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

Abstract

Context. The Spitzer GLIMPSE and MIPSGAL surveys have revealed a wealth of details about the Galactic plane in the infrared (IR) with orders of magnitude higher sensitivity, higher resolution, and wider coverage than previous IR observations. The structure of the interstellar medium (ISM) is tightly connected to the countless star-forming regions. We use these surveys to study the energetics and dust properties of the Eagle Nebula (M 16), one of the best known star-forming regions. Aims. We present MIPSGAL observations of M 16 at 24 and 70 μm and combine them with previous IR data. The mid-IR image shows a shell inside the well-known molecular borders of the nebula, as in the ISO and MSX observations from 15 to 21 μm. The morphologies at 24 and 70 μm are quite different, and its color ratio is unusually warm. The far-IR image resembles the one at 8 μm that enhances the structure of the molecular cloud and the "pillars of creation". We use this set of IR data to analyze the dust energetics and properties within this template for Galactic star-forming regions. Methods. We measure IR spectral energy distributions (SEDs) across the entire nebula, both within the inner shell and the photodissociation regions (PDRs). We use the DUSTEM model to fit these SEDs and constrain the dust temperature, the dust-size distribution, and the radiation field intensity relative to that provided by the star cluster NGC 6611 (χ/χ 0 ). Results. Within the PDRs, the inferred dust temperature (∼35 K), the dust-size distribution, and the radiation field intensity (χ/χ 0 < 1) are consistent with expectations. Within the inner shell, the dust is hotter (∼70 K). Moreover, the radiation field required to fit the SED is larger than that provided by NGC 6611 (χ/χ 0 > 1). We quantify two solutions to this problem: (1) The size distribution of the dust in the shell is not that of interstellar dust. There is a significant enhancement of the carbon dust-mass in stochastically heated very small grains. (2) The dust emission arises from a hot (∼10 6 K) plasma where both UV and collisions with electrons contribute to the heating. Within this hypothesis, the shell SED may be fit for a plasma pressure p/k ∼ 5 × 10 7 K cm −3 . Conclusions. We suggest two interpretations for the M 16 inner shell: (1) The shell matter is supplied by photo-evaporative flows arising from dense gas exposed to ionized radiation. The flows renew the shell matter as it is pushed out by the pressure from stellar winds. Within this scenario, we conclude that massive-star forming regions such as M 16 have a major impact on the carbon dustsize distribution. The grinding of the carbon dust could result from shattering in grain-grain collisions within shocks driven by the dynamical interaction between the stellar winds and the shell. (2) We also consider a more speculative scenario where the shell is a supernova remnant. In this case, we would be witnessing a specific time in the evolution of the remnant where the plasma pressure and temperature would enable the remnant to coo...

Star formation history of Canis Major OB1

“…the ∼10,000 X-ray sources detected as young stars clearly belong to two distinct, equally numerous groups: (i) a clustered population, with centrally concentrated distributions of stars (including cases of "clusters of clusters"), and a fairly homogeneous distributed fainter population. This is consistent with two main modes of star formation: localized cluster formation from dense molecular cores, and more widely spread star formation from smaller condensations (e.g., "pillars" eroded by the hydrodynamical feedback from winds and/or supernova explosions from massive stars (M ⋆ > 8M ⊙ ), as in the Eagle Nebula (Guarcello et al 2010;Flagey et al 2011). The various clusters, as well as the distributed young stars in Carina nebula, also show a large spread in ages (∼1-10 Myr depending on the location within the nebula).…”

Radiation-pressure-driven dust waves inside bursting interstellar bubbles

“…This emission could either originate from small stochastically heated particles, often referred to as very small grains (VSGs), or big grains (BGs) in thermal equilibrium with the radiation field (e.g., Paladini et al 2012). Whereas several authors attribute the 24 µm emission inside ionized regions to the increase of VSGs compared with BGs (Paradis et al 2011;Flagey et al 2011), the analysis of the IR arc in IC 434 revealed that the increased heating by stellar photons from the nearby stars, σ Ori AB, can explain the MIR emission (Ochsendorf et al 2014). Here, we sidestep this problem and note that the emission from 24 µm and Hα/radio throughout the bubble interiors shown in Fig.…”

Ionization compression impact on dense gas distribution and star formation

“…Recently, Flagey et al (2011) have measured dust spectral energy distributions (SEDs) thanks to Spitzer data. They show that the SED cannot be accounted for by interstellar-dust heating by UV radiation, but an additional source of radiation is needed to match the mid-infrared flux.…”