Engineering topics

Bjorkman, K. S.

Publications and source records attributed to Bjorkman, K. S..

24 records · Page 2

NASA NTRS1991

A study of the asymmetric wind of P Cygni

Results of optical linear spectropolarimetric measurements of the hypergiant P Cygni are presented. The data indicate that both the intrinsic polarization and the position angle vary with time. It is found that, prior to removing the interstellar polarization vector, the polarization varies by 0.642 percent, while the position angle takes on values between 27.0 and 39.8 deg. The wavelength dependence of the polarization curve also changes with time, indicating that the intrinsic polarization curve is not always constant with the wavelength. After a model is determined for interstellar polarization, it is found that the amplitude of the intrinsic polarization of P Cyg varies from 0.04 to 0.48 percent, with a change of 0.12 percent on successive nights. It is inferred from this large variation in the polarization on such a short timescale that the scattering material lies close to the star. Several possible geometries to describe the circumstellar material of P Cygni are presented.

Taylor, M.↗

NASA NTRS1989

Highly ionized stellar winds in Be stars. II - Winds in B6-B9.5e stars

The results of a UV survey of stellar winds and circumstellar shells in 40 B6-B9.5e stars covering luminosity classes V-III are presented. A graph is presented of the region from 1520-1560 A, which includes both the Si II UV multiplet 2 transitions and the C IV resonance transition for selected Be stars in the sample. The detection of shortward-shifted discrete component absorption features in nine of the program stars suggests that the material is produced in a stellar wind, which attains velocities of at least a few hundred km/s.

Grady, C. A.↗

NASA NTRS1988

Recurrent shell infall events in a B0.5e star: HD 58978 1979-1988

Infall from the circumstellar envelope onto the bright B0.5 IVe star, HD 58978 was studied. The IUE data indicate that the star was surrounded by a low and moderately ionized circumstellar shell at least 12 times between 1979 and 1988. During 6 of these episodes, the signatures of cool circumstellar material were redshifted with respect to the photosphere by 20 to 80 km/sec. The data indicate that the transition from infall to minimal shell absorption can occur in under 10 days, and are consistent either with infall phases lasting up to 6 months, or with infall episodes shorter than 10 to 15 days. The long term behavior of the shell episodes is compared with variability in the stellar wind.

Grady, C. A.↗

NASA NTRS1988

Stellar winds in two massive Herbig Be stars

A survey of the IUE archival spectra of Herbig Be stars, revealed 2 stars, HD 200775 and HD 259431, with winds which differ from those seen in more evolved stars and from the winds observed in the cooler Herbig Ae stars. The winds are strong, show a wide range of ionization from C IV to Mg II and Si II, and exhibit pronounced ionization gradients as a function of radial velocity. These winds are present at v sin i less than or = 100 km/sec, suggesting that winds from massive pre-main sequence Be stars extend to higher latitudes than those observed in more evolved stars. The presence of well-developed discrete absorption components, together with stronger winds than can be accounted for based on the stellar luminosity, suggests that mechanisms other than simple radiation pressure are important in initiating and driving stellar winds from these stars.

Grady, C. A.↗

NASA NTRS1987

Radio detection of the Be star Psi Persei

The results of a VLA survey for radio emission from a sample of Be stars having strong excess emission in the IRAS bands are presented. Radio emission was detected from one star, Psi Persei. The radio flux density, together with nearly simultaneous ultraviolet and near-infrared observations, argue strongly against a spherically symmetric model for the circumstellar envelope, thus favoring equatorial disk models. In general, it is found that the spectral index of the excess emission increases between the radio and far-infrared, suggesting either increased acceleration of the outflow at large distances from the star, partial recombination of the outer regions of the disk, or divergence of the equatorial disk at large radii.

Taylor, A. R.↗

NASA NTRS1987

Highly ionized stellar winds in Be stars - The evidence for aspect dependence

The paper presents the results of an ultraviolet survey of stellar winds in 62 Be and 43 normal B stars covering spectral types B0.5-B5 and luminosity classes V-III. Excess wind absorption in Be stars over that detected in normal B stars is found in C IV and Si IV, confirming results of previous surveys. The stars showing excess wind absorption also show one or more shortward-shifted discrete absorption components, suggesting that much of the excess wind absorption is in the form of discrete absorption components. Evidence is presented that the high-velocity and highly ionized stellar wind observed in some Be stars is a function of latitude.

Grady, C. A.↗