Observational Study of Extremely Luminous Type Ia Supernova 2009dc

極めて明るいIa型超新星SN2009dcの観測的研究

Yamanaka Masayuki

In this thesis, we examined an extremely luminous Type Ia supernova (SNe Ia) SN 2009dc. Type Ia supernovae are luminous ones among whole types of supernova and they are believed to be a result of thermonuclear explosion in a white dwarf, a degenerated compact star, of which the mass reaches near the Chandrasekhar-limit-mass, about 1.4 times solar mass. Since the luminosity of Type Ia supernovae are homogeneous, they are used as a standard candle of cosmologically distant objects. In recent years, two extremely luminous Type Ia supernovae (SNe 2003fg and 2006gz) have been discovered. They pointed a discrepancy against the explosion model; ≳ 1.2 times solar mass of 56Ni (energy source) and thus over 1.4 times solar mass of total ejecta should be required. However, the observational material on them was restricted and poor, and their nature was still unclear.

Here we present new observational results of the recently-discovered SN 2009dc. This supernova is also classified as an extremely luminous Type Ia supernova. From our early-phase observation, we derived a decline rate of the light curve is Δm15 (B) = 0.65 ± 0.03, which is one of the slowest decline rate among those of SNe Ia. The peak V-band absolute magnitude is estimated to be Mv = -19.90 ± 0.15 mag if no host extinction is assumed. It reaches MV = -20.19 ± 0.19 mag if we assume the host extinction of Av = 0.29 mag. SN 2009dc clearly belongs to the extremely luminous class both in optical and near-infrared wavelengths. We estimate the ejected 56Ni mass of 1.3 ± 0.3 M⊙ for the no host extinction case (and of 1.8+ 0.4 Me for the host extinction of Av = 0.29 mag). Using an analytic model, we successfully reproduced the (quasi-) bolometric light curve until 150 days after the B-band maximum. This indicates that the peak luminosity could be explained by 56Ni dacay. The C II λ6580 absorption line remains visible until a week after the B-band maximum brightness, in contrast to its early disappearance in SN 2006gz. The line velocity of Si II λ6355 is about 8000 km s-1 around the B-band maximum, being considerably slower than that of SN 2006gz. The velocity of the C II line is similar to or slightly less than that of the Si II line around the maximum. The presence of the carbon line suggests that thick unburned carbon and oxygen layer remains after the explosion. Spectropolarimetric study indicates that the explosion is not aspherical but rather symmetric along the line of sight. All these observational facts suggest that the amount of the ejecta is larger than that of normal SNe Ia and the progenitor's mass is over the Chandrasekhar-limit-mass.

In addition to the early-phase observation, we obtained photometric and spectroscopic data at a year after the B-band maximum. This late-phase optical photometry shows much fainter luminosity than that expected from early-phase data. A similar result has been marginally obtained for SN 2006gz. On the other hand, the observed late-phase light curve cannot be reproduced with the model. We suggest that the late-phase luminosity decreases due to an increase of optical opacity with dust formation in the ejected material. This model is supported by the observational facts of the presence of carbon-rich ejecta in early-phase spectra and redder color index at late-phase. We also confirm the existence of [Ni II] and [Ca II] lines whose radial velocities relative to the radial velocity of the host galaxy are -600 km s-1 and +300 km s-1, respectively. This indicates that a significant amount of intermediate-mass elements like calcium exists near the explosion core together with iron-group elements. All the observational facts would be explained by super-Chandrasekhar white dwarf model, except for the possible mixing in the inner core. We propose that the progenitor's mass exceeded the Chandrasekhar-limiting mass possibly due to hyper rotation of the progenitor white dwarf.

Astronomy. Space sciences [ 440 ]

eng

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広島大学