We present an overview of the first data release (DR1) and first-look science from the Green Bank Ammonia Survey (GAS). GAS is a Large Program at the Green Bank Telescope to map all Gould Belt star-forming regions with A V 7 mag visible from the northern hemisphere in emission from NH 3 and other key molecular tracers. This first release includes the data for four regions in Gould Belt clouds: B18 in Taurus, NGC 1333 in Perseus, L1688 in Ophiuchus, and Orion A North in Orion. We compare the NH 3 emission to dust continuum emission from Herschel, and find that the two tracers correspond closely. NH 3 is present in over 60 % of lines-of-sight with A V 7 mag in three of the four DR1 regions, in agreement with expectations from previous observations. The sole exception is B18, where NH 3 is detected toward ∼ 40 % of lines-of-sight with A V 7 mag. Moreover, we find that the NH 3 emission is generally extended beyond the typical 0.1 pc length scales of dense cores. We produce maps of the gas kinematics, temperature, and NH 3 column densities through forward modeling of the hyperfine structure of the NH 3 (1,1) and (2,2) lines. We show that the NH 3 velocity dispersion, σ v , and gas kinetic temperature, T K , vary systematically between the regions included in this release, with an increase in both the mean value and spread of σ v and T K with increasing star formation activity. The data presented in this paper are publicly available.

We use gas temperature and velocity dispersion data from the Green Bank Ammonia Survey and core masses and sizes from the James Clerk Maxwell Telescope Gould Belt Survey to estimate the virial states of dense cores within the Orion A molecular cloud. Surprisingly, we find that almost none of the dense cores are sufficiently massive to be bound when considering only the balance between selfgravity and the thermal and non-thermal motions present in the dense gas. Including the additional pressure binding imposed by the weight of the ambient molecular cloud material and additional smaller pressure terms, however, suggests that most of the dense cores are pressure confined.

We present deep NH 3 observations of the L1495-B218 filaments in the Taurus molecular cloud covering over a 3 degree angular range using the K-band focal plane array on the 100m Green Bank Telescope. The L1495-B218 filaments form an interconnected, nearby, large complex extending over 8 pc. We observed NH 3 (1,1) and (2,2) with a spectral resolution of 0.038 km/s and a spatial resolution of 31 . Most of the ammonia peaks coincide with intensity peaks in dust continuum maps at 350 µm and 500 µm. We deduced physical properties by fitting a model to the observed spectra. We find gas kinetic temperatures of 8 − 15 K, velocity dispersions of 0.05 − 0.25 km/s, and NH 3 column densities of 5×10 12 − 1×10 14 cm −2 . The CSAR algorithm, which is a hybrid of seeded-watershed and binary dendrogram algorithms, identifies a total of 55 NH 3 structures including 39 leaves and 16 branches. The masses of the NH 3 sources range from 0.05 M to 9.5 M . The masses of NH 3 leaves are mostly smaller than their corresponding virial mass estimated from their internal and gravitational energies, which suggests these leaves are gravitationally unbound structures. 9 out of 39 NH 3 leaves are gravitationally bound and 7 out of 9 gravitationally bound NH 3 leaves are associated with star formation. We also found that 12 out of 30 gravitationally unbound leaves are pressure-confined. Our data suggest that a dense core may form as a pressure-confined structure, evolve to a gravitationally bound core, and undergo collapse to form a protostar.

We use Green Bank Ammonia Survey observations of NH 3 (1,1) and (2,2) emission with 32 FWHM resolution from a ∼ 10 pc 2 portion of the Cepheus-L1251 molecular cloud to identify hierarchical dense gas structures. Our dendrogram analysis of the NH 3 data results in 22 top-level structures, which reside within 13 lower-level, parent structures. The structures are compact (0.01 pc R ef f 0.1 pc) and are spatially correlated with the highest H 2 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 NH 3 column density, -2derived from detailed modeling of the NH 3 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 (2 0 − 1 0 ) and HC 5 N (9 − 8) emission from the region reveal broader line widths and centroid velocity offsets when compared to the NH 3 (1,1) emission in some cases, likely due to these carbon-based molecules tracing the turbulent outer layers of the dense cores.

We perform a virial analysis of starless dense cores in three nearby star-forming regions : L1688 in Ophiuchus, NGC 1333 in Perseus, and B18 in Taurus. Our analysis takes advantage of comprehensive kinematic information for the dense gas in all of these regions made publicly available through the Green Bank Ammonia Survey Data Release 1, which used to estimate internal support against collapse. We combine this information with ancillary data used to estimate other important properties of the cores, including continuum data from the James Clerk Maxwell Telescope Gould Belt Survey for core identification, core masses, and core sizes. Additionally, we used Planck and Herschel -based column density maps for external cloud weight pressure, and Five College Radio Astronomy Observatory 13 CO observations for external turbulent pressure. Our self-consistent analysis suggests that many dense cores in all three star-forming regions are not bound by gravity alone, but rather require additional pressure confinement to remain bound. Unlike a recent, similar study in Orion A, we find that turbulent pressure represents a significant portion of the external pressure budget. Our broad conclusion emphasizing the importance of pressure confinement in dense core evolution, however, agrees with earlier work.