Although small in number, massive stars are critical to the formation and evolution of galaxies. They shape the interstellar medium of
galaxies through their strong winds and ultra-violet radiation, are a
major source of the heavy elements enriching the interstellar medium,
and are the progenitors of core-collapse supernovae and gamma-ray
bursts, which are among the most energetic explosions in the Universe
and mark the death of a massive star. Still, our understanding of the
connection between massive stars and supernovae from observations is
fairly limited. In this dissertation, I present new observational
evidence that shows the importance of metallicity, mass-loss, and
binarity in the lives and deaths of massive stars.
We investigate how the different types of supernovae are relatively
affected by the metallicity of their host galaxy. We take advantage of
the large number of spectra of star-forming galaxies obtained by the
Sloan Digital Sky Survey and their overlap with supernova host
galaxies. We find strong evidence that type Ib/c supernovae are
occurring in higher-metallicity host galaxies than type II
supernovae. We discuss various implications of our findings for
understanding supernova progenitors and their host galaxies, including
interesting supernovae found in low-metallicity hosts.
We present the discovery of the progenitors of SN 2008S and the luminous
transient in NGC 300 in archival data obtained with the Spitzer Space
Telescope. They are deeply dust-enshrouded massive stars, with extremely
red mid-infrared colors compared to other massive stars, and relatively
low bolometric luminosities ≈5x104 L☉. We discuss the implications of
these findings for the evolution and census of “low-mass” massive
stars (i.e., ~8-12 M☉), and we connect it with theoretical discussions
of electron-capture supernovae near this mass range, explosive birth of
massive white dwarfs, and massive star outbursts.
We present a Spitzer low-resolution mid-infrared spectrum of the
luminous transient in NGC 300. The spectrum shows that the transient is
very luminous in the mid-infrared and most of the pre-existing
progenitor dust survived the explosion. Furthermore, the spectrum shows
strong, broad emission features that are observed in Galactic
carbon-rich proto-planetary nebulae. These observations support our
conclusions of an explosive event on a massive carbon-rich AGB or
post-AGB star as the origin of the transient in NGC 300 and SN 2008S.
We present extensive ugriz YHJKs photometry and optical spectroscopy of SN
2005gj. These data show that SN 2005gj is the second possible case,
after SN 2002ic, of a thermonuclear explosion in a dense circumstellar
environment. The interaction of the supernova ejecta with the dense
circumstellar medium is stronger than in SN 2002ic.
Finally, we present the discovery of a peculiar eclipsing binary in a
variability survey of the dwarf galaxy Holmberg IX undertaken with the
Large Binocular Telescope. The binary has a period of 271 days, and is
composed of two yellow supergiants that are overflowing their Roche
lobes. Such systems must be rare, and indeed we only note one similar
system in the Small Magellanic Cloud. We propose that these systems may
be the progenitors of supernovae that appeared to have yellow-supergiant
progenitors.