Sky & Telescope - August 2021 - 33

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STRANGE BEHAVIOR RW Aurigae varied sporadically until late 2010, when it began an unusual drop. The system's brightness then plummeted
in 2014, as shown by these data collected by various AAVSO observers. Vertical lines mark when the team acquired X-ray observations from the two
stars (the two 2017 observations were only two days apart and are combined into a single line).
episode. If by 2017 those planetary fragments had fallen onto
the star, that could explain the huge jump in iron emission.
There's another possibility, though. Protoplanetary disks
often have striking patterns of bright and dark rings. The
most tantalizing explanation for this tree-ring pattern is that
baby planets collect gas and dust as they orbit their parent
star, carving out dark lanes in the bright disk. But rings can
also form thanks to pressure traps, high-pressure regions that
can collect dust and prevent it from accreting onto the star.
Pressure traps can form where gas in the disk freezes or the
disk is too dense for the star's radiation to ionize it strongly.
(To make matters more complicated, there's also evidence
that coalescing planets can cause pressure traps to arise.)
If dust piles up in a pressure trap, it could reach the density
implied by the X-ray observations. A gravitational nudge from
the close passage of RW Aurigae B could disrupt the pressure
trap and send the dust on a collision course with the star.
This might explain both the increase in the density and the
increase in iron emission, if the dust is iron rich.
Both explanations are exciting. We have indirect evidence
of planetesimals accreting onto a star from observations of
white dwarfs, but this could be our fi rst opportunity to see it
happening in real time. Conversely, if a pressure trap is the
cause, then this system could help us better understand the
role these traps play in planet formation and accretion. But
how do we tell which scenario is actually playing out?
Looking Ahead
Unsurprisingly, it's likely to be challenging. " We don't have
enough resolution to see occultations by the planetesimals, "
says Vanaverbeke. " The amplitude of the occultations would
be very small - that would not be observable, I think. So
right now, really choosing between accretion of protoplanets
and ejection and accretion of dust that is inside the disk and
shepherded by tidal accretion will probably have to be because
we see chemical differences. "
Those chemical differences - things like new elements or
ionization levels appearing in the data - might be discernible
when the dimming episodes end and RW Aurigae returns to a
" normal " state. If the absorbing material around RW Aurigae
A disperses, the soft X-ray emission that was all but absent
in the 2015 and 2017 observations will shine through once
again. That could mean that other chemical species might
make an appearance, hopefully clarifying the situation.
Seeing those chemical markers " could tell you something
about the composition of those grains, and then you can go
to a planetary scientist and say, 'I found silicon, iron, and
magnesium in a ratio of one to one to one, does it look like a
planetary core or not?' " says Günther.
The time scales for the two possibilities might be different
as well. In the pressure-trap scenario, the dust grains are
separated by size, and grains of different sizes should take different
amounts of time to fall onto the star. This could mean
that the star is just beginning to accrete the material caught
in the pressure trap, and RW Aurigae A's accretion rate might
increase over time. On the other hand, in the planetesimalcollision
scenario, there might be a wide range of grain sizes
that all fall onto the star at the same time.
It's not quite that cut-and-dry, though; as always, protoplanetary
disks are complicated. Fragments of planets left
over after a collision might themselves undergo collisions,
resulting in smaller pieces on orbits of varying eccentricity,
which would muddy the calculation. Turbulence and magnetic
fi elds - the two things most likely to keep modelers up
at night - have the potential to play a complicating role in
either scenario.
Günther, Dubois, and Vanaverbeke plan to keep an eye on
the system to see how long it takes to go back to normal - if
that ever happens. " RW Aurigae has surprised us so often, it's
entirely possible that something unexpected happens, " Günther
says. " And I don't know what happens after that! "
 KERRY HENSLEY is a planetary scientist and science writer
based in Boston.
sk yandtelescope.org * AUGUST 2021 33
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V (visible) band
Visual measurements (no filter)
GREGG DINDERMAN / S&T, SOURCES: AAVSO AND
GÜNTHER ET AL. / ASTRONOMICAL JOURNAL 2018
Magnitude

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Sky & Telescope - August 2021

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