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White dwarf star pulled onto the neutron star: University of Warwick researchers
explain mystery of the loneliest supernovas. Compact binary star
systems that have been thrown far from their host galaxy when one
star of that pair became a neutron star, go through a second trauma
when the remaining white dwarf star is eventually pulled onto the
neutron star. Credit: Artist's impression is free to use with story
but © Mark A. Garlick / space-art.co.uk / University of Warwick
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A research team led by astronomers and
astrophysicists at the University of Warwick have found that some of
the Universe's loneliest supernovae are likely created by the
collisions of white dwarf stars into neutron stars.
Dr Joseph Lyman from the University of
Warwick is the lead researcher on the paper, The progenitors of
calcium-rich transients are not formed in situ, published today by
the journal Monthly Notices of the Royal Astronomical Society (to
appear on astro-ph 8 August 2014).
"Our paper examines so-called
`calcium-rich' transients" says Dr Lyman. "These are
luminous explosions that last on the timescales of weeks, however,
they're not as bright and don't last as long as traditional
supernovae, which makes them difficult to discover and study in
detail".
Previous studies had shown that calcium
comprised up to half of the material thrown off in such explosions
compared to only a tiny fraction in normal supernovae. This means
that these curious events may actually be the dominant producers of
calcium in our universe.
"One of the weirdest aspects is
that they seem to explode in unusual places. For example, if you look
at a galaxy, you expect any explosions to roughly be in line with the
underlying light you see from that galaxy, since that is where the
stars are" comments Dr Lyman. "However, a large fraction of
these are exploding at huge distances from their galaxies, where the
number of stellar systems is miniscule.
"What we address in the paper is
whether there are any systems underneath where these transients have
exploded, for example there could be very faint dwarf galaxies there,
explaining the weird locations. We present observations, going just
about as faint as you can go, to show there is in fact nothing at the
location of these transients - so the question becomes, how did they
get there?"
Calcium-rich transients observed to
date can be seen tens of thousands of parsecs away from any potential
host galaxy, with a third of these events at least 65 thousand light
years from a potential host galaxy.
The researchers used the Very Large
Telescope in Chile and Hubble Space Telescope observations of the
nearest examples of these calcium rich transients to attempt to
detect anything left behind or in the surrounding area of the
explosion.
The deep observations taken allowed
them to rule out the presence of faint dwarf galaxies or globular
star clusters at the locations of these nearest examples.
Furthermore, an explanation for core-collapse supernovae, which
calcium-rich transients resemble, although fainter, is the collapse
of a massive star in a binary system where material is stripped from
the massive star undergoing collapse. The researchers found no
evidence for a surviving binary companion or other massive stars in
the vicinity, allowing them to reject massive stars as the
progenitors of calcium rich transients.
Professor Andrew Levan from the
University of Warwick's Department of Physics and a researcher on the
paper said:
"It was increasingly looking like
hypervelocity massive stars could not explain the locations of these
supernovae. They must be lower mass longer lived stars, but still in
some sort of binary systems as there is no known way that a single
low mass star can go supernova by itself, or create an event that
would look like a supernova." The researchers then compared
their data to what is known about short-duration gamma ray bursts
(SGRBs). These are also often seen to explode in remote locations
with no coincident galaxy detected. SGRBs are understood to occur
when two neutron stars collide, or when a neutron star merges with a
black hole – this has been backed up by the detection of a
'kilonova' accompanying a SGRB thanks to work led by Professor Nial
Tanvir, a collaborator on this study. Although neutron star and black
hole mergers would not explain these brighter calcium rich
transients, the research team considered that if the collision was
instead between a white dwarf star and neutron star, it would fit
their observations and analysis as it:
Would provide enough energy to generate
the luminosity of calcium rich transients.
The presence of a white dwarf would
provide a mechanism to produce calcium rich material.
The presence of the Neutron star could
explain why this binary star system was found so far from a host
galaxy.
Dr Lyman said: "What we therefore propose is
these are systems that have been ejected from their galaxy. A good
candidate in this scenario is a white dwarf and a neutron star in a
binary system. The neutron star is formed when a massive star goes
supernova. The mechanism of the supernova explosion causes the
neutron star to be `kicked' to very high velocities (100s of km/s).
This high velocity system can then escape its galaxy, and if the
binary system survives the kick, the white dwarf and neutron star
will merge causing the explosive transient."
The researchers note that such merging
systems of white dwarfs and neutron stars are postulated to produce
high energy gamma-ray bursts, motivating further observations of any
new examples of calcium rich transients to confirm this.
Additionally, such merging systems will contribute significant
sources of gravitational waves, potentially detectable by upcoming
experiments that will shed further light on the nature of these
exotic systems.
R.P. Church and M.B. Davis of the Lund
University Observatory, Department of Astronomy and Theoretical
Physics and N.R.Tanvir of the Department of Physics and Astronomy,
University of Leicester made significant contributions to the work in
addition to the University of Warwick researchers.
The work used observations made with
the ESO Telescopes at the Paranal Observatory under programme ID
092.D-0420 and the NASA/ESA Hubble Space Telescope, with obtained
from the data archive at the Space Telescope Science Institute.
The University of Warwick acknowledges
the support from the UK Science and Technology Facilities Council
(grant ID ST/I001719/1).
Contact: Tom Frew
A.T.Frew@warwick.ac.uk
44-024-765-75910
University of Warwick
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