Letter to the Editor

Tiny H I clouds in the local ISM

¹ ASTRON, Postbox 2, 7990 AA Dwingeloo, The Netherlands e-mail: braun@astron.nl
² National Radio Astronomy Observatory, Socorro, NM 87801, USA e-mail: nkanekar@aoc.nrao.edu

Received: 22 April 2005
Accepted: 9 May 2005

Abstract

We report deep, high spectral resolution WSRT 21 cm observations of four high latitude compact radio sources, that have revealed a new population of tiny, discrete clouds in the diffuse ISM, with peak optical depths %,  column densities of 0.4– cm^-2 and core temperatures of 20–80 K. Imaging detections confirm these low column densities and imply linear core dimensions of a few thousand AU, assuming a distance of 100 pc. The physical origin of these tiny  structures and their distribution in the ISM is at present unknown. Further observations will be required to determine whether they are a ubiquitous component of the ISM.