The Projected Rotational Velocity Distribution of a Sample of OB stars from a Calibration based on Synthetic He I lines

TL;DR

This study develops a calibration method using synthetic He I lines to accurately measure the projected rotational velocities of OB stars, providing insights into their rotational velocity distribution.

Contribution

It introduces a new calibration based on synthetic He I line profiles for determining vsini in OB stars, validated with a large stellar sample.

Findings

Calibration effectively estimates vsini for O9-B5 stars.

vsini distribution in Cep OB2 aligns with other associations.

Synthetic spectra provide reliable vsini measurements.

Abstract

We derive projected rotational velocities (vsini) for a sample of 156 Galactic OB star members of 35 clusters, HII regions, and associations. The HeI lines at $\lambda\lambda$4026, 4388, and 4471A were analyzed in order to define a calibration of the synthetic HeI full-widths at half maximum versus stellar vsini. A grid of synthetic spectra of HeI line profiles was calculated in non-LTE using an extensive helium model atom and updated atomic data. The vsini's for all stars were derived using the He I FWHM calibrations but also, for those target stars with relatively sharp lines, vsini values were obtained from best fit synthetic spectra of up to 40 lines of CII, NII, OII, AlIII, MgII, SiIII, and SIII. This calibration is a useful and efficient tool for estimating the projected rotational velocities of O9-B5 main-sequence stars. The distribution of vsini for an unbiased sample of early B stars in the unbound association Cep OB2 is consistent with the distribution reported elsewhere for other unbound associations.