Massive stars lose mass during their lifetimes by means of radiation pressure acting on resonance lines of abundant ions in their outer atmospheres and forming a stellar wind. Morrison's research aims to disocver how the characteristics of the stellar wind depend on stellar characteristics such as temperature, gravity, and chemical composition. To this end, the winds are studied through observations of ultraviolet resonance lines with the IUE and other satellites, while the basic stellar characteristics are estimated from a model-atmosphere analysis of spectral line shapes and strengths obtained here at Ritter and at other observatories on the ground.
After the end of hydrogen burning in their cores, massive stars leave the main sequence, evolving to medium-temperature supergiant stars. In order to learn more about their evolution, Morrison is studying the highly time-variable atmospheres of these stars. Some supergiant stars may, however, be low-mass stars that have already left the asymptotic giant branch, in a stage of evolution in which their spectra mimic those of massive supergiant stars. A famous example is the pulsating star UU Herculis, which has been well studied photometrically but not spectroscopically. Morrison has begun a radial-velocity study of this star and of similar stars.
J.A. Grigsby, N.D. Morrison, and L. S. Anderson, "Non-LTE, line-blanketed model atmospheres for late O- and early B-type stars," Astrophys. J. Supp. Ser. 78, 205-237, 1992.
J.A. Grigsby and N.D. Morrison, "The Threshold for Stellar Winds in Hot Main-Sequence Stars," Astrophys. J. 442, 794-811, 1995.
G. Rauw, N. D. Morrison, J.-M. Vreux, E. Gosset and C. L. Mulliss, "The Spectral Variability of HD 192639 and Its Implications for the Star's Wind Structure," Astron. Astrophys., 366, 585-597, 2001.
W. J. Fischer and N. D. Morrison, "Spectrum Variability of the A-Type Supergiant Star HD 223960," Publ. Astron. Soc. Pacific, 113, 821-828, 2001. (PDF)
N. Markova, N. Morrison, I. Kolka, and H. Markov, "P Cygni in a Short S Dor Phase. Spectroscopic and Photometric Evidences," Astron. Astrophys., 376, 898-905, 2001.
J. P. Wisniewski, N. D. Morrison, K. S. Bjorkman, A. S. Miroshnichenko, A. C. Gault, J. L. Hoffman, M. R. Meade, and J. M. Nett, "Spectroscopic and Spectropolarimetric Observations of V838 Monocerotis," Astrophys. J., 588, 486-493, 2003.
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