Sciency Thoughts: The Gibbs Family of Asteroids.

Asteroid families are groups of objects thought to have a common origin, typically the catastrophic break-up of a parent body at some remote point in the past, which has left a population of asteroids with similar orbital and physical properties. It has been proposed that some young asteroid families might have a relationship with Active Asteroids, or Main Belt Comets, bodies with asteroid-like orbits within the Main Asteroid Belt but which produce comet-like ejecta.

Main Belt Comets are thought to do this in a similar way to regular comets, by the sublimation of ice into gas (there is no liquid phase in a vacuum) at or close to the surface of the body, releasing clouds of dusty particles, which form a visible halo around the object. However since each time this occurs some of the ice is lost, and cannot be replaced, and since bodies in the Main Asteroid Belt must encounter solar energy sufficient to melt ice much more frequently than bodies on typical cometary orbits, all Main Belt Comets must be relatively young objects. Such objects could potentially form in two ways; a regular comet could somehow be captured into an asteroid-like orbit, or an asteroid could be broken apart, revealing ice that was previously buried deep enough within the body to protect it from solar radiation.

In this later case there is a clear potential for a relationship between such a body and a young asteroid family with its origin in the break-up of a larger body. To date two Main Belt Comets have been assigned to asteroid families; 133P/Elst-Pizarro is thought to be a member of the Beagle Asteroid Family, which are thought to have shared a common origin less than 10 million years ago, and P/2006 VW139 is thought to be a member of an unnamed asteroid family, which is about 7.5 million years old. Cleary there is potential for the discovery of associations between Main Belt Comets and asteroid families, and the discovery of asteroid families associated with Main Belt Comets. However caution needs to be used in such a search, as not all Active Asteroids are Main Belt Comets; asteroids may sometimes produce ejecta for other reasons, such as collisions or the partial breakup of a body due to the tidal influence of planets (smaller versions of the sort of events that are thought to lead to the formation of asteroid families).

In a paper published on the arXiv database at Cornell University Library on 13 January 2014 (check), Bojan Novaković of the Department of Astronomy at the University of Belgrade, Henry Hsieh of the Institute for Astronomy at the University of Hawaii, Alberto Cellino of the Osservatorio Astrofisico di Torino, Marco Micheli also of the Institute for Astronomy at the University of Hawaii and Marco Pedani of the Fundación Galileo Galilei describe the results of the search for an asteroid family connected with the recently discovered Active Asteroid P/2012 F5 (Gibbs).

P/2012 F5 (Gibbs) was discovered in 2012 by a team of scientists led by Alex Gibbs of the Mount Lemon Survey. It was briefly thought to be a Main Belt Comet; the designation P/ implies a Periodic Comet (a comet with an orbital period of less than 200 years) 2012 F5 implies the fifth asteroid (asteroid 5) discovered in the second half of March 2012 (period 2012 F) and (Gibbs) indicates the name of the discoverer. This was due to an active period in 2012, thought to be the result of cometary activity, though two separate studies since this time have now suggested that the ejecta initially discovered was the result of a collision, and that P/2012 F5 (Gibbs) should be considered to be an asteroid rather than a comet.

Novaković et al. began by calculating precise details of the orbit of P/2012 F5 (Gibbs). To this end they collected data from 125 observations made of the body after its discovery, plus 17 observations from Pan-STARRS1 survey data that predated it’s discovery, and in addition made 7 new observations with the 3.6 m Canada-France-Hawaii Telescope and the University of Hawaii 2.2 m telescope on Mauna Kea, for a total of 3.6 years of observations, though 37 observations from around the time when P/2012 F5 (Gibbs) was most active were rejected, due to problems in determining the precise location of the body’s core within its coma.

Using this data they calculated P/2012 F5 (Gibbs) to have an average distance from the Sun (semi-major axis) of 3.005 AU (i.e. 3.005 times the average distance between the Earth and the Sun, or roughly twice the average distance at which Mars orbits the Sun), and an orbital eccentricity of 0.0417 (i.e. the centre of the body’s orbit is 4.17% of the orbit’s diameter from the Sun) (check), so that its orbit takes it from 2.88 AU from the Sun at perihelion to 3.13 AU from the Sun at aphelion. This orbit is inclined at 9.74° to the plane of the Solar System.

The calculated orbit of P/2012 F5 (Gibbs). JPL Small Body Database Browser.

Having calculated this Novaković et al. began looking for asteroids with similar orbital properties. This search yielded a set of eight asteroids with very similar properties to P/2012 f5 (Gibbs); (20674) 1999 VT1, (140429) 2001 TQ96, (177075) 2003 FR36, (249738) 2000 SB159, (25134) 1998 SC17, (321490) 2009 SH54, 2007 RT138 and 2002 TF325. These nine bodies form a distinct cluster, easily distinguished from other objects in the vicinity. The cluster is close spatially to the Eos Family of Asteroids, but has distinct orbital properties with no overlap. A small number of asteroids with similar orbits could potentially be a coincidence, but more than five such objects is statistically unlikely, and Novaković et al. calculate that there is less than a 0.1% chance of nine such bodies having similar orbital properties coincidentally.

Novaković et al. next tried to calculate the orbits of the nine asteroids backwards to attempt to find a common point of origin. This method can only work if a cluster of bodies is young (less than 10 million years) and none of the bodies has been seriously perturbed since the origin of the cluster, but in the event all nine bodies could be traced back to a single point of origin 1.5 million years ago, strongly supporting a common origin for the cluster.

The calculated orbit of (321490) 2009 SH54. JPL Small Body Database Browser.

Only two members of the cluster have been analysed spectrographically, (140429) 2001 TQ96 and (177075) 2003 FR36, both have been tentatively described as Q-class asteroids, though with probabilities of only 32% and 13% respectively. Bodies forming a single cluster would typically be expected to belong to a single class of body, so potentially all the bodies might be Q-class asteroids. This is interesting because Q-type asteroids are thought to have young surfaces, which have not been subject to much space-weathering, as is the case with the more common S-type asteroids.

The precise time for which a body remains a Q-type asteroid before being resurfaced by the solar winds is unclear; none of the Karin Family of Asteroids, which are thought to be about 7 million years old, is a Q-type asteroid, though the majority of Near Earth Asteroids are, possibly indicating that these bodies are being resurfaced by some process. It has been suggested that solar winds could redden a new Q-type asteroid to an S-type asteroid in about a million years, which would be problematic if the Gibbs cluster of asteroids is about 1.5 million years old, although this figure could be inaccurate, or the speed of weathering could be dependent on the distance from the Sun, allowing asteroids at a distance of about 3 AU to retain a ‘young’ surface for longer.

The calculated orbit of (140429) 2001 TQ96. JPL Small Body Database Browser.

The precise size of the objects in the Gibbs cluster is hard to determine as the precise albedo (the proportion of the light that reaches them which they reflect) of the bodies is unknown, though P/2012 F5 (Gibbs) is thought to be the smallest object, between 1 and 2 km in diameter, while (20674) 1999 VT1 is clearly the largest, between 9 and 17.1 km in diameter, which makes it about three times as large as the next largest body, (140429) 2001 TQ96, which is between 3 and 5.9 km in diameter.

Novaković et al. estimate that the parent body for the Gibbs cluster was between 12 and 24 km in diameter. This suggests that (20674) 1999 VT1 could contain around 90% of the original mass of the parent body, and that the event which led to the formation of the cluster could be seen as somewhere between a catastrophic collision and a cratering event.

The calculated orbit of (20674) 1999 VT1. JPL Small Body Database Browser.

Since the study began it has become apparent that P/2012 F5 (Gibbs) is unlikely to be a Main Belt Comet, and likely that the production of the coma around the body in 2012 was the result of a one-off collisional event. Follow up observations of seven of the other bodies failed to show any sign of comet-type activity from them either, thus while the study has succeeded in identifying a new asteroid family, it has failed to confirm a relationship between Main Belt Comets and asteroid families.

Nevertheless Novaković et al. feel that the family is worthy of further study, and that gaining better spectrographic data on its members would be particularly useful. They further note that (20674) 1999 VT1, the largest body in the group and therefore potentially the best subject for observations, will be at opposition (directly opposite the Sun when seen from Earth; the point when it is both closest to Earth and likely to reflect the most light back towards it) in September 2014.

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Friday, 25 July 2014

The Gibbs Family of Asteroids.