arXiv subjects

E. Caffau

Publications and source records attributed to E. Caffau.

At least 127 records · Page 7Linked to original sources

arXiv2013

Oxygen spectral line synthesis: 3D non-LTE with CO5BOLD hydrodynamical model atmospheres

In this work we present first results of our current project aimed at combining the 3D hydrodynamical stellar atmosphere approach with non-LTE (NLTE) spectral line synthesis for a number of key chemical species. We carried out a full 3D-NLTE spectrum synthesis of the oxygen IR 777 nm triplet, using a modified and improved version of our NLTE3D package to calculate departure coefficients for the atomic levels of oxygen in a CO5BOLD 3D hydrodynamical solar model atmosphere. Spectral line synthesis was subsequently performed with the Linfor 3D code. In agreement with previous studies, we find that the lines of the oxygen triplet produce deeper cores under NLTE conditions, due to the diminished line source function in the line forming region. This means that the solar oxygen IR 777 nm lines should be stronger in NLTE, leading to negative 3D NLTE-LTE abundance corrections. Qualitatively this result would support previous claims for a relatively low solar oxygen abundance. Finally, we outline several further steps that need to be taken in order to improve the physical realism and numerical accuracy of our current 3D-NLTE calculations.

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arXiv2013

Carbon-enhanced metal-poor stars: the most pristine objects?

Carbon-enhanced metal poor stars (CEMP) form a significant proportion of the metal-poor stars, their origin is not well understood. Three very metal-poor C-rich turnoff stars were selected from the SDSS survey, observed with the ESO VLT (UVES) to precisely determine the element abundances. In turnoff stars (unlike giants) the carbon abundance has not been affected by mixing with deep layers and is therefore easier to interpret. The analysis was performed with 1D LTE static model atmospheres. When available, non-LTE corrections were applied to the classical LTE abundances. The 3D effects on the CH and CN molecular bands were computed using hydrodynamical simulations of the stellar atmosphere (CO5BOLD) and are found to be very important. To facilitate a comparison with previous results, only 1D abundances are used in the discussion. The abundances (or upper limits) of the elements enable us to place these stars in different CEMP classes. The carbon abundances confirm the existence of a plateau at A(C)= 8.25 for [Fe/H] \geq -3.4. The most metal-poor stars ([Fe/H] < -3.4) have significantly lower carbon abundances, suggesting a lower plateau at A(C) \approx 6.5. Detailed analyses of a larger sample of very low metallicity carbon-rich stars are required to confirm (or refute) this possible second plateau and specify the behavior of the CEMP stars at very low metallicity.

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arXiv2013

Signs of atmospheric inhomogeneities in cool stars from 1D-NLTE analysis of iron lines

For the well studied halo star HD 122563 and the four stars in the globular cluster NGC 6397, we determine NLTE abundances of iron using classical plane-parallel model atmospheres. Each star reveals a discrepancy in abundances between the Fe I lines arising from the ground state and the other Fe I lines, in qualitative agreement with the 3D-LTE line formation predictions, however, the magnitude of the observed effect is a factor of 2 smaller compared with the predicted one. When ignoring the Fe I low-excitation lines, the NLTE abundances from the two ionization stages, Fe I and Fe II, are consistent in each investigated star. For the subgiants in NGC 6397, this is only true when using the cooler effective temperature scale of Alonso et al. (1999). We also present full 3D-LTE line formation calculations for some selected iron lines in the solar and metal-poor 4480/2/-3 models and NLTE calculations with the corresponding spatial and temporal average <3D> models. The use of the <3D> models is justified only for particular Fe I lines in particular physical conditions. Our NLTE calculations reproduce well the centre-to-limb variation of the solar Fe I 7780 A line, but they are unsuccessful for Fe I 6151 A. The metal-poor <3D> model was found to be adequite for the strong Fe I 5166 A (Eexc = 0) line, but inadequite in all other investigated cases.

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arXiv2013

Fluorine Abundances of Galactic Low-Metallicity Giants

With abundances and 2σ upper limits of fluorine (F) in seven metal-poor field giants, nucleosynthesis of stellar F at low metallicity is discussed. The measurements are derived from the HF(1-0) R9 line at 23358Å using nearinfrared K-band high-resolution spectra obtained with CRIRES at the Very Large Telescope. The sample reaches lower metallicities than previous studies on F of field giants, ranging from [Fe/H] = -1.56 down to -2.13. Effects of three-dimensional model atmospheres on the derived F and O abundances are quantitatively estimated and shown to be insignificant for the program stars. The observed F yield in the form of [F/O] is compared with two sets of Galactic chemical evolution models, which quantitatively demonstrate the contribution of Type II supernova (SN II) ν-process and asymptotic giant branch/Wolf-Rayet stars. It is found that at this low-metallicity region, models cannot well predict the observed distribution of [F/O], while the observations are better fit by models considering an SN II ν-process with a neutrino energy of E_ν = 3 x 10^53 erg. Our sample contains HD 110281, a retrograde orbiting low-α halo star, showing a similar F evolution as globular clusters. This supports the theory that such halo stars are possibly accreted from dwarf galaxy progenitors of globular clusters in the halo.

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arXiv2013

Is the Sun Lighter than the Earth? Isotopic CO in the Photosphere, Viewed through the Lens of 3D Spectrum Synthesis

We consider the formation of solar infrared (2-6 micron) rovibrational bands of carbon monoxide (CO) in CO5BOLD 3D convection models, with the aim to refine abundances of the heavy isotopes of carbon (13C) and oxygen (18O,17O), to compare with direct capture measurements of solar wind light ions by the Genesis Discovery Mission. We find that previous, mainly 1D, analyses were systematically biased toward lower isotopic ratios (e.g., R23= 12C/13C), suggesting an isotopically "heavy" Sun contrary to accepted fractionation processes thought to have operated in the primitive solar nebula. The new 3D ratios for 13C and 18O are: R23= 91.4 +/- 1.3 (Rsun= 89.2); and R68= 511 +/- 10 (Rsun= 499), where the uncertainties are 1 sigma and "optimistic." We also obtained R67= 2738 +/- 118 (Rsun= 2632), but we caution that the observed 12C17O features are extremely weak. The new solar ratios for the oxygen isotopes fall between the terrestrial values and those reported by Genesis (R68= 530, R6= 2798), although including both within 2 sigma error flags, and go in the direction favoring recent theories for the oxygen isotope composition of Ca-Al inclusions (CAI) in primitive meteorites. While not a major focus of this work, we derive an oxygen abundance of 603 +/- 9 ppm (relative to hydrogen; 8.78 on the logarithmic H= 12 scale). That the Sun likely is lighter than the Earth, isotopically speaking, removes the necessity to invoke exotic fractionation processes during the early construction of the inner solar system.

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arXiv2013

Convective line shifts for the Gaia RVS from the CIFIST 3D model atmosphere grid

To derive space velocities of stars along the line of sight from wavelength shifts in stellar spectra requires accounting for a number of second-order effects. For most stars, gravitational redshifts, convective blueshifts, and transverse stellar motion are the dominant contributors. We provide theoretical corrections for the net velocity shifts due to convection expected for the measurements from the Gaia Radial Velocity Spectrometer (RVS). We used a set of three-dimensional time-dependent simulations of stellar surface convection computed with CO5BOLD to calculate spectra of late-type stars in the Gaia RVS range and to infer the net velocity offset that convective motions will induce in radial velocities derived by cross-correlation. The net velocity shifts derived by cross-correlation depend both on the wavelength range and spectral resolution of the observations. Convective shifts for Gaia RVS observations are less than 0.1 km/s for late-K-type stars, and they increase with stellar mass, reaching about 0.3 km/s or more for early F-type dwarfs. This tendency is the result of an increase with effective temperature in both temperature and velocity fluctuations in the line-forming region. Our simulations also indicate that the net RVS convective shifts can be positive (i.e. redshifts) in some cases. Overall, the blueshifts weaken slightly with increasing surface gravity, and are enhanced at low metallicity. Gravitational redshifts amount up to 0.7 km/s and dominate convective blueshifts for dwarfs, but become much weaker for giants.

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arXiv2012

First stars XVI. STIS/HST abundances of heavy-elements in the uranium-rich star CS 31082-001

Detailed abundances of the elements produced by r-process nucleosynthesis in various circumstances are our best observational clues to their origin, since the site(s) of r-element production is(are) still not known with certainty. A small fraction of extremely metal-poor (EMP) stars exhibit excesses of heavy neutron-capture elements produced in the r-process, and CS 31082-001 is among the 4 well-known r-process-enhanced EMP stars. Observations with HST/STIS provide abundances for elements observable only from the UV region. Here we aim to supplement the optical data with abundances from near-UV spectroscopy of the first and second peak of the r-elements, which are crucial to giving insight into the nucleosynthesis of the elements beyond iron. The UVES spectrum provided additional measurements, thereby improving the previous results. The spectra were analyzed with the OSMARCS LTE model atmosphere and with a consistent approach based on the spectrum synthesis code Turbospectrum to derive abundances of heavy elements in CS 31082-001, using updated oscillator strengths from the recent literature. We computed synthetic spectra for all lines of the elements of interest, checking for proper intensities and possible blends. We combined the abundances of heavy elements derived in previous works with the derivation of abundances from all reliable new list of lines, for the first and second peaks of r-elements. We were able to derive new abundances for 23 n-elements, 6 of them - Ge, Mo, Lu, Ta, W, and Re - were not available in previous works, making this star the most complete r-II object studied, with a total of 37 detections of n-capture elements. We also present the first NLTE+3D lead abundance in this star. The results provide improved constraints on the nature of the r-process.

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arXiv2012

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars II. Spectral line formation in the atmosphere of a giant located near the RGB tip

We investigate the role of convection in the formation of atomic and molecular lines in the atmosphere of a red giant star. For this purpose we study the formation properties of spectral lines that belong to a number of astrophysically important tracer elements, including neutral and singly ionized atoms, and molecules. We focus our investigation on a prototypical red giant located close to the red giant branch (RGB) tip (Teff=3660K, logg=1.0, [M/H]=0.0). We used two types of model atmospheres, 3D hydrodynamical and classical 1D, calculated with the CO5BOLD and LHD stellar atmosphere codes, respectively. Both codes share the same atmospheric parameters, chemical composition, equation of state, and opacities, which allowed us to make a strictly differential comparison between the line formation properties predicted in 3D and 1D. The influence of convection on the spectral line formation was assessed with the aid of 3D-1D abundance corrections, which measure the difference between the abundances of chemical species derived with the 3D hydrodynamical and 1D classical model atmospheres. We find that convection plays a significant role in the spectral line formation in this particular red giant. The derived 3D-1D abundance corrections rarely exceed \pm0.1 dex when lines of neutral atoms and molecules are considered, which is in line with the previous findings for solar-metallicity red giants located on the lower RGB. The situation is different with lines that belong to ionized atoms, or to neutral atoms with high ionization potential. In both cases, the corrections for high-excitation lines (χ>8 eV) may amount to Δ_3D-1D ~ -0.4 dex. The 3D--1D abundance corrections generally show a significant wavelength dependence; in most cases they are smaller in the near-infrared, at 1600-2500 nm.

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arXiv2012

Velocity and abundance precisions for future high-resolution spectroscopic surveys: a study for 4MOST

In preparation for future, large-scale, multi-object, high-resolution spectroscopic surveys of the Galaxy, we present a series of tests of the precision in radial velocity and chemical abundances that any such project can achieve at a 4m class telescope. We briefly discuss a number of science cases that aim at studying the chemo-dynamical history of the major Galactic components (bulge, thin and thick disks, and halo) - either as a follow-up to the Gaia mission or on their own merits. Based on a large grid of synthetic spectra that cover the full range in stellar parameters of typical survey targets, we devise an optimal wavelength range and argue for a moderately high-resolution spectrograph. As a result, the kinematic precision is not limited by any of these factors, but will practically only suffer from systematic effects, easily reaching uncertainties <1 km/s. Under realistic survey conditions (namely, considering stars brighter than r=16 mag with reasonable exposure times) we prefer an ideal resolving power of R~20000 on average, for an overall wavelength range (with a common two-arm spectrograph design) of [395;456.5] nm and [587;673] nm. We show for the first time on a general basis that it is possible to measure chemical abundance ratios to better than 0.1 dex for many species (Fe, Mg, Si, Ca, Ti, Na, Al, V, Cr, Mn, Co, Ni, Y, Ba, Nd, Eu) and to an accuracy of about 0.2 dex for other species such as Zr, La, and Sr. While our feasibility study was explicitly carried out for the 4MOST facility, the results can be readily applied to and used for any other conceptual design study for high-resolution spectrographs.

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arXiv2012

Lithium and sodium in the globular cluster M4. Detection of a Li-rich dwarf star: preservation or pollution?

Context. The abundance inhomogeneities of light elements observed in Globular Clusters (GCs), and notably the ubiquitous Na-O anti-correlation, are generally interpreted as evidence that GCs comprise several generations of stars. There is an on-going debate as to the nature of the stars which produce the inhomogeneous elements, and investigating the behavior of several elements is a way to shed new light on this problem. Aims. We aim at investigating the Li and Na content of the GC M 4, that is known to have a well defined Na-O anti-correlation. Methods. We obtained moderate resolution (R=17 000-18 700) spectra for 91 main sequence (MS)/sub-giant branch stars of M 4 with the Giraffe spectrograph at the FLAMES/VLT ESO facility. Using model atmospheres analysis we measured lithium and sodium abundances. Results. We detect a weak Li-Na anti-correlation among un-evolved MS stars. One star in the sample, # 37934, shows the remarkably high lithium abundance A(Li)=2.87, compatible with current estimates of the primordial lithium abundance. Conclusions. The shallow slope found for the Li-Na anti-correlation suggests that lithium is produced in parallel to sodium. This evidence, coupled with its sodium-rich nature, suggests that the high lithium abundance of star # 37934 may originate by pollution from a previous generations of stars. The recent detection of a Li-rich dwarf of pollution origin in the globular cluster NGC 6397 may also point in this direction. Still, no clear cut evidence is available against a possible preservation of the primordial lithium abundance for star # 37934.

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arXiv2012

Planetary companions around the metal-poor star HIP 11952

Aims. We carried out a radial-velocity survey to search for planets around metal-poor stars. In this paper we report the discovery of two planets around HIP 11952, a metal-poor star with [Fe/H]= -1.9 that belongs to our target sample. Methods. Radial velocity variations of HIP 11952 were monitored systematically with FEROS at the 2.2 m telescope located at the ESO La Silla observatory from August 2009 until January 2011. We used a cross-correlation technique to measure the stellar radial velocities (RV). Results. We detected a long-period RV variation of 290 d and a short-period one of 6.95 d. The spectroscopic analysis of the stellar activity reveals a stellar rotation period of 4.8 d. The Hipparcos photometry data shows intra-day variabilities, which give evidence for stellar pulsations. Based on our analysis, the observed RV variations are most likely caused by the presence of unseen planetary companions. Assuming a primary mass of 0.83 M\odot, we computed minimum planetary masses of 0.78 MJup for the inner and 2.93 MJup for the outer planet. The semi-major axes are a1 = 0.07 AU and a2 = 0.81 AU, respectively. Conclusions. HIP 11952 is one of very few stars with [Fe/H]< -1.0 which have planetary companions. This discovery is important to understand planet formation around metal-poor stars

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arXiv2012

An upper limit on the sulphur abundance in HE 1327-2326

Context: Star HE 1327-2326 is a unique object, with the lowest measured iron abundance ([Fe/H] ~ -6) and a peculiar chemical composition that includes large overabundances of C, N, and O with respect to iron. One important question is whether the chemical abundances in this star reflect the chemical composition of the gas cloud from which it was formed or if they have been severely affected by other processes, such as dust-gas winnowing. Aims: We measure or provide an upper limit to the abundance of the volatile element sulphur, which can help to discriminate between the two scenarios. Methods: We observed HE 1327-2326 with the high resolution infra-red spectrograph CRIRES at the VLT to observe the S I lines of Multiplet 3 at 1045 nm. Results: We do not detect the S I line. A 3sigma$upper limit on the equivalent width (EW) of any line in our spectrum is EW<0.66 pm. Using either one-dimensional static or three-dimensional hydrodynamical model-atmospheres, this translates into a robust upper limit of [S/H]<-2.6. Conclusions: This upper limit does not provide conclusive evidence for or against dust-gas winnowing, and the evidence coming from other elements (e.g., Na and Ti) is also inconclusive or contradictory. The formation of dust in the atmosphere versus an origin of the metals in a metal-poor supernova with extensive "fall-back" are not mutually exclusive. It is possible that dust formation distorts the peculiar abundance pattern created by a supernova with fall-back, thus the abundance ratios in HE 1327-2326 may be used to constrain the properties of the supernova(e) that produced its metals, but with some caution.

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arXiv2012

Lithium abundances in extremely metal-poor turn-off stars

We discuss the current status of the sample of Lithium abundances in extremely metal poor (EMP) turn-off (TO) stars collected by our group, and compare it with the available literature results. In the last years, evidences have accumulated of a progressive disruption of the Spite plateau in stars of extremely low metallicity. What appears to be a flat, thin plateau above [Fe/H]\sim-2.8 turns, at lower metallicities, into a broader distribution for which the plateau level constitutes the upper limit, but more and more stars show lower Li abundances. The sample we have collected currently counts abundances or upper limits for 44 EMP TO stars between [Fe/H]=-2.5 and -3.5, plus the ultra-metal poor star SDSS J102915+172927 at [Fe/H]=-4.9. The "meltdown" of the Spite plateau is quite evident and, at the current status of the sample, does not appear to be restricted to the cool end of the effective temperature distribution. SDSS J102915+172927 displays an extreme Li depletion that contrasts with its otherwise quite ordinary set of [X/Fe] ratios.

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arXiv2012

6Li detection in metal-poor stars: can 3D model atmospheres solve the second lithium problem?

The presence of 6Li in the atmospheres of metal-poor halo stars is usually inferred from the detection of a subtle extra depression in the red wing of the 7Li doublet line at 670.8 nm. However, the intrinsic line asymmetry caused by convective flows in the photospheres of cool stars is almost indistinguishable from the asymmetry produced by a weak 6Li blend on a (presumed) symmetric 7Li profile. Previous determinations of the 6Li/ 7Li isotopic ratio based on 1D model atmospheres, ignoring the convection-induced line asymmetry, must therefore be considered as upper limits. By comparing synthetic 1D LTE and 3D non-LTE line profiles of the Li 670.8 nm feature, we quantify the differential effect of the convective line asymmetry on the derived 6Li abundance as a function of effective temperature, gravity, and metallicity. As expected, we find that the asymmetry effect systematically reduces the resulting 6Li/7Li ratios. Depending on the stellar parameters, the 3D-1D offset in 6Li/7Li ranges between -0.005 and -0.020. When this purely theoretical correction is taken into account for the Asplund 2006 sample of stars, the number of significant 6Li detections decreases from 9 to 5 (2 sigma criterion), or from 5 to 2 (3 sigma criterion). We also present preliminary results of a re-analysis of high-resolution, high S/N spectra of individual metal-poor turn-off stars, to see whether the "second Lithium problem" actually disappears when accounting properly for convection and non-LTE line formation in 3D stellar atmospheres. Out of 8 stars, HD84937 seems to be the only significant (2 sigma) detection of 6Li. In view of our results, the existence of a 6Li plateau appears questionable.

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arXiv2012

Amplitudes of solar-like oscillations in red-giant stars: Evidences for non-adiabatic effects using CoRoT observations

A growing number of solar-like oscillations has been detected in red giant stars thanks to CoRoT and Kepler space-crafts. The seismic data gathered by CoRoT on red giant stars allow us to test mode driving theory in physical conditions different from main-sequence stars. Using a set of 3D hydrodynamical models representative of the upper layers of sub- and red giant stars, we computed the acoustic mode energy supply rate (Pmax). Assuming adiabatic pulsations and using global stellar models that assume that the surface stratification comes from the 3D hydrodynamical models, we computed the mode amplitude in terms of surface velocity. This was converted into intensity fluctuations using either a simplified adiabatic scaling relation or a non-adiabatic one. From L and M (the luminosity and mass), the energy supply rate Pmax is found to scale as (L/M)^2.6 for both main-sequence and red giant stars, extending previous results. The theoretical amplitudes in velocity under-estimate the Doppler velocity measurements obtained so far from the ground for red giant stars by about 30%. In terms of intensity, the theoretical scaling law based on the adiabatic intensity-velocity scaling relation results in an under-estimation by a factor of about 2.5 with respect to the CoRoT seismic measurements. On the other hand, using the non-adiabatic intensity-velocity relation significantly reduces the discrepancy with the CoRoT data. The theoretical amplitudes remain 40% below, however, the CoRoT measurements. Our results show that scaling relations of mode amplitudes cannot be simply extended from main-sequence to red giant stars in terms of intensity on the basis of adiabatic relations because non-adiabatic effects for red giant stars are important and cannot be neglected. We discuss possible reasons for the remaining differences.

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arXiv2012

Chemical abundances of distant extremely metal-poor unevolved stars

Aims: The purpose of our study is to determine the chemical composition of a sample of 16 candidate Extremely Metal-Poor (EMP) dwarf stars, extracted from the Sloan Digital Sky Survey (SDSS). There are two main purposes: in the first place to verify the reliability of the metallicity estimates derived from the SDSS spectra; in the second place to see if the abundance trends found for the brighter nearer stars studied previously also hold for this sample of fainter, more distant stars. Methods: We used the UVES at the VLT to obtain high-resolution spectra of the programme stars. The abundances were determined by an automatic analysis with the MyGIsFOS code, with the exception of lithium, for which the abundances were determined from the measured equivalent widths of the Li I resonance doublet. Results: All candidates are confirmed to be EMP stars, with [Fe/H]<= -3.0. The chemical composition of the sample of stars is similar to that of brighter and nearer samples. We measured the lithium abundance for 12 stars and provide stringent upper limits for three other stars, for a fourth star the upper limit is not significant, owing to the low signal-to noise ratio of the spectrum. The "meltdown" of the Spite plateau is confirmed, but some of the lowest metallicity stars of the sample lie on the plateau. Conclusions: The concordance of the metallicities derived from high-resolution spectra and those estimated from the SDSS spectra suggests that the latter may be used to study the metallicity distribution of the halo. The abundance pattern suggests that the halo was well mixed for all probed metallicities and distances. The fact that at the lowest metallicities we find stars on the Spite plateau suggests that the meltdown depends on at least another parameter, besides metallicity. (abridged)

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arXiv2012

Barium abundance in red giants of NGC 6752. Non-local thermodynamic equilibrium and three-dimensional effects

(Abridged) Aims: We study the effects related to departures from non-local thermodynamic equilibrium (NLTE) and homogeneity in the atmospheres of red giant stars in Galactic globular cluster NGC 6752, to assess their influence on the formation of Ba II lines. Methods: One-dimensional (1D) local thermodynamic equilibrium (LTE) and 1D NLTE barium abundances were derived using classical 1D ATLAS stellar model atmospheres. The three-dimensional (3D) LTE abundances were obtained for 8 red giants on the lower RGB, by adjusting their 1D LTE abundances using 3D-1D abundance corrections, i.e., the differences between the abundances obtained from the same spectral line using the 3D hydrodynamical (CO5BOLD) and classical 1D (LHD) stellar model atmospheres. Results: The mean 1D barium-to-iron abundance ratios derived for 20 giants are <[Ba/Fe]>_{1D NLTE} = 0.05 \pm0.06 (stat.) \pm0.08 (sys.). The 3D-1D abundance correction obtained for 8 giants is small (~+0.05 dex), thus leads to only minor adjustment when applied to the mean 1D NLTE barium-to-iron abundance ratio for the 20 giants, <[Ba/Fe]>_{3D+NLTE} = 0.10 \pm0.06(stat.) \pm0.10(sys.). The intrinsic abundance spread between the individual cluster stars is small and can be explained in terms of uncertainties in the abundance determinations. Conclusions: Deviations from LTE play an important role in the formation of barium lines in the atmospheres of red giants studied here. The role of 3D hydrodynamical effects should not be dismissed either, even if the obtained 3D-1D abundance corrections are small. This result is a consequence of subtle fine-tuning of individual contributions from horizontal temperature fluctuations and differences between the average temperature profiles in the 3D and 1D model atmospheres: owing to the comparable size and opposite sign, their contributions nearly cancel each other.

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arXiv2012

An extremely primitive halo star

The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium1, almost all other elements were created in stars and supernovae. The mass fraction, Z, of elements more massive than helium, is called "metallicity". A number of very metal poor stars have been found some of which, while having a low iron abundance, are rich in carbon, nitrogen and oxygen. For theoretical reasons and because of an observed absence of stars with metallicities lower than Z=1.5E-5, it has been suggested that low mass stars (M<0.8M\odot, 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 1.5E-8\leqZ\leq1.5E-6, although competing theories claiming the contrary do exist. Here we report the chemical composition of a star with a very low Z\leq6.9E-7 (4.5E-5 of that of the Sun) and a chemical pattern typical of classical extremely metal poor stars, meaning without the enrichment of carbon, nitrogen and oxygen. This shows that low mass stars can be formed at very low metallicity. Lithium is not detected, suggesting a low metallicity extension of the previously observed trend in lithium depletion. Lithium depletion implies that the stellar material must have experienced temperatures above two million K in its history, which points to rather particular formation condition or internal mixing process, for low Z stars.

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