Table 4
Spectral type indices selected.
| Index name | Index reference | Sensitivity range | Coefficients(a) | Type of fit | rmse (SpT) | |||
|---|---|---|---|---|---|---|---|---|
| c0 | c1 | c2 | c3 | |||||
| H2OB | (1) | M5-L4 | 5.808 | 64.156 | −108.775 | 44.736 | 3rd degree | 0.99 |
| L9-T8 | 29.479 | -20.416 | – | – | linear | 1.00 | ||
| K1 | (2) | M5-L3 | 6.707 | 23.623 | – | – | linear | 0.77 |
| H2O-C | (3) | M4-L2 | 37.130 | -30.240 | – | – | linear | 0.79 |
| H2O-H | (4) | M5-L3 | 38.348 | -33.438 | – | – | linear | 0.98 |
| T0-T8 | 31.521 | -19.174 | – | – | linear | 0.68 | ||
| H2O (1.5 μm) | (5) | M5-L4 | -32.696 | 51.745 | −14.011 | – | 2nd degree | 0.97 |
| ωD | (6) | M4-L6 | 4.547 | 29.312 | −11.631 | – | 2nd degree | 0.94 |
| T1-T8 | 15.649 | 6.877 | −0.837 | – | 2nd degree | 0.66 | ||
| sHJ | (7) | T2.5-T8 | 18.803 | 10.554 | −8.353 | 2.982 | 3rd degree | 0.25 |
| CH4A | (1) | T0-T8 | 28.918 | -11.717 | 0.220 | – | 2nd degree | 0.37 |
| CH4B | (1) | T0-T5 | 27.750 | -8.969 | – | – | linear | 0.56 |
| ω0 | (6) | T1-T8 | 14.897 | 7.534 | −1.007 | – | 2nd degree | 0.60 |
| K2 | (2) | T0-T8 | 18.861 | -5.297 | −0.108 | – | 2nd degree | 0.56 |
| CH4-B | (3) | T3-T8 | 28.871 | -11.882 | 4.908 | – | 2nd degree | 0.23 |
| CH4-C | (3) | T0-T5 | 26.477 | -8.668 | – | – | linear | 0.52 |
| CH4-H | (4) | T3-T8 | 28.985 | -11.037 | 3.981 | – | 2nd degree | 0.27 |
| CH4-K | (4) | T0-T6 | 27.176 | -9.550 | 0.473 | – | 2nd degree | 0.52 |
Notes. (a)The spectral type is calculated from the polynomial fits as: 
(with n = 1 for linear, n = 2 for the second-degree and n = 3 for the third-degree polynomial fit).
References. (1) McLean et al. (2003), (2) Tokunaga & Kobayashi (1999), (3) Burgasser et al. (2002), (4) Burgasser et al. (2006a), (5) Geballe et al. (2002), (6) Zhang et al. (2018), (7) Testi et al. (2001).
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