Observation Reports

Reports

Detailed Updates on Edith by David Dunham

Mar 25 Update: New astrometry favors southern U.S.A. path for Edith occultation

On Thursday morning, March 28th, a naked-eye star, 5.9-magnitude Z.C. 2305 = SAO 184113, about 2 deg. southeast of Delta Scorpii, will be occulted by the 95-km asteroid (517) Edith. Not only is this the brightest star predicted to be occulted by an asteroid in the U.S.A. during the last few years, but also the relative motion is unusually slow. This causes the event to be longer than usual, with an expected central duration of 33 seconds, and has also permitted more astrometry than usual. But the astrometry is complicated by the 9 magnitude difference between the star and asteroid.

In order to measure the size and shape of this asteroid, and check for possible satellites (like Dactyl) in its vicinity, we need as many people watching the star as possible from clear areas of most of the U.S.A. that morning. YOU and your friends can contribute to this effort. PLEASE SPREAD THE WORD about this bright star occultation, and encourage as many others as possible in your area to observe, since observation might be made even with a pair of binoculars. Those without TimeKubes or other shortwave receivers can use tape recorders or camcorders (only for the audio, although if sensitive enough when aimed into the eyepeice of a telescope, a videorecording of the event would be valuable) to obtain the duration of the occultation (from calling out D and R at the disappearance and reappearance, respectively), and if an actual occultation occurs, they can leave the recording going for ten or twenty minutes and telephone (and record) the US Naval Obervatory master clock at 1-900-410-8463 (or 202-653-1800 in the DC area) twice to determine the rate of their recording.

If an occultation occurs, the star could drop 9 mags. in brightness. But be prepared for a smaller drop in brightness (or two drops) in case the star turns out to be a very close double star. The J2000 coordinates of the star are R.A. 16h 06m 06.4s, Dec. -23 deg. 36' 23", about 2 degrees southeast of Delta Scorpii. The B1950 corrdinates are R.A. 16h 03m 07.1s, Dec. -23 deg. 28' 20".

Finder Charts Available by Clicking on Links

There is a finder chart on p. 73 of the March issue of Sky and Telescope, and a more detailed Chart in the Planetary Occultation Supplement for 1996 for North American Observers distributed with the last issue of Occultation Newsletter in December. A large scale finder chart here shows the night sky with horizons for each of the four time zones to simplify finding the star in Scorpius. Two additional finder charts are included here showing GSC stars to magnitude 13.5, one showing a ten day orbit segment, the other a thirty day segment, both centered on the occultation.

Future updates will be posted at this Web site, and will also be put on the IOTA occultation phone message at 301-474-4945. When possible, the updates will also be distributed by e-mail, but efforts must concentrate on only the updated region. Information on basic timing techniques will be supplied by e-mail upon request, or can be accessed from IOTA's lunar event web site at URL http://www.sky.net/~robinson/iotandx.htm.

Considering all of the available astrometry, my best estimate of the Edith occultation path is that it will be 4.3 path widths south of my path shown on p. 68 of the February issue of Sky and Telescope, with a correction to the time of 16.5 minutes late. This path passes over Bermuda at 9:00 UT; over southern Georgia at 9:08; Jackson, Miss. at 9:12; the center about 30 mi. north of Dallas, Tex., at 9:15; over Albuquerque, N.M., at 9:20; over the Grand Canyon at 9:22; the center just south of Lake Tahoe at 9:24; and the center about 25 miles south of Eureka, Calif., at 9:26 UT.

The formal uncertainty in the path is +/-0.7 path widths, but with the possible systematic errors that might be present, I think that the real error could be +/-2 or 3 path widths, or even more; observers throughout the U.S.A. south of the nominal path shown in the Feb. S&T map are encouraged to observe. The plot on the Web site mentioned above also shows the northern limit in case of a 3 path width north shift, and the southern limit in case of a 3 path width south shift; the actual path could even be outside of that range. The time is probably accurate to +/-3 minutes, but I would recommend start observing at least 7 minutes before the closest approach time at your location. G. Garradd and R. McNaught have time on the ANU 40" telescope on March 26 and 27, so hopefully they will be able to obtain some more good astrometry then for a more accurate final prediction. That telescope has a longer focal length than the 12-inch telescope used for Garradd's observations reported below.

A computation by Edwin Goffin's considering all of the Edith observations, a different interpretation from mine, puts the path essentially coincident with the nominal path, but with a later time, like my update.

Other observations have been made of both the star and asteroid, also separate short exposures for the star, by Jan Manek at Ondrejov Observatory, Czech Republic (65 cm telescope) and by Warren Offutt, Cloudcroft, New Mex. (60 cm telescope). The path computed from Manek's data is far to the north, at about lat. +55 deg. across Labrador and Quebec, then farther north, while the path from Offutt's data crosses Brazil at lat. -12 deg. (8:58 - 9:10 UT) and Peru at lat. -14 deg. (9:11 - 9:13 UT). The declination residuals for Edith for both are within 0.2" from Garradd's residuals; the large path differences are caused mainly by different declinations of the star determined by the different observers, especially Offutt's, whose declination is 1.5" north of the PPM declination. This gives an idea of the difficulty of determining the star's position relative to the faint GSC reference stars that are used for the Edith positions. Unlike Garradd, Offutt and Manek were able to force solutions for both the star and Edith using the same reference stars. Nevertheless, I think that Garradd's positions are the best, since he used a larger number of reference stars and was observing at much higher altitudes above the horizon than the other observers. A sizeable error in the star declination can remain, emphasizing the rule that observers rather far from the redicted path should also observe in the too frequent case of larger-than-expected errors. Of course, the time error could also be larger than expected.

The main change from my March 23rd prediction is caused by G. Garradd's March 24th 12 new observations at Loomberah, N.S.W. Australia, which give a southern path, refuting the previous northward extrapolation. I have also used a new orbit by Edwin Goffin published in MPC 26147 last December.

There is still a strong linear trend in the Right Ascension residuals, so I have extrapolated them for my new prediction. I have averaged Garradd's 40 declination residuals from his observations on March 15-24. I decided not to use his 8 March 12th observations, since they are so much farther south of the others; Edith was farthest from the target star then, and probably different reference stars used for the reduction caused the difference. If they were included, they would move the path 1.9 path-widths south.

Geocentric closest approach information is given in the table below for the different solutions is given in the table below; my best estimate of the path is given on the last line. I will add path-width shifts to this table later. Probably tomorrow, I will compute some detailed track data and a station track sort to help plan coverage for this event, especially by mobile observers.

Reporting Form

Two additional finder charts are available here showing GSC stars to magnitude 13.5, one showing a ten day orbit segment, the other a thirty day segment, both centered on the occultation.

Coordinates in the path using all of Garradd's data are given below, but keep in mind the large uncertainties such that the path could end up being much farther north. Future updates will be posted at the Web site, and will also be put on the IOTA occultation phone message at 301-474-4945. When possible, they will also be distributed by e-mail, but efforts must concentrate on only the updated region. Please spread the word about this possible occultation, the brightest star that will be occulted by an asteroid in North America, to others in your area who might observe it. Information on basic timing techniques will be supplied by e-mail upon request, or can be accessed from IOTA's lunar event web site at the URL for IOTA's Grazing Occultation Section .