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IAUC 8938: C/2008 H1; V2491 Cyg

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IAUC number

                                                  Circular No. 8938
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://www.cfa.harvard.edu/iau/cbat.html  ISSN 0081-0304
Phone 617-495-7440/7244/7444 (for emergency use only)

     An apparently asteroidal object discovered by the LINEAR
project (discovery observation below) and posted on the Minor
Planet Center's 'NEOCP' webpage, was first reported as cometary by
E. Reina L., Hospitalet, Spain, on Apr. 19.0 UT (0.25-m f/3.3
Schmidt-Cassegrain reflector), who noted a 25" coma of total mag
16.6 and a 43" tail in p.a. 233 deg.  E. Guido and G. Sostero,
observing remotely from Mayhill, NM, U.S.A., on Apr. 19.5 (0.25-m
f/3.4 reflector), report that stacking twenty unfiltered 60-s CCD
exposures in strong moonlight showed the object to be diffuse in
comparison to stars of similar brightness.  J. M. Aymami, Tiana,
Spain (0.13-m f/5.9 refractor), remarked on a suspected coma in
p.a. 209 deg on Apr. 19.8.  At the same time R. Apitzsch, Wildberg,
Germany (0.35-m f/4.2 reflector) noted a tail to the southwest.

     2008 UT             R.A. (2000) Decl.       Mag.
     Apr. 18.34722   17 02 01.68   +79 15 56.0   18.1

The available astrometry, the preliminary parabolic orbital
elements (T = 2008 Mar. 11.781 TT, q = 2.78957 AU, Peri. = 94.644
deg, Node = 33.724 deg, i = 75.770 deg, equinox 2000.0), and an
ephemeris appear on MPEC 2008-H09.

     R. J. Rudy, D. K. Lynch, and R. W. Russell, The Aerospace
Corporation; C. E. Woodward, University of Minnesota; and K. Covey,
Center for Astrophysics, report on SpeX observations (wavelength
range 0.8-2.5 microns) of this nova (cf. IAUC 8934) obtained at the
Infrared Telescope Facility on Apr. 17.6 UT.  In the five days
since Lynch et al. (IAUC 8935) observed V2491 Cyg on Apr. 12, it
declined in brightness by a factor of 3.  The strengths of the
emission lines compared to the continuum have generally increased,
as has the excitation of the emission-line spectrum.  The neutral
helium triplet and singlet at 1.08 and 2.06 microns, respectively,
doubled in size relative to the hydrogen lines.  Although the C I
and N I lines decreased, the O I lines at 0.84 and 1.13 microns,
which are fluorescently excited by Lyman_beta, are now the
strongest emission features in the infrared spectrum.  Their
increase has permitted more accurate measurements of their relative
strengths, which indicate a reddening of E(B-V) = 0.43, a value
slightly greater than that reported by Lynch et al.  Photometric
magnitudes on Apr. 17:  J = 7.7, H = 7.8, K = 7.4.

                      (C) Copyright 2008 CBAT
2008 April 23                  (8938)            Daniel W. E. Green

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