Cesium detected in the atmosphere of a hot white dwarf

By analyzing data from the Far Ultraviolet Spectroscopic Explorer (FUSE) telescope, an international team of astronomers has discovered cesium in the atmosphere of a hot white dwarf known as HD 149499B. The discovery, reported Nov. 3 on preprint server arXiv, marks the first time this element has been identified in the atmosphere of a hot white dwarf.

White dwarfs (WD) are stellar cores left behind after a star has exhausted its nuclear fuel. Due to their high gravity, they are known to have pure hydrogen or pure helium atmospheres. However, a small fraction of the WDs show traces of heavier elements.

To date, only 18 trans-iron elements (with an atomic number greater than 28) have been identified so far in the atmospheres of hot white dwarfs. In general, these trans-iron elements were found to be overabundant in WDs.

Now, a group of astronomers led by Pierre Chayer of the Space Telescope Science Institute in Baltimore, Maryland, reports the discovery of cesium (Cs), with atomic number 55, in the atmosphere of HD 149499B – a bright white dwarf rich in helium with an effective temperature of 49,500 K. The detection is the result of observations of this star with FUSE between 2000 and 2006.

“We report the first detection of cesium (Z = 55) in the atmosphere of a white dwarf. About ten Cs absorption lines _IVcs _Vand Cs _VII were identified in the far-ultraviolet spectroscopic explorer spectrum of the He-rich white dwarf HD 149499B,” the researchers wrote in the paper.

A total of thirteen cesium lines were detected in the FUSE spectrum with equivalent widths between 2.3 and 26.9 mÅ. The astronomers added that all these transitions come from low energy levels ranging from tens of thousands to a few tens of thousands of cm.⁻¹.

According to the study authors, the detection of cesium in the atmosphere of HD 149449B is not surprising, given that the list of trans-iron elements identified in the atmospheres of WDs has grown considerably since the detection of germanium in 2005.

The abundance of cesium in the atmosphere of HD 149449B has been calculated to be -5.45 (abundance ratio of cesium to helium), or -3.95 in terms of mass fraction. This result makes cesium the most abundant trans-iron element observed in HD 149499B.

By summarizing the results, the researchers are trying to find the most plausible hypothesis explaining the presence of cesium in the atmosphere of this white dwarf.

“Radiative levitation is the most plausible natural phenomenon to explain its occurrence…Although radiative levitation complicates the interpretation of the source of the trans-iron elements, it allows the accumulation of large abundances, and therefore, the detection of elements that otherwise would not be detected”, conclude the authors of the article.

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