Subdwarf B stars as possible surviving companions in Type Ia supernova remnants

Abstract: Although type Ia supernovae (SNe Ia) are so important in many astrophysical fields, a debate on their progenitor model is still endless. Searching the surviving companion in a supernova remnant (SNR) may distinguish different progenitor models, since a companion still exists in the remnant for the single-degenerate (SD) model, but does not for the double degenerate (DD) model. However, some recent surveys do not discover the surviving companions in the remnant of SN 1006 and Kepler's supernova, which seems to disfavor the SD model. Such a result could be derived from an incorrect survey target. Here, based on the common-envelope wind SD model for SNe Ia, in which the initial binary system consists of a white dwarf (WD) and a main-sequence (MS) star, we found that the companion at the moment of supernova explosion may be a MS, red-giant (RG) or subdwarf B (sdB) star if a spin-down timescale of less than $10^{\rm 7}$ yr is assumed. We show the properties of the companions at the moment of supernova explosion, which are key clues to search the surviving companions in SNRs. Here, we suggest that the sdB star may be the surviving companion in some SNRs, even if the progenitor systems are the WD + MS systems. The SNe Ia with the sdB companions may contribute to all SNe Ia as much as 22\%.