Title:Signatures of accretion: Pa alpha emission in HD 100546

Abstract:
Pa alpha is one of the intrinsically most valuable diagnostics of accretion in 
YSOs. However, it is based in a wavelength region (1.875 microns) which is 
heavily affected by absorption from water vapour in the earth''s atmosphere and 
thus generally only observed from space. We have taken advantage of the recent 
installation of a dedicated Particulate Water Vapour (PWV) monitor on Paranal 
observatory to use the CRIRES instrument at the VLT to observe the Pa alpha 
line in the Herbig Be star HD 100546 during an episode of particularly low PWV 
conditions, normally only present at much higher altitude sites. This 
constitutes the first detection of this line from the ground. We use the Pa 
alpha line to determine the accretion parameters of HD 100546 and present a 
comparison of this novel method to measure accretion with other methods.