Central Bureau for Astronomical Telegrams
IAUC 6636: C/1995 O1
The following International Astronomical Union Circular may be linked-to from your own Web pages, but must not otherwise be redistributed (see these notes on the conditions under which circulars are made available on our WWW site).
Read IAUC 6637
.dvi or
.ps format.
Circular No. 6636
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, 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)
URL http://cfa-www.harvard.edu/cfa/ps/cbat.html
Phone 617-495-7244/7440/7444 (for emergency use only)
COMET C/1995 O1 (HALE-BOPP)
Z. Sekanina, Jet Propulsion Laboratory, reports: "Available
information on the sodium tail (IAUC 6631, 6634), combined with
the photoionization lifetime of 0.56 day for sodium atoms at r =
0.97 AU from the sun (inferred as an upper limit to their
destruction lifetime; Spinrad and Miner 1968, Ap.J. 153, 355) and
with the radiation-pressure acceleration of 52 cm/s**2 at the same
r, or beta = 83 (Wurm 1963, Icarus 2, 29), shows that sodium atoms
could not be released directly from the nucleus or from dust grains
of any size (visible or invisible). The first option is eliminated
because of the short destruction lifetime, the second -- in
addition -- because the accelerations on all particulates in comets
are beta_dust < 3, more than one order of magnitude lower than
required. Thus, a sodium tail of this kind would in the best case
trace the leading boundary of the dust tail. The most probable
parents consistent with observation are sodium-bearing molecular
species with relatively long dissociation lifetimes, which could
have been initially released from the comet in dust particles but
separated soon after their ejection. The observed width of the
sodium tail is consistent with the above photoionization lifetime
at an expansion velocity of < 7 km/s. If this velocity is provided
entirely by the parent's dissociation, the implied dissociation
energy is several eV, and Spinrad and Miner list a number of
molecules that would generally fit this constraint. The combined
imaging and spectroscopic observations from Apr. 19.90 require
beta_parent = 91 (coincident within errors with the sodium's beta
value) and the parent's dissociation lifetime of tau_parent = 2.2
days to explain the tail's presence in p.a. 56 deg at a distance of
1.5 deg from the nucleus. The calculated line-of-sight velocity is
+114 km/s, close to the observed value. On Apr. 20.85 UT at 0.7
deg from the nucleus, the implied tau_parent = 1.5 days (for the
same value of beta_parent) and the line-of-sight velocity is
calculated to be 57 km/s after correction for the comet's
contribution, in excellent agreement with observation. For Apr.
16.88, the tail's length of 6.6 deg would require tau_parent = 4.8
days for beta = 91, but the calculated p.a. is then 47 deg. A
better fit is provided by beta approximately 450 at tau_parent =
2.2 days, in which case p.a. = 51 deg, in good agreement with
observation. In any case, the constraint implied by the tail's
maximum length of 6.6 deg is [tau (beta)**0.5]_parent </= 50r**2
days, which could be tested by a continuing monitoring of the
tail's appearance (after accounting for production variations with
r)."
(C) Copyright 1997 CBAT
1997 April 25 (6636) Daniel W. E. Green
Read IAUC 6637
.dvi or
.ps format.
Our Web policy. Index to the CBAT/MPC/ICQ pages.
