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 ≳300 K. To address whether such hot conditions are common among low-mass protostars, we observe CH3CN transitions at 1, 2, and 3 mm with the NOrthern Extended Millimeter Array toward seven low-mass and one intermediate-mass protostar (L bol ∼ 2–300L ⊙), as CH3CN 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 CH3OH obtained with the same observations is always lower (∼135–250 K), suggesting that CH3CN and CH3OH have a different spatial distribution. A comparison of the column densities at 1 and 3 mm shows a stronger increase at 3 mm for CH3CN than for CH3OH. Since the dust opacity is lower at longer wavelengths, this indicates that CH3CN is enhanced in the hot gas compared to CH3OH. If this CH3CN enhancement is the result of carbon-grain sublimation, these results suggest that Earth’s initial formation conditions may not be rare.