Jingqi Miao

Triggered star formation in bright-rimmed clouds: the Eagle nebula revisited

A three dimensional Smoothed Particle Hydrodynamical (SPH) model has been extended to study the radiative driven implosion effect of massive stars on the dynamical evolutions of surrounding molecular clouds. The new elements in the upgraded code are the inclusion of Lyman continuum in the incident radiation flux and the trea tment of hydrogen ionisation process; introducing ionisation heating & recombination cooling effects; and adding a proper description of the magnetic and turbulent pressures to the i nternal pressure of the molecular cloud. This extended code provides a realistic model to trace not only the dynamical evolution of a molecular cloud, but also can be used to model the kinematics of the ionisation & shock fronts and the photo-evaporating gas surrounding the molecular cloud, which the previous code is unable to deal with. The application of this newly developed model to the structure of the middle Eagle Nebula finger suggests that the shock induced by the ionising rad iation at the front side of the head precedes an ionisation front moving towards the center of the core, and that the core at the fingertip is at transition stage evolving toward a state of indu ced star formation. The dynamical evolution of the velocity field of the simulated cloud struct ure is discussed to illustrate the role of the self-gravity and the different cloud morphologies which appear at different stages in the evolutionary process of the cloud. The motion of the ionisation front and the evaporating gas are also investigated. The modelled gas evaporation rate is consistent with that of current other models and the density, temperature and chemical profiles ar e agreement with the observed values. The relative lifetimes of different simulated cloud morphologies suggests a possible answer to the question of why more bright-rimmed clouds are observed to possess a flat-core than an elongated-core morphology.

A Radio and mid-infrared survey of northern bright-rimmed clouds

We have carried out an archival radio, optical and infrared wavelength imaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA Sky Survey (NVSS) archive, images from the Digitised Sky Survey (DSS) and the Midcourse Space eXperiment (MSX). The data characterise the physical properties of the Ionised Boundary Layer (IBL) of the BRCs. We have classified the radio detections as: that associated with the ionised cloud rims; that associated with possible embedded Young Stellar Objects (YSOs); and that unlikely to be associated with the clouds at all. The stars responsible for ionising each cloud are identified and a comparison of the expected ionising flux to that measured at the cloud rims is presented. A total of 25 clouds display 20 cm radio continuum emission that is associated with their bright optical rims. The ionising photon flux illuminating these clouds, the ionised gas pressure and the electron density of the IBL are determined. We derive internal molecular pressures for 9 clouds using molecular line data from the literature and compare these pressures to the IBL pressures to determine the pressure balance of the clouds. We find three clouds in which the pressure exerted by their IBLs is much greater than that measured in the internal molecular material. A comparison of external pressures around the remaining clouds to a global mean internal pressure shows that the majority of clouds can be expected to be in pressure equilibrium with their IBLs and hence are likely to be currently shocked by photoionisation shocks. We identify one source which shows 20 cm emission consistent with that of an embedded high-mass YSO and confirm its association with a known infrared stellar cluster. This embedded cluster is shown to contain early-type B stars, implying that at least some BRCs are intimately involved in intermediate to high mass star formation.

Searching for signs of triggered star formation toward IC 1848

We have carried out an in-depth study of three bright-rimmed clouds SFO 11, SFO 11NE and SFO 11E associated with the HII region IC 1848, using observations carried out at the James Clerk Maxwell Telescope (JCMT) and the Nordic Optical Telescope (NOT), plus archival data from IRAS, 2MASS and the NVSS. We show that the overall morphology of the clouds is reasonably consistent with that of radiative-driven implosion (RDI) models developed to predict the evolution of cometary globules. There is evidence for a photoevaporated flow from the surface of each cloud and, based upon the morphology and pressure balance of the clouds, it is possible that D-critical ionisation fronts are propagating into the molecular gas. The primary O star responsible for ionising the surfaces of the clouds is the 06V star HD 17505. Each cloud is associated with either recent or ongoing star formation: we have detected 8 sub-mm cores which possess the hallmarks of protostellar cores and identify YSO candidates from 2MASS data. We infer the past and future evolution of the clouds and demonstrate via a simple pressure-based argument that the UV illumination may have induced the collapse of the dense molecular cores found at the head of SFO 11 and SFO 11E.

AN INVESTIGATION ON THE MORPHOLOGICAL EVOLUTION OF BRIGHT-RIMMED CLOUDS

A new radiative driven implosion (RDI) model based on smoothed particle hydrodynamics technique is developed and applied to investigate the morphological evolutions of molecular clouds under the effect of ionizing radiation. This model self-consistently includes the self-gravity of the cloud in the hydrodynamical evolution, the UV radiation component in the radiation transferring equations, the relevant heating and cooling mechanisms in the energy evolution, and a comprehensive chemical network. The simulation results reveal that under the effect of ionizing radiation, a molecular cloud may evolve through different evolutionary sequences. Depending on its initial gravitational state, the evolution of a molecular cloud does not necessarily follow a complete morphological evolution sequence from type A?B?C, as described by previous RDI models. When confronted with observations, the simulation results provide satisfactory physical explanations for a series of puzzles derived from bright-rimmed clouds observations. The consistency of the modeling results with observations shows that the self-gravity of a molecular cloud should not be neglected in any investigation on the dynamical evolution of molecular clouds when they are exposed to ionizing radiation.

A radio and mid-infrared survey of northern bright-rimmed clouds

We have carried out an archival radio, optical and infrared wavelength imaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA Sky Survey (NVSS) archive, images from the Digitised Sky Survey (DSS) and the Midcourse Space eXperiment (MSX). The data characterise the physical properties of the Ionised Boundary Layer (IBL) of the BRCs. We have classified the radio detections as: that associated with the ionised cloud rims; that associated with possible embedded Young Stellar Objects (YSOs); and that unlikely to be associated with the clouds at all. The stars responsible for ionising each cloud are identified and a comparison of the expected ionising flux to that measured at the cloud rims is presented. A total of 25 clouds display 20 cm radio continuum emission that is associated with their bright optical rims. The ionising photon flux illuminating these clouds, the ionised gas pressure and the electron density of the IBL are determined. We derive internal molecular pressures for 9 clouds using molecular line data from the literature and compare these pressures to the IBL pressures to determine the pressure balance of the clouds. We find three clouds in which the pressure exerted by their IBLs is much greater than that measured in the internal molecular material. A comparison of external pressures around the remaining clouds to a global mean internal pressure shows that the majority of clouds can be expected to be in pressure equilibrium with their IBLs and hence are likely to be currently shocked by photoionisation shocks. We identify one source which shows 20 cm emission consistent with that of an embedded high-mass YSO and confirm its association with a known infrared stellar cluster. This embedded cluster is shown to contain early-type B stars, implying that at least some BRCs are intimately involved in intermediate to high mass star formation.

Near-IR Imaging Polarimetry toward a Bright-rimmed Cloud: Magnetic Field in SFO 74

We have made near-infrared (JHKs) imaging polarimetry of a bright-rimmed cloud (SFO 74). The polarization vector maps clearly show that the magnetic field in the layer just behind the bright rim is running along the rim, quite different from its ambient magnetic field. The direction of the magnetic field just behind the tip rim is almost perpendicular to that of the incident UV radiation, and the magnetic field configuration appears to be symmetric as a whole with respect to the cloud symmetry axis. We estimated the column and number densities in the two regions (just inside and far inside the tip rim), and then derived the magnetic field strength, applying the Chandrasekhar-Fermi method. The estimated magnetic field strength just inside the tip rim, ~90 uG, is stronger than that far inside, ~30 uG. This suggests that the magnetic field strength just inside the tip rim is enhanced by the UV radiation induced shock. The shock increases the density within the top layer around the tip, and thus increases the strength of the magnetic field. The magnetic pressure seems to be comparable to the turbulent one just inside the tip rim, implying a significant contribution of the magnetic field to the total internal pressure. The mass-to-flux ratio was estimated to be close to the critical value just inside the tip rim. We speculate that the flat-topped bright rim of SFO 74 could be formed by the magnetic field effect.

AN INVESTIGATION OF THE TYPE M MORPHOLOGICAL STRUCTURE OF IC59: A NEW MODEL FOR BRIGHT RIM CLOUDS?

We report the results from a smoothed particle hydrodynamics simulation designed to model recent observational data on the nebula and Bright Rim Cloud IC59. We further examine, in the context of radiative-driven implosion (RDI) models, the possible formation mechanisms of the morphological structure of IC59. The results of the simulation reveal the existence of a new, fourth morphological state for Bright Rim Clouds (BRCs)—which we propose to call a Type M BRC morphology. We discuss the necessary conditions for the appearance of Type M BRCs, based on analytical and numerical simulations. The simulated physical properties from our model are consistent with the available observations of IC59. We further show that the prospect of RDI triggered star formation in all Type M BRCs is not supported by the simulations.

Evolution of prolate molecular clouds at H II boundaries - I. Formation of fragment-core structures

The evolution of a prolate cloud at an H II boundary is investigated using smoothed particle hydrodynamics. The prolate molecular clouds in our investigation are set with their semi-major axis perpendicular to the radiative direction of a plane-parallel ionizing extreme ultraviolet (EUV) flux. Simulations on three high-mass prolate clouds reveal that EUV radiation can trigger distinctive high-density core formation embedded in a final linear structure. This contrasts with results of the previous work in which only an isotropic far-ultraviolet interstellar background flux was applied. A systematic investigation on a group of prolate clouds of equal mass but different initial densities and geometric shapes finds that the distribution of the cores over the final linear structure changes with the initial conditions of the prolate cloud and the strength of the EUV radiation flux. These highly condensed cores may either scatter over the full length of the final linear structure or form two groups of high-density cores at two foci, depending on the value of the ionizing radiation penetration depth dEUV, the ratio of the physical ionizing radiation penetration depth to the minor axis of the cloud. Data analysis on the total mass of the high-density cores and the core formation time finds that the potential for EUV radiation triggered star formation efficiency is higher in prolate clouds with shallow ionization penetration depth and intermediate major-to-minor axial ratio, for the physical environments investigated. Finally, it is suggested that the various fragment-core structures observed at H II boundaries may result from the interaction between ionizing radiation and pre-existing prolate clouds of different initial geometrical and physical conditions.

Evolution of prolate molecular clouds at H ii boundaries – II. Formation of BRCs of asymmetrical morphology

A systematic investigation on the evolution of a prolate cloud at an H II boundary is conducted using smoothed particle hydrodynamics in order to understand the mechanism for a variety of irregular morphological structures found at the boundaries of various H II regions. The prolate molecular clouds in this investigation are set with their semimajor axes at inclinations between 0° and 90° to a plane-parallel ionizing radiation flux. A set of four parameters, the number density n, the ratio of major to minor axis γ, the inclination angle Φ and the incident flux FEUV, are used to define the initial state of the simulated clouds. The dependence of the evolution of a prolate cloud under radiation-driven implosion (RDI) on each of the four parameters is investigated. It is found that (i) in addition to the well-studied standard type A, B or C bright-rimmed clouds (BRCs), many other types such as asymmetrical BRCs, filamentary structures and irregular horse-head structures could also be developed at H II boundaries with only simple initial conditions; (ii) the final morphological structures are very sensitive to the four initial parameters, especially to the initial density and the inclination; (iii) the previously defined ionizing radiation penetration depth can still be used as a good indicator of the final morphology. Based on the simulation results, the formation time-scales and masses of the early RDI-triggered star formation from clouds of different initial conditions are also estimated. Finally a unified mechanism for the various morphological structures found in many different H II boundaries is suggested.

Filamentary structure formation in the Interstellar Radiation Field (ISRF)

A new mechanism is proposed for the formation of filament/core structure by ISRF and clumpy molecular cloud interaction. The derived characterizes of the filament/core network is consistent with that produced by the compressive forcing turbulence model.