CU5-DU13: Ground-based support observations for Gaia absolute photometric calibration

BOLOGNA: C. Cacciari, G. Altavilla, M. Bellazzini, A. Bragaglia, E. Diolaiti, L. Federici, F. Fusi Pecci, P. Montegriffo, E. Pancino, E. Rossetti

BARCELONA: C. Jordi, G. Anglada, J.M. Carrasco, F. Figueras

GRONINGEN: S. Trager [A. Helmi, E. Tolstoy]

Preliminary assessment - 1

Definition of requirements (see GAIA-C5-TN-OABO-MBZ-001-1): general description of criteria for

● selection of SpectroPhotometric Standard Stars (SPSS) – number, spectral types, magnitude range, spatial distribution

● selection of observing modes – spectrophotometry, spectroscopy & photometry, spectral resolution, etc.

Definition of observing protocol (draft, in preparation by OABO)

Definition of tests for pilot program (draft, in preparation by OABO)

Definition & construction of database for SPSS obs. (draft, in preparation by OABO)

Preliminary assessment - 2

Simulations by the Barcelona team (see GAIA-C5-TN-UB-JMC-039-1, draft): S/N estimates for BP/RP spectra and crowding due to FoV overlapping vs.

● magnitude ● interstellar extinction ● n. of observations ● calibration errors

quantitative estimate of limiting magnitude, position on the sky and spectral type of candidate SPSS

aiming at S/N > 100 over entire spectral range for both BP and RP

Preliminary assessment - 3

Total n. of uncrowded transits i.e. observations at the end of the mission as a function of position in the sky (equatorial coordinates)

Average n. of

observations on the full sky is 80

Preliminary assessment - 4

end of mission SNR for B5, A0, F0, G2, K5 stars with V=10, 13, 16. Assumed zero reddening, 80 observations and calibration error =10 mmag

Work in progress: SPSS selection - 1

Evaluation and selection criteria for SPSS (see GAIA-C5-TN-OABO-MBZ-001-1):

● as featurless as possible (WDs, hot sd, very metal-poor) ● need for spectral type coverage metal-poor red stars ● initial estimate of magnitude range V=10-16 ● position on the sky: for a mean value of 80 transits stars at V=16 do not reach S/N=100 in the bluest part of BP and RP

(exact fraction depending on spectral type) stars at V=13 require more than the average 80 transits to reach

S/N=100 in the bluest range for stars brighter than V=13 no requirements on the number of transits are

needed

Need to restrict magnitude range to V=10-14

Work in progress: SPSS selection - 2 Construction of database for Gaia SPSS:

Scan of literature HST CalSpec primary SPSS ● based on 3 pillar stars + best existing Vega cal. (~ 2% vis &

4% ir) [Bohlin & Gilliland 2004] ● entire wavelength coverage ● 1% internal photometric accuracy ● incomplete spectral type coverage (few red stars) ● few stars with high enough S/N in BP & RP (mag. + sky

location)

Altogether, including other suitable sources in addition to HST CalSpec, < 40 stars are candidate primary SPSS

Need to expand with additional secondary SPSS to reach ~ 100 stars (exact number TBD based on simulations & pilot program)

Work in progress: ground-based observations -1

The observing campaign: Both primary and secondary SPSS need to be (re)observed from the ground to ensure homogeneous treatment (spectral coverage, resolution, telluric lines removal, data reduction, etc.) and same zero-point of the flux scale

The observing criteria (to match BP/RP characteristics):

Spectrophotometry: ideal, but difficult (ph. conditions are rare) Spectroscopy & Photometry ● Spectral coverage 330-1050 nm, possibly in no more than 2 steps

with partial overlap ● Spectral resolving power R ~ 500-1000 ● Aiming at S/N=100 per res. element over the entire range ● Use of standard (Johnson UBV + Cousins RI) filters ● Aiming at 1% accuracy in flux

Work in progress: ground-based observations -2

The observing campaign needs to be preceded by a pilot program

● to perform all necessary tests to ensure feasibility and homogeneity of procedures and quality of the final database

● to start building the database of primary SPSS

Test Methods: ● calibration (e.g. SP vs S&P) ● observing (strategies, e.g. are all UBVRI needed?) ● data reduction (e.g. essential calibration frames)

Test Quality: ● night, run, telescope, instrument (e.g. systematics) ● site (e.g. photometric conditions, extinction curve)

Work in progress: ground-based observations -3 The pilot program

Evaluation of sites/telescopes/instruments (see GAIA-C5-TN-OABO-LF-001-1, draft)

● Calar Alto: CAFOS@CAHA ● Canary Islands: ISIS@WHT; IDS@INT; ALFOSC@NOT;

LRS@TNG ● ESO: EMMI@NTT; FORS2@UT1

► LRS@TNG, CAFOS@CAHA and FORS2@UT1 are ranked highest for pilot program, ranking may change after pilot program results

At least three facilities need to be used for the observing campaign (for risk management policy)

Work in progress: ground-based observations - 4

Observing proposal for pilot program ready for submission at Calar Alto (deadline Sept. 15)

Observing proposal for pilot program in preparation for submission at TNG (deadline around Oct. 15)

Possible proposal for ESO considered for spring 2007 deadline (depending on coordination with other CUs, TBD)