Exploring the nature of new main-belt comets with the 10.4 m GTC telescope: (300163) 2006 VW139

¹ Instituto de Astrofísica de Canarias (IAC), C/vía Láctea s/n, 38205 La Laguna, Tenerife Spain
e-mail: jlicandr@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205, La Laguna Tenerife, Spain
³ Instituto de Astrofísica de Andalucía – CSIC, Granada, Spain
⁴ Departmento de Edafología y Geología, Universidad de La Laguna (ULL), Tenerife, Spain
⁵ INAF, Osservatorio Astronomico di Arcetri, Italy
⁶ GTC Project, 38205 La Laguna, Tenerife, Spain

Received: 23 July 2012
Accepted: 20 November 2012

Abstract

Aims. We aim to study the dust ejected by main-belt comet (MBC) (300163) 2006 VW₁₃₉ to obtain information on the ejection mechanism and the spectral properties of the object. This will help to see if they are compatible with those of “normal” comets.

Methods. Broad-band images in the g and r band as well as a low-resolution spectrum in the 0.35–0.9 μm region were obtained with the GTC telescope (Roque de los Muchachos Observatory, La Palma, Spain). Images were analyzed to produce a color map and derive a lower limit of the absolute magnitude H. A Monte Carlo scattering model was used to derive dust properties such as mass loss rates and ejection velocities as a function of time. The reflectance spectrum was compared to that of the well-studied MBC 133P/ Elst-Pizarro. The spectrum was also used to search for CN emission and to determine the upper limit of the CN production rate.

Results. The reflectance spectrum of 2006 VW₁₃₉ is typical of a C-class asteroid, with a spectral slope S'_V = 0.5 ± 1.0%/1000 Å. It is similar to the spectrum of 133P and other MBCs. No CN emission is detected in the spectrum. A CN production rate upper limit of 3.76 × 10²³ s^-1 is derived. Images show that the MBC present a narrow almost linear tail that extends up to 40 000 km in the anti-solar direction and more than 80 000 km in the direction of the object’s orbital plane. The color of the tail is slightly redder than the Sun (S′ between 3−6%/1000 Å). The Monte Carlo dust tail model derived the mass loss rates and ejection velocity as a function of time, and the results show that the activity onset occurs shortly after perihelion, and lasts about 100 days; the total ejected mass is ~2 × 10⁶ kg.

Conclusions. The spectrum of 2006 VW₁₃₉ suggests that it is not a “normal” comet. The spectrum is typical of the other observed MBCs. Even if no CN emission is detected, the more likely activation mechanism is water-ice sublimation. Like other well studied MBCs, (300163) 2006 VW₁₃₉ is likely a primitive C-class asteroid that has a water-ice subsurface depth reservoir that has recently been exposed to sunlight or to temperatures that produce enough heat to sublime the ice.