K0IV

MK43 standard:        del Eri (=HD 23249) - K0IV
     		      eta Cep (=HD 198149) - K0IV 
     		      46 LMi - K0III-IV
		      eta Ser - K0III-IV
JM53 standard:        del Eri (=HD 23249) - K0IV
                      eta Cep (=HD 198149) - K0IV
                      HD 89862 (HIP 50892) - non-standard K0IV
MK73 dagger standard: eta Ser (=HR 6869) - K0III-IV
Houk88 standard:      del Eri (=HR 1136 = HD 23249) - K0IV (from JM53)
Keenan89 standard:    eta Cep (=HD 198149) - K0IV 
                      HD 23249 - K0+IV
GrayNstars:           eta Cep (=HD 198149) - K0IV

eta Cep (K0IV), del Eri (K0+IV), omi Col are the brightest K0IVs in
the Bright Star Catalog 5th ed. (Hoffliet91)

From Hipparcos (1997) stars classified as "K0IV" within 75 pc:
<B-V>(K0IV) = 0.906 (+-0.017 sem, +-0.077 stdev; N=35)
<Mv>(K0IV)  = 3.417 (+-0.236 sem, +-1.512 stdev; N=35)
(obviously mix of Mv ~ 5-6 MS stars, but mostly Mv ~ 1.6-4 true subgiants)

Plotting the distributions of absolute magnitudes for K-type stars in
XHIP, the minimum in the absolute magnitudes separation K dwarfs from
subgiants appears near Mv ~ 4.2, so stars intrinsically fainter than
this should probably be clipped as misclassified dwarfs.

Only 27 of these are more than a 1 mag above the Wright04 main sequence.
Among these:
<B-V>(K0IV,delta(Mv)<-1) = 0.931 (+- 0.015 sem, +-0.075 stdev; N=27)
<Mv>(K0IV,delta(Mv)<-1)  = 2.998 (+- 0.173 sem, +-0.890 stdev; N=27)

Among Gray03 and Gray06 stars within 40 pc:
B-V(K0IV) = 0.919 (+- 0.032 sem, +- 0.055 stdev; N=5)
V-I(K0IV) = 0.938 (+- 0.036 sem, +- 0.065 stdev; N=5)

For Hipparcos stars within 75pc with parallax S/N > 8 (N=16):
<B-V>(K0IV)  = 0.870 (+-0.020 sem, +-0.054 stdev)
<V-I>(K0IV)  = 0.886 (+-0.013 sem, +-0.058 stdev)
<V-J>(K0IV)  = 1.486 (+-0.031 sem, +-0.105 stdev)
<V-H>(K0IV)  = 1.875 (+-0.051 sem, +-0.167 stdev)
<V-Ks>(K0IV)  = 2.005 (+-0.059 sem, +-0.172 stdev)
<J-H>(K0IV)  = 0.392 (+-0.011 sem, +-0.061 stdev)
<H-K>(K0IV)  = 0.125 (+-0.010 sem, +-0.034 stdev)
<Bt-Vt>(K0IV)= 1.019 (+-0.024 sem, +-0.065 stdev)
<V-Vt>(K0IV) =-0.092 (+-0.008 sem, +-0.026 stdev)

Teff(K0IV) =  4916 K ; var. stan. eta Ser (K0III-IV)
Teff(K0IV) = 4974 K ; pri. stan. eta Cep (see below)
Teff(K0IV) =  5051 K ; var. stan. HD 23249 (see below)
Teff(K0IV) = 5108 K ; exemplar HD 22713 = 21 Eri (see below)
=> adopt Teff(K0IV) = 4970 K [last updated 7/15/2017]

(B-V)(K0IV) = 0.898 ; median for 8 XHIP K0IV stars d<50pc (eta Cep & HIP 2422 define middle)
(B-V)(K0IV) =  0.901 ; exemplar HD 177389 = HIP 94150
(B-V)(K0IV) = 0.912 ; pri. stan. eta Cep = HD 198149
(B-V)(K0IV) = 0.915 ; K0+IV variant stan. del Eri = HD 23249 
(B-V)(K0IV) =  0.921 ; exemplar HD 22713 = 21 Eri
(B-V)(K0IV) = 0.924 ; poly fit to FGK subgiant standards & exemplars
(B-V)(K0IV)  = 0.9241 ; variant stan. K0III-IV HD 168723 = eta Ser
(B-V)(K0IV) = 0.946(+-0.021 sem,+-0.060 stdev) ; SIMBAD median K0IV d<75pc, V<6.5(N=10)
=> adopt (B-V)(K0IV) = 0.92 [updated 2/32/2022]

Mv(K0IV) =  3.74 ; K0+IV variant stan. del Eri = HD 23249 
Mv(K0IV) =  3.27 ; exemplar HD 22713 = 21 Eri
Mv(K0IV) = 3.2   ; GrayCorbally09 
Mv(K0IV) = 2.96  ; quadratic trend G0IV-K2IV standards+exemplars (for K0IV)
Mv(K0IV) =  2.64 ; pri. stan. eta Cep = HD 198149 
Mv(K0IV) =  2.47 ; exemplar HD 177389 = HIP 94150
Mv(K0IV) = 2.38(+-0.24 sem,+-0.62 stdev) ; SIMBAD median K0IV d<75pc, V<6.5(N=10)
Mv(K0IV) =   1.89 ; variant stan. K0III-IV HD 168723 = eta Ser 
=> adopt Mv(K0IV) = 2.96 [updated 1/28/2022]

Note: Sandage16 bounds the subgiants as 1.6 < Mv < 3.4 and types between G2 to K4,
based on Mt. Wilson work from Adams35 and Stromberg30


# Primary Standard

HD 198149 = eta Cep = HR 7957 = GJ 807 = HIP 102422 = LHS 3578 (J2045+6150)
  G7IV: Adams35
  G8IV: Lindblad22,Bidelman85(G8)
  G9.5IV-V: Morgan38
 *K0IV: Keenan41,MK43,JM53,Keenan53,Jaschek64(most),Cannon(K0),Keenan89,Gray03
  K0III-IV: Roman52
  
  Best standard, but far north. In the XHIP catalog, it is also the
  nearest K0IV star. PASTEL: <Teff> = 4988 K (N=24), <[Fe/H]> = -0.13
  (N=11). Most precise recent values: Teff=5003+-15K, logg=3.42+-0.06,
  [Fe/H]=-0.13+-0.02(Niedzielski16).  V=3.41(HIP),
  V=3.420+-0.013(Mermilliod91), B-V=0.912+-0.002(HIP),
  V-I=0.94+-0.04(HIP).  plx=70.10+-0.11mas(HIP2),
  plx=69.6638+-0.2998mas(GaiaDR2) => Mv=2.64(HIP2).

# Variant Standard

HD 23249 = del Eri = 23 Eri = HR 1136 = GJ 150 = Rana (J0343-0945, V=3.5)
  G9: Bidelman85
  K0IV: Adams35,MK43,Moore50
 *K0+IV: Keenan80,Keenan89
  K1IV: Keenan41,Moorer50,Gray03
  K1III-IV: Gray06
  K2IV: Ljunggren66
    
  Was mislabeled in SIMBAD for years as RS CVn-type, but this appears
  to be due to photometric campaign in 1980s that used eps Eri (itself
  an an active fast-rotating star) as photometric standard. So there
  is zero evidence to support RS CVn/chromosperically active binary
  status for del Eri. vsini<2 km/s apparently, from Glebocki+00
  catalog. B-V=0.915 (negligibly redder than primary standard eta
  Cep), revised HIP data => Mv=3.74, 2.50 mag above MS. PASTEL: <Teff>
  = 5051 K (N=22), <[Fe/H]> = 0.13.

HD 168723 = eta Ser = HR 6869 (V=3.26)
  G8: Bidleman85
  G8III/IV: Adams35
  G8IV: Roman52(lines weak)
  G9IV: Morgan38
 *K0III-IV: MK43,MK73(dagger),Keenan80,Keenan89,Gray03
  K0IV: Moore50,1966ArA.....3..501L
  K1IV: Keenan41

  V=3.23(HIP), B-V=0.941+-0.006(HIP),
  V-I=0.96+-0.02(HIP). plx=53.93+-0.18mas(HIP2) => Mv=1.89.  PASTEL:
  <Teff>=4930K(N=27), <[Fe/H]>=-0.19(N=23), <logg>=3.08(N=23).

HD 28 = 33 Psc 
  K0IV: Adams35
 *K0III-IV: Keenan89
  K0IIIb_CN-0.5: Keenan80
  K1III: Roman52

  Sandage16 lists as one of the original 90 Mt. Wilson subgiants.
  Mv=1.63.

# Standards

46 LMi
  K0III-IV: MK43


# Exemplar

HD 89862 (J1023+5631, V=8.2)
 *K0IV: JM53(non-stan.),Jaschek64(all)
  K0III-IV: Jaschek78(Yoss61)
  K0V: Popper43
  K0: Cannon
  
  Everyone seems to agree on "K0" subtype, but not considered a
  standard by anyone.  B-V=0.938, revised HIP data => Mv=3.05, 3.28
  mag above MS, at base of giant branch. No PASTEL data.

HD 177389 = HIP 94150
  G4: Moore32
  G8/K0III/IV: Houk75
 *K0IV: Gray06
  K0III: 1943HarCi.449....1H

  V=5.31(HIP), B-V=0.901+-0.001(HIP), V-I=0.90+-0.00(HIP).
  plx=27.0405+-0.0890mas(GaiaDR2) => Mv=2.47.  PASTEL: <Teff> =
  5061K(N=3), <logg>=3.49(N=3), <[Fe/H]>=-0.05(N=3).

HD 22713 = 21 Eri = HR 1111 = HIP 17027 
 *K0IV: Houk99
  K1V: Adams35
  K2III: Harlan81 (Skiff comment: "dwarf, not giant")

  V=5.97(HIP), B-V=0.921+-0.004(HIP), V-I=0.94+-0.04(HIP).
  plx=28.7688+-0.0506mas(GaiaDR2) => Mv=3.27.  PASTEL: <Teff> =
  5108K(N=2), <logg>=3.73(N=2), <[Fe/H]>=+0.055(N=2).

HD 171627 = HIP 91273
 *K0IV: 1943HarCi.449....1H,McCuskey64,Houk82
  K1V: Evans57

# Other Stars

HD 191026 = HIP 99031
  G3V: Adams35
  G5V: 1932MeUpp..55....1S,1952UppAn...3e....S
  G8V: Malmquist60
  G8IV: Annear53,Ljunggren61
 *G8.5IVa: Keenan89
  G9IV: 1972AbaOB..43..109W, Bidelman85(G9)
  K0IV: Roman55,Gray03
  K1IV: 1962JO.....45...57B

HD 191026 = 27 Cyg = HR 7689
  G8V: 1960UppAn...4h...1M
  G8IV: Adams35,1961UppAn...4j...1L
 *G8.5IVa: Keenan89(non-stan)
  G9: Bidelman85
  G9IV: 1972AbaOB..43..109W
  K0IV: Roman55,Gray03
  K1IV: 1962JO.....45...57B

  Not good agreement on types! 

# COMMENTS ON COLORS OF K SUBGIANTS VS. K DWARFS

Hipparcos catalog has 20 K subgiants with d<75pc and parallax S/N>8.
Slash and dashed spectral types were rejected. For this sample:

<(B-V)KIV> - <(B-V)KV> = 0.004+-0.007 (stdev=0.044; N=20)

For individual subtypes:
<(B-V)K0IV> - <(B-V)K0V> =  0.018+-0.021 (stdev=0.053; N=7)
<(B-V)K1IV> - <(B-V)K1V> =  0.004+-0.016 (stdev=0.037; N=7)
<(B-V)K2IV> - <(B-V)K2V> =  0.006+-0.002 (stdev=0.002; N=2)
<(B-V)K3IV> - <(B-V)K3V> = -0.013+-0.026 (stdev=0.045; N=3)

Conclusion: Mean B-V color differences between subgiants and dwarfs of
a given K class *subtype* are negligible. [later note: need to reevaluate as
it is clear that many spectral type IV objects in literature among early
Ks are misclassified dwarfs, which are much more numerous]

B-V   SpT (B-V)V (B-V)-(B-V)V
0.870  K0  0.816  0.054
0.900  K0  0.816  0.084
0.838  K0  0.816  0.022
0.833  K0  0.816  0.017
0.778  K0  0.816  -0.038
0.760  K0  0.816  -0.056
0.845  K0  0.816  0.029
0.787  K1  0.847  -0.06
0.840  K1  0.847  -0.007
0.871  K1  0.847  0.024
0.927  K1  0.847  0.08
0.860  K1  0.847  0.013
0.853  K1  0.847  0.006
0.820  K1  0.847  -0.027
0.900  K2  0.893  0.007
0.897  K2  0.893  0.004
0.925  K3  0.990  -0.065
1.000  K3  0.990  0.01
1.005  K3  0.990  0.015
1.080  K5  1.134  -0.054