arXiv subjects

Takashi Nagao

Publications and source records attributed to Takashi Nagao.

28 records · Page 2Linked to original sources

arXiv2018

Multi-band polarization of Type IIP supernovae due to light echo from circumstellar dust

Type IIP supernovae (SNe IIP) often show relatively high continuum polarization ($\sim 1$\%) in the late phase. This polarization feature is generally believed to be due to an inner aspherical core revealed in the late phase, while this polarization feature can also be contributed by the effect of polarized-scattered echoes by circumstellar (CS) dust around the SN. In this paper, we propose a unique method to distinguish polarization from the SN ejecta and from the light echo. We quantitatively examine wavelength dependence of the polarization created by the scattered echoes for various geometries and amounts of CS dust. It is found that the polarization in the $U$-band has characteristic features, i.e., the polarization emerges at an earlier phase with higher polarization degree than that in longer wavelengths. These are due to the rapid evolution of the $U$-band light curve as well as higher optical depth of dust in shorter wavelengths. Except for the $U$ band, the polarization increases after the plateau phase, and the polarization degree is generally higher for shorter wavelengths. These polarimetric features can be easily distinguished from the polarization expected from an aspherical core, which predicts almost no wavelength dependence. Moreover, we show that multi-band polarimetric observations for SNe IIP can constrain a parameter space in the CS dust mass and distance from the SNe. We thus encourage multi-band polarimetric observations for SNe IIP.

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arXiv2018

Polarization as a probe of dusty environments around Type Ia supernovae: radiative transfer models for SN 2012dn

Geometry of circumstellar (CS) medium around supernovae (SNe) provides important diagnostics to understand the nature of their progenitors. In this paper, properties of CS dust around SN 2012dn, a super-Chandrasekhar candidate Type Ia supernova (SC-SN), have been studied through detailed three dimensional radiation transfer simulations. With the detected near-infrared excess from SN 2012dn, we show that it has a disk-like dusty CS environment whose mass is roughly consistent with a branch of an accreting white dwarf system (the single degenerate scenario). We show that a similar system should produce polarization signals up to $\sim 8$ \% in the $B$ band, depending on the viewing direction if polarimetric observation is performed. We predict that the maximum polarization is reached around $\sim 60$ days after the $B$-band maximum. We show that the temporal and wavelength dependence of the polarization signals, together with other unique features, can be easily distinguished from the interstellar polarization and intrinsic SN polarization. Indeed, the small polarization degree observed for normal Type Ia SNe (SNe Ia) can constrain a parameter space in the CS dust mass and distribution. We thus encourage multi-band polarimetric observations for SNe Ia, especially for outliers including SC-SNe for which some arguments for the single degenerate scenario exist but the polarization data are very rare so far.

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arXiv2017

Circumstellar Light Echo as a Possible Origin of the Polarization of Type IIP Supernovae

Type IIP supernovae (SNe IIP) are the most common class of core-collapse SNe. They often show rapid increase of polarization degree in the late phase. This time evolution is generally believed to originate from the emergence of an inner aspherical core, while an effect of polarized-scattered echoes by circumstellar (CS) dust around the SN may also substantially contribute to this polarization feature. In this study, we examine the effects of the scatted echoes on the SN polarization through radiative transfer simulations for various geometry and amount of CS dust. It has been found that asymmetrically-distributed CS dust, which is generally inferred for red supergiants, could reproduce the observed polarization features. We have applied our results to SNe 2004dj and 2006ov, deriving the geometry and amount of CS dust to explain their observed polarization features in this scenario. For both SNe, the blob-like or bipolar distribution of CS dust rather than the disk-like distribution is favored. The derived dust mass $M_{\mathrm{dust}}$ in the blob model (the bipolar CS dust model) for SNe 2004dj and 2006ov are $\sim 7.5 \times 10^{-4}$ M$_{\odot}$ ($\sim 8.5 \times10^{-4}$ M$_{\odot}$) and $\sim 5.2 \times 10^{-4}$ M$_{\odot}$ ($\sim 1.3 \times10^{-3}$ M$_{\odot}$), respectively. Even in the case where this process would not play a dominant role in the observed polarization signals, this effect should in principle contribute to it, the strength of which depends on the nature of CS dust. Therefore, this effect must be taken into account in discussing multi-dimensional structure of an SN explosion through polarimetric observations.

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arXiv2016

The origin of the near-infrared excess in SN Ia 2012dn: Circumstellar dust around the super-Chandrasekhar supernova candidate

The nature of progenitors of the so-called super-Chandrasekhar candidate Type Ia supernovae (SC-SNe Ia) has been actively debated. Recently, Yamanaka et al. (2016) reported a near-infrared (NIR) excess for SN 2012dn, and proposed that the excess originates from an echo by circumstellar (CS) dust. In this paper, we examine a detailed distribution of the CS dust around SN 2012dn, and investigate implications of the CS dust echo scenario for general cases of SC-SNe Ia. We find that a disk/bipolar CS medium configuration reproduces the NIR excess fairly well, where the radial density distribution is given by a stationary mass loss. The inner radius of the CS dust is $0.04$ pc. The mass-loss rate of the progenitor system is estimated to be $1.2 \times 10^{-5}$ and $3.2 \times 10^{-6}$ M$_{\odot}$ yr$^{-1}$ for the disk and bipolar CS medium configurations, respectively, which adds another support for the single degenerate scenario. Our models limit SN 2009dc, another SC-SN Ia, to have a dust mass less than $0.16$ times that of SN 2012dn. While this may merely indicate some variation on the CS environment among SC-SNe Ia, this could raise another interesting possibility. There could be two classes among SC-SNe Ia; the brighter SC-SNe Ia in a clean environment (SN 2009dc) and the fainter SC-SNe Ia in a dusty environment (SN 2012dn).

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arXiv2016

OISTER Optical and Near-Infrared Observations of the Super-Chandrasekhar Supernova Candidate SN 2012dn: Dust Emission from the Circumstellar Shell

We present extensively dense observations of the super-Chandrasekhar supernova (SC SN) candidate SN 2012dn from $-11$ to $+140$ days after the date of its $B$-band maximum in the optical and near-infrared (NIR) wavelengths conducted through the OISTER ToO program. The NIR light curves and color evolutions up to 35 days after the $B$-band maximum provided an excellent match with those of another SC SN 2009dc, providing a further support to the nature of SN 2012dn as a SC SN. We found that SN 2012dn exhibited strong excesses in the NIR wavelengths from $30$ days after the $B$-band maximum. The $H$ and $K_{s}$-band light curves exhibited much later maximum dates at $40$ and $70$ days after the $B$-band maximum, respectively, compared with those of normal SNe Ia. The $H$ and $K_{s}$-band light curves subtracted by those of SN 2009dc displayed plateaued evolutions, indicating a NIR echo from the surrounding dust. The distance to the inner boundary of the dust shell is limited to be $4.8 - 6.4\times10^{-2}$ pc. No emission lines were found in its early phase spectrum, suggesting that the ejecta-CSM interaction could not occur. On the other hand, we found no signature that strongly supports the scenario of dust formation. The mass loss rate of the pre-explosion system is estimated to be $10^{-6}-10^{-5}$ M$_{\odot}$ yr$^{-1}$, assuming that the wind velocity of the system is $10-100$ km~s$^{-1}$, which suggests that the progenitor of SN 2012dn could be a recurrent nova system. We conclude that the progenitor of this SC SN could be explained by the single-degenerate scenario.

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arXiv2015

Constraining the Amount of Circumstellar Matter and Dust around Type Ia Supernovae through Near-Infrared Echoes

The circumstellar (CS) environment is key to understanding progenitors of type Ia supernovae (SNe Ia), as well as the origin of a peculiar extinction property toward SNe Ia for cosmological application. It has been suggested that multiple scatterings of SN photons by CS dust may explain the non-standard reddening law. In this paper, we examine the effect of re-emission of SN photons by CS dust in the infrared (IR) wavelength regime. This effect allows the observed IR light curves to be used as a constraint on the position/size and the amount of CS dust. The method was applied to observed near-infrared (NIR) SN Ia samples; meaningful upper limits on the CS dust mass were derived even under conservative assumptions. We thereby clarify a difficulty associated with the CS dust scattering model as a general explanation for the peculiar reddening law, while it may still apply to a sub-sample of highly reddened SNe Ia. For SNe Ia in general, the environment at the interstellar scale appears to be responsible for the non-standard extinction law. Furthermore, deeper limits can be obtained using the standard nature of SN Ia NIR light curves. In this application, an upper limit of Mdot ~10^{-8}-10^{-7} Msun/yr (for the wind velocity of ~10 km/s) is obtained for a mass loss rate from a progenitor up to ~0.01 pc, and Mdot ~10^{-7}-10^{-6} Msun/yr up to ~0.1 pc.

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arXiv2013

Superflares on Solar Type Stars Observed with Kepler I. Statistical Properties of Superflares

By extending our previous study by Maehara et al. (2012), we searched for superflares on G-type dwarfs (solar type stars) using Kepler data for a longer period (500 days) than that (120 days) in our previous study. As a result, we found 1547 superflares on 279 G-type dwarfs, which are much more than previous 365 superflares on 148 stars. Using these new data, we studied the statistical properties of occurrence frequency of superflares, and basically confirmed the previous results, i.e., the occurrence frequency (dN/dE) of superflares vs flare energy (E) shows power-law distribution with dN/dE \propto E^{-α}, where α~ 2. It is interesting that this distribution is roughly on the same line as that for solar flares. In the case of the Sun-like stars (with surface temperature 5600-6000K and slowly rotating with a period longer than 10 days), the occurrence frequency of superflares with energy of 10^34 -10^35 erg is once in 800-5000 years. We also studied long term (500 days) stellar brightness variation of these superflare stars, and found that in some G-type dwarfs the occurrence frequency of superflares was extremely high, ~ 57 superflares in 500 days (i.e., once in 10 days). In the case of Sun-like stars, the most active stars show the frequency of one superflares (with 10^34 erg) in 100 days. There is evidence that these superflares have extremely large starspots with a size about 10 times larger than that of the largest sunspot. We argue that the physical origin of extremely high occurrence frequency of superflares in these stars may be attributed to the existence of extremely large starspots.

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arXiv2013

High Dispersion Spectroscopy of the Superflare Star KIC6934317

We conducted the high-resolution spectroscopic observation with Subaru/HDS for a G-type star (KIC6934317). We selected this star from the data of the Kepler spacecraft. This star produces a lot of superflares, and the total energy of the largest superflare on this star is ~10^3 times larger (~2.2*10^35 erg) than that of the most energetic flare on the Sun (~10^32 erg). The core depth and emission flux of Ca II infrared triplet lines and Ha line show high chromospheric activity in this star, in spite of its low lithium abundance and the small amplitude of the rotational modulation. Using the empirical relations between emission flux of chromospheric lines and X-ray flux, this star is considered to show much higher coronal activity than that of the Sun. It probably has large starspots which can store a large amount of magnetic energy enough to give rise to superflares. We also estimated the stellar parameters, such as effective temperature, surface gravity, metallicity, projected rotational velocity (v sin_i), and radial velocity. KIC6934317 is then confirmed to be an early G-type main sequence star. The value of v sin_i is estimated to be ~1.91 km s^-1. In contrast, the rotational velocity is calculated to be ~20 km s^-1 by using the period of the brightness variation as the rotation period. This difference can be explained by its small inclination angle (nearly pole-on). The small inclination angle is also supported by the contrast between the large superflare amplitude and the small stellar brightness variation amplitude. The lithium abundance and isochrones implies that the age of this star is more than about a few Gyr, though a problem why this star with such an age has a strong activity remains unsolved.

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arXiv2013

Superflares on Solar-Type Stars Observed with Kepler II. Photometric Variability of Superflare-Generating Stars : A Signature of Stellar Rotation and Starspots

We performed simple spot-model calculations for quasi-periodic brightness variations of solar-type stars showing superflares, by using Kepler photometric data. Most of superflare stars show quasi-periodic brightness modulations with the typical period of one to a few tens of days. Our results indicate that these brightness variations of superflare stars can be explained by the rotation of the star with fairly large starspots. Using the result of the period analysis, we investigated the relation between the energy and frequency of superflares and the rotation period. Stars with relatively slower rotation rates can still produce flares that are as energetic as those of more rapidly rotating stars, although the average flare frequency is lower for more slowly rotating stars. We found that the energy of superflares are related to the total coverage of starspots. The correlation between the spot coverage and the flare energy in superflares is similar to that in solar flares. These results suggest that the energy of superflares can be explained by the magnetic energy stored around starspots.

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arXiv2012

Can Superflares Occur on Our Sun?

Recent observations of solar type stars with the Kepler satellite by Maehara et al. have revealed the existence of superflares (with energy of 10^33 - 10^35 erg) on Sun-like stars, which are similar to our Sun in their surface temperature (5600 K - 6000 K) and slow rotation (rotational period > 10 days). From the statistical analysis of these superflares, it was found that superflares with energy 10^34 erg occur once in 800 years and superflares with 10^35 erg occur once in 5000 years on Sun-like stars. In this paper, we examine whether superflares with energy of 10^33 - 10^35 erg could occur on the present Sun through the use of simple order-of-magnitude estimates based on current ideas relating to the mechanisms of the solar dynamo.

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