A “diamond-ring” star: the unusual morphology of a young (multiple?) object ^*,**

¹ Universidade de Lisboa – Faculdade de Ciências, Centro de Astronomia e Astrofísica da Universidade de Lisboa, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal e-mail: [yun;nuno]@oal.ul.pt
² Instituto de Ciencias del Espacio (CSIC) and Institut d'Estudis Espacials de Catalunya, Facultat de Física, Planta 7a, Universitat de Barcelona, Avenida Diagonal 647, 08028 Barcelona, Spain e-mail: torrelles@ieec.fcr.es

Received: 6 February 2007
Accepted: 26 March 2007

Abstract

Aims.Our new near-infrared J and -band images taken towards IRAS 06468-0325 revealed the presence of extended emission with very unusual and enigmatic morphology: that of a diamond-ring, that is, a ring or torus with a bright source overlaping the edge of it (not inside it). We report the discovery of this source, explore its nature, and propose possible interpretations of its morphological structure.

Methods.We observed IRAS 06468-0325 obtaining optical and infrared images through s and L' filters, K-band low-resolution spectroscopy, together with millimetre line observations of CO(1–0), ¹³CO(2–1), C¹⁸O(2–1), and CS(2–1).

Results.Morphologically, IRAS 06468-0325 has two components: a bright, close to point-like source (the diamond) and a sharp-edge ring-like structure (the ring). The source is not detected in the optical wavelengths shorter than the I-band. The diamond is seen in all the imaging bands observed. The ring-like structure in IRAS 06468-0325 is clearly seen in the I, J, H, and Ks. It is not detected in the L'-band image. Infrared colours of the diamond are compatible with excess circumstellar emission and a young stellar nature. Strongly non-gaussian, moderately bright CO(1–0) and ¹³CO(2–1) lines are seen towards IRAS 06468-0325, at v_LSR of 30.5 km s^-1 (corresponding to a kinematic distance of 3 kpc). Very weak C¹⁸O(2–1) and CS(2–1) lines were detected. K-band spectra of the diamond and of the ring are similar both in the slope of the continuum and in the presence of lines supporting the idea that the ring is reflected light from the diamond.

Conclusions.With the current data, a few different scenarios are possible to explain the morphology of this object. However, the available data seem to favour the idea that the morphology of IRAS 06468-0325 corresponds to a young, stellar multiple system in a transient stage where a binary co-exists with a circumbinary disc, similar to the case of GG Tau. In this case, the sharpness of the well-defined ring may be due to tidal truncation from dynamic interactions between components in a binary or multiple stellar system. IRAS 06468-0325 may be an important rare case that illustrates a short-lived stage of the process of binary or multiple star formation.