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358 results on '"Ayres, Thomas R."'

103. Key Problems in Cool-Star Astrophysics

Author

Pagano, Isabella, primary, Ayres, Thomas R., additional, Lanzafame, Alessandro C., additional, Linsky, Jeffrey L., additional, Montesinos, Benjamín, additional, and Rodonò, Marcello, additional

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109. DIVISION IV: COMMISSION 36: THEORY OF STELLAR ATMOSPHERES

Author

Puls, Joachim, primary, Hubeny, Ivan, additional, Asplund, Martin, additional, Allard, France, additional, Prieto, Carlos Allende, additional, Ayres, Thomas R., additional, Carlsson, Mats, additional, Gustafsson, Bengt, additional, Kudritzki, Rolf-Peter, additional, and Ryabchikova, Tatiana A., additional

Published
2013
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111. Ultraviolet emission lines of Si II in cool star and solar spectra.

Author

Laha, Sibasish, Keenan, Francis P., Ferland, Gary J., Ramsbottom, Catherine A., Aggarwal, Kanti M., Ayres, Thomas R., Chatzikos, Marios, van Hoof, Peter A. M., and Williams, Robin J. R.

Subjects
SOLAR radiation, SOLAR spectra, STELLAR spectra, COOL stars (Astronomy)
Abstract

Recent atomic physics calculations for Si II are employed within the CLOUDY modelling code to analyse Hubble Space Telescope (HST) STIS ultraviolet spectra of three cool stars, β Geminorum, α Centauri A and B, as well as previously published HST/GHRS observations of α Tau, plus solar quiet Sun data from the High Resolution Telescope and Spectrograph. Discrepancies found previously between theory and observation for line intensity ratios involving the 3s23p 2PJ-3s3p2 4PJ' intercombination multiplet of Si II at ~ 2335 Å are significantly reduced, as are those for ratios containing the 3s23p 2PJ-3s3p2 2DJ' transitions at ~1816 Å. This is primarily due to the effect of the new Si II transition probabilities. However, these atomic data are not only very different from previous calculations, but also show large disagreements with measurements, specifically those of Calamai et al. for the intercombination lines. New measurements of transition probabilities for Si II are hence urgently required to confirm (or otherwise) the accuracy of the recently calculated values. If the new calculations are confirmed, then a long-standing discrepancy between theory and observation will have finally been resolved. However, if the older measurements are found to be correct, then the agreement between theory and observation is simply a coincidence and the existing discrepancies remain. [ABSTRACT FROM AUTHOR]

Published
2016
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120. Non-LTE CO, revisited

Author

Ayres, Thomas R and Wiedemann, Gunter R

Subjects
Astrophysics
Abstract

A more extensive and detailed non-LTE simulation of the Delta v = 1 bands of CO than attempted previously is reported. The equations of statistical equilibrium are formulated for a model molecule containing 10 bound vibrational levels, each split into 121 rotational substates and connected by more than 1000 radiative transitions. Solutions are obtained for self-consistent populations and radiation fields by iterative application of the 'Lambda-operator' to an initial LTE distribution. The formalism is used to illustrate models of the sun and Arcturus. For the sun, negligible departures from LTE are found in either a theoretical radiative-equilibrium photosphere with outwardly falling temperatures in its highest layers or in a semiempirical hot chromosphere that reproduces the spatially averaged emission cores of Ca II H and K. The simulations demonstrate that the puzzling 'cool cores' of the CO Delta V = 1 bands observed in limb spectra of the sun and in flux spectra of Arcturus cannot be explained simply by non-LTE scattering effects.

Published
1989
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121. A spectral dissection of the ultraviolet emissions of Capella

Author

Ayres, Thomas R

Subjects
Astronomy
Abstract

Phase-resolved observations of Capella were conducted with the IUE to separate the spectrum of the core-helium-burning primary from that of the fast-rotating, chromospherically active secondary. Characteristics of the primary and secondary spectra are presented. The UV emissions of the primary are compared to those of the four K0 giants of the Hyades, and the field K giants Beta Cet and Beta Gem. The Capella primary is found to be similar to the two active Hyads, Gamma Tauri and Theta Tauri, suggesting that if Capella is coeval with the Hyades nucleus, the age of the cluster might be less than half that commonly assumed.

Published
1988
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122. Redshifts of high-temperature emission lines in the far-ultraviolet spectra of late-type stars. II - New, precise measurements of dwarfs and giants

Author

Ayres, Thomas R, Jensen, Eberhard, and Engvold, Oddbjorn

Subjects
Astrophysics
Abstract

Results are presented from an IUE SWP camera investigation of the occurrence of gasdynamic flows, analogous to the downdrafts of 10 to the 5th K material observed over magnetic active regions of the sun, among stars of late spectral type. The SWP calibration spectra study conducted documents the existence of local, small, persistent distortions of the echelle wavelength scales that are of unknown origin. Attention is given to the enormous widths of the stellar high-excitation emission lines in both the dwarfs and the giants, with respect to the comparatively small, subsonic Doppler shifts; the widths are typically an order of magnitude greater than the redshifts.

Published
1988
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123. Far-ultraviolet observations of the Delta-Scuti variable Beta Cassiopeiae

Author

Ayres, Thomas R and Bennett, Jeffrey O

Subjects
Astrophysics
Abstract

FUV (115-200-nm) observations of Beta Cas, obtained at resolutions 5 and 0.15 A by the IUE during September 1984, are reported and analyzed. The data are presented graphically and compared with results for Procyon, a sharp-lined F star similar in temperature to Beta Cas. A sharp factor-of-two increase in H I Ly-alpha flux on a time scale comparable to its Delta Sct pulsation period was observed, but the data were insufficient to determine whether the increase was flarelike or pulsation-related. The outer edges of the Beta Cas emission are shown to be significantly wider than those of Procyon.

Published
1987

128. COMMISSION 36: THEORY OF STELLAR ATMOSPHERES

Author

Spite, Monique, primary, Landstreet, John D., additional, Asplund, Martin, additional, Ayres, Thomas R., additional, Balachandran, Suchitra C., additional, Dravins, Dainis, additional, Hauschildt, Peter H., additional, Kiselman, Dan, additional, Nagendra, K. N., additional, Sneden, Christopher, additional, Tautvaišiené, Grazina, additional, and Werner, Klaus, additional

Published
2007
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130. α TrA Junior

Author

Ayres, Thomas R., primary, Brown, Alexander, additional, and Harper, Graham M., additional

Published
2007
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149. Key Problems in Cool-Star Astrophysics.

Author

De Castro, Ana I. Gómez, Wamsteker, Willem, Pagano, Isabella, Ayres, Thomas R., Lanzafame, Alessandro C., Linsky, Jeffrey L., Montesinos, Benjamín, and Rodonò, Marcello

Abstract

Selected key problems in cool-star astrophysics are reviewed, with emphasis on the importance of new ultraviolet missions to tackle the unresolved issues. UV spectral signatures are an essential probe of critical physical processes related to the production and transport of magnetic energy in astrophysical plasmas ranging, for example, from stellar coronae, to the magnetospheres of magnetars, and the accretion disks of protostars and Active Galactic Nuclei. From an historical point of view, our comprehension of such processes has been closely tied to our understanding of solar/stellar magnetic activity, which has its origins in a poorly understood convection-powered internal magnetic dynamo. The evolution of the Sun's dynamo, and associated magnetic activity, affected the development of planetary atmospheres in the early solar system, and the conditions in which life arose on the primitive Earth. The gradual fading of magnetic activity as the Sun grows old likewise will have profound consequences for the future heliospheric environment. Beyond the Sun, the magnetic activity of stars can influence their close-in companions, and vice versa. Cool star outer atmospheres thus represent an important laboratory in which magnetic activity phenomena can be studied under a wide variety of conditions, allowing us to gain insight into the fundamental processes involved. TheUV range is especially useful for such studies because it contains powerful diagnostics extending from warm (∼104 K) chromospheres out to hot (1-10 MK) coronae, and very highresolution spectroscopy in the UV has been demonstrated by the GHRS and STIS instruments on HST but has not yet been demonstrated in the higher energy EUV and X-ray bands. A recent example is the use of the hydrogen Lya resonance line — at 110 000 resolution with HST STIS — study, for the first time, coronal winds from cool stars through their interaction with the interstellar gas. These winds cannot be detected from the ground, for lack of suitable diagnostics; or in the X-rays, because the outflowing gas is too thin. A2m class UV space telescope with high resolution spectroscopy and monitoring capabilitieswould enable important newdiscoveries in cool-star astronomy among the stars of the solar neighborhood out to about 150 pc. A larger aperture facility (4-6 m) would reach beyond the 150 pc horizon to fainter objects including young brown dwarfs and pre-main sequence stars in star-forming regions like Orion, and magnetic active stars in distant clusters beyond the Pleiades and α Persei. This would be essential, as well, to characterize the outer atmospheres of stars with planets, that will be discovered by future space missions like COROT, Kepler, and Darwin. [ABSTRACT FROM AUTHOR]

Published
2006
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150. COmospheres and Beyond.

Author

Käufl, Hans Ulrich, Siebenmorgen, Ralf, Moorwood, Alan F. M., and Ayres, Thomas R.

Abstract

There is an unexpectedly cool atmospheric layer on top of the Sun's photosphere, likely present in virtually all stars of solar temperature and cooler. The main diagnostic of this mysterious layer is the $\varDelta v$ = 1 (fundamental) rovibrational band system of carbon monoxide in the thermal infrared, near 5 μm. It is thought that strong radiative cooling by the CO bands, themselves, is directly responsible for the depressed thermal structure of the cold layer, thus the moniker "COmosphere." Here, three different views of stellar COmospheres are presented: a survey of the 5 μm CO bands in late-type stars with the PHOENIX cryogenic echelle spectrometer at the Kitt Peak 2.1 m telescope; recent studies of the off-limb CO emissions in the Sun exploiting the McMath-Pierce Fourier transform spectrometer (FTS); and time-resolved integral field spectroscopy of solar surface dynamics using the Imaging Infrared Spectrograph (IRIS) and a new IR adaptive optics system, also on the McMath-Pierce at Kitt Peak. [ABSTRACT FROM AUTHOR]

Published
2005
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