Iron abundance in the atmosphere of Arcturus

Abundance of iron in the atmosphere of Arcturus has been determined from the profiles or regions of the profiles of the weak lines sensitive to iron abundance. The selected lines of Fe I and Fe II were synthesized with the MARCS theoretical models of the atmosphere. From the observed profiles of lines available with a high spectral resolution in the atlas by Hinkle and Wallace (2005), the values of the iron abundance $A = 6.95 \pm 0.03$ and the radial-tangential macroturbulent velocity $5.6 \pm 0.2$ km/s were obtained for Arcturus. The same physical quantities were found for the Sun as a star; they are $7.42 \pm 0.02$ and $3.4 \pm 0.3$ km/s, respectively. For Arcturus, the iron abundance relative to the solar one was determined with the differential method as [Fe/H] $=-0.48 \pm 0.02$.Comment: 14 pages, 4 figures, 4 tables, published in Kinematika i Fizika Nebesnykh Tel, 2015, Vol. 31, no. 4, P. 21-38 (in Russian); Kinematics and Physics of Celestial Bodies, 2015, Vol. 31, no. 4, P. 172-183 (in English

Temperature stratification of the atmosphere of Arcturus

A brief overview of the results of the investigations of the red giant star Arcturus is given. One-dimensional LTE modeling of the atmospheres of Arcturus and the Sun as a star is carried out on the basis of synthesis of the extended wings of the H and K Ca II lines. It is found that the local continuum in this spectral region is underestimated by an average of 12% in the atlases of Arcturus. The average deficit in UV absorption amounts to 43% for Arcturus whereas it is 9% for the Sun. For Arcturus the correction factor to the continuum opacity at the wavelengths of 390.0, 392.5, 395.0, 398.0, and 400.0 nm equals 2.20, 1.90, 1.70, 1.55, and 1.45. The model atmosphere of Arcturus obtained from the best-fit of the wings of the H and K Ca II lines corresponds to the model atmosphere with the fundamental parameters T_eff = 4286 K, log g = 1.66, and [Fe/H]=-0.52 derived by Ramirez and Allende Prieto (2011). The temperature stratification of Arcturus' atmosphere is presented in tabular form. To obtain more accurate temperature stratification in the future, we need a high spectral resolution spectrum calibrated to absolute fluxes with high accuracy.Comment: 21 pages, 10 figures, published in Kinematika i Fizika Nebesnykh Tel, 2013, Vol. 29, no. 4, P. 31-60 (in Russian); Kinematics Phys. Celestial Bodies 2013, Vol. 29, no. 4, P. 176-194 (in English

The wings of Ca II H and K as photospheric diagnostics and the reliability of one-dimensional photosphere modeling

The extended wings of the Ca II H and K lines provide excellent diagnostics of the temperature stratification of the photosphere of the Sun and of other cool stars, thanks to their LTE opacities and source functions and their large span in formation height. The aim of this study is to calibrate the usage of the H and K wings in one-dimensional interpretation of spatially averaged spectra and in deriving per-pixel stratifications from resolved spectra. I use multi-dimensional simulations of solar convection to synthesize the H and K wings, derive one-dimensional models from these wings as if they were observed, and compare the resulting models to the actual simulation input. I find that spatially-averaged models constructed from the synthesized wings generally match the simulation averages well, except for the deepest layers of the photosphere where large thermal inhomogeneities and Planck-function nonlinearity gives large errors. The larger the inhomogeneity, the larger the latter. The presence of strong network fields increases such inhomogeneity. For quiet photospheric conditions the temperature excesses reach about 200 K. One-dimensional stratification fits of discrete structures such as granulation and small-scale magnetic concentrations give satisfactory results with errors that are primarily due to steep temperature gradients and abrupt changes of temperature with depth. I conclude that stratification modeling using the H and K wings is a useful technique for the interpretation of solar high-resolution observations.Comment: 22 pages, 16 figures, Accepted for publication in Solar Physic

Two-dimensional MHD models of solar magnetogranulation. Dynamics of magnetic elements

We present the results of a statistical analysis of the Doppler shifts and the asymmetry parameters of V profiles of the Fe I 630.25 nm line produced by 2D MHD simulations of solar granulation. The realism of the simulations tested using the magnetic ratio of Fe I 524.71 and 525.02 nm lines. The Stokes spectra were synthesized in snapshots with a mixed polarity field having a mean magnetic flux density of 0.2 mT and mean unsigned field strength of 35 mT. We found that downflows with a velocity of 0.5 km/s predominate, on the average, in areas with some network magnetic elements at the disk center. In separate strong fluxtubes the average velocity is equal to 3 km/s and the maximum velocity is 9 km/s. In weak diffuse magnetic fields upflows dominate. Their average velocity is 0.5 km/s and maximal one is 3 km/s. The V-profile asymmetry depends on the spatial resolution. The V profiles synthesized with high spatial resolution (35 km) have average amplitude and area asymmetries -1%, 1%, respectively. The asymmetry scatter is \pm70% for weak profiles and \pm10% for strong ones. The profiles with low spatial resolution (700 km) have average amplitude and area asymmetries 3%, -2\%, respectively. Low spatial resolution is a reason why the amplitude asymmetry is always positive and greater than the area asymmetry in V profiles observed. We found weak correlation between the asymmetry of V profiles and velocity. Upflows cause negative asymmetry, on the average, and downflows cause positive asymmetry. We examined center-to-limb variations of vertical velocity in magnetic elements. Beginning from cos theta = 0.9, the average velocity abruptly increases from 0.5 to 2 km/s and then slightly varies closer to the limb. We found nonlinear oscillations of vertical velocity with power peaks in the 5-minute and 3-minute bands.Comment: 20 pages, 10 figures, published in Kinematika i Fizika Nebesnykh Te

Abundance sensitive points of line profiles in the stellar spectra

Many abundance studies are based on spectrum synthesis and $\chi$-squared differences between the synthesized and an observed spectrum. Much of the spectra so compared depend only weakly on the elemental abundances. Logarithmic plots of line depths rather than relative flux make this more apparent. We present simulations that illustrate a simple method for finding regions of the spectrum most sensitive to abundance, and also some caveats for using such information. As expected, we find that weak features are the most sensitive. Equivalent widths of weak lines are ideal features, because of their sensitivity to abundances, and insensitivity to factors that broaden the line profiles. The wings of strong lines can also be useful, but it is essential that the broadening mechanisms be accurately known. The very weakest features, though sensitive to abundance, should be avoided or used with great caution because of uncertainty of continuum placement as well as numerical uncertainties associated with the subtraction of similar numbers.Comment: 6 pages, 10 figures, 3 tables, accepted for publication in MNRA

Formation depths of Fraunhofer lines

We have summed up our investigations performed in 1970--1993. The main task of this paper is clearly to show processes of formation of spectral lines as well as their distinction by validity and by location. For 503 photospheric lines of various chemical elements in the wavelength range 300--1000 nm we list in Table the average formation depths of the line depression and the line emission for the line centre and on the half-width of the line, the average formation depths of the continuum emission as well as the effective widths of the layer of the line depression formation. Dependence of average depths of line depression formation on excitation potential, equivalent widths, and central line depth are demonstrated by iron lines.Comment: 25 pages, 8 figures, 1 table, published in The Preprints of The Main Astronomical Observatory of the National Academy of Sciences of Ukraine, MAO-97-1P, p.3-35, 199

Evolution of solar magnetic tubes and its manifestation in Stokes parameters

Basic scenarios and mechanisms for the formation and decay of small-scale magnetic elements and their manifestation in synthesized Stokes profiles of the Fe I 15648.5 A infrared line are considered in the context of two-dimensional modeling of nonstationary magnetogranulation on the Sun. The stage of convective collapse is characterized by large redshifts in the V profiles accompanied by complete Zeeman splitting of the I profiles. This is due to intense downward flows of material, which facilitates the concentration of longitudinal field with an amplitude of about several kG in the tube. The dissipation of strong magnetic structures is characterized by blueshifts in the Stokes profiles, which result from upward fluxes that decrease the magnetic field in the tube. Typical signatures during key stages in the evolution of compact magnetic elements should be detectable via observations with sufficiently high spatial and temporal resolution.Comment: 14 pages, 6 figures, published in Astronomy Reports, 2000, vol. 44, Issue 10, p.701-71

Numerical simulation of the interaction between solar granules and small-scale magnetic fields

We have carried out numerical simulation based on the equations of radiation magnetohydrodynamics to study the interaction of solar granules and small-scale magnetic fields in photospheric regions with various magnetic fluxes. Four sequences of 2D time-dependent models were calculated for photospheric regions with average vertical magnetic fluxes of 0, 10, 20, and 30 mT. The models exhibit no substantial variations in their temperature structure with varying average field strength, while the density and gas pressure profiles display gross changes. The solar granulation brightness field also varies substantially with magnetic flux. The contribution of the small-scale component to the intensity power spectrum increases with average field strength, whereas the large-scale component (of about a granule size) contributes less, the total rms intensity fluctuations being approximately the same. Thus the observed decrease in rms intensity fluctuations with growing average magnetic flux can be interpreted as smoothing of the small-scale component in the power spectrum by the modulation transfer function of the telescope.Comment: 15 pages, 6 figures, pudlished in Kinematika i Fizika Nebesnykh Tel (in Russian); Kinematics Phys. Celest. Bodies (in English

Fourier analysis of Fe I lines in the spectra of the Sun, {\alpha} Centauri A, Procyon, Arcturus, and Canopus

We used spectral observations of Fe I line profiles with a 200 000 resolution to determine micro and macroturbulent velocities in the atmospheres of the Sun as a star, {\alpha} Cen A, Procyon ({\alpha} CMi), Arcturus ({\alpha} Boo), and Canopus ({\alpha} Car). Isotropic microturturbulent velocities (V_mi) and radial-tangential macroturbulent velocities (V_ma,RT) were found to be a quite suitable approximation to the velocity field in the atmospheres of all stars studied except Canopus. The average velocities V_mi and V_ma,RT are 0.8 +/- 0.1 and 2.6 +/- 0.3 km/s for the Sun as a star, 0.8 +/- 0.2 and 2.9 +/- 0.4 km/s for {\alpha} Cen A, 0.8 +/- 0.3 and 5.9 +/- 0.2 km/s for Procyon, 1.0 +/- 0.2 and 4.6 +/- 0.3 km/s for Arcturus. The velocity field in the atmosphere of Canopus can be described by an anisotropic radial-tangential distribution of microturbulence with V_mi,RT = 2.1 km/s and anisotropic distribution of macroturbulence with V_ma,rad = 17 +/- 2 km/s and V_ma,tan = 1.3 +/- 1.0 km/s. From Fourier analysis of broadening and shapes of three spectral lines of Fe I, we have derived the rotation velocity V_e sini = 3.5 +/- 0.2 km/s for Canopus.Comment: 15 pages, 3 figures, 4 tables, published by Kinematics Phys. Celest. Bodies (in English) and Kinematika Fiz. Nebesn. Tel (in Russian)

Convective shifts of iron lines in the spectrum of the solar photosphere

The influence of the convective structure of the solar photosphere on the shifts of spectral lines of iron was studied. Line profiles in the visible and infrared spectrum were synthesized with the use of 2-D time-dependent hydrodynamic solar model atmospheres. The dependence of line shifts on excitation potential, wavelength, and line strength was analyzed, along with the depression contribution functions. The line shifts were found to depend on the location of the line formation region in convective cells and the difference between the line depression contributions from granules and intergranular lanes. In visible spectrum the weak and moderate lines are formed deep in the photosphere. Their effective line formation region is located in the central parts of granules, which make the major contribution to the absorption of spatially unresolved lines. The cores of strong lines are formed in upper photospheric layers where is formed reversed granulation due to convection reversal and physical conditions change drastically there. As a consequence the depression contributions in the strong line from intergranular lanes with downflows substantially increase. This accounts for smaller blue shifts of strong lines. In infrared spectrum the observed decrease in the blue line shifts is explained by the fact that their effective line formation regions lie higher in the photosphere and extend much further into the reversed granulation region due to the line opacity rise with the increase of line wavelength. Additionally the effective line formation depths of the synthesized visible and infrared Fe I lines and their dependence on line parameters is discussed.Comment: 13 pages, 5 figures, pudlished in Kinematika i Fizika Nebesnykh Tel (in Russian), Kinematics Phys. Celest. Bodies (in English