64,136 research outputs found
Testing the asymptotic relation for period spacings from mixed modes of red giants observed with the Kepler mission
Dipole mixed pulsation modes of consecutive radial order have been detected
for thousands of low-mass red-giant stars with the NASA space telescope Kepler.
Such modes have the potential to reveal information on the physics of the deep
stellar interior. Different methods have been proposed to derive an observed
value for the gravity-mode period spacing, the most prominent one relying on a
relation derived from asymptotic pulsation theory applied to the gravity-mode
character of the mixed modes. Our aim is to compare results based on this
asymptotic relation with those derived from an empirical approach for three
pulsating red-giant stars. We developed a data-driven method to perform
frequency extraction and mode identification. Next, we used the identified
dipole mixed modes to determine the gravity-mode period spacing by means of an
empirical method and by means of the asymptotic relation. In our methodology,
we consider the phase offset, , of the asymptotic
relation as a free parameter. Using the frequencies of the identified dipole
mixed modes for each star in the sample, we derived a value for the
gravity-mode period spacing using the two different methods. These differ by
less than 5%. The average precision we achieved for the period spacing derived
from the asymptotic relation is better than 1%, while that of our data-driven
approach is 3%. Good agreement is found between values for the period spacing
derived from the asymptotic relation and from the empirical method.
Full abstract in PDF file.Comment: 14 pages, 13 figures, accepted for publication in A&
A Socio-Informatic Approach to Automated Account Classification on Social Media
Automated accounts on social media have become increasingly problematic. We
propose a key feature in combination with existing methods to improve machine
learning algorithms for bot detection. We successfully improve classification
performance through including the proposed feature.Comment: International Conference on Social Media and Societ
CoRoT and Kepler results: Solar-like oscillators
The space-borne observatories CoRoT (Convection Rotation and planetary
Transits) and Kepler have provided photometric time series data of
unprecedented precision for large numbers of stars. These data have
revolutionized the fields of transiting exoplanets and asteroseismology. In
this review some important asteroseismic results obtained using data from the
CoRoT and Kepler space missions concerning stars that show solar-like
oscillations are discussed. These results comprise, among others, measurements
of the location of the base of the convection zone and helium second-ionization
zone in main-sequence stars, the presence (or not) of core-helium burning in
red-giant stars, as well as differential rotation in these stars.Comment: Invited review for Advances in Space Researc
Combining tower mixing ratio and community model data to estimate regional-scale net ecosystem carbon exchange by boundary layer inversion over 4 flux towers in the U.S.A.
We evaluated an idealized boundary layer (BL) model with simple parameterizations using vertical transport information from community model outputs (NCAR/NCEP Reanalysis and ECMWF Interim Analysis) to estimate regional-scale net CO2 fluxes from 2002 to 2007 at three forest and one grassland flux sites in the United States. The BL modeling approach builds on a mixed-layer model to infer monthly average net CO2 fluxes using high-precision mixing ratio measurements taken on flux towers. We compared BL model net ecosystem exchange (NEE) with estimates from two independent approaches. First, we compared modeled NEE with tower eddy covariance measurements. The second approach (EC-MOD) was a data-driven method that upscaled EC fluxes from towers to regions using MODIS data streams. Comparisons between modeled CO2 and tower NEE fluxes showed that modeled regional CO2 fluxes displayed interannual and intra-annual variations similar to the tower NEE fluxes at the Rannells Prairie and Wind River Forest sites, but model predictions were frequently different from NEE observations at the Harvard Forest and Howland Forest sites. At the Howland Forest site, modeled CO2 fluxes showed a lag in the onset of growing season uptake by 2 months behind that of tower measurements. At the Harvard Forest site, modeled CO2 fluxes agreed with the timing of growing season uptake but underestimated the magnitude of observed NEE seasonal fluctuation. This modeling inconsistency among sites can be partially attributed to the likely misrepresentation of atmospheric transport and/or CO2gradients between ABL and the free troposphere in the idealized BL model. EC-MOD fluxes showed that spatial heterogeneity in land use and cover very likely explained the majority of the data-model inconsistency. We show a site-dependent atmospheric rectifier effect that appears to have had the largest impact on ABL CO2 inversion in the North American Great Plains. We conclude that a systematic BL modeling approach provided new insights when employed in multiyear, cross-site synthesis studies. These results can be used to develop diagnostic upscaling tools, improving our understanding of the seasonal and interannual variability of surface CO2 fluxes
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