HST images of NGC 6537 were obtained on 12 Sept. 1997. A total of 18 images were obtained with WFPC2 in program 6502 (B.Balick, P.I.). Most of the images are not ideal for our purpose, since they were taken with filters which include strong nebular lines. One image was taken with the filter F547M, which passes visible light containing only weak nebular lines and the nebular continuum. The exposure time was 40 seconds. Some of the other images had longer exposure times, up to 500 seconds. The results are interesting: no central star is visible in any of the images including that taken with the F547M filter. This is corraborated by Balick (private communication, 2000) who states that none of the images, including the F547M image, `show any trace of a central star'.
To obtain a quantitative estimate of the upper limit to the magnitude we have proceeded as follows. NGC 6537 has been compared to Me2-1, which has been chosen because the two nebulae are very similar, except that the central star of the latter nebula is clearly seen. Me2-1 is slightly larger (diameter about 9") than the central region of NGC 6537 where almost all the emission is concentrated (diameter about 7"). Me2-1 has a total H flux of -11.34 (log erg cm-2s-1) compared to -11.66 for NGC 6537. The average surface brightness is therefore very similar, but because the central regions of NGC 6537 (where presumably the central star is located) is clearly less bright than its outer regions, the central surface brightness in NGC 6537 is less than it is in Me2-1.
Me2-1 was measured by the WFPC2 on 23 Feb. 1994 in program 5107 (J. Westphal,P.I.) and has been calibrated and analysed by Wolff et al. (2000). One of the images measured was taken with an F547M filter and is entirely equivalent to the image taken of NGC 6537, except that the exposure time was somewhat longer: 100 seconds. The visual magnitude of the central star was found to be 18.40, and the ratio of the peak stellar flux in the visual to the surrounding nebular continuum is 20. A star of this magnitude is thus easily seen. If a star could be recognized when its peak flux is 50% of the nearby nebular continuum, its flux would be lower than that of Me2-1 by a factor of 40. This is 4 magnitudes, and applies for the central star of NGC 6537 if the nebular continuum is the same as for Me2-1. This is approximately true, since, as shown above, the H surface brightness of both nebulae is about the same. Because the central regions of NGC 6537 have a lower surface brightness, the star could be somewhat fainter. A possible compensation for this is the somewhat shorter exposure time of the WFPC2 image of NGC 6537. We estimate that the central star has mv=22.4 or fainter.
This result could have been anticipated. The ground based measurement was made with a seeing of approximatel 0.8-1.0"(Jacoby and Kaler, 1989), while the WFPC2 measurement has a resolution of 0.1", an improvement of between a factor of 64-100(about 4.5-5 magnitudes). Since Jacoby and Kaler have an upper limit of mv=18.78, we might expect an upper limit to the WFPC2 measurement of between 23.3 and 23.8 magnitudes. That we do not go so faint may have two origins. First, Jacoby and Kaler had a longer measurement: 600 seconds. Second, we may have been conservative in estimating that a 50% change in the continuum level is necessary to recognize the central star. The effect of the longer exposure time is difficult to estimate, because the nebular background increases just as fast as the starlight. We maintain the estimate of mv=22.4.
© European Southern Observatory (ESO) 2000
Online publication: October 24, 2000