Anne B. Underhill

C IV 5806 IN WOLF-RAYET STARS

It is shown that the observed differences between the equivalent widths of the C IV λ5806 line in WC and WN stars can be understood using the fractional populations of the levels of the C atom predicted using a simple theory and the solar value for NC/NH. The dominant factor causing the difference in line strength between WC and WN types is the electron temperature. The electron density is a second significant factor. The line-emitting regions (LERs) of WN stars seem to have lower density than the LERs of WC spectra.

Concerning the interpretation of the ultraviolet radiation from B stars

Measurements of the shape of the ultraviolet spectrum from B stars are compared with the theoretical spectra predicted from a homogeneous series of eight model atmospheres which are known to be close to a state of radiative equilibrium and to give a good representation of the ordinarily observed spectral region. The broad-band photometer measurements of Byram, Chubb, and Friedman in the region λ 1314 indicate that the stars become brighter in the ultraviolet as their temperature increases. The theoretical spectra reproduce this trend. However, the theoretical spectra are about three times as bright at λ 1314 relative to their brightness at λ 5560 as is observed.The spectral observations at 50Å resolution of Stecher and Milligan of six absorption-line stars are compared in detail with theoretical spectra. The observed shape of the spectrum is reproduced well by the models from λ 2600 to longer wavelengths. At wavelengths shorter than λ 2600 Å, the observed fluxes from B stars are less than the predicted fluxes. At λ 2000 the deficiency is between a factor two and a factor four. The spectrum of ɛ Canis Majoris is observed to have a different shape from that found for four other early-type stars. In the case of ɛ Canis Majoris the deficiency at λ 2000 is about a factor 13.The proper manner in which to compare theory and observation is discussed and some astrophysical terminology is explained. Theoretical fluxes, ℱλ, are given in Table 1 for eight early B type model atmospheres at wavelengths between the Lyman limit and λ 6251. These fluxes have been computed without consideration of the opacity due to line blanketing. It is shown that line blanketing can probably account for the differences noted between predicted and observed ultra-violet spectra of B stars. It is not necessary at present to invoke unusual sources of opacity in the stellar atmosphere or in the space between the star and the earth in order to explain the observations. Spectra of B stars in the λ 2000 region at sufficient resolution to show the line spectrum would clarify the problem.

A SPECTROSCOPIC ORBIT FOR THE O8 IF STAR 9 SGE

We performed a period search of the 109 published radial velocities of 9 Sge (HD 188001) obtained since 1921 and found a dominant period of 78.74 day plus its first harmonic. We confirm earlier suggestions that the O8 If star 9 Sge is an SB1 binary of small amplitude (K ~ 11 km s -1). Our improved set of orbital elements indicate the companion may be a small star whose mass is probably in the range 1.2 - 1.9 solar mass. This permits a non-degenerate dwarf companion as early as A-type, which would not be detectable in contrast with the bright primary star.

The riddle of the Wolf-Rayet stars

The history of Wolf-Rayet stars since their discovery is noted and a brief review is given of the major studies of these rare emission-line stars. One goal of the paper is to determine what the spectra of Wolf-Rayet stars imply concerning the physical state of the atmospheres of Wolf-Rayet stars. A second goal is to determine what the relationship is, if any, between a particular Wolf-Rayet subtype and the evolutionary state of a massive star. The characteristic features of Wolf-Rayet spectra are reviewed and the typical masses, radii, effective temperatures, and rates of mass loss of Population I Wolf-Rayet stars are noted. The use of modeling as a tool for determining the physical state of a Wolf-Rayet atmosphere is reviewed and evaluations are given of the analyses which have been published of Wolf-Rayet spectra. It is concluded that Wolf-Rayet stars appear to have effective temperatures in the range from 25,000K to 30,000K, normal (solar) composition, a density of the order of 1010cm-3 in the emission-line forming regions, and an electron temperature in the range from 50,000K to 105K in the case of WC stars, but greater than 105K in WN stars. The mantles of Wolf-Rayet stars are very extensive, at least 1014cm in radius.

Filaments, Disks, and Jets for of and Wr Stars

It is noted that the spectra of Wolf-Rayet stars can be well described in terms of emission-line formation in a moderate density wind, a ring-like disk, and magnetically supported filaments which link the disk to the central star. The emission-line spectra of Of, O(f), and O((f)) stars likewise can be understood in terms of jets, filaments, and disks. Some examples of Of spectra are described and it is noted that expanding-spherical-wind models do not appear to be able to describe accurately all which is observed. When interpreting the spectra of Wolf-Rayet and O stars, it is useful to think in terms of the type of model which has proved useful for interpreting the emission-line spectra of Herbig Ae/Be and T Tauri stars.

Why Wolf-Rayet Stars should not be Considered to be Evolved Cores of Massive Stars

Maeder uses three criteria to define the Wolf-Rayet stage of stellar evolution, namely (1) anomalous surface abundances like those in nuclear processed material, (2) an effective temperature higher than 30,000 K, and (3) a dense wind with a rate of mass loss greater than or equal to 10−5 M ⊙ yr−1. These are properties which can all be determined from the observations. I think that spectroscopic observations from X rays to radio wavelengths of Wolf-Rayet stars show that Wolf-Rayet stars do not have these properties. Rather the observations show that Wolf-Rayet stars have properties typical of young stellar objects barely arrived on the main sequence and still surrounded by a line-emitting region (LER) which geometrically resembles the LERs of young low-mass stellar objects such as Herbig Ae/Be stars and T Tauri stars.

A NEW PUZZLE REGARDING THE DIFFUSE INTERSTELLAR BANDS

In the case of 5 O stars out of 14, the average difference between the line-of-sight velocity component shown by the diffuse interstellar bands (DIBs) and that shown by the interstellar Na I lines exceeds the range ± 10 km s -1. For 9 O stars this difference lies in the range ± 6 km s-1. What does this mean? Because the typical standard deviation of vDIB from three DIBs is 3.8 ± 1.2 km s-1 while that of vNaI from two interstellar lines is 1.8 ± 1.3 km s-1, the larger values of vDIB - vNaI are meaningful. They imply that the grains or molecules carrying the DIBs move with the interstellar Na atoms along the lines of sight to only some O stars.

Lambda Persei : an unusual B9 IV star

Spectrograms of Lambda Per obtained at 30 A/mm in the red region show weak emission peaks in the core of the H-alpha absorption line. The V/R for the emission components changed from about 1.0 on October 28 1990 UT to less than 1.0 on October 29, 1990 UT. The midpoints of absorption lines from He I, Si II, Fe II, and the midpoints of the H-alpha profile measured at residual intensities between 0.80 and 0.96 are shifted slightly shortward of the wavelengths suggested by the catalog radial velocities of Lambda Per and by the wavelength of the sharp central absorption core of H-alpha. It is plausible that the emission features originate in plage areas on the surface of Lambda Per, and that the aspect of the plages presented to the observer changes as the star rotates. Plages imply the presence of surface magnetic fields.

V444 Cygni and CQ Cephei, Key Wolf-Rayet Binary Stars

The fundamental properties of the Wolf-Rayet components of V444 Cygni and CQ Cephei such as mass, effective temperature, and luminosity are presented. These properties are consistent with these stars being young objects recently arrived on the main sequence. The properties of evolved, peeled-down model stars are not consistent with the observed properties of V444 Cygni and CQ Cephei.

Evidence That Wolf-Rayet Stars are Pre-Main Sequence Objects

The evidence is reviewed that Population I Wolf-Rayet stars have solar abundances, that they are surrounded by remnant disks formed from their natal cloud, and that their rate of mass loss is moderate. These properties are consistent with Wolf-Rayet stars being young objects recently arrived on the main sequence rather than the evolved, peeled-down remnants of massive stars.