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Gas and Dust in Comet Hale-Bopp

23 Dicembre 1996

This series of four images shows the appearance of Comet Hale Bopp in early November 1996. At this time it was approaching the Sun in the sky, and these images are some of the last made by a major astronomical telescope in 1996. They were made by Hermann Boehnhardt of the Astronomical Institute of the Ludwig-Maximilian-University in Munich (Germany).

The images were taken at the Danish 1.5-metre telescope with the DFOSC multi-mode instrument on 2 November 1996 around 0 UT. The detector was a LORAL CCD chip (2052 x 2052 pix). The orientation is such that North is up and East is to the left. The pixel size is 0.4 arcsec. At the time of the exposures the comet was just 20 deg above the horizon and still in the twilight sky. The heliocentric distance was 2.49 AU (372 million km) and the Earth distance was 3.05 AU (456 million km). Thus, 1 pix = 885 km. The exposure time was 300 sec (41a+b; CN-filter) and 5 sec (41c+d; R-filter), respectively.

There are four images. The first was obtained in the light of CN-molecules and has been flat-fielded. The field measures about 13.5 x 13.5 arcmin. The second is a Laplace-filtered subframe (width 15 pix) of the first; the field is just over 2 arcmin square. The third was obtained through a red filtre and mostly shows sunlight reflected from the dust in the coma. The fourth is a Laplace-filtered subframe (width 15 pix) of the third; the field is a little less than 3 arcmin square.

In the R filter, it is possible to identify a total of 7 jets at PA = 13, 85, 185, 230, 267, 300 and 332 deg (there is an indication of an 8th very weak structure at PA = 135 deg). The PA (position angle) was measured from North over West (i.e. clockwise from the top on these images). In the CN band, the S/N ratio is much worse and there only seems to be counterparts for the 4 brightest jets in R, but most likely the jets at the the other position angles are not detected.

A comparison of the flat-fielded CN (41a) and R (41c) images suggests that the CN-jets are somewhat wider and smoother than their counterparts in R. This may be a consequence of the gas expansion in the CN jets while the dust as seen in R seems to be better collimated.

If one compares the November R image with the August 1996 image, it is evident that the jet appearances have changed significantly, i.e. the relative intensity varies over a long time scale (order of weeks). However, the position angles remain almost constant. Particularly nice examples are the jets at PA = 13, 185, 267 and 300 deg.

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