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Astron. Astrophys. 318, 908-924 (1997)

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4. HST images of the expanding envelope

The detailed description of the HST images of the expanding envelope taken on days 467, 689, 726 and 818 as well as data reduction procedures were published by Paresce et al. (1995). The main feature is the bright elliptical ring best visible in [FORMULA] and [O III] lines, which linearly expands along the semi-major axis by 0.297 mas/day and along the semi-minor axis (defined by two bright knots) by 0.218 mas/day. According to Paresce et al. (1995) the major axis of the bright ring defines polar direction!

In our rediscussion we used the images given in Table 5. All the images were treated using the standard procedures described by Nota et al. (1994) as well as using the point spread functions derived from observations of bright single stars through the same filters. The standard IRAF and MIDAS packages were used for further processing. The images from the same observing run, where the bright ring was well defined, were merged. Since the ring was not uniform in brightness, we chose 3 tresholds corresponding to different intensities. The central part of the image was removed. It is apparent that the ring in all cases was well defined by an ellipse.


Table 5. Journal of HST images

In our interpretation the bright ring is identical with the circular equatorial ring. In this case the elliptical shape of the ring is caused by the projection of the circular ring onto the celestial sphere. An inclination angle i of the equatorial ring plane against the celestial sphere can be calculated as follows: The radius vector of the point of the ring r at azimuth angle A is given by:


where [FORMULA] is the true radius of the ring (equal to semimajor axis of the ellipse) and [FORMULA] is the azimuth angle of the ascending node measured from the north point of the ring eastward. Fitting of the isophotes by a general least square method using the simplex downhill method and Eq. (3) for the best coordinates of the ring centre, we obtained i and [FORMULA] as given in Fig. 4. We also measured the projected angular distances of the blobs from the center of the nebula. Using these values we constructed the diagram on Fig. 7 formally similar to Fig. 6 in the paper by Paresce et al. (1995). However, by the inspection of Fig. 4 we see that the minor axis is not defined by two bright knots as supposed by Paresce et al. (1995). These knots designated by crosses are in fact the polar blobs and move independently with respect to the ring. Comparison of the images in Fig. 4 also shows that i and [FORMULA] of the ring change with time. If we suppose the linear increase of i and decrease of [FORMULA] as it is depicted in Figs. 8 and   9, we can easily predict their values during the expansion of the envelope.

[FIGURE] Fig. 4. Parameters of the equatorial ring and polar blobs

The first HST image of Nova V 1974 Cyg taken on day 467, in time before installation of COSTAR, is highly problematic (see Fig. 5). Our processing of the image by the same method as in previous three images led to the value of [FORMULA] = 67 [FORMULA] 5. If this were true, the orientation of the ring has changed in time for an unbelievably large amount and in an unpredictable way. However, the flipped image gives [FORMULA] = 112 [FORMULA] 5, in good agreement with the predicted values (see Fig. 8). Therefore we believe that the sign of matrix of the orientation of the image in the header of the pre-COSTAR image is incorrect. A further problem arises with the inclination angle. In the image which we have processed only the brightest parts of the ring were taken and the ring was not well defined. Moreover as it is easily seen from Fig. 7, the length of the semi-major axis is underestimated. So the value of i = 39 [FORMULA] 5 could only be the upper limit of the possible inclination angle of the ring. For the determination of i we used also the image processed in STScI by Paresce (1994) and available in the HST ftp server. We delineated the contour of the ring by identifying the pixels with the same intensity at the edge of the ring and measuring their positions given by radius vector against the azimuth angle. The resulting value is i = 33 [FORMULA] 5 as it is shown in Fig. 6. The mean value of the inclination angle found by these two methods is i = 36 [FORMULA] 5 [FORMULA] 2 [FORMULA] 1. This value is in agreement with the predicted value found by the extrapolation of i from other 3 images in Fig. 9.

[FIGURE] Fig. 5. Parameters of the equatorial ring and polar blobs on day 467
[FIGURE] Fig. 6. Determination of the inclination angle from the HST image taken on day 467.
[FIGURE] Fig. 7. The expansion rates of the equatorial ring and polar blobs. The values in the brackets were not used for the fits.
[FIGURE] Fig. 8. Time dependence of the position angle of the expanding equatorial ring. The values in the brackets were not used for the fit.
[FIGURE] Fig. 9. Time dependence of the inclination of the expanding equatorial ring. The value in the brackets was not used for the fit.

Among the available HST images of the expanding envelope of the nova, the last two (May 17, 1994 = day 818) are the most remarkable. Fig. 10 was obtained by superposition of the HST images in [Ne V] and [O III] lines. We interpret it as an evidence for a magnetic force shaping of the inner envelope up to a distance of about 450 AU from the nova caused by the interplay between the dipole magnetic field of the underlying white dwarf and magnetized plasma. The "light particles" of the outflowing plasma in the polar region follow the magnetic lines of force forming about 10 arc-like meridional streams (flux tubes) resembling a water fountain. The points of intersection of these streams with the expanding spherical lower density inner envelope are clearly seen as the bright spots within the shell (Fig. 11). If we suppose that the bright spots are located on a circle, we can calculate the position of the magnetic field axis on day 818 using the same method as in the case of the equatorial ring. The resulting value is i = 51 [FORMULA] 0. It is clear that the polar axis of the expanding envelope is defined by the symmetry axis of the fountain. This direction is nearly perpendicular to the direction of the major axis of the ring, which according to Paresce et al. (1995) defines the polar direction.

[FIGURE] Fig. 10. HST image of the magnetic fountain in the inner envelope
[FIGURE] Fig. 11. Magnetic fountain in the inner envelope

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998