Two teams of astronomers have finally found the long-sought progenitor star to a Type Ic supernova by sifting through Hubble archival data.
An artist’s concept of a blue supergiant star that once existed inside a cluster of young stars in the spiral galaxy NGC 3938. Image credit: NASA / ESA / J. Olmsted, STScI.
A Type Ic supernova is thought to detonate after a massive star has shed or been stripped of its outer layers of hydrogen and helium. These stars are among the most massive known — at least 30 times more massive than our own Sun. Even after shedding some of their material late in life, they remain very large and bright.
So it was a mystery as to why astronomers had not been able to nab one of these stars in pre-explosion images.
Finally, in 2017, they got lucky — a nearby star exploded as a Type Ic supernova.
Two teams of astronomers pored through the archive of Hubble images to uncover the presumed precursor star in pre-explosion photos taken in 2007.
The supernova, catalogued as SN 2017ein, appeared near the center of NGC 3938, a spiral galaxy located roughly 65 million light-years away.
This discovery could yield important insights into stellar evolution, including how the masses of stars are distributed when they are born in batches.
“Finding a bona fide progenitor of a Type Ic supernova is a big prize of progenitor searching. We now have for the first time a clearly detected candidate object,” said Caltech astronomer Dr. Schuyler Van Dyk, leader of one of the teams and first author of a paper published in the Astrophysical Journal.
This Hubble image of the spiral galaxy NGC 3938 shows the location of SN 2017ein, in a spiral arm near the bright core. Image credit: NASA / ESA / S. Van Dyk, Caltech / W. Li, University of California.
The second team, headed by University of California, Santa Cruz’s Dr. Charles Kilpatrick, observed SN 2017ein in June 2017 in infrared images from the OH-Suppressing Infrared Imaging Spectrograph (OSIRIS) on the 10-m Keck-I telescope at W.M. Keck Observatory in Hawaii.
The astronomers then analyzed the same archival Hubble photos as Dr. Van Dyk’s team to uncover the possible source. An analysis of the object’s colors shows that it is blue and extremely hot.
“This supernova occurred in a crowded part of its host galaxy. When we looked at a pre-explosion Hubble image, the stars appeared closely packed together,” said Dr. Kilpatrick, first author of a paper published in the Monthly Notices of the Royal Astronomical Society.
“We were fortunate that the supernova was nearby and very bright, about 5 to 10 times brighter than other Type Ic supernovas, which may have made the progenitor easier to find.”
Because the object is blue and exceptionally hot, both teams suggest two possibilities for the source’s identity.
The progenitor could be a single hefty star between 45 and 55 times more massive than our Sun. Another idea is that it could have been a massive binary-star system in which one of the stars weighs between 60 and 80 solar masses and the other roughly 48 solar masses. In this latter scenario, the stars are orbiting closely and interact with each other.
The more massive star is stripped of its hydrogen and helium layers by the close companion, and eventually explodes as a supernova. The possibility of a massive double-star system is a surprise.
“This is not what we would expect from current models, which call for lower-mass interacting binary progenitor systems,” Dr. Van Dyk said.
_____
Schuyler D. Van Dyk et al. 2018. SN 2017ein and the Possible First Identification of a Type Ic Supernova Progenitor. ApJ 860, 90; doi: 10.3847/1538-4357/aac32c
Charles D. Kilpatrick et al. 2018. A Potential Progenitor for the Type Ic Supernova 2017ein. MNRAS 480 (2): 2072-2084; doi: 10.1093/mnras/sty2022
