The following are examples of my research activities in the astronomy and astrophysics of OB star photospheres:

Figure 1: Sensitivity of UV Si II and III lines to temperature (right) and luminosity (left) in B stars. Symbols: filled = IV & V, half filled = III, belted circles = II, open circles = Ib & Iab, crosses = Ia (from Massa 1989).

Figure 2: Example of the progression of photospheric velocity gradients in luminous B star. The spectra are aligned on their relatively weak C II lines and it is evident that the strong Si III triplets become progressively asymmetric and blue shifted as the stellar luminosity increases (from Massa et al. 1992)

Research activities:

I have been interested in the physics of OB star winds for a number of years. Most recently, I have been collaborating with Prinja and Fullerton on several studies of wind variability in B supergiant winds. Some of my work in this field includes:

Demonstration of the effects of rotational in the late O star HD 93521 (Massa 1995). Because the peculiarities in both the wind and photospheric lines of HD 93521 are consistent with those expected from rapid rotation, this analysis probably provides the best case for a gravitationally reddened star.
Demonstration that radial velocity gradients are present in the photospheres of luminous B stars (Massa et al. 1992). By taking advantage of the enormous strength of the UV silicon lines in B stars and the wide range of line strengths, we provided conclusive evidence for velocity gradients in the line forming regions of luminous B stars.
The establishment of a silicon based UV spectroscopic classification system for B stars (Massa 1989). By confining the classification criteria to silicon liens, this approach provides an excellent tie in to Walborn's optical system.
Determination of the intrinsic UV continua of O stars (Massa et al. 1985, Massa & Conti 1981). By concentrating on open clusters containing O stars and using the cluster main sequence B stars to determine the extinction and test its variability, this work remains the only consistent determination of the UV continua of O stars.
Quantified the magnitude of the error introduced to dereddened UV continuum incurred by applying a ``mean'' extinction curve (Massa 1987). This analysis established that extremely large errors can be introduced by dereddening a stellar continuum with a mean curve.

Some relevant publications:

``HD 93521, zeta Oph, and the effects of Rapid Rotation on the Atmospheres and Winds of O9.5 V Stars'', D. Massa 1995, ApJ, 438, 376

``S Mus B Revisited'', N.R. Evans, D. Massa & T.J. Teays 1994 AJ, 108, 2251

``Photospheric Velocity Gradients in B1 Supergiants'', D. Massa, S.N. Shore & D. Wynne 1992 A&A, 264, 169

``The UV Silicon Spectra of Early B Stars'', D. Massa 1989, A&A, 224, 131

``The Variability of UV Extinction-Curve Shapes and Its Impact Upon Dereddened UV Energy Distributions'', D. Massa 1987, AJ, 94, 1675

``The Intrinsic Ultraviolet Continua of O Stars'', D. Massa & B.D.Savage 1985, ApJ, 299, 905

``Main-Sequence B Stars with Strong Winds in the Core of NGC 6231'', D. Massa, B.D. Savage, & J.P. Cassinelli 1984, ApJ, 287, 814

``IUE Observations of Eight OB Stars in NGC 2244: UV Continua and Extinction'', D. Massa & P.S. Conti 1981, ApJ, 248, 201

``Vector Space Methods of Photometric Analysis. II. Refinement of the MK Grid for B Stars'', D. Massa 1980, AJ, 85, 1644

``Vector Space Methods of Photometric Analysis: Applications to O Stars and Interstellar Reddening'', D. Massa & C.F. Lillie 1978, ApJ, 221 833

``The influence of Rotation and Stellar Winds upon the Be Phenomenon'', D. Massa 1975, PASP, 87, 777

Derck Massa,
Emergent IT