Density estimates for two sightlines in the Taurus Molecular Cloud, using C₂

Gaining insight into the growth of dust grains within the interstellar medium (ISM) is crucial to understand the development and formation of new planets. Direct measurements of grain growth require observations of hydrogen gas densities which are currently only possible with the James Webb Space Telescope (JWST). Using spectro-polarimetry ground-based telescopes may, however, provide a way to indirectly measure the gas densities of interstellar clouds. Observations of polarized starlight from the Taurus Molecular Cloud (TMC) have revealed evidence of grain coagulation, even in regions of lower extinction (A_V~5-6 mag) than is commonly found. We present a comparison of the gas densities along two lines of sight through the TMC; the first towards HD 283809, the second towards SWIW 100. Although the lines of sight exhibit similar extinction values, they have distinctly different polarization peaks (λ_max). To determine the temperatures and column densities of hydrogen (n_H) along the lines of sight, we analyzed the absorption spectra for the 1-0 and 2-0 vibrational bands of the Phillips system in diatomic carbon (C₂). While our current C₂ data for the high λ_max target, SWIW 100, only reach J=10 in the rotation ladder, the densities derived for the two lines of sight are, within the uncertainties, equal. This unexpected result necessitates the need for additional analysis of other molecules, which will provide further insight into the gas density of the cloud. Finally, further observations of SWIW 100 should provide enough data to detect absorption lines up to J=16.