Astron. Astrophys. 332, 849-856 (1998)

1. Introduction

The Chamaeleon Infrared Nebula (Cha IRN) is a bipolar reflection nebula in the Cha I dark cloud (Schwartz & Henize 1983). The source is associated with IRAS 11072-7727. In the near infrared, the typical appearance of a bipolar nebula with the two lobes separated by a dark plane points to the presence of a circumstellar disk. Cohen & Schwartz (1984) performed far-infrared measurements on Cha IRN, which revealed an elongated structure in north-south direction at 52 µm and 100 µm, ascribed to the coolest outer parts of the disk. They estimated the visual extinction towards the central object as mag, the object itself being assumed to be either an A7 main sequence dwarf or a T Tauri star of photospheric type K7. From fitting a 65K blackbody to the spectral energy distribution (SED) and thereby determining the emitting area, they concluded that the disk possesses a major axis of ( 2800 AU at a distance of 140 pc after Rydgren 1980) and is viewed almost edge-on at an inclination of . The new HIPPARCOS data on optically visible stars near Cha IRN suggest that the actual distance is closer to 190 pc. This distance is adopted throughout the paper.

Ageorges et al. (1996) performed a high-resolution polarization study in the near infrared (NIR) and concluded that Cha IRN consists of a central object surrounded by a disk with a radius of AU and a thickness of AU, which is inclined by , assuming a distance of 140 pc. Their model for the circumstellar environment consists of a combination of a Toomre-like disk, a free-falling spherical envelope, and a bipolar cavity. The assumption concerning the central object was that of a star with K and . However, aiming more at the determination of the geometry of the system, they fitted the polarization maps solely at one single wavelength. As intensity variations across the spectrum are disregarded by this procedure, the nature of the central object is not very well constrained.

Gledhill et al. (1996) argue from a tilt between the bipolar nebula axis and the polar axis of the disk as concluded from the polarization pattern that the system must possess two disks with a tilt of with respect to each other. The dense inner disk would then be responsible for a highly non-isotropic illumination of the outer molecular disk and the outflow and thus provide the polarization. They suggest the tilt to be caused by tidal forces from a young binary system inside. A similar mechanism to produce non-isotropic illumination is considered for FU Orionis stars by Bell & Chick (1997).

This paper presents new imaging and spectroscopic data on the Cha IRN to learn more about this fascinating object. In Sect. 2 we describe the observations and the data reduction process. This covers the near-infrared images, obtained with IRAC2b at ESO's 2.2 m telescope, speckle data taken at the NTT using SHARP I, and spectroscopic data obtained using IRSPEC at ESO's NTT and ISOPHOT-S. In Sect. 3 we use the imaging data to discuss morphological properties of the nebula. These data support the hypothesis that a very young binary system is located inside the Cha IRN. Additionally, we derive the grain composition of the surrounding dust from the spectra. Here we found indications for water and, only very marginally, ammonia ice together with clear evidence for CO and CO₂. Features from PAH emission are completely lacking and the silicate absorption band at 10 µm is not detected. Finally in Sect. 4, we present the results obtained using our radiative transfer model to fit the SED, the IR spectra and the mid-infrared appearance. Combined with our new NIR images of Cha IRN, this approach results in a self-consistent model of the source.

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
helpdesk.link@springer.de