The activity and size of the nucleus of Comet Hale-Bopp (C/1995 O1)

Science

Volumn 275, Issue 5308, 1997, Pages 1900-1904

  Weaver, H A ^a   Feldman, P D ^a   A'Hearn, M F ^b   Arpigny, C ^c   Brandt, J C ^d   Festou, M C ^e   Haken, M ^b   McPhate, J B ^a   Stern, S A ^f   Tozzi, G P ^g  

^a JOHNS HOPKINS UNIVERSITY   (United States)

^b University of Maryland   (United States)

^c UNIVERSITY OF LIÈGE   (Belgium)

^d UNIVERSITY OF COLORADO   (United States)

^e OBSERVATOIRE MIDI PYRÉNÉES   (France)

^f SOUTHWEST RESEARCH INSTITUTE   (United States)

^g INAF OSSERVATORIO ASTROFISICO DI ARCETRI   (Italy)

Author keywords

[No Author keywords available]

Indexed keywords

CARBON DIOXIDE; CARBON DISULFIDE; ICE; WATER;

ARTICLE; ASTRONOMY; BRIGHTNESS; DEVICE; DUST; GAS; PRIORITY JOURNAL; SPECTROSCOPY; ULTRAVIOLET RADIATION;

CARBON DIOXIDE; CARBON DISULFIDE; COSMIC DUST; METEOROIDS; SPECTRUM ANALYSIS; WATER;

COMET HALE-BOPP; COMETARY NUCLEUS; HUBBLE TELESCOPE;

EID: 0030620507     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.275.5308.1900     Document Type: Article

References (38)

1. A. Hale and T. Bopp, IAU Circular 6187 (1995).
2. D. Jewitt, M. Senay, H. Matthews, Science 271, 1110 (1996).
3. N. Biver et al., Nature 380, 137 (1996).
4. -1 and one planetary mode CCD having 0.0455 arc sec per pixel. The point spread function (PSF) of the planetary mode has a fullwidth at half-maximum of ∼0.070 arc sec, which projects to 250 km at a geocentric distance of 5 AU. Further information on the WFPC2 is available at http://www.stsci.edu/ftp/instrument_news/WFPC2/wfpc2_top.html
5. The FOS is a versatile instrument that can provide moderate and low resolution spectra over the wavelength range from 1140 Å to 9000 Å by employing multiple gratings and two one-dimensional digicon detectors coated with different photocathodes. During our observations, we used the G190H and G270H gratings, which together covered the wavelength range from 1570 Å to 3300 Å at a resolving power of ∼1000 for the cometary emissions having a sharply-peaked spatial distribution. Further information on the FOS is available http://www.stsci.edu/ftp/instrument_news/FOS/topfos.html
6. p is the geometric albedo.
7. P. L. Lamy and I. Toth Astron. Astrophys. 293, L43 (1995).
8. P. L. Lamy, I. Toth, H. A. Weaver, Astron. Astrophys., in preparation.
9. -2.
10. D. C. Jewitt, in Comets in the Post-Halley Era, R. L. Newburn, M. Neugebauer, J. Rahe, Eds. (Kluwer, Dordrecht, 1991).
11. All of our non-trailed HST images taken in planetary mode show a brightness cusp at the peak pixel that we interpret as the signature of the nucleus. However, if icy grains dominated the dust population within a few hundred kilometers of the nucleus and evaporated into lower albedo particles outside this region, then the brightness cusp could be due to coma rather than the nucleus. On the other hand, a continuous degradation of dust albedo with cometocentric distance would steepen the coma surface brightness profile, causing us to underestimate the size of the nucleus. Similarly, dust fragmentation in the coma could cause us to underestimate or overestimate the size of the nucleus, depending on the exact nature of the fragmentation.
12. H. U. Keller et al., Astron. Astrophys. 187, 807 (1987).
13. While the signal-to-noise ratio in the peak pixel is large (over 100), the error in estimating the size of the nucleus is determined by systematic effects. In particular, the derived size depends on the extrapolation of the coma into the unresolved region. Light scattered from a 42-km nucleus only contributes ∼37% of the brightness in the peak pixel, so the coma dominates the observed light distribution even in the core of the image. The nucleus has to be large in order to detect its signature in the presence of such a bright coma, and a Halley-sized object would be undetectable with HST.
14. M. F. A'Hearn et al., Icarus 118, 223 (1995).
15. M. R. Kidger et al., Astrophys. J. Lett., in preparation.
16. H. A. Weaver, IAU Circular 6376 (1996).
17. _, P. D. Feldman, M. C. Festou, M. F. A'Hearn, H. U. Keller, Icarus 47, 449 (1981).
18. D. G. Schleicher and M. F. A'Hearn, Astrophys. J. 331, 1058 (1988); F. P. Schloerb, Astrophys. J. 332, 524 (1988). "Quenched" means that the populations of the two Λ-doubled components of the lowest rotational level are equally populated due to collisions.
19. D. G. Schleicher and M. F. A'Hearn, Astrophys. J. 331, 1058 (1988); F. P. Schloerb, Astrophys. J. 332, 524 (1988). "Quenched" means that the populations of the two Λ-doubled components of the lowest rotational level are equally populated due to collisions.
20. 2 s.
21. 2 s.
22. W. M. Jackson, J. B. Halpern, P. D. Feldman, J. Rahe, Astron. Astrophys. 107, 385 (1982).
23. 2 s.
24. J. J. Cowan and M. F. A'Hearn, Moon and Planets 21, 155 (1979).
25. 2O-dominated one.
26. J. Crovisier et al., paper on the radio observations of Hale-Bopp presented at the 28th Annual Meeting of the Division of Planetary Sciences of the American Astronomical Society, Tucson, AZ, 22 to 26 October 1996.
27. H. A. Weaver et al., Astrophys. J. 422, 374 (1994).
28. -1, B is the observed brightness of the Cameron (1, 0) band in rayleighs, and 9 is the diameter of the equivalent circular aperture of 2.4 arc sec.
29. -0.5 in order to be consistent with the assumptions made in analyzing the HST data.
30. (0.4)(0.035)φ. A different choice for the phase law is unlikely to affect our conclusions, as Hale-Bopp was observed over a narrow range in phase angle.
31. p = 0.04 (Table 1 and Fig. 4).
32. -0.5.
33. C. Arpigny et al., Atlas of Cometary Spectra (Kluwer, Dordrecht, 1997).
34. -1.
35. We ruled out pointing error as the source of the observed temporal variation by verifying that the flux measured in spectra taken immediately before the outburst was essentially identical to that measured during observations spanning up to ∼6.5 hours earlier. A solar-type star entering the aperture and having a visual magnitude of ∼11.5 could have produced the observed increase in the continuum, but no such object was present in the cometary field. Furthermore, another object entering the aperture would not explain the observed increase in CS emission. During the outburst, the spatial distribution of the continuum became markedly more peaked towards the nucleus, which is consistent with a dust outburst originating at the nucleus of the comet.
36. -0.5 [N. T. Bobrovnikov, Astron. J. 59, 357 (1954)], which gives a Q ratio of 7 and implies that the total dust production rate averaged over the outburst was eight times the quiescent value.
37. 2O molecules.
38. Support for this work was provided by NASA through grant numbers GO-05844 and GO-06663 from the Space Telescope Science Institute (STScl), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. C. Arpigny gratefully acknowledges financial support by the Belgian Fonds National de la Recherche Scientifique. We thank A. Lubenow and A. Storrs of the STScl for expertly planning the HST observations of Hale-Bopp. We also express our gratitude to W. Wamsteker and H. Andernach for their strong support of the IUE Hale-Bopp program.