I. Physical Studies of Distant Comets. II. Morphologies of Planetary Nebulae

Citation

Jewitt, David Clifford (1983) I. Physical Studies of Distant Comets. II. Morphologies of Planetary Nebulae. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/f9ge-m194. https://resolver.caltech.edu/CaltechETD:etd-10132005-145503

Abstract

Part 1:

Broadband observations of comets P/Stephan-Oterma (1980g), Bowell (1980b) and Panther (1980u) in the visual (0.5 ≾ λ(µm) ≾ 0.9) and infrared (1.2 ≾ λ(µm) ≾ 20) wavelength regions are reported together with measurements in the 1.5 to 2.4 µm wavelength range having 5% spectral resolution. The visual data indicate the existence of solid grains in extended halos around the nuclei of the three comets. The visual photometric profiles of comets P/Stephan-Oterma and Panther are interpreted as evidence that grains around Panther and those close to the nucleus of P/Stephan-Oterma are sublimating. Broadband near infrared and thermal infrared measurements of comet Panther suggest the presence of 2 to 4 µm radius particles in the coma. The particles within a 5.8 x 10⁶ diameter region centered on the comet have a total cross section of 10⁸m² and a near infrared geometric albedo of about 14%. Comet Bowell presents a total cross section of 3 x 10⁸m² within a 1.2 x 10⁷m region centered on the comet and its coma grains also have an albedo of 14%.

The near infrared spectrum of P/Stephan-Oterma is a featureless solar-reflection continuum. The near infrared spectra of Bowell and Panther exhibit features which are similar in the two comets. The spectral features are not due to H₂O, CH₄ or CO₂ ices nor to emissions from gases released from the nuclei nor to reflection from mineral grains of known composition in the comae. The spectrum of solid ammonia provides the best match to the near infrared; it is nevertheless significantly different from the comet spectra.

The synthesis of the visual data with the infrared data is attempted in terms of a model involving a mantle of volatile material on the nuclei of Bowell and Panther, but not on P/Stephan-Oterma. The composition of the mantle cannot be exactly specified from the existing data but a complex molecule incorporating the N-H bond may be present.

Part 2:

An extensive series of observations of comet Bowell at optical and infrared wavelengths is summarized and interpreted in terms of a model of the outgassing of the nucleus. The observations indicate that the optical coma consists of large grains. The outer edge of the coma is expanding from the nucleus at 0.9 ± 0.2 ms^-1: extrapolation of the expansion suggests that the coma was ejected from the nucleus when the comet was at a heliocentric distance R ~ 10 AU. At this distance, water ice sublimation would be negligible. Models of the brightness of the comet as a function of R are compared with the measured brightness variation. Between R = 5.3 and 3.4 AU, the apparent cometary brightness changed by a factor of less than 20. Models which account for the equilibrium sublimation of volatiles from the nucleus predict much larger brightness variations. In particular, the models confirm that the sublimation of water ice does not control the activity of comet Bowell. The observations are consistent with the presence of an inert nucleus surrounded by a dissipating grain coma ejected impulsively at R ~ 10 AU. Strong OH production observed near R = 3.4 AU could be a result of the photodissociation of water molecules released from the coma grains by sublimation. However, near infrared spectra of the grains show no evidence for the presence of OH. Instead, several absorptions similar to the overtones of NH bond vibrations are observed. It is noted that the outgassing properties of the comet are similar to those of proton-bombarded laboratory ice specimens. Both the impulsive coma ejection and the peculiar near infrared spectrum may result from the presence of unstable compounds produced in the nucleus by cosmic ray bombardment. The activity of comet Bowell is quite unlike that exhibited by comets at small R, but may be representative of the properties of the distant comets.

Part 3:

The spatial distribution of [CI] 9823, 9850 Å emission in NGC 6720 is reported. Like [01], the [CI] radiation appears enhanced in the region of the bright filaments. A few percent of the carbon atoms in the filaments are neutral. This neutral fraction is consistent with ionization equilibrium calculations made under the assumption of complete shielding of direct stellar radiation by hydrogen. The observed carbon lines are excited by photoelectrons produced from hydrogen by the nebular diffuse radiation field. The [CI] observations confirm that the filaments in NGC 6720 are regions of locally enhanced shielding.

Part 4:

The preliminary results of a CCD survey designed to detect and investigate faint halos around planetary nebulae are described. A TI 800 x 800 pixel CCD was used to take deep exposures of 44 planetary nebulae. The exposures were mostly obtained through an Ha filter at the Cassegrainian focus of the Palomar 1.5 m telescope. Spatial resolutions of 1 to 2 arcsecond were obtained across 400 arcsecond wide fields. The images, which are in many cases considerably deeper than any previously taken, reveal numerous planetary nebula halos. About 2/3 of the studied nebulae possess extensive outer halos. In some nebulae the mass in the halo is comparable to the mass contained in the primary HII region. We have used the data to place constraints on the mode of origin of the halos. It is likely that the halos originate either by dynamical separation of a single ejected shell of gas, or by the ejection of two or more such shells from the central star. It is less likely that the halos are caused by excitation of the pre-planetary stellar wind and highly improbable that the halos represent reflection nebulae.

Thesis Files