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arXiv · 1609.04465

Early observations of the nearby type Ia supernova SN 2015F

R. Cartier·M. Sullivan·R. Firth·G. Pignata·P. Mazzali·K. Maguire·M. J. Childress·I. Arcavi·C. Ashall·B. Bassett·S. M. Crawford·C. Frohmaier·L. Galbany·A. Gal-Yam·G. Hosseinzadeh·D. A. Howell·C. Inserra·J. Johansson·E. K. Kasai·C. McCully·S. Prajs·S. Prentice·S. Schulze·S. J. Smartt·K. W. Smith·M. Smith·S. Valenti·D. R. Young
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Abstract

We present photometry and time-series spectroscopy of the nearby type Ia supernova (SN Ia) SN 2015F over $-16$ days to $+80$ days relative to maximum light, obtained as part of the Public ESO Spectroscopic Survey of Transient Objects (PESSTO). SN 2015F is a slightly sub-luminous SN Ia with a decline rate of $\Delta m15(B)=1.35 \pm 0.03$ mag, placing it in the region between normal and SN 1991bg-like events. Our densely-sampled photometric data place tight constraints on the epoch of first light and form of the early-time light curve. The spectra exhibit photospheric C II $\lambda 6580$ absorption until $-4$ days, and high-velocity Ca II is particularly strong at $<-10$ days at expansion velocities of $\simeq$23000\kms. At early times, our spectral modelling with syn++ shows strong evidence for iron-peak elements (Fe II, Cr II, Ti II, and V II) expanding at velocities $>14000$ km s$^{-1}$, suggesting mixing in the outermost layers of the SN ejecta. Although unusual in SN Ia spectra, including V II in the modelling significantly improves the spectral fits. Intriguingly, we detect an absorption feature at $\sim$6800 \AA\ that persists until maximum light. Our favoured explanation for this line is photospheric Al II, which has never been claimed before in SNe Ia, although detached high-velocity C II material could also be responsible. In both cases the absorbing material seems to be confined to a relatively narrow region in velocity space. The nucleosynthesis of detectable amounts of Al II would argue against a low-metallicity white dwarf progenitor. We also show that this 6800 \AA\ feature is weakly present in other normal SN Ia events, and common in the SN 1991bg-like sub-class.

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R. Cartier, M. Sullivan, R. Firth, G. Pignata, P. Mazzali, K. Maguire, M. J. Childress, I. Arcavi, C. Ashall, B. Bassett, S. M. Crawford, C. Frohmaier, L. Galbany, A. Gal-Yam, G. Hosseinzadeh, D. A. Howell, C. Inserra, J. Johansson, E. K. Kasai, C. McCully, S. Prajs, S. Prentice, S. Schulze, S. J. Smartt, K. W. Smith, M. Smith, S. Valenti, D. R. Young. 2016-09-14. Early observations of the nearby type Ia supernova SN 2015F. https://doi.org/10.1093/mnras/stw2678

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