Boundaries in the H-R Diagram

There are three fundamental boundaries in the H-R diagram that have been determined from observations.

1. The Granulation Boundary runs nearly vertically through the middle of the diagram. On the cool (right hand) side, stars have velocity fields in their photospheres that are solar-like. There are systematic changes across the cool half of the diagram, with generally declining strength toward cooler and fainter stars. On the hot half (left hand side) of the diagram, the velocity fields are often larger and have a different character. The observations delineating the Granulation Boundary come from the asymmetries in spectral line profiles. Asymmetries are usually discussed using the bisectors of spectral lines.
2. The Rotation Boundary parallels the Granulation Boundary and separates the rapid rotators on the hot side from the slow rotators on the cool side. Magnetic braking is almost certainly the cause of the slow rotation, along with changes in moment of inertia as stars evolve. As stars evolve from the main sequence across the Rotation Boundary, the loss of angular momentum in at least the convective envelope is very abrupt. On the cool side of the boundary, rotation depends only on position in the H-R diagram, a result thought to result from the control of rotation by the parameters of the convection through the Rotostat Mechanism.
3. The Coronal Boundary maps the loss of x-ray emission which comes from the coronae of stars. Most likely the coronae disappear when a star evolves into this portion of the diagram because rotation becomes very slow; too slow to drive the dynamo generation of magnetic fields that are needed to support coronae. Rotation dies away from magnetic braking and the increase in moment of inertia. The existence of the Coronal Boundary has been challenged recently.

References:

• K.G. Strassmeier, F.C. Fekel, D.F. Gray, A.P. Hatzes, J.H.M.M. Schmitt, & S.K. Solanki, 1997, Tenth Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, Cambridge, MA, in press, Evolved stars: what happens to activity off the main sequence.
• David F. Gray 1991 in Angular Momentum of Young Stars, (Kluwer: Dordrecht), S. Catalona & J.R. Stauffer, eds., p. 183, Rotation of Evolved Stars.
• Lectures on Spectral-Line Analysis: F, G, and K Stars, (The Publisher: Arva), 1988, a hard-bound book available now from me for a $15 shipping fee.
• David F. Gray 1986, International Astronomical Union General Assembly, New Delhi, India, in Highlights of Astronomy, (D. Reidel: Dordrecht), J.-P. Swings, ed., pp. 411, Convection as a Regulator of Dynamos.

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