The progenitor of LP40-365 could be a binary
star system like the one shown in this animation. Here, an ultra-massive
and compact dead star called a white dwarf (shown as a small white
star) is accreting matter from its giant companion (the larger red
star). The material escapes from the giant and forms an accretion disk
around the white dwarf. Once enough material is accreted onto the white
dwarf, a violent thermonuclear runaway tears it apart and destroys the
entire system. The giant star and the surviving fragment of the white
dwarf are flung into space at tremendous speeds. The surviving white
dwarf shrapnel hurtles towards our region of the Galaxy, where its
radiation is detected by ground based telescopes. Copyright Russell Kightley (http://scientific.pictures), used with permission.
An international team of astronomers led by Stephane Vennes at the Astronomical Institute in the Czech Republic have identified a white dwarf moving faster than the escape velocity of the Milky Way. This high velocity star is thought to be shrapnel thrown away millions of years ago from the site of an ancient, peculiar Type Ia supernova explosion. The team used telescopes located in Arizona, the Canary Islands and Maunakea’s GRACES, a high resolution spectrograph that combines the large aperture of the Gemini North telescope with Espadons, the high resolution spectropolarimeter at CFHT, via a 250m optical fiber link.
Type Ia supernovae play an important role important in our
understanding of the Universe. They act as standard candles,
astronomical objects for which astronomers have a decent estimate of
their intrinsic brightness or luminosity. Astronomers can estimate the
true total luminosity of a Type Ia supernovae and use that information
to determine the distance. Despite astronomers’ understanding of the
luminosity and distance relationship for Type Ia supernovae, very little
is known about the explosions themselves. Astronomers build models
aimed at a deeper understanding of the engine powering these explosions.
One of these models suggest that at the heart of a Type Ia supernova
is a compact star known as a white dwarf. If the white dwarf has a
close companion star, over time the gravity of the white dwarf may
attract gas from the other star. This continuous feeding compresses the
white dwarf to such a high density and temperature that the white dwarf
is engulfed in a thermonuclear explosion. It is thought that nothing
survives this kind of explosion. However, a new class of models called
"subluminous type 1a supernova also known as a Type Iax” can leave a
partially burnt remnant that is instantly ejected at high velocity.
"Such a cataclysmic binary star has never been caught feeding and
getting just ready for the explosion," commented Stephane Vennes,
leading author of the Science article. "All we ever witness is the
aftermath of the explosion, that is the bright flash in the distant
Universe that even outshines the galaxy hosting that event. But now,
with the discovery of a surviving remnant of the white dwarf itself, we
have direct clues to the nature of the most important actor involved in
these events."
The team studied the white dwarf star LP40-365 for two-years with
telescopes located in Arizona, the Canary Islands, and Hawaii. The new
star was first identified with the National Science Foundation's (NSF)
Mayall four-meter telescope at Kitt Peak National Observatory in
Arizona. "We selected this object for observation with the spectrograph
at the four-meter telescope because of its large apparent motion across
the celestial sphere. Thousands of objects like this one are known, but
the sky was partly cloudy on that night and we had to go for the
brightest star available which turned out to be LP40-365," said team
member Adela Kawka, underpinning the importance of serendipity in
astronomy. "We alerted team members J.R. Thorstensen and E. Alper at
Dartmouth College, and P. Nemeth at the Karl Remeis Observatory for
urgent follow-up observations."
A final, crowning data set was obtained with the help of team member
Viktor Khalack at the Université de Moncton using a unique instrument,
GRACES on Maunakea. GRACES is a collaboration between the
Canada-France-Hawaii Telescope and the NSF Gemini Observatory. When
GRACES is in use, CFHT’s spectropolarimeter Espadons receives light fed
by an optical fiber hooked to its neighbor on the summit, the
eight-meter Gemini North telescope. “GRACES provides astronomers the
best of both worlds, the light collecting power of the Gemini
observatory combined with a state of the art instrument like Espadons.
The combination packs a powerful punch and creates opportunities for
discoveries like this one” says Nadine Manset, the GRACES instrument
scientist at CFHT.
After collecting the data, the team used state of the art computer
codes for analysis. The analysis proved the compact nature of the star
and its exotic chemical composition. "The extreme peculiarity of the
atmosphere required a lengthy and complex model atmosphere analysis
which crunched several weeks of computing time. But the results proved
very exciting. Such a peculiar atmosphere devoid of hydrogen and helium
is rare indeed," commented team member Peter Nemeth. The analysis also
revealed an extraordinary Galactic trajectory. "The extremely high
velocity of this star puts it on a path out of the Milky Way with no
return ever," said team member Lilia Ferrario.
Supernova models and simulations did entertain the possibility of
observing surviving stellar remnants in the aftermath of a supernova
explosion. The unique object LP40-365 is the first observational
evidence for surviving bound remnants of failed supernovae and therefore
it is an invaluable object to improve our understanding of these
cosmological standard candles.
Many more of these objects are lurking in the Milky Way and awaiting
discovery. The recent ESA/Gaia mission may well help us discover many
more of these objects and help us understand how a little white dwarf
star can survive supernova explosions.
Additional information
Official press release
Paper
Contact Information:
Mary Beth Laychak
Outreach Program Manager
Canada-France-Hawaii Telescope
65-1238 Mamalahoa Hwy
Kamuela, HI 96743
808-885-3121
laychak@cfht.hawaii.edu
Science contact
Stephane Vennes
Astronomical Institute
The Czech Academy of Sciences
Fricova 298
251 65 Ondrejov
Czech Republic
+420 323620217
vennes@asu.cas.cz
