Summer Term 2019

The H₃⁺ and H₄⁺ species both exhibit a rich potential energy surface topology, which affects their collisional and laser induced dynamics. We have investigated the charge transfer and atom interchange in a series of fast-beam experiments.
Photodissociation experiments were conducted to investigate the role of the avoided crossing seam between the ground and first excited potential energy surface of H₃⁺[1]. Three-dimensional imaging of dissociation products was used to determine the kinetic energy release and branching ratio among the fragmentation channels. Vibrational distributions were measured by dissociative charge transfer of H₂⁺ products. It is found that the photodissociation of hot H₃⁺ in the near ultraviolet produces coldH₂⁺, but hot H₂. Modelling the wavepacket dynamics along the repulsive potential energy surface accounts for the repopulation of the ground potential energy surface. The important role of the H₃⁺ avoided crossing seam for the astrophysically relevant H₂⁺ + H₂ charge transfer reactions [2] will be underlined.

The partially deuterated isotopologues of H₃⁺, i.e. H₂D⁺ and D₂H⁺, have pure rotational spectra that can be excited at prestellar core temperatures, allowing for the total H₃⁺ abundance to be inferred, provided the deuterating reactions of H₃⁺ are well understood. Here, we present laboratory measurements of the rate coefficients for the reactions of atomic D with H₃⁺, H₂D⁺, and D₂H⁺. The measurements were performed using a dual-source, merged fast beams apparatus [3]. In addition, high-level quantum ab initio calculations have been carried out to model the zero-point-energy corrected energy profile and the shape of the potential energy barrier, allowing an evaluation of tunneling effects. From the combination of our experimental and theoretical results, we derive thermal rate coefficients in the temperature range relevant for prestellar cores [4].

[1] X. Urbain, A. Dochain, R. Marion, T. Launoy, and J. Loreau 2019 Phil. Trans. R. Soc. A (submitted).
[2] X. Urbain, N. de Ruette, V.M. Andrianarijaona, M.F. Martin, L. Fernández Menchero, L. Errea, L. Méndez, I. Rabadán, B. Pons 2013 Phys. Rev. Lett. 111 203201.
[3] A. P. O'Connor, X. Urbain, J. Stützel, K. A. Miller, N. de Ruette, M. Garrido, and D. W. Savin 2015 ApJ Suppl. 219 6.
[4] P.-M. Hillenbrand, K. P. Bowen, J. Liévin, X. Urbain, and D. W. Savin 2019 ApJ (submitted).

Star formation is the key astrophysical process that converts baryonic matter into the stars and planets that form the backbone of galaxy structure. Many details of the star formation process can only be investigated in the Milky Way. Such research thus provides us with important information about the evolution of the gas and the stars in galaxies over cosmic time.

I will discuss two studies of star formation in the Milky Way that seek to advance our understanding of cosmic star formation. First, I will talk about research into Galactic Center molecular clouds with ALMA and other interferometers. This informs us about star formation under extreme conditions, resembling those found in starburst galaxies. Second, I will present results from the LEGO Large Program on the IRAM 30m–telescope, which obtains the most comprehensive wide–field spectroscopic views of molecular clouds available to date. Such work shows us how we can characterize spatially unresolved gas in other galaxies, and it reveals the astrochemistry of star and planet formation at an unprecedented level of detail.

Surveys of the Milky Way plane in a variety of gas or dust tracers have helped us develop an understanding of how the physical conditions of the interstellar medium vary throughout the Galaxy. We are now able to investigate what role Galactic environmental factors might play in giving rise to the conditions conducive to star formation. I will discuss a recent study conducted using the Herschel Infrared Galactic Plane Survey (Hi-GAL), which has catalogued over 100000 compact objects, or "clumps", throughout the plane and characterised their properties by modelling their spectral energy distributions (SEDs). We find that although the spiral arms of our Galaxy are where clumps are primarily (though not exclusively) concentrated, the clumps found in arms are not significantly more likely to be star-forming than interarm clumps. We also find a notable gradient in the prevalence of star formation in clumps with Galactocentric radius. I will also discuss our studies of "giant" 100-parsec scale molecular filaments in the Galaxy which enable us to link the clump scales up to Galactic scales. A comprehensive profile of the Milky Way from parsec to kiloparsec scales provides us with the appropriate context in which we can understand star formation in other galaxies.