NGC/IC Project Restoration Effort

===== NGC 537 = NGC 523, and the surroundings. The night of 13 September 1784 was not a good one for WH's clock readings. With one exception (H II 224 = NGC 404), all eight objects for which he used Beta Andromedae as a comparison star are off in RA, and -- as it has turned out -- by different amounts. In the sweep, which I finally got to in July 2016, WH noted uncertainty in the RAs of five of the objects (NGC 495, 496, 499, 515, and 517), and recorded transit times for four others (NGC 515, 517, 536 and 537) only to a full minute of time (Dreyer, as we'll see below, was well aware of all of this). In addition, his descriptions -- with a few exceptions -- are scanty, so identifying his nebulae has proved problematical over the years. Here is the story, roughly in chronological order, as I first presented it years ago. Note that Wolfgang has recently (July 2014) reanalyzed the field in light of WH's sweeping path; a summary of his results is at the end of this note. WH's seven questionable objects (III 167 through III 173; NGC 515, 517, 513, 523, 536, 552, and 553, respectively -- yes, NGC 513 is out of order) all appeared in his sweep within 3 minutes of each other. Given the rush, he determined the positions for only five of them, lumping four together into two pairs, and treating the remaining three individually. In addition, Dreyer (as I pointed out above) noted that WH recorded three transits -- III 167/8, 170, and 171 -- to only a full minute of time. Finally, WH himself noted the final two as "a little doubtful." Of all this uncertainty, only that phrase made it into WH's first catalogue of nebulae where these were presented. JH has only five nebulae here. He claimed one (h120) to be the same as his father's III 171, and the western of that pair (h118) to be a nova. Auwers, and later d'Arrest, agreed with JH in making H III 171 = h 120, but noted the difficulties in WH's RAs for some of the nebulae. d'Arrest in particular pointed out discrepancies of about 40 seconds of time between his own RA's and WH's in several cases, and found what he thought was a new double nebula in the field (NGC 523). However, while assembling the GC, JH reinterpreted the field and chose to regard the nebulae that his father discovered as separate objects from his own. Dreyer, too, was aware of the problem when he compiled the NGC, and attempted to sort things out based primarily on d'A's observations. It's clear, however, that he was a bit uncertain about the state of the field as he wrote NGC notes for some of the objects, and commented again on all of them in his 1912 edition of WH's Scientific Papers. How can we make sense out of the two Herschels' observations? Let's start by assuming that WH's nebulae are properly ordered by RA, and that their polar distances (Declinations) are also relatively correct. Doing this, and looking at JH's and d'A's later observations, we can make some tentative identifications for NGC 513, 515, 517, and 536. Plotting the difference in RA (WH minus "true") for these, we see that as the time went on, WH's RA's got worse. Plotting a straight line through the data points, and putting a mark at WH's RA for III 170 = NGC 537 suggests an RA correction of about 0.9 minutes of time for it. This moves the RA back to within 0.2 arcmin of NGC 523, and confirms Dreyer's suspicion in the NGC Notes that WH's number belongs on this NGC number. Adding this point to the plot actually suggests that the slope might be even steeper. But what about N536 = III 171? Did WH really see that, or did he perhaps see its brighter, higher surface brightness companion, N529, which precedes it by about 40 seconds? (N536's two fainter companions found by Lord Rosse, N531 and N542, have problems of their own; they have a seperate note here under N531). Assuming WH in fact did see the western of the two objects, we can then draw a new line through the points on the plot (this steeper relationship suggests that WH's clock was running at about half speed, which it, of course, was not!) in a desperate attempt to recover his final two objects, N552 and N553. If we correct WH's RA accordingly, the position of these two objects falls close to CGCG 502-084 and an equally bright 15th magnitude star just west of it. Finally, I note that -- with the exception of NGC 513, the first object in the series -- all of WH's declinations here are 3-4 arcmin too large. Correcting those puts his positions near enough the galaxies I've noted in the table below. This lends a bit more support to the hypothesis I've sketched out. In the end, then, I'm suggesting these identifications for the nebulae in the area (the CGCG names added for verification): RA (2000.0) Dec NGC WH JH d'A CGCG III WH JH 01 24 26.81 +33 47 58.2 513 169 111 --- --- 521-020 01 24 38.50 +33 28 21.8 515 167 113 167 113 502-077 01 24 43.80 +33 25 46.5 517 168 114 168 114 502-079 01 25 20.76 +34 01 29.9 523=537 170 --- (Nova) 521-022 01 25 40.29 +34 42 46.8 529 171 118 --- 118 521-023 01 26 21.76 +34 42 11.1 536 --- 120 171 120 521-025 01 26 10.01 +33 24 20.5 552 172 --- --- --- --- = * 01 26 12.52 +33 24 18.5 553 173 --- --- --- 502-084 The careful reader will have already seen that the RA's for N552 and N553 are smaller than that for N536. This adds more weight to the idea that Herschel saw N529 rather than N536. A postscript: Both Auwers and d'Arrest comment on WH's insecure RA's for these objects. However, d'A apparently goes on to suggest that some of JH's RA's are off, too. But they aren't, so I clearly need to take the time to translate d'A's comments (that remains to be done as of July 2016). ----- There it stood until the summer of 2014 when Wolfgang went back to CH's fair copy of the sweep and reanalyzed it, making careful and justified assumptions about WH's sweeping methods related to the size of his field through his eyepiece. Rather than suggest a clock running at the wrong rate, Wolfgang showed that the crowding of the nebulae contributed to several inexactly recorded times. His detailed report will appear in his forthcoming book on WH's nebulae, but his conclusions are as follows: RA (2000.0) Dec NGC WH JH d'A CGCG III WH JH 01 24 26.81 +33 47 58.2 513 169 111 --- --- 521-020 01 24 38.50 +33 28 21.8 515 167 113 167 113 502-077 01 24 43.80 +33 25 46.5 517 168 114 168 114 502-079 01 25 20.76 +34 01 29.9 523=537 170 --- (Nova) 521-022 01 25 40.29 +34 42 46.8 529 --- 118 --- 118 521-023 01 26 21.76 +34 42 11.1 536 171 120 171 120 521-025 01 26 57.0 +33 33 28 552 172 --- --- --- --- = * 01 27 03.0 +33 32 55 553 173 --- --- --- --- = ** Note that only two of the NGC identifications are changed from my initial sorting out, and that only one of the WH numbers had to be moved for this particular group of nebulae. Wolfgang also looked closely at other of the Herschels' nebulae in the area. I'll comment on those as needed.

===== NGC 537 See observing notes for NGC 523.