Hamburg Observatory, Hamburg, Germany
The nebula He 2-104 (RA=140833.5, D= 511219.1, PK 315+91, PN G 315.4+09.4), known as the Southern Crab since the discovery of its large bipolar structure by Schwarz et al.(1989), originally discovered in 1964 by Henize (1967) and classified also as a symbiotic star (SS, Allen, 1984), is a typical example of objects classified simultaneously as PN and SS.
The nebula was observed with the 0.9 m tel.+ CCD at La Silla in Apr./May 1995 (scale: 1 pixel = 0.443). The shape of it, clearly visible in the light of [OIII] 5007A line, is bipolar with bright central arcs together with two main condensations. These condensations (N1, S1) lie exactly at the ``poles'' of the nebula. The dimensions of the main structure are 80x34 in p.a. 120.8; the total extension of the nebula may be 105 if the outer faint condensations N13 and S13 are also taken into account. The space between the main condensations N1 and S1 and the arcs is filled out by some matter. If the radiation of this is in the continuum, which seems to be probable, its nature would be dust. Further investigation is planned in order to prove this.
We identified altogether 26 condensations or clouds. They appear in pairs, which means that to each northern condensation a southern counterpart can be found. The condensations are nearly symmetrical towards the centre of the nebula. A common origin of the pairs of condensations is very probable. The presence of condensations in [OIII] 5007A indicates their high excitation state.
The central object is not starlike but somewhat extended: only 0.40 in X- and 0.49 in Y-direction if FWHM of it is compared with those of the surrounding stars.
Adopting the distance d=800 pc we get 0.41 pc for the total extent of the nebula, and 0.035 x 0.029 pc for its inner core. We believe that the name ``symbiotic proto-PN'' (Schwarz et al., 1989) is justified for He 2-104.
The nebula He 2-104 (RA = 140833.5,
total extension of the nebula may be 105 if the outer faint
N13 and S13 are also taken into account. We identified
altogether 26 mostly faint condensations or clouds inside the
nebula. The position of these condensations is marked in
the northern part condensations N1-N13 (the letter "N" should be
read together with the numbers 1-13), in the southern part condensations
S1-S13 (the letter "S" should again be read together with
the numbers 1-13). The list of the condensations, giving their
positions (in arcsec) relative to the central object, is presented
in Table 1. In the case of some condensations their FWHM in both
directions (X corresponds to right ascension, Y corresponds to
declination) are also given in the table. The mean values FWHM
(from 27 surrounding stars; x-direction: 1.52, y-direction: 1.78)
were subtracted from the observed values:
FWHM = FWHM FWHM.
The condensations are substantially larger than the seeing disc of the stars: the mean value in x-direction is 3.6, the mean value in y-direction is 4.0. They are mostly either circular or elongated along the nebular axis. Setting the central intensity of [OIII] 5007A to 100, the most prominent condensations N1 and S1 would be bright about 0.15 and the brightness of the other condensations 40% or even less than this. The presence of condensations in [OIII] 5007A indicates their high excitation state.
The condensations appear in pairs, which means that to each northern condensation a southern counterpart can be found. The association between the northern and the southern condensations may be in principle rather uncertain and is at present only preliminary, because we do not know the velocity of the condensations. The association Nn with Sn (n=113) is our proposal. We would like to mention the result of Corradi and Schwarz (1993) concerning the expansion velocity of the polar blobs (identical with our bright condensations N1+N2 and S1+S2). They found V250 km.s having adopted the inclination of the nebula i > or = 60 . In order to propose the outflow model of the nebula we would need the 2-D velocity field of it.
The condensations lie nearly symmetrically towards the centre of the nebula (x=0, y=0). The mean point of all 13 pairs differs only very little from the centre of the nebula: x=+0.10.2, y=0.80.4. It is very probable that the condensations expand from the centre and that the pairs of condensations have a common origin.
An independent check for the existence of nebular condensations was based on star counts: we counted altogether 82 stars in the field of He 2-104. The star density in our photometric system was therefore 118 arcsec for 1 star outside the nebula and 108 arcsec in the visible part of the nebula (those parts of the nebula which correspond to the arcs and to the central region of the nebula were subtracted). The density of all observed objects inside the nebula is more than twice as high as the density of stars only, so that there is no doubt about the existence of the nebular condensations.
The structure of the central arcs is dotted with several condensations as seen in the paper of Corradi, Schwarz (1993, Fig.1). The arcs extend in all four directions to about 24 arcsec from the centre of the nebula. They have no sharp ends and their width is between 2.3 and 5.0. The "opening" angle (the angle between two arcs measured at the centre in the direction of the nebular axis) of the northwest arc is 116 (p.a. of the arcs 243 and 359), that of the southeast arc is also 116 (p.a. of the arcs 63 and 179). The direction to the individual condensations deviates from the nebular axis up to about 38 degrees (= phiCON- phiA, see Table 1). The mean value is only 4.7 degrees larger than phi and it nearly corresponds to the nebular axis.
The space between the main body of the nebula (the arcs) and the condensations N1 and S1 is filled out by some matter (see Fig.2). If the radiation of this is in the continuum, which seems to be probable, its nature would be dust. Further investigation is planned in order to prove this. In that case dust would be present not only in the assumed equatorial torus, but also in the polar directions.
The central object is not starlike but somewhat extended: only 0.40 in X- and 0.49 in Y-direction, respectively, if FWHM of it is compared with those of the surrounding stars. The central nebula looks on our deep image nearly like a square; it was resolved into a second crab-like pattern by Burgarella et al. (1991) and shows two oval loops of size 5 x 8 (see also Corradi, Schwarz, 1993).
If we adopt the distance d=800 pc of He 2-104 (Schwarz et al., 1989) we get
0.41 pc for the total extent of the nebula, and 0.035 (towards the poles) x 0.029 pc for the dimension of its inner core. The idea of high-velocity
jets and the calculated dimensions of the inner nebulosity are both compatible with the
common conception of small proto-PN, so that we believe that the name
``symbiotic proto-PN'' (Schwarz et al., 1989), considering also the
M-component of the central star (see Whitelock, 1988), is justified for
|N1||interacting with N2|
|N2||interacting with N1, separate condensation?|
|N4||association with S4 uncertain, may also be with S10|
|N6||together with a star|
|N10||very faint and small; association with S10 uncertain, may also be with S4|
|S1||interacting with S2|
|S2||interacting with S1, separate condensation?|
|S4||association with N4 uncertain, may also be with N10|
|S10||association with N10 uncertain, may also be with N4|
* Based on data collected at the European Southern Observatory, La Silla, Chile.
Fig.1: Bipolar shape of He 2-104 with 26 condensations in pairs: read N1...N13, S1...13. [OIII] 5007A filter.
Fig.2: He 2-104 in the light of [OIII] 5007A; the space between the main nebular body and the condensations N1+N2 and S1+S2 is filled out by some matter, probably dust. Position in pixels, scale: 1 pixel = 0.443'' orientation as Fig.1 (north at the top, east on the right).