Sky & Telescope - December 2018 - 33

Absolute Magnitude in Gaia Data

LP 40-365
D6-1
US 708

US 708 in Ursa Major
LP 40-365 in Ursa Minor

Main
Main sequence
sequence

D6-3

D6-3 in Draco

D6-2
White
White
dwarfs
dwarfs

D6-2 in Pegasus
Small
Magellanic
Cloud

bluer

Color

D6-1 in Apus
Large
Magellanic
Cloud

redder

HERT ZSPRUNG -RUSSELL DIAG R A M: K . SHEN E T A L. / ASTR OPHYSICAL
JOUR NAL 2018; A LL-SK Y DIAG R A M: LE A H TISCIONE / S&T, BACKG ROUND: ESA
/ G AIA / DPAC, ACK NOWLEDG EMEN T: UNIV ERSIT Y OF LISBON; SUPER NOVA
REMN A N T: ROBERT FESEN (DA RT M OU TH COLLEG E )

p Left: The Hertzsprung-Russell diagram, as measured here by the Gaia mission, shows stars' luminosities versus their color. Most stars fall along the
main sequence, while white dwarfs bunch together at lower luminosities. The five hypervelocity stars are larger and brighter than typical white dwarfs
but smaller and dimmer than main-sequence stars of the same temperature. Right: This all-sky diagram shows where the five runaway stars are
located; the Milky Way's plane runs horizontally across the middle, with its core at center.

stars in the galaxy," Shen says. They travel at roughly 2,300
and 2,400 km/s, respectively - more than 5 million mph, or
0.8% the speed of light. At that rate you could zip from Earth
to Pluto in a month.
The three high-speed stars all arose in the same way, he
thinks, in systems with two white dwarfs. The less massive
white dwarf dumped gas onto the more massive one, where
helium exploded on its surface. That explosion then triggered
a far more lethal one: It ignited the massive white dwarf's
carbon core, blowing up that star and freeing the other,
which dashed away at high speed. Quite a way to get out of
an unhappy marriage.
The names of the three newly discovered stars reﬂect
their origin. White dwarfs are degenerate stars, made of
matter so dense it behaves like a metal rather than a gas.
The more massive white dwarf in the binaries suffered two
explosions, and the ﬁ rst explosion occurred because the
transfer of gas from star to star had been violent. So this
picture bears the alliterative term "dynamically driven
double degenerate double detonation," explaining the "D6"
in the stars' names.
One of the stars, D6-2 in Pegasus, is rushing away from a
supernova remnant that Robert Fesen (Dartmouth College)
and colleagues spotted in the same constellation three years
ago. The star would have been at the center of the supernova
remnant 90,000 years ago, dating the catastrophe that kicked
the star away. "That was a really nice piece of
evidence that these stars actually come from
supernovae," Shen says. Plus, while core-collapse
supernovae spring from young stars, which
inhabit the Milky Way's disk,
u SMOKING GUN This hydrogenalpha image captures part of the
supernova remnant that the hypervelocity star D6-2 is fleeing.

this supernova remnant is more than 1,000 light-years below
the galactic plane, pointing to an origin from an older star, an
exploding white dwarf.
"We got very lucky," Shen says, "that these three stars
turned out to be much brighter than we expected. If they
hadn't been so bright, I think we would have seen none."
The stars are so luminous because the supernova explosions
that freed them also made them expand, causing them to
be larger than ordinary white dwarfs. Over time, he says,
the bloated stars will shrink and heat up, perhaps coming to
resemble US 708, the ﬁrst and the hottest of the new breed of
hypervelocity stars. Then they will start to cool and fade as
white dwarfs normally do.

Haste Makes Waste
The exquisite new Gaia data must harbor other hypervelocity
stars that exploding white dwarfs have ﬂung away. But, Geier
warns, "we - the people looking for extreme objects - have
to be very careful." With 1.7 billion stars in the catalog, the
Gaia data must have errors. "Nothing is perfect," he explains.
"There must be outliers in there.
Those outliers can appear to have
very extreme properties which are
just not real."
With that caution in mind,
astronomers will continue their
quest for the galaxy's swiftest stars.
A larger sample of such stars should
provide more insight into how
exploding white dwarfs catapult stars out of the
Milky Way and into the vast expanse of intergalactic space that will soon engulf them.

¢ KEN CROSWELL is the author of eight books,
including The Alchemy of the Heavens and The
Lives of Stars.
sk yandtelescope.com * DECE MBE R 2 018

http://www.skyandtelescope.com

Sky & Telescope - December 2018

Table of Contents for the Digital Edition of Sky & Telescope - December 2018