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14 December, 2014

One of Most Accurate Measurements of the Size of a Trans-Neptunian Object from a Stellar Occultation

Stellar occultations by minor bodies have been widely used to measure their sizes and constrain their shapes. The technique itself is quite simple to implement; it is only necessary to measure the lightcurve of the system minor body plus star before, during, and after the minor body crosses the line of sight to the star. Measuring the duration of the dimming in the lightcurve and knowing the speed at which the minor body moves it is then possible to measure the length of that chord in distance units. If many such chords could be measured of the same event using different telescopes on Earth, then it is possible to obtain the shape of the body.

To predict when and where such events will occur is difficult as highly accurate positions of both the star and the minor body are required, the latter being the main source of uncertainty. For this reason, a large international campaing works to predict and observe occultations by Trans-Neptunian Objects (TNOs) and Centaurs. TNOs are the most primitive objects of the solar system and, due to their distance from the Sun, the most difficult to measure reliable sizes via other techniques (such as modeling of thermal data). Moreover, the duration of the eclipsing events varies from a few to almost 30 seconds, depending on the size and distance from the observer, and hence, high-speed cameras are necessary to obtain the data.

One such event was observed at the William Herschel Telescope during the first hours of the night April 26, 2012 using ULTRACAM. ULTRACAM is a very fast camera able to obtain short exposures with very short overheads, which is a critical instrumental constraint due to the high time resolution demanded. The occulting body was (119951) 2002 KX14, a so-called classical TNO in a low-inclination, low-eccentricity orbit with a previous thermal estimate of its diameter of 455 km. Using the superb data provided by ULTRACAM on the WHT, astronomers were able to obtain a lightcurve with very sharp edges, showing no evidence of atmosphere that could have refracted the light coming from the star, and a total eclipse duration of 20.87±0.05 s, which translates into a chord of 415±1 km, one of the most accurate measurements obtained so far of a chord.

Observed lightcurve in the i' filter of the system star plus kx14. The total flux before and after the event is normalized to enhance the depth of the event.

Unfortunately only one chord of the event was measured, which provides only a lower bound for its size. Furthermore, observations using other telescopes were unsuccesful. Despite this, from a few reasonable estimates of the density of 2002 KX14, 0.8 gr/cm3, and of its projected shape, elliptical, astronomers were able to set constraints on the effective size: 365 +30 -21 km. The size of 2002 KX14 precludes the existence of volatiles other than possible water ice on its surface, which also agrees with the observed spectra.

More information:

A. Alvarez-Candal et al., 2014, "Stellar occultation by (119951) 2002 KX14 on April 26, 2012", A&A, 571, A48. [ Paper ]

Outreach video "A Stellar Occultation by 2002 KX14".

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Last modified: 15 December 2014