Astron. Astrophys. 361, L29-L32 (2000)

4. Interpretation of the observed structures

The near-infrared emission is dominated by the central intensity peak, that contains of the total flux in our image. This peak probably marks the location of the central protostar S140 IRS1. An alternative explanation might be that the protostar is too deeply embedded to be directly visible in our image, and the intensity peak might be light from the protostar that is scattered at the walls of the outflow cavity. However, the observed polarization pattern which is centro-symmetric around the intensity peak strongly suggests that the location of the illuminating source actually is at the intensity peak of our image. The observed ratio of diffuse versus direct light together with typical optical properties of circumstellar dust (e.g. Preibisch et al. 1993) suggests an extinction of mag ( mag) along the line-of-sight to the protostar, i.e., the dominant intensity peak seems to be a highly extincted image of the central protostar. The possibly elongated structure of the central source might be caused by bright emission from the innermost part of the larger extended structure, or might indicate that the central object is a binary star.

The most striking feature in our image is the bright extended and very clumpy structure pointing from IRS1 to the south-east. Interestingly, the position angle of this structure of matches the direction of the blue-shifted CO outflow very well: the ¹²CO map of Minchin et al. (1993) gives a position angle of for the blue-shifted outflow lobe. This strongly suggests that the elongated feature in our image is related to the blue-shifted CO outflow. The axis of the CO outflow is known to be closer to the line-of-sight than to the plane of the sky (Minchin et al. 1993), i.e. the south-eastern lobe of the outflow is oriented roughly towards us, while the north-western lobe is pointing away from us. This suggests the following interpretation of the observed features:

The central object IRS1 is deeply embedded in a dense circumstellar envelope or perhaps a thick circumstellar disk. The outflow has cleared a cavity in the circumstellar material and what we see as the bright extended structure south-east of IRS1, is light from the central protostar that is scattered by dust grains at the inner wall of this outflow cavity into our direction. The very clumpy appearance of the emission suggests that the surface of the cavity wall is not smooth but highly structured. This might be the signature of the violent interaction between the outflowing material and the circumstellar envelope.

Our image traces the diffuse emission out to at least about away from the central source. This suggests that the size of the circumstellar envelope or disk around IRS1 is at least AU. The asymmetric general shape of our image, i.e. the fact that we do not see a counter lobe north-west of IRS1, is easily explained as a geometrical effect. The red-shifted outflow component, pointing in the north-west direction, has probably cleared a similar cavity. That cavity, however, is not visible in our near-infrared images because it is pointing away from us and the light predominantly escapes in a direction away from us.

© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000
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