Collapsing Enormous Stars

One of the big puzzles in astrophysics is how supermassive black holes (SMBHs) managed to grow to the large sizes we’ve observed in the very early universe. In a recent study, a team of researchers examines the possibility that they were formed by the direct collapse of supermassive stars.

Formation Mystery

SMBHs billions of times as massive as the Sun have been observed at a time when the universe was less than a billion years old. But that’s not enough time for a stellar-mass black hole to grow to SMBH-size by accreting material — so another theory is needed to explain the presence of these monsters so early in the universe’s history. A new study, led by Tatsuya Matsumoto (Kyoto University, Japan), poses the following question: what if supermassive stars in the early universe collapsed directly into black holes?

Previous studies of star formation in the early universe have suggested that, in the hot environment of these primordial times, stars might have been able to build up mass much faster than they can today. This could result in early supermassive stars roughly 100,000 times more massive than the Sun. But if these early stars end their lives by collapsing to become massive black holes — in the same way that we believe massive stars can collapse to form stellar-mass black holes today — this should result in enormously violent explosions. Matusmoto and collaborators set out to model this process, to determine what we would expect to see when it happens!

Energetic Bursts

The authors modeled the supermassive stars prior to collapse and then calculated whether a jet, created as the black hole grows at the center of the collapsing star, would be able to punch out of the stellar envelope. They demonstrated that the process would work much like the widely-accepted collapsar model of massive-star death, in which a jet successfully punches out of a collapsing star, violently releasing energy in the form of a long gamma-ray burst (GRB).

Because the length of a long GRB is thought to be proportional to the free-fall timescale of the collapsing star, the collapse of these supermassive stars would create much longer GRBs than are typical of massive stars today. Instead of the typical long-GRB length of ~30 seconds, these ultra-long GRBs would be 10⁴–10⁶ seconds.

Interestingly, we have already detected a small number of ultralong GRBs; they make up the tail end of the long GRB duration distribution. Could these detections be signals of collapsing supermassive stars in the early universe? According to the authors’ estimates, we could optimistically expect to detect roughly one of these events per year — so it’s entirely possible!

Citation

Tatsuya Matsumoto et al 2015 ApJ 810 64. doi:10.1088/0004-637X/810/1/64