Astrophysical Maser Flares from Shock Impact

Figure 1. A sequence of synthetic images in maser line emission of an astrophysical cloud being compressed by a 7.5km/s shock moving vertically upwards. The percentage figures represent the fraction of the cloud compressed. Axes are in the scaled units of the model, and the colour bar to the right of each figure is the log (base 10) of the maser intensity in units of the saturation intensity; all panels are on the same scale.

One mechanism that has been put forward as a generator of maser flares in massive star-forming regions is the impact of a shock of modest speed on a cloud with a typical size of a few astronomical units. The sequence of diagrams on the left show the effect on images of such a cloud if struck by a 7.5km/s shock that moves up the z-axis (bottom to top). There is a frame change at the 50% shocked point from a view where the shock appears to move, at lower precentages, to a view where the remains of the uncompressed cloud flows into a stationary shock at the cloud mid-point (for shocked proportions > 50%). The shock is modelled as isothermal, with a compression factor proportional to the square of the Mach number. In this case, the Mach number is 3, giving a density enhancement of 9 in the shocked part of the cloud, which increases from 10% of the cloud in the top diagram to 80% in the bottom panel. Compression is applied to the node distribution of the model prior to triangulation. The colour palette is the base-10 logarithm of the maser specific intensity relative to the saturation intensity so the brightest parts of the image increase from about 0.001 of the saturation intensity in the undisturbed cloud to of order 104 times this parameter in the 80% panel, so that the shock increases the brightest pixel intensities by of order 10 million times. The effect on the flux density, averaged over the whole cloud is lower, but this impact still represents a very convincing mechanism for generating high-contrast maser flares. If one model unit on the axes of each panel is equal to half an astronomical unit, then the rise time of the flare is approximately the time taken for the shock to cover 1 astronomical unit, or 231 days. The effectiveness of the flare mechanism relies on a switch, due to the shock, from a weakly saturated initial cloud (approximately spherical) to a quite strongly saturated, and highly oblate, final cloud.
Data from DiRAC is available for (original) optical depths ranging from 0.1-30.0, and a number of different shock speeds.