Determining the Sources of the Zodiacal Cloud Using Relative Velocities of Dust Particles From High-Resolution Spectroscopy

The zodiacal cloud is the Solar System debris disk in which the Earth’s orbit is located. The dust that comprises the cloud comes from cometary, asteroidal, and (to some extent) interstellar and other source populations, but the relative ratios have proven hard to determine. However, asteroidal and cometary particles typically have quite different types of orbits, with asteroidal particles having more circular and lower inclination orbits than cometary particles. Accordingly, the relative velocities of these groups of particles with respect to Earth are also quite different, and measurements of these relative velocities can help distinguish between the sources. The spectrum of the zodiacal light contains solar absorption lines that are Doppler-shifted by moving dust particles. The profiles of the shifted absorption lines therefore provide information about the velocities of the dust particles, which are the critical data needed to determine their origin. It is possible to determine these dust particle velocities by observing the zodiacal light using the Wisconsin H-alpha Mapper (WHAM) — a specialized Fabry-Perot spectrometer used to study wide-scale, diffuse sources.

Focusing on a pair of scattered solar Mg I Fraunhofer lines, we have recently begun a three-year observing campaign with WHAM, taking advantage of otherwise unused dawn/dusk time. The long baseline of observations will eventually allow us to determine not only seasonal effects, such as the azimuthal asymmetries of the cloud which define its structure, but also start to differentiate temporal variations over the solar cycle, which will allow us to ascertain the contribution of interstellar source particles.

To interpret the observations, we create synthetic Doppler-shifted spectra based upon the results of dynamical simulations of the dust particles composing the zodiacal cloud. We determine the orbital parameters of dust particles from a range of sources by tracking the dynamical evolution of the particles from their source regions into the inner Solar System. We then produce synthetic observations of how such orbital distributions of dust particles would shift and modify the profiles of the observed spectral lines. Comparing these synthetic spectra to the actual observations will allow us to better constrain the structure and sources of the dust composing the zodiacal cloud.

Here we present an overview of this new project and our work on producing the synthetic spectra, following the method of Hirschi and Beard (1987) and Ipatov (2008).