Tiny-Scale Atomic Structure and the Cold Neutral Medium—Review and Recap

Almost a decade ago I wrote an article with the same title as this. It focused on the physical properties of the Tiny-Scale Atomic Structure (TSAS) as discrete structures of the Cold Neutral Medium (CNM). To be observable, tiny discrete structures that don’t grossly violate pressure equilibrium need two attributes: low temperatures and geometrical anisotropy. Here I update that article. I discuss thermal and pressure equilibrium, ionization, optical lines, H2 abundance, and evaporation. 1. Canonical Values for TSAS as Discrete Structures Following Heiles (1997; H97), we use currently-available observations of the tinyscale atomic structure (TSAS) to describe its properties; the observations have improved, so the inferred properties are now somewhat different. The TSAS is seen in 21-cm line absorption so must be part of the Cold Neutral Medium (CNM). The observations consist of time-variable 21-cm absorption lines against pulsars, VLBI images of the 21-cm line in absorption against extended radio sources, and optical absorption line observations against background stars. The canonical values for important properties come directly from these observations and have no theoretical or otherwise imaginary input, except for the subjective judgment and weighting that we must perform to collapse a variety of measured values into a single number that we can use as a canonical value. We provide values for the following properties: 1. TSAS column density: N(HI)TSAS,18 = 0.75T15 , (1) where N(HI)18 is the HI column density in units of 10 18 cm−2 and T15 the temperature in units of 15 Kelvins (§3.1. justifies this canonical value of 15 K). The T -dependence arises because the 21-cm line opacity ∝ 1 T . The numerical value 0.75 is 4 times larger than the canonical value adopted by H97 and is based on Weisberg & Stanimirovic (this meeting). 2. The TSAS scale length in the plane of the sky (denoted by the subscript ⊥): LTSAS,⊥ = 30 AU . (2)