The VLT-FLAMES Tarantula Survey

Abstract

Context. The initial distribution of spin rates of massive stars is a fingerprint of their elusive formation process. It also sets a key initial condition for stellar evolution and is thus an important ingredient in stellar population synthesis. So far, most studies have focused on single stars. Most O stars are, however, found in multiple systems. Aims. By establishing the spin-rate distribution of a sizeable sample of O-type spectroscopic binaries and by comparing the distributions of binary subpopulations with one another and with that of presumed-single stars in the same region, we aim to constrain the initial spin distribution of O stars in binaries, and to identify signatures of the physical mechanisms that affect the evolution of the spin rates of massive stars. Methods. We use ground-based optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS) to establish the projected equatorial rotational velocities ( e sin i) for components of 114 spectroscopic binaries in 30 Doradus. The e sin i values are derived from the full width at half maximum (FWHM) of a set of spectral lines, using a FWHM vs. e sin i calibration that we derive based on previous line analysis methods applied to single O-type stars in the VFTS sample. Results. The overall e sin i distribution of the primary stars resembles that of single O-type stars in the VFTS, featuring a lowvelocity peak (at e sin i < 200 km s −1 ) and a shoulder at intermediate velocities (200 < e sin i < 300 km s −1 ). The distributions of binaries and single stars, however, differ in two ways. First, the main peak at e sin i ∼ 100 km s −1 is broader and slightly shifted towards higher spin rates in the binary distribution than that of the presumed-single stars. This shift is mostly due to short-period binaries (P orb ∼ < 10 d). Second, the e sin i distribution of primaries lacks a significant population of stars spinning faster than 300 km s −1 , while such a population is clearly present in the single-star sample. The e sin i distribution of binaries with amplitudes of radial velocity variation in the range of 20 to 200 km s −1 (mostly binaries with P orb ∼ 10−1000 d and/or with q < 0.5) is similar to that of single O stars below e sin i ∼ < 170 km s −1 . Conclusions. Our results are compatible with the assumption that binary components formed with the same spin distribution as single stars, and that this distribution contains few or no fast-spinning stars. The higher average spin rate of stars in short-period binaries may either be explained by spin-up through tides in such tight binary systems, or by spin-down of a fraction of the presumed-single stars and long-period binaries through magnetic braking (or by a combination of both mechanisms). Most primaries and secondaries of SB2 systems with P orb ∼ < 10 d appear to have similar rotational velocities. This is in agreement with tidal locking in close binaries where the components have similar radii. The lack of very rapidly spinning stars among binary systems supports the ide...

Physical properties of seven binary and higher-order multiple OB systems

“…Our results for v sin i values seem to confirm the conclusion of Ramírez-Agudelo et al (2015) about the lack of very high rotational velocities among O-type binary systems, at least for cases where we have some idea about the orbital inclinations.…”

The single star path to Be stars

“…Finally we note that a key difference between Be stars produced by the two channels concerns their potential binary companions. Since the initial rotational velocity distribution for single stars and stars in binaries appears to be similar (Ramírez-Agudelo et al 2015), we would expect a significant fraction of Be stars formed through the single star channel (i.e., without accretion-induced spin-up) to have unevolved main sequence companions. However, essentially no such stars are known.…”

The spin rates of O stars in WR + O binaries – I. Motivation, methodology, and first results from SALT

“…Therefore, one needs a large number of systems from which one can extract statistical information from v. sin i to deduce < ve >. This is precisely the methodology adopted in studies of rotation speeds of single field O stars (Ramírez-Agudelo et al 2013), and in studies of O + O star binaries (Ramírez-Agudelo et al 2015). In this study we have succeeded in measuring the v. sin i of eight O stars in WR + O binaries, thereby raising the sample size with well-measured spin rates from two to ten.…”

“…To determine the value for v. sin i for the stars in our sample, we followed the process as outlined in detail by Ramírez-Agudelo et al (2013) and Ramírez-Agudelo et al (2015). This involved measuring the broadening of these stars' HeI and HeII lines via the full width at half maximum (FWHM).…”