Sodium in Cometary Comae at Various Distances from the Sun

Comets are among the most primitive bodies in the Solar System, preserving primordial materials from the early formation of the Solar System. Among various species observed in cometary comae, sodium is relatively lesser understood due to the high environmental temperature required for its sublimation, which restricts the observable samples to near-Sun comets. Here we present an analysis of five comets using a combination of narrowband imaging and high-resolution spectroscopy.

 

Coronagraph Imaging

In early July 2020, comet C/2020 F3 (NEOWISE) approached the Sun for the first time in nearly 4,500 years. Comet NEOWISE was observed by the Planetary Science Institute’s Io Input/Output Facility (IoIO; Morgenthaler et al., 2019) 35-cm coronagraph from July 7-16. Narrowband filters were used to isolate the dust and sodium gas emissions in the comae, and this apparition at only 0.33 AU from the Sun allowed some of the best quality images of a cometary sodium tail taken to date, as seen in Figure 1.  For inner solar system comets, sodium is often the brightest emission line available to ground-based telescopes thanks to its efficient resonant scattering of sunlight near the maximum of the solar irradiance spectrum. Strong radiation pressure from resonant scattering shapes sodium gas into a vast anti-sunward tail.

Figure 1: Narrowband filtered images of continuum dust reflectance and sodium D line emission in C/2020 F3 (NEOWISE). At 86° phase angle, the sodium tail here appears nearly orthogonal to the line of sight.

Structure in cometary sodium emissions is shaped by the collisional entrainment in the bulk outflow from the nucleus, additional dusty sources, the temperature of the atoms, absorption of intervening sunlight in the dense coma regions, and radiation pressure. At a certain radius from the nucleus, Na atoms collisionally decouple from the bulk outflow velocity with a thermal distribution. While it is challenging to uniquely parameterize the collisional radius, outflow velocity, and gas temperature, additional insight can be gained from emission line profiles at ample spectral resolutions of R >100,000 thanks to Doppler broadening.

 

Resolved Linewidth Measurements

During the IoIO imaging campaign, C/2020 F3 (NEOWISE) was also observed with the Lowell Discovery Telescope’s Extreme Precision Spectrometer (EXPRES; Jurgenson et al., 2016), at a resolving power of R = 150,000. At a phase angle of 108° and heliocentric distance of 0.46 AU, forward modeling of the line profiles that accounts for hyperfine structure retrieves effective temperatures ranging 1750 to 1950 K. The RMS velocity of this Doppler broadening is equivalent to 1.13 to 1.19 km/s, which suggests that collisional coupling to the bulk water outflow velocity is the dominant component in the emission line shape.

Figure 2: Outflow velocity at five comets with varying heliocentric distances as derived from resolved Na D emissions. Comets near 90° phase angle roughly follow the r^-0.5 relationship outlined by Budzien et al. (1994). C/2023 A3 was measured at both ~0.4 and 0.85 AU and shows variation consistent with this trend. Comets with phase angle >20° from quadrature are shown as outlines.

The water outflow velocity in comets is well known to vary with heliocentric distance. The often-cited relationship of v_H2O = 0.85r^-0.5 applied by Budzien et al. (1994) is a compromise between hydrodynamical models, which suggest 0.7r^-0.5 (Gombosi et al., 1986), and radio observations of OH and HCN at 1P/Halley, which suggest 1.1r^-0.5 (Combi, 1989). By comparing Na linewidths in a survey of comets at varying heliocentric distance, we find that the thermal velocity derived from Na linewidths produces a similar relationship. Figure 2 shows linewidth-derived outflow velocity for five comets at a range of heliocentric distances. All spectra were obtained with EXPRES, with the exception of the C/2023 A3 data point at 0.85 AU, which was obtained using the Large Binocular Telescope’s Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI; Strassmeier et al., 2015). Na D2 speeds appear systematically higher, implying that the line core may exceed unity optical depth. Comets 96P, 2P, and C/2023 A3 were observed at phase angles >20° from quadrature, which caused distortion of the line shape due to projection effects along the tail, resulting in outflow speeds higher than the overall trend.

 

Monte-Carlo Modeling

Figure 3: Preliminary results of Monte-Carlo modeling sodium gas in the coma of C/2020 F3 (NEOWISE), assuming here a collisional radius of 10,000 km, bulk outflow speed of 1.5 km/s and gas temperature of 150 K.

Phase angle distortion of the emission line profiles can be accounted for with Monte-Carlo modeling, and with nearly concurrent imaging and spectroscopy in the case of C/2020 F3 (NEOWISE), this approach has good potential to uniquely quantify the parameters that control sodium dynamics in its coma. We adapt the Monte-Carlo model of C/2012 S1 (ISON) by Schmidt et al. (2015), and this presentation will describe attempts to self-consistently simulate both spatial structure in the tail and Doppler broadening near the optocenter, using C/2020 F3 (NEOWISE) as a case study. A preliminary model run is shown in Figure 3, though it does not yet include emission from within the collisional radius and emission from dusty sources, which will broaden the tail. Work thus far suggests that spectrally resolved sodium emissions could offer a proxy for the water outflow velocity, enabling a new method at optical wavelengths.