Tuesday 25 December 2018

Looking for colour changes on the surface of 174P/Echeclus following a cometary outburst.

The Centaurs are a population of Solar System bodies on paths that cross the orbits of the major planets of the Outer Solar System. A body is defined as a Centaur if its orbit brings it no closer to the Sun than the planet Jupiter, but its average distance from the Sun is less than that of Neptune. The orbits of Centaurs are inherently unstable, due to their frequent close encounters with the major planets, and objects are thought to typically remain a part of this population for no more than ten million years, with new Centaurs being recruited from the Solar System’s Scattered Disk by the gravity of Neptune, and older members of the group either being knocked out of the Solar System altogether or progressing inwards to become Jupiter Family Comets (a comet with a period of under 20 years; the time a comet takes to orbit the sun is referred to as a 'period', the term 'year' being reserved for planets).

The Trans-Neptunian Objects that make up the Scattered Disk are typically reddish in colour, which is attributed to the presence of hydrocarbons on their surfaces that have been irradiated by the Sun for billions of years, while Jupiter Family Comets tend to be neutral in colour. The Centaurs can be split into two populations in terms of colour, one reddish like the Trans-Neptunian Objects, the other Neutral in colour like the Jupiter Family Comets. As Centaurs migrate towards the Inner Solar System, they undergo bouts of cometary activity, in which chunks of the icy material that make up the bodies are heated by the Sun to above their sublimation point (comets are made up of different ices, including water, carbon dioxide and carbon monoxide, which sublimate – pass directly from a solid to a gaseous state – at different temperatures), causing any dust or other material above them to be lost into space, forming a coma (cometary halo) around the body. This process of coma-formation has been directly observed on Jupiter Family Comet 67P/Churyumov-Gerasimenko by the European Space Agency's Rosetta Spacecraft, and is thought to be the driving mechanism of colour change in the Centaurs, as repeated bursts of cometary activity lead to the red hydrocarbon dust on the surface being lost, and the exposure of neutrally coloured ices beneath.

174P/Echeclus is a Centaur with a perihelion (point on its orbit at which it is closest to the Sun) of 5.82 AU (i.e. 5.82 times as far from the Sun as the planet Earth, and just outside the orbit of Jupiter) and an average distance from the Sun of 10.68 AU (10.68 times as far from the Sun as the Earth and slightly outside the orbit of Saturn). It was discovered in March 2000, and since then has been observed to undergo a series of cometary outbursts, the first in December 2005, then two in May 2011, one in August 2016 and one in December 2017.

In a paper published on the  arXiv database at Cornell University Library on 29 November 2018, Tom Seccull and Wesley Fraser of the Astrophysics Research Centre at Queen's University Belfast, Thomas Puzia of the Institute of Astrophysics at the Pontificia Universidad Católica de Chile, Alan Fitzsimmons, also of the Astrophysics Research Centre at Queen's University Belfast, and Guido Cupani of the Osservatorio Astronomico di Trieste, present the results of a spectrographic analysis of 174P/Echeclus, based upon observations of the object made before and after the August 2016 outburst with the X-Shooter Spectrograph mounted on the European Southern Observatory's Very Large Telescope in Chile.

The X-Shooter Spectrograph is a medium resolution echelle spectrograph with three arms, that can make simultaneous observations at three separate parts of the light spectrum, 0.30-0.56 μm (near UV to blue), 0.55-1.02 μm (visible light) and 1.02-2.48 μm (near infrared).

This instrument was used to observe 174P/Echeclus on two occasions, on 3 August 2014, when it had been inactive for over three years, and on 7-8 October 2016, roughly six weeks after the August 2016 outburst, when the coma from that outburst was still present. On both occasions stars in the field with known light spectra were used to calibrate the spectrograph, with HD 198289, HIP 107708, and HIP 105408 included in the 2014 observations and HIP 107708 and HD 16017 in the October 2016 observations (the presence of HIP 107708 in both sets of observations provides extra support for the validity of the calibration).

A debiased, flat-fielded, 45 second r' exposure of Echeclus and its coma, observed with X-Shooter's Acquisition and Guiding Camera on 7 October 2016. The dashed ring has a radius of 26’’ (~ 1 x 10⁵ km at Echeclus) and is centred on the nucleus, marking the point at which the radially averaged surface brightness of the coma blends into that of the background sky. The small black rectangle marks the size and average orientation of X-Shooter's slit while obtaining spectra of Echeclus on 7 October 2016. Seccull et al. (2018). 

Seccull et al. could find no difference between the light spectra of 174P/Echeclus in August 2014 and October 2016. This is does not present any difficulty for the theory that the outbursts progressively change the colour of Centaurs, as it could indicate that the outburst occurred over an area too small to be observed from the Earth, or pointing away from us at the time of the observations; this observation neither supports nor contradicts the theorem.

The coma generated by the August 2016 outburst still largely surrounded 174P/Echeclus at the time of the October 2016 observations, and therefore was also captured by these observations. Surprisingly, this coma was markedly blue in colour, not just compared to the (reddish) parent body, but compared to the visual spectrum of the Sun. This could be a result of sorting within the coma, as larger hydrocarbon molecules tend to be bluer than smaller ones, and become bluer as they are broken down by solar irradiation, and hydrocarbon molecules are known to make up about 50% of the material within cometary halos. This would have the effect that as smaller hydrocarbon molecules are lost from the coma into surrounding space the coma would become dominated by larger molecules and their break-down product, and therefore become progressively bluer. Should enough material from sorted, large-molecule comas settle back onto the parent body, this could potentially have the effect of neutralising the red colour of that body, providing an explanation for the apparent colour change seen in Centaurs. However, it cannot be ruled out that the blue colour observed is the result of the angle from which the coma was observed, a result of light refraction within the cometary halo.

A zoomed contour plot of the top image after it was smoothed with a gaussian filter. It has linear scaling, and shows the observed morphology of Echeclus' coma, which is similar to that observed in our g' filter image on the same night. The lowest contour is set at one standard deviation of the background noise above the median background level. The + symbol marks a background source that is unrelated to Echeclus. Seccull et al. (2018). 

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