Astron. Astrophys. 343, 841-846 (1999)

3. Resolved nebulae

In Table 2, we present a compilation of all the optically resolved nebulae around symbiotic stars known up to October 1998 with references to the discovery papers and to the most relevant imaging works published. The extended nebulae around CH Cyg, HM Sge, and V1016 Cyg, with apparent sizes of up to fifty times larger than previously known, were discovered during the present survey (NOT, 4.6.1996), and their first images were presented in Corradi & Schwarz (1997). No images are shown here, and the reader is referred to the preliminary results in the paper above, or to individual spatiokinematical studies which are to be published soon (Corradi et al. 1999).

Table 2. Nebulae resolved at optical wavelengths.
Notes:
1 Bang et al. 1992;
2 Corradi & Schwarz 1993;
3 Corradi & Schwarz 1995;
4 Corradi & Schwarz 1997;
5 Corradi et al. 1999;
6 Fuensalida et al. 1988;
7 Hack & Paresce 1993;
8 Hollis et al. 1989;
9 Hollis et al. 1990;
10 Hollis et al. 1997a;
11 Kohoutek 1997;
12 Lampland 1923;
13 Michalitsianos et al. 1988;
14 Munari & Patat 1993;
15 Paresce 1990;
16 Paresce & Hack 1994;
17 Schild & Schmid 1997;
18 Schwarz 1991;
19 Schwarz & Corradi 1992;
20 Schwarz et al. 1989;
21 Solf & Ulrich 1985;
22 Solf 1992;
23 Van Winckel et al. 1994;
24 This paper.

Discovery of a nebula around H 2-2 was reported by Kohoutek (1997). The nebula is 1:004 long, elongated along P.A.= and observed in the [OIII ] light but curiously not in the H+[NII ] filter. Further observations with subarcsec seeing are needed to obtain better information on this nebula.

A bibliographical compilation of the radio detections of resolved symbiotic nebulae is presented in Table 3.

Table 3. Nebulae resolved at radio wavelengths.
Notes:
1 Boboltz et al. 1997;
2 Chigo & Cohen 1981;
3 Dougherty et al. 1995;
4 Dougherty et al. 1996;
5 Eyres et al. 1995;
6 Eyres et al. 1996a;
7 Eyres et al. 1996b;
8 Hjellming 1985;
9 Hjellming et al. 1986;
10 Hollis et al. 1985;
11 Hollis et al. 1986a;
12 Hollis et al. 1986b;
13 Hollis et al. 1987;
14 Hollis et al. 1997b;
15 Kafatos et al. 1983;
16 Kafatos et al. 1989;
17 Kenny et al. 1991;
18 Kenny et al. 1993;
19 Kenny et al. 1996;
20 Lehto & Johnson 1992;
21 Newell 1981;
22 Porcas et al. 1986;
23 Purton et al. 1983;
24 Seaquist & Taylor 1987;
25 Sopka et al. 1982;
26 Taylor et al. 1986;
27 Taylor 1988;
28 Taylor et al. 1989.

3.1. H 1-36

H 1-36 is partially resolved in our optical images. Its FWHM is slightly but systematically larger than that of nearby field stars. We were very careful in considering possible instrumental effects, such as the variation of the point-spread function through the field (but comparison stars well distributed around the symbiotic system were taken, and some small systematic effects taken into account), or possible non-linearity of the CCD (but different exposure times and luminosities of the comparison stars give the same results). Moreover, similar results are obtained for images taken at different epochs, under various seeing conditions, and with different telescopes (the ESO 3.5m NTT and 1.54m Danish). In Table 4, we list the FWHM of H 1-36 and the average value for the comparison stars both along the X and Y directions in the frames and in the different filters. The surprising results is that the nebula appears resolved also in a continuum image, suggesting that it might be a reflection nebula. The deconvolved diameter of the nebula around H 1-36 was estimated using the recepies in Bedding & Zijlstra (1994). It is computed to be between 0:009 and 1:005, depending on the intrinsic geometry of the nebular model (hollow shell, disc, or uniform sphere).

Table 4. Images of H 1-36

At a distance of 7.5 kpc (Whitelock 1987), this optical nebula would have a diameter between 7000 and 11000 AU. Note that H 1-36, according to several studies (Allen 1983, Ivison et al. 1994), is an extreme symbiotic system having a binary separation of 1000 AU. The two components of the system are possibly resolved in the radio (Taylor 1988). The size of the optical nebula, a few times the binary separation, would agree with the prediction by Taylor & Seaquist (1984). Further optical observations at higher spatial resolution (HST) of this object are however needed to investigate the properties of the nebula (emission? reflection?).

3.2. Uncertain cases: MWC 560

Kohoutek (1997) also suggested the possible detection of a resolved nebula around MWC 560, with a deconvolved diameter smaller than half an arcsec. The difference in FWHM between the symbiotic system and the field stars, however, is only marginal (see his Table 2), and we do not consider it as significant. Further observations, under good seeing conditions, will provide more precise information on this object.

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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