Sky & Telescope - February 2020 - 60

The Stars That Shouldn't Be
MAKING A CARBON DWARF Little red dwarfs become carbon stars when a companion showers them with the element.

1. Long ago, a blue star a few times as massive as the Sun and
a red dwarf were born together. Both stars fuse hydrogen
into helium at their centers. (Stars are not to scale.)

2. The blue star runs out of hydrogen at its core and expands
into a red giant. The helium at its core eventually ignites to
make carbon and oxygen.

scenario explaining the star's odd nature came later,
when Ulrich Heber (University of Erlangen-Nuremberg, Germany) and colleagues discovered a hot
white dwarf in Cancer named PG 0824+289 that
had a carbon dwarf companion. The pair presumably was a younger version of G77-61.
Meanwhile, in Seattle, a graduate student named
Paul Green, whom Margon was advising, hoped to
determine the Milky Way's mass by ﬁnding carbon
giants in the outer halo and measuring their speeds.
Because these stars are luminous, Green reasoned
that those that look dim should be far away, possibly
more than 100,000 light-years distant.
Instead, Green was shocked to ﬁnd that three
faint, previously known carbon stars had detectable proper motions, a sign the stars weren't bright
giants at the edge of the galaxy but instead dim
dwarfs much nearer to the Sun. "So right away," he
says, "I got scared that my thesis was in jeopardy."
And indeed it was. Thwarted by the scarcity of
carbon giants in the distant halo, he devoted his dissertation to dwarf carbon stars instead. After moving
to the Harvard-Smithsonian Center for Astrophysics,
he spent years mining the Sloan Digital Sky Survey.
In 2013, he reported 729 new carbon dwarfs, far
outnumbering all previous discoveries put together.

3. Some of the carbon enters the giant's atmosphere. (The
carbon might turn the star into a carbon giant like R Leporis.)
During this stage the star sheds lots of mass. Some falls onto
the red dwarf companion, changing it into a carbon dwarf.

4. The giant star casts off its atmosphere, exposing its hot core:
a white dwarf.

5. Over billions of years, the white dwarf cools and fades so
that observers can no longer see it.

FEBRUARY 2 02 0 * SK Y & TELESCOPE

With so many carbon dwarfs now known - "The
carbon dwarfs vastly, vastly outnumber the carbon
giants," says Jay Farihi (University College London)
- astronomers can explore them further. For one
thing, all carbon dwarfs seem to have white dwarf
companion stars, just as theory predicts. In 2018,
Lewis Whitehouse (also University College London),
Farihi, Green, and their colleagues reported detecting companion stars orbiting 21 of the 28 carbon
dwarfs the scientists monitored. "Our results are
consistent with a 100% binary fraction," Farihi says.
Ironically, many carbon dwarfs belong to the very
population Green had originally wanted to study:
the stellar halo, whose oldest members date back to
the Milky Way's birth. Although the stellar halo constitutes a pittance of the galaxy's total stellar mass,
it carries an enormous amount of information about
the distant past.
Because we live in the Milky Way's disk, most
nearby stars are disk stars. But halo stars stand out
by their kinematics - that is, their motions relative
to us. The Sun and the other disk stars revolve fast
around the galactic center, whereas halo stars don't.
As we pass them by, they therefore appear to move
fast relative to us. Dahn's team recognized from the
start that G77-61 had the kinematic hallmarks of
the halo.

TERRI DUBÉ / S&T

Stellar Time Capsules

Sky & Telescope - February 2020

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