Menkalinan – spectroscopic binary star

Menkalinan (beta Aurigae) is a spectroscopic eclipsing binary star system. The two components appear as single star to the visual observations, but their orbit plane is almost aligned with our line of sight. As a result, they appear to eclipse each other as they orbit, and the system brightness varies slightly when seen from Earth.

Both components of this system are quite similar to each other. They are A1 type subgiants that contain metallic-lines, so they full spectral type is A1m IV. Each component is about 2.8 times larger than Sun, 2.4 times more massive and about 48 times more luminous. They are quite close to each other – about a fifth of the Sun-Mercury distance. Their shapes have been distorted by gravitational interaction, so the stars are distinctly egg-shaped. This star system is doing full orbit every 47.5 hours and is 81 light years away from us.

Menkalinan spectra

I have made four spectroscopic observations of this system during four subsequent nights. I used 1800 lpmm grating, so the effective resolution was about 5000 and the hydrogen alpha line shape could be determined.

Menkalinan beta Aurigae spectrum
Menkalinan beta Aurigae spectrum

You can immediately notice, that there are few lines split into two – the most prominent is hydrogen alpha line at 6563A. These absorption lines comes from both Menkalinan components. They appear at different wavelengths due to Doppler effect. The line shifted to blue originates in the component that is moving towards us. The red shifted line is from the component that at given moment is moving away from us. The same situation is for smaller lines from silicon and iron.

Menkalinan beta Aurigae spectrum enlarged
Menkalinan beta Aurigae spectrum enlarged

Orbital velocity

At the enlarged spectrum above there is enlarged part of spectrum around hydrogen alpha line. Small insets show the actual position and moving direction of the binary system components at given moments of time. The separation of the split line depressions may be used to calculate the actual difference in the speed of stars movement in the line of sight.

The velocity difference in the line of sight depends on the current position of the components in the orbit. When we know the eclipse phase (basing on photometric observations) we may calculate the maximum speed difference. I assumed that orbits are circular and calculated this speed value. The averaged maximum speed is 215.6 km/s and it is very close to the published value of 219 km/s.

Radial velocity

There are more absorption lines besides hydrogen alpha and marked metallic. But these other lines are not split, because they do not come from the star system, but from Earth atmosphere components, mainly water and oxygen. That lines can be used to precisely calibrate such high resolution spectrum and determine the radial speed of the Menkalinan binary system.

Menkalinan radial velocity measurements
Menkalinan radial velocity measurements

I used three telluric lines to calibrate spectrum and then measured actual position of the hydrogen alpha line. Averaged result was 6563.08 angstrom, and that shift gives 10.5 km/s radial speed. This value needs to be corrected for the Earth and rotational speed and the end result of radial velocity for Menkalinan is -15.9 km/s, that is very close to published -15.75 km/s.

Clear skies!

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