Evidence for Multiple Outbursts from the Cepheus A Molecular Outflow

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Abstract

We report evidence for multiple episodes of outflow activity in the Cepheus A star-forming region. We present new, high signal-to-noise CSO observations of 12CO J = 3 2, 13CO J = 3 → 2, and CS J = 7 → 6 emission. We also present new, interferometric and single-dish observations of 12CO J = 1 → 0 emission toward the Cepheus A molecular outflow. Using line core velocity centroid maps, we argue that the multiple self-absorption features in the CO J = 3 → 2 line profiles are tracing cool shells of material swept-up by an episodic outflow. We present the results of a flexible three-dimensional LTE outflow model and radiative transfer code that best explain the observations as tracing multiple generations of outflow. The difference in the dynamical timescales between the "old" and "new" swept-up shells gives an estimate of ∼1.6 x 105 yr between the two generations of outbursts. The high-resolution 12CO J = 1 → 0 maps obtained by combining single-dish observations with interferometric data clearly show a shell-like morphology at low velocities. This cool shell appears to encompass the hot, extremely high velocity (EHV) winds seen in the J = 3 → 2 transition. The interferometric observations show that the current generation of outflow is being powered by the object Cepheus A-HW 2. There is also evidence for redirection of the blueshifted lobe of the current generation of outflow, possibly by the extended NH₃ structure Cep A-3. We present a model of the outflow geometry that can explain most of the observed structures in Cepheus A. The rotating, dense core traced by the CS observations is ∼0.32 pc in diameter and has an estimated dynamical mass of 330 M_sun. The velocity structure of the core suggests that it is being disrupted by the high-velocity winds driving the molecular outflow. This new technique of extracting information from self-absorbed line profiles could be used to study other deeply embedded protostellar systems. Since outflows are believed to be intimately tied to accretion, such studies could lead to constraints on mass accretion models for young stellar objects.

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