Recent observations indicate that some class 0 sources have accretion rates orders of magnitude higher than those of class I. We investigated the conditions for the high accretion rates of some class 0 sources by numerical calculations, modelling an external trigger. For no external trigger, we find that the maximum value of the accretion rate is determined by the ratio α of the gravitational energy to the thermal one within a flat inner region of the cloud core. The accretion rate reaches ∼10−4 M⊙ yr−1 if the cloud core has α≳ 2. For an external trigger, we find that the maximum value of the accretion rate is proportional to the momentum given to the cloud core. The accretion rate reaches ≳10−4 M⊙ yr−1 with a momentum of ∼0.1 M⊙ km s−1 when the initial central density of the cloud core is ∼10−18 g cm−3. A comparison between recent observational results for pre‐stellar cores and our model with no triggered collapse indicates that the flat inner regions of typical pre‐stellar cores are not large enough to cause accretion rates of ∼10−4 M⊙ yr−1. Our results show that the triggered collapse of the cloud core is more preferable for the origin of the high accretion rates of class 0 sources than no triggered collapse.

Context. Molecular clouds near H ii regions tend to harbor more luminous protostars.Aims. We investigate whether a radiation-driven implosion mechanism enhances the luminosity of protostars near regions of high ionizing fluxes. Methods. We performed numerical simulations to model collapse of cores exposed to UV radiation from O stars. We investigated the dependence of mass loss rates on the initial density profiles of cores and variation of UV fluxes. We derived simple analytic estimates of accretion rates and final masses of protostars. Results. The radiation-driven implosion mechanism can increase accretion rates of protostars by 1-2 orders of magnitude. On the other hand, mass loss due to photo-evaporation is not high enough to have a significant impact on the luminosity. The increase in accretion rate results in luminosity 1-2 orders of magnitude higher than those of protostars that form without external triggering.Conclusions. Radiation-driven implosion can help explain the observed higher luminosity of protostars in molecular clouds near H ii regions.

A two dimensional hydrochemical hybrid code, KM2, is constructed to deal with astrophysical problems that would require coupled hydrodynamical and chemical evolution. The code assumes axisymmetry in cylindrical coordinate system, and consists of two modules: a hydrodynamics module and a chemistry module. The hydrodynamics module solves hydrodynamics using a Godunov-type finite volume scheme and treats included chemical species as passively advected scalars. The chemistry module implicitly solves non-equilibrium chemistry and change of the energy due to thermal processes with transfer of external ultraviolet radiation. Self-shielding effects on photodissociation of CO and H 2 are included. In this introductory paper, the adopted numerical method is presented, along with code verifications using the hydrodynamics module, and a benchmark on the chemistry module with reactions specific to a photon-dominated region (PDR). Finally, as an example of the expected capability, the hydrochemical evolution of a PDR is presented based on the PDR benchmark.

The bright-rimmed cloud SFO 22 was observed with the 45 m telescope of Nobeyama Radio Observatory in the 12 CO (J = 1-0), 13 CO (J = 1-0), and C 18 O (J = 1-0) lines, where well-developed head-tail structure and small line widths were found. Such features were predicted by radiation-driven implosion models, suggesting that SFO 22 may be in a quasi-stationary equilibrium state. We compare the observed properties with those from numerical models of a photo-evaporating cloud, which include effects of magnetic pressure and heating due to strong far-ultraviolet (FUV) radiation from an exciting star. The magnetic pressure may play a more important role in the density structures of bright-rimmed clouds, than the thermal pressure that is enhanced by the FUV radiation. The FUV radiation can heat the cloud surface to near 30 K, however, its effect is not enough to reproduce the observed density structure of SFO 22. An initial magnetic field of 5 µG in our numerical models produces the best agreement with the observations, and its direction can affect the structures of bright-rimmed clouds.