  The LIGO-Virgo collaboration recently reported a puzzling event, GW190814,
with component masses of 23 and 2.6 solar masses. Motivated by the relatively
small rate of such a coalescence 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
Msun 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 (less than 0.1 solar)
environments. Our model provides a number of predictions. (1) The spin of the
secondary in GW190814-like systems is 0.6 to 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 Msun, whereas the range between ~3.5 and ~5 Msun stays
empty (provided that stellar evolution does not generate such BHs in the "mass
gap"). (3) About 90% (10%) of GW190814-like events have an eccentricity of e >
2x10^{-3} (> 0.1) near gravitational wave frequency of 10 mHz. (4) A
significant fraction (> 10%) of bNS mergers should have signatures of a massive
tertiary at a distance of a few AU in the gravitational waveform. (5) There are
10^5 undetected radio-quiet bNS systems with a massive BH tertiary in the Milky
Way.
