Comet Hale-Bopp in outburst: Imaging the dynamics of icy particles with HST/NICMOS

D. W. McCarthy, S. R. Stolovy, H. Campins, S. Larson, N. H. Samarasinha, S. D. Kern

Research output: Contribution to journalArticle

7 Scopus citations

Abstract

Comet Hale-Bopp was imaged at wavelengths from 1.87 to 2.22 μm by HST/NICMOS in post-perihelion observations starting on UT 1997 August 27.95. Diffraction-limited (∼0over(., ″)2) images were obtained at high signal-to-noise (∼1500) to probe the composition and dynamics of the inner coma and also the size and activity of the nucleus. The velocities of several unusual morphological features over a 1.7 h period, indicate that a significant outburst occurred 7.4 h prior to these images while the comet was at a heliocentric distance of 2.49 AU. Similar features are also apparent after re-analysis of pre-perihelion ground-based images. The inner coma (radius ≲ 2500   km) is dominated by an "arc" feature, which expanded and became more diffuse with time. This feature can be modeled as the bright central portion of a "jet of outburst" from a near-equatorial region of the nucleus. Less prominent, time-variable linear and circular morphologies are also apparent. The expansion rates of both the arc feature and the circular morphologies imply a common origin and also suggest a grain size distribution with two broad maxima. In addition, several static linear features extend to the edge of the field of view (21,100 km). Radial brightness profiles are highly asymmetric and only approach a ρ-1 decline at distances ≥15,000 km. Images in a narrow-band filter at 2.04 μm exhibit a ∼4% absorption feature relative to nearly simultaneous images at wavelengths of 2.22, 1.90, and 1.87 μm. This absorption is attributed to H2O ice in the coma grains. The spatial distribution and expansion velocity of the absorption at 2.04 μm indicate that these grains are associated with the outburst. The constancy of the absorption feature indicates no appreciable sublimation over 1.7 h. The unresolved nucleus has a flux density consistent with a 40 ± 10   km diameter assuming a 4% geometric albedo.

Original languageEnglish (US)
Pages (from-to)184-195
Number of pages12
JournalIcarus
Volume189
Issue number1
DOIs
StatePublished - Jul 1 2007

Keywords

  • Comet Hale-Bopp
  • Comets
  • Ices
  • Infrared observations
  • composition

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    McCarthy, D. W., Stolovy, S. R., Campins, H., Larson, S., Samarasinha, N. H., & Kern, S. D. (2007). Comet Hale-Bopp in outburst: Imaging the dynamics of icy particles with HST/NICMOS. Icarus, 189(1), 184-195. https://doi.org/10.1016/j.icarus.2007.01.019