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

ColumnTypeUCDUnitDescription
TYCvarcharmeta.main, meta.idThe recommended designation for the star derived from appending the TYC1, TYC2, and TYC3 values for the star
POSFLAGcharA 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.
RAdoublepos.eq.ra
meta.main
degObserved Tycho-2 Right Ascension, ICRS
DEdoublepos.eq.dec
meta.main
degObserved Tycho-2 Declination, ICRS
pmRAfloatpos.pm
pos.eq.ra
mas/yrThe proper motion in the RA direction, in milliarcseconds per year, i.e., RA*cos(Dec)
pmDEfloatpos.pm
pos.eq.dec
mas/yrThe proper motion in the declination direction, in milliarcseconds per year.
eRAsmallintstat.errormasStatistical error of Ra
eDEsmallintstat.errormasThe standard error in the mean declination, in milliarcseconds. This error is based on an error model.
epmRAfloatstat.errormasStatistical error of RA*cos(dec),at mean epoch
epmDEfloatstat.errormasStatistical error of Dec at mean epoch
EpRAmfloatJulian yearsThe epoch of the mean right ascension in Julian years.
EpDEmfloatJulian yearsThe epoch of the mean declination in Julian years.
NumsmallintThe number of positions used in order to compute the mean position and proper motion.
q_RAmdegfloatThe 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_DEmdegfloatThe 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_pmRAfloatThe 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_pmDEfloatThe 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.
BTfloatphot.mag
em.opt.B
magTycho-2 BT magnitude
eBTfloatstat.errormagStatitstical error of BT
VTfloatphot.mag
em.opt.V
magThe Tycho-2 VT magnitude, blank when no magnitude is available. Either BT (bt_mag) or VT (vt_mag) is always given.
eVTfloatstat.errorThe standard error in the VT magnitude.
proxsmallintA proximity indicator, being the angular distance in units of 100 mas to the nearest entry in the Tycho-2 main catalog or first supplement.
TYC1charmeta.id.part
meta.main
TYC1 from TYC or GSC
HIPCCDMvarcharmeta.idHipparcos number
RAddoublepos.eq.ra
meta.main
degThe 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.
DEddoublepos.eq.dec
meta.main
degThe 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.
EpRA2floatyearsThe epoch of the observed right ascension (observed_ra), in years.
EpDE2floatyearsThe epoch of the observed declination (observed_dec), in years.
eRAdfloatstat.errorThe standard error in the observed right ascension, in milliarcseconds, i.e., RA*cos(Dec). This error is based on an error model.
eDEdfloatstat.errorThe standard error in the observed declination, in milliarcseconds. This error is based on an error model.
solflgcharA 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.
corrfloatThe 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.

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