Smoothed particle hydrodynamics simulations of expanding H II regions

Abstract

Context. Ionizing radiation plays a crucial role in star formation at all epochs. In contemporary star formation, ionization abruptly raises the pressure by more than three orders of magnitude; the temperature increases from ∼10 K to ∼10 4 K, and the mean molecular weight decreases by a factor of more than 3. This may result in positive feedback, either by compressing pre-existing clouds and rendering them unstable, or by sweeping up gravitationally unstable shells. It may also result in negative feedback (by dispersing residual dense gas). Ionizing radiation from OB stars is also routinely invoked as a means of injecting kinetic energy into the interstellar medium and as a driver of sequential self-propagating star formation in galaxies. Aims. We describe a new algorithm for including the dynamical effects of ionizing radiation in SPH simulations, and we present several examples of how the algorithm can be applied to problems in star formation. Methods. We use the HEALPix software to tessellate the sky and to solve the equation of ionization equilibrium along a ray towards each of the resulting tesserae. We exploit the hierarchical nature of HEALPix to make the algorithm adaptive, so that fine angular resolution is invoked only where it is needed, and the computational cost is kept low. Results. We present simulations of (i) the spherically symmetric expansion of an Hii region inside a uniform-density, non-selfgravitating cloud; (ii) the spherically symmetric expansion of an Hii region inside a uniform-density, self-gravitating cloud; (iii) the expansion of an off-centre Hii region inside a uniform-density, non-self-gravitating cloud, resulting in rocket acceleration and dispersal of the cloud; and (iv) radiatively driven compression and ablation of a core overrun by an Hii region. Conclusions. The new algorithm provides the means to explore and evaluate the role of ionizing radiation in regulating the efficiency and statistics of star formation.

The prevalence of star formation as a function of Galactocentric radius

“…A change in SFF with location may indicate a spatial variation in environmental factors influencing the probability that a clump will form stars. Such factors may include the availability of dense molecular gas (Roman-Duval et al 2016), turbulent pressure (Wolfire et al 2003), local magnetic-field strength (Heiles & Troland 2005), or the presence of a triggering agent such as a wind-or radiation-driven bubble (Bertoldi 1989;Bisbas et al 2009). Reid et al 2014;Reid et al 2016).…”

Ionisation Feedback in Star and Cluster Formation Simulations

“…The situation in the latest years has been rapidly improving, however, with more sophisticated implementations of ionised radiation both in grid-based codes (e.g. Mellema et al 2006;Peters et al 2010) Miao et al 2006;Dale et al 2007;Bisbas et al 2009). Klessen et al (2009) andMac Low et al (2007) present recent reviews of the numerical methods employed.…”

SPH Radiative Hydrodynamics Methods

“…Another important example is the positive/negative radiative feedback by massive stars in local star formation processes (e.g. Bisbas et al 2009, Gritschneder et al 2009). In order to tackle these problems, a careful treatment of photoionization as well as a radiation transfer solver fully coupled with hydrodynamics is required.…”

“…On the other hand, a few codes (Alvarez et al 2006, Bisbas et al 2009) utilize a public domain code called HEALPix (Górski et al 2005). HEALPix is a software which produces a subdivision of a spherical surface in which each pixel covers the same surface area as every other pixel.…”

“…Another approach to overcome the difficulty of the ionizing photon transfer is called "Ifront tracking method" or "Strömgren volume approximation" (Alvarez et al 2006, Dale et al 2007, Yoshida et al 2007, Bisbas et al 2009). This method basically tries to follow the position of ionization front (I-front) for every direction (Fig.1).…”