A lunar eclipse took place this morning (UT), and reports about the event, including observations of Hale-Bopp, are expected soon. From now on, the Comet will be best visible in the evening as it moves higher in the north-western sky. For detailed information about viewing conditions, check the Sky & Telescope Hale-Bopp On-line Viewing Guide; a Sky Chart for late March with horizon is particularly useful.
The magnitude estimates by experienced visual observers are still slightly fainter than -1; from the compratively few reports which have been published during the past two days, it does seem that a certain leveling-off, as is to be expected, is now taking place.
Lots of interesting news have been received over the week-end; I expect that it will be ready for display here at about 10:00 UT this morning (Monday, March 24). In the meantime, the information below which was placed here on Friday, March 21, will stay on.
On IAU Circular 6594 (March 19), Zdenek Sekanina (JPL, Pasadena) writes that excellent dust models have been obtained to match the diurnal evolution of the bright jet monitored by L. Jorda et al. on Feb. 28 (IAUC 6583) and the system of nearly-concentric halos observed by S. M. Larson and C. W. Hergenrother on Feb. 8. It is confirmed that the nucleus is not in a state of pure spin. The two features are ... from different sources on the nucleus... both being activated only between local sunrise and sunset.
During the past days, there have been rumours circulating in amateur circles about a possible splitting of the nucleus - in one report published on the Web, a photo from a medium-size telescope is displayed which purportedly shows six pieces. However, at least until this moment, there have been no confirming reports from major professional telescopes which presumably have obtained more detailed images in the meantime. It is therefore rather unlikely that the reported effect is real - it is most probably due to the presence of very complex structures in the coma and the relatively low angular resolution on the published image. Moreover, visual observers with good-quality optical instruments (and there are many of them now!) would undoubtedly have seen any splitting, had it really happened.
Exciting new spectroscopic results have been announced. On IAU Circular 6591 (March 19), J. E. Wink at Institut de Radio Astronomie Millimetrique (IRAM; Grenoble, France) and collaborators from Observatoire de Paris, Observatoire de Bordeaux (France) and the Joint Astronomy Centre (Hilo, Hawaii, USA) report the detection of SO2 in Hale-Bopp on March 18 at the IRAM interferometer at Plateau de Bure (France), using its five antennae in a single-dish mode: The SO2 11(1,11)-10(0,10) rotational line at 221.965 GHz was detected... Assuming thermal excitation at 80 K, the derived SO2 production rate is about 6 x 1027 molecules/sec.... This is the first detection of SO2 in a comet, one of the sources of SO (Lis et al., IAUC 6573). Comparison of observations of SO ... and H2S... lines, made simultaneously in both single-dish and interferometric modes at IRAM on Mar. 13.3, shows that the bulk of SO does not come directly from the nucleus, in contrast to H2S.
Preparations for a special Hale-Bopp session at the time of the General Assembly of the International Astronomical Union in Kyoto (Japan) in August 1997 are underway. The Scientific Organising Committee will be chaired by Martha Hanner and Mike A'Hearn. It will include short, invited presentations and posters. It is suggested that interested GA participants with potential contributions make themselves known to them without delay.
News have been received about the activities of the Ulysses Comet Watch which is using data from instruments onboard the NASA/ESA Ulysses spacecraft that study the properties of the solar wind, i.e. the fast particles emitted by the Sun. In the last two years, Ulysses has investigated the steady, high-speed winds at high latitudes, and alternating, slow and fast winds near the Sun's equator. Hale-Bopp will soon enter the lower latitude zone, where the disturbed solar wind resides, and where dramatic changes in the comet's plasma tail are expected to occur.
The Ulysses Comet Watch group will provide worldwide observations of the comet as it descends from the polar regions of the Sun. Images from more than 200 observers around the world will be posted on the Ulysses Comet Watch home page. Observations will continue to be posted well after the comet makes its closest approach to the Sun on April 1.
The Ulysses group is watching for changes in the comet's blueish plasma tail, which consists of ionized gas being emitted by the comet and picked up by the magnetic field being swept along by the solar wind. Cometary plasma tails change constantly due to their interactions with slow and high-speed solar wind. One of the more dramatic changes that can occur is the abrupt disconnection of the tail, which then drifts away. Often a new tail will form within hours. The comet, by acting as a `solar wind sock,' can be used to map the conditions in different latitudes of the solar wind.
The most recent reports indicate that the brightness continues to rise, as expected now somewhat more slowly. The magnitude is close to -0.5. The tail appearance is characterised by the excellent separation between the blueish ion tail and the more reddish dust tail; this is due to the particular spatial geometry (current position of the Sun, the comet's orbit and the Earth).
Narrow-band photometry was obtained by T. Farnham and colleagues with the 31-inch telescope at Lowell Observatory (Arizona, USA) on Mar 6.5 and 7.5 UT when the heliocentric distance was about 1.02 AU. When comparing these results with those obtained from Lowell by D. Schleicher and others during the previous 7 weeks at (at 1.54-1.52, 1.34, 1.23-1.20, and 1.09 AU) it appears that the gas production has now leveled off and dust production is increasing at a slower rate.... than in mid-January to mid-February.... (from a message submitted to the IAU Circulars on March 15).
On IAU Circular 6587 (March 14), J. E. Wink and colleagues report that they have detected the millimeter-wave continuum with the IRAM Plateau de Bure Interferometer at 3 mm (resolution 3.5 arcsec) and at 1 mm (resolution 1.5 arcsec) on Mar. 9 and 11. On Mar. 11.4 UT... the flux can be accounted for by thermal radiation at blackbody temperature 380 K from a 1600-km2 area, which corresponds to a sphere of diameter 45 km..... It should be noted that this is not necessarily the true size of the nucleus (it may refer to the densest part of the dust cloud), but this measurement does set an upper limit to its size.
