  There are only a few known main belt (MB) asteroid families with ages greater
than 2 Gyr (Bro\v{z} et al. 2013, Spoto et al. 2015). Estimates based on the
family producing collision rate suggest that the lack of >2 Gyr-old families
may be due to a selection bias in current techniques used to identify families.
Family fragments disperse in their orbital elements, semi-major axis, $a$,
eccentricity, $e$, and inclination, $i$, due to secular resonances, close
encounters with massive asteroids and the non-gravitational Yarkovsky force.
This causes the family fragments to be indistinguishable from the background of
the main belt making them more difficult to identify with the hierarchical
clustering method (HCM) with increasing family age. The discovery of the
Eulalia and new Polana families in the inner belt relied on new techniques
because Yarkovsky spreading made them too disperse to be identified using the
classical HCM. The technique used to discover the new Polana and Eulalia
families is modified to identify asteroid families by searching for
correlations between $a$ and asteroid diameter, $D$, or absolute magnitude,
$H$. A group of asteroids is identified as a collisional family if its boundary
in the $a$ vs. 1/$D$ or $a$ vs. $H$ planes has a characteristic V-shape which
is due to the size dependent Yarkovsky spreading. The V-shape boundary is
identified with two separate techniques. The first identifies a border by
measuring the drop between the number of objects inside and outside of the
border. The second identifies the V-shape border by measuring the number
density of objects in $a$ vs. 1/$D$,$H$ space. The V-shape techniques are
demonstrated on known families Erigone, Vesta, Koronis, and families difficult
to identify by HCM such as Flora, Baptistina, new Polana, Eulalia and Karin.
Application of the technique in a search for >2 Gyr-old families throughout the
MB is discussed.
