Abstract:
We report the beginning of activity for comet C
/
2015 ER61
(
PANSTARRS
)
, the
fi
rst instance of watching a long-
period comet turn on. Pre-discovery observations and observations from the
NEOWISE
space telescope suggest
that the nucleus is large, with a radius of
R
N
∼
9 km, assuming an albedo of 0.025. Our photometric data follows
the comet from
r
=
8.9 to 4.8 au as it moved into solar conjunction in 2016 July. Our sublimation model shows
that activity began near
r
=
8.8 au
(
true anomaly, TA
=
−
139
°
)
in early 2015, driven by CO
2
sublimation, which
peaked in 2016 April at
r
=
5.1 au
(
TA
=
−
127
°
)
. Appreciable water sublimation began around
r
=
5.0 au. Our
sublimation model is consistent with an active water sublimation area of 1% of the surface
(
equivalent to
10.2 km
2
)
, and an active surface area for CO
2
sublimation of 0.029%
(
0.3 km
2
)
. The CO
2
production rate at
r
=
4.66 au as measured by
NEOWISE
is
(
8.4
±
2
)
×
10
25
s
−
1
.IfCO
2
-ice had been present on the surface, dust
dragged from the surface by sublimation would have been observed much farther out
—
as far as 20 au. Our thermal
models suggest that the CO
2
ice was present at a depth of 0.4 m. The comet came out of solar conjunction in 2016
December
and, unless it brightens signi
fi
cantly, is unlikely to have water production rates much higher than a few
×
10
28
s
−
1
.
