Efficient Searches for r-Process-Enhanced, Metal-Poor Stars

Neutron-capture-enhanced, metal-poor stars are of central importance to developing an understanding of the operation of the r-process in the early Galaxy, thought to be responsible for the formation of roughly half of all elements beyond the iron peak. A handful of neutron-capture-rich, metal-poor stars with [Fe/H] < -2.0 have already been identified, including the well known r-process-enhanced stars CS 22892-052 and CS 31082-001. However, many questions of fundamental interest can only be addressed with the assemblage of a much larger sample of such stars, so that general properties can be distinguished. We describe a new effort, HERES: The Hamburg/ESO R-Process-Enhanced Star survey, nearing completion, which will identify on the order of 5-10 additional highly r-process-enhanced, metal-poor stars, and in all likelihood, a similar or greater number of mildly r-process-enhanced, metal-poor stars in the halo of the Galaxy. HERES is based on rapid "snapshot" spectra of over 350 candidate halo giants with [Fe/H] < -2.0, obtained at moderately high resolution, and with moderate signal-to-noise ratios, using the UVES spectrograph on the European VLT 8m telescope.Comment: Contributed paper to The Eighth Nuclei in the Cosmos conference, to appear (in refereed form) in Nuclear Physics

The solar photospheric silicon abundance according to CO5BOLD: Investigating line broadening, magnetic fields, and model effects

Context. In this work, we present a photospheric solar silicon abundance derived using CO5BOLD model atmospheres and the LINFOR3D spectral synthesis code. Previous works have differed in their choice of a spectral line sample and model atmosphere as well as their treatment of observational material, and the solar silicon abundance has undergone a downward revision in recent years. We additionally show the effects of the chosen line sample, broadening due to velocity fields, collisional broadening, model spatial resolution, and magnetic fields. Aims. Our main aim is to derive the photospheric solar silicon abundance using updated oscillator strengths and to mitigate model shortcomings such as over-broadening of synthetic spectra. We also aim to investigate the effects of different line samples, fitting configurations, and magnetic fields on the fitted abundance and broadening values. Methods. CO5BOLD model atmospheres for the Sun were used in conjunction with the LINFOR3D spectral synthesis code to generate model spectra, which were then fit to observations in the Hamburg solar atlas. We took pixel-to-pixel signal correlations into account by means of a correlated noise model. The choice of line sample is crucial to determining abundances, and we present a sample of 11 carefully selected lines (from an initial choice of 39 lines) in both the optical and infrared, which has been made possible with newly determined oscillator strengths for the majority of these lines. Our final sample includes seven optical Si i lines, three infrared Si i lines, and one optical Si ii line. Results. We derived a photospheric solar silicon abundance of log ϵSi = 7.57 ± 0.04, including a - 0.01 dex correction from Non-Local Thermodynamic Equilibrium (NLTE) effects. Combining this with meteoritic abundances and previously determined photospheric abundances results in a metal mass fraction Z/X = 0.0220 ± 0.0020. We found a tendency of obtaining overly broad synthetic lines. We mitigated the impact of this by devising a de-broadening procedure. The over-broadening of synthetic lines does not substantially affect the abundance determined in the end. It is primarily the line selection that affects the final fitted abundance

On the Formation Height of the SDO/HMI Fe 6173 Doppler Signal

The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) is designed to study oscillations and the mag- netic field in the solar photosphere. It observes the full solar disk in the Fe I absorption line at 6173\AA . We use the output of a high-resolution 3D, time- dependent, radiation-hydrodynamic simulation based on the CO5BOLD code to calculate profiles F({\lambda},x,y,t) for the Fe I 6173{\AA} line. The emerging profiles F({\lambda},x,y,t) are multiplied by a representative set of HMI filter transmission profiles R_i({\lambda},1 \leq i \leq 6) and filtergrams I_i(x,y,t;1 \leq i \leq 6) are constructed for six wavelengths. Doppler velocities V_HMI(x,y,t) are determined from these filtergrams using a simplified version of the HMI pipeline. The Doppler velocities are correlated with the original velocities in the simulated atmosphere. The cross- correlation peaks near 100 km, suggesting that the HMI Doppler velocity signal is formed rather low in the solar atmosphere. The same analysis is performed for the SOHO/MDI Ni I line at 6768\AA . The MDI Doppler signal is formed slightly higher at around 125 km. Taking into account the limited spatial resolution of the instruments, the apparent formation height of both the HMI and MDI Doppler signal increases by 40 to 50 km. We also study how uncertainties in the HMI filter-transmission profiles affect the calculated velocities.Comment: 15 pages, 11 Figure

Stokes Diagnostis of 2D MHD-simulated Solar Magnetogranulation

We study the properties of solar magnetic fields on scales less than the spatial resolution of solar telescopes. A synthetic infrared spectropolarimetric diagnostics based on a 2D MHD simulation of magnetoconvection is used for this. We analyze two time sequences of snapshots that likely represent two regions of the network fields with their immediate surrounding on the solar surface with the unsigned magnetic flux density of 300 and 140 G. In the first region we find from probability density functions of the magnetic field strength that the most probable field strength at logtau_5=0 is equal to 250 G. Weak fields (B < 500 G) occupy about 70% of the surface, while stronger fields (B 1000 G) occupy only 9.7% of the surface. The magnetic flux is -28 G and its imbalance is -0.04. In the second region, these parameters are correspondingly equal to 150 G, 93.3 %, 0.3 %, -40 G, and -0.10. We estimate the distribution of line-of-sight velocities on the surface of log tau_5=-1. The mean velocity is equal to 0.4 km/s in the first simulated region. The averaged velocity in the granules is -1.2 km/s and in the intergranules is 2.5 km/s. In the second region, the corresponding values of the mean velocities are equal to 0, -1.8, 1.5 km/s. In addition we analyze the asymmetry of synthetic Stokes-V profiles of the Fe I 1564.8 nm line. The mean values of the amplitude and area asymmetry do not exceed 1%. The spatially smoothed amplitude asymmetry is increased to 10% while the area asymmetry is only slightly varied.Comment: 24 pages, 12 figure

The yields of r-process elements and chemical evolution of the Galaxy

The supernova yields of r-process elements are obtained as a function of the mass of their progenitor stars from the abundance patterns of extremely metal-poor stars on the left-side [Ba/Mg]-[Mg/H] boundary with a procedure proposed by Tsujimoto and Shigeyama. The ejected masses of r-process elements associated with stars of progenitor mass $M_{ms}\leq18M_{\odot}$ are infertile sources and the SNe II with 20$M_{\odot}\leq M_{ms}\leq 40M_{\odot}$are the dominant source of r-process nucleosynthesis in the Galaxy. The ratio of these stars 20$M_{\odot}\leq M_{ms}\leq40M_{\odot}$ with compared to the all massive stars is about $\sim$18%. In this paper, we present a simple model that describes a star's [r/Fe] in terms of the nucleosynthesis yields of r-process elements and the number of SN II explosions. Combined the r-process yields obtained by our procedure with the scatter model of the Galactic halo, the observed abundance patterns of the metal-poor stars can be well reproducedComment: 7 pages, 6 figures, Accepted for publication in Astrophysics and Space Scienc

The Gaia-ESO Public Spectroscopic Survey: Motivation, implementation, GIRAFFE data processing, analysis, and final data products

Context. The Gaia-ESO Public Spectroscopic Survey is an ambitious project designed to obtain astrophysical parameters and elemental abundances for 100 000 stars, including large representative samples of the stellar populations in the Galaxy, and a well-defined sample of 60 (plus 20 archive) open clusters. We provide internally consistent results calibrated on benchmark stars and star clusters, extending across a very wide range of abundances and ages. This provides a legacy data set of intrinsic value, and equally a large wide-ranging dataset that is of value for the homogenisation of other and future stellar surveys and Gaia's astrophysical parameters. Aims. This article provides an overview of the survey methodology, the scientific aims, and the implementation, including a description of the data processing for the GIRAFFE spectra. A companion paper introduces the survey results. Methods. Gaia-ESO aspires to quantify both random and systematic contributions to measurement uncertainties. Thus, all available spectroscopic analysis techniques are utilised, each spectrum being analysed by up to several different analysis pipelines, with considerable effort being made to homogenise and calibrate the resulting parameters. We describe here the sequence of activities up to delivery of processed data products to the ESO Science Archive Facility for open use. Results. The Gaia-ESO Survey obtained 202 000 spectra of 115 000 stars using 340 allocated VLT nights between December 2011 and January 2018 from GIRAFFE and UVES. Conclusions. The full consistently reduced final data set of spectra was released through the ESO Science Archive Facility in late 2020, with the full astrophysical parameters sets following in 2022. A companion article reviews the survey implementation, scientific highlights, the open cluster survey, and data products

The Gaia-ESO Public Spectroscopic Survey: Implementation, data products, open cluster survey, science, and legacy

Context. In the last 15 years different ground-based spectroscopic surveys have been started (and completed) with the general aim of delivering stellar parameters and elemental abundances for large samples of Galactic stars, complementing Gaia astrometry. Among those surveys, the Gaia-ESO Public Spectroscopic Survey, the only one performed on a 8m class telescope, was designed to target 100 000 stars using FLAMES on the ESO VLT (both Giraffe and UVES spectrographs), covering all the Milky Way populations, with a special focus on open star clusters. Aims. This article provides an overview of the survey implementation (observations, data quality, analysis and its success, data products, and releases), of the open cluster survey, of the science results and potential, and of the survey legacy. A companion article reviews the overall survey motivation, strategy, Giraffe pipeline data reduction, organisation, and workflow. Methods. We made use of the information recorded and archived in the observing blocks; during the observing runs; in a number of relevant documents; in the spectra and master catalogue of spectra; in the parameters delivered by the analysis nodes and the working groups; in the final catalogue; and in the science papers. Based on these sources, we critically analyse and discuss the output and products of the Survey, including science highlights. We also determined the average metallicities of the open clusters observed as science targets and of a sample of clusters whose spectra were retrieved from the ESO archive. Results. The Gaia-ESO Survey has determined homogeneous good-quality radial velocities and stellar parameters for a large fraction of its more than 110 000 unique target stars. Elemental abundances were derived for up to 31 elements for targets observed with UVES. Lithium abundances are delivered for about 1/3 of the sample. The analysis and homogenisation strategies have proven to be successful; several science topics have been addressed by the Gaia-ESO consortium and the community, with many highlight results achieved. Conclusions. The final catalogue will be released through the ESO archive in the first half of 2022, including the complete set of advanced data products. In addition to these results, the Gaia-ESO Survey will leave a very important legacy, for several aspects and for many years to come

The broadening of strong lines of Ca+, Mg+ and Ba+ by collisions with neutral hydrogen atoms

The theory of Anstee & O'Mara is extended to the line broadening of transitions of singly ionized atoms by collisions with neutral hydrogen atoms. The theory is used to calculate broadening cross-sections for strong lines of singly ionized calcium, magnesium and barium. The broadening cross-sections calculated are compared with both theoretical and empirical results of other workers

The broadening of d-f and f-d transitions by collisions with neutral hydrogen atoms

The theory of Anstee & O'Mara for the broadening of spectral lines owing to collisions by neutral hydrogen atoms is extended to d-f and f-d transitions of neutral atoms. Width cross-sections are tabulated against effective principal quantum numbers for the upper and lower states of the transition, for a relative collision velocity of 10 ms. The cross-sections are fitted to a variation with velocity of v, and results for the velocity parameter a are similarly tabulated. The data are tested by application to d-f spectral lines in the solar spectrum. The derived abundances are consistent with those for meteorites

Line broadening cross sections for the broadening of transitions of neutral atoms by collisions with neutral hydrogen

Line broadening cross sections for the broadening of spectral lines by collisions with neutral hydrogen atoms have been tabulated by Anstee & O'Mara (1995), Barklem & O'Mara (1997) and Barklem, O'Mara & Ross (1998) for s-p, p-s, p-d, d-p, d-f and f-d transitions. To make these data more accessible to the end user, fortran code which interpolates in these tabulations has been prepared and placed on the World Wide Web. It should be easy to incorporate this code into existing spectrum synthesis programs or to use it in a stand-alone mode to compute line broadening cross sections for specific transitions. The use of the code is demonstrated by its application to two transitions of astrophysical interest