What happens when we stop assuming what can't be assumed?
Astrophotography by GrandpaJunkrat - ----- Will a star be born?
Cosmology from my armchair..............................
I do it metric .............................................................. Location:
Sneek
bastion@protonmail.com The Netherlands
Bortle 7 - 9
Star evolution
The Central Role of H3+ in Pre‑stellar Chemistry
In the cold interiors of dense molecular cloud cores, the ion H3+ occupies a uniquely fundamental position in the chemical evolution that precedes star formation. Its importance arises from the fact that it is the first and most abundant molecular ion produced under the physical conditions typical of pre‑stellar environments.
The formation of H3+begins when cosmic-ray (CR) particles, highly energetic particles from space, most likely consisting of free protons, penetrate the cloud and ionize molecular hydrogen, initiating the sequence:
H2 + CR -> H2+ + e-
H2+ + H2 -> H3+ + H
Because molecular hydrogen is by far the dominant constituent of the cloud, this pathway ensures that H3+ becomes the principal ion in regions where ultraviolet photons cannot penetrate. Once formed, H3+ acts as the universal proton donor in the gas phase. Its ability to transfer a proton with virtually no activation barrier makes it the driver of nearly all ion–molecule chemistry at temperatures around 10 K. Through reactions such as
H3+ + CO -> HCO+ + H2
H3+ + N2 -> N2H+ + H2
it seeds the formation of the major molecular ions that characterize dense gas. These ions, in turn, regulate the ionization fraction of the cloud, which controls the coupling between the gas and the magnetic field and thereby influences the dynamical evolution of the collapsing core.
The significance of H3+ becomes even more pronounced once CO begins to freeze out onto dust grains. In the early stages of core evolution, CO is abundant in the gas phase and efficiently destroys H3+. When CO depletes, this destruction channel is suppressed, allowing H3+ to accumulate to much higher abundances. Under these conditions, the exothermic fractionation reaction
H3+ + HD -> H2D+ + H2
becomes dominant, because the reverse reaction is energetically forbidden at low temperatures. The resulting enhancement of H2D+ initiates a cascade of deuterium‑rich chemistry, including the formation of N2D+ through
H2D+ + N2 -> N2D+ + H2
These deuterated ions are among the most reliable tracers of the coldest, densest, and most chemically evolved pre‑stellar cores. Their presence signals that the core has entered the final phase before gravitational collapse, a stage in which the chemistry is dominated by the descendants of H3+.
In this way, H3+ serves simultaneously as the chemical starting point, the regulator of ionization, and the gateway to the extreme deuteration that marks the onset of star formation. Without H3+, the chemical and dynamical evolution of pre‑stellar cores would be fundamentally different. Its central role links cosmic‑ray ionization, dust‑grain physics, molecular depletion, and the earliest stages of gravitational collapse into a single coherent framework that underpins the entire pre‑Class 0 evolutionary sequence.
From Molecular Cloud to the Onset of Class 0
Star formation begins long before a protostar exists. The earliest phase unfolds inside cold, quiescent molecular clouds composed primarily of molecular hydrogen and sub‑micron dust grains. These clouds, typically at temperatures of 10-15 K, are shaped by turbulence, magnetic fields, and external shocks. Over time, these processes generate localized density enhancements. As these condensations grow, they decouple from the ambient turbulence and evolve into gravitationally bound dense cores. Within such a core, the temperature continues to fall while the density rises to values of 104-105 cm3, creating the physical conditions required for the onset of pre‑stellar chemistry.
At these densities, dust grains begin to dominate the microphysics. Gas‑phase CO molecules collide with dust surfaces and freeze out, forming icy mantles. This freeze‑out is a pivotal transition: CO is the primary destroyer of the fundamental ion H3+. As long as CO remains in the gas phase, the reaction
H3+ + CO -> HCO+ + H2
efficiently removes H3+ from the chemistry. Once CO is depleted onto grains, this destruction pathway collapses, allowing H3+ to accumulate. The survival of H3+ triggers the characteristic deuteration chemistry of the pre‑stellar phase. The key fractionation reaction,
H3+ +HD→H2D+ +H2
becomes strongly favored at low temperatures because the backward reaction is endothermic. As a result, H2D+ becomes abundant and initiates further deuterium transfer. One of the most important channels is
H2D++N2→N2D++H2
which produces N2D+, a robust tracer of the coldest and densest gas. The rise of H2D+ and N2D+ marks the chemical signature of a core that has entered the final pre‑collapse regime. Because CO is frozen out, radiative cooling becomes more efficient, the thermal pressure drops, and the core approaches gravitational instability.
As the collapse begins, the density increases rapidly while the temperature remains low in the outer regions. The central region, however, slowly warms as compressional heating overtakes radiative cooling. The chemistry remains dominated by deuterated ions until the density becomes high enough that the gas becomes opaque to its own cooling radiation. At this stage, the collapse slows and the first hydrostatic object forms: the First Hydrostatic Core (FHSC). This transient structure, with central temperatures approaching 2000 K, represents the final stage before true protostellar birth. Once molecular hydrogen begins to dissociate, the FHSC loses pressure support and undergoes a second, rapid collapse.
This event marks the formation of a Class 0 protostar. The newborn object remains deeply embedded within its infalling envelope, invisible at optical wavelengths and dominated by cold dust emission. A small rotationally supported disk begins to form, and the first bipolar outflows break through the surrounding material. Sublimation of CO and other volatiles near the warm inner regions reintroduces gas‑phase molecules, initiating the earliest stages of warm chemistry. Yet the system is still far from the Class I phase; the envelope remains massive, the protostar is heavily obscured, and the accretion rate is at its peak.
The narrative of pre‑stellar evolution therefore ends at the moment the protostar forms but before the envelope disperses. The transition from a chemically regulated, CO‑depleted pre‑stellar core to a deeply embedded Class 0 source encapsulates the earliest and most fundamental processes of star formation, governed jointly by dust physics, ion‑molecule chemistry, and gravitational collapse.
Chemical Differentiation in Pre‑Stellar Cores
Chemical differentiation, often referred to as chemical zoning, is one of the most striking signatures of the pre‑stellar phase. As a dense core evolves toward gravitational collapse, its interior becomes chemically stratified, forming a layered structure in which different molecules dominate at different radii. This zoning is not an incidental feature but a natural consequence of the interplay between density, temperature, freeze‑out, ionization, and dynamical evolution. Observations of well‑studied cores such as L1544 show that the chemical structure resembles an onion: concentric shells of distinct molecular abundances that trace the physical conditions and evolutionary state of the core.
In the outer envelope, where densities are modest and temperatures remain above the threshold for efficient freeze‑out, carbon‑bearing species such as CO, C¹⁸O, CS, and CCS remain abundant in the gas phase. These molecules trace the more diffuse, slowly contracting regions of the core. As the density increases toward the center, the timescale for gas–grain collisions becomes short enough that CO and other heavy molecules rapidly deplete onto dust grains. This freeze‑out produces a pronounced drop in the gas‑phase abundance of CO by more than an order of magnitude in the innermost 10⁴ AU, as confirmed by both observations and dynamical‑chemical models. The removal of CO from the gas phase has profound consequences: it suppresses the primary destruction pathway of H₃⁺, allowing deuterated ions such as H₂D⁺ and N₂D⁺ to become abundant. These ions, in turn, drive the chemistry of the coldest and densest regions.
Nitrogen‑bearing species behave differently. Molecules such as NH₃ and N₂H⁺ show little or no depletion in the central region, and in some cases their abundances even rise toward the core center. This resilience is linked to the relatively slow freeze‑out of N₂ compared to CO, as well as the enhanced production of N₂H⁺ in CO‑depleted gas. As a result, NH₃ and N₂H⁺ become excellent tracers of the high‑density, low‑velocity plateau at the core center — the region where infall motions are slowest and the collapse is just beginning. Their centrally peaked distributions contrast sharply with the hollowed‑out profiles of CO, CS, and CCS, which trace the faster‑moving inner envelope rather than the quiescent core.
This chemical zoning is not merely a passive imprint of physical conditions; it actively influences the collapse. The ionization fraction — controlled by the abundances of ions such as H₃⁺, HCO⁺, and N₂H⁺ — regulates the efficiency of ambipolar diffusion, the process by which neutral gas decouples from magnetic fields. As the chemistry evolves, the ionization fraction changes, altering the rate at which the core can contract. Models show that modifying only the chemical parameters, such as sticking probabilities or recombination rates, can change the collapse timescale by factors of two to three. Thus, the chemical structure and the dynamical evolution are inseparably linked.
The resulting radial abundance profiles provide a powerful diagnostic tool. Centrally depleted species trace the infalling envelope, while centrally enhanced species trace the static or slowly contracting core. By comparing synthetic line profiles with observations, one can infer the velocity field, the degree of freeze‑out, and the evolutionary stage of the core. Chemical differentiation therefore serves as a natural probe of both the physical and chemical history of pre‑stellar cores, offering a window into the earliest stages of star formation before the emergence of a Class 0 protostar.
V[TCT2004] VLA 3B Centimetric Radio Source 21 43 06.462 +66 06 55.22
VLA 3B —
De sterkste en meest compacte Class‑0 radiobron in NGC 7129.
Dit is een proto‑object: een instortende kern met een ionized jet‑basis.
Zichtbaar in C‑band en X‑band, soms Ku‑band.
Past perfect bij jouw idee van “conceptie”.
VLA 3A —
Companion‑bron van 3B, waarschijnlijk deel van hetzelfde collapse‑complex.
Vaak gezien als een binary protostellar embryo.
Moleculaire outflow‑signatuur in CO.
VLA 2 —
Een bredere, diffuse radiobron.
Past bij een Class‑0/early Class‑I object met een brede moleculaire outflow.
Sterk in L‑band door free–free emissie.
VLA 1 —
Een zwakkere maar duidelijke ionized jet‑basis.
Vaak geassocieerd met H₂‑shocks in de nabijheid.
Mogelijk een transitional Class‑0 → Class‑I bron.
VLA 4 —
Compacte radiobron, minder goed bestudeerd.
Past bij een collapse‑fragment in het stervormingsfilament.
Mogelijk een toekomstige protostar.
Chemical Segregation in the Early Stages of Star Formation https://edoc.ub.uni-muenchen.de/35897/1/Giers_Katharina.pdf
Mapping Interstellar Ice Inventory toward Class 0 Protostars in Star-forming Region Orion A with JWST Data https://arxiv.org/abs/2606.09776
Self-Consistent Theoretical Models of Collapsing Pre-Stellar Cores Valery I. Shematovich - doi:10.1017/S1743921306007022 https://www.researchgate.net/publication/231919010_Self-Consistent_Theoretical_Models_of_Collapsing_Pre-Stellar_Cores
