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 ($L{\rm{bol}} \sim2-300 L_\odot$), 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.