Structure and geometry of L1204/S140

L1204/S140 is a classical well studied edge-on photon dominated region (PDR). It consists of the dark cloud L1204, pc (Crampton & Fisher 1974), which is bordered in the south-west by an H  region. This H  region is commonly thought to be excited by the B0 V star HD 211880 located well outside the cloud. The interface between the cloud and H  region is visible as a bright rim called S140 (Sharpless 1959), located at 7' NE of the exciting star. At 1' north-east of this interface lies a dense core with mag which is associated with a cluster of infrared objects (Beichman et al. 1979). These have been identified as embedded early B-type stars (Evans et al. 1989) and are commonly denoted as S140IRS. One of them, IRS1, is associated with a bipolar molecular outflow (Bally & Lada 1983) along a NW-SE axis.

 
Figure 1: POSS II R-band image of L1204/S140 (, centred on S140IRS1) with superposed contours of the integrated CO(2-1) emission (HPBW=3'; Plume et al. 1994). Darker grey scales correspond to higher brightnesses

The S140IRS region has intensively been studied in molecular line emission at high angular resolution. Only recently studies of the large scale structure of L1204 cloud have been performed in C (Emery et al. 1996), C and CO (Plume et al. 1994) emission and provide important information on the translucent carbon transition region. These studies reveal a large atomic carbon (C, C) abundance over the whole cloud even at large distances from the interface region. This can be understood from a non-homogeneous cloud structure, where the UV photons can penetrate deep into the cloud, and has been modelled successfully by Spaans & van Dishoeck (1996), who also investigated geometry effects, e.g. if the illuminating star and L1204 would not lie in the same plane on the sky. Direct information on the translucent cloud structure and geometry are lacking, but can be obtained from deep optical images (see Sect. 2). We therefore started a programme to get deep R, I, and H CCD images of the L1204/S140 complex using the 1.06m Cassegrain telescope of Bonn University at Hoher List Observatory. In combination with a focal reducer (f/3.5) a field size of 28' (circular) is obtained with a pixel resolution of 0.8'' on a 2k2k CCD. Below we present first results from deep H imaging.

 
Figure 2: Deep continuum subtracted H image of L1204/S140 of an area around S140IRS1, taken at Hoher List Observatory

Figure 1 shows part of a digitised POSS II R-band film (smoothed to 4'' resolution) centred on S140IRS with superposed contours of the integrated CO(2-1) emission (3' resolution; Plume et al. 1994). Note that the CO emission appears to extend into the H  region. This is not real, due to the low resolution of CO the sharp edge is unresolved and therefore appears displaced. The R-band image clearly reveals the dark cloud L1204, the bright rim S140 at as well as the diffuse extended H  region around the exciting B-type star at ). However, the sensitivity for the H line in the R-band is rather low and there is contamination with diffusely scattered red light from dust. Figure 2 shows the continuum subtracted H CCD image. A wealth of small-scale structures is visible on the H image. Brightness variations in the diffuse H emission are real and caused by extinction of dust within the H  region. The edge of the dark cloud is well delineated, note also the eruption-like structures in S140 which are typical of photoevaporation. It is also clearly visible that the H  emission extends over the face of the cloud. Figure 3 shows a NE-SW cut crossing S140. The H intensity drops steep at the interface, but remains significant even at the position of S140IRS. Thus, the illumination is somewhat face-on.

The [C ] emission in the S140 complex has been measured with ISO-LWS by Emery et al. (1996). They found a strong increase in [C ] intensity at the interface (see their Fig. 3), and the emission remains strong at large distances into the cloud. The derived UV (912-2000 Å) intensity at the interface is about . This is well matched by the flux of a B0V star at the projected distance of 2 pc (7'). Further into the L1204 cloud the [C ] emission requires a constant , which would be possible if the star is located out of the plane of the cloud. In addition, for a clumpy cloud structure, there may be a contribution of UV flux from the embedded sources. Smirnov et al. (1995) have measured the H166 recombination line and derived cm. A crude estimate of the Strömgren radius from the H image gives pc (for a B0V photon flux) which yields an average electron density consistent with theirs. It is interesting that H166 emission was found even in the direction of L1204, this could mean that the H  region may extend behind the cloud. A detailed analysis of the deep optical images will be presented elsewhere (Stark, Reif & Bagschik in prep).

 
Figure 3: H intensity along a NE-SW cut through S140IRS and HD 211880

© Copyright Astronomical Society of Australia 1997