This artist’s concept shows the nova system HM Sagittae (HM Sge), where a white dwarf star is pulling material from its red giant companion. This forms a blazing hot disk around the dwarf, which can unpredictably undergo a spontaneous thermonuclear explosion as the infall of hydrogen from the red giant grows denser and reaches a tipping point. These fireworks between companion stars are fascinating to astronomers by yielding insights into the physics and dynamics of stellar evolution in binary systems. NASA, ESA, Leah Hustak (STScI)
Astronomers have used new data from
NASA's Hubble
Space Telescope and the retired SOFIA (Stratospheric Observatory for Infrared
Astronomy) as well as archival data from other missions to revisit one of the
strangest binary star systems in our galaxy – 40 years after it burst onto the
scene as a bright and long-lived nova. A nova is a star that suddenly increases
its brightness tremendously and then fades away to its former obscurity,
usually in a few months or years.
Between April and September 1975,
the binary system HM Sagittae (HM Sge) grew 250 times brighter. Even more
unusual, it did not rapidly fade away as novae commonly do, but has maintained
its luminosity for decades. Recently, observations show that the system has
gotten hotter, but paradoxically faded a little.
HM Sge is a particular kind of
symbiotic star where a white dwarf and a bloated, dust-producing giant
companion star are in an eccentric orbit around each other, and the white dwarf
ingests gas flowing from the giant star. That gas forms a blazing hot disk
around the white dwarf, which can unpredictably undergo a spontaneous
thermonuclear explosion as the infall of hydrogen from the giant grows denser
on the surface until it reaches a tipping point. These fireworks between
companion stars fascinate astronomers by yielding insights into the physics and
dynamics of stellar evolution in binary systems.
“When I first saw the new data, I went – 'wow this is what Hubble UV
spectroscopy can do!' – I mean it's spectacular, really spectacular.
RAVI SANKRIT
Astronomer
"In 1975 HM Sge went from being a
nondescript star to something all astronomers in the field were looking at, and
at some point that flurry of activity slowed down," said Ravi Sankrit of
the Space Telescope Science Institute (STScI) in Baltimore. In 2021, Steven
Goldman of STScI, Sankrit and collaborators used instruments on Hubble and
SOFIA to see what had changed with HM Sge in the last 30 years at wavelengths
of light from the infrared to the ultraviolet (UV).
The 2021 ultraviolet data from Hubble showed a strong emission line of
highly ionized magnesium that was not present in earlier published spectra from
1990. Its presence shows that the estimated temperature of the white dwarf and
accretion disk increased from less than 400,000 degrees Fahrenheit in 1989 to
greater than 450,000 degrees Fahrenheit now. The highly ionized magnesium line
is one of many seen in the UV spectrum, which analyzed together will reveal the
energetics of the system, and how it has changed in the last three decades.
"When I first saw the new data," Sankrit said, "I went – 'wow this is what Hubble UV spectroscopy can do!' – I mean it's spectacular, really spectacular."
A Hubble Space Telescope image of the symbiotic star
Mira HM Sge. Located 3,400 light-years away in the constellation Sagitta, it
consists of a red giant and a white dwarf companion. The stars are too close
together to be resolved by Hubble. Material bleeds off the red giant and falls
onto the dwarf, making it extremely bright. This system first flared up as a
nova in 1975. The red nebulosity is evidence of the stellar wind. The nebula is
about one-quarter light-year across.
NASA, ESA, Ravi Sankrit (STScI), Steven Goldman
(STScI); Image Processing: Joseph DePasquale (STScI)
With data from NASA's flying
telescope SOFIA, which retired in 2022, the team was able to detect the water,
gas, and dust flowing in and around the system. Infrared spectral data shows
that the giant star, which produces copious amounts of dust, returned to its
normal behavior within only a couple years of the explosion, but also that it
has dimmed in recent years, which is another puzzle to be explained.
With SOFIA astronomers were able to
see water moving at around 18 miles per second, which they suspect is the speed
of the sizzling accretion disk around the white dwarf. The bridge of gas
connecting the giant star to the white dwarf must presently span about 2
billion miles.
The team has also been working with
the AAVSO (American Association of Variable Star Observers), to collaborate
with amateur astronomers from around the world who help keep telescopic eyes on
HM Sge; their continued monitoring reveals changes that haven't been seen since
its outburst 40 years ago.
"Symbiotic stars like HM Sge
are rare in our galaxy, and witnessing a nova-like explosion is even rarer.
This unique event is a treasure for astrophysicists spanning decades,"
said Goldman.
The initial
results from the
team's research were published in the Astrophysical Journal, and Sankrit is presenting research focused on the UV
spectroscopy at the 244th meeting of the American Astronomical Society in
Madison, Wisconsin.
The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and 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, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.
Source: Hubble Finds Surprises Around a Star That Erupted 40 Years Ago - NASA Science
No comments:
Post a Comment