Herschel/HIFI spectroscopic observations of CO J = 10 - 9 , CO J = 16 - 15 and [ C ii ] towards HD 100546 are presented .
The objective is to resolve the velocity profile of the lines to address the emitting region of the transitions and directly probe the distribution of warm gas in the disk .
The spectra reveal double-peaked CO line profiles centered on the systemic velocity , consistent with a disk origin .
The J = 16 - 15 line profile is broader than that of the J = 10 - 9 line , which in turn is broader than those of lower J transitions ( 6 - 5 , 3 - 2 , observed with APEX ) , thus showing a clear temperature gradient of the gas with radius .
A power-law flat disk model is used to fit the CO line profiles and the CO rotational ladder simultaneously , yielding a temperature of T _ { 0 } = 1100 \pm 350 K ( at r _ { 0 } = 13 AU ) and an index of q = 0.85 \pm 0.1 for the temperature radial gradient .
This indicates that the gas has a steeper radial temperature gradient than the dust ( mean q _ { dust } \sim 0.5 ) , providing further proof of the thermal decoupling of gas and dust at the disk heights where the CO lines form .
The [ C ii ] line profile shows a strong single-peaked profile red-shifted by 0.5 km s ^ { -1 } compared to the systemic velocity .
We conclude that the bulk of the [ C ii ] emission has a non-disk origin ( e.g. , remnant envelope or diffuse cloud ) .