The
three panels represent moments before, during, and after the faint
supernova iPTF14gqr, visible in the middle panel, appeared in the
outskirts of a spiral galaxy located 920 million light years away. The
massive star that died in the supernova left behind a neutron star in a
very tight binary system. These dense stellar remnants will ultimately
spiral into each other and merge in a spectacular explosion, giving off
gravitational and electromagnetic waves. Credit: SDSS/Caltech/Keck
The death of a massive star and the birth of a compact neutron star binary
A
Caltech-led team of researchers has observed the peculiar death of a
massive star that exploded in a surprisingly faint and rapidly fading
supernova. These observations suggest that the star has an unseen
companion, gravitationally siphoning away the star's mass to leave
behind a stripped star that exploded in a quick supernova. The explosion
is believed to have resulted in a dead neutron star orbiting around its
dense and compact companion, suggesting that, for the first time,
scientists have witnessed the birth of a compact neutron star binary
system.
The research was led by graduate student Kishalay De and
is described in a paper appearing in the October 12 issue of the
journal
Science. The work was done primarily in the laboratory of
Mansi Kasliwal
(MS '07, PhD '11), assistant professor of astronomy. Kasliwal is the
principal investigator of the Caltech-led Global Relay of Observatories
Watching Transients Happen (GROWTH) project.
When a massive
star—at least eight times the mass of the sun—runs out of fuel to burn
in its core, the core collapses inwards upon itself and then rebounds
outward in a powerful explosion called a supernova. After the explosion,
all of the star's outer layers have been blasted away, leaving behind a
dense neutron star—about the size of a small city but containing more
mass than the sun. A teaspoon of a neutron star would weigh as much as a
mountain.
During a supernova, the dying star blasts away all of
the material in its outer layers. Usually, this is a few times the mass
of the sun. However, the event that Kasliwal and her colleagues
observed, dubbed iPTF 14gqr, ejected matter only one fifth of the mass
of the sun.
"We saw this massive star's core collapse, but we saw
remarkably little mass ejected," Kasliwal says.
"We call this an
ultra-stripped envelope supernova and it has long been predicted that
they exist. This is the first time we have convincingly seen core
collapse of a massive star that is so devoid of matter."
The fact that the star exploded at all implies that it must have previously been
enveloped in lots of material, or its core would never have become
heavy enough to collapse. But where, then, was the missing mass?
The
researchers inferred that the mass must have been stolen—the star must
have some kind of dense, compact companion, either a white dwarf,
neutron star, or black hole—close enough to gravitationally siphon away
its mass before it exploded. The neutron star that was left behind from
the supernova must have then been born into orbit with that dense
companion. Observing iPTF 14gqr was actually observing the birth of a
compact neutron star binary. Because this new neutron star and its
companion are so close together, they will eventually merge in a
collision similar to the
2017 eventthat produced both gravitational waves and electromagnetic waves.
Not
only is iPTF 14gqr a notable event, the fact that it was observed at
all was fortuitous since these phenomena are both rare and short-lived.
Indeed, it was only through the observations of the supernova's early
phases that the researchers could deduce the explosion's origins as a
massive star.
"You need fast transient surveys and a
well-coordinated network of astronomers worldwide to really capture the
early phase of a supernova," says De. "Without data in its infancy, we
could not have concluded that the explosion must have originated in the
collapsing core of a massive star with an envelope about 500 times the
radius of the sun."
The event was first seen at Palomar
Observatory as part of the intermediate Palomar Transient Factory
(iPTF), a nightly survey of the sky to look for transient, or
short-lived, cosmic events like supernovae. Because the iPTF survey
keeps such a close eye on the sky, iPTF 14gqr was observed in the very
first hours after it had exploded. As the earth rotated and the Palomar
telescope moved out of range, astronomers around the world collaborated
to monitor iPTF 14gqr, continuously observing its evolution with a
number of telescopes that today form the GROWTH network of
observatories.
The
Zwicky Transient Facility,
the successor of iPTF at Palomar Observatory, is examining the sky even
more broadly and frequently in the hopes of catching more of these rare
events, which make up only one percent of all observed explosions. Such
surveys, in partnership with coordinated follow-up networks like
GROWTH, will enable astronomers to better understand how compact binary
systems evolve from binary massive stars.
The research was
primarily funded by the National Science Foundation under the PIRE
GROWTH project. A full list of funding sources and co-authors can be
found in the Science study, titled "A hot and fast
ultra-stripped supernova that likely formed a compact neutron star
binary." In addition to De and Kasliwal, other Caltech co-authors are
Gary Doran of the Jet Propulsion Laboratory; graduate student Gina
Duggan; Shri Kulkarni, George Ellery Hale Professor of Astronomy and
Planetary Science; and Russ Laher and Frank Masci of Caltech's Infrared
Processing and Analysis Center.
For more about GROWTH, visit: http://growth.caltech.edu.
Written by Lori Dajose
Contact:
Whitney Clavin
(626) 395-1856
wclavin@caltech.edu
