117 research outputs found
The 1996-1997 Fading of V651 Mon, the Binary Central Star of the Planetary Nebula NGC 2346
V651 Mon is the binary central star of the bipolar planetary nebula NGC 2346.
The star showed the second-ever deep fading in 1996-1997, which was presumably
caused by obscuration by a dust cloud in the planetary nebula, as was proposed
to explain the 1981-1985 event. The entire duration of the 1996-1997 event was
\~400 d, remarkably shorter than the 1981-1985 event, suggesting that the
obscuring body was smaller or had a larger tangential velocity. The most
remarkable feature in this event was the presence of a sharply defined
transient clearing (brightening). From the time-scale of the variation, we
propose an upper limit of the projected scale of several times ~10^11 cm of the
structure responsible for the brightening. This observation provides the first
evidence for a sharply defined, small lucent structure within the obscuring
body around the central star of NGC 2346Comment: 4 pages, 3 figures (using a non-standard style file
On the Rebrightenings of Classical Novae during the Early Phase
We report on the spectral evolution of 6 classical novae, V1186 Sco, V2540
Oph, V4745 Sgr, V5113 Sgr, V458 Vul, and V378 Ser, based on the low-resolution
spectra obtained at the Fujii-Bisei Observatory and the Bisei Astronomical
Observatory, Japan. In the light curves, these 6 novae show several
rebrightenings during the early phase lasting ~10 days after the first maximum
in fast novae, and ~100 days in slow novae. The early spectra of all of these
novae had emission lines with a P-Cygni profile at the maximum brightness. The
absorption component of the P-Cygni profiles then disappeared after the
maximum, and reappeared when the novae brightened to the next maximum. We
suggest that the re-appearance of the absorption component at the rebrightening
is attributable to re-expansion of the photosphere after it once shifts
sufficiently inside. From the light curves, we found that the time intervals of
the rebrightenings of these 6 novae show a similar systematic trend, which is
applicable to all types of novae: fast and slow, and Fe II type and hybrid
type. Moreover, we note the difference between the spectra at the
rebrightenings during the early phase and at the rebrightening in V2362 Cyg,
and at the oscillation during the transition phase in V1494 Aql, which means
difference of the physical mechanism of the rebrightening during the early
phase and the later oscillations.Comment: 11 pages, 15 figures, accepted for publication in PAS
1998 Superoutburst of the Large-Amplitude SU UMa-Type Dwarf Nova WX Ceti
We observed the 1998 November superoutburst of WX Cet, a dwarf nova
originally proposed as a WZ Sge-like system. The observation established that
WX Cet is an SU UMa-type dwarf nova with a mean superhump period of 0.05949(1)
d, which is 2.1% longer than the reported orbital period. The lack of early
superhumps at the earliest stage of the superoutburst, the rapid development of
usual superhumps, and the possible rapid decay of late superhumps seem to
support that WX Cet is a fairly normal large-amplitude SU UMa-type dwarf nova,
rather than a WZ Sge-type dwarf nova with a number of peculiarities. However, a
period increase of superhumps at a rate dot(P)/P = (+8.5+/-1.0) x 10^-5 was
observed, which is one of the largest dot(P)/P ever observed in SU UMa-type
dwarf novae. A linear decline of light, with a rate of 0.10 mag/d, was observed
in the post-superoutburst stage. This may be an exemplification of the decay of
the viscosity in the accretion disk after the termination of a superoutburst,
mechanism of which is proposed to explain a variety of post-superoutburst
phenomena in some SU UMa-type dwarf novae.Comment: 8 pages, 8 figures, accepted for publication in Publ. Astron. Soc.
Japa
Photometric Studies of a WZ Sge-Type Dwarf Nova Candidate, ASAS160048-4846.2
We report on our time-resolved CCD photometry during the 2005 June
superoutburst of a WZ Sge-type dwarf nova candidate, ASAS 160048-4846.2. The
ordinary superhumps underwent a complex evolution during the superoutburst. The
superhump amplitude experienced a regrowth, and had two peaks. The superhump
period decreased when the superhump amplitude reached to the first maximum,
successively gradually increased until the second maximum of the amplitude, and
finally decreased again. Investigating other SU UMa-type dwarf novae which show
an increase of the superhump period, we found the same trend of the superhump
evolution in superoutbursts of them. We speculate that the superhump regrowth
in the amplitude has a close relation to the increase of the superhump period,
and all of SU UMa-type dwarf novae with a superhump regrowth follow the same
evolution of the ordinary superhumps as that of ASAS 160048-4846.2.Comment: 7 pages, 4 figure
Statistical properties of superflares on solar-type stars based on 1-min cadence data
We searched for superflares on solar-type stars using Kepler data with 1 min
sampling in order to detect superflares with short duration. We found 187
superflares on 23 solar-type stars whose bolometric energy ranges from the
order of erg to erg. Some superflares show multiple peaks
with the peak separation of the order of - seconds which is
comparable to the periods of quasi-periodic pulsations in solar and stellar
flares. Using these new data combined with the results from the data with 30
min sampling, we found the occurrence frequency () of superflares as a
function of flare energy () shows the power-law distribution () with for erg which is
consistent with the previous results. The average occurrence rate of
superflares with the energy of erg which is equivalent to X100 solar
flares is about once in 500-600 years. The upper limit of energy released by
superflares is basically comparable to a fraction of the magnetic energy stored
near starspots which is estimated from the photometry. We also found that the
duration of superflares () increases with the flare energy () as . This can be explained if we assume the time-scale of
flares is determined by the Alfvn time.Comment: Accepted for for publication in Earth, Planets and Spac
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