A young double stellar cluster in an HII region, emerging from its parent molecular cloud

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@oal.ul.pt
² Nordic Optical Telescope, Apdo 474, 38700 Santa Cruz de La Palma, Spain e-mail: akaas@not.iac.es
³ Instituto de Astrofísica de Andalucía, CSIC, Apdo 3004, 18080-Granada, Spain e-mail: [delgado;emilio]@iaa.es

Received: 16 November 2007
Accepted: 22 February 2008

Abstract

Aims. We investigate the star formation occurring in the region towards IRAS 07141-0920 contained in the HII region Sh2-294 (S294). We report the discovery
of a new young double stellar cluster, and we describe its properties.

Methods. High-resolution optical and Hα images obtained with ALFOSC mounted on the Nordic Optical Telescope (NOT), near-infrared  images obtained with NOTCam at the Nordic Optical Telescope, and VLT/ISAAC images obtained through the H₂ (2.12 μm) filter were all used to make photometric and morphological studies of the point sources and the nebula seen towards Sh2-294. Identification of PMS members was done using three different methods: comparison with isochrones in optical colour-magnitude diagrams and detection of either near-IR excess or Hα emission.

Results. The optical images reveal an emission nebula rich in morphological details. The nebula is composed mainly of ultraviolet scattered light and of Hα emission with local morphological features conveying an impression of strong dynamical complexity. Contrasting with the bright parts of the nebula, opaque patches are seen. They appear elongated, filamentary, or clumpy. Our optical photometry confirms that the identification of the illuminator of the nebula is likely to be a B0.5V star located at a distance of about 3.2 kpc. Our high-resolution near-IR images reveal an embedded cluster, extending about 2 pc and exhibiting sub-clustering: a denser, more condensed, subcluster surrounding the optical high-mass B0.5V illuminator star; and a more embedded, optically invisible, subcluster located towards the eastern, dark part of the nebula and including the luminous MSX source G224.1880+01.2407, a massive protostellar candidate that could be the origin of jets and extended features seen at 2.12 μm. The double cluster appears to be clearing the remaining molecular material of the parent cloud, creating patches of lower extinction and allowing some of the least reddened members to be detected in the optical images. We find 12 main-sequence (MS) and 143 pre-main sequence(PMS) members. The most massive star fits a 4 Myr, post-MS isochrone. The age of the optically selected PMS population is estimated to be 7-8 Myr. The IR-excess population shows sub-clustering on scales as small as 0.23 pc and is probably much younger.