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.

************************************************************
