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. The initiation of this process is among the least understood steps of star formation. 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. Much of our insight into the structure of starless cloud cores comes from observations of the millimeter dust emission. 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. Moreover, dust studies do not provide direct insight into the dynamics of these cores nor into their chemistry.

Likewise, molecular observations are also known to provide a biased view of starless cores. This reflects the condensation of species onto ice mantles at high densities. However, for reasons that are not fully understood, nitrogen-bearing species, in particular ammonia, do not seem to participate in this freeze-out. Deuterated species, including ammonia isotopologues or N₂D+, form the second exception. High deuteration of gas-phase species is, in effect, a direct result of the freeze-out, and coincidental disappearance of ortho-H₂ from the gas phase, without which no deuteration would happen. This in turn drives up the gas-phase abundance and fractionation of H₃+.

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. This source is characterized by some of the highest deuteration levels seen in the interstellar medium. The change in the NH₂D line velocity and width across the core provides clear evidence of an interaction with the outflow, traced by the high-velocity water emission. The most quiescent, cold gas, characterized by narrow line widths is found in the NE part of the core. 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 SW part of the core is still interacting with the outflow.

Strong N₂D+ and ND₃ emission is detected with the ACA. The ACA data also reveal the presence a compact dust continuum source, with a mean size of ∼1100 au, a high central density of (1−2) × 10⁷ cm-3, and a mass of 0.2–0.4 Mo. The dust emission peak is displaced to the south with respect to the molecular emission, as well as the single-dish dust continuum peak, suggesting that the northern, quiescent part of the core is characterized by spatially extended continuum emission, which is largely resolved out by the interferometer. There is no clear evidence of fragmentation in the quiescent part of the core, which could lead to a second generation of star formation, although a weak dust continuum source is detected in this region in the ACA data.

There is no evidence in the deuterated ammonia data for the turbulent velocity to vary with radius, as seen in some cores. The 0.4 km s-1 FWHM line width roughly corresponds to the H₂ thermal line width at 7 K and is 3–4 times larger than the expected thermal line width of NH₂D, ND₃, or N₂D+. This shows that the line broadening is mainly non-thermal and that sonic or somewhat sub-sonic turbulent motions are dominant even in the northern, quiescent part of the core. This is different from the typical prestellar cores in Taurus, where line widths are essentially thermal, and is perhaps related to the interaction with the outflow.

These new observations demonstrate the utility of the fundamental rotational transitions of deuterated ammonia as a tracer of the deeply embedded, prestellar phase of star formation. These lines are accessible to the current ground-based submillimeter facilities, in particular ALMA, offering new insights into the early phases of the star formation process.

Reference
Star Formation and Feedback : A Molecular Outflow—Prestellar Core Interaction in L1689N. D.C. Lis, H.A. Wootten, M. Gerin, L. Pagani, E. Roueff, F.F.S. van der Tak, C. Vastel, C.M. Walmsley,
The Astrophysical Journal, in press,