Jet formation from massive young stars: Magnetohydrodynamics versus radiation pressure
Bhargav Vaidya , Christian Fendt , Henrik Beuther  and Oliver Porth
 Max Planck Institute for Astronomy, Heidelberg
Observations indicate that outflows from massive young stars are more collimated during their early
evolution compared to later stages.
Our paper investigates various physical processes that impacts the
outflow dynamics, i.e. its acceleration and collimation.
We perform axisymmetric MHD simulations particularly considering the radiation
pressure exerted by the star and the disk.
We have modified the PLUTO code to include radiative forces
in the line-driving approximation.
We launch the outflow from the innermost disk region ($r < 50$,AU)
by magneto-centrifugal acceleration.
In order to disentangle MHD effects from radiative forces, we start the
simulation in pure MHD, and later switch on the radiation force.
We perform a parameter study considering different stellar masses (thus luminosity),
magnetic flux, and line-force strength.
For our reference simulation - assuming a $30msun$ star, we find substantial de-collimation of 35%
due to radiation forces.
The opening angle increases from $20^circ$ to $32^circ$ for stellar masses from
$20msun$ to $60msun$.
A small change in the line-force parameter $alpha$ from 0.60 to 0.55 changes the opening angle
by $sim 8^circ$.
We find that it is mainly the stellar radiation which affects the jet
dynamics. Unless the disk extends very close to the star, its pressure is too small to have much impact.
Essentially, our parameter runs with different stellar mass can be understood
as a proxy for the time evolution of the star-outflow system.
Thus, we have shown that when the stellar mass (thus luminosity) increases
(with age), the outflows become less collimated.
Reference: Accepted for publication in ApJ
Status: Manuscript has been accepted