The underluminous Type Ia Supernova 2005bl and the class of objects similar to SN 1991bg

TL;DR

This paper studies the underluminous Type Ia supernova 2005bl, comparing it with similar peculiar supernovae, analyzing its spectral and photometric properties, and discussing implications for progenitor models.

Contribution

It provides detailed photometric and spectroscopic analysis of SN 2005bl, confirming its similarity to SN 1991bg and exploring its spectral features and progenitor implications.

Findings

SN 2005bl is underluminous with rapid decline and red colors.

Spectra show carbon presence and low Fe content, indicating low burning efficiency.

Correlations between decline rate and luminosity are confirmed.

Abstract

Optical observations of the Type Ia supernova (SN Ia) 2005bl in NGC 4070, obtained from -6 to +66 d with respect to the B-band maximum, are presented. The photometric evolution is characterised by rapidly-declining light curves and red colours at peak and soon thereafter. With M_B,max = -17.24 the SN is an underluminous SN Ia, similar to the peculiar SNe 1991bg and 1999by. This similarity also holds for the spectroscopic appearance, the only remarkable difference being the likely presence of carbon in pre-maximum spectra of SN 2005bl. A comparison study among underluminous SNe Ia is performed, based on a number of spectrophotometric parameters. Previously reported correlations of the light-curve decline rate with peak luminosity and R(Si) are confirmed, and a large range of post-maximum Si II lambda6355 velocity gradients is encountered. 1D synthetic spectra for SN 2005bl are presented, which confirm the presence of carbon and suggest an overall low burning efficiency with a significant amount of leftover unburned material. Also, the Fe content in pre-maximum spectra is very low, which may point to a low metallicity of the precursor. Implications for possible progenitor scenarios of underluminous SNe Ia are briefly discussed.