"Small and time-variable structures in the interstellar medium of the Milky Way"

We present FLAMES observations towards seven Galactic and Magellanic open clusters 
and UVES and McDonald twin-epoch measurements towards more than 100 early-type stars in 
order to search for small- and tiny-scale (time-variable) structures in the interstellar medium of 
the Milky Way. The lines studied are CaI, CaII and NaI. Our ultimate aim is to determine if 
previously observed time-variability is real, and if so, if it is caused by actual variations in 
column density over time, differences in ionisation, physical geometry or as a result of the 
power-spectrum of the ISM.

We find ubiquitous structure on small-scales (~15 arcseconds to 25 arcminutes) in CaII with 
variations in line strength of around 10 percent on small (pc) scales, which is compared with a 
simple model of the ISM. However, only a few percent of sightlines show variation (AU-
scale structure) in CaI and/or NaI on timescales of 6 to 20 years.

Jonathan Smoker, Catherine McEvoy, Philip Dufton, Keith Smith, Francis Keenan, Michael 
Kennedy, Andrew Fox and David Lambert