Structured velocity field in the inner envelope of B335: ALMA observations of rare CO isotopologues

Abstract

Context. Studies of Class 0 objects allow to characterize the dynamical processes taking place at the onset of the star formation process and to determine the physical mechanisms responsible for the outcome of the collapse. Observations of dense gas tracers allow for the characterization of key kinematics of the gas that are directly involved in the star formation process, such as infall, outflow, and rotation. Aims. This work is aimed at investigating the molecular line velocity profiles of the Class 0 protostellar object B335 and attempts to place constraints on the infall motions happening in the circumstellar gas of the object. Methods. We present observations of C17O (1–0), C18O (1–0), and 12CO (2–1) transitions along with an analysis of spectral profiles at envelope radii between 100 and 860 au. Results. C17O emission presents a double-peaked line profile distributed in a complex velocity field. Both peaks present an offset of 0.2–1 km s−1 from the systemic velocity of the source in the probed area. The optical depth of the C17O emission has been estimated and found to be less than 1, suggesting that the two velocity peaks trace two distinct velocity components of the gas in the inner envelope. Conclusions. After discarding possible motions that could produce the complex velocity pattern, such as rotation and outflow, we conclude that infall motions are responsible for producing the velocity field. Because inside-out symmetric collapse cannot explain those observed profiles, it is suggested that these are produced by non-isotropic accretion from the envelope into the central source along the outflow cavity walls.

Magnetically regulated collapse in the B335 protostar? II. Observational constraints on gas ionization and magnetic field coupling

“…The imaging parameters used to obtain all the spectral cubes and their final characteristics are shown in Table 1. Even though the characteristics and properties of the C 17 O (J=1-0) maps are slightly different than the ones presented in Cabedo et al (2021), due to the differences in the imaging process, the line profiles show the same characteristics, i.e., line profiles are double-peaked and present the same velocity pattern at different offsets from the center of the source, confirming that the imaging process has no large effect on the shape of the line profile, and that the two velocity components can still be observed.…”

“…Final images of the data were generated from the calibrated visibilities using the tCLEAN algorithm within CASA, using Briggs weighting with a robust parameter of 1 for all the tracers. Since we want to compare our data to the C 17 O (J=1-0) emission presented in Cabedo et al (2021), we adjust our imaging parameters to obtain matching beam maps with similar angular and spectral resolution. For DCO + (J=3-2) and H 13 CO + (J=3-2) we restricted the baselines to a common u,v-range, between 9 and 140 kλ, and finally smoothed them to the same angular resolution.…”

“…Since all the molecules present a double-peaked profile with similar velocity patterns as the ones observed in Cabedo et al (2021), namely two different velocity components with varying intensity across the source spatial extent, we use two independent velocity components to fit simultaneously the line profiles. Following Cabedo et al (2021), we modeled the spectrum at each pixel by using the HfS fitting program (Estalella 2017).…”

“…Since all the molecules present a double-peaked profile with similar velocity patterns as the ones observed in Cabedo et al (2021), namely two different velocity components with varying intensity across the source spatial extent, we use two independent velocity components to fit simultaneously the line profiles. Following Cabedo et al (2021), we modeled the spectrum at each pixel by using the HfS fitting program (Estalella 2017). We obtained maps of peak velocity and velocity dispersion for each velocity component, deriving the corresponding uncertainties from the χ 2 goodness of fit (see Estalella 2017, for a thorough description of the derivation of uncertainties).…”

“…Asymmetric double-peaked line profiles observed in the molecular emission of the gas toward the envelope have been interpreted as optically thick lines tracing infalling motions, and have been extensively used to derive mass infall rates from 10 −7 to ∼3×10 −6 M yr −1 at radii of 100-2000 au (Zhou et al 1993;Yen et al 2010;Evans et al 2015). However, new observations of optically thin emission from less abundant molecules revealed the presence of two velocity components tracing non-symmetric motions at these envelope scales, which could contribute significantly to the double-peaked line profiles (Cabedo et al 2021). These results have suggested that simple spherically symmetric infall models might not be adequate to describe the collapse of the B335 envelope, and tentatively unveiled the existence of preferential accretion streamlines along outflow cavity walls.…”