The Tycho-2 Catalogue is an astrometric reference catalogue 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 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 catalogues, all reduced to the Hipparcos celestial coordinate system. Tycho-2 supersedes in most applications Tycho-1, as well as the ACT (Cat. I/246) and TRC (Cat. I/250) catalogues based on Tycho-1. Supplement-1 lists stars from the Hipparcos and Tycho-1 Catalogues which are not in Tycho-2. Supplement-2 lists 1146 Tycho-1 stars which are probably either false or heavily disturbed. For more information, please consult the Tycho-2 home page: http://www.astro.ku.dk/~erik/Tycho-2

Columns description:

Tycho 2 identifier. The TYC identifier is constructed from the GSC region number
(TYC1), the running number within the region (TYC2) and a component
identifier (TYC3) which is normally 1. Some non-GSC running numbers
were constructed for the first Tycho Catalogue and for Tycho-2.
The recommended star designation contains a hyphen between the
TYC numbers, e.g. TYC 1-13-1.

[1,120404]? Hipparcos number

[1,9537]+= TYC1 from TYC or GSC

The TYC identifier is constructed from the GSC region number
(TYC1), the running number within the region (TYC2) and a component
identifier (TYC3) which is normally 1. Some non-GSC running numbers
were constructed for the first Tycho Catalogue and for Tycho-2.
The recommended star designation contains a hyphen between the
TYC numbers, e.g. TYC 1-13-1.

[1,12121] TYC2 from TYC or GSC

[1,3] TYC3 from TYC

These are the IDs as published in tycho2. In TYCHO-2 objects were identified by
3 numbers (TYC1,TYC2 and TYC3) and we have combined these into one complete
number given by (TYC1*1000000.0d0)+(TYC2*10.0d0)+(TYC3*1.0d0)

[T] Tycho-1 star

’ ’ = no Tycho-1 star was found within 0.8 arcsec (quality 1-8)
or 2.4 arcsec (quality 9).
’T’ = this is a Tycho-1 star. The Tycho-1 identifier is given in the
beginning of the record. For Tycho-1 stars, resolved in
Tycho-2 as a close pair, both components are flagged as
a Tycho-1 star and the Tycho-1 TYC3 is assigned to the
brightest (VT) component.
The HIP-only stars given in Tycho-1 are not flagged as Tycho-1 stars.

Observed Tycho-2 Right Ascension, ICRS

Observed Tycho-2 Declination, ICRS

Observed Tycho-2 Right Ascension, ICRS

Observed Tycho-2 Declination, ICRS

Mean Right Asc, ICRS, epoch=J2000

The mean position is a weighted mean for the catalogues contributing
to the proper motion determination. This mean has then been brought to
epoch 2000.0 by the computed proper motion. See Note(2) above for
details. Tycho-2 is one of the several catalogues used to determine
the mean position and proper motion. The observed Tycho-2 position is
given in the fields RAdeg and DEdeg.

Mean Decl, ICRS, at epoch=J2000

Proper motion in RA*cos(dec)

Some Hipparcos stars (having a positive number in the HIP column)
have no proper motions; these are virtually all in multiple systems.

Proper motion in Dec

Epoch-1990 of RAdeg

Epoch-1990 of de_deg

ep_ra_m : [1915.95,1992.53] mean epoch of RA. The mean epochs are given in Julian years. (float, Time[year])

Mean epoch of RA. The mean epochs are given in Julian years.

ep_de_m : [1911.94,1992.01] mean epoch of Dec. The mean epochs are given in Julian years. (float, Time[year])

Mean epoch of Dec. The mean epochs are given in Julian years.

Number of positions used

e_ra_deg : s.e.RA*cos(dec), of observed Tycho-2 RA. The errors are based on error models. (double, Angle[mas])

s.e.RA*cos(dec), of observed Tycho-2 RA. The errors are based on error models.

s.e. of observed Tycho-2 Dec. The errors are based on error models.

Correlation (ra_deg, de_deg)

e_ra_mdeg : [3,183] s.e. RA*cos(dec),at mean epoch. The errors are based on error models. (double, Angle[mas])

s.e. RA*cos(dec),at mean epoch. The errors are based on error models.

e_de_mdeg : [1,184] s.e. of Dec at mean epoch. The errors are based on error models. (double, Angle[mas])

s.e. of Dec at mean epoch. The errors are based on error models.

e_pm_ra : [0.2,11.5] s.e. prop mot in RA*cos(dec).The errors are based on error models. (float, Angular Velocity[mas/year])

s.e. prop mot in RA*cos(dec). The errors are based on error models.

e_pm_de : [0.2,10.3] s.e. of proper motion in Dec. The errors are based on error models. (float, Angular Velocity[mas/year])

s.e. of proper motion in Dec. The errors are based on error models.

Goodness of fit for mean RA

This goodness of fit is the ratio of the scatter-based and the
model-based error. It is only defined when Num $>2$. Values
exceeding 9.9 are truncated to 9.9.

Goodness of fit for mean Dec

Goodness of fit for pmDE

Goodness of fit for pmRA

[ PX] mean position flag

’ ’ = normal mean position and proper motion.
’P’ = 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.
’X’ = no mean position, no proper motion.

[ DP] type of Tycho-2 solution

’ ’ = 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.

CCDM component identifier for HIP stars

The CCDM component identifiers for double or multiple Hipparcos stars
contributing to this Tycho-2 entry. For photocentre solutions, all
components within 0.8 arcsec contribute. For double star solutions any
unresolved component within 0.8 arcsec contributes. For single star
solutions, the predicted signal from close stars were normally
subtracted in the analysis of the photon counts and such stars
therefore do not contribute to the solution. The components are given
in lexical order.

Proximity indicator

Distance in units of 100 mas to the nearest entry in the Tycho-2
main catalogue or supplement. The distance is computed for the
epoch 1991.25. A value of 999 (i.e. 99.9 arcsec) is given if the
distance exceeds 99.9 arcsec.

Tycho-2 BT magnitude

Blank when no magnitude is available. Either BTmag or VTmag is
always given. Approximate Johnson photometry may be obtained as:
V = VT -0.090*(BT-VT)
B-V = 0.850*(BT-VT)
Consult Sect 1.3 of Vol 1 of ”The Hipparcos and Tycho Catalogues”,
ESA SP-1200, 1997, for details.

Tycho-2 VT magnitude

s.e. of BT

s.e. of VT