Studying the small scale ISM structure with supernovae

Abstract

<i>Aims. <i/>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.<i>Methods. <i/>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.<i>Results. <i/>The calculations show that isolated, small (<i>≤<i/>100 AU) clouds with column densities exceeding a few 10<sup>11<sup/> cm<sup>-2<sup/> 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 ~10<sup>12<sup/> cm<sup>-2<sup/> on a scale of 1000 AU.<i>Conclusions. <i/>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.