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

Abstract

Investigating the chemical complexity of the interstellar medium (ISM) is key for understanding its physical nature and evolution. In this work, we study parsec-scale interstellar dust clouds in the neutral ISM of the Milky Way using two different probes: dust depletion and dust extinction. We examine their relationship to investigate the distribution of metals and dust in the Solar neighbourhood, and how they are related to the Local Bubble. We use measurements of dust depletion for individual gas clouds along sight lines of sight towards bright O/B stars within 1.1 kpc of the Sun, derived from UV absorption-line spectra. We combine these with parsec-scale 3D dust extinction density maps out to 1.25 kpc. We assume a correlation between dust depletion and dust extinction density, which we use to imply 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. We identify peaks in the dust extinction curves, and then associate the stronger peaks with the strongest dust depletion components. Independent distance measurements along the line of sight towards one of our targets, theta1 Ori C, validates our results. In our sample, the minimum distance between clouds that have significantly different chemical properties is ~ 100pc, giving 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.