The formation process of massive stars is still not well understood (cf. Garay & Lizano 1999 for a recent review). The evolutionary timescales for massive stars are very short, as they start to burn hydrogen while still accreting material from the surrounding protostellar cloud. Young massive stars also affect their environment strongly by driving very powerful winds, molecular outflows, and jets, and also by emitting intense UV radiation. An especially interesting aspect of massive star formation is the observational result that many massive protostars drive very energetic outflows that are often more massive than the central protostar (cf. Churchwell 1997). To reach a better understanding of the physics of massive star formation, observations with high spatial resolution are crucial in order to disentangle the numerous different physical processes taking place simultaneously. This is the motivation for our high-resolution study of the massive protostellar object S140 IRS1, well known for driving a massive bipolar molecular outflow.
S140 is an HII region at the south-east edge of the L1204 dark cloud and part of a cloud complex located at the edge of a prominent infrared emission ring, known as the Cepheus ring. This ring is probably the result of a supernova explosion and stellar winds from massive stars in the open cluster NGC 7160, close to the center of the ring (cf. Kun et al. 1987.)
About north-east of the S140 HII region, Rouan et al. (1977) detected strong far-infrared emission at a position at which no objects could be seen in visible light. Harvey et al. (1978) found that the spectral energy distribution of this infrared source, called S140 IRS, strongly increases between 2 µm and 100 µm and estimated an infrared luminosity of (for a presumed distance of 1 kpc). Beichman et al. (1979) carried out 20 µm observations of this region and were able to resolve the infrared emission into three individual sources. At 20 µm, the dominant source IRS1 is 7-10 times brighter than the two other sources IRS2 and IRS3. The luminosity of IRS1 was estimated to be (Lester et al. 1986), suggesting it to be a deeply embedded ( mag; Harker et al. 1997) early B-type star with a mass of about . However, these estimates of the stellar parameters appear to be quite uncertain. A determination of the extinction towards IRS1 based on 3 µm ice-band spectroscopy by Brooke et al. (1996) yielded a column density of corresponding to an extinction of mag. This would suggest a somewhat higher luminosity and thus also a higher mass for IRS1.
A strong molecular CO outflow in the S140 IRS region was first detected by Blair et al. (1978). The total outflow mass was estimated to be (Bally & Lada 1983), i.e. about 6 times more than the mass of the central protostar. Minchin et al. (1993) studied the CO molecular line emission in the S140 region with the JCMT and found a bipolar outflow morphology. S140 IRS1 lies just in the middle between the blue- and red-shifted outflow lobes; since the other two infrared sources IRS2 and IRS3 are clearly not located on the outflow axis, IRS1 can reliably be assumed to be the source of this outflow. The position angle of the outflow axis is about . Since the blue- and red-shifted outflow lobes overlap strongly, the outflow axis is believed to be rather close to the line of sight.
In previous studies, near-infrared images of S140 were obtained for example by Harker et al. (1997) and Yao et al. (1998). Near-infrared polarization maps were reported by Lenzen (1987), Whitney et al. (1997), and Yao et al. (1998). These observations, however, were seeing-limited and therefore did not have the resolution required to study the inner environment ( AU) of the central source S140 IRS1. In order to get a better insight into the nature of this interesting object, we have carried out bispectrum speckle interferometry and speckle polarimetry of S140 IRS1.
© European Southern Observatory (ESO) 2000
Online publication: October 2, 2000