Observation of light echoes around very young stars^*

¹ Instituto de Astrofísica de Andalucía – CSIC, Apt 3004, 18080 Granada, Spain e-mail: duffard@iaa.es
² Department of Physics and Astronomy, Goucher College, 1021 Dulaney Valley Rd., Baltimore, Maryland 21204, USA
³ Astroimagen. C. Abad y Lasierra, 58 Bis – 62, 07800 Ibiza, Islas Baleares, Spain
⁴ Observatoire de Paris, LESIA-UMR CNRS 8109, 5 place Jules Janssen, 92195 Meudon Cedex, France
⁵ Complejo Astronómico El Leoncito (CASLEO-CONICET) and San Juan National University, Avda. de España 1512 sur, J5402DSP, San Juan, Argentina
⁶ Instituto de Astronomía y Física del Espacio, Universidad de Buenos Aires, CONICET, CC 67-Suc 28, C1428ZAA, Ciudad Autónoma de Buenos Aires, Argentina

Received: 16 March 2010
Accepted: 1 June 2010

Abstract

Aims. The goal of the paper is to present new results on light echoes from young stellar objects.

Methods. Broad band CCD images were obtained over three months at one-to-two week intervals for the field of NGC 6726, using the large field-of-view remotely-operated telescope on top of Cerro Burek.

Results. We detected scattered light echoes around two young, low-amplitude, irregular variable stars. Observations revealed not just one, but multiple light echoes from brightness pulses of the T Tauri star S CrA and the Herbig Ae/Be star R CrA. Analysis of S CrA's recurring echoes suggests that the star is located 138 ± 16 pc from Earth, making these the closest echoes ever detected. The environment that scatters the stellar light from S CrA is compatible with an incomplete dust shell or an inclined torus some 10 000 AU in radius and containing ~2 × 10^-3  of dust. The cause of such concentration at ~10 000 AU from the star is unknown. It could be the remnant of the envelope from which the star formed, but the distance of the cloud is remarkably similar to the nominal distance of the Oort cloud to the Sun, leading us to also speculate that the dust (or ice) seen around S CrA might have the same origin as the Solar System Oort cloud.