The Best Occultation of a Star by Pluto ever Predicted for North America will occur Sun. March 18 am - Updated 2007 Mar. 16, 18h UT
Useful observations of the faint star in Sagittarius might be made with telescopes with apertures as small as 10 inches
Extensive station list with local circumstances, and CCD observing tips, added
1st Astro Meteo Weather Forecast
The best occultation of a star by Pluto ever predicted for North America will occur between 10:40 and 11:00 UT Sunday morning, March 18, in northern and possibly central Mexico, the western and central U.S.A., & southwestern Canada. Information from this occultation will help for studying the time history of Pluto's atmosphere (which is expected to freeze out and collapse sometime during the next several years as Pluto's distance from the Sun increases) and for planning observations by the New Horizons spacecraft that is enroute to the Pluto system. Although this event is the best so far for North America, the star is comparable in brightness to Pluto, with mag. V = 14.9 and R = 14.1. Since the motion is relatively slow, useful astrometric observations of the occultation might be made with telescopes of 10-inch aperture or greater, especially with integrating video cameras like the Stellacam II, and I think that the signal to noise ratio might be good enough to measure Pluto's atmosphere with telescopes of 16-inch aperture or greater with video (especially integrating) observations, where the integrations might be done over 4, 8, or even 16 frames. Integrations can also be done with software after the fact, with probably a little less sensitivity (especially if the tracking is not perfect), but then you might not see the target in real time to be absolutely sure you have it (but that might be ensured with brighter stars in the field of view). I think that it will be possible to make useful observations from where astronomical twilight is just beginning, in Manitoba, Minnesota, Iowa, Missouri, Arkansas, and Louisiana, and certainly west of there, where the sky will be dark; east of those places, the twilight may be too bright, although I think Wisconsin, Illinois, and Mississippi have a chance. Low altitude will hamper observations in Washington State and southern British Columbia. The star is UCAC2 25823784 at J2000 RA 17h 55m 05.70s, Dec -16 deg. 28' 34.4", in northwestern Sagittarius about 2.5 deg. north of M23 and 0.7 deg. south-southwest of 6th-mag. SAO 160915 at J2000 RA 17h 56m 19.1s, Dec. -15 deg. 48' 45". A central occultation would last about 6 minutes. Maps, detailed finder charts, observing tips, and much other information about this event are on Bruno Sicardy's Web site. Due to the great distance, there's still a fair amount of uncertainty in where the actual path will be, as can be seen from the different predictions portrayed on Sicardy's Web site, but at least the uncertainty is relatively small compared to Pluto's diameter so that observers throughout the "lower 48" USA and northern Mexico have a very high chance for an occultation. But with the current information, the location of the central line can't be predicted well enough to guarantee a "central flash" that will occur at the center due to focusing by the atmosphere. Observations of the central flash will be especially valuable for studying Pluto's atmosphere, so as many observatories as possible are encouraged to try to record the event so that some of them might record the central flash, since the phenomenon will be strong only for about 100 km distance from the actual central line, which could be anywhere over a range of about 2000 km on the ground, anywhere from the north-central USA to northern Mexico, possibly as far south as the southernmost part of Baja California. Let us know your plans to try to observe this occultation (size and location of telescope that will be used, and recording method) to try to coordinate coverage of the event, to try to prevent duplication of fixed-site chords by mobile observers and maximize the chance for observations of the central flash. Now that Pluto and the star are close enough together, accurate astrometric observations with long focal-length telescopes may be possible (especially after the lunar phase decreases enough) to refine the prediction of the central line, but so far, this has not been done, as far as I know, and there are only a few days left. Even if you don't observe the central flash, your observation will help to at least determine the accurate astrometric parameters of the occultation, which are needed to determine the Plutocentric latitudes of the recordings of the star's fluctuations in the atmosphere that can be made from major observatories, and your observation could capture some unique aspect of Pluto's atmosphere; we are interested in how the atmosphere varies with different latitudes around Pluto. I've only checked Accuweather predictions for Texas; it shows that there will be only scattered cirrus over most of the State, with slightly thicker cirrus only in the far south. A more detailed forecast will become available about 16h UT on Friday March 16th with the first Canadian weather forecast for astronomy (used by Clear Sky Clock). STATION LIST This is the 1st version of this list, made 2007 March 15 at 7h UT, but now with several updates made up to 18h UT of March 16. Several unconfirmed stations that will probably be clouded out have been removed from the annotated (short) list. Here is an annotated (confirmed observers indicated) abbreviated version of a larger station list. If your station is not in either of these lists, send an email message to me and to Derek Breit at breit_ideas@hotmail.com. Dave Herald provided the Occult input file for this event several months ago. I have modified it so that the central line is about halfway between the "Swiss" and "Brazilian" predictions with Pluto offset as described on Bruno Sicardy's Web site. But as noted above, the actual location of the center of the occultation path (from which the central flash can be observed) is poorly known, since it is not certain that the Pluto ephemeris offset will be the same this month as it was for the previous event and Sicardy believes that the error could be 1000 km or even a little more in the sky-plane (and then twice as much on the ground). There's only a small chance that we might get some recent astrometry, now that the two objects are in the same CCD field for long-focal-length astrometric telescopes, to improve the prediction. So for now, the central line defined by my prediction should be considered only as a reference line from which different observatories can be compared. The list is useful since you can use it to tell the predicted time of closest approach of Pluto's center to the star. Note that for all observers, the event will occur late during 10h UT, which is 5 am CDT, 4 am MDT, and 3 am PDT (= MST for Arizona). In the annotated (shorter) list, I've marked stations in the space between the distance (from my reference predicted central line, in km) and the observer's (or location's) name with "A" for a major observatory where observations are planned and that should be able to determine rather detailed information about Pluto's atmosphere from the observations; * for observations with smaller telescopes that should at least be able to tell if the central flash occurred or not, and help determine astrometric parameters of the occultation; and m for like *, but observer is mobile. A ? indicates no confirmation, but it would be useful if someone could observe there. "W" is a confirmed major observatory that will probably be clouded out, according to the current Astro Meteo weather forecast (see below), and "w" denotes other confirmed observers (* or m) who will likely be clouded out. A blank (no mark) is a station where no observation will probably be made, but if someone does plan to observe there, please let me know; I'll remove most of them in future updates. Let me know if you plan to observe; I'll post updated versions of this here until tomorrow (Friday) March 16 up to about 23h UT. After that, I will not be able to update this Web site, but will be able to update the list for e-mail distribution up to Saturday morning at 14h UT, and Derek Breit can post that on his Web site (which will then supersede this one). The predicted central occultation by Pluto's surface is 333 seconds, but the Pluto atmospheric variations will extend this at least to 400 seconds, and it is recommended that observers record at least for a 16-minute period centered on the predicted closest approach time (that is, from 8 min. before it to 8 min. after). Besides the UT of closest approach, the altitudes of the Star and of the Sun at that time are also given. WEATHER FORECAST The first Canadian weather forecast for astronomy (Astro Meteo) is now available for this event; I've put the 47h prognosis forecast maps in this Power Point file. They show that it will be cloudy across southwestern Canada and over most of the US Pacific Northwest, and also very cloudy over North Dakota, Minnesota, Wisconsin, Iowa, Tennessee, Kentucky, and most of Illinois, Arkansas, Louisiana, and New Mexico, and cloudy in eastern Nebraska, parts of Missouri (Kansas City is near the edge of a clear area extending south & west), and there will be marine clouds in most of the populous coastal areas of California (over the L.A. basin and west of the coastal mountains). It shows clouds over the mountains of western Texas that could threaten McDonald Obs., and a patch of clouds over central Texas, from San Antonio to Tyler and extending south some distance from San Antonio; scattered clouds are predicted for much of western and southern Texas. Perhaps more disturbing for Texas is the high humidity forecast for most areas east of Big Bend; with that, the usual adiabatic cooling clouds from the Gulf could be worse than Astro Meteo forecasts. Accuweather is forecasting clear skies in most of these areas early in the morning, but with a 60% to 80% chance of it clouding up at all central and southern Texas locations an hour or two before the Pluto occultation. Accuweather forecasts good conditions for eastern and central (mountains) Baja California. Before going to Texas, I'll check with the next Astro Meteo forecast that will become available at about 4h UT March 17, and will check Accuweather and the National Weather Service forecaster to see how bad the situation might be there; I'll decide whether to try the event in Texas, or possibly switch to southern Baja California, then, but probably will not be able to update this Web site. Check Derek Breit's Web site for later news. Also, click here for the later Astro Meteo forecasts; the 35h prognosis maps will become available about 4h UT March 17, 23h maps at 16h UT March 17, and the last 11h maps at 4h UT March 18, about 7h before the event (by then, also consult IR weather satellite images such as those available at http://www.rap.ucar.edu/weather/satellite but if the weather satellite is in eclipse, there will be a few-hour gap in its coverage that could occur when we most want it). The 47h prognosis Astro Meteo maps are also in the following .gif files: Eastern cloud cover, Western cloud cover, transparency, surface humidity, and surface winds. CCD Observing Tips John Broughton observed a similar occultation by Pluto with a relatively small telescope at his home in Australia about a year ago; he offers the following advice for CCD observers: Intrepid observers keen on recording this event will be looking at their observing options. High star density in that region and prediction uncertainty categorically rules out the option of CCD drift-scan. However, valuable data can be obtained by way of CCD photometry. I observed last June's 15th magnitude occultation from near the northern limit and was able to time a 68-second duration between the half-light immersion and emmersion. 1-second accuracy was achieved from 15-second exposures due to the quality of the data - noise levels were approximately 15 times smaller than the magnitude drop. My results are here. It was only after the event I thought of a practical way to increase the frame-rate and at the same time obtain an appropriate 2 arc- second pixel scale; a combination of 8x8 binning and eyepiece projection. Two weeks later, a 14th magnitude Leukothea occultation was observed by that method. My results for that event are here. Clear skies and may the path be with you! __________________________________________ David Dunham, 2007 Mar. 16, 18h UT Phones home 301-474-4722; office 240-228-5609; cell 301-526-5590 e-mail david.dunham@jhuapl.edu home e-mail: dunham@starpower.net .