Sky and Telescope - February 2016 - 14

News Notes

STELLAR I White Dwarf Eats Rock, Another Star Might Have Destroyed Comet
Strange dips in the light from two stars
observed by the Kepler space telescope
reveal what might be disintegrating bodies.
The ﬁrst star is a white dwarf designated 1145+017. White dwarfs generally
have hydrogen and helium atmospheres,
with the heavier elements (such as carbon
and silicon) sunk far below. But roughly
a third have atmospheres mysteriously
tainted by these heavy elements. Debris
disks also encircle a few. So astronomers
suspect that these white dwarfs are covered in the crumbs of munched planets.
As Andrew Vanderburg (HarvardSmithsonian Center for Astrophysics)
and colleagues report in the October
22nd Nature, they've now found around
1145+017 at least one potential disintegrating planetesimal - or maybe even six
or more, all about the size of the asteroid
Ceres or smaller. The object revealed
itself by passing repeatedly between us
and the white dwarf, creating a weird
transit pattern in the white dwarf's light.
When ﬁrst observed with Kepler in 2014,
the star's light dipped only a bit but stayed
low for about an hour. But when observed

nearly a year later with multiple groundbased telescopes, the star's brightness
dropped by a whopping 40% for 5 minutes. The transits also weren't symmetric.
The object orbits the white dwarf
closely in about 4½ hours and trails a
comet-like tail. Spectroscopic measurements reveal heavy elements in the white
dwarf's atmosphere, too, and the star has
a dusty debris disk, further adding to
evidence that the "pollution" comes from
a torn-up world.
The signal from the star KIC 8462852
is a far more curious case. Citizen scientists with the Planet Hunters program
ﬁrst spotted a dip of 0.5% that lasted for an
incredible 4 days. Subsequent dips proved
ragged and irregular, sometimes shallow
but sometimes blocking up to 20% of the
star's light, with unpredictable timing.
Tabetha Boyajian (Yale) and colleagues delved deeply into the mystery
in an analysis published October 17th in
Monthly Notices of the Royal Astronomical
Society, but without ﬁnding an answer.
The signals aren't from the 12th-magnitude, yellow F star itself - no hint of

giant starspots, throbbing pulsations, or
other quirks. Nor are there signs of debris
clouds from smashed-up asteroids or
planets. A large, random assortment of
cometary debris spread out along a single
orbit might work, though a shatteredcomet model has trouble producing some
of the starlight dips' quirks. The American Association of Variable Star Observers has issued an alert requesting more
observations of this star.
In the spirit of fun, the team is
contemplating one more hypothesis,
not mentioned in the paper: a partially
completed "Dyson sphere," a hypothetical mega-structure constructed around a
star by an advanced alien civilization to
capture the energy the star radiates. Boyajian has now teamed up with exoplanet
specialist Jason Wright (Penn State)
and SETI researcher Andrew Siemion
(University of California, Berkeley) to
conduct a radio search for transmissions
that might be leaking out from the aliens'
construction site. An independent SETI
search has turned up nothing yet.

IN BRIEF

the needed ultraviolet photons. All told,
there was enough radiation to totally reionize
the universe by about 750 million years after
the Big Bang, the team reports in an upcoming Astrophysical Journal.

Dipierro (University of Milan, Italy) and colleagues found that planets with a few tenths
of Jupiter's mass could also create the gaps
- but only if they clear away the dust and not
the gas. Astronomers will need to devise an
observing technique to unambiguously detect
that gas in order to conﬁrm the theory.

Small Galaxies Helped Light Universe.
Hubble observations conﬁrm that much of
the ultraviolet light that broke up the early
universe's hydrogen atoms came from the
smallest galaxies. This breakup, called the
era of reionization, occurred within the universe's ﬁrst billion years. Astronomers think
that almost all of the ultraviolet radiation
responsible came from star-forming galaxies,
and simulations suggest that dwarf galaxies
in particular might have contributed 30%
of the ultraviolet energy needed. As part of
the Frontier Fields project (S&T: Jan. 2015, p.
20), Hakim Atek (Federal Polytechnic School
of Lausanne, Switzerland) and colleagues
went hunting for early, faint galaxies using
the gravitational magnifying power of three
massive galaxy clusters. They detected 252
galaxies from about 650 to 950 million years
after the Big Bang and conﬁrmed that such
systems contributed about 20% to 60% of

Februar y 2016 sky & telescope

■ CAMILLE M. CARLISLE

Planets Hiding Around HL Tauri? A bull'seye disk of dust and gas surrounds the young
star HL Tauri, with ringlike gaps 15, 30, and 70
astronomical units from the star. Such gaps
normally signal forming planets. But Leonardo Testi (European Southern Observatory,
Germany) and colleagues found nothing when
they used the Large Binocular Telescope in
Arizona to look at the farthest-out gap (the
others were blocked by the telescope's coronagraph) - even though they should have
been able to detect the infrared glow from
the expected super-Jupiter, they report in the
October 20th Astrophysical Journal Letters.
Yet in the October 11th Monthly Notices of
the Royal Astronomical Society, Giovanni

■ CAMILLE M. CARLISLE & J. KELLY BEATTY

■ MONICA YOUNG

Giant Scope Reopens Eye. McDonald
Observatory's Hobby-Eberly Telescope (HET)
has completed its upgrade, begun in 2009
(S&T: Jan. 2015, p. 60). The facility now sports
new mechanics, software, and a complex
optical system that increases its eﬀective
aperture from 9.2 to 10 meters; in eﬀective
area, HET is tied with the Southern African
Large Telescope for third place in the world's
largest optical telescopes. At the end of 2016,
the instrument will begin a three-year observing project to document the universe's expansion rate over cosmic time. ✦
■ CAMILLE M. CARLISLE

Sky and Telescope - February 2016

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