Time-varying sodium absorption in the Type Ia supernova 2013gh^⋆

¹ Department of Physics, The Oskar Klein Centre, Stockholm University, Albanova, 106 92 Stockholm, Sweden
e-mail: raphael.ferretti@fysik.su.se
² Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 7610001 Rehovot, Israel
³ Department of Terrestrial Magnetism, Carnegie Institute of Washington, Washington, DC 20015, USA
⁴ Cahill Center for Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
⁵ Astrophysics Science Division, NASA Goddard Space Flight Center, Mail Code 661, Greenbelt, MD 20771, USA
⁶ Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
⁷ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
⁸ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
⁹ Department of Astronomy, The Oskar Klein Center, Stockholm University, Albanova, 10691 Stockholm, Sweden
¹⁰ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
¹¹ Las Cumbres Observatory Global Telescope Network, 6740 Cortona Dr., Suite 102, Goleta, CA 93117, USA
¹² Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
¹³ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
¹⁴ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
¹⁵ Humboldt-Universität zu Berlin, Institut für Physik, Newtonstrasse 15, 12589 Berlin, Germany
¹⁶ Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 50B-4206, Berkeley, CA 94720, USA
¹⁷ Department of Physics, University of California, One Shields Avenue, Davis, CA 95616, USA
¹⁸ UCO/Lick Observatory, Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
¹⁹ Los Alamos National Laboratory, MS D436, Los Alamos, NM 87545, USA

Received: 19 February 2016
Accepted: 2 May 2016

Abstract

Context. Temporal variability of narrow absorption lines in high-resolution spectra of Type Ia supernovae (SNe Ia) is studied to search for circumstellar matter. Time series which resolve the profiles of absorption lines such as Na I D or Ca II H&K are expected to reveal variations due to photoionisation and subsequent recombination of the gases. The presence, composition, and geometry of circumstellar matter may hint at the elusive progenitor system of SNe Ia and could also affect the observed reddening law.

Aims. To date, there are few known cases of time-varying Na I D absorption in SNe Ia, all of which occurred during relatively late phases of the supernova (SN) evolution. Photoionisation, however, is predicted to occur during the early phases of SNe Ia, when the supernovae peak in the ultraviolet. We attempt, therefore, to observe early-time absorption-line variations by obtaining high-resolution spectra of SNe before maximum light.

Methods. We have obtained photometry and high-resolution spectroscopy of SNe Ia 2013gh and iPTF 13dge, to search for absorption-line variations. Furthermore, we study interstellar absorption features in relation to the observed photometric colours of the SNe.

Results. Both SNe display deep Na I D and Ca II H&K absorption features. Furthermore, small but significant variations are detected in a feature of the Na I D profile of SN 2013gh. The variations are consistent with either geometric effects of rapidly moving or patchy gas clouds or photoionisation of Na I gas at R ≈ 10¹⁹ cm from the explosion.

Conclusions. Our analysis indicates that it is necessary to focus on early phases to detect photoionisation effects of gases in the circumstellar medium of SNe Ia. Different absorbers such as Na I and Ca II can be used to probe for matter at different distances from the SNe. The nondetection of variations during early phases makes it possible to put limits on the abundance of the species at those distances.