PI: N. Schneider
Ionizing radiation from OB stars forms elongated structures in the neutral gas at the interface between H II regions and molecular clouds. Early studies, mainly in the optical and IR, revealed features that were called ‘elephant trunk’, ‘speck’, ‘teardrop’, and ‘cometary’ globules. Later, the famous ‘pillars of creation’ in M16 and EGGs (evaporating gaseous globules) were discovered with the HST, and since then more and more of these UV-illuminated features have been detected also at FIR wavelengths.
Typical characteristics of these features are a bright rim at the edge of the cloud and, in most cases, a head-tail structure pointing towards the illuminating source, an intrinsic velocity gradient, and a concentration of mass at the tip that often forms stars.
However, the precise physical processes leading to the formation of pillars and globules, and the stars inside them, are not yet well understood. Classical approaches with Rayleigh–Taylor instabilities and (non-)accelerating ionization fronts do not manage to explain all aspects of pillar formation. The importance of turbulence and gravity — major ‘ingredients’ for molecular cloud and star formation in general — is becoming more and more recognized, and sophisticated (magneto-)hydrodynamic simulations begin to successfully model pillar/globule formation.
A dedicated project to fully simulate the formation of pillars is being undertaken at the CEA in Saclay, using the HERACLES code, which comprises hydrodynamics with various equations of state, radiative transfer, gravity, non-equilibrium thermo-chemistry, cooling and heating, and thermal conduction. The HERACLES output (density, velocity, temperature cubes) also serves as input for the radiative-transfer photon-dominated region (PDR) code KOSMA-tau, developed by the University of Cologne, to produce ‘observables’ such as integrated line intensities of atomic and molecular lines, average (column) density, total mass in one beam, FUV-field intensity, and abundances.
In order to better understand the formation of pillars and globules, and star formation within them, we applied for spectroscopic observations with Herschel. We then intend to fully model a number of pillars/globules in different physical environments (UV-field strength, initial density, initial clumpiness, etc.), using both HERACLES and KOSMA-tau, and to compare the results to observations. Within the context of the Herschel ‘Open Time’ project (“Pillars of creation: physical origin and connection to star formation”, PI: N. Schneider), three molecular cloud regions were observed in spectroscopic mode (PACS and SPIRE). They are located in Cygnus, Rosette, and M16, and show nice examples of nascent and evolved pillars and globules and other features.
All regions were mapped in the major atomic cooling lines of [C II] at 158 micron, [O I] 63 and 145 micron in the PACS range spectroscopy mode (spectrally unresolved), and in the spectrally resolved HIFI OTF mode (only [C II] 158 micron). High-J CO and HCO+ lines and the two atomic [C I] lines are observed using the SPIRE FTS. This data is currently being exploited, and PDR modelling has started.