The peculiar Ca-rich SN 2019ehk: Evidence for a Type IIb core-collapse supernova from a low mass stripped progenitor. (arXiv:2009.02347v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+De_K/0/1/0/all/0/1">Kishalay De</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fremling_U/0/1/0/all/0/1">U. Christoffer Fremling</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gal_Yam_A/0/1/0/all/0/1">Avishay Gal-Yam</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Yaron_O/0/1/0/all/0/1">Ofer Yaron</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kasliwal_M/0/1/0/all/0/1">Mansi M. Kasliwal</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kulkarni_S/0/1/0/all/0/1">S. R. Kulkarni</a>
The nature of the peculiar `Ca-rich’ SN 2019ehk in the nearby galaxy M100
remains unclear. Its origin has been debated as either a stripped core-collapse
supernova or a thermonuclear helium detonation event. Here, we present very
late-time photometry of the transient obtained with the Keck I telescope at
$approx280$ days from peak light. Using the photometry to perform accurate
flux calibration of a contemporaneous nebular phase spectrum, we measure an [O
I] luminosity of $(0.19-1.08)times10^{38}$ erg s$^{-1}$ and [Ca II] luminosity
of $(2.7-15.6)times10^{38}$ erg s$^{-1}$ over the range of the uncertain
extinction along the line of sight. We use these measurements to derive lower
limits on the synthesized oxygen mass of $approx0.004-0.069$ M$_odot$. The
oxygen mass is a sensitive tracer of the progenitor mass for core-collapse
supernovae, and our estimate is consistent with explosions of very low mass CO
cores of $1.45-1.5$ M$_odot$, corresponding to He core masses of
$approx1.8-2.0$ M$_odot$. We present high quality peak light optical spectra
of the transient and highlight features of hydrogen in both the early (`flash’)
and photospheric phase spectra, that suggest the presence of $gtrsim0.02$
M$_odot$ of hydrogen in the progenitor at the time of explosion. The presence
of H, together with the large [Ca II]/[O I] ratio ($approx10-15$) in the
nebular phase is consistent with SN 2019ehk being a Type IIb core-collapse
supernova from a stripped low mass ($approx9-9.5$ M$_odot$) progenitor,
similar to the Ca-rich SN IIb iPTF 15eqv. These results provide evidence for a
likely class of `Ca-rich’ core-collapse supernovae from stripped low mass
progenitors in star forming environments, distinct from the thermonuclear
Ca-rich gap transients in old environments.
The nature of the peculiar `Ca-rich’ SN 2019ehk in the nearby galaxy M100
remains unclear. Its origin has been debated as either a stripped core-collapse
supernova or a thermonuclear helium detonation event. Here, we present very
late-time photometry of the transient obtained with the Keck I telescope at
$approx280$ days from peak light. Using the photometry to perform accurate
flux calibration of a contemporaneous nebular phase spectrum, we measure an [O
I] luminosity of $(0.19-1.08)times10^{38}$ erg s$^{-1}$ and [Ca II] luminosity
of $(2.7-15.6)times10^{38}$ erg s$^{-1}$ over the range of the uncertain
extinction along the line of sight. We use these measurements to derive lower
limits on the synthesized oxygen mass of $approx0.004-0.069$ M$_odot$. The
oxygen mass is a sensitive tracer of the progenitor mass for core-collapse
supernovae, and our estimate is consistent with explosions of very low mass CO
cores of $1.45-1.5$ M$_odot$, corresponding to He core masses of
$approx1.8-2.0$ M$_odot$. We present high quality peak light optical spectra
of the transient and highlight features of hydrogen in both the early (`flash’)
and photospheric phase spectra, that suggest the presence of $gtrsim0.02$
M$_odot$ of hydrogen in the progenitor at the time of explosion. The presence
of H, together with the large [Ca II]/[O I] ratio ($approx10-15$) in the
nebular phase is consistent with SN 2019ehk being a Type IIb core-collapse
supernova from a stripped low mass ($approx9-9.5$ M$_odot$) progenitor,
similar to the Ca-rich SN IIb iPTF 15eqv. These results provide evidence for a
likely class of `Ca-rich’ core-collapse supernovae from stripped low mass
progenitors in star forming environments, distinct from the thermonuclear
Ca-rich gap transients in old environments.
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