Over The Horizon

MACHO Events and Multiple Comets

For 60 years astronomers have been seeking ways to explain the yawning gap between the gravitational mass of galaxies, calculated from various observations, and the amount of matter actually detectable in them as stars, gas and dust. The observable mass comes to only 2-10% of the mass determined by radial velocities and rotation curves.

One idea suggests that in the halos of galaxies there exist many massive, compact objects. These might be faint white dwarf stars, neutron stars or black holes formed in an earlier stage of a galaxy's evolution, or they might be super-massive nuclear particles ("cold dark matter") that don't normally interact with the "normal" matter making up the rest of a galaxy.

The only way that a "massive compact halo object", or MACHO, might currently be detected is by its effect on light. If a MACHO passes between us and a distant star, its gravity will bend the star's diverging light rays, briefly focusing them in our direction. The star will appear to brighten but, because gravity bends all light equally, its colour will not change.

This makes a "gravitational microlensing" event different from most other stellar variations, where colour and brightness change with the star's temperature. Also a microlensed star will show a characteristic rise and return to former brightness, as the MACHO passes in front of it.

A large number of stars have to be monitored to detect such an event since the chance of a MACHO crossing our line of sight is very small.

A suspected gravitational microlensing event has been under scrutiny in recent weeks. It was first detected by a team comprising Mt Stromlo astronomers and the Lawrence Livermore National Laboratory's Center for Particle Astrophysics. They noted that a star had brightened by 0.85 magnitude during the last week of August. Measures of the star in red and yellow light showed the key microlensing feature of an unchanging colour. Spectra of the star indicate that it is a normal reddish K0 star, unlikely to be brightening for any internal reason. Monitoring continues.

Also in August, Don Machholz of California found a new comet (his second) with a short orbital period of 6.81 years. Reports now indicate that P/Machholz 2 is not alone.

Two weeks later on August 28, Michael Jager of Austria noticed a second comet close to the first, moving parallel to it, and about two magnitudes fainter. Five days later Czech and US observers reported a third comet near to the second and one magnitude fainter than it. Several other observers soon reported fourth and fifth components, even smaller.

Two earlier multiple comets observed (P/Brooks 2 1889 V, and P/Shoemaker-Levy 9 which smashed into Jupiter recently) were both broken up by tidal forces during close approaches to Jupiter. P/Machholz 2 does cross Jupiter's orbit, but it hasn't made a close approach recently. There's something else odd about the comet -- it has been passing the sun at Venus's distance every 6.8 years for the past several orbits, yet has only now been discovered. It was even more favourably placed for discovery at its last return, in 1987-8, yet nobody saw it.

We suggest that P/Machholz 2 has been inactive for some time due to the formation of a thick crust around the nucleus. A recent collision with a small (perhaps boulder-sized) asteroid has punched a hole in the crust, exposing a fresh icy surface to sunlight and reactivating the comet. The satellite comets are fragments of icy material dislodged by the impact. An obvious conclusion is that the small comets will disappear quickly, as their sources of gas and dust evaporate.

If the primary nucleus has only the impact crater as its active region then northern comet observers may see an obvious jet of gas rotating like a garden sprinkler, turning on and off as it is exposed to sunlight. Sadly the comet is likely to be much too faint for any activity to be seen when it emerges into our sky in late September.

Alan Gilmore, Pam Kilmartin
Mt John Observatory