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At least 541 records · Page 30
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NASA NTRS1986

Chromospheric dust formation, stellar masers and mass loss

A multistep scenario which describes a plausible mass loss mechanism associated with red giant and related stars is outlined. The process involves triggering a condensation instability in an extended chromosphere, leading to the formation of cool, dense clouds which are conducive to the formation of molecules and dust grains. Once formed, the dust can be driven away from the star by radiation pressure. Consistency with various observed phenomena is discussed.

Stencel, R. E.↗
NASA NTRS1986

Modelling extended chromospheres

Attention is given to the concept that the warm, partially ionized plasma (presently called chromosphere) associated with such stars as Alpha Boo and Rho Per extends outwards at least several photospheric radii. Calculations are presented for the Mg II K line in light of two input model atmospheres. Specific predictions are deduced from the results obtained by each of the two models.

Linsky, J. L.↗
NASA NTRS1986

UBV photometry of ten southern hemisphere active-chromosphere stars

High-precision UBV photometry of ten southern hemisphere active-chromosphere stars with strong Ca II H and K and/or H-alpha emission has been obtained. Eight of these stars showed variability during June 1985. Complete or partial light curves are presented for the stars, and these data, as well as mean V magnitudes and colors, are compared with the results of other investigators. In a number of cases, significant changes in photometric amplitude are found, which may serve to track the formation and evolution of active regions on these stars.

Bopp, Bernard W.↗
NASA NTRS1986

Evidence for explosive chromospheric evaporation in a solar flare observed with SMM

SMM soft X-ray data and Sacramento Peak Observatory H-alpha observations are combined in a study of the impulsive phase of a solar flare. A blue asymmetry, indicative of upflow motions, was observed in the coronal Ca XIX line during the soft X-ray rise phase. H-alpha redshifts, indicative of downward motions, were observed simultaneously in bright flare kernels during the period of hard X-ray emission. It is shown that, to within observational errors, the impulsive phase momentum transported by the upflowing soft X-ray plasma is equivalent to that of the downward moving chromospheric material.

Zarro, D. M.↗
NASA NTRS1987

Evidence for gentle chromospheric evaporation during the gradual phase of large solar flares

The Multichannel Subtractive Double Pass Spectrograph of the Meudon solar tower is used to obtain high spatial resolution H-alpha line profiles during the gradual phase of three solar flares. In all cases, small blueshifts lasting for several hours are observed in the flare ribbons. By contrast, the region between the two ribbons exhibits large redshifts that are typical of H-alpha post flare loops. The blueshifts in the ribbons is interpreted as upward chromospheric flows of 0.5-10 km/s, and the possible ambiguities of the interpretation are discussed. A preliminary analysis indicates that such upflows are sufficient to supply the greater than 10 to the 16th g of mass needed to maintain a dense H-alpha postflare loop system in the corona.

Schmieder, B.↗
NASA NTRS1987

Periodic photospheric and chromospheric modulation in Alpha Orionis (Betelgeuse)

The bright cool supergiant Alpha Orionis has been monitored spectroscopically and photometrically over the past three years (1984-1986) in the optical and the ultraviolet wavelength regions. A 420-day periodic modulation of the flux is observed in the optical and ultaviolet continua, and in the Mg II line emission cores. Periodic photospheric pulsations are the most likely explanation of these observations. This identification is based on the large amplitude of the variation, the correlation of the continuum and chromospheric fluxes, and the length of the observed period. Pulsation may heat and extend the atmosphere of Alpha Ori and initiate the mass flow from the star.

Dupree, A. K.↗
NASA NTRS1987

Chromospherically active stars. III - HD 26337 = EI Eri: An RS CVn candidate for the Doppler-imaging technique

Spectroscopic observations of the chromospherically active G5 IV single-lined binary HD 26337 = EI Eri are presented. An orbital period of 1.94722 days is found for the star. It has moderately strong Ca II H and K emission and strong ultraviolet emission features, while H-alpha is a weak absorption feature that is variable in strength. The inclination of the system is 46 + or - 12 deg, and the unseen secondary is probably a late K or early M dwarf. The v sin i of the primary is 50 + or - 3 km/s, resulting in a minimum radius of 1.9 + or - 0.1 solar radius. The star is within the required limits for Doppler imaging. The primary is close to filling its Roche lobe, resulting in a strong constraint that the mass ratio is 2.6 or greater, with a primary mass of at least 1.4 solar mass. The distance to the system is estimated at 75 pc.

Fekel, Francis C.↗
NASA NTRS1987

The rapidly oscillating Ap stars as a test of stellar chromospheric heating mechanisms

The discovery of a new class of magnetic stars, the rapidly pulsating Ap stars, provides an unprecedented tool for the study of Alfvenic heating in the outer atmospheres of main sequence stars. In this note, it is noted that detection of rotationally-modulated chromospheric emission line fluxes, especially the Ca II, Mg II, and C IV lines, can provide a novel method for investigating the role of Alfvenic heating in stellar atmospheres.

Shore, S. N.↗
NASA NTRS1987

A solar chromosphere and spicule model based on far-infrared limb observations

Techniques developed for LTE radiative transfer problems in a rough atmosphere were used to compute a model chromosphere containing spicules consistent with high-resolution solar limb observations from 100 microns to 2.6 mm. The model consists of a smooth, plane-parallel temperature minimum region extending from the photosphere to a height of 1000 km and randomly distributed cylindrical spicules above this height. It is found that the observed limb brightness profiles are well fitted by spicules with electron temperatures on the order of 7000 K.

Braun, D.↗
NASA NTRS1986

Evolution of the chromospheric and coronal activity of intermediate mass stars

Recent ultraviolet and X-ray observations pertaining to the outer atmospheric structure of intermediate mass (4-6 solar masses) stars and the evolution of their structure are presented. A distance-limited (d equal to or less than 200 pc) IUE ultraviolet survey of early K bright giants shows that C IV emission commonly is present. These stars are almost evenly split between stars showing hybrid-chromospheric and coronal outer atmospheric structures. Exosat observations have been obtained for three hybrid stars, of which only Alpha TrA, the nearest, is detected. The temperature of the emitting plasma is likely to be about 10 to the 6th K. Observtions of six K II stars made with the Einstein satellite show no detections. The general conclusion from the available X-ray data is that early K bright giants are not strong X-ray sources.

Brown, Alexander↗
NASA NTRS1988

Chromospherically active stars. IV - HD 178450 = V478 Lyr: An early-type BY Draconis type binary

It is shown that the variable star HD 178450 = V478 Lyr is a chromospherically active G8 V single-lined spectroscopic binary with a period of 2.130514 days. This star is characterized by strong UV emission features and a filled-in H-alpha absorption line which is variable in strength. Classified as an early-type BY Draconis system, it is similar to the BY Dra star HD 175742 = V775 Her. The unseen secondary of HD 178450 has a mass of about 0.3 solar masses and is believed to be an M2-M3 dwarf.

Fekel, Francis C.↗
NASA NTRS1987

Rotational modulation and flares on RS CVn and BY Dra stars. IV - The spatially resolved chromosphere of AR Lacertae

The authors observed the RS Canum Venaticorum system AR Lacertae systematically over an orbital period with the International Ultraviolet Explorer in October 1983. Contemporaneous radio observations were obtained at the VLA. The spectra of the Mg II k emission line were analyzed using a Doppler imaging technique. In this way, three discrete regions of emission were identified in the outer atmosphere of the K star-two 'plages' and a chromospheric brightening that was related to a radio flare. The widths of the plage profiles indicate that the two plages together cover about 2 percent of the visible stellar hemisphere, and their v sin i values indicate that they lie close to the equator of the K star. The Mg II k surface flux in the plages is about five times the mean Mg II k surface flux of the K star.

Walter, F. M.↗
NASA NTRS1987

Closed magnetic structures in the chromosphere and in the transition region

Using simultaneous observations of the same solar regions in the lines H-alpha and C IV 1548 A, schematic models of closed magnetic lines have been derived from dynamical constraints. It is concluded that the magnetic loops are closed at higher levels above facular than above nonfacular regions. This result remains valid whatever are the assumed density models and even if the 3-min oscillations are taken into account. The center-to-limb behavior is well predicted by taking into account the relative opacity in the chromosphere and transition region.

Malherbe, J. M.↗
NASA NTRS1988

A catalog of chromospherically active binary stars

A catalog of 168 chromospherically active binary stars is presented, including the class of RS CVn, BY Dra binaries, and other binaries which show strong Ca II H and K emission in their spectra. The catalog contains information on the photometric, spectroscopic, orbital, and physical properties of the systems as well as the space motions and positions. Summarized in a candidate list are in additional 37 stars having similar characteristics but which are not definitely known binaries or have not had H and K emission observed. The catalog contains stars in both the northern and the southern sky.

Strassmeier, K. G.↗
NASA NTRS1987

Chromospheric activity in open clusters

Spectra of Ca II H and K are being taken for stars of similar mass in the Hyades, the Pleiades, and NGC 752. These spectra will be used to create indices of chromospheric activity on these stars. The dispersion in these indices will then be compared to model dispersions which take into account stellar inclination, position of active regions, and filling factor. Only a few observations have been made to date. These show that a high signal to noise is achievable over reasonable exposure times. Modeling has only just begun.

Pilger, E. J.↗
NASA NTRS1988

Asymmetry in zeta Auriage chromospheres

Asymmetry in the ultraviolet spectra of zeta-Aur, similar to that reported in optical observations, was studied using IUE data. A plot of the integrated flux of zeta Aurigae from 1625 to 1675 A as a function of absolute phase shows no significant difference between the ingress and egress phases. A plot of the integrated flux from 1625 to 1675 A for 22 Vul as a function of absolute phase for both ingress and egress confirms that the atmospheric eclipse is asymmetric in 22 Vul. The eclipse in 22 Vul begins symmetrically but departs from symmetry at a phase greater than 0.05. The pronounced dip at ingress suggests a feature in the chromosphere.

Ahmad, I. A.↗
NASA NTRS1988

Mg II flux and profile variability of hybrid-chromosphere stars

Results from an IUE investigation of the variability seen in the Mg II emission line fluxes and wind absorption components of hybrid-chromosphere stars are presented. Real Mg II flux variability is shown with most of the variation being long-term. This variability appears to be due to global changes in the Mg II surface activity. Major changes in the wind absorption component are not seen.

Veale, Anthony↗
NASA NTRS1989

Chromospherically active stars. VI - HD 136901 = UV CrB: A massive ellipsoidal K giant single-lined spectroscopic binary

The variable star HD 136901 = UV CrB is a chromospherically active K2 III single-lined spectroscopic binary with an orbital period of 18.665 days. It has modest-strength Ca H and K emission and UV features, while H-alpha is a strong absorption feature containing little or no emission. The inclination of the system is 53 + or - 12 deg. The v sin i of the primary is 42 + or - 2 km/s, resulting in a minimum radius of 15.5 + or - 0.8 solar. When compared with the Roche lobe radius, this results in a mass ratio of 2.90 or larger. Additional constraints indicate that the secondary has a mass between 0.85 and 1.25 solar. Thus, the mass of the primary is at least 2.5 solar and probably is in the range 2.5-4 solar.

Fekel, Francis C.↗