The Runaway Stars: AE Aurigae, 53 Arietis, μ Col

                          RA                    DEC               m            Space Velocity          Distance
AEAurigae    5h 16m 18.15s    +34° 18' 44"       +5.6            109 km/sec               446 pc
53Arietis       3h 7m 25.69s      +17° 52' 48"       +5.9              26 km/sec               231 pc
μCol             5h 45m 59.89s     -32° 18'23.0"     +5.3            116 km/sec               397 pc

Summary:  New proper motion and parallax data from the Hipparcos satellite for the runaway stars AE Aur, 53 Art, and μ Col  indicate that the single event ejection theory causing their rapid outward ejection away from the center of Orion
cannot be confirmed. The data suggests that the pair AE Auriga and μCol had a separation of 40 ±10 parsecs some 4.4
million years ago, while 53 Arietis was at least 125 ± 25 parsecs from the pair during this kinematic time. The
Hipparcos and other data indicate that AE Aur and μCol have nearly equal space velocities with respect to the LSR and       are traveling at nearly right angles to each other.
 

In the 1950's and early 1960's proper motion surveys identified several high velocity stars. Several of these high
velocity stars were studied in detail by the Dutch astronomer Adrian Blaauw, in particular AE Aurigae, 53 Arietis and μ
Col. From the proper motion, radial velocity and other data available at the time, Blaauw (1961) postulated that all three
left the constellation of Orion possibly in a single event which occurred some three million years ago. Since then each
has traveled across an intervening constellation to reach its present day position in the constellations Auriga, Aries and
Columbia respectively.

Blaauw (1961) proposed the Binary Supernova Scenario (BSS) in which the pair AE Aur and μ Col originally may
have been members of a multiple star system similar to the Trapezium, with one of the members exploding as a
Supernova. Having nothing left to orbit, the remaining stars were thrown away like a slingshot, thus expelled away.
Blaauw 1993 showed that AE Aur and μCol, having similar spectral types, masses and speeds, may have originated
from the Ori OBI region by the Cluster Ejection Scenario (CES).

AE Aur and μ Col computed to be at a minimum distance (50 ± 10 parsecs) some 4.4 million ± 500,000 years ago. The fact that these to stars have a nearly equal space velocities (109 km/sec and 117 km/sec respectively) suggests that they may have
been ejected from the same event (Blaauw 1961, Blaauw, 1993). However, 53 Ari is at a much greater distance from the
of the three stars over the estimated age of the OBI association and Orion Bubble.
 

In the Binary-Supernova Scenario (BSS) discussed by Blaauw 1961, Blaauw 1993, both AE Aur and μCol were
members of a multiple system in which one member went supernova. A kick, estimated to be on the order of 100 km/sec
would give rise to an uncertain amount of mass transfer between the two stars. The explosion would also not necessarily
have the energy needed to break apart the binary (Leonard and Dewey 1992). Within the range of uncertainty, it appears
that the data here shows that AE Aur and μ Col could not have been once members of the same binary system, even
going back to the 4.4 Myr kinematic age. These two stars could however, have been members of separate binary or
multiple systems each being ejected in about the same kinematic time of 4.4 Mys ago.
The runaway star 53 Ari has a much lower space velocity of 26 km/sec. Its minimum distance from p. Col is 50 ± 20 pc
which occured at 4.4 Myr ago. These two were closer than that of AE Aur and p. Col 4.4 Myr ago. Even though 53 Ari
and p. Col were closer in space than AE Aur and p. Col were 4.4 Myr ago, their distances are too far apart to have been
members of the same binary systems. However the cluster ejection scenario (CES) originally proposed by Poveda et al.
1967, may account for the runaways outward motion away from the OBI subgroup Ib. In the CES, dynamical
interaction of stars within a young cluster would cause the stars to be thrown off. Problems arise here as no compact
parent cluster can be identified (tracing the motion of the runaways backward) in such a short kinematic time of a few
million years.

Discussion

Much speculation has been suggested to account for the high escape velocities of these runaway objects. The runaway
stars AE Aurigae seem to have left the M42 region about 2.7 million years ago along with 53 Arietis and μ Col. The
explosion of a supernova, while it seems a plausible explanation of the escape theory does not appear to provide the
energy for such a "slingshot" effect. In monte carlo simulations of supernova explosions of binary systems, Leonard and
Dewey (1992) suggested that less than 10% of O-type primaries would be runaways. They found that asymmetric kicks
of 100-200 km/sec would not be enough to disrupt the binary systems, thus most of the runaways are expected to have
neutron star companions. Brown, et al. (1995) proposed a series of supernova and stellar winds to account for the current
size and expansion velocity of the Orion-Eridanus Bubble. This bubble's estimated age (Brown, et al. 1995) is 5.8 million
years, which is less than the estimated age of the Ori subgroup Ib, namely 4 Myr (Schaller et al. 1992). This suggestion
seems plausible since in a high star formation area such as Orion, many supernova explosions would be expected to
occur. This explanation is somewhat problematic for 53 Arietis, since it is was listed as spectroscopic binary. Now it is
known that 53 Arietis is a Beta Canis type variable with a 3h 40m period, and the variable velocity seen in the spectral lines can be accounted for this way, rather than from orbital motion.
 

Another issue with the runaways is the ages of the subgroups. The above conclusions of the runaway stars were based on old proper motion and distance data for each AE Aurigae, 53Aries and μCol. Now enter the HIPPARCOS satellite data. Using the Hipparcos estimates for proper motion and distance, it now appears that AE Aurigae was not expelled from the same region as the other 2 stars. As can see from the table below, AE Aurigae was nearly 200 pc further away than the other stars when the alleged ejection event occurred.
                

                                      Current Distance                                 Prior Distance
AE Aurigae                           446 pc                                        375 pc (2.2 million yrs)

53 Arietis                              397 pc                                        181 pc (2.2 million yrs)
 

μ Col                                     231pc                                         191 pc (5 million yrs)


Future astronomers have their work cut out for them. A much longer baseline may detect expansion of Barnard's Loop
away from the central Orion region by direct comparison of old and new plates of the area. Similarly, more accurate
parameters of the runaway stars may lead to revised estimates for their previous positions and distances with respect to
the Orion complex.
 

Figure 1. Path of runaway stars showing originating point from 2.2 million years for AE Aurigae, μ Col, and 5 million
years for 53 Aries. Hipparcos proper motion data used.

REFERENCES

Brown, A.G.A., Geus, E.J., Zeeuw, P.T. de, 1994, The Orion OB1 Association: I. Stellar Content, Astronomy & Astrophysics, 289, pp. 101-120.

Brown, A.G.A., Hartmann, D., Burton, W.B., 1995, The Orion OB1 Association: II. The Orion-Eridanus Bubble, Astronomy & Astrophysics, 290, pp. 903

Blaauw, A., 1993, Massive Runaway Stars, Massive Stars: Their Lives in the Interstellar Medium, ASP Conference Series, Vol 35, p.207-219.

Blaauw, A., 1961, On the Origin of the O- and B- Type Stars with High Velocities (The "Run-Away" Stars), and some Related Problems, Bulletin of the Astronomical Institutes of the Netherlands, Vol XV, No. 505, p. 265-290.