2,568 research outputs found
Evidence of Rocky Planetesimals Orbiting Two Hyades Stars
The Hyades is the nearest open cluster, relatively young and containing
numerous A-type stars; its known age, distance, and metallicity make it an
ideal site to study planetary systems around 2-3 Msun stars at an epoch similar
to the late heavy bombardment. Hubble Space Telescope far-ultraviolet
spectroscopy strongly suggests ongoing, external metal pollution in two remnant
Hyads. For ongoing accretion in both stars, the polluting material has
log[n(Si)/n(C)] > 0.2, is more carbon deficient than chondritic meteorites, and
is thus rocky. These data are consistent with a picture where rocky
planetesimals and small planets have formed in the Hyades around two
main-sequence A-type stars, whose white dwarf descendants bear the scars. These
detections via metal pollution are shown to be equivalent to infrared excesses
of Lir/L* ~ 1e-6 in the terrestrial zone of the stars.Comment: 7 pages, 3 figures, 2 tables, accepted to MNRA
XMM-Newton observations of EF Eridani: the textbook example of low-accretion rate polars
Archival X-ray observations of EF Eridani obtained in a low state revealed
distinct X-ray detections at a luminosity L_X ~ 2 10^{29} erg/s, three orders
of magnitude below its high state value. The plasma temperature was found to be
as low as kT \loa 2 keV, a factor 10 below the high state. The X-ray/UV/IR
spectral energy distribution suggests faint residual accretion rather than
coronal emission as being responsible for the low-state X-ray emission. EF Eri
thus showed a clear transition from being shock-dominated in the high state to
be cyclotron-dominated in the low state. From the optical/UV spectral energy
distribution we re-determine the photospheric temperature of the white dwarf to
\~10000K. Contrary to earlier claims, WD model atmospheres produce sufficient
UV flux to reproduce the published GALEX flux and orbital modulation.Comment: A&A, in pres
SDSS White Dwarf mass distribution at low effective temperatures
The DA white dwarfs in the Sloan Digital Sky Survey, as analyzed in the
papers for Data Releases 1 and 4, show an increase in surface gravity towards
lower effective temperatures below 11500 K. We study the various possible
explanations of this effect, from a real increase of the masses to
uncertainties or deficiencies of the atmospheric models. No definite answer is
found but the tentative conclusion is that it is most likely the current
description of convection in the framework of the mixing-length approximation,
which leads to this effect.Comment: to appear in the proceedings of the 16th European Workshop on White
Dwarfs, Barcelona, 200
Looping : an old idea revisited
The purpose of this paper was to examine the existing literature and research available on the topic of looping, and to synthesize the information into a comprehensive report. This paper includes the historical background of looping, an explanation of the basics of looping, the examination of the teacher-student relationship, and the advantages and disadvantages of looping
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