Identification of hot gas around low-mass protostars

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 ≳ 300 K. To address whether such hot conditions are common amongst low-mass protostars, we observe CH_3CN transitions at 1, 2 and 3 mm with the NOrthern Extended Millimeter Array (NOEMA) toward seven low-mass and one intermediate-mass protostar (L_bol∼2-300 L_⊙), as CH_3CN 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_3OH obtained with the same observations is always lower (∼135-250 K), suggesting that CH_3CN and CH_3OH 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_3CN than for CH_3OH. Since the dust opacity is lower at longer wavelengths, this indicates that CH_3CN is enhanced in the hot gas compared to CH_3OH. If this CH_3CN enhancement is the result of carbon-grain sublimation, these results suggests that Earth's initial formation conditions may not be rare.