Sky and Telescope - May 2016 - 14

News Notes

STELLAR I Spotting "Twins" of Eta Carinae

BLACK HOLES I The

NASA / ESA / HUBBLE HERITAGE TEAM / R. KHAN (GSFC AND ORAU)

Disappearing Quasar

Eta Twin-4

26 million light-years away: M51, M83,
M101, and NGC 6943. Together, these
galaxies produced 20 of the Type II (corecollapsing, big-star-killing) supernovae
seen in the last century. And here is
where the astronomers found ﬁve objects
that look just like Eta Car.
"Look just like" is used loosely, Khan
explained January 6th during a press
conference at the American Astronomical Society's biannual meeting. The ﬁve
objects are too distant to be resolved.
But the light coming from these sources
behaves just like that from Eta Car. The
sources are relatively faint in nearinfrared and visible light, as seen by
the Hubble Space Telescope, but they're
bright at mid-infrared wavelengths, as
seen with the Spitzer Space Telescope.
Moreover, their brightness "ﬂattens out"
in this mid-infrared range, in a manner
that's a spot-on match for star-enveloping dust that's been warmed to between
400 and 600 kelvin. Thus, the team
members are pretty sure they're seeing
light from massive, evolved stars, as
they also explain in the December 20th
Astrophysical Journal Letters.

Astronomers tracking a distant quasar
over a span of 13 years reported January
8th at the American Astronomical Society
meeting in Kissimmee, Florida, that all
signs of the quasar have disappeared.
The quasar, known as SDSS
J1011+5442, was ﬁrst detected in 2002.
A follow-up spectrum collected the next
year showed all the signs of hot gas
feeding a typical, ferociously gobbling
supermassive black hole sitting at the
center of a galaxy.
The quasar's brightness declined
steadily over the next decade, far more
systematically than usually happens with
such objects. When Jessie Runnoe (Penn
State University) and colleagues observed
the quasar again as part of the Time
Domain Spectroscopic Survey in 2015,
they took a visible-light spectrum of the
beast's disk of gas. They found nothing
- almost all signs of the quasar had vanished. Instead, they saw only a relatively
ordinary galaxy. The team published
its results in the January 11th Monthly
Notices of the Royal Astronomical Society.
This isn't the ﬁrst case of a disappearing quasar; astronomers now know of
several "changing look" active galaxies (as
they're collectively known).
The best explanation for J1011+5442's
disappearance is a diet, in which the black
hole cut its feeding rate by a factor of 10.
The accretion disk should still surround
it, though. The outermost part of the disk,
which is responsible for the light, would
normally take 800 years to gradually
empty out and fade, Runnoe says. Instead,
she suggests that rather than clear its
plate completely in the few short years
they've been observing, the quasar swallowed the nearest, hottest gas from the
inner accretion disk. This could happen
quickly, in a month or two. The hot, inner
gas would have emitted ultraviolet light as
it swirled toward the black hole, irradiating the outer disk to make it glow. So
when the ultraviolet beacon near the black
hole went dark, the outer disk would have
lost its visible-light shine as well. ✦

■ CAMILLE M. CARLISLE

■ MONICA YOUNG

The galaxy M83 hosts two potential
"twins" of Eta Carinae. These candidates (insets, not resolved) seem to
have warm, dusty shrouds much like
the one that envelopes Eta Car.

Eta Twin-5

2 arcseconds

Astronomers looking for clones of the
huge, unstable star Eta (η) Carinae have
potentially found ﬁve in other galaxies.
Eta Car is one of the most massive
stars in the Milky Way, estimated to
have 120 Suns' worth of material. It
pumps out more than 5 million times
the energy than the Sun does, but it and
its companion star hide inside a doubledumbbell of dust and gas called the
Homunculus Nebula.
Astronomers haven't seen any stars
quite like Eta Car, but they want to. Such
massive, evolved stars created many of
our galaxy's heavy elements, and these
megastars might also be the culprits
behind what are called superluminous
supernovae, stellar explosions that are
strangely bright compared with their kin.
Observers haven't had much luck
looking for Eta Carinae's twins, though
they've turned up several candidates.
Rubab Khan (NASA Goddard) and his
team are narrowing the search by looking
in other galaxies for dust-enshrouded
stars whose visible and infrared light
closely matches Eta Car's.
The team looked at four bright, starforming spiral galaxies that lie 15 to
14

May 2016 sky & telescope
Sky and Telescope - May 2016

Table of Contents for the Digital Edition of Sky and Telescope - May 2016
