Triggered/sequential star formation? A multi-phase ISM study around the prominent IRDC G18.93-0.03
Bibliographic record
Abstract
Context. Triggered star formation has been discussed for many years, and evidence for the formation of stars and cores triggered by Hii regions is under debate. Aims. We investigate the imprints of an expanding Hii region on a pre-existing starless clumps. Methods. We selected an infrared dark filament spanning 0.8°. One portion of this filament, G18.93-0.03 is a prominent dust complex, with the molecular clump G18.93/m being IR dark from near-IR wavelength up to 160 μm. Spitzer composite images show an IR bubble spatially associated with G18.93-0.03. We use GRS 13CO and IRAM 30 m H13CO+ data to disentangle the large and small scale spatial structure of the region. From ATLASGAL submm data we calculate the gas mass, while we use the H13CO+ line width to estimate its virial mass. To study the IR properties of G18.93/m we use Herschel data and produce temperature maps from fitting the spectral energy distribution. With the MAGPIS 20 cm and SuperCOSMOS Hα data we trace the ionized gas, and the VGPS Hisurvey provides information on the atomic hydrogen gas. Results. We show that the bubble is spatially associated with G18.93, located at a kinematic near distance of 3.6 kpc. The total gas mass of ~870 M⊙ splits up into 6 sub-clumps, of which G18.93/m is the most massive with 280 M⊙. The virial analysis shows that it may be gravitationally bound and has neither Spitzer young stellar objects nor mid-IR point sources within. Therefore we call it pre-stellar. Fitting the spectral energy distribution reveals a temperature distribution that decreases towards its center, but heating from the ionizing source puts it above the general ISM temperature. We find that the bubble is filled by Hii gas, ionized by an O8.5 star. Between the ionizing source and the IR dark clump G18.93/m we find a layered structure of hydrogen phases, from ionized to atomic to molecular gas, revealing a photon dominated region. Furthermore, we identify an additional velocity component within the bubble’s 8 μm emission rim at the edge of the infrared dark cloud and speculate that it might be shock induced by the expanding Hii region. Conclusions. While the elevated temperature allows for the build-up of larger fragments, and the shock induced velocity component may lead to additional turbulent support, the density profile of G18.93/m does not show signatures of the expanding bubble. While the first two signatures favor high-mass star formation, we do not find conclusive evidence that the massive clump G18.93/m is prone to collapse because of the expanding Hii region.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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".