JAC Style guide: template
 Comet Hale-Bopp (C/1995 O1)
The authors are collaborating in the search for supervolatiles in active comets. Supervolatiles (ices more
volatile than water ice) have long been suspected as drivers of the activity in distant comets, but only
recently has technology advanced to the point where meaningful attempts at direct observation are possible.
They used the JCMT to search for gaseous emissions in the bright, distant comet Hale-Bopp. Emission
from the 230 GHz CO[2-1] rotational emission line was first suspected on UT 1995 Sept 5 and 7, but
appeared partially confused with galactic CO lines. Subsequent observations on Sept. 19 and 20 and Oct 16
showed the line free of contamination. The discovery was reported (Matthews et al. 1995; see Figure 1)
and soon confirmed by observations at IRAM (Rauer et al. 1995). The line appears blue-shifted relative to
the instantaneous radial velocity of the comet by about 350 m/s. The blue-shift results from sunward
ejection of the carbon monoxide in response to solar heating of the nucleus. The area of the line is variable,
and corresponds to a CO production rate in the range 1000 kg/s to 2000 kg/s (1 to 2 tonne/s). Such
prodigious outgassing rates are not normally attained by comets until they are much closer to the sun.
The gross properties of the line are very similar to those observed earlier at the JCMT in comet
P/Schwassmann-Wachmann 1 (Senay and Jewitt 1994). The activity of both comets, at distances ~6 AU
from the sun, is readily explained by the outgassing of CO. At 6 AU, any water ice in the nuclei will be
frozen solid, and cannot contribute to the observed cometary activity. Whereas the orbit of
P/SW1 is nearly circular, giving no opportunity for the comet to be heated enough for water to ever
sublimate, the orbit of Hale-Bopp is highly elliptical. We expect that water ice in Hale-Bopp will begin to
sublimate strongly when the comet is 3 to 4 AU from the sun. It will reach this distance early in the summer
of 1996. Thereafter, water may rival or surpass CO as the driver of outgassing. Hale-Bopp presents an
unprecedented opportunity to study the development of activity in a comet that is still far from the sun. It
should provide exciting new constraints on the volatile abundance and outgassing modes of comets.
Figure 1: CO spectra from Comet Hale-Bopp.
The spectra were taken at the James Clerk Maxwell Telescope on Mauna Kea, using the RxA2 receiver, and
are described in a paper to appear in the February 23 issue of SCIENCE by Jewitt, Senay and
Matthews.
References:
D. Jewitt, M. Senay, and H. Matthews (1996), Observations of Carbon Monoxide in Comet Hale-
Bopp, Science, in press (February 23 issue).
H. Matthews, D. Jewitt, and M. Senay. (1995). IAU Circular 6234 (September 21).
M. Senay and D. Jewitt (1994), Activity in a Distant Comet: First Detection of Carbon Monoxide,
Nature, 371, 229-231.
H. Rauer, D. Despois, R. Moreno, G. Paubert, N. Biver, D. Bockelee-Morvan, P. Colom, J. Crovisier, and
L. Jorda (1995). IAU Circular 6236 (September 25).
Henry Matthews, JAC,
David Jewitt & Matt Senay, IfA, Univ. of Hawaii
for further adventures of Dave with Submillimetre Astronomy please look at:
Dave's Submillimetre page
Last Modification Date 1996/04/08 - Last Modification Author: Graeme Watt (gdw)
|
