Catching the Birth of a Dark Molecular Cloud for the First Time

Abstract

The majority of hydrogen in the interstellar medium (ISM) is in atomic form. The transition from atoms to molecules and, in particular, the formation of the H${}_2$ molecule, is a key step in cosmic structure formation en route to stars. Quantifying H${}_2$ formation in space is difficult, due to the confusion in the emission of atomic hydrogen (HI) and the lack of a H${}_2$ signal from the cold ISM. Here we present the discovery of a rare, isolated dark cloud currently undergoing H${}_2$ formation, as evidenced by a prominent "ring" of HI self-absorption. Through a combined analysis of HI narrow self-absorption, CO emission, dust emission, and extinction, we directly measured, for the first time, the [HI]/[H${}_2$] abundance varying from 2% to 0.2%, within one region. These measured HI abundances are orders of magnitude higher than usually assumed initial conditions for protoplanetary disk models. None of the fast cloud formation model could produce such low atomic hydrogen abundance. We derived a cloud formation timescale of 6$\times$10${}^6$ years, consistent with the global Galactic star formation rate, and favoring the classical star formation picture over fast star formation models. Our measurements also help constrain the H${}_2$ formation rate, under various ISM conditions.

arXiv comment7 pages, 2 figures, accepted by ApJ

References in corpus (27)

• The Relation Between Optical Extinction and Hydrogen Column Density in the Galaxy
• Dense Cores in Dark Clouds. XIV. N2H+(1-0) maps of dense cloud cores
• A KInetic Database for Astrochemistry (KIDA)
• The Millennium Arecibo 21-CM Absorption Line Survey. II. Properties of the Warm and Cold Neutral Media
• Star Formation in a Crossing Time
• Simulating the formation of molecular clouds. II. Rapid formation from turbulent initial conditions
• Interstellar Carbon in Translucent Sightlines
• Environmental Dependence of the Kennicutt-Schmidt Relation in Galaxies
• Chemistry in Disks. IV. Benchmarking gas-grain chemical models with surface reactions
• HI-to-H2 Transitions and H I Column Densities in Galaxy Star-Forming Regions
• The GALFA-HI Survey: Data Release 1
• Molecular Cloud Formation Behind Shock Waves
• The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime
• Abundances and Behavior of 12CO, 13CO, and C2 in Translucent Sight Lines
• HI Narrow Line Absorption in Dark Clouds
• Oxygen depletion in dense molecular clouds: a clue to a low O2 abundance?
• HI Narrow Self-Absorption in Dark Clouds: Correlations with Molecular Gas and Implications for Cloud Evolution and Star Formation
• FUSE Observations of Atomic Abundances and Molecular Hydrogen in the Leading Arm of the Magellanic Stream
• Cold and Warm Atomic Gas around the Perseus Molecular Cloud. II. The Impact of High Optical Depth on the HI Column Density Distribution and Its Implication for the HI-to-H2 Transition
• The impact of baryon physics on the structure of high-redshift galaxies
• The Local Leo Cold Cloud and New Limits on a Local Hot Bubble
• HI, CO, and Planck/IRAS dust properties in the high-latitude-cloud complex, MBM 53, 54, 55 and HLCG 92-35; Possible evidence for an optically thick HI envelope around the CO clouds
• Atomic and Molecular Carbon as a Tracer of Translucent Clouds
• Survival of interstellar molecules to prestellar dense core collapse and early phases of disk formation
• HI-to-H2 Transition Layers in the Star-Forming Region W43
• Molecular hydrogen formation on grain surfaces
• Formation of molecular hydrogen on analogues of interstellar dust grains: experiments and modelling

Cited by in corpus (12)

• The HI/OH/Recombination line survey of the inner Milky Way (THOR): data release 2 and HI overview
• Cloud formation in the atomic and molecular phase: HI self absorption (HISA) towards a Giant Molecular Filament
• Sheets, filaments and clumps - high resolution simulations of how the thermal instability can form molecular clouds
• Pilot HI Survey of Planck Galactic Cold Clumps with FAST
• The CO-dark molecular gas in the cold HI arc
• A FAST Survey of HINSA in PGCCs Guided by HC3N
• HI Self-absorption toward the Cygnus X North: From Atomic Filament to Molecular Filament
• Chemical variations across the TMC-1 boundary: molecular tracers from translucent phase to dense phase
• Rotation of Two Micron All Sky Survey Clumps in Molecular Clouds
• Tracing the Formation of Molecular Clouds in a Low-Metallicity Galaxy: A HI Narrow Self-Absorption Survey of the Large Magellanic Cloud
• Effect of surface H${}_2$ on molecular hydrogen formation on interstellar grains
• HI Narrow-Line Self-Absorptions Toward the High-Mass Star-Forming Region G176.51+00.20