Jim Loudon Observatory
Appulse Observations


Richard E. Hill
JIM LOUDON OBSERVATORY
10130 E. King Manor
Tucson, AZ 85730
Longitude:110.77554 deg. W or 110 deg. 46' 31.9"
Latitude: 32.18006 deg. N or 32 deg. 10' 48.2"
Height above sealevel: 2875 ft.

DON'T MISS A NEAR MISS!
INTRODUCTION:

For any given point on the surface of the earth asteroidal occultations are rare to say the least, often more rare than solar eclipses for that location. True, there are thousands of asteroids, and millions of available stars but the available angular surface area is vastly less than the 707 square arc minutes of available angular surface area of the sun or moon. To look at it from the other side, the occultation path of even the larger asteroids (in angular size) is often smaller than that of the most solar eclipses. This seems rather disappointing at first. To get any information from such an occultation one would have to travel some distance it would seem. But many of these paths pass close to us resulting in an appulse, or close approach of a solar system object to a star, in this case the object being an asteroid. OBSERVATIONS:

In 1984 the Jim Loudon Observatory, then the La Estrella Obs., was outfitted with two micrometers for measurement of high proper motion stars and multiple star systems. This work has continued for years until, in 1991, it was suggested that micrometer measurment of appulses may have some value in the astronometry of the ZC catalogue and asteroids. The first effort was made in March of that year with an appulse of 2 Pallas and AGK3+06 1336. (Dunham 1990) For this event two telescopes were outfitted with micrometers and used by one observer, the author, simultaneously. One micrometer, a filar micrometer, was mounted on a C14 while a static-filar micrometer was mounted on a C5 to test if this device, mostly used for high proper motion measures, could effectively measure appulses.

The occultation path ran from Tierra del Fuego through the Yucatan and then virtually up the length of the Mississippi River and into Hudson Bay making it available to a large portion of the U.S. A later prediction moved the track further east into the Atlantic Ocean. These two tracks differed by some 1500 km. This wide discrepency was undoubtedly due to uncertanties in the star position, not Pallas' orbit!

Seeing on March 29/30, was around one arc second in humid skies due to recent rains. The event took place near the meridian as seen at Loudon Obs. which, when combined with the excellent seeing, made conditions ideal except for a full moon. Pallas was quickly located and observations were begun almost three hours before the predicted central event. The asteroid approached the 9th mag. star from the SSE and in only a few minutes motion could be detected. Measures were made as with multiple star systems (Aitken, 1935) the only difference being that single measures of PA and separation were used rather than averages of multiple measures. Later, when the asteroid was visibly moving past the star, it was no longer possible to use all the standardized procedures. Five minutes either side of closest point of approach (CPA) the asteroid was moving so fast that it was difficult to get the separation quickly enough so the position angle (PA) was still valid! Within one minute of CPA motion could clearly be seen as the Pallas glided past the star. It was then completely impossible to make any reasonable measures. (But it was fun and exciting to watch!) Observations were continued for an hour and a half after CPA in order to refine the data to more accurately determine CPA.

The observations with the static filar micrometer showed a greater error than the filar micrometer measures. This was to be expected due to the smaller aperture of the instrument and greater inherent inaccuracy in estimating separations. Nevertheless, the observations were good enough so the CPA separation was within 10% of that determined with the other telescope.

A month later another opportunity was available when Vesta had a near miss with AGK+20 416 on 11 April, 1991. (Dunham 1990) This time only the filar micrometer was used on the C14. The conditions for this event were not nearly as good with seeing around 5 arc sec. but the moon (just past 3rd Quarter) was absent. The appulse happened just after the end of twilight with Vesta in the Hyades which led to an hour angle of nearly 5h at the begining of the observing period and well over 5h at the end. The predicted event track ran from the state of Washington through Iowa and ended in Illinois. But realistically, only the westernmost states would see anything due to the very low altitude.

Measures were taken at shorter intervals in hopes of statistically improving the observation. The asteroid approached the star from the WSW and again was readily found but the low altitude made it difficult to get even as many as a dozen points. RESULTS:

A line of regression through the Pallas data showed two separate slopes, one for the observations within 5 min. of CPA and another for observations further out. This is easily explained throught the method used in taking measures. Standard procedure is to first get the PA, then separation, and note time. During the time between taking the PA and setting the separation the astroid would move a few tenths of a second. Thus ingress measures would fall a bit closer to the star and egress would tend to fall farther away. For this reason a separate line was not determined for the innermost observations.

For Pallas the CPA was determined to be 2.1 arc sec. at a PA of 76.13 deg. At no time were star and asteroid NOT separated clearly. Time of CPA was 0600 UT +/- 20s. At CPA the visible motion was dramatic and needs to be seen to be appreciated.

The line of regression drawn through the data of the Vesta appulse showed the scatter of the data to be about the same as the Pallas event, even in the markedly worse conditions. This can only be attributed to the greater care taken in making the measures and the recently acquired knowledge of what to expect. The CPA worked out to be only 0.57 arc sec. at a PA of 348.3 deg. occurring at 03:24:55 UT +/-5s. Notice that the CPA was far below what the conditions would have allowed and it was a simple matter to determine that the occultation was a clean miss for Loudon Obs. CONCLUSION:

These first attempts at this technique have shown the following:

During the early stages of the appulse the greater part of the motion is in separation amounting to only a small fraction of the total separation for any given measure, an observer would be well advised to make as many ingress measures as patience and eye strain will allow!

With reasonable care one can reduce errors below 0.1 arc sec. making the results valuable for astrometry of the asteroid or star.

Combining the two above statements, it should be possible, several hours before the event, to decide between two discordant predictions as in the case of Pallas. With a network of observers ready observational capability could be maximized and more asteroidal occultaions could be successfully observed! For example, from these two events it is pretty clear that the Vesta occultation occurred fairly close to the predicted track while the Pallas occultation (with the widely differing predictions) probably did occur along the oceanic path.

The value of these observations has been much discussed. It is felt that the greater value may be with the fainter asteroids.(Marsden 1993) However, many star positions have been found in error when preparing for occultations (as with the Pallas event?). This could prove to be an additional tool in the determination of such errors. Unfortunately, these errors are often found only after the event was missed at a location where it was expected. A greater value for such work may be in observations done a few hours to a few days before the event to help determine shifts in the occultation track on the Earth's surface.



A plot of the Vesta and Pallas appulses done as described above.

REFERENCES: Aitken, R.G., 1935, THE BINARY STARS, McGraw-Hill Book Co. Inc., NY. (Reprinted by Dover, 1963) Dunham, J.B., Ed., 1990, Occultation Newsletter, V.5, No.1 (and Asteroidal Suppliment to same). Marsden, B., 1993, private communication.