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Stars come in a range of sizes, ranging from less than 1/10 up to more than 50 times the mass of our Sun. The more massive stars are very hot and bright, emitting copious quantities of ultraviolet (UV) radiation, which will heat and ionize the surrounding gas. Most stars are born in large groups, known as stellar clusters, which typically contain a handful of high-mass stars together with hundreds to thousands of lower-mass stars. The young, low-mass stars, each still surrounded by the dusty accretion disk from which it formed, are exposed to the ultraviolet radiation of the nearby massive stars. This is what gives rise to the proplyd phenomenon.
The cartoon shows the structure of a typical proplyd. The star is surrounded by a dense disk of dust and molecular hydrogen. Non-ionizing UV radiation penetrates to the surface of the disk, heating and dissociating the hydrogen molecules, which flow away from the disk as a dense, warm (~1000 K), slow (~3 km/s), neutral wind.
The column density of the neutral flow is too large for the ionizing UV radiation to penetrate down to its base. Instead, an ionization front (IF) forms at a radius of a few times the disk radius. On passing through the IF, the gas is heated to ~10,000 K and accelerates up to ~10 km/s. It leaves the IF as a transonic ionized photoevaporation flow, which accelerates up to about 40 km/s and it becomes more ionized and less dense at ever increasing radii. The back pressure of the ionized gas drives a shock into the neutral flow from the disk, forming a dense neutral shell just inside the ionization front.
The above description applies to the side of the proplyd that faces the dominant ionizing star. However, there is also a diffuse component to the UV radiation field due to other stars, dust scattering, and recombination of gas in the nebula. This diffuse field is relatively more important for the non-ionizing UV radiation, which, combined with the relative insensitivity of the disk evaporation to the strength of the UV field, leads to the neutral flow being roughly isotropic (the same in all directions). The ionizing UV field, on the other hand, is totally dominated by the emission from one high-mass star, known as θ1 Ori C. A roughly hemispherical ionization front forms on the side of the proplyd that faces this star and this effectively shields a cylindrical volume of the neutral wind, hence forming the proplyd tail, which points directly away from θ1 Ori C.
The weak diffuse ionizing field does manage to ionize part of the shadow region, causing the tail to taper toward its tip. The proplyds closest to θ1 Ori C have the longest, slenderest tails, whereas for farther-out proplyds, where the diffuse field is relatively stronger, the tails are short and stubby.
Although proplyds have now been found in many star-forming regions in our own and other galaxies, by far the richest source is still the Orion Nebula, where they were first discovered. The objects we now know as proplyds were first identified from spectroscopic observations in the late 1970's and studied in depth in the 1980's using radio-wavelength interferometers. However, it was not until they were observed with the Hubble Space Telescope (HST) in the early 1990's that their full beauty and phenomenological complexity was truly appreciated.
The above color images of typical Orion proplyds are constructed from narrow-band emission line imaging obtained using the WFPC2 instrument on the HST. Nearly all the emission seen in the images comes from the proplyd's ionized photoevaporation flow. Bluer emission indicates more highly ionized gas, which is generally seen at larger radii.
The photoevaporation flow from the proplyd's ionization front does not expand into a vacuum but, instead, will eventually interact with its environment. Just what this environment consists of will depend on the position of the proplyd within the nebula (see diagram at left).
As mentioned above, the excitation and ionization of the Orion nebula is dominated by one high-mass star. This star also has a powerful stellar wind. Those proplyds closest to θ1 Ori C lie in the supersonic part of the stellar wind, while those a little farther out may lie in a hot, shocked, subsonic wind bubble. Beyond that, some proplyds my lie outside the domain of influence of the stellar wind entirely, where, instead, they will interact with the champagne flow from the nebula's principal ionization front.
Most of the evidence for environmental interaction comes from the closest-in proplyds that interact with the supersonic wind from θ1 Ori C. Faint blue arcs of emission are seen between the proplyds and the ionizing star. These are interpreted as bowshocks, which are formed where the slow dense photoevaporation flow from the proplyd collides with the fast tenuous wind from a massive star. There are even indications that the global properties of the stellar wind at large radii are modified by the "mass-loading" due to its interaction with the proplyd photoevaporation flows.
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Last modified: Tue Feb 18 09:26:54 CST 2003