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J1923+3715 · ISIS blue arm · 2010-07-03

81.8S/N per pixel
2.12%residual rms
1.32 Åresolution
?spectra combined
iHe-sdBclass

The fit

fitTeff [K]log glog n(He)/n(H)Zreduced χ²
three parameters23202 ± 1534.917 ± 0.019-0.59 ± 0.02held 01.36
four parameters23371 ± 1434.952 ± 0.023-0.64 ± 0.030.83 ± 0.141.30

Observed minus model scatters by 2.12% of the continuum over the fitted lines, worst on He I 4472 (2.9%), Hζ (2.9%). 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+0.268
log g ↔ Z+0.338
helium ↔ Z-0.364
Teff ↔ log g+0.344

Core shifts, line by line

Hβ, Hγ and Hδ give -8.5 km/s between them, so the star's radial velocity is +8.5 km/s; they scatter by 4.5 km/s about it.

lineλ0shift [Å][km/s]
Hβ4861.32-0.06-3.7
Hγ4340.46-0.21-14.5
Hδ4101.73-0.10-7.3
Hε3970.07+0.33+24.9
Hζ3889.05-0.02-1.5
Hη3835.38-0.14-10.9
Hθ3797.90+0.08+6.3
Hι3770.63-0.01-0.8
Hκ3750.15+0.04+3.2
He II 46864685.70+2.16+138.2beyond any plausible radial velocity
He II 45424541.59-38.00-2508.4model slid -38.0 Å onto a noise minimum
He II 42004199.84-0.42-30.0
He I 44724471.60+0.30+20.1
He I 40264026.29+0.31+23.1
He I 47134713.28-0.06-3.8
He I 49224921.93-0.03-1.8
He I 50165015.68+0.03+1.8

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.

0.501.00observed3p model4p model-0.000-500+50Hβ1.9%
0.501.00observed3p model4p model-0.050.000.05-500Hγ1.9%
0.501.00observed3p model4p model-0.100.000.10-500+50Hδ1.8%
0.501.00observed3p model4p model-0.100.000.10-200+20+40Hε1.7%
0.501.00observed3p model4p model-0.100.000.10-200+20Hζ2.3%
1.00observed3p model4p model-0.050.000.05-200+20Hη2.4%
0.801.00observed3p model4p model-0.050.000.05-200+20Hθ2.3%
0.801.00observed3p model4p model-0.000-100+10Hι0.5%
0.901.00observed3p model4p model-0.0500.0000.050-100+10Hκ0.6%
0.951.00observed3p model4p model-0.000-200+20He II 46861.7%
0.951.001.05observed3p model4p model-0.0500.0000.050-200+20He II 45420.3%
0.951.001.05observed3p model4p model-0.050.000.05-200+20He II 42000.3%
0.600.801.00observed3p model4p model-0.100.000.10-200+20He I 44722.4%
0.600.801.00observed3p model4p model-0.050.000.05-200+20He I 40262.4%
0.801.00observed3p model4p model-0.050.000.05-200+20He I 47131.0%
0.801.00observed3p model4p model-0.000-200+20He I 49222.0%
0.801.00observed3p model4p model-0.050.000.05-200+20He I 50161.1%

Where it came from

objectJ1923+3715
telescopeWHT
instrumentISIS blue arm
date2010-07-03
exposure [s]900.01
airmass1.013781
observerAerts
pixels3900
range [Å]3552.39 5260.52
dispersion [Å/px]0.4381
S/N estimate81.8
FWHM from Ca II K [Å]1.32
Ca II K shift [Å]-0.03
W(He II 4686) [Å]-0.040
W(He I 4471) [Å]2.906
source fileJ1923.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.