We present new millimeter observations of the ionized wind from the massive young stellar object LkH \mathbf { \alpha } 101 , made with the IRAM interferometer and 30m telescope .
Several recombination lines , including higher order transitions , were detected for the first time at radio wavelengths in this source . From three \alpha –transitions we derive an accurate value for the stellar velocity and , for the first time , an unambiguous expansion velocity of the wind which is 55 km s ^ { -1 } , much slower than reported previously , and the mass loss rate is 1.8 10 ^ { -6 } M _ { \odot } yr ^ { -1 } .
The wideband continuum spectra and the interferometer visibilities show that the density of the wind falls off more steeply than compatible with constant–velocity expansion .
We argue that these properties indicate that the wind is launched from a radially narrow region of the circumstellar disk , and we propose that slow speed and a steep density gradient are characteristic properties of the evolutionary phase where young stars of intermediate and high mass clear away the gaseous component of their accretion disks . The recombination lines are emitted close to local thermal equilibrium , but the higher order transitions appear systematically broader and weaker than expected , probably due to impact broadening . Finally , we show that LkH \mathbf { \alpha } 101 shares many properties with MWC 349 , the only other stellar wind source where radio recombination lines have been detected , some of them masing .
We argue that LkH \mathbf { \alpha } 101 evades masing at millimeter wavelengths because of the disk ’ s smaller size and unfavorable orientation .
Some amplification may however be detectable at shorter wavelengths .