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IAUC 7461: delta Sco; C/1999 S4

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                                                  Circular No. 7461
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/iau/cbat.html  ISSN 0081-0304
Phone 617-495-7244/7440/7444 (for emergency use only)

     J. Fabregat, University of Valencia; and P. Reig, Foundation
for Research and Technology Hellas and University of Crete,
Heraklion, write:  "This star is undergoing a major optical and
H-alpha outburst.  Spectra (resolution 0.3 nm, range 580-760 nm)
taken with the Skinakas Observatory 1.3-m telescope reveal the
H-alpha line in emission.  Measured equivalent widths are -0.34 +/-
0.02 nm on July 19.89 UT and -0.33 +/- 0.02 nm (FWHM 0.69 +/- 0.01
nm) on July 20.90.  We made observations after S. Otero, Buenos
Aires, reported a gradual brightening (visual mag estimates:  June
30.03, 2.24; July 4.08, 2.23; 16.09, 2.06; 20.02, 2.04).  In 1990,
Cote and van Kerkwijk (1993, A.Ap. 274, 870) observed weak emission
on the flanks of the H-alpha line core in absorption, and they
proposed the classification of delta Sco as a Be star.  All other
observations in the literature present a normal absorption-line
spectrum, while our observations constitute the first detection of
emission lines in delta Sco, confirming its Be-star nature."

     H. Weaver, Johns Hopkins University, on behalf of the Hubble
Space Telescope (HST) ToO comet team, reports the following results:
"HST images of the comet show a dramatic increase in activity on
July 5, with the flux in a 0".15 square aperture increasing by a
factor of about 1.5 in just under 4 hr (from July 5.776 to 5.940
UT).  During HST observations one day later (July 6.717-6.889), the
activity levels were decreasing and were about 3 times lower for
the final observation, compared to the peak value from the previous
day.  The flux in the last HST image (on July 7.961) was about 7
times lower than the peak value measured on July 5.  On July 7, at
least one 'fragment' is seen 0".85 (460 km, projected) from the
nucleus in the tailward direction, and a sharp tailward spike of
emission is observed, reminiscent of the morphology observed during
the outburst in C/1996 B2 (Hyakutake) in late-March 1996.  HST
spectroscopic data taken on July 5 with STIS show evidence for
emissions from CO, C, H, O, and possibly S.  S_2, CS, and OH were
detected during STIS observations on July 6, and OH, CS, NH, and
possibly S_2 were detected on July 7.  Preliminary production rates
are 5 x 10**26 (July 5), 1.4 x 10**24 (July 6), and 1.2 x 10**29
(July 6) for CO, S_2, and H_2O, respectively, but the CO and S_2
values could change by a factor of about 2 or so, pending final
analysis.  Nevertheless, one firm conclusion is that CO is strongly
depleted in C/1999 S4, relative to the observed abundances in
C/1996 B2 and C/1995 O1."

                      (C) Copyright 2000 CBAT
2000 July 20                   (7461)            Daniel W. E. Green

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