We present high resolution Na i D spectroscopy of the \beta Pic disk , and the resonantly scattered sodium emission can be traced from less than 30 AU to at least 140 AU from the central star .
This atomic gas is co-existent with the dust particles , suggestive of a common origin or source .
The disk rotates towards us in the south-west and away from us in the north-east .
The velocity pattern of the gas finally provides direct evidence that the faint linear feature seen in images of the star is a circumstellar disk in Keplerian rotation .
From modelling the spatial distribution of the Na i line profiles we determine the effective dynamical mass to be ( 1.40 \pm 0.05 ) M _ { \odot } , which is smaller than the stellar mass , 1.75 M _ { \odot } .
We ascribe this difference to the gravity opposing radiation pressure in the Na i lines .
We argue that this is consistent with the fact that Na is nearly completely ionised throughout the disk ( Na i/Na < 10 ^ { -4 } ) .
The total column density of sodium gas is N ( Na ) = 10 ^ { 15 } { cm } ^ { -2 } .