Li Gap Research Articles

LITHIUM ABUNDANCES OF THE SUPER-METAL-RICH OPEN CLUSTER NGC 6253

High-resolution CTIO 4-m/HYDRA spectroscopy of the super-metal-rich open cluster NGC 6253 ([Fe/H]=+0.43+/-0.01) has been used to study the stellar lithium (Li) abundances near the cluster's turnoff. NGC 6253 greatly expands the range of [Fe/H] for clusters that have a Li abundance analysis. This is important for studying the complicated effects of, and potential correlations with, stellar Fe abundance on surface Li abundance. Comparisons to the younger and less-metal-rich Hyades and to the similarly-aged but solar-metallicity M67 show that NGC 6253's Li abundances are qualitatively consistent with the prediction, from Standard Stellar Evolution Theory, that higher-metallicity stars have a greater Li depletion. Comparison with M67 provides evidence that the more-metal-rich NGC 6253 had a higher initial Li, which is consistent with expectations from models of Galactic Li production. NGC 6253 is also compared to the intermediate-aged NGC 3680, NGC 752, and IC 4651 open clusters. Comparison of the Li-gap positions in all six clusters shows: a) the gap's position in Teff is independent of metallicity, but b) higher-metallicity clusters have their gaps in higher-mass stars. In addition, the Li gap's position is shown not to evolve with age, which provides an important constraint for the non-standard depletion mechanisms that may create the Li gap.

AmFm and lithium gap stars

<i>Aims. <i/>A thorough study of the effects of mass loss on internal and surface abundances of A and F stars is carried out in order to constrain mass loss rates for these stars, as well as further elucidate some of the processes which compete with atomic diffusion.<i>Methods. <i/>Self-consistent stellar evolution models of 1.3 to 2.5 stars including atomic diffusion and radiative accelerations for all species within the OPAL opacity database were computed with mass loss and compared to observations as well as previous calculations with turbulent mixing.<i>Results. <i/>Models with unseparated mass loss rates between 5 × 10<sup>-14<sup/> and 10<sup>-13<sup/> reproduce observations for many cluster AmFm stars as well as Sirius A and <i>o<i/> Leonis. These models also explain cool Fm stars, but not the Hyades lithium gap. Like turbulent mixing, these mass loss rates reduce surface abundance anomalies; however, their effects are very different with respect to internal abundances. For most of the main-sequence lifetime of an A or F star, surface abundances in the presence of such mass loss depend on separation which takes place between log <i>Δ<i/><i>M<i/>/<i>M<i/><sub><sub>*<sub/><sub/> = -6 and -5.<i>Conclusions. <i/>The current observational constraints do not allow us to conclude that mass loss is to be preferred over turbulent mixing (induced by rotation or otherwise) in order to explain the AmFm phenomenon. Internal concentration variations which could be detectable through asteroseismic tests should provide further information. If atomic diffusion coupled with mass loss are to explain the Hyades Li gap, the wind would need to be separated.

WIYN/Hydra Detection of Lithium Depletion in F Stars of the Young Open Cluster M35 and Implications for the Development of the Lithium Gap

We report the discovery of significant depletion of on the surfaces of F dwarf stars in the 150 Myr old open cluster M35, analagous to a feature in the 700 Myr old Hyades that has been referred to as the Li gap. We have caught the gap in the act of forming: using high-resolution, high signal-to-noise ratio WIYN/Hydra observations, we detect in all but a few M35 F stars; the maximum depletion lies at least 0.6-0.8 dex below minimally depleted (or undepleted) stars. The M35 depletion region (1) is quite wide, with clear depletion seen from 6000 to 6700 K or hotter, (2) shows a significant dispersion in abundance at all Teff, even with stars of the same Teff, and (3) contains undepleted stars (as well as depleted ones) in the (narrow) classical Hyades gap region, which itself shows no undepleted stars. All of these M35 depletion properties support rotationally induced slow mixing as the primary physical mechanism that forms the gap and argue against other proposed mechanisms, particularly diffusion and steady main-sequence mass loss. When viewed in the context of the M35 depletion properties, the Hyades gap may well be wider than is usually recognized.

The electronic band structure of Li2O:testing theoretical predictions using electron momentumspectroscopy

Using the technique of electron momentum spectroscopy (EMS) wehave measured the oxygen 2p- and 2s-derived valence bands and lithium 1s-derived core level in lithium oxide. All three setsof bands have been measured in a single experiment allowing theenergy gap between the bands to be determined. At theΓ point the O(2p)-O(2s) band gap is measured to be16.1±0.2 eV, and the O(2s)-Li(1s) band gap is34.3±0.2 eV. We can also determine bandwidths since EMS measures the fullband structure directly, resolved both in energy and momentum.As expected, the O(2s) and Li(1s) bands are essentiallynon-dispersing, while the O(2p) has an observed width of 1.6±0.2 eV. The experiment is compared with calculationsusing the linear combination of atomic orbitals approach. At the Hartree-Fock (HF)level these calculationsoverestimate the gap between the valence bands and the width of theO(2p) band. The three density functional methods usedgive a reduced intervalence band gap and bandwidth. The hybridgradient corrected method, PBE0 (where PBE standsfor `Perdew-Burke-Ernzerhof'), gives the closest agreement forthe band gap at 16.7 eV, while the gradient corrected method, PBE, gives the best value for the bandwidth at 2.0 eV. At alllevels the O(2s)-Li(1s) gap is underestimated; HF gives theclosest agreement at 31.8 eV.

Beryllium in the Hyades F and G Dwarfs from Keck HIRES Spectra

Although there are extensive observations of Li in field stars of all types and in both open and (recently) globular cluster stars, there are relatively few observations of Be. Because Be is not destroyed as easily as Li, the abundances of Li and Be together can tell us more about the internal physical processes in stars than either element can alone. We have obtained high-resolution (45,000) and high signal-to-noise ratio (typically 90 per pixel) spectra of the Be II resonance lines in 34 Hyades F and G dwarfs with the Keck I telescope and HIRES. In addition we took a spectrum of the daytime sky to use as a surrogate for the solar spectrum so we could determine the value for Be in the Sun, analyzed in the same manner as that for the stars. We have adopted stellar temperatures and some Li abundances for these stars from the literature. For most of the F dwarfs we have rederived Li abundances. The Be abundances have been derived with the spectrum synthesis method. We find that Be is depleted, but detected, in the Li gap in the F stars reaching down to values of A(Be) = 0.60 dex, or a factor of nearly 7 below the meteoritic Be abundance (a factor of 3.5 below the solar value of Chmielewski et al.). There is little or no depletion of Be in stars cooler than 6000 K, in spite of the large depletions (0.5-2.5 dex) in Li. The mean value of A(Be) for the 10 coolest stars is 1.33 ± 0.06, not far from the meteoritic value of 1.42. The pattern in the Be abundances—a Be dip and undepleted Be in the cool stars—is well matched by the predictions of slow mixing due to stellar rotation. We have interpolated the calculations of Deliyannis and Pinsonneault for Be depletion due to rotational mixing to the age of the Hyades; we find excellent agreement of the predictions with the observed Be abundances but less good agreement with the observed Li abundances. Some of our Hyades stars have photometrically determined rotation periods, but there is no relation between Be and rotation period. (Generally, the lower mass stars have less Li and longer periods, which may indicate greater spin-down and thus more Li depletion relative to Be.) The Li and Be abundances are correlated for stars in the temperature range of 5850-6680 K, similar to results from earlier work on Li and Be in F and G field stars. This indicates that the depletions are not just correlated—the only claim that can be made for the field stars—but are probably occurring together during main-sequence evolution. The Hyades G dwarfs have more Be than the Sun; their initial Be may have been larger or they may not be old enough to have depleted much Be. For those Hyades stars that seem to have little or no depletion of Li or Be, the Li/Be ratio is found to be 75 ± 30; the meteoritic ratio Li/Be is 78. The Hyades ratio is a representative value for the initial ratio in the material out of which the Hyades cluster was formed.

Beryllium in F and G Field Dwarfs from High‐Resolution Canada‐France‐Hawaii Telescope Spectra

It is important to add observations of Be to the huge arsenal of Li observations in order to identify the mechanisms operating in stellar interiors that alter the surface composition of the light elements. Beryllium is more resistant to destruction than is Li, so information on the abundances of both Li and Be reveals more information on the internal processes than either element does alone. We have made observations of Be II at 3131 A in 46 solar-type stars from the Canada-France-Hawaii Telescope with high spectral resolution and high signal-to-noise ratios (S/N). Our Li I 6707 A data for 39 of these stars come from our high-resolution, high-S/N observations with the University of Hawai`i 88 inch (2.2 m) telescope and coude spectrograph and Keck I High Resolution Echelle Spectrometer and, for six stars, from the literature. Most of the stars in our sample are F and G dwarfs with Teff between 6100 and 6600 K and with [Fe/H] between -0.6 and +0.2. The abundances of Be have been determined through spectrum synthesis, while Li has been analyzed as a blend to find the Li abundance. We find a large range in both Li and Be in these stars; for Be it is at least 2.5 dex and for Li at least 3 dex. However, there is an excellent correlation between Li and Be, as discovered by Deliyannis et al. from a smaller sample. We find that in the range of Teff of 5850 K (near the Li peak in open clusters) to 6680 K (at the bottom of the Li gap as defined by the Hyades), Li and Be appear to be depleted together. The slope of this remarkable logarithmic relation is 0.36: as Li is reduced by a factor of 10, Be is reduced by only 2.2 times. There is some scant evidence for a change in the slope between the cooler stars and the hotter stars such that the cooler stars deplete more Li relative to Be than the hotter stars. These results are well matched by models that incorporate rotationally induced slow mixing of the stellar surface material with the deeper layers of the star.

Constraints on Stellar Hydrodynamics from Abundance Anomalies of LiBeB and Metals

The availability of large atomic data bases has made it possible to calculate stellar evolution models taking into detailed account the atomic diffusion of all important contributors to opacity. The radiative accelerations and the opacity are continuously calculated during evolution taking the abundance changes of 28 species into account. This leads to the first self-consistent stellar evolution models for A and F stars. In A and F stars an iron-peak convection zone appears.The calculated abundance anomalies are very similar to those observed in AmFm stars in open clusters except that they are larger by a factor of about 3. To reduce the calculated anomalies to the observed ones, an additional source of turbulence (or some other hydrodynamical process) must be introduced. The mixed zone must extend about 5 times deeper than the iron convection zone. Detailed comparisons to a few AmFm stars have been carried out.The LiBeB abundances observed in clusters give additional information. The abundances of the 28 species offer considerable constraints on the models. Various potential turbulence models have been introduced in a stellar evolution code and results of evolutionary calculations for Li gap stars are discussed in the light of the constraints offered by the abundances of LiBeB and metals. The radiative accelerations of LiBeB have also been recalculated taking the effect of changing metal abundances into account. This modifies the expected Li gap in the absence of turbulence.

Consistent Evolution of F Stars: Diffusion, Radiative Accelerations, and Abundance Anomalies

Consistent stellar evolution models of F stars (1.1-1.5 M☉) are calculated with radiative forces, opacities, and diffusion for all elements included in OPAL's opacity tables. The opacities and radiative forces are continuously recomputed during evolution from OPAL's monochromatic data (~1.5 Gbyte) in order to include all effects of abundance changes due to diffusion and nuclear evolution. TOPbase is also used for radiative accelerations. Iron surface overabundances occur in stars more massive than 1.3 M☉. Local overabundances of iron peak elements increase the Rosseland opacity in a region at the base of the convection zone by a factor of 3-6; this increases the mass of the convective zone by up to a factor of 5. It is important to follow Cr, Mn, and Ni independently of Fe, since they peak at different temperatures within the star. The predicted abundance anomalies are much larger than observed in most F-type stars of open clusters. This suggests that atomic diffusion is not the only process responsible for the Li gap in open clusters. The predicted iron peak element overabundances indicate trends that are compatible with those observed in Fm stars. They however tend to be larger than the observed overabundances, leaving room for some perturbing hydrodynamical process. The present models, devoid of free parameters, are a necessary step in constraining the additional hydrodynamical processes required to better reproduce observed surface abundances. Since the abundances of 28 elements are calculated, one may have 27 constraints on stellar hydrodynamics, once the relative abundances of all species have been determined observationally.

Correlated Depletion of Lithium and Beryllium in F Stars

It has been known for over a decade that Hyades F stars have severely depleted their abundances (the Li gap), in sharp contrast to the predictions of the standard stellar evolution theory. We began a and Be survey aimed at identifying the physical mechanism that creates the gap. We present here the first results of that survey, which include high-resolution (R = 48,000-120,000) and high signal-to-noise ratio observations in 24 stars of the I λ6707.8 and/or the Be II λ3131 doublets taken at the University of Hawaii 2.2 m, Canada-France-Hawaii 3.6 m, and Keck I 10 m telescopes. Our program stars with detections in both and Be define a clear trend that suggests (1) the surface and Be abundance depletions are correlated and (2) surface diminishes more rapidly than surface Be. Our results suggest that correlated and Be depletion is a normal process that F stars undergo. The Li-Be trend argues strongly against the mass-loss and diffusion mechanisms and strongly supports slow mixing as the cause of the surface light-element deficiencies. Moreover, models with rotationally induced mixing are in better agreement with the data than are models with wave-driven mixing.

Beryllium in Lithium‐deficient F and G Stars

We present the results of an extensive search, conducted at the Canada-France-Hawaii 3.6-m telescope, for beryllium (Be) in the atmospheres of lithium-deficient F and G dwarfs. We also report revised lithium (Li) estimates for the entire sample using previously published equivalent widths and updated, consistently calculated stellar parameters. Abundances derived from an LTE analysis of the Li and Be line-forming regions confirm the suspicion that F stars which deplete Li by factors of 10-200 may also be beryllium deficient. Photospheric Be concentrations range from near meteoritic levels in G dwarfs to factors of 10-100 below this assumed initial abundance in hotter stars. Moreover, significant Be deficiencies appear in stars that populate a 600 K wide effective temperature window centered on 6500 K. This Be abundance gap is reminiscent of the Li gap observed in open clusters. Also, the discovery of 12 probable "110 Herculis" stars, objects that exhibit a depleted, but detected, surface concentration of both Li and Be, provides a powerful means of differentiating between the possible physical processes responsible for observed light element abundance patterns. Indeed, the Be data presented here, in conjunction with the newly calculated Li abundances, lead to the following conclusions regarding the hypothesized, light element depletion scenarios: Mass loss cannot account for stars with severely depleted (but detected) Li and moderate Be deficiencies. The predicted timescales for surface depletion due to microscopic diffusion are too long for significant Li and Be deficiencies to develop in cool (Teff ≤ 6200) stars; nevertheless, underabundances are observed in these stars. Diffusion theory also predicts Li and Be depletion rates to be comparable, but it is evident that Li and Be depletion proceed at different speeds. Models of mixing induced by internal gravity waves cannot explain mild Be deficiencies in cool dwarfs. A key meridional circulation prediction regarding the efficiency and severity of Li and Be dilution is shown to be fallible. However, rotationally induced mixing, a turbulent blending of material beneath the surface convection zone due to the onset of instabilities from superficial angular momentum loss, predicts both the observed light element depletion morphology as well as the existence of 110 Her analogs. These "Yale" mixing models provide, therefore, the most plausible explanation, of those presented, for the observed Li and Be abundances.

110 Herculis: A Possible Prototype for Simultaneous Lithium and Beryllium Depletion, and Implications for Stellar Interiors

Standard stellar evolution predicts that late F stars should have retained their initial surface lithium abundance because their convection zones are too shallow to destroy it at their base. Yet observations reveal Li depletions, sometimes quite severe (the Boesgaard Li gap). Several physical mechanisms, which are not usually included in stellar evolution calculations, have been proposed to account for the Li depletions. These mechanisms include mass loss, microscopic diffusion, and various types of slow mixing driven by either waves or rotation. Identifying which of these (if any) might really be at work not only is of vital interest to advancing our knowledge of stellar interiors but also may have serious implications in other contexts, such as cosmology. We bring attention to beryllium (Be) observations in late F stars, and propose that they are crucial for discriminating between scenarios. Particularly important is the star 110 Her, which is depleted in surface Be by about a factor of 5-10, and yet enough surface Li has survived to be detected. The Li depletion is stronger than that of Be, about a factor of 100-200. Depleting surface Be without having depleted all of the surface Li (and depleting Li more strongly than Be) requires specific circumstances; we discuss how this Li/Be depletion pattern, if representative, would severely constrain or eliminate most of the proposed mechanisms. One mechanism, rotationally induced mixing, predicts relative Li and Be depletions that agree well with what is observed. It is important to establish whether or not the Li/Be depletion pattern observed in 110 Her is representative.

Diffusion of Lithium and Beryllium in Evolving, Nonrotating Main-Sequence A and F Stars

view Abstract Citations (84) References (85) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Diffusion of Lithium and Beryllium in Evolving, Nonrotating Main-Sequence A and F Stars Richer, Jacques ; Michaud, Georges Abstract The evolution of the internal distribution of lithium and beryllium in the envelopes of nonrotating Population I A and F stars is computed for the whole main-sequence lifetime of the stars. The calculations include the effects of changing envelope structure due to core evolution and helium gravitational settling, convective mixing, nuclear burning, and diffusion in presence of radiative forces and dragging by the main gas constituents. The new OPAL opacities are used. Since adjustable parameters were calibrated using the Sun, the calculations presented here are entirely from first principles. The formation of a stable subsurface reservoir (sometimes two reservoirs in the case of Be) and later dredge-up from that reservoir are identified as the main mechanism controlling the surface abundance of lithium and beryllium in these stellar models. This phenomenon should be a generic one among hot stars, for many other trace elements, including Fe. The position and depth of the calculated Li abundance gap is consistent with the observed Li gap in several open clusters. It is however, narrower than the observed gap. Similar calculations using the 1977 opacities of Huebner et al., and independently calibrated, do not produce a deep enough gap. A "beryllium gap" is also predicted and is consistent with the observations in the Hyades. Publication: The Astrophysical Journal Pub Date: October 1993 DOI: 10.1086/173237 Bibcode: 1993ApJ...416..312R Keywords: DIFFUSION; STARS: ABUNDANCES; STARS: EVOLUTION; STARS: INTERIORS; STARS: PECULIAR full text sources ADS | data products SIMBAD (8)

Oxygen abundances in F-type stars of the Hyades and the Ursa Major group

We have derived the oxygen abundances of 50 F-type main-sequence stars, belonging to the Hyades open cluster, the Ursa Major group, and the field, using an NLTE analysis of the infrared triplet lines of O I at λλ7771-7775. The stars of the middle F-range (6500<T eff <6900 K) have previously been shown to exhibit a marked reduction in lithium abundance compared with both cooler and hotter objects in the same systems: the Li gap, for which several explanations have been proposed. Our results here show a substantial measure of uniformity in the oxygen abundances over the range in T eff between 5800 and 7400 K. The data might indicate a small dip (<0.1 dex) in the oxygen abundance for stars located in the Li gap

Particle Transport Processes

AbstractThe effect of gravitational settling and radiation driven diffusion on the evolution of stars near the main sequence is reviewed. New simplified formulae for calculating diffusion are proposed that improve on previous such formulae. The reliability of available diffusion coefficients is discussed and areas where further work is needed are identified. Newly available opacity calculations are used to estimate the effects of radiative acceleration on Fe.The size of the modifications to the evolution are shown to be modest: a reduction of order 10% on the evolutionary age of globular clusters. There are indications from the Li abundance in the high Teff halo stars that some turbulence is present below the convection zone. Models calibrated using solar properties reproduce the Teff at which the Li gap is observed in F stars as well as its depth without any arbitrary parameter. Similarly the presence of AmFm stars is explained over the Teff range where they are observed.

Oxygen Abundances in LI-Depleted F-Type Stars

AbstractWe have obtained the oxygen abundance in a sample of 44 F-type dwarfs in the Ursa Major group and the Hyades open cluster, using an NLTE analysis of the O I infrared triplet at λλ 7771-7775 Å. These clusters show the “lithium gap” in the F spectral range, and it has been suggested that if this phenomenon were due to microscopic diffusion, their Li-depleted stars could also show oxygen and/or nitrogen depletion. Our results here show a substantial measure of uniformity in the oxygen abundances. The data might indicate a very small dip (&lt; 0.1 dex) in the O abundance for stars located in the Li gap. Microscopic diffusion seems to be the only known mechanism able to produce an oxygen dip. If this mechanism were responsible for the Li gap, it would have to account for a depletion of up to two orders of magnitude in Li and, at the same time, less than 0.1 dex in oxygen. We note that turbulent mixing could modify a microscopic diffusion pattern in order to yield the observed abundances of the two elements.

Li in Clusters: A and Am Stars

AbstractA and Am stars were observed in open clusters for their Li content. The Li abundance is therefore studied on the hot side of the Li gap along isochrones. It is discussed in relation to evolutionary model atmospheres including particle transport processes.

Li depletion in F stars by internal gravity waves

view Abstract Citations (169) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Li Depletion in F Stars by Internal Gravity Waves Garcia Lopez, Ramon J. ; Spruit, Hendrik C. Abstract It is argued that internal gravity waves generated by the convective envelope of a star may be effective in producing a weak mixing in its radiative interior. The spectrum of wavelengths and frequencies generated is estimated. Only the largest horizontal wavelengths and the lowest frequencies contribute significantly to the wave energy flux and the mixing at the Li-burning depth. At this layer, the combination of the dependence on spectral type, both for the wave energy flux and the shear rate induced by the waves, would produce a distribution of Li abundances similar to that of the Li gap found among F-type stars of several open clusters, and the field. Quantitative agreement with the observed location and time scale of the gap is obtained, but only by increasing the intensity of the waves generated by a factor 15 above straightforward mixing-length estimates. The ratio of mixing length to scale height needed to get the gap in the right spectral range is 1.6. The blue edge of the gap is predicted to be sharp. Publication: The Astrophysical Journal Pub Date: August 1991 DOI: 10.1086/170356 Bibcode: 1991ApJ...377..268G Keywords: Abundance; F Stars; Gravity Waves; Internal Waves; Lithium; Stellar Interiors; Depletion; Stellar Composition; Stellar Envelopes; Stellar Gravitation; Astrophysics; CONVECTION; GRAVITATION; STARS: ABUNDANCES; STARS: INTERIORS; WAVE MOTIONS full text sources ADS | data products SIMBAD (1)

The lithium abundance in stars

The observations of Li abundances in pre-Main-Sequence, Main-Sequence (Population I and II), sub-giant, and giant stars are reviewed in order to show how Li can be used as a constraint on stellar hydrodynamics and in particular on particle transport processes in stars. Important observational results include the tight Li abundance dependence on T eff in the Hyades, the time dependence of the Li abundance below T eff = 6000 K, the presence of a Li gap at T eff = 6700 K in young clusters and the large Li abundance in some peculiar giants. The observed abundances are compared to models which include progressively more physical processes. The ‘standard’ stellar evolution model is compatible with the upper envelope of the observations in young clusters such as the Pleiades and α Per. The observed Li underabundances is then caused by Li burning on the pre-Main Sequence. The large abundance spread observed is not understood. It does not appear to be simply related to rotation since the Pleiades stars rotate more slowly but have larger Li abundances than many stars of α Per. The Li abundance gap observed in clusters is not explained by the ‘standard’ model. Models involving diffusion seem to explain it in a natural way, though meridional circulation could also be involved. Evolutionary effects and the interaction between diffusion and meridional circulation should, however, be taken more fully into account in those models. The Li abundances in giants show that additional destruction processes are involved beyond those included in the ‘standard’ evolutionary models. Meridional circulation is compatible with most of those observations, without any arbitrary parameter being adjusted. While turbulence is nearly certainly present in stars, it is poorly understood and we suggest that it should be invoked to explain only those phenomena that the better understood processes cannot explain. Its description always involves arbitrary parameters. Turbulence appears to be required to explain the Li abundances in the Sun and in G stars of the Hyades and older clusters. In halo stars, the observed Li abundance has probably been reduced from the original by a factor of 2 so that the original abundance was probably equal to log N(Li) = 2.5. More calculations are needed to better establish this value. The large Li abundances observed in some peculiar giants are not understood.

Diffusion, Meridional Circulation and Mass Loss in Main Sequence and Horizontal Branch Stars

Photospheric abundances are used to determine the importance of atomic diffusion, meridional circulation, mass loss and turbulence in main sequence and horizontal branch stars. Atomic diffusion leads to the appearance of the Li gap and the AmFm and HgMn phenomena at approximately the Teff at which they are observed. It leads to a 12 to 25% reduction in the age of halo stars. The Li abundance in Halo stars is probably 50% of the original abundance. Atomic diffusion competes with meridional circulation. The V sin i at which the HgMn and AmFm phenomena disappear give an argument in favour of the meridional circulation model of Tassoul and Tassoul. Mass loss is probably present in AmFm stars and cooler F stars but only at the rate of 10-15 M⊙ yr-1. In many objects, the turbulent particle diffusion coefficient is at most 10 times larger than the atomic diffusion coefficient.

Beryllium abundances in the lithium-deficient Hyades F stars

Spectra of the Be II doublet at 3130.4 and 3131.1 A have been obtained in eight Hyades F dwarfs, including stars in the Hyades Li gap found by Boesgaard and Trippico (1986). Abundances were determined in the standard way, but a 'differential spectroscopy' procedure was also used which involved comparing the Hyades stars with a standard star via a least-squares algorithm to give the best fit between the object and standard spectra in the region immediately around the Be doublet. This procedure was adapted to rapidly rotating stars by convolving with the standard spectrum with a rotation profile of the correct width. A synthesis of the spectrum was done also with 20 spectral lines in the region of the Be blend. There is virtually no change in the Be content in those stars across the temperature region where there is a large deficiency in Li. The Be/H values are about 10 to the -11th, near the solar value of 1.4 x 10 to the -11th. 15 references.