# Probing the Conditions for the Hı-to-H$_{2}$ Transition in the Interstellar Medium

**Authors:** Gyueun Park, Min-Young Lee, Shmuel Bialy, Blakesley Burkhart, J. R. Dawson, Carl Heiles, Di Li, Claire Murray, Hiep Nguyen, Anita Hafner, Daniel R. Rybarczyk, Snežana Stanimirović  
**arXiv:** [2307.09513](https://arxiv.org/abs/2307.09513)  
**Date:** 2023-07-18  
**Summary page:** https://www.summarizepaper.com/en/paper/2307.09513v1/

## Key points

- Investigates the conditions for the HI-to-H$_{2}$ transition in the solar neighborhood
- Analyzes HI emission and absorption measurements towards 58 Galactic lines of sight (LOSs)
- Uses $^{12}$CO(1$-$0) (CO) and dust data for analysis
- Decomposes gas into atomic and molecular phases based on accurate column densities of CNM and WNM
- Finds that observed LOSs are mostly HI-dominated, with CO-dark H$_{2}$ being a major component of dark gas in the solar neighborhood
- CNM is more closely associated with CO than WNM
- Identifies specific characteristics of CO-bright molecular gas formation, such as low spin temperature, high peak optical depth, CNM fraction, and V-band dust extinction
- Compares observed CNM properties with Sternberg et al.'s H$_{2}$ formation model
- Suggests clumpy CNM with small volume filling factor or missing processes may play a role in HI-to-H$_{2}$ transition
- Paper is organized into sections providing background information, data analysis, comparison of observed properties to model predictions, and discussion/summary of results.

## Summary

This paper investigates the conditions for the HI-to-H$_{2}$ transition in the solar neighborhood by analyzing HI emission and absorption measurements towards 58 Galactic lines of sight (LOSs), along with $^{12}$CO(1$-$0) (CO) and dust data. The authors first decompose the gas into atomic and molecular phases based on accurate column densities of the cold and warm neutral medium (CNM and WNM). They find that the observed LOSs are mostly HI-dominated, with CO-dark H$_{2}$ being a major component of the dark gas in the solar neighborhood. To understand the formation of CO-bright molecular gas, the authors analyze the kinematic association between HI and CO. They find that CNM is more closely associated with CO than WNM. By isolating CNM components within CO line widths, they identify specific characteristics such as a spin temperature below 200 K, peak optical depth above 0.1, CNM fraction of approximately 0.6, and V-band dust extinction exceeding 0.5 mag. These results suggest that CO-bright molecular gas tends to form in environments with high column densities where CNM becomes colder and more abundant. The authors compare their observed CNM properties with the steady-state H$_{2}$ formation model proposed by Sternberg et al., inferring that CNM must be clumpy with a small volume filling factor. Alternatively, missing processes in the model such as cosmic rays and gas dynamics may play an important role in the HI-to-H$_{2}$ transition. The paper is organized as follows: Section 2 provides background information by summarizing two relevant studies conducted by Nguyen et al. (2019) and S14 (Sternberg et al.). Sections 3 and 4 present data from HI emission/absorption measurements, $^{12}$CO(1-$0) observations (CO), as well as dust data used for analysis; these sections also investigate environmental conditions along GNOMES lines of sight (LOSs). Section 5 describes results from decomposing gas into different atomic/molecular phases based on accurate column densities of cold/warm neutral medium (CNM/WNM). Sections 6 & 7 compare observed HI & CO properties to each other; they also compare these properties to predictions from S14 model - providing observational & theoretical perspectives on formation of CO bright molecular gas. Finally, Sections 8 & 9 discuss & summarize results obtained in this study.