Enhanced star formation through the high-temperature formation of H2 on carbonaceous dust grains Francesco Grieco 1,2 ,Patrice Theulé 3 , Ilse De Looze 2 , François Dulieu 1 1 CY Cergy-Paris Université,Observatoire de Paris,PSL University,Sorbonne Université,CNRS, LERMA,Cergy(France). 2 University of Ghent,Department of Physics and Astronomy,Ghent(Belgium). 3 Aix Marseille Université,CNRS,CNES,LAM,Marseille(France). Molecular hydrogen is the most abundant molecule in the Universe and its formation has implications on star formation rates over cosmic times. It is the cooling agent needed to initiate the cloud collapse regulating the star formation efficiency.[1,2]The following results can contribute to changing the H/H2 photodissociation front location and the respective size of PDR(PhotoDissociation Region), HII and molecular regions in a classical PDR picture. [3]The dominant H2 formation route depends on dust grains as catalysts, such as small carbonaceous grains, including PAHs(Polycyclic Aromatic Hydrocarbons), that have been shown to increase the H2 formation rates.[4,5] H2 formation on PAHs was thought to reduce above dust temperatures of 50K and H atom recombination was believed to be highly efficient only below 20K. Until now, laboratory and theoretical works have suggested that H2 cannot form on grains with temperatures above 100K and they do not provide a direct measurement of the recombination efficiency at dust temperatures >20K.[6,7,8,9]Here we report direct laboratory measurements of the high efficiency formation of H2 at temperatures up to 250K on a carbonaceous surface mimicking interstellar dust. We observe a plateau above 100K(20%), elevated values(30%) between 30K-80K, a maximum(45%) around 20K, a sharp decrease(20%) at 10K. This efficiency includes accretion, diffusion and reaction steps. The H2 formation pathway on surfaces can therefore be much more efficient than previously estimated, over an extended range of temperatures. H2 could start contributing to the cooling of warmer gas(T~50-250K) having a huge impact on our understanding of H2 formation in nearby galaxies and the availability of H2 reservoirs for star formation in high-redshift galaxies, in which significant dust masses have been built up and the CMB(Cosmic Microwave Background) pushes the dust temperatures to >20K.[10]This study will enable an estimation of the contribution of PAHs to interstellar H2 formation at higher temperature. Correctly accounting for H2 formation over cosmic times is a key ingredient to interpret the James Webb Space Telescope observations of the PAH grain population, the H2 line emission in local PDRs and nearby galaxies, and to study the formation of the first generations of stars in
Early Universe galaxies.[11,12] More info in Grieco et al.[13] References 1. Glover, S. C. O. & Clark, P. C. MNRAS. 437, 9–20(2014). 2. Bigiel, F. et al. Astron. J. 136, 2846(2008). 3. Tielens, A. G. G. M. & Hollenbach, D. Astrophys. J. 291, 722(1985). 4. Lipshtat, A. & Biham, O. MNRAS. 362, 666–670(2005). 5. Congiu, E., Matar, E., Kristensen, L. E., Dulieu, F. & Lemaire, J. L. MNRAS Lett. 397, L96–L100(2009). 6. Cazaux, S. & Spaans, M. Astrophys. J. 611, 40–51(2004). 7. Pirronello, V., Biham, O., Liu, C., Shen, L. & Vidali, G. Astrophys. J. 483, L131–L134(1997). 8. Cazaux, S. et al. Sci. Rep. 6, 19835(2016).
9. Wakelam, V. et al. Mol. Astrophys. 9, 1–36(2017). 10. Cunha, E. da et al. Astrophys. J. 766, 13(2013). 11. Berné, O. et al. Preprint at https://doi.org/10.1088/1538-3873/ac604c(2022). 12. Finkelstein et al. ApJ 940, 559(2022). 13. Grieco et al. Nature Astronomy(in press,2023).
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