Keck HIRES spectroscopy of SkyMapper commissioning survey candidate extremely metal-poor stars

Abstract

We present results from the analysis of high-resolution spectra obtained with the Keck HIRES spectrograph for a sample of 17 candidate extremely metal-poor (EMP) stars originally selected from commissioning data obtained with the SkyMapper telescope. Fourteen of the stars have not been observed previously at high dispersion. Three have [Fe/H] −3.0 while the remainder, with two more metal-rich exceptions, have −3.0 [Fe/H] −2.0 dex. Apart from Fe, we also derive abundances for the elements C, N, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, and Zn, and for n-capture elements Sr, Ba, and Eu. None of the current sample of stars is found to be carbon-rich. In general our chemical abundances follow previous trends found in the literature, although we note that two of the most metal-poor stars show very low [Ba/Fe] (∼ −1.7) coupled with low [Sr/Ba] (∼ −0.3). Such stars are relatively rare in the Galactic halo. One further star, and possibly two others, meet the criteria for classification as a r-I star. This study, together with that of Jacobson et al. (2015), completes the outcomes of the SkyMapper commissioning data survey for EMP stars.

The SkyMapper DR1.1 search for extremely metal-poor stars

“…The majority of these EMP candidates have been observed at high resolution with the MIKE echelle spectrograph on the Magellan Clay 6.5m telescope and the abundance analyses will be presented in a forthcoming paper . When combined with the high-dispersion follow-up of SkyMapper commissioning-era EMP candidates discussed in Jacobson et al (2015) and Marino et al (2019), the results will represent by far the largest sample of consistently selected and analysed element abundance measurements for extremely metal-poor stars. The most metal-poor star discovered in this survey to date is SMSS J160540.18-144323.1 which has [Fe/H] f itter < -4.75 and [C/Fe] likely exceeding +2 dex.…”

“…This 'commissioning-era' survey, which is superceded by the current work, led to the discovery of the most ironpoor star known SMSS J031300.36-670839.3 (usually abbreviated to SMSS J0313-6708), which has [Fe/H] < -6.5 (3D, NLTE) (Keller et al 2014;Bessell et al 2015;Nordlander et al 2017). A total of 139 additional Galactic halo EMP candidates from the commissioning-era survey were also followed-up at high dispersion to provide detailed abundance information: 44 stars were shown to have [Fe/H] -3.0 and 3 have [Fe/H] ≈ -4.0 (Jacobson et al 2015;Marino et al 2019). Full descriptions of the derived abundances and abundance ratios for these stars are given and discussed in Jacobson et al (2015) and Marino et al (2019).…”

“…A total of 139 additional Galactic halo EMP candidates from the commissioning-era survey were also followed-up at high dispersion to provide detailed abundance information: 44 stars were shown to have [Fe/H] -3.0 and 3 have [Fe/H] ≈ -4.0 (Jacobson et al 2015;Marino et al 2019). Full descriptions of the derived abundances and abundance ratios for these stars are given and discussed in Jacobson et al (2015) and Marino et al (2019). SkyMapper commissioning-era photometry was also employed by Howes et al (2016) in a search for EMP stars in the Galactic Bulge.…”

Stellar signatures of inhomogeneous big bang nucleosynthesis

“…In this section, we compare the abundances obtained with AlterBBN at high η to some observed abundances in distant metal-poor stars [46] (see Fig. 3, blue dashed areas).…”

“…4. Comparison of AlterBBN europium abundance (black line, summed over the isotopes) as a function of η with metalpoor halo star data [46] (blue dashed areas), dwarf spheroidal faint galaxy data [47] (green dashed areas), and two sets of rprocess enhanced dwarf galaxy data: Reticulum II [42] (purple areas) and Tucana III [48] (orange areas). We also represent the 1σ spread of the data.…”