scholarly journals The relationship between photometric and spectroscopic oscillation amplitudes from 3D stellar atmosphere simulations

2021 ◽  
Author(s):  
Yixiao Zhou ◽  
Thomas Nordlander ◽  
Luca Casagrande ◽  
Meridith Joyce ◽  
Yaguang Li ◽  
...  
Keyword(s):  
Radial Velocity ◽  
Three Dimensional ◽  
Quantitative Relationship ◽  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Wavelength Shift ◽  
The Sun ◽  
Theoretical Derivation ◽  
Velocity Amplitude ◽  
Good Agreement

Abstract We establish a quantitative relationship between photometric and spectroscopic detections of solar-like oscillations using ab initio, three-dimensional (3D), hydrodynamical numerical simulations of stellar atmospheres. We present a theoretical derivation as proof of concept for our method. We perform realistic spectral line formation calculations to quantify the ratio between luminosity and radial velocity amplitude for two case studies: the Sun and the red giant ε Tau. Luminosity amplitudes are computed based on the bolometric flux predicted by 3D simulations with granulation background modelled the same way as asteroseismic observations. Radial velocity amplitudes are determined from the wavelength shift of synthesized spectral lines with methods closely resembling those used in BiSON and SONG observations. Consequently, the theoretical luminosity to radial velocity amplitude ratios are directly comparable with corresponding observations. For the Sun, we predict theoretical ratios of 21.0 and 23.7 ppm/[m s−1] from BiSON and SONG respectively, in good agreement with observations 19.1 and 21.6 ppm/[m s−1]. For ε Tau, we predict K2 and SONG ratios of 48.4 ppm/[m s−1], again in good agreement with observations 42.2 ppm/[m s−1], and much improved over the result from conventional empirical scaling relations which gives 23.2 ppm/[m s−1]. This study thus opens the path towards a quantitative understanding of solar-like oscillations, via detailed modelling of 3D stellar atmospheres.

scholarly journals 3D non-LTE line formation of neutral carbon in the Sun

2019 ◽  
Vol 624 ◽  
pp. A111 ◽  
Author(s):  
A. M. Amarsi ◽  
P. S. Barklem ◽  
R. Collet ◽  
N. Grevesse ◽  
M. Asplund
Keyword(s):  
Three Dimensional ◽  
Solar Spectrum ◽  
Neutral Hydrogen ◽  
Stellar Atmospheres ◽  
Impact Excitation ◽  
Solar Photosphere ◽  
Late Type ◽  
Current Standard ◽  
Standard Value ◽  
Good Agreement

Carbon abundances in late-type stars are important in a variety of astrophysical contexts. However C I lines, one of the main abundance diagnostics, are sensitive to departures from local thermodynamic equilibrium (LTE). We present a model atom for non-LTE analyses of C I lines, that uses a new, physically-motivated recipe for the rates of neutral hydrogen impact excitation. We analyse C I lines in the solar spectrum, employing a three-dimensional (3D) hydrodynamic model solar atmosphere and 3D non-LTE radiative transfer. We find negative non-LTE abundance corrections for C I lines in the solar photosphere, in accordance with previous studies, reaching up to around 0.1 dex in the disk-integrated flux. We also present the first fully consistent 3D non-LTE solar carbon abundance determination: we infer log ɛC = 8.44 ± 0.02, in good agreement with the current standard value. Our models reproduce the observed solar centre-to-limb variations of various C I lines, without any adjustments to the rates of neutral hydrogen impact excitation, suggesting that the proposed recipe may be a solution to the long-standing problem of how to reliably model inelastic collisions with neutral hydrogen in late-type stellar atmospheres.

scholarly journals Chromospheric Surface Structures on EI Eridani and HD 199178

1991 ◽  
Vol 130 ◽  
pp. 330-332
Author(s):  
James E. Neff
Keyword(s):  
Spatial Distribution ◽  
Effective Temperature ◽  
Three Dimensional ◽  
Stellar Atmospheres ◽  
Surface Structures ◽  
High Dispersion ◽  
The Sun ◽  
Late Type ◽  
Magnetic Topology ◽  
Radial Dimension

Several groups at this meeting are presenting maps of the spatial distribution of either brightness or effective temperature in the photospheres of rapidly-rotating, late-type stars. It is generally believed that structure seen in these maps traces the magnetic topology, in analogy with the Sun. We expect the structure of the outer atmospheres (i.e., chromosphere and corona) of these stars to be even more directly tied to the magnetic topology; the magnetic structure is three-dimensional. In order to probe the radial dimension of stellar atmospheres, we need to combine maps of the spatial distribution of emission from chromospheres and coronae with these detailed photospheric maps.Along with collaborators at Armagh, Catania, Boulder, Paris, Helsinki, and Stony Brook, I have been obtaining high-dispersion ultraviolet spectra of several rapidly-rotating, late-type stars using the IUE spacecraft. I discuss results for two stars, El Eridani and HD 199178, for which photospheric maps are presented elsewhere at this conference.

scholarly journals Three-dimensional simulations of scattering polarization and the Hanle effect in MHD chromospheric models

2014 ◽  
Vol 10 (S305) ◽  
pp. 360-367 ◽  
Author(s):  
J. Štěpán
Keyword(s):  
Recent Progress ◽  
Spectral Synthesis ◽  
Three Dimensional ◽  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Line Formation ◽  
Spatially Resolved ◽  
Synthetic Spectra ◽  
Hanle Effect ◽  
Three Dimensional Simulations

AbstractScattering line polarization and the Hanle effect are among the most important mechanisms for diagnostics of the solar and stellar atmospheres. The fact that real stellar atmospheres are horizontally inhomogeneous makes the spectral synthesis and interpretation very challenging because the effect of thermodynamic fluctuations on spectral line polarization is entangled with the action of magnetic fields. This applies to the spatially resolved as well as to the averaged spectra. The necessary step towards the interpretation of such spectra is to study the line formation in sufficiently realistic 3D MHD models and compare the synthetic spectra with observations. This paper gives an overview of recent progress in the field of 3D NLTE synthesis of polarized spectral lines resulting from investigations with the radiative transfer code PORTA.

Differential Fe I Line Shifts as Convective Signatures in R = 40 000 Échelle Spectra?

1999 ◽  
Vol 170 ◽  
pp. 286-290
Author(s):  
Dag Gullberg
Keyword(s):  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Wavelength Shift ◽  
Cross Correlations

AbstractThe convection in stellar atmospheres causes spectral lines to be more or less blue-shifted. The differential wavelength shift between two groups of spectral lines could be used to characterize the convection. This paper describes such an effort using cross-correlations between R ≈ 40 000 échelle spectra and laboratory wavelength templates.

scholarly journals Simulation of the small-scale magnetism in main-sequence stellar atmospheres

2018 ◽  
Vol 614 ◽  
pp. A78 ◽  
Author(s):  
R. G. Salhab ◽  
O. Steiner ◽  
S. V. Berdyugina ◽  
B. Freytag ◽  
S. P. Rajaguru ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Flux ◽  
Spectral Type ◽  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Small Scale ◽  
Positive Contribution ◽  
The Sun ◽  
Free Atmosphere ◽  
Paper Properties

Context. Observations of the Sun tell us that its granular and subgranular small-scale magnetism has significant consequences for global quantities such as the total solar irradiance or convective blueshift of spectral lines. Aims. In this paper, properties of the small-scale magnetism of four cool stellar atmospheres, including the Sun, are investigated, and in particular its effects on the radiative intensity and flux. Methods. We carried out three-dimensional radiation magnetohydrodynamic simulations with the CO5BOLD code in two different settings: with and without a magnetic field. These are thought to represent states of high and low small-scale magnetic activity of a stellar magnetic cycle. Results. We find that the presence of small-scale magnetism increases the bolometric intensity and flux in all investigated models. The surplus in radiative flux of the magnetic over the magnetic field-free atmosphere increases with increasing effective temperature, Teff, from 0.47% for spectral type K8V to 1.05% for the solar model, but decreases for higher effective temperatures than solar. The degree of evacuation of the magnetic flux concentrations monotonically increases with Teff as does their depression of the visible optical surface, that is the Wilson depression. Nevertheless, the strength of the field concentrations on this surface stays remarkably unchanged at ≈1560 G throughout the considered range of spectral types. With respect to the surrounding gas pressure, the field strength is close to (thermal) equipartition for the Sun and spectral type F5V but is clearly sub-equipartition for K2V and more so for K8V. The magnetic flux concentrations appear most conspicuous for model K2V owing to their high brightness contrast. Conclusions. For mean magnetic flux densities of approximately 50 G, we expect the small-scale magnetism of stars in the spectral range from F5V to K8V to produce a positive contribution to their bolometric luminosity. The modulation seems to be most effective for early G-type stars.

scholarly journals Spectroscopic Radial Velocities: Photospheric Lineshifts Calibrated By Hipparcos

1998 ◽  
Vol 11 (1) ◽  
pp. 564-564
Author(s):  
D. Gullberg ◽  
D. Dravins
Keyword(s):  
Radial Velocity ◽  
High Precision ◽  
Open Clusters ◽  
Radial Velocities ◽  
Radial Motion ◽  
Spectral Lines ◽  
Gravitational Redshift ◽  
The Sun

Wavelengths of stellar spectral lines depend not only on the star’s motion. Until recently, accurate studies of shifts not caused by radial motion were feasible only for the Sun. Solar lineshifts are interpreted as gravitational redshift (636 m/s) and convective blueshifts (~ 400 m/s; caused by velocity-brightness correlations). In other stars, such effects may be greater (Dravins & Nordlund 1990). Accurate astrometric radial velocities are now available from Hipparcos (Dravins et al. 1997a; 1997b), permitting studies of such shifts also in some other stars. For such stars in the open clusters of Hyades, Ursa Major and Coma Berenices, a spectroscopic program is in progress, analyzing wavelength shifts in groups of lines with different strengths, excitation potentials, etc., using the ELODIE high-precision radial-velocity instrument (Baranne et al. 1996) at Haute-Provence Observatory.

scholarly journals Tomography of cool giant and supergiant star atmospheres

2018 ◽  
Vol 610 ◽  
pp. A29 ◽  
Author(s):  
K. Kravchenko ◽  
S. Van Eck ◽  
A. Chiavassa ◽  
A. Jorissen ◽  
B. Freytag ◽  
...  
Keyword(s):  
Spectral Line ◽  
Three Dimensional ◽  
Velocity Fields ◽  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Line Of Sight ◽  
Line Formation ◽  
Tomographic Method ◽  
1D Model ◽  
Supergiant Star

Context. Cool giant and supergiant star atmospheres are characterized by complex velocity fields originating from convection and pulsation processes which are not fully understood yet. The velocity fields impact the formation of spectral lines, which thus contain information on the dynamics of stellar atmospheres. Aim. The tomographic method allows to recover the distribution of the component of the velocity field projected on the line of sight at different optical depths in the stellar atmosphere. The computation of the contribution function to the line depression aims at correctly identifying the depth of formation of spectral lines in order to construct numerical masks probing spectral lines forming at different optical depths. Methods. The tomographic method is applied to one-dimensional (1D) model atmospheres and to a realistic three-dimensional (3D) radiative hydrodynamics simulation performed with CO5BOLD in order to compare their spectral line formation depths and velocity fields. Results. In 1D model atmospheres, each spectral line forms in a restricted range of optical depths. On the other hand, in 3D simulations, the line formation depths are spread in the atmosphere mainly because of temperature and density inhomogeneities. Comparison of cross-correlation function profiles obtained from 3D synthetic spectra with velocities from the 3D simulation shows that the tomographic method correctly recovers the distribution of the velocity component projected on the line of sight in the atmosphere.

scholarly journals C I 5380 Å: A SPECTRAL LINE WITH LOW INFLUENCE FROM STARSPOTS DYNAMICS AND MAGNETIC CYCLES

2012 ◽  
Vol 18 ◽  
pp. 178-181
Author(s):  
D. SOUTO ◽  
J. D. DO NASCIMENTO
Keyword(s):  
Radial Velocity ◽  
Activity Cycle ◽  
Magnetic Activity ◽  
Spectral Lines ◽  
The Sun ◽  
Total Irradiance ◽  
Modulation Signal ◽  
Solar Magnetic Activity ◽  
Activity Cycles ◽  
Magnetic Cycles

In the Sun-as-a-star Project, the sun was observed spectroscopically and photometrically for more than 25 years in order to determine variability and luminosity changes. This project detected systematic longterm decrease in the total irradiance as a consequence of the solar magnetic activity cycle (scale of years) and variability on solar activity from a time scale of days-months. The solar magnetic activity cycles could mimic the radial velocity modulation signal of a long-period companion in several spectral lines. This effect is an important limitation for the exoplanet searches programs using the radial velocity technique. The Lomb-Scargle periodogram analysis of the Sun-as-a-star spectroscopic data shows that the photospheric line C I 5380 Å and other 11 lines seems to not show significant influence from the rotational or cromospheric magnetic activity modulation. Thus, our analysis suggest that C I 5380 Å line could be used in programs that require extremely line stability.

scholarly journals Temporal evolution and correlations of optical activity indicators measured in Sun-as-a-star observations

2019 ◽  
Vol 627 ◽  
pp. A118 ◽  
Author(s):  
J. Maldonado ◽  
D. F. Phillips ◽  
X. Dumusque ◽  
A. Collier Cameron ◽  
R. D. Haywood ◽  
...  
Keyword(s):  
Radial Velocity ◽  
Optical Activity ◽  
Temporal Evolution ◽  
Solar Rotation ◽  
Active Regions ◽  
Stellar Atmospheres ◽  
Solar Photosphere ◽  
The Sun ◽  
Stellar Activity ◽  
Velocity Variations

Context. Understanding stellar activity in solar-type stars is crucial for the physics of stellar atmospheres as well as for ongoing exoplanet programmes. Aims. We aim to test how well we understand stellar activity using our own star, the Sun, as a test case. Methods. We performed a detailed study of the main optical activity indicators (Ca II H & K, Balmer lines, Na I D1 D2, and He I D3) measured for the Sun using the data provided by the HARPS-N solar-telescope feed at the Telescopio Nazionale Galileo. We made use of periodogram analyses to study solar rotation, and we used the pool variance technique to study the temporal evolution of active regions. The correlations between the different activity indicators as well as the correlations between activity indexes and the derived parameters from the cross-correlation technique are analysed. We also study the temporal evolution of these correlations and their possible relationship with indicators of inhomogeneities in the solar photosphere like sunspot number or radio flux values. Results. The value of the solar rotation period is found in all the activity indicators, with the only exception being Hδ. The derived values vary from 26.29 days (Hγ line) to 31.23 days (He I). From an analysis of sliding periodograms we find that in most of the activity indicators the spectral power is split into several “bands” of periods around 26 and 30 days. They might be explained by the migration of active regions between the equator and a latitude of ∼30°, spot evolution, or a combination of both effects. A typical lifetime of active regions of approximately ten rotation periods is inferred from the pooled variance diagrams, which is in agreement with previous works. We find that Hα, Hβ, Hγ, Hϵ, and He I show a significant correlation with the S index. Significant correlations between the contrast, bisector span, and the heliocentric radial velocity with the activity indexes are also found. We show that the full width at half maximum, the bisector, and the disc-integrated magnetic field correlate with the radial velocity variations. The correlation of the S index and Hα changes with time, increasing with larger sun spot numbers and solar irradiance. A similar tendency with the S index and radial velocity correlation is also present in the data. Conclusions. Our results are consistent with a scenario in which higher activity favours the correlation between the S index and the Hα activity indicators and between the S index and radial velocity variations.

scholarly journals Observations of Stellar Spectra Related to Extended Atmospheres

1973 ◽  
Vol 51 ◽  
pp. 117-133
Author(s):  
K. O. Wright
Keyword(s):  
Binary Systems ◽  
Variable Stars ◽  
Stellar Atmospheres ◽  
Spectral Lines ◽  
Emission Lines ◽  
Atmospheric Effects ◽  
The Sun ◽  
Be Stars ◽  
Primary Star ◽  
Additional Information

Although the Struve Symposium was planned to discuss conditions in binary stellar systems, it is necessary to consider the observations of extended atmospheres in single stars in order to compare them with those of binary systems. The Sun, being the closest star, has the first chromosphere that was observed, and we can assume that the many features observed on the Sun will be much enhanced in giant stars with more extended atmospheres. Among the early-type stars, the hydrogen emission lines in the spectra of Be stars are known to vary and this suggests the atmospheres are unstable. Similar, even more pronounced effects, are seen in the spectra of shell stars such as 48 Librae. The supergiant stars have also been shown to have spectral lines that change, often quite rapidly, with time. Smolinski has observed in the spectra of some F and G-type supergiants much-broadened absorption lines that also vary with time. Wilson's systematic studies of the H and K emission lines in late-type stars have greatly extended our knowledge of stellar chromospheres, and the ionized magnesium lines at 2800 Å should give additional information as more extra-terrestrial observations are obtained. Numerous observations of the ζ Aur stars have been obtained near eclipse; ζ Aur and 31 Cyg seem to be similar systems and show similar phenomena – a rise in temperature outward from the photosphere and an extensive, variable calcium chromosphere, but a viable theory for these chromospheres has not yet been given. VV Cep and 32 Cyg probably are larger and more complex systems. The emission lines in the ultraviolet spectra of the M-type stars may be indicators of coronae in these cooler atmospheres. Most variable stars seem to have extended atmospheres and, when detailed studies are made, show changes in both light and spectrum. Shock-wave phenomena are undoubtedly important in the analysis of many of these stars, as also are fluorescent, selective-excitation processes. Flare stars, though generally dwarfs, are of great interest in studies of stellar atmospheres.Extended atmospheres can be detected from observed gross deviations from local thermodynamic equilibrium, as in the study of emission lines, large velocity fields, and dilution effects. It is probable that the presence of a companion affects the atmosphere of the primary star in close double stars, but in stars like ζ Aur and 31 Cyg, the atmospheric effects produced near eclipse may be among the best clues for the interpretation of extended stellar atmospheres.

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