The accuracy of Giraffe measurements of radial velocity in young clusters Richard Jackson – Keele...

The accuracy of Giraffe measurements ofradial 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 distributionor two?

Gamma Velorum cluster

RV (km/s)

Fre

quen

cy

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 Teff


Jeffries, Jackson, Cottaar et al. 2013

Measured uncertaintyof repeat observations

2/RV

Spectra vs SNR and vsini

0

0.1

0.2

0.3

0.4

-4 -2 0 2 4

Normalised uncertainty

Gaussian

Gaussianwith tail

Derive a normalised uncertainty -independent of SNR and vsini

Empirical “Poisson” uncertainty in RV - for short term repeats

SNR/])C/sin(1[B

2/2iv

RV


Measured uncertaintyof repeat observations

2/RV

Spectra vs SNR and vsini

sigma = 2.6 (1 + (vsini/46)^2)/SNR

0

0.5

1

1.5

0.00 0.05 0.10 0.15 0.201/SNR

SIG

MA

_R

Vvsini=0 km/s

vsini=20 km/s

vsini=50 km/s

vsini=80 km/s

HR15N

0

0.1

0.2

0.3

0.4

-4 -2 0 2 4


Gaussian

Gaussianwith tail

Reduce to normalised uncertaintyindependent of SNR and vsini


8463 repeats

in 8 clusters

Gama2Vel

Cha_I

rho_oph

NGC2264

NGC2547

NGC2516

NGC6633

IC4665

+ field stars

(Corot sample)

Fix Bavav=5.7=5.7


SNR/])C/sin(1[B 2iv

Uncertainty normalised to

where B varies with Teff

and B/C2 constant

Fix C = 26.5

RV

/2

x S

NR

logTeff

RV

/2

x S

NR

/(1

+vs

ini2/

C2

)

Find B(Teff)

RV

/2

x S

NR

/(1

+vs

ini2/C

2)

Uncertainty in wavelength calibration - for long term repeats

-0.6

0.0

0.6

0 20 40 60 80 100 120

Spectrum number across image (NSPEC_OLD)

Change in

wave

length

calib

ratio

n (

km/s

)

Change day 1 to 2 plate 1 Change day 1 to 2 plate 2


Simcal 0.22 & 0.23

Uncertainty 0.08 & 0.09

Simcal -0.10 & -0.17


Uncertainty in wavelength calibration - for long term repeats

-0.6

0.0

0.6

0 20 40 60 80 100 120

Spectrum number across image (NSPEC_OLD)

Change in

wave

length

calib

ratio

n (

km/s

)



Simcal 0.22 & 0.23


Simcal -0.10 & -0.17


-1

-0.5

0

0.5

1

-0.25 1.75 3.75 5.75 7.75 9.75 11.75 13.75

SIM

CA

L (

km

/s)

HR 15N Plate 2 HR10 plate 2

HR15N plate 1 HR10 Plate 1

Other filters

1 day

SIMCAL offset in wavelengthscale 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


22222 SNR/])C/sin(1[BA

2/

iv

RV

Wavelength term

Poisson term

Poisson uncertainty(2 spectra per OB)

Cu

mu

lativ

e p

rob

abi

lity


Total uncertaintybetween OBs(2047 repeats)

A = 0.28 km/s

B = 5.70 km/s

C = 26.5 km/s




2/

iv

RV

Wavelength term

Poisson term


Cu

mu

lativ

e p

rob

abi

lity


Total uncertaintybetween OBs2047 repeats

A = 0.28 km/s

B = 5.70 km/s

C = 26.5 km/s

RV / (normalisation factor)

Fra

ctio

n o

f po

pula

tion

Tail of empiricaluncertaintybetween OBs

Tail of Poissonuncertainty




2/

iv

RV

Wavelength term

Poisson term


Cu

mu

lativ

e p

rob

abi

lity


Total uncertaintybetween OBs2047 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

HIP 050139HIP 106147HIP 000616HIP 026973HIP 029295HIP 077348HIP 026973HIP 108065HIP 031415HIP 032045HIP 038747HIP 017147HIP 032103HIP 045283HIP 104318HIP 051007HIP 065859HIP 105439HIP 066032HIP 020616HIP 093373HIP 005176HIP 038747

23 standards

RV of standards from Soubiran et al. 2013rms uncertainty 0.04km/s

-0.2

0.0

0.2

0.4

0.6

HR10 HR15 HR21

Filter

RV

- R

V_

cata

log

ue

Velclass

Casu CCF

Standard stars

Absolute accuracy of RVs observed in HR10, HR15N and HR21

HIP 050139HIP 106147HIP 000616HIP 026973HIP 029295HIP 077348HIP 026973HIP 108065HIP 031415HIP 032045HIP 038747HIP 017147HIP 032103HIP 045283HIP 104318HIP 051007HIP 065859HIP 105439HIP 066032HIP 020616HIP 093373HIP 005176HIP 038747

23 standards

RV of standards from Soubiran et al. 2013rms uncertainty 0.04km/s

-0.2

0.0

0.2

0.4

0.6

HR10 HR15 HR21

Filter

RV

- R

V_

cata

log

ue

Velclass

Casu CCF

Velclass

-2

-1

0

1

2

0 50 100

SNR

delta

RV

HR10 - HR21

Standard stars Field stars

RV = 0.32 km/s

Casu CCF

-2

-1

0

1

2

0 50 100

SNR

de

lta R

V

HR10 - HR21

RV = 0.09 km/s

Data set GE_SD_CR~1500 Corot targets

Velclass

-2

-1

0

1

2

3.5 3.6 3.6 3.7 3.7 3.8 3.8

logTeff

RV

HR

10 -

HR

21 (

km/s

)

field stars Standards

Casu CCF

-2

-1

0

1

2

3.5 3.6 3.7 3.8

logTeff

RV

HR

10

- H

R2

1(k

m/s

)


Casu CCF

-2

-1

0

1

2

0 200 400 600

SNR

RV

HR

10

- H

R2

1 (

km/s

)

Field stars Standards

Variation in absolute RV between HR10 and HR21 filters

Apparent bias in Velclass RVs (for HR21) a function of SNR and/or Teff?

Velclass

-2

-1

0

1

2

0 200 400 600

SNR

RV

HR

10

- H

R2

1 (

km/s

)


HIP066032 (~5000K)

4.0

4.2

4.4

4.6

4.8

0 100SNR

RV

(km

/s)

HR10 HR15N

HR21 Standard

Possible causes of difference in RV measured by Velclass and CCF method

CCFmethod

RV

CC

F

Velclassmethod

RV

CH

I-S

QR

Measured difference in RV Differences in templates used

0.0

0.2

0.4

0.6

0.8

1.0

0.4 0.9 1.4

Colour (B-V)

RV

_velc

lass

- R

V_ccf (k

m/s

)

3500

4500

5500

6500

3500 4500 5500 6500

Teff Velclass

Te

ff C

AS

U-C

CF

Target HIP066032 (Spt K2) RV template hi-res synthetic 50-4.0-0.0 (COELHO 2005)

0

2

4

6

0 2 4 6

logG Velclass

log

G C

AS

U-C

CF

HIP066032 (~5000K)

4.0

4.2

4.4

4.6

4.8

0 50 100 150SNR

RV

(km

/s)

HR10 HR15NHR21 Standard

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 Teff, 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 Teff and/or SNR in all filters.


The accuracy of Giraffe measurements of radial velocity in young clusters Richard Jackson – Keele...

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