iPTF 16hgs: A Double-peaked Ca-rich Gap Transient in a Metal-poor, Star-forming Dwarf Galaxy

Creators: De, Kishalay; Kasliwal, Mansi M.; Cantwell, Therese; Cao, Yi; Cenko, S. Bradley; Gal-Yam, Avishay; Johansson, Joel; Kong, Albert; Kulkarni, Shrinivas R.; Lunnan, Ragnhild; Masci, Frank; Matuszewski, Matt; Mooley, Kunal P.; Neill, James D.; Nugent, Peter E.; Ofek, Eran O.; Perrott, Yvette; Rebbapragada, Umaa D.; Rubin, Adam; O'Sullivan, Donal; Yaron, Ofer

Abstract

Calcium-rich gap transients represent an intriguing new class of faint and fast-evolving supernovae that exhibit strong [Ca ii] emission in their nebular phase spectra. In this paper, we present the discovery and follow-up observations of a faint and fast-evolving transient, iPTF 16hgs, that exhibited a double-peaked light curve. Exhibiting a Type Ib spectrum in the photospheric phase and an early transition to a [Ca ii] dominated nebular phase, iPTF 16hgs shows properties consistent with the class of Ca-rich gap transients, with two important exceptions. First, while the second peak of the light curve is similar to other Ca-rich gap transients, the first blue and fast-fading peak (declining over ≈2 days) is unique to this source. Second, we find that iPTF 16hgs occurred in the outskirts (projected offset of ≈6 kpc ≈ 1.9 R_(eff)) of a low-metallicity (≈0.4 Z_⊙), star-forming, dwarf spiral galaxy. Deep limits from late-time radio observations suggest a low-density environment for the source. If iPTF 16hgs shares explosion physics with the class of Ca-rich gap transients, the first peak can be explained by the presence of 0.01 M_⊙ of ^(56)Ni in the outer layers the ejecta, reminiscent of some models of He-shell detonations on WDs. However, if iPTF 16hgs is physically unrelated to the class, the first peak is consistent with shock cooling emission (of an envelope with a mass of ≈0.08 M_⊙ and radius of ≈13 R_⊙) in a core-collapse explosion of a highly stripped massive star in a close binary system.

Additional Information

© 2018. The American Astronomical Society. Received 2018 June 26; revised 2018 September 2; accepted 2018 September 4; published 2018 October 15. We thank the referee Hagai Perets for a careful reading of the manuscript and providing valuable feedback that has improved its content. We thank C. Steidel, E. Kirby, K. Shen, T. Moriya, L. Bildsten, D. Kasen, A. Horesh, and N. Stone for valuable discussions. We thank Ken Shen, Takashi Moriya, and Stuart Sim for providing the comparison models presented in this paper. We thank Q. Ye, N. Blagorodnova, V. Ravi, S. Adams, and R. Lau for assisting with the observations presented in the paper. The Intermediate Palomar Transient Factory project is a scientific collaboration among the California Institute of Technology, Los Alamos National Laboratory, the University of Wisconsin, Milwaukee, the Oskar Klein Center, the Weizmann Institute of Science, the TANGO Program of the University System of Taiwan, and the Kavli Institute for the Physics and Mathematics of the Universe. This work was supported by the GROWTH (Global Relay of Observatories Watching Transients Happen) project funded by the National Science Foundation under PIRE Grant No. 1545949. GROWTH is a collaborative project among California Institute of Technology (USA), University of Maryland College Park (USA), University of Wisconsin Milwaukee (USA), Texas Tech University (USA), San Diego State University (USA), Los Alamos National Laboratory (USA), Tokyo Institute of Technology (Japan), National Central University (Taiwan), Indian Institute of Astrophysics (India), Indian Institute of Technology Bombay (India), Weizmann Institute of Science (Israel), The Oskar Klein Centre at Stockholm University (Sweden), Humboldt University (Germany), Liverpool John Moores University (UK). Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. These results made use of the Discovery Channel Telescope at Lowell Observatory. Lowell is a private, non-profit institution dedicated to astrophysical research and public appreciation of astronomy and operates the DCT in partnership with Boston University, the University of Maryland, the University of Toledo, Northern Arizona University, and Yale University. The upgrade of the DeVeny optical spectrograph has been funded by a generous grant from John and Ginger Giovale. We thank the staff of the Mullard Radio Astronomy Observatory for their invaluable assistance in the commissioning and operation of AMI, which is supported by Cambridge University and the European Research Council under grant ERC-2012-StG-307215 LODESTONE. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. We thank the staff of the GMRT that made these observations possible. The GMRT is run by the National Center for Radio Astrophysics of the Tata Institute of Fundamental Research. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Y.C.P. is supported by a Trinity College JRF. Facilities: PO 1.2 m - , PO 1.5 m - , DCT - , Hale (DBSP - , CWI) - , Keck-I (LRIS) - , Swift (XRT - , UVOT) - , AMI - , VLA - , uGMRT - . Software: HEAsoft (Arnaud 1996), BOXFIT code (van Eerten et al. 2010), pyMCZ code (Bianco et al. 2016), FAST code (Kriek et al. 2009), STARLIGHT code (Cid Fernandes et al. 2005), AMI-REDUCE (Davies et al. 2009), CASA (McMullin et al. 2007), Astropy (The Astropy Collaboration et al. 2018).

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