1 THE MINOR PLANET BULLETIN OF THE MINOR PLANETS SECTION OF THE BULLETIN ASSOCIATION OF LUNAR AND PLANETARY OBSERVERS VOLUME 31, NUMBER 1, A.D. 2004 JANUARY-MARCH 1. ASTROMETRIC PROGRAM FOR JR refers to asteroids nearby to the 3:1 mean motion resonance NEAR-EARTH OBJECT SOURCES with Jupiter, MC are the Mars Crossers, NU are objects in the ν 6 secular resonance. Sergio Foglia UAI Minor Planets Section Recorder References F. Bisleri 11, I-20148 Milano, Italy € [email protected] Morbidelli et. al. (2003). Understanding the distribution on Near Earth Objects. http://www.obs-nice.fr/morby/ESA/ esa.html (Received: 24 September Revised: 6 October) MPC Orbit Database. ftp://ftp-cfa.harvard.edu/pub/MPCORB/ 2003 Sep. 17, Minor Planet Center A new astroometric program is proposed for asteroids located in likely source regions supplying the near-Earth Proper Elements of Minor Planets. http://hamilton.dm object population. .unipi.it/astdys/ 2003 Sep. 17, Asteroid Dynamic Site According to Morbidelli et al. (2003), near-Earth objects (NEOs) come mainly from 5 sources with the following contributions to the population: the ν 6 resonance region at the inner border of the asteroid main belt (37 ± 8 %), the 3:1 resonance region in the middle of the asteroid main belt (23 ± 8 %), the Intermediate Mars-Crossing (IMC) population (25 ± 3 %), the Outer Belt (OB) population€ (8 ± 1 %), and the population of dormant Jupiter Family Comets (JFC) (6 ± 4 %). Astrometric measurements of objects in these source regions would be very useful to increase knowledge about the NEO source population and solar system dynamics. Thus a new astrometric program for NEO source bodies involving amateur astronomers is suggested. For this program, most objects are usually brigther than 18.0 V magnitude and thus are within the range of equipment for many observers to obtain good measurements. Our goal is to suggest an observing program named Near Earth Figure 1. Depiction of asteroids for the NEOSAP program. Light Objects Source Astrometric Program (NEOSAP) involving the grey bordered rectangles are the Mars Crossers, dark grey following objects: Mars Crossers having q < 1.52 and Q > 1.52, bordered rectangles are objects in the ν 6 secular resonance, and where q and Q are perihelion and aphelion distances in AU; filled rectangles are asteroids nearby to the 3:1 mean motion asteroids in the ν 6 secular resonance; asteroids nearby to the 3:1 resonance with Jupiter. mean motion resonance with Jupiter. € Interested observers will find ephemeris and other information at the following€ URL: http://www.uai.it/sez_ast/neosap.htm Table I: Numbers of NEOSAP asteroids. Astrometric measurements must be sent to the Minor Planet Center in the usual way; no other action is required by observers JR 717 and the listed URL serves only to provide information and MC 726 ephemerides for recommended targets. MC + JR 5 MC + NU 10 NU 116 Figure 1 shows the distribution of these objects in the plane of the Total 1574 orbital elements semi-major axis (a) and inclination (i). Table I gives numbers of NEOSAP asteroids in the different categories: Minor Planet Bulletin 31 (2004) 2 ROTATION PERIOD AND LIGHTCURVE OF ASTEROID We were lucky enough to observe a well defined peak every night 1635 BOHRMANN we observed. I measured the time differences between peaks on different nights to generate a list of possible alias periods and then Christine M. Simpson to narrow these periods down to a rotation period of 11.73 ±0.01 Whitin Observatory hours. From this period it can be deduced that 2.0 rotations Wellesley College elapsed between the peaks on Sept. 6 and Sept. 7; 22.5 rotations 106 Central Street elapsed between the peaks on Sept. 6 and Sept. 17; and 31.0 Wellesley, MA 02481 rotations elapsed between the peaks on Sept. 6 and Sept. 21. I then translated our differential magnitudes for each night onto the (Received: 14 October Revised: 6 November) time scale for Sept. 6 modulo the period. This result is shown in Figure 1, showing an amplitude near 0.28 magnitudes. I also have observations of 1635 Bohrmann for two more nights during this Koronis family asteroid 1635 Bohrmann was observed 15 day interval and for three nights after this interval. I am over the course of 15 days in September, 2003 at Whitin planning to use these data and other observations during the Observatory in Wellesley, Massachusetts. A period of current apparition to get a solar phase angle curve for the asteroid. 11.73 ± 0.01 hours was determined with an amplitude of about 0.28 mag. Acknowledgements I would like to thank Steve Slivan for advising me on this project Introduction and observing with me on Sept. 21. Also, many thanks go to Jeff Regester for fixing the CCD coolant chiller when it shorted. Asteroid 1635 Bohrmann is a member of the Koronis family of asteroids, located in the main asteroid belt. Asteroid families like References Koronis are thought to be the fragments of larger bodies broken apart in collisions. This formation method would suggest random Binzel, R. P. (2003). “Spin control for asteroids.” Nature 425, rotation periods and spin vector orientations, but recent 131-132. observations and analysis of some of the larger members of the Koronis family have shown that there are statistically significant Slivan, S. M. (2003). “A Web-Based Tool to Calculate clusters of rotation periods and spin vector orientations (Slivan et Observability of Koronis Program Asteroids.” The Minor Planet al. 2002). Just this year Vokrouhlicky´ et al. (2003) have proposed Bulletin 30, 71-72. thermal re-radiation or the “YORP effect” as an explanation for these phenomena (see also Binzel 2003). Bohrmann is a smaller Slivan, S. M., R. P. Binzel, L. D. Crespo da Silva, M. Kaasalainen, member of the Koronis family, and our observations, combined M. M. Lyndaker, and M. Krco. (2002). “Spin Vectors in the with future observations, will be used to determine Bohrmann’s Koronis Family: Comprehensive Results from Two Independent spin vector orientation. Information for observing 1635 Analyses of 213 Rotation Lightcurves.” Icarus 162, 285-307. Bohrmann and other Koronis family asteroids can be found at http://www.koronisfamily.com (Slivan 2003). Vokrouhlicky´, D., D. Nesvorny´, and W. F. Bottke. (2003). “The vector alignments of asteroid spins by thermal torques.” Nature Observations and Analysis 425, 147-151. I observed 1635 Bohrmann for four nights in September of 2003 from Whitin Observatory at Wellesley College in Wellesley, Massachusetts. A 1024 square pixel CCD camera was used to image the asteroid through a V filter at the Cassegrain focus of the 0.61m Sawyer telescope. The field of view was approximately 16 arcminutes square. Observations were made during clear (1635) Bohrmann 2.65 UT Sept 6 conditions, except for those made during the first hour of UT Sept 7 2.7 UT Sept 17 observations on September 21 when there were some clouds UT Sept 21 2.75 moving in and out. The exposure time for each observation was Magnitude 240s. IRAF software packages were used to analyze the data. All V 2.8 images were corrected for bias, dark and flat field effects. 2.85 Instrumental For each night of data, I selected a non-variable comparison star in 2.9 the field that was brighter than the asteroid. The instrumental magnitude of the comparison star was subtracted from the 2.95 Differential instrumental magnitude of the asteroid to get a differential 3 instrumental magnitude for each image. I then selected a non- -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 UT hours on 2003 Sept 6 variable check star in our field similar in brightness to the asteroid. For each image I found a differential instrumental magnitude for Figure 1: Composite lightcurve for (1635) Bohrmann for a this check star with respect to the same comparison star used for rotation period of 11.73 ± 0.01hours. These data are light time the asteroid differential magnitude. The standard deviation of this corrected. The error bars are a one-sigma estimation of the differential instrumental magnitude was then used to estimate the uncertainty with respect to the local comparison star used for each uncertainty in the differential instrumental magnitude of our night. asteroid. Minor Planet Bulletin 31 (2004) 3 LIGHTCURVE ANALYSIS OF KORONIS FAMILY Table 1: Observing session details. ASTEROID 1635 BOHRMANN Observer Date Obs Phase Robert D. Stephens Stephens Sept 17 85 3.3 Santana Observatory Stephens Sept 18 86 2.9 11355 Mount Johnson Court Stephens Sept 19 21 2.4 Rancho Cucamonga, CA 91737 USA Stephens Sept 20 94 2.0 [email protected] Stephens Sept 21 33 1.5 Stephens Sept 22 62 1.1 Brian D. Warner Warner Sept 29 79 1.9 Palmer Divide Observatory Warner Oct 02 59 3.2 17995 Bakers Farm Road Total 519 Colorado Springs, CO 80908 [email protected] analysis routine developed by Alan Harris (Harris et al., 1989). (Received: 15 October) This program allows combining data from different observers and adjusting the zero points to compensate for different equipment and comparison stars. All observations were corrected for light The lightcurve for 1635 Bohrmann was obtained by the time. All observations were unfiltered. Dark frames and flat authors in September and October 2003. The rotational fields were used to calibrate the images. period was determined to be 11.730 ± 0.005 hours with an amplitude of 0.28 ± 0.03 mag. Bohrmann was discovered on March 7, 1924 by Karl Reinmuth at Heidelberg. It is named in honor of Alfred Bohrmann who was at the Königstuhl Observatory from 1924 to 1969.