Table information for 'gaia.dr2light'

General

Table Description:

This is a “light” version of the full Gaia DR2 gaia_source table, containing the original astrometric and photmetric columns with just enough additional information to let careful researchers notice when data is becomes uncertain and the full error model should be consulted. The full DR2 is available from numerous places in the VO (in particular from the TAP services ivo://uni-heidelberg.de/gaia/tap and ivo://esavo/gaia/tap).

This table also includes a column containing the Renormalized Unit Weight Error RUWE (GAIA-C3-TN-LU-LL-124-01), a robust measure for the consistency of the solution.

On this TAP service, there is the table gdr2dist.main containing distances computed by Bailer-Jones et al (2018AJ....156...58B). If in doubt, use these instead of the parallaxes provided here.

This table is available for ADQL queries and through the TAP endpoint.

Resource Description:

This schema contains data re-published from the official Gaia mirrors (such as ivo://uni-heidelberg.de/gaia/tap) either to support combining its data with local tables (the various Xlite tables) or to make the data more accessible to VO clients (e.g., epoch fluxes).

Other Gaia-related data is found in, among others, the gdr2dist, gdr3mock, gdr3spec, gedr3auto, gedr3dist, gedr3mock, and gedr3spur schemas.

For a list of all services and tables belonging to this table's resource, see Information on resource 'Selections from Gaia Data Release 2 (DR2)'

Citing this table

@MISC{vo:gaia_dr2light,
  year=2018,
  title={Gaia DR2 source catalogue "light"},
  author={{GAIA} Collaboration},
  url={http://dc.zah.uni-heidelberg.de/tableinfo/gaia.dr2light},
  howpublished={{VO} resource provided by the {GAVO} Data Center}
}

Copyright, License, Acknowledgements

If you use public Gaia DR2 data in a paper, please take note of ESAC's guide on how to acknowledge and cite it.

Columns

Sorted alphabetically. [Sort by DB column index]

Columns that are parts of indices are marked like this.

VOResource

VO nerds may sometimes need VOResource XML for this table.

Notes

• Note gof

  This is the “gaussianized chi-square”, which for good fits should approximately follow a normal distribution with zero mean value and unit standard deviation. Values exceeding, say, +3 thus indicate a bad fit to the data.

  This statistic is computed according to the formula

  gof = sqrt(9ν/2) [(χ²/ν)^(1/3) + 2/(9ν) − 1],

  where χ² (astrometric_chi2_al) is the AL chi-square statistic and ν = (astrometric_n_good_obs_al − 5) is the number of degrees of freedom for a source update. Here, 5 is the number of astrometric parameters. Note that only “good” (i.e. not strongly downweighted) observations are included in χ² and ν.

  An alternative indicator of bad fits is the astrometric excess noise. In AGIS the source update deals with bad fits by adding astrometric excess noise to the formal observation noise. This reduces the weight of the observations and inflates the covariance of the estimated astrometric parameters correspondingly. However, the chi-square values used to calculate astrometric_gof_al do not take into account the astrometric excess noise, and astrometric_gof_al can therefore always be used as a goodness-of-fit indicator of the source solution in AGIS.

• Note id

  For the contents of Gaia DR2, the source ID 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 33 - 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

    to 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

  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).
• Note phot

  Note that several systematics have been found for both Gaia G and BP photometry on the level of 10s of mmags. See 2018A&A...616A..17A and 2018A&A...616A...4E for a description of the problem as well as some proposed empirical corrections. For sources brighter than 16.5, see also 2018MNRAS.479L.102C and 2018A&A...617A.138W, where the latter also discusses systematics in BP.

• Note ruwe

  The Renormalized Unit Weight Error (RUWE) is defined in L. Lindegren: Re-normalising the astrometric chi-square in Gaia DR2 (Gaia tech note GAIA-C3-TN-LU-LL-124-01). This is essentially the astrometric_chi2 from the DR2 release, but corrected for its strong dependency on magnitude and colour. For quality cuts, this is the recommended quantity to use. Lindegren suggests using RUWE<1.4 as a quality cut for “good” solutions based on the shape of the distribution.

  The values given here were calculated independently of DPAC and do not use interpolation on the grids provided by Lindegren et al. Thus, minor differences from the “official” RUWE values are expected. As Lindegren et al point out, such differences should not matter in science applications, though.