Tirela: an unusual asteroid family in the outer main belt

¹ Observatório Nacional, Rua Gal. José Cristino, 77, 20921-400 Rio de Janeiro, Brasil
² UFRJ/Observatório do Valongo, Ladeira Pedro Antônio, 43, 20080-090 Rio de Janeiro, Brasil e-mail: thais.mothe@astro.ufrj.br
³ Southwest Research Institute, 1050 Walnut St., Suite 400, Boulder, Colorado 80302, USA e-mail: davidn@boulder.swri.edu

Received: 29 May 2008
Accepted: 15 September 2008

Abstract

Context. Earlier visible spectroscopic classification of (1400) Tirela suggests that this object is a D-type. As the Tirela family has high eccentricities and inclinations, its parent-body could have been implanted in the main belt during the late heavy bombardment, a scenario recently proposed for some D-type asteroids. Spectra of other members of the family have not been available until now, except for colors from the Sloan Digital Sky Survey (filters u, g, r, i, and z) for some of them. These colors also suggest that the family has slopes similar to D-types.

Aims. Thus, we wanted to spectroscopically characterize the Tirela family. Due to the faintness of the objects, we used some relatively big telescopes in some observing runs.

Methods. We obtained visible spectra of 10 faint Tirela family members at telescopes TNG and GEMINI and the NIR spectrum of two members of the family at the IRTF telescope. These spectra were classified and comparisons were made with meteorites and minerals in an attempt to infer the mineralogical surface composition of the members.

Results. The Tirela family was found to be composed of Ld-type asteroids. Comparisons with minerals suggest that the combined visible+near-infrared spectrum of (1400) Tirela show some similarities to the spectrum of the pure mineral pigeonite.

Conclusions. We conclude that the family is not a D-type, but comparisons with minerals suggest similarities between the absorptions found in Tirela with those found in the mineral pigeonite.