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

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 1.5For 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.

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
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.