Identification of hot gas around low-mass protostars

Abstract

The low carbon content of Earth and primitive meteorites compared to the Sun and interstellar grains suggests that carbon-rich grains were destroyed in the inner few astronomical units of the young solar system. A promising mechanism to selectively destroy carbonaceous grains is thermal sublimation within the soot line at $\gtrsim$ 300 K. To address whether such hot conditions are common amongst low-mass protostars, we observe CH${}_3$CN transitions at 1, 2 and 3 mm with the NOrthern Extended Millimeter Array (NOEMA) toward seven low-mass and one intermediate-mass protostar ($L_{\mathrm{bol}}\sim 2-300L_{\odot }$), as CH${}_3$CN is an excellent temperature tracer. We find $>$ 300 K gas toward all sources, indicating that hot gas may be prevalent. Moreover, the excitation temperature for CH${}_3$OH obtained with the same observations is always lower ($\sim$135-250 K), suggesting that CH${}_3$CN and CH${}_3$OH have a different spatial distribution. A comparison of the column densities at 1 and 3 mm shows a stronger increase at 3 mm for CH${}_3$CN than for CH${}_3$OH. Since the dust opacity is lower at longer wavelengths, this indicates that CH${}_3$CN is enhanced in the hot gas compared to CH${}_3$OH. If this CH${}_3$CN enhancement is the result of carbon-grain sublimation, these results suggests that Earth's initial formation conditions may not be rare.

19 pages, 9 figures, 4 tables (plus 13 pages appendix with 10 figures, 4 tables). Accepted for publication in ApJ

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