Abundances Of Si Research Articles

INFRARED AND X-RAY SPECTROSCOPY OF THE Kes 75 SUPERNOVA REMNANT SHELL: CHARACTERIZING THE DUST AND GAS PROPERTIES

We present deep Chandra observations and Spitzer Space Telescope infrared (IR) spectroscopy of the shell in the composite supernova remnant (SNR) Kes 75 (G29.7-0.3). The remnant is composed of a central pulsar wind nebula and a bright partial shell in the south that is visible at radio, IR, and X-ray wavelengths. The X-ray emission can be modeled by either a single thermal component with a temperature of ~ 1.5 keV, or with two thermal components with temperatures of 1.5 and 0.2 keV. Previous studies suggest that the hot component may originate from reverse-shocked SN ejecta. However, our new analysis shows no definitive evidence for enhanced abundances of Si, S, Ar, Mg, and Fe, as expected from supernova (SN) ejecta, or for the IR spectral signatures characteristic of confirmed SN condensed dust, thus favoring a circumstellar or interstellar origin for the X-ray and IR emission. The X-ray and IR emission in the shell are spatially correlated, suggesting that the dust particles are collisionally heated by the X-ray emitting gas. The IR spectrum of the shell is dominated by continuum emission from dust with little, or no line emission. Modeling the IR spectrum shows that the dust is heated to a temperature of ~ 140 K by a relatively dense, hot plasma, that also gives rise to the hot X-ray emission component. The density inferred from the IR emission is significantly higher than the density inferred from the X-ray models, suggesting a low filling factor for this X-ray emitting gas. The total mass of the warm dust component is at least 0.013 solar masses, assuming no significant dust destruction has occurred in the shell. The IR data also reveal the presence of an additional plasma component with a cooler temperature, consistent with the 0.2 keV gas component. Our IR analysis therefore provides an independent verification of the cooler component of the X-ray emission.

Suzaku Observations of Metal Distributions in the Intracluster Medium of the Centaurus Cluster

We report the first observations of metal distributions in the intracluster medium of the Centaurus cluster up to $\sim\ $ 0.17 $\ r_{180}$ with Suzaku. Radial profiles of the O, Mg, Si, S, Ar, Ca, and Fe were determined at the outer region of the cluster, and their variations appear to be similar to each other. Within the cool core region ( $r$$<$ 0.045 $\ r_{180}$ ), all of the metal distributions sharply increased toward the center. In the central region ( $r$$<$ 0.015 $\ r_{180}$ ), the abundances of Si, S, Ar, Ca, and Fe were 1.5–1.8 solar, while those of O and Mg were approximately 1 solar. The derived abundance ratios of O and Mg to Fe were slightly lower than those of a set of other clusters. In contrast, the calculated mass-to-light ratios (MLRs) for O, Mg, and Fe were larger than those of the other clusters. For the outer region of the cool core ( $r$$>$ 0.07 $\ r_{180}$ ), all of the abundances were almost constant at 0.5 solar. The derived MLRs were comparable to those of the other clusters. This suggests that the cD galaxy of the Centaurus cluster efficiently supplies more Fe than the other clusters.

Magnetic field and atmospheric chemical abundances of the magnetic Ap star HD 318107

New spectra have been obtained with the ESPaDOnS spectropolarimeter supplemented with unpolarised spectra from the ESO UVES, UVES-FLAMES, and HARPS spectrographs of the very peculiar large-field magnetic Ap star HD 318107, a member of the open cluster NGC 6405. The available data provide sufficient material with which to re-analyse the first-order model of the magnetic field geometry and to derive abundances of Si, Ti, Fe, Nd, Pr, Mg, Cr, Mn, O, and Ca. The magnetic field structure was modelled with a low-order colinear multipole expansion, using coefficients derived from the observed variations of the field strength with rotation phase. The abundances of several elements were determined using spectral synthesis. After experiments with a very simple model of uniform abundance on each of three rings of equal width in co-latitude and symmetric about the assumed magnetic axis, we decided to model the spectra assuming uniform abundances of each element over the stellar surface. The new magnetic field measurements allow us to refine the rotation period of HD 318107 to P = 9.7088 +/- 0.0007 days. Appropriate magnetic field model parameters were found that very coarsely describe the (apparently rather complex) field moment variations. Spectrum synthesis leads to the derivation of mean abundances for the elements Mg, Si, Ca, Ti, Cr, Fe, Nd, and Pr. All of these elements except for Mg and Ca are strongly overabundant compared to the solar abundance ratios. There is considerable evidence of non-uniformity, for example in the different values of B_z found using lines of different elements. The present data set, while limited, is nevertheless sufficient to provide a useful first-order assessment of both the magnetic and surface abundance properties of HD 318107, making it one of the very few magnetic Ap stars of well-known age for which both of these properties have been studied.

DA white dwarfs in Sloan Digital Sky Survey Data Release 7 and a search for infrared excess emission

We present HST ultraviolet spectroscopy of the white dwarfs PG0843+516, PG1015+161, SDSS1228+1040, and GALEX1931+0117, which accrete circumstellar planetary debris formed from the destruction of asteroids. Combined with optical data, a minimum of five and a maximum of eleven different metals are detected in their photospheres. With metal sinking time scales of only a few days, these stars are in accretion/diffusion equilibrium, and the photospheric abundances closely reflect those of the circumstellar material. We find C/Si ratios that are consistent with that of the bulk Earth, corroborating the rocky nature of the debris. Their C/O values are also very similar to those of bulk Earth, implying that the planetary debris is dominated by Mg and Fe silicates. The abundances found for the debris at the four white dwarfs show substantial diversity, comparable at least to that seen across different meteorite classes in the solar system. PG0843+516 exhibits significant over-abundances of Fe and Ni, as well as of S and Cr, which suggests the accretion of material that has undergone melting, and possibly differentiation. PG1015+161 stands out by having the lowest Si abundance relative to all other detected elements. The Al/Ca ratio of the planetary debris around different white dwarfs is remarkably similar. This is analogous to the nearly constant abundance ratio of these two refractory lithophile elements found among most bodies in the solar system. Based on the detection of all major elements of the circumstellar debris, we calculate accretion rates of ~1.7e8g/s ~1.5e9g/s. We detect additional circumstellar absorption in the SiIV 1394,1403 doublet in PG0843+516 and SDSS1228+1040, reminiscent to similar high-ionisation lines seen in white dwarfs in cataclysmic variables. We suspect that these lines originate in hot gas close to the white dwarf, well within the sublimation radius.

CHEMICAL ABUNDANCES OF THE SECONDARY STAR IN THE BLACK HOLE X-RAY BINARY V404 CYGNI

We present a chemical abundance analysis of the secondary star in the black hole binary V404 Cygni, using Keck I/HIRES spectra. We adopt a $\chi^2$-minimization procedure to derive the stellar parameters, taking into account any possible veiling from the accretion disk. With these parameters we determine the atmospheric abundances of O, Na, Mg, Al, Si, Ca, Ti, Fe, and Ni. The abundances of Al, Si, and Ti appear to be slightly enhanced when comparing with average values in thin-disk solar-type stars. The O abundance, derived from optical lines, is particularly enhanced in the atmosphere of the secondary star in V404 Cygni. This, together with the peculiar velocity of this system as compared with the Galactic velocity dispersion of thin-disk stars, suggests that the black hole formed in a supernova or hypernova explosion. We explore different supernova/hypernova models having various geometries to study possible contamination of nucleosynthetic products in the chemical abundance pattern of the secondary star. We find reasonable agreement between the observed abundances and the model predictions. However, the O abundance seems to be too high regardless of the choice of explosion energy or mass cut, when trying to fit other element abundances. Moreover, Mg appears to be underabundant for all explosion models, which produces Mg abundances roughly 2 times higher than the observed value.

Analysis of MESSENGER Gamma-Ray Spectrometer data from the Mercury flybys

During its three flybys of Mercury, the MESSENGER spacecraft made the first detection of gamma-ray emission from the planet's surface. With a closest approach distance of ∼200 km, the flybys provided an opportunity to measure elemental abundances of Mercury's near-equatorial regions, which will not be visited at low altitude during MESSENGER's orbital mission phase. Despite being limited by low planetary photon flux, sufficient counts were accumulated during the first two flybys to estimate bounds on abundances for some elements having relatively strong gamma-ray spectral peaks, including Si, Fe, Ti, K, and Th. Only for Si is the standard deviation σ sufficiently small to conclude that this element was detected with 99% confidence. Iron and potassium are detected at the 2− σ (95% confidence) level, whereas only upper bounds on Ti and Th can be determined. Relative to a Si abundance assumed to be 18 weight percent (wt%), 2− σ upper bounds have been estimated as 9.7 wt% for Fe, 7.0 wt% for Ti, 0.087 wt% for K, and 2.2 ppm for Th. The relatively low upper bound on K rules out some previously suggested models for surface composition for the regions sampled. Upper bounds on Fe/Si and Ti/Si ratios are generally consistent with Ti and Fe abundances estimated from the analysis of measurements by the MESSENGER Neutron Spectrometer during the flybys but are also permissive of much lower concentrations.

Zircon U–Pb ages, Hf–O isotopes and whole-rock Sr–Nd–Pb isotopic geochemistry of granitoids in the Jinshajiang suture zone, SW China: Constraints on petrogenesis and tectonic evolution of the Paleo-Tethys Ocean

The Jinshajiang suture zone, located in the eastern part of the Tethyan tectonic domain, is noticeable for a large-scale distribution of Late Jurassic to Triassic granitoids. These granitoids were genetically related to the evolution of the Paleo-Tethys Ocean. Beiwu, Linong and Lunong granitoids occur in the middle zone of the Jinshajiang Suture Zone, and possess similar geochemical features, indicating that they share a common magma source. SIMS zircon U–Pb dating reveals that the Beiwu, Linong and Lunong granitic intrusions were emplaced at 233.9 ± 1.4 Ma (2σ), 233.1 ± 1.4 Ma (2σ) and 231.0 ± 1.6 Ma (2σ), respectively. All of these granitoids are enriched in abundances of Si (SiO 2t = 65.2–73.5 wt.%), and large-ion lithophile elements (LILEs), but depleted in high field strength elements contents (HFSEs, e.g., Nb, Ta, and Ti). In addition, they have low P 2O 5 contents (0.06–0.11 wt.%), A/CNK values ([molecular Al 2O 3/(CaO + Na 2O + K 2O)], mostly < 1.1) and 10,000 Ga/Al ratios (1.7–2.2), consistent with the characteristics of I-type granites. In terms of isotopic compositions, these granitoids have high initial 87Sr/ 86Sr ratios (0.7078–0.7148), Pb isotopic compositions [( 206Pb/ 204Pb) t = 18.213–18.598, ( 207Pb/ 204Pb) t = 15.637–15.730 and ( 208Pb/ 204Pb) t = 38.323–38.791], zircon δ 18O values (7.3‰–9.3‰) and negative ε Nd(t) values (− 5.1 to − 6.7), suggesting they were predominantly derived from the continental crust. Their Nb/Ta ratios (average value = 8.6) are consistent with those of the lower continental crust (LCC). However, variable ε Hf(t) values (− 8.6 to + 2.8) and the occurrences of mafic microgranular enclaves (MMEs) suggest that mantle-derived melts and lower crustal magmas were involved in the generation of these granitoids. Moreover, the high Pb isotopic ratios and elevated zircon δ 18O values of these rocks indicate a significant contribution of the upper crustal composition. We propose a model in which the Beiwu, Linong and Lunong granitoids were generated under a late collisional or post-collisional setting. It is possible that this collision was completed before Late Triassic. Decompression induced mantle-derived magmas to be underplated and provided the heat for the anatexis of the crust. Hybrid melts including mantle-derived and the lower crustal magmas were then generated. The hybrid melts thereafter ascended to a shallow depth and resulted in some degree of sedimentary rock assimilation. Such three-component mixing magma source and subsequent fractional crystallization could be responsible for the formation of the Beiwu, Linong and Lunong granitoids.

BLUE LUMINOUS STARS IN NEARBY GALAXIES—UIT 005: A POSSIBLE LINK TO THE LUMINOUS BLUE VARIABLE STAGE

(abridged) A detailed study of the blue supergiant UIT005 (B2-2.5Ia+) in M33 is presented. The results of our quantitative spectral analysis indicate that the star is a very luminous, log(L/Lsun)~5.9 dex, and massive, M~50 Msun, object, showing a very high nitrogen-to-oxygen ratio in its surface (N/O~8, by mass). Based on the derived Mg and Si abundances, we argue that this high N/O ratio cannot be the result of an initial low O content due to its location on the disk of M33, known to present a steep metallicity gradient. In combination with the He abundance, the most plausible interpretation is that UIT005 is in an advanced stage of evolution, showing in its surface N enrichment and O depletion resulting from mixing with CNO processed material from the stellar interior. A comparison with the predictions of current stellar evolutionary models indicates that there are significant discrepancies, in particular with regard to the degree of chemical processing, with the models predicting a much lower degree of O depletion than observed. At the same time, the mass-loss rate derived in our analysis is an order of magnitude lower than the values considered in the evolutionary calculations. Based on a study of the surrounding stellar population and the nearby cluster NGC588, using WFPC2 photometry, we suggest that UIT005 could be in fact a runaway star from this cluster.

Elemental abundances of low-mass stars in the young clusters 25 Orionis andλ Orionis

Aims. We aim to derive the chemical pattern of the young clusters 25 Orionis and lambda Orionis through homogeneous and accurate measurements of elemental abundances. Methods. We present FLAMES/UVES observations of a sample of 14 K-type targets in the 25 Ori and lambda Ori clusters; we measure their radial velocities, in order to confirm cluster membership. We derive stellar parameters and abundances of Fe, Na, Al, Si, Ca, Ti, and Ni using the code MOOG. Results. All the 25 Ori stars are confirmed cluster members without evidence of binarity; in lambda Ori we identify one non-member and one candidate single-lined binary star. We find an average metallicity [Fe/H]=-0.05+/-0.05 for 25 Ori, where the error is the 1sigma standard deviation from the average. lambda Ori members have a mean iron abundance value of 0.01+/-0.01. The other elements show close-to-solar ratios and no star-to-star dispersion. Conclusions. Our results, along with previous metallicity determinations in the Orion complex, evidence a small but detectable dispersion in the [Fe/H] distribution of the complex. This appears to be compatible with large-scale star formation episodes and initial non-uniformity in the pre-cloud medium. We show that, as expected, the abundance distribution of star forming regions is consistent with the chemical pattern of the Galactic thin disk.

ALPHA ENHANCEMENT AND THE METALLICITY DISTRIBUTION FUNCTION OF PLAUT’S WINDOW

We present Fe, Si, and Ca abundances for 61 giants in Plaut's Window (l=-1,b=-8.5) and Fe abundances for an additional 31 giants in a second, nearby field (l=0,b=-8) derived from high resolution (R~25,000) spectra obtained with the Blanco 4m telescope and Hydra multifiber spectrograph. The median metallicity of red giant branch (RGB) stars in the Plaut field is ~0.4 dex lower than those in Baade's Window, and confirms the presence of an iron abundance gradient along the bulge minor axis. The full metallicity range of our (biased) RGB sample spans -1.5<[Fe/H]<+0.3, which is similar to that found in other bulge fields. We also derive a photometric metallicity distribution function for RGB stars in the (l=-1,b=-8.5) field and find very good agreement with the spectroscopic metallicity distribution. The radial velocity and dispersion data for the bulge RGB stars are in agreement with previous results of the BRAVA survey, and we find evidence for a decreasing velocity dispersion with increasing [Fe/H]. The [alpha/Fe] enhancement in Plaut field stars is nearly identical to that observed in Baade's window, and suggests that an [alpha/Fe] gradient does not exist between b=-4 and -8. Additionally, a subset of our sample (23 stars) appear to be foreground red clump stars that are very metal--rich, exhibit small metallicity and radial velocity dispersions, and are enhanced in alpha elements. While these stars likely belong to the Galactic inner disk population, they exhibit [alpha/Fe] ratios that are enhanced above the thin and thick disk.

Lunar X-ray fluorescence observations by the Chandrayaan-1 X-ray Spectrometer (C1XS): Results from the nearside southern highlands

The Chandrayaan-1 X-ray Spectrometer (C1XS) flown on-board the first Indian lunar mission Chandrayaan-1, measured X-ray fluorescence spectra during several episodes of solar flares during its operational period of ∼9 months. The accompanying X-ray Solar Monitor (XSM) provided simultaneous spectra of solar X-rays incident on the Moon which are essential to derive elemental chemistry. In this paper, we present the surface abundances of Mg, Al, Si, Ca and Fe, derived from C1XS data for a highland region on the southern nearside of the Moon. Analysis techniques are described in detail including absolute X-ray line flux derivation and conversion into elemental abundance. The results are consistent with a composition rich in plagioclase with a slight mafic mineral enhancement and a Ca/Al ratio that is significantly lower than measured in lunar returned samples. We suggest various possible scenarios to explain the deviations.

Peculiarities of physical property distribution on the surface of CP Star HD182255

The star HD182255 reveals spectral variability in rotation, irregular short-term variability, as well as long-term variations in the spectra. The He and Si abundances vary in antiphase. According to the theory, silicon accumulates in the regions between the poles, where the field lines are horizontal. Hence the observed behavior of He and Si may indicate the presence of magnetic field, where one of the poles passes through the central meridian in phase Φ = 0 as the star rotates. Given a general weakening of He over the entire surface, in Φ = 0 its abundance is higher, which may be linked with the presence of wind from the magnetic pole. The star definitely belongs to the He-w type. The study of the structure of the upper atmosphere based on the hydrogen line profiles shows that it varies over the years. Our estimates of the magnetic field from the Zeeman spectra are within the measurement errors. According to the spectroscopic characteristics the star can be considered weakly magnetic.

An estimate for the bulk composition of juvenile upper continental crust derived from glacial till in the North American Cordillera

We use a compilation of bulk chemical analyses for 2771 rocks and 3487 till samples (silt + clay fraction) from British Columbia, Canada to construct an estimate of juvenile upper crust composition in the North American Cordillera for comparison with upper crust estimates for older continental crust. Principle component analyses demonstrate that glacial tills are a robust natural average of their surface rock provenance. The chemical weathering during the generation and deposition of tills is small, yet they preserve the chemical weathering history of their source regions. Our results show that the upper continental crust of the Cordillera is broadly similar to other estimates in terms of the abundance of Si, Al, Mg, Ca, Na, Zr, Y, Sr, Ba, Th and U. The till-based upper crustal estimate, however, shows consistent and marked enrichment in Fe, Ti, Mn, P and HREE and depletion in LREE and K. These attributes are not obviously attributed to any analytical or sampling bias; surface rock averages and modern river sediments from British Columbia show identical patterns of enrichment and depletion as the tills. We attribute the unique character of our Cordilleran juvenile upper crust estimate to a significant component of basaltic soils in till generation, as compared to upper crustal estimates from more mature denuded terrains. Nonetheless, our crustal estimate remains overall ‘andesitic’, similar to those from Precambrian shields, supporting the notion that reworking of old crust is volumetrically far more important than production of juvenile crust in the growth of the North American continent.

The Elemental Abundance Distributions of Milky Way Satellite Galaxies

The chemical compositions of the stars in Milky Way (MW) satellite galaxies reveals the history of gas flows and star formation (SF) intensity. This talk presented a Keck/DEIMOS spectroscopic survey of the Fe, Mg, Si, Ca, and Ti abundances of nearly 3000 red giants in eight MW dwarf satellites. The metallicity and alpha-to-iron ratio distributions obey the following trends: (1) The more luminous galaxies are more metal-rich, indicating that they retained gas more efficiently than the less luminous galaxies. (2) The shapes of the metallicity distributions of the more luminous galaxies require gas infall during their SF lifetimes. (3) At [Fe/H] < -1.5, [alpha/Fe] falls monotonically with increasing [Fe/H] in all MW satellites. One interpretation of these trends is that the SF timescale in any MW satellite is long enough that Type Ia supernovae exploded for nearly the entire SF lifetime.

An Abundance Analysis of the Primary Star of the Peculiar Eclipsing Binary $\epsilon$ Aurigae out of the Eclipsing Phase

Abstract A detailed abundance analysis of the primary star of $\epsilon$ Aur before an eclipse was carried out using a very high signal-to-noise ratio optical-region spectrum. An A7 Iab supergiant HD 81471, presumably a member of the Vela OB1 association, was used as a reference. We obtained atmospheric parameters (T$_{\rm eff}$, log g, and $\xi_{\rm t}$) to be (8025 K, 1.0 and 10 km s$^{-1}$) and (8050 K, 1.0 and 7 km s$^{-1}$) for $\epsilon$ Aur and HD 81471, respectively. The abundances of Mg, Si, S, Ca, Sc, Ti, Cr, and Fe were very close to the solar abundances in both stars. Light elements C and O were under-abundant, while N and Na were over-abundant in both stars, after correcting for non-LTE effects. Definite under-abundances of Sr were detected in both stars. Slight, but definite, over-abundances were found in $\epsilon$ Aur for the s-process elements Y, Zr, and Ba, when compared with the results of HD 81471. Enhancements in the abundances of N, Na, and s-process elements might indicate the occurences of thermal dredge-up and the s-process nucleosynthesis in $\epsilon$ Aur during the past evolution.

Metallicity of the Fossil Group NGC 1550 Observed with Suzaku

Abstract We studied the temperature and metal abundance distributions of the intra-cluster medium (ICM) in a group of galaxies, NGC 1550, observed with Suzaku. NGC 1550 is classified as a fossil group, which has few bright member galaxies, except for the central galaxy. Thus, such a type of galaxy is important to investigate how metals are enriched to the ICM. With the Suzaku XIS instrument, we directly measured not only the Si, S, and Fe lines, but also the O and Mg lines, and obtained those abundances to an outer region of $\sim\ $0.5$r_{180}$ for the first time, and confirmed that the metals in the ICM of such a fossil group are indeed extending to a large radius. We found steeper gradients for the Mg, Si, S, and Fe abundances, while O showed an almost flat abundance distribution. Abundance ratios of $\alpha$-elements to Fe were similar to those of the other groups and poor clusters. We calculated the number ratio of type II to type Ia supernovae for the ICM enrichment to be 2.9$\ \pm\ $0.5 within 0.1$r_{180}$; the value is consistent with those for the other groups and poor clusters observed with Suzaku. We also calculated the metal mass-to-light ratios (MLRs) for Fe, O, and Mg with the $B$-band and $K$-band luminosities of the member galaxies of NGC 1550. The derived MLRs are comparable to those of the NGC 5044 group in the $r$$&amp;lt;$ 0.1$r_{180}$ region, while those of NGC 1550 are slightly higher than those of NGC 5044 in the outer region.

ESTABLISHING A CONNECTION BETWEEN ACTIVE REGION OUTFLOWS AND THE SOLAR WIND: ABUNDANCE MEASUREMENTS WITH EIS/ HINODE

One of the most interesting discoveries of the X-ray Telescope and EUV Imaging Spectrometer (EIS) on board the Hinode solar observatory is the presence of persistent high temperature high speed outflows from the edges of active regions. Measurements by EIS indicate that the outflows reach velocities of 50 km/s with spectral line asymmetries approaching 200 km/s. It has been suggested that these outflows may lie on open field lines that connect to the heliosphere, and that they could potentially be a significant source of the slow speed solar wind. A direct link has been difficult to establish, however. In this letter, we use EIS measurements of spectral line intensities that are sensitive to changes in the relative abundance of Si and S as a result of the first ionization potential (FIP) effect, to measure the chemical composition in the outflow regions of AR 10978 over a period of 5 days in December 2007. We find that Si is always enhanced over S by a factor of 3--4. This is generally consistent with the enhancement factor of low FIP elements measured in-situ in the slow solar wind by non-spectroscopic methods. Plasma with a slow wind-like composition was therefore flowing from the edge of the active region for at least 5 days. Furthermore, on December 10--11, when the outflow from the western side was favorably oriented in the Earth direction, the Si/S ratio was found to match the value measured a few days later by ACE/SWICS. These results provide strong observational evidence for a direct connection between the solar wind, and the coronal plasma in the outflow regions.

CHEMICAL CLUES ON THE FORMATION OF PLANETARY SYSTEMS: C/O VERSUS Mg/Si FOR HARPS GTO SAMPLE

Theoretical studies suggest that C/O and Mg/Si are the most important elemental ratios in determining the mineralogy of terrestrial planets. The C/O ratio controls the distribution of Si among carbide and oxide species, while Mg/Si gives information about the silicate mineralogy. We present a detailed and uniform study of C, O, Mg and Si abundances for 61 stars with detected planets and 270 stars without detected planets from the homogeneous high-quality unbiased HARPS GTO sample, together with 39 more planet-host stars from other surveys. We determine these important mineralogical ratios and investigate the nature of the possible terrestrial planets that could have formed in those planetary systems. We find mineralogical ratios quite different from those of the Sun, showing that there is a wide variety of planetary systems which are not similar to Solar System. Many of planetary host stars present a Mg/Si value lower than 1, so their planets will have a high Si content to form species such as MgSiO$_{3}$. This type of composition can have important implications for planetary processes like plate tectonics, atmospheric composition or volcanism.

MULTI-ELEMENT ABUNDANCE MEASUREMENTS FROM MEDIUM-RESOLUTION SPECTRA. II. CATALOG OF STARS IN MILKY WAY DWARF SATELLITE GALAXIES

We present a catalog of Fe, Mg, Si, Ca, and Ti abundances for 2961 red giant stars that are likely members of eight dwarf satellite galaxies of the Milky Way (MW): Sculptor, Fornax, Leo I, Sextans, Leo II, Canes Venatici I, Ursa Minor, and Draco. For the purposes of validating our measurements, we also observed 445 red giants in MW globular clusters and 21 field red giants in the MW halo. The measurements are based on Keck/DEIMOS medium-resolution spectroscopy combined with spectral synthesis. We estimate uncertainties in [Fe/H] by quantifying the dispersion of [Fe/H] measurements in a sample of stars in monometallic globular clusters. We estimate uncertainties in Mg, Si, Ca, and Ti abundances by comparing our medium-resolution spectroscopic measurements to high-resolution spectroscopic abundances of the same stars. For this purpose, our DEIMOS sample included 132 red giants with published high-resolution spectroscopy in globular clusters, the MW halo field, and dwarf galaxies. The standard deviations of the differences in [Fe/H] and [alpha/Fe] (the average of [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe]) between the two samples is 0.15 and 0.16, respectively. This catalog represents the largest sample of multi-element abundances in dwarf galaxies to date. The next papers in this series draw conclusions on the chemical evolution, gas dynamics, and star formation histories from the catalog presented here. The wide range of dwarf galaxy luminosity reveals the dependence of dwarf galaxy chemical evolution on galaxy stellar mass.

Chemical pattern across the young associations ONC and OB1b

Context. Abundances of iron-peak and alpha-elements are poorly known in Orion, and the available measurements yield contradictory results. Aims. We measure accurate and homogeneous elemental abundances of the Orion subgroups ONC and OB1b, and search for abundance differences across the Orion complex. Methods. We present FLAMES/UVES spectroscopic observations of 20 members of the ONC and OB1b. We measured radial velocity, veiling, effective temperature using two spectroscopic methods, and determined the chemical abundances of Fe, Na, Al, Si, Ca, Ti, and Ni using the code MOOG. We also performed a new consistent analysis of spectra previously analyzed by our group. Results. We find three new binaries in the ONC, two in OB1b, and three non-members in OB1b (two of them most likely being OB1a/25 Ori members). Veiling only affects one target in the ONC, and the effective temperatures derived using two spectroscopic techniques agree within the errors. The ONC and OB1b are characterized by a small scatter in iron abundance, with mean [Fe/H] values of -0.11+/-0.08 and -0.05+/-0.05, respectively. We find a small scatter in all the other elemental abundances. We confirm that P1455 is a metal-rich star in the ONC. Conclusions. We conclude that the Orion metallicity is not above the solar value. The OB1b group might be slightly more metal-rich than the ONC; on the other hand, the two subgroups have similar almost solar abundances of iron-peak and alpha-elements with a high degree of homogeneity.