Hubble revisits 40-year-old nova mystery of a strange star

The Hubble Telescope revisits a star system that is still unusually hot

If we could look at the wonderful spiral structure of our planet milky way The galaxy is from much higher up, compressing millions of years into seconds, and we’ll see short bursts of light, like camera flashes exploding at a stadium event. These are novae, where there is a burning star, A White dwarf, swallowing gas from its bloated, red giant companion orbiting it. One of the strangest such events occurred in 1975, when a nova called HM Sagittae became 250 times brighter. It never faded as novae usually do, but it maintained its brightness for decades. The latest Hubble observations show that the system has gotten hotter, but paradoxically has faded a bit.

Hubble Space Telescope image of the symbiotic star Mira HM Sge. Located 3,400 light-years away in the Sagittarius constellation, it consists of a red giant and a companion white dwarf. The stars are too close together for Hubble to resolve. Matter bleeds from the red giant and falls onto the dwarf, making it extremely bright. This system first erupted as a nova in 1975. The red haze is evidence of stellar winds. The nebula is about a quarter of a light-year across. Image credit: NASA, ESA, Ravi Sankrit (STScI), Steven Goldman (STScI), Joseph DePasquale (STScI)

Hubble Space Telescope finds surprises about a star that exploded 40 years ago

Astronomers have revisited one of the strangest binary star systems in our galaxy — 40 years after it burst onto the scene as a bright, long-lived nova — using new data from… NASAThe Hubble Space Telescope and the retired SOFIA (Stratospheric Observatory for Infrared Astronomy) as well as archival data from other missions. A nova is a star that suddenly increases dramatically in brightness and then fades to its previous obscurity, usually within a few months or years.

The unusual behavior of HM Sge

Between April and September 1975, the binary system HM Sagittae (HM Sge) became 250 times brighter. Even more surprisingly, it did not fade as quickly as novae usually do, but maintained its brightness for decades. Recently, observations show that the system has become hotter, but paradoxically has faded slightly.

HM Sge is a particular type of symbiotic star in which a white dwarf and a bulging, dust-producing giant companion star are in eccentric orbit around each other, and the white dwarf swallows gas pouring out of the giant star. This gas forms a hot, burning disk around the white dwarf, which can unexpectedly undergo a spontaneous thermonuclear explosion as the hydrogen falling from the giant to the surface increases in intensity until it reaches a tipping point. These accompanying interstellar fireworks dazzle astronomers by providing insight into the physics and dynamics of stellar evolution in binary systems.

“When I first saw the new data, I said, ‘Wow, this is what Hubble UV spectroscopy can do!’ — I mean it’s amazing, really amazing.

— Ravi Sankrit, astronomer

Notable changes in 2021

“In 1975, HM Sge went from a nondescript star to something all the astronomers in the field were looking at, and at some point the flurry of activity slowed,” said Ravi Sankrit of the Space Telescope Science Institute (STScI) in Baltimore. In 2021, Stephen Goldman of STScI, Sancrete and collaborators used instruments on Hubble and Sofia To see what has changed with HM Sge in the past 30 years at wavelengths of light from infrared to ultraviolet (UV).

The 2021 ultraviolet data from Hubble showed a strong emission line of highly ionized magnesium that was not present in previously published spectra from 1990. Its presence shows that the estimated temperature of the white dwarf and accretion disk has increased from less than 400,000 degrees. F in 1989 to more than 450,000 degrees Fahrenheit now. The highly ionized magnesium line is one of many lines appearing in the UV spectrum, which analyzing together will reveal the energy of the system, and how it has changed in the past three decades.

“When I first saw the new data, I said, ‘Wow, this is what Hubble UV spectroscopy can do!’” Sankrit said. – I mean it’s amazing, really amazing.

SOFIA flies over the snow-capped Sierra Nevada Mountains with its telescope door open during a test flight. Sophia is a modified Boeing 747SP. SOFIA achieved full operational capability in 2014 and concluded its final science flight on September 29, 2022. Image credit: NASA/Jim Ross

Data from SOFIA

Using data from NASA’s SOFIA flying telescope, which was retired in 2022, the team was able to detect the flow of water, gas and dust in and around the system. Infrared spectroscopic data show that the giant star, which produces copious amounts of dust, returned to its normal behavior within just two years of the explosion, but it has also become fainter in recent years, another mystery to explain.

Using SOFIA, astronomers were able to see the water moving at about 18 miles per second, which they suspect is the speed of the whizzing accretion disk around the white dwarf. The gas bridge currently connecting the giant star to the white dwarf should extend about 2 billion miles.

The team is also working with AAVSO (American Association of Variable Star Observers), collaborating with amateur astronomers from around the world who help maintain telescopic eyes on HM Sge; Their ongoing monitoring reveals changes not seen since its eruption 40 years ago.

Hubble Space Telescope image of the symbiotic star Mira HM Sge with a compass and scale bar. Located 3,400 light-years away in the Sagittarius constellation, it consists of a red giant and a companion white dwarf. The stars are too close together for Hubble to resolve. Matter bleeds from the red giant and falls onto the dwarf, making it extremely bright. This system first erupted as a nova in 1975. The red haze is evidence of stellar winds. The nebula is about a quarter of a light-year across. Image credit: NASA, ESA, Ravi Sankrit (STScI), Steven Goldman (STScI)

The rarity and importance of HM Sge

“Symbiotic stars like HM Sge are rare in our galaxy, and witnessing a nova-like explosion is even rarer. This unique event has been a treasure trove for astrophysicists for decades,” Goldman said.

The preliminary results of the team’s research were published in a journal Astrophysical JournalSancret is presenting research focused on ultraviolet spectroscopy at the 244th meeting of the American Astronomical Society in Madison, Wisconsin.

Reference: “Multi-wave study of the symbiotic myra HM Sge with SOFIA and HST” by Steven R. Goldman, Ravi Sankrit, Edward Montiel, Sean Garner, Nathan Welthuis, and Nicole Karnath, 11 January 2024, Astrophysical Journal.
doi: 10.3847/1538-4357/ad12c9

the Hubble Space Telescope It has been operating for more than three decades and continues to make groundbreaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and the European Space Agency (ESA).European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, Colorado, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, Maryland, operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.