Oxygen and helium in stripped-envelope supernovae

¹ Department of Astronomy, The Oskar Klein Center, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
e-mail: fremling@caltech.edu
² Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
³ Las Cumbres Observatory, 6740 Cortona Dr., Suite 102, Goleta, CA 93117, USA
⁴ Department of Physics, Broida Hall, University of California, Santa Barbara, CA 93106-9530, USA
⁵ NASA Goddard Space Flight Center, Mail Code 661, Greenbelt, MD 20771, USA
⁶ Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
⁷ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
⁸ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁹ Miller Senior Fellow, Miller Institute for Basic Research in Science, University of California, Berkeley, CA 94720, USA
¹⁰ Benoziyo Center for Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel
¹¹ Astronomy Department, University of Washington, Box 351580, U.W., Seattle, WA 98195-1580, USA
¹² Racah Institute of Physics, Hebrew University, Jerusalem 91904, Israel
¹³ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstraße 15, 12489 Berlin, Germany
¹⁴ National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
¹⁵ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 50B-4206, Berkeley, CA 94720, USA
¹⁶ Astrophysics Research Institute, Liverpool John Moores University, IC2 Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁷ Department of Astronomy, San Diego State University, San Diego, CA 92182, USA
¹⁸ Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
¹⁹ Laboratoire de Physique Nucléaire et de Hautes Énergies, Université Pierre et Marie Curie Paris 6, Université Paris Diderot Paris 7, CNRS-IN2P3, 4 place Jussieu, 75252 Paris Cedex 05, France

Received: 3 August 2017
Accepted: 3 July 2018

Abstract

We present an analysis of 507 spectra of 173 stripped-envelope (SE) supernovae (SNe) discovered by the untargeted Palomar Transient Factory (PTF) and intermediate PTF (iPTF) surveys. Our sample contains 55 Type IIb SNe (SNe IIb), 45 Type Ib SNe (SNe Ib), 56 Type Ic SNe (SNe Ic), and 17 Type Ib/c SNe (SNe Ib/c). We have compared the SE SN subtypes via measurements of the pseudo-equivalent widths (pEWs) and velocities of the He I λλ5876, 7065 and O I λ7774 absorption lines. Consistent with previous work, we find that SNe Ic show higher pEWs and velocities in O I λ7774 compared to SNe IIb and Ib. The pEWs of the He I λλ5876, 7065 lines are similar in SNe Ib and IIb after maximum light. The He I λλ5876, 7065 velocities at maximum light are higher in SNe Ib compared to SNe IIb. We identify an anticorrelation between the He I λ7065 pEW and O I λ7774 velocity among SNe IIb and Ib. This can be interpreted as a continuum in the amount of He present at the time of explosion. It has been suggested that SNe Ib and Ic have similar amounts of He, and that lower mixing could be responsible for hiding He in SNe Ic. However, our data contradict this mixing hypothesis. The observed difference in the expansion rate of the ejecta around maximum light of SNe Ic (V_m = √2E_k/M_ej ≈ 15 000 km s⁻¹) and SNe Ib (V_m ≈ 9000 km s⁻¹) would imply an average He mass difference of ∼1.4 M_⊙, if the other explosion parameters are assumed to be unchanged between the SE SN subtypes. We conclude that SNe Ic do not hide He but lose He due to envelope stripping.