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IAUC 6634: C/1995 O1; 29P

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                                                  Circular No. 6634
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
URL http://cfa-www.harvard.edu/cfa/ps/cbat.html
Phone 617-495-7244/7440/7444 (for emergency use only)

     G. Cremonese, Astronomical Observatory, Padua; H. Rauer, Observatoire
de Paris, Meudon; and A. Fitzsimmons, Queen's University, Belfast,
on behalf of the European Hale-Bopp Team, report further observations
pertaining to the neutral-sodium tail recently discovered in C/1995 O1
(IAUC 6631; c.f. http://ing.iac.es/PR/Caption_ING397.html): "Spectra were
obtained on Apr. 19.90 UT with the Utrecht Echelle Spectrograph at the
4.2-m William Herschel Telescope at the Observatorio del Roque de los
Muchachos, La Palma, with a 4".0 by 1".1 slit positioned 1.5 deg along the
sodium tail at p.a. 56 deg (this position angle being determined by
simultaneous imaging with the CoCAM instrument).  The location of the
slit corresponds to a cometocentric distance of 1.1 x 10**7 km along the
tail.  A series of spectral windows were observed between 500 nm and
800 nm at a resolution of 7 km/s.  In a 15-min exposure, strong Na D
lines were detected (their strength relative to the sky lines being
approximately 1.3:1), redshifted at a line-of-sight velocity of
+105 km/s, which corresponds to a cometocentric velocity of approximately
95 km/s along the tail.  The lines are resolved with an intrinsic FWHM
of about 11 km/s.  These data confirmed that neutral Na is present in
the newly-detected third type of cometary tail.  Emissions from H2O+, NH2
and C2 (among other molecules) were not detected at this tail position.
The same instrumentation was used on Apr. 20.85 UT to obtain spectra at
10" from the nucleus and at 0.7 deg (cometocentric distance 5.0 x 10**6
km) along the Na tail.  Once again, Na D-line emission associated with
the comet was observed, at cometocentric velocities of 0 and 58 km/s,
respectively.  From the measured velocity distribution and surface
brightness along the tail, it would appear that the data are consistent
with a model in which neutral sodium atoms are released from
still-unidentified sources within the coma and then accelerated in the
antisolar direction by simple fluorescence.  At the same time as the
Apr. 19.90 spectroscopic data were being secured, CCD imaging was performed
by J. Licandro, Instituto de Astrofisica de Canarias, with the 1-m
Jacobus Kapteyn Telescope through a narrow-band filter (lambda_0 =
587.7 nm, FWHM = 4.9 nm) and an adjacent continuum filter.  A comparison
between these images over the 5' field of view did not reveal any
features that could be identified as peculiar to neutral Na."

     Visual m1 estimates: Mar. 10.93 UT, 13.5 (K. Hornoch, Lelekovice,
Czech Republic, 0.35-m refl.); 28.93, 14.2 (J. Carvajal, Teruel, Spain,
0.44-m refl.); Apr. 1.92, [13.5 (K. Sarneczky, Raktanya, Hungary,
0.44-m refl.).

                      (C) Copyright 1997 CBAT
1997 April 21                  (6634)              Brian G. Marsden

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