A far-ultraviolet-driven photoevaporation flow observed in a protoplanetary disk

Most low-mass stars form in stellar clusters that also contain massive stars, which are sources of far-ultraviolet (FUV) radiation. Theoretical models predict that this FUV radiation produces photodissociation regions (PDRs) on the surfaces of protoplanetary disks around low-mass stars, which affects planet formation within the disks. We report James Webb Space Telescope and Atacama Large Millimeter Array observations of a FUV-irradiated protoplanetary disk in the Orion Nebula. Emission lines are detected from the PDR; modeling their kinematics and excitation allowed us to constrain the physical conditions within the gas. We quantified the mass-loss rate induced by the FUV irradiation and found that it is sufficient to remove gas from the disk in less than a million years. This is rapid enough to affect giant planet formation in the disk.

Fragmentation channels of non-fullerene cationic carbon clusters

The unimolecular fragmentation channels of highly excited small cationic carbon clusters have been measured with a time-of-flight mass spectrometer after photofragmentation. The dominant channel is loss of the neutral trimer, for all CN+N = 10-27 clusters except for N = 11, 12 which decay by monomer emission, and C25+ which shows competing loss of C2 and C3. The results permit to quantify the role of the rotational entropy in the competition between monomer and trimer decays with the help of energies calculated with density functional theory.

Formation of the methyl cation by photochemistry in a protoplanetary disk

Forty years ago, it was proposed that gas-phase organic chemistry in the interstellar medium can be initiated by the methyl cation CH3+ (refs. 1-3), but so far it has not been observed outside the Solar System4,5. Alternative routes involving processes on grain surfaces have been invoked6,7. Here we report James Webb Space Telescope observations of CH3+ in a protoplanetary disk in the Orion star-forming region. We find that gas-phase organic chemistry is activated by ultraviolet irradiation.

Aromatic and Acetylenic C-H or C-D Stretching Bands Anharmonicity Detection of Phenylacetylene by UV Laser-Induced Vibrational Emission

The anharmonic infrared (IR) emission spectra of phenylacetylene C₆H₅CCH and an isotopologue C₆H₅CCD induced by 193 nm UV excitation have been investigated in the gas phase. The study has been operated with a homemade IR spectrometer enabling to record time- and wavelength-resolved spectra between 2.5 and 4.5 μm, emitted all along the collisional cooling. The analysis is supported by a kinetic Monte Carlo simulation in the vibrational harmonic approximation. For both species, the anharmonic shifts of the acetylenic C-H or C-D stretching modes and the aromatic C-H stretching modes are studied for band positions and bandwidths in terms of the internal energy. For C₆H₅CCD, the internal energy dependence of the emission intensity band ratio is investigated and rationalized. This work demonstrates the potential of time-resolved IR emission spectroscopy to explore anharmonicity of astrophysically relevant molecules.

Radiative relaxation in isolated large carbon clusters: Vibrational emission versus recurrent fluorescence

Recurrent fluorescence (RF) from isolated carbon clusters containing between 24 and 60 atoms is theoretically investigated as a function of internal energy, cluster size, and structural features. The vibrational relaxation kinetics and the associated IR emission spectra are determined by means of a Monte Carlo approach with vibrational density of states computed in the harmonic approximation. RF is generally found to be highly competitive with vibrational emission. The behaviors predicted for clusters of various sizes and archetypal structures indicate that the IR emission spectra are strongly influenced by RF, an energy gap law being obtained for the evolution of the RF rate constant depending on the electronic excitation state. The present results are relevant to the photophysics of the interstellar medium and could contribute to elucidating the carriers of the extended red emission bands and the continuum emission lying below the aromatic infrared bands believed to originate from mixed aromatic–aliphatic compounds.