Testo della Circolare n 6631, nella quale viene annunciata la scoperta della coda di sodio della cometa Hale-Bopp, inviata dal dottor Gabriele Cremonese al Central Bureau for Astronomical Telegrams dell'International Astronomical Union (Unione Astronomica Internazionale), presso lo Smithsonian Astrophysical Observatory di Cambridge, Massachusetts (U.S.A.) :

On IAU Circular 6631 (April 18), an exciting discovery wasannounced by Gabriele Cremonese (Astronomical Observatory,Padua, Italy), on behalf of the European Hale-Bopp team. It concerns the detection of an extensiveneutral sodium (Na) tail, i.e. a third type of cometary tail (thetwo first being the dust and the ion tails). He writes:

The structure is visible on images obtained by Don Pollaco (Isaac Newton Group) with the wide-field CoCAM instrument (aperture 50 mm, 2000 x1000 EEV CCD, 26 arcsec/pixel) at the Observatorio del Roque de losMuchachos (La Palma, Spain).

The following description is based on two10-min exposures obtained through a Na D filter on Apr. 16.88 UT undermediocre sky conditions. The filter (lamda_0 = 589.2 nm, FWHM = 2.1nm) has no leak at the 0.02 percent level over the entire sensivitityrange (300 - 1100 nm) of the CCD.

The images display two maintails. The first is a very linear feature 6.6 deg long and less than(or equal to) 10 arcmin wide with parallel edges over its entirelength and a very sharp south edge at p.a. 52 deg (3 deg north of theprojected antisolar direction).

The second is a broad diffusenorthern tail extending to the edge of the frame, i.e., to more than6.5 deg. While the second tail coincides with the dust tail as seenin an adjacent continuum filter, the first tail has no suchcounterpart, and we identify it as a neutral Na tail.

In particular,near-simultaneous imaging with an H2O+ filter (lambda_0 = 619.3nm,FWHM = 4.5 nm) displays an ion tail of entirely different and morecomplex morphology with a main ray at p.a. 46 deg. We believe thatthis is the first clear separation of a neutral tail of a comet fromthe dust and ion tails.

The existence of this third type of cometarytail was confirmed in additional CoCAM images obtained on Apr. 17.8UT.

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