The nature of the LMC protostar na-IR

Th. Henning ¹, R. Klein ¹, S. J. Chan ², E. L. Fitzpatrick ³, R. Siebenmorgen ⁴, & B. Stecklum ⁵

¹ Astrophysikalisches Institut und Universitäts-Sternwarte (AIU) Jena, Schillergäßchen 2-3, D-07745 Jena, Germany

² University of Cambridge, UK

³ Villanova University, USA

⁴ ISO Science Operations Centre, Astrophysics Division of ESA, Spain

⁵ Thüringer Landessternwarte Tautenburg, Germany

We present an ISO-SWS spectrum pointed on the protostar na-IR in the Large Magellanic Cloud (LMC). The spectrum covers the full wavelength range of SWS on board of ISO (2.5 - 45m) where most of the dust features are located. The protostar was detected by Epchtein et al. (A&A, 140, 67, 1984) by photometric observations. We also present near- and mid-infrared images of the protostar. For the first time, the protostar is seen in the NIR as a compact point source in the north-west of the HII region na. Further indication for na-IR being a protostar is the presence of an H₂O maser.

The SWS spectrum of na-IR shows the unidentified infrared bands (UIBs) and silicate absorption on a steeply rising continuum plus fine structure lines from the HII region. The comparison of the UIBs with those of other sources demonstrates that the features are similar to the UIBs signatures of galactic compact HII regions determined by a high radiation field exciting the UIBs. Whether this radiation field is produced by the protostar itself or by the surrounding HII blobs is not yet clear.

Together with a flux measurement at 1.3mm, the spectral energy distribution (SED) is known over a sufficient broad wavelength range to allow one-dimensional radiative transfer modeling of the continuum radiation. The protostar has a luminosity of L and the mass of the cloud is about 50,000M. The central object with a temperature of about 50.000K is deeply embedded (A 50mag) in its parental cloud. The environment must be carbon-rich. Otherwise the silicate absorption should be much stronger. The step rise of the SED is reproduced using a rather flat radial density distribution ( ).