Poster Abstract
The observed number of black holes in a binary with a high mass Main Sequence companion is very small as compared to the observed number of Wolf Rayet stars in a similar binary configuration, whereas the latter is believed to be the progenitors of the former. We study the origin of this discrepancy based on the detectability and lifetime of black holes in a binary system. A black hole is 'observable' if there is an accretion disk around the black hole and it emits X-Ray radiation from the accretion disk which we can measure using X-Ray telescopes. We determine whether an accretion disk can form around a black hole using a criterion for the formation of an accretion disk that depends on the masses of the individual components, the orbital separation, and wind velocity of the Main Sequence companion. We find that, largely due to the very high wind velocity of Main Sequence stars, an accretion disk cannot form around the black hole for the majority of the Main Sequence lifetime of its companion. It is only during the very late stage of core hydrogen burning of the companion, when the stellar radius increases, that its wind velocity decreases significantly. Noting that the usually measured wind velocities of Main Sequence stars are very high, of the order of ~2000 km/s, we argue that one possible solution to the low number of observed black hole binaries might be the absence of accretion disks around those compact objects due to high wind velocities of their companions. We also found that previous literature estimating the criterion for the formation of accretion disks underestimated the wind velocities in Main Sequence stars, by a factor of 2-3.
Plain text summary
In the Solar neighbourhood, the observed number of black holes (BHs) in a binary (only one, Cyg X-1) with a high mass Main Sequence (MS) companion is very small as compared to the observed number of Wolf Rayet (WR) stars in a similar binary configuration (∼80, Van der Hucht+2001), whereas the latter is believed to be the progenitors of the former. Langer+2020 predicted to find ∼3 out of every 100 massive MS star in a binary to harbour a BH as its binary companion. The typical lifetime of a WR+MS binary system (∼0.4 Myrs) is also smaller than that of a BH+MS binary. So, where are all these BHs and why don’t we see them? We investigate the origin of this discrepancy based on the detectability and lifetime of BHs in a binary system with an MS companion. Since a BH does not emit any light of its own, it is ‘observable’ only if there is an accretion disk around it and X-Ray radiation is emitted from the accretion disk which we can measure using X-Ray telescopes here from Earth/space. We determine whether an accretion disk can form around a BH using a criterion for the formation of an accretion disk that depends on the masses of the individual components, the orbital separation, and wind velocity of the MS companion. Assuming that the matter accreted from the stellar wind of the MS companion, having specific angular momentum given by Shapiro+1976, goes into a Keplerian orbit (with radius R_disk) around the BH, an accretion disk can form if the R_disk is greater than the Innermost Stable Circular Orbit radius (R_ISCO) of the BH. We find that, largely due to the very high wind velocity of massive MS stars, an accretion disk cannot form around the BH for the majority of the MS lifetime of its companion. It is only towards the end of core hydrogen burning of the MS companion when its stellar radius increases and the wind velocity decreases significantly, can an accretion disk can form around the BH. When an accretion disk can form, the BH+MS binary will be X-Ray active, and hence 'observable'. Accounting for the relative lifetime of the X-ray active BH+MS phases with that of the lifetime of WR+MS systems, we estimate to ‘observe’ ∼12 BH+MS systems for 17 well-observed WR+MS progenitor systems in the Solar neighbourhood, which is still inconsistent with observations! However, noting that the usually measured wind velocities of Main Sequence stars are consistently very high, of the order of ∼2000 km/s (Lamers+1995, Vink+2001), we recalculate our accretion disk criterion for the BH+MS binaries using a fixed v_wind = 2000 km/s near the BH throughout the MS lifetime of the companion and find that accretion disks cannot form around BHs from all except two of the 17 WR+MS progenitor systems (Fig. 2). Comparing our accretion disk criteria with previously published work (Iben+1996, Vanbeveren+2020), we find that earlier work has underestimated the wind velocities in MS stars by a factor of 2-3, thereby predicting much larger X-Ray active lifetime of BH+MS systems. We propose that a possible solution (see also Vanbeveren+2020) to the low number of observed BH+MS binaries as compared to WR+MS binaries might be due to the absence of accretion disks around those compact objects, owing to the high wind velocities of MS stars, making them undetectable. One then has to look at variations in the radial velocity of spectroscopic massive MS binaries to indirectly detect the presence of such BHs (see Langer+2020 for more details) in binaries with MS stars.
Poster Title
Black holes and where to find them: The curious case of missing stellar mass black holes
