CCD PHOTOMETRY OF ASTEROID 16 PSYCHE

A. William Neely NF/ Observatory Rt. 15 Box 760 San Lorenzo, NM 88041 EMAIL neely@astro.wnmu.edu CCD observations in V of the asteroid 16 Venetia were made at the NF/ Observatory during the 1992 apparition. The synodic rotational period was 4.196 .06 hours. The lightcurve amplitude was m=.19 .02. This data agrees well with previous studies. Introduction Asteroid 16 Psyche is listed as a type M asteroid (Tholen, 1989). It has a diameter of 264km 4km and albedo of .10 with an absolute magnitude of 5.99V.(Tedesco, 1989). It has been studied previously. Switches between the primary and secondary extrema occurring in some solutions have been a matter of debate (Magnusson, 1989). Therefore, continued monitoring has been important. During February and March 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 with opposite quadrants silvered allowing frame transfer of a 512x512 pixel image. The pixel size is 1.2 arc seconds. The chip and amplifier noise is 25 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). 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 comparison star was calibration was unsuccessful because of bad weather during calibration nights. 90 second integrations were obtained for all data points. Dark fields were taken before each exposure. Approximately 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: February 17, February 22, March 6, March 12, and March 13. Observational circumstances for 16 Psyche are shown in Table 1. The synodic rotational period was calculated using a minimum phase error method similar to Stellingwerf(1978). This method determined a period of 4.196 .005 hours with two unequal minima and two unequal maxima per rotation. This agrees with many published results.(Lagervist, 1989). The phase corrected data is shown in Figure 1. The maximum magnitude difference was m =.19 .02. 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. Additional thanks to Larry Miller for his improvements on PCVISTA and to Fred Treasure who kept the radio link up through a very difficult lightning season. References Harris, A. W. (1992),"Photoelectric Photometer Opportunities". The Minor Planet Bulletin. 19, 1, 5-6. Lagervist, C. I., Harris, A. W., Zappala, V.,"Asteroid Lightcurve Parameters". In Asteroids II (R. Binzel, T. Gehrels and M. Shapley Matthews, Ed.), pp. 1162-1179. University of Arizona Press, Tucson. Magnusson, P., Barucci, M. A., Drummond, J. D., Lumme, K., Ostro, S. J., Surdej, J. Taylor, R. C., Zappala, V., "Determinations of Pole Orientations and Shapes of Asteroids". In Asteroids II (R. Binzel, T. Gehrels and M. Shapley Matthews, Ed.), pp. 66-98. University of Arizona Press, Tucson. 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 16 Psyche Date Long RA(2000) Dec AU to Earth VMag 2/17/92 158 42 10 59.1 7 11 2.236 4.8 10.6 2/22/92 159 20 10 56.2 7 33 2.226 2.2 10.5 3/06/92 161 36 10 45.3 8 50 2.229 1.8 10.5 3/12/92 162 24 10 41.5 9 17 2.244 4.1 10.6 3/13/92 162 34 10 40.8 9 22 2.248 4.4 10.6