On the Timescale for the Formation of Protostellar Cores in Magnetic Interstellar Clouds
Bibliographic record
Abstract
We revisit the problem of the formation of dense protostellar cores caused by ambipolar diffusion within magnetically supported molecular clouds and derive an analytical expression for the core formation timescale. The resulting expression is similar to the canonical expression ≃ τ /τ ni ~ 10τ ff (where τ ff is the free-fall time and τ ni is the neutral-ion collision time), except that it is multiplied by a numerical factor (μ c0 ), where μ c0 is the initial central mass-to-flux ratio normalized to the critical value for gravitational collapse. (μ c0 ) is typically ~1 in highly subcritical clouds (μ c0 ≪ 1), although certain conditions allow (μ c0 ) ≫ 1. For clouds that are not highly subcritical, (μ c0 ) can be much less than unity, with (μ c0 ) → 0 for μ c0 → 1, which significantly reduces the time required to form a supercritical core. This, along with recent observations of clouds with mass-to-flux ratios close to the critical value, may reconcile the results of ambipolar diffusion models with statistical analyses of cores and young stellar objects that suggest an evolutionary timescale ~1 Myr for objects of mean density ~10 4 cm -3 . We compare our analytical relation to the results of numerical simulations and also discuss the effects of dust grains on the core formation timescale.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.006 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.002 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.003 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".