Waves on the surface of the Orion molecular cloud

Abstract

One key signature of an instability is wave-like structures in the gas, which have hitherto not been seen. Here we report the presence of 'waves' at the surface of the Orion cloud near where massive stars are forming. The waves appear to be a Kelvin-Helmholtz instability, arising during the expansion of the nebula, as gas heated and ionised by massive stars is blown over pre-existing molecular gas. 1 Because it is the closest massive star forming region (d~414 pc 8 ), the Orion nebula provides a unique opportunity to study at small spatial scales, combining different wavelengths, the feedback action of the HII region and plasma on the cloud 9,10 . Figure 1 presents the carbon monoxide (CO) millimetre map of the cloud, that has a spatial coverage and angular resolution enabling morphological comparison with high angular resolution mid-infrared and X-Ray images.In the southern part of the nebula, the growing bubble of gas ionised by massive stars (HII region) has pushed the surrounding molecular gas towards the far and near side of the nebula (respectively red and blue in Fig. 1.b.). The X ray plasma bubble 12 , fills the southern cavity of molecular gas ( Fig. 1.b.). In the infrared, the cloud surfaces exhibit «ripples » in several regions. This is particularly obvious in the Southwest region, (Fig. 1.c.) where a series of 5 surprisingly regular wavelets (the Ripples hereafter) enshrouding an elongated molecular cloud, subject to a strong velocity gradient (7-9 km.s -1 .pc -1 , Fig. 1.c and 2.a), are seen.A possibility would be that the CO structure results from the outflow of a young protostar, and the IR emission from the associated Herbig-Haro objects 13,14,15 . However, we do not find any evidence of an embedded protostar able to drive an outflow in this region, neither in the Spitzer IRAC/MIPS images nor in the SIMBAD database. Nor do we find evidence of wings in the line profiles of 12 CO emission (Fig. 2a), which are characteristic of outflows 16 . Examining the Spitzer infrared spectrograph (IRS) archival spectrum of the second Ripple (Fig. 2c), we find no trace of highly ionised species ([NeIII], [FeII]), that would attest to the presence of a high velocity Herbig-Haro shock 13,14,15 (the [SIII] emission detected here is spatially associated to the HII region).The comparison between the spatial distribution of Polycyclic Aromatic Hydrocarbons (PAHs), H2 0-0 S(1) pure rotational line intensity, and 12 CO (2-1) emissions in an East to West cut across the Ripples (marked 2 in Fig. 2b) shows a stratification with PAHs 2 more to the East, followed by H2 and then CO emission (Fig. 2d). This stratification is expected in UV driven Photo-Dissociation Regions (PDR) which lie at the boundary between HII regions formed by massive stars and molecular gas 17,18 , but not in shocks.Furthermore, the H2 S(1) intensity (~7x10 -8 W.m -2 .sr -1 ) is compatible with the predictions of PDR models 19 reproducing the conditions of the observed region: dense gas (above 10 4 cm -3 ) highly irradiated (10 3 times the interstellar...

Herschelfar-infrared observations of the Carina Nebula complex

“…One possible explanation for this pattern is that we see waves at the surface of the molecular cloud, similar to the pattern discovered in the Orion Nebula by Berné et al (2010).…”

“…According to the models of Berné et al (2010), this longer wavelength suggests that a plasma of density ∼1 cm −3 with a flow velocity of ∼100 km s −1 could be causing the instability. Such values appear quite possible in the case of the Carina Nebula superbubble.…”

The enigmatic nature of the circumstellar envelope and bow shock surrounding Betelgeuse as revealed byHerschel

“…In the case of the CO waves (or ripples) detected on the surface of the Orion molecular cloud, Berné et al (2010) argues that ultraviolet radiation has created a (small) insulating photo-ablative layer, allowing the development of a KH instability with a wavelength at least one order of magnitude longer than the insulating layer. A speculative idea is that the multiple arcs found around Betelgeuse are undulations created by photochemical effects stimulating the growth of some instability with wavelength around the observed width of the arcs, which are then traced by the smaller dust grains that tend to follow the gas instabilities.…”

Blast-wave driven Kelvin-Helmholtz shear layers in a laser driven high-energy-density plasma

“…A recent astronomical observation of KelvinHelmholtz instability occurring during the expansion of hot, ionized nebular gas sweeping over denser gas in the Orion molecular cloud is a striking example of the role played by KH in astrophysics Berne et al (2010).…”