An Interferometric View of H-MM1. I. Direct Observation of NH3 Depletion

Abstract

Spectral lines of ammonia, NH${}_3$, are useful probes of the physical conditions in dense molecular cloud cores. In addition to advantages in spectroscopy, ammonia has also been suggested to be resistant to freezing onto grain surfaces, which should make it a superior tool for studying the interior parts of cold, dense cores. Here we present high-resolution NH${}_3$ observations with the Very Large Array (VLA) and Green Bank Telescope (GBT) towards a prestellar core. These observations show an outer region with a fractional NH${}_3$ abundance of X(NH${}_3$) = (1.975$\pm$0.005)$\times 10^{-8}$ ($\pm 10\%$ systematic), but it also reveals that after all, the X(NH${}_3$) starts to decrease above a H${}_2$ column density of $\approx 2.6\times 10^{22}$ cm${}^{-2}$. We derive a density model for the core and find that the break-point in the fractional abundance occurs at the density n(H${}_2$) $\sim 2\times 10^5$ cm${}^{-3}$, and beyond this point the fractional abundance decreases with increasing density, following the power law $n^{-1.1}$. This power-law behavior is well reproduced by chemical models where adsorption onto grains dominates the removal of ammonia and related species from the gas at high densities. We suggest that the break-point density changes from core to core depending on the temperature and the grain properties, but that the depletion power law is anyway likely to be close to $n^{-1}$ owing to the dominance of accretion in the central parts of starless cores.

arXiv commentAccepted for publication in AJ. 18 Pages, 16 Figures, 3 Tables

References in corpus (45)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Astropy: A Community Python Package for Astronomy
• emcee: The MCMC Hammer
• Binary companions of evolved stars in APOGEE DR14: Search method and catalog of ~5,000 companions
• Dust Grain Size Distributions and Extinction in the Milky Way, LMC, and SMC
• Cold Dark Clouds: The Initial Conditions for Star Formation
• Characterizing interstellar filaments with Herschel in IC5146
• The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background: I. Limits and Detections
• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• The global dust SED: Tracing the nature and evolution of dust with DustEM
• Observing the gas temperature drop in the high-density nucleus of L 1544
• Complete Depletion in prestellar cores
• Formation of Complex Molecules in Prestellar Cores: a Multilayer Approach
• Direct observation of a sharp transition to coherence in Dense Cores
• An Extinction Threshold for Protostellar Cores in Ophiuchus
• A protostellar system fed by a streamer of 10,500 au length
• Depletion and low gas temperature in the L183 prestellar core : the N2H+ - N2D+ tool
• The Green Bank Ammonia Survey (GAS): First Results of NH3 mapping the Gould Belt
• The JCMT Gould Belt Survey: First results from the SCUBA-2 observations of the Ophiuchus molecular cloud and a virial analysis of its prestellar core population
• Isotopic fractionation of carbon, deuterium and nitrogen : a full chemical study
• Gaia-DR2 confirms VLBA parallaxes in Ophiuchus, Serpens and Aquila
• The structure and stability of molecular cloud cores in external radiation fields
• Nitrogen chemistry and depletion in starless cores
• The Central 1000 au of a Pre-stellar Core Revealed with ALMA. II. Almost Complete Freeze-out
• High sensitivity maps of molecular ions in L1544: I. Deuteration of N2H+ and HCO+ and first evidence of N2D+ depletion
• Droplets I: Pressure-Dominated Sub-0.1 pc Coherent Structures in L1688 and B18
• Spin-state chemistry of deuterated ammonia
• Why does ammonia not freeze out in the center of pre-stellar cores?
• Mantle formation, coagulation and the origin of cloud/core-shine: II. Comparison with observations
• Extended emission of D2H+ in a prestellar core
• Deuteration of ammonia in the starless core Ophiuchus/H-MM1
• CN in prestellar cores
• Gas and dust temperature in pre-stellar cores revisited: New limits on cosmic-ray ionization rate
• SOC program for dust continuum radiative transfer
• Modeling deuterium chemistry in starless cores: full scrambling versus proton hop
• Efficient methanol production on the dark side of a prestellar core
• LOC program for line radiative transfer
• On the importance of scattering at 8 microns: Brighter than you think
• Neutral vs Ion Linewidths in Barnard 5: Evidence for Penetration by MHD Waves
• Transition from Coherent Cores to Surrounding Cloud in L1688
• Depletion and fractionation of nitrogen in collapsing cores
• Ubiquitous $\mathrm{N}{\mathrm{H}}_3$ supersonic component in L1688 coherent cores
• Magnetic Field Structure of Dense Cores using Spectroscopic Methods
• Observations of 6 - 200 μm emission of the Ophiuchus cloud LDN 1688
• VLA and NOEMA view of the Bok Globule CB 17: the starless nature of a proposed FHSC candidate

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• A Survey of Deuterated Ammonia in the Cepheus Star-Forming Region L1251
• Star formation timescale in the molecular filament WB 673
• Mapping Synthetic Observations to Prestellar Core Models: An Interpretable Machine Learning Approach
• The first interferometric measurements of NH${}_2$D/NH${}_3$ ratio in hot corinos
• PRODIGE VI -- Envelope to Disk with NOEMA: VI. The Missing Sulfur Problem
• Probing the Physics of Star-Formation (ProPStar) III. No evidence for dissipation of turbulence down to 20 mpc (4 000 au) scale
• The evolution of CH in Planck Galactic Cold Clumps
• Asymmetric accretion through a streamer onto the pre-stellar core H-MM1
• Deuterium fractionation in cold dense cores in the low-mass star forming region L1688
• A low cosmic-ray ionisation rate in the prestellar core Ophiuchus/H-MM1. Mapping of the molecular ions ortho-H2D+, N2H+, and DCO+
• Hunting pre-stellar cores with APEX: overview
• Chemical constraints on the dynamical evolution of the cold core L694
• Role of NH3 Binding Energy in the Early Evolution of Protostellar Cores
• Chemistry of Dark Molecular Clouds