Engineering topics

Hartmann, L.

Publications and source records attributed to Hartmann, L..

61 records · Page 4

NASA NTRS1979

Ultraviolet observations of AM Herculis with IUE

Ultraviolet observations of the binary X-ray source AM Herculis from the IUE satellite show strong emission lines of ions from O I to N V, probably originating in photoionized gas, and a continuum which is partially eclipsed in phase with the X-ray eclipse. The emission lines have broad (about 600 km/s) and sharp (about 80 km/s) components at different velocities, as has been seen in optical lines. The continuum is interpreted as two components, a blackbody (blackbody kT of 25-30 eV) which accounts for the X-ray emission below 0.5 keV and the eclipsed part of the UV continuum, and a component which is never eclipsed, whose spectrum is roughly proportional to the inverse of frequency. Strong ultraviolet emission due to optically thick cyclotron emission, which has been predicted theoretically, is not observed. This presents a severe difficulty for the theory of accretion onto the magnetic pole of a white dwarf.

Raymond, J. C.↗

NASA NTRS1979

Studies of interstellar absorption lines with IUE

The paper deals with a survey of interstellar absorption lines in the wavelength region from 1150 to 3200 A toward 25 stars. The spectra of 10 of these stars show very strong narrow absorption lines of Si IV and C IV. Lines of N V appear in eight directions. The N V lines can arise in the same gas that is responsible for the ubiquitous interstellar lines of O VI, observed previously with a Copernicus spectrometer, but the lines of Si IV and C IV must be formed in a cooler gas. The measured lines of Si IV and C IV can be produced by normal H II regions photoionized by the observed stars.

Black, J. H.↗

NASA NTRS1979

Chromospheres and coronae of late-type stars

IUE short wavelength (1150-2000 A) spectra of late-type dwarfs, giant, and supergiant stars show a variety of emission features varying in excitation from about 10,000 K to about 3 x 10 to the 5th K. High excitation species are found most strongly in main sequence stars. Dwarf stars with active chromospheres (e.g., Xi Boo) and flare stars (EQ Peg) show enhancement of surface flux as compared to the quiet sun; binary systems of W UMa and RS CVn types show even larger surface fluxes. The enhancement increases with temperature of formation much like a solar active region. The presence of high excitation species in the giant and supergiant stars suggests that parameters other than effective temperature and luminosity are important in establishing a transition region and corona.

Dupree, A. K.↗

NASA NTRS1979

IUE observations of the X-ray source HZ Herculis/Her X-1

The results of a campaign of monitoring HZ Her with the IUE satellite are presented. The spectrum and light variation outside of eclipse are fairly consistent with the Milgrom and Salpeter (1975) model for emission from the X-ray heated photosphere of the primary. Near eclipse there is UV flux in excess of the prediction of the heated photosphere model. This suggests an additional emitting component whose relative importance decreases toward shorter wavelengths. The excess flux near 1500A appears to vary with 35 day phase in a manner consistent with the model of Gerend and Boynton (1976) in which the flux contributed by the accretion disk is modulated by precession.

Gursky, H.↗

NASA NTRS1979

Ultraviolet observations of globular clusters

Ultraviolet spectra of six globular clusters (M15, M92, NGC 1851, NGC 6624, 47 Tuc, and NGC 6752) have been taken with the International Ultraviolet Explorer Satellite in order to investigate the surface brightness distributions in the central regions. The ultraviolet emission indicates composite stellar spectra. Short wavelength emission (1200-1900 A) arises from blue horizontal branch stars; long wavelength emission (2100-3000 A) is characteristic of late type horizontal branch and giant stars. The surface brightness distribution is more compact at short wavelengths than at long wavelengths, suggesting a segregation of horizontal branch stars. A dense core appears to be present in X-ray emitting clusters and absent in those clusters without X-ray sources. The X-ray source in NGC 6624 may have been detected at short wavelengths, (1300-1900 A).

Gursky, H.↗

NASA NTRS1978

Long-term variability of dMe stars

Photographic B magnitudes of the four spot stars BY Dra (dM0e), CC Eri (K7 Ve), AU Mic (dM2.5e), and YZ CMi (dM4.5e) are reported which considerably extend the time baseline of the photoelectric data on these stars. The long-term variation of the stars in yearly mean light is investigated. It is found that both BY Dra and CC Eri exhibit long-term B-magnitude variations with amplitudes of about 0.3 to 0.4 magnitude and times between maximum and minimum light near 30 yr. No apparent variability is observed for AU Mic and YZ CMi. The possibility is discussed that the variability of BY Dra and CC Eri is connected with a starspot cycle, for which the period would be around 60 yr.

Phillips, M. J.↗

NASA NTRS1978

Nebular observations and stellar coronae

Copernicus UV observations of O VI in early-type stars are used to infer the existence of coronae or hot stellar winds. Attention is given to the optical doublet of O VI at wavelengths 3811 and 3834, which is seen in the spectra of some central stars in planetary nebulae, and taken to be evidence of a coronal wind. It is noted (from observations of NGC 6751) that hot gas in the coronal model produces radiation at energies above 54 eV, which can affect He II Zanstra temperatures. He(+) ionizing radiation in population I early-type stars is also taken as an indication of hot winds.

Hartmann, L.↗