# Obtaining TESS input catalog (TIC) IDs for Gaia source_ids?

The TESS input catalog (TIC) "uses the GAIA DR2 catalog as a base" according to the catalog's website. The Gaia DR2 database provides source_ids for each observation of a target. I want to get the TIC number for a target based on the Gaia source_id. Is there an accepted way of doing this that I'm missing?

To be more specific, I can get a list of Cepheids from Gaia DR2 using their archive and the ADQL command select * from gaiadr2.vari_cepheid. The first result has the Gaia source_id as 4052922352453886976. How can I find the TIC number for this target star? Is the best way of doing this using the RA and DEC of the Gaia observation and finding the closest match in the TIC? Since the TIC is based on the Gaia DR2, I would assume there would be a direct corresponding target and finding the closest RA and DEC match would be unnecessary.

You can query TIC by Gaia ID. Using the MAST Portal, MAST Catalogs, TESS Input v7, Advanced Search form, I specified:

GAIA ID: 4052922352453886976

and got a single result:

TIC ID (ID):            50559830
RA (ra):                275.945292 (18:23:46.870)
Dec (dec):              -25.672353 (-25:40:20.47)
TESS Mag. (Tmag):       13.579
Version (version):      20180610
UCAC ID (UCAC):         322-168973
2MASS ID (TWOMASS):     18234686-2540204
GAIA ID (GAIA):         4052922352453886976
GAIA G Mag. (GAIAmag):  14.5068


Since the MAST portal currently provides TICv7, the Gaia ID is probably from Gaia DR1. The TIC release notes say Gaia DR2 data are new in TICv8.

9  GAIA  The matched Gaia DR2 identifier has been updated in TICv8. Gaia IDs in TICv8 are included for stars that are found in the Gaia-provided Gaia-2MASS look-up table. These identifiers have been updated by the Gaia team from DR1 to DR2, and may not be consistent between TICv7 and TICv8.

• From the Gaia table description the source_id encodes "...the provenance (data processing centre where it was created)...". Does this mean that each TIC target could correspond to multiple source_ids? That is, can multiple Gaia observations be of the same target? With this, the TIC table may only link a single source_id and so some source_ids may correspond to targets in the TIC table, but may not show up there? – Shiania White May 19 at 17:38
• If the TIC paper and some test queries don't answer that question, try the TESS help desk? – Mike G May 19 at 19:05
• Thank you for the reference. For others reading this, from the TIC Paper, "Gaia IDs in TIC-6 are included for stars which are found in the Gaia-provided Gaia-2MASS look-up table. For TIC stars with more than one associated Gaia magnitude, the ID of the brightest matching Gaia source is provided." So it would seem that multiple Gaia source_ids will correspond to a single TIC ID, but only the brightest will be shown. So if the source_id is not the brightest one, the above method will not work, and RA/DEC will still be needed (unless I'm missing something). – Shiania White May 19 at 22:36