The IACOB project

Abstract

Context. The empirical distribution of projected rotational velocities (v sin i) in massive O-type stars is characterised by a dominant slow velocity component and a tail of fast rotators. It has been proposed that binary interaction plays a dominant role in the formation of this tail. Aims. We perform a complete and homogeneous search for empirical signatures of binarity in a sample of 54 fast-rotating stars with the aim of evaluating this hypothesis. This working sample has been extracted from a larger sample of 415 Galactic O-type stars that covers the full range of v sin i values. Methods. We used new and archival multi-epoch spectra in order to detect spectroscopic binary systems. We complemented this information with Gaia proper motions and TESS photometric data to aid in the identification of runaway stars and eclipsing binaries, respectively. We also benefitted from additional published information to provide a more complete overview of the empirical properties of our working sample of fast-rotating O-type stars. Results. The identified fraction of single-lined spectroscopic binary (SB1) systems and apparently single stars among the fast-rotating sample is ∼18% and ∼70%, respectively. The remaining 12% correspond to four secure double-line spectroscopic binaries (SB2) with at least one of the components having a v sin i > 200 km s−1 (∼8%), along with a small sample of 2 stars (∼4%) for which the SB2 classification is doubtful: these could actually be single stars with a remarkable line-profile variability. When comparing these percentages with those corresponding to the slow-rotating sample, we find that our sample of fast rotators is characterised by a slightly larger percentage of SB1 systems (∼18% vs. ∼13%) and a considerably smaller fraction of clearly detected SB2 systems (8% vs. 33%). Overall, there seems to be a clear deficit of spectroscopic binaries (SB1+SB2) among fast-rotating O-type stars (∼26% vs. ∼46%). On the contrary, the fraction of runaway stars is significantly higher in the fast-rotating domain (∼33–50%) than among those stars with v sin i < 200 km s−1. Lastly, almost 65% of the apparently single fast-rotating stars are runaways. As a by-product, we discovered a new over-contact SB2 system (HD 165921) and two fast-rotating SB1 systems (HD 46485 and HD 152200) Also, we propose HD 94024 and HD 12323 (both SB1 systems with a v sin i < 200 km s−1) as candidates for hosting a quiescent stellar-mass black hole. Conclusions. Our empirical results seem to be in good agreement with the assumption that the tail of fast-rotating O-type stars (with v sin i > 200 km s−1) is mostly populated by post-interaction binary products. In particular, we find that the final statistics of identified spectroscopic binaries and apparent single stars are in good agreement with newly computed predictions obtained with the binary population synthesis code BPASS and earlier estimations obtained in previous studies.

Tracing the evolution of short-period binaries with super-synchronous fast rotators

“…The position of primaries of the selected systems on the Hertzsprung-Russell (HR) diagram is presented in Fig. 1 together with a sample of 50 Galactic fast-rotating O stars from Britavskiy et al (2023) for reference. In addition, we calculated the evolutionary tracks that represent the main-sequence evolutionary stage of a nonrotating single star at solar metallicity (Z = 0.02).…”

“…stellar lifetimes (i.e., HD 165246). Interestingly, according to Mahy et al (2022), more long-period and highly eccentric binaries among the initial sample of SB1 fast rotators from Britavskiy et al (2023) could possibly host a neutron star. However, according to their spectroscopic disentangling simulations, only the secondaries with M 2 3 M can be detected in the available spectra.…”

“…Such binary interactions may also play a dominant role in shaping the observed distribution of the surface rotation rate, especially for the tail of fast rotators because of tides, mass transfer, or mergers. This idea has been proposed multiple times, both from a theoretical perspective (Packet 1981;Pols & Marinus 1994;de Mink et al 2013;Vinciguerra et al 2020) and observational studies (Blaauw 1993;Ramírez-Agudelo et al 2015;Cazorla et al 2017;Britavskiy et al 2023). One example is the famous category of Be stars, which are fast-rotating stars with decretion disks (Frémat et al 2005;Rivinius et al 2013).…”

“…Here, we study known Galactic fast-rotating OB stars that have been identified in single-lined spectroscopic binaries, specifically HD 25631, HD 191495, and HD 46485 (SB1;Britavskiy et al 2023;Nazé et al 2023, see also Table 1). Taking into account their high surface rotational velocities (v sin i > 200 km s −1 ), short orbital periods (P < 7 days), and very unequal mass ratios, we aim to understand what causes such a high rotation, and to determine what the effect is (if any) of past and future binary interactions.…”

Extreme mass ratios and fast rotation in three massive binaries

“…The initial rotation rate could in addition further increase during main sequence evolution (Ekström et al 2008). An alternative scenario, with some empirical support (Holgado et al 2022;Britavskiy et al 2023), relies on interactions within a binary system (Podsiadlowski et al 1992;van Bever & Vanbeveren 1997;de Mink et al 2013). There are several ways in which the rotation of a star may be affected by its companion.…”