This paper presents the second data release of the Hyper Suprime-Cam Subaru Strategic Program, a wide-field optical imaging survey using the 8.2 m Subaru Telescope. The release includes data from 174 nights of observation through 2018 January. The Wide layer data cover about 300 deg$^2$ in all five broad-band filters ($grizy$) to the nominal survey exposure (10 min in $gr$ and 20 min in $izy$). Partially observed areas are also included in the release; about 1100 deg$^2$ is observed in at least one filter and one exposure. The median seeing in the i-band is ${0_{.}^{\prime \prime }6}$, demonstrating the superb image quality of the survey. The Deep (26 deg$^2$) and UltraDeep (4 deg$^2$) data are jointly processed and the UltraDeep-COSMOS field reaches an unprecedented depth of $i\sim 28$ at $5 \, \sigma$ for point sources. In addition to the broad-band data, narrow-band data are also available in the Deep and UltraDeep fields. This release includes a major update to the processing pipeline, including improved sky subtraction, PSF modeling, object detection, and artifact rejection. The overall data quality has been improved, but this release is not without problems; there is a persistent deblender problem as well as new issues with masks around bright stars. The user is encouraged to review the issue list before utilizing the data for scientific explorations. All the image products as well as catalog products are available for download. The catalogs are also loaded into a database, which provides an easy interface for users to retrieve data for objects of interest. In addition to these main data products, detailed galaxy shape measurements withheld from Public Data Release 1 (PDR1) are now available to the community. The shape catalog is drawn from the S16A internal release, which has a larger area than PDR1 (160 deg$^2$). All products are available at the data release site, https://hsc-release.mtk.nao.ac.jp/.

We run a suite of hydrodynamics simulations of tidal disruption events (TDEs) of a white dwarf (WD) by a black hole (BH) with a wide range of WD/BH masses and orbital parameters. We implement nuclear reactions to study nucleosynthesis and its dynamical effect through release of nuclear energy. The released nuclear energy effectively increases the fraction of unbound ejecta. This effect is weaker for a heavy WD with 1.2 M , because the specific orbital energy distribution of the debris is predominantly determined by the tidal force, rather than by the explosive reactions. The elemental yield of a TDE depends critically on the initial composition of a WD, while the BH mass and the orbital parameters also affect the total amount of synthesized elements. Tanikawa et al. (2017) find that simulations of WD-BH TDEs with low resolution suffer from spurious heating and inaccurate nuclear reaction results. In order to examine the validity of our calculations, we compare the amounts of the synthesized elements with the upper limits of them derived in a way where we can avoid uncertainties due to low resolution. The results are largely consistent, and thus support our findings. We find particular TDEs where early self-intersection of a WD occurs during the first pericentre passage, promoting formation of an accretion disc. We expect that relativistic jets and/or winds would form in these cases because accretion rates would be super-Eddington. The WD-BH TDEs result in a variety of events depending on the WD/BH mass and pericentre radius of the orbit.

Rapidly evolving transients form a new class of transients that show shorter timescales of light curves than those of typical core-collapse and thermonuclear supernovae. We performed a systematic search for rapidly evolving transients using 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 five rapidly evolving transients. Our samples are found in a wide range of redshifts (0.3 ≤ z ≤ 1.5) and peak absolute magnitudes (−17 ≥ M i  ≥ −20). The light-curve properties 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 8000–18,000 K. We show that some of the transients require power sources other than the radioactive decays of 56Ni because of their high peak luminosities and short timescales. The host galaxies of all of the samples are star-forming galaxies, suggesting a massive star origin for the rapidly evolving transients. The event rate is roughly estimated to be ∼4000 events yr−1 Gpc−3, which is about 1% of core-collapse supernovae.

We study properties of the emission from thermonuclear explosions in a helium white dwarf (WD) tidal disruption event (TDE). The helium WD is not only tidally disrupted but is detonated by the tidal compression and by succeeding shocks. We focus on the emission powered by radioactive nuclei in the unbound ejecta of the TDE debris. We consider a TDE where a 0.2 M helium WD is disrupted by a 10 2.5 M intermediate-mass black hole (IMBH). We perform hydrodynamic simulations coupled with nuclear reactions, post-process detailed nucleosynthesis calculations, and then radiative transfer simulations. We thus derive multi-band light curves and spectra. The helium WD TDE shows rapid (∆t 1mag 5-10 d) and relatively faint (L peak 10 42 erg s −1 ) light curves, because the ejecta mass and 56 Ni mass are low (0.12 M and 0.03 M , respectively). The spectra show strong calcium and Fe-peak features and very weak silicon features, reflecting the peculiar elemental abundance. The key feature is the Doppler shift of the spectral lines up to ±12, 000 km s −1 , depending on the viewing angle, due to the bulk motion of the ejecta. Our model matches well with some rapid transients. The particular model presented here does not match with observed supernovae Iax, calcium-rich transients, nor .Ia explosion candidates, either in the spectra or light curves. However, we expect a large variety of the observational signatures once a wide range of the WD/BH masses and orbital parameters are considered. This study helps to search for WD TDEs with current and upcoming surveys, and to identify IMBHs as disrupters in the TDEs.

We report our observations of HSC16aayt (SN 2016jiu), which was discovered by the Subaru/Hyper Suprime-Cam (HSC) transient survey conducted as part of the Subaru Strategic Program. 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.