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Astron. Astrophys. 361, 1112-1120 (2000)

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2. Observational material

2.1. Imagery

The best imagery of NGC 1501 available in the literature goes back to Minkowski (1968), who obtained H[FORMULA] and [OIII] interference filter plates with the Hale Telescope at Mount Palomar, revealing the intricate nebular structure (but see also Chu et al., 1987, and Manchado et al., 1996a).

The superior spatial resolution of HST hasn't yet been exploited for NGC 1501; at the moment, only four broad-band, quite underexposed frames of the nebula (taken by Howard Bond on September 7, 1995, during a snapshot survey for companions of PNe nuclei) are contained in the HST archives; because of the low surface brightness of the emitting gas, they little add to Minkowski's H[FORMULA] plate published in the frontispiece of the IAU Symposium N. 34 proceedings.

On September 1999, a broad-band R image of NGC 1501 (exposure time 600s, seeing 0.76") was obtained with the Optical Imager Galileo (OIG) mounted on the 3.58m Italian National Telescope (TNG, Roque de los Muchachos, La Palma, Canary Islands) and equipped with a mosaic of two thinned, back-illuminated EEV42-80 CCDs with 2048x4096 pixels each (pixel size=13.5 microns; pixel scale in 2x2 binned mode=0.144 " pix-1). The CCD data array was processed in the usual way using standard IRAF packages.

The appearance of NGC 1501 in the R band, shown in Fig. 1, is almost entirely due to H[FORMULA] emission; the contamination of [NII] ([FORMULA][FORMULA]6548 and 6584 Å) and of HeI ([FORMULA]5876 Å) amounts to 4% and 3%, respectively; other fainter lines contribute for less than 2% (see Kaler, 1976, Stanghellini et al., 1994, and the next sub-section).

[FIGURE] Fig. 1. Broad-band R image of NGC 1501 (exposure time 600s, seeing 0.76") obtained with the 3.58m Italian National Telescope (TNG); North is up and East is on the left. At higher contrast a very faint, diffuse and roundish envelope appears, extending up to 34" from the central star.

Our R image reveals the presence of a diffuse, roundish envelope framing the main nebular body and extending up to a distance of 34" from the central star (this halo is too faint to be reproduced in Fig. 1).

The application of a soft Lucy-Richardson restoration (point spread function=seeing, 10 iterations; Fig. 2) to the original R frame highlights the bubbly structure of NGC 1501, rich of filaments and condensations ("bearing a resemblance to the convolutions of the brain" is the suggestive description given by Francis G. Pease in 1917).

[FIGURE] Fig. 2. The same R frame of Fig. 1 after the application of a soft Lucy-Richardson restoration (point spread function=seeing, 10 iterations), showing in great detail the tangled H[FORMULA] structure of NGC 1501. The haloes around stars projected onto the nebula are artifacts due to the deconvolution.

2.2. High resolution spectra

On December 1998, [FORMULA][FORMULA]4500-8000 Å spatially resolved, long-slit spectra of NGC 1501 (+ flat fields + Th-Ar calibrations + comparison star spectra) were secured with the Echelle spectrograph (Sabbadin & Hamzaoglu, 1981, 1982b) attached to the Cassegrain focus of the 182cm Asiago telescope and equipped with a Thompson 1024x1024 pixels CCD.

We selected four position angles: 100o (apparent major axis, after Curtis, 1918), 10o (perpendicular to the apparent major axis), 55o and 145o (intermediate positions). All spectra were centred on the exciting star; we used the stellar continuum as a position marker and to correct the data for seeing and guiding uncertainties. The slit-width was 0.200 mm (2.5" on the sky), corresponding to a spectral resolution of 13.5 km s-1 (1.5 pixel). The calibration in wavelength and flux was performed in the standard way using the IRAF Echelle packages.

H[FORMULA] and [FORMULA]5007 Å of [OIII], the dominant emissions in our spectra, present the same structure in every detail (but not in the line width: as normally observed, the first is broader than the second, because of the larger thermal motions). Besides H[FORMULA] and [FORMULA]5007 Å (and the correlated lines H[FORMULA] and [FORMULA]4959 Å), only a few, faint emissions were detected, due to the low surface brightness of the emitting gas; they all mimic the H[FORMULA] and [OIII] structure. The line intensities, integrated over the whole nebula and the entire velocity profile, are listed in Table 1; only relative fluxes are reported here, since the observational conditions (the sky was stable only at intervals) prevent the accurate calibration into absolute fluxes.


Table 1. Integrated line intensities

The interstellar extinction was derived by comparing the observed [FORMULA] intensity ratio with the dereddened value given by Brocklehurst (1971) for [FORMULA] and [FORMULA]; we obtain [FORMULA], in agreement with the values of 1.1 and 0.96 reported by Kaler (1976) and Stanghellini et al. (1994), respectively.

The close resemblance of the H[FORMULA] and [OIII] emission structure and the weakness (or the absence) of low excitation lines (in particular, [NII] at [FORMULA][FORMULA]6548 and 6584 Å and [SII] at [FORMULA][FORMULA]6717 and 6731 Å) indicate that NGC 1501 is an optically thin, density bounded PN.

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Online publication: October 10, 2000