Central Bureau for Astronomical Telegrams
IAUC 9256: C/2011 L4
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Circular No. 9256
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
New postal address: Hoffman Lab 209; Harvard University;
20 Oxford St.; Cambridge, MA 02138; U.S.A.
CBATIAU@EPS.HARVARD.EDU ISSN 0081-0304
URL http://www.cbat.eps.harvard.edu/index.html
Prepared using the Tamkin Foundation Computer Network
COMETS C/2011 L4 (PANSTARRS) AND C/2012 F6 (LEMMON)
C. E. Woodward, University of Minnesota; R. W. Russell and
D. L. Kim, The Aerospace Corporation; B. Cabreira, NASA Infrared
Telescope Facility (IRTF); M. L. Sitko, Space Science Institute; D.
H. Wooden, Ames Research Center, NASA; M. S. Kelley, University of
Maryland; and D. E. Harker, Center for Astrophysics and Space
Science, University of California, San Diego, report thermal-
infrared data of comets C/2011 L4 on Apr. 7.88 UT and C/2012 F6 on
Apr. 7.97, obtained with the The Aerospace Corporation Broadband
Array Spectrograph System (BASS) on the NASA IRTF during daylight.
Comet C/2011 L4 (cf. CBET 9254) exhibited a 10-micron silicate
emission feature that was smooth, with a feature-to-continuum ratio
of about 1.4, where the temperature of the underlying continuum is
about 312 K. A weak crystalline olivine feature at 11.3 microns
was evident. Spectrophotometry of C/2011 L4 occurred over a two-
day period, and temporal variations are being assessed. Daytime
observations, with seeing and image motions, complicated
photometric calibration of the absolute level of the spectral
energy distribution (SED). However, BASS observes all wavelengths
from 2 to 13 microns simultaneously; thus the shape and structure
of the 10-microns silicate feature are much less sensitive to the
seeing and image motion than the absolute photometric values of the
spectrophotometric data points. Comet C/2012 F6 (cf. CBET 9255)
also exhibited a 10-micron silicate feature with feature-to-
continuum ratio of about 1.3, with the dust continuum exhibiting a
higher temperature of about 352 K. The silicate feature in C/2012
F6 is more trapezoidal in shape, with the longer-wavelength end of
the feature (near 12 microns) likely being a bit stronger than the
bluer-end (near 9.0 micron). No narrow 11.3-micron crystalline-
olivine component is present. In contrast, C/2011 L4 exhibits
more of a 9.7-micron peak, with a second rise in the silicate
feature at 11.3 microns that is indicative of a small crystalline-
olivine component. Approximate [N]-band magnitudes in a 4".4
circular aperture of -0.44 (C/2011 L4) and -0.28 (C/2012 F6) are
derived from the SEDs, with fairly large uncertainties in the
absolute brightness. Scattered sunlight at shorter wavelengths
was clearly seen in the spectra of both comets.
Additional visual total-magnitudes estimates for C/2011 L4:
Mar. 13.09 UT, 1.3 (C. W. Hergenrother, Tucson, Arizona, naked eye);
17.81, 2.2 (J. J. Gonzalez, Leon, Spain, naked eye); 22.81, 2.8 (O.
Skilbrei, Honefoss, Norway, naked eye); 26.81, 3.6 (B. H. Granslo,
Oslo, Norway, 10x50 binoculars); 30.81, 4.4 (J. J. Gonzalez,
Zamora, Spain, 10x50 binoculars); Apr. 8.87, 5.5 (Granslo); 13.97,
6.1 (Granslo).
(C) Copyright 2013 CBAT
2013 April 17 (9256) Daniel W. E. Green
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