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Ayres, T. R.

Publications and source records attributed to Ayres, T. R..

At least 37 records · Page 2
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NASA NTRS1983

Outer atmospheres of cool stars. XIII - Capella at critical phases

The results are presented of a program of high-dispersion spectroscopy of Capella at critical orbital phases utilizing the International Ultraviolet Explorer. The considered program had the objective to establish as carefully as possible the relative ultraviolet emission levels of the Capella primary and secondary in order to obtain an understanding regarding the nature of their chromospheres and coronae. The results of the program confirm the earlier study of the Capella by Ayres and Linsky (1980), which was limited to orbital phases 0.50 and 0.75. Both programs found that the Capella secondary is considerably brighter than the primary, in surface flux, in all of the prominent far-ultraviolet emissions, particularly the high-excitation CII-CIV lines. In addition, the present study has revealed several puzzling aspects of the system. It is concluded that the Capella primary is comparatively bright, for a yellow giant, in far-ultraviolet and perhaps also in soft X-ray normalized fluxes. The implications of this conclusion are evaluated.

Ayres, T. R.↗
NASA NTRS1983

The narrow ultraviolet emission lines of the red dwarf Au Microscopii (dM1.6e)

It is pointed out that the red dwarfs are the smallest, coolest, faintest, least massive, but most common of normal main-sequence stars. The dMe (H-alpha emission) subclass of the red dwarfs exhibits the largest median soft X-ray to bolometric luminosity ratio of any group of late-type stars. In connection with the present investigation, attention is given to the first high-dispersion spectra of the chromospheric (6000 K) and higher temperature (up to 100,000 K) emissions of a dMe star, AU Microscopii in the far-ultraviolet (1150-2000 A) and middle-ultraviolet (2000-3000 A) bands accessible to the International Ultraviolet Explorer (IUE). AU Mic is one of the most luminous of lower main-sequence stars in C IV and soft X-ray emission.

Ayres, T. R.↗
NASA NTRS1982

Evolution of chromospheres and coronae in solar mass stars - A far-ultraviolet and soft X-ray comparison of Arcturus /K2 III/ and Alpha Centauri A /G2 V/

IUE far-UV and Einstein Observatory soft X-ray observations for the red giant Arcturus and the nearby yellow dwarf Alpha-Centauri A, which are archetypes of solar mass stars in different stages of evolution, are compared. Evidence is found for neither coronal soft X-ray emission from the red giant, at surface flux levels of only 0.0006 that detected previously for the yellow dwarf, nor C II and IV resonance line emission at surface flux levels of only 0.02 those of the yellow dwarf. The resonance line upper limits and previous detections of the C II intersystem UV multiplet 0.01 near 2325 A provide evidence for an Arcturus outer atmosphere that is geometrically extended, tenuous and cool. The red giant has, in addition, a prominent cool stellar wind. An extensive tabulation of line identifications, widths and fluxes for the IUE far-UV echelle spectra of the two stars is given, and two competing explanations for the Wilson-Bappu effect are discussed.

Ayres, T. R.↗
NASA NTRS1982

High resolution EUV spectroscopy of 56 Pegasi (KO 2p + wd)

The hypothesis that X-ray and UV emission by 56 Pegasi (K0II p + wd) originates in a conventional chromosphere, transition region and corona, as opposed to Schindler's hypothesis of accretion of the cool stellar wind onto the white dwarf companion, is discussed. Analysis of IUE data indicates that within the uncertainties associated with determining the stellar radius and abundances, the chromosphere, transition region, and corona of 56 Peg are similar to that of Beta Draconis both in structure and energy balance. The accretion hypothesis is not a unique explanation of the observed spectral properties and the white dwarf companion may not play a direct role. Furthermore, 56 Peg, according to current values of V-R and luminosity class, is on the nonsolar side of the Linsky-Haisch dividing line whereas Beta Draconis is on the solar side.

Jordan, C.↗
NASA NTRS1982

Outer atmospheres of cool stars. X - HR 1099 at quadrature

IUE high-dispersion, far-UV spectra of the active-chromosphere RS CVn binary HR 1099 are reported. The emission features produced by such high-temperature species as C II and C IV are found to be very bright, to exhibit structure, change significantly in the one-week interval separating the two exposures, and generally follow the radial velocity motion of the K subgiant primary, while the less massive G dwarf secondary appears only weakly in the composite spectrum. It is concluded that chromospheric and transition region emission in RS CVn binaries is a stellar rather than system phenomenon, and the structure evident in some of the emission line shapes is interpreted as a patchy brightness distribution on and above the K star surface that is spread out in velocity by the rapid rotation.

Ayres, T. R.↗
NASA NTRS1982

Capella revisited

The highlights of two studies of Capella, a spectroscopic binary, undertaken during the third and fourth years of the IUE are given. The first program consists of high dispersion spectroscopy at critical phases of the Capella orbit. The second program is a two month monitoring effort to search for ultraviolet modulations induced by the rotation of magnetic active regions onto and off of the visible hemisphere of the Capella secondary.

Ayres, T. R.↗
NASA NTRS1982

High dispersion far ultraviolet spectra of cool stars

Recent far ultraviolet high dispersion spectra of two cool supergiant stars, Beta Dra (G2 Ib) and Alpha Ori (M2 Iab) are examined in the context of current questions regarding stellar chromospheres, coronae and mass loss. These stars show very different outer atmosphere structure. Beta Dra has a geometrically thin transition region with bright emission lines of 100,000 K plasma that are red-shifted, indicating downflow in magnetic flux tubes. By contrast, Alpha Ori has a cool extended chromosphere and circumstellar envelope with large mass loss.

Stencel, R. E.↗
NASA NTRS1981

The cool half of the H-R diagram in soft X-rays

The results of an Einstein Observatory program to map the occurrence of hot coronae (T greater than 1 million K) in the cool half of the Hertzsprung-Russell diagram are reported. F-M dwarfs, and late F through early K star giants characterized by 10,000 K chromospheric and 20,000-200,000 K FUV emission lines were studied in one region, while a second region study included red giants later than K2 III and supergiants later than G5 Ib with weaker chromospheric emission and no high temperature species. Program goals comprised determination of the C IV division as seen in soft X-rays, and identification of stellar parameters which distinguish strong from weak coronal X-ray sources. A summary of target stars, X-ray fluxes, and UV emission profiles is provided, and coronal emissions, comparisons of C IV and wind boundaries, hybrid-spectrum supergiants, the energy balance of stellar outer atmospheres, stellar rotation and coronae, and evolutionary considerations are discussed, along with lines of future research.

Ayres, T. R.↗
NASA NTRS1981

Far-ultraviolet fluorescence of carbon monoxide in the red giant Arcturus

Evidence is presented that many of the weak features observed with International Ultraviolet Explorer (IUE) in the far-ultraviolet (1150-2000 A) spectrum of the archetype red giant Arcturus (K2 III) are A-X fourth positive bands of carbon monoxide excited by chromospheric emissions of O I, C I, and H I. The appearance of fluorescent CO bands near the wavelength of commonly used indicators of high-temperature (T greater than 20,000 K) plasma, such as C II at wavelength 1335 and C IV at wavelength 1548, introduces a serious ambiguity in diagnosing the presence of hot material in the outer atmospheres of the cool giants by means of low-dispersion IUE spectra.

Ayres, T. R.↗
NASA NTRS1981

Outer atmospheres of cool stars. IX - A survey of ultraviolet emission from F-K dwarfs and giants with IUE

Low-dispersion ultraviolet spectra (1150-2000 A) of a representative sample of cool stars, including dwarfs and giants of spectral types F-K, obtained with the IUE, are examined. The observation and the absolute calibration procedures are described. Correlation diagrams are constructed that compare chromospheric and transition-region emission line strengths and broadband coronal soft X-ray fluxes. The transition-region and coronal emission in the G-K dwarfs and G giants is well correlated with the Mg II (wavelength 2800) doublet emission strength, which is symptomatic of chromospheric energy losses. The power-law slopes are steeper than unity, particularly for soft X-rays. The implications of the correlations are discussed with respect to the weakening or disappearance of transition regions and hot coronae in the cool half of the red-giant branch and possible chromospheric and coronal heating mechanisms. It is proposed that the weakness of outer atmospheres in the red giants compared with the yellow giants can be understood as a consequence of stellar evolution, since it is possible that stars of slightly different spectral type in the giant branch have very different main-sequence progenitors.

Ayres, T. R.↗
NASA NTRS1981

Fourier Transform Spectrometer observations of solar carbon monoxide. I - The fundamental and first overtone bands in the quiet sun

Measurements of the 2200/cm fundamental and 4300/cm first overtone vibration-rotation band systems of solar carbon monoxide, were obtained with the Fourier Transform Spectrometer of the McMath telescope at Kitt Peak. The overtone measurements were taken at the east, north, and west heliocentric limbs, and at disk center. Observations of the strong fundamental bands were obtained at disk center and near the north limb. The low core brightness temperatures of the strongest fundamental carbon monoxide lines near the limb, reported previously by Noyes (1972) and Hall (1974), are confirmed. The possibility that thermal inhomogeneities might be responsible for the unusual behavior of the fundamental carbon dioxide lines have been examined. The somewhat discordant behavior of the fundamental lines at disk center compared with the north limb seems to favor a limb shadowing effect. The first overtone limb equivalent widths and the best-fit thermal and microvelocity models indicate a solar carbon abundance of 0.004 (on the scale with A sub H = 1) for an oxygen-to-carbon abundance ratio of 2.

Ayres, T. R.↗
NASA NTRS1981

An IUE's eye view of cool-star outer atmospheres

Three topics are discussed which together demonstrate the power of the IUE to probe the occurrences of chromospheres and coronas in the cool half of the HR diagram. These are: (1) the complementary low dispersion and echelle observing modes; (2) Mg II h and k: chromospheric cooling and width luminosity correlation; and (3) empirical correlations among chromospheric, transition region, and coronal emission. The spectra of alpha Centauri (G2 V + K1 V) and Capella (G6 III + F9 III) are compared with that of the Sun and recent low dispersion surveys of cool star emission in the 1150 A to 2000 A short wavelength region are summarized.

Ayres, T. R.↗
NASA NTRS1981

The Mg 2 h and k lines in a sample of dMe and dM stars

Both Mg II h and k line fluxes are presented for a sample of 4 dMe and 3 dM stars obtained with the IUE satellite in the long wavelength, low dispersion mode. The observed fluxes are converted to stellar surface flux units and the importance of chromospheric non radiative heating in this sample of M dwarf stars is intercompared. In addition, the net chromospheric radiative losses due to the Ca II H and K lines in those stars in the sample for which calibrated Ca II H and K line data exist are compared. Active region filling factors which likely give rise to the observed optical and ultraviolet chromospheric emission are estimated. The implications of the results for homogeneous, single component stellar model chromospheres analyses are discussed.

Giampapa, M. S.↗
NASA NTRS1980

Does H/-/ truly cool the solar chromosphere

The controversial problem of H(-) radiative cooling in the solar chromosphere is examined. It is found, in agreement with Praderie and Thomas (1972, 1976), that H(-) is a substantial source of radiative heating in the outer atmosphere, especially when departures from LTE are important. The role of H(-) as a chromospheric heating agent must be considered carefully before net radiative cooling rates can be assessed from empirical chromospheric models, or calculations of nonradiative heating, for example by acoustic waves, can be pursued meaningfully.

Ayres, T. R.↗
NASA NTRS1980

Outer atmospheres of cool stars. V - IUE observations of Capella - The rotation-activity connection

UV spectra of Capella (G6 III + F9 III) obtained with the IUE are analyzed. High-resolution emission-line profiles taken near the elongation at phase 78 days suggest that virtually all of the emission in transition-region lines and most of the emission in chromospheric lines comes from the late-F secondary of the system. It is suggested that the origin of the extraordinary activity levels on these otherwise very similar stars can be traced to the one property that is obviously different, i.e., rotation. The Capella primary is a normal sharp-line slow rotating giant, whereas the secondary has broader lines and is a rapid rotator for a late-type giant.

Ayres, T. R.↗