4,898 research outputs found
Evidence for a merger of binary white dwarfs: the case of GD 362
GD 362 is a massive white dwarf with a spectrum suggesting a H-rich
atmosphere which also shows very high abundances of Ca, Mg, Fe and other
metals. However, for pure H-atmospheres the diffusion timescales are so short
that very extreme assumptions have to be made to account for the observed
abundances of metals. The most favored hypothesis is that the metals are
accreted from either a dusty disk or from an asteroid belt. Here we propose
that the envelope of GD 362 is dominated by He, which at these effective
temperatures is almost completely invisible in the spectrum. This assumption
strongly alleviates the problem, since the diffusion timescales are much larger
for He-dominated atmospheres. We also propose that the He-dominated atmosphere
of GD 362 is likely to be the result of the merger of a binary white dwarf.Comment: 4 pages, 3 figures. Accepted for publication in Astrophysical Journal
Letter
A Characterization of the Brightness Oscillations During Thermonuclear Bursts From 4U 1636-536
The discovery of nearly coherent brightness oscillations during thermonuclear
X-ray bursts from six neutron-star low-mass X-ray binaries has opened up a new
way to study the propagation of thermonuclear burning, and may ultimately lead
to greater understanding of thermonuclear propagation in other astrophysical
contexts, such as in Type Ia supernovae. Here we report detailed analyses of
the ~580 Hz brightness oscillations during bursts from 4U 1636-536. We
investigate the bursts as a whole and, in more detail, the initial portions of
the bursts. We analyze the ~580 Hz oscillations in the initial 0.75 seconds of
the five bursts that were used in a previous search for a brightness
oscillation at the expected ~290 Hz spin frequency, and find that if the same
frequency model describes all five bursts there is insufficient data to require
more than a constant frequency or, possibly, a frequency plus a frequency
derivative. Therefore, although it is appropriate to use an arbitrarily
complicated model of the ~580 Hz oscillations to generate a candidate waveform
for the ~290 Hz oscillations, models with more than two parameters are not
required by the data. For the bursts as a whole we show that the
characteristics of the brightness oscillations vary greatly from burst to
burst. We find, however, that in at least one of the bursts, and possibly in
three of the four that have strong brightness oscillations throughout the
burst, the oscillation frequency reaches a maximum several seconds into the
burst and then decreases. This behavior has not been reported previously for
burst brightness oscillations, and it poses a challenge to the standard burning
layer expansion explanation for the frequency changes.Comment: 18 pages including three figures, uses aaspp4.sty, submitted to The
Astrophysical Journal on April
The Formation of Cataclysmic Variables with Brown Dwarf Secondaries
The present-day formation of cataclysmic variables (CVs) with brown dwarf
(BD) secondaries (0.013 M_sun < M_sec < 0.075 M_sun) is investigated using a
population synthesis technique. Results from the latest, detailed models for
BDs have been incorporated into the population synthesis code. For our models,
we find that ZACVs with BD secondaries have orbital periods in the range 46 min
to 2.5 hrs. We also find that ZACVs with BD secondaries comprise 18% of the
total, present-day ZACV population. In addition, we find that 80% of ZACVs with
BD secondaries have orbital periods < 78 minutes. This implies that 15% of the
present-day ZACV population should have orbital periods shorter than the
observed orbital period minimum for CVs. We also investigate the dependence of
the present-day formation rate of CVs with BD secondaries on the assumed value
of the common envelope efficiency parameter, alpha_CE, for three different
assumed mass ratio distributions in ZAMS binaries. Surprisingly, we find that
the common envelope process must be extremely inefficient (alpha_CE < 0.1) in
order for CVs with BD secondaries not to be formed. Finally, we find that the
progenitor binaries of ZACVs with BD secondaries have ZAMS orbital separations
< 3 AU and ZAMS primary masses between ~1-10 M_sun, with ~75% of the primary
masses less than ~1.6 M_sun. Interestingly, these ranges in orbital separation
and primary mass place the majority of the progenitor binaries within the
so-called ``brown dwarf desert.''Comment: preprint 27 pages 4 figures; to appear in ApJ April 1, 200
Prevalence and distribution of endodontic treatments and apical periodontitis in an Italian population sample
This study was undertaken to investigate for the first time the distribution and prevalence of apical periodontitis and its determinants in an Italian population. A total of 312 participants (191 women, 121 men) received free clinical examination and full-mouth digital X-ray. Data regarding the oral health status of the patients were collected such as the reason for the visit and the incidence of lesions. We examined 8101 teeth, 6.59% of which had been submitted to endodontic treatment and 15.02% of which were lower molars. Lower molars were reported missing more frequently, in detail 16.24%. Moreover 3.89% of the lower molars had developed an apical lesion, whereas only 0.08% of the lower incisors. Periapical lesions were present in 0.29% of not endodontically treated teeth, while in endodontically treated teeth lesions were present in 17.04%. Additionally, 15.65% of the endodontically treated teeth showed incongruous (long or short) seals. A prevalence of women (61.21%) over men (38.78%) was reported indicating that women attend to their oral health more. Endodontic treatment success and failure in over- or under-filled teeth was evaluated with success being verified in 97% of appropriately treated teeth; the success rate decreases in inappropriately treated teeth (by 72.73% in over-filled and 77.71% under-filled teeth). The prevalence of teeth with periapical lesions amounted to 1.35%. Finally, endodontically treated teeth with periapical lesions amounted to 17.04%. Successful treatment can be achieved when disease is intercepted at an early stage while prevention still remains better than cur
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