Galactic Halo Stars Research Articles

The origin of nitrogen

Recent new high-precision abundance data for Galactic halo stars suggest important primary nitrogen production in very metal-poor massive stars. Here, we compute a new model for the chemical evolution of the Milky Way aimed at explaining these new abundance data. The new data can be explained by adopting: a) the stellar yields obtained from stellar models that take into account rotation; and b) an extra production of nitrogen in the very metal-poor massive stars. In particular, we suggest an increase of nearly a factor of 200 in 14N for a star of 60 and 40 for a star of 9 , for metallicities below , with respect to the yields given in the literature for and rotational velocity of 300 km s-1. We show that once we adopt the above prescriptions, our model is able to predict high N/O abundance ratios at low metallicities and still explains the nitrogen abundances observed in thin disk stars in the solar vicinity. The physical motivation for a larger nitrogen production in massive stars in very metal-poor environments could be the fact that some stellar models as well as observational data suggest that at low metallicities stars rotate faster. If this is the case, such large nitrogen production seen in the pristine phases of the halo formation would not necessarily happen in Damped Lyman-α systems which have metallicities always above , and could have been pre-enriched. We also compute the abundance gradient of N/O along the Galactic disk and show that a negative gradient is predicted once we adopt stellar yields where rotation is taken into account. The latter result implies that intermediate mass stars contribute less to the primary nitrogen than previously thought.

The U/Th production ratio and the age of the Milky Way from meteorites and Galactic halo stars

Some heavy elements (with atomic number A > 69) are produced by the 'rapid' (r)-process of nucleosynthesis, where lighter elements are bombarded with a massive flux of neutrons. Although this is characteristic of supernovae and neutron star mergers, uncertainties in where the r-process occurs persist because stellar models are too crude to allow precise quantification of this phenomenon. As a result, there are many uncertainties and assumptions in the models used to calculate the production ratios of actinides (like uranium-238 and thorium-232). Current estimates of the U/Th production ratio range from approximately 0.4 to 0.7. Here I show that the U/Th abundance ratio in meteorites can be used, in conjunction with observations of low-metallicity stars in the halo of the Milky Way, to determine the U/Th production ratio very precisely (0.57(+0.037)(-0.031). This value can be used in future studies to constrain the possible nuclear mass formulae used in r-process calculations, to help determine the source of Galactic cosmic rays, and to date circumstellar grains. I also estimate the age of the Milky Way (14.5(+2.8)(-2.2)Gyr in a way that is independent of the uncertainties associated with fluctuations in the microwave background or models of stellar evolution.

"Sculptor-ing" the Galaxy? The Chemical Compositions of Red Giants in the Sculptor Dwarf Spheroidal Galaxy

We have used high-resolution, high signal-to-noise spectra obtained with the VLT and UVES to determine abundances of 17 elements in 4 red giants in the Sculptor dwarf spheroidal galaxy. Our [Fe/H] values range from --2.10 to --0.97, confirming previous findings of a large metallicity spread. We have combined our data with similar data for five Sculptor giants studied recently to form one of the largest samples of high resolution abundances yet obtained for a dwarf spheroidal galaxy, covering essentially the full known metallicity range. These properties allow us to establish trends of [X/Fe] with [Fe/H] for many elements, X. The trends are significantly different from the trends seen in galactic halo and globular cluster stars. We compare our Sculptor sample to their most similar Galactic counterparts and find substantial differences remain even with these stars. The many discrepancies in the relationships between [X/Fe] as seen in Sculptor compared with Galactic field stars indicates that our halo cannot be made up in bulk of stars similar to those presently seen in dwarf spheroidal galaxies like Sculptor. These results have serious implications for the Searle-Zinn and hierarchical galaxy formation scenarios. We also find that the most metal-rich star in our sample is a heavy element-rich star. A very high percentage of such heavy element stars are now known in dwarf spheroidals compared to the halo, further mitigating against the formation of the halo from such objects.

Improved Laboratory Transition Probabilities for Ptiand Application to the Platinum Abundances of BD +17o3248 and the Sun

Radiative lifetimes, accurate to ±5%, have been measured for 58 odd-parity levels of Pt I using laser-induced fluorescence. The lifetimes were combined with branching fractions measured using grating and Fourier transform spectrometry to determine transition probabilities for 127 lines of Pt I. The new Pt lifetime measurements were found to be in good agreement with previous but less extensive measurements based on laser-induced fluorescence. The new branching fraction measurements were found to be in fair agreement with one earlier study. Absolute atomic transition probabilities from the new measurements were used to determine the platinum abundance in the metal-poor Galactic halo star BD +17°3248. An attempt to refine the solar photospheric abundance of platinum was unsuccessful; the single Pt I line used in an earlier abundance determination was found to be even more severely blended than expected from earlier work.

The Measurement of Radiative Lifetimes Using Laser-Induced Fluorescence: Experimental Review and Astrophysical Application

One of the standard methods for determining atomic transition probabilities is to combine branching fractions measured using Fourier-transform spectrometry with radiative lifetimes measurements using laser-induced fluorescence (LIF). This combination of techniques provides an efficient method for measuring large sets of accurate, absolute transition probabilities. The radiative lifetimes, which provide the overall scaling for the transition probabilities, can be measured routinely to ± 5% accuracy using time-resolved LIF. Although the time-resolved LIF technique we use does not achieve the accuracy of fast-beam LIF, the time-resolved technique does enable us to make measurements at a far greater rate (hundreds of level lifetimes per year). Care must be taken, however, to understand and control the systematic effects in time-resolved LIF measurements to maintain ± 5% accuracy over a wide dynamic range and hundreds of lifetime measurements.Over the last 25 years, we have measured lifetimes for 47 spectra using time-resolved LIF. Our atomic beam source can produce a slow beam of neutral and singly ionized atoms of nearly any element. Lifetimes from 2 ns to ~2 µs can be measured for energy levels ranging from 15,000 to ~60,000 cm-1. In this review we will describe our method of measuring radiative lifetimes with an emphasis on possible errors and techniques used for controlling them. The electronic bandwidth, linearity, and overall fidelity of the fast photomultiplier, cable connections, and transient waveform digitizer are concerns. Possible errors from atomic collisions, radiation trapping, Zeeman quantum beats, hyperfine quantum beats, atoms/ions escaping from the observation region before radiating, and from radiative cascade through lower levels must be understood and controlled.We will then present a recent example of the application of our transition probability data to abundance determinations in the sun and in metal-poor halo stars (Den Hartog E A et al 2003 Astrophys. J. Suppl. 148 543). Our results have significantly reduced the uncertainty of abundance determinations for many elements including the rare-earths: La, Nd, Eu, Tb, Dy, Ho, Tm, and Lu. Better laboratory data have made it possible to cleanly separate the enhanced abundance of r(apid)-process neutron capture elements in metal poor Galactic halo stars. The long-term scientific payoff from this attention to detail in laboratory experiments is a better understanding of the r-process, of the Galactic chemical evolution, of the age of the oldest stars in the Galaxy, and indeed of the origins of the non-primordial chemical elements.

Subaru/HDS Abundances in Three Giant Stars in the Ursa Minor Dwarf Spheroidal Galaxy

Abstract With the HDS (High Dispersion Spectrograph) on the Subaru Telescope, we obtained high-resolution optical region spectra of three red giant stars (COS 4, COS 82, and COS 347) in the Ursa Minor dwarf spheroidal galaxy. The chemical abundances in these stars were analyzed for 26 elements, including $\alpha$-, iron-peak, and neutron-capture elements. All three stars show low abundances of $\alpha$-elements (Mg, Si, and Ca), and two stars (COS 82 and COS 347) show high abundances of Mn compared to Galactic halo stars of similar metallicity. One star (COS 4) has been confirmed to be very metal deficient ($\mathrm{[Fe/H]} =-2.7$) and found to show anomalously low abundances of Mn, Cu, and Ba. In another star, COS 82 ($\mathrm{[Fe/H]} =-1.5$), we have found a large excess of heavy neutron-capture elements with a general abundance pattern similar to the scaled solar system $r$-process abundance curve. These observational results are rather puzzling: low abundances of $\alpha$-elements and high abundance of Mn seem to suggest a significant contribution of SNe Ia at low metallicity, while there is no hint of an $s$-process (i.e., AGB stars) contribution, even at $\mathrm{[Fe/H]} =-1.5$, suggesting a peculiar nucleosynthetic history of the UMi dSph galaxy.

Probing Primordial and Pre-Galactic Lithium with High-Velocity Clouds

The pre-Galactic abundance of lithium offers a unique window into non-thermal cosmological processes. The primordial Li abundance is guaranteed to be present and probes big bang nucleosynthesis (BBN), while an additional Li component is likely to have been produced by cosmic rays accelerated in large scale structure formation. Pre-Galactic Li currently can only be observed in low metallicity Galactic halo stars, but abundance measurements are plagued with systematic uncertainties due to modeling of stellar atmospheres and convection. We propose a new site for measuring pre-Galactic Li: low-metallicity, high-velocity clouds (HVCs) which are likely to be extragalactic gas accreted onto the Milky Way, and which already have been found to have deuterium abundances consistent with primordial. A Li observation in such an HVC would provide the first extragalactic Li measurement, and could shed new light on the apparent discrepancy between BBN predictions and halo star Li abundance determinations. Furthermore, HVC Li could at the same time test for the presence of non-primordial Li due to cosmic rays. The observability of elemental and isotopic Li abundances is discussed, and candidate sites identified.

Nucleosynthesis, Reionization, and the Mass Function of the First Stars

We critique the hypothesis that the first stars were very massive stars (VMSs; M > 140 M☉). We review the two major lines of evidence for the existence of VMSs: (1) that the relative metal abundances of extremely metal-poor Galactic halo stars show evidence of VMS enrichment and (2) that the high electron-scattering optical depth (τe) to the cosmic microwave background found by the Wilkinson Microwave Anisotropy Probe (WMAP) requires VMSs for reionization in a concordance ΛCDM cosmology. The yield patterns of VMSs exploding as pair-instability supernovae are incompatible with the Fe-peak and r-process abundances in halo stars. Models including Type II supernovae and/or hypernovae from zero-metallicity progenitors with M = 8-40 M☉ can better explain the observed trends. We use the nucleosynthesis results and stellar evolution models to construct an initial mass function (IMF) for reionization. With a simple metal transport model, we estimate that halo enrichment curtails metal-free star formation after ~108 yr at z ~ 20. Because the lifetime-integrated ionizing photon efficiency of metal-free stars peaks at ~120 M☉ and declines at higher mass, an IMF with an approximate lower bound at M ~ 10-20 M☉ and no VMS can maximize the ionizing photon budget and still be consistent with the nucleosynthetic evidence. An IMF devoid of low-mass stars is justified independently by models of the formation of primordial stars. Using a semianalytic model for H I and He II reionization, we find that such an IMF can reproduce τe 0.10-0.14, consistent with the range from WMAP, without extreme astrophysical assumptions, provided that metal-free star formation persists 107-108 yr after star formation begins. Because stars in the mass range 50-140 M☉ are the most efficient sources of ionizing photons but are expected to collapse to black holes without releasing metals, this IMF effectively decouples early metal enrichment and early ionization. Such an IMF may allow the unique properties of the zero-metallicity IMF to persist longer than they would in the pure VMS case and to contribute significantly to the global ionizing photon budget before halo self-enrichment and/or interhalo metal transport truncates metal-free star formation. We conclude, on the basis of these results, that VMSs are not necessary to meet the existing constraints commonly taken to motivate them.

Hierarchical Structure Formation and Chemical Evolution of Galaxies

We present an analytical and phenomenological model for metal enrichment in halos based on hierarchical structure formation. This model assumes that astration of normal stellar populations along with Type II supernovae (SNe II) already occurs at very high redshift. It focuses on the regime of [Fe/H] -1, in which SNe Ia are major contributors, is also discussed in general. For halos that are not disrupted by SN II explosions, the chemical evolution of the gas and stars is explicitly determined by the rate of gas infall as compared with the astration rate and the corresponding rate of metal production by SNe II per H atom in the gas. This model provides a good description of the data on [Fe/H] for damped Lyα systems over a wide range of redshift, 0.5 < z < 5. For all halos not disrupted by SN II explosions, if there is a cessation of gas infall, the metallicities of the stars follow a bimodal distribution. This distribution is characterized by a sharp peak at the value of [Fe/H] corresponding to the time of infall cessation and by a broad peak at a higher value of [Fe/H] corresponding to the subsequent period of astration during which the bulk of the remaining gas forms stars. Such a distribution can be compared with that observed for the Galactic halo stars. If the gas in a halo is rapidly lost on cessation of infall, then an assemblage of stars with a very sharply defined [Fe/H] value will be left behind. This assemblage of stars may be accreted by a larger system and become a globular cluster of the larger system. We discuss the masses and metallicities of the globular clusters in this model considering the criterion for the onset of astration and possible disruption of low-mass halos by SN II explosions. The implications of possible globular clusters with very low metallicities of [Fe/H] ≾-3 are also discussed.

Stellar Chemical Signatures and Hierarchical Galaxy Formation

To compare the chemistries of stars in the Milky Way dwarf spheroidal (dSph) satellite galaxies with stars in the Galaxy, we have compiled a large sample of Galactic stellar abundances from the literature. When kinematic information is available, we have assigned the stars to standard Galactic components through Bayesian classification based on Gaussian velocity ellipsoids. As found in previous studies, the [α/Fe] ratios of most stars in the dSph galaxies are generally lower than similar metallicity Galactic stars in this extended sample. Our kinematically selected stars confirm this for the Galactic halo, thin-disk, and thick-disk components. There is marginal overlap in the low [α/Fe] ratios between dSph stars and Galactic halo stars on extreme retrograde orbits (V < -420 km s-1), but this is not supported by other element ratios. Other element ratios compared in this paper include r- and s-process abundances, where we find a significant offset in the [Y/Fe] ratios, which results in a large overabundance in [Ba/Y] in most dSph stars compared with Galactic stars. Thus, the chemical signatures of most of the dSph stars are distinct from the stars in each of the kinematic components of the Galaxy. This result rules out continuous merging of low-mass galaxies similar to these dSph satellites during the formation of the Galaxy. However, we do not rule out very early merging of low-mass dwarf galaxies, since up to one-half of the most metal-poor stars ([Fe/H] ≤ -1.8) have chemistries that are in fair agreement with Galactic halo stars. We also do not rule out merging with higher mass galaxies, although we note that the LMC and the remnants of the Sgr dwarf galaxy are also chemically distinct from the majority of the Galactic halo stars. Formation of the Galaxy's thick disk by heating of an old thin disk during a merger is also not ruled out; however, the Galaxy's thick disk itself cannot be comprised of the remnants from a low-mass (dSph) dwarf galaxy, nor of a high-mass dwarf galaxy like the LMC or Sgr, because of differences in chemistry.

Binary Statistics Among Population II Stars

AbstractPopulation II stars are old, metal-poor, Galactic halo stars with high proper motion. We have carried out a visual binary survey of 164 halo stars in the solar neighborhood (median distance 100 pc), using infrared speckle interferometry, adaptive optics, and wide field direct imaging. The sample is based on the lists of Population II stars of Carney et al. (1994) and Norris (1986), with reliable distances from HIPPARCOS measurements.At face value, we found 33 binaries, 6 triples, and 1 quadruple system. When we limit ourselves to K-band flux ratios larger than 0.1 (to avoid background contamination), the numbers drop to 9 binaries and 1 triple, corresponding to a binary frequency of 6–7% above our angular resolution limit of about 0.1 arcsec. If we count all systems with K-band flux ratios greater than 0.01, we obtain 15 more binaries and 3 more triples, corresponding to a binary frequency for projected separations in excess of 10 AU of around 20 %. This is to be compared with the frequency of spectroscopic binaries (up to a period of 3000 days) of Population II stars of about 15 % (Latham et al. 2002). We also determined a semi-major axis distribution for our visual Population II binary and triple systems, which appears to be remarkably different from that of Population I stars. Second epoch-observations must help confirm the reality of our results.

Accretion of Dust Grains as a Possible Origin of Metal-poor Stars with Low /Fe Ratios

The origin of low α/Fe ratios in some metal-poor stars, the so-called low-α stars, is discussed. It is found that most of low-α stars in the Galaxy are on the main sequence. This strongly suggests that these stars suffered from external pollution. It is also found that the Zn/Fe abundance ratios of low-α stars both in the Galaxy and in dwarf spheroidal galaxies are lower than the average value of Galactic halo stars, whereas damped Lyα absorbers have higher ratios. This implies that some low-α stars accreted matter that was depleted from gas onto dust grains. To explain the features in these low-α stars, we have proposed that metal-poor stars harboring planetary systems are the origin of these low-α stars. Stars engulfing a small fraction of planetesimals enhance the surface content of Fe to exhibit low α/Fe ratios on their surfaces, while they are on the main sequence, because dwarfs have shallow surface convection zones where the engulfed matter is mixed. After the stars leave the main sequence, the surface convection zones become deeper, reducing the enhancement of Fe. Eventually, when the stars ascend to the tip of the red giant branch, they engulf giant planets to become low-α stars again as observed in dwarf spheroidal galaxies. We predict that low-α stars with low Mn/Fe ratios harbor planetary systems.

Improved Laboratory Transition Probabilities for Nd ii and Application to the Neodymium Abundances of the Sun and Three Metal‐poor Stars

Radiative lifetimes, accurate to � 5%, have been measured for 168 odd-parity levels of Nd ii using laser-induced fluorescence. The lifetimes are combined with branching fractions measured using Fouriertransform spectrometry to determine transition probabilities for over 700 lines of Nd ii. This work is the largest-scale laboratory study to date of Nd ii transition probabilities using modern methods. This improved data set is used to determine Nd abundances for the Sun and three metal-poor giant stars with neutroncapture enhancement: CS 22892� 052, HD 115444, and BD +17 � 3248. In all four stars the line-to-line scatter is considerably reduced from earlier published results. The solar photospheric abundance is determined to be log � ðNd Þ¼ 1:45 � 0:01ð� ¼ 0:05Þ, which is in excellent agreement with meteoric data. The ratio of Nd/Eu is virtually identical in all three metal-poor Galactic halo stars. Furthermore, the newly determined stellar Nd abundances, in comparison with other heavy neutron-capture elements, are consistent with an r-process–only origin early in the history of the Galaxy. These more accurate Nd abundance determinations might help to constrain the predicted solar system r-process abundances, and suggest other elements for further neutron-capture abundance studies. Subject headings: atomic data — stars: abundances — Sun: abundances On-line material: machine-readable tables

Observations of neutron-capture elements in the early galaxy

Neutron-capture elements in low metallicity Galactic halo stars vary widely both in overall contents and detailed abundance patterns. This review discusses recent observational results of the n-capture elements, discussing the implications for early Galactic nucleosynthesis of: (a) the star-to-star “bulk” variations in the n-capture/Fe abundance ratios; (b) the distinct signature of rapid n-capture synthesis events in many (most?) of the lowest metallicity stars; (c) the existence of metal-poor stars heavily enriched in the products of slow n-capture synthesis reactions; and (d) the now-routine detection of radioactive thorium (and even uranium in one and possibly two cases) in the spectra of metal-poor stars.

Highly Ionized Gas in the Galactic Halo: AFUSESurvey of OviAbsorption toward 22 Halo Stars

Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of 22 Galactic halo stars are studied to determine the amount of O VI in the Galactic halo between ~0.3 and ~10 kpc from the Galactic midplane. Strong O VI ?1031.93 absorption was detected toward 21 stars, and a reliable 3 ? upper limit was obtained toward HD 97991. The weaker member of the O VI doublet at 1037.62 ? could be studied toward only six stars because of stellar and interstellar blending problems. The measured logarithmic total column densities vary from 13.65 to 14.57 with log N = 14.17 ? 0.28 (1 ?). The observed columns are reasonably consistent with a patchy exponential O VI distribution with a midplane density of 1.7 ? 10-8 cm-3 and scale height between 2.3 and 4 kpc. We do not see clear signs of strong high-velocity components in O VI absorption along the Galactic sight lines, which indicates the general absence of high-velocity O VI within 2-5 kpc of the Galactic midplane. This result is in marked contrast to the findings of Sembach et al., who reported high-velocity O VI absorption toward ~60% of the complete halo sight lines observed by FUSE. The line centroid velocities of the O VI absorption do not reflect Galactic rotation well. The O VI velocity dispersions range from 33 to 78 km s-1, with an average of b = 45 ? 11 km s-1 (1 ?). These values are much higher than the value of ~18 km s-1 expected from thermal broadening for gas at T ~ 3 ? 105 K, the temperature at which O VI is expected to reach its peak abundance in collisional ionization equilibrium. Turbulence, inflow, and outflow must have an effect on the shape of the O VI profiles. Kinematical comparisons of O VI with Ar I reveal that eight of 21 sight lines are closely aligned in LSR velocity (|?VLSR| ? 5 km s-1), while nine of 21 exhibit significant velocity differences (|?VLSR| ? 15 km s-1). This dual behavior may indicate the presence of two different types of O VI-bearing environments toward the Galactic sight lines. The correlation between the H I and O VI intermediate-velocity absorption is poor. We could identify the known H I intermediate-velocity components in the Ar I absorption but not in the O VI absorption in most cases. Comparison of O VI with other highly ionized species suggests that the high ions are produced primarily by cooling hot gas in the Galactic fountain flow and that turbulent mixing also has a significant contribution. The role of turbulent mixing varies from negligible to dominant. It is most important toward sight lines that sample supernova remnants like Loops I and IV. The average N(C IV)/N(O VI) ratios for the nearby halo (this work) and complete halo (Savage et al.) are similar (~0.6), but the dispersion is larger in the sample of nearby halo sight lines. We are able to show that the O VI enhancement toward the Galactic center region that was observed in the FUSE survey of complete halo sight lines (Savage et al.) is likely associated with processes occurring near the Galactic center by comparing the observations toward the nearby HD 177566 sight line to those toward extragalactic targets.

Integrated spectroscopy of bulge globular clusters and fields

An empirical calibration is presented for the synthetic Lick indices (e.g. Mg2, <Fe>, Hβ, etc.) of Simple Stellar Population (SSP) models that for the first time extends up to solar metallicity. This is accomplished by means of a sample of Milky Way globular clusters (GCs) whose metallicities range from ~ to , thanks to the inclusion of several metal rich clusters belonging to the Galactic bulge (e.g., NGC 6553 and NGC 6528). This metallicity range approaches the regime that is relevant for the interpretation of the integrated spectra of elliptical galaxies. It is shown that the spectra of both the globular clusters and the Galactic bulge follow the same correlation between magnesium and iron indices that extends to elliptical galaxies, showing weaker iron indices at given magnesium indices with respect to the predictions of models that assume solar-scaled abundances. This similarity provides robust empirical evidence for enhanced [ α/Fe] ratios in the stellar populations of elliptical galaxies, since the globular clusters are independently known to have enhanced [ α/Fe] ratios from spectroscopy of individual stars. We check the uniqueness of this α-overabundance solution by exploring the whole range of model ingredients and parameters, i.e. fitting functions, stellar tracks, and the initial mass function (IMF). We argue that the standard models (meant for solar abundance ratios) succeed in reproducing the Mg-Fe correlation at low metallicities () because the stellar templates used in the synthesis are Galactic halo stars that actually are α-enhanced. The same models, however, fail to predict the observed Mg-Fe pattern at higher metallicities () (i.e., for bulge clusters and ellipticals alike) because the high-metallicity templates are disk stars that are not α-enhanced. We show that the new set of SSP models which incorporates the dependence on the [ α/Fe] ratio (Thomas et al. 2003) is able to reproduce the Mg and Fe indices of GCs at all metallicities, with an α-enhancement α/Fe=+0.3, in agreement with the available spectroscopic determinations. The Hβ index and the higher-order Balmer indices are well calibrated, provided the appropriate morphology of the Horizontal Branch is taken into account. In particular, the Balmer line indices of the two metal rich clusters NGC 6388 and NGC 6441, which are known to exhibit a tail of warm Horizontal Branch stars, are well reproduced. Finally, we note that the Mg indices of very metal-poor () populations are dominated by the contribution of the lower Main Sequence, hence are strongly affected by the present-day mass function of individual globular clusters, which is known to vary from cluster to cluster due to dynamical effects.

Galactic Halo Stars in Phase Space: A Hint of Satellite Accretion?

The present-day chemical and dynamical properties of the Milky Way bear the imprint of the Galaxy's formation and evolutionary history. One of the most enduring and critical debates surrounding Galactic evolution is that regarding the competition between satellite and monolithic collapse; the apparent strong correlation between orbital eccentricity and metallicity of halo stars was originally used as supporting evidence for the latter. While modern-day unbiased samples no longer support the claims for a significant correlation, recent evidence has been presented by Chiba & Beers for the existence of a minor population of high-eccentricity metal-deficient halo stars. It has been suggested that these stars represent the signature of a rapid (if minor) collapse phase in the Galaxy's history. Employing velocity and integrals of motion phase-space projections of these stars, coupled with a series of N-body/smoothed particle hydrodynamic chemodynamical simulations, we suggest that an alternative mechanism for creating such stars may be the recent accretion of a polar orbit dwarf galaxy.

High-resolution Keck I spectroscopy of Galactic halo post-asymptotic giant branch stars

Absolute and differential abundance analyses have been performed from high-resolution, high signal-to-noise ratio optical (Keck I) spectra for three evolved Galactic halo stars, namely PG 1704 + 222, HD 341617 and LS IV -04 01. Their derived atmospheric parameters indicate that all three objects are undergoing a post-asymptotic giant branch (post-AGB) phase of evolution. A differential abundance analysis reveals HD 341617 as having a mild carbon deficiency of 0.74 dex, possibly due to the star having evolved off the AGB before the onset of the third dredge-up. Although such carbon underabundances are typical of hot post-AGB objects, the same trend is not observed in PG 1704 + 222, where the carbon abundance is found to be consistent with those derived for nitrogen and oxygen. Hence, a dredge-up scenario need not be invoked to explain the chemical composition of PG 1704 + 222. For LS IV -04 01 no iron deficiency is apparent relative to magnesium and silicon, and hence a gas-dust separation event in the AGB progenitor need not be invoked for this star.

Distribution of the disrupted satellite galaxies as a function of metallicity

Recent observations show that some dwarf satellite galaxies fall into the gravitational well of the Galaxy while being disrupted by the tidal force. As a consequence, the Galactic halo suers contamination from this process, the stellar populations of the tidal disrupted satellites make an additional contribution and alter the intrinsic metallicity distribution of the halo stars. The distribution of this kind of disrupted satellite galaxy as a function of metallicity is investigated in this paper. The model is limited to one class of dwarf spherical satellite galaxy, that is assumed to have similar abundance patterns to the Galactic eld halo stars. We discuss their distributions through the links between minor merger processes in the history of the Galaxy and the observed distribution of metal-poor eld halo stars. The upper limit of 35% of the metal-poor halo eld stars was established by merging of this kind of satellite galaxy with characteristic mass Msat, and adopting the mass-metallicity relation among dwarf spheroidal galaxy, the distribution of the disrupted satellite galaxy as a function of metallicity is derived to reproduce the observed metallicity distribution of the extremely metal-poor halo eld stars in the Galaxy. The problem of missing metal-free halo stars in the Galaxy is explained in this model as well.

The Prompt Inventory from Very Massive Stars and Elemental Abundances in L[CLC]y[/CLC]α Systems

It has been proposed that very massive stars (VMSs) dominated heavy-element production until a threshold corresponding to [Fe/H] ≈ -3 was reached. This results in a prompt (P) inventory of elements, the abundances of which were determined from observations of Galactic halo stars with [Fe/H] ≈ -3. We calculate Ω_P(E) from the P inventory for a large number of elements in the intergalactic medium (IGM). Using the available data on Ω(E_(ion)) for C IV, O VI, and Si IV in Lyα systems, we find that the ionization fractions calculated from Ω(E_(ion))/Ω_P(E) are, within reasonable uncertainties, compatible with the values estimated from ionization models for Lyα systems. This agreement appears to hold from z ~ 0.09 to ~4.6, indicating that the bulk of the baryonic matter remains dispersed with a fixed chemical composition. We conclude that the P inventory was established in the epoch prior to z ~ 4.6. The dispersal of processed baryonic matter to the general IGM is considered to be the result of energetic VMS explosions that disrupted baryonic aggregates until the threshold was reached to permit normal astration. The formation of most galaxies is considered to have occurred subsequent to the achievement of this metallicity threshold in the IGM.