More static maps of path

On Tuesday morning, February 12th, the 6.6-mag. star SAO 138233 in far s.e. Leo will be occulted for about 3 seconds by the 33-km asteroid (916) America in a path expected to cross central Florida at 10:40 UT (5:40 am EST), s. Texas at 10:42 UT (4:42 am CST), 10:43 UT over n. Mexico, and 10:44 UT (2:44 am.PST) over Baja California Norte. It’s the 7th brightest asteroidal occultation in North America this year, but the first of three bright asteroidal occultations this month, with the much brighter events next week (see below), one along a similar path to this one. Bright occultations like this provide an opportunity to obtain high signal-to-noise (more believable) observations of occultations by possible satellites of the asteroid, so observers out to 10 times the width of the occultation path should try to observe. That is the wide range shown between the two dark gray lines on the map near the bottom of the special page that Steve Messner has set up for this event at http://www.occultationpages.com/rasc/20190212_916America.html .That page includes other useful maps of the path, showing more detail of it across central Florida and southern Texas, augmenting Steve Preston’s  prediction page with finder charts of different scales for locating the target star available at http://www.asteroidoccultation.com/2019_02/0212_916_62644.htm .The weather forecast looks rather good for the path region, with mostly clear skies expected across central Florida (the central line passes over Ft. Pierce, Sebring, and Bradenton) and a strong “blue norther” descending rapidly on Texas (where the expected center is near Refugio, George West, and Cotulla) to clear out earlier clouds almost all the way to the coast. But in Texas, only 3 hours before the event, most of the path area is expected to still be overcast, so the actual situation will depend on how quickly the clear air mass from the northwest moves in; hopefully, it will be early, but if it’s late, clouds could still be in the path. The forecast also looks good across northern Mexico east of Baja Calif., so hopefully, Salvador Aguirre, signed up with Occult Watcher (OW) to observe from Hermosillo, will be able to record the occultation. Tom Campbell has a site declared on OW, perhaps the first of a few remote stations that he may try to run. Because the asteroid is rather small, the event rank is 67, meaning that an observer at the predicted central line has a 48% chance of an occultation. More observers are needed to try this event, to try to cover the path and the 1-sigma prediction uncertainty zone (between the two red lines on the maps; the 1-sigma error in this case is 3/4ths of a path-width), to give a good chance for getting a few chords across the asteroid.

The star is HIP 56061 = SAO 138233 at J2000 RA 11h 29m 24.2s, Dec -0 deg. 50¢ 56², appearing closer to s.w. Virgo than Leo, in which the star is located. SAO 138233 is 5 deg. s.w. of 3.6-mag. beta Virginis (Zavijava) and 1.8 deg. west of 4.3-mag. upsilon Leonis. A star of similar brightness (7.1-mag. SAO 138220) lies only 0.3 deg. west of the target star. For a central occultation, the star will disappear for 3 seconds with a spectacular 9-mag. drop to the 16th mag. of America. But if the spectral type K2 star is an unknown close binary, the drop could be less, or in quick steps. The angular diameter of the star should be 0.0015 arc second (²), while America will subtend 0.024². The disappearance and reappearance of the star should be quick, taking 0.2 second to cover and uncover the star’s disk for a central event, but longer for near-grazing conditions. There will be partial occultation zones 2.5 km wide at the northern and southern limits of the actual occultation path, which should be about 40 km wide. The 45% sunlit waxing Moon will be below the horizon. Finder charts of different scales and other event details are on S. Preston’s prediction page with link given above. Let us or IOTA’s free Occult Watcher asteroidal occultation planning tool (http://www.occultwatcher.net/ ) know your plans; we would prefer not to duplicate your observation with one of IOTA’s mobile sites, so we can optimize observational coverage of America with the most detail possible.

David Dunham, IOTA, dunham@starpower.net, cell 301-526-5590
Binoculars occultation of 6.6-mag. star by asteroid (916) America
Tues. am, Feb. 12, cen FL, sTX, nMex