arXiv subjects

Takashi J. Moriya

Publications and source records attributed to Takashi J. Moriya.

At least 109 records · Page 6Linked to original sources

arXiv2020

Systematic investigation of the effect of 56Ni mixing in the early photospheric velocity evolution of stripped-envelope supernovae

Mixing of 56Ni, whose nuclear decay energy is a major luminosity source in stripped-envelope supernovae, is known to affect the observational properties of stripped-envelope supernovae such as light-curve and color evolution. Here we systematically investigate the effect of 56Ni mixing on the photospheric velocity evolution in stripped-envelope supernovae. We show that 56Ni mixing significantly affects the early photospheric velocity evolution. The photospheric velocity, which is often used to constrain the ejecta mass and explosion energy, significantly varies by just changing the degree of 56Ni mixing. In addition, the models with a small degree of 56Ni mixing show a flattening in the early photospheric velocity evolution, while the fully mixed models show a monotonic decrease. The velocity flattening appears in both helium and carbon+oxygen progenitor explosions with a variety of ejecta mass, explosion energy, and 56Ni mass. Some stripped-envelope supernovae with early photospheric velocity information do show such a flattening. We find that Type Ib SN 2007Y, which has early photospheric velocity information, has a signature of a moderate degree of 56Ni mixing in the photospheric velocity evolution and about a half of the ejecta is mixed in it. The immediate spectroscopic follow-up observations of stripped-envelope supernovae shortly after the explosion providing the early photospheric evolution give an important clue to constrain 56Ni mixing in the ejecta.

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arXiv2020

A systematic study on the rise time-peak luminosity relation for bright optical transients powered by wind shock breakout

A number of astrophyical transients originating from stellar explosions are powered by the collision of the ejected material with the circumstellar medium, which efficiently produces thermal radiation via shock dissipation. We investigate how such interaction-powered transients are distributed in the peak bolometric luminosity vs the rise time phase space. Taking the advantage of less time-consuming one-dimensional simulations with spherical symmetry, we calculated more than 500 models with different circumstellar mass and radius, ejecta mass and energy, and chemical compositions. The peak bolometric luminosity, the total radiated energy, and the rise time of the interaction-powered emission are measured for each simulated light curve. We consider how these characteristic quantities are determined as a function of the model parameters and discuss possible implications for the observed populations of (potential) interaction-powered transients, such as type IIn supernovae and fast blue optical transients.

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arXiv2020

Calcium-rich Transient SN 2019ehk in A Star-Forming Environment: Yet Another Candidate for An Ultra-Stripped Envelope Supernova

We present optical and near-infrared observations of SN~Ib~2019ehk. We show that it evolved to a Ca-rich transient according to its spectral properties and evolution in late phases. It, however, shows a few distinguishable properties from the canonical Ca-rich transients: a short-duration first peak in the light curve, high peak luminosity, and association with a star-forming environment. Indeed, some of these features are shared with iPTF14gqr and iPTF16hgs, which are candidates for a special class of core-collapse SNe (CCSNe): the so-called ultra-stripped envelope SNe, i.e., a relatively low-mass He (or C+O) star explosion in a binary as a precursor of double neutron star binaries. The estimated ejecta mass ($0.43 M_\odot$) and explosion energy ($1.7 \times 10^{50} $~erg) are consistent with this scenario. The analysis of the first peak suggests existence of dense circumstellar material in the vicinity of the progenitor, implying a CCSN origin. Based on these analyses, we suggest SN 2019ehk is another candidate for an ultra-stripped envelope SN. These ultra-stripped envelope SN candidates seem to form a subpopulation among Ca-rich transients, associated with young population. We propose that the key to distinguishing this population is the early first peak in their light curves.

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arXiv2020

Late-Phase Spectropolarimetric Observations of Superluminous Supernova SN 2017egm to Probe the Geometry of the Inner Ejecta

We present our spectropolarimetric observations of SN 2017egm, a Type I superluminous supernova (SLSN-I) in a nearby galaxy NGC 3191, with Subaru telescope at 185.0 days after the g-band maximum light. This is the first spectropolarimetric observation for SLSNe at late phases. We find that the degree of the polarization in the late phase significantly changes from that measured at the earlier phase. The spectrum at the late phase shows a strong Ca emission line and therefore we reliably estimate the interstellar polarization component assuming that the emission line is intrinsically unpolarized. By subtracting the estimated interstellar polarization, we find that the intrinsic polarization at the early phase is only ~ 0.2 %, which indicates an almost spherical photosphere, with an axial ratio ~ 1.05. The intrinsic polarization at the late phase increases to ~ 0.8 %, which corresponds to the photosphere with an axial ratio 1.2. A nearly constant position angle of the polarization suggests the inner ejecta are almost axisymmetric. By these observations, we conclude that the inner ejecta are more aspherical than the outer ejecta. It may suggest the presence of a central energy source producing aspherical inner ejecta.

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arXiv2020

The HSC-SSP Transient Survey: Implications from Early Photometry and Rise Time of Normal Type Ia Supernovae

With a booming number of Type Ia supernovae (SNe Ia) discovered within a few days of their explosions, a fraction of SNe Ia that show luminosity excess in the early phase (early-excess SNe Ia) have been confirmed. In this article, we report early-phase observations of seven photometrically normal SNe Ia (six early detections and one deep non-detection limit) at the COSMOS field through a half-year transient survey as a part of the Hyper Suprime-Cam Subaru Strategic Program (HSC SSP). In particular, a blue light-curve excess was discovered for HSC17bmhk, a normal SN Ia with rise time longer than 18.8 days, during the first four days after the discovery. The blue early excess in optical wavelength can be explained not only by interactions with a non-degenerate companion or surrounding dense circumstellar matter but also radiation powered by radioactive decays of $^{56}$Ni at the surface of the SN ejecta. Given the growing evidence of the early-excess discoveries in normal SNe Ia that have longer rise times than the average and a similarity in the nature of the blue excess to a luminous SN Ia subclass, we infer that early excess discovered in HSC17bmhk and other normal SNe Ia are most likely attributed to radioactive $^{56}$Ni decay at the surface of the SN ejecta. In order to successfully identify normal SNe Ia with early excess similar to that of HSC17bmhk, early UV photometries or high-cadence blue-band surveys are necessary.

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arXiv2020

Rapidly Evolving Transients from the Hyper Suprime-Cam SSP Transient Survey

Rapidly evolving transients form a new class of transients which show shorter timescales of the light curves than those of typical core-collapse and thermonuclear supernovae. We performed a systematic search for rapidly evolving transients using the deep data taken with the Hyper Suprime-Cam Subaru Strategic Program Transient Survey. By measuring the timescales of the light curves of 1824 transients, we identified 5 rapidly evolving transients. Our samples are found in a wide range of redshifts (0.3 $\le$ z $\le$ 1.5) and peak absolute magnitudes ($-$17 $\ge$ $M_i$ $\ge$ $-$20). The properties of the light curves are similar to those of the previously discovered rapidly evolving transients. They show a relatively blue spectral energy distribution, with the best-fit blackbody of 8,000 - 18,000 K. We show that some of the transients require power sources other than the radioactive decays of $^{56}$Ni because of their high peak luminosities and short timescales. The host galaxies of all the samples are star-forming galaxies, suggesting a massive star origin for the rapidly evolving transients. The event rate is roughly estimated to be $\sim$4,000 events yr$^{-1}$ Gpc$^{-3}$, which is about 1 $\%$ of core-collapse supernovae.

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arXiv2020

Probing the extragalactic fast transient sky at minute timescales with DECam

Searches for optical transients are usually performed with a cadence of days to weeks, optimised for supernova discovery. The optical fast transient sky is still largely unexplored, with only a few surveys to date having placed meaningful constraints on the detection of extragalactic transients evolving at sub-hour timescales. Here, we present the results of deep searches for dim, minute-timescale extragalactic fast transients using the Dark Energy Camera, a core facility of our all-wavelength and all-messenger Deeper, Wider, Faster programme. We used continuous 20s exposures to systematically probe timescales down to 1.17 minutes at magnitude limits $g > 23$ (AB), detecting hundreds of transient and variable sources. Nine candidates passed our strict criteria on duration and non-stellarity, all of which could be classified as flare stars based on deep multi-band imaging. Searches for fast radio burst and gamma-ray counterparts during simultaneous multi-facility observations yielded no counterparts to the optical transients. Also, no long-term variability was detected with pre-imaging and follow-up observations using the SkyMapper optical telescope. We place upper limits for minute-timescale fast optical transient rates for a range of depths and timescales. Finally, we demonstrate that optical $g$-band light curve behaviour alone cannot discriminate between confirmed extragalactic fast transients such as prompt GRB flashes and Galactic stellar flares.

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arXiv2019

Supernova ejecta interacting with a circumstellar disk. I. two-dimensional radiation-hydrodynamic simulations

We perform a series of two-dimensional radiation-hydrodynamic simulations of the collision between supernova ejecta and circumstellar media (CSM). The hydrodynamic interaction of a fast flow and the surrounding media efficiently dissipates the kinetic energy of the fast flow and considered as a dominant energy source for a specific class of core-collapse supernovae. Despite some observational evidence for aspherical ejecta and/or CSM structure, multi-dimensional effects in the ejecta-CSM interaction are relatively unexplored. Our numerical simulations equipped with an adaptive mesh refinement technique successfully reproduce hydrodynamic instabilities developing around the ejecta-CSM interface. We also investigate effects of disk-like CSM on the dynamical evolution of supernova ejecta and bolometric light curves. We find that emission powered by ejecta-disk interaction exhibits significant viewing angle dependence. For a line of sight close to the symmetry axis, the observer directly sees the supernova ejecta, leading to a short brightening timescale. For an observer seeing the emission through the CSM disk, thermal photons diffuse throughout the CSM and thus the light curve is severely smeared out.

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arXiv2019

iPTF14hls as a variable hyper-wind from a very massive star

The origin of iPTF14hls, which had Type IIP supernova-like spectra but kept bright for almost two years with little spectral evolution, is still unclear. We here propose that iPTF14hls was not a sudden outburst like supernovae but rather a long-term outflow similar to stellar winds. The properties of iPTF14hls, which are at odds with a supernova scenario, become natural when interpreted as a stellar wind with variable mass-loss rate. Based on the wind hypothesis, we estimate the mass-loss rates of iPTF14hls in the bright phase. We find that the instantaneous mass-loss rate of iPTF14hls during the 2-year bright phase was more than a few Msun/yr ("hyper-wind") and it reached as much as 10 Msun/yr. The total mass lost over two years was about 10 Msun. Interestingly, we find that the light curve of iPTF14hls has a very similar shape to that of eta Carinae during the Great Eruption, which also experienced a similar but less extreme brightening accompanied by extraordinary mass loss, shedding more than 10 Msun in 10 years. The progenitor of iPTF14hls is less than 150 Msun if it still exists, which is similar to eta Carinae. The two phenomena may be related to a continuum-driven extreme wind from very massive stars.

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arXiv2019

A rapidly declining transient discovered with Subaru/Hyper Suprime-Cam

We perform a high-cadence transient survey with Subaru Hyper Suprime-Cam (HSC), which we call the Subaru HSC survey Optimized for Optical Transients (SHOOT). We conduct HSC imaging observations with time intervals of about one hour on two successive nights, and spectroscopic and photometric follow-up observations. A rapidly declining blue transient SHOOT14di at $z=0.4229$ is found in observations on two successive nights with an image subtraction technique. The rate of brightness change is $+1.28^{+0.40}_{-0.27}~{\rm mag~day^{-1}}$ ($+1.83^{+0.57}_{-0.39}~{\rm mag~day^{-1}}$) in the observer (rest) frame and the rest-frame color between $3400$ and $4400~\unicode[.8,0]{x212B}$ is $M_{\rm 3400\unicode[.8,0]{x212B}}-M_{\rm 4400\unicode[.8,0]{x212B}}=-0.4$. The nature of the object is investigated by comparing its peak luminosity, decline rate, and color with those of transients and variables previously observed, and those of theoretical models. None of the transients or variables share the same properties as SHOOT14di. Comparisons with theoretical models demonstrate that, while the emission from the cooling envelope of a Type IIb supernova shows a slower decline rate than SHOOT14di, and the explosion of a red supergiant star with a dense circumstellar wind shows a redder color than SHOOT14di, the shock breakout at the stellar surface of the explosion of a $25M_{\odot}$ red supergiant star with a small explosion energy of $\leq0.4\times10^{51}$ erg reproduces the multicolor light curve of SHOOT14di. This discovery shows that a high-cadence, multicolor optical transient survey at intervals of about one hour, and continuous and immediate follow-up observations, is important for studies of normal core-collapse supernovae at high redshifts.

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arXiv2019

VTC J095517.5+690813: A radio transient from an accretion-induced collapse of a white dwarf?

We investigate a possibility that a recently reported radio transient in M81, VTC J095517.5+690813, is caused by an accretion-induced collapse of a white dwarf. It became bright in radio but no associated optical transient was discovered. The accretion-induced collapse is predicted to be radio bright but optically faint, satisfying the observed property. We compare predicted radio emission from the accretion-induced collapse with that of VTC J095517.5+690813 and show that it can be reasonably explained by the accretion-induced collapse. Although it is difficult to firmly conclude that VTC J095517.5+690813 is an accretion-induced collapse, our study shows that radio-bright transients without an optical counterpart could still be related to stellar deaths.

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arXiv2019

Circumstellar properties of Type Ia supernovae from the helium star donor channel

We investigate predicted circumstellar properties of Type Ia supernova progenitor systems with non-degenerate helium star donors. It has been suggested that systems consisting of a carbon+oxygen white dwarf and a helium star can lead to Type Ia supernova explosions. Binary evolution calculations for the helium star donor channel predict that such a progenitor system is in either a stable helium-shell burning phase or a weak helium-shell flash phase at the time of the Type Ia supernova explosion. By taking the binary evolution models from our previous study, we show that a large fraction of the progenitor systems with a helium star donor have low enough density to explain the current non-detection of radio emission from Type Ia supernovae. Most of the progenitor systems in the weak helium-shell flash phase at the time of the Type Ia supernova explosions, which may dominate the prompt (short delay time) Type Ia supernova population, have both low circumstellar density and a faint helium star donor to account for the non-detection of radio emission and a pre-explosion companion star in SN 2011fe and SN 2014J. We also find some progenitor systems that are consistent with the properties of the companion star candidate identified at the explosion location of Type Iax SN 2012Z.

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arXiv2019

Observational signature of circumstellar interaction and $\rm ^{56}$Ni-mixing in the Type II Supernova 2016gfy

The optical and ultra-violet broadband photometric and spectroscopic observations of the Type II supernova (SN) 2016gfy are presented. The $V$-band light curve (LC) shows a distinct plateau phase with a slope, $s_2$ $\sim$ 0.12 mag / 100 d and a duration of 90 $\pm$ 5 d. Detailed analysis of SN 2016gfy provided a mean $\rm^{56}Ni$ mass of 0.033 $\pm$ 0.003 $\rm M_{\odot}$, a progenitor radius of $\sim$ 350-700 $\rm R_{\odot}$, a progenitor mass of $\sim$ 12-15 $\rm M_{\odot}$ and an explosion energy of 0.9-1.4$\rm \times 10^{51}\ erg\ s^{-1}$. The P-Cygni profile of H$\rm α$ in the early phase spectra ($\sim$ 11-21 d) shows a boxy emission. Assuming that this profile arises from the interaction of the SN ejecta with the pre-existing circumstellar material (CSM), it is inferred that the progenitor underwent a recent episode (30-80 years prior to the explosion) of enhanced mass loss. Numerical modeling suggests that the early LC peak is reproduced better with an existing CSM of 0.15 $\rm M_{\odot}$ spread out to $\sim$ 70 AU. A late-plateau bump is seen in the $VRI$ LCs during $\sim$ 50-95 d. This bump is explained as a result of the CSM interaction and/or partial mixing of radioactive $\rm ^{56}Ni$ in the SN ejecta. Using strong-line diagnostics, a sub-solar oxygen abundance is estimated for the supernova H II region (12 + log(O/H) = 8.50 $\pm$ 0.11), indicating an average metallicity for the host of a Type II SN. A star formation rate of $\sim$ 8.5 $\rm M_{\odot}\ yr^{-1}$ is estimated for NGC 2276 using the archival $GALEX$ FUV data.

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arXiv2019

HSC16aayt: Slowly evolving interacting transient rising for more than 100 days

We report our observations of HSC16aayt (SN 2016jiu), which was discovered by the Subaru/Hyper Suprime-Cam (HSC) transient survey conducted as part of Subaru Strategic Program (SSP). It shows very slow photometric evolution and its rise time is more than 100 days. The optical magnitude change in 400 days remains within 0.6 mag. Spectra of HSC16aayt show a strong narrow emission line and we classify it as a Type IIn supernova. The redshift of HSC16aayt is 0.6814 +/- 0.0002 from the spectra. Its host galaxy center is at 5 kpc from the supernova location and HSC16aayt might be another example of isolated Type IIn supernovae, although the possible existence of underlying star forming activity of the host galaxy at the supernova location is not excluded.

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arXiv2019

Fallback accretion powered supernova light curves based on a neutrino-driven explosion simulation of a 40 Msun star

We present synthetic light curves of fallback-powered supernovae based on a neutrino-driven explosion of a 40 Msun zero-metallicity star with significant fallback accretion onto a black hole that was previously simulated by Chan et al. (2018) until shock breakout. Here, we investigate the light curve properties of the explosion after shock breakout for various fallback models. Without extra power from fallback accretion, the light curve is that of a Type IIP supernova with a plateau magnitude of around -14 mag and a plateau duration of 40 days. With extra power for the light curve from fallback accretion, however, we find that the transient can be significantly more luminous. The light-curve shape can be SN 1987A-like or Type IIP-like, depending on the efficiency of the fallback engine. If the accretion disk forms soon after the collapse and more than 1% of the accretion energy is released as the central engine, fallback accretion powered supernovae become as luminous as superluminous supernovae. We suggest that Type II superluminous supernovae with broad hydrogen features could be related to such hydrogen-rich supernovae powered by fallback accretion. In the future, such superluminous supernovae powered by fallback accretion might be found among the supernovae from the first stars in addition to pair-instability supernovae and pulstational pair-instability supernovae.

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arXiv2019

The Hyper Suprime-Cam SSP Transient Survey in COSMOS: Overview

We present an overview of a deep transient survey of the COSMOS field with the Subaru Hyper Suprime-Cam (HSC). The survey was performed for the 1.77 deg$^2$ ultra-deep layer and 5.78 deg$^2$ deep layer in the Subaru Strategic Program over 6- and 4-month periods from 2016 to 2017, respectively. The ultra-deep layer shows a median depth per epoch of 26.4, 26.3, 26.0, 25.6, and 24.6 mag in $g$, $r$, $i$, $z$, and $y$ bands, respectively; the deep layer is $\sim0.6$ mag shallower. In total, 1,824 supernova candidates were identified. Based on light curve fitting and derived light curve shape parameter, we classified 433 objects as Type Ia supernovae (SNe); among these candidates, 129 objects have spectroscopic or COSMOS2015 photometric redshifts and 58 objects are located at $z > 1$. Our unique dataset doubles the number of Type Ia SNe at $z > 1$ and enables various time-domain analyses of Type II SNe, high redshift superluminous SNe, variable stars, and active galactic nuclei.

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arXiv2019

SN 2017czd: A Rapidly Evolving Supernova from a Weak Explosion of a Type IIb Supernova Progenitor

We present optical and near-infrared observations of the rapidly evolving supernova (SN) 2017czd that shows hydrogen features. The optical light curves exhibit a short plateau phase ($\sim 13$ days in the $R$-band) followed by a rapid decline by $4.5$ mag in $\sim 20 \mathrm{days}$ after the plateau. The decline rate is larger than those of any standard SNe, and close to those of rapidly evolving transients. The peak absolute magnitude is $-16.8$ mag in the $V$-band, which is within the observed range for SNe IIP and rapidly evolving transients. The spectra of SN 2017czd clearly show the hydrogen features and resemble those of SNe IIP at first. The H$α$ line, however, does not evolve much with time and it becomes similar to those in SNe IIb at decline phase. We calculate the synthetic light curves using a SN IIb progenitor which has 16 M$_{\odot}$ at the zero-age main sequence and evolves in a binary system. The model with a low explosion energy ($5\times 10^{50}$ erg) and a low ${}^{56}$Ni mass ($0.003 \mathrm{M}_{\odot}$) can reproduce the short plateau phase as well as the sudden drop of the light curve as observed in SN 2017czd. We conclude that SN 2017czd might be the first identified weak explosion from a SN IIb progenitor. We suggest that some rapidly evolving transients can be explained by such a weak explosion of the progenitors with little hydrogen-rich envelope.

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arXiv2019

Searches for Population III pair-instability supernovae: Predictions for ULTIMATE-Subaru and WFIRST

ULTIMATE-Subaru (Ultra-wide Laser Tomographic Imager and MOS with AO for Transcendent Exploration on Subaru) and WFIRST (Wide Field Infra-Red Survey Telescope) are the next generation near-infrared instruments that have a large field-of-view. They allow us to conduct deep and wide transient surveys in near-infrared. Such a near-infrared transient survey enables us to find very distant supernovae that are redshifted to the near-infrared wavelengths. We have performed the mock transient surveys with ULTIMATE-Subaru and WFIRST to investigate their ability to discover Population III pair-instability supernovae. We found that a 5-year 1 deg2 K-band transient survey with the point-source limiting magnitude of 26.5 mag with ULTIMATE-Subaru may find about 2 Population III pair-instability supernovae beyond the redshift of 6. A 5-year 10 deg2 survey with WFIRST reaching 26.5 mag in the F184 band may find about 7 Population III pair-instability supernovae beyond the redshift of 6. We also find that the expected numbers of the Population III pair-instability supernova detections increase about a factor of 2 if the near-infrared transient surveys are performed towards clusters of galaxies. Other supernovae such as Population II pair-instability supernovae would also be detected in the same survey. This study demonstrates that the future wide-field near-infrared instruments allow us to investigate the explosions of the first generation supernovae by performing the deep and wide near-infrared transient surveys.

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