# 3.21 ucac4_original_valid

UCAC4 Catalogue
Reference paper:
Zacharias N., et al. 2013, AJ 145, 44
Original catalogue:
DVD sent by author.

Columns description:

objt : Object type (int)

The object type flag is used to identify possible problems with a star or the source of data.
The object type flag has the following meaning:

• 0 = good, clean star, no known problem

• 1 = largest flag of any image = near overexposed star

• 2 = largest flag of any image = possible streak object

• 3 = high proper motion (HPM) star, match with external PM file

• 4 = actually use external HPM data instead of UCAC4 observ.data (accuracy of positions varies between catalogs)

• 5 = poor proper motion solution, report only CCD epoch position

• 6 = substitute poor astrometric results by FK6/Hip/Tycho-2 data

• 7 = added supplement star (no CCD data) from FK6/Hip/Tycho-2 data, and 2 stars added from high proper motion surveys

• 8 = high proper motion solution in UCAC4, star not matched with PPMXL

• 9 = high proper motion solution in UCAC4, discrepant PM to PPMXL

cdf : Combined double flag (int)

The cdf flag is a combined double star flag used to indicate the type/quality of double star fit. It is a combination of 2 flags:

cdf = $(10\cdot dsf)+dst$

where

dsf = double star flag = overall classification

• 0 = single star

• 1 = component #1 of ”good” double star

• 2 = component #2 of ”good” double star

• 3 = blended image

dst = double star type, from pixel data image profile fits, largest value of all images used for this star

• 0 = no double star, not sufficient #pixels or elongation

• to even call double star fit subroutine

• 1 = elongated image but no more than 1 peak detected

• 2 = 2 separate peaks detected $\to$ try double star fit

• 3 = secondary peak found on each side of primary

• 4 = case 1 after successful double fit (small separ. blended image)

• 5 = case 2 after successful double fit (most likely real double)

• 6 = case 3 after successful double fit (brighter secondary picked)

Caution: often a dsf= 1 or 2 image is paired with a dsf= 3. If for a star any of the several images reveals a ”blended image”, that higher dsf=3 flag is carried into the output file. This can happen for a regular double star with unique components 1 and 2. A flag dsf=3 means this could be component 1 or 2 but at least on one CCD frame a blended image was detected. This blend could be with the other component, or a spurious image or artifact. The double star flags need to be interpreted with caution; anything but a zero means ”likely some double star component or blended image”.

ucac4_oid : Unique Numeric Identifier (long)

Incremental unique numeric identifier (increasing with declination).
This field was not in the original UCAC4 catalogue, but was added for cross-match purposes.

ucac4_identifier : source Id in original catalog (string)

Official UCAC4 star ID, in the following format:
UCAC4-zzz-nnnnnn
where zzz is the 3-digit zone number (from 001 to 900, each 0.2 deg wide) and nnnnnn the 6-digit running record number along the zone file.

ra : RA, ICRS (double, Angle[deg])

Right ascension with respect to the ICRS at epoch J2000.0.
The weighted mean catalog position was updated using the provided proper motions.
NB For objects with no proper motions, the positions are at the central epoch (which actually is UCAC4 mean observation epoch). There are 4 982 212 stars with no proper motions.

dec : DEC, ICRS (double, Angle[deg])

Declination with respect to the ICRS, at epoch J2000.0.
The weighted mean catalog position was updated using the provided proper motions.
NB For objects with no proper motions, the positions are at the central epoch (which actually is UCAC4 mean observation epoch). There are 4 982 212 stars with no proper motions.

ra_error_mean_epoch : s.e. at central epoch in RAcosDEC (double, Angle[mas])

There is not 0 mas value; data less than 1 mas have been set to 1 mas. Original data larger than 255 mas have been set to 255.
In order to be able to calculate positional errors at any epoch, the central epoch, i.e. the weighted mean epoch of the data (UCAC + early epoch other catalogs) is given. At the central epoch (which varies from star to star and is also different for RA and Dec) the positional error has its smallest value; the one given in the catalog for ”sigma position”.
If the astrometric data for a star was substituted from an external catalog like Hipparcos, Tycho or High proper motion data, a mean error in position and proper motion depending on the catalog and magnitude of the star was adopted.
For stars with no proper motions only the observed CCD position and its error is reported.

dec_error_mean_epoch : s.e. at central epoch in DEC (double, Angle[mas])

There is not 0 mas value; data less than 1 mas have been set to 1 mas. Original data larger than 255 mas have been set to 255.
In order to be able to calculate positional errors at any epoch, the central epoch, i.e. the weighted mean epoch of the data (UCAC + early epoch other catalogs) is given. At the central epoch (which varies from star to star and is also different for RA and Dec) the positional error has its smallest value; the one given in the catalog for ”sigma position”.
If the astrometric data for a star was substituted from an external catalog like Hipparcos, Tycho or High proper motion data, a mean error in position and proper motion depending on the catalog and magnitude of the star was adopted.
For stars with no proper motions only the observed CCD position and its error is reported.

pmra : proper motion in RAcosDEC (double, Angular Velocity[mas/year])

Proper motion in RA*cos(Dec).

pmde : proper motion in DEC (double, Angular Velocity[mas/year])

Proper motion in Declination.

pmra_error : s.e. in proper motion in RAcosDEC (double, Angular Velocity[mas/year])

For astrometric data copied from the FK6, Hipparcos and Tycho-2 catalogs a mean error in positions was adopted depending on input catalog and the brightness of the star rather than giving the individual star’s error quoted in those catalogs.

pmde_error : s.e. in proper motion in DEC (double, Angular Velocity[mas/year])

For astrometric data copied from the FK6, Hipparcos and Tycho-2 catalogs a mean error in positions was adopted depending on input catalog and the brightness of the star rather than giving the individual star’s error quoted in those catalogs.

mean_epoch_ra : Central epoch for mean RA (double, Time[Julian Years])

Central Epoch for mean RA.
Probably epoch is a Julian epoch expressed as Julian Years with reference J2000.0. For stars with proper motions, the central epoch, i.e. the weighted mean epoch of the data (UCAC + early epoch other catalogs) is given. At the central epoch (which varies from star to star and is also different for RA and Dec) the positional error has its smallest value; the one given in the catalog for ”sigma position”.
For stars with no proper motions, the central Epoch is the UCAC4 mean observation epoch.

mean_epoch_dec : Central epoch for mean DEC (double, Time[Julian Years])

Central Epoch for mean DEC.
Probably epoch is a Julian epoch expressed as Julian Years with reference J2000.0. For stars with proper motions, the central epoch, i.e. the weighted mean epoch of the data (UCAC + early epoch other catalogs) is given. At the central epoch (which varies from star to star and is also different for RA and Dec) the positional error has its smallest value; the one given in the catalog for ”sigma position”.
For stars with no proper motions, the central Epoch is the UCAC4 mean observation epoch.

magm : fit model magnitude (double, Magnitude[mag])

Fit model magnitude.
Systematic errors are expected to be below 0.1 mag for magm,maga photometric results obtained from the UCAC CCD pixel data. The aperture photometry is considered more robust, particularly for ”odd” cases, while the model fit magnitude is expected to be more accurate for ”well behaved” stars.

maga : aperture magnitude (double, Magnitude[mag])

Aperture magnitude.
Systematic errors are expected to be below 0.1 mag for magm,maga photometric results obtained from the UCAC CCD pixel data. The aperture photometry is considered more robust, particularly for ”odd” cases, while the model fit magnitude is expected to be more accurate for ”well behaved” stars.

sigmag : error of magnitude (double, Magnitude[mag])

For many stars a photometric error based on the scatter from individual observations of that star on different CCD frames could be obtained. A model error was also attempted to be assigned, based on the S/N ratio. The error quoted here is the larger of the two.
If the error exceeds 0.9 mag, the error was set to 0.9 mag in catalog data.

n_epochs : number of epochs used for proper motions (short)

Number of catalogs (epoch) used for proper motions.