Lunar & Planetary Laboratory
University of Arizona
Tucson, Arizona 85721

Dr. William B. Hubbard
(520) 621-6942


Richard E. Hill
(520) 621-4077


(NOTE! - This is arranged by date with the first postings appearing first. For the latest information go to the end of this page. For earlier and possibly outdated information on this event go here)

(The following is condensed from email of Bruno Sicardy and David Dunham acting on mail they received from Claudio Martinez in Argentina who first identified this event.) The occultation will take place on Sept. 8, 2001, and the most favourable region of visibility will be South America (Ecuador, Peru, etc...). The event could also be visible low above the horizon from Southern France and Spain. A GIF file of the currently predicted path is reproduced below. The star is very bright (V= 7.2, and even brighter in the IR), vs. 13.9 for Titania, so that the contrast will be very good. It is an Hipparcos star (#106829), so that predictions should be good too. But the prediction could change somewhat as we approach the time of occultation. One goal of this observing effort would be to detect an atmosphere of this satellite. It is 800 km in radius, bigger than Charon, and 60% the size of Triton. On the other hand, it is quite warmer: ~ 60 K from IR observations (Grundy et al, Icarus 142, 536, 1999). This is a very rare and bright event, and we should not miss a good opportunity to make some science with it. As you can see from the attached map, Colombia, Venezuela, Iberia, and France are all within the predicted path for this occultation by Titania, the largest satellite of Uranus. It will occur on 2001 Sept. 8 U.T. at about 2h U.T., or the evening of Sept. 7 local time in South America. Using a diameter of 1580 km = 0.114" for Titania, a central occultation would last 76 seconds with a 6.4-mag. drop. The star can be located with the help of finder charts for Uranus; it is at J2000 RA 21h 38m 14.0s, Dec -14 deg. 54' 36".
Original occultation path plot from David Herald, 3/01. Go here for the latest, updated plot!

Here is further information on the star: Aliases: HD 205829 BD-15 6027 GC 30276 GSC 05799-01022 HIC 106829 HIP 106829 IRAS 21354-1508 PMC 90-93 6284 PPM 239128 SAO 164538 SKY# 41252 SRS 16903 TYC 5799 1022 1 uvby98 100205829 YZ 105 8137 ZC 3167 ICRS 2000.0 coordinates 21 38 13.9595 -14 54 35.958 [11.30 6.30 94] FK5 2000.0/2000.0 coordinates 21 38 13.96 -14 54 36.0 [11.30 6.30 94] FK4 1950.0/1950.0 coordinates 21 35 29.60 -15 08 10.6 [65.39 36.80 92] Galactic coordinates: 38.01 -43.58 Proper motion (mas/yr) [error ellipse] 26.77 29.50 [ 1.27 .72 93] B magn, V magn, Peculiarities 8.24, 7.20 Spectral type: K0III Parallaxes (mas) 5.89 [.91]
The lightcurves below show computed occultation signatures of static Titania atmospheres of pure nitrogen with surface pressures of 0.1 (blue) and 1 microbar (black). The atmospheres are assumed to be at a constant temperature of 60 K. The 1-microbar atmosphere would be easily detected. The 0.1-microbar atmosphere could be detected with good photometry (at a level of about +/- 3% for 1-sec integrations). This could be achieved with a large aperture or by combining a large number of independent observations. Both approaches are desirable. If the atmosphere is in a state of blow-off, like a cometary atmosphere, the atmospheric density might vary proportional to 1/r^2, much more slowly than an exponential static atmosphere. The occultation signature of this case would be a much more gradual decline in flux, as shown in green in the figure below. To be detectable, the base density of such an atmosphere would have to be at least equal to the density of a 1-microbar atmosphere at 60 K. All this assumes an occulted star with a radius about 15 times the Sun's, with a distance for the given parallax of 170 pc. We now estimate a projected stellar diameter at Uranus equal to about 11 km, or 0.54 s in time.

Dear Titanians, You will find linked to this page three JPEG files showing synthetic light curves for the Titania occultation, for three hypothetical pressures at the surface: 15, 1 and 0.1 mubar. All the curves assume N2 atmosphere with scale height H= 47 km (corresponding to T= 60K). Each figure is supposed to have 4 panels (will be fixed later). The pressure printed in the lower left corner is the pressure at the surface. The quantity "b" is the impact parameter, i.e. the distance of closest approach of the observer to the center of Titania's shadow.

Remarks: ------- - Detecting a 15 mubar atmosphere (like Triton) is pretty obvious. Note that a detection is possible even with b= 1000 km, i.e. more than 200 km OUTSIDE the geometrical radius of the satellite (790 km). Note that a grazing occultation (b= 790 km) is pretty obvious. Hence the importance to observe even if one is outside the predicted geometrical path. - A 1 mubar atmosphere is pretty obvious too. Note that grazing occultations (b= 700-750 km) detect more easily an atmosphere that central ones. This is because the drop of signal due to the atmosphere last longer and has a shallower slope for grazing events than for central ones. Again grazing observers are welcome! - A 0.1 mubar atmosphere is probably the limit of what we can do. Once again grazing occultations are more sensitive to an atmosphere than central events. Note that the drop due to the atmosphere just before the drop due to the physical limb is about 7%, not much! - I am thinking about another method to detect a faint atmosphere. Refraction "shrinks" a little bit the physical shadow of Titania. For p= 0.1 mubar, this shrinking amounts to about 4 km radially. Thus, even if we cannot see the faint 7% atmospheric drop, we can at least measure the radius of the shadow, using the timings at various stations. The shadow moves at about 20 km s-1, thus we need an sub-second accuracy for the absolute timing at each station, on order for this method to work. Also, we need a good sampling for the light curve, since the star is going to smooth the signal over ~ 0.5 sec anyway. Best regards Bruno Sicardy

From: Thu Jul 26 10:23:06 2001 Dear Titania observers, I found in a double star list (see a system which nicely simulate (IN THE VISIBLE) Uranus and Titania during the occultation: SAO 10937 (HIP 207 and 208), RA: 00h 02m 36.087 sec DE: +66d 05' 56.28" (J2000) with separation 15.2 arsec, V= 5.9, type G8 III, V= 7.2, type A2 V, for each star respectively. It could be useful for tests of saturation, scattered light, scale, etc... and easy because it is in Cassiopea. Note however that the fainter star is rather blue, while the occulted star is read (type K0 III) so that the test will be misleading outside the V band. Cheers, Bruno
From Bruno.... Some remarks about the observations in Venezuela: ->The most important is to get a lightcurve of the event with sufficient time resolution. Titania is going 20 km/sec with respect to the star, projected onto the the plane of the sky. Thus we need 1sec resolution or better in order to resolve an atmosphere, which is likely to have a scale height of no more than 30-40 km. ->Fast CCD images or video, or any fast imager with reasonable linearity are ok. A fast aperture photometer could be tricky becaus Uranus will be close and bright (V= 5.7 at 16 arcsec from the star, V= 7.1). Scattered light from the planet would contaminate the aperture, and make the interpretation of the lightcurve difficult after the event. The best would be at have some IOTA cameras in Venezuela, or some good video camera whose signal could be digitized afterward. ->It is important to record the all event, not just the ingress and egress. If the atmosphere is thick enough, the occultation will be rather smooth and the star may not disappear completely even when it is the geometrically behind Titania. ->It is important to note that there is still an uncertainty which corresponds to about one radius or more of Titania. In other words, the gray stripe on the diagrams (track of the shadow on the Earth) may still shift north or south by half its width or even more. So, it would be wise to observe even if you are outside the nominal track. ->I do not think that spectra will be very useful, since the atmos. will be so tenuous that absorption lines will be impossible to detect. ->We do NOT know if Titania has an atmosphere. This is a prospective experiment. The problem is the star scintillation in the Earth atmos, NOT the photon noise, since the star is so bright. Thus, a big telescope like the 1-m is good not because it collects more photons, but because it reduces the scintillation wrt a 20cm telescope. Under good conditions, we can detect down to 10^{-7} bars at the surface of the satellite. ->To make tests, you can use a double star which simulates nicely Uranus + the star at the moment of the occultation: SAO 10937 (HIP 207 and 208 in Cassiopea, RA: 00h 02m 36.087 sec DE: +66d 05' 56.28" (J2000) with separation 15.2 arsec, V= 5.9, type G8 III, V= 7.2, type A2 V, for each star respectively. It could be useful for tests of saturation, scattered light, scale, etc... Note however that the occulted star is red, while the fainter star in the binary above is blue, so tests could be misleading outside the V band. Cheers Bruno Sicardy Observatoire de Paris 92195 Meudon Cedex Principal France <+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+><+> Dear Titanians, I received last week astrometric results from Bordeaux, giving some O-C's for the Uranian system. Michel Rapaport obtained 11 positions for Uranus, 7 for Titania and 7 for Oberon, wrt Hipparcos stars, with the meridian instrument. The average O-C obtained from Titania and Oberon positions is: O-C alpha*cos(delta)= -0.065" O-C delta = -0.057" which yields the track shown in the attached ps file. This new prediction is north of, but otherwise essentially the same as the one previously derived from Bill Owen's observations at Table Mountain, and from which I derived the tracks posted on Rik Hill's site on July 20 (see I did not include the O-C of Uranus in the present track, since the planet is 3.8" across, and thus with a more poorly defined photocenter than the satellite. In any case the O-C of Uranus would not change much the present track. There is still a problem of error bars: the Bordeaux observations have typical dispersions of < ~ 50 mas, i.e. about one Titania radius. Thus, the track shown in the attached ps file can still be off by +/- one Titania radius, i.e. one Titania diameter full width. In other words, all the observations (Ron Stone, Bill Owen, Michel Rapaport) are pretty consistent, but none of them really pin down the error bars below +/- one Titania radius. Averaging all these results is tricky because some systematic errors could linger in each of these observations. What would be needed now is having Titania and the star on the same plate and make direct differential astrometry. The two bodies are about 1 deg apart now. Can anybody do that ???????????? Cheers Bruno

From: Bruno SICARDY Subject: Titania astrometry update Here is a brief account of the latest astrometric measurements of the Uranian system: - Bordeaux results (Michel Rapaport): O-C alpha * cos(delta) = -0.075 arcsec, O-C delta= -0.030 arcsec. Then the occultation is grazing in southern Portugal, Madeira, and the shadow cuts in half Ecuador. Venezuela and Aruba are in full shadow. - NOFS: O-C= -0.119, -0.079 arcsec in alpha and delta, resp. Than Portugal, Madeira, Venezuela, aruba and Ecuador and in full shadow. - Table Mountain: (Bill Owen) O-C= -0.153, -0.034 arcsec. Portugal, Madeira and Ecuador are outside the shadow. Venezuela and Aruba are in. This later results is at contrast with all the other previous measurements, essentially because of a relatively high offset in alpha (-0.175 arcsec instead of more typical values of -0.070 arcsec). There is no explanation for this divergent result, according to Bill Owen. Thus the shadow is still wandering around with no definite convergence... Madeira, Southern Portugal and Southern Ecuador are dangerously close to the southern limit of the shadow. But remember that grazing occns are best for detecting an atmosphere. Note that all the astrometric masurements indicate that the occn by Titania will occur SOONER than predicted by the nominal JPL Titania ephemeris. It should occur around 1:54-1:55 UT sept. 8 in Europe, and around 2:00-2:01 UT in S. America. Be prepared! (and correct the diagram in Sky and Telescope, Sept. issue, p. 95

Created by: Richard E. Hill.
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