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IAUC 6394: C/1995 O1; C/1996 B2

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                                                  Circular No. 6394
Central Bureau for Astronomical Telegrams
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)
Phone 617-495-7244/7440/7444 (for emergency use only)

     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."

     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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