Thomas R. Ayres

The photospheric solar oxygen project: I. Abundance analysis of atomic lines and influence of atmospheric models

Context: The solar oxygen abundance has undergone a major downward revision in the past decade, the most noticeable one being the update including 3D hydrodynamical simulations to model the solar photosphere. Up to now, such an analysis has only been carried out by one group using one radiation-hydrodynamics code. Aims: We investigate the photospheric oxygen abundance considering lines from atomic transitions. We also consider the relationship between the solar model used and the resulting solar oxygen abundance, to understand whether the downward abundance revision is specifically related to 3D hydrodynamical effects. Methods: We performed a new determination of the solar photospheric oxygen abundance by analysing different high-resolution high signal-to-noise ratio atlases of the solar flux and disc-centre intensity, making use of the latest generation of CO5BOLD 3D solar model atmospheres. Results: We find 8.73 ? log (N_O/N_H) +12 ? 8.79. The lower and upper values represent extreme assumptions on the role of collisional excitation and ionisation by neutral hydrogen for the NLTE level populations of neutral oxygen. The error of our analysis is ± (0.04± 0.03) dex, the last being related to NLTE corrections, the first error to any other effect. The 3D ?granulation effects? do not play a decisive role in lowering the oxygen abundance. Conclusions: Our recommended value is log (N_O/N_H) = 8.76 ± 0.07, considering our present ignorance of the role of collisions with hydrogen atoms on the NLTE level populations of oxygen. The reasons for lower O abundances in the past are identified as (1) the lower equivalent widths adopted and (2) the choice of neglecting collisions with hydrogen atoms in the statistical equilibrium calculations for oxygen. This paper is dedicated to the memory of Hartmut Holweger.

Goddard high-resolution spectrograph observations of the local interstellar medium and the deuterium/hydrogen ratio along the line of sight toward Capella

HST Goddard High-Resolution Spectrograph observations of the 1216, 2600, and 2800 A spectral regions are analyzed for the spectroscopic binary system Capella, obtained at orbital phase 0.26 with 3.27-3.57 km/s resolution and high SNR. The column densities of H I, D I, Mg II, and Fe II for the local interstellar medium along this 12.5 pc line of sight, together with estimates of the temperature and turbulent velocity are inferred. It is inferred that the atomic deuterium/hydrogen ratio by number is 1.65(+0.07, -0.18) x 10 exp -5 for this line of sight. Galactic evolution calculations indicate that the primordial D/H ratio probably lies in the range of (1.5-3) x (D/H)LISM. If H0 = 80 km/s Mpc, as recent evidence suggests, then the baryonic density in units of the Einstein-de Sitter closure density is 0.023-0.031. Thus the universe is argued to expand forever, unless nonbaryonic matter greatly exceeds the amount of baryonic matter.

Evolution of the solar ionizing flux

A young magnetically active Sun, with enhanced ionizing radiations and an elevated solar wind, might have contributed to erosion of the primordial atmosphere of Mars (which is particularly vulnerable to dissociative recombination and sputtering by solar wind pickup ions). Spacecraft and ground-based observations of solar-type dwarfs in young galactic clusters have yielded a unified view of the early evolution and subsequent systematic decline of magnetic activity with age. Rotational braking by the coronal wind ultimately quenches the spin-catalyzed dynamo at the heart of stellar magnetism. Decay of the 10 6 -10 7 K corona is much faster than the 10 4 K chromosphere, but XUV emissions of both can be predicted reliably, and photoionization of key planetary atmospheric constituents can be modeled. The early Martian atmosphere (age 1 Gyr) probably was subjected to photoionization rates ∼5 times contemporary peak values (sunspot maximum), possibly more if the ages of galactic clusters have been underestimated.

Multiwavelength Observations of Short-Timescale Variability in NGC 4151. IV. Analysis of Multiwavelength Continuum Variability

For pt.III see ibid., vol.470, no.1, p.349-63 (1996). Combines data from the three preceding papers in order to analyze the multi wave-band variability and spectral energy distribution of the Seyfert 1 galaxy NGC 4151 during the 1993 December monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in medium-energy (~1.5 keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA=6%) variations (uncorrelated with those at lower energies) were seen at soft gamma-ray energies of ~100 keV. No significant variability was seen in softer (0.1-1 keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased from 9% to 1% as the wavelength increased from 1275 to 6900 Aring. These data do not probe extreme ultraviolet (1200 Aring to 0.1 keV) or hard X-ray (250 keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of lsim0.15 day between 1275 Aring and the other ultraviolet bands, lsim0.3 day between 1275 Aring and 1.5 keV, and lsim1 day between 1275 and 5125 Aring. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-wave-band AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well fitted with a very steep, red power law (ales-2.5)

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. XI. Intensive monitoring of the ultraviolet spectrum of NGC 7469

From 1996 June 10 to July 29, the International Ultraviolet Explorer monitored the Seyfert 1 galaxy NGC 7469 continuously in an attempt to measure time delays between the continuum and emission-line fluxes. From the time delays, one can estimate the size of the region dominating the production of the UV emission lines in this source. We find the strong UV emission lines to respond to continuum variations with time delays of about 23-31 for Lyα, 27 for C IV λ1549, 19-24 for N V λ1240, 17-18 for Si IV λ1400, and 07-10 for He II λ1640. The most remarkable result, however, is the detection of apparent time delays between the different UV continuum bands. With respect to the UV continuum flux at 1315 A, the flux at 1485 A, 1740 A, and 1825 A lags with time delays of 021, 035, and 028, respectively. Determination of the significance of this detection is somewhat problematic since it depends on accurate estimation of the uncertainties in the lag measurements, which are difficult to assess. We attempt to estimate the uncertainties in the time delays through Monte Carlo simulations, and these yield estimates of ~007 for the 1 σ uncertainties in the interband continuum time delays. Possible explanations for the delays include the existence of a continuum-flux reprocessing region close to the central source and/or a contamination of the continuum flux with a very broad time-delayed emission feature such as the Balmer continuum or merged Fe II multiplets.

Local Interstellar Medium Properties and Deuterium Abundances for the Lines of Sight toward HR 1099, 31 Comae, β Ceti, and β Cassiopeiae*

We analyze Goddard High-Resolution Spectrograph data to infer the properties of local interstellar gas and the deuterium/hydrogen (D/H) ratio for lines of sight toward four nearby late-type stars-H ...

Fourier transform spectrometer observations of solar carbon monoxide. II - Simultaneous cospatial measurements of the fundamental and first-overtone bands, and Ca II K, in quiet and active regions

Fourier transform spectrometry has yielded simultaneous cospatial measurements of important diagnostics of thermal structure in the high solar photosphere and low chromosphere. It is noted that the anomalous behavior of the fundamental bands of CO in quiet areas near the limb is accentuated in an active region plage observed close to the limb. The difference between the core temperatures of the CO fundamental bands in a plage and a nearby quiet region at the limb is larger than the corresponding brightness temperature differences in the inner wings of the Ca II line measured in a quiet region and several plages closer to the disk center. Numerical simulations indicate that the disparate behavior of the CO bands with respect to Ca II K cannot be reconciled with existing single component thermal structure models; a two-component atmosphere is required.

A Far Ultraviolet Spectroscopic Explorer Survey of Late‐Type Dwarf Stars

We describe the 910-1180 Aspectra of seven late-type dwarf stars obtained with the Far Ultraviolet Spec- troscopic Explorer (FUSE) satellite. The stars include Altair (A7 IV), Procyon (F5 IV-V), � Cen A (G2 V), AB Dor (K1 V), � Cen B (K2 V), � Eri (K2 V), and AU Mic (M0 V). We present line identifications, fluxes, Doppler shifts, and widths. Doppler shifts are measured with respect to heliocentric wavelength scales deter- mined from interstellar absorption lines, and are compared with transition region line shifts seen in Hubble Space Telescope (HST) ultraviolet spectra. For the warmer stars the O vi lines extend the trend of increasing redshift with line formation temperature, but for the cooler stars the O vi line redshifts are essentially zero. The Ciii and Ovi lines of most stars in the sample are best fit with two Gaussians, and we confirm the correla- tion of increasing importance of the broad component with increasing stellar activity. The nonthermal veloc- ities of the narrow component are subsonic and exhibit a trend toward larger velocities with decreasing surface gravity, while the nonthermal velocities of the broad components show no obvious trend with stellar gravity. The C iii and O vi lines of Altair show unique broad horned profiles. Two flares were observed on AU Mic. One shows increasing continuum flux to shorter wavelengths, which we interpret as free-free emis- sion from hot plasma, and relatively narrow, redshifted C iii and O vi emission. The other shows very broad line profiles. Subject headings: stars: activity — stars: chromospheres — stars: late-type — techniques: spectroscopic — ultraviolet: stars

The solar photospheric abundance of carbon - Analysis of atomic carbon lines with the CO5BOLD solar model

Context. The analysis of the solar spectra using hydrodynamical simulations, with a specific selection of lines, atomic data, and method for computing deviations from local thermodynamical equilibrium, has led to a downward revision of the solar metallicity, Z. We are using the latest simulations computed with the CO5BOLD code to reassess the solar chemical composition. Our previous analyses of the key elements, oxygen and nitrogen, have not confirmed any extreme downward revision of Z, as derived in other works based on hydrodynamical models. Aims. We determine the solar photospheric carbon abundance with a radiation-hydrodynamical CO5BOLD model and compute the departures from local thermodynamical equilibrium by using the Kiel code. Methods. We measured equivalent widths of atomic C i lines on high-resolution, high signal-to-noise ratio solar atlases of disccentre intensity and integrated disc flux. These equivalent widths were analysed with our latest solar 3D hydrodynamical simulation computed with CO5BOLD. Deviations from local thermodynamic equilibrium we computed in 1D with the Kiel code, using the average temperature structure of the hydrodynamical simulation as a background model. Results. Our recommended value for the solar carbon abundance relies on 98 independent measurements of observed lines and is A(C) = 8.50 ± 0.06. The quoted error is the sum of statistical and systematic errors. Combined with our recent results for the solar oxygen and nitrogen abundances, this implies a solar metallicity of Z = 0.0154 and Z/X = 0.0211. Conclusions. Our analysis implies a solar carbon abundance that is about 0.1 dex higher than what was found in previous analyses based on different 3D hydrodynamical computations. The difference is partly driven by our equivalent width measurements (we measure, on average, larger equivalent widths than the other work based on a 3D model), in part because of the different properties of the hydrodynamical simulations and the spectrum synthesis code. The solar metallicity we obtain from the CO5BOLD analyses is in slightly better agreement with the constraints of helioseismology than the previous 3D abundance results.

Thermal bifurcation in the solar outer atmosphere

Two distinct plasma thermal states are possible in the solar outer atmosphere, owing to the bifurcated character of the low-temperature cooling function at small optical depths. In radiative equilibrium, the plasma is strongly cooled to temperatures well below 4000 K by surface emission in the ..delta..V = 1 fundamental vibration-rotation bands of carbon monoxide. However, when significant mechanical energy deposition is present in addition to the radiative heating component, the only effective cooling channel available to stabilize the plasma is optically thin emission in the recombination continuum of H/sup -/. As a result, thermal equilibrium in a mechanically heated atmospheric zone can be attained only for temperatures above T/sub crit/roughly-equal4900 K because H/sup -/ is itself a net radiative heating agent for temperature cooler than T/sub crit/. Thermal bifurcation effects in the solar outer atmosphere are encouraged by the likelihood that mechanical energy deposition is significantly enhanced in small-scale, discrete structures, namely, magnetic flux tubes.