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Published January 1, 2021 | Version v1
Journal article Open

Calcium-rich Transient SN 2019ehk in a Star-forming Environment: Yet Another Candidate for a Precursor of a Double Neutron-star Binary

Creators

  • 1. Kyoto Univ, Dept Astron, Sakyo Ku, Kitashirakawa Oiwake Cho, Kyoto 6068502, Japan
  • 2. Kyoto Univ, Okayama Observ, 3037-5 Honjo, Okayama 7190232, Japan
  • 3. Tohoku Univ, Grad Sch Sci, Astron Inst, Aoba Ku, Sendai, Miyagi 9808578, Japan
  • 4. Hiroshima Univ, Dept Phys Sci, Kagamiyama 1-3-1, Higashihiroshima 7398526, Japan
  • 5. Univ Tokyo, Grad Sch Sci, Inst Astron, 2-21-1 Osawa, Mitaka, Tokyo 1810015, Japan
  • 6. European Southern Observ, Karl Schwarzschild Str 2, D-85748 Garching, Germany
  • 7. Univ Tokyo, Inst Adv Study, Kavli Inst Phys & Math Universe WPI, 5-1-5 Kashiwanoha, Kashiwa, Chiba 2778583, Japan

Description

We present optical and near-infrared observations of SN 2019ehk, which was initially reported as a Type Ib supernova (SN). We show that it evolved to a Ca-rich transient according to its spectral properties and evolution in late phases. However, it shows a few properties distinct from those of the canonical Ca-rich transients: a short-duration first peak in the light curve, high peak luminosity, and association with a star-forming environment. Indeed, some of these features are shared with iPTF14gqr and iPTF16hgs, which are candidates for a special class of core-collapse SNe: the so-called ultra-stripped envelope SNe, i.e., a relatively low-mass He (or C+O) star explosion in a binary as a precursor of short-period double neutron star (NS) binaries. The estimated ejecta mass (0.4M (circle dot)) and explosion energy (1.7 x 10(50) erg) are consistent with this scenario. The analysis of the first peak suggests the existence of dense circumstellar material in the vicinity of the progenitor, implying a CCSN origin. Based on this analysis, we suggest SN 2019ehk is another candidate for a low-mass He star explosion. It might create a double NS binary, but with a wide separation. These candidates for low-mass stripped envelope SNe, including ultra-stripped envelope SN candidates, seem to form a subpopulation among Ca-rich transients, associated with young population. We propose that the key to distinguishing this population is the early first peak in their light curves.

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