Milky Way: Riddles of scandium lines solved?

Strange surplus of three metals was obviously based on optical illusion

Milky Way Center recorded in the infrared - is there another star chemistry there? © NASA / JPL-Caltech / S. Stolovy (SSC / Caltech)
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Mysterious spectral signature: In the center of the Milky Way, some stars seem to contain inexplicably scandium, vanadium, and yttrium-anomalous spectral signatures of these elements. But now supplementary observations reveal that the supposed excess of scandium is based on an optical illusion - a disruptive effect that leads to thickened lines of these elements in cool stars, as the astronomers report. Why this is so, but remains unclear for the time being.

Although the Milky Way is our home galaxy, we do not know all their secrets. Especially her center, which is covered by dense clouds of dust, has only been explored in parts so far. Although it has been known for some time that there are many old stars there. But the discovery of gigantic gas bubbles, a strange x-structure of stars and an excess of gamma radiation in the Milky Way Center also surprised astronomers.

Exotic chemistry in the Milky Way Center?

Astronomers discovered another peculiarity last year: spectroscopic analysis of some red giants around the galactic center revealed unusually thick and strong lines for the elements scandium, vanadium, and yttrium. "In some cases, the analyzes yielded ten times as many of these elements as in our sun, " explain Brian Thorsbro of Lund University and his colleagues.

But these spectral signatures contradicted what models predict for chemistry and physics in red giants. "If the values ​​are confirmed, that would be a chemical signature for the specific environment of the galactic center - and potentially very significant, " say the astronomers. Because that could mean that stars generate more of these metals than anywhere else in the galaxy, and that the formation of these atoms in their interior may be different than thought.

Spectral lines of scandium in the visible range. The stellar anomalies have been measured by astronomers in the near infrared. public domain

What is behind it?

So the big question is whether this ridiculous scandium excess is real, or maybe it's just a series of effects. "In addition to the frequency of an element, other parameters also influence the strength of a line in the spectrum, " explain the researchers. Especially with near-infrared wavelengths, the excitation of the atoms and thus the temperature also play a role. display

Could this be behind the anomalously strong Scandium and Co signatures? To verify this, Thorsbro and his team have determined the elemental spectra and temperatures of seven near-solar red giants using a spectrometer at the Keck Observatory in Hawaii. These values ​​compared with those of the already known from the Milky Way Center.

Temperature instead of position

The Surprising Result: Some of the near-sun red giants showed as striking a scandium excess as their "counterparts" in the galactic center. Instead of the position of the star giants their temperature seemed to be the decisive factor: "The diagrams seem to prove that the amount of scandium simply increases with decreasing temperature, " the researchers report.

Interesting only: "There is no astrophysical basis for such a temperature dependency of the amount of scandium in a star, " emphasize Thorsbro and his colleagues. "We can therefore pretty much assume that there is something behind these spectra other than a real scandium excess."

Impact instead of element set

According to astronomers, the unusually pronounced lines of scandium, vanadium and yttrium are a kind of optical illusion a direct impact, which under 3.600 Kelvin star formation temperature may be particularly thick and possibly Superimposed spectral lines leads. "These strong lines are therefore an intrinsic property of the line formation process under these conditions, " says Thorsbro and his colleagues.

How exactly this spectral delusion comes about and why, however, is still unclear to astronomers. "These lines should therefore not be used to estimate the amount of scandium in cool stars until we understand better how these lines are formed, " they state. (The Astrophysical Journal, 2018; doi: 10.3847 / 1538-4357 / aadb97)

(Lund University, 11.10.2018 - NPO)