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 CH _3 CN transitions at 1, 2, and 3 mm with the NOrthern Extended Millimeter Array toward seven low-mass and one intermediate-mass protostar ( L _bol ∼ 2–300 L _⊙ ), 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 (∼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 suggest that Earth’s initial formation conditions may not be rare.

Keywords

• Astrochemistry
• Protostars