44 research outputs found
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 and the
radial-tangential macroturbulent velocity km/s were obtained for
Arcturus. The same physical quantities were found for the Sun as a star; they
are and km/s, respectively. For Arcturus, the
iron abundance relative to the solar one was determined with the differential
method as [Fe/H] .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 -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