Hubble Space Telescope WFPC2 Imaging of M16: Photoevaporation and Emerging Young Stellar Objects

Abstract

We present Hubble Space Telescope WFPC2 images of elephant trunks in the H II region M16. There are three principle results of this study. First, the morphology and stratified ionization structure of the interface between the dense molecular material and the interior of the H II region is well understood in terms of photoionization of a photoevaporative flow. Photoionization models of an empirical density profile capture the essential features of the observations, including the extremely localized region of [S II] emission at the interface and the observed offset between emission peaks in lower and higher ionization lines. The details of this structure are found to be a sensitive function both of the density profile of the interface and of the shape of the ionizing continuum. Interpretation of the interaction of the photoevaporative flow with gas in the interior of the nebula supports the view that much of the emission from H II regions may arise in such flows. Photoionization of photoevaporative flows may provide a useful paradigm for interpreting a wide range of observations of H II regions. Second, we report the discovery of a population of small cometary globules that are being uncovered as the main bodies of the elephant trunks are dispersed. Several lines of evidence connect these globules to ongoing star formation, including the association of a number of globules with stellar objects seen in IR images of M16 or in the continuum HST images themselves. We refer to these structures as evaporating gaseous globules, or "EGGs." These appear to be the same type of object as the nebular condensations seen previously in M42. The primary difference between the two cases is that in M16 we are seeing the objects from the side, while in M42 the objects are seen more nearly face-on against the backdrop of the ionized face of the molecular cloud. We find that the "evaporating globule" interpretation naturally accounts for the properties of objects in both nebulae, while avoiding serious difficulties with the competing "evaporating disk" model previously applied to the objects in M42. More generally, we find that disk-like structures are relatively rare in either nebula. Third, the data indicate that photoevaporation may have uncovered many EGGs while the stellar objects in them were still accreting mass, thereby freezing the mass distribution of the protostars at an early stage in their evolution. We conclude that the masses of stars in the cluster environment in M16 are generally determined not by the onset of stellar winds, as in more isolated regions of star formation, but rather by disruption of the star forming environment by the nearby 0 stars.

Tracing the energetics and evolution of dust withSpitzer: a chapter in the history of the Eagle Nebula

“…The gas pressure inferred from Hubble observations of the optical line emission from the faint end of the photo-evaporation flows arising from pillar I is p/k ∼ 10 7 K cm −3 (see Fig. 7b, abscissa 0 in Hester et al 1996). This value sets an upper limit on the ambient pressure around the flows, which is lower than the pressure required for the collisional heating solution.…”

“…The Eagle Nebula (M 16) is a nearby (d = 2.0 ± 0.1 kpc, Hillenbrand et al 1993) massive-star forming region that has become an icon since the publication of spectacular Hubble Space Telescope (HST) images of its ionized gas emission (Hester et al 1996). As one of the nearest star-forming regions and one of the most well-observed across the electromagnetic spectrum, the Eagle Nebula is a reference source.…”

Using binary statistics in Taurus-Auriga to distinguish between brown dwarf formation processes

“…Dynamical interactions among fragments or protostars in a massive core could lead to the ejection of some of them, which prematurely halts their accretion (Reipurth & Clarke 2001;Boss 2001;Bate et al 2002Bate et al , 2003Goodwin et al 2004;Umbreit et al 2005;Bate 2011). Photoionizing radiation from massive OB stars might contribute by removing much of the envelope and disk of a low-mass protostar (Hester et al 1996;Kroupa & Bouvier 2003a;Whitworth & Zinnecker 2004). Alternatively, turbulent compression and fragmentation of gas in a molecular cloud produces collapsing cores over a wide range of masses (Padoan & Nordlund 2002, 2004Hennebelle & Chabrier 2008;Elmegreen 2011).…”

A search for pre- and proto-brown dwarfs in the dark cloud Barnard 30 with ALMA

“…In low-mass star-forming regions, which typically form objects in groups or loose associations, the main brown dwarf formation mechanisms are turbulent fragmentation (Padoan & Nordlund 2004;Hennebelle & Chabrier 2008) and ejection from multiple protostellar systems and/or fragmented disks (Reipurth & Clarke 2001;Bate et al 2002;Matzner & Levin 2005;Whitworth & Stamatellos 2006). However, in the surroundings of high-mass stars, where objects typically form in a more tightly packed manner, there are additional mechanisms, namely photo-evaporation of cores near massive stars (e.g., Hester et al 1996;Whitworth & Zinnecker 2004) that have already been observed in young clusters (see e.g., Bouy et al 2009;Hodapp et al 2009), and gravitational fragmentation of dense filaments formed in a nascent cluster (e.g., Bonnell et al 2008;Bate 2012).…”