Methanol CH3 OH is a small organic molecule of particular importance in astrophysics. It has recently been detected in its gaseous form in protoplanetary disks ([1], [2]) and its formation is supposed to be the first step toward a complex organic chemistry. Its presence in these star forming regions is still not fully understood. It is generally believed to form on the surface of cold (T < 100 K) dust grains, in condensed phase, diluted in the main constituents of the icy mantles, H2O and/or CO. As it cannot thermally desorb from these ices, a non thermal process should explain its gas phase presence. In protoplanetary disks, it is expected that X-rays emitted by the central young stellar object could trigger CH3OH ejection from the ices into the gas phase, hence explaining the detections. This process, known as X-ray photodesorption, can participate to the overall gas to ice ratio of methanol in these cold regions and needs to be quantify.

Experimental study of X-ray photodesorption from methanol containing ices has been achieved in the range of 500-570 eV by coupling the Ultrahigh Vacuum SPICES setup to the SEXTANTS beamline of the SOLEIL synchrotron facility in Paris Saclay. The diversity and richness of the desorbing molecules from the studied ices is probed by mass spectrometry. This allows to derive quantitative X-ray photodesorption yields (expressed in molecule desorbed by incident photon) that are representative of the efficiency of X-rays to desorb a given molecule. The critical influence of several parameters on these yields, such as the ice composition (mainly the influence of CO and H2O molecules) and the incident photon energy, allows to shed light on the chemical and physical mechanisms at play in these X-ray irradiated organic ices. In this context, I will present the main findings of these experiments and I will discuss their astrophysical implications.

References

[1] Wash et al., First detection of gas phase methanol in a protoplanetary disk, The Astrophysical Journal Letters , 823, L10 (2016)

[2] Carney et al., Upper limits on CH3OH in the HD 163296 protoplanetary disk, A&A , A124 (2019)

[3] R.Basalgète et al., Complex Organic Molecules in protoplanetary disks : X ray photodesorption from methanol containing ices Part 1 : Pure methanol ices, A&A , 647, A35 (2021)

[4] R.Basalgète et al., Complex Organic Molecules in protoplanetary disks : X ray photodesorption from methanol containing ices Part 2 : Mixed methanol CO and methanol H2O ices, A&A, 647, A36 (2021)

Comment la complexité chimique évolue au cours du processus conduisant à la formation d'un Soleil et de son système planétaire? La richesse chimique d'un système planétaire de type solaire est-elle partiellement héritée des premiers stades ou y  a-t-il une réinitialisation chimique complète? Des preuves récentes suggèrent que la formation des planètes commence probablement déjà dans les jeunes disques protostellaires (phase de Classe I ~ 10⁵ yr). Par conséquent, l'étude de leurs compositions chimiques représente une étape clé dans notre compréhension du matériau initial disponible pour la formation des planètes.
Je me concentrerai sur la complexité chimique observée dans les proto-étoiles de Classe I. En particulier, je montrerai les résultats que nous avons obtenus des Large Programs ASAI et SOLIS et, plus récemment de FAUST, le premier Large Program ALMA axé sur l'astrochimie. En utilisant des molécules deutérées et des molécules organiques complexes, je comparerai la richesse chimique observée dans les proto-étoiles de Classe I avec celles observées dans les proto-étoiles plus jeunes de Classe 0 et dans les comètes, représentant le matériau le plus vierge de notre système solaire. Je présenterai les possibles tendances évolutives afin d'étudier le scénario d'héritage. Je présenterai également les perspectives futures sur les études des proto-étoiles de Classe I et l'exploration moléculaire en vue de l'avènement de SKA.

Olivia Chitarra, Jean-thibaut Spaniol, Thomas Hearne, Jean-Christophe Loison, Marie-Aline Martin-Drumel and Olivier Pirali

To enable the detection of new species in the interstellar medium (ISM),
rotational spectroscopy, occurring in the centimeter and millimeter-wave
region, is very effective and enabled the detection of almost 250 molecules to
date. Recently, molecules of increasing complexity have been detected [1], and
several hypothesis concerning their formation in the gas phase involve
reactions with open shell, radical species. The detection of these highly
reactive species would give important insights about the chemistry happening in
the ISM. To support the search of those molecules, laboratory spectroscopy is
essential.

We led new high-resolution studies in order to complete the spectroscopic
knowledge on 3 different radicals in the millimeter-wave region (from 140 GHz
up to 900 GHz). These species were produced by H abstraction from their
precursor (acetonitrile, CH₃CN, and methanol, CH₃OH) using
F atoms, produced by a microwave discharge. Their pure rotational spectra were
recorded using a frequency multiplication chain spectrometer in the 140-850 GHz
spectral range, following a method described in Ref. [2].

Using these precursors, we were able to produce the cyanomethyl (CH₂CN),
the methoxy (CH₃O), and the hydroxymethyl (CH₂OH)
radicals. For those three species, previous investigations only measured
transitions up to 300 GHz [3, 4, 5] and intense at room temperature. However,
several lines of astrophysical interest (intense under cold conditions) were
not measured above 300 GHz preventing their detection by radiotelescope
covering the sub-millimeter-wave region. We completed the spectroscopic
characterization above 300 GHz yielding to an improvement in their
spectroscopic parameters. The insterstellar search for these species can now be
undertaken in cold to warn environments of the ISM up to 900 GHz.

References:

[1]: Cernicharo, J. et al., Astronomy &amp; Astrophysics Letters, 41156 (2021)

[2]: Chitarra, O. et al., Astronomy &amp; Astrophysics, 644, A 123 (2020)

[3]: Saito, S. and Yamamoto, S., The Journal of Chemical Physics, 107, 1732 (1997)

[4]: Endo, Y. et al., The Journal of Chemical Physics, 81, 122 (1984)

[5]: Bermudez, C. et al., Astronomy &amp; Astrophysics, 598, A9 (2017)

CF⁺ as a new tool to study PDRs

Benjamin Desrousseaux¹, François Lique¹, Javier R. Goicoechea², Ernesto Quintas-Sanchez³ and Richard Dawes³

¹ Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) - UMR 6251, F-35000 Rennes, France.

² Instituto de Física Fundamental (CSIC). Calle Serrano 121-123, 28006, Madrid, Spain.

³ Department of Chemistry, Missouri University of Science and Technology, Rolla, Missouri 65409, United States

Abstract

  The detection of CF⁺ in interstellar clouds potentially allows astronomers to infer the elemental fluorine abundance and the ionization fraction in ultraviolet-illuminated molecular gas. Because local thermodynamic equilibrium (LTE) conditions are hardly fulfilled in the interstellar medium (ISM), the accurate determination of the CF⁺ abundance requires one to model its non-LTE excitation via both radiative and collisional processes. CF⁺ being mostly detected in warm molecular gas, where the H₂ ortho-to-para ratio is found to be large, obtaining rate coefficients for collisional excitation with both spin isomers of \ch{H2} can be crucial.

 In this work, we carried out close-coupling calculations of the rotational (de)excitation cross sections for collisions between CF⁺ and both para- and ortho-H₂ for transitions between the 22 first rotational levels of CF⁺. Collisional energies up to 1500 cm^-1 were explored in order to determine rate coefficients for temperatures up to 150 K. As already observed for a wide variety of ion-molecule collisions, para- and ortho-H₂ rate coefficients are found to be similar in magnitude.

 Then, we present non-LTE excitation models that reveal population inversion in physical conditions typical of photodissociation regions (PDRs). We successfully applied these models to fit the CF⁺ emission lines previously observed toward the Orion Bar and Horsehead PDRs, constraining gas density and temperature at the surface of the Horsehead and Orion Bar PDRs. The radiative transfer models achieved with these new rate coefficients allow the use of CF⁺ as a powerful probe to study molecular clouds exposed to strong stellar radiation fields.

X²-value as a function of the H₂ column density and gas kinetic temperature for the Orion Bar PDR. The solid black lines represent confidence contour levels of 63.3%, 90.0%, 99.0% and 99.9%. The set of parameters {T, n(H₂)} which give minimum X² is indicated with a white + symbol.

References

Desrousseaux, B., Quintas-Sanchez, E., Dawes, R., and Lique, F. J. Phys. Chem. A, 123, 45 (2019).

Desrousseaux, B., Lique, F., Goicoechea, J. R., Quintas-Sanchez, E., and Dawes, R. A&A, 645, A8 (2021).

Nitriles are key “ingredients” in pre-biotic reactions leading to the formation of aminoacides, i.e. life building-blocks. Understanding where and how these molecules form is crucial to determine 1) how universal the seeds of pre-biotic chemistry are  and 2) what type of molecules can be inherited by nascent planets. Complex nitriles, such as HC3N and CH3CN, are observed in a wide variety of astrophysical environments, including at relatively high abundances in photon-dominated regions (PDRs) and the UV exposed atmospheres of planet-forming disks. Mounting evidence suggest that the gas-phase of planet-forming disks is oxygen-poor. We recently investigated whether elevated abundances of complex nitriles in UV-irradiated environments could be explained by simple gas-phase PDR chemistry in C-rich environment, i.e. when the elemental C/O ratio is higher than the solar value (~0.5). We find that a C/O ratio higher than ~0.9 can explain the observed nitrile abundances both in PDRs and disk atmospheres, increasing predicted abundances by several orders of magnitude compared to standard C/O assumptions. I will present these new results and show how the C/O ratio appears to be a key variable for complex organic molecule abundances in photon-dominated regions across a wide range of scales.

With the aim of characterizing the role played by magnetic fields in the formation of young protostars, several recent studies have revealed unprecedented features toward high angular resolution ALMA dust polarization observations of Class 0 protostellar cores. Especially, the dust polarization has been found to be enhanced along the irradiated cavity walls of bipolar outflows, but also in region most likely linked with the infalling envelope, in the form of filamentary structure being potential magnetized accretion streamer.  These observations allow us to investigate the physical processes involved in the Radiative Alignment Torques (RATs) acting on dust grains from the core to disk scales. Synthetic observations of non-ideal magneto-hydrodynamic simulations of protostellar cores implementing RATs, show that the ALMA values of grain alignment efficiency lie among those predicted by a perfect alignment of grains, and are significantly higher than the ones obtained with the standard RAT alignment of paramagnetic grains. Ultimately, our results suggest dust alignment mechanism(s) are efficient at producing polarized dust emission in the local conditions typical of Class 0 protostars. However, further study leading to a better characterization of dust grain characteristics, or additional grain alignment mechanisms, will be required to investigate the cause of strong polarized dust emission located in regions of the envelope where alignment conditions are not favorable. A new attempt aiming at a better characterization of the dust grain alignment mechanism(s) occurring in young star forming objects, is to investigate the chemistry going on in those cores, and see if the local physical conditions (irradiation, temperature) can reconcile both molecular line emission emanating from the outflow cavity walls of PDR-like conditions, and dust polarization observations.

Molecular clouds are surrounded by an atomic layer where hydrogen is in atomic form (H) instead of molecular form (H2). As one looks deeper into the cloud, the position where H2 becomes more abundant than HI is called the H/H2 transition. This transition controls the fraction of molecular gas, which constitutes the mass reservoir for star formation, as evidenced by the Schmidt-Kennicutt law. Theoretical descriptions of this H/H2 transition have been proposed in the case where the gas is assumed static (e.g. Sternberg et al. 2014). In star forming regions, however, Herschel and spatially-resolved ALMA observations (Joblin et al. 2018, Wu et al 2018, Goicoechea et al. 2016) have revealed important dynamical effects at the PDR edges of molecular clouds, which could be explained by photo-evaporation (Bron et al. 2018) resulting in an advance of the ionization front into the neutral gas, or, equivalently, to neutral gas being advected through the PDR.

In the present paper, we extend the analytic theory of the H/H2 transition to include the dynamics of the gas induced by photo-evaporation and find its consequences on the total atomic hydrogen column density at the surface of clouds in presence of a strong UV field, and on the properties of the H/H2 transition. We also include H2 formation on grains, H2 photodissociation, H2 self-shielding and metallicity. The advection of gas through the H/H2 transition caused by photoevaporation reduces the width of the atomic region compared to static models. The atomic region may disappear if the ionization front velocity exceeds a certain value, leading the H/H2 transition and the ionization front to merge. We provide analytical expressions to determine the total HI column density. Finally, we compared our results to observations of PDR illuminated by O-stars, for which we conclude that the dynamical effects are strong. Some H2 is then expected to be closer to the edge, in a hot and FUV-rich region, where H2 lines are more intense. These lines will be the focus of upcoming James Webb Space Telescope (JWST) observations.

Despite observational clues of disk-mediated accretion, molecular outflows, and large samples showing that massive stars are often located in multiple stellar systems, the mechanisms behind these statements remain poorly understood.

The accretion question has been tackled first. Massive stars still accrete when going onto the main-sequence, hence they start radiating, and at a much higher luminosity as their mass increases. The radiation barrier problem (i.e. radiative force halting accretion in 1D, Larson, 1971) has motivated the implementation of radiative transfer in simulations, confirmed disk-mediated accretion as a solution to the radiation barrier problem and the formation of radiative cavities in multi-dimensional simulations. Nevertheless, several studies have shown the formation of Rayleigh-Taylor instabilities in these cavities, acting as a second accretion channel. Using dedicated radiation-hydrodynamical simulations and a so-called “hybrid” radiative transfer method which treats stellar radiation and ambient radiation separately to accurately compute the radiative force, I bring numerical and analytical arguments to the debate (MR+20).

The outflow and multiplicity questions require more realistic environmental conditions and additional physics. Here, I present the first radiation-magnetohydrodynamical (RMHD) simulations of massive star formation including the newly-implemented hybrid radiative transfer method and non-ideal MHD (ambipolar diffusion). Using these, we identify the magnetic or radiative origin of massive protostellar outflows and compare their properties (e.g., opening angle) with observational constraints (MR+21b, to be subm.). Including initial turbulence, we investigate the regulating mechanism of disk formation and the dependence of stellar multiplicity on the environment (MR+21a, subm.). Finally, we address the question of the disk-outflow-magnetic fields alignment.

Part of the ionizing continuum (Lyman continuum, LyC) produced by young stars can leak out of the host galaxy and ionize its surroundings. At high redshift, such LyC-leaking galaxies are among the best candidates to fully account for reionization (e.g. Robertson et al. 2013). However, direct measurements are extremely difficult as the LyC photons are easily absorbed by neutral gas on the specific line of sight. Instead, indirect tracers have been used to probe the structure of the interstellar medium (ISM) (e.g. Lyman alpha line, absorption lines) but such methods are also sensitive to line of sight selection effects.
Using integrated emission lines in the optical and infrared domain palliates viewing angle dependencies; it is a promising method to make the most of observations with current ground-based facilities (e.g., ALMA) and upcoming space missions (e.g., JWST) that will grant access to many spectroscopic tracers up to redshift above 7. However, a complex modeling step is much needed to take into account the available tracers originating in different phases and to consider a multi-component topology which matches the ISM signatures of known leaking galaxies (Ramambason et al. 2020).
Such complex representative models are crucial to investigate morphology-dependent questions such as the impact of the metal and dust content on the gas distribution and mass in the different reservoirs and the porosity to ionizing radiation. Local low-metallicity galaxies, with primordial-like physical conditions and with numerous emission lines available, are ideal test-cases to benchmark this new method and explore its predictive power in high-redshift galaxies for which only a few tracers are often observed.
To constrain the parameters of this representative galaxy model, I co-developped MULTIGRIS (Lebouteiller & Ramambason. in prep) a new Bayesian code using MCMC sampling. Among the various applications, MULTIGRIS can produce probability density functions of physical parameters, either primary (density, ionization parameter, stellar population age etc...) or secondary (ionizing photon escape fraction, dust mass, H2 mass etc...). I will present the first results obtained on the Dwarf Galaxy Survey (Madden et al. 2013), a sample of local, low-metallicity galaxies using combinations of Cloudy models (Ferland et al. 2007). We build upon previous results from Cormier et al. (2019) to quantify the larger porosity of the interstellar medium for low-metallicity galaxies, with the inferred topology having more density bounded regions, leading to photons escaping the HII regions. We explore dependencies of ionizing
photons escape fraction on our model parameters and discuss promising line ratios for future local- and high-redshift studies.

The LHyRICA project is a collaboration between astrophysicists and AI experts. We propose, using this approach, to correct our impaired view of the origin of the IMF by leaving out the current <i>a-priori </i>astrophysical hypothesis made on core structure (close to elliptical Gaussian), its kinematics (homogeneously bound) and its link to its parental cloud (clear separation and thus independent evolution). Using the full multi- and hyperspectral data of Herschel (Gould Belt &amp; HOBYS surveys) and ALMA (project ALMA-IMF) and deep convolutional learning techniques (DeepHyperX; Audebert et al. 2019), we aim to obtain a new taxonomy of astrophysical objects, more precise than the simple core/filaments classification. We believe that it may revolutionise our understanding of stellar cluster formation.

In this presentation, I will show our first results of a core extraction technique based on deep convolutional networks. These first results, obtained on (multi-)fractal simulations of the ISM and Herschel’s observations of the Taurus cloud, show that a full connected convolutional network can be trained to recognise more than a thousand core-like structures mixed to a variety of environments, i.e. from the diffuse ISM to highly structured filaments. Moreover, the efficiency of a trained network, in terms of computational power and time, seems to surpass most of the common core extraction tools used at the moment.