Context. The primordial nature of the Spite plateau is at odds with the WMAP satellite measurements, implying a primordial Li production at least three times higher than observed. It has also been suggested that A(Li) might exhibit a positive correlation with metallicity below [Fe/H] ∼ −2.5. Previous samples studied comprised few stars below [Fe/H] = −3. Aims. We present VLT-UVES Li abundances of 28 halo dwarf stars between [Fe/H] = −2.5 and −3.5, ten of which have [Fe/H] < −3. Methods. We determined stellar parameters and abundances using four different T eff scales. The direct infrared flux method was applied to infrared photometry. Hα wings were fitted with two synthetic grids computed by means of 1D LTE atmosphere models, assuming two different self-broadening theories. A grid of Hα profiles was finally computed by means of 3D hydrodynamical atmosphere models. The Li i doublet at 670.8 nm has been used to measure A(Li) by means of 3D hydrodynamical NLTE spectral syntheses. An analytical fit of A(Li) 3D,NLTE as a function of equivalent width, T eff , log g, and [Fe/H] has been derived and is made available. Results. We confirm previous claims that A(Li) does not exhibit a plateau below [Fe/H] = −3. We detect a strong positive correlation with [Fe/H] that is insensitive to the choice of T eff estimator. From a linear fit, we infer a steep slope of about 0.30 dex in A(Li) per dex in [Fe/H], which has a significance of 2-3σ. The slopes derived using the four T eff estimators are consistent to within 1σ. A significant slope is also detected in the A(Li)-T eff plane, driven mainly by the coolest stars in the sample (T eff < 6250), which appear to be Li-poor. However, when we remove these stars the slope detected in the A(Li)-[Fe/H] plane is not altered significantly. When the full sample is considered, the scatter in A(Li) increases by a factor of 2 towards lower metallicities, while the plateau appears very thin above [Fe/H] = −2.8. At this metallicity, the plateau lies at A(Li) 3D,NLTE = 2.199 ± 0.086. Conclusions. The meltdown of the Spite plateau below [Fe/H] ∼ −3 is established, but its cause is unclear. If the primordial A(Li) were that derived from standard BBN, it appears difficult to envision a single depletion phenomenon producing a thin, metallicity independent plateau above [Fe/H] = −2.8, and a highly scattered, metallicity dependent distribution below. That no star below [Fe/H] = −3 lies above the plateau suggests that they formed at plateau level and experienced subsequent depletion.

We present a detailed analysis of all the known hot DQ white dwarfs in the Fourth Data Release of the Sloan Digital Sky Survey (SDSS) recently found to have carbon-dominated atmospheres. Our spectroscopic and photometric analysis reveals that these objects all have effective temperatures between $18,000 and 24,000 K. The surface composition is found to be completely dominated by carbon, as revealed by the absence of H and He i k4471 lines (or a determination of trace amounts in a few cases). We find that the surface gravity of all objects but one seems to be ''normal'' and around log g ¼ 8:0, while one is likely near log g ¼ 9:0. The presence of a weak magnetic field is directly detected by spectropolarimetry in one object and is suspected in two others. We propose that these strange stars could be cooled-down versions of the weird PG 1159 star H1504+65 and form a new family of hydrogen-and heliumdeficient objects following the post-AGB phase. Finally, we present the results of full nonadiabatic calculations dedicated specifically to each of the hot DQs that show that only SDSS J142625.70+575218.4 is expected to exhibit luminosity variations. This result is in excellent agreement with recent observations by Montgomery et al., who find that J142625.70+575218.4 is the only pulsator among six hot DQ white dwarfs surveyed in 2008 February.

Context. The origin of carbon-enhanced metal-poor stars enriched with both s and r elements is highly debated. Detailed abundances of these types of stars are crucial to understand the nature of their progenitors. Aims. The aim of this investigation is to study in detail the abundances of SDSS J1349-0229, SDSS J0912+0216 and SDSS J1036+1212, three dwarf CEMP stars, selected from the Sloan Digital Sky Survey. Methods. Using high resolution VLT/UVES spectra (R ∼ 30 000) we determine abundances for Li, C, N, O, Na, Mg, Al, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni and 21 neutron-capture elements. We made use of CO 5 BOLD 3D hydrodynamical model atmospheres in the analysis of the carbon, nitrogen and oxygen abundances. NLTE corrections for Ci and Oi lines were computed using the Kiel code. Results. We classify SDSS J1349-0229 and SDSS J0912+0216 as CEMP-r+s stars. SDSS J1036+1212 belongs to the class CEMPno/s, with enhanced Ba, but deficient Sr, of which it is the third member discovered to date. Radial-velocity variations have been observed in SDSS J1349-0229, providing evidence that it is a member of a binary system. Conclusions. The chemical composition of the three stars is generally compatible with mass transfer from an AGB companion. However, many details remain difficult to explain. Most notably of those are the abundance of Li at the level of the Spite plateau in SDSS J1036+1212 and the large over-abundance of the pure r-process element Eu in all three stars.

White dwarfs represent the endpoint of stellar evolution for stars with initial masses between approximately 0.07 M ⊙ and 8-10 M ⊙ , where M ⊙ is the mass of the Sun (more massive stars end their life as either black holes or neutron stars). The theory of stellar evolution predicts that the majority of white dwarfs have a core made of carbon and oxygen, which itself is surrounded by a helium layer and, for ∼80 per cent of known white dwarfs, by an additional hydrogen layer 1−3 . All white dwarfs therefore have been traditionally found to belong to one of two categories: those with a hydrogen-rich atmosphere (the DA spectral type) and those with a helium-rich atmosphere (the non-DAs). Here we report the discovery of several white dwarfs with atmospheres primarily composed of carbon, with little or no trace of hydrogen or helium. Our analysis shows that the atmospheric parameters found for these stars do not fit satisfactorily in any of the currently known theories of post-asymptotic giant branch evolution, although these objects might be the cooler counterpart of the unique and extensively studied PG1159 star H1504+65 (refs 4-7). These stars, together with H1504+65, might accordingly form a new evolutionary sequence that follow the asymptotic giant branch.Traces of carbon are typically observed as either neutral carbon lines or molecular C 2 Swan bands (defining the DQ spectral type) in cool helium-rich white dwarfs with effective temperatures (T eff ) below ∼ 13,000 K. The presence of carbon in the atmospheres of these objects has been explained successfully by a model in which carbon is dredged-up from the underlying carbon/oxygen core by the deep helium convection zone 8 . This model predicts a maximum contamination of carbon at an effective temperature of ∼10,000 K (corresponding approximately to the temperature at which the surface convection zone is maximal) before gradually decreasing with lower temperature, in agreement with atmospheric analysis determinations 9 . We note that although some of these objects show a very high level of

Context. The old Galactic halo stars hold the fossil record of the interstellar medium chemical composition at the time of their formation. Most of the stars studied so far are relatively near to the Sun, this prompts the study of more distant stars, both to increase the size of the sample and to search for possible variations of abundance patterns at greater distances. 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.

UVO 0825+15 is a hot bright helium-rich subdwarf which lies in K2 Field 5 and in a sample of intermediate helium-rich subdwarfs observed with Subaru/HDS. The K2 light curve shows low-amplitude variations, whilst the Subaru spectrum shows Pbiv absorption lines, indicative of a very high lead overabundance. UVO 0825+15 also has a high proper motion with kinematics typical for a thick disk star. Analyses of ultraviolet and intermediate dispersion optical spectra rule out a short-period binary companion, and provide fundamental atmospheric parameters of T eff = 38 900±270 K, log g/cm s −2 = 5.97 ± 0.11, log n He /n H = −0.57 ± 0.01, E B−V ≈ 0.03, and angular radius θ = 1.062±0.006×10−11 radians (formal errors). The high-resolution spectrum shows that carbon is > 2 dex subsolar, iron is approximately solar and all other elements heavier than argon are at least 2 -4 dex overabundant, including germanium, yttrium and lead. Approximately 150 lines in the blue-optical spectrum remain unidentified. The chemical structure of the photosphere is presumed to be determined by radiatively-dominated diffusion. The K2 light curve shows a dominant period around 10.8 h, with a variable amplitude, its first harmonic, and another period at 13.3 h. The preferred explanation is multi-periodic non-radial oscillation due to g-modes with very high radial order, although this presents difficulties for pulsation theory. Alternative explanations fail for lack of radial-velocity evidence. UVO 0825+15 represents the fourth member of a group of hot subdwarfs having helium-enriched photospheres and 3-4 dex overabundances of trans-iron elements, and is the first lead-rich subdwarf to show evidence of pulsations.

Context. The stable element hafnium (Hf) and the radioactive element thorium (Th) were recently suggested as a suitable pair for radioactive dating of stars. The applicability of this elemental pair needs to be established for stellar spectroscopy. Aims. We aim at a spectroscopic determination of the abundance of Hf and Th in the solar photosphere based on a CO 5 BOLD 3D hydrodynamical model atmosphere. We put this into a wider context by investigating 3D abundance corrections for a set of Gand F-type dwarfs. Methods. High-resolution, high signal-to-noise solar spectra were compared to line synthesis calculations performed on a solar CO 5 BOLD model. For the other atmospheres, we compared synthetic spectra of CO 5 BOLD 3D and associated 1D models. Results. For Hf we find a photospheric abundance A(Hf) = 0.87 ± 0.04, in good agreement with a previous analysis, based on 1D model atmospheres. The weak Th ii 401.9 nm line constitutes the only Th abundance indicator available in the solar spectrum.It lies in the red wing of a Ni-Fe blend exhibiting a non-negligible convective asymmetry. Accounting for the asymmetry-related additional absorption, we obtain A(Th) = 0.08 ± 0.03, consistent with the meteoritic abundance, and about 0.1 dex lower than obtained in previous photospheric abundance determinations. Conclusions. Only for the second time, to our knowledge, has a non-negligible effect of convective line asymmetries on an abundance derivation been highlighted. Three-dimensional hydrodynamical simulations should be employed to measure Th abundances in dwarfs if similar blending is present, as in the solar case. In contrast, 3D effects on Hf abundances are small in G-to mid F-type dwarfs and sub-giants, and 1D model atmospheres can be conveniently used.

We describe an opacity sampled version of the LTE model atmosphere code sterne with revised continuous opacities. In particular, we examine the results of replacing the existing treatment of the continuous opacities which followed the method of Kurucz (1970, SAOSR, No. 309) and Peach (1970, MNRAS, 73, 1), with photoionization cross-sections from the Opacity Project and the IRON Project, and the effects of substituting an opacity distribution function method for treating the line opacities with an opacity sampling method. In studying the application of these new models to atmospheres with hydrogen-rich and extreme chemical compositions, we find the new opacity treatment increases the temperature in the line forming region by up to 3000 K for extremely hydrogen-poor stars and up to 1250 K for hydrogen-rich stars with T eff = 30 000 K and log g = 3.00. This translates into systematically cooler effective temperature measurements for these types of stars.