A new study has accurately estimated the abundance of Helium in our Sun for the first time. This could be a major step in assessing the opacity of the Sun’s photosphere.

Astronomers have traditionally assumed the abundance of Helium in the photosphere of Sun-like stars to be one tenth of that of Hydrogen by extrapolating from hotter stars, or from the outer atmosphere of the Sun (solar corona, solar wind), or from seismology studies of the interior of the Sun. None of these methods are based on direct observations of the photosphere due to the absence of Helium spectral lines.

An accurate and reliable measurement of the abundance of the element Helium in the photosphere of our Sun remains a challenge for astronomers to this day. The abundance of various elements in our Sun, or in any other star, is estimated from their absorption spectral lines. Since Helium does not produce any observable spectral lines from the visible surface, or the photosphere, of the Sun, its abundance has usually been estimated through indirect means.

Indian Institute of Astrophysics (IIA), an autonomous institute of the Department of Science and Technology (DST), has used Magnesium and Carbon features in the observed high-resolution spectrum of the Sun to accurately calculate the abundance of Helium in our Sun, in a recent study. This study published as a paper in “Astrophysical Journal, has been carried out by Satyajeet Moharana, B.P. Hema, and Gajendra Pandey, all from the Indian Institute of Astrophysics, based on an earlier novel method developed by the latter two authors. Moharana is also a student at IISER Berhampur.

“Using a novel and consistent technique, whereby the spectral lines of neutral Magnesium and Carbon atoms in conjunction with the lines from the Hydrogenated molecules of these two elements are carefully modelled, we are able to constrain the relative abundance of Helium in the Sun’s photosphere now”, said Satyajeet Moharana, the first author of the published study and currently a PhD scholar at KASI, South Korea.

Fig: Abundance of carbon (from CI, CH and C2 lines) and magnesium (from Mg I and MgH lines) for different Helium/Hydrogen ratios.

 

“We analysed the lines of neutral Magnesium and the subordinate lines of MgH molecule, and the neutral Carbon and the subordinate lines of CH and C2 molecules, from the photospheric spectrum of the Sun”, said B.P. Hema. This was done by a careful calculation of the various parameters involved in the formation of the spectral lines. They then subjected the data to Equivalent Width analyses and spectrum syntheses.

“The abundance of Magnesium derived from its neutral atomic line must necessarily agree with the abundance derived from its hydrogenated molecular line”, she explained. Similarly, the abundance of Carbon derived from its neutral atomic line must agree with that derived from its molecular lines. The estimate of the abundance of these two elements from each of their lines depends, in turn, on the abundance of Hydrogen. Since Helium is the second most abundant element in the Sun after Hydrogen, the abundance of Helium is linked to the abundance of Hydrogen. This is the basic principle of this method.

“For example,”, explains Moharana, “if Helium was assumed to be slightly more abundant, this would proportionately decrease the abundance of Hydrogen, which will decrease the opacity of the Sun’s photosphere and decrease the availability of Hydrogen to form molecules with Magnesium and Carbon”. For a metal hydride (e.g. MgH or CH) line, a combined effect of the reduced continuum absorption and the line’s reduced absorption strength demands an increased metal abundance to fit the same observed line strength.

“In our analysis, we calculated the expected abundance of Mg and C for various values of the relative abundance of Helium to Hydrogen, from the atomic and molecular lines”, said Gajendra Pandey. For the Mg and C abundances to match their respective atomic and molecular features, the Helium to Hydrogen ratio that we infer are consistent with a value of 0.1.  

“Our derived He/H ratios are in fair agreement with the results obtained through various helioseismological studies, signifying the reliability and accuracy of our novel technique in determining the solar helium-to-hydrogen ratio. This study also confirms that the widely assumed and adopted (He/H) ratio of 0.1 is in fair agreement with our measurements.”, said B.P. Hema.

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