Astronomers Discovered First Heavy Metal Hot Subdwarf Star In SB 744 (Planetary Science)

A team of international astronomers reported the discovery of first heavy-metal hot subdwarf composite binary SB 744. Using radial velocity follow-up they revealed that, the hot subdwarf star in SB 744 have a substantial overabundances of lead and fluorine in its atmosphere. Their study recently appeared in Arxiv.

Hot subdwarf stars are 0.5 M core helium burning stars on the extreme horizontal branch (EHB). Unlike normal horizontal branch stars they retain only a very thin hydrogen envelope after their evolution on the red giant branch (RGB). The envelope is lost under unclear circumstances, precisely by the time they ignite helium burning in the core. The precise timing and high binary fraction hints a yet unknown connection between mass loss and binary evolution.

The spectral properties of hot subdwarfs show a nearly continuous sequence from 25000 K B-type subdwarfs (sdB) to the hottest sdO type stars exceeding 40000 K. However, there are parallel sequences well separated by helium abundance, and, along these sequences, multiple groups can be identified. This granulation in parameter space correlates with other properties, such as binarity and most likely indicate different formation pathways that lead to different sub-classes of hot subdwarfs. The spectra of the intermediate helium-rich sub-class show a mix of He i and ii lines (sdOB type) and their location in the parameter space differ from the canonical picture of sdB and sdO stars. A relation between sdB and sdO stars can be outlined by stellar evolution, where sdB stars are the progeny of post-EHB sdO stars. The similar binary properties of sdB and sdO types confirm this evolutionary link. However, the sdOB subclass shows a very low binary fraction, implying a different formation history. This hypothesis got further support from the studies which demonstrated that heavy-metal over-abundances in several sdOB stars. The trans-iron metal abundances reach over 10 000 times the solar values in these stars. Such heavy metals are produced by the s-process during intermediate evolution within the He-shell burning environments of low metallicity stars. However, whether these sdOB stars have extra amounts of heavy-metals or the observed abundances are the result of diffusion, which places metals selectively into a thin photospheric layer, is not yet clear.

SB 744 (also MCT 0146-2651) is a bright (V=12.31 mag) hot subdwarf composite spectrum binary. It was first identified
by Slettebak & Brundage as the 744th object in their catalog of early type stars near the south Galactic Pole. It is a relatively well studied object, its first quantitative spectral analyses date back to the 1980s. However, the binary nature of the system is still in question.

Fig 1: The orbital period versus the eccentricity and mass ratio for all known composite sdB binaries with solved orbits. The main group is shown in blue filled circles, the secondary group in orange open circles, and SB 744 is highlighted as a red square. The orbital parameters of SB 744 fit with those of the main group. © Nemeth et al.

Now, a team of international astronomers confirmed that SB 744 is a typical composite binary with orbital parameters similar to those of other composite subdwarf binaries. Later, with the help of stellar evolution models, they determined the mass of hot subdwarf and its cool companion which was found to be 0.47 M and 0.72 M, respectively.

In addition, they analyzed the optical spectra with homogeneous atmospheric models to derive surface parameters of the binary members from a direct wavelength space decomposition and independently measured the atmospheric properties of the cool companion.

“While lead has been observed in a dozen of sdOB stars, SB 744 is a first such star that shows a measurable Fluorine abundance”

— they said

They found that, it is an old, population II system, that has gone through dramatic events. The hot subdwarf star in SB 744 belongs to the heavy-metal subclass of sdOB stars and have a substantial overabundances of lead and fluorine in its atmosphere. But, it also has the iron and helium abundance lowest among the known heavy-metal sdOB stars.

“The companion does not show extra overabundances, and it seems like a normal low metallicity MS star.”

— they said.

The presence of fluorine implies that SB 744 was once a hierarchical triple system and the inner binary has merged in the near past. As an alternative scenario, single-star evolution through late core helium flash and atmospheric mixing can also produce the observed fluorine abundances. The atmospheric metal over-abundances currently observed are perhaps the results of a combination of mixing processes during formation and radiative support.

“The mass of the sdOB is consistent with the canonical mass and is likely formed from a low-mass progenitor, or a merger.”

— they said.

Moreover, by using the GALPY software package they calculated the Galactic orbit of SB 744 and found that it is a Halo object, like all other heavy-metal hot subdwarfs discovered to date.

Fig 2: The Galactic orbit of SB 744 projected onto the Galactic plane. The trajectory over a time of 2 Gyr and the current position (red dot) is shown. © Németh et al.

Finally, SB 744 demonstrated that heavy-metal overabundances occur in metal-poor stars, where efficient atomic diffusion can support the Fluorine and Lead abundance in thin photospheric layers.

“The period of atmospheric overabundances are probably temporary, short lived phases compared to stellar evolution. This explains why not all similar stars show heavy-metal over-abundances, which are most likely in connection with mixing events following a He-shell subflash.”

— they added.

“This study will take the investigations of heavy-metal subdwarfs to a new level and implies that more such subdwarfs may be hidden in composite spectrum binaries.”, they concluded.

Featured image credit: Getty Images


Reference: P. Németh, J. Vos, F. Molina, and A. Bastian, “The first heavy-metal hot subdwarf composite binary SB 744”, will appear in Astronomy and Astrophysics, preprint:
arXiv:2107.03270


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