Central Bureau for Astronomical Telegrams
IAUC 6394: C/1995 O1; C/1996 B2
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Circular No. 6394
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
BMARSDEN@CFA.HARVARD.EDU or DGREEN@CFA.HARVARD.EDU (science)
Phone 617-495-7244/7440/7444 (for emergency use only)
COMET C/1995 O1 (HALE-BOPP)
J. Crovisier, E. Gerard, P. Colom, N. Biver, D. Bockelee-
Morvan, and H. Rauer, Observatoire de Paris-Meudon, report the
detection of the 18-cm lines of OH with the Nancay radio telescope
from the average of spectra observed on Apr. 12-19 and May 1-8:
"The line areas at 1667 MHz are 0.028 +/- 0.007 and 0.027 +/- 0.007
Jy km sE-1, respectively. This would correspond to a water-
production rate of 4.2 x 10E28 molecules/s, using our usual OH
excitation and space distribution model, which assumes outgassing
from the nucleus. This rate is three times larger than that
estimated for Apr. 7 from ultraviolet observations (IAUC 6376).
This discrepancy may be due to shortcomings of the distribution
model and might be explained by OH coming from icy grains."
COMET C/1996 B2 (HYAKUTAKE)
J. P. Pye and R. G. West, Leicester University; M. Harden and
M. Ricketts, Rutherford Appleton Laboratory; K. Dennerl, J.
Englhauser, J. Schmitt, and J. Trumper, Max-Planck-Institut fur
Extraterrestrische Physik, Garching; and C. Lisse, M. Mumma, and R.
Petre, NASA Goddard Space Flight Center, report the discovery of
extreme-ultraviolet (EUV) emission from comet C/1996 B2 with the
ROSAT Wide Field Camera (WFC), simultaneously with the x-ray
measurements reported by Lisse et al. on IAUC 6373: "The WFC
measurements were made with the S1A filter, nominal bandpass 90-206
eV (14-6 nm). An image can be found at
http://ledas-www.star.le.ac.uk/rosat-goc/comet/. Bright, diffuse
emission is seen, roughly coincident with the x-rays and sunward of
the nucleus, and strongly time-variable in a similar manner. The
peak EUV surface brightness is of order 0.0004 count sE-1 arcminE-2
as measured, or of order 0.003 count sE-1 arcminE-2 when corrected
to WFC 'at launch' efficiency. The WFC images also show fainter
diffuse emission extending beyond the edge of the HRI field-of-view
(i.e., at about 15'-40' from the center of the comet nucleus),
forming an arc around the bright central region. The ratio of WFC
to HRI countrates shows that the spectrum in the EUV/x-ray band is
rather 'soft' and steeply increasing towards low photon energies.
This would appear to rule out soft-x-ray line-fluorescence as the
dominant emission mechanism. For assumed spectral shapes of a
power law or thermal bremsstrahlung emission, the ratio indicates a
photon index in the approximate range 2.0-3.0, or a temperature of
about 0.1-0.4 keV."
(C) Copyright 1996 CBAT
1996 May 9 (6394) Daniel W. E. Green
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