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### B.1.11 Gaia

Functions related to astrometry suitable for use with data from the Gaia astrometry mission.

The methods here are not specific to the Gaia mission, but the parameters of the functions and their units are specified in a form that is convenient for use with Gaia data, in particular the `gaia_source` catalogue available from http://gea.esac.esa.int/archive/ and copies or mirrors.

There are currently three main sets of functions here:

- position and velocity vector calculation and manipulation
- distance estimation from parallaxes
- astrometry propagation to different epochs

Position and velocity vectors

Functions are provided for converting the astrometric parameters contained in the Gaia catalogue to ICRS Cartesian position (XYZ) and velocity (UVW) vectors. Functions are also provided to convert these vectors between ICRS and Galactic or Ecliptic coordinates. The calculations are fairly straightforward, and follow the equations laid out in section 1.5.6 of The Hipparcos and Tycho Catalogues, ESA SP-1200 (1997) and also section 3.1.7 of the Gaia DR2 documentation (2018).

These functions will often be combined; for instance to calculate the position and velocity in galactic coordinates from Gaia catalogue values, the following expressions may be useful:

        xyz_gal = icrsToGal(astromXYZ(ra,dec,parallax))
        uvw_gal = icrsToGal(astromUVW(array(ra,dec,parallax,pmra,pmdec,radial_velocity)))

though note that these particular examples simply invert parallax to provide distance estimates, which is not generally valid. Note also that these functions do not attempt to correct for solar motion. Such adjustments should be carried out by hand on the results of these functions if they are required.

Functions for calculating errors on the Cartesian components based on the error and correlation quantities from the Gaia catalogue are not currently provided. They would require fairly complicated invocations. If there is demand they may be implemented in the future.

Distance estimation

Gaia measures parallaxes, but some scientific use cases require the radial distance instead. While distance in parsec is in principle the reciprocal of parallax in arcsec, in the presence of non-negligable errors on measured parallax, this inversion does not give a good estimate of distance. A thorough discussion of this topic and approaches to estimating distances for Gaia-like data can be found in the papers

- C.A.L.Bailer-Jones, "Estimating distances from parallaxes", PASP 127, p994 (2015) 2015PASP..127..994B
- T.L.Astraatmadja and C.A.L.Bailer-Jones, "Estimating Distances from Parallaxes. II. Performance of Bayesian Distance Estimators on a Gaia-like Catalogue", ApJ 832, a137 (2016) 2016ApJ...832..137A
- X.Luri et al., "Gaia Data Release 2: Using Gaia Parallaxes", A&A in press (2018) arXiv:1804.09376

The functions provided here correspond to calculations from Astraatmadja & Bailer-Jones, "Estimating Distances from Parallaxes. III. Distances of Two Million Stars in the Gaia DR1 Catalogue", ApJ 833, a119 (2016) 2016ApJ...833..119A based on the Exponentially Decreasing Space Density prior defined therein. This implementation was written with reference to the Java implementation by Enrique Utrilla (DPAC).

These functions are parameterised by a length scale L that defines the exponential decay (the mode of the prior PDF is at r=2L). Some value for this length scale, specified in parsec, must be supplied to the functions as the `lpc` parameter.

Note that the values provided by these functions do not match those from the paper Bailer-Jones et al. "Estimating Distances from Parallaxes IV: Distances to 1.33 Billion stars in Gaia Data Release 2", accepted for AJ (2018) arXiv:1804.10121. The calculations of that paper differ from the ones presented here in several ways: it uses a galactic model for the direction-dependent length scale not currently available here, it pre-applies a parallax correction of -0.029mas, and it uses different uncertainty measures and in some cases (bimodal PDF) a different best distance estimator.

Epoch Propagation

The Gaia source catalogue provides, for at least some sources, the six-parameter astrometric solution (Right Ascension, Declination, Parallax, Proper motion in RA and Dec, and Radial Velocity), along with errors on these values and correlations between these errors. While a crude estimate of the position at an earlier or later epoch than that of the measurement can be made by multiplying the proper motion components by epoch difference and adding to the measured position, a more careful treatment is required for accurate propagation between epochs of the astrometric parameters, and if required their errors and correlations. The expressions for this are set out in section 1.5.5 (Volume 1) of The Hipparcos and Tycho Catalogues, ESA SP-1200 (1997) (but see below), and the code is based on an implementation by Alexey Butkevich and Daniel Michalik (DPAC). A correction is applied to the SP-1200 treatment of radial velocity uncertainty following Michalik et al. 2014 2014A&A...571A..85M because of their better handling of small radial velocities or parallaxes.

The calculations give the same results, though not exactly in the same form, as the epoch propagation functions available in the Gaia archive service.

`polarXYZ( phi, theta, r )`
  Converts from spherical polar to Cartesian coordinates.

  - Parameters:
    - `phi` (floating point): longitude in degrees
    - `theta` (floating point): latitude in degrees
    - `r` (floating point): radial distance
  - Return value
    - (array of floating point): 3-element vector giving Cartesian coordinates
  - Examples:
    - `polarXYZ(ra, dec, distance_estimate)`
    - `polarXYZ(l, b, 3262./parallax)` - calculates vector components in units of light year in the galactic system, on the assumption that distance is the inverse of parallax
`astromXYZ( ra, dec, parallax )`
  Calculates Cartesian components of position from RA, Declination and parallax. This is a convenience function, equivalent to:

          polarXYZ(ra, dec, 1000./parallax)

  Note that this performs distance scaling using a simple inversion of parallax, which is not in general reliable for parallaxes with non-negligable errors. Use at your own risk.

  - Parameters:
    - `ra` (floating point): Right Ascension in degrees
    - `dec` (floating point): Declination in degrees
    - `parallax` (floating point): parallax in mas
  - Return value
    - (array of floating point): 3-element vector giving equatorial space coordinates in parsec
  - Examples:
    - `astromXYZ(ra, dec, parallax)`
    - `icrsToGal(astromXYZ(ra, dec, parallax))`
`icrsToGal( xyz )`
  Converts a 3-element vector representing ICRS (equatorial) coordinates to galactic coordinates. This can be used with position or velocity vectors.

  The input vector is multiplied by the matrix AG', given in Eq. 3.61 of the Gaia DR2 documentation, following Eq. 1.5.13 of the Hipparcos catalogue.

  The output coordinate system is right-handed, with the three components positive in the directions of the Galactic center, Galactic rotation, and the North Galactic Pole respectively.

  - Parameters:
    - `xyz` (array of floating point): 3-element vector giving ICRS Cartesian components
  - Return value
    - (array of floating point): 3-element vector giving Galactic Cartesian components
  - Example:
    - `icrsToGal(polarXYZ(ra, dec, distance))`
`galToIcrs( xyz )`
  Converts a 3-element vector representing galactic coordinates to ICRS (equatorial) coordinates. This can be used with position or velocity vectors.

  The input vector is multiplied by the matrix AG, given in Eq. 3.61 of the Gaia DR2 documentation, following Eq. 1.5.13 of the Hipparcos catalogue.

  The input coordinate system is right-handed, with the three components positive in the directions of the Galactic center, Galactic rotation, and the North Galactic Pole respectively.

  - Parameters:
    - `xyz` (array of floating point): 3-element vector giving Galactic Cartesian components
  - Return value
    - (array of floating point): 3-element vector giving ICRS Cartesian components
  - Example:
    - `galToIcrs(polarXYZ(l, b, distance))`
`icrsToEcl( xyz )`
  Converts a 3-element vector representing ICRS (equatorial) coordinates to ecliptic coordinates. This can be used with position or velocity vectors.

  The transformation corresponds to that between the coordinates `(ra,dec)` and `(ecl_lon,ecl_lat)` in the Gaia source catalogue (DR2).

  - Parameters:
    - `xyz` (array of floating point): 3-element vector giving ICRS Cartesian components
  - Return value
    - (array of floating point): 3-element vector giving ecliptic Cartesian components
  - Example:
    - `icrsToEcl(polarXYZ(ra, dec, distance))`
`eclToIcrs( xyz )`
  Converts a 3-element vector representing ecliptic coordinates to ICRS (equatorial) coordinates. This can be used with position or velocity vectors.

  The transformation corresponds to that between the coordinates `(ecl_lon,ecl_lat)` and `(ra,dec)` in the Gaia source catalogue (DR2).

  - Parameters:
    - `xyz` (array of floating point): 3-element vector giving ecliptic Cartesian coordinates
  - Return value
    - (array of floating point): 3-element vector giving ICRS Cartesian coordinates
  - Example:
    - `eclToIcrs(polarXYZ(ecl_lon, ecl_lat, distance))`
`astromUVW( astrom6 )`
  Calculates Cartesian components of velocity from quantities available in the Gaia source catalogue. The output is in the same coordinate system as the inputs, that is ICRS for the correspondingly-named Gaia quantities.

  The input astrometry parameters are represented by a 6-element array, with the following elements:

       index  gaia_source name  unit    description
       -----  ----------------  ----    -----------
         0:   ra                deg     right ascension
         1:   dec               deg     declination
         2:   parallax          mas     parallax
         3:   pmra              mas/yr  proper motion in ra * cos(dec)
         4:   pmdec             mas/yr  proper motion in dec
         5:   radial_velocity   km/s    barycentric radial velocity

  The units used by this function are the units used in the `gaia_source` table.

  This convenience function just invokes the 7-argument `astromUVW` function using the inverted parallax for the radial distance, and without invoking the Doppler correction. It is exactly equivalent to:

          astromUVW(a[0], a[1], a[3], a[4], a[5], 1000./a[2], false)

  Note this naive inversion of parallax to estimate distance is not in general reliable for parallaxes with non-negligable errors.

  - Parameters:
    - `astrom6` (array of floating point): vector of 6 astrometric parameters as provided by the Gaia source catalogue
  - Return value
    - (array of floating point): 3-element vector giving equatorial velocity components in km/s
  - Examples:
    - `astromUVW(array(ra, dec, parallax, pmra, pmdec, radial_velocity))`
    - `icrsToGal(astromUVW(array(ra, dec, parallax, pmra, pmdec, radial_velocity)))`
`astromUVW( ra, dec, pmra, pmdec, radial_velocity, r_parsec, useDoppler )`
  Calculates Cartesian components of velocity from the observed position and proper motion, radial velocity and radial distance, with optional light-time correction. The output is in the same coordinate system as the inputs, that is ICRS for the correspondingly-named Gaia quantities.

  The radial distance must be supplied using the `r_parsec` parameter. A naive estimate from quantities in the Gaia source catalogue may be made with the expression `1000./parallax`, though note that this simple inversion of parallax is not in general reliable for parallaxes with non-negligable errors.

  The calculations are fairly straightforward, following Eq. 1.5.74 from the Hipparcos catalogue. A (usually small) Doppler factor accounting for light-time effects can also optionally be applied. The effect of this is to multiply the returned vector by a factor of `1/(1-radial_velocity/c)`, as discussed in Eq. 1.2.21 of the Hipparcos catalogue.

  Note that no attempt is made to adjust for solar motion.

  - Parameters:
    - `ra` (floating point): Right Ascension in degrees
    - `dec` (floating point): Declination in degrees
    - `pmra` (floating point): proper motion in RA * cos(dec) in mas/yr
    - `pmdec` (floating point): proper motion in declination in mas/yr
    - `radial_velocity` (floating point): radial velocity in km/s
    - `r_parsec` (floating point): radial distance in parsec
    - `useDoppler` (boolean): whether to apply the Doppler factor to account for light-time effects
  - Return value
    - (array of floating point): 3-element vector giving equatorial velocity components in km/s
  - Examples:
    - `astromUVW(ra, dec, pmra, pmdec, radial_velocity, dist, true)`
    - `icrsToGal(astromUVW(ra, dec, pmra, pmdec, radial_velocity, 1000./parallax, false))`
`epochProp( tYr, astrom6 )`
  Propagates the astrometry parameters, supplied as a 6-element array, to a different epoch.

  The input and output astrometry parameters are each represented by a 6-element array, with the following elements:

       index  gaia_source name  unit    description
       -----  ----------------  ----    -----------
         0:   ra                deg     right ascension
         1:   dec               deg     declination
         2:   parallax          mas     parallax
         3:   pmra              mas/yr  proper motion in ra * cos(dec)
         4:   pmdec             mas/yr  proper motion in dec
         5:   radial_velocity   km/s    barycentric radial velocity

  The units used by this function are the units used in the `gaia_source` table.

  Null values for `parallax`, `pmra`, `pmdec` and `radial_velocity` are treated as if zero for the purposes of propagation. The documentation of the equivalent function in the Gaia archive comments "This is a reasonable choice for most stars because those quantities would be either small (parallax and proper motion) or irrelevant (radial velocity). However, this is not true for stars very close to the Sun, where appropriate values need to be taken from the literature (e.g. average velocity field in the solar neighbourhood)."

  The effect is that the output represents the best estimates available for propagated astrometry; proper motions, parallax and RV are applied if present, but if not the output values are calculated or simply copied across as if those quantities were zero.

  - Parameters:
    - `tYr` (floating point): epoch difference in years
    - `astrom6` (array of floating point): astrometry at time t0, represented by a 6-element array as above (a 5-element array is also permitted where radial velocity is zero or unknown)
  - Return value
    - (array of floating point): astrometry at time `t0+tYr`, represented by a 6-element array as above
  - Example:
    - `epochProp(-15.5, array(ra,dec,parallax,pmra,pmdec,radial_velocity))` - calculates the astrometry at 2000.0 of gaia_source values that were observed at 2015.5
`epochPropErr( tYr, astrom22 )`
  Propagates the astrometry parameters and their associated errors and correlations, supplied as a 22-element array, to a different epoch.

  The input and output astrometry parameters with associated error and correlation information are each represented by a 22-element array, with the following elements:

       index  gaia_source name      unit    description
       -----  ----------------      ----    -----------
         0:   ra                    deg     right ascension
         1:   dec                   deg     declination
         2:   parallax              mas     parallax
         3:   pmra                  mas/yr  proper motion in RA * cos(dec)
         4:   pmdec                 mas/yr  proper motion in Declination
         5:   radial_velocity       km/s    barycentric radial velocity
         6:   ra_error              mas     error in right ascension
         7:   dec_error             mas     error in declination
         8:   parallax_error        mas     error in parallax
         9:   pmra_error            mas/yr  error in RA proper motion * cos(dec)
        10:   pmdec_error           mas/yr  error in Declination proper motion
        11:   radial_velocity_error km/s    error in barycentric radial velocity
        12:   ra_dec_corr                   correlation between ra and dec
        13:   ra_parallax_corr              correlation between ra and parallax
        14:   ra_pmra_corr                  correlation between ra and pmra
        15:   ra_pmdec_corr                 correlation between ra and pmdec
        16:   dec_parallax_corr             correlation between dec and parallax
        17:   dec_pmra_corr                 correlation between dec and pmra
        18:   dec_pmdec_corr                correlation between dec and pmdec
        19:   parallax_pmra_corr            correlation between parallax and pmra
        20:   parallax_pmdec_corr           correlation between parallax and pmdec
        21:   pmra_pmdec_corr               correlation between pmra and pmdec

  Note the correlation coefficients, always in the range -1..1, are dimensionless.

  Null values for `parallax`, `pmra`, `pmdec` and `radial_velocity` are treated as if zero for the purposes of propagation. The documentation of the equivalent function in the Gaia archive comments "This is a reasonable choice for most stars because those quantities would be either small (parallax and proper motion) or irrelevant (radial velocity). However, this is not true for stars very close to the Sun, where appropriate values need to be taken from the literature (e.g. average velocity field in the solar neighbourhood)."

  The effect is that the output represents the best estimates available for propagated astrometry; proper motions, parallax and RV are applied if present, but if not the output values are calculated or simply copied across as if those quantities were zero.

  This transformation is only applicable for radial velocities determined independently of the astrometry, such as those obtained with a spectrometer. It is not applicable for the back-transformation of data already propagated to another epoch.

  This is clearly an unwieldy function to invoke, but if you are using it with the gaia_source catalogue itself, or other similar catalogues with the same column names and units, you can invoke it by just copying and pasting the example shown in this documentation.

  - Parameters:
    - `tYr` (floating point): epoch difference in years
    - `astrom22` (array of floating point): astrometry at time t0, represented by a 22-element array as above
  - Return value
    - (array of floating point): astrometry at time t0+tYr, represented by a 22-element array as above
  - Example:
    - `epochPropErr(-15.5, array( ra,dec,parallax,pmra,pmdec,radial_velocity, ra_error,dec_error,parallax_error,pmra_error,pmdec_error,radial_velocity_error, ra_dec_corr,ra_parallax_corr,ra_pmra_corr,ra_pmdec_corr, dec_parallax_corr,dec_pmra_corr,dec_pmdec_corr, parallax_pmra_corr,parallax_pmdec_corr, pmra_pmdec_corr))` - calculates the astrometry with all errors and correlations at 2000.0 for gaia_source values that were observed at 2015.5.
`rvMasyrToKms( rvMasyr, plxMas )`
  Converts from normalised radial velocity in mas/year to unnormalised radial velocity in km/s.

  The output is calculated as `AU_YRKMS * rvMasyr / plxMas`, where `AU_YRKMS=4.740470446` is one Astronomical Unit in km.yr/sec.

  - Parameters:
    - `rvMasyr` (floating point): normalised radial velocity, in mas/year
    - `plxMas` (floating point): parallax in mas
  - Return value
    - (floating point): radial velocity in km/s
`rvKmsToMasyr( rvKms, plxMas )`
  Converts from unnormalised radial velocity in km/s to normalised radial velocity in mas/year.

  The output is calculated as `rvKms * plxMas / AU_YRKMS`, where `AU_YRKMS=4.740470446` is one Astronomical Unit in km.yr/sec.

  - Parameters:
    - `rvKms` (floating point): unnormalised radial velocity, in mas/year
    - `plxMas` (floating point): parallax in mas
  - Return value
    - (floating point): radial velocity in mas/year
`distanceEstimateEdsd( plxMas, plxErrorMas, lPc )`
  Best estimate of distance using the Exponentially Decreasing Space Density prior. This estimate is provided by the mode of the PDF.

  - Parameters:
    - `plxMas` (floating point): parallax in mas
    - `plxErrorMas` (floating point): parallax error in mas
    - `lPc` (floating point): length scale in parsec
  - Return value
    - (floating point): best distance estimate in parsec
`distanceBoundsEdsd( plxMas, plxErrorMas, lPc )`
  Calculates the 5th and 95th percentile confidence intervals on the distance estimate using the Exponentially Decreasing Space Density prior.

  Note this function has to numerically integrate the PDF to determine quantile values, so it is relatively slow.

  - Parameters:
    - `plxMas` (floating point): parallax in mas
    - `plxErrorMas` (floating point): parallax error in mas
    - `lPc` (floating point): length scale in parsec
  - Return value
    - (array of floating point): 2-element array giving the 5th and 95th percentiles in parsec of the EDSD distance PDF
`distanceQuantilesEdsd( plxMas, plxErrorMas, lPc, qpoints, ... )`
  Calculates arbitrary quantiles for the distance estimate using the Exponentially Decreasing Space Density prior.

  Note this function has to numerically integrate the PDF to determine quantile values, so it is relatively slow.

  - Parameters:
    - `plxMas` (floating point): parallax in mas
    - `plxErrorMas` (floating point): parallax error in mas
    - `lPc` (floating point): length scale in parsec
    - `qpoints` (floating point, one or more): one or more required quantile cut points, each in the range 0..1
  - Return value
    - (array of floating point): array with one element for each of the supplied `qpoints` giving the corresponding distance in parsec
  - Examples:
    - `distanceQuantilesEdsd(parallax, parallax_error, 1350, 0.5)[0]` calculates the median of the EDSD distance PDF using a length scale of 1.35kpc
    - `distanceQuantilesEdsd(parallax, parallax_error, 3000, 0.01, 0.99)` returns a 2-element array giving the 1st and 99th percentile of the distance estimate using a length scale of 3kpc
`distanceToModulus( distPc )`
  Converts a distance in parsec to a distance modulus. The formula is `5*log10(distPc)-5`.

  - Parameters:
    - `distPc` (floating point): distance in parsec
  - Return value
    - (floating point): distance modulus in magnitudes
`modulusToDistance( distmod )`
  Converts a distance modulus to a distance in parsec. The formula is `10^(1+distmod/5)`.

  - Parameters:
    - `distmod` (floating point): distance modulus in magnitudes
  - Return value
    - (floating point): distance in parsec
`AU_YRKMS`
  This quantity is A_v, the Astronomical Unit expressed in km.yr/sec. See the Hipparcos catalogue (ESA SP-1200) table 1.2.2 and Eq. 1.5.24.

`PC_AU`
  Parsec in Astronomical Units, equal to 648000/PI.

`PC_YRKMS`
  Parsec in units of km.yr/sec.

`C_KMS`
  The speed of light in km/s (exact).

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