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angles and those on alternate nights nearly the same phase in the
lightcurve, with a slow circulation toward the right (increasing
phase). New all-night observations of both sides of the lightcurve
were obtained every 4 to 8 days to achieve complete coverage
with abundant overlap. The most prominent feature is a single
deep minimum near phase 0.23. As the rotational phase presented
during the observable part of the night slowly circulated, three sets
of observations of this feature were obtained that served to anchor
the period determination; however the 46.551 hour period also fits
the shallow features well. The possibility that the period might be
twice as long was investigated and can be ruled out entirely. On a
lightcurve phased to 93.11 h, the two halves looked very similar to
each other and to the lightcurve phased to 46.551 h. A shape
sufficiently irregular to produce this lightcurve yet invariant over a
180-degree rotation is unlikely for real asteroids. An
approximately 93-hour period is compatible with the 0.22 mag
amplitude lightcurve obtained by Bembrick and Allen (2005) only
if there are four maxima and minima per cycle. While such a curve
is possible, even with amplitudes of about 0.2 mag, they are rare
and require extraordinary evidence. Such evidence is lacking in
this case.
131 Vala. Pilcher (2008) published a symmetric bimodal
lightcurve phased to 10.359 h based on observations obtained
2007 October and November that showed an amplitude 0.09 ±
0.02 mag. However, the result was not considered secure since the
high degree of symmetry and small amplitude suggested a pole-on
viewing aspect with a period near 5.18 h and monomodal
lightcurve. Investigation at the following opposition was
recommended to resolve the ambiguity. The first night of
observation, 2009 Feb. 7, confirmed the pole-on hypothesis,
showing a bimodal lightcurve bimodal with P = 5.18 h.
Lightcurves for the 2007 and 2009 observations are presented here
with the 2007 data phased to a period of 5.179 ± 0.001 h. The new
observations on five nights 2009 Feb. 7 – Mar. 14 show a period
5.1812 ± 0.0001 h, amplitude 0.28 ± 0.03 mag.
271 Penthesilea. Harris et al. (2008) list no previous photometric
measurements. Observations on seven nights 2009 Jan. 1 – Feb.
17 show a period of 18.787 ± 0.001 h, amplitude 0.32 ± 0.04 mag.
Acknowledgements
The author thanks Alan W. Harris for helpful suggestions on the
data analysis for 157 Dejanira.
References
Bembrick, C. S., and Allen, B., (2005) “120 Lachesis – A Very
Slow Rotator.” Minor Planet Bul. 32, 45-46.
Harris, A. W., Warner, B. D., and Pravec, P., “Asteroid Lightcurve
Data File, Revised Mar. 10, 2008.”
http://www.MinorPlanetObserver.com/astlc/default.htm.
Pilcher, F. (2008) “Period Determinations for 84 Klio, 98 Ianthe,
102 Miriam, 112 Iphigenia, 131 Vala, and 650 Amalasuntha.”
Minor Planet Bul. 35, 71-72.
Warner, B. D., (2005) “Asteroid Lightcurve Analysis at the
Palmer Divide Observatory – Spring 2005.” Minor Planet Bul. 32,
90-92.
If one obtains a nearly symmetric bimodal lightcurve of small
amplitude, for which the two halves look almost the same, there is
a good possibility that the target asteroid is in near-polar aspect
with a rotation period half that of the bimodal lightcurve. If
possible the object should be observed at a future opposition at
which the phase angle bisector longitude is at near right angles to
the original observations. If the two lightcurves show significantly
different amplitudes, then the viewing aspect for the apparition
with the lower amplitude was very probably pole-on.
157 Dejanira. Warner (2005) obtained lightcurves on six nights
2005 Apr. 22-May 13 and found a period of 15.819 h, amplitude
0.52 mag. One set of new observations was obtained on five
nights, 2008 Dec. 21 – 2009 Jan. 8, at phase angles 24.4° to 19.7°
with full phase coverage and showed a synodic period 15.817 ±
0.001 h, amplitude 0.48 ± 0.02 mag., in excellent agreement with
Warner (2005). A second set of observations on three nights 2009
Feb. 11–23 at phase angles 9.9° to 11.4°, also with full phase
coverage, showed a synodic period 15.828 ± 0.001 h, amplitude
0.38 ± 0.02 mag. This smaller amplitude at smaller phase angle is
usually encountered for asteroids because of less shadowing. A
synodic period increase on the later dates when the prograde rate
of motion of the phase angle bisector is much smaller than in
December/January implies retrograde rotation. Because real errors
in rotation periods are often several times larger than formal
errors, this suggestion should be considered provisional. The
observations on all eight nights evaluated as a single set produce a
mean synodic period of 15.825 ± 0.001 h.
Minor Planet Bulletin 36 (2009)