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.

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

References in corpus (18)

• Cold Dark Clouds: The Initial Conditions for Star Formation
• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• Observing the gas temperature drop in the high-density nucleus of L 1544
• Formation of Complex Molecules in Prestellar Cores: a Multilayer Approach
• 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
• Isotopic fractionation of carbon, deuterium and nitrogen : a full chemical study
• The Central 1000 au of a Pre-stellar Core Revealed with ALMA. II. Almost Complete Freeze-out
• Why does ammonia not freeze out in the center of pre-stellar cores?
• CN in prestellar cores
• SOC program for dust continuum radiative transfer
• LOC program for line radiative transfer
• 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
• VLA and NOEMA view of the Bok Globule CB 17: the starless nature of a proposed FHSC candidate

Cited by in corpus (20)

• The ALMA Survey of 70 $\mu \mathrm{m}$ Dark High-mass Clumps in Early Stages (ASHES). VII: Chemistry of Embedded Dense Cores
• Nitrogen fractionation in ammonia and its insights on nitrogen chemistry
• First detection of CHD${}_2$OH towards pre-stellar cores
• Tracking the ice mantle history in the Solar-type Protostars of NGC 1333 IRAS 4
• Broadband spectroscopy of astrophysical ice analogues: II. Optical constants of CO and CO${}_2$ ices in the terahertz and infrared ranges
• Hunting pre-stellar cores with APEX: IRAS16293E (Oph464)
• 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