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## TYCHO2 - Tycho-2 Catalog of the 2.5 Million Brightest Stars |
HEASARC Archive |

The Tycho-2 catalog is an astrometric reference catalog containing positions and proper motions as well as two-color 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 catalog (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 https://cdsarc.cds.unistra.fr/ftp/cats/I/259/suppl_1.dat.gz) lists stars from the Hipparcos and Tycho-1 catalogs which are not in Tycho-2. Supplement-2 (not part of this HEASARC database but available at https://cdsarc.cds.unistra.fr/ftp/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 catalog 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 catalog 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 10For more information on the original catalog, please consult the Tycho-2 home page at http://www.astro.ku.dk/~erik/Tycho-2^{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 HEASARC implementation of the Tycho-2 catalog, please consult the "HEASARC Implementation" section of this help.

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.

**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
photocenter 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 photocenter 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' = photocenter 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.