Radiation Pressure Feedback in Massive Star Formation - Circumventing the radiation pressure barrier and stable radiation-pressure-dominated outflow cavities -

Rolf Kuiper

Context:

During their evolution, massive stars quickly become so luminous that their radiation pressure onto the environment exceeds their gravitational attraction. Hence, radiation pressure plays a major role in shaping the circumstellar environment.

Methods:

In 1D, 2D, and 3D self-gravity radiation hydrodynamics simulations of various collapsing pre-stellar cores of gas and dust, we determine the impact of the radiation pressure on the accretion disk and the bipolar outflow. The evolution of the stellar environment is resolved down to the order of 1 AU (logarithmically decreasing towards larger distances to the star) and the stellar irradiation feedback is computed by use of a highly accurate frequency-dependent ray-tracing (RT) approach (Kuiper et al. 2010, A&A 511) plus flux-limited diffusion (FLD) for thermal dust emission. The evolution of the pre-stellar environment is computed for several 100 kyr (up to a maximum of 14 free-fall times), including the whole accretion phase of the forming star.

Results:

a) Recently (in Kuiper et al. 2012, A&A 537) - we depicted the importance of the accuracy of the RT step in revealing the sustained stability of radiation-pressure-dominated outflow cavities during the formation of massive stars. In contrast, making use of the gray FLD approximation highly underestimates the absorption probability in the cavity shell. As a result, the FLD approximation for stellar irradiation feedback leads to a configuration prone to the so-called radiative Rayleigh-Taylor instability (Krumholz et al., 2009, Science).

b) Our parameter scan of varying initial core masses performed in different geometries demonstrates, how the well-known radiation pressure problem in the formation of massive stars can be circumvented via classical disk accretion: The computation of the radiative feedback and the shielding property of the inner disk region requires to resolve the dust sublimation front around the forming massive star. Not including the dust sublimation front artificially terminates the disk accretion epoch, such as in Yorke & Sonnhalter (2002), ApJ 569. The formation of long-living massive accretion disks enforces a strong anisotropy of the thermal radiation field, enabling steady accretion through the shielded disk region (Kuiper et al. 2010, ApJ 722). This so-called flashlight effect is even amplified by the optically thick gas around forming massive protostars (Kuiper & Yorke, ApJ 2012). In 3D the self-gravity of the massive accretion disk drives a sufficiently high angular momentum transport enabling the accretion flow to overcome the residual radiation pressure (Kuiper et al. 2011, ApJ 732).

Conclusions:

a) Treating the stellar irradiation in the gray FLD approximation underestimates the radiative forces acting on the cavity shell. This can artificially lead to situations unstable to the radiative Rayleigh-Taylor instability. The proper treatment of direct stellar irradiation by massive stars is crucial for the stability of radiation-pressure-dominated cavities.

b) Summing up, the various simulation series draw a consistent picture of the formation of the most massive (> 100 Msol) stars including classical disk accretion and radiation pressure driven outflow launching.