The accuracy of Giraffe measurements of radial velocity in young clusters Richard Jackson – Keele University in collaboration with Rob Jeffries and Amy Dobson - Keele Jim Lewis and Sergey Koposov - Casu

Lithium rich ? One distribution or two? Gamma Velorum cluster RV (km/s) Frequency To study cluster kinematic substructure We need to know (1)Typical uncertainty in RV (using MAD) (2)Tail of the uncertainty distribution in RV (3) Any bias in RV with SNR and/or T eff The accuracy of Giraffe measurements of radial velocity in young clusters Jeffries, Jackson, Cottaar et al. 2013

Measured uncertainty of repeat observations Spectra vs SNR and vsiniDerive a normalised uncertainty -independent of SNR and vsini Empirical “Poisson” uncertainty in RV - for short term repeats

Normalised uncertainty Measured uncertainty of repeat observations Spectra vs SNR and vsiniReduce to normalised uncertainty independent of SNR and vsini Empirical “Poisson” uncertainty in RV - for short term repeats

8463 repeats in 8 clusters Gama2Vel Cha_I rho_oph NGC2264 NGC2547 NGC2516 NGC6633 IC field stars (Corot sample) av =5.7 Fix B av =5.7 Empirical “Poisson” uncertainty in RV - for short term repeats Uncertainty normalised to where B varies with T eff and B/C 2 constant Fix C = 26.5  RV/  2 x SNR logT eff  RV/  2 x SNR/(1+vsini 2/C 2) Find B(T eff )  RV/  2 x SNR/(1+vsini 2 /C 2 )

Uncertainty in wavelength calibration - for long term repeats Change day 1 to 2 plate 1 Change day 1 to 2 plate 2 Change day 2 to 3 plate 1 Change day 2 to 3 plate 2 Simcal 0.22 & 0.23 Uncertainty 0.08 & 0.09 Simcal & Uncertainty 0.15 & 0.08

Uncertainty in wavelength calibration - for long term repeats Change day 1 to 2 plate 1 Change day 1 to 2 plate 2 Change day 2 to 3 plate 1 Change day 2 to 3 plate 2 Simcal 0.22 & 0.23 Uncertainty 0.08 & 0.09 Simcal & Uncertainty 0.15 & 0.08 SIMCAL offset in wavelength scale varies through night - In similar way for all filters. Appears to be a “mechanical offset” - independent of SNR & vsini

Total uncertainty in RV - for long term repeats between OBs Normalised uncertainty Wavelength term Poisson term Poisson uncertainty (2 spectra per OB) Cumulative probability Normalised uncertainty Total uncertainty between OBs (2047 repeats) A = 0.28 km/s B = 5.70 km/s C = 26.5 km/s

Total uncertainty in RV - for long term repeats between OBs Normalised uncertainty Wavelength term Poisson term Poisson uncertainty (2 spectra per OB) Cumulative probability Normalised uncertainty Total uncertainty between OBs 2047 repeats A = 0.28 km/s B = 5.70 km/s C = 26.5 km/s  RV / (normalisation factor) Fraction of population Tail of empirical uncertainty between OBs Tail of Poisson uncertainty

Total uncertainty in RV - for long term repeats between OBs Normalised uncertainty Wavelength term Poisson term Poisson uncertainty (2 spectra per OB) Cumulative probability Normalised uncertainty Total uncertainty between OBs 2047 repeats A = 0.28 km/s B = 5.70 km/s C = 26.5 km/s Comparison empirical and Velclass uncertainties -Velclass uncertainties are ~  2 higher.

Absolute accuracy of RVs observed in HR10, HR15N and HR21 23 standards RV of standards from Soubiran et al rms uncertainty 0.04km/s Standard stars

Absolute accuracy of RVs observed in HR10, HR15N and HR21 23 standards RV of standards from Soubiran et al rms uncertainty 0.04km/s Standard stars Field stars  RV = 0.32 km/s  RV = 0.09 km/s Data set GE_SD_CR ~1500 Corot targets

Variation in absolute RV between HR10 and HR21 filters Apparent bias in Velclass RVs (for HR21) a function of SNR and/or T eff ?

Possible causes of difference in RV measured by Velclass and CCF method CCF method Velclass method Measured difference in RV Differences in templates used Target HIP (Spt K2) RV template hi-res synthetic (COELHO 2005)

1.Comparison of repeat measurements of RV in young clusters shows that the measurement uncertainty can be normalised to a simple function of SNR and vsini with a weaker dependence stellar properties. 2.The measurement uncertainty at higher SNR is dominated by a fixed uncertainty of ~0.28km/s due to changes in wavelength calibration between setups. 3.Analysis of RVs of standard stars shows a bias of ~0.4km/s for measurements made using HR21 (but no significant bias for HR10 and HR15N). The most likely cause is a mismatch between target spectra and synthetic RV templates. 4.Where possible stellar properties used to select RV templates should be fixed for each target - based on the best available estimates of T eff, logG and Fe/H. 5.Following the planned change to GAIA/Phoenix synthetic spectra revised RVs should be re-analysed for evidence of bias with T eff and/or SNR in all filters. The accuracy of Giraffe measurements of radial velocity in young clusters