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TYCHO2 - Tycho-2 Catalog of the 2.5 Million Brightest Stars

HEASARC
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Overview

The Hipparcos and Tycho Catalogues are the primary products of the European Space Agency's astrometric mission, Hipparcos. The satellite, which operated for four years, returned high quality scientific data from November 1989 to March 1993.

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.

Supplement-1 (not part of this HEASARC database but available at ftp://cdsarc.u-strasbg.fr/cats/I/259/suppl_1.dat.gz) lists stars from the Hipparcos and Tycho-1 Catalogues which are not in Tycho-2. Supplement-2 (not part of this HEASARC database but available at ftp://cdsarc.u-strasbg.fr/cats/I/259/suppl_2.dat.gz) lists 1146 Tycho-1 stars which are probably either false or heavily disturbed.

The principal characteristics of the Tycho-2 Catalogue are summarized below. By means of proper motions the positions are transferred to the year 2000.0, the epoch of the catalog. The median values of internal standard errors are given:

    Mean satellite observation epoch      ~J1991.5
    Epoch of the Tycho-2 Catalogue         J2000.0
    Reference system                       ICRS
      coincidence with ICRS (1)             +/-0.6 mas
      deviation from inertial (1)           +/-0.25 mas/yr
    Number of entries                      2,539,913
    Astrometric standard errors (2)
      V_T < 9 mag                           7 mas
      all stars, positions                   60 mas
      all stars, proper motions              2.5 mas/yr
    Photometric std. errors (3) on V_T
      V_T < 9 mag                           0.013 mag
      all stars                              0.10 mag
    Star density
      b= 0 deg                             150 stars/sq.deg.
      b= +/-30 deg                          50 stars/sq.deg.
      b= +/-90 deg                          25 stars/sq.deg.
    Completeness to 90 per cent            V ~ 11.5 mag
    Completeness to 99 per cent            V ~ 11.0 mag
    Number of Tycho observations           ~300 10^6^

    Note (1): about all 3 axes
    Note (2): ratio of external to internal standard errors is ~1.0
              for positions and for proper motions. Systematic errors
              are less than 1 mas and 0.5 mas/yr
    Note (3): ratio of photometric external to internal standard errors
              at V_T > 9 mag is below 1.5
For more information on the original catalog, please consult the Tycho-2 home page at http://www.astro.ku.dk/~erik/Tycho-2

For more information on the HEASARC implementation of the Tycho-2 Catalog, please consult the HEASARC_Implementation section of this help.


Catalog Bibcode

2000A&A...355L..27H

References

The Tycho-2 Catalogue of the 2.5 Million Brightest Stars,
Hog E., Fabricius C., Makarov V.V., Urban S., Corbin T., Wycoff G.,
Bastian U., Schwekendiek P., and Wicenec A.
2000, A&A, 355, L27.

Construction and Verification of the Tycho-2 Catalogue,
Hog E., Fabricius C., Makarov V.V., Bastian U., Schwekendiek P.,
Wicenec A., Urban S., Corbin T., and Wycoff G.,
2000, A&A, 357, 367.

Provenance

This database table was created at the HEASARC in June 2000 based on the ADC/CDS Catalog I/259, using the file tyc2.dat. Galactic coordinates (calculated by converting the observed ICRS Equatorial positions) were added to this HEASARC database table in August 2005.

HEASARC Implementation

All positions in the original Tycho-2 Catalog were given in the ICRS system (which is essentially J2000 equator), with the parameters mean_ra and mean_dec being the position at epoch 2000.0 while the parameters observed_ra and observed_dec being the position at the epochs RA_epoch and Dec_epoch, respectively, when observed by Tycho. The HEASARC parameters RA and Dec are equivalent to the observed_ra and observed_dec values (except they are in sexagesimal format rather than the original decimal degrees) if the user specifies 2000 for the equinox of RA and Dec: if the user specifies a different equinox, the Browse software precesses RA and Dec the appropriate amount. Since the Browse precession software is not of astrometric accuracy, the precessed RA and Dec values will only be accurate to ~1". For all uses where the original Tycho-2 accuracy is required, one should use the parameters mean_ra and mean_dec, or observed_ra and observed_dec, rather than the HEASARC parameters RA and dec.

Parameters

Name
The recommended designation for the star derived from appending the TYC1, TYC2, and TYC3 values for the star (given as fields in the original table) after the prefix 'TYC', where TYC1 is the Guide Star Catalog region number, TYC2 is the running number within the region, and TYC3 is a Tycho specific component number. Double star components will either have two different running numbers, or the same running number and different component numbers. In about 4500 cases, new consistent running numbers within GSC regions were constructed for Tycho-2.

Position_Flag
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.

Mean_RA
The mean right ascension in decimal degrees in the ICRS system and J2000 equinox, given in the original catalog to a precision of 10^-8 degrees. Tycho-2 is one of the several catalogs used to determine the mean position and proper motion. The mean position is a weighted mean from the catalogs contributing to the proper motion determination. This mean has then been brought to the epoch of 2000.0 using the computed proper motion. The parameter position_flag (q.v.) contains information on how the mean_ra was obtained. The observed Tycho-2 position is given in the fields observed_ra and observed_dec.

Mean_Dec
The mean declination in decimal degrees in the ICRS system and J2000 equinox, given in the original catalog to a precision of 10^-8 degrees. Tycho-2 is one of the several catalogs used to determine the mean position and proper motion. The mean position is a weighted mean from the catalogs contributing to the proper motion determination. This mean has then been brought to the epoch of 2000.0 using the computed proper motion. The parameter position_flag (q.v.) contains information on how the mean_dec was obtained. The observed Tycho-2 position is given in the fields observed_ra and observed_dec.

PM_RA
The proper motion in the RA direction, in milliarcseconds per year, i.e., RA*cos(Dec).

PM_Dec
The proper motion in the declination direction, in milliarcseconds per year.

Mean_RA_Error
The standard error in the mean right ascension, in milliarcseconds, i.e., RA*cos(Dec). This error is based on an error model.

Mean_Dec_Error
The standard error in the mean declination, in milliarcseconds. This error is based on an error model.

PM_RA_Error
The standard error in the proper motion in the RA direction, in milliarcseconds per year, i.e., RA*cos(Dec). This error is based on an error model.

PM_Dec_Error
The standard error in the proper motion in the declination direction, in milliarcseconds per year. This error is based on an error model.

Mean_RA_Epoch
The epoch of the mean right ascension in Julian years.

Mean_Dec_Epoch
The epoch of the mean declination in Julian years.

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

GoF_Mean_RA
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.

GoF_Mean_Dec
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.

GoF_PM_RA
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.

GoF_PM_Dec
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_Mag
The Tycho-2 BT magnitude, blank when no magnitude is available. Either BT (bt_mag) or VT (vt_mag) is always given. Approximate Johnson photometric magnitudes may be obtained as follows:

      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 more details.

Bt_Mag_Error
The standard error in the BT magnitude.

Vt_Mag
The Tycho-2 VT magnitude, blank when no magnitude is available. Either BT (bt_mag) or VT (vt_mag) is always given. Approximate Johnson photometric magnitudes may be obtained as follows:

      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 more details.

Vt_Mag_Error
The standard error in the VT magnitude.

Proximity
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. (Stars in the second supplement, i.e., false Tycho-1 stars, were ignored). The distance is computed for the epoch 1991.25. A value of 999 (i.e. 99.9 arcsec) is given if the angular distance exceeds 99.9 arcsec.

Tycho1_Star
A flag which specifies whether or not the star was in the original Tycho Catalog (Tycho-1), where ' ' means that no Tycho-1 star was found within 0.8 arcsec (for the Tycho-1 quality 1-8 stars) or 2.4 arcsec (for the Tycho-1 quality 9 stars), and 'T' means that this is a Tycho-1 star. For Tycho-1 stars which are resolved in Tycho-2 as a close pair, both components are flagged as a Tycho-1 star and the Tycho-1 TYC3 component number is assigned to the component that is brightest in VT. The HIP-only stars given in Tycho-1 are not flagged as Tycho-1 stars.

HIP_Number
The Hipparcos (HIP) number corresponding to the entry.

CCDM_Components
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.

RA
The Right Ascension of the star, in the specified equinox, at the epoch specified by the parameter ra_epoch. The HEASARC parameters RA and Dec are equivalent to the observed_ra and observed_dec values (except they are in sexagesimal format rather than the original decimal degrees) if the user specifies 2000 for the equinox of RA and Dec: if the user specifies a different equinox, the Browse software precesses RA and Dec the appropriate amount. Since the Browse precession software is not of astrometric accuracy, the precessed RA and Dec values will only be accurate to ~1". For all uses where the original Tycho-2 accuracy is required, one should use the parameters mean_ra and mean_dec, or observed_ra and observed_dec, rather than the HEASARC parameters RA and dec.

Dec
The declination of the star, in the specified equinox, at the epoch specified by the parameter dec_epoch. The HEASARC parameters RA and Dec are equivalent to the observed_ra and observed_dec values (except they are in sexagesimal format rather than the original decimal degrees) if the user specifies 2000 for the equinox of RA and Dec: if the user specifies a different equinox, the Browse software precesses RA and Dec the appropriate amount. Since the Browse precession software is not of astrometric accuracy, the precessed RA and Dec values will only be accurate to ~1". For all uses where the original Tycho-2 accuracy is required, one should use the parameters mean_ra and mean_dec, or observed_ra and observed_dec, rather than the HEASARC parameters RA and dec.

LII
The Galactic longitude of the star, in degrees. The HEASARC parameters LII and BII were created by converting the observed_ra and observed_dec values to the Galactic coordinate system. Since the Browse coordinate system transformation software is not of astrometric accuracy, the LII and BII values will only be accurate to ~1". For all uses where the original Tycho-2 accuracy is required, one should use the parameters mean_ra and mean_dec, or observed_ra and observed_dec, rather than the HEASARC parameters LII and BII.

BII
The Galactic latitude of the star, in degrees. The HEASARC parameters LII and BII were created by converting the observed_ra and observed_dec values to the Galactic coordinate system. Since the Browse coordinate system transformation software is not of astrometric accuracy, the LII and BII values will only be accurate to ~1". For all uses where the original Tycho-2 accuracy is required, one should use the parameters mean_ra and mean_dec, or observed_ra and observed_dec, rather than the HEASARC parameters LII and BII.

Observed_RA
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.

Observed_Dec
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.

Observed_RA_Epoch
The epoch of the observed right ascension (observed_ra), in years.

Observed_Dec_Epoch
The epoch of the observed declination (observed_dec), in years.

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

Observed_Dec_Error
The standard error in the observed declination, in milliarcseconds. This error is based on an error model.

Solution_Type
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.
  

RA_Dec_Corr
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.


Contact Person

Questions regarding the TYCHO2 database table can be addressed to the HEASARC User Hotline.

Page Author: Browse Software Development Team
Last Modified: 16-Aug-2005