Linear spectropolarimetry of 35 Type Ia supernovae with VLT/FORS: an analysis of the Si$$\small {II}$$ line polarization

Journal Article · 30 August 2019 · Monthly Notices of the Royal Astronomical Society

DOI: https://doi.org/10.1093/mnras/stz2322 · OSTI ID:1572871

Cikota, Aleksandar  ^[1]; Patat, Ferdinando ^[2]; Wang, Lifan ^[3]; Wheeler, J. Craig  ^[4]; Bulla, Mattia  ^[5]; Baade, Dietrich ^[2]; Höflich, Peter ^[6]; Cikota, Stefan ^[7]; Clocchiatti, Alejandro ^[8]; Maund, Justyn R.  ^[9]; Stevance, Heloise F.  ^[9]; Yang, Yi ^[10]

---

1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Physics Division; European Southern Observatory, Garching bei München (Germany)
2. European Southern Observatory, Garching bei München (Germany)
3. Texas A&M Univ., College Station, TX (United States). Dept. of Physics
4. Univ. of Texas, Austin, TX (United States). Dept. of Astronomy
5. Stockholm Univ. (Sweden). Dept. of Physics, Oskar Klein Centre
6. Florida State Univ., Tallahassee, FL (United States). Dept. of Physics
7. Univ. of Zagreb (Croatia). Dept. of Applied Physics, Faculty of Electrical Engineering and Computing
8. Univ. Católica de Chile, Santiago (Chile). Inst. of Astrophysics; Millennium Inst. of Astrophysics, Santiago (Chile)
9. Univ. of Sheffield (United Kingdom). Dept. of Physics and Astronomy
10. Weizmann Inst. of Science, Rehovot (Israel). Dept. of Particle Physics and Astrophysics

Spectropolarimetry enables us to measure the geometry and chemical structure of the ejecta in supernova explosions, which is fundamental for the understanding of their explosion mechanism(s) and progenitor systems. We collected archival data of 35 Type Ia supernovae (SNe Ia), observed with Focal Reducer and Low-Dispersion Spectrograph (FORS) on the Very Large Telescope at 127 epochs in total. We examined the polarization of the Si ιι λ6355 Å line ($$p_{\rm Si\, \small {II}}$$) as a function of time, which is seen to peak at a range of various polarization degrees and epochs relative to maximum brightness. We reproduced the $$\Delta m_{15}\!-\!p_{\rm Si\, \small {II}}$$ relationship identified in a previous study, and show that subluminous and transitional objects display polarization values below the $$\Delta m_{15}\!-\!p_{\rm Si\, \small {II}}$$ relationship for normal SNe Ia. We found a statistically significant linear relationship between the polarization of the Si ii λ6355 Å line before maximum brightness and the Si ιι line velocity and suggest that this, along with the $$\Delta m_{15}\!-\!p_{\rm Si\, \small {II}}$$ relationship, may be explained in the context of a delayed-detonation model. In contrast, we compared our observations to numerical predictions in the $$\Delta m_{15}\!-\!v_{\rm Si\, \small {II}}$$ plane and found a dichotomy in the polarization properties between Chandrasekhar and sub-Chandrasekhar mass explosions, which supports the possibility of two distinct explosion mechanisms. A subsample of SNe displays evolution of loops in the q–u plane that suggests a more complex Si structure with depth. This insight, which could not be gleaned from total flux spectra, presents a new constraint on explosion models. To conclude, we compared our statistical sample of the Si ιι polarization to quantitative predictions of the polarization levels for the double-detonation, delayed-detonation, and violent-merger models.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1572871
• Journal Information: Monthly Notices of the Royal Astronomical Society, Journal Name: Monthly Notices of the Royal Astronomical Society Journal Issue: 1 Vol. 490; ISSN 0035-8711
• Publisher: Royal Astronomical SocietyCopyright Statement
• Country of Publication: United States
• Language: English