This table is a subset of gaia_source comprising those stars in the Hipparcos and Tycho-2 Catalogues for which a full 5-parameter astrometric solution has been possible in Gaia Data Release 1. This is possible because the early Hipparcos epoch positions break some degeneracies due to the limited Gaia time coverage.
This table contains a substantial fraction of the around 2.5 million stars in the Hipparcos and Tycho-2 catalogue. Many stars have been excluded due to several reasons, such as saturation, cross-match errors or bad astrometric solution.
Columns description:
Hipparcos identifier (this field will be empty if the source was not in the Hipparcos catalogue).
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.
To see if Hipparcos or Tycho measurements where used as a prior constraint in the astrometic solution for a given source please see astrometric_priors_used.
The data in the MDB will be described by means of a ”Solution identifier” parameter. This will be a numeric field attached to each table row that can be used to unequivocally identify the version of all the subsystems that where used in the generation of the data as well as the input data used. Each DPC generating the data will have the freedom to choose the Solution identifier number, but they must ensure that given the Solution identifier they can provide detailed information about the ”conditions” used to generate the data: versions of the software, version of the data used…
A unique single numerical identifier of the source.
For the contents of Gaia DR1, which does not include Solar System objects, the source ID consists consists of a 64-bit integer, least significant bit = 1 and most significant bit = 64, comprising:
a HEALPix index number (sky pixel) in bits 36 – 63; by definition the smallest HEALPix index number is zero.
a 2-bit Data Processing Centre code in bits 34 – 35; for example MOD(source_id / 4294967296, 8) can be used to distinguish between sources initialised via the Initial Gaia Source List by the Torino DPC (code = 0) and sources otherwise detected and assigned by Gaia observations (code $>0$)
a 25-bit plus 7 bit sequence number within the HEALPix pixel in bits 1 – 32 split into:
a 25 bit running number in bits 8 – 32; the running numbers are defined to be positive, i.e. never zero (except in the case of forced empty windows)
a 7-bit component number in bits 1 – 7
one spare bit in bit 33
This means that the HEALpix index level 12 of a given source is contained in the most significant bits. HEALpix index of 12 and lower levels can thus be retrieved as follows:
HEALpix level 12 = source_id / 34359738368
HEALpix level 11 = source_id / 137438953472
HEALpix level 10 = source_id / 549755813888
HEALpix level n = source_id / 2 ^ 35 * 4 ^ (12 – level).
Additional details can be found in the Gaia DPAC public document Source Identifiers — Assignment and Usage throughout DPAC (document code GAIA–C3–TN–ARI–BAS–020) available from http://www.cosmos.esa.int/web/gaia/public-dpac-documents
Random index which can be used to select smaller subsets of the data that are still representative. The column contains a random permutation of the numbers from 0 to N-1, where N is the number of rows.
The random index can be useful for validation (testing on 10 different random subsets), visualization (displaying 1% of the data), and statistical exploration of the data, without the need to download all the data.
Reference epoch to which the astrometic source parameters are referred, expressed as a Julian Year in TCB.
Barycentric right ascension $\alpha $ of the source in ICRS at the reference epoch ref_epoch
Standard error ${\sigma}_{\alpha *}\equiv {\sigma}_{\alpha}\mathrm{cos}\delta $ of the right ascension of the source in ICRS at the reference epoch ref_epoch.
Barycentric declination $\delta $ of the source in ICRS at the reference epoch ref_epoch
Standard error ${\sigma}_{\delta}$ of the declination of the source in ICRS at the reference epoch ref_epoch
Absolute barycentric stellar parallax $\varpi $ of the soure at the reference epoch ref_epoch
Standard error ${\sigma}_{\varpi}$ of the stellar parallax at the reference epoch ref_epoch
Proper motion in right ascension ${\mu}_{\alpha *}$ of the source in ICRS at the reference epoch ref_epoch. This is the projection of the proper motion vector in the direction of increasing right ascension.
pmra_error : Standard error of proper motion in right ascension direction (double, Angular Velocity[mas/year] )
Standard error ${\sigma}_{\mu \alpha *}$ of the proper motion vector in right ascension at the reference epoch ref_epoch
Proper motion in declination ${\mu}_{\delta}$ of the source at the reference epoch ref_epoch. This is the projection of the proper motion vector in the direction of increasing declination.
pmdec_error : Standard error of proper motion in declination direction (double, Angular Velocity[mas/year] )
Standard error ${\sigma}_{\mu \delta}$ of the proper motion in declination at the reference epoch ref_epoch
ra_dec_corr : Correlation between right ascension and declination (float, Dimensionless[see description])
Correlation between right ascension and declination, in dimensionless units [-1:+1]
ra_parallax_corr : Correlation between right ascension and parallax (float, Dimensionless[see description])
Correlation between right ascension and parallax, in dimensionless units [-1:+1]
ra_pmra_corr : Correlation between right ascension and proper motion in right ascension (float, Dimensionless[see description])
Correlation between right ascension and proper motion in right ascension, in dimensionless units [-1:+1]
ra_pmdec_corr : Correlation between right ascension and proper motion in declination (float, Dimensionless[see description])
Correlation between right ascension and proper motion in declination, in dimensionless units [-1:+1]
dec_parallax_corr : Correlation between declination and parallax (float, Dimensionless[see description])
Correlation between declination and parallax, in dimensionless units [-1:+1]
dec_pmra_corr : Correlation between declination and proper motion in right ascension (float, Dimensionless[see description])
Correlation between declination and proper motion in right ascension, in dimensionless units [-1:+1]
dec_pmdec_corr : Correlation between declination and proper motion in declination (float, Dimensionless[see description])
Correlation between declination and proper motion in declination, in dimensionless units [-1:+1]
parallax_pmra_corr : Correlation between parallax and proper motion in right ascension (float, Dimensionless[see description])
Correlation between parallax and proper motion in right ascension, in dimensionless units [-1:+1]
parallax_pmdec_corr : Correlation between parallax and proper motion in declination (float, Dimensionless[see description])
Correlation between parallax and proper motion in declination, in dimensionless units [-1:+1]
pmra_pmdec_corr : Correlation between proper motion in right ascension and proper motion in declination (float, Dimensionless[see description])
Correlation between proper motion in right ascension and proper motion in declination, in dimensionless units [-1:+1]
Total number of AL observations (= CCD transits) used in the astrometric solution of the source, independent of their weight. Note that some observations may be strongly downweighted (see
astrometric_n_bad_obs_al).
Total number of AC observations (= CCD transits) used in the astrometric solution of the source, independent of their weight. Note that some observations may be strongly downweighted (see
astrometric_n_bad_obs_ac). Nearly all sources having G $$ will have AC observations from 2d windows, while fainter than that limit only $\sim 1$% of stars (the so–called ‘calibration faint stars’) are assigned 2d windows resulting in AC observations.
Number of AL observations (= CCD transits) that were not strongly downweighted in the astrometric solution of the source. Strongly downweighted observations (with downweighting factor $$) are instead counted in astrometric_n_bad_obs_al. The sum of astrometric_n_good_obs_al and astrometric_n_bad_obs_al equals
astrometric_n_obs_al, the total number of AL observations used in the astrometric solution of the source.
Number of AC observations (= CCD transits) that were not strongly downweighted in the astrometric solution of the source. Strongly downweighted observations (with downweighting factor $$) are instead counted in astrometric_n_bad_obs_ac. The sum of astrometric_n_good_obs_ac and astrometric_n_bad_obs_ac equals
astrometric_n_obs_ac, the total number of AC observations used in the astrometric solution of the source.
Number of AL observations (= CCD transits) that were strongly downweighted in the astrometric solution of the source, and therefore contributed little to the determination of the astrometric parameters. An observation is considered to be strongly downweighted if its downweighting factor $$, which means that the absolute value of the astrometric residual exceeds 4.83 times the total uncertainty of the observation, calculated as the quadratic sum of the centroiding uncertainty, excess source noise, and excess attitude noise.
Number of AC observations (= CCD transits) that were strongly downweighted in the astrometric solution of the source, and therefore contributed little to the determination of the astrometric parameters. An observation is considered to be strongly downweighted if its downweighting factor $$, which means that the absolute value of the astrometric residual exceeds 4.83 times the total uncertainty of the observation, calculated as the quadratic sum of the centroiding uncertainty, excess source noise, and excess attitude noise.
In the TGAS solution astrometric_delta_q ($\mathrm{\Delta}Q$) indicates the discrepancy between the Hipparcos proper motion and the TGAS proper motion. A large value of $\mathrm{\Delta}Q$ could indicate non-linear motion (e.g. in a binary).
The precise definition is
$\mathrm{\Delta}Q=\left[\begin{array}{cc}\hfill \mathrm{\Delta}{\mu}_{\alpha *}\hfill & \hfill \mathrm{\Delta}{\mu}_{\delta}\hfill \end{array}\right]{\left({\overrightarrow{C}}_{\text{pm,\hspace{0.17em}T}}+{\overrightarrow{C}}_{\text{pm,\hspace{0.17em}H}}\right)}^{-1}\left[\begin{array}{c}\hfill \mathrm{\Delta}{\mu}_{\alpha *}\hfill \\ \hfill \mathrm{\Delta}{\mu}_{\delta}\hfill \end{array}\right]$
where $\mathrm{\Delta}{\mu}_{\alpha *}={\mu}_{\alpha *,\mathrm{T}}-{\mu}_{\alpha *,\mathrm{H}}$, $\mathrm{\Delta}{\mu}_{\delta}={\mu}_{\delta ,\mathrm{T}}-{\mu}_{\delta ,\mathrm{H}}$, with T and H indicating values from the Gaia DR1 (TGAS) solution and Hipparcos catalogue. ${\overrightarrow{C}}_{\text{pm,\hspace{0.17em}T}}$ and ${\overrightarrow{C}}_{\text{pm,\hspace{0.17em}H}}$ are the corresponding $2\times 2$ covariance matrices.
In order to compute $\mathrm{\Delta}Q$ the two sets of proper motions must use the same reference frame and the same reference epoch. Thus, the proper motion components as given in the Hipparcos catalogue were rotated to the Gaia DR1 reference frame, and then propagated to the Gaia reference epoch.
This is the excess noise ${\u03f5}_{i}$ of the source. It measures the disagreement, expressed as an angle, between the observations of a source and the best-fitting standard astrometric model (using five astrometric parameters). The assumed observational noise in each observation is quadratically increased by ${\u03f5}_{i}$ in order to statistically match the residuals in the astrometric solution. A value of 0 signifies that the source is astrometrically well-behaved, i.e. that the residuals of the fit statistically agree with the assumed observational noise. A positive value signifies that the residuals are statistically larger than expected.
The significance of ${\u03f5}_{i}$ is given by astrometric_excess_noise_sig ($D$). If $D\le 2$ then ${\u03f5}_{i}$ is probably not significant, and the source may be astrometrically well-behaved even if ${\u03f5}_{i}$ is large.
The excess noise ${\u03f5}_{i}$ may absorb all kinds of modelling errors that are not accounted for by the observational noise (image centroiding error) or the excess attitude noise. Such modelling errors include LSF and PSF calibration errors, geometric instrument calibration errors, and part of the high-frequency attitude noise. These modelling errors are particularly important in the early data releases, but should decrease as the astrometric modelling of the instrument and attitude improves over the years.
Additionally, sources that deviate from the standard five-parameter astrometric model (e.g. unresolved binaries, exoplanet systems, etc.) may have positive ${\u03f5}_{i}$. Given the many other possible contributions to the excess noise, the user must study the empirical distributions of ${\u03f5}_{i}$ and $D$ to make sensible cutoffs before filtering out sources for their particular application.
In Gaia DR1, the excess source noise has the same interpretation as described above for both the primary (TGAS) and secondary data sets. It measures the disagreement between the five-parameter model and the observations, augmented by the different priors used. Thus, in TGAS the excess noise may be increased if the proper motion seen during the 14 months of Gaia observations are not in agreement with the proper motion inferred from the Tycho-2/Gaia comparison. In the secondary solution the excess noise may be increased if the Gaia observations indicate a proper motion or parallax several times larger than the prior uncertainty.
The excess source noise is further explained in Sects. 3.6 and 5.1.2 of
(1).
A dimensionless measure ($D$) of the significance of the calculated astrometric_excess_noise (${\u03f5}_{i}$). A value $D>2$ indicates that the given ${\u03f5}_{i}$ is probably significant.
For good fits in the limit of a large number of observations, $D$ should be zero in half of the cases and approximately follow the positive half of a normal distribution with zero mean and unit standard deviation for the other half. Consequently, $D$ is expected to be greater than 2 for only a few percent of the sources with well-behaved astrometric solutions.
In the early data releases ${\u03f5}_{i}$ will however include instrument and attitude modelling errors that are statistically significant and could result in large values of ${\u03f5}_{i}$ and $D$. The user must study the empirical distributions of these statistics and make sensible cutoffs before filtering out sources for their particular application.
The excess noise significance is further explained in Sect. 5.1.2 of
(1).
Flag indicating if this source was used as a primary source (true) or secondary source (false). Only primary sources contribute to the estimation of attitude, calibration, and global parameters. The estimation of source parameters is otherwise done in exactly the same way for primary and secondary sources.
Relegation factor of the source calculated as per Eq. (118) in (1) used for the primary selection process.
Mean astrometric weight of the source in the AL direction.
The mean astrometric weight of the source is calculated as per Eq. (119) in (1).
Mean astrometric weight of the source in the AC direction
The mean astrometric weight of the source is calculated as per Eq. (119) in (1).
Type of prior used in the astrometric solution:
0: No prior used
1: Galaxy Bayesian Prior for parallax and proper motion
2: Galaxy Bayesian Prior for parallax and proper motion relaxed by factor 10
3: Hipparcos prior for position
4: Hipparcos prior for position and proper motion
5: Tycho2 prior for position
6: Quasar prior for proper motion
The Galaxy Bayesian Prior is defined in (2), where it is denoted ${\sigma}_{\varpi ,F90}$ (for the parallax) and ${\sigma}_{\mu ,F90}=\mathcal{R}{\sigma}_{\varpi ,F90}$, with $\mathcal{R}=10$ yr${}^{-1}$ (for proper motion). The Galaxy Bayesian Prior relaxed by a factor 10 is $10{\sigma}_{\varpi ,F90}$ and $10{\sigma}_{\mu ,F90}$, respectively.
For Gaia DR1 the only types of priors used are 2 (for the secondary data
set), 3 (for the Hipparcos subset of the primary data set), or 5 (for
the non-Hipparcos subset of the primary data set). Type 6 was used for
internal calibration purposes and alignment of the reference frame, but
the corresponding astrometric results are in general not published.
This field indicates the number of observations (detection transits) that have been matched to a given source during the last internal crossmatch revision.
During data processing, this source happened to been duplicated and one source only has been kept. This may indicate observational, cross-matching or processing problems, or stellar multiplicity, and probable astrometric or photometric problems in all cases.
In DR1, for close doubles with separations below some 2 arcsec, truncated windows have not been processed, neither in astrometry and photometry. The transmitted window is centred on the brighter part of the acquired window, so the brighter component has a better chance to be selected, even when processing the fainter transit. If more than two images are contained in a window, the result of the image parameter determination is un-predictable in the sense that it might refer to either (or neither) image, and no consistency is assured.
The scanDirectionStrength and scanDirectionMean quantify the distribution of AL scan directions across the source. scanDirectionStrength[$k-1$] ($k=1,2,3,4$) is the absolute value of the trigonometric moments ${m}_{k}=\u27e8\mathrm{exp}(ik\theta )\u27e9$, where $\theta $ is the position angle of the scan and the mean value is taken over the nObs[0] AL observations contributing to the astrometric parameters of the source. $\theta $ is defined in the usual astronomical sense: $\theta =0$ when the FoV is moving towards local North, and $\theta ={90}^{\circ}$ towards local East.
The scanDirectionStrength is a number between 0 and 1, where 0 means that the scan directions are well spread out in different directions, while 1 means that they are concentrated in a single direction (given by scanAngleMean).
The different orders $k$ are statistics of the scan directions modulo ${360}^{\circ}/k$. For example, at first order ($k=1$), $\theta ={10}^{\circ}$ and $\theta ={190}^{\circ}$ count as different directions, but at second order ($k=2$) they are the same. Thus, scanDirectionStrength[0] is the degree of concentration when the sense of direction is taken into account, while scanDirectionStrength[1] is the degree of concentration without regard to the sense of direction. A large value of scanDirectionStrength[3] indicates that the scans are concentrated in two nearly orthogonal directions.
The scanDirectionStrength and scanDirectionMean quantify the distribution of AL scan directions across the source. scanDirectionStrength[$k-1$] ($k=1,2,3,4$) is the absolute value of the trigonometric moments ${m}_{k}=\u27e8\mathrm{exp}(ik\theta )\u27e9$, where $\theta $ is the position angle of the scan and the mean value is taken over the nObs[0] AL observations contributing to the astrometric parameters of the source. $\theta $ is defined in the usual astronomical sense: $\theta =0$ when the FoV is moving towards local North, and $\theta ={90}^{\circ}$ towards local East.
The scanDirectionStrength is a number between 0 and 1, where 0 means that the scan directions are well spread out in different directions, while 1 means that they are concentrated in a single direction (given by scanAngleMean).
The different orders $k$ are statistics of the scan directions modulo ${360}^{\circ}/k$. For example, at first order ($k=1$), $\theta ={10}^{\circ}$ and $\theta ={190}^{\circ}$ count as different directions, but at second order ($k=2$) they are the same. Thus, scanDirectionStrength[0] is the degree of concentration when the sense of direction is taken into account, while scanDirectionStrength[1] is the degree of concentration without regard to the sense of direction. A large value of scanDirectionStrength[3] indicates that the scans are concentrated in two nearly orthogonal directions.
The scanDirectionStrength and scanDirectionMean quantify the distribution of AL scan directions across the source. scanDirectionStrength[$k-1$] ($k=1,2,3,4$) is the absolute value of the trigonometric moments ${m}_{k}=\u27e8\mathrm{exp}(ik\theta )\u27e9$, where $\theta $ is the position angle of the scan and the mean value is taken over the nObs[0] AL observations contributing to the astrometric parameters of the source. $\theta $ is defined in the usual astronomical sense: $\theta =0$ when the FoV is moving towards local North, and $\theta ={90}^{\circ}$ towards local East.
The scanDirectionStrength is a number between 0 and 1, where 0 means that the scan directions are well spread out in different directions, while 1 means that they are concentrated in a single direction (given by scanAngleMean).
The different orders $k$ are statistics of the scan directions modulo ${360}^{\circ}/k$. For example, at first order ($k=1$), $\theta ={10}^{\circ}$ and $\theta ={190}^{\circ}$ count as different directions, but at second order ($k=2$) they are the same. Thus, scanDirectionStrength[0] is the degree of concentration when the sense of direction is taken into account, while scanDirectionStrength[1] is the degree of concentration without regard to the sense of direction. A large value of scanDirectionStrength[3] indicates that the scans are concentrated in two nearly orthogonal directions.
The scanDirectionStrength and scanDirectionMean quantify the distribution of AL scan directions across the source. scanDirectionStrength[$k-1$] ($k=1,2,3,4$) is the absolute value of the trigonometric moments ${m}_{k}=\u27e8\mathrm{exp}(ik\theta )\u27e9$, where $\theta $ is the position angle of the scan and the mean value is taken over the nObs[0] AL observations contributing to the astrometric parameters of the source. $\theta $ is defined in the usual astronomical sense: $\theta =0$ when the FoV is moving towards local North, and $\theta ={90}^{\circ}$ towards local East.
The scanDirectionStrength is a number between 0 and 1, where 0 means that the scan directions are well spread out in different directions, while 1 means that they are concentrated in a single direction (given by scanAngleMean).
The different orders $k$ are statistics of the scan directions modulo ${360}^{\circ}/k$. For example, at first order ($k=1$), $\theta ={10}^{\circ}$ and $\theta ={190}^{\circ}$ count as different directions, but at second order ($k=2$) they are the same. Thus, scanDirectionStrength[0] is the degree of concentration when the sense of direction is taken into account, while scanDirectionStrength[1] is the degree of concentration without regard to the sense of direction. A large value of scanDirectionStrength[3] indicates that the scans are concentrated in two nearly orthogonal directions.
scan_direction_mean_k1 : Mean position angle of scan directions across the source (float, Angle[deg])
The scanDirectionStrength and scanDirectionMean quantify the distribution of AL scan directions across the source. scanDirectionMean[$k-1$] ($k=1,2,3,4$) is $1/k$ times the argument of the trigonometric moments ${m}_{k}=\u27e8\mathrm{exp}(ik\theta )\u27e9$, where $\theta $ is the position angle of the scan and the mean value is taken over the nObs[0] AL observations contributing to the astrometric parameters of the source. $\theta $ is defined in the usual astronomical sense: $\theta =0$ when the FoV is moving towards local North, and $\theta ={90}^{\circ}$ towards local East.
scanDirectionMean[$k-1$] is an angle between $-{180}^{\circ}/k$ and $+{180}^{\circ}/k$, giving the mean position angle of the scans at order $k$.
The different orders $k$ are statistics of the scan directions modulo ${360}^{\circ}/k$. For example, at first order ($k=1$), $\theta ={10}^{\circ}$ and $\theta ={190}^{\circ}$ count as different directions, but at second order ($k=2$) they are the same. Thus, scanDirectionMean[0] is the mean direction when the sense of direction is taken into account, while scanDirectionMean[1] is the mean direction without regard to the sense of the direction. For example, scanDirectionMean[0] = 0 means that the scans preferentially go towards North, while scanDirectionMean[1] = 0 means that they preferentially go in the North-South direction, and scanDirectionMean[4] = 0 that they preferentially go either in the North-South or in the East-West direction.
scan_direction_mean_k2 : Mean position angle of scan directions across the source (float, Angle[deg])
The scanDirectionStrength and scanDirectionMean quantify the distribution of AL scan directions across the source. scanDirectionMean[$k-1$] ($k=1,2,3,4$) is $1/k$ times the argument of the trigonometric moments ${m}_{k}=\u27e8\mathrm{exp}(ik\theta )\u27e9$, where $\theta $ is the position angle of the scan and the mean value is taken over the nObs[0] AL observations contributing to the astrometric parameters of the source. $\theta $ is defined in the usual astronomical sense: $\theta =0$ when the FoV is moving towards local North, and $\theta ={90}^{\circ}$ towards local East.
scanDirectionMean[$k-1$] is an angle between $-{180}^{\circ}/k$ and $+{180}^{\circ}/k$, giving the mean position angle of the scans at order $k$.
The different orders $k$ are statistics of the scan directions modulo ${360}^{\circ}/k$. For example, at first order ($k=1$), $\theta ={10}^{\circ}$ and $\theta ={190}^{\circ}$ count as different directions, but at second order ($k=2$) they are the same. Thus, scanDirectionMean[0] is the mean direction when the sense of direction is taken into account, while scanDirectionMean[1] is the mean direction without regard to the sense of the direction. For example, scanDirectionMean[0] = 0 means that the scans preferentially go towards North, while scanDirectionMean[1] = 0 means that they preferentially go in the North-South direction, and scanDirectionMean[4] = 0 that they preferentially go either in the North-South or in the East-West direction.
scan_direction_mean_k3 : Mean position angle of scan directions across the source (float, Angle[deg])
The scanDirectionStrength and scanDirectionMean quantify the distribution of AL scan directions across the source. scanDirectionMean[$k-1$] ($k=1,2,3,4$) is $1/k$ times the argument of the trigonometric moments ${m}_{k}=\u27e8\mathrm{exp}(ik\theta )\u27e9$, where $\theta $ is the position angle of the scan and the mean value is taken over the nObs[0] AL observations contributing to the astrometric parameters of the source. $\theta $ is defined in the usual astronomical sense: $\theta =0$ when the FoV is moving towards local North, and $\theta ={90}^{\circ}$ towards local East.
scanDirectionMean[$k-1$] is an angle between $-{180}^{\circ}/k$ and $+{180}^{\circ}/k$, giving the mean position angle of the scans at order $k$.
The different orders $k$ are statistics of the scan directions modulo ${360}^{\circ}/k$. For example, at first order ($k=1$), $\theta ={10}^{\circ}$ and $\theta ={190}^{\circ}$ count as different directions, but at second order ($k=2$) they are the same. Thus, scanDirectionMean[0] is the mean direction when the sense of direction is taken into account, while scanDirectionMean[1] is the mean direction without regard to the sense of the direction. For example, scanDirectionMean[0] = 0 means that the scans preferentially go towards North, while scanDirectionMean[1] = 0 means that they preferentially go in the North-South direction, and scanDirectionMean[4] = 0 that they preferentially go either in the North-South or in the East-West direction.
scan_direction_mean_k4 : Mean position angle of scan directions across the source (float, Angle[deg])
The scanDirectionStrength and scanDirectionMean quantify the distribution of AL scan directions across the source. scanDirectionMean[$k-1$] ($k=1,2,3,4$) is $1/k$ times the argument of the trigonometric moments ${m}_{k}=\u27e8\mathrm{exp}(ik\theta )\u27e9$, where $\theta $ is the position angle of the scan and the mean value is taken over the nObs[0] AL observations contributing to the astrometric parameters of the source. $\theta $ is defined in the usual astronomical sense: $\theta =0$ when the FoV is moving towards local North, and $\theta ={90}^{\circ}$ towards local East.
scanDirectionMean[$k-1$] is an angle between $-{180}^{\circ}/k$ and $+{180}^{\circ}/k$, giving the mean position angle of the scans at order $k$.
The different orders $k$ are statistics of the scan directions modulo ${360}^{\circ}/k$. For example, at first order ($k=1$), $\theta ={10}^{\circ}$ and $\theta ={190}^{\circ}$ count as different directions, but at second order ($k=2$) they are the same. Thus, scanDirectionMean[0] is the mean direction when the sense of direction is taken into account, while scanDirectionMean[1] is the mean direction without regard to the sense of the direction. For example, scanDirectionMean[0] = 0 means that the scans preferentially go towards North, while scanDirectionMean[1] = 0 means that they preferentially go in the North-South direction, and scanDirectionMean[4] = 0 that they preferentially go either in the North-South or in the East-West direction.
Number of observations (CCD transits) that contributed to the G mean flux and mean flux error.
Mean flux in the G-band.
Error on the mean flux in the G-band.
Mean magnitude in the G band. This is computed from the G-band mean flux applying the magnitude zero-point in the Vega scale.
Flag indicating if variability was identified in the photometric G band:
source not processed and/or exported to catalogue
Source not identified as variable
source identified and processed as variable, see tables:
phot_variable_summary,
phot_variable_time_series_gfov,
phot_variable_time_series_gfov_statistical_parameters,
and cepheid or rrlyrae for more details.
Note that for this data release only a small subset of (variable) sources was processed and/or exported, so for many (known) variable sources this flag is set to ”NOT AVAILABLE”. No ”CONSTANT” sources were exported either.
Galactic Longitude of the object at reference epoch ref_epoch, see ESA, 1997, ’The Hipparcos and Tycho Catalogues’, ESA SP-1200, Volume 1, Section 1.5.3, for the conversion details.
Galactic Latitude of the object at reference epoch ref_epoch, see ESA, 1997, ’The Hipparcos and Tycho Catalogues’, ESA SP-1200, Volume 1, Section 1.5.3, for the conversion details.
Ecliptic Longitude of the object at reference epoch ref_epoch, see ESA, 1997, ’The Hipparcos and Tycho Catalogues’, ESA SP-1200, Volume 1, Section 1.5.3, for the conversion details.
Ecliptic Latitude of the object at reference epoch ref_epoch, see ESA, 1997, ’The Hipparcos and Tycho Catalogues’, ESA SP-1200, Volume 1, Section 1.5.3, for the conversion details.