Long-rising Type II supernovae from Palomar Transient Factory and Caltech Core-Collapse Project

¹ The Oskar Klein Centre, Department of AstronomyStockholm University, AlbaNova, 10691 Stockholm, Sweden
e-mail: francesco.taddia@astro.su.se
² Department of Particle Physics & Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel
³ Department of Astronomy, San Diego State University, San Diego, CA 92182-1221, USA
⁴ Astronomy Department, California Institute of Technology, Pasadena, California 91125, USA
⁵ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
⁶ Las Cumbres Observatory Global Telescope, 6740 Cortona Dr, Suite 102, Goleta, CA 93117, USA
⁷ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
⁸ The Observatories, Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA
⁹ Spitzer Science Center, California Institute of Technology, M/S 314-6, Pasadena, CA 91125, USA
¹⁰ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
¹¹ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
¹² Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H4, Canada
¹³ Department of Astronomy, University of Texas, Austin, TX 78712, USA
¹⁴ Los Alamos National Laboratory, MS D436, Los Alamos, NM 87545, USA

Received: 23 November 2015
Accepted: 23 January 2016

Abstract

Context. Supernova (SN) 1987A was a peculiar hydrogen-rich event with a long-rising (~84 d) light curve, stemming from the explosion of a compact blue supergiant star. Only a few similar events have been presented in the literature in recent decades.

Aims. We present new data for a sample of six long-rising Type II SNe (SNe II), three of which were discovered and observed by the Palomar Transient Factory (PTF) and three observed by the Caltech Core-Collapse Project (CCCP). Our aim is to enlarge this small family of long-rising SNe II, characterizing their differences in terms of progenitor and explosion parameters. We also study the metallicity of their environments.

Methods. Optical light curves, spectra, and host-galaxy properties of these SNe are presented and analyzed. Detailed comparisons with known SN 1987A-like events in the literature are shown, with particular emphasis on the absolute magnitudes, colors, expansion velocities, and host-galaxy metallicities. Bolometric properties are derived from the multiband light curves. By modeling the early-time emission with scaling relations derived from the SuperNova Explosion Code (SNEC) models of MESA progenitor stars, we estimate the progenitor radii of these transients. The modeling of the bolometric light curves also allows us to estimate other progenitor and explosion parameters, such as the ejected ⁵⁶Ni mass, the explosion energy, and the ejecta mass.

Results. We present PTF12kso, a long-rising SN II that is estimated to have the largest amount of ejected ⁵⁶Ni mass measured for this class. PTF09gpn and PTF12kso are found at the lowest host metallicities observed for this SN group. The variety of early light-curve luminosities depends on the wide range of progenitor radii of these SNe, from a few tens of R_⊙ (SN 2005ci) up to thousands (SN 2004ek) with some intermediate cases between 100 R_⊙ (PTF09gpn) and 300 R_⊙ (SN 2004em).

Conclusions. We confirm that long-rising SNe II with light-curve shapes closely resembling that of SN 1987A generally arise from blue supergiant (BSG) stars. However, some of them, such as SN 2004em, likely have progenitors with larger radii (~300 R_⊙, typical of yellow supergiants) and can thus be regarded as intermediate cases between normal SNe IIP and SN 1987A-like SNe. Some extended red supergiant (RSG) stars such as the progenitor of SN 2004ek can also produce long-rising SNe II if they synthesized a large amount of ⁵⁶Ni in the explosion. Low host metallicity is confirmed as a characteristic of the SNe arising from compact BSG stars.