Multi-epoch high-resolution spectroscopy of SN 2011fe

Linking the progenitor to its environment^⋆

¹ European Organization for Astronomical Research in the Southern Hemisphere (ESO), Karl-Schwarzschild-Str. 2, 85748, Garching b. München, Germany
e-mail: fpatat@eso.org
² Astrochemistry Laboratory and the Goddard Center for Astrobiology, Mailstop 691, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20770, USA
³ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium
⁴ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁵ Fundación Galileo Galilei - Telescopio Nazionale Galileo, Rambla José Ana Fernández Pérez 7, 38712 Breña Baja, TF, Spain
⁶ Astrophysics Research Center, School of Mathematics and Physics, Queens University Belfast, Belfast, BT7 1NN, UK
⁷ CENTRA – Centro Multidisciplinar de Astrofísica, Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
⁸ INAF, Osservatorio Astronomico di Padova, via dell’Osservatorio 8, 36012 Asiago (VI), Italy
⁹ INAF, Osservatorio Astronomico di Padova, v. Osservatorio n. 5, 35122 Padua, Italy
¹⁰ Albanova University Center, Department of Physics, Stockholm University Roslagstullsbacken 21, 106 91 Stockholm, Sweden

Received: 1 December 2011
Accepted: 12 July 2012

Abstract

Aims. The nearby Type Ia supernova (SN) 2011fe has provided an unprecedented opportunity for deriving some of the properties of its progenitor. This work provides additional and independent information on the circumstellar environment in which the explosion took place.

Methods. We obtained high-resolution spectroscopy of SN 2011fe for 12 epochs, from 8 to 86 days after the estimated date of explosion, testing in particular the time evolution of Ca ii and Na i.

Results. Three main absorption systems are identified from Ca ii and Na i, one associated to the Milky Way, one probably arising within a high-velocity cloud, and one most likely associated to the halo of M101. The total (Galactic and host galaxy) reddening, deduced from the integrated equivalent widths (EW) of the Na i lines, is E_B − V ≲ 0.05 mag. The host galaxy absorption is dominated by a component detected at the same velocity measured from the 21-cm H i line at the projected SN position (~180 km s^-1). During the  ~3 months covered by our observations its EW peak-to-peak variation is 15.6 ± 6.5 mÅ. This small and marginally significant change is shown to be compatible with the geometric effects produced by the rapid SN photosphere expansion coupled to the patchy fractal structure of the interstellar medium (ISM). The observed behavior is fully consistent with ISM properties similar to those derived for our own Galaxy, with evidences for structures on scales  ≲ 100 AU.

Conclusions. SN 2011fe appears to be surrounded by a “clean” environment. The lack of blueshifted, time-variable absorption features is fully consistent with the progenitor being a binary system with a main-sequence, or even another degenerate star.