Stellar Surface Convection, Line Asymmetries, and Wavelength Shifts (original) (raw)
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Subsurface convection zones in hot massive stars and their observable consequences
Astronomy & Astrophysics, 2009
Context. We study the convection zones in the outer envelope of hot massive stars which are caused by opacity peaks associated with iron and helium ionization. Aims. We determine the occurrence and properties of these convection zones as function of the stellar parameters. We then confront our results with observations of OB stars. Methods. A stellar evolution code is used to compute a grid of massive star models at different metallicities. In these models, the mixing length theory is used to characterize the envelope convection zones. Results. We find the iron convection zone (FeCZ) to be more prominent for lower surface gravity, higher luminosity and higher initial metallicity. It is absent for luminosities below about 10 3.2 L ⊙ , 10 3.9 L ⊙ , and 10 4.2 L ⊙ for the Galaxy, LMC and SMC, respectively. We map the strength of the FeCZ on the Hertzsprung-Russell diagram for three metallicities, and compare this with the occurrence of observational phenomena in O stars: microturbulence, non-radial pulsations, wind clumping, and line profile variability.
A Technique for the Study of Stellar Convection: The Visible Solar Flux Spectrum
Publications of the Astronomical Society of the Pacific, 1999
Convective motions in the solar photosphere produce line asymmetries that are traditionally studied using line bisectors observed with very high spectral resolution and signal-to-noise ratio. We study relative line core shifts as a surrogate for bisectors, using the most current laboratory line positions of 298 Fe I lines in the sun's visible ux spectrum and a new technique for determining line position. We show that this technique can be applied at signi cantly lower spectral resolutions and signal-to-noise ratios than has been done using bisectors. These line core shifts show a clear dependence on both line strength and wavelength. The possibility of a dependence on the lower excitation energy is also examined.
Redshifts of high-temperature emission lines in the far-ultraviolet spectra of late-type stars
The Astrophysical Journal, 1983
High-dispersion IUE spectra of six late-type stars exhibit small but statistically significant differential redshifts of high-temperature emission lines, like Si iv and C iv, with respect to low-temperature fines like S T and O I. A well-exposed, small-aperture spectrum of the active chromosphere binary Capella (a Aurigae A: G6 III + F9 III) establishes that the high-temperature lines are redshifted in an absolute sense with respect to the accurately determined photospheric velocity of the system at single-fine phase 0.50. We discuss several possible explanations for the stellar redshifts, including a warm wind (10 5 K) in which apparent redshifts are produced in optically thick fines by an accelerating outflow, and the downflowing component of a vertical circulation system for which the up-leg portion of the flow is too cool, too hot, or too tenuous to be visible in Si iv and C iv. If the second scenario is true, the stellar redshifts may provide an important phenomenological fink to the downflows observed in 10 5 K species over magnetic active regions on the Sun.
Intrinsic spectral blueshifts in rapidly rotating stars?
Astronomy and Astrophysics, 2003
Spectroscopic radial velocities for several nearby open clusters suggest that spectra of (especially earliertype) rapidly rotating stars are systematically blueshifted by 3 km s −1 or more, relative to the spectra of slowly rotating ones. Comparisons with astrometrically determined radial motions in the Hyades suggests this to be an absolute blueshift, relative to wavelengths naively expected from stellar radial motion and gravitational redshift. Analogous trends are seen also in most other clusters studied (Pleiades, Coma Berenices, Praesepe, α Persei, IC 2391, NGC 6475, IC 4665, NGC 1976 and NGC 2516. Possible mechanisms are discussed, including photospheric convection, stellar pulsation, meridional circulation, and shock-wave propagation, as well as effects caused by template mismatch in determining wavelength displacements. For early-type stars, a plausible mechanism is shock-wave propagation upward through the photospheric line-forming regions. Such wavelength shifts thus permit studies of certain types of stellar atmospheric dynamics and -irrespective of their cause -may influence deduced open-cluster membership (when selected from common velocity) and deduced cluster dynamics (some types of stars might show fortuitous velocity patterns).
Wavelength Shifts in Solar-Type Spectra
Spectral-line displacements away from the wavelengths naively expected from the Doppler shift caused by stellar radial motion may originate as convective shifts (correlated velocity and brightness patterns in the photosphere), as gravitational redshifts, or perhaps be induced by wave motions. Absolute lineshifts, in the past studied only for the Sun, are now accessible also for other stars thanks to astrometric determination of stellar radial motion, and spectrometers with accurate wavelength calibration. Comparisons between spectroscopic apparent radial velocities and astrometrically determined radial motions reveal greater spectral blueshifts in F-type stars than in the Sun (as theoretically expected from their more vigorous convection), further increasing in A-type stars (possibly due to atmospheric shockwaves). An important near-future development to enable a further analysis of stellar surface structure will be the study of wavelength variations across spatially resolved stella...
Granulation in K-type dwarf stars
Astronomy and Astrophysics, 2008
Aims. We seek to detect and quantify the effects of surface convection (granulation) on the line spectra of K-dwarfs as a first step towards a rigorous testing of hydrodynamic models for their atmospheres. Methods. Very high resolution (R ≃ 160, 000 − 210, 000), high signal-to-noise ratio (S /N 300) spectra of nine bright K-dwarfs were obtained with the 2dcoudé spectrograph on the 2.7 m Telescope at McDonald Observatory to determine wavelength shifts and asymmetries of Fe i lines. Spectra of the same stars acquired with the High Resolution Spectrograph (R ≃ 120, 000) on the 9.2 m Hobby Eberly Telescope were used as radial velocity templates to calibrate the wavelength scale of the 2dcoudé spectra.
Turbulence and magnetic spots at the surface of hot massive stars
Proceedings of The International Astronomical Union, 2010
Hot luminous stars show a variety of phenomena in their photospheres and in their winds which still lack clear physical explanations at this time. Among these phenomena are non-thermal line broadening, line profile variability (LPVs), discrete absorption components (DACs), wind clumping and stochastically excited pulsations. Cantiello et al. (2009) argued that a convection zone close to the surface of hot, massive stars, could be responsible for some of these phenomena. This convective zone is caused by a peak in the opacity due to iron recombination and for this reason is referred as the "iron convection zone" (FeCZ). 3D MHD simulations are used to explore the possible effects of such subsurface convection on the surface properties of hot, massive stars. We argue that turbulence and localized magnetic spots at the surface are the likely consequence of subsurface convection in early type stars.
Investigating the origin of cyclical spectral variations in hot, massive stars
Proceedings of the International Astronomical Union, 2013
OB stars are known to exhibit various types of wind variability, as detected in their ultraviolet spectra, amongst which are the ubiquitous discrete absorption components (DACs). These features have been associated with large-scale azimuthal structures extending from the base of the wind to its outer regions: corotating interaction regions (CIRs). There are several competing hypotheses as to which physical processes may perturb the star's surface and generate CIRs, including magnetic fields and non radial pulsations (NRPs), the subjects of this paper with a particular emphasis on the former. Although large-scale magnetic fields are ruled out, magnetic spots deserve further investigation, both on the observational and theoretical fronts.
Constraints on stellar convection from multi-colour photometry of ? Scuti stars
Astronomy and Astrophysics, 2003
In δ Scuti star models, the calculated amplitude ratios and phase differences for multi-colour photometry exhibit a strong dependence on convection. These observables are tools for determination of the spherical harmonic degree, ℓ, of the excited modes. The dependence on convection enters through the complex parameter f , which describes bolometric flux perturbation. We present a method of simultaneous determination of f and harmonic degree ℓ from multi-colour data and apply it to three δ Scuti stars. The method indeed works.
2013
Context. The convective envelopes of cool main-sequence stars harbour magnetic fields with a complex global and local structure. These fields affect the near-surface convection and the outer stellar atmospheres in many ways and are responsible for the observable magnetic activity of stars. Aims. Our aim is to understand the local structure in unipolar regions with moderate average magnetic flux density. These correspond to plage regions covering a substantial fraction of the surface of the Sun (and likely also the surface of other Sun-like stars) during periods of high magnetic activity. Methods. We analyse the results of 18 local-box magnetohydrodynamics simulations covering the upper layers of the convection zones and the photospheres of cool main-sequence stars of spectral types F to early M. The average vertical field in these simulations ranges from 20 to 500 G. Results. We find a substantial variation of the properties of the surface magnetoconvection between main-sequence stars of different spectral types. As a consequence of a reduced efficiency of the convective collapse of flux tubes, M dwarfs lack bright magnetic structures in unipolar regions of moderate field strength. The spatial correlation between velocity and the magnetic field as well as the lifetime of magnetic structures and their sizes relative to the granules vary significantly along the model sequence of stellar types.