## 5. A hierarchical triple systemAt a projected separation of only 3" (1400 AU), VLA 1 and 2
are likely to be somehow related. Yet, the two sources are rather
different, VLA 1 (which appears to be a very close binary) being
very reddened and having significant outflow, while VLA 2 shows
no optical evidence of outflow and is much less reddened. Most
puzzling is the location of both stars We can explain these features if we assume that the three stars
were born together and until recently formed a non-hierarchical triple
system located inside the dark lane. It is well known from the
statistical theory of three-body interactions (e.g. Monaghan 1976,
Anosova 1986, Valtonen & Mikkola 1991) that such systems are
inherently unstable, and within about a hundred crossing-times will
eject a member, while the remaining components contract to form a
closer binary. The ejected member may or may not escape entirely,
depending on the details of the interaction. If the crossing time is
, where R is a characteristic length
scale for the system in AU, M is the total system mass in
and
is in yr (Anosova 1986), and we
assume the non-hierarchical triple members were formed within 150 AU
of each other with a total mass of 2
(e.g. 1.0, 0.6, 0.4 ), then
statistically a member is ejected after 22,000 yr. This is comparable
with the dynamical age of the giant HH 111 outflow of about 25,000 yr,
if one assumes a mean flow velocity of 150 km s Assuming VLA 2 is moving with a mean transverse velocity of
2 km s That the above numbers fit together so well is obviously fortuitous given the stochastic nature of the process, and they merely serve to show that a plausible choice of parameters could be consistent with the observed facts. Astrometry a few centuries from now could test this conjecture. It is interesting to note that VLA 2/Star B is now
located © European Southern Observatory (ESO) 1999 Online publication: December 2, 1999 |