Brian A. Skiff
Lowell Observatory
1400 West Mars Hill Road
Flagstaff AZ 86001-4499
USA
e-mail: bas@lowell.edu
AbstractPhotoelectric photometry of stars surrounding NGC 5005, NGC 5033, NGC 5055 (Messier 63), and NGC 5363/5364 is presented. The fields are contained in the Thompson & Bryan "Supernova Search Charts and Handbook". Most of the stars have not been measured previously, and serve to fill in charts lacking comparison stars. IntroductionThe Thompson & Bryan (1990) "Supernova Search Charts and Handbook" provide for many amateurs the only convenient method of making reliable searches for supernovae in bright galaxies. Although many of the 300 or so galaxies included on the charts have a sequence of magnitudes for field stars surrounding them, a surprising number have none or only one or a few bright stars measured in this way. This makes determining magnitudes for real or suspected supernovae difficult using visual techniques. In order to help ameliorate this problem, I have begun making observing stars on charts that are "blank" in the sense that they lack stars with measured magnitudes. ObservationsThe observations presented here were done on two telescopes with different photometric systems. The fields near NGC 5005, 5033, and 5363/64 were observed with the Lowell 53cm photometric telescope, located on Mars Hill, in Flagstaff, Arizona. The photometer is fitted with intermediate- bandwidth Strömgren filters. I observed the stars in the b and y filters of this four-color system. The y filter is centered at the same effective wavelength as the traditional Johnson V, but has a narrower passband (only ~200Å versus ~1000Å for Johnson V). The transformation from instrumental y to standard V magnitudes is in general no more difficult than when using an ordinary V filter. The b-y color serves as a temperature indicator for stars just as the more familiar B-V color does in the Johnson system. The stars were observed through a 29-arcsec aperture. Each star was measured on two consecutive photometric nights, 24 & 25 June 1993 UT. The transformations to the standard system were derived from linear fits in magnitude and color using extinction coefficients measured on each night. The results for the standard stars, mostly secondary standards, are given in Table 1. The two primary standards observed are indicated with an asterisk. The large number of standard stars involved resulted from full nights of observing on several projects using a single solution for the whole night. Column 'n' shows the number of observations for each star. The final columns show the differences for each observed mean value from the assumed values in the sense "observed minus standard". The means of these observations differ from the assumed values on average by +0.001 ± 0.006 mag in V and -0.000 ± 0.005 mag. in b-y, indicating a good match to the standard system across the full range of star colors. |
Star V (obs) b-y n delV/del(b-y) HD109995 7.602 0.042 1 +001/-006 HD122563* 6.213 0.641 2 +011/+008 HD122866 6.161 0.016 2 -001/-004 HD125489 6.197 0.114 2 +007/+005 HD129975 8.366 0.962 1 +003/-007 HD134064 6.041 0.044 2 -002/+012 HD136381 6.289 0.009 2 -002/+001 HD137006 6.114 0.155 1 +001/+000 HD139308 7.779 0.800 3 +000/-001 HD140850 8.814 1.099 1 -002/-003 HD143761* 5.405 0.391 4 +002/-003 HD149382 8.941 -0.146 3 -003/-004 HD160233 9.095 0.025 2 +000/+000 HD160471 6.179 1.164 1 -006/+002 HD161817 6.971 0.141 1 -011/+004 HD162596 6.342 0.711 1 +000/-006 HD172829 8.474 1.387 1 +014/-002 |
| The observations of stars near NGC 5055 (Messier 63) were made using the 1.1-m Hall telescope, located at Lowell's dark-sky Anderson Mesa site. (The freedom from anthropogenic sources of light hardly mattered, since there was substantial moonlight on both observing nights.) The telescope has an f/8 Ritchey-Chretien focus, and was used with a photometer having Johnson B and V filters. The stars were observed through a 27-arcsec aperture. The two nights on this telescope, 26 & 27 June 1993 UT, were not "beautimous" stable photometric nights, but the reductions of the standard stars indicates the results are sufficiently accurate for the purposes of visual observing. Some stars in other fields were observed on the same nights that had already been measured at the 53cm telescope, and the consistency of the results for those stars gives further confidence that the present results are at least satisfactory, if not utterly reliable. The transformations to the standard system were again made using linear equations, but with estimated extinction values. The results for the standard stars, drawn from the lists of Landolt (1983), are shown in Table 2 the same way as in Table 1. The means of the standard-star observations differ from the assumed values on average by -0.003 ± 0.022 mag. in V and -0.001 ± 0.007 mag. in B-V. This suggests that although the two nights were not stable---so the V magnitudes are somewhat uncertain---the transparency changes were slow enough that the separate V and B observations (about 1 minute apart) led to reliable B-V colors. |
Star V (obs) B-V n delV/del(B-V) HD109995 7.628 0.052 2 +029/+001 HD128866 9.092 0.714 2 +004/+013 BD+0 3222 9.807 1.369 1 +022/+007 HD129727 9.483 0.377 3 -001/-003 HD139308 7.788 1.276 1 +009/+001 HD140850 8.817 1.671 4 +011/+001 HD149382 8.940 -0.278 3 -004/+003 HD149646 8.202 0.554 2 -006/-005 HD160233 9.070 -0.059 1 -025/-005 BD-0 3353 9.296 1.455 1 -036/-007 HD161427 8.978 0.788 1 -039/-014 |
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The magnitudes and colors for the stars in the galaxy fields are given in Tables 3 and 4. Table 3 shows the Strömgren photometry for stars in three fields; NGC 5363 and NGC 5364 appear on the same Thompson & Bryan chart. Identifications and positions are given for all the stars. Next come the averaged V magnitude and b-y color. The second line of each entry shows the standard deviation of the pair of observations. For the brighter stars observed with the 53cm telescope, the results are of quite high precision, whereas fainter stars are more uncertain. This results directly from the low count rates encountered when measuring these relatively faint stars with a small telescope and filters passing only a narrow region of the spectrum. The large uncertainty for the star near NGC 5364 may have arisen from poor compensation for the background light of the galaxy; it is not likely that the star is variable. Spectral types are listed for the brighter stars as available from the literature. For the fainter stars, the spectral types are estimates (in lower-case letters) based on the b-y colors. For all three fields, it happens that the GSC magnitude scale is fairly close to standard V. This is not usually the case! The GSC magnitudes for stars near Messier 81, for example, are systematically about half a magnitude too bright as well as possessing a trend as a function of color. This led to problems during the outburst of supernova 1993J in that galaxy: some observers used the raw GSC magnitudes while others used magnitudes on the standard system from the literature. In a few cases, the table also shows V and b-y magnitudes available from other sources. |
Galaxy RA (2000) Dec V b-y n spec Remarks
Star name
NGC 5005 13 10 56 +37 03.6 GSC 2534-0098
HD 114428 13 10 02.9 +37 09 07 8.594 0.339 2 G5 see note
.001 .005
GSC 2534-0604 13 11 15.1 +37 09 24 12.200 0.402 2 g0v
.034 .025
GSC 2534-0815 13 10 51.9 +37 08 07 13.257 0.448 2 g5v
.008 .023
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HD 114428 Olsen (1993): V=8.613, b-y=0.334.
HD 121605 = ADS 9060: sep. 1".0, p.a. 113, 1990.4; Heintz 1992. Olsen
(1983): V=7.977, b-y=0.313.
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Table 4 shows the results for the NGC 5055 field. The format is the same as Table 3. In this field the Thompson & Bryan chart shows magnitudes from the AAVSO chart along with a single star measured photoelectrically. Many stars in the field have been measured by Deming et al. (1973); their results are listed in the remarks for the stars in common. I measured the bright star HD115270 on one night as a consistency check; Harold Corwin, who observed the star originally, supplied his observation (from a publication in preparation). Our results match within our mutual uncertainties (mostly mine!), but the V magnitudes differ significantly from that of Deming et al. They used a fairly large aperture (40 arcsec), and may not have been able to subtract out the galaxy background light sufficiently, and so the star came out too bright. Our B-V color (all three sources agree) shows that the HD spectral type for this star cannot be correct. A dwarf of type K0 has a B-V of about 0.8, while a more common K0 giant star should have a B-V near 1.0. Our B-V rules out both of these, and is indicative instead of a mid-F dwarf star. Corwin's B-V and U-B colors (0.01), indicate an F6 to F8 dwarf (cf. FitzGerald 1970). In fact, Deming et al. note that the spectral type is F6, although they do not say how this was determined---possibly it was "invented" from the B-V color alone. The new observations also match all the chart magnitudes closely except for the faintest star, which I make to be substantially brighter than indicated. (This was evident visually in the telescope as well.) The GSC magnitudes average a few tenths of a magnitude brighter than V in this field, which is fairly typical. The GSC magnitude for the faintest star also suggests an error on the Thompson & Bryan chart. |
Galaxy RA (2000) Dec V B-V n spec Remarks
Star name
NGC 5055 13 15 49 +42 01.8
HD 115270 13 15 30.4 +42 03 13 9.310 0.480 1 f5v see note
GSC 3024-1166 13 15 08.6 +42 02 13 10.836 0.776 2 k0v see note
.011 .009
GSC 3024-0126 13 16 36.1 +42 05 38 12.974 0.554 2 f8v see note
.003 .010
GSC 3024-1200 13 15 45.8 +41 54 28 13.983 0.731 2 g8v see note
.032 .071
GSC 3024-0557 13 15 15.2 +42 04 03 14.373 0.608 2 g0v
.004 .029
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HD 115270 HD spectral type in error. Corwin: V=9.30, B-V=0.47; Deming
et al.: V=9.24, B-V=0.48.
GSC 3024-1166 Deming et al.: V=10.85, B-V=0.77
GSC 3024-0126 Deming et al.: V=12.99, B-V=0.50
GSC 3024-1200 Deming et al.: V=13.95, B-V=0.84
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This work was facilitated by the use of the SIMBAD bibliographic database, maintained by the Centre de Données astronomiques at Observatoire de Strasbourg, France. |
Deming D., Rust, B. W., & Olson, E. C. 1973. The light curve of supernova
1971I, Publ. Astron. Soc. Pac., 85, 321.
FitzGerald, M. P. 1970. The intrinsic colors of stars and two-color
reddening lines, Astron. & Astrophys., 4, 234.
Landolt, A. U. 1983. UBVRI photometric standards around the celestial
equator, Astron. J., 88, 439.
Olsen, E. H. 1983. Four-color and H-beta photometry of A5 to G0 stars
brighter than 8m.3, Astron. & Astrophys. Suppl., 54, 55.
Olsen, E. H. 1993. Stromgren four-color uvby photometry of G5-type HD stars
brighter than mv=8.6, Astron. & Astrophys. Suppl., 102, 89.
Thompson, G. D., & Bryan, J. T. 1990. The Supernova Search Charts
and Handbook, Cambridge University Press.
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