Hyperfine excitation of NS+^+ due to para-H2_2(j = 0) impact

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Constructing potential energy surface for carbon-chain containing systems using the Radial Angular Network with Gradual Expansion method

International audienceInvestigating molecular excitation induced by collisions requires the prior determination of accurate analytical potential energy surfaces for the colliding partners. For carbon-chain molecules, such as cyanopolyynes, this has been a longstanding challenge, resulting in the absence of rate coefficients for HC$_5$N, HC$_7$N, HC$_9$N, and others, induced by collisions with He. To overcome this bottleneck, we introduce a new approach: the Radial Angular Network with Gradual Expansion (RANGE). This method jointly connects the construction of ab initio interaction potentials with the determination of their analytical forms. We use the HC3N-He molecular complex as a reference to assess the reliability of our method, given that its analytical potential has been derived using various methods. Additionally, we apply the RANGE approach to construct the analytical representation of the interaction potential for HC$_5$N-He and HC$_7$N-He. The analysis of the analytical potentials reveals three systematic trends: (i) the anisotropy increases with the length of the carbon chain, (ii) the number of local minima correlates with the number of carbon atoms, and (iii) the shallowest local minimum is consistently located at approximately 30 cm-1 below the dissociation limit of the complex. Using the time-independent quantum mechanical close-coupling formalism, we briefly estimate the propensity rules governing the excitation of HC$_3$N, HC$_5$N, and HC$_7$N induced by collisions with He. Consequently, the three collisional systems exhibit the same propensity rule, favoring ∆ j = 2 transitions

Hyperfine resolved rate coefficients of HCNH+ /H13CNH+ induced by collision with He and H2 : towards HCNH+ abundance modelling and H13CNH+ line intensity prediction

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First close-coupling study of the excitation of a large cyclic molecule: collision of c-C5H6 with He

International audienceRecent astronomical observations revealed an increasing molecular complexity in the interstellar medium through the detection of a series of large cyclic carbon species. To correctly interpret these detections, a complex analysis is necessary that takes into account the non-local thermodynamic equilibrium (non-LTE) conditions of the emitting media (e.g. when energy level populations deviate from a Boltzman distribution). This requires proper state-to-state collisional data for the excitation and de-excitation processes of the molecular levels. Cyclopentadiene (c-C5H6), which was recently detected in multiple cold interstellar clouds, is extensively studied in many aspects due to its large importance for chemistry in general. At the same time, there are no collisional data available for this species, which are necessary for a more precise interpretation of the corresponding detections. In this work, we first provide an accurate 3D rigid-rotor interaction potential for the [c-C5H6 + He] complex from high-level of ab initio theories, which has been used to study their inelastic collision by the exact close coupling quantum scattering method. To the best of our knowledge, this is the first study where this method is systematically applied to treat the dynamics of molecular collisions involving more than ten atoms. We also analyse the collisional propensity rules and the differences in contrast to calculations, where the approximate coupled states scattering methods is used

Size and Shape Constraints of (486958) Arrokoth from Stellar Occultations

International audienceWe present the results from four stellar occultations by (486958) Arrokoth, the flyby target of the New Horizons extended mission. Three of the four efforts led to positive detections of the body, and all constrained the presence of rings and other debris, finding none. Twenty-five mobile stations were deployed for 2017 June 3 and augmented by fixed telescopes. There were no positive detections from this effort. The event on 2017 July 10 was observed by the Stratospheric Observatory for Infrared Astronomy with one very short chord. Twenty-four deployed stations on 2017 July 17 resulted in five chords that clearly showed a complicated shape consistent with a contact binary with rough dimensions of 20 by 30 km for the overall outline. A visible albedo of 10% was derived from these data. Twenty-two systems were deployed for the fourth event on 2018 August 4 and resulted in two chords. The combination of the occultation data and the flyby results provides a significant refinement of the rotation period, now estimated to be 15.9380 ± 0.0005 hr. The occultation data also provided high-precision astrometric constraints on the position of the object that were crucial for supporting the navigation for the New Horizons flyby. This work demonstrates an effective method for obtaining detailed size and shape information and probing for rings and dust on distant Kuiper Belt objects as well as being an important source of positional data that can aid in spacecraft navigation that is particularly useful for small and distant bodies