Sky & Telescope - August 2021 - 32

Star Sleuths
possible, " says Dubois. His teammates Ludwig Logie and Steve
Rau maintain the telescope while Siegfried Vanaverbeke, a
physicist by training, analyzes the data.
In 2014, the team began a program of variable star observations
in order to collaborate with professional astronomers.
Altogether, the group has provided data and analysis to 27
scientifi c publications.
Chandra Sheds (X-Ray) Light on the Matter
The decades of ground-based observations of RW Aurigae
have given astronomers hints as to the underlying causes of
this system's behavior, but the visual-wavelength data don't
tell the whole story - especially because the binary pair isn't
resolved in the AAVSO observations. To learn more, the team
and their collaborators collected X-ray data, which can give
insight into the material between us and the star as well as
the conditions very close to the star. (Much of the X-ray emission
from a young star comes from gas accreting onto the star
from its surrounding disk or from the hot, tenuous corona.)
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" I think the synergy between space telescopes and groundbased
telescopes is the part of the work that is new and is a
real breakthrough, " says Vanaverbeke.
The team analyzed data from the Chandra X-ray Observatory
taken in 2013, 2015, and twice in 2017. The secondary
star, RW Aurigae B, was fairly consistent over the years:
The fl ux rose and fell, but the overall shape of its spectrum
remained the same. Not so for the primary, RW Aurigae A. In
2013, when the system's visual brightness was about average,
the star's coronal X-ray emission peaked around 1 keV (rather
low-energy, or " soft, " for X-rays). But in subsequent observations,
when the optical emission was dim, the soft X-ray
emission that was so prominent in 2013 practically vanished.
This suggests that there was a huge increase in the amount of
material between the star and the telescope - jumping by a
factor of 70 in 2015 and by a factor of a few hundred in 2017.
In 2017, things got weirder: The star's X-ray emission
shifted to higher energies, and a new feature shot up. Previous
observations of RW Aurigae A showed an emission line at
6.7 keV, arising from iron atoms stripped of all but two electrons.
The 2017 observations were instead dominated by an
emission line at a slightly lower energy (6.63 keV), indicating
that it came from cooler iron than the 6.7-keV line.
" Cooler iron should emit very few X-rays, so in order to
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Capella
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Menkalinan
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AURIGA
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M36
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RW Aur
TAURUS
Elnath
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RW AURIGAE This binary sits on the southern edge of Auriga, the Charioteer. Its brightness usually varies from
about magnitude 9 to 12.5. The star won't be well placed for evening observing for a few months yet; to see it in
summer, you'll need to rise well before dawn.
32 AUGUST 2021 * SKY & TELESCOPE
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get that much iron emission from this cooler iron you need
to have a lot of it, " Günther says. The team estimates that
the iron abundance in RW Aurigae A's corona is an order of
magnitude higher than it is in the Sun. " And then you start
questioning where that iron comes from. It essentially must
have fallen into the star, because it wasn't there before and
stars don't have large reservoirs of iron that suddenly come
out without affecting the rest of the spectrum. "
So the amount of material around RW Aurigae A has rocketed
up by one or two orders of magnitude, and its corona is
suddenly rich in iron. What's behind this bizarre behavior?
Planetesimals or Pressure Traps?
One possibility is that in the chaos of formation, collisions
between planets growing around RW Aurigae A generated
enormous clouds of debris. This dust could be responsible for
absorbing the soft X-ray
emission that's missing
in the later observations
and, if it's iron-rich, could
cause the enhanced iron
in the corona as well.
In this scenario, two
RW Aurigae
planets or planetesimals
may have collided in 2011,
producing a large brightness
dip seen by observers.
The fragments from that
crash could have collided
again around 2015,
causing another dimming
Star magnitudes
IMAGE: POSS-II / STSCI / CALTECH / PALOMAR OBSERVATORY;
CHART: GREGG DINDERMAN / S&T

Sky & Telescope - August 2021

Table of Contents for the Digital Edition of Sky & Telescope - August 2021