Testing the comet nature of main belt comets. The spectra of 133P/Elst-Pizarro and 176P/LINEAR

TL;DR

This study analyzes the spectra of main belt comets 133P and 176P, finding they resemble B-type asteroids and Themis family members, with no detectable CN gas emission, suggesting they are likely asteroid fragments with retained volatiles.

Contribution

The paper provides the first visible spectra of 133P and 176P, confirming their asteroid-like nature and association with the Themis family, challenging the idea of their cometary origin.

Findings

Spectra of 133P and 176P resemble B-type asteroids.

No CN gas emission detected, with a low upper limit for production.

Spectral similarities support their classification as Themis family asteroids.

Abstract

We present the visible spectrum of MBCs 133P/Elst-Pizarro and 176P/LINEAR, as well as three Themis family asteroids: (62) Erato, (379), Huenna and (383) Janina, obtained in 2007 using three telescopes at "El Roque de los Muchachos"' Observatory, in La Palma, Spain, and the 8m Kueyen (UT2) VLT telescope at Cerro Paranal, Chile. The spectra of 133P and 176P resemble best those of B-type asteroid and are very similar to those of Themis family members and are significantly different from the spectrum of comet 162P/Siding-Spring and most of the observed cometary nuclei. CN gas emission is not detected in the spectrum of 133P. We determine an upper limit for the CN production rate Q(CN) = $= 2.8 \times 10^{21}$ mol/s, three orders of magnitude lower than the Q(CN) of Jupiter family comets observed at similar heliocentric distances. The spectra of 133P/Elst-Pizarro and 176P/LINEAR confirm that they are likely members of the Themis family of asteroids, fragments that probably retained volatiles, and unlikely have a cometary origin in the trans-neptunian belt or the Oort cloud.