The SW part of the core is still interacting with the outflow. The shock associated with the outflow could have already propagated through this part of the core resulting in narrow, undisturbed line profiles in the cold, compressed post-shock gas, blueshifted with respect to the systemic velocity of the ambient cloud. The most quiescent, cold gas, characterized by narrow line widths is found in the NE part of the core. The change in the NH 2D line velocity and width across the core provides clear evidence of an interaction with the outflow, traced by the high-velocity water emission. This source is characterized by some of the highest deuteration levels seen in the interstellar medium.
Herschel, ALMA Compact Array (ACA), and Caltech Submillimeter Observatory (CSO) observations have now provided new insights into the structure of the prestellar core in L1689N, which has been suggested to be interacting with a molecular outflow driven by the nearby solar type protostar IRAS 16293-2422. La structure hyperfine ajustée est indiquée en rouge. (En bas à gauche) Spectres N2D+ et ND3 observés avec ACA (panneaux gauche et droit, respectivement) vers les pics d’émission respectifs. (En haut à gauche) Intensité intégrée de la raie de N2D+ dans L1689N observée avec ACA (image en couleurs et contours blancs), avec superposition des contours noirs de l’émission ND3. Les flèches rouges et bleues marquent les directions du flot compact en molécule CO. La proto-étoile proche de type solaire IRAS 16293-2422, origine du flot moléculaire, est représentée par les contours noirs de l’émission excitée de SO. (A droite) Image couleur de l’intensité intégrée de la raie d’émission de l’eau dans L1689N observée avec HIFI, traçant le flot moléculaire, avec les contours blancs superposés de l’intensité intégrée de l’émission NH2D, qui révèle l’emplacement du noyau pré-stellaire. This in turn drives up the gas-phase abundance and fractionation of H 3+. High deuteration of gas-phase species is, in effect, a direct result of the freeze-out, and coincidental disappearance of ortho-H 2 from the gas phase, without which no deuteration would happen. Deuterated species, including ammonia isotopologues or N 2D+, form the second exception. However, for reasons that are not fully understood, nitrogen-bearing species, in particular ammonia, do not seem to participate in this freeze-out. This reflects the condensation of species onto ice mantles at high densities. Likewise, molecular observations are also known to provide a biased view of starless cores. Moreover, dust studies do not provide direct insight into the dynamics of these cores nor into their chemistry. However, continuum observations yield only a partial picture of the cloud structure, since coagulation of dust is a key process at the high densities of inner starless cores, which changes the grain opacity coefficient and effectively hides much of the mass of the dust from view.
Much of our insight into the structure of starless cloud cores comes from observations of the millimeter dust emission. Yet, it is key to understanding some of its most fundamental aspects, such as the initial mass function, the binarity fraction and its dependence on stellar mass, and the star formation efficiency. The initiation of this process is among the least understood steps of star formation. Low-mass star formation is known to occur exclusively in the shielded interiors of molecular cloud cores, when gravity wrests control from supporting thermal, magnetic and turbulent pressures and collapse ensues.
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