A stellar relic from the early Milky Way

Abstract

The chemical composition of the most metal-deficient stars reflects the composition of the gas from which they formed. These old stars provide crucial clues to the star formation history and the synthesis of chemical elements in the early Universe. They are the local relics of epochs otherwise observable only at very high redshifts 1,2 ; if totally metal-free ("population III") stars could be found, this would allow the direct study of the pristine gas from the Big Bang. Earlier searches for such stars found none with an iron abundance less than 1/10,000 that of the Sun 3,4 , leading to the suggestion 5,6 that low-mass stars could only form from clouds above a critical iron abundance. Here we report the discovery of a low-mass star with an iron abundance as low as 1/200,000 of the solar value. This discovery suggests that population III stars could still exist, that is, that the first generation of stars also contained long-lived low-mass objects. The previous failure to find them may be an observational selection effect.The star HE 0107−5240, at coordinates right ascension R.A.(2000.0) = 01 h 09 m 29.1 s and declination δ = −52 • 24 34 , is a giant star of the Galactic halo population with apparent magnitude B = 15.86. It was found during medium-resolution spectroscopic follow-up observations of candidate metal-poor stars selected from the Hamburg/ESO objective prism survey (HES) 7,8 . This survey, which covers the entire southern high-galactic-latitude sky to an apparent magnitude limit of B ≈ 17.5, extends the total survey volume for metal-poor stars in the Galaxy by almost one order of magnitude compared to the total volume explored by previous spectroscopic surveys.A medium-resolution (δλ ≈ 0.2 nm) spectrum of HE 0107−5240 was obtained by M. Spring Observatory 2.3-m telescope on 12 November 2001. The Ca II K (λ = 393.4 nm) line was barely visible in that spectrum, indicating that the star was likely to be extremely metal deficient. Shortly thereafter, a high-resolution, high signal-to-noise ratio spectrum was obtained with the 8-m Unit Telescope 2 (UT2) of the Very Large Telescope at the European Southern Observatory (ESO), Paranal, Chile. Figure 1: A portion of the spectrum of HE 0107−5240, shown compared to the spectrum of CD −38 • 245, the previously most iron-poor giant star known. Both spectra were obtained with VLT-UT2, and the UltravioletVisual Echelle Spectrograph (UVES). We note the strong molecular CH and C 2 lines and extremely weak lines of Fe I in the spectrum of HE 0107−5240. The spectra used in our analysis have a resolution of R = λ/∆λ = 40, 000, and a signal-to-noise ratio of more than 100 per pixel at λ > 400.0 nm. The covered wavelength ranges are 329.0-452.0 nm, 478.0-576.0 nm, and 583.0-681.0 nm.We derive an effective temperature T eff = 5100 ± 150 K for HE 0107−5240 by means of broad-band visual and infrared photometry. The absence of Fe II lines, through the Fe I/Fe II ionisation equilibrium, constrains the star to have a logarithmic surface gravity of log(g) > 2.0 dex, while main-seq...

An extremely primitive star in the Galactic halo

“…The mass fraction, Z, of elements more massive than helium, is called "metallicity". A number of very metal poor stars have been found 2,3 , some of which, while having a low iron abundance, are rich in carbon, nitrogen and oxygen 4,5,6 . For theoretical reasons 7,8 and because of an observed absence of stars with metallicities lower than Z=1.5×10 5 , it has been suggested that low mass stars (M‹0.8M ⊙ , the ones that survive to the present day) cannot form until the interstellar medium has been enriched above a critical value, estimated to lie in the range 1.5×10 8 ≤Z≤1.5×10 6[8] , although competing * Gliese Fellow…”

The new record holder for the most iron-poor star: HE 1327–2326, a dwarf or subgiant with [Fe/H[=−5.4

“…Until recently only one star was known to have an iron abundance with [Fe/H] < −4.0 (HE 0107−5240 with [Fe/H] = −5.2; Christlieb et al 2002). Now, a second star, HE 1327−2326, has been found with [Fe/H] = −5.4 (Frebel et al 2005, Aoki et al in preparation).…”

Detailed Nucleosynthesis Yields from the Explosion of Massive Stars

“…However, with the latest generation of telescopes and with space based telescopes, such as the Hubble Space Telescope (HST), a much larger range of n-capture elements can be examined. The most metal-poor stars observed to date, HE0107-5240 (Christlieb et al 2002) and HE1327-2326 (Frebel et al 2005), with metallicities below [Fe/H]< −5 are enriched in C, N, and O but very poor in n-capture elements. However, the detection of Sr/Fe (exceeding 10 times the solar ratio) in the most metal-poor star (HE1327-2326, Frebel et al 2005) suggests the existence of a primary process producing elements beyond Fe and Zn.…”