Astron. Astrophys. 321, L21-L24 (1997)

3. The structure of the nebula

The processed N-band and [Ne II ] 12.8 m images of HR Car and its surrounding nebula are shown in figures 1 and 2, respectively. In the N-band the HR Car nebula has a slightly bipolar structure, as have many LBV nebulae. The distribution, however, is not point-symmetric around the star. The N-band image shows the star as a clear central peak. The integrated flux of the central source is 1.5 Jy, which compares well with an estimated stellar flux of 1 Jy from extrapolating the the ISO-SWS spectrum (Lamers et al. 1996), both numbers having an error of at least 25 percent. A somewhat "patchy" arc is seen E to SE of the star and a brighter blob NW of the star. The total fluxes of the regions NW and SE respectively are almost equal. Assuming the dust emission to be optically thin at 10 m and equal dust temperatures in both regions, this implies that more or less the same amount of dust is present at the SE and the NW of the star, but that towards the NW it is more confined. The assumption of optically thin dust emission is very reasonable at 10 m, given the optical extinction and IR colors of the object (see Hutsemékers & van Drom 1991b), but the assumption of equal temperatures cannot be tested using our images alone. A fainter extended nebula is also seen, to across, which shows no clear sign of bipolarity. This more extended region is probably the source of the colder dust seen with IRAS and ISO. The total integrated flux of the compact nebula alone is 6.5 Jy and the total flux of the region is 11.7 Jy, with an uncertainty of about 20 percent.

The average projected distance of the SE-arc is about and the blob NW of the star has a projected distance of about 1 5. Assuming the distance to HR Car to be 5.4 kpc (Hutsemékers & van Drom 1991b) corresponds to a projected distance of 0.026 pc. This means that the NW-peak is at a projected distance of only 0.04 pc.

Fig. 1. Image of the HR Car nebula in the N-band. North is up and East is to the left. The first contour is at 10 mJy/ and contours increase by steps of 40 mJy/ .

Fig. 2. Image of the HR Car nebula in the [Ne II ] 12.8 m-band. North is up and East is to the left. The first contour is at 25 mJy/ and contours increase by steps of 33 mJy/ .

The [Ne II ] 12.8 m image shows a somewhat different picture. Due to the narrow band-width of the filter, the quality of the [Ne II ] image is less than the N-band image, yet we are confident that all the spatially extended features visible in the presented image are real. The central star is still seen as the brightest source, with an estimated flux density of 1.8 1 Jy. This is within the uncertainty of the extrapolated continuum flux of 0.8 Jy from ISO-SWS, but a possible excess could be due to [Ne II ] line emission from the present day stellar wind. Contrary to what is seen in the N-band image the nebula as seen in the [Ne II ] band shows no clear bipolarity; virtually no emission is seen to the NW of the star but the region SE of the star shows a similar arc-like structure as the N-band image, but is slightly closer to the star; the ionized [Ne II ] region appears to be on the inside of the dusty nebula. Its structure is also more "patchy" than the continuum N-band image. This may be caused by the fact that the [Ne II ] line flux is proportional to , whereas the dust flux is a linear function of the density. The integrated flux of the region is 11.9 Jy with an uncertainty of about 50 percent.

In principle the signal in the [Ne II ]-band could be due to continuum dust-emission rather than to [Ne II ] 12.8 m line emission. The ratio of the observed flux in the nebula of the [Ne II ] over the N-band filter is slightly higher than the ratio of the respective filter throughputs, by a factor of 1.3, but due to the large uncertainty in the [Ne II ] flux, it cannot be ruled out that a significant portion of the flux measured through the [Ne II ] filter is in fact continuum flux. Also, high resolution [Ne II ] 12.8 m spectroscopy of the HR Car nebula, which will be discussed elsewhere more extensively, gives only an upper limit to the nebular continuum flux at 12.8 m, which is too high to be able to dismiss the possibility that we observe mainly continuum radiation through the [Ne II ] filter rather than line radiation.

It should be noted that the images taken through the [Ne II ] filter and the N-band filter do not coincide spatially, which indicates that we are seeing different material through the different filters. It also indicates that optical depth effects play a role; the central star ionizes only part of the nebula.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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