Exactly What Is Stellar ‘Radial Velocity’?

Lindegren, L.; Dravins, Dainis; Madsen, S.N.

doi:10.1017/s0252921100048387

Cited by 10 publications

(7 citation statements)

References 7 publications

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“…The FTS atlas is on an absolute radial velocity scale as the relative motion between Sun and Earth is known precisely and has been corrected for the solar gravitational redshift of 633 m s −1 (for light intercepted on Earth, Lindegren et al 1999). The line list stems from Asplund et al (2009) and is augmented with additional lines from Asplund et al (2000a) The necessary laboratory wavelengths to place the measured line shifts and bisectors on a velocity scale comes from Nave et al (1994) for Fe  and from Johansson (priv.…”

Section: Observationsmentioning

confidence: 99%

How realistic are solar model atmospheres?

Pereira

Asplund

Collet

et al. 2013

A&A

114

128

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Context.Recently, new solar model atmospheres have been developed to replace classical 1D local thermodynamical equilibrium (LTE) hydrostatic models and used to for example derive the solar chemical composition. Aims. We aim to test various models against key observational constraints. In particular, a 3D model used to derive the solar abundances, a 3D magnetohydrodynamical (MHD) model (with an imposed 10 mT vertical magnetic field), 1D NLTE and LTE models from the PHOENIX project, the 1D MARCS model, and the 1D semi-empirical model of Holweger & Müller. Methods. We confronted the models with observational diagnostics of the temperature profile: continuum centre-to-limb variations (CLVs), absolute continuum fluxes, and the wings of hydrogen lines. We also tested the 3D models for the intensity distribution of the granulation and spectral line shapes. Results. The predictions from the 3D model are in excellent agreement with the continuum CLV observations, performing even better than the Holweger & Müller model (constructed largely to fulfil such observations). The predictions of the 1D theoretical models are worse, given their steeper temperature gradients. For the continuum fluxes, predictions for most models agree well with the observations. No model fits all hydrogen lines perfectly, but again the 3D model comes ahead. The 3D model also reproduces the observed continuum intensity fluctuations and spectral line shapes very well. Conclusions. The excellent agreement of the 3D model with the observables reinforces the view that its temperature structure is realistic. It outperforms the MHD simulation in all diagnostics, implying that recent claims for revised abundances based on MHD modelling are premature. Several weaknesses in the 1D hydrostatic models (theoretical and semi-empirical) are exposed. The differences between the PHOENIX LTE and NLTE models are small. We conclude that the 3D hydrodynamical model is superior to any of the tested 1D models, which gives further confidence in the solar abundance analyses based on it.

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Section: Observationsmentioning

confidence: 99%

How realistic are solar model atmospheres?

Pereira

Asplund

Collet

et al. 2013

A&A

114

128

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“…Although this quantity approximately corresponds to radial velocity, its precise interpretation is model dependent and one should therefore avoid calling it 'radial velocity'. The term radial-velocity measure was proposed by Lindegren et al (1999), and accepted in the later IAU resolution, emphasising both its connection with the traditional spectroscopic method and the fact that it is not quite the radial velocity in the classical sense.…”

Section: The Spectroscopic Parameter: Barycentric Radial-velocity Mea...mentioning

confidence: 99%

The fundamental definition of “radial velocity”

Lindegren¹,

Dravins²

2003

A&A

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163

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Abstract. Accuracy levels of metres per second require the fundamental concept of "radial velocity" for stars and other distant objects to be examined, both as a physical velocity, and as measured by spectroscopic and astrometric techniques. Already in a classical (non-relativistic) framework the line-of-sight velocity component is an ambiguous concept, depending on whether, e.g., the time of light emission (at the object) or that of light detection (by the observer) is used for recording the time coordinate. Relativistic velocity effects and spectroscopic measurements made inside gravitational fields add further complications, causing wavelength shifts to depend, e.g., on the transverse velocity of the object and the gravitational potential at the source. Aiming at definitions that are unambiguous at accuracy levels of 1 m s −1 , we analyse different concepts of radial velocity and their interrelations. At this accuracy level, a strict separation must be made between the purely geometric concepts on one hand, and the spectroscopic measurement on the other. Among the geometric concepts we define kinematic radial velocity, which corresponds most closely to the "textbook definition" of radial velocity as the line-of-sight component of space velocity; and astrometric radial velocity, which can be derived from astrometric observations. Consistent with these definitions, we propose strict definitions also of the complementary kinematic and astrometric quantities, namely transverse velocity and proper motion. The kinematic and astrometric radial velocities depend on the chosen spacetime metric, and are accurately related by simple coordinate transformations. On the other hand, the observational quantity that should result from accurate spectroscopic measurements is the barycentric radial-velocity measure. This is independent of the metric, and to first order equals the line-ofsight velocity. However, it is not a physical velocity, and cannot be accurately transformed to a kinematic or astrometric radial velocity without additional assumptions and data in modelling the process of light emission from the source, the transmission of the signal through space, and its recording by the observer. For historic and practical reasons, the spectroscopic radial-velocity measure is expressed in velocity units as cz B , where c is the speed of light and z B is the observed relative wavelength shift reduced to the solar-system barycentre, at an epoch equal to the barycentric time of light arrival. The barycentric radial-velocity measure and the astrometric radial velocity are defined by recent resolutions adopted by the International Astronomical Union (IAU), the motives and consequences of which are explained in this paper.Key words. techniques: radial velocities -techniques: spectroscopic -astrometry -reference systems -stars: kinematicsmethods: data analysis The need for stringent definitionsRadial velocity is an omnipresent concept in astronomy, and a quantity whose precision of determination has improved significantly in recent...

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“…In this work, we follow this IAU definition of "astrometric radial velocity". The difference with respect to alternative possible definitions is on the order of v 2 r /c, with c = speed of light (Lindegren et al 1999;Lindegren & Dravins, in preparation). Most population I objects (including all clusters and associations considered in this paper) have low velocities, |v r | < 50 km s −1 , resulting in only very small differences, <10 m s −1 , between possible alternative definitions.…”

Section: Calculation Of Astrometric Radial Velocitiesmentioning

confidence: 99%

Astrometric radial velocities

Madsen¹,

Dravins²,

Lindegren³

2002

A&A

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122

174

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Radial motions of stars in nearby moving clusters are determined from accurate proper motions and trigonometric parallaxes, without any use of spectroscopy. Assuming that cluster members share the same velocity vector (apart from a random dispersion), we apply a maximum-likelihood method on astrometric data from Hipparcos to compute radial and space velocities (and their dispersions) in the Ursa Major, Hyades, Coma Berenices, Pleiades, and Praesepe clusters, and for the Scorpius-Centaurus, α Persei, and "HIP 98321" associations. The radial motion of the Hyades cluster is determined to within 0.4 km s −1 (standard error), and that of its individual stars to within 0.6 km s −1. For other clusters, Hipparcos data yield astrometric radial velocities with typical accuracies of a few km s −1. A comparison of these astrometric values with spectroscopic radial velocities in the literature shows a good general agreement and, in the case of the best-determined Hyades cluster, also permits searches for subtle astrophysical differences, such as evidence for enhanced convective blueshifts of F-dwarf spectra, and decreased gravitational redshifts in giants. Similar comparisons for the Scorpius OB2 complex indicate some expansion of its associations, albeit slower than expected from their ages. As a by-product from the radial-velocity solutions, kinematically improved parallaxes for individual stars are obtained, enabling Hertzsprung-Russell diagrams with unprecedented accuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), its main sequence resembles a thin line, possibly with wiggles in it. Although this main sequence has underpopulated regions at certain colours (previously suggested to be "Böhm-Vitense gaps"), such are not visible for other clusters, and are probably spurious. Future space astrometry missions carry a great potential for absolute radial-velocity determinations, insensitive to the complexities of stellar spectra.

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Exactly What Is Stellar ‘Radial Velocity’?

Cited by 10 publications

References 7 publications

How realistic are solar model atmospheres?

How realistic are solar model atmospheres?

The fundamental definition of “radial velocity”

Astrometric radial velocities

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