The Observation and Analysis of Stellar Photospheres

Gray, David F.

doi:10.1017/cbo9781316036570

Cited by 1,586 publications

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“…Two models are currently used to describe the line broadening carried by macroturbulent motions: isotropic Gaussian and anisotropic with radial and tangential components, which each have a Gaussian-dependent velocity distribution (Gray 1975(Gray , 1992. Although the anisotropic model depends on at least three free parameters, these authors assume that both models are characterized by a unique dispersion of macroturbulent velocities σ mt , which is somewhat correlated with the stellar effective temperature.…”

Section: Preliminary Commentsmentioning

confidence: 99%

Critical study of the distribution of rotational velocities of Be stars

Zorec

Frémat

Souza

et al. 2016

A&A

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Context. Among intermediate-mass and massive stars, Be stars are the fastest rotators in the main sequence (MS) and, as such, these stars are a cornerstone to validate models of structure and evolution of rotating stars. Several phenomena, however, induce under-or overestimations either of their apparent Vsin i, or true velocity V. Aims. In the present contribution we aim at obtaining distributions of true rotational velocities corrected for systematic effects induced by the rapid rotation itself, macroturbulent velocities, and binarity. Methods. We study a set of 233 Be stars by assuming they have inclination angles distributed at random. We critically discuss the methods of Cranmer and Lucy-Richardson, which enable us to transform a distribution of projected velocities into another distribution of true rotational velocities, where the gravitational darkening effect on the Vsin i parameter is considered in different ways. We conclude that iterative algorithm by Lucy-Richardson responds at best to the purposes of the present work, but it requires a thorough determination of the stellar fundamental parameters. Results. We conclude that once the mode of ratios of the true velocities of Be stars attains the value V/V c 0.77 in the main-sequence (MS) evolutionary phase, it remains unchanged up to the end of the MS lifespan. The statistical corrections found on the distribution of ratios V/V c for overestimations of Vsin i, due to macroturbulent motions and binarity, produce a shift of this distribution toward lower values of V/V c when Be stars in all MS evolutionary stages are considered together. The mode of the final distribution obtained is at V/V c 0.65. This distribution has a nearly symmetric distribution and shows that the Be phenomenon is characterized by a wide range of true velocity ratios 0.3 V/V c 0.95. It thus suggests that the probability that Be stars are critical rotators is extremely low. Conclusions. The corrections attempted in the present work represent an initial step to infer indications about the nature of the Be-star surface rotation that will be studied in the second paper of this series.

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Section: Preliminary Commentsmentioning

confidence: 99%

Critical study of the distribution of rotational velocities of Be stars

Zorec

Frémat

Souza

et al. 2016

A&A

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“…Differential rotation provides not only constraints on the image space in terms of the resulting sharpness of the image but also on the data space because the differential rotation alters the velocity distribution of the individual local line profiles over the visible surface and thus produces a modification of the disk-integrated flux profile (see, e.g. Gray 2005). The data space approach has been extensively used in the past for determining the differential rotation of a number of stars (e.g.…”

Section: Differential Rotation Inversionmentioning

confidence: 99%

The magnetic field topology of the weak-lined T Tauri star V410 Tauri

Carroll

Strassmeier

Rice

et al. 2012

A&A

112

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Aims. In a follow-up investigation we present Zeeman-Doppler maps of the weak-lined T Tauri star (WTTS) V410 Tau. As a rapid rotating star and a typical WTTS the stellar surface of V410 Tau is accessible to surface imaging techniques and allows us to detect and reconstruct the major magnetic surface features on this pre-main sequence star. Methods. The polarized signals we are measuring are on the order of 10 −4 to 10 −3 and are hidden well below the noise level of a single observation. A new line profile reconstruction technique based on a singular value decomposition (SVD) allows us to extract the weak polarized line profiles (Stokes V) as well as the intensity profiles (Stokes I). One of the key features of the line profile reconstruction is that the SVD line profiles are amenable to radiative transfer modeling within our Zeeman-Doppler Imaging code iMap. The code also utilizes a new iterative regularization scheme which is independent of any additional surface constraints. To provide more stability a vital part of our inversion strategy is to invert both Stokes I and Stokes V profiles to simultaneously reconstruct the temperature and magnetic field surface distribution of V410 Tau. A new image-shear analysis is also implemented to allow the search for image and line profile distortions induced by a differential rotation of the star. Results. The magnetic field structure we obtain for V410 Tau shows a good spatial correlation with the surface temperature and is dominated by a strong field within the cool polar spot. The Zeeman-Doppler maps exhibit a large-scale organization of both polarities around the polar cap in the form of a twisted bipolar structure. The magnetic field reaches a value of almost 2 kG within the polar region but smaller fields are also present down to lower latitudes. The pronounced non-axisymmetric field structure and the nondetection of a differential rotation for V410 Tau supports the idea of an underlying α 2 -type dynamo, which is predicted for WTTS.

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“…Following Strassmeier et al (2000) we use V − R colors computed via B − V colors from the color-color relation of Gray (1992), since this minimizes the effects of the rotational modulation of star spots (Strassmeier et al 1994). In order to calculate the activity index properly, we need to take into account the contribution of the photosphere.…”

Section: Activity Indexmentioning

confidence: 99%

Binarity, activity and metallicity among late-type stars

Dall¹,

Bruntt²,

Strassméier

2005

A&A

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We present the first in a series of papers that attempt to investigate the relation between binarity, magnetic activity, and chemical surface abundances of cool stars. In the current paper, we lay out and test two abundance analysis methods and apply them to two well-known, active, single stars, HD 27536 (G8IV-III) and HD 216803 (K5V), presenting photospheric fundamental parameters and abundances of Li, Al, Ca, Si, Sc, Ti, V, Cr, Fe, Co and Ni. The abundances from the two methods agree within the errors for all elements except calcium in HD 216803, which means that either method yields the same fundamental model parameters and the same abundances. Activity is described by the radiative loss in the Ca ii H and K lines with respect to the bolometric luminosity, through the activity index R HK . Binarity is established by very precise radial velocity (RV) measurements using HARPS spectra. The spectral line bisectors are examined for correlations between RV and bisector shape to distinguish between the effects of stellar activity and unseen companions. We show that HD 27536 exhibit RV variations mimicking the effect of a low-mass (m ∼ 4 M J ) companion in a relatively close (a ∼ 1 AU) orbit. The variation is strongly correlated with the activity, and consistent with the known photometric period P = 306.9 d, demonstrating a remarkable coherence between R HK and the bisector shape, i.e. between the photosphere and the chromosphere. We discuss the complications involved in distinguishing between companion and activity induced RV variations.

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The Observation and Analysis of Stellar Photospheres

Cited by 1,586 publications

References 0 publications

Critical study of the distribution of rotational velocities of Be stars

Critical study of the distribution of rotational velocities of Be stars

The magnetic field topology of the weak-lined T Tauri star V410 Tauri

Binarity, activity and metallicity among late-type stars

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