arXiv:astro-ph/0412665 v1 29 Dec 2004
THE ORION NEBULA IN THE MID-IR
M. Robberto, S. V. W. Beckwith, N. Panagia et al.
A big fat paper on mid-IR imaging of Orion. Mostly seems to concentrate on the BNKL region but has much interesting stuff on the dust arcs around the Trapezium too.
They analyse the shapes of the arcs in terms of an interacting winds model but seem to be pretty confused about it.
They calculate the relation between R_0/D and R_90/D for the CRW thin-shell model (first mistake: they do not take account of the fact that the "wind" from the proplyds is not isotropic). They find that all solutions fall on a thin line - although LV1, 4, and 5 are close to this line (with R_0/D = 0.2, R_90/D = 0.4), LV2, 3 and Ney-Allen are nowhere near (all have R_90/D ~= 0.4, with R_0/D = 0.3-0.5). In other words, they are much more open than they have a right to be.
However, they have ignored the fact that, when the los is "inside" the bowshock (inc <= theta_infty), then the visible arc is not the tangent to the bow surface at all (see my LV1 paper in RMAA). Whether this can explain the more circular shapes remains to be see. This would be a very easy project to do with Fulgencio. Unfortunately, the figures on arxiv are very low quality. Maybe they would send me a fits file of the data if I asked them for it.
They also don't seem to realise that in LV1 we are seeing the interaction of 3 winds. And, furthermore, they seem intent on explaining the Ney-Allen nebula around th1D as also due to a wind-wind interaction. They don't even mention the usual model for Ney-Allen, which has th1D being close to OMC-1 and heating dust in the PDR there. Very odd.
Another strange thing is that they seem to make no comment on the dust temperatures around the Trapezium. The only spectra they show are for features around BNKL. This is because their narrow band images were only taken of that region, so they only have wideband 10 and 20 micron images of the trapezium.
Comparing their images with HST images, there does seem to be some correspondence between some of their filaments and the elephant trunk structures. There is also a diffuse ball of IR emission around th1C, which is rather interesting. Its radius is about 20", a bit larger than the region containing the proplyd arcs. Could this be showing the boundary of the inner shock in th1C's fast wind. That would be exciting if true.
The reason there is no diffuse emission beyond the shock is that the grains will have been all destroyed by sputtering in the shock (is this reasonable for a 1000 km/s shock). But would there be diffuse emission in the supersonic wind region? This would have to be dust from the proplyd flows, and that won't be moving at 1000 km/s, so even if it then passes into the shocked wind region, it won't be going through a 1000 km/s shock. Rather, it will be at whatever speed it gets up to in the bowshock winds - maybe 100 km/s. This dosen't seem high enough to destroy the grains.
Their region 45" south of te trapezium (fig 4) looks very much like the ridges around HST3.
Gary says:
There were two papers by chick woodward showing that the optical depth
to dust at 10 microns is smaller than a pdr. He first argued that the
nebula was some sort of sheet close to D. I wonder if D happens to
be close to the veil?
A can't find the Woodward papers. D is not close to the veil.
I found that Mc Caughrean & Stauffer 1994 detect the Ney-Allen nebula at 2 microns