Towards a Unified View of Inhomogeneous Stellar Winds in Isolated Supergiant Stars and Supergiant High Mass X-Ray Binaries

Abstract

Massive stars, at least ∼ 10 times more massive than the Sun, have two key properties that make them the main drivers of evolution of star clusters, galaxies, and the Universe as a whole. On the one hand, the outer layers of massive stars are so hot that they produce most of the ionizing ultraviolet radiation of galaxies; in fact, the first massive stars helped to re-ionize the Universe after its Dark Ages. Another important property of massive stars are the strong stellar winds and outflows they produce. This mass loss, and finally the explosion of a massive star as a supernova or a gamma-ray burst, provide a significant input of mechanical and radiative energy into the interstellar space. These two properties together make massive stars one of the most important cosmic engines: they trigger the star formation and enrich the interstellar medium with heavy elements, that ultimately leads to formation of Earth-like rocky planets and the development of complex life. The study of massive star winds is thus a truly multidisciplinary field and has a wide impact on different areas of astronomy.In recent years observational and theoretical evidences have been growing that these winds are not smooth and homogeneous as previously assumed, but rather populated by dense "clumps". The presence of these structures dramatically affects the mass loss rates derived from the study of stellar winds. Clump properties in isolated stars are nowadays inferred mostly through indirect methods (i.e., spectroscopic observations of line profiles in various wavelength regimes, and their analysis based on tailored, inhomogeneous wind models). The limited characterization of the clump physical properties (mass, size) obtained so far have led to large uncertainties in the mass loss rates from massive stars. Such uncertainties limit our understanding of the role of massive star winds in galactic and cosmic evolution.Supergiant high mass X-ray binaries (SgXBs) are among the brightest X-ray sources in the sky. A large number of them consist of a neutron star accreting from the wind of a massive companion and producing a powerful X-ray source. The characteristics of the stellar wind together with the complex interactions between the compact object and the donor star determine the observed X-ray output from all these systems. Consequently, the use of SgXBs for studies of massive stars is only possible when the physics of the stellar winds, the compact objects, and accretion mechanisms are combined together and confronted with observations. This detailed review summarises the current knowledge on the theory and observations of winds from massive stars, as well as on observations and accretion processes in wind-fed high mass X-ray binaries. The aim is to combine in the near future all available theoretical Unified view of of inhomogeneous stellar winds in supergiant stars and HXMB 3 diagnostics and observational measurements to achieve a unified picture of massive star winds in isolated objects and in binary systems.

Evidence for different accretion regimes in GRO J1008−57

“…c) The emission geometry of GRO J1008−57 is unknown and, thus, we assume isotropic emission of each pole into its hemisphere, i.e., g = 2π (the theoretical expectations apply to one pole only). According to Martínez-Núñez et al (2017), who estimate the effects of light bending (e.g., Kraus 2001), the real luminosity for a typical fan beam accretion geometry can differ by ∼25% relative to the derived luminosity assuming isotropic emission. Due to the higher accretion rate in BeXRBs compared to low mass X-ray binaries (LMXBs), which usually emit via a pencil beam geometry (see, e.g., Basko & Sunyaev 1975), the fan beam is a justified geometry for GRO J1008−57.…”

The stellar and wind parameters of six prototypical HMXBs and their evolutionary status

“…The wind-fed HMXBs are of particular interest (for recent reviews see Martínez-Núñez et al 2017 andSander 2018). Here the compact object is situated in and accretes solely from the wind of the massive star, usually a supergiant.…”

The super-orbital modulation of supergiant high-mass X-ray binaries

“…The donor star in this system has the same spectral type as J Pup and the estimated distance is 6−26 kpc (see Martínez-Núñez et al 2017, and references therein). Given the typical parameters of a B0.5 Ib star (Sect.…”

“…The smaller structures, referred to as "clumps", are typically endowed with an increased density of a factor of 10 compared to a smooth wind and can be as large as ∼0.1 R * , where R * is the OB supergiant radius. Clumps are usually invoked to interpret the stochastic X-ray variability displayed by SgXBs on time scales of 10−1000 s (see, e.g., Martínez-Núñez et al 2017, and references therein). The existence of larger structures in the OB supergiant winds was suggested in the early 80 s (Mullan 1984), and confirmed by the detection of discrete absorption components (DACs; see, e.g., Underhill 1975).…”

“…Two groups are typically distinguished; the classical systems, which show a virtually persistent high X-ray luminosity, and the supergiant fast X-ray transients (SFXTs), which feature a still highly debated and peculiarly prominent variability in X-rays (Martínez-Núñez et al 2017). The bulk of the X-ray radiation from the SgXBs can be reasonably well explained as being due to the accretion of the stellar wind from the OB supergiant onto a highly magnetized neutron star (NS), with no evidence supporting the presence of longlived accretion disks (Bozzo et al 2008;Shakura et al 2012;Romano et al 2015;Hu et al 2017).…”