Saturday 1 June 2013

The Main-Belt Comet P/2012 T1 (PANSTARRS).

Main-Belt Comets are bodies within the Main Asteroid Belt that show some similarities with comets, notably the production of a halo (dust tail) during the part of their orbit that takes them closest to the Sun. The tail of a typical comet is a distinctive object; comets have icy surfaces which evaporate away as they pass through the Inner Solar System, carry a trail of mineral specks and chunks of ice that form the visible tail. For most of a comets life it is in the Outer Solar System, far from the Sun's influence and produces no tail. Main Belt Comets spend their entire lives between the orbits of Mars and Jupiter. They are thought to be compositionally similar to regular comets, but only to produce much smaller halos at the innermost part of their orbit's, when they are warmed marginally more by the Sun. Such objects are less obvious than regular comets, and have only been known about since 1996.

In a paper published on the arXiv database at Cornell University Library on 23 May 2013, a team of scientists led by Henry Hsieh of the Institute for Astronomy at the University of Hawaii describe a new Main-Belt Comet, discovered on 6 October 2012 by the Pan-STARRS1 survey telescope on the Haleakala volcano in Hawaii, and named P/2012 T1 (PANSTARRS); where 'P/' implies a periodic comet, '2012' is the year of discovery, 'T1' implies the object was the first such object discovered between 1 and 16 October that year, and '(PANSTARRS)' is the discoverer (historically this would be a single astronomer, but this is not appropriate for large modern instruments operated by teams of scientists).

Image of P/2012 T1 (PANSTARRS). The comet is a point, while the elongate objects are stars; this is because the telescope is tracking the moving comet, making the (non-moving) stars in the background appear to be in motion. G.V. Schiaparelli Astronomical Observatory.

After the initial discovery follow-up observations of the new comet were made with University of Hawaii (UH) 2.2 m and the 10 m Keck I telescopes, both on Mauna Kea, the 6.5 m Baade and Clay Magellan telescopes at Las Campanas in Italy, the 2.0 m Faulkes Telescope South at Siding Spring in New South Wales, the 1.8 m Perkins Telescope at Lowell Observatory in Arizona, and the Southern Astrophysical Research Telescope on Cerro Pachon in Chile.

P/2012 T1 (PANSTARRS) roughly doubled in brightness in the 40 days following its discovery (between 6 October and mid-November 2012), remained constant in brightness till late December, then declined in brightness by 60% over the period till mid-February. The steady brightening over the initial part of this cycle and steady dimming over the second part is taken to be representative of sublimation from the surface of P/2012 T1 (PANSTARRS), rather than a sudden ejection of matter following a collision.

Orbital diagram for P/2012 T1 (PANSTARRS). The point at the centre is the Sun, the four inner rings, the orbits of Mercury, Venus, Earth and Mars respectively, the outermost ring showing only at the corners of the diagram is the orbit of Jupiter. The point marked 'P' is the comet's perihelion, the point at which it is closest to the Sun; the point marked 'A' is it's aphelion,  when it is furthest from the Sun. The points (1-6) represent observations of the comet; (1) 2012 October 6-8, (2) 2012 October 12-25, (3) 2012 November 8-14, (4) 2012 December 18-20, (5) 2013 January 8, and (6) 2013 February 4. The scale is in Astronomical Units (AU), where 1 AU is the average distance between the Earth and the Sun. Hsieh et al. (2013).

Hsieh et al. calculate that P/2012 T1 (PANSTARRS) ejected water molecules at an average rate of 5 × 10²⁵ mol s-¹ during the eruptive part of it's cycle, which is equivalent to just under 2 100 000 000 megatonnes of water per  day. All the ejected material appears to be water; a spectrographic analysis could find no sign of hydrated minerals; this does not imply that the temperatures are high enough to sublimate water (turn directly from a solid to a gas; liquids cannot exist in a vacuum), it is more likely that it is due to the sublimation of carbon dioxide (at a lower temperature) creating a halo of water-ice molecules (snowflakes) freed from the matrix.

P/2012 T1 (PANSTARRS) sits within a cluster of asteroids known as the Lixiaohua Family, which are believed to have originated in the breakup of a larger body about 155 million years ago, though it is not possible to tell if it shares a common origin with these objects or whether its position is coincidental; it could be an object from the Outer Solar System that has only recently been captures or it could potentially be a member of the much older Thetis group of asteroids.

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