version: 2019 Jun 11: fainter northern/southern Landolt stars added 30 Jul 1998, see Fig. 1 of Florczak et al. 1998 Icarus 133, 233 for ratioed spectra of HD 1835, HD 28099, and HD 44594. 21 Dec 1994, the tables are now about 105 columns wide. ****************************************************************************** Near-Solar MK Standards and Photometric Standards of Similar Color Brian A. Skiff Lowell Observatory 1400 West Mars Hill Road Flagstaff AZ 86001-4499 bas@lowell.edu Introduction From the perspective of a stellar astronomer, the use of solar analogs in solar-system astronomy has been lacking in both rigor and consistency. Comparison stars of solar color have often been chosen randomly and without due consideration as to color or spectral output. The confrontation between spectral type (usually in the MK system) and photometric color is usually not considered at all. The problem (from the stellar perspective) arises because the MK system describes the appearance of spectra as recorded on blue-sensitive plates from around 3600A to 5000A, based on an assortment of temperature-, gravity-, and abundance-sensitive line ratios (not line depths nor colors). Outside that wavelength region all bets are off, although the "MK Process" can be applied with new classification criteria. The MK system is defined by line ratios not photometric colors; determining MK types from photometric colors does not involve the MK Process, and hence is not legitimate in the philosophical sense. From the astrophysical viewpoint, it is desirous to know the color(s) of the Sun for a number of reasons. But since this has yet to be determined with any certainty by direct observation, the only practical recourse is to stars of identical spectral type. There is a rich and contentious literature about the color of the Sun and how close the "solar twins" and "solar analogs" really are. The only substantial differences in the visible region are in the near-UV, where slight metal-abundance differences affect blanketing (and hence colors) in a noticeable way. From 4000A longward there don't seem to be great differences among favorite analogs like van Bueren 64 in the Hyades and 16 Cygni B, although the former is perhaps a fifth the age of the Sun and has triple its chromospheric activity---and is a photometric variable, V911 Tauri. Finally, since at present absolute flux calibrations are no better than 1-2%, there does seem to be an irreducible limit to the accuracy to which you can match stellar output on an absolute scale. In hopes of being able to provide observers working at large telescopes with a set of stars similar in color to the Sun, I have taken the somewhat naive and brute-force approach of making a list of all the near-solar MK standards and assessing what their photometric colors are, assuming a priori that these stars must have properties close to those of the Sun itself, at least in the blue and yellow parts of the spectrum. My intention is not to contribute to the discussion about the solar colors, but to ameliorate the practical problem of finding close solar analogs for use at the telescope in the study of solar-system objects (e.g., subtracting out a solar spectrum or solar colors from asteroid and comet data). Stars with MK Types Close to the Sun Table 1 is the latest version of the list of standards of MK spectral type between G1V and G3V inclusive. The types are extracted from Keenan and Yorka's (1988) list of standards. The standards list is also published together with additional classifications in Keenan & McNeil (1989). The list of MK standards is augmented with J2000 positions, BVRI magnitudes and colors, plus Stromgren b-y and v-b. Several people to whom this list was originally sent complained that nobody can actually observe these stars simply because they're too bright. I then went through Keenan's latest and possibly final list (Keenan & McNeil 1989) to find other stars he's classified but that did not qualify as standards (Table 2). All these stars are also bright, but add weight to the mean photometric colors for stars similar in spectral type to the Sun. Photometric Solar Analogs I next scanned Landolt's lists of equatorial standards to find stars that nearly match in all four BVRI colors the means for the bright MK standards. For only two of these rather fainter stars (see Table 5) are Stromgren indices available. Note that the mean colors for these stars (Table 8) are very close to those for the MK standards. The standard deviations of the means for the MK, Landolt, and Taylor samples are all about +/- 0.02 for B-V and b-y, +/-0.03 for v-b, and +/- 0.01 for the redder colors. Both the named variables (HD 1835 and HD 28099 = van Bueren 64) have full amplitudes in V of 3-5%, but their color variations do not exceed 0.5%, so should be no less usable as solar analogs than the other stars. The variations are due to rotation of spots, and both stars have periods of about five days. U-B is neglected partly because of uncertainties inherent in the U-B color system and because U-B observations nominally tied to UBV standards are often fraught with systematic errors. In addition, small differences in metal abundance affect U-B more strongly than B-V. In lieu of U-B, I have included (for the MK standards at least) the ersatz Stromgren color v-b, derived from the usual indices: (b-y) + m1. This provides a color slope between 4100A (v) and 4700A (b), i.e. intermediate between U-B and B-V. Metal-poor stars of solar temperature show here as well by having v-b less than about 0.60. Although these stars are essentially unreddened, the color match DOES NOT guarantee anything about the spectral types of the stars! The separate lists (Tables 3 - 5) are included below in order of decreasing reliability, although for most purposes they're all similarly good. I also went through Taylor's (1986) list of stars for which there was extensive, multi-source photometry that is not strictly on the Johnson-Cousins system. He determined transformations for these and published the revised numbers as a supplement to the Landolt stars (Table 6). The stars from all four lists provide adequate coverage in R.A. and Dec., and lie mostly in the range 9 < V < 12. Positions are from astrometric catalogues or the GSC; spectral types are from variously unreliable sources extracted from SIMBAD, and should be assumed to have large uncertainties compared to the MK standards. In other words, it doesn't mean much if the types are or are not G2V. In the Landolt lists, the column headed "m/n" indicates the number of measures (m) on (n) nights (this is the reverse of his scheme). Discussion & Desiderata In the process of putting this file together I reviewed the literature on the solar color problem. At a resolution of ~1A, there's no such thing as a perfect "solar twin" that anyone's found. HD 44594 (which appears in Table 2) and 16 Cygni A & B (both 1988 MK standards in Table 1) come remarkably close, however, and are the closest true solar twins analyzed in detail so far. See Cayrel de Strobel & Bentolila (1989) and Friel et al. (1993) for discussions of these three stars. Friel et al. predict the solar B-V = 0.651 +/- 0.008 based on a comparison of temperature-sensitive line ratios between the Sun and 16 Cyg A & B. From a similar analysis of the Sun itself, Gray (1992) predicts solar B-V = 0.656 +/-0.005 and b-y = 0.414 +/- 0.003. Since these stars have temperatures within a few tens of kelvins of the Sun, if you can observe 6th-magnitude stars, then measuring whichever of these three is available is the obvious prescription. From analysis of both published photometry and spectrophotometry for Hyades and field dwarfs, Taylor (1992) derives solar R-I = 0.337 +/- 0.002, close to the mean value for the MK standards. At ~10A resolution, there are many stars that come pretty close, essentially the MK standards in Table 1 below. Obviously, a simple way to check on the sensitivity of any particular instrumental set-up to slight differences among the solar analogs is to ratio a conventional pair of them to see if the differences are even observable. The best example is again the pair 16 Cygni A & B, which differ by 1.5 subclasses (a very small increment in this region of MK space!), and appear to bracket the Sun in temperature according to high-dispersion analysis. The best discussion of near neighbors to the Sun in UBV space is given by Neckel (1986). His sample includes stars within 0.02 of his assumed colors of the Sun (U-B = 0.195, B-V = 0.650), which can't be far off from the truth. Since he took data from the fourth Yale Bright Star Catalogue at face value, it would be good to do this again with higher-quality data. I plan to obtain Stromgren b-y for all the northern stars observable with the Lowell 53-cm photometric telescope not already observed by Olsen (1983,1993). For what it's worth, the spectral type of the Sun (G2V) is not in doubt, since it has been essentially defined to have that type. (It is also a 1988 MK standard!) See Keenan (1991) for a discussion, including a direct re-determination of the Sun's type at classification dispersion. The references there enable that controversy to be traced back into the literature. The list of possible solar analogs could be expanded by including, for instance some ECAS standards and stars in the various IHW lists. But none of those stars not already listed below are well-observed for MK type or photometric color; those given below are probably sufficient for now. Having found a number of suitably faint (and since they're well-observed standards, presumably constant) stars closely matching the solar analogs in several colors, it would now be useful to make spectrophotometric observations of these stars directly against the canonical solar analogs often used by planetary scientists, viz. 16 Cygni A & B and van Bueren 64 in the Hyades. At the spectral resolution required for solar-system observations, do these fainter stars have negligible differences in output compared to the MK standards? **** All VRI Colors are on the Cousins System **** Table 1. Near-Solar 1988 MK Standards (* on b-y color indicates uvby standard) Name HD MK RA (2000) Dec V B-V b-y U-B V-R R-I V-I zeta-2 Ret 20807 G1V 3 18 12.8 -62 30 23 5.232 0.60 0.383* 0.01 0.344 0.327 0.671 39 Ser 142267 G1V Fe-1 15 53 12.1 +13 11 48 6.084 0.60 0.392 0.00 0.342 -- 10307 G1.5V 1 41 47.2 +42 36 48 4.961 0.62 0.390 0.12 0.343 0.325 0.668 -- 20619 G1.5V 3 19 01.9 -02 50 36 7.052 0.645 0.406 0.11 0.365 0.345 0.710 -- 53705 G1.5V 7 03 57.3 -43 36 29 5.561 0.65 0.395 0.05 0.379 0.351 0.730 16 Cyg A 186408 G1.5Vb 19 41 49.0 +50 31 30 5.976 0.648 0.410* 0.20 0.357 0.341 0.698 Sun -- G2V -- 76151 G2V 8 54 17.9 -05 26 04 6.008 0.667 0.418* 0.191 0.363 0.333 0.696 xi UMa B 98230B G2V 11 18 10.9 +31 31 45 4.85: 0.62: 0.40 -- 45184 G2Va 6 24 43.9 -28 46 48 6.374 0.62 0.394 18 Sco 146233 G2Va 16 15 37.3 -08 22 10 5.494 0.65 0.404 0.17 0.353 0.335 0.688 vB 64 (Hyad) 28099 G2+V 4 26 40.1 +16 44 49 8.10v 0.657 0.413 0.20 0.376 0.341 0.717 9 Cet 1835 G2.5V 0 22 51.8 -12 12 34 6.39v 0.66 0.416 0.23 0.365 0.335 0.700 psi Ser 140538 G2.5V 15 44 01.8 +02 30 55 5.868 0.68 0.424 0.24 0.38 0.36 0.74 zeta-1 Ret 20766 G2.5V Hdelta1 3 17 46.2 -62 34 31 5.513 0.64 0.404* 0.08 0.369 0.351 0.720 16 Cyg B 186427 G3V 19 41 52.0 +50 31 03 6.237 0.660 0.416* 0.21 0.363 0.343 0.706 20 LMi 86728 G3Va Hdelta1 10 01 00.7 +31 55 25 5.374 0.66 0.416* 0.28 0.332 Table 2. Stars with Near-Solar MK types (not standards) from Keenan & McNeil 1989 Name HD MK RA (2000) Dec V B-V b-y U-B V-R R-I V-I -- 96700 G1V Fe-0.5 11 07 54.4 -30 10 28 6.517 0.60 0.395 0.07 0.34 0.33 0.67 -- 144585 G2V 16 07 03.4 -14 04 17 6.314 0.66 0.410 0.245 -- 44594 G2Va 6 20 06.1 -48 44 28 6.611 0.66 0.408 0.19 -- 144873 G2.5Vb 16 06 40.0 +34 06 11 8.558 (0.69) 0.425 -- 102365 G3V 11 46 31.1 -40 30 01 4.894 0.66 0.412 0.10 0.37 0.36 0.73 -- 1461 G3V Fe-0.5 0 18 41.9 -08 03 11 6.467 0.68 0.423 0.29 0.37 0.34 0.71 Note: HD 144873 Tycho-2 B-V shown; colors suggest MK type is slightly too early. Mean colors for MK and photometric standards: B-V b-y U-B V-R R-I V-I MK stds 0.647 0.407 0.16 0.362 0.341 0.704 Landolt 0.636 - 0.364 0.347 Taylor 0.647 - 0.13 0.357 0.347 0.704 ------------------------------------------------------------------------------------------- Landolt 2013 AJ 146, 131: fainter northern standards with solar color (candidates) Name RA (2000) Dec V B-V U-B V-R R-I V-I m/n GD 2C 0 07 32.4 +33 20 15 13.314 0.619 0.081 0.360 0.357 0.718 17/ 8 GD 8C 0 39 37.1 +31 37 03 13.299 0.636 0.140 0.371 0.358 0.724 14/ 7 SA 26-264 6 44 14.1 +44 46 42 15.092 0.631 0.097 0.376 0.347 0.715 12/ 6 GD 98B 8 57 09.8 +40 18 35 14.001 0.630 0.115 0.359 0.346 0.708 16/ 8 KUV 348-14C 10 05 18.5 +38 48 11 15.141 0.658 0.118 0.346 0.367 0.712 2/ 1 SA 32-105 12 56 29.2 +43 54 07 13.372 0.632 0.138 0.364 0.341 0.706 13/ 6 GD 336B 14 31 46.3 +37 07 47 12.985 0.646 0.155 0.368 0.348 0.716 15/ 8 PG 1648+536E 16 49 48.2 +53 31 30 15.152 0.668 0.172 0.407 0.326 0.736 2/ 1 SA 38-297 18 50 27.0 +45 07 56 14.395 0.610 0.137 0.346 0.350 0.696 2/ 1 ------------------------------------------------------------------------------------------- Landolt 2009 AJ 137, 4186: fainter equatorial standards with solar color (candidates) Name RA (2000) Dec V B-V U-B V-R R-I V-I m/n Feige 24C 2 35 18.3 +03 42 40 13.546 0.668 0.188 0.382 0.367 0.749 19/10 Rubin 152F 7 29 53.8 -02 04 52 14.564 0.635 0.069 0.382 0.315 0.689 15/ 8 SA 104-430 12 41 50.3 -00 25 52 13.858 0.652 0.131 0.364 0.363 0.727 15/ 8 SA 104-350 12 43 14.2 -00 33 21 13.634 0.673 0.165 0.383 0.353 0.736 14/ 7 Mark A2 20 43 55.0 -10 45 31 14.540 0.666 0.096 0.379 0.371 0.751 21/10 SA 113-163 21 42 35.4 +00 16 46 14.540 0.658 0.106 0.380 0.355 0.735 15/ 7 G 93-48D 21 52 10.2 +02 21 26 13.664 0.636 0.120 0.368 0.362 0.724 21/ 9 ------------------------------------------------------------------------------------------- Landolt 2007 AJ 133, 2502: fainter southern standards with solar color (candidates) LB 1735B 4 31 22.8 -53 36 37 14.542 0.675 0.126 0.379 0.361 0.741 17/ 9 MCT 0550-4911A 5 52 04.1 -49 10 49 14.255 0.606 0.020 0.351 0.340 0.690 14/ 9 WD 1056-384C 10 58 11.1 -38 41 51 13.326 0.649 0.164 0.367 0.357 0.724 13/ 8 LSE 259F 16 53 55.5 -55 59 26 13.580 0.615 0.052 0.374 0.375 0.749 17/ 9 MCT 2019-4339B 20 22 46.7 -43 28 11 13.923 0.671 0.208 0.369 0.347 0.716 19/11 MCT 2019-4339F 20 23 04.0 -43 31 22 13.936 0.647 0.097 0.369 0.360 0.729 20/11 JL 82C 21 35 45.1 -72 50 13 13.440 0.612 0.041 0.357 0.358 0.715 19/10 JL 117A 22 54 13.3 -72 23 04 14.061 0.629 0.072 0.362 0.354 0.717 6/ 3 ------------------------------------------------------------------------------------------- Table 3. Landolt 1992 equatorial standards with near-solar colors Name RA (2000) Dec V B-V U-B V-R R-I V-I m/n MK SA 92-276 0 56 26.6 +00 41 51 12.036 0.629 0.067 0.368 0.357 0.726 21/11 G5V SA 97-249 5 57 07.6 +00 01 12 11.733 0.648 0.100 0.369 0.353 0.723 69/62 G5V SA 98-682 6 52 16.5 -00 19 41 13.749 0.632 0.098 0.366 0.352 0.717 13/ 7 Rubin 149B 7 24 17.6 -00 33 06 12.642 0.662 0.151 0.374 0.354 0.728 15/10 PG 0918+029C 9 21 42.3 +02 46 36 13.537 0.631 0.087 0.367 0.357 0.722 21/10 SA 102-1081* 10 57 04.0 -00 13 13 9.903 0.664 0.255 0.366 0.333 0.698 58/50 G5IV SA 110-361 18 42 45.0 +00 08 05 12.425 0.632 0.035 0.361 0.348 0.709 30/17 SA 115-271* 23 42 41.8 +00 45 13 9.695 0.615 0.101 0.353 0.349 0.701 77/55 G2V Notes: SA 102-1081 BD+00 2717. SA 115-271 BD-00 4557. ------------------------------------------------------------------------------------------- Table 4. Landolt 1983a equatorial standards with near-solar colors Name RA (2000) Dec V B-V U-B V-R R-I V-I m/n MK SA 107-684* 15 37 18.1 -00 09 50 8.433 0.619 0.072 0.356 0.353 0.708 14/ 7 G3V SA 113-276* 21 42 27.4 +00 26 20 9.074 0.647 0.181 0.357 0.336 0.692 12/ 6 G5V Notes: SA 107-684 HD 139287. SA 113-276 fainter of 26" pair. ------------------------------------------------------------------------------------------- Table 5. Landolt 1983b "Equipment Stability" standards with near-solar colors Name RA (2000) Dec V B-V U-B V-R R-I V-I m/n MK SA 140-84 0 03 38.1 -28 41 47 11.961 0.668 0.136 0.379 0.363 0.741 9/ 6 SA 196-1801 17 11 08.2 -60 06 29 12.755 0.649 0.105 0.399 0.368 0.767 10/ 7 HD 159656* 17 37 53.9 -42 34 03 7.168 0.641 0.130 0.357 0.332 0.685 20/12 G2/3V HD 219018* 23 12 38.6 +02 41 10 7.708 0.620 0.116 0.355 0.339 0.694 10/ 6 G1V SA 115-268* 23 42 30.7 +00 52 11 12.487 0.637 0.076 0.361 0.353 0.714 9/ 6 G5 Notes: HD 159656 Olsen 1993: V=7.148, b-y=0.412. HD 219018 Olsen 1993: V=7.712, b-y=0.397. SA 115-268 NSV 14676. ------------------------------------------------------------------------------------------- Table 6. Taylor 1986 "supplemental" standards with near-solar colors Name RA (2000) Dec V B-V U-B V-R R-I V-I MK remarks SA 93-101 1 53 18.4 +00 22 23 9.734 0.653 0.14 0.354 0.337 0.691 G5V HD 11532 SA 102-370 10 56 33.8 -01 10 41 11.229 0.622 0.05 0.365 0.350 0.715 G2V SA 103-487 11 55 11.1 -00 23 38 11.874 0.663 0.19 0.367 0.353 0.720 G5V SA 103-204 11 57 27.0 -00 56 54 11.189 0.663 0.10 0.353 0.358 0.711 G7V BD-00 2514 SA 105- 56 13 38 41.9 -01 14 17 9.975 0.665 0.15 0.357 0.334 0.691 G5V BD-00 2719 SA 107-998 15 38 16.3 +00 15 22 10.436 0.636 0.16 0.358 0.352 0.710 G3IV BD+00 3383 SA 112-1333 20 43 12.0 +00 26 13 9.977 0.626 0.09 0.348 0.342 0.690 G2V BD-00 4074 ------------------------------------------------------------------------------------------- Table 7. Landolt 1973 standards with near-solar colors Name RA (2000) Dec V B-V U-B m/n MK SA 101-321 9 55 40.4 -00 18 51 12.85 0.65 0.14 10/ 6 G7: brighter of 30" pair SA 101-329 9 56 18.8 -00 26 27 11.99 0.64 0.13 12/ 7 G7V: SA 104-483 12 44 16.7 -00 27 33 12.08 0.67 0.18 14/ 5 G5V SA 114-654 22 41 26.1 +01 10 11 11.83 0.66 0.19 13/ 6 G0 Notes: HD 142267 = NSV 7313 HD 89230 = NSV 5165 HD 146233 = NSV 7577 HD 28099 = V911 Tau HD 1835 = BE Cet References: Cayrel de Strobel, G., and Bentolila, C. 1989, A&A 211, 324. Friel, E., Cayrel de Strobel, G., Chmielewski, Y., Spite, M., Lebre, A., and Bentolila, C. 1993, A&A 274, 825. Gray, D. F. 1992, PASP 104, 1035. Keenan, P. C., and McNeil, R. C. 1989, ApJ Suppl 71, 245. Keenan, P. C., and Yorka, S. B. 1988, Bull. Inf. Centre de Donnees astronomiques [nee Stellaire] de Strasbourg 35, 37. Landolt, A. U. 1983a, AJ 88,439. Landolt, A. U. 1983b, AJ 88, 853. Landolt, A. U. 1992, AJ 104, 340. Neckel, H. 1986, A&A 169, 194; minor erratum in A&A 176,372. Olsen, E. H. 1983, A&A Suppl 54, 55. Olsen, E. H. 1993, A&A Suppl 102, 89. Taylor, B. J. 1986, ApJ Suppl 60,577; Table 6. Taylor, B. J. 1992, PASP 104, 500. ========== Stromgren b,y photometry for some of the Landolt stars (BSkiff): Name b-y BD+00 2717 0.411, n=1 HD 139287 0.404, n=2 BD+00 3383 0.438, n=2 BD-00 4557 0.401, n=2 ************************************************************ We've looked at four Landolt stars from MDM thus far. They are: 98-978 (10.572/0.609/0.094/0.349/0.322/0.671/G3V, m=46,n=38 L92) 102-1081 107-684 107-998 -- Bobby ************************************************************ Figure 1 of Florczak et al. 1998 Icarus 133, 233, shows ratios of the stars HD 28099 (vB64), HD 1835, and HD 44594 at roughly 10A resolution. Longward of 5000A, they look pretty flat. ************************************************************