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Oxygen and helium in stripped-envelope supernovae
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We present a spectral analysis of 513 spectra of 176 stripped-envelope (SE) supernovae (SNe) discovered by the untargeted Palomar Transient Factory (PTF) and intermediate PTF (iPTF) surveys. Our sample contains 44 Type Ib SNe, 55 Type IIb SNe, 59 Type Ic SNe and 18 Type Ib/c SNe. We aim to identify differences among the SE SN subtypes in order to put constraints on their progenitors via measurements of the pseudo equivalent widths (pEWs) and velocities of the He I λλ5876,7065, and O I λ7774 lines. 

We find that Type Ic SNe show higher pEWs and velocities in their O I λ7774 absorption signatures compared to Type IIb and Type Ib SNe. The pEWs of the He I λλ5876,7065 absorption lines are similar in Type Ib and Type IIb SNe, with a few Type Ib SNe showing stronger absorptions prior to maximum light in the He I λ5876 line. The He I λλ5876,7065 velocities at maximum light are higher.

We have also identified an anti-correlation between He I λ7065 pEW and O I λ7774 velocity among Type IIb and Type Ib SNe. If both Type Ib and Type Ic SNe had similar amounts of He, and if low mixing was responsible for hiding He in Type Ic SNe, systematically lower velocities and lower oxygen line strengths are predicted for Type Ic SNe. This is contrary to what we have observed. If He is not hidden, it must have been lost in Type Ic SNe. The anti-correlation for Type Ib and Type IIb SNe then suggests that varying amounts of He can be present the time of explosion. The observed difference in the average O I λ7774 velocity around maximum light of Type Ic (~9800 km s-1) and Type Ib SNe (~7900 km s-1) can be explained by the removal of ~0.9 solar masses from the envelope, on average, if the other explosion parameters are assumed to be unchanged between the subtypes.

Keyword [en]
supernovae
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-145015OAI: oai:DiVA.org:su-145015DiVA: diva2:1119800
Available from: 2017-07-04 Created: 2017-07-04 Last updated: 2017-07-20Bibliographically approved
In thesis
1. Stripped-envelope supernovae discovered by the Palomar Transient Factory
Open this publication in new window or tab >>Stripped-envelope supernovae discovered by the Palomar Transient Factory
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is based on research made by the intermediate Palomar Transient Factory [(i)PTF]. The focus is on stripped-envelope (SE) supernovae (SNe) discovered by (i)PTF, and it is closely tied to the research on the SE SN iPTF13bvn, that occurred in the nearby galaxy NGC 5806. This SN was initially thought to have been the explosion of a very massive Wolf-Rayet star, but we have shown that this is very likely not the case. We suggest instead that iPTF13bvn originated from a binary system where the envelope was stripped off from the SN progenitor by tidal forces from a companion (Paper I). PTF12os exploded in the same galaxy as iPTF13bvn, and our analysis shows that PTF12os and  iPTF13bvn were very similar, and that both were also remarkably similar to the Type IIb SN 2011dh, in terms of their light-curves and spectra. In Paper II, hydrodynamical models were used to constrain the explosion parameters of iPTF13bvn, PTF12os and SN 2011dh; finding 56Ni masses in the range 0.063-0.075 solar masses (Ms), ejecta masses in the range 1.85-1.91 Ms, and kinetic energies in the range 0.54-0.94 x 1051 erg. Furthermore, using nebular models and late-time spectroscopy we were able to constrain the Zero-Age Main Sequence (ZAMS) mass to ~ 12 Ms, for iPTF13bvn and ≤ 15 Ms for PTF12os. In current stellar evolution models, stars with these masses on the ZAMS cannot lose their envelopes and become SE SNe without binary interactions. In Paper III we investigate a peculiar SE SN, iPTF15dtg; this SN lacks both hydrogen and helium and shows a double-peaked LC with a broad main LC peak. Using hydrodynamical modeling we show that iPTF15dtg had a very large ejecta mass (~ 10 Ms), resulting from an explosion of a very massive star (~ 35 Ms). The initial peak in the LC can be explained by the presence of extended material around the star, likely due to an episode of strong mass-loss experienced by the progenitor prior to the explosion. In Paper IV we perform a statistical study of the spectra of all 176 SE SNe (Type IIb, Ib and Ic) discovered by (i)PTF. The spectra of Type Ic SNe show O absorption features that are both stronger and broader (indicating faster expansion velocities) compared to Type IIb and Type Ib SNe. These findings along with very weak He absorption support the traditional picture with Type Ic SNe being heavily stripped of their He envelopes prior to the explosions, and argue against alternative explanations, such as differences in explosive mixing of 56Ni among the SE SN subtypes.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2017. 110 p.
Keyword
supernovae, transients
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-145016 (URN)978-91-7649-929-0 (ISBN)978-91-7649-930-6 (ISBN)
Public defence
2017-09-08, sal FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.

Available from: 2017-08-16 Created: 2017-07-04 Last updated: 2017-08-17Bibliographically approved

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