Are you Yuichi Soejima?

Claim your profile

Publications (7)9.76 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We systematically surveyed period variations of superhumps in SU UMa-type dwarf novae based on newly obtained data and past publications. In many systems, the evolution of the superhump period is found to be composed of three distinct stages: an early evolutionary stage with a longer superhump period, a middle stage with systematically varying periods, and a final stage with a shorter, stable superhump period. During the middle stage, many systems with superhump periods of less than 0.08 d show positive period derivatives. We present observational characteristics of these stages and give greatly improved statistics. Contrary to an earlier claim, we found no clear evidence for a variation of period derivatives among different superoutbursts of the same object. We present an interpretation that the lengthening of the superhump period is a result of the outward propagation of an eccentricity wave, which is limited by the radius near the tidal truncation. We interpret that late-stage superhumps are rejuvenated excitation of a 3:1 resonance when superhumps in the outer disk are effectively quenched. The general behavior of the period variation, particularly in systems with short orbital periods, appears to follow a scenario proposed in Kato, Maehara, and Monard (2008, PASJ, 60, L23). We also present an observational summary of WZ Sge-type dwarf novae. Many of them have shown long-enduring superhumps during a post-superoutburst stage having longer periods than those during the main superoutburst. The period derivatives in WZ Sge-type dwarf novae are found to be strongly correlated with the fractional superhump excess, or consequently with the mass ratio. WZ Sge-type dwarf novae with a long-lasting rebrightening or with multiple rebrightenings tend to have smaller period derivatives, and are excellent candidates for those systems around or after the period minimum of evolution of cataclysmic variables.
    Publications- Astronomical Society of Japan 12/2009; · 2.44 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We carried out an international spectroscopic observation campaign of the dwarf nova GW Librae (GW Lib) during the 2007 superoutburst. Our observation period covered the rising phase of the superoutburst, maximum, slowly decaying phase (plateau), and long fading tail after the rapid decline from the plateau. The spectral features dramatically changed during the observations. In the rising phase, only absorption lines of H$\alpha$, H$\beta$, and H$\gamma$ were present. Around the maximum, the spectrum showed singly-peaked emission lines of H$\alpha$, He I 5876, He I 6678, He II 4686, and C III/N III as well as absorption lines of Balmer components and He I. These emission lines significantly weakened in the latter part of the plateau phase. In the fading tail, all the Balmer lines and He I 6678 were in emission, as observed in quiescence. We find that the center of the H$\alpha$ emission component was mostly stable over the whole orbital phase, being consistent with the low inclination of the system. Comparing with the observational results of WZ Sge during the 2001 superoutburst, the same type of stars as GW Lib seen with a high inclination angle, we interpret that the change of the H$\alpha$ profile before the fading tail phase is attributed to a photoionized region formed at the outer edge of the accretion disk, irradiated from the white dwarf and inner disk.
    Publications- Astronomical Society of Japan 05/2009; · 2.44 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We systematically surveyed period variations of superhumps in SU UMa-type dwarf novae based on newly obtained data and past publications. In many systems, the evolution of superhump period are found to be composed of three distinct stages: early evolutionary stage with a longer superhump period, middle stage with systematically varying periods, final stage with a shorter, stable superhump period. During the middle stage, many systems with superhump periods less than 0.08 d show positive period derivatives. Contrary to the earlier claim, we found no clear evidence for variation of period derivatives between superoutburst of the same object. We present an interpretation that the lengthening of the superhump period is a result of outward propagation of the eccentricity wave and is limited by the radius near the tidal truncation. We interpret that late stage superhumps are rejuvenized excitation of 3:1 resonance when the superhumps in the outer disk is effectively quenched. Many of WZ Sge-type dwarf novae showed long-enduring superhumps during the post-superoutburst stage having periods longer than those during the main superoutburst. The period derivatives in WZ Sge-type dwarf novae are found to be strongly correlated with the fractional superhump excess, or consequently, mass ratio. WZ Sge-type dwarf novae with a long-lasting rebrightening or with multiple rebrightenings tend to have smaller period derivatives and are excellent candidate for the systems around or after the period minimum of evolution of cataclysmic variables (abridged). Comment: 239 pages, 225 figures, PASJ accepted
    05/2009;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigated the superhump evolution, analysing optical photometric observations of the 2000 February-March, the 2002 October-November, and the 2006 September superoutbursts of SW UMa. The superhumps evolved in the same way after their appearance during the 2000 and the 2002 superoutbursts, and probably during the 2006 one. This indicates that the superhump evolution may be governed by the invariable binary parameters. We detected a periodicity in light curve after the end of the 2000 superoutburst without phase shift, which seems to be the remains of the superhumps. We found QPOs at the end stage of the 2000 and the 2002 superoutbursts, but failed to find extraordinarily large-amplitude QPOs called `super-QPOs' which previously have been observed in SW UMa. Comment: 17 pages, 19 figures
    Publications- Astronomical Society of Japan 05/2009; · 2.44 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 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 of the object 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 the first maximum, subsequently increased gradually until the second maximum of the amplitude, and finally decreased again. Investigating other SU UMa-type dwarf novae, which show an increase in 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 direct relation to the increase in superhump period, and all SU UMa-type dwarf novae with a superhump regrowth follow the same evolution of the ordinary superhumps as that of ASAS 160048-4846.2.
    Publications- Astronomical Society of Japan 04/2009; 61. · 2.44 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We discovered the first outburst of V455 And (=HS2331+3905) on Sep. 4 2007 at V=14.59, which has been considered a good candidate for the WZ Sge-type dwarf nova. A time-resolved photometry campaign was soon coordinated and covered the whole of the superoutburst from the rising phase to the slow fading tail. At the time of our outburst detection, the object was in the rising phase at a rate of 10 mag/day. We found that there were no orbital humps nor early superhumps during the rapid rising phase of superoutburst. Early superhumps with a period of 0.05626(1) days appeared around the maximum. Ordinary superhumps appeared 9 days after the maximum. The superhump period was 0.057202(1) days which was 1.6 decline from the plateau 19 days after the maximum. After the rapid fading, the object showed a slow fading trend at a rate of 0.03 mag/day. During the early phase of the slow fading tail, superhumps with a period of 0.05728(3) days were observed. The object showed no rebrightenings. We will present the results from our observation campaign on this object.
    01/2009;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We carried out intensive spectroscopic observations of two WZ Sge-type dwarf novae, GW Lib, and V455 And during their superoutbursts in 2007, at 6 observatories. The observations covered the whole of both superoutbursts from the very maximum to the fading tail. We found evidence of the winds having a speed of $\sim$1000 km s$^{-1}$ which blew in GW Lib during the rising phase. The evolution of the hydrogen, helium, and carbon lines suggests flaring of the accretion disk and emergence of the temperature inversion layer on the disk.
    08/2008;