Discussion

In Figure 1 one sees that not only bright-rimmed clouds and cometary globules but also most reflection clouds have elongated tails. Figure 2 shows the surface distribution of all the objects in Table 1 together with the directions of their tails. The tails of nearby clouds are apparently aligned in a common direction. More interestingly all the clouds as a whole point roughly to a common center, which is located somewhere in the Belt or Sword region. (As for the huge CC-A the extension of its axis passes far north, but the bright part of its jagged outline appears to point to the Belt region.) The same trend was also found by Bally (1991) in his CO (J=1-0) and 0.5ex13CO (J=1-0) survey in the outer Orion region. This suggests that these structures have been formed by a common action, which is most probably the UV radiation from the past and/or present O stars in the Ori OB1 Association. In particular for many clouds in the Belt region the shaping center is the O7V star Ori, which is the exciting star of IC 434.

In the Belt region one finds a spatial sequence of the three categories of remnant clouds. Bright-rimmed clouds including the Horsehead Nebula (object no. 52) are located closest to Ori. To the northwest of the star there are several cometary globules, among which Ori I-2 (object no. 40A) is the most prominent. Further to the northwest a group of reflection clouds (typified by object no. 27) are found. This beautiful arrangement strongly suggests the evolutionary sequence of remnant clouds starting from bright-rimmed clouds through cometary globules to reflection clouds. However, remnant clouds which are located far from the O star(s) may directly become cometary globules or reflection clouds.

As already mentioned there is plenty of evidence for recent or ongoing star formation in bright-rimmed clouds and cometary globules. For selected reflection clouds in the Belt region we have carried out CO ( J=1-0), 0.5ex13CO (J=1-0) and C0.5ex18O (J=1-0) observations as well as IR imaging. The molecular nature of the clouds has been confirmed and young stellar objects have been found associated with some of them. These results will be reported in a forthcoming paper (Nakano et al. 1997).

One of the foci in recent studies of star formation is the so-called dispersed T Tauri stars, which are found far outside molecular clouds. Three explanations have so far been proposed as to their origin or nature: run-away T Tau stars (Sterzik and Durisen 1995), star formation in high-velocity cloudlets (Figelson 1996) and young main-sequence field stars (Briceño et al. 1997). In the Orion region Sterzik et al (1996) identified many such candidates by using the ROSAT All Sky Survey. The objective-prism Schmidt surveys of our group (Nakano, Kogure and Wiramihardja 1995 and references therein) have also detected altogether over 1000 H emission stars in a similar sky area to that of the present study. We suspect that retarded star formation in remnant clouds can explain the presence of dispersed T Tauri stars at least in the peripheries of Orion OB1 and other OB associations.

 
Figure 1: Identification charts for the objects listed in Table 1. These are reproduced from the SERC Equatorial EJ Sky Atlas (indicated by EJ), the Second Palomar Sky Survey B Atlas (indicated by B) or red plates taken with the UK Schmidt telescope (indicated by R). The magnification follows the color band. North is up and east to the left.

 
Figure 2: Surface distribution of the objects in Table 1. Ticks indicate the directions of their tails.

© Copyright Astronomical Society of Australia 1997