[Paper Review] Rotational velocities of A-type stars. III. Velocity distributions

F. Royer, J. Zorec+1 more|arXiv (Cornell University)|Oct 26, 2006

Stellar, planetary, and galactic studiesPhysics and Astronomy59 references128 citations

TL;DR

This study analyzes rotational velocities of ~1,100 single A-type stars using robust statistical deconvolution to correct for projection effects and measurement errors. It reveals bimodal equatorial velocity distributions in late B and early A-type stars, suggesting pre-main sequence angular momentum loss and redistribution, while intermediate and late A-type stars show a striking lack of slow rotators, likely due to evolutionary effects.

ABSTRACT

Aim - In this work, a sample of vsini of B9 to F2-type main sequence single stars has been built from highly homogeneous vsini parameters determined for a large sample cleansed from objects presenting the Am and Ap phenomenon as well as from all known binaries. The aim is to study the distributions of rotational velocities in the mass range of A-type stars for normal single objects. Methods - Robust statistical methods are used to rectify the vsini distributions from the projection effect and the error distribution. The equatorial velocity distributions are obtained for an amount of about 1100 stars divided in six groups defined by the spectral type, under the assumption of randomly orientated rotational axes. Results - We show that late B and early A-type main-sequence stars have genuine bimodal distributions of true equatorial rotational velocities due probably to phenomena of angular momentum loss and redistribution the star underwent before reaching the main sequence. A striking lack of slow rotators is noticed among intermediate and late A-type stars. The bimodal-like shape of their true equatorial rotational velocity distributions could be due to evolutionary effects.

Motivation & Objective

• To investigate the true equatorial rotational velocity distributions of single A-type stars, correcting for projection and measurement effects.
• To determine whether rotational velocity distributions in A-type stars are unimodal or multimodal, particularly in the early main sequence phase.
• To assess whether the observed distributions reflect stellar formation characteristics or internal structural properties, especially around the transition between convective and radiative envelopes.
• To identify signatures of angular momentum loss and redistribution in the rotational velocity distributions of A-type stars.

Proposed method

• The study uses a sample of ~1,100 single A-type stars, excluding Am, Ap stars and binaries, to ensure homogeneity.
• It applies kernel density estimation with a Gaussian kernel and Sheather-Jones bandwidth selection to smooth the observed v sin i distributions.
• The Lucy iterative deconvolution technique is used to correct for the effects of measurement errors and inclination projection, using both error and inclination distributions as deconvolution kernels.
• The method involves iterative refinement of the density estimate until convergence, using a Kolmogorov-Smirnov test at a 1% significance level to stop iterations.
• Variability bands are computed to assess the significance of modes in the deconvolved distributions, ensuring statistical reliability of bimodal features.
• The analysis is performed separately for six spectral type groups, with results transformed to equatorial velocity distributions under the assumption of random rotational axis orientations.

Experimental results

Research questions

• RQ1 Do single late B- and early A-type main-sequence stars exhibit unimodal or multimodal rotational velocity distributions after correcting for projection and measurement effects?
• RQ2 What causes the bimodal-like structure in the equatorial velocity distributions of intermediate and late A-type stars, and is it linked to evolutionary processes?
• RQ3 Is the observed lack of slow rotators among intermediate and late A-type stars due to intrinsic stellar evolution or selection effects?
• RQ4 To what extent do pre-main sequence angular momentum loss and redistribution processes leave imprints on the rotational velocity distributions of A-type stars?
• RQ5 Can the bimodal nature of rotational velocity distributions in early A-type stars be attributed to formation-related processes rather than internal structural properties?

Key findings

• Late B and early A-type main-sequence stars exhibit genuine bimodal distributions of true equatorial rotational velocities, indicating that pre-main sequence angular momentum loss and redistribution processes significantly shape their rotational properties.
• The bimodal distribution is attributed to the combined effects of angular momentum loss and redistribution during pre-main sequence evolution, rather than to observational biases.
• Intermediate and late A-type stars show a pronounced lack of slow rotators, suggesting that evolutionary processes—possibly related to the transition between convective and radiative envelopes—suppress the formation or survival of slow rotators.
• The observed rotational velocity distributions in intermediate and late A-type stars are consistent with a bimodal-like shape, likely driven by evolutionary effects rather than formation characteristics.
• The statistical deconvolution method successfully isolates the intrinsic rotational velocity distributions, revealing features obscured by projection and measurement errors.
• The variability bands confirm that the observed modes in the deconvolved distributions are statistically significant, supporting the robustness of the bimodal and unimodal features identified.