Lawrence M. Chernin

Properties of jet-driven molecular outflows

We present a critical review of the observed properties of molecular outflows from young stellar objects and show that these properties can be explained by a model in which the molecular flow is driven solely by a collimated jet. Many of the best studied outflows have several characteristics in common, including (1) bipolarity, (2) collimation, (3) partly empty lobes, (4) most material at low velocities, (5) extremely high velocity features, and (6) average momentum directed nearly parallel to the axis of the flow, with small transverse components. From consideration of the conservation of vector momentum, we deduce that energy-driven flows cannot produce the observed distribution of the momentum direction

Momentum Transfer by Astrophysical Jets

We have used 3-D smoothed particle hydrodynamical simulations to study the basic properties of the outflow that is created by a protostellar jet in a dense molecular cloud. The dynamics of the jet/cloud interaction is strongly affected by the cooling in the shocked gas behind the bow shock at the head of the jet. We show that this cooling is very rapid, with the cooling distance of the gas much less than the jet radius. Thus, although ambient gas is initially driven away from the jet axis by the high thermal pressure odf the post-shock gas, rapid cooling reduces the pressure and the outflow subsequently evolves in a momentum-conserving snowplow fashion. The velocity of the ambient gas is high in the vicinity of the jet head, but decreases rapidly as more material is swept up. Thus, this type of outflow produces extremely high velocity clumps of post shock gas which resemble the features seen in outflows. We have investigated the transfer of momentum from the jet to the ambient medium as a function of the jet parameters. We show that a low Mach number (<6) jet slows down rapidly because it entrains ambient material along its sides. On the other hand, the beam of a high Mach number jet is separated from the ambient gas by a low density cocoon of post-shock gas, and this jet transfers momentum to the ambient medium principally at the bow-shock. In high Mach number jets, as those from young stellar objects, the dominant interaction is therefore at the bow shock at the head of the jet.

Properties of swept-up molecular outflows

Models of momentum-conserving, molecular outflows swept up by radially directed stellar winds from young stellar objects are analyzed, and the new constraint that the models should reproduce the observed spectral line shapes of outflows, which trace the amount of mass as a function of velocity, is introduced. The models can easily reproduce the spatial shape of a typical outflow and the appearance of the highest velocity at the edge of the flow, but they have difficulty in reproducing the observed spectral line shape

The HCO+ Molecular Outflow in NGC 2071

We present high angular resolution and multitransition HCO+ observations toward the NGC 2071 molecular outflow. Comparison of the high-velocity (HV) HCO+ and the near-IR H2 in the molecular outflow shows a clear correlation. At high HCO+ flow velocities the spatial coincidence is especially remarkable. In addition, the HV HCO+ presents clear morphological and kinematical differences with the CO outflow. These differences appear not only in the HV HCO+ emission associated with the H2 but in the overall outflow. There is a clear HCO+ emission enhancement, relative to CO, at increasing flow velocities. This enhancement is probably due to an abundance enhancement produced by a velocity-dependent chemistry in the shocks. An overabundance of CH in low Mach shocks may cause the HCO+ abundance enhancement. Because of the short cooling time for H2, the correlation between the HCO+ and the H2 implies that HCO+ emission can provide a useful tool to study in detail the current interactions of protostellar winds with the dense ambient medium. At the position of the extremely high velocity (EHV) CO component in the red lobe we detect HCO+ (J = 3 → 2) emission within the velocity range of the EHV CO gas. This emission is roughly compatible with the expected HCO+ emission associated with EHV gas arising from behind dissociative shocks.

A nearly unipolar CO outflow from the HH 46-47 system

We have made observations of CO in the molecular outflow centered on the HH 46-47 system, which is at the northern end of the dense Bok globule 210-6A. The molecular outflow is asymmetric, with the redshifted part of the flow being more extensive and massive than the blueshifted part, although the appearance of the HH objects suggests that the underlying flow is symmetric. Using the CO J=2-1 and J=3-2 lines we find that the total mass and momentum of the molecular outflow are 0.3 and 1.9 M ⊙ km s −1 , respectively, with the redshifted CO flow accounting for ∼80% of these values.

Observations of SO and SiO in the outflow from NGC 2071

We have detected SO J K =6 5 -5 4 and SiO J=5-4 emission from the lobes of the NGC 2071 outflow. In the middle of the northern lobe there is a broad high-velocity feature with velocities of up to 30 km s −1 relative to that of the ambient cloud. The spatial and velocity distributions of SO and SiO are unlike those of any other molecule in the outflow and we suggest that this is due to a velocity-selective enhancement caused by shocks

CO J=6–5 Observations of Protostellar Outflows

Although the CO J=1−0 transition is the most frequently used tracer of molecular outflows from protostars, CO J=6−5 is a better probe of warm, dense gas in outflows, since the upper state energy of the J=6 level is 110 K above ground in comparison to 5.5 K for J=1. In this short contribution we compare CO J=6−5 to lower J observations of the NGC 2071 outflow, and concentrate the discussion on the outflow structure, overall density and temperature, and the extremely high velocity (EHV) CO features. We present data here for the NGC 2071 outflow, but in an associated journal article we will also present data on several other outflows. The CO J=6-5 data were obtained in December 1994 at the Caltech Submillimeter Observatory (CSO) on Mauna Kea, Hawaii.