Properties of extragalactic dust inferred from linear polarimetry of Type Ia Supernovae

摘要

Aims: The aim of this paper is twofold: 1) to investigate the properties ofextragalactic dust and compare them to what is seen in the Galaxy; 2) toaddress in an independent way the problem of the anomalous extinction curvesreported for reddened Type Ia Supernovae (SN) in connection to the environmentsin which they explode. Methods: The properties of the dust are derived from thewavelength dependence of the continuum polarization observed in four reddenedType Ia SN: 1986G, 2006X, 2008fp, and 2014J. [...] Results: All four objectsare characterized by exceptionally low total-to-selective absorption ratios(R_V) and display an anomalous interstellar polarization law, characterized byvery blue polarization peaks. In all cases the polarization position angle iswell aligned with the local spiral structure. While SN~1986G is compatible withthe most extreme cases of interstellar polarization known in the Galaxy,SN2006X, 2008fp, and 2014J show unprecedented behaviours. The observeddeviations do not appear to be connected to selection effects related to therelatively large amounts of reddening characterizing the objects in the sample.Conclusions: The dust responsible for the polarization of these four SN is mostlikely of interstellar nature. The polarization properties can be interpretedin terms of a significantly enhanced abundance of small grains. The anomalousbehaviour is apparently associated with the properties of the galacticenvironment in which the SN explode, rather than with the progenitor systemfrom which they originate. For the extreme case of SN2014J, we cannot excludethe contribution of light scattered by local material; however, the observedpolarization properties require an ad hoc geometrical dust distribution.