[Paper Review] 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|arXiv (Cornell University)|Nov 14, 2013

Stellar, planetary, and galactic studies50 references3 citations

TL;DR

This study revises projected rotational velocities (v sin i) in ~200 Northern Galactic O and early B-type stars by accounting for macroturbulent and microturbulent line broadening using a novel FT+goodness-of-fit (GOF) methodology in IDL. It finds that previous v sin i measurements below 120 km s⁻¹ must be systematically revised downward by ~25 ± 20 km s⁻¹ due to unaccounted macroturbulence, significantly altering the low-v sin i distribution in massive stars.

ABSTRACT

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 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.

Motivation & Objective

• To reassess projected rotational velocities (v sin i) in Galactic O and early B-type stars using high-quality spectroscopy.
• To quantify the impact of macroturbulent and microturbulent broadening on v sin i measurements.
• To develop and validate a robust FT+goodness-of-fit (GOF) methodology for line-broadening analysis in OB stars.
• To investigate the reliability of v sin i measurements in the low-velocity regime, particularly when microturbulence is neglected.
• To assess the implications of revised v sin i distributions for massive star evolutionary models.

Proposed method

• A user-friendly IDL tool was developed based on a combined Fourier Transform (FT) and goodness-of-fit (GOF) methodology for line-broadening analysis in OB stars.
• The method models line profiles using a radial-tangential formulation of macroturbulent velocity (v_m), which better matches observed profiles than isotropic Gaussians.
• The GOF approach was validated against FT results, showing excellent agreement when using the radial-tangential macroturbulent profile.
• The analysis was applied to a sample of ~200 Galactic OB stars, measuring v sin i and v_m simultaneously.
• The method was tested for sensitivity to noise, showing the GOF approach is less sensitive than FT in lower signal-to-noise data.
• The study warns against combining v_m measurements from different literature sources if they assume different macroturbulent profile formulations.

Experimental results

Research questions

• RQ1 To what extent does macroturbulent broadening affect the accuracy of projected rotational velocity (v sin i) measurements in O and early B-type stars?
• RQ2 How do different assumptions about macroturbulent profile shape (e.g., isotropic vs. radial-tangential) impact v sin i and v_m determinations?
• RQ3 What is the effect of neglecting microturbulence on v sin i measurements, especially in the low-velocity regime?
• RQ4 Can the FT+GOF methodology reliably disentangle rotational broadening from microturbulent and macroturbulent contributions in high-resolution spectra?
• RQ5 How do revised v sin i values affect the observed distribution of rotational velocities in massive stars, particularly in the low-v sin i range?

Key findings

• Macroturbulent broadening is ubiquitous in O-type stars and early-B supergiants and giants, not just in evolved B-type stars as previously thought.
• Previous v sin i measurements in stars with v sin i ≤ 120 km s⁻¹ must be systematically revised downward by approximately 25 ± 20 km s⁻¹ due to unaccounted macroturbulence.
• The revised low-v sin i peak in the distribution is now centered at 40–60 km s⁻¹, resolving part of the long-standing discrepancy in the observed low-rotation population.
• The radial-tangential macroturbulent profile provides a significantly better fit than the isotropic Gaussian profile, which systematically underestimates v sin i.
• Microturbulence may impose an upper limit on the reliability of v sin i and v_m measurements when not included in the analysis, particularly for stars with narrow lines.
• The study highlights the need for improved modeling of intrinsic line profiles and further investigation into the role of microturbulence in low-v sin i stars.