arXiv subjects

Izumi Hachisu

Publications and source records attributed to Izumi Hachisu.

At least 91 records · Page 5Linked to original sources

arXiv2006

Photometric Observation and Numerical Simulation of Early Superhumps in BC UMa during the 2003 Superoutburst

We carried out time-resolved photometric observations of an SU UMa type dwarf nova, BC UMa, during its superoutburst in February 2003. We detect early superhumps (or outburst orbital humps) during the first two days of the outburst. The period of early superhumps is 0.06258(13) d and the amplitude is 0.04 mag. After the early superhump phase, common superhumps with an amplitude of 0.3 mag and a period of 0.064466(16) d developed. The change rate of superhump periods was positive through the superoutburst. The superhump period excess is 3% and we derive a mass ratio of 0.13. This is twice as large as that of WZ Sge, suggesting that the mechanism of early superhumps in BC UMa is not the 2:1 resonance which was proposed in WZ Sge. We have modelled early superhump light curves including irradiation effects of the accretion disk and secondary star by the white dwarf and accretion disk. The observed early superhumps can be reproduced when two-armed spirals appear on the accretion disk. We have found the long term data taken from AAVSO, VSOLJ and VSNET shows BC UMa has normal outbursts with a maximum magnitude of V~13 and two types of superoutbursts: one has a short duration (around 10 days) and faint maximum magnitude (V~12.5), the other has a long duration (around 20 days) and bright maximum (V~11-11.5). BC UMa is the first example of dwarf novae showing the two types of superoutbursts. The supercycle of BC UMa is between 600 days and 1000 days. This is shorter than WZ Sge type dwarf novae and longer than normal SU UMa type stars. This phenomenon suggests that BC UMa is an intermediate dwarf nova between WZ Sge and SU UMa.

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arXiv2006

A Universal Decline Law of Classical Novae

We calculate many different nova light curves for a variety of white dwarf masses and chemical compositions, with the assumption that free-free emission from optically thin ejecta dominates the continuum flux. We show that all these light curves are homologous and a universal law can be derived by introducing a ``time scaling factor.'' The template light curve for the universal law has a slope of the flux, F \propto t^{-1.75}, in the middle part (from ~2 to ~6 mag below the optical maximum), but it declines more steeply, F \propto t^{-3.5}, in the later part (from ~6 to ~10 mag). This break on the light curve is due to a quick decrease in the wind mass-loss rate. The nova evolutions are approximately scaled by the time of break. Once the time of break is observationally determined, we can derive the various timescales of novae such as the period of a UV burst phase, the duration of optically thick wind phase, and the turnoff date of hydrogen shell-burning. We have applied our template light curve model to the three well-observed novae, V1500 Cyg, V1668 Cyg, and V1974 Cyg. Our theoretical light curves show excellent agreement with the optical y and infrared J, H, K light curves. The WD mass is estimated, from the light curve fitting, to be 1.15 M_\sun for V1500 Cyg, 0.95 ~M_\sun for V1668 Cyg, and 0.95-1.05 M_\sun for V1974 Cyg.

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arXiv2006

The Hydrogen Burning Turn-off of RS Ophiuchi 2006

We report a coordinated multi-band photometry of the RS Oph 2006 outburst and highlight the emission line free y-band photometry that shows a mid-plateau phase at y ~ 10.2 mag from day 40 to day 75 after the discovery followed by a sharp drop of the final decline. Such mid-plateau phases are observed in other two recurrent novae, U Sco and CI Aql, and are interpreted as a bright disk irradiated by the white dwarf. We have calculated theoretical light curves based on the optically thick wind theory and have reproduced the observed light curves including the mid-plateau phase and the final sharp decline. This final decline is identified with the end of steady hydrogen shell-burning, which turned out the day ~80. This turnoff date is consistent with the end of a supersoft X-ray phase observed with Swift. Our model suggests a white dwarf mass of 1.35 \pm 0.01 M_\sun, which indicates that RS Oph is a progenitor of Type Ia supernovae. We strongly recommend the y-filter observation of novae to detect both the presence of a disk and the hydrogen burning turn-off.

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arXiv2006

Prediction of Supersoft X-ray Phase during the 2006 Outburst of RS Ophiuchi

RS Ophiuchi is one of the well-observed recurrent novae and also a candidate progenitor of Type Ia supernovae. Its sixth recorded outburst was discovered on 12 February 2006. Detection of a supersoft X-ray phase will provide a firm confirmation of hydrogen shell-burning on the white dwarf and its turn-on/turn-off dates strongly constrain a mass range of the white dwarf, which clarify whether or not RS Oph becomes a Type Ia supernova. For a timely detection of its supersoft X-ray phase, we have calculated outburst evolution of RS Oph based on the optically thick wind theory and predicted a supersoft X-ray phase for the 2006 outburst: it will most probably start on day 80-90 and continue until day 110-120 after the optical maximum. Its duration is so short as only a month or so. We strongly recommend multiple observations during April, May, and June of 2006 to detect turn-on and turn-off times of the supersoft X-ray phase.

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arXiv2006

Progenitors of Type Ia Supernovae: Circumstellar Interaction, Rotation, and Steady Hydrogen Burning

Among the important issues in identifying the progenitor system of Type Ia supernovae (SNe Ia), we focus mostly on circumstellar interaction in SN 2002ic, and give brief discussion on the controversial issues of the effects of rotation in merging double degenerates and steady hydrogen shell burning in accreting white dwarfs. SN 2002ic is a unique supernova which shows the typical spectral features of SNe Ia near maximum light, but also apparent hydrogen features that have been absent in SNe Ia. Based on the hydrodynamical models of circumstellar interaction in SN Ia, we suggest that circumstellar medium is aspherical (or highly clumpy) and contains ~1.3 Msun. Possible progenitor systems of SN 2002ic are discussed.

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arXiv2005

A Modeling of the Super-Eddington Luminosity in Nova Outbursts: V1974 Cygni

We have modeled nova light curves exceeding the Eddington luminosity. It has been suggested that a porous structure develops in nova envelopes during the super Eddington phase and the effective opacity is much reduced for such a porous atmosphere. Based on this reduced opacity model, we have calculated envelope structures and light curves of novae. The optically thick wind model is used to simulate nova winds. We find that the photospheric luminosity and the wind mass-loss rate increase inversely proportional to the reducing factor of opacities, but the wind velocity hardly changes. We also reproduce the optical light curve of V1974 Cygni (Nova Cygni 1992) in the super-Eddington phase, which lasts 13 days from the optical peak 1.7 mag above the Eddington luminosity.

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arXiv2005

Toward a unified light curve model for multi-wavelength observations of V1974 Cygni (Nova Cygni 1992)

We present a unified model for optical, ultraviolet (UV), and X-ray light curves of V1974 Cygni (Nova Cygni 1992). Based on an optically thick wind model of nova outbursts, we have calculated light curves and searched for the best fit model that is consistent with optical, UV, and X-ray observations. Our best fit model is a white dwarf (WD) of mass 1.05 M_\sun with a chemical composition of X=0.46, C+N+O=0.15, and Ne = 0.05 by mass weight. Both supersoft X-ray and continuum UV 1455 Ålight curves are well reproduced. Supersoft X-rays emerged on day ~ 250 after outburst, which is naturally explained by our model: our optically thick winds cease on day 245 and supersoft X-rays emerge from self-absorption by the winds. The X-ray flux keeps a constant peak value for ~ 300 days followed by a quick decay on day ~ 600. The duration of X-ray flat peak is well reproduced by a steady hydrogen shell burning on the WD. Optical light curve is also explained by the same model if we introduce free-free emission from optically thin ejecta. A t^{-1.5} slope of the observed optical and infrared fluxes is very close to the slope of our modeled free-free light curve during the optically thick wind phase. Once the wind stops, optical and infrared fluxes should follow a t^{-3} slope, derived from a constant mass of expanding ejecta. An abrupt transition from a t^{-1.5} slope to a t^{-3} slope at day ~ 200 is naturally explained by the change from the wind phase to the post-wind phase on day ~ 200. The development of hard X-ray flux is also reasonably understood as shock-origin between the wind and the companion star. The distance to V1974 Cyg is estimated to be ~ 1.7 kpc with E(B-V)= 0.32 from the light curve fitting for the continuum UV 1455 Å.

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arXiv2004

The nature of premaximum halts of classical nova outbursts: V723 Cassiopeiae and V463 Scuti

We present a new interpretation of long premaximum halts of nova outbursts. For V723 Cas (Nova Cas 1995) and V463 Sct (Nova Sct 2000), we have reproduced light curves, excluding the brightness maxima, starting from the long premaximum halt through the late decay phase of the outbursts using a steady-state optically thick wind model. When the hydrogen-rich envelope of the white dwarf (WD) is massive enough, the star expands to \sim 100 R_\sun or over and its surface temperature decreases to below 7000 K. At this supergiant mimicry stage, the changes in both the photospheric radius and the temperature are small against the large increase in the envelope mass. These changes cause a saturation in visual magnitude that lasts a long time before it begins to decline. This saturation is known as the premaximum halt of a classical nova outburst. The visual magnitude during the saturation is close to the bolometric magnitude, which is an upper limit for a given WD mass. Since the WD masses are estimated to be 0.59 M_\sun for V723 Cas and 1.1 M_\sun for V463 Sct by fitting the decline rate of nova light curves, we can determine the absolute magnitude of premaximum halts. It is a refined Eddington luminosity. Thus, the premaximum halt of a nova works as a standard candle.

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arXiv2004

Mass accumulation efficiency in helium shell flashes for various white dwarf masses

We have calculated the mass accumulation efficiency during helium shell flashes on white dwarfs (WDs) of mass 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, and $1.35 M_{\odot}$ for the helium accretion rates $\log\dot M_{\rm He} \ (M_{\odot}$ yr$^{-1}) = -7.4$ to -5.8. This efficiency is a crucial factor for binary evolutions of Type Ia supernovae. For less massive WDs ($< 0.8 \ M_{\odot}$) no wind mass loss occurs and all the accreted mass accumulates on the WD if the Roche lobe size is large enough. The efficiency takes the minimum values in between 1.1 and 1.2 $M_{\odot}$ WD for a given mass accretion rate and increases in both less and more massive WDs. The mass accumulation efficiency is larger than 0.5 for $\log\dot M_{\rm He} \geq -6.72$ in all the WD masses.

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arXiv2004

Circumstellar Interaction of Type Ia Supernova SN 2002ic

SN 2002ic is a unique supernova which shows the typical spectral features of Type Ia supernovae (SNe Ia) near maximum light, but also apparent hydrogen features that have been absent in SNe Ia. We have calculated hydrodynamical models for the interaction between the SN Ia ejecta and the H-rich circumstellar medium (CSM) to reproduce the observed features of SN 2002ic. Based on our modeling, we suggest that CSM is aspherical (or highly clumpy) and contains about 4-5 solar masses. Possible progenitor systems of SN 2002ic are discussed.

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arXiv2004

Detection of two-armed spiral shocks on the accretion disk of the eclipsing fast nova V1494 Aquilae

We have modeled the unusual orbital light curve of V1494 Aquilae (Nova Aquilae 1999 No.2) and found that such an unusual orbital light curve can be reproduced when there exist two-armed, spiral shocks on the accretion disk. V1494 Aql is a fast classical nova and found to be an eclipsing system with the orbital period of 0.1346138 days in the late phase of the nova outburst. Its orbital light curve shows a small bump at orbital phase 0.2, a small dip at 0.3, sometimes a small bump at 0.4, and a large bump at 0.6-0.7 outside eclipse. Such a double- or triple-wave pattern outside eclipse has never been observed even though overall patterns look like some supersoft X-ray sources or eclipsing polars. We have calculated orbital light curves including the irradiation effects of the accretion disk and the companion by the hot white dwarf. These unusual patterns can be reproduced when we assume two-armed spiral shocks on the accretion disk. Especially, triple-wave patterns are naturally obtained. This result strongly suggests the existence of two-armed spiral shocks on the accretion disk in the late phase of the nova outburst.

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arXiv2004

Numerical Simulation of the Surface Flow on the Companion Star in a Close Binary System II. Construction of Doppler maps and application to Galactic supersoft X-ray sources

We perform three-dimensional numerical simulations of surface flows on the companion star in a semi-detached binary system and construct the corresponding Doppler maps. The mass ratio of the binary system, $q=M_2/M_1$, considered here is $q=0.05, 0.33, 0.5, 1, 2$, and 3. For all cases, we obtain the H-, L1-, and L2-eddies, as found in our previous work, and confirm that the flow pattern does not heavily depend on the mass ratio. We propose that this kind of problem be dubbed ``{\it stellar meteorology}.'' The Doppler maps at the position of the companion show a structure tilted towards clockwise direction and presenting deviations from the critical Roche surface due to the L1-eddy and the L2-eddy on the companion star. We apply our results to the Galactic supersoft X-ray source RX J0019.8+2156 and try to attribute the low radial velocity component of the emission lines of He II $λ4686$ observed recently to the irradiated spot on the surface of the companion rather than that of the white dwarf or the accretion disc. Based on the comparison between the observations and our constructed Doppler map, we estimate the mass of the companion star in RX J0019.8+2156 to be $\sim 2 M_{\odot}$ assuming the mass of the white dwarf star to be around $0.6 M_{\odot}$.

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arXiv2003

V445 Puppis -- Helium nova on a massive white dwarf

The 2000 outburst of V445 Puppis shows unique properties, such as absence of hydrogen, enrichment of helium and carbon, slow development of the light curve with a small amplitude that does not resemble any classical novae. This object has been suggested to be the first example of helium novae. We calculate theoretical light curves of helium novae and reproduce the observational light curve of V445 Pup. Modeling indicates a very massive white dwarf (WD), more massive than 1.3 Msun. The companion star is possibly either a helium star or a helium-rich main-sequence star. We estimate the ignition mass as several times 10^{-5} Msun, the corresponding helium accretion rate as several times 10^{-7} Msun yr^{-1}, and the recurrence period as several tens of years. These values suggest that the WD is growing in mass and ends up either a Type Ia supernova or an accretion induced collapse to a neutron star.

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arXiv2003

Evolution of Rotating Accreting White Dwarfs and the Diversity of Type Ia Supernovae

Type Ia supernovae (SNe Ia) have relatively uniform light curves and spectral evolution, which make SNe Ia useful standard candles to determine cosmological parameters. However, the peak brightness is not completely uniform, and the origin of the diversity has not been clear. We examine whether the rotation of progenitor white dwarfs (WDs) can be the important source of the diversity of the brightness of SNe Ia. We calculate the structure of rotating WDs with an axisymmetric hydrostatic code. The diversity of the mass induced by the rotation is ~0.08 Msun and is not enough to explain the diversity of luminosity. However, we found the following relation between the initial mass of the WDs and their final state; i.e., a WD of smaller initial mass will rotate more rapidly before the supernova explosion than that of larger initial mass. This result might explain the dependence of SNe Ia on their host galaxies.

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arXiv2003

A limit cycle model for long-term optical variations of V Sagittae: The second example of accretion wind evolution

V Sagittae shows quasi-periodic optical high (soft X-ray off) and low (soft X-ray on) states with the total period of ~300 days. A binary model is presented to explain orbital light curves both for the high and low states as well as the transition mechanism between them. The binary model consists of a white dwarf (WD), a disk around the WD, and a lobe-filling main-sequence (MS) companion. In the optical high state, the mass transfer rate to the WD exceeds the critical rate of ~1 x 10^{-6} Msun/yr, and the WD blows an optically thick, massive wind. Surface layers of the disk are blown in the wind and the disk surface extends to the companion or over. As a result, optical luminosity of the disk increases by a magnitude because of its large irradiation effect. The massive wind completely obscures soft X-rays. This corresponds to the optical high/soft X-ray off state. The transition between optical high and low states is driven by an attenuation of the mass transfer from the secondary. As the mass supply stops, the WD wind weakens and eventually stops. The disk shrinks to a Roche lobe size and the optical magnitude drops. This phase corresponds to the optical low/soft X-ray on state. This cycle is repeated like a limit cycle. The WD can grow in mass at the critical rate and eventually reach the Chandrasekhar mass limit. This process is called ``accretion wind evolution,'' which is a key evolutionary process in a recently developed evolutionary scenario of Type Ia supernovae. This evolutionary process was first confirmed in the LMC supersoft X-ray source RX J0513.9$-$6951. Thus, V Sge is the second example of accretion wind evolution.

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arXiv2003

RX J0513.9-6951: The first example of accretion wind evolution, a key evolutionary process to Type Ia supernovae

A new self-sustained model for long-term light curve variations of RX J0513.9-6951 is proposed based on an optically thick wind model of mass-accreting white dwarfs (WDs). When the mass accretion rate to a WD exceeds the critical rate of \sim 1 x 10^{-6} M_\sun yr^{-1}, optically thick strong winds begin to blow from the WD so that a formation of common envelope is avoided. The WD can accrete and burn hydrogen-rich matter atop the WD at the critical rate. The excess matter transferred to the WD above the critical rate is expelled by winds. This is called the accretion wind evolution. This ejection process, however, occurs intermittently because the mass transfer is attenuated by strong winds: the strong winds collide with the secondary surface and strip off the very surface layer of the secondary. The matter stripped-off is lost from the binary system. Properly formulating this mass stripping effect and the ensuing decay of mass transfer rate, we are able to reproduce, in a self-sustained manner, the transition between the optical high/X-ray off and optical low/X-ray on states of RX J0513.9-6951. Thus RX J0513.9-6951 is the first example of the accretion wind evolution, which is a key evolutionary process in a recently developed evolutionary path to Type Ia supernovae.

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arXiv2003

A new clue to the transition mechanism between optical high and low states of the supersoft X-ray source RX J0513.9-6951, implied from the recurrent nova CI Aquilae 2000 outburst model

We have found a new clue to the transition mechanism between optical high/X-ray off and optical low/X-ray on states of the LMC supersoft X-ray source RX J0513.9-6951. A sharp ~1 mag drop is common to the CI Aql 2000 outburst. These drops are naturally attributed to cessation of optically thick winds on white dwarfs. A detailed light-curve analysis of CI Aql indicates that the size of a disk drastically shrinks when the wind stops. This causes ~1-2 mag drop in the optical light curve. In RX J0513.9-6951, the same mechanism reproduces sharp ~1 mag drop from optical high to low states. We predict this mechanism also works on the transition from low to high states. Interaction between the wind and the companion star attenuates the mass transfer and drives full cycles of low and high states.

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arXiv2003

The Recurrent Nova U Scorpii in the 1999 Outburst: the First Detection of a Significant Orbital-Period Change

In this paper we present and discuss our time-resolved photometry of an eclipsing recurrent nova, U Sco, during an outburst in 1999, which was conducted from immediately after the optical maximum to the final fading toward the quiescence. In the first-ever complete light-curve, a few primary and secondary eclipses of the binary system were detected, and the timings of the minima were determined. We found that the eclipses showed no totality during the outburst. The depth of the primary eclipses was 0.4-0.8 mag, much shallower than that in quiescence. In the plateau phase, very little irradiation (< 0.1 mag) was observed in the orbital light curve, which implies the existence and a large flaring rim of the accretion disk during the outburst. The minima of the eclipses were detected at earlier orbital phases for the predicted ephemerides. Thus, we obtained an orbital period change of the binary system as \dot{P}/P = -1.7 (+/- 0.7) x 10^{-6} yr^{-1} from the O-C. Assuming that this period change is a result of the conservative mass transfer between the component stars, its mass-transfer rate reaches \dot{M} = 2.4 (+/- 1.0) x 10^{-6} M_solar yr^{-1} for a 1.37 M_solar white dwarf and a 2.0 M_solar mass-donor companion, which is too high to cause shell flashes, even on a massive white dwarf. Therefore, this large rate of the period change strongly indicates a non-conservative mass transfer in the binary system.

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