We summarize in Table 4
all our results for the 277 LMC PNe.
The 4 candidates (Sa108, Sa119, Sa125 and Sa127) have been excluded (see
Sect. 3.1).
Excluded also are WS03 (not confirmed as a PN candidate, see SMP and
Fehrenbach et al. 1978), and N016, N047 (extended), N099 (point like) VLE
objects (Monk et
al. 1988), but coordinates are available.
The PNe are listed, first ordered by catalog names and then, within each
catalogue, by increasing Right Ascension.
Column 1 indicates the selected name, with one cross-identification (first
SMP, if available, then, in decreasing order of priority, Sanduleak, Jacoby,
Henize, Westerlund, Lindsay, etc...).
The complementary cross-identifications can then be found, if necessary, in
the original papers.
In Col. 2 Right Ascension and declination coordinates are given
in equinox J2000 and B1950.
Column 3 is a number which gives a rough idea of the position error (1 for
precision better than 02, 2 between 02 and 03,
3 between 03 and 05, and finally 4 for coordinates less
precise than 05).
In parentheses, respectively, the number of position determinations (a maximum
of 4), and the number of positions kept for the final averaged coordinate.
The derived B magnitude are listed in Col. 4 (the number of plates used is
given in parentheses).
Because of intense crowding, some measurements were impossible.
The second product of this work is the complete set of finding charts for the
LMC PNe.
As already mentioned, only a fraction of the known PNe had published
finding charts, and these were not always of usable quality.
We therefore produced a homogeneous set of charts for all the objects.
Plates 1-25 give the 280 finding charts taken from the IIIa-J plates of
the ESO/SERC Southern Sky Atlas.
Only a few exceptions (SMP098, Sa104, Mo01, Mo02, Mo49 and Mo54) are taken
from Red plates.
Field sizes are all and North is up and East to the
left.
Note that, as a by-product, accurate positions are now also available for the standard stars.
As an example of possibilities opened by the knowledge of precise coordinates for PNe, we have looked for associations with IRAS sources, as given in the catalogue of Schwering & Israel (1990). While this is of obvious interest for the study of the PNe, it is however not illustrating the best possibilities of our catalogue, as the IRAS positions themselves are not known with an extreme accuracy. The uncertainty of the IRAS positions is given in Table 3 (click here) as the size of the error-ellipse from the Point Source Catalogue (PSC), when an association exists, as explained in the Catalogue of Schwering & Israel. The main criterion for an association of a PN with an IRAS source is the coincidence in position. We have, somewhat arbitrarily (but nevertheless based on the size of the average error-ellipse) separated sources in 3 categories: association closer than in distance, between and , and between and . With the density of sources in the LMC, at these larger distances however, many unrelated sources would enter the catalogue. A second criterion has thus been applied, based on the expected IR flux distribution for PNe. Following experience from galactic PNe (Pottasch et al. 1988), a very broad selection on flux densities can be used to select PNe: 25 / 12 > 2 (to eliminate stars) and 25 / 60 > 0.2 to eliminate galaxies. We have applied this criterion for those objects were the distance of the IRAS source to the optical PN was larger than , to eliminate many spurious associations. For the closer associations, we have left all of them in Table 3 (click here), but will apply the criterion subsequently to assess the reality of the identification. When the flux was not well estimated due to confusion, the source was kept, pending further identification (a C is then printed instead of a flux value).
Table 3: Possible IRAS associations
The results are given in Table 3,
where we list the serial number
from Schwering's list, in Col. 1, the PN name (Col. 2), the IRAS
PSC cross-identification when appropriate (Col. 3) and the
associated error ellipse (Cols. 4-6).
If no PSC identification could be done the typical errors are in
both RA. and Dec. (Schwering & Israel 1990).
Columns 7 to 9 give respectively the optical and IR positions (in J2000
equinox) and the distance between the two determinations (in arcsec).
Columns 10 to 13 show the IR fluxes at 12, 25, 60 and , while in
column 14, a ``p'' indicates a point-source.
More details about the IRAS data can be found in the original reference
(Schwering & Israel 1990).
Zilstra et al. (1994) published a list of 12 IRAS sources associated with LMC
PNe.
Five of them do not show up here: SMP006, SMP061, SMP098, MG45 and
Mo18, which indicates a distance between IRAS and optical position larger
than 2'.
As we used only the catalogue of Schwering & Israël
(1990), based on a special observation in the Deep Sky Mapping
mode, (while Zilstra et al. used indifferently this list or the Faint Source
Catalogue (FSC)), this difference has probably to be ascribed to a
discrepancy between the two catalogues, which needs further investigation.
Applying now the flux ratio criteria in addition to the association with
separations smaller than , we end with a ``quality criterion'' (Q)
for the association, given in the last column of Table 3.
Quality 1 is a secure association, both because of small distances and
adequate flux ratios, while quality 2 is less secure (flux ratios at the
limit, or unclear due to confusion, and larger separation between optical and
IR sources).
Quality 3 is a doubtful association, while 0 means rejection.
In the latter category enter SMP013, SMP043, SMP064, SMP069 and MG58 because
the IR flux distribution indicates a stellar source (for SMP064, the IRAS
source is clearly associated to the cluster NGC1984), while for SMP092 and
Mo19 the flux distribution would indicate a cold galaxy.
If the two latter objects are indeed PNe, which is confirmed for SMP092
(Meatheringham & Dopita 1991b), then they have in fact an optically thick
dust shell, with a cooler exciting source, which is known to mimic the
flux distribution of a galaxy.
Further spectroscopy will be necessary.
The same remark applies to some of the quality 3 associations in the first
part of Table 3,
which could be the same type objects, or lower
excitation HII regions (SMP024, SMP029, SMP036, SMP075).
The known (high) excitation class for the first three PNe suggests an IR
excess due to a dust shell.
Note that some objects are already classified as Very Low Excitation
objects.
It should be noted also that, because of the metallicity which is different
in the Clouds compared to our Galaxy, the color criteria for the PNe might
also be slightly modified.
We end with 15 associations of quality 2 or better, closer than
(among them 7 in the list of Zijlstra et al. 1994).
These objects are worth further study.
In the case of associations with larger separation, a close inspection of the
finding charts reveals in almost all cases a better identification for
the IRAS source than the PN.