[Paper Review] Rotational velocities of A-type stars I. Measurement of vsini in the southern hemisphere

F. Royer, M. Gerbaldi+2 more|ArXiv.org|Oct 22, 2001

Stellar, planetary, and galactic studiesPhysics and Astronomy40 references71 citations

TL;DR

This study presents a new measurement of projected rotational velocities (v sin i) for 525 A-type stars in the southern hemisphere using Fourier transform analysis of high-resolution spectroscopic data from ESO's La Silla Observatory. The key finding is that v sin i values are systematically 10–12% higher than those in the widely used Slettebak et al. (1975) calibration, likely due to unresolved binaries and outdated model atmospheres in the earlier sample.

ABSTRACT

Within the scope of a Key Programme determining fundamental parameters of stars observed by HIPPARCOS, spectra of 525 B8 to F2-type stars brighter than V=8 have been collected at ESO. Fourier transforms of several line profiles in the range 4200-4500 A are used to derive vsini from the frequency of the first zero. Statistical analysis of the sample indicates that measurement error is a function of vsini and this relative error of the rotational velocity is found to be about 6% on average. The results obtained are compared with data from the literature. There is a systematic shift from standard values from Slettebak et al. (1975), which are 10 to 12% lower than our findings. Comparisons with other independent vsini values tend to prove that those from Slettebak et al. are underestimated. This effect is attributed to the presence of binaries in the standard sample of Slettebak et al., and to the model atmosphere they used.

Motivation & Objective

• To measure v sin i for a statistically significant sample of B8 to F2-type stars brighter than V=8 in the southern hemisphere.
• To assess the accuracy and systematic errors of existing v sin i measurements, particularly the widely used Slettebak et al. (1975) calibration.
• To investigate whether the observed scatter in v sin i for late B and early A stars is due to measurement error or physical evolutionary effects.
• To provide an independent, calibration-free measurement of v sin i using the Fourier transform method to avoid biases from prior calibrations.

Proposed method

• Acquired high-resolution spectroscopic data (4200–4500 Å) for 525 A-type stars using ESO's telescopes at La Silla Observatory.
• Applied the Fourier transform technique to stellar line profiles to determine the frequency of the first zero, which correlates with v sin i.
• Used the method of Ramella et al. (1989) to derive v sin i directly from the Fourier transform without relying on FWHM-to-v sin i calibrations.
• Performed statistical analysis to quantify measurement error as a function of v sin i, finding a mean relative error of ~6%.
• Compared results with literature values, including Slettebak et al. (1975), Abt & Morrell (1995), and others, to identify systematic offsets.
• Analyzed standard stars with known v sin i to validate the method and assess discrepancies in the literature.

Experimental results

Research questions

• RQ1 How do the v sin i measurements of A-type stars in the southern hemisphere compare to the widely used Slettebak et al. (1975) calibration?
• RQ2 What is the true measurement uncertainty of v sin i when derived via the Fourier transform method, and how does it vary with rotational velocity?
• RQ3 Are the observed scatter in v sin i values for late B and early A stars due to measurement error or physical evolutionary trends?
• RQ4 To what extent are previous v sin i values underestimated due to unresolved binary systems or outdated model atmospheres?
• RQ5 How consistent are the new v sin i measurements with independent determinations from cross-correlation and spectral synthesis techniques?

Key findings

• The mean relative measurement error for v sin i is approximately 6%, with higher uncertainty at lower rotational velocities.
• The new v sin i measurements are systematically 10–12% higher than those reported in Slettebak et al. (1975), indicating a significant underestimation in the standard calibration.
• The discrepancy with Slettebak et al. (1975) is attributed to the presence of unresolved binary stars in their sample and the use of outdated model atmospheres.
• Independent measurements from Abt & Morrell (1995), Wolff & Simon (1997), and others show consistent trends, supporting the conclusion that Slettebak et al. values are underestimated.
• For standard stars like Sirius, ν Pup, and ϵ Aqr, the new measurements are significantly higher than those in the SCBWP catalogue, with discrepancies up to 30%.
• The study confirms that the apparent scatter in v sin i for late B and early A stars is not solely due to measurement error, suggesting possible physical evolution effects, though no strong evolutionary trend is confirmed for stars above 1.6 M☉.