From UC Santa Cruz: “Kepler telescope captures extraordinary observations of a star’s death throes”

UC Santa Cruz

From UC Santa Cruz

November 30, 2018
Tim Stephens

Unprecedented images of a Type Ia supernova, from the moment of explosion through the rise and fall of the light curve, show an unexpected early rise in brightness.

The supernova—known as SN 2018oh—is located in a spiral galaxy called UGC 4780 in the constellation Cancer at a distance of more than 170 million light years.

An exploding star in another galaxy has been documented with unprecedented precision thanks to the Kepler Space Telescope’s K2 Supernova Cosmology Experiment, one of the telescope’s final missions before running out of fuel late last month.

Kepler’s observations of the supernova known as SN 2018oh showed an unexpected fast rise in brightness that may be an important clue to understanding the progenitors of Type Ia supernovae, which cosmologists use to study the expansion of the universe and dark energy.

An international team led by astronomers at the University of California, Santa Cruz, conducted an analysis of SN 2018oh focusing on the first week after the explosion. Their paper, accepted for publication in Astrophysical Journal Letters, is one of a series of papers analyzing SN 2018oh.

Kepler’s observations of the supernova known as SN 2018oh showed an unexpected fast rise in brightness that may be an important clue to understanding the progenitors of Type Ia supernovae, which cosmologists use to study the expansion of the universe and dark energy.

“This is an incredibly exciting discovery,” said Georgios Dimitriadis, a postdoctoral researcher at UC Santa Cruz who led the analysis. “When I downloaded the data and started looking at it in detail, my jaw dropped.”

“The observations are exquisite, because we have images from Kepler every 30 minutes, starting from before the explosion all the way past its peak brightness. And it’s scientifically interesting because the increase in brightness deviates from the expected behavior,” said Ryan Foley, assistant professor of astronomy and astrophysics at UC Santa Cruz.

The supernova was also extensively monitored by ground-based facilities which provided important complementary observations, including the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1) at Haleakala Observatory, Hawaii, and the Dark Energy Camera (DECam) at Cerro Tololo Inter-American Observatory in Chile.

Pannstars telescope, U Hawaii, Mauna Kea, Hawaii, USA

Dark Energy Survey

Dark Energy Camera [DECam], built at FNAL

NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

The light curve of the supernova shows how its brightness changed over time. A typical supernova gets steadily brighter for almost three weeks, then gradually fades away. SN 2018oh, however, brightened very quickly right after explosion before settling into the normal progression. Because of the fast brightening, SN 2018oh was about 3 times brighter than a typical supernova a few days after explosion.

“This early bump in the light curve requires an extra source of light, and the question is where does that come from,” Foley said.

Dimitriadis said the team investigated three possible explanations.

“We know a Type Ia supernova results from the explosion of a white dwarf that acquires extra mass given to it from a companion star,” he explained. “But we don’t know what kind of star donates this extra mass.”

One possibility is that the white dwarf accretes matter from a star like our sun. This scenario could give rise to extra light (the bump in the light curve) when the shock wave from the exploding white dwarf runs into the companion star. As the supernova flows around the companion star, it creates an area of extremely hot material on the star which emits light in addition to the light from the supernova.

“In that scenario, we would expect the observation of excess light to be very dependent on the viewing angle, which may explain why it has not been seen in all supernova observations,” Foley said.

Another prediction of this scenario is that the excess light would be blue, because of the high shock temperatures. The researchers obtained critical color information for SN 2018oh from ground-based observations. “We observed blue colors at the time of the flux excess, a key clue in understanding what was causing the extra light,” Dimitriadis said.

The scenario where the supernova runs into its nearby companion star should produce blue light similar to what was seen from the ground. However, the researchers did not rule out other possible explanations. The light from a supernova comes from the radioactive decay of heavy elements such as nickel–56, which tend to be in the center of the star. If nickel accumulates on the surface during the explosion, however, its radioactive decay could also generate excess light at an early stage of the supernova. It could even produce a “double detonation” in which a small explosion on the surface triggers a second explosions that consumes the entire star.

Another possibility is excess light being emitted when the shock wave from the supernova heats a large shell of material just above the surface of the star. According to Foley, the color information from early ground-based images is critical to distinguishing between these different scenarios.

“The blue color, in particular, agrees with the scenario in which the supernova interacts with a companion star, and is harder to explain with either nickel on the surface or the heating of circumstellar material,” he said.

This is significant because it favors one of the two general models for Type Ia supernovae that astrophysicists have been debating for decades. In the “single-degenerate” model, the white dwarf accretes matter from a normal companion star until it reaches a certain limit and explodes. In the “double-degenerate” model, the excess mass results from the merger of two white dwarfs.

“The interaction with a companion star is a prediction of the single-degenerate model, whereas the other two scenarios for the excess light could fit with either model,” Foley said. “This supernova is consistent with the single-degenerate model, but there are other supernovae where there is strong evidence against a normal companion star, so it remains an open question.”

Dimitriadis adds that his team continues to observe the supernova, searching for additional clues about how it exploded. He says, “This is an important problem, and we will keep chipping away at it.”

SN 2018oh is located in a spiral galaxy called UGC 4780 in the constellation Cancer at a distance of more than 170 million light years. This galaxy was included as a target for monitoring by NASA’s Kepler Space Telescope as part of the K2 Supernova Cosmology Experiment. The supernova was discovered in February 2018 by the All Sky Automated Survey for Supernovae (ASAS-SN). Early images were obtained by the Pan-STARRS1 telescope and the CTIO Mayall telescope with DECam.

“This study was a large collaborative effort involving 150 scientists from a wide range of specialties,” Dimitriadis said. “A lot of credit goes to the people who worked on the Kepler telescope and gave it extra life with the K2 mission. Kepler was not designed to observe supernovae, and we had important contributions from exoplanet scientists because they know the instrument best.”

The coauthors of the paper include scientists from more than 50 institutions, including UC Santa Cruz, Space Science Telescope Institute, and UC Berkeley. This work was supported in part by NASA, the Gordon and Betty Moore Foundation, the Packard Foundation, the National Science Foundation, and the Heising-Simons Foundation.

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UCSC Lick Observatory, Mt Hamilton, in San Jose, California, Altitude 1,283 m (4,209 ft)


UCO Lick Shane Telescope
UCO Lick Shane Telescope interior
Shane Telescope at UCO Lick Observatory, UCSC

Lick Automated Planet Finder telescope, Mount Hamilton, CA, USA

Lick Automated Planet Finder telescope, Mount Hamilton, CA, USA

UC Santa Cruz campus
The University of California, Santa Cruz, opened in 1965 and grew, one college at a time, to its current (2008-09) enrollment of more than 16,000 students. Undergraduates pursue more than 60 majors supervised by divisional deans of humanities, physical & biological sciences, social sciences, and arts. Graduate students work toward graduate certificates, master’s degrees, or doctoral degrees in more than 30 academic fields under the supervision of the divisional and graduate deans. The dean of the Jack Baskin School of Engineering oversees the campus’s undergraduate and graduate engineering programs.

UCSC is the home base for the Lick Observatory.

Lick Observatory's Great Lick 91-centimeter (36-inch) telescope housed in the South (large) Dome of main building
Lick Observatory’s Great Lick 91-centimeter (36-inch) telescope housed in the South (large) Dome of main building

Search for extraterrestrial intelligence expands at Lick Observatory
New instrument scans the sky for pulses of infrared light
March 23, 2015
By Hilary Lebow
The NIROSETI instrument saw first light on the Nickel 1-meter Telescope at Lick Observatory on March 15, 2015. (Photo by Laurie Hatch) UCSC Lick Nickel telescope

Astronomers are expanding the search for extraterrestrial intelligence into a new realm with detectors tuned to infrared light at UC’s Lick Observatory. A new instrument, called NIROSETI, will soon scour the sky for messages from other worlds.

“Infrared light would be an excellent means of interstellar communication,” said Shelley Wright, an assistant professor of physics at UC San Diego who led the development of the new instrument while at the University of Toronto’s Dunlap Institute for Astronomy & Astrophysics.

Wright worked on an earlier SETI project at Lick Observatory as a UC Santa Cruz undergraduate, when she built an optical instrument designed by UC Berkeley researchers. The infrared project takes advantage of new technology not available for that first optical search.

Infrared light would be a good way for extraterrestrials to get our attention here on Earth, since pulses from a powerful infrared laser could outshine a star, if only for a billionth of a second. Interstellar gas and dust is almost transparent to near infrared, so these signals can be seen from great distances. It also takes less energy to send information using infrared signals than with visible light.

Frank Drake, professor emeritus of astronomy and astrophysics at UC Santa Cruz and director emeritus of the SETI Institute, said there are several additional advantages to a search in the infrared realm.

“The signals are so strong that we only need a small telescope to receive them. Smaller telescopes can offer more observational time, and that is good because we need to search many stars for a chance of success,” said Drake.

The only downside is that extraterrestrials would need to be transmitting their signals in our direction, Drake said, though he sees this as a positive side to that limitation. “If we get a signal from someone who’s aiming for us, it could mean there’s altruism in the universe. I like that idea. If they want to be friendly, that’s who we will find.”

Scientists have searched the skies for radio signals for more than 50 years and expanded their search into the optical realm more than a decade ago. The idea of searching in the infrared is not a new one, but instruments capable of capturing pulses of infrared light only recently became available.

“We had to wait,” Wright said. “I spent eight years waiting and watching as new technology emerged.”

Now that technology has caught up, the search will extend to stars thousands of light years away, rather than just hundreds. NIROSETI, or Near-Infrared Optical Search for Extraterrestrial Intelligence, could also uncover new information about the physical universe.

“This is the first time Earthlings have looked at the universe at infrared wavelengths with nanosecond time scales,” said Dan Werthimer, UC Berkeley SETI Project Director. “The instrument could discover new astrophysical phenomena, or perhaps answer the question of whether we are alone.”

NIROSETI will also gather more information than previous optical detectors by recording levels of light over time so that patterns can be analyzed for potential signs of other civilizations.

“Searching for intelligent life in the universe is both thrilling and somewhat unorthodox,” said Claire Max, director of UC Observatories and professor of astronomy and astrophysics at UC Santa Cruz. “Lick Observatory has already been the site of several previous SETI searches, so this is a very exciting addition to the current research taking place.”

NIROSETI will be fully operational by early summer and will scan the skies several times a week on the Nickel 1-meter telescope at Lick Observatory, located on Mt. Hamilton east of San Jose.

The NIROSETI team also includes Geoffrey Marcy and Andrew Siemion from UC Berkeley; Patrick Dorval, a Dunlap undergraduate, and Elliot Meyer, a Dunlap graduate student; and Richard Treffers of Starman Systems. Funding for the project comes from the generous support of Bill and Susan Bloomfield.