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(vsnet 651, Brian Skiff)
Kato-san has asked me about inconsistencies among comparison stars for the prototype dwarf nova SS Cygni. Apparently the magnitudes on charts from various organizations are somewhat inconsistent.
I have done a literature survey (using SIMBAD) and find that indeed the published UBV photometry for the field leaves something to be desired. The several sources show scatter of ~0.15 mag. in both V and B-V, with possible zero-point and/or color terms evident. Since I do not have a current AAVSO chart for the field, I do not know if it includes other unpublished results. (Perhaps Charles Scovil could fill us in on the decision process that led to the current AAVSO sequence.) I likewise may not have caught every publication with useful information about the comparison stars. For instance, there may be additional useful photometry for stars close to SS Cyg measured when Nova Cyg 1978 erupted near here. However, I did not do a search in these papers, since the observed stars are probably mostly rather far from SS Cyg.
In the SS Cyg field, the two main sources of UBV photometry are Lenouvel and Daguillon (1956) and Zuckermann (1961). The L&D sequence was adopted in part by the Space Telescope crowd as part of the calibration for the GSC; the stars appear as sequence P235 in the Guide Star Photometric Catalogue (Lasker et al. 1988) with two additional fainter stars. There are additional stars in the field observed by Grant (Grant & Abt 1959), Mallama & Skillman (1979), Hopp (1979), Howarth & Bailey (1980), and finally some very faint stars close to the variable by Misselt (1996). My opinion is that none of these sources is utterly reliable at face value, and so without additional data it is not obvious that one source is more nearly correct than the others.
The data from these sources is summarized in the table below, along with HD, BD, and GSC identifications and J2000 positions either from the PPM catalogue or the GSC. The column 's' indicates the source of the photometry in each line, with the codes explained at the bottom of the table. The letter names in the remarks are mostly those assigned by Lenouvel & Daguillon, which seem to have been adopted by most subsequent observers. The list includes the two brightest stars observed by Misselt since he includes V-R colors, which will be useful for calibrating color terms with CCDs.
Obviously, even the V magnitudes for these stars show too much scatter, although for simply determining whether SS Cyg is "up" or "down" from visual inspection, even this scatter does not matter much. But because things such as duration of outbursts and so on are important in the study of this intensively-studied object, a proper sequence is more important than normal. Since the sequence is one of the few for dwarf novae that is bright enough for me to observe without difficulty, I will start work on this at the next opportunity with the Lowell 53cm telescope.
In purely visual observing, the advantages of retaining a fixed sequence of comparison stars is significant, even when it is not perfect. The BAA's use for many decades of the early sequence for SS Cyg by Parkhurst & Daniel (1900)---which isn't so far from the standard V system---made later analysis internally highly consistent. But it is clear now that amateur visual observing of variables is gradually being supplanted by CCDs, as is evident from the progression of vsnet reports since the service began less than two years ago. Thus having well-established magnitudes (and colors) for comparison stars on the standard system is becoming imperative. And, of course, once a sequence is properly observed, it won't ever have to be done again!
As a final note, the position for SS Cyg given below derives from the large number of observations (~100!) made by the Carlsberg Meridian Circle in the last decade. The position is for both equinox and epoch J2000, with the proper motion derived from the Carlsberg observations taken into account. The star also appears with concordant positions in the GSC (GSC 3196-0723) and in the USNO "UJ1.0" CD-ROM catalogue.
Will report back when I get some new results for these stars, which I will try to get to during the coming dark run (no promises!).
\Brian Skiff (bas@lowell.edu)
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Summary of published photometry in the field of SS Cygni
Name RA (2000) Dec V B-V s sp remarks
SS Cygni 21 42 42.8 +43 35 10
BD+42 4190 21 43 08.6 +43 35 01 8.37 1.33 1 K2 = c
8.51 1.33 2
8.47 1.20 3
HD 207020 21 44 44.5 +43 45 47 9.10 0.40 5 B8 = e
BD+43 4012 21 41 44.1 +43 59 14 9.27 0.14 7 A2
BD+43 4020 21 42 41.3 +43 43 06 9.47 1.01 1 = f
9.62 1.00 2
9.57 0.95 3
BD+42 4186 21 42 20.3 +43 34 03 9.70 0.40 1 F0 = g
9.86 0.36 2
9.72 0.54 3
9.80 0.38 4
GSC 3196-1019 21 42 13.8 +43 47 45 10.29 0.41 1 = h
GSC 3196-1824 21 42 29.7 +43 54 09 10.73 0.23 1 = k
10.86 0.18 2
GSC 3196-0857 21 42 27.1 +43 33 45 10.80 0.55 1 = l
10.92 0.51 2
10.73 0.47 3
10.88 0.52 4
GSC 3196-0815 21 43 00.3 +43 41 23 11.01 1.15 2 = m
GSC 3196-1015 21 43 34.5 +43 36 25 11.32 1.10 5 = H&B 49
GSC 3196-0763 21 43 20.1 +43 36 38 11.44 0.60 2 = n
11.44 0.67 5
GSC 3196-0829 21 42 35.0 +43 39 17 11.91 0.59 2 = o
GSC 3196-1143 21 42 33.2 +43 34 03 12.05 1.86 6 = Misselt 1
V-R=1.06
GSC 3196-0831 21 42 45.0 +43 40 28 12.31 0.62 2 = p
GSC 3196-0941 21 42 43.4 +43 34 24 13.36 0.46 6 = Misselt 6
V-R=0.27
1 = Zuckermann
2 = Lenouvel & Daguillon
3 = Hopp
4 = Grant & Abt
5 = Howarth & Bailey
6 = Misselt
7 = Mallama & Skillman
Grant, G., and Abt, H. 1959, ApJ 129, 323.
Hopp, U. 1979, IBVS 1633.
Howarth, I., and Bailey, J. 1980, JBAA 90, 265.
Lasker, B. et al. 1988, ApJ Suppl. 68, 1.
Lenouvel, F., and Daguillon, J. 1956, Journ. Obs. 39, 1.
Mallama, A., and Skillman, D. 1979, PASP 91, 99.
Misselt, K. 1996, PASP 108, 146.
Parkhurst, J. A., and Daniel, Z. 1900, ApJ 12, 259.
Zuckermann, M.-C. 1961, Ann. Ap. 24, 431.