Papers by francesca primas
Astronomy and Astrophysics, 2004
In the framework of the ESO Large Programme "First Stars", very high-quality spectra of some 70 v... more In the framework of the ESO Large Programme "First Stars", very high-quality spectra of some 70 very metalpoor dwarfs and giants were obtained with the ESO VLT and UVES spectrograph. These stars are likely to have descended from the first generation(s) of stars formed after the Big Bang, and their detailed composition provides constraints on issues such as the nature of the first supernovae, the efficiency of mixing processes in the early Galaxy, the formation and evolution of the halo of the Galaxy, and the possible sources of reionization of the Universe. This paper presents the abundance analysis of an homogeneous sample of 35 giants selected from the HK survey of , emphasizing stars of extremely low metallicity: 30 of our 35 stars are in the range −2.7 < [Fe/H] < −4.1, and 22 stars have [Fe/H] < −3.0. Our new VLT/UVES spectra, at a resolving power of R ∼ 45, 000 and with signal-to-noise ratios of 100-200 per pixel over the wavelength range 330 -1000 nm, are greatly superior to those of the classic studies of and . The immediate objective of the work is to determine precise, comprehensive, and homogeneous element abundances for this large sample of the most metal-poor giants presently known. In the analysis we combine the spectral line modeling code "Turbospectrum" with OSMARCS model atmospheres, which treat continuum scattering correctly and thus allow proper interpretation of the blue regions of the spectra, where scattering becomes important relative to continuous absorption (λ < 400 nm). We obtain detailed information on the trends of elemental abundance ratios and the star-to-star scatter around those trends, enabling us to separate the relative contributions of cosmic scatter and observational/analysis errors. Abundances of 17 elements from C to Zn have been measured in all stars, including K and Zn, which have not previously been detected in stars with [Fe/H] < −3.0. Among the key results, we discuss the oxygen abundance (from the forbidden [OI] line), the different and sometimes complex trends of the abundance ratios with metallicity, the very tight relationship between the abundances of certain elements (e.g., Fe and Cr), and the high [Zn/Fe] ratio in the most metal-poor stars. Within the error bars, the trends of the abundance ratios with metallicity are consistent with those found in earlier literature, but in many cases the scatter around the average trends is much smaller than found in earlier studies, which were limited to lower-quality spectra. We find that the cosmic scatter in several element ratios may be as low as 0.05 dex. The evolution of the abundance trends and scatter with declining metallicity provides strong constraints on the yields of the first supernovae and their mixing into the early ISM. The abundance ratios found in our sample do not match the predicted yields from pair-instability hypernovae, but are consistent with element production by supernovae with progenitor masses up to 100 M ⊙ . Moreover, the composition of the ejecta that have enriched the matter now contained in our very metal-poor stars appears surprisingly uniform over the range −4.0 ≤ [Fe/H] < −3.0. This would indicate either that we are observing the products of very similar primordial bursts of high-mass stars, or that the mixing of matter from different bursts of early star formation was extremely rapid. In any case, it is unlikely that we observed the ejecta from individual (single) supernovae (as has often been concluded in previous work), as we do not see scatter due to different progenitor masses. The abundance ratios at the lowest metallicities (−4.0 ≤ [Fe/H] ≤ −3.0) are compatible with those found by and later studies. However, when elemental ratios are plotted with respect to Mg, we find no clear slopes below [Mg/H] = -3, but rather, a plateau-like behaviour defining a set of initial yields.
Astronomy & Astrophysics, 2014
Context. Several models compete to explain the abundance properties of stellar populations in glo... more Context. Several models compete to explain the abundance properties of stellar populations in globular clusters. One of the main constraints is the present-day ratio of first-and second-generation stars that are currently identified based on their sodium content. Aims. We propose an alternative interpretation of the observed sodium distribution, and suggest that stars with low sodium abundance that are counted as members of the first stellar generation could actually be second-generation stars. Methods. We compute the number ratio of second-generation stars along the Na distribution following the fast rotating massive star model using the same constraints from the well-documented case of NGC 6752 as in our previous developments. Results. We reproduce the typical percentage of low-sodium stars usually classified as first-generation stars by invoking only secondary star formation from material ejected by massive stars and mixed with original globular cluster material in proportions that account for the Li-Na anti-correlation in this cluster. Conclusions. Globular clusters could be totally devoid of first-generation low-mass stars today. This can be tested with the determination of the carbon isotopic ratio and nitrogen abundance in turn-off globular cluster stars. Consequences and related issues are briefly discussed.
Neutron-Capture Elements in Extremely Metal-Poor Giants
The abundances of the neutron capture elements (Sr Ba LaCe Eu...) in a sample of more than 30 ext... more The abundances of the neutron capture elements (Sr Ba LaCe Eu...) in a sample of more than 30 extremely metal-poor giants ([Fe/H]=1mA). The trends of the ratios [Sr/Fe] [Ba/Fe]... with metallicity are shown and the scatters compared to the scatter observed for iron-peak elements. Consequences for the formation of these elements and the the galactic evolution are discussed.
Proceedings of the International Astronomical Union, 2009
The primordial lithium abundance inferred from WMAP and standard Big Bang nucleosysnthesis is app... more The primordial lithium abundance inferred from WMAP and standard Big Bang nucleosysnthesis is approximately three times higher than the plateau value measured in old metal-poor Population II stars, suggesting that these stars have undergone atmospheric Li depletion. To constrain the physics responsible for such depletion, we conducted a homogeneous analysis of a large sample of stars in the metal-poor globular cluster NGC 6397, covering all evolutionary phases from below the main-sequence turnoff to high up the red-giant branch (RGB). The dwarf, turnoff, and early subgiant stars form a thin abundance plateau, with a sharpe edge in the middle of the subgiant branch, where Li dilution caused by the inward extension of the convective envelope starts (the beginning of the so-called first dredge up). A second steep abundance drop is seen at the RGB bump, again highlighting the need for the onset of nonstandard mixing in this evolutionary phase. Moreover, by also measuring the sodium abundances of the targets, we have gained insight into the degree of pollution by early cluster self-enrichement, and may separate highly polluted, Li-poor and Na-rich stars from stars formed from pristine material. Our observational findings strictly limit both the extent of lithium surface depletion, which in turn constrains the efficiency of mixing below the outer convection zone, and the resulting spread in lithium abundance in metal-poor turn-off stars.
Proceedings of the International Astronomical Union, 2006
As part of a study of the detailed abundance patterns in extremely metal-poor stars, we have comp... more As part of a study of the detailed abundance patterns in extremely metal-poor stars, we have compared our samples of giants and dwarfs with two samples of dwarfs measured by different teams. For most elements the abundances are in good agreement, but for C, Na, and Al we show that the atmospheric abundances are different in dwarfs and in giants. For C the difference could be explained by "atmospheric effects" or by the influence of the first dredge-up, but for Na and Al deep mixing inside the stars must be invoked. Until now, such deep mixing has not been observed in metal-poor field stars. An excess scatter in [Mg/Fe] in giants remains unexplained.
Proceedings of the International Astronomical Union, 2007
Proceedings of the International Astronomical Union, 2008
The members of IAU Commission 29 Stellar Spectra are actively engaged in the quantitative analysi... more The members of IAU Commission 29 Stellar Spectra are actively engaged in the quantitative analysis of spectra of various types of stars. With large and medium size telescopes equipped with high resolution spectrographs LTE and Non-LTE analysis of spectra of all types stars are being carried out. Spectra of stars in our Galaxy, in globular and open clusters, stars in LMC and SMC and in nearby galaxies are being studied. Accurate chemical composition analysis of various types of stars has been carried out during the past three years. Now the analysis of stellar spectra covers the wavelength range from X-ray region to IR and sub-millimeter range. Recently stellar spectra are being analysed using time-dependent, 3D, hydrodynamical model atmospheres to derive accurate stellar abundances.
The Astrophysical Journal, 2005
We present new abundance determinations of neutron-capture elements Ge, Zr, Os, Ir, and Pt in a s... more We present new abundance determinations of neutron-capture elements Ge, Zr, Os, Ir, and Pt in a sample of 11 metal-poor (À3:1 ½Fe /H À1:6) Galactic halo giant stars, based on Hubble Space Telescope UV and Keck I optical high-resolution spectroscopy. The stellar sample is dominated by r-process-rich stars such as the well-studied CS 22892À052 and BD +17 3248 but also includes the r-process-poor, bright giant HD 122563. Our results demonstrate that abundances of the third r-process peak elements Os, Ir, and Pt in these metal-poor halo stars are very well correlated among themselves and with the abundances of the canonical r-process element Eu (determined in other studies), thus arguing for a common origin or site for r-process nucleosynthesis of heavier (Z > 56) elements. However, the large (and correlated) scatters of [Eu, Os, Ir, Pt /Fe] suggest that the heaviest neutron-capture r-process elements are not formed in all supernovae. In contrast, the Ge abundances of all program stars track their Fe abundances, very well. An explosive process on iron peak nuclei (e.g., the -rich freezeout in supernovae), rather than neutron capture, appears to have been the dominant synthesis mechanism for this element at low metallicities: Ge abundances seem completely uncorrelated with Eu. The correlation (with very small scatter) of Ge and Fe abundances suggests that Ge must have been produced rather commonly in stars, even at early times in the Galaxy, over a wide range of metallicity. The Zr abundances show much the same behavior as Ge with (perhaps) somewhat more scatter, suggesting some variations in abundance with respect to Fe. The Zr abundances also do not vary cleanly with Eu abundances, indicating a synthesis origin different than that of heavier neutron-capture elements. Detailed abundance distributions for CS 22892À052 and BD +17 3248, combining the new elemental determinations for Os-Pt and recently published Nd and Ho measurements, show excellent agreement with the solar system r-process curve from the elements Ba to Pb. The lighter n-capture elements, including Ge, in general fall below the same solar system r-process curve that matches the heavier elements.
The Astronomical Journal, 2003
We have used the Ultra-Violet Echelle Spectrograph (UVES) on Kueyen (UT2) of the VLT to take spec... more We have used the Ultra-Violet Echelle Spectrograph (UVES) on Kueyen (UT2) of the VLT to take spectra of 15 individual red giant stars in the centers of four nearby dwarf spheroidal galaxies: Sculptor, Fornax, Carina and Leo I. We measure the abundance variations of numerous elements in these low mass stars with a range of ages (1−15 Gyr old). This means that we can effectively measure the chemical evolution of these galaxies with time.
New Astronomy Reviews, 2001
The Astrophysical Journal, 2007
We present new intermediate-band Strömgren photometry based on more than 300 u, v, b, y images of... more We present new intermediate-band Strömgren photometry based on more than 300 u, v, b, y images of the Galactic globular cluster ω Cen. Optical data were supplemented with new multiband nearinfrared (NIR) photometry (350 J, H, K s images). The final optical-NIR catalog covers a region of more than 20 × 20 arcmin squared across the cluster center. We use different optical-NIR color-color planes together with proper motion data available in the literature to identify candidate cluster red giant (RG) stars. By adopting different Strömgren metallicity indices we estimate the photometric metallicity for ≈ 4, 000 RGs, the largest sample ever collected. The metallicity distributions show multiple peaks ([Fe/H] phot = −1.73 ± 0.08, −1.29 ± 0.03, −1.05 ± 0.02, −0.80 ± 0.04, −0.42 ± 0.12 and −0.07 ± 0.08 dex) and a sharp cut-off in the metal-poor tail ([Fe/H] phot −2 dex) that agree quite well with spectroscopic measurements. We identify four distinct sub-populations, namely metalpoor MP, [Fe/H] ≤ −1.49), metal-intermediate (MI, −1.49 < [Fe/H] ≤ −0.93), metal-rich (MR, −0.95 < [Fe/H] ≤ −0.15) and solar metallicity (SM, [Fe/H] ≈ 0). The last group includes only a small fraction of stars (∼ 8 ± 5%) and should be confirmed spectroscopically. Moreover, using the difference in metallicity based on different photometric indices, we find that the 19 ± 1% of RGs are candidate CN-strong stars. This fraction agrees quite well with recent spectroscopic estimates and could imply a large fraction of binary stars. The Strömgren metallicity indices display a robust correlation with α-elements ([Ca+Si/H]) when moving from the metal-intermediate to the metal-rich regime ([Fe/H] −1.7 dex). 1 Based on observations collected in part with the 1.54m Danish Telescope and with the NTT@ESO Telescope operated in La Silla, and in part with the VLT@ESO Telescope operated in Paranal. The Strömgren data were collected with DFOSC2@Danish (proprietary data), while the NIR data were collected with SOFI@NTT, proposals: 66.D-0557 and 68D-0545 (proprietary data), 073.D-0313 and 59.A-9004 (ESO Science archive) and with ISAAC@VLT, proposal 075.D-0824 (proprietary data).
Astronomy and Astrophysics, 2007
Context. Determinations of beryllium abundance in stars, together with lithium, provide a key too... more Context. Determinations of beryllium abundance in stars, together with lithium, provide a key tool to investigate the so far poorly understood extra-mixing processes at work in stellar interiors. Aims. We measured Be in three open clusters, complementing existing Be surveys, and aiming at gathering a more complete empirical scenario of the evolution of Be as a function of stellar age and temperature. Methods. We analyzed VLT/UVES spectra of members of NGC 2516, the Hyades, and M 67 to determine their Be and Li abundances. In the first two clusters we focused on stars cooler than 5400 K, while the M 67 sample includes stars warmer than 6150 K, as well as two subgiants and two blue stragglers. We also computed the evolution of Be for a 0.9 M ⊙ star based on standard evolutionary models. Results. We find different behaviours for stars in different temperature bins and ages. Stars warmer than 6150 K show Be depletion and follow a Be vs. Li correlation, while Be is undepleted in stars in the ∼ 6150 − 5600 K range. NGC 2516 members cooler than 5400 K have not depleted any Be, while older Hyades of similar temperature show some depletion. Be is severely depleted in the subgiants and blue stragglers. Conclusions. The results for warm stars are in agreement with those of previous studies, supporting the hypothesis that mixing in this temperature regime is driven by rotation. The same holds for the two subgiants that have evolved from the "Li gap". This mechanism is instead not the dominant one for solar-type stars. Be depletion of cool Hyades cannot simply be explained by the effect of increasing depth of the convective zone. Finally, the different Be content of the two blue stragglers suggests that they have formed by two different processes (i.e., collisions vs. binary merging).
Arxiv preprint astro-ph/ …, 2006
Large surveys of very metal-poor stars have revealed in recent years that a large fraction of the... more Large surveys of very metal-poor stars have revealed in recent years that a large fraction of these objects were carbon-rich, analogous to the more metal-rich CH-stars. The abundance peculiarities of CH-stars are commonly explained by mass-transfer from a more evolved companion. In an effort to better understand the origin and importance for Galactic evolution of Fe-poor, C-rich stars, we present abundances determined from high-resolution and high signal-to-noise spectra obtained with the UVES instrument attached to the ESO/VLT. Our analysis of carbon-enhanced objects includes both CH stars and more metal-poor objects, and we explore the link between the two classes. We also present preliminary results of our ongoing radial velocity monitoring.
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Papers by francesca primas