Mariko Kato

Low-Metallicity Inhibition of Type Ia Supernovae and Galactic and Cosmic Chemical Evolution

We introduce a metallicity dependence of the Type Ia supernova (SN Ia) rate into the Galactic and cosmic chemical evolution models. In our SN Ia progenitor scenario, the accreting white dwarf (WD) blows a strong wind to reach the Chandrasekhar mass limit. If the iron abundance of the progenitors is as low as [Fe/H] -1, then the wind is too weak for SNe Ia to occur. Our model successfully reproduces the observed chemical evolution in the solar neighborhood. We make the following predictions that can test this metallicity effect:(1) SNe Ia are not found in the low iron abundance environments such as dwarf galaxies and the outskirts of spirals. (2) The cosmic SN Ia rate drops at z~1-2 because of the low iron abundance, which can be observed with the Next Generation Space Telescope. At z1-2, SNe Ia can be found only in the environments where the timescale of metal enrichment is sufficiently short as in starburst galaxies and elliptical galaxies. The low-metallicity inhibition of SNe Ia can shed new light on the following issues: (1) The limited metallicity range of the SN Ia progenitors would imply that evolution effects are relatively small for the use of high-redshift SNe Ia to determine the cosmological parameters. (2) WDs of halo populations are poor producers of SNe Ia, so that the WD contribution to the halo mass is not constrained from the iron abundance in the halo. (3) The abundance patterns of globular clusters and field stars in the Galactic halo lack of SN Ia signatures in spite of their age difference of several Gyr, which can be explained by the low-metallicity inhibition of SNe Ia. (4) It could also explain why the SN Ia contamination is not seen in the damped Lyα systems over a wide range of redshift.

A theoretical light-curve model for the 1999 outburst of U Scorpii

A theoretical light curve for the 1999 outburst of U Scorpii is presented in order to obtain various physical parameters of the recurrent nova. Our U Sco model consists of a very massive white dwarf (WD) with an accretion disk and a lobe-filling, slightly evolved, main-sequence star (MS). The model includes a reflection effect by the companion and the accretion disk together with a shadowing effect on the companion by the accretion disk. The early visual light curve (with a linear phase of t approximately 1-15 days after maximum) is well reproduced by a thermonuclear runaway model on a very massive WD close to the Chandrasekhar limit (MWD=1.37+/-0.01 M middle dot in circle), in which optically thick winds blowing from the WD play a key role in determining the nova duration. The ensuing plateau phase (t approximately 15-30 days) is also reproduced by the combination of a slightly irradiated MS and a fully irradiated flaring-up disk with a radius approximately 1.4 times the Roche lobe size. The cooling phase (t approximately 30-40 days) is consistent with a low-hydrogen content of X approximately 0.05 of the envelope for the 1.37 M middle dot in circle WD. The best-fit parameters are the WD mass of MWD approximately 1.37 M middle dot in circle, the companion mass of MMS approximately 1.5 M middle dot in circle (0.8-2.0 M middle dot in circle is acceptable), the inclination angle of the orbit (i approximately 80&j0;), and the flaring-up edge, the vertical height of which is approximately 0.30 times the accretion disk radius. The duration of the strong wind phase (t approximately 0-17 days) is very consistent with the BeppoSAX supersoft X-ray detection at t approximately 19-20 days because supersoft X-rays are self-absorbed by the massive wind. The envelope mass at the peak is estimated to be approximately 3x10-6 M middle dot in circle, which is indicates an average mass accretion rate of approximately 2.5x10-7 M middle dot in circle yr-1 during the quiescent phase between 1987 and 1999. These quantities are exactly the same as those predicted in a new progenitor model of Type Ia supernovae.

A Model for the Quiescent Phase of the Recurrent Nova U Scorpii.

A theoretical light curve is constructed for the quiescent phase of the recurrent nova U Scorpii in order to resolve the existing distance discrepancy between the outbursts (d approximately 6 kpc) and the quiescences (d approximately 14 kpc). Our U Sco model consists of a very massive white dwarf (WD), an accretion disk (ACDK) with a flaring-up rim, and a lobe-filling, slightly evolved, main-sequence star (MS). The model properly includes an accretion luminosity of the WD, a viscous luminosity of the ACDK, and a reflection effect of the MS and the ACDK irradiated by the WD photosphere. The B light curve is well reproduced by a model of 1.37 M middle dot in circle WD + 1.5 M middle dot in circle MS (0.8-2.0 M middle dot in circle MS is acceptable) with an ACDK having a flaring-up rim and the inclination angle of the orbit i approximately 80&j0;. The calculated color is rather blue (B-V approximately 0.0) for a suggested mass accretion rate of 2.5x10-7 M middle dot in circle yr-1, thus indicating a large color excess of E(B-V) approximately 0.56 with the observational color of B-V=0.56 in quiescence. Such a large color excess corresponds to an absorption of AV approximately 1.8 and AB approximately 2.3, which reduces the distance to 6-8 kpc. This is in good agreement with the distance estimation of 4-6 kpc for the latest outburst. Such a large intrinsic absorption is very consistent with the recently detected period change of U Sco, which is indicating a mass outflow of approximately 3x10-7 M middle dot in circle yr-1 through the outer Lagrangian points in quiescence.

A Theoretical Light-Curve Model for the Recurrent Nova V394 Coronae Australis

A theoretical light curve for the 1987 outburst of V394 Coronae Australis (V394 CrA) is modeled to obtain various physical parameters of this recurrent nova. We then apply the same set of parameters to a quiescent phase and confirm that these parameters give a unified picture of the binary. Our V394 CrA model consists of a very massive white dwarf (WD), with an accretion disk (ACDK) having a flaring-up rim, and a lobe-filling, slightly evolved, main-sequence star (MS). The model includes irradiation effects of the MS and the ACDK by the WD. The early visual light curve (t ~ 1-10 days after the optical maximum) is well reproduced by a thermonuclear runaway model on a very massive WD close to the Chandrasekhar limit (1.37 ± 0.01 M☉). The ensuing plateau phase (t ~ 10-30 days) is also reproduced by the combination of a slightly irradiated MS and a fully irradiated flaring-up disk with a radius ~1.4 times the Roche lobe size. The best-fit parameters are the WD mass ~1.37 M☉, the companion mass ~1.5 M☉ (0.8-2.0 M☉ is acceptable), the inclination angle of the orbit i ~ 65°-68°, and the flaring-up rim ~0.30 times the disk radius. The envelope mass at the optical peak is estimated to be ~6 × 10-6 M☉, which indicates an average mass accretion rate of ~1.5 × 10-7 M☉ yr-1 during the quiescent phase between the 1949 and 1987 outbursts. In the quiescent phase, we properly include the accretion luminosity of the WD and the viscous luminosity of the ACDK as well as the irradiation effects of the ACDK and MS by the WD. The observed light curve can be reproduced with a disk size of 0.7 times the Roche lobe size and a rather slim thickness of 0.05 times the accretion disk size at the rim. About 0.5 mag sinusoidal variation of the light curve requires a mass accretion rate higher than ~1.0 × 10-7 M☉ yr-1, which is consistent with the above estimation from the 1987 outburst. These newly obtained quantities are exactly the same as those predicted in a new progenitor model of Type Ia supernovae.

A New Interpretation for the Second Peak of T Coronae Borealis Outbursts: A Tilting Disk around a Very Massive White Dwarf

A new interpretation for the second peak of T Coronae Borealis outbursts is proposed based on a thermonuclear runaway (TNR) model. The system consists of a very massive white dwarf (WD) with a tilting accretion disk and a lobe-filling red giant. The first peak of the visual light curve of T CrB outbursts is well reproduced by the TNR model on a WD close to the Chandrasekhar mass (MWD1.35 M), while the second peak is reproduced by the combination of the irradiated M giant and the irradiated tilting disk. The derived fitting parameters are the WD mass MWD~1.35 M, the M giant companion mass MRG~0.7 M (0.6-1.0 M☉ is acceptable), the inclination angle of the orbit i~70°, and the tilting angle of the disk iprec~35°. These parameters are consistent with the recently derived binary parameters of T CrB.

Light Curve Analysis of Novae

Light curve fittings for the decay phase of 7 novae are shown. Theoretical light curves are obtained using the optically thick wind theory with the new opacity. Good quality light curves enable us to determine the white dwarf mass, chemical composition and the distance to the star. Multiwavelength observation is very helpful in obtaining these parameters. The theoretical light curve for the helium nova is also given.

The origin of diversity of type ia supernovae and environmental effects

Observations suggest that the properties of Type Ia supernovae (SNe Ia) may depend on environmental characteristics, such as the morphology, metallicity, and age of the host galaxies. The influence of these environmental properties on the resulting SNe Ia is studied in this Letter. First, it is shown that the carbon mass fraction X(C) in the C + O white dwarf SN Ia progenitors tends to be smaller for a lower metallicity environment and an older binary system. It is then suggested that the variation of X(C) causes the diversity in the brightness of SNe Ia: a smaller X(C) leads to a dimmer SN Ia. Further studies of the propagation of the turbulent flame are necessary to confirm this relation. Our model for the SN Ia progenitors then predicts that when the progenitors belong to an older population or to a low-metallicity environment, the number of bright SNe Ia is reduced, so that the variation in brightness among the SNe Ia is also smaller. Thus, our model can explain why the mean SN Ia brightness and its dispersion depend on the morphology of the host galaxies and on the distance of the SN from the center of the galaxy. It is further predicted that at higher redshift (z 1), both the mean brightness of SNe Ia and its variation should be smaller in spiral galaxies than in elliptical galaxies. These variations are within the range observed in nearby SNe Ia. Insofar as the variation in X(C) is the most important cause for the diversity among SNe Ia, the light-curve shape method that is currently used to determine the absolute magnitude of SNe Ia can also be applied to high-redshift SNe Ia.