Blowing in the wind: The dust wave around σ Orionis AB

¹ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden The Netherlands
e-mail: ochsendorf@strw.leidenuniv.nl
² Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200D, bus 2401, 3001 Leuven, Belgium
³ Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, PO Box 94249, 1090 GE Amsterdam, The Netherlands
⁴ Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse, France

Received: 18 October 2013
Accepted: 27 January 2014

Abstract

Observations obtained with the Spitzer Space Telescope and the WISE satellite have revealed a prominent arc-like structure at 50′′ (≃0.1 pc) from the O9.5V/B0.5V system σ Ori AB. We measure a total dust mass of 2.3 ± 1.5 × 10^-5  M_⊙. The derived dust-to-gas mass ratio is ≃0.29 ± 0.20. We attribute this dust structure to the interaction of radiation pressure from the star with dust carried along by the IC 434 photo-evaporative flow of ionized gas from the dark cloud L1630. We have developed a quantitative model for the interaction of a dusty ionized flow with nearby (massive) stars where radiation pressure stalls dust, piling it up at an appreciable distance (>0.1 pc), and force it to flow around the star. The model demonstrates that for the conditions in IC 434, the gas will decouple from the dust and will keep its original flow lines. Hence, we argue that this dust structure is the first example of a dust wave created by a massive star moving through the interstellar medium. Our model shows that for higher gas densities, coupling is more efficient and a bow wave will form, containing both dust and gas. Our model describes the physics of dust waves and bow waves and quantitatively reproduces the optical depth profile at 70 μm. Dust waves (and bow waves) stratify dust grains according to their radiation pressure opacity, which reflects the size distribution and composition of the grain material. It is found that in the particular case of σ Ori AB, dust is able to survive inside the ionized region. Comparison of our model results with observations implies that dust-gas coupling through Coulomb interaction is less important than previously thought, challenging our understanding of grain dynamics in hot, ionized regions of space. We describe the difference between dust (and bow) waves and classical bow shocks created by the interaction of a stellar wind with the interstellar medium. The results show that for late O-type stars with weak stellar winds, the stand-off distance of the resulting bow shock is very close to the star, well within the location of the dust wave. In general, we conclude that dust waves and bow waves should be common around stars showing the weak-wind phenomenon, i.e., stars with log(L/L_⊙)< 5.2, and that these structures are best observed at mid-IR to FIR wavelengths, depending on the stellar spectral type. In particular, dust waves and bow waves are most efficiently formed around weak-wind stars moving through a high density medium. Moreover, they provide a unique opportunity to study the direct interaction between a (massive) star and its immediate surroundings.