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Overview of Active OB Stars

In contrast to their classical ideal as constant, spherical, radiative envelopes, the hot, luminous, OB-type stars often exhibit signatures of activity, with associated variability and structure on a range of temporal and spatial scales. For example, spectral monitoring shows that even “normal” OB stars commonly exhibit variable Discrete Absorption Components (DACs) in UV lines formed in their stellar wind. And certain special classes – e.g. Be, B[e], LBV, Bp stars – are in effect defined by their particular signatures of activity. A common element is often the emission and/or absorption by circumstellar material. Thus a general theme here is to consider the physical mechanisms that can drive material off the nominally tightly bound hydrostatic stellar surface. Specifically I discuss the dynamical roles played by radiation, rotation, pulsation, and magnetic fields, and how these, individually or in combination, can eject the circumstellar clouds, disks, and/or mass outflows that seem to be at the root of many observed signatures of hot-star activity. In emphasizing a framework of physical commonalities, an overall goal is to foster exchanges among researchers specializing in the various classes of active OB stars.