A metallicity study of 1987A-like supernova host galaxies^⋆,⋆⋆

¹ The Oskar Klein Centre, Department of AstronomyStockholm University, AlbaNova, 10691 Stockholm, Sweden
e-mail: francesco.taddia@astro.su.se
² INAF – Osservatorio Astronomico di Padova, Vicolo dellOsservatorio 5, 35122 Padova, Italy
³ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁴ The Oskar Klein Centre, Department of Physics, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
⁵ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark

Received: 14 July 2013
Accepted: 26 August 2013

Abstract

Context. The origin of the blue supergiant (BSG) progenitor of Supernova (SN) 1987A has long been debated, along with the role that its sub-solar metallicity played. We now have a sample of SN 1987A-like events that arise from the rare core collapse (CC) of massive (~20 M_⊙) and compact (≲100 R_⊙) BSGs.

Aims. The metallicity of the explosion sites of the known BSG SNe is investigated, as well as the association of BSG SNe to star-forming regions.

Methods. Both indirect and direct metallicity measurements of 13 BSG SN host galaxies are presented, and compared to those of other CC SN types. Indirect measurements are based on the known luminosity-metallicity relation and on published metallicity gradients of spiral galaxies. In order to provide direct metallicity measurements based on strong line diagnostics, we obtained spectra of each BSG SN host galaxy both at the exact SN explosion sites and at the positions of other H ii regions. We also observed these hosts with narrow Hα and broad R-band filters in order to produce continuum-subtracted Hα images. This allows us to measure the degree of association between BSG SNe and star-forming regions, and to compare it to that of other SN types.

Results. BSG SNe are found to explode either in low-luminosity galaxies or at large distances from the nuclei of luminous hosts. Therefore, their indirectly measured metallicities are typically lower than those of SNe IIP and Ibc. This result is confirmed by the direct metallicity estimates, which show slightly sub-solar oxygen abundances (12 + log (O/H) ~ 8.3–8.4 dex) for the local environments of BSG SNe, similar to that of the Large Magellanic Cloud (LMC), where SN 1987A exploded. However, we also note that two objects of our sample (SNe 1998A and 2004em) were found at near solar metallicity. SNe IIb have a metallicity distribution similar to that of our BSG SNe. Finally, we find that the degree of association to star-forming regions is similar among BSG SNe, SNe IIP and IIn.

Conclusions. Our results suggest that LMC metal abundances play a role in the formation of some 1987A-like SNe. This would naturally fit in a single star scenario for the progenitors. However, the existence of two events at nearly solar metallicity suggests that also other channels, e.g. binarity, contribute to produce BSG SNe.