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H3+ in the diffuse interstellar medium.

Philosophical transactions. Series A, Mathematical, physical, and engineering sciences·2006

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Molecular Diffusion in Plasma Membranes of Primary Lymphocytes Measured by Fluorescence Correlation Spectroscopy

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The most diffuse molecular gas in the galaxy.

Harvey S Liszt¹

¹National Radio Astronomy Observatory , 520 Edgemont Road Charlottesville, Virginia 22903-2475, United States.

The Journal of Physical Chemistry. A

|February 9, 2013

Summary

Even in warm interstellar gas, molecular hydrogen (H2) and other molecules form. However, observed H2 fractions in the Milky Way exceed model predictions, suggesting density variations obscure warmer gas contributions.

Area of Science:

Astrochemistry
Interstellar Medium Physics

Background:

Interstellar molecules, particularly H2, are typically found in cold, dense regions.
Warm interstellar medium (ISM) conditions (>1000 K, high electron fraction) can support some gas-phase molecule formation.
Endothermic reactions are energetically feasible in warm gas, influencing molecular abundances.

Purpose of the Study:

To investigate the formation of interstellar molecules, including H2, CH+, SH+, and OH+, in the warm ISM.
To compare predicted molecular abundances from warm gas chemistry with observational data.
To understand the discrepancy between observed and modeled H2 fractions in the Milky Way.

Main Methods:

Utilizing gas-phase chemical models to simulate molecule formation in warm ISM conditions.

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Analyzing reaction pathways, including endothermic reactions, relevant to warm gas chemistry.

Comparing model outputs with observational data of molecular abundances in the Milky Way.

Main Results:

Warm gas chemistry successfully reproduces the abundance patterns of SH+, CH+, and OH+.
However, the overall abundances of these molecules, and especially H2, are not accurately reproduced by warm gas models.
Observed H2 fractions in low-density sightlines are significantly higher than predicted by warm gas-phase or grain-surface formation processes.

Conclusions:

Warm interstellar medium chemistry contributes to the formation of certain molecules but cannot fully explain observed abundances.
The discrepancy suggests that density inhomogeneities within the ISM may play a crucial role in obscuring the molecular signatures of warmer gas.
Further research is needed to reconcile models with observations, potentially incorporating complex density structures.