The IACOB project: I. Rotational velocities in Northern Galactic O and early B-type stars revisited. The impact of other sources of line-broadening

S. Simón-Díaz & A. Herrero

Instituto de Astrofísica de Canarias

Stellar rotation is an important parameter in the evolution of massive stars. Accurate and reliable measurements of projected rotational velocities in large samples of OB stars are crucial to confront the predictions of stellar evolutionary models with observational constraints.

We reassess previous determinations of projected rotational velocities (vsini) in Galactic OB stars using a large, high quality spectroscopic dataset, and a strategy which account for other sources of broadening appart from rotation affecting the diagnostic lines.

We present a versatile and user friendly IDL tool --- based on a combined Fourier Transform (FT) + goodness of fit (GOF) methodology --- for the line-broadening characterization in OB-type stars. We use this tool to (a) investigate the impact of macroturbulent and microturbulent broadenings on vsini measurements, and (b) determine vsini in a sample of ~200 Galactic OB-type stars, also characterizing the amount of macroturbulent broadening (vmacro) affecting the line profiles.

We present observational evidence illustrating the strengths and limitations of the proposed FT+GOF methodology for the case of OB stars. We confirm previous statements (based on indirect arguments or smaller samples) that the macroturbulent broadening is ubiquitous in the massive star domain. We compare the newly derived vsini in the case of O stars and early-B Supergiants and Giants (where the effect of the macroturbulent broadening is found to be larger) with previous determinations not accounting for this extra line-broadening contribution, and show that those cases with vsini < 120 km/s need to be systematically revised downwards by ~25 (+/-20) km/s. We suggest that microturbulence may impose an upper limit below which vsini and vmacro could be incorrectly derived by means of the proposed methodology as presently used, and discuss the implications of this statement on the study of relatively narrow line massive stars.

An investigation of impact of the revised vsini distributions on the predictions by massive star evolutionary models is now warranted. Also, the reliability of vsini measurements in the low vsini regime, using a more precise description of the intrinsic profiles used for the line-broadening analysis, needs to be further investigated.

Reference: A&A
Status: Manuscript has been accepted


Comments: Accepted for publication in A&A (19 pages, 15 figures, 6 tables). Tables A1-A5 will be make available in the final edited version of the paper (or under request to SS-D)