed by LSCE. The FLUXNET eddy covariance data processing and harmonization was carried out by the European Fluxes Database Cluster, AmeriFlux Management Project, and Fluxdata project of FLUXNET, with the support of CDIAC and ICOS Ecosystem Thematic Center, and the OzFlux, ChinaFlux, and AsiaFlux offices. Open access funding enabled and organized by Projekt DEAL
Evaluating stereo and motion cues for visualizing information nets in three dimensions
DA white dwarfs from the LSS-GAC survey DR1: the preliminary luminosity and mass functions and formation rate
Modern large-scale surveys have allowed the identification of large numbers
of white dwarfs. However, these surveys are subject to complicated target
selection algorithms, which make it almost impossible to quantify to what
extent the observational biases affect the observed populations. The LAMOST
(Large Sky Area Multi-Object Fiber Spectroscopic Telescope) Spectroscopic
Survey of the Galactic anti-center (LSS-GAC) follows a well-defined set of
criteria for selecting targets for observations. This advantage over previous
surveys has been fully exploited here to identify a small yet
well-characterised magnitude-limited sample of hydrogen-rich (DA) white dwarfs.
We derive preliminary LSS-GAC DA white dwarf luminosity and mass functions. The
space density and average formation rate of DA white dwarfs we derive are
0.83+/-0.16 x 10^{-3} pc^{-3} and 5.42 +/- 0.08 x 10^{-13} pc^{-3} yr^{-1},
respectively. Additionally, using an existing Monte Carlo population synthesis
code we simulate the population of single DA white dwarfs in the Galactic
anti-center, under various assumptions. The synthetic populations are passed
through the LSS-GAC selection criteria, taking into account all possible
observational biases. This allows us to perform a meaningful comparison of the
observed and simulated distributions. We find that the LSS-GAC set of criteria
is highly efficient in selecting white dwarfs for spectroscopic observations
(80-85 per cent) and that, overall, our simulations reproduce well the observed
luminosity function. However, they fail at reproducing an excess of massive
white dwarfs present in the observed mass function. A plausible explanation for
this is that a sizable fraction of massive white dwarfs in the Galaxy are the
product of white dwarf-white dwarf mergers.Comment: 23 pages, 14 figures and 5 tables. Accepted for publication by MNRA
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Different Verbal Learning Strategies in Autism Spectrum Disorder: Evidence from the Rey Auditory Verbal Learning Test
The Rey Auditory Verbal Learning Test, which requires the free recall of the same list of 15 items over 5 trials, was administered to a group of highfunctioning adolescents and adults with autism spectrum disorder (PDD) and a group of matched typical individuals. Overall levels of free recall were comparable in the two groups, as were the rates of learning over trials. Both groups also subjectively organised their recall to a similar extent. However, the serial position curve of the PDD participants, although similar to that of the comparison group on the first trial, became flatter on subsequent trials and was characterised by a slower growth in the primacy effect. Growth in the middle and recency portions of the curve was comparable in both groups. The findings are discussed in the light of current models of serial position effects and their implications for memory in ASD
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