Even 13 years after publication, the Hipparcos Catalogue still generates exiting new science. In a recent investigation - published in the Astrophysical Journal, Andreas Irrgang and collaborators have revealed the nature of the hypervelocity-runaway star HIP60350. The authors conclude that the star is more likely than not moving too fast for the Galaxy's gravitational forces to hold on to it, after having been ejected at high speed from its birth place some 15 Myr ago.
Runaway stars are massive, young stars found far from their birthplaces, the spiral arms in the Galaxy. Two explanations are generally put forward to explain such oddities: a supernova explosion disrupting a binary system and sending the surviving star off with a large velocity (Blaauw 1961) or a close, dynamical interaction between (binary) stars in a young, compact star cluster (Poveda et al. 1967).
HIP60350 is a known runaway OB star. It is a young, B-type star (≈5 solar masses, age ≈15 Myr) only 15 degrees away from the Galactic pole at a distance of some 3 kpc. Spectroscopic data, together with proper motions from Floor van Leeuwen's new release of the Hipparcos Catalogue, allowed the authors to trace the three-dimensional motion of the star through the Galaxy back in time. They repeated this exercise 50,000 times, each time with a slightly different starting condition, to sample the distribution of observational errors. The results indicate that HIP60350 has most likely a three-dimensional velocity above 500 kms/s, i.e., exceeding the local Galactic escape velocity. The data are fully consistent with the star originating, some ≈15 Myr ago, in the Crux-Scutum spiral arm.
At the moment, the quality of the observational data - in particular the stellar parallax (distance) - is insufficient to pinpoint the precise origin of the star within the spiral arm. At least 5 open clusters are consistent with being the birth place of HIP60350. Gaia will observe this 11-th magnitude star several dozen times and will determine its parallax to within 10 muas. As a result, the distance to the star will be known with a precision of around 3%, which will allow to significantly reduce the spread of the 50,000 orbits and to unambiguously determine the birth cluster.
The left panel shows an intersection of the Galactic disc (XY plane) and the distribution of the "end points" of the 50,000 back-traced orbits. Additionally, the trajectories of the best open-cluster candidates in which the star was most likely born are depicted; these are Ruprecht 127, NGC5606, NGC5617, Collinder 347, and Moffat 1. The flight timeís colour code is given in the lower left corner. A right-handed, non-rotating frame of reference with the Galactic centre at the origin, Galactic north pole in the positive Z-direction, and the Sunís current position at (-8.0, 0, 0) kpc is used. The ellipses mark the 1-sigma region with (solid) or without (dashed, dotted) accounting for uncertainties in distance, respectively. The gray-shaded regions schematically represent the locus of two spiral arms ≈15 Myr ago. The right panel shows the histogram of the distribution of (un)bound trajectories. The abscissa is the difference between the current space velocity and the local escape velocity. The majority of orbits are unbound.
A larger version of the figure is available in the following link.