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DISTANCE R (1000 km)

DISTANCE R (1000 km)

Fig. 17. Gas velocity measured in situ at comet Halley by the Giotto neutral mass spectrometer [134]

The temperature and gas velocity can also be determined by ground-based observations, although at a much lower spatial resolution and only for sufficiently bright comets. These observations are made in the infrared (IR) and at radio wavelengths range.

At IR wavelengths, recent developments of new telescopes and instruments allowed detecting molecules such as CO with long-slit spectroscopy. With high spectral and spatial resolution observations of CO lines in the near-IR range, it has been possible to study the heating processes in the coma with increasing nucleocentric distance in bright comets (e.g., [66]).

At radio wavelengths (mm and sub-mm wavelengths), velocities and temperatures of parent molecules can be measured by high-spectral resolution observations. At these wavelengths, emission lines can be fully resolved and allow us to measure the Doppler shift of the line and also to analyze the line shape. However, the beam size of radio observations is usually very large and does not allow us to spatially resolve the coma (although some spatial resolution is obtained by mosaics of pointings or interferometers). The measured emission signal results from the inner to the intermediate coma, where densities of the observed molecules are high. As illustrated in Fig. 12, the velocities and temperatures vary within this coma region. Therefore, the measurements of gas velocity in the radio range correspond to a kind of "weighted" average of the inner coma. Measurements of the gas expansion velocities of comets near 1 AU give velocities around 1kms_1, as we would expect in the intermediate coma. Generally, for comets at rh < 3AU velocities around 0.6-1.8kms_1 are found, and the expansion velocity increases with decreasing heliocentric distance [141,189,192]. In addition, the investigation of OH emissions in several comets indicates a dependence on the cometary gas production rate [31]. Analysis of H2O, HCN, and OH emission profiles results in somewhat different velocities. Line asymmetries are often observed in radio lines (e.g., Fig. 18) and are interpreted as indicators for asymmetric outgassing.

C/1995 Ol Hale-Bopp: CO(2-l) at 230.5 GHz: Hay 2000-M«r.2001

C/1995 Ol Hale-Bopp: CO(2-l) at 230.5 GHz: Hay 2000-M«r.2001

Fig. 18. Radio emission line of CO observed during the approach of comet Hale-Bopp to perihelion [24]. The line shape is asymmetric, indicative for anisotropic outgassing at the comet

Bright comets allow us to measure the variation of gas velocity with heliocentric distance. Observations of comet Hale-Bopp [24,25] provided observational evidence for a scaling law of the expansion velocity, based on measurements extending beyond rh = 8 AU. Somewhat different scaling laws were found pre- and post-perihelion (Fig. 19). On average, the velocity scaled as:

In summary, the recent observations of u at radio wavelengths suggest that a scaling law like u = ar-b (with a near 1 kms-1 and an exponent of b « 0.5)

Fig. 19. Gas velocity over heliocentric distance determined from measurements at radio wavelengths of comet Hale-Bopp [25]

provides a reasonable extrapolation of the gas expansion velocity to large rh when production rates need to be determined in comets for which no direct measurement of u can be made.

Remote observations from ground at radio wavelengths usually provide insufficient resolution to study the nucleocentric temperature profile, but allow us to derive an "average" rotational temperature (see Sect. 4) determined by the conditions in the collisional zone covered in the field-of-view (FOV) of the observations. In principle, as the line excitation is mainly caused by collisions, the derived Trot corresponds to Tkin at the last collision of the molecules.

Again, comet Hale-Bopp was the first comet that allowed measurements of Trot (e.g., [25]) over a wide range of heliocentric distances (Fig. 20). Rotational temperatures in the coma dropped from about 130 K at perihelion to about 10 K at 7-8 AU. Again, the evolution can be approximated by a power law:

Gas and Dust Jets

In many images of comae, sunward outgassing is dominating the gas flow and sunward and tailward asymmetries are often observed. Furthermore, gas and dust jet structures are present. Major gas jets are believed to originate from localized regions on the cometary surface with enhanced sublimation activity. Such increased sublimation can be caused by local differences in the ice/dust content, differences in chemical ice composition and locally different heat flow efficiencies into the nucleus interior. In addition, surface topography

Comet C/1995 01 (Hale-Bopp)

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