C.C.D. Photometry of 16 Psyche

Robert W. Brown
A. William Neely
NF/ Observatory
Rt. 15 Box 760
San Lorenzo, New Mexico, 88041


br> CCD observations in V of the asteroid 16 Psyche were made at the NF/Observatory at two different oppositions. Exposures were made on nine nights in May and June, 1993. Additional exposures were made on U.D. August 14, 1994. Spin vector of asteroid was determined using the amplitude-magnitude method and was found to be at latitude Ī9Ż and longitude 222Ż. This is in variance with some previously published results. Additionally, a Vmax of 5.80 at the minimum assumed aspect angle was derived. This is 0.2 magnitude less than reported from the 1973 opposition (Tedesco et al, 1985).

16 Psyche is a well studied main belt asteroid listed as M-class with a rotational period of 4.196 hours. Lightcurve amplitude varies from 0.03 to 0.42 (Lagerkvist et al.). Previously published pole position for spin vector have varied in ecliptic longitudes 215Ż to 225Ż, and ecliptic latitudes 4Ż to 55Ż (Magnusson, P. “Pole Determinations of Asteroids”). Orbital velocity of 16 Psyche has made observations at opposition possible only at four heliocentric longitudes (near 70Ż, 160Ż, 220Ż & 310Ż). This fact has increased uncertainty on pole position.calculations. A more accurate pole latitude designation for 16 Psyche is likely from the ten nights of favorable data taken at N/F Observatory.

C.C.D. 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 floor noise is 27 electrons. Full well is 100,000 electrons. The telescope is a 0.45-m Newtonian. Control of the observatory was done by radio link and has been described elsewhere (Neely, 1989). Differential photometry was used between the asteroid and comparison star within the same field. Exposures were made for 80 seconds. Data reduction and analysis was accomplished using 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, and an annulus was used for computing and subtracting the sky background. The V magnitudes were all transformed to the standard system.

Nine nights of observations in April ,May, June, 1993:
16 Psyche was imaged with differential comparison stars on U.D dates 4/27, 5/1, 5/2, 5/3, 5/11, 6/4, 6/5, 6/6 & 6/7. During this apparition, a heliocentric longitude of Ň228Ż presents the lowest photometric amplitude variation. Star 4383-0434 (Guide Star Catalogue) was imaged on May 3, 1993 and used for calibration of the comparison stars. Time variable extinction was calculated by plotting a curve of the calibration star and comparison stars . Star 4383-0434 had been extensibly studied for calibration in photometry of S.N. 1993j (Vaucouleurs, 1994) and has a good color match with 16 Psyche. Additional photometry of comparison stars in all fields were used to callibrate instrumental V magnitude to the standard magnitude system.

One night of observations, U.D. August 14, 1994:
On this date, 16 Psyche was imaged at heliocentric longitude 314Ż and presented a high photometric amplitude. Calibration frames were made using Landholdt 112-275. Airmass between comparison and calibration stars were negligible .

The observational circumstances for two apparitions are shown in table #1 and include Vmax reduced to standard distance to determine V(1,0). It should be noted that on U.D. 5/11/93, 16 Psyche was at heliocentric longitude of 228Ż 14’ and yielded an absolute magnitude of 5.80 which is brighter than previous measurements. It is possible that data contained within this paper represents the most favorable observational circumstances to determine aspect angle. Using data from observations cited and previously published sources (Taylor et al., Tedesco et al. 1982, Tedesco et al. 1984, Lupishko et al., Pfleiderer et al., Zhou et al. and Weidenschilling et al..), pole position was calculated by amplitude-magnitude method . A north pole longitude of Ň222Ż is in general agreement with past research. However, north pole latitude are much harder to determine. Previously published pole positions vary as much as 51Ż in latitude. Latitude designations widely vary for a number of reasons. First, the ecliptic plane and pole latitude are in apparent proximity. Due to this proximity, small errors in the variation of V max amplitude produces wide results using the amplitude-magnitude method. For instance, a latitude of 37Ż could be explained by an amplitude variation of .07. Additional latitudes have been derived using the epoch method. Although the epoch method has the advantage of not being phase angle dependent, two varying solutions for pole position are inherently produced. Opportunities to gather data for pole position of 16 Psyche have been limited due to an orbital period of 4.99 years and synodic period of Ň1.25 years. Using the amplitude-magnitude method demands a phase angle of <15Ż(Magnusson et al.), yet the orbital period offers only four heliocentric longitudes to gather data. A synodic orbital period of Ň1.25 years,coupled with a pole latitude proximate to the ecliptic offers an observed amplitude at true minimum in five year increments. Using the epoch method, Tedesco et al. indicates a pole longitude, latitude of 223Ż and 37Ż respectively, but chose not to use data taken by Lupishko et al. (1980) on May 5, 1978 due to incongruities in the reported lightcurve (Tedesco et al.). However, a minimum variation in amplitude of 0.03 Ī0.005 reported by this paper is in agreement with Lupishko et al.(1980). A designation of 8.7ŻĪ5Ż in pole latitude is further strengthened by the superimposed lightcurves of three nights of data shown in figure #1. Finally on 8/14/94, 16 Psyche offered opposition at heliocentric longitude 314Ż 19’. A Vmag of 0.32 Ī0.05 was observed on this date. Shape of 16 Psyche was derived from this Vmag and the minimum amplitude variation of 0.03 Ī0.005 mentioned above. The result are as follows: a/b=1.35, b/c=1.29, a/c=1.76.

Table 1
Observational Circumstances
U.D. Long. Earth-Ast. (A.U) Sun- Ast. (A.U.) Phase Vmax V(1,0)
4/27/94 226Ż 01’ 2.26 3.25 3.4Ż 10.56 . . .
5/01/93 226Ż 39’ 2.25 3.25 2.8Ż 10.54 . . .
5/02/93 226Ż 49’ 2.25 3.25 2.6Ż 10.38 . . .
5/03/93 226Ż 58’ 2.25 3.25 2.4Ż 10.32 . . .
5/11/93 * 228Ż 14’ 2.25 3.24 2.0Ż 10.27 5.80
6/04/93 232Ż 04’ 2.32 3.23 9.8 10.80 . . .
6/05/93 232Ż 14’ 2.33 3.23 10.1 10.78 . . .
6/06/93 232Ż 24’ 2.34 3.23 10.5 10.79 . . .
6/07/93 232Ż 33’ 2.34 3.23 10.9Ż 10.85 . . .
8/14/94 314Ż 19’ 1.71 2.71 2.0Ż 9.62 6.13
*U.D. 5/11/94 used for extrapolation of V(1,0)

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