We report multi-epoch VLBI H$_2$O maser observations towards the compact cluster of YSOs close to the Herbig Be star LkH$alpha$ 234. This cluster includes LkH$alpha$ 234 and at least nine more YSOs that are formed within projected distances of $sim$10 arcsec ($sim$9,000 au). We detect H$_2$O maser emission towards four of these YSOs. In particular, our VLBI observations (including proper motion measurements) reveal a remarkable very compact ($sim$0.2 arcsec = $sim$180 au), bipolar H$_2$O maser outflow emerging from the embedded YSO VLA 2. We estimate a kinematic age of $sim$40 yr for this bipolar outflow, with expanding velocities of $sim$20 km s$^{-1}$ and momentum rate $dot M_w V_w$ $simeq$ $10^{-4}-10^{-3}$ M$_{odot}$ yr$^{-1}$ km s$^{-1}$$times (Omega$/$4pi)$, powered by a YSO of a few solar masses. We propose that the outflow is produced by recurrent episodic jet ejections associated with the formation of this YSO. Short-lived episodic ejection events have previously been found towards high-mass YSOs. We show now that this behaviour is also present in intermediate-mass YSOs. These short-lived episodic ejections are probably related to episodic increases in the accretion rate, as observed in low-mass YSOs. We predict the presence of an accretion disk associated with VLA 2. If detected, this would represent one of the few known examples of intermediate-mass stars with a disk-YSO-jet system at scales of a few hundred au.
تحميل البحث