V445 Pup (2000) is a unique object identified as a helium nova. Color indexes during the outburst are consistent with those of free-free emission. We present a free-free emission-dominated ...light-curve model of V445 Pup on the basis of the optically thick wind theory. Our light-curve fitting shows that (1) the white dwarf (WD) mass is very massive (image M sub(image)) and (2) half of the accreted matter remains on the WD, both of which suggest that the WD mass is increasing. Therefore, V445 Pup is a strong candidate for a Type Ia supernova progenitor. The estimated distance to V445 Pup is now consistent with recent observational suggestions, image kpc. A helium star companion is consistent with the brightness of image mag just before the outburst, if it is a slightly evolved hot star with the mass of image M sub(image). We then emphasize importance of observations in the near- future quiescent phase after the thick circumstellar dust dissipates away, especially its color and magnitude, to specify the nature of the companion star. We have also calculated helium ignition masses for helium shell flashes against various helium accretion rates and discussed the recurrence period of helium novae.
Phytochromes are believed to be solely responsible for red and far-red light perception, but this has never been definitively tested. To directly address this hypothesis, a phytochrome triple mutant ...(phyAphyBphyC) was generated in rice (Oryza sativa L. cv. Nipponbare) and its responses to red and far-red light were monitored. Since rice only has three phytochrome genes (PHYA, PHYB and PHYC), this mutant is completely lacking any phytochrome. Rice seedlings grown in the dark develop long coleoptiles while undergoing regular circumnutation. The phytochrome triple mutants also show this characteristic skotomorphogenesis, even under continuous red or far-red light. The morphology of the triple mutant seedlings grown under red or far-red light appears completely the same as etiolated seedlings, and they show no expression of the light-induced genes. This is direct evidence demonstrating that phytochromes are the sole photoreceptors for perceiving red and far-red light, at least during rice seedling establishment. Furthermore, the shape of the triple mutant plants was dramatically altered. Most remarkably, triple mutants extend their internodes even during the vegetative growth stage, which is a time during which wild-type rice plants never elongate their internodes. The triple mutants also flowered very early under long day conditions and set very few seeds due to incomplete male sterility. These data indicate that phytochromes play an important role in maximizing photosynthetic abilities during the vegetative growth stage in rice.
Abstract
IW And stars are a recently recognized subgroup of dwarf novae which are characterized by (often repetitive) slowly rising standstills terminated by brightening, but the exact mechanism for ...this variation is not yet identified. We have identified BO Cet, which had been considered as a novalike cataclysmic variable, as a new member of IW And stars based on its behavior in 2019–2020. In addition to this, the object showed dwarf nova-type outbursts in 2020–2021, and superhumps that had periods $7.8\%$ longer than the orbital one developed during at least one long outburst. This object has been confirmed as an SU UMa-type dwarf nova with an exceptionally long orbital period (0.1398 d). BO Cet is thus the first cataclysmic variable showing both SU UMa-type and IW And-type features. We obtained a mass ratio (q) of 0.31–0.34 from the superhumps in the growing phase (stage A superhumps). At this q, the radius of the 3 : 1 resonance, responsible for tidal instability and superhumps, and the tidal truncation radius are very similar. We interpret that in some occasions this object showed IW And-type variation when the disk size was not large enough, but that the radius of the 3 : 1 resonance could be reached as a result of thermal instability. We also discuss that there are SU UMa-type dwarf novae above q = 0.30, which is above the previously considered limit (∼0.25) derived from numerical simulations and that this is possible since the radius of the 3 : 1 resonance is inside the tidal truncation radius. We constrained the mass of the white dwarf larger than 1.0 M⊙, which may be responsible for the IW And-type behavior and the observed strength of the He ii emission. The exact reason, however, why this object is unique in that it shows both SU UMa-type and IW And-type features is still unsolved.
We observed the 2012–2013 superoutburst of the newly identified transient SSS J122221.7–311523, and found that this object showed two successive superoutbursts. Superhumps grew in amplitude during ...the second superoutburst, and showed a characteristic feature in their period change reflecting the growth of superhumps. Assuming that the periods of the superhumps in the growing stage 0.07721(1) d and the post-superoutburst stage 0.07673(3) d represent the dynamical precession rates at the disk radius of the 3:1 resonance and at the disk radius immediately after the superoutburst, respectively, we found that this object has a very small mass ratio of
$q = M_2/M_1 \lt 0.05$
. The possible orbital period from quiescent data suggests
$q = 0.045$
, which is the smallest among mass ratios of hydrogen-rich cataclysmic variables. The long orbital period and low
$q$
make this object a perfect candidate for a period bouncer. We suggest that the peculiar feature of the double superoutburst is a result of a low
$q$
, and may be characteristic of period bouncers.
Abstract
ASASSN-14dx showed an extraordinary outburst whose features are a small outburst amplitude (∼2.3 mag) and long duration (>4 yr). Because we found a long observational gap of 123 d before the ...outburst detection, we propose that the main outburst plateau was missed and that this outburst is just a “fading tail” often seen after the WZ Sge-type super-outbursts. In order to distinguish between WZ Sge and SU UMa-type dwarf novae (DNe), we investigated Gaia Data Release 2 (DR2) statistically. We applied a logistic regression model and succeeded in classifying by using absolute Gaia magnitudes MG and Gaia colors GBP–GRP. Our new classifier also suggests that ASASSN-14dx is the best candidate of a WZ Sge-type DN. We estimated distances from the Earth of known WZ Sge stars by using Gaia DR2 parallaxes. The result indicates that ASASSN-14dx is the third-nearest WZ Sge star (next to WZ Sge and V455 And), and hence the object can show the third-brightest WZ Sge-type super-outburst whose maximum is V = 8–9 mag.
Abstract
We report on time-resolved photometry of the 2015 February–March superoutburst of QZ Virginis. The superoutburst consisted of a separated precursor, main superoutburst, and rebrightening. We ...detected superhumps with a period of 0.061181(42) d between the precursor and main superoutburst. Based on analyses of period changes and amplitudes of superhumps, the observed superhumps were identified as growing superhumps (stage A superhumps). The duration of the stage A superhumps was about 5 d, unusually long for SU UMa-type dwarf novae. Using the obtained stage A superhump period, we estimated the mass ratio of QZ Vir to be 0.108(3). This value suggests that QZ Vir is an SU UMa-type dwarf nova evolving toward the period minimum. Based on the present and previous observations regarding long-lasting stage A superhumps, the time scale for stage A superhumps is likely to be determined by the mass ratio of the system and the temperature of the accretion disk.
We report on the spectral evolution of the enigmatic, very slow nova V5558 Sgr, based on low-resolution spectra obtained at the Fujii-Bisei Observatory and the Bisei Astronomical Observatory, Japan ...during the period of 2007 April 6 to 2008 May 3. V5558 Sgr shows a pre-maximum halt, and then several flare-like rebrightenings, which is similar to another very slow nova, V723 Cas. In our observations, the spectral type of V5558 Sgr evolved from the He/N type toward the Fe II type during a pre-maximum halt, and then toward the He/N type again. This course of spectral transition was observed for the first time in the long history of nova research. In the rebrightening stage after the initial brightness maximum, we could identify many emission lines accompanied by a stronger absorption component of the P Cyg profile at the brightness maxima. We found that the velocity of the P Cyg absorption component, measured from the emission peak, decreased at the brightness maxima. Furthermore, we compared the spectra of V5558 Sgr with V723 Cas, and other novae that exhibited several rebrightenings during the early phase.
Abstract
We report on our photometric observations of the 2016 superoutburst of ASASSN-16eg. This object showed a WZ Sge-type superoutburst with prominent early superhumps with a period of ...0.075478(8) d and a post-superoutburst rebrightening. During the superoutburst plateau, it showed ordinary superhumps with a period of 0.077880(3) d and a period derivative of 10.6(1.1) × 10−5 in stage B. The orbital period (Porb), which is almost identical with the period of the early superhumps, is exceptionally long for a WZ Sge-type dwarf nova. The mass ratio (q = M2/M1) estimated from the period of developing (stage A) superhumps is 0.166(2), which is also too large for a WZ Sge-type dwarf nova. This suggests that the 2 : 1 resonance can be reached in such high-q systems, contrary to our expectation. Such conditions are considered to be achieved if the mass-transfer rate is much lower than those in typical SU UMa-type dwarf novae that have comparable orbital periods to ASASSN-16eg, and a resultant accumulation of a large amount of matter on the disk is realized at the onset of an outburst. We examined other candidates for long-period WZ Sge-type dwarf novae for their supercycles, which are considered to reflect the mass-transfer rate, and found that V1251 Cyg and RZ Leo have longer supercycles than those of other WZ Sge-type dwarf novae. This result indicates that these long-period objects including ASASSN-16eg have a low mass-transfer rate in comparison to other WZ Sge-type dwarf novae.
In 2015 March, the notable WZ Sge-type dwarf nova AL Com exhibited an unusual outburst with a recurrence time of ∼ 1.5 yr, which is the shortest interval of superoutbursts among WZ Sge-type dwarf ...novae. Early superhumps in the superoutburst light curve were absent, and a precursor was observed at the onset of the superoutburst for the first time in WZ Sge-type dwarf novae. The present superoutburst can be interpreted as a result of the condition that the disk radius barely reached the 3:1 resonance radius, but did not reach the 2:1 resonance one. Ordinary superhumps immediately grew following the precursor. The initial part of the outburst is indistinguishable from those of superoutbursts of ordinary SU UMa-type dwarf novae. This observation supports the interpretation that the 2:1 resonance suppresses a growth of ordinary superhumps. The estimated superhump period and superhump period derivative are P
sh = 0.0573185(11) d and P
dot = +1.5(3.1) × 10−5, respectively. These values indicate that the evolution of ordinary superhumps is the same as that in past superoutbursts with much larger extent. Although the light curve during the plateau stage was typical for an SU UMa-type dwarf nova, this superoutburst showed a rebrightening, together with a regrowth of the superhumps. The overall light curve of the rebrightening was the almost the same as those observed in previous rebrightenings. This implies that the rebrightening type is inherent in the system.
We observed the first-ever recorded outburst of PM J03338+3320, the cataclysmic variable selected by proper-motion survey. The outburst was composed of a precursor and the main superoutburst. The ...precursor outburst occurred at least 5 d before the maximum of the main superoutburst. Despite this separation, long-period superhumps were continuously seen between the precursor and main superoutburst. The period of these superhumps is longer than its orbital period by 6.0(1)% and can be interpreted to reflect the dynamical precession rate at the 3 : 1 resonance for a mass ratio of 0.172(4). These superhumps smoothly evolved into those in the main superoutburst. These observations provide the clearest evidence that the 3 : 1 resonance is triggered by the precursor outburst, even if it is well separated, and the resonance eventually causes the main superoutburst as predicted by the thermaltidal instability model. The presence of superhumps well before the superoutburst cannot be explained by alternative models (the enhanced mass-transfer model and the pure thermal instability one) and the present observations clearly support the thermaltidal instability model.
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