High-Resolution Spectroscopy of Ursa Major Moving Group Stars

We use new and extant literature spectroscopy to address abundances and membership for UMa moving group stars. We first compare the UMa, Coma, and Hyades H-R diagrams via a homogeneous set of isochrones, and find that these three aggregates are essentially coeval. Our spectroscopy of cool UMa dwarfs reveals striking abundance anomalies--trends with Teff, ionization state, and excitation potential--like those recently seen in young cool M34, Pleaides, and Hyades dwarfs. In particular, the trend of rising 7774 Ang-based OI abundance with declining Teff is markedly subdued in UMa compared to the Pleiades, suggesting a dependence on age or metallicity. Despite disparate sources of Li data,our homogeneous analysis indicates that UMa members evince remarkably small scatter in the Li-Teff plane for Teff>5200 K. Significant star-to-star scatter suggested by previous studies is seen for cooler stars. Comparison with the consistently determined Hyades Li-Teff trend reveals differences qualitatively consistent with this cluster's larger [Fe/H] (and perhaps slightly larger age). However, quantitative comparison with standard stellar models indicates the differences are smaller than expected, suggesting the action of a fourth parameter beyond age, mass, and [Fe/H] controlling Li depletion.Comment: To appear in Publ. Astron. Soc. Pacif. (September 2005

Stellar Oxygen Abundances III. The Oxygen Abundance of the very Metal Poor Halo Star BD -13 deg 3442

A spectrum of the very metal poor ((Fe/H) approximately -3) halo star BD -13 deg 3442 is presented and used to determine this star\u27s oxygen abundance. Our determination makes BD -13 deg 3442 the most metal poor dwarf (though a somewhat evolved one) with an O abundance determination. The O abundance (determined from the 7774 A O I triped) and (O/Fe) ratio is compared to that of two other metal-poor stars. The (O/Fe) ratio of BD -13 deg 3442 is found to be approximately 0.35 dex larger than that of the other two halo stars. Possible implications of this result are discussed

Lithium in the Young Cluster NGC 2264

We use moderate-resolution, moderate signal-to-noise ratio spectroscopy to derive Li abundances for six F and G stars in the young (few Myr) cluster NGC 2264. These are combined with consistently determined abundances of five NGC 2264 G and K stars having published Li data. The mean non-LTE abundance, calculated with photometric temperatures, is log N(Li) = 3.27 ± 0.05. This is essentially identical to the meteoritic value, providing no evidence of Galactic Li enrichment over the past 4.6 Gyr—at least to the extent that the meteoritic value represents a typical cosmic value at that time. The scatter of 0.16 dex is well within the expected uncertainties and does not indicate any unexpected differential Li depletion. Our mean Li abundance is 0.2–0.3 dex larger than that in the hotter stars of IC 2602 (30 Myr) and the Pleiades (100 Myr), which have consistently determined abundances. This might indicate prior modest differential enrichment, very recent Galactic Li enrichment that is not a global process, or the increasing effect of Li depletion at the young ages of these clusters; such depletion cannot be satisfactorily understood in terms of extant standard or rotational stellar models. Li is not overabundant in the cluster short-period binary W134, a result consistent with the predictions of tidal theory and rotational stellar models. The flatness of Li with Teff (mass) persists to 4000 K (~0.5 M⊙), a morphology in agreement with both standard and rotational stellar models having ages 4 Myr. We note that some spectral type–based Teff scales lead to Li abundances in all of the five cooler cluster stars that are 0.3–0.8 dex larger than abundances in the six hotter stars. Interestingly, such behavior also is inferred for the near-initial Li-Teff (mass) morphology of IC 2602 and the Pleiades when model depletion factors are mapped onto their observed current abundances. No physical origin of such an abundance pattern, which might suggest an initial cluster Li abundance over a factor of 2 larger than meteoritic, is identified. Rather, we believe that it is caused by lingering deficiencies in the model depletion predictions and errant spectral type–based relative Teff values; comparison of independent spectral classifications and the effects of their differences on the derived Li abundances provide direct support for the latter. This underscores the need for accurate relative spectroscopic Teff values derived for a larger number of cluster stars from higher quality data. Finally, radial velocities are derived for our NGC 2264 stars. Our heliocentric estimate of 24 km s-1 is in fine agreement with recent determinations from early-type cluster stars, but shows ample scatter. Some candidate pre–main-sequence spectroscopic binaries are noted, including a multiple-lined star not included in the Li study

Abundance Ratios in a Common Proper Motion Pair: Chemical Evidence of Accreted Substructure in the Halo Field?

Elemental abundances are presented for the metal-poor ([Fe/H] =-1.50) common proper motion pair HD 134439 and HD 134440. The abundances for the two stars are in very good agreement, with the neutral species showing only a small difference (˜0.05 dex) which is well within the statistical and Teff uncertainties. The essentially identical abundances, kinematics, and parallaxes of the two stars indicate that they share a common history. This history, however, appears to be different than other metal-poor stars. Suggestions, based on kinematic evidence, that these two-stars are representative of a distinct accretion event are corroborated by our abundance ratios, which indicate [Mg/Fe], [Si/Fe], and [Ca/Fe] are consistently some ˜0.3 dex lower than the vast majority of metal-poor field stars. Such underabundances have been predicted in environments like dwarf Spheroidals and the Magellanic Clouds. Moreover, our abundance ratio deficiencies are consistent with those recently observed in the the anomalously young globular clusters Rup 106 and Pal 12, which have been alleged to have been accreted from the Magellanic Clouds. The [Fe/H] and retrograde motion of the common proper motion pair are characteristic of the subset of Galactic globular clusters suggested by Rodgers & Paltoglou [ApJ, 283, L5 (1984)] to have been coalesced from satellite galaxies. We also call attention to the metal-poor subgiant BD+03 740 as another possible representative of an accreted or chaotically formed member of the halo field. If recent Fe analyses of this star are correct, then [Mg/Fe] and [0/Fe] are 0.5 dex lower than in other metal-poor field stars. This star also has a relatively low photometrically inferred age; relative youth has been noted as a possible characteristic of accreted field populations, and is qualitatively consistent with the young ages of the purportedly accreted globular clusters Rup 106, Pal 12, Ter 7, and Arp 2. Additionally, the revised [O/Fe] ratio for BD+03 740 would suggest a large spread, perhaps 0.7 dex, in [0/Fe] of field stars of very low [Fe/H]; this itself might provide strong evidence of some degree of chaotic halo formation in independent fragments. If, on the other hand, earlier Fe analyses of this star are correct, [Mg/Fe] and [O/Fe] for this star are unremarkable; however, the low gravity estimates from earlier studies would then suggest that BD+03 740 is a \u3c=3 Gyr field star with [Fe/H] ˜-3. Further spectroscopic study of this interesting object is needed to determine if it may be similar to the metal-poor ([Fe/H] = - 3.1) high velocity star CS 22873-139, which Preston [M 108, 2267 (1994)] has argued is \u3c=8 Gyr in age. Finally, our abundance ratios for RD 134439 and RD 134440 suggest that low [alphaFe] may be a characteristic of accreted halo systems including the anomalously young globulars. However, as has been noted by others, the low alpha-element abundances apparently cannot explain differences between photometric and Ca II-based metallicity estimates for these clusters, nor the variation in these differences between Rup 106 and Pal 12

Accretion from Circumstellar Discs and the λ Boo Phenomenon

This paper examines the suggestion by Venn & Lambert that the λ Boo phenomenon may be caused by accretion of depleted circumstellar gas. The IRAS data base was searched for sources corresponding to those in a catalogue of over 100 λ Boo candidates. A relatively small fraction (\u3c 20 per cent) of the catalogue stars that were detected by IRAS show infrared excesses like those seen in other early-type stars. However, this is expected, due to non-genuine λ Boo entries in the catalogue and finite circumstellar disc lifetimes. Additionally, it is argued that the amount of accreted depleted gas required to cause λ Boo abundance patterns is so small that any associated dust is not necessarily detectable by thermal emission in the infrared or submillimetre regions. The proportions of bona fide and non-λ Boo stars showing infrared excesses differ at the 84 per cent confidence level, which is oniy suggestive evidence for accretion. A simple model, where accretion of depleted gas once in a circumstellar disc×s ‘central hole’ occurs, gives accretion rates and masses consistent with semi-empirical values inferred from observed abundances in λ Boo stars. These values are, however, oniy weakly constrained. The accretion rates are also consistent with Charbonneau×s independent estimate

Galactic [O/Fe] and [C/Fe] Ratios: The Influence of New Stellar Parameters

We consider the effects of recent NLTE gravities and Fe abundances on stellar [O/Fe] and [C/Fe] ratios. The NLTE parameters greatly reduce or eliminate the well-known discrepancy between CH- and C I–based C abundances in metal-poor stars and previously seen trends of atomic-based [C/Fe] and [O/Fe] with Teff. With the NLTE parameters, the metal-poor molecular-based [C/Fe] ratio maintains its increase with declining [Fe/H]; this may also be demonstrated by the revised atomic-based ratios. [O/Fe] values derived from OH and O I features are considerably reduced and typically show improved agreement but are 0.1–0.2 dex larger than those exhibited by the Lick-Texas syndicate\u27s recent [O I]–based giant determinations. The revised [O/Fe] ratios still show an increase down to at least [Fe/H] ~ -2; we suggest that recent field giant data show an increase with similar slope. Even adopting uniform NLTE parameters, study-to-study abundance differences can be significant; moreover, different NLTE studies yield differing gravities and Fe abundances even after taking Teff differences into account. Comparison of metal-poor giant gravities and cluster abundances with isochrones, trigonometric gravities, and near-turnoff cluster abundances yields conflicting indications about whether the evolved gravities might be underestimated as suggested for metal-poor dwarfs. Regardless, we argue that even extreme gravity revisions do not affect the [O/Fe]-[Fe/H] relation derived from the extant results. Combining what we believe the most reliable giant and dwarf data considered here, we find [O/Fe] = -0.184(±0.022) × [Fe/H] + 0.019 with an rms scatter of only 0.13 dex; there is no indication of a break or slope change at intermediate [Fe/H]. The gentle slope is in very reasonable agreement with some chemical evolution models employing yields with small mass and metallicity dependences. Finally, two notes are made concerning Na abundance-spatial position and element-to-element correlations in M13 giants

Fe I and Fe II Abundances of Solar-Type Dwarfs in the Pleiades Open Cluster

We have derived Fe abundances of 16 solar-type Pleiades dwarfs by means of an equivalent width analysis of Fe I and Fe II lines in high-resolution spectra obtained with the Hobby - Eberly Telescope and High Resolution Spectrograph. Abundances derived from Fe II lines are larger than those derived from Fe I lines (herein referred to as over-ionization) for stars with Teff < 5400 K, and the discrepancy (deltaFe = [Fe II/H] - [Fe I/H]) increases dramatically with decreasing Teff, reaching over 0.8 dex for the coolest stars of our sample. The Pleiades joins the open clusters M 34, the Hyades, IC 2602, and IC 2391, and the Ursa Major moving group, demonstrating ostensible over-ionization trends. The Pleiades deltaFe abundances are correlated with Ca II infrared triplet and Halpha chromospheric emission indicators and relative differences therein. Oxygen abundances of our Pleiades sample derived from the high-excitation O I triplet have been previously shown to increase with decreasing Teff, and a comparison with the deltaFe abundances suggests that the over-excitation (larger abundances derived from high excitation lines relative to low excitation lines) and over-ionization effects that have been observed in cool open cluster and disk field main sequence (MS) dwarfs share a common origin. Star-to-star Fe I abundances have low internal scatter, but the abundances of stars with Teff < 5400 K are systematically higher compared to the warmer stars. The cool star [Fe I/H] abundances cannot be connected directly to over-excitation effects, but similarities with the deltaFe and O I triplet trends suggest the abundances are dubious. Using the [Fe I/H] abundances of five stars with Teff > 5400 K, we derive a mean Pleiades cluster metallicity of [Fe/H] = +0.01 +/- 0.02.Comment: 32 pages, 7 figures, 7 tables; accepted by PAS

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 3330 Ang region. While our Hyades N abundances show a modest 0.2 dex trend over a 800 K Teff 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 Teff 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 no larger 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.Comment: Accepted for publication in the Publications of the Astronomical Society of the Pacifi

A Spectroscopic Analysis of the Eclipsing Short-Period Binary v505 Per and the Origin of the Lithium Dip

As a test of rotationally-induced mixing causing the well-known Li dip in older mid-F dwarfs in the local Galactic disk, we utilize high-resolution and -S/N Keck/HIRESspectroscopy to measure the Li abundance in the components of the1 Gyr, [Fe/H]=-0.15 eclipsing short-period binary V505 Per. We find A(Li)=2.7+/-0.1 and 2.4+/-0.2 in the Teff=6500 and 6450 K primary and secondary components, respectively. Previous Teff determinations and uncertainties suggest that each component is located in the midst of the Li dip. If so, their A(Li) are >=2-5 times larger than A(Li) detections and upper limits observed in the similar metallicity and intermediate-age open clusters NGC 752 and 3680, as well as the more metal-rich and younger Hyades and Praesepe. These differences are even larger if the consistent estimates of the scaling ofinitial Li with metallicity inferred from nearby disk stars, open clusters, and recent Galactic chemical evolution models are correct. Our results suggest, independently of complementary evidence based on Li/Be ratios, Be/B ratios, and Li in subgiants evolving out of the Li dip, that main-sequence angular momentum evolution is the origin of the Li dip. Specifically, our stars' A(Li) indicates tidal synchronization can be sufficiently efficient and occur early enough in short-period binary mid-F stars to reduce the effects of rotationally-induced mixing and destruction of Li occuring during the main-sequence in otherwise similar stars that are not short-period tidally-locked binaries.Comment: Accepted for publication in Publications of the Astronomical Society of the Pacific (July 2013 volume

Alkali-Activity Correlations in Open Clusters

We present a census of correlations between activity measures and neutral resonance lines of the alkali elements Li i and K i in open clusters and star-forming regions. The majority of very young associations and star formation regions show no evidence of Li-activity correlations, perhaps because their chromospheric activity indicators have a dominant origin in accretion processes with implied disk-clearing timescales in the range of a few times 106 to $4 ;107 yr. Alkali-alkali and/or alkali-activity correlations are newly noted within IC 2391, M34, and perhaps Blanco 1 and NGC 6475. Global X-ray luminosities are not as robust indicators as traditional optical indicators of alkali-activity correlations, nor are Li i–K i relations. Intracluster alkali-activity correlations are not global but are seen only within different intracluster subsamples, evincing rich behavior. Li- and K-activity correlations appear to go hand in hand, likely suggesting that at least some part of intracluster Li variance is not due to real differential Li depletion. Although up to$90% of the star-to-star variance in Li i and K i within such a subsample can be related to that in optical chromospheric emission, significant Li dispersion above observational scatter may remain even after accounting for this. We suggest, for example, that at least three independent mechanisms (including a possible intracluster age spread) influence the distribution in the M34 Li-Teff plane. We argue that Li-activity correlations are not illusory manifestations of a physical Li-rotation connection. Although an unexpected corre-lation between Li, chromospheric emission, and the k6455 Ca i feature in cool M34 dwarfs indicates that the role of ‘‘activity’’ is played by spots/plages, we note that the alkali-activity correlations are qualitatively opposite in sign to other abundance anomalies being rapidly delineated in active, young, cool stars