Studying the small scale ISM structure with supernovae

¹ European Organization for Astronomical Research in the Southern Hemisphere (ESO), K. Schwarzschild-str. 2, 85748 Garching b. München, Germany e-mail: fpatat@eso.org
² Herschel Science Centre, European Space Astronomy Centre, ESA, PO Box 78, 28691 Madrid, Spain
³ Institute of Astronomy, K.U. Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁴ Department of Physics and Astronomy. University of Oklahoma, Norman, OK 73019, USA
⁵ Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755-3528, Germany

Received: 23 December 2009
Accepted: 25 February 2010

Abstract

Aims. In this work we explore the possibility of using the fast expansion of a type Ia supernova photosphere to detect extra-galactic ISM column density variations on spatial scales of ~100 AU on time scales of a few months.

Methods. We constructed a simple model which describes the expansion of the photodisk and the effects of a patchy interstellar cloud on the observed equivalent width of D lines. Using this model we derived the behavior of the equivalent width as a function of time, spatial scale and amplitude of the column density fluctuations.

Results. The calculations show that isolated, small (≤100 AU) clouds with column densities exceeding a few 10¹¹ cm^-2 would be easily detected. In contrast, the effects of a more realistic, patchy ISM become measurable in a fraction of cases, and for peak-to-peak variations larger than ~10¹² cm^-2 on a scale of 1000 AU.

Conclusions. The proposed technique provides a unique way to probe the extra-galactic small scale structure, which is out of reach for any of the methods used so far. The same tool can also be applied to study the sub-AU Galactic ISM structure.