ASASSN-18tb: A Most Unusual Type Ia Supernova Observed by TESS and SALT. (arXiv:1903.08665v1 [astro-ph.HE])

ASASSN-18tb: A Most Unusual Type Ia Supernova Observed by TESS and SALT. (arXiv:1903.08665v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Vallely_P/0/1/0/all/0/1">P. J. Vallely</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fausnaugh_M/0/1/0/all/0/1">M. Fausnaugh</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jha_S/0/1/0/all/0/1">S. W. Jha</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tucker_M/0/1/0/all/0/1">M. A. Tucker</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Eweis_Y/0/1/0/all/0/1">Y. Eweis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shappee_B/0/1/0/all/0/1">B. J. Shappee</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kochanek_C/0/1/0/all/0/1">C. S. Kochanek</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stanek_K/0/1/0/all/0/1">K. Z. Stanek</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chen_P/0/1/0/all/0/1">Ping Chen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dong_S/0/1/0/all/0/1">Subo Dong</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Prieto_J/0/1/0/all/0/1">J. L. Prieto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sukhbold_T/0/1/0/all/0/1">T. Sukhbold</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Thompson_T/0/1/0/all/0/1">Todd A. Thompson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Brimacombe_J/0/1/0/all/0/1">J. Brimacombe</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Stritzinger_M/0/1/0/all/0/1">M. D. Stritzinger</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Holoien_T/0/1/0/all/0/1">T. W.-S. Holoien</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Buckley_D/0/1/0/all/0/1">D. A. H. Buckley</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gromadzki_M/0/1/0/all/0/1">M. Gromadzki</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bose_S/0/1/0/all/0/1">Subhash Bose</a>

We present photometric and spectroscopic observations of the unusual Type Ia
supernova ASASSN-18tb, including a series of SALT spectra obtained over the
course of nearly six months and the first observations of a supernova by the
Transiting Exoplanet Survey Satellite (TESS). We confirm a previous observation
by Kollmeier et al. (2019) showing that ASASSN-18tb is the first relatively
normal Type Ia supernova to exhibit clear broad ($sim1000$ km s$^{-1}$)
H$alpha$ emission in its nebular phase spectra. We find that this event is
best explained as a sub-Chandrasekhar mass explosion with $M_{Ni} approx
0.27; rm{M}_odot$. Despite the strong H$alpha$ signature at late times, we
find that the early rise of the supernova shows no evidence for deviations from
a single-component power-law and is best fit with a moderately shallow
power-law of index $1.40pm0.03$. We find that the H$alpha$ luminosity remains
approximately constant after its initial detection at phase +37 d, and that the
H$alpha$ velocity evolution does not trace that of the Fe~III$~lambda4660$
emission. These suggest that the H$alpha$ emission arises from circumstellar
medium (CSM) rather than swept up material from a non-degenerate companion.
However, ASASSN-18tb is strikingly different from other known CSM-interacting
Type Ia supernovae in a number of significant ways. Those objects typically
show an H$alpha$ luminosity two orders of magnitude higher than what is seen
in ASASSN-18tb, pushing them away from the empirical light-curve relations that
define “normal” Type Ia supernovae. Conversely, ASASSN-18tb exhibits a fairly
typical light curve and luminosity for an underluminous or transitional SN Ia,
with $M_R approx -18.1$ mag. Moreover, ASASSN-18tb is the only SN Ia showing
H$alpha$ from CSM interaction to be discovered in an early-type galaxy.

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