CCD PHOTOMETRY OF 441 BATHILDE
A. William Neely
NF/ Observatory
Rt. 15 Box 760
San Lorenzo, NM 88051
EMAIL neely@astro.wnmu.edu
CCD observations in V of the asteroid 441 Bathilde were made at the
NF/ Observatory during the 1992 apparition. The synodic rotational
period was 8.571 ñ.006 hours. This is in variance with a
previously published period of 10.35 hours (Harris 1980). The
lightcurve amplitude was m=.19 ñ.02.
Introduction
Asteroid 441 Bathilde is listed as a type M asteroid (Tholen,
1989). It has a diameter of 73.2km ñ2.5km and albedo of .14 with
an absolute magnitude of 8.40V.(Tedesco, 1989). During November and
December of 1992 the asteroid was in a favorable position and
phase. The asteroid was listed as a "Photoelectric Photometer
Opportunity" in the Minor Planet Bulletin(Harris, 1992). The
photometric studies were conducted by A. William Neely.
Observations
The CCD exposures were made through a Johnson V filter on a CRAF-
Cassini 1024x1024 CCD. The pixel size is 1.2 arc seconds. The chip
and amplifier noise is 27 electrons. Full well is 100,000
electrons. The telescope is a .45m newtonian. Control of the
observatory was by digital radio link, and has been described
elsewhere (Neely, 1989). The images were stored on tape and
analyzed with PCVISTA (written by Michael Richmond at Berkeley).
After the centroids of the asteroid and comparison stars were
found, an aperture of 8 arc seconds was integrated, then a annulus
was subtracted for sky.
The comparison and check stars were within the frame of the CCD
image. This reduced the differential atmospheric extinction to
negligible levels and allowed data collection on nights with
marginal seeing. The uncertainties were primarily related to the
bightness of the comparison stars. Since the field is only 10 arc-
minutes, well-matched comparisons are a matter of luck. The
uncertainties on a given night were between .02 and .04 magnitudes.
The comparison stars were calibrated with a Landolt field, cluster
PG 0220 (Landolt, 1983), for UD November 15, 25, 30, and December
2, 1992. All-sky photometry was done to determine a first order
extinction term to correct for airmass differences. The Landholt
field was imaged at zero hour angle to minimize airmass and
extinction corrections. The asteroid magnitudes were averaged and
referenced to the calibrated-comparison stars to calculate a V
magnitude for the asteroid. The uncertainties were .06 mag.
150 second integrations were obtained for all data points. Dark,
bias, and flat fields were taken at the beginning of the night. Up
to 60 exposures were taken per night. The dark and bias fields
were subtracted at the time of readout and the frame was
flatfielded before being stored on tape for later analysis. All
the flatfields were "twilight flats".
Results
CCD observations of the asteroid were made on 1992 UD's: November
13, 15, 16, 25, 30, and December 2. Observational circumstances
for 441 Bathilde are shown in Table 1. The last column gives the
average V magnitude of the asteroid.
The synodic rotational period was calculated using a minimum phase
error method similar to Stellingwerf(1978). Minima and maxima of
the indivdual nightly curves were fitted using a least squares
calculation. The best fits were then compared back to the original
nights. This method determined a period of 8.571 hours ñ.006 with
two unequal maxima per rotation. A fit was attempted to the 'Harris
period' of 10.35 hours without success. The phase corrected data
from 6 nights is shown in Figure 1.
Acknowledgements
This work was supported by a grant from NASA administered by
American Astronomical Society and a grant from the Theodore Dunham
Jr. Fund for Astronomical Research.
References
Harris, A. W. (1992),"Photoelectric Photometer Opportunities". The
Minor Planet Bulletin. 19, 4, 37.
Harris, A. W., Young, J. W.: 1980, Icarus 43, 20 - 32.
Landholt, A. U. (1983),"UBVRI Stars Around the Celestial Equator".
Astronomy J. 88, 439-460.
Neely, A. W. (1989),"The Automatic Radio Linked Telescope (ARLT) at
the NF/ Observatory". In Remote Access Automatic Telescopes(Hayes,
D. and Genet R. M., Ed.), pp. 141-150. Fairborn Press, Tucson.
Stellingwerf, R. F. (1978),"Period Determination using Phase
Dispersion Minimization". Ap. J. 224, 953.
Tedesco, E. P. (1989),"Asteroid Magnitudes, UBV Colors, and IRAS
Albedos and Diameters". In Asteroids II (R. Binzel, T. Gehrels and
M. Shapley Matthews, Ed.), pp. 1090-1138. University of Arizona
Press, Tucson.
Tholen, D. J. (1989). "Asteroid Taxonomic Classifications". In
Asteroids II (R. Binzel, T. Gehrels and M. Shapley Matthews, Ed.),
pp. 1139-1150. University of Arizona Press, Tucson.
Table I. Observational circumstances
for 441 Bathilde
Date Long RA(2000) Dec AU to Earth Ph VMag
11/13/92 63 41 04 36.6 24 21 1.66 7.1
11/15/92 64 10 04 35.0 24 13 1.651 6.0 11.74
11/16/92 64 24 04 34.1 24 08 1.642 5.7
11/25/92 66 34 04 25.9 23 23 1.623 2.1 11.66
11/30/92 67 46 04 21.2 22 56 1.62 1.0 11.61
12/02/92 68 14 04 19.3 22 45 1.62 2.5 11.79
Average magnitudes were calibrated to PG0220 ñ .04.