Evidence for a Chandrasekhar-mass explosion in the Ca-strong 1991bg-like type Ia supernova 2016hnk^⋆

¹ PITT PACC, Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260, USA
e-mail: llgalbany@pitt.edu, lluisgalbany@gmail.com
² Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain
³ Department of Physics, Florida State University, Tallahassee, FL 32306, USA
⁴ CENTRA/COSTAR, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
⁵ European Southern Observatory, Karl-Schwarzschild-Straeß2, 85748 Garching bei München, Germany
⁶ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching bei München, Germany
⁷ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁸ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁹ Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK, USA
¹⁰ Aix Marseille Univ., CNRS, CNES, Laboratoire d’Astrophysique de Marseille (LAM), Marseille, France
¹¹ Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU UMI 3386, and Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
¹² Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Road 5, Hai Dian District, Beijing 100871, PR China
¹³ The Oskar Klein Centre, Department of Physics, Stockholm University, AlbaNova 106 91, Stockholm, Sweden
¹⁴ Las Cumbres Observatory, 6740 Cortona Dr. Suite 102, Goleta, CA 93117, USA
¹⁵ University of California, Santa Barbara, Department of Physics, Broida Hall, Santa Barbara, CA 93111, USA
¹⁶ The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA
¹⁷ Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, Casilla 603, La Serena, Chile
¹⁸ Department of Astronomy, School of Physics, Peking University, Yi He Yuan Road 5, Hai Dian District, Beijing 100871, PR China
¹⁹ Capodimonte Observatory, INAF-Naples, Salita Moiariello 16, 80131 Naples, Italy
²⁰ Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²¹ School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
²² Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Turku 20014, Finland
²³ Harvard-Smithsonian Center for Astrohysics, 60 Garden Street, Cambridge, MA 02138, USA
²⁴ University of California, La Jolla, San Diego, CA 92093, USA
²⁵ School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
²⁶ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
²⁷ Department of Physics and Astronomy, Rutgers the State University of New Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854, USA
²⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²⁹ Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik, Philosophenweg 12, 69120 Heidelberg, Germany
³⁰ Heidelberger Institut für Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
³¹ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
³² Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
³³ School of Physics, Trinity College Dublin, Dublin 2, Ireland
³⁴ University of Texas at Austin, 1 University Station C1400, Austin, TX 78712-0259, USA
³⁵ Department of Physics and Astronomy, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907, USA
³⁶ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile
³⁷ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
³⁸ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA
³⁹ Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
⁴⁰ Department of Physics, BITS Pilani K.K. Birla Goa Campus, India
⁴¹ Vanderbilt University, Department of Physics & Astronomy, Nashville, TN 37235, USA
⁴² Department of Physics, University of California, Davis, CA 95616, USA
⁴³ Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁴⁴ Physics Department/Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084, PR China
⁴⁵ Department of Astronomy, University of Texas at Austin, Austin, TX 78712, USA
⁴⁶ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
⁴⁷ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, PR China

Received: 20 March 2019
Accepted: 5 August 2019

Abstract

Aims. We present a comprehensive dataset of optical and near-infrared photometry and spectroscopy of type Ia supernova (SN) 2016hnk, combined with integral field spectroscopy (IFS) of its host galaxy, MCG -01-06-070, and nearby environment. Our goal with this complete dataset is to understand the nature of this peculiar object.

Methods. Properties of the SN local environment are characterized by means of single stellar population synthesis applied to IFS observations taken two years after the SN exploded. We performed detailed analyses of SN photometric data by studying its peculiar light and color curves. SN 2016hnk spectra were compared to other 1991bg-like SNe Ia, 2002es-like SNe Ia, and Ca-rich transients. In addition, we used abundance stratification modeling to identify the various spectral features in the early phase spectral sequence and also compared the dataset to a modified non-LTE model previously produced for the sublumnious SN 1999by.

Results. SN 2016hnk is consistent with being a subluminous (M_B = −16.7 mag, s_BV=0.43 ± 0.03), highly reddened object. The IFS of its host galaxy reveals both a significant amount of dust at the SN location, residual star formation, and a high proportion of old stellar populations in the local environment compared to other locations in the galaxy, which favors an old progenitor for SN 2016hnk. Inspection of a nebular spectrum obtained one year after maximum contains two narrow emission lines attributed to the forbidden [Ca II] λλ7291,7324 doublet with a Doppler shift of 700 km s⁻¹. Based on various observational diagnostics, we argue that the progenitor of SN 2016hnk was likely a near Chandrasekhar-mass (M_Ch) carbon-oxygen white dwarf that produced 0.108 M_⊙ of ⁵⁶Ni. Our modeling suggests that the narrow [Ca II] features observed in the nebular spectrum are associated with ⁴⁸Ca from electron capture during the explosion, which is expected to occur only in white dwarfs that explode near or at the M_Ch limit.