Browsing by Author "Carpenter, J."

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    High-resolution ALMA observations of compact discs in the wide-binary system Sz 65 and Sz 66
    (2024) Miley, J. M.; Carpenter, J.; Booth, R.; Jennings, J.; Haworth, T. J.; Vioque, M.; Andrews, S.; Wilner, D.; Benisty, M.; Huang, J.; Perez, L.; Guzman, V.; Ricci, L.; Isella, A.
    Context. Substructures in disc density are ubiquitous in the bright extended discs that are observed with high resolution. These substructures are intimately linked to the physical mechanisms driving planet formation and disc evolution. Surveys of star-forming regions find that most discs are in fact compact, less luminous, and do not exhibit these same substructures. It remains unclear whether compact discs also have similar substructures or if they are featureless. This suggests that different planet formation and disc evolution mechanisms operate in these discs. Aims. We investigated evidence of substructure within two compact discs around the stars Sz 65 and Sz 66 using high angular resolution observations with ALMA at 1.3 mm. The two stars form a wide-binary system with 6 ''.36 separation. The continuum observations achieve a synthesised beam size of 0 ''.026 x 0 ''.018, equivalent to about 4.0 x 2.8 au, enabling a search for substructure on these spatial scales and a characterisation of the gas and dust disc sizes with high precision. Methods. We analysed the data in the image plane through an analysis of reconstructed images, as well as in the uv plane by non-parametrically modelling the visibilities and by an analysis of the (CO)-C-12 (2-1) emission line. Comparisons were made with highresolution observations of compact discs and radially extended discs. Results. We find evidence of substructure in the dust distribution of Sz 65, namely a shallow gap centred at approximate to 20 au, with an emission ring exterior to it at the outer edge of the disc. Ninety percent of the measured continuum flux is found within 27 au, and the distance for (CO)-C-12 is 161 au. The observations show that Sz 66 is very compact: 90% of the flux is contained within 16 au, and 90% of the molecular gas flux lies within 64 au. Conclusions. While the overall prevalence and diversity of substructure in compact discs relative to larger discs is yet to be determined, we find evidence that substructures can exist in compact discs.
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    The extremely sharp transition between molecular and ionized gas in the Horsehead nebula
    (2023) Hernandez-Vera, C.; Guzman, V. V.; Goicoechea, J. R.; Maillard, V.; Pety, J.; Le Petit, F.; Gerin, M.; Bron, E.; Roueff, E.; Abergel, A.; Schirmer, T.; Carpenter, J.; Gratier, P.; Gordon, K.; Misselt, K.
    Massive stars can determine the evolution of molecular clouds by eroding and photo-evaporating their surfaces with strong ultraviolet (UV) radiation fields. Moreover, UV radiation is relevant in setting the thermal gas pressure in star-forming clouds, whose influence can extend across various spatial scales, from the rims of molecular clouds to entire star-forming galaxies. Probing the fundamental structure of nearby molecular clouds is therefore crucial to understand how massive stars shape their surrounding medium and how fast molecular clouds are destroyed, specifically at their UV-illuminated edges, where models predict an intermediate zone of neutral atomic gas between the molecular cloud and the surrounding ionized gas whose size is directly related to the exposed physical conditions. We present the highest angular resolution (similar to 0 ''.5, corresponding to 207 au) and velocity-resolved images of the molecular gas emission in the Horsehead nebula, using CO J = 3-2 and HCO+ J = 4-3 observations with the Atacama Large Millimeter/submillimeter Array (ALMA). We find that CO and HCO+ are present at the edge of the cloud, very close to the ionization (H+/H) and dissociation fronts (H/H-2), suggesting a very thin layer of neutral atomic gas (<650 au) and a small amount of CO-dark gas (AV = 0.006-0.26 mag) for stellar UV illumination conditions typical of molecular clouds in the Milky Way. The new ALMA observations reveal a web of molecular gas filaments with an estimated thermal gas pressure of P-th = (2.3 - 4.0) x 10(6) K cm(-3), and the presence of a steep density gradient at the cloud edge that can be well explained by stationary isobaric photo-dissociation region (PDR) models with pressures consistent with our estimations. However, in the HII region and PDR interface, we find P-th,P-PDR > P-th,P-HII, suggesting the gas is slightly compressed. Therefore, dynamical effects cannot be completely ruled out and even higher angular observations will be needed to unveil their role.