Complement to VII/239A

NGC 5106

is probably NGC 5100. But there are some problems remaining. Here is the story:

WH saw the nebula on just one night, and recorded only the right ascension, not the north polar distance. The GC and NGC note the polar distance as "v doubtful." The NGC carries a brief note: "Not found by Tempel (A.N. 2522); at least there was no 2nd class nebula near the place." Dreyer has a longer note in his edition of WH's complete papers: "Sw. 108. 'A vS and F neb sp 59 Virginis [= SAO 119847]. Its A.R. is about 13h 06 1/4m. While I looked into the finder to determine its situation, I lost it, but shall endeavour to find it another night.' The transit is given as 13h 10m, that of 31 Bootis (Sweep 111) as 14h 30 1/4m. It is probably = Marth 255 (NGC 5100), 35s p, 30' n of the assumed place of II. 22; no neb in H's place." In his NGC corrections based on the WH edition, Dreyer simply says, "II.22 must be = 5100."

There is no problem with NGC 5100. Marth's position is very close to Bigourdan's which is, in turn, very close to the GSC's. It has a small companion which is mentioned in the CGCG, MCG, and UGC, though was apparently not seen by Marth or Bigourdan. And N5100 is indeed 35s p, and 30' n of the NGC position. All this supports Dreyer's contention of the equality of the two NGC numbers.

However, JH makes no mention of how he determined the polar distance which he used in GC for N5106 (Dreyer simply copied the PD into NGC). If we assume that JH used the PD of 31 Bootis for II.22, then the GC/NGC PD is more than 30 arcmin in error – it should read 81 18.1 rather than the 80 46.5 it does. Also, regardless of the PD, WH's RA places II.22 south-following 59 Virginis, not south-preceding as he states.

So, questions linger around this object. Unless WH's or JH's unpublished notes can shed some light on this, we have to regard the identity of N5106 with N5100 as provisional.



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