Cesium found in the atmosphere of a white dwarf for the first time

(ORDO NEWS) — An international team of astronomers led by Pierre Chayer of the Space Telescope Research Institute (STScI) has detected cesium (Cs) for the first time in the atmosphere of the hot white dwarf HD 149499B. The discovery is reported in a preprint published on the arXiv website.

HD 149499B is a helium-rich, bright white dwarf with an effective temperature of 49,500 kelvins, about 37 parsecs (120 light-years) away.

Observations of the star were carried out between 2000 and 2006 using the FUSE (Far Ultraviolet Spectroscopic Explorer) orbiting space telescope. Astronomers have identified a dozen absorption lines for Cs IV, Cs V, and Cs VI.

The abundance of caesium-55 in the atmosphere of HD 149449B was determined to be -5.45 (the logarithm of the ratio of cesium to helium), corresponding to a mass fraction of -3.95. This makes cesium the most abundant element heavier than iron observed in this white dwarf.

The discovery of cesium in HD 149449B’s atmosphere is not surprising, the scientists say, as the list of trans-iron elements found in white dwarfs has expanded significantly since the discovery of germanium more than 15 years ago, with cesium coming in at number nineteen.

The content of cesium in HD 149449B is 125 thousand times greater than that of the Sun. Since white dwarfs have strong gravity, there must be a mechanism that prevents this chemical element from settling on the surface of the star.

In a hot white dwarf, this mechanism is radiative levitation, a phenomenon that occurs when atoms of heavy elements absorb photons, pushing them away.

Stellar models predict that trans-iron elements result from the slow capture of neutrons by lighter elements in low-to-intermediate-mass main-sequence stars.

Determining the exact source of cesium is difficult due to diffuse processes such as gravitational settling and radiative levitation that erase the abundance history of the elements in stellar atmospheres.

However, the slow neutron capture hypothesis predicts that the element technetium should be found in white dwarfs.

According to the hypothesis, this element moves from deeper regions of the atmosphere and shows abundance if the acceleration due to radiation is equal to the acceleration of gravity. Technetium eventually disappears from the atmosphere, but its disappearance can be delayed by radiation levitation.

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