Abundances of Rarely Detected s-process Elements Derived from the Ultraviolet Spectrum of the s-process-enhanced Metal-poor Star HD 196944

TL;DR

HD 196944, a CEMP-s star, shows a rich heavy-element inventory detectable in the ultraviolet, enabling an expanded census of $Z>30$ elements. The abundances are well reproduced by s-process nucleosynthesis in a low-mass AGB companion diluted by about $1$ dex, with minimal contributions to Ga, Ge, and As and a non-detection of Bi, highlighting NLTE uncertainties for several elements. The UV data extend the heavy-element inventory beyond optical studies and provide robust tests of AGB nucleosynthesis in metal-poor binaries. The work also emphasizes NLTE caveats and points to future UV-capable facilities to apply these methods to larger samples of CEMP-s stars.

Abstract

We present an analysis of the heavy-element abundances of HD 196944, a carbon-enhanced metal-poor (CEMP) star enriched with elements produced by the slow neutron-capture process (s-process). We obtained a new high-resolution ultraviolet (UV) spectrum of this star, the UV-brightest known CEMP-s star, with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. This spectrum extends deeper into the UV (2029 < lambda < 2303 Angstroms) than previous studies of any CEMP-s star. When combined with previous UV and optical analysis, a total of 35 elements heavier than zinc (Z = 30) can be detected in HD 196944, and upper limits are available for nine other heavy elements. The abundances can be well fit by models of s-process nucleosynthesis operating in a low-mass companion star that evolved through the asymptotic giant branch phase and transferred heavy elements to HD 196944. This s-process event did not contribute substantially to the Ga, Ge, or As abundances (31 <= Z <= 33). Our results demonstrate that UV spectroscopy can greatly expand the inventory of heavy elements detectable in CEMP-s stars.

Figures (4)

• Figure 1: Sections of the STIS/E230H spectra of [HD 196944]HD 196944 around lines of interest. The filled squares represent the observed spectrum. The solid blue line represents a synthetic spectrum with the best-fit abundance for each line of interest, and the light-blue bands represent a change in this abundance by a factor of $\pm$ 2 (0.3 dex). Blue lines without this light blue band denote upper limits. The solid black line represents a synthetic spectrum with no contributions from the species of interest.
• Figure 2: Sections of the STIS/E230H spectra of [HD 196944]HD 196944 around lines of interest. Symbols are the same as in Figure \ref{['specplot1']}.
• Figure 3: Sections of the STIS/E230H spectra of [HD 196944]HD 196944 around lines of interest. Symbols are the same as in Figure \ref{['specplot1']}.
• Figure 4: Comparison between the [HD 196944]HD 196944 abundance pattern and model predictions for the s-process operating in an AGB star with initial mass = 3.0 $M_{\odot}$, $Z$ = 0.0001, and partial mixing zone (PMZ) = 0.001 $M_{\odot}$karakas10lugaro12. The Solar s-process pattern from bisterzo11 (for Ga--Br) and bisterzo14 (for Rb--Bi) are shown in orange studded lines for comparison. The comparison is shown in terms of $\log\varepsilon$ abundances (top), [X/Fe] ratios (middle), and differences between the [HD 196944]HD 196944 pattern and the predicted s-process pattern (bottom).