Disk Field Stars Research Articles

Origin of neutron-capture elements with the Gaia-ESO survey: the evolution of s- and r-process elements across the Milky Way

ABSTRACT We investigate the origin of neutron-capture elements by analysing their abundance patterns and radial gradients in the Galactic thin disc. We adopt a detailed two-infall chemical evolution model for the Milky Way, including state-of-the-art nucleosynthesis prescriptions for neutron-capture elements. We consider r-process nucleosynthesis from merging neutron stars (MNS) and magneto-rotational supernovae (MR-SNe), and s-process synthesis from low- and intermediate-mass stars (LIMS) and rotating massive stars. The predictions of our model are compared with data from the sixth data release of the Gaia-ESO survey, from which we consider 62 open clusters with age ≳ 0.1 Gyr and ∼1300 Milky Way disc field stars. We conclude that: (i) the [Eu/Fe] versus [Fe/H] diagram is reproduced by both prompt and delayed sources, with the prompt source dominating Eu production; (ii) rotation in massive stars significantly contributes to the first peak s-process elements, but MNS and MR-SNe are necessary to match the observations; and (iii) our model slightly underpredicts Mo and Nd, while accurately reproducing the [Pr/Fe] versus [Fe/H] trend. Regarding the radial gradients, we find that: (i) our predicted [Fe/H] gradient slope agrees with observations from Gaia-ESO and other high-resolution spectroscopic surveys; (ii) the predicted [Eu/H] radial gradient slope is steeper than the observed one, regardless of how quick the production of Eu is, prompting discussion on different Galaxy-formation scenarios and stellar radial migration effects; and (iii) elements in the second s-process peak as well as Nd and Pr exhibit a plateau at low-Galactocentric distances, likely due to enhanced enrichment from LIMS in the inner regions.

The Gaia-ESO survey: Placing constraints on the origin of r-process elements

Context. A renewed interest in the origin of r-process elements has been stimulated by the multi-messenger observation of the gravitational event GW170817, with the detection of both gravitational waves and electromagnetic waves corresponding to the merger of two neutron stars. Such a phenomenon has been proposed as one of the main sources of the r-process. However, the origin of the r-process elements at different metallicities is still under debate. Aims. We aim at investigate the origin of the r-process elements in the Galactic thin-disc population. Methods. From the sixth internal data release of the Gaia-ESO, we have collected a large sample of Milky Way (MW) thin- and thick-disc stars for which abundances of Eu, O, and Mg are available. The sample consists of members of 62 open clusters (OCs), located at a Galactocentric radius between ∼5 kpc and ∼20 kpc in the disc, in the metallicity range [ − 0.5, 0.4], and covering an age interval from 0.1 to 7 Gy, and about 1300 Milky Way disc field stars in the metallicity range [ − 1.5, 0.5]. We compare the observations with the results of a chemical evolution model, in which we varied the nucleosynthesis sources for the three elements considered. Results. Our main result is that Eu in the thin disc is predominantly produced by sources with short lifetimes, such as magneto-rotationally driven SNe. There is no strong evidence for additional sources at delayed times. Conclusions. Our findings do not imply that there cannot be a contribution from mergers of neutron stars in other environments, as in the halo or in dwarf spheroidal galaxies, but such a contribution is not needed to explain Eu abundances at thin-disc metallicities.

FSR 1776: A new globular cluster in the Galactic bulge?

Context. Recent near-IR surveys have uncovered a plethora of new globular cluster (GC) candidates towards the Milky Way bulge. These new candidates need to be confirmed as real GCs and properly characterised. Aims. We investigate the physical nature of FSR 1776, a very interesting star cluster projected towards the Galactic bulge. This object was originally classified as an intermediate-age open cluster, and has recently been re-discovered independently and classified as a GC candidate (Minni 23). Firstly, our aim is to confirm its GC nature; secondly, we determine its physical parameters. Methods. The confirmation of the cluster existence is checked using the radial velocity (RV) distribution of a MUSE data cube centred at FSR 1776. The cluster parameters are derived from isochrone fitting to the RV-cleaned colour-magnitude diagrams (CMDs) from visible and near-IR photometry taken from VVV, 2MASS, DECAPS, and Gaia all together. Results. The predicted RV distribution for the FSR 1776 coordinates, considering only contributions from the bulge and disc field stars, is not enough to explain the observed MUSE RV distribution. The extra population (12% of the sample) is FSR 1776 with an average RV of −103.7 ± 0.4 km s−1. The CMDs reveal that it is 10 ± 1 Gyr metal-rich population with [Fe/H]phot ≈ +0.2 ± 0.2, [Fe/H]spec = +0.02 ± 0.01(σ = 0.14 dex), located at the bulge distance of 7.24 ± 0.5 kpc with AV ≈ 1.1 mag. The mean cluster proper motions are (⟨μα⟩,⟨μδ⟩) = (−2.3 ± 1.1, −2.6 ± 0.8) mas yr−1. Conclusions. FSR 1776 is an old GC located in the Galactic bulge with a super-solar metallicity, among the highest for a Galactic GC. This is consistent with predictions for the age–metallicity relation of the bulge, being FSR 1776 the probable missing link between typical GCs and the metal-rich bulge field. High-resolution spectroscopy of a larger field of view and deeper CMDs are now required for a full characterisation.

The Gaia-ESO Survey: The N/O abundance ratio in the Milky Way

Context. The abundance ratio N/O is a useful tool to study the interplay of galactic processes, for example star formation efficiency, timescale of infall, and outflow loading factor. Aims. We aim to trace log(N/O) versus [Fe/H] in the Milky Way and to compare this ratio with a set of chemical evolution models to understand the role of infall, outflow, and star formation efficiency in the building up of the Galactic disc. Methods. We used the abundances from IDR2-3, IDR4, IDR5 data releases of the Gaia-ESO Survey both for Galactic field and open cluster stars. We determined membership and average composition of open clusters and we separated thin and thick disc field stars. We considered the effect of mixing in the abundance of N in giant stars. We computed a grid of chemical evolution models, suited to reproduce the main features of our Galaxy, exploring the effects of the star formation efficiency, infall timescale, and differential outflow. Results. With our samples, we map the metallicity range −0.6 ≤ [Fe/H] ≤ 0.3 with a corresponding −1.2 ≤ log(N/O) ≤ −0.2, where the secondary production of N dominates. Thanks to the wide range of Galactocentric distances covered by our samples, we can distinguish the behaviour of log(N/O) in different parts of the Galaxy. Conclusions. Our spatially resolved results allow us to distinguish differences in the evolution of N/O with Galactocentric radius. Comparing the data with our models, we can characterise the radial regions of our Galaxy. A shorter infall timescale is needed in the inner regions, while the outer regions need a longer infall timescale, coupled with a higher star formation efficiency. We compare our results with nebular abundances obtained in MaNGA galaxies, finding in our Galaxy a much wider range of log(N/O) than in integrated observations of external galaxies of similar stellar mass, but similar to the ranges found in studies of individual H II regions.

Observing multiple populations in globular clusters with the ESO archive: NGC 6388 reloaded

The metal-rich and old bulge globular cluster (GC) NGC 6388 is one of the most massive Galactic GCs (M ~ 106 M⊙). However, the spectroscopic properties of its multiple stellar populations rested only on 32 red giants (only 7 of which observed with UVES, the others with GIRAFFE), given the difficulties in observing a rather distant cluster, heavily contaminated by bulge and disc field stars. We bypassed the problem using the resources of the largest telescope facility ever: the European Southern Observatory (ESO) archive. By selecting member stars identified by other programmes, we derive atmospheric parameters and the full set of abundances for 15 species from high-resolution UVES spectra of another 17 red giant branch stars in NGC 6388. We confirm that no metallicity dispersion is detectable in this GC. About 30% of the stars show the primordial composition of first-generation stars, about 20% present an extremely modified second-generation composition, and 50% have an intermediate composition. The stars are clearly distributed in the Al–O and Na–O planes in three discrete groups. We find substantial hints that more than a single class of polluters is required to reproduce the composition of the intermediate component in NGC 6388. In the heavily polluted component the sum Mg+Al increases as Al increases. The sum Mg+Al+Si is constant, and is the fossil record of hot H-burning at temperatures higher than about 70 MK in the first-generation polluters that contributed to form multiple populations in this cluster.

DISCOVERY OF A METAL-POOR FIELD GIANT WITH A GLOBULAR CLUSTER SECOND-GENERATION ABUNDANCE PATTERN

ABSTRACT We report on the detection, from observations obtained with the Apache Point Observatory Galactic Evolution Experiment spectroscopic survey, of a metal-poor ([Fe/H] = −1.3 dex) field giant star with an extreme Mg–Al abundance ratio ([Mg/Fe] = −0.31 dex; [Al/Fe] = 1.49 dex). Such low Mg/Al ratios are seen only among the second-generation population of globular clusters (GCs) and are not present among Galactic disk field stars. The light-element abundances of this star, 2M16011638-1201525, suggest that it could have been born in a GC. We explore several origin scenarios, studying the orbit of the star in particular to check the probability of its being kinematically related to known GCs. We performed simple orbital integrations assuming the estimated distance of 2M16011638-1201525 and the available six-dimensional phase-space coordinates of 63 GCs, looking for close encounters in the past with a minimum distance approach within the tidal radius of each cluster. We found a very low probability that 2M16011638-1201525 was ejected from most GCs; however, we note that the best progenitor candidate to host this star is GC ω Centauri (NGC 5139). Our dynamical investigation demonstrates that 2M16011638-1201525 reaches a distance from the Galactic plane and minimum and maximum approaches to the Galactic center of R min < 0.62 kpc and R max < 7.26 kpc in an eccentric (e ∼ 0.53) and retrograde orbit. Since the extreme chemical anomaly of 2M16011638-1201525 has also been observed in halo field stars, this object could also be considered a halo contaminant, likely to have been ejected into the Milky Way disk from the halo. We conclude that 2M16011638-20152 is also kinematically consistent with the disk but chemically consistent with halo field stars.

Quantitative chemical tagging, stellar ages and the chemo-dynamical evolution of the Galactic disc

The early science results from the new generation of high-resolution stellar spectroscopic surveys, such as GALAH and the Gaia-ESO survey, will represent major milestones in the quest to chemically tag the Galaxy. Yet this technique to reconstruct dispersed coeval stellar groups has remained largely untested until recently. We build on previous work that developed an empirical chemical tagging probability function, which describes the likelihood that two field stars are conatal, that is, they were formed in the same cluster environment. In this work we perform the first ever blind chemical tagging experiment, i.e., tagging stars with no known or otherwise discernable associations, on a sample of 714 disc field stars with a number of high quality high resolution homogeneous metal abundance measurements. We present evidence that chemical tagging of field stars does identify coeval groups of stars, yet these groups may not represent distinct formation sites, e.g. as in dissolved open clusters, as previously thought. Our results point to several important conclusions, among them that group finding will be limited strictly to chemical abundance space, e.g. stellar ages, kinematics, colors, temperature and surface gravity do not enhance the detectability of groups. We also demonstrate that in addition to its role in probing the chemical enrichment and kinematic history of the Galactic disc, chemical tagging represents a powerful new stellar age determination technique.

Nitrogen Abundances and the Distance Moduli of the Pleiades and Hyades

Recent reanalyses of Hipparcos parallax data confirm a previously noted discrepancy with the Pleiades distance modulus estimated from main-sequence fitting in the color-magnitude diagram. One proposed explanation of this distance modulus discrepancy is a Pleiades He abundance that is significantly larger than the Hyades value. We suggest that, based on our theoretical and observational understanding of Galactic chemical evolution, nitrogen abundances may serve as a proxy for helium abundances of disk stars. Utilizing high-resolution near-UV Keck/HIRES spectroscopy, we determine N abundances in the Pleiades and Hyades dwarfs from NH features in the λ3330 region. While our Hyades N abundances show a modest ∼0.2 dex trend over a 800 K T eff range, we find the Pleiades N abundance (by number) is 0.13 ± 0.05 dex lower than in the Hyades for stars in a smaller overlapping T eff range around 6000 K; possible systematic errors in the lower Pleiades N abundance result are estimated to be at the ≤ 0.10 dex level. Our results indicate [N/Fe] ∼ 0 for both the Pleiades and Hyades, consistent with the ratios exhibited by local Galactic disk field stars in other studies. If N production is a reliable tracer of He production in the disk, then our results suggest the Pleiades He abundance is smaller than that in the Hyades. This finding is supported by the relative Pleiades–Hyades C, O, and Fe abundances interpreted in the current context of Galactic chemical evolution and is resistant to the effects on our derived N abundances of a He abundance difference like that needed to explain the Pleiades distance modulus discrepancy. A physical explanation of the Pleiades distance modulus discrepancy does not appear to be related to He abundance.

Disrupting primordial planet signatures: the close encounter of two single-planet exosystems in the Galactic disc

During their main sequence lifetimes, the majority of all Galactic Disc field stars must endure at least one stellar intruder passing within a few hundred AU. Mounting observations of planet-star separations near or beyond this distance suggest that these close encounters may fundamentally shape currently-observed orbital architectures and hence obscure primordial orbital features. We consider the commonly-occurring fast close encounters of two single-planet systems in the Galactic Disc, and investigate the resulting change in the planetary eccentricity and semimajor axis. We derive explicit 4-body analytical limits for these variations and present numerical cross-sections which can be applied to localized regions of the Galaxy. We find that each wide-orbit planet has a few percent chance of escape and an eccentricity that will typically change by at least 0.1 due to these encounters. The orbital properties established at formation of millions of tight-orbit Milky Way exoplanets are likely to be disrupted.

Lithium abundances along the red giant branch: FLAMES-GIRAFFE spectra of a large sample of low-mass bulge stars

Context: A small number of K-type giants on the red giant branch (RGB) is known to be very rich in lithium (Li). This fact is not accounted for by standard stellar evolution theory. The exact phase and mechanism of Li enrichment is still a matter of debate. Aims: Our goal is to probe the abundance of Li along the RGB, from its base to the tip, to confine Li-rich phases that are supposed to occur on the RGB. Methods: For this end, we obtained medium-resolution spectra with the FLAMES spectrograph at the VLT in GIRAFFE mode for a large sample of 401 low-mass RGB stars located in the Galactic bulge. The Li abundance was measured in the stars with a detectable Li 670.8 nm line by means of spectral synthesis with COMARCS model atmospheres. A new 2MASS (J-K) - Teff calibration from COMARCS models is presented in the Appendix. Results: Thirty-one stars with a detectable Li line were identified, three of which are Li-rich according to the usual criterion ($\log\epsilon({\rm Li})>1.5$). The stars are distributed all along the RGB, not concentrated in any particular phase of the red giant evolution (e.g. the luminosity bump or the red clump). The three Li-rich stars are clearly brighter than the luminosity bump and red clump, and do not show any signs of enhanced mass loss. Conclusions: We conclude that the Li enrichment mechanism cannot be restricted to a clearly defined phase of the RGB evolution of low-mass stars ($M\sim1M_{\sun}$), contrary to earlier suggestions from disk field stars.

A CHEMICAL ABUNDANCE STUDY OF RED GIANTS IN OPEN CLUSTERS NGC 2204 AND NGC 2243

Detailed element abundances have been determined for 10-13 stars each in the open clusters (OCs) NGC 2204 and NGC 2243 based on Hydra multi-object echelle spectra obtained with the CTIO 4 m telescope. We have found average cluster metallicities of [Fe/H] = –0.23 ± 0.04 and –0.42 ± 0.05 for NGC 2204 and NGC 2243, respectively, from an equivalent width analysis. NGC 2243 is the most metal-poor cluster at its Galactocentric radius and is one of the most metal-poor OCs currently known. These two clusters lie ~1 kpc below the Galactic plane; it is therefore worthwhile to compare their abundance patterns to those of clusters both closer to and further from the plane. To that end, we combined the results of the current study with those of clusters from our previous work as well as from the literature. To minimize systematic differences between different studies, element abundances of many outer disk OCs as well as thin and thick disk field stars have been placed on our abundance scale. Plots of [X/Fe] versus [Fe/H] for NGC 2204, NGC 2243, other clusters from the literature, and thin and thick disk field stars show NGC 2204 and NGC 2243 to have element abundance patterns comparable to those of other clusters regardless of distance from the plane or center of the Galaxy. Similarly, no individual cluster or group of clusters far from the Galactic mid-plane can be identified as belonging to the thick disk based on their abundance patterns.

Dynamical friction of star clusters against disc field stars in galaxies: implications on stellar nucleus formation and globular cluster luminosity functions

We numerically investigate orbital evolution of star clusters (SCs) under the influence of dynamical friction by field stars of their host disc galaxies embedded in dark matter haloes. We find that SCs with masses larger than ∼2 x 10 5 M ⊙ can show significant orbital decay within less than 1 Gyr due to dynamical friction by disc field stars in galaxies with disc masses (M d ) less than 10 9 M ⊙ . We also find that orbital decay of SCs due to dynamical friction is more remarkable in disc galaxies with smaller M d and higher mass ratios of discs to dark matter haloes. The half-number radii (R h,sc ) and mean masses within R h,sc of the SC systems (SCSs) in low-mass disc galaxies with M d ≤ 10 9 M ⊙ are found to evolve significantly with time owing to dynamical friction of SCs. More massive SCs that can spiral-in to the central regions of discs can form multiple SCSs with smaller velocity dispersions so that they can merge with one another to form single stellar nuclei with their masses comparable to ∼0.4 per cent of their host disc masses. Based on these results, we suggest that luminosity functions for more massive globular clusters (GCs) with masses larger than 2 x 10 5 M⊙ can steepen owing to transformation of the more massive GCs into single stellar nuclei through GC merging in less luminous galaxies. We also suggest that the half-number radii of GC systems can evolve owing to dynamical friction only for galaxies with their total masses smaller than ∼10 10 M⊙.

O and Na abundance patterns in open clusters of the Galactic disk

<i>Aims. <i/>A global O-Na abundance anti-correlation is observed in globular clusters, which is not present in the Galactic field population. Open clusters are thought to be chemically homogeneous internally. We aim to explore the O and Na abundance pattern among the open cluster population of the Galactic disk.<i>Methods. <i/>We combine open cluster abundance ratios of O and Na from high-resolution spectroscopic studies in the literature and normalize them to a common solar scale. We compare the open cluster abundances against the globular clusters and disk field.<i>Results. <i/>We find that the different environments show different abundance patterns. The open clusters do not show the O-Na anti-correlation at the extreme O-depletion/Na-enhancement as observed in globular clusters. Furthermore, the high Na abundances in open clusters do not match the disk field stars. If real, it may be suggesting that the dissolution of present-day open clusters is not a significant contribution to building the Galactic disk. Large-scale homogeneous studies of clusters and field will further confirm the reality of the Na enhancement.

The initial luminosity and mass functions of the Galactic open clusters

We aim at the construction of luminosity and mass functions for Galactic open clusters, based on integrated magnitudes and tidal masses. We also aim at studying the evolution of these functions, with the ultimate purpose of deriving the initial luminosity and mass distributions of star clusters, independent of model assumptions regarding the cluster mass-to-light ratio. Finally we aim at a new determination of the percentage of field stars that have originated in open clusters. The integrated magnitudes are computed from individual photometry of cluster members selected from the ASCC-2.5catalogue. The cluster masses we assumed to be the estimated tidal mass recently published by us elsewhere. Analysis of the cluster brightness distribution as a function of apparent integrated magnitudes shows that the cluster sample drawn from the ASCC-2.5is complete down to apparent integrated magnitude IV = 8, with 440 clusters and compact associations above this completeness limit. This, on average, corresponds to a completeness area in the solar neighbourhood with an effective radius of about 1 kpc. The observed luminosity function can be constructed in a range of absolute integrated magnitudes mag, i.e. about 5 mag deeper than in the most nearby galaxies. It increases linearly from the brightest limit to a turnover at about . The slope of this linear portion is , which agrees perfectly with the slope deduced for star cluster observations in nearby galaxies. The masses of the Galactic clusters span a range from a few to . The mass function of these clusters can be fit as a linear function with log mass for , and shows a broad maximum between and 2.5. For , the linear part of the upper cluster mass function has a slope , again in agreement with data on extragalactic clusters. We regard this agreement as indirect evidence that the tidal masses for Galactic clusters and the luminosity-based masses for extragalactic clusters are on the same scale. Considering the evolution of the cluster mass function now reveals a slight but significant steepening of the slope with increasing age from at to at . This indicates that open clusters are formed with a flatter (initial) mass distribution than the overall observed cluster mass distribution averaged over all ages. Interestingly, the luminosity function of open clusters does not show the same systematic steepening with age as the mass function does. We find that the initial mass function of open clusters (CIMF) has a two-segment structure with the slopes in the range and in the range . The average mass of open clusters at birth is , which should be compared to the average observed mass of about . The average cluster formation rate derived from the comparison of initial and observed mass functions is . Multiplying by the age of the Galactic disc ( Gyr) the predicted surface density of Galactic disc field stars originating from dissolved open clusters amounts to which is about 40% of the total surface density of the Galactic disc in the solar neighbourhood. Thus, we conclude that almost half of all field stars were born in open clusters, a much higher fraction than previously thought.

Chemical Composition in the Globular Cluster M71 from Keck HIRES Spectra of Turnoff Stars

We have made observations with the Keck I telescope and HIRES at a resolution of ~45,000 of five nearly identical stars at the turnoff of the metal-rich globular cluster M71. We derive stellar parameters and abundances of several elements. Our mean Fe abundance, [Fe/H] = -0.80 ± 0.02, is in excellent agreement with previous cluster determinations from both giants and near-turnoff stars. There is no clear evidence for any star-to-star abundance differences or correlations in our sample. Abundance ratios of the Fe peak elements (Cr, Ni) are similar to Fe. The turnoff stars in M71 have remarkably consistent enhancements of 0.2-0.3 dex in [Si/Fe], [Ca/Fe], and [Ti/Fe], like the red giants. Our [Mg/Fe] ratio is somewhat lower than that suggested by other studies. We compare our mean abundances for the five M71 stars with field stars of similar metallicity [Fe/H]: eight with halo kinematics and 17 with disk kinematics. The abundances of the α-fusion products (Mg, Si, Ca, Ti) agree with both samples but seem a closer match to the disk stars. The Mg abundance in M71 is at the lower edge of the disk and halo samples. The neutron-capture elements, Y and Ba, are enhanced relative to solar in the M71 turnoff stars. Our ratio [Ba/Fe] is similar to that of the halo field stars but a factor of 2 above that for the disk field stars. The important [Ba/Y] ratio is significantly lower than M71 giant values; the precluster material may have been exposed to a higher neutron flux than the disk stars or self-enrichment has occurred subsequent to cluster star formation. The Na content of the M71 turnoff stars is remarkably similar to that in the disk field stars but more than a factor of 2 higher than the halo field star sample. We find [Na/Fe] = +0.14 ± 0.04 with a spread less than half of that found in the red giants in M71. Excluding Mg, the lack of intracluster α-element variations (turnoff vis-a-vis giants) suggests that the polluting material needed to explain the abundance patterns in M71 did not arise from explosive nucleosynthesis but in a more traditional s-process environment such as AGB stars. The determination of light s-peak abundances should reveal whether this pollution occurred before or after cluster formation.

Galactic Stellar and Substellar Initial Mass Function11The page charges for this Review were partially covered by a generous gift from a PASP supporter.

ABSTRACTWe review recent determinations of the present‐day mass function (PDMF) and initial mass function (IMF) in various components of the Galaxy—disk, spheroid, young, and globular clusters—and in conditions characteristic of early star formation. As a general feature, the IMF is found to depend weakly on the environment and to be well described by a power‐law form for m≳1 M⊙ and a lognormal form below, except possibly for early star formation conditions. The disk IMF for single objects has a characteristic mass around mc ∼ 0.08 M⊙ and a variance in logarithmic mass σ ∼ 0.7, whereas the IMF for multiple systems has mc ∼ 0.2 M⊙ and σ ∼ 0.6. The extension of the single MF into the brown dwarf regime is in good agreement with present estimates of L‐ and T‐dwarf densities and yields a disk brown dwarf number density comparable to the stellar one, nBD ∼ n* ∼ 0.1 pc−3. The IMF of young clusters is found to be consistent with the disk field IMF, providing the same correction for unresolved binaries, confirming the fact that young star clusters and disk field stars represent the same stellar population. Dynamical effects, yielding depletion of the lowest mass objects, are found to become consequential for ages ≳130 Myr. The spheroid IMF relies on much less robust grounds. The large metallicity spread in the local subdwarf photometric sample, in particular, remains puzzling. Recent observations suggest that there is a continuous kinematic shear between the thick‐disk population, present in local samples, and the genuine spheroid one. This enables us to derive only an upper limit for the spheroid mass density and IMF. Within all the uncertainties, the latter is found to be similar to the one derived for globular clusters and is well represented also by a lognormal form with a characteristic mass slightly larger than for the disk, mc ∼ 0.2–0.3 M⊙, excluding a significant population of brown dwarfs in globular clusters and in the spheroid. The IMF characteristic of early star formation at large redshift remains undetermined, but different observational constraints suggest that it does not extend below ∼1 M⊙. These results suggest a characteristic mass for star formation that decreases with time, from conditions prevailing at large redshift to conditions characteristic of the spheroid (or thick disk) to present‐day conditions. These conclusions, however, remain speculative, given the large uncertainties in the spheroid and early star IMF determinations.These IMFs allow a reasonably robust determination of the Galactic present‐day and initial stellar and brown dwarf contents. They also have important galactic implications beyond the Milky Way in yielding more accurate mass‐to‐light ratio determinations. The mass‐to‐light ratios obtained with the disk and the spheroid IMF yield values 1.8–1.4 times smaller than for a Salpeter IMF, respectively, in agreement with various recent dynamical determinations. This general IMF determination is examined in the context of star formation theory. None of the theories based on a Jeans‐type mechanism, where fragmentation is due only to gravity, can fulfill all the observational constraints on star formation and predict a large number of substellar objects. On the other hand, recent numerical simulations of compressible turbulence, in particular in super‐Alfvénic conditions, seem to reproduce both qualitatively and quantitatively the stellar and substellar IMF and thus provide an appealing theoretical foundation. In this picture, star formation is induced by the dissipation of large‐scale turbulence to smaller scales through radiative MHD shocks, producing filamentary structures. These shocks produce local nonequilibrium structures with large density contrasts, which collapse eventually in gravitationally bound objects under the combined influence of turbulence and gravity. The concept of a single Jeans mass is replaced by a distribution of local Jeans masses, representative of the lognormal probability density function of the turbulent gas. Objects below the mean thermal Jeans mass still have a possibility to collapse, although with a decreasing probability.

Radial velocities of early-type stars in the Perseus OB2 association

We present radial velocities for 29 B- and A-type stars in the field of the nearby association Perseus OB2. The velocities are derived from spectra obtained with AURELIE, via cross correlation with radial velocity standards matched as closely as possible in spectral type. The resulting accuracy is ~2 - 3 km s$^{-1}$. We use these measurements, together with published values for a few other early-type stars, to study membership of the association. The mean radial velocity (and measured velocity dispersion) of Per OB2 is 23.5 \pm 3.9 km s$^{-1}$, and lies ~15 km s$^{-1}$ away from the mean velocity of the local disk field stars. We identify a number of interlopers in the list of possible late-B- and A-type members which was based on Hipparcos parallaxes and proper motions, and discuss the colour-magnitude diagram of the association.

Galactic Chemical Evolution of Lithium: Interplay between Stellar Sources

In this paper we study the evolution of 7Li in the Galaxy considering the contributions of various stellar sources: Type II supernovae, novae, red giant stars, and asymptotic giant branch (AGB) stars. We present new results for the production of 7Li in AGB stars via the hot bottom burning process, based on stellar evolutionary models. In the light of recent observations of dense circumstellar shells around evolved stars in the Galaxy and in the Magellanic Clouds, we also consider the impact of a very high mass-loss rate episode (superwind) before the evolution off the AGB phase on the 7Li enrichment in the interstellar medium. We compare the Galactic evolution of 7Li obtained with these new 7Li yields (complemented with a critical reanalysis of the role of supernovae, novae and giant stars) with a selected compilation of spectroscopic observations including halo and disk field stars as well as young stellar clusters. We conclude that even allowing for the large uncertainties in the theoretical calculation of mass-loss rates at the end of the AGB phase, the superwind phase has a significant effect on the 7Li enrichment of the Galaxy.

Abundance Patterns in the Draco, Sextans, and Ursa Minor Dwarf Spheroidal Galaxies

The Keck I telescope has been used to obtain HIRES spectra for red giants belonging to the Draco, Sextans and Ursa Minor dwarf spheroidal (dSph) galaxies. An analysis of these spectra is presented, along with abundance ratios for more than 20 elements. The resulting database of element abundances for 17 stars is the most extensive yet assembled for stars in dSph environments. Our main findings are summarized as follows: (1) There is unambiguous evidence for a large internal spread in metallicity in all three galaxies: our program stars span a range of [Fe/H] = 1.53, 1.40 and 0.73 dex in Draco, Sextans and Ursa Minor, respectively. (2) The abundance patterns among the dSph stars are remarkably uniform, suggesting that all three galaxies have similar nucleosynthetic histories. (3) A comparison of the measured abundance ratios for our sample of dSph stars with published values for Galactic halo and disk field stars suggests that the dSph galaxies have 0.02 < [alpha/Fe] < 0.13 dex, whereas the halo field star sample has [alpha/Fe] ~ 0.28 dex over the same range in metallicity. (4) The most metal-rich dSph stars in our sample have [Y/Fe] abundances which are significantly lower than those measured for halo field stars of similar metallicity, while the measured [Ba/Eu] ratios for the dSph stars suggest that the early chemical evolution of these galaxies was dominated by the r-process. Taken together, these results suggest that the Galactic halo is unlikely to have assembled, in its entirety, through the disruption of dwarf galaxies similar to the low-luminosity dSphs studied here. (ABRIDGED).

Hubble Space Telescope WFPC2 Photometry of M33: Properties of the Halo Star Clusters and Surrounding Fields*

We present CCD photometry for 10 fields centered on halo star clusters in the nearby spiral galaxy M33. The data are based on Hubble Space Telescope Wide-Field Planetary Camera 2 observations in the F555W (~V) and F814W (~I) filters. The resultant color-magnitude diagrams (CMDs) extend ~1.5 mag below the horizontal branch (HB). From these diagrams, we draw the following conclusions: Eight of the 10 M33 halo clusters possess HBs that are located entirely redward of the RR Lyrae instability strip. For nine of these clusters, we are able to use the slope of the red giant branch to measure metal abundances; we find their metal abundances to be in the range -1.64 ≥ [Fe/H] ≥ -0.65 with a mean of [Fe/H] = -1.27 ± 0.11. Utilizing this information, we are able to place these objects in the [Fe/H]-HB type diagram from which we conclude that the halo clusters in M33 suffer from the second-parameter effect; this suggests that another parameter, in addition to metal abundance, is affecting the morphology of their HBs. We present several arguments in favor of the hypothesis that this second parameter is cluster age. If this is indeed the case, while two of these clusters are likely to be as old as Galactic globular clusters at their metallicity, the remaining seven clusters have a mean age of ~7 Gyr. Using the two clusters in which the magnitude of the RR Lyrae variables can be inferred and adopting the RR Lyrae luminosity-metallicity relation favored by Chaboyer, we calculate a distance modulus of (m - M)0 = 24.84 ± 0.16 for M33. Our CMDs of the fields onto which the clusters are projected reveal the M33 disk population to be quite complicated. Our main conclusion regarding the disk field stars is that the youngest population is concentrated toward the center of M33.