Confronting the Superbubble Model with X‐Ray Observations of 30 Doradus C

Abstract

We present an analysis of XMM-Newton observations of the superbubble 30 Dor C and compare the results with the predictions from the standard wind-blown bubble model. We find that the observed X-ray spectra cannot be fitted satisfactorily with the model alone and that there is evidence for nonthermal X-ray emission, which is particularly important at k4 keV. The combination of the bubble model and a power law gives a reasonable fit to the observed spectra. The thermal pressure and central temperature of the bubble are 3:3 ; 10 À11 dynes cm À2 and 7:4 ; 10 6 K, respectively, and we infer that for a bubble age of t $ 4 ; 10 6 yr the ambient density is n 0 ' 38 cm À3 , the mechanical luminosity is L mech $ 10 37 ergs s À1 , and the coefficient of thermal conductivity is $0.05 of the Spitzer value. The total unabsorbed 0.1-10 keV luminosities of the eastern and western parts of the bubble are '3 ; 10 36 and '5 ; 10 36 ergs s À1 , respectively. The unabsorbed 0.1-10 keV luminosity of the bubble model is $4 ; 10 36 ergs s À1 , and so the power-law component contributes between 1 3 and 1 2 of the total unabsorbed luminosity in this energy band. The nature of the hard nonthermal emission is not clear, although recent supernovae in the bubble may be responsible. We expect that about one or two core-collapse supernovae could have occurred and are required to explain the enrichment of the hot gas, as evidenced by the overabundance of -elements by a factor of k3, compared to the mean value of $0.5 solar for the interstellar medium in the Large Magellanic Cloud. As in previous studies of various superbubbles, the amount of energy currently present in 30 Dor C is significantly less than the expected energy input from the enclosed massive stars over their lifetime. We speculate that a substantial fraction of the input energy may be radiated in the far-infrared by dust grains, which are mixed with the hot gas because of the thermal conduction and/or dynamic mixing.

Outflows in Infrared‐Luminous Starbursts at z < 0.5. II. Analysis and Discussion

“…Simple formulas for thermal evaporation in the absence of magnetic fields (Cowie et al 1981) and cloud destruction by hydrodynamic ablation (Klein et al 1994;Poludnenko et al 2002) show that small clouds of size 1 pc and density 100 cm À2 can survive for a time of order 10 6 yr in a wind fluid of density 10 À3 cm À2 and temperature 3 ; 10 7 K. This is not much smaller than the wind lifetime (5 ; 10 6 5 ; 10 7 yr) of a 5 kpc shell with a constant velocity of 100-1000 km s À1 . Various effects can increase cloud lifetimes, including (1) the presence of azimuthal magnetic fields, which make cloud evaporation less likely, as they decrease conduction relative to the classical Spitzer value (as observed in 30 Dor; Smith & Wang 2004); (2) conversely, increased heat conduction, which may inhibit hydrodynamic instabilities ( Marcolini et al 2005); (3) clouds stop evaporating when they are crushed to high optical depth (Ferrara & Shchekinov 1993); and (4) Kelvin-Helmholtz instabilities are suppressed by ablation of small bumps in the cloud (Schiano et al 1995). Cloud lifetimes can be decreased, however, by interactions among nearby clouds (Poludnenko et al 2004).…”

Supernova remnants, planetary nebulae and superbubbles: Prospects for new XMM‐Newton observations

“…The hot gas is enriched in α-elements (O, Ne, Mg, Si, S, Ar, Ca) by a factor of about 3 while the group of Fe-like elements is consistent with the LMC abundances. The additional ≃ 5 M ⊙ of oxygen in the hot gas require 2-3 high mass (≥ 20 M ⊙ ) recent core-collapse supernovae (Smith & Wang 2004). Nonthermal emission has been detected as well in this superbubble (Bamba et al 2004).…”

“…Why the amount of energy currently present in superbubbles is significantly less than the energy input expected from the enclosed massive stars over their lifetime (Cooper et al 2004)? What is the origin of the nonthermal emission observed in some superbubbles (Smith & Wang 2004)? Are superbubbles a relevant source for the acceleration of galactic cosmic rays (Parizot et al 2004)?…”

“…Nonthermal emission has been detected as well in this superbubble (Bamba et al 2004). However, the origin of this emission is still an open question: synchrotron from TeV accelerated electrons, Inverse Compton or nonthermal bremsstrahlung (Smith & Wang 2004)?…”

The violent interstellar medium of the Magellanic Cloud System

“…A remarkable feature of the X-ray emission from N 51D is that its X-ray spectrum requires a nonthermal component to explain the emission in 1−3 keV. Nonthermal X-ray emission has also been detected in the LMC superbubbles 30 Dor C (Bamba et al 2004;Smith & Wang 2004) and N 11 (Maddox et al 2008). N 51D does not have detectable molecular clouds around its superbubble shell rim; however, there is still on-going star formation.…”