On the formation of GW190814

The LIGO-Virgo collaboration recently reported a puzzling event, GW190814, with component masses of 23 and 2.6 M_⊙. Motivated by the relatively small rate of such a coalescence (1- $23\rm \, Gpc^{-3}\, yr^{-1}$ ) and the fact that the mass of the secondary is close to the total mass of known binary neutron star (bNS) systems, we propose that GW190814 was a second-generation merger from a hierarchical triple system; i.e. the remnant from the bNS coalescence was able to merge again with the 23 M_⊙ black hole (BH) tertiary. We show that this occurs at a sufficiently high probability provided that the semimajor axis of the outer orbit is less than a few au at the time of bNS coalescence. It remains to be explored whether the conditions for the formation of such tight triple systems are commonly realized in the Universe, especially in low-metallicity (≲0.1 Z_⊙) environments. Our model provides a number of predictions. (1) The spin of the secondary in GW190814-like systems is 0.6-0.7. (2) The component mass distribution from a large sample of LIGO sources should have a narrow peak between 2.5 and ∼3.5 M_⊙, whereas the range between ∼3.5 and ∼5 M_⊙ stays empty (provided that stellar evolution does not generate such BHs in the 'mass gap'). (3) About 90 per cent (10 per cent) of GW190814-like events have an eccentricity of e ≳ 2 × 10^-3 (≳0.1) near gravitational wave frequency of $10\,$ mHz. (4) A significant fraction ( ${\gtrsim}10\, \rm {per\, cent}$ ) of bNS mergers should have signatures of a massive tertiary at a distance of a few au in the gravitational waveform. (5) There are 10⁵ undetected radio-quiet bNS systems with a massive BH tertiary in the Milky Way.