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Early observations of the nearby type Ia supernova SN 2015F

Cartier, R and Sullivan, M and Firth, R and Pignata, G and Mazzali, PA and Maguire, K and Childress, MJ and Arcavi, I and Ashall, C and Bassett, B and Crawford, SM and Frohmaier, C and Galbany, L and Gal-Yam, A and Hosseinzadeh, G and Howell, DA and Inserra, C and Johansson, J and Kasai, EK and McCully, C and Prajs, S and Prentice, SJ and Schulze, S and Smartt, SJ and Smith, KW and Smith, M and Valenti, S and Young, DR (2017) Early observations of the nearby type Ia supernova SN 2015F. Monthly Notices of the Royal Astronomical Society, 464 (4). pp. 4476-4494. ISSN 0035-8711

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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.

Item Type: Article
Additional Information: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Uncontrolled Keywords: astro-ph.SR; astro-ph.SR; astro-ph.CO
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Divisions: Astrophysics Research Institute
Publisher: Oxford University Press
Related URLs:
Date Deposited: 16 Dec 2016 09:12
Last Modified: 06 Sep 2017 20:36
DOI or Identification number: 10.1093/mnras/stw2678
URI: http://researchonline.ljmu.ac.uk/id/eprint/5105

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