Connecting the dusty dots: Dust depletion and extinction of local interstellar clouds

Investigating the chemical complexity of the interstellar medium (ISM) is key for understanding its physical nature and evolution. In this work, we studied parsec-scale interstellar dust clouds in the neutral ISM of the Milky Way using two different probes: dust depletion and dust extinction. We examined their relationship to investigate the distribution of metals and dust in the solar neighbourhood and how they are related to the Local Bubble. We used dust depletion measurements for individual gas clouds along eight lines of sight towards bright O/B stars within 1.1 kpc of the Sun, derived from UV absorption-line spectra. We combined these with parsec-scale 3D dust extinction density maps out to 1.25 kpc. Based on the well-known relationship between gas and dust in the ISM, we assumed a correlation between dust depletion and dust extinction density. This assumption allowed us to infer that the absorption components are spatially associated with the peaks in dust extinction density and to pinpoint the likely locations of the gas clouds in physical space. Using the Python package scipy find_peaks , we identified peaks in the dust extinction curves, and then associated the stronger peaks with the strongest dust depletion components. Independent distance measurements along the line of sight towards one of our targets, þeta^1 Ori C, validates our result and supports the reliability of our method. In our sample, the minimum distance between clouds with significantly different chemical properties (in terms of dust depletion) is ∼ 100pc. This gives an indication on the physical scale on which chemical mixing remains incomplete in the ISM of the Milky Way. For five of the eight targets, we report dust depletion values for gas clouds associated with the Local Bubble. Additionally, we find a velocity gradient that is consistent with the expansion of the Local Bubble, further supporting our methodology. Overall, we show that it is possible to use complementary information from dust depletion and dust extinction to build more detailed maps of ISM metal and dust distributions.