‘Messy’ star makes binary companion go supernova

Astronomers have discovered evidence of a star in a binary system going supernova as a result of material shed by its stellar companion, making crucial and difficult observations of the donor star.

When a star of adequate size, runs out of fuel for nuclear fusion and the resulting radiation pressure can no longer counter the gravitational force pulling it inwards, its core collapses and it violently sheds its outer layers — it goes supernova. But there is another way that astronomers have theorised such a violent explosion of matter can be caused — when in a binary system and its companion spills material on to its surface.

The material shed in a supernova explosion reveals a great deal about the exploding star but gives little away about the companion star which donated the excess material. This leaves astronomers forced to sift for clues as to the nature and composition of the donor star.

An X-ray/infrared composite image of G299, a Type Ia supernova remnant in the Milky Way Galaxy approximately 16,000 light years away (http://chandra.harvard.edu/photo/2015/g299/)

Through repeated observations of SN 2015cp — a supernova 545 million light years away, astronomers have detected hydrogen-rich debris that the companion star had shed prior to the explosion, it was announced on 10th January at the 2019 American Astronomical Society meeting in Seattle.

Melissa Graham, a University of Washington astronomer and lead author on the accompanying paper to be published in The Astrophysical Journal, presented the findings: “The presence of debris means that the companion was either a red giant star or similar star that, prior to making its companion go supernova, had shed large amounts of material.

Material ejected by the aforementioned supernova smashed into this previously shed material producing ultraviolet radiation. This was observed two years later by the Hubble space telescope and several other observatories. By looking for evidence of debris impacts months or years after a supernova in a binary star system, Graham’s team believes that astronomers could determine whether the companion had been a messy red giant or a relatively neat and tidy star.

The team made this discovery as part of a wider study of a particular type of supernova known as a Type Ia supernova, which occurs when a carbon-oxygen white dwarf star explodes suddenly due to the activity of a binary companion. Carbon-oxygen white dwarfs are small, dense and — for stars — quite stable. They form from the collapsed cores of larger stars and, if left undisturbed, can persist for billions of years.

Type 1a supernovae are often referred to as standard candles, their stability is used by astronomers to estimate distances to other galaxies and astronomical objects. They can also be used to estimate the expansion rate of the Universe and thus to infer the existence of dark energy.

Yet scientists are not certain what kinds of companion stars could trigger a Type Ia event. Plenty of evidence indicates that, for most Type Ia supernovae, the companion was likely another carbon-oxygen white dwarf, which would leave no hydrogen-rich debris in the aftermath.

Theoretical models have shown that stars like red giants could also trigger a Type Ia supernova, which could leave hydrogen-rich debris that would be hit by the explosion. Out of the thousands of Type Ia supernovae studied to date, only a small fraction was later observed impacting hydrogen-rich material shed by a companion star. Prior observations of at least two Type Ia supernovae detected glowing debris months after the explosion. Scientists weren’t sure if those events were isolated occurrences or signs that Type Ia supernovae could have many different kinds of companion stars.

Graham says: “All of the science to date that has been done using Type Ia supernovae, including research on dark energy and the expansion of the universe, rests on the assumption that we know reasonably well what these ‘cosmic lighthouses’ are and how they work.

“It is very important to understand how these events are triggered, and whether only a subset of Type Ia events should be used for certain cosmology studies.”

Using data from the Hubble space telescope, Graham’s team looked for ultraviolet emissions from 70 Type Ia supernovae approximately one to three years following the initial explosion. As Graham explains: “By looking years after the initial event, we were searching for signs of shocked material that contained hydrogen, which would indicate that the companion was something other than another carbon-oxygen white dwarf.”

An image of SN 1994D (lower left), a Type Ia supernova detected in 1994 at the edge of galaxy NGC 4526 (https://www.spacetelescope.org/images/opo9919i/)

In the case of SN 2015cp, a supernova first detected in 2015, the scientists found what they were searching for. In 2017, 686 days after the supernova exploded, Hubble picked up an ultraviolet glow of debris. This debris was far from the supernova source — at least 100 billion kilometres, or 62 billion miles, away. For reference, Pluto’s orbit takes it a maximum of 7.4 billion kilometres from our sun.

By comparing SN 2015cp to the other Type Ia supernovae in their survey, the researchers estimate that no more than 6% of Type Ia supernovae have such a litterbug companion. Repeated, detailed observations of other Type Ia events would help cement these estimates, Graham says.

The Hubble Space Telescope was essential for detecting the ultraviolet signature of the companion star’s debris for SN 2015cp. In the fall of 2017, the researchers arranged for additional observations of SN 2015cp by the W.M. Keck Observatory in Hawaii, the Karl G. Jansky Very Large Array in New Mexico, the European Southern Observatory’s Very Large Telescope and NASA’s Neil Gehrels Swift Observatory, among others. These data proved crucial in confirming the presence of hydrogen and are presented in a companion paper lead by Chelsea Harris, a research associate at Michigan State University.

In 2017, 686 days after the initial explosion, the Hubble Space Telescope recorded an ultraviolet emission (blue circle) from SN 2015cp, which was caused by supernova material impacting hydrogen-rich material previously shed by a companion star. Yellow circles indicate cosmic ray strikes, which are unrelated to the supernova (Graham ML et al)

Graham the difficulty of such operations and the role of pure luck in the proceedings: “The discovery and follow-up of SN 2015cp’s emission really demonstrate how it takes many astronomers, and a wide variety of types of telescopes, working together to understand transient cosmic phenomena.

“It is also a perfect example of the role of serendipity in astronomical studies: If Hubble had looked at SN 2015cp just a month or two later, we wouldn’t have seen anything.”

Graham, a senior fellow with the UW’s DIRAC Institute and a science analyst with the Large Synoptic Survey Telescope, or LSST, continues: “In the future, as a part of its regularly scheduled observations, the LSST will automatically detect optical emissions similar to SN 2015cp — from hydrogen impacted by material from Type Ia supernovae.

“It’s going to make my job so much easier!”

Original research: Graham ML et al. “Delayed Circumstellar Interaction for Type Ia SN 2015cp Revealed by an HST Ultraviolet Imaging Survey”