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Frodo WHT

J19271 Blue · ISIS blue arm · 2009-07-15

81.9S/N per pixel
1.72%residual rms
3.37 Åresolution
?spectra combined
sdBclass

The fit

fitTeff [K]log glog n(He)/n(H)Zreduced χ²
three parameters35053 ± 3425.973 ± 0.039-3.42 ± 0.54held 01.19
four parameters37103 ± 8875.973 ± 0.042-3.52 ± 0.460.86 ± 0.251.14

Observed minus model scatters by 1.72% of the continuum over the fitted lines, worst on Hε (2.4%), Hβ (1.8%). That number, not the signal-to-noise, is what says whether the spectrum can carry a fourth parameter.

Correlations in the four-parameter fit

paircorrelation
Teff ↔ Z+0.839
log g ↔ Z+0.151
helium ↔ Z-0.402
Teff ↔ log g+0.115

Core shifts, line by line

Hβ, Hγ and Hδ give +47.2 km/s between them, so the star's radial velocity is -47.2 km/s; they scatter by 9.4 km/s about it.

lineλ0shift [Å][km/s]
Hβ4861.32+0.98+60.4
Hγ4340.46+0.57+39.4
Hδ4101.73+0.57+41.7
Hε3970.07+1.04+78.5
Hζ3889.05+1.20+92.5
Hη3835.38+1.89+147.7beyond any plausible radial velocity
Hθ3797.90+1.89+149.2beyond any plausible radial velocity
Hι3770.63+1.18+93.8
He II 46864685.70-1.22-78.1
He I 44724471.60+6.21+416.3model slid +6.2 Å onto a noise minimum
He I 40264026.29-8.56-637.4model slid -8.6 Å onto a noise minimum
He I 49224921.93+9.14+556.7model slid +9.1 Å onto a noise minimum

Each line is centred independently: the fitter finds its core with a parabola and slides the model there. The sign is the fitter's, opposite to the star's motion. Lines far from the Balmer mean above are not measuring a velocity — near the Balmer jump the series crowds and pulls the apparent core blueward, and a weak line has no core to find at all.

Spectrum

shaded: the intervals the fit used ·  │ a cancelled pixeldrag on either panel to zoom · dbl-click or Esc to reset · arrows to pan

The whole reduced spectrum. Shaded bands are the intervals the fit used; drag on either panel to zoom.

Line by line

residuals

Offset from the line centre in Ångström across, normalised flux up. The strip under each profile is observed minus model for whichever fit the buttons select; they all switch together and share a vertical scale.

0.600.801.00observed3p model4p model-0.050.000.05-500+50Hβ1.4%
0.801.00observed3p model4p model-0.000-500+50Hγ1.6%
0.801.00observed3p model4p model-0.0200.0000.020-500+50Hδ1.3%
0.801.00observed3p model4p model-0.100.000.10-200+20+40Hε0.7%
0.801.00observed3p model4p model-0.000-200+20Hζ0.5%
0.901.00observed3p model4p model-0.050.000.05-200+20Hη0.4%
0.900.951.00observed3p model4p model-0.000-200+20Hθ0.5%
0.951.00observed3p model4p model-0.000-100+10Hι0.3%
1.00observed3p model4p model-0.000-200+20He II 46861.7%
1.00observed3p model4p model-0.0200.0000.020-200+20He I 44720.4%
1.00observed3p model4p model-0.000-200+20He I 40260.1%
1.00observed3p model4p model-0.000-200+20He I 49220.1%

Where it came from

objectJ19271 Blue
telescopeWHT
instrumentISIS blue arm
date2009-07-15
exposure [s]1800.01
airmass1.23344
observerConny Aerts
pixels4040
range [Å]3081.74 6566.65
dispersion [Å/px]0.8628
S/N estimate81.9
FWHM from Ca II K [Å]3.37
Ca II K shift [Å]-0.12
W(He II 4686) [Å]0.123
W(He I 4471) [Å]0.011
source filemeanallW.fits

The resolution is measured from the interstellar Ca II K line, which is not in the star, so its width is the instrumental profile. It is an upper limit: an unresolved interstellar line can still be several velocity components. The same fit gives the wavelength shift, which is the zero point only in so far as the cloud sits near rest.