## TYCHO2.tycho2

#### Description

The Tycho-2 Catalogue is an astrometric reference catalog containing positions and proper motions as well as two-colour photometric data for the 2.5 million brightest stars in the sky. The Tycho-2 positions and magnitudes are based on precisely the same observations as the original Tycho Catalogue (hereafter Tycho-1; see CDS Cat. I/239) collected by the star mapper of the ESA Hipparcos satellite, but Tycho-2 is much bigger and slightly more precise, owing to a more advanced reduction technique. Components of double stars with separations down to 0.8 arcsec are included. Proper motions precise to about 2.5 mas/yr are given as derived from a comparison with the Astrographic Catalogue and 143 other ground-based astrometric catalogs, all reduced to the Hipparcos celestial coordinate system. Tycho-2 supersedes in most applications Tycho-1, as well as the ACT (CDS Cat. I/246) and the TRC (CDS Cat. I/250) catalogs based on Tycho-1.

#### Columns

Column | Type | UCD | Unit | Description |
---|---|---|---|---|

TYC | varchar | meta.main, meta.id | The recommended designation for the star derived from appending the TYC1, TYC2, and TYC3 values for the star | |

POSFLAG | char | A flag describing how the mean position was constructed: ' ' means the mean position and proper motion were constructed in the normal way, 'P' that the mean position, proper motion, etc., refer to the photocentre of two Tycho-2 entries, where the BT magnitudes were used in weighting the positions, and 'X' that there is no mean position or proper motion available. | ||

RA | double | pos.eq.ra meta.main | deg | Observed Tycho-2 Right Ascension, ICRS |

DE | double | pos.eq.dec meta.main | deg | Observed Tycho-2 Declination, ICRS |

pmRA | float | pos.pm pos.eq.ra | mas/yr | The proper motion in the RA direction, in milliarcseconds per year, i.e., RA*cos(Dec) |

pmDE | float | pos.pm pos.eq.dec | mas/yr | The proper motion in the declination direction, in milliarcseconds per year. |

eRA | smallint | stat.error | mas | Statistical error of Ra |

eDE | smallint | stat.error | mas | The standard error in the mean declination, in milliarcseconds. This error is based on an error model. |

epmRA | float | stat.error | mas | Statistical error of RA*cos(dec),at mean epoch |

epmDE | float | stat.error | mas | Statistical error of Dec at mean epoch |

EpRAm | float | Julian years | The epoch of the mean right ascension in Julian years. | |

EpDEm | float | Julian years | The epoch of the mean declination in Julian years. | |

Num | smallint | The number of positions used in order to compute the mean position and proper motion. | ||

q_RAmdeg | float | The goodness-of-fit parameter for the mean right ascension. This goodness of fit is the ratio of the scatter-based error to the model-based error. It is only defined when the number of positions used (num_positions) is greater than 2. Values exceeding 9.9 for the goodness of fit have been truncated to 9.9. | ||

q_DEmdeg | float | The goodness-of-fit parameter for the mean declination. This goodness of fit is the ratio of the scatter-based error to the model-based error. It is only defined when the number of positions used (num_positions) is greater than 2. Values exceeding 9.9 for the goodness of fit have been truncated to 9.9. | ||

q_pmRA | float | The goodness-of-fit parameter for the proper motion in right ascension. This goodness of fit is the ratio of the scatter-based error to the model-based error. It is only defined when the number of positions used (num_positions) is greater than 2. Values exceeding 9.9 for the goodness of fit have been truncated to 9.9. | ||

q_pmDE | float | The goodness-of-fit parameter for the proper motion in declination. This goodness of fit is the ratio of the scatter-based error to the model-based error. It is only defined when the number of positions used (num_positions) is greater than 2. Values exceeding 9.9 for the goodness of fit have been truncated to 9.9. | ||

BT | float | phot.mag em.opt.B | mag | Tycho-2 BT magnitude |

eBT | float | stat.error | mag | Statitstical error of BT |

VT | float | phot.mag em.opt.V | mag | The Tycho-2 VT magnitude, blank when no magnitude is available. Either BT (bt_mag) or VT (vt_mag) is always given. |

eVT | float | stat.error | The standard error in the VT magnitude. | |

prox | smallint | A proximity indicator, being the angular distance in units of 100 mas to the nearest entry in the Tycho-2 main catalog or first supplement. | ||

TYC1 | char | meta.id.part meta.main | TYC1 from TYC or GSC | |

HIPCCDM | varchar | meta.id | Hipparcos number | |

RAd | double | pos.eq.ra meta.main | deg | The Right Ascension of the star, in the ICRS system (J2000 equinox), at the epoch specified by the parameter ra_epoch, and given in decimal degrees. This parameter was given in the original catalog to a precision of 10^-8 degrees. |

DEd | double | pos.eq.dec meta.main | deg | The declination of the star, in the ICRS system (J2000 equinox), at the epoch specified by the parameter dec_epoch, and given in decimal degrees. This parameter was given in the original catalog to a precision of 10^-8 degrees. |

EpRA2 | float | years | The epoch of the observed right ascension (observed_ra), in years. | |

EpDE2 | float | years | The epoch of the observed declination (observed_dec), in years. | |

eRAd | float | stat.error | The standard error in the observed right ascension, in milliarcseconds, i.e., RA*cos(Dec). This error is based on an error model. | |

eDEd | float | stat.error | The standard error in the observed declination, in milliarcseconds. This error is based on an error model. | |

solflg | char | A flag describing the type of Tycho-2 solution, where:' ' = normal treatment, close stars were subtracted when possible. 'D' = double star treatment. Two stars were found. The companion is normally included as a separate Tycho-2 entry, but may have been rejected. 'P' = photocentre treatment, close stars were not subtracted. This special treatment was applied to known or suspected doubles which were not successfully (or reliably) resolved in the Tycho-2 double star processing. | ||

corr | float | The correlation coefficient of the observed right ascension and declination. The observed Tycho-2 position is derived from a series of one-dimensional observations, taken in a number of different orientations. In general, therefore, the observed right ascension and declination are correlated quantities. |