3,832 research outputs found
A Detailed Model Atmosphere Analysis of Cool White Dwarfs in the Sloan Digital Sky Survey
We present optical spectroscopy and near-infrared photometry of 126 cool
white dwarfs in the Sloan Digital Sky Survey (SDSS). Our sample includes high
proper motion targets selected using the SDSS and USNO-B astrometry and a dozen
previously known ultracool white dwarf candidates. Our optical spectroscopic
observations demonstrate that a clean selection of large samples of cool white
dwarfs in the SDSS (and the SkyMapper, Pan-STARRS, and the Large Synoptic
Survey Telescope datasets) is possible using a reduced proper motion diagram
and a tangential velocity cut-off (depending on the proper motion accuracy) of
30 km/s. Our near-infrared observations reveal eight new stars with significant
absorption. We use the optical and near-infrared photometry to perform a
detailed model atmosphere analysis. More than 80% of the stars in our sample
are consistent with either pure hydrogen or pure helium atmospheres. However,
the eight stars with significant infrared absorption and the majority of the
previously known ultracool white dwarf candidates are best explained with mixed
hydrogen and helium atmosphere models. The age distribution of our sample is
consistent with a Galactic disk age of 8 Gyr. A few ultracool white dwarfs may
be as old as 12-13 Gyr, but our models have problems matching the spectral
energy distributions of these objects. There are only two halo white dwarf
candidates in our sample. However, trigonometric parallax observations are
required for accurate mass and age determinations and to confirm their
membership in the halo.Comment: ApJ Supplements, in pres
A new extremely low-mass white dwarf in the NLTT catalogue
We report on the discovery of the extremely low-mass, hydrogen-rich white
dwarf, NLTT 11748. Based on measurements of the effective temperature
(8540+/-50 K) and surface gravity (log g = 6.20+/-0.15) obtained by fitting the
observed Balmer line profiles with synthetic spectra, we derive a mass of
0.167+/-0.005 M_solar. This object is one of only a handful of white dwarfs
with masses below 0.2 M_solar that are believed to be the product of close
binary evolution with an episode of Roche lobe overflow onto a degenerate
companion (neutron star or white dwarf). Assuming membership in the halo
population, as suggested by the kinematics and adopting a cooling age of 4.0 -
6.3 Gyrs for the white dwarf, we infer a progenitor mass of 0.87 - 0.93
M_solar. The likely companion has yet to be identified, but a search for radial
velocity variations may help constrain its nature.Comment: Accepted for publication in A&A Letter
Feshbach Resonance and Growth of a Bose-Einstein Condensate
Gross-Pitaevskii equation with gain is used to model Bose Einstein
condensation (BEC) fed by the surrounding thermal cloud. It is shown that the
number of atoms continuously injected into BEC from the reservoir can be
controlled by applying the external magnetic field via Feshbach resonance.Comment: 4 page
Design of master and slave modules on battery management system for electric vehicles
Nowadays, electric vehicle usage and the use of LiFePO4 batteries in electric vehicles gradually increase. However, there are important features to be considered to use these batteries safely and efficiently. Incorrect use of these batteries can lead to burning, explosion or shortening of the life of batteries.
In this paper, a Battery Management System (BMS) for lithium based batteries is designed that operates more efficiently and communicates with UART between master and slave modules and can communicate via CAN protocol with external devices. Micro controller based control and protection equipment is designed that help to measure and monitor the voltage, temperature and current values of the batteries. They protect the battery cells from the conditions such as over charge, over discharge, high current, high temperature. BMS balances battery cell voltages during charging process with passive cell voltage balancing. In addition to the main controller module in the BMS, slave controller modules have been added to provide high resolution voltage and temperature tracking. A modular BMS has been devised which can be used in groups of batteries of different voltage values thanks to electrically isolated slave control modules
More Network Conscious than Ever? Challenges, strategies and analytic labor of users in the Facebook Environment
Cataloged from PDF version of article.As is widely observed, social network sites (SNS) constitute a new environment of interaction where users encounter various challenges that they usually do not encounter in other environments. This study aims to provide an in-depth understanding of how users deal with the challenges in this unique environment, paying particular attention to the ways in which they examine and reflect on their social ties and networks. On the basis of 36 semistructured interviews with Facebook users, the article presents the hypothesis that participants of SNS develop a tendency to become highly observant and inquisitive about their networks and are frequently involved in an activity that the authors call analytic labor. © 2013 International Communication Association
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