The Green Bank Ammonia Survey: Observations of Hierarchical Dense Gas Structures in Cepheus-L1251

Jared Keown ,Yancy L Shirley ,F O Alves ,A Chacón-Tanarro ,Hope How-Huan Chen ,Alyssa A Goodman ,Ayushi Singh ,Helen Kirk ,Adam Ginsburg ,Rachel Friesen ,Michael Chun-Yuan Chen ,Stella S R Offner ,James Di Francesco ,E Redaelli ,Christopher D Matzner ,Héctor G Arce ,Jaime E Pineda ,A Punanova ,Erik Rosolowsky ,Young Min Seo ,P G Martín ,P Caselli ,Philip C Myers

doi:10.3847/1538-4357/aa93ec

Abstract

Abstract We use Green Bank Ammonia Survey observations of NH3 (1, 1) and (2, 2) emission with 32″ FWHM resolution from a ∼10 pc2 portion of the Cepheus-L1251 molecular cloud to identify hierarchical dense gas structures. Our dendrogram analysis of the NH3 data results in 22 top-level structures, which reside within 13 lower-level parent structures. The structures are compact and are spatially correlated with the highest H2 column density portions of the cloud. We also compare the ammonia data to a catalog of dense cores identified by higher-resolution (18.″2 FWHM) Herschel Space Observatory observations of dust continuum emission from Cepheus-L1251. Maps of kinetic gas temperature, velocity dispersion, and NH3 column density, derived from detailed modeling of the NH3 data, are used to investigate the stability and chemistry of the ammonia-identified and Herschel-identified structures. We show that the dust and dense gas in the structures have similar temperatures, with median T dust and T K measurements of 11.7 ± 1.1 K and 10.3 ± 2.0 K, respectively. Based on a virial analysis, we find that the ammonia-identified structures are gravitationally dominated, yet may be in or near a state of virial equilibrium. Meanwhile, the majority of the Herschel-identified dense cores appear to be not bound by their own gravity and instead confined by external pressure. CCS (20 − 10) and HC5N emission from the region reveal broader line widths and centroid velocity offsets when compared to the NH3 (1, 1) emission in some cases, likely due to these carbon-based molecules tracing the turbulent outer layers of the dense cores.

Highlights

Recent large-scale surveys of dust continuum emission from nearby star-forming regions have provided unprecedented insights into the structure of molecular clouds
We modeled our virial analysis on the methodology adopted by Friesen et al (2016), who analyzed dendrogram-identified NH3 structures in the Serpens South region
Despite the lack of large-scale magnetic field measurements in Cepheus, our virial analysis of L1251, which showed that most the ammonia-identified structures in the region are gravitationally bound when magnetic pressure is neglected and a power-law density profile is assumed for the structures, matches the results of Friesen et al (2016)

Summary

Introduction

Recent large-scale surveys of dust continuum emission from nearby star-forming regions have provided unprecedented insights into the structure of molecular clouds. The Gould Belt Legacy surveys on the Herschel Space Observatory (HGBS; Andre et al 2010) and James Clerk Maxwell Telescope (JGBS; Ward-Thompson et al 2007) have fueled these advancements, providing photometric censuses of nearby (< 500 pc) Galactic molecular clouds. They have revealed that filaments pervade both active (Andre et al 2010; Men’shchikov et al 2010) and quiescent (Miville-Deschenes et al 2010; Ward-Thompson et al 2010) molecular clouds. Without adequate spectral line measurements for the prestellar cores, it is difficult to determine whether these structures are truly gravitationally bound objects or rather they are pressure-confined by the molecular cloud within which they reside

Results

Discussion

Conclusion