arXiv subjects

Mariko Kato

Publications and source records attributed to Mariko Kato.

At least 73 records · Page 4Linked to original sources

arXiv2011

Quiet Novae with Flat Maximum -- No Optically Thick Winds

I will explain why most novae show a sharp optical peak in the light curve, whereas a small number of novae, such as PU Vul, shows a long-lasted flat optical maximum. Re-examination of occurrence condition of optically thick winds clarifies that hydrostatic evolutions, in which optically thick winds are suppressed, are realized during nova outbursts on low mass white dwarfs (WDs) (less than about 0.7 Mo). In such a case, a nova outburst evolves very slowly, because of no strong mass-ejection, and stays at low temperature stage a long time, which results in a long-lasted flat optical peak. This explains outburst nature of symbiotic nova PU Vul, that shows flat optical maximum lasted as long as eight years with no spectral indication of wind mass-loss. On the other hand, in ordinary nova outbursts, strong optically thick winds inevitably occur that carry out most of the envelope matter very quickly. Thus the light curve decays immediately after the optical peak, which results in a sharp peak of optical light curve.

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arXiv2011

A Light-Curve Model of the Symbiotic Nova PU Vul (1979) -- A Very Quiet Explosion with Long-Lasted Flat Peak

We present a light curve model of the symbiotic nova PU Vul (Nova Vulpeculae 1979) that shows a long-lasted flat peak with no spectral indication of wind mass-loss before decline. Our quasi-evolution models consisting of a series of static solutions explain both the optical flat peak and ultraviolet (UV) light curve simultaneously. The white dwarf mass is estimated to be ~0.6 Mo. We also provide a new determination of the reddening, E(B-V) = 0.43 +/- 0.05, from UV spectral analysis. Theoretical light curve fitting of UV 1455 A provides the distance of d=3.8 +/- 0.7 kpc.

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arXiv2010

Supersoft X-ray Phase of Single Degenerate Type Ia Supernova Progenitors in Early Type Galaxies

In the single degenerate (SD) scenario for Type Ia supernova (SN Ia) progenitors, an accreting white dwarf (WD) is expected to undergo a supersoft X-ray source (SSS) phase. Recently, Gilfanov & Bogdan (2010, hereafter GB10) claimed that observed X-ray fluxes of early type galaxies would be too low to be consistent with the prediction of the SD scenario based on rather simple assumptions. We present realistic evolutionary models of SD systems and calculate durations of SSS phases. In most cases, accreting WDs spend a large fraction of time in the optically thick wind phase and the recurrent nova phase rather than the SSS phase. Thus the SSS phase lasts only for a few hundred thousand years. This is by a factor of ~10 shorter than those adopted by GB10 where the SN~Ia progenitor WD was assumed to spend most of its life as a SSS. The theoretical X-ray luminosity of the SSS has a large uncertainty because of the uncertain atmospheric model of mass-accreting WDs and absorption of soft X-rays by the companion star's cool wind material. We thus adopt an average of the observed fluxes of existing symbiotic SSSs, i.e., ~0.4 x 10^{36} erg s^{-1} for 0.3--0.7 keV. Using these SSS duration and soft X-ray luminosity, we show that the observed X-ray flux obtained by GB10 is rather consistent with our estimated flux in early type galaxies based on the SD scenario. This is a strong support for the SD scenario as a main-contributor of SNe Ia in early type galaxies.

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arXiv2009

A Prediction of Supersoft X-Ray Phase of Classical Nova V5583 Sagittarii

We have observed the fast nova V5583 Sagittarii with five B, V, y, R_C, and I_C bands, and found that these multi-band light curves are almost identical with those of V382 Vel 1999 until at least 100 days after outburst. A supersoft X-ray phase of V382 Vel was detected with BeppoSAX about six months after outburst. V5583 Sgr outbursted a few days ago the discovery on 2009 August 6.5 UT near its optical peak. From a complete resemblance between these two nova light curves, we expect a supersoft X-ray phase of V5583 Sgr six months after outburst. Detection of supersoft X-ray turn-on/turnoff dates strongly constrain the evolution of a nova and, as a result, mass range of the WD. For a timely observation of a supersoft X-ray phase of V5583 Sgr, we have calculated nova outburst evolution based on the optically thick wind theory, which predicts the supersoft X-ray phase: it will most probably start between days 100 and 140 and continue until days 200-240 after outburst. We strongly recommend multiple observations during 2009 December, and 2010 January, February, and March to detect the turn-on and turnoff times of the supersoft X-ray phase of V5583 Sgr.

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arXiv2009

A Prediction Formula of Supersoft X-ray Phase of Classical Novae

On the basis of the recently developed universal decline law of classical novae, we propose prediction formulae for supersoft X-ray on and off times, i.e., t_{X-on} = (10 \pm 1.8) t_3 days and t_{X-off} = (5.3 \pm 1.4) (t_3)^{1.5} days for 8 < t_3 < 80 days. We have determined the absolute magnitude of our free-free emission model light curves and derived maximum magnitude vs. rate of decline (MMRD) relations. Our theoretical MMRD relations are governed by two parameters, one is the white dwarf (WD) mass and the other is the initial envelope mass at a nova outburst; this second parameter explains the scatter of MMRD points of individual novae. Our theoretical MMRD relations are also in good agreement with the well-known empirical formulae. We also show another empirical relation of M_V(15) ~ -5.7 \pm 0.3 based on the absolute magnitude of our model light curves, i.e., the absolute magnitude at 15 days after optical maximum is almost common among various novae. We analyzed ten nova light curves, in which a supersoft X-ray phase was detected, and estimated their WD masses. The models best reproducing simultaneously the optical and supersoft X-ray observations are ONeMg WDs with 1.28 \pm 0.04 M_\sun (V598 Pup), 1.23 \pm 0.05 M_\sun (V382 Vel), 1.15 \pm 0.06 M_\sun (V4743 Sgr), 1.13 \pm 0.06 M_\sun (V1281 Sco), 1.2 \pm 0.05 M_\sun (V597 Pup), 1.06 \pm 0.07 M_\sun (V1494 Aql), 1.04 \pm 0.07 M_\sun (V2467 Cyg), 1.07 \pm 0.07 M_\sun (V5116 Sgr), 1.05 \pm 0.05 M_\sun (V574 Pup), and a CO WD with 0.93 \pm 0.08 M_\sun (V458 Vul). The newly proposed relationships are consistent with the emergence or decay epoch of the supersoft X-ray phase of these ten novae. Finally, we discuss the mechanism of shock-origin hard X-ray component in relation to the emergence of companion star from the WD envelope.

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arXiv2009

Accreting White Dwarfs as Supersoft X-ray Sources

I review various phenomena associated with mass-accreting white dwarfs (WDs) in the view of supersoft X-ray sources. When the mass-accretion rate is low (\dot M_{acc} < a few \times 10^{-7} M_\sun yr^{-1}), hydrogen nuclear burning is unstable and nova outbursts occur. A nova is a transient supersoft X-ray source (SSS) in its later phase which timescale depends strongly on the WD mass. The X-ray turn on/off time is a good indicator of the WD mass. At an intermediate mass-accretion rate an accreting WD becomes a persistent SSS with steady hydrogen burning. For a higher mass-accretion rate, the WD undergoes "accretion wind evolution" in which the WD accretes matter from the equatorial plane and loses mass by optically thick winds from the other directions. Two SSS, namely RX J 0513-69 and V Sge, are corresponding objects to this accretion wind evolution. We can specify mass increasing WDs from light-curve analysis based on the optically thick wind theory using multiwavelength observational data including optical, IR, and supersoft X-rays. Mass estimates of individual objects give important information for the binary evolution scenario of type Ia supernovae.

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arXiv2009

A Universal Decline Law of Classical Novae. IV. V838 Her (1991): A Very Massive White Dwarf

We present a unified model of optical and ultraviolet (UV) light curves for one of the fastest classical novae, V838 Herculis (Nova Herculis 1991), and estimate its white dwarf (WD) mass. Based on an optically thick wind theory of nova outbursts, we model the optical light curves with free-free emission and the UV 1455 Ålight curves with blackbody emission. Our models of 1.35 \pm 0.02 M_\sun WD reproduce simultaneously the optical and UV 1455 Åobservations. The mass lost by the wind is ΔM_{wind} \sim 2 \times 10^{-6} M_\sun. We provide new determinations of the reddening, E(B-V) = 0.53 \pm 0.05, and of the distance, 2.7 \pm 0.5 kpc.

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arXiv2009

Multiplicity of Nova Envelope Solutions and Occurrence of Optically Thick Winds

We revisit the occurrence condition of optically thick winds reported by Kato (1985) and Kato and Hachisu (1989) who examined mathematically nova envelope solutions with an old opacity and found that optically thick winds are accelerated only in massive white dwarfs (WDs) of >~ 0.9 Mo. With the OPAL opacity we find that the optically thick wind occurs for >~ 0.6 Mo WDs and that the occurrence of winds depends not only on the WD mass but also on the ignition mass. When the ignition mass is larger than a critical value, winds are suppressed by a density-inversion layer. Such a static solution can be realized in WDs of mass ~0.6-0.7 Mo. We propose that sequences consisting only of static solutions correspond to slow evolutions in symbiotic novae like PU Vul because PU Vul shows no indication of strong winds in a long-lasted flat peak followed by a very slow decline in its light curve.

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arXiv2009

Optical and Supersoft X-ray Light Curve Models of Classical Nova V2491 Cygni: A New Clue to the Secondary Maximum

V2491 Cygni (Nova Cygni 2008 No.2) was detected as a transient supersoft X-ray source with the Swift XRT as early as 40 days after the outburst, suggesting a very massive white dwarf (WD) close to the Chandrasekhar limit. We present a unified model of near infrared, optical, and X-ray light curves for V2491 Cyg, and have estimated, from our best-fit model, the WD mass to be 1.3 \pm 0.02 M_\sun with an assumed chemical composition of the envelope, X=0.20, Y= 0.48, X_{CNO} =0.20, X_{Ne} =0.10, and Z = 0.02 by mass weight. We strongly recommend detailed composition analysis of the ejecta because some enrichment of the WD matter suggests that the WD mass does not increase like in RS Oph, which is a candidate of Type Ia supernova progenitors. V2491 Cyg shows a peculiar secondary maximum in the optical light curve as well as V1493 Aql and V2362 Cyg. Introducing magnetic activity as an adding energy source to nuclear burning, we propose a physical mechanism of the secondary maxima.

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arXiv2008

Optical light curves of RS Oph (2006) and hydrogen burning turnoff

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 early decline, mid-plateau phase, and final decline. The final decline is identified with the end of steady hydrogen shell-burning, which turned out at about 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 turnoff. Observational data of y magnitudes are provided here together with other multi-wavelength light curve data.

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arXiv2008

Supersoft X-ray Light Curve of RS Oph -- The White Dwarf Mass is Now Increasing

The recurrent nova RS Ophiuchi, one of the candidates for Type Ia supernova progenitors, underwent the sixth recorded outburst in February 2006. We report a complete light curve of supersoft X-ray that is obtained for the first time. A numerical table of X-ray data is provided. The supersoft X-ray flux emerges about 30 days after the optical peak and continues until about 85 days when the optical flux shows the final decline. Such a long duration of supersoft X-ray phase can be naturally understood by our model in which a significant amount of helium layer piles up beneath the hydrogen burning zone during the outburst, suggesting that the white dwarf mass is effectively growing up. We have estimated the white dwarf mass in RS Oph to be 1.35 \pm 0.01 M_\sun and its growth rate to be about (0.5-1) \times 10^{-7} M_\sun yr^{-1} in average.

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arXiv2008

The Delay Time Distribution of Type Ia Supernovae and the Single Degenerate Model

We present a theoretical delay time distribution (DTD) of Type Ia supernovae on the basis of our new evolutionary models of single degenerate (SD) progenitor systems. Our model DTD has almost a featureless power law shape (\propto t^{-n} with n \approx 1) for the delay time from t \sim 0.1 to 10 Gyr. This is in good agreement with the recent direct measurement of DTD. The observed featureless property of the DTD has been suggested to be favorable for the double degenerate (DD) scenario but not for the SD scenario. If the mass range of the companion star to the white dwarf (WD) were too narrow in the SD model, its DTD would be too limited around the companion's main-sequence lifetime to be consistent with the observed DTD. However, this is not the case in our SD model that consists of the two channels of WD + RG (red giant) and WD + MS (main-sequence star). In these channels, the companion stars have a mass range of \sim 0.9-3 M_\sun (WD+RG) and \sim 2- 6 M_\sun (WD+MS). The combined mass range is wide enough to yield the featureless DTD. We emphasize that the SD scenario should include two important processes: the optically thick winds from the mass-accreting WD and the mass-stripping from the companion star by the WD wind.

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arXiv2008

A Universal Decline Law of Classical Novae. III. GQ Mus 1983

We present a unified model of infrared (IR), optical, ultraviolet (UV), and X-ray light curves for the 1983 outburst of GQ Muscae (Nova Muscae 1983) and estimate its white dwarf (WD) mass. Based on an optically thick wind model of nova outbursts, we model the optical and IR light curves with free-free emission, and the UV 1455 Åand supersoft X-ray light curves with blackbody emission. The best fit model that reproduces simultaneously the IR, optical, UV 1455 Å, and supersoft X-ray observations is a 0.7 \pm 0.05 M_\sun WD for an assumed chemical composition of the envelope, X=0.35-0.55, X_{CNO} =0.2-0.35, and Z = 0.02, by mass weight. The mass lost by the wind is estimated to be ΔM_{wind} \sim 2 \times 10^{-5} M_\sun. We provide a new determination of the reddening, E(B-V) = 0.55 \pm 0.05, and of the distance, \sim 5 kpc. Finally, we discuss the strong UV flash that took place on JD 2,445,499 (151 days after the outburst).

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arXiv2008

Helium Nova on a Very Massive White Dwarf -- A Light Curve Model of V445 Puppis (2000) Revised

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 (M_WD \gtrsim 1.35 M_\sun), and (2) a half of the accreted matter remains on the WD, both of which suggest that the increasing WD mass. Therefore, V445 Pup is a strong candidate of Type Ia supernova progenitor. The estimated distance to V445 Pup is now consistent with the recent observational suggestions, 3.5 < d < 6.5 kpc. A helium star companion is consistent with the brightness of m_v=14.5 mag just before the outburst, if it is a little bit evolved hot (\log T (K) \gtrsim 4.5) star with the mass of M_He \gtrsim 0.8 M_\sun. 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.

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arXiv2008

Young and Massive Binary Progenitors of Type Ia Supernovae and Their Circumstellar Matter

We present new evolutionary models for Type Ia supernova (SN Ia) progenitors, introducing mass-stripping effect on a main-sequence (MS) or slightly evolved companion star by winds from a mass-accreting white dwarf (WD). The mass-stripping attenuates the rate of mass transfer from the companion to the WD. As a result, quite a massive MS companion can avoid forming a common envelope and increase the WD mass up to the SN Ia explosion. Including the mass-stripping effect, we follow binary evolutions of various WD + MS systems and obtain the parameter region in the initial donor mass - orbital period plane where SNe Ia occur. The newly obtained SN Ia region extends to donor masses of 6-7 M_\sun, although its extension depends on the efficiency of mass-stripping effect. The stripped matter would mainly be distributed on the orbital plane and form very massive circumstellar matter (CSM) around the SN Ia progenitor. It can explain massive CSM around SNe Ia/IIn(IIa) 2002ic and 2005gj as well as tenuous CSM around normal SN Ia 2006X. Our new model suggests the presence of very young (\lesssim 10^8 yr) populations of SNe Ia, being consistent with recent observational indications of young population SNe Ia.

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arXiv2007

Thermal Stability of White Dwarfs Accreting Hydrogen-rich Matter and Progenitors of Type Ia Supernovae

We revisit the properties of white dwarfs accreting hydrogen-rich matter by constructing steady-state models, in which hydrogen shell burning consumes hydrogen at the same rate as the white dwarf accretes it. We obtain such steady-state models for various accretion rates and white dwarf masses. We confirm that these steady models are thermally stable only when the accretion rate is higher than \sim 10^{-7} M_sun/yr. We show that recent models of ``quiescent burning'' in the ``surface hydrogen burning'' at a much wider range of accretion rates results from the too large zone mass in the outer part of the models; hydrogen burning must occur in a much thinner layer. A comparison of the positions on the HR diagram suggests that most of the luminous supersoft X-ray sources are white dwarfs accreting matter at rates high enough that the hydrogen burning shell is thermally stable. Implications on the progenitors of Type Ia supernovae are discussed.

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arXiv2007

Supersoft X-ray Light Curve of RS Ophiuchi (2006)

One of the candidates for Type Ia supernova progenitors, the recurrent nova RS Ophiuchi underwent the sixth recorded outburst in February 2006, and for the first time a complete light curve of supersoft X-ray has been obtained. It shows the much earlier emergence and longer duration of a supersoft X-ray phase than expected before. These characteristics can be naturally understood when a significant amount of helium layer piles up beneath the hydrogen burning zone during the outburst, suggesting that the white dwarf (WD) is effectively growing up in mass. We have estimated the WD mass in RS Oph to be 1.35 \pm 0.01 M_sun and the growth rate of the WD mass to be at an average rate of about 1 \times 10^{-7} M_sun yr^{-1}. The white dwarf will probably reach the critical mass for Type Ia explosion if the present accretion continues further for a few to several times 10^{5} years.

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arXiv2007

A Universal Decline Law of Classical Novae. II. GK Persei 1901 and Novae in 2005

Optical and infrared light-curves of classical novae are approximately homologous among various white dwarf (WD) masses and chemical compositions when free-free emission from optically thin ejecta dominates the continuum flux of novae. Such a homologous template light-curve is called a ``universal decline law.'' The timescale of the light curve depends strongly on the WD mass but weakly on the chemical composition, so we are able to roughly estimate the WD mass from the light-curve fitting. We have applied the universal decline law to the old nova GK Persei 1901 and recent novae that outbursted in 2005. The estimated WD mass is 1.15 M_sun for GK Per, which is consistent with a central value of the WD mass determined from the orbital velocity variations. The other WD masses of 10 novae in 2005 are also estimated to be 1.05 M_sun (V2361 Cyg), 1.15 M_sun (V382 Nor), 1.2 M_sun (V5115 Sgr), 0.7 M_sun (V378 Ser), 0.9 M_sun (V5116 Sgr), 1.25 M_sun (V1188 Sco), 0.7 M_sun (V1047 Cen), 0.95 M_sun (V476 Sct), 0.95 M_sun (V1663 Aql), and 1.30 M_sun (V477 Sct), within a rough accuracy of +- 0.1 M_sun. Four (V382 Nor, V5115 Sgr, V1188 Sco, and V477 Sct) of ten novae in the year 2005 are probably neon novae on an O-Ne-Mg WD. Each WD mass depends weakly on the chemical composition (especially the hydrogen content X in mass weight), i.e., the obtained WD masses increase by +0.5(X-0.35) M_sun for the six CO novae and by +0.5(X-0.55) M_sun for the four neon novae above. Various nova parameters are discussed in relation to its WD mass.

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