Browsing by Author "Gaudel, Mathilde"

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    Bias versus variance when fitting multi-species molecular lines with a non-LTE radiative transfer model Application to the estimation of the gas temperature and volume density
    (2024) Roueff, Antoine; Pety, Jerome; Gerin, Maryvonne; Segal, Leontine E.; Goicoechea, Javier R.; Liszt, Harvey S.; Gratier, Pierre; Beslic, Ivana; Einig, Lucas; Gaudel, Mathilde; Orkisz, Jan H.; Palud, Pierre; Santa-Maria, Miriam G.; Magalhaes, Victor de Souza; Zakardjian, Antoine; Bardeau, Sebastien; Bron, Emeric; Chainais, Pierre; Coude, Simon; Demyk, Karine; Guzman, Viviana V.; Hughes, Annie; Languignon, David; Levrier, Francois; Lis, Dariusz C.; Le Bourlot, Jacques; Le Petit, Franck; Peretto, Nicolas; Roueff, Evelyne; Sievers, Albrecht; Thouvenin, Pierre-Antoine
    Context. Robust radiative transfer techniques are requisite for efficiently extracting the physical and chemical information from molecular rotational lines. Aims. We study several hypotheses that enable robust estimations of the column densities and physical conditions when fitting one or two transitions per molecular species. We study the extent to which simplifying assumptions aimed at reducing the complexity of the problem introduce estimation biases and how to detect them. Methods. We focus on the CO and HCO+ isotopologues and analyze maps of a 50 square arcminutes field. We used the RADEX escape probability model to solve the statistical equilibrium equations and compute the emerging line profiles, assuming that all species coexist. Depending on the considered set of species, we also fixed the abundance ratio between some species and explored different values. We proposed a maximum likelihood estimator to infer the physical conditions and considered the effect of both the thermal noise and calibration uncertainty. We analyzed any potential biases induced by model misspecifications by comparing the results on the actual data for several sets of species and confirmed with Monte Carlo simulations. The variance of the estimations and the efficiency of the estimator were studied based on the Cramer-Rao lower bound. Results. Column densities can be estimated with 30% accuracy, while the best estimations of the volume density are found to be within a factor of two. Under the chosen model framework, the peak (CO)-C-12 (1 - 0) is useful for constraining the kinetic temperature. The thermal pressure is better and more robustly estimated than the volume density and kinetic temperature separately. Analyzing CO and HCO+ isotopologues and fitting the full line profile are recommended practices with respect to detecting possible biases. Conclusions. Combining a non-local thermodynamic equilibrium model with a rigorous analysis of the accuracy allows us to obtain an efficient estimator and identify where the model is misspecified. We note that other combinations of molecular lines could be studied in the future.
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    Gas kinematics around filamentary structures in the Orion B cloud
    (2023) Gaudel, Mathilde; Orkisz, Jan H.; Gerin, Maryvonne; Pety, Jerome; Roueff, Antoine; Marchal, Antoine; Levrier, Francois; Miville-Deschenes, Marc-Antoine; Goicoechea, Javier R.; Roueff, Evelyne; Le Petit, Franck; Magalhaes, Victor de Souza; Palud, Pierre; Santa-Maria, Miriam G.; Vono, Maxime; Bardeau, Sebastien; Bron, Emeric; Chainais, Pierre; Chanussot, Jocelyn; Gratier, Pierre; Guzman, Viviana; Hughes, Annie; Kainulainen, Jouni; Languignon, David; Le Bourlot, Jacques; Liszt, Harvey; Oberg, Karin; Peretto, Nicolas; Sievers, Albrecht; Tremblin, Pascal
    Context. Understanding the initial properties of star-forming material and how they affect the star formation process is key. From an observational point of view, the feedback from young high-mass stars on future star formation properties is still poorly constrained.Aims. In the framework of the IRAM 30m ORION-B large program, we obtained observations of the translucent (2 <= A(V) < 6 mag) and moderately dense gas (6 <= A(V) < 15 mag), which we used to analyze the kinematics over a field of 5 deg(2) around the filamentary structures.Methods. We used the Regularized Optimization for Hyper-Spectral Analysis (ROHSA) algorithm to decompose and de-noise the (CO)-O-18(1-0) and (CO)-C-13(1-0) signals by taking the spatial coherence of the emission into account. We produced gas column density and mean velocity maps to estimate the relative orientation of their spatial gradients.Results. We identified three cloud velocity layers at different systemic velocities and extracted the filaments in each velocity layer. The filaments are preferentially located in regions of low centroid velocity gradients. By comparing the relative orientation between the column density and velocity gradients of each layer from the ORION-B observations and synthetic observations from 3D kinematic toy models, we distinguish two types of behavior in the dynamics around filaments: (i) radial flows perpendicular to the filament axis that can be either inflows (increasing the filament mass) or outflows and (ii) longitudinal flows along the filament axis. The former case is seen in the Orion B data, while the latter is not identified. We have also identified asymmetrical flow patterns, usually associated with filaments located at the edge of an H II region.Conclusions. This is the first observational study to highlight feedback from H II regions on filament formation and, thus, on star formation in the Orion B cloud. This simple statistical method can be used for any molecular cloud to obtain coherent information on the kinematics.