ALBERT

Description: We show that if young low-mass stars are subject to vigorous X-ray driven disc winds, then such winds may be rendered detectable in cluster environments through their interaction with ionizing radiation from massive stars. In particular we argue that in the Orion nebula cluster (ONC) one expects to see of order tens of ‘X-ray proplyds’ (i.e. objects with offset ionization fronts detectable through optical imaging) in the range 0.3–0.6 pc from 1 C Ori (the dominant O star in the ONC). Objects at this distance lie outside the central ‘FUV zone’ in the ONC where proplyd structures are instead well explained by neutral winds driven by external far-ultraviolet (FUV) emission from 1 C Ori. We show that the predicted numbers and sizes of X-ray proplyds in this region are compatible with the numbers of proplyds observed and that this may also provide an explanation for at least some of the far flung proplyds observed in the Carina nebula. We compare the sizes of observed proplyds outside the FUV region of the ONC with model predictions based on the current observed X-ray luminosities of these sources (bearing in mind that the current size is actually set by the X-ray luminosity a few hundred years previously, corresponding to the flow time to the ionization front). We discuss whether variability on this time-scale can plausibly explain the proplyd size data on a case-by-case basis. We also calculate the predicted radio free–free emission signature of X-ray proplyds and show that this is readily detectable. Monitoring is, however, required in order to distinguish such emission from non-thermal radio emission from active coronae. We also predict that it is only at distances more than a parsec from 1 C Ori that the free–free emission signature of such offset ionized structures would be clearly distinguishable from an externally driven ionized disc wind. We argue that the fortuitous proximity of massive stars in the ONC can be used as a beacon to light up internally driven X-ray winds and that this represents a promising avenue for observational tests of the X-ray photoevaporation scenario.

Description: The lack of observed transition discs with inner gas holes of radii greater than ~50 au implies that protoplanetary discs dispersed from the inside out must remove gas from the outer regions rapidly. We investigate the role of photoevaporation in the final clearing of gas from low mass discs with inner holes. In particular, we study the so-called ‘thermal sweeping’ mechanism which results in rapid clearing of the disc. Thermal sweeping was originally thought to arise when the radial and vertical pressure scalelengths at the X-ray heated inner edge of the disc match. We demonstrate that this criterion is not fundamental. Rather, thermal sweeping occurs when the pressure maximum at the inner edge of the dust heated disc falls below the maximum possible pressure of X-ray heated gas (which depends on the local X-ray flux). We derive new critical peak volume and surface density estimates for rapid radiative clearing which, in general, result in rapid dispersal happening less readily than in previous estimates. This less efficient clearing of discs by X-ray driven thermal sweeping leaves open the issue of what mechanism (e.g. far-ultraviolet heating) can clear gas from the outer disc sufficiently quickly to explain the non-detection of cold gas around weak line T Tauri stars.