Birgitta Nordström

First stars. I. The extreme r-element rich, iron-poor halo giant CS 31082-001 - Implications for the r-process site(s) and radioactive cosmochronology

We present a high-resolution (R = 75 000, S/N 500) spectroscopic analysis of the bright (V = 11.7), extreme halo giant CS 31082-001 ((Fe/H) = 2.9), obtained in an ESO-VLT Large Programme dedicated to very metal-poor stars. We nd CS 31082-001 to be extremely rich in r-process elements, comparable in this respect only to the similarly metal-poor, but carbon-enriched, giant CS 22892-052. As a result of the extreme overabundance of the heaviest r-process elements, and negligible blending from CH and CN molecular lines, a reliable measurement is obtained of the U II line at 386 nm, for the rst time in a halo star, along with numerous lines of Th II, as well as lines of 25 other r-process elements. Abundance estimates for a total of 43 elements (44 counting Hydrogen) are reported in CS 31082-001, almost half of the entire periodic table. The main atmospheric parameters of CS 31082- 001 are as follows: Te = 4825 50 K, logg =1 :5 0: 3( cgs), (Fe/H) = 2.9 0:1 (in LTE), and microturbulence 1.8 0.2 km s 1 . Carbon and nitrogen are not signicantly enhanced relative to iron. As usual in giant stars, Li is depleted by dilution (log(Li/H) = 0.85). The -elements show the usual enhancements with respect to iron, with (O/Fe) = 0:6 0:2 (from (O I) 6300 A), (Mg/Fe) = 0:45 0:16, (Si/Fe) = 0:24 0:1, and (Ca/Fe) = 0:41 0:08, while (Al/Fe) is near 0.5. The r-process elements show unusual patterns: among the lightest elements (Z 40), Sr and Zr follow the Solar r-element distribution, but Ag is down by 0.8 dex. All elements with 56 Z 72 follow the Solar r-element pattern, reduced by about 1.25 dex. Accordingly, the (r/Fe) enhancement is about +1.7 dex (a factor of 50), very similar to that of CS 22892-052. Pb, in contrast, seems to be below the shifted Solar r-process distribution, possibly indicating an error in the latter, while thorium is more enhanced than the lighter nuclides. In CS 31082-001, log(Th/Eu) is 0:22 0:07, higher than in the Solar System ( 0.46) or in CS 22892-052 ( 0.66). If CS 31082-001 and CS 22892-052 have similar ages, as expected for two extreme halo stars, this implies that the production ratios were dierent by about 0.4 dex for the two objects. Conversely, if the Th/Eu production ratio were universal, an age of 15 Gyr for CS 22892-052 would imply a negative age for CS 31082-001. Thus, while a universal production ratio for the r-process elements seems to hold in the interval 56 Z 72, it breaks down in the actinide region. When available, the U/Th is thus preferable to Th/Eu for radioactive dating, for two reasons: (i) because of its faster decay rate and smaller sensitivity to observational errors, and (ii) because the inital production ratio of the neighboring nuclides 238 Ua nd 232 Th is more robustly predicted than the 151 Eu/ 232 Th ratio. Our current best estimate for the age of CS 31082-001 is 14:0 2: 4G yr. However, the computed actinide production ratios should be veried by observations of daughter elements such as Pb and Bi in the same star, which are independent of the subsequent history of star formation and nucelosynthesis in the Galaxy.

Measurement of stellar age from uranium decay

The ages of the oldest stars in the Galaxy indicate when star formation began, and provide a minimum age for the Universe. Radioactive dating of meteoritic material and stars relies on comparing the present abundance ratios of radioactive and stable nuclear species to the theoretically predicted ratios of their production. The radioisotope 232Th (half-life 14 Gyr) has been used to date Galactic stars, but it decays by only a factor of two over the lifetime of the Universe. 238U (half-life 4.5 Gyr) is in principle a more precise age indicator, but even its strongest spectral line, from singly ionized uranium at a wavelength of 385.957 nm, has previously not been detected in stars. Here we report a measurement of this line in the very metal-poor star CS31082-0018, a star which is strongly overabundant in its heavy elements. The derived uranium abundance, log(U/H) = -13.7 ± 0.14 ± 0.12 yields an age of 12.5 ± 3 Gyr, though this is still model dependent. The observation of this cosmochronometer gives the most direct age determination of the Galaxy. Also, with improved theoretical and laboratory data, it will provide a highly precise lower limit to the age of the Universe.

First Stars II. Elemental abundances in the extremely metal-poor star CS 22949-037 ? A diagnostic of early massive supernovae

CS 22949-037 is one of the most metal-poor giants known ((Fe=H) 4:0), and it exhibits large overabundances of carbon and nitrogen (Norris et al.). Using VLT-UVES spectra of unprecedented quality, regarding resolution and S=N ratio, covering a wide wavelength range (from= 350 to 900 nm), we have determined abundances for 21 elements in this star over a wide range of atomic mass. The major new discovery is an exceptionally large oxygen enhancement, (O=Fe)= 1:97 0:1, as measured from the (O I) line at 630.0 nm. We find an enhancement of (N/Fe) of 2:56 0:2, and a milder one of (C=Fe)= 1:17 0:1, similar to those already reported in the literature. This implies Z? = 0:01 Z. We also find carbon isotopic ratios 12 C/ 13 C= 42: 0a nd 13 C/ 14 N= 0:03 +0:035 0:015 , close to the equilibrium value of the CN cycle. Lithium is not detected. Na is strongly enhanced ((Na=Fe)=+2:1 0:2), while S and K are not detected. The silicon-burning elements Cr and Mn are underabundant, while Co and Zn are overabundant ((Zn=Fe)=+0:7). Zn is measured for the first time in such an extremely metal-poor star. The abundances of the neutron-capture elements Sr, Y, and Ba are strongly decreasing with the atomic number of the element: (Sr=Fe) +0:3, (Y=Fe) 0:1, and (Ba=Fe) 0:6. Among possible progenitors of CS 22949-037, we discuss the pair- instability supernovae. Such very massive objects indeed produce large amounts of oxygen, and have been found to be possible sources of primary nitrogen. However, the predicted odd/even eect is too large, and the predicted Zn abundance much too low. Other scenarios are also discussed. In particular, the yields of a recent model (Z35Z) from Heger and Woosley are shown to be in fair agreement with the observations. The only discrepant prediction is the very low abundance of nitrogen, possibly curable by taking into account other eects such as rotationally induced mixing. Alternatively, the absence of lithium in our star, and the values of the isotopic ratios 12 C/ 13 Ca nd 13 C/ 14 N close to the equilibrium value of the CN cycle, suggest that the CNO abundances now observed might have been altered by nuclear processing in the star itself. A 30-40 M supernova, with fallback, seems the most likely progenitor for CS 22949-037.

First stars IX - Mixing in extremely metal-poor giants. Variation of the 12C/13C, [Na/Mg] and [Al/Mg] ratios

Context: .Extremely metal-poor (EMP) stars preserve a fossil record of the composition of the ISM when the Galaxy formed. It is crucial, however, to verify whether internal mixing has modified their surface composition, especially in the giants where most elements can be studied. Aims: .We aim to understand the CNO abundance variations found in some, but not all EMP field giants analysed earlier. Mixing beyond the first dredge-up of standard models is required, and its origin needs clarification. Methods: .The 12C/^{13C} ratio is the most robust diagnostic of deep mixing, because it is insensitive to the adopted stellar parameters and should be uniformly high in near-primordial gas. We have measured 12C and ^{13C} abundances in 35 EMP giants (including 22 with {[Fe/H] < -3.0}) from high-quality VLT/UVES spectra analysed with LTE model atmospheres. Correlations with other abundance data are used to study the depth of mixing. Results: .The 12C/^{13C} ratio is found to correlate with [C/Fe] (and Li/H), and clearly anti-correlate with [N/Fe], as expected if the surface abundances are modified by CNO processed material from the interior. Evidence for such deep mixing is observed in giants above {log L/L? = 2.6}, brighter than in less metal-poor stars, but matching the bump in the luminosity function in both cases. Three of the mixed stars are also Na- and Al-rich, another signature of deep mixing, but signatures of the ON cycle are not clearly seen in these stars. Conclusions: .Extra mixing processes clearly occur in luminous RGB stars. They cannot be explained by standard convection, nor in a simple way by rotating models. The Na- and Al-rich giants could be AGB stars themselves, but an inhomogeneous early ISM or pollution from a binary companion remain possible alternatives.

Thickness of the soft tissue layers and articular disk in temporomandibular joints with deviations in form

115 right temporomandibular joints from Swedish subjects, aged 1 day to 93 years, were examined. Twenty-four of the 102 adult joints exhibited deviations in form. Twenty-two of these affected joints were examined histologically regarding the thickness of the soft tissue in the lateral and mediocentral parts of the condyle and temporal component as well as in the lateral, laterocentral, mediocentral and medial parts of the disk. The thickness of the soft tissue was measured microscopically on sagittal sections in the anterior, middle and posterior parts of the joint components. The soft tissue layers were thickest in the lateral parts of the condyle superiorly (0.6 mm) laterally on the anterior and posterior-inferior slopes of the articular tubercle of the temporal component (0.6 and 0.5 mm) and posteriorly in the mediocentral portion of the disk (3.2 mm). The measurements raised the question of shrinkage of the actual tissues. Therefore a small methodological investigation was undertaken to estimate the s...

Pieces of the puzzle: ancient substructure in the galactic disc

We search for signatures of past accretion events in the Milky Way in the recently published catalogue by Nordstrom et al., containing accurate spatial and kinematic information as well as metallicities for 13 240 nearby stars. To optimize our strategy, we use numerical simulations and characterize the properties of the debris from disrupted satellites. We find that stars with a common progenitor should show distinct correlations between their orbital parameters; in particular, between the apocentre (A) and pericentre (P), as well as their z-angular momentum (L-z). In the APL space, such stars are expected to cluster around regions of roughly constant eccentricity. The APL space for the Nordstrom catalogue exhibits a wealth of substructure, much of which can be linked to dynamical perturbations induced by spiral arms and the Galactic bar. However, our analysis also reveals a statistically significant excess of stars on orbits of common ( moderate) eccentricity, analogous to the pattern expected for merger debris. Besides being dynamically peculiar, the 274 stars in these substructures have very distinct metallicity and age distributions, providing further evidence of their extragalactic provenance. It is possible to identify three coherent groups among these stars, that, in all likelihood, correspond to the remains of disrupted satellites. The most metal-rich group ([Fe/H] >= -0.45 dex) has 120 stars distributed into two stellar populations of similar to 8 Gyr (33 per cent) and similar to 12 Gyr (67 per cent) of age. The second group with similar to -0.6 dex has 86 stars and shows evidence of three populations of 8 Gyr ( 15 per cent), 12 Gyr (36 per cent) and 16 Gyr (49 per cent) of age. Finally, the third group has 68 stars, with typical metallicity around -0.8 dex and a single age of similar to 14 Gyr. The identification of substantial amounts of debris in the Galactic disc whose origin can be traced back to more than one satellite galaxy, provides evidence of the hierarchical formation of the Milky Way. (Less)

First stars X. The nature of three unevolved carbon-enhanced metal-poor stars

Context. On the order of 20% of the very metal-poor stars in the Galaxy exhibit large carbon enhancements. It is important to establish which astrophysical sites and processes are responsible for the elemental abundance patterns of this early Galactic population. Aims. We seek to understand the nature of the progenitors of three main-sequence turnoff Carbon-Enhanced Metal-Poor (CEMP) stars, CS 31080-095, CS 22958-042, and CS 29528-041, based on a detailed abundance analysis. Methods. From high-resolution VLT/UVES spectra (R similar to 43 000), we determine abundances or upper limits for Li, C, N, O, and other important elements, as well as C-12/C-13 isotopic ratios. Results. All three stars have -3.30 <= [Fe/H]<= -2.85 and moderate to high CNO abundances. CS 22958-042 is one of the most carbon-rich CEMP stars known ([C/Fe] = +3.2), while CS 29528-041 (one of the few N-enhanced metal-poor stars known) is one of the most nitrogen rich ([N/Fe] = +3.0). Oxygen is very high in CS 31080-095 ([O/Fe] = +2.35) and in CS 22958-042 ([O/Fe] = +1.35). All three stars exhibit [Sr/Fe] < 0; Ba is not detected in CS 22958-042 ([Ba/Fe] < -0.53),but it is moderately enhanced ([Ba/Fe] similar to 1) in the other two stars. CS 22958-042 displays one of the largest sodium overabundances yet found in CEMP stars ([Na/Fe] = +2.8). CS 22958-042 has C-12/C-13 = 9, similar to most other CEMP stars without enhanced neutron-capture elements, while C-12/C-13 = 40 in CS 31080-095. CS 31080-095 and CS 29528-041 have A(Li) similar to 1.7, below the Spite Plateau, while Li is not detected in CS 22958-042. Conclusions. CS 22958-042 is a CEMP-no star, but the other two stars are in no known class of CEMP star and thus either constitute a new class or are a link between the CEMP-no and CEMP-s classes, adding complexity to the abundance patterns for CEMP stars. We interpret the abundance patterns in our stars to imply that current models for the presumed AGB binary progenitors lack an extra-mixing process, similar to those apparently operating in RGB stars. (Less)

First stars IV. CS 29497-030: Evidence for operation of the s-process at very low metallicity

We present an abundance analysis of the very metal-poor, carbon-enhanced star CS 29497-030. Our results indicate that this unusually hot turno star (Te = 6650 K, logg= 3.5) has a metallicity (Fe/H)= 2:8, and exhibits large overabun- dances of carbon ((C/Fe)=+2.38), nitrogen ((N/Fe)=+1.88), and oxygen ((O/Fe)=+1.67). This star also exhibits a large enhancement in its neutron-capture elements; the pattern follows that expected to arise from the s-process. In particular, the Pb abundance is found to be very high with respect to iron ((Pb/Fe)=+3.5), and also with respect to the second peak s-process elements (e.g., Ba, La, Ce, Nd), which fits into the newly introduced classification of lead (Pb) stars. The known spectroscopic binary status of this star, along with the observed s-process abundance pattern, suggest that it has accreted matter from a com- panion, which formerly was an Asymptotic Giant-Branch (AGB) star. In a preliminary analysis, we have also identified broad absorption lines of metallic species that suggest a large axial rotational velocity for this star, which may be the result of spin-up associated with the accretion of material from its previous AGB companion. In addition, this star is clearly depleted in the light element Li. When considered along with its rather high inferred temperature, these observations are consistent with the expected properties of a very low metallicity halo blue straggler.

New strong evidence for the importance of convective overshooting in intermediate-mass stars

Major differences between current series of stellar evolution calculations concern their opacities and treatment of convection. Accurate mass, radius, luminosity, and abundance data from eclipsing binaries now allow significant conclusions on these differences: binary stars with small convective cores are very well fitted by standard models using Los Alamos (but not Cox-Stewart) opacities. However, at just slightly larger masses, the moderately evolved binaries clearly require convective overshooting for a satisfactory fit. Precise radial velocities for F-type turnoff stars in IC 4651 and NGC 3680 not only confirm the signatures of overshooting observed in these clusters but also show that any such firm conclusions require proper identification of binaries and nonmembers. It is concluded that standard (and some overshooting) models are inconsistent with current precise data for intermediate-mass stars. Among the consequences of rejecting the older models, it is pointed out that ages for stars younger than roughly 4 Gyr will increase by up to 50-100 percent. 27 refs.

Lead abundance in the uranium star CS 31082-001

In a previous paper we were able to measure the abundance of uranium and thorium in the very-metal poor halo giant BPS CS 31082-001, but only obtained an upper limit for the abundance of lead (Pb). We have got from ESO 17 h of additional exposure on this star in order to secure a detection of the minimum amount of lead expected to be present in CS 31082-001, the amount arising from the decay of the original content of Th and U in the star. We report here this successful detection. We find an LTE abundance log (Pb/H) + 12 = −0.55 ± 0.15 dex, one dex below the upper limits given by other authors for the similar stars CS 22892-052 and BD +17 ◦ 3248, also enhanced in r-process elements. From the observed present abundances of Th and U in the star, the expected amount of Pb produced by the decay of 232 Th, and 238 U alone, over 12-15 Gyr is −0.73±0.17 dex. The decay of 235 Ui s more difficult to estimate, but is probably slightly below the contribution of 238 U, making the contribution of the 3 actinides only slightly below, or even equal to, the measured abundance. The contribution from the decay of 234 Uh as was not included, for lack of published data. In this sense our determination is a lower limit to the contribution of actinides to lead production. We comment this result, and we note that if a NLTE analysis, not yet possible, doubles our observed abundance, the decay of the 3 actinides will still represent 50 per cent of the total lead, a proportion higher than the values considered so far in the literature.