Cyanopolyyne Chemistry in the L1544 Prestellar Core: New Insights from GBT Observations

Abstract

We report a comprehensive study of the cyanopolyyne chemistry in the prototypical prestellar core L1544. Using the 100 m Robert C. Byrd Green Bank Telescope, we observe three emission lines of HC _3 N, nine lines of HC _5 N, five lines of HC _7 N, and nine lines of HC _9 N. HC _9 N is detected for the first time toward the source. The high spectral resolution (∼0.05 km s ^−1 ) reveals double-peak spectral line profiles with the redshifted peak a factor 3–5 brighter. Resolved maps of the core in other molecular tracers indicate that the southern region is redshifted. Therefore, the bulk of the cyanopolyyne emission is likely associated with the southern region of the core, where free carbon atoms are available to form long chains, thanks to the more efficient illumination of the interstellar field radiation. We perform a simultaneous modeling of the HC _5 N, HC _7 N, and HC _9 N lines to investigate the origin of the emission. To enable this analysis, we performed new calculation of the collisional coefficients. The simultaneous fitting indicates a gas kinetic temperature of 5–12 K, a source size of 80″, and a gas density larger than 100 cm ^−3 . The HC _5 N:HC _7 N:HC _9 N abundance ratios measured in L1544 are about 1:6:4. We compare our observations with those toward the well-studied starless core TMC-1 and with the available measurements in different star-forming regions. The comparison suggests that a complex carbon chain chemistry is active in other sources and is related to the presence of free gaseous carbon. Finally, we discuss the possible formation and destruction routes in light of the new observations.

Keywords

• Astrochemistry
• Star formation
• Interstellar medium
• Interstellar molecules
• Chemical abundances