26 AUSTRALIAN SKY & TELESCOPE May | June 2017
DIMITRI VERASET AL. /
MONTHLY NOTICES OF THE RAS
, OCT. 25, 2016precise brightness measurements that are required for this.
Virgo wasn’t observable until later in the year, and, as
luck would have it, the northern winter of 2015-16 was so
cloudy that I could not start collecting data until March.
Fortunately, advanced amateurs in other parts of the world
started taking data in December. One was Bruce Gary, a
leader in the amateur exoplanet-transit world. His early
and influential book Exoplanet Observing for Amateurs had
inspired me to get started in this exciting field.
In December 2015, Gary and Tom Kaye confirmed that the
star is undergoing multiple chaotic transits. I took the light
curve on the previous page on March 29, 2016. It showed that
by then the system had evolved into a new state. The brief,
initial sharp fade for 1 or 2 minutes likely corresponds to the
planet itself, but it was much deeper than expected for such
a small body. Apparently an opaque or semi-opaque cloud of
material surrounded it. Following this ‘primary’ transit came
a clearer spell and then an even deeper dimming lasting some25 minutes: a large cloud of debris, which later expanded in
size and thinned out. The rest of the orbit showed smaller
rapid obscurations, indicating that an irregular, dusty debris
field completely rings WD 1145. Broken-off chunks of the
main body seem to be shedding debris and dust of their own,
in a cascading process. Some of the objects seem to have
slightly longer orbital periods of up to 4.9 hours.
Meanwhile, the comparison stars in my imaging field held
very steady: to about 0.01 magnitude from frame to frame, or
1% of their brightnesses. So the seemingly ‘noisy’ fluctuations
you see from minute to minute in the light curves are mostly
real — not instrumental noise or atmospheric effects.
By comparison, in a recording I made of a normal
exoplanet crossing the star HAT-P-26 last April, the smooth,
steady dimming of only 7 millimagnitudes (a 0.7% percent
brightness change) stands out clearly from my system’s
noise. That planet is roughly Neptune-size and crosses a
main-sequence star of spectral type K1. This was one of the
observations that Dennis Conti of the American Association
of Variable Star Observers (AAVSO) is co-ordinating among
amateurs to support a major Hubble study of 15 exoplanets.
Compare that to the wild fluctuations of WD 1145
and you see what drew Andrew’s initial attention to this
object. Instead of the typical transit drop of just a few
millimagnitudes, we have seen WD 1145 being obscured by
up to 40%!
The light curves at right show five more of my observing
runs from the following two months. Most span a large part
of the system’s 4.5-hour ‘year’. No two are alike, indicating
the debris field’s fast evolution. We are watching the end of a
world in real time.
Given the planet’s small size, this situation is not likely to
continue much longer. Eventually everything should grind
itself down to small particles, settle into flat rings like Saturn’s,
and gradually spiral into the star via ‘friction’ with the star’s
brilliant radiation. Indeed, the last observation on May 31st
showed reduced activity. Was the show winding down?
Nope! When Virgo came back into view in December
2016, Bruce Gary was the first to find that new debris was
causing big dips again, which I subsequently confirmed.
We will collect many more observations over time to
follow the continuing evolution of the WD 1145 system. It’s
possible that more than one initial body has broken up, or is
breaking up, or is about to.Join us!
Scientists like Vanderburg, Croll and Rappaport depend
on close professional-amateur collaborations. Pro-amROCHE APPROACH
In this gravitational
simulation, a 2,000-km
body in a slightly eccentric
orbit dips within the Roche
limit around its star. The
star is far off to the left,
and the direction of the
body’s orbital motion is
upward. The numbers are
completed orbits. Gray
particles are the object's
rocky mantle; denser green
particles formed its core.Next year the TESS satellite should start
revealing a flood of nearby exoplanets. Some
will be especially ripe for pro-am followups.2000km0.0 0.30.64.03.2WRECKAGE AT A WHITE DWARF