Teff Range Research Articles

A Systematic Study of Departures from Chemical Equilibrium in the Atmospheres of Substellar Mass Objects

We present a systematic study of the spectral consequences of departures from chemical equilibrium in the atmospheres of L and T dwarfs, and for even cooler dwarfs. The temperature/pressure profiles of the nonequilibrium models are fully consistent with the nonequilibrium chemistry. Our grid of nonequilibrium models includes spectra for effective temperatures from 200 to 1800 K, three surface gravities, four possible values of the coefficient of eddy diffusion in the radiative zone, and three different CO/CH4 chemical reaction prescriptions. We also provide clear and cloudy model variants. We find, in keeping with previous studies, that there are essentially only two spectral regions where the effects of departures from chemical equilibrium can influence the predicted spectrum. These are in the M (~4-5 μm) and N (8-14 μm) bands due to CO and NH3, respectively. The overabundance of CO translates into flux suppressions of at most ~40% between effective temperatures of 600 and 1800 K. The effect is largest around Teff ≈ 1100 K. The underabundance of ammonia translates into flux enhancements of no more than ~20% for the Teff range from 300 to 1800 K, with the largest effects at the lowest values of Teff. The magnitude of the departure from chemical equilibrium increases with decreasing gravity, with increasing eddy diffusion coefficient, and with decreasing speed of the CO/CH4 reaction. Although these effects are modest, they lead to better fits with the measured T dwarf spectra. Furthermore, the suppression in the M band due to nonequilibrium enhancements in the CO abundance disappears below ~500 K, and is only partial above ~500 K, preserving the M-band flux as a useful diagnostic of cool atmospheres and maintaining its importance for searches for the cooler brown dwarfs beyond the T dwarfs.

Behavior of Li abundances in solar-analog stars

Context. It is known that the surface lithium abundances of field solar-analog G dwarfs show a large dispersion of >2 dex (among which our Sun is located at the lower end) despite the similarity of stellar parameters, and planet-host stars tend to show comparatively lower Li abundances in the narrow Teff range. Aims. To investigate the reason for these phenomena, an extensive study of Li abundances and their dependence on stellar parameters was carried out for a homogeneous sample of 118 selected solar analogs based on high-dispersion spectra obtained at Okayama Astrophysical Observatory. Methods. The atmospheric parameters were spectroscopically determined by using the equivalent widths of Fe i and Fe ii lines, the ages/masses were estimated from stellar evolutionary tracks, and the width of the macrobroadening (rotation plus macroturbulence) function as well as Li abundances (ALi) were established by spectrum-fitting analyses. Results. The resulting ALi vs. Teff relation revealed a characteristic inverse-triangle-like distribution enclosed by two clear-cut boundaries (the slanted one running from ∼5900 K to ∼5800 K and the vertical one at ∼5700 K), while the Sun is located around its lowest apex. More significantly, ALi in this region of large dispersion was found to closely correlate with the macrobroadening width (vr+m), which is considered to be the most important parameter. Conclusions. With a reasonable assumption that the difference of rotational velocity is mainly responsible for the variety of vr+m ,w e may conclude that the stellar angular momentum plays the decisive role in determining the surface Li abundances of solar-analog stars in the Teff range of ∼5900–5700 K. The low-Li tendency of planet-host stars may thus be interpreted in terms of rotational characteristics.

New response functions for absorption-line indices from high-resolution spectra

Using a huge library of 1-Å resolution spectra over a wide range of log Teff, log g and [Fe/H] and for both solar and α-enhanced abundance ratios [α/Fe], we derive the response functions (s) which are commonly used to correct indices with solar α-enhanced ratios to indices with α-enhanced>0. The s vary not only with the type of star but also with metallicity. With the aid of this and the fitting functions (s), we derive the indices for single stellar populations. The new indices for single stellar populations are used to derive the age, metallicity, and degree of enhancement first for sample of Galactic globular clusters for which these key parameters have been independently derived from the colour-magnitude diagram and/or spectroscopic studies, and second to study the classical two index diagram.

Pulsational Instabilities in Accreting White Dwarfs

The detection of g-mode pulsations in accreting white dwarfs (WDs) in cataclysmic variables (CVs) with a large range of effective temperatures (Teff) has shown these WDs to be a more diverse class than their isolated counterparts, the ZZ Ceti and DB pulsators. The simplest contrast of CV to isolated pulsators is an envelope of solar-like composition (of various helium enrichments if the donor is evolved) rather than pristine hydrogen or helium. A range of WD masses is expected, from low-mass He core WDs to massive WDs. We investigate the impact of this diversity on the range of Teff for which g-modes are unstable. Motivated by earlier theoretical studies, we compare a fiducial g-mode period to the thermal time at the base of the convection zones created by H and first He (H/He I) ionization or second He (He II) ionization zones. We find that (for solar composition envelopes), relative to a fiducial WD mass 0.6 M☉, the blue edge for a 0.4 M☉ He core WD shifts downward by ≈1000 K, while that for a massive ≈1.2 M☉ WD shifts upward by ≈2000 K. Surprisingly, increasing Y by only 10% relative to solar creates an intermediate instability strip near 15,000 K.

L and T Dwarf Models and the L to T Transition

Using a model for refractory clouds, a novel algorithm for handling them, and the latest gas-phase molecular opacities, we have produced a new series of L and T dwarf spectral and atmosphere models as a function of gravity and metallicity, spanning the \teff range from 2200 K to 700 K. The correspondence with observed spectra and infrared colors for early- and mid-L dwarfs and for mid- to late-T dwarfs is good. We find that the width in infrared color-magnitude diagrams of both the T and L dwarf branches is naturally explained by reasonable variations in gravity and, therefore, that gravity is the second parameter of the L/T dwarf sequence. We investigate the dependence of theoretical dwarf spectra and color-magnitude diagrams upon various cloud properties, such as particle size and cloud spatial distribution. In the region of the L$\to$T transition, we find that no one cloud-particle-size and gravity combination can be made to fit all the observed data. Furthermore, we note that the new, lower solar oxygen abundances of Allende-Prieto, Lambert, & Asplund (2002) produce better fits to brown dwarf data than do the older values. Finally, we discuss various issues in cloud physics and modeling and speculate on how a better correspondence between theory and observation in the problematic L$\to$T transition region might be achieved.

The Correlation of Lithium and Beryllium in F and G Field and Cluster Dwarf Stars

Although Li has been extensively observed in main-sequence field and cluster stars, there are relatively fewer observations of Be. We have obtained Keck HIRES spectra of 36 late F and early G dwarfs in order to study the Li-Be correlation we found previously in the temperature regime of 5900-6650 K. The sample size for this temperature range with detectable and (usually) depleted Li and Be is now 88, including Li and Be abundances in both cluster and field stars. Therefore we can now investigate the influence of other parameters such as age, temperature, and metallicity on the correlation. The Be spectra at 3130 8 were taken over six nights from 1999 November to 2002 January and have a spectral resolution of � 48,000 and a median signal-to-noise ratio (S/N) of 108 pixel � 1 . We obtained Li spectra of 22 stars with the University of Hawaii 88 inch (2.2 m) telescope and coudespectrograph with a spectral resolution of � 70,000 and a median S/N of 110 pixel � 1 . We have rede- termined the effective temperatures for all the stars and adopted other parameters from published data or empirical relations. The abundances of both Li and Be in the stars we observed were determined from spectrum synthesis with MOOG 2002. The previously observed Li equivalent widths for some of our Be stars were used with the new temperatures and MOOG 2002 in the ''blends'' mode. For the 46 field stars from this and earlier studies we find a linear relation between A(Li) andA(Be) with a slope of 0:375 � 0:036. Over the Teff range 5900-6650 K, we find the modest scatter about the Be-Li relation to be significantly correlated with Teff and perhaps also (Fe/H). Dividing the sample into two temperature regimes of 6300-6650 K (corresponding to the cool side of the Li-Be dip) and 5900-6300 K (corresponding to the Li ''plateau'') reveals possible small differences in the slopes for the two groups, 0:404 � 0:034 and 0:365 � 0:049, respectively. When we include the cluster stars (Hyades, Pleiades, Praesepe, UMa Group, and Coma), the slope for the full temperature range (88 stars) is essentially the same, at 0:382 � 0:030, as for the field stars alone. For the hotter temperature group of 35 Li-Be dip stars in the field and in clusters the slope is higher, at 0:433 � 0:036, while for the cooler star group (54 stars) the slope is 0:337 � 0:031, different by more than 1 � . This small difference in the slope is predicted by the theory of rotationally induced mixing. The four stars with (Fe/H) less than � 0.4 are all below the best-fit relation, i.e., there is more Be depletion at a given A(Li) or less Be ab initio. The youngest stars, i.e., Pleiades, have less depletion of both Li and Be. This too is predicted by rotationally induced slow mixing. Combining the Be results from both field and cluster stars, we find that there are stars with undepleted Be, i.e., near the meteoritic values of 1.42 dex, at all temperatures from 5500 to 6800 K. Depletions of Be of up to and even exceeding 2 orders of magnitude are common between 6000 and 6700 K.

Photometry of Eclipsing Binary Stars in the Large and Small Magellanic Clouds

AbstractThe light curves for three eclipsing binaries in the Magellanic Clouds have been obtained using CCD uVJIC photometry. One target in the LMC, MACHO*05:36:48.7−69:17:00, is an eccentric system, e = 0.20, with a period of 3.853534 ± 0.000005 d. Initial solutions indicate a primary component in the range Teff,1 = 20,000−35,000K and the secondary Teff,2 1000−2000K cooler than the primary, with inclinations ranging i = 84.2° − 86.0°. Two targets in the SMC, MOA J005018.4-723855 and MOA J005623.5−722123, have periods of 1.8399±0.0004 and 2.3199 ± 0.0003 days respectively. Both have circular orbits with the former being a semi-detached system.

Really Cool Stars and the Star Formation History at the Galactic Center

We present λ/Δλ = 550 to 1200 near infrared H and K spectra for a magnitude limited sample of 79 asymptotic giant branch and cool supergiant stars in the central ≈5 pc (diameter) of the Galaxy. Using a set of similar spectra obtained for solar neighborhood stars with the same range in Teff and Mbol for the Galactic center (GC) sample as calibrators, we construct the Hertzsprung–Russell diagram for the GC sample. Using an automated maximum likelihood routine, we derive a coarse star formation history of the GC. We find that roughly 75 % of the stars that have formed in the central few pc are older than 5 Gyr. In particular, the best fitting star formation history yields star formation rates in four age bins of 0.01 to 0.10 Gyr, 0.10 to 1.0 Gyr, 1.0 to 5.0 Gyr, and 5 Gyr to 12 Gyr, of 39.5 ± 11 × 10−4M⊙ yr−1, 3.6 ± 2 × 10−4M⊙ yr−1, 5.7 ± 2 × 10−4M⊙ yr−1, and 13.4 ± 5 × 10−4M⊙ yr−1, respectively. The GC has been forming stars over the last 100 Myr at a rate roughly four times higher than the average rate over 12 Gyr, though the total mass formed in this recent epoch is less than three percent of the total mass formed over all times.

Beyond the T Dwarfs: Theoretical Spectra, Colors, and Detectability of the Coolest Brown Dwarfs

We explore the spectral and atmospheric properties of brown dwarfs cooler than the latest known T dwarfs. Our focus is on the yet-to-be-discovered free-floating brown dwarfs in the Teff range from ~800 to ~130 K and with masses from 25 to 1 MJ. This study is in anticipation of the new characterization capabilities enabled by the launch of the Space Infrared Telescope Facility (SIRTF) and the eventual launch of the James Webb Space Telescope (JWST). In addition, it is in support of the continuing ground-based searches for the coolest substellar objects. We provide spectra from ~0.4 to 30 μm, highlight the evolution and mass dependence of the dominant H2O, CH4, and NH3 molecular bands, consider the formation and effects of water ice clouds, and compare our theoretical flux densities with the putative sensitivities of the instruments on board SIRTF and JWST. The latter can be used to determine the detection ranges from space of cool brown dwarfs. In the process, we determine the reversal point of the blueward trend in the near-infrared colors with decreasing Teff (a prominent feature of the hotter T dwarf family), the Teff's at which water and ammonia clouds appear, the strengths of gas-phase ammonia and methane bands, the masses and ages of the objects for which the neutral alkali metal lines (signatures of L and T dwarfs) are muted, and the increasing role as Teff decreases of the mid-infrared fluxes longward of 4 μm. These changes suggest physical reasons to expect the emergence of at least one new stellar class beyond the T dwarfs. Furthermore, studies in the mid-infrared could assume a new, perhaps transformational, importance in the understanding of the coolest brown dwarfs. Our spectral models populate, with cooler brown dwarfs having progressively more planet-like features, the theoretical gap between the known T dwarfs and the known giant planets. Such objects likely inhabit the Galaxy, but their numbers are as yet unknown.

Discovery of the Most Lithium-rich Dwarf: Diffusion in Action

Using WIYN/Hydra observations, we report the first discovery of a super-Li-rich dwarf, the star J37 of the Hyades-aged cluster NGC 6633. This star's gargantuan surface abundance of A(Li) = 4.29 ± 0.07 [A(X) ≡ 12 + log(NX/NH)] is a factor of 10 larger than the abundances observed in meteorites and in minimally depleted young open cluster stars. J37 may be the smoking gun for the action of diffusion, models of which predict the existence of striking overabundances due to radiative acceleration (the Li peak) in the extremely narrow Teff range of 6900-7100 K. The low-C, high-Fe, and higher Ni abundances in J37 also support the diffusion model, although other elemental abundances do not agree as well with the model predictions. We have discounted other Li-enrichment scenarios (spots, asymptotic giant branch contamination, planetesimal accretion). While slow mixing seems to be the universally dominant Li-depleting mechanism during the main-sequence evolution of F, G, and K dwarfs, the F dwarf J37 illustrates that diffusion can occur in solar-type stars under the right circumstances.

Radiative accelerations on Ne in the atmospheres of late B stars

Radiative accelerations on Ne are calculated for the atmospheres of main sequence stars with 11000 < Teff < 15000 K, corresponding to the range of the HgMn stars. The calculations take into account neon fine structure as well as shadowing of neon lines using the entire Kurucz line list, bound-bound, bound-free, and free-free opacity of H, He, and C as well as some NLTE effects. NLTE effects modify the radiative acceleration by a factor of order 100 in the outer atmosphere. The dependence of the radiative acelerations on the Ne abundance, Teff, and gravity is studied. Radiative accelerations are well below gravity in the entire range of Teff and it is predicted that in stable atmospheres Ne should sink and be observed as underabundant. This agrees with recent observations of low Ne abundances in HgMn stars.

Abundances in Stars from the Red Giant Branch Tip to Near the Main-Sequence Turnoff in M71. III. Abundance Ratios

We present [Fe/H] abundance results that involve a sample of stars with a wide range in luminosity, from luminous giants to stars near the turnoff in a globular cluster. Our sample of 25 stars in M71 includes 10 giant stars more luminous than the RHB, three horizontal branch stars, nine giant stars less luminous than the RHB, and three stars near the turnoff. We analyzed both Fe I and Fe II lines in high-dispersion spectra observed with HIRES at the W. M. Keck Observatory. We find that the [Fe/H] abundances from both Fe I and Fe II lines agree with each other and with earlier determinations. Also the [Fe/H] obtained from Fe I and Fe II lines is constant within the rather small uncertainties for this group of stars over the full range in Teff and luminosity, suggesting that non-LTE effects are negligible in our iron abundance determination. In this globular cluster, there is no difference among the mean [Fe/H] of giant stars located at or above the RHB, RHB stars, giant stars located below the RHB and stars near the turnoff.

A photoionization model of the compact HII region G29.96-0.02

We present a detailed photoionization model of G29.96-0.02 (hereafter G29.96), one of the brightest Galactic Ultra Compact HII (UCHII) regions in the Galaxy. This source has been observed extensively at radio and infrared wavelengths. The most recent data include a complete ISO (SWS and LWS) spectrum, which displays a remarkable richness in atomic fine-structure lines. The number of observables is twice as great as the number available in previous studies. In addition, most atomic species are now observed in two ionization stages. The radio and infrared data on G29.96 are best reproduced using a nebular model with two density components: a diffuse (n(e) similar to 680 cm(-3)) extended (similar to1 pc) component surrounding a compact (similar to0.1 pc) dense (n(e) similar to 57 000 cm(-3)) core. The properties of the ionizing star were derived using state-of-the-art stellar atmosphere models. CoStar models yield an effective temperature of similar to30(+2)(-1) kK whereas more recent non-LTE line blanketed atmospheres with stellar winds indicate somewhat higher values, T-eff similar to 32-38 kK. This range in T-eff is compatible with all observational constraints, including near-infrared photometry and bolometric luminosity. The range 33-36 kK is also compatible with the spectral type O5-O8 determined by Watson & Hanson (1997) when recent downward revisions of the effective temperature scale of O stars are taken into account. The age of the ionizing star of G29.96 is found to be a few 10(6) yr, much older than the expected lifetime of UCHII regions. Accurate gas phase abundances are derived with the most robust results being Ne/S = 7.5 and N/O = 0.43 (1.3 and 3.5 times the solar values, respectively).

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.

Abundances in Stars from the Red Giant Branch Tip to Near the Main-Sequence Turnoff in M71. II. Iron Abundance

We present [Ffe/H] abundance results that involve a sample of stars with a wide range in luminosity from luminous giants to stars near the turnoff in a globular cluster. Our sample of 25 stars in M71 includes 10 giant stars more luminous than the RHB, 3 horizontal branch stars, 9 giant stars less luminous than the RHB, and 3 stars near the turnoff. We analyzed both Fe I and Fe II lines in high dispersion spectra observed with HIRES at the W. M. Keck Observatory. We find that the [Fe/H] abundances from both Fe I and Fe II lines agree with each other and with earlier determinations. Also the [Fe/H] obtained from Fe I and Fe II lines is constant within the rather small uncertainties for this group of stars over the full range in Teff and luminosity, suggesting that NLTE effects are negligible in our iron abundance determination. In this globular cluster, there is no difference among the mean [Fe/H] of giant stars located at or above the RHB, RHB stars, giant stars located below the RHB and stars near the turnoff.

Neon abundances in normal late-B and mercury--manganese stars

ABSTRA C T We make new non-local thermodynamic equilibrium calculations to deduce the abundances of neon from visible-region echelle spectra of selected Ne i lines in seven normal stars and 20 HgMn stars. We find that the best strong blend-free Ne line that can be used at the lower end of the effective temperature Teff range is l6402, although several other potentially useful Ne i lines are found in the red region of the spectra of these stars. The mean neon abundance in the normal starsOlog Aa 8:10U is in excellent agreement with the standard abundance of neon (8.08). However, in HgMn stars neon is almost universally underabundant, ranging from marginal deficits of 0.1‐0.3 dex to underabundances of an order of magnitude or more. In many cases, the lines are so weak that only upper limits can be established. The most extreme example found is y Her with an underabundance of at least 1.5 dex. These underabundances are qualitatively expected from radiative acceleration calculations, which show that Ne has a very small radiative acceleration in the photosphere, and that it is expected to undergo gravitational settling if the mixing processes are sufficiently weak and there is no strong stellar wind. According to theoretical predictions, the low Ne abundances place an important constraint on the intensity of such stellar winds, which must be less than 10 214 M( yr 21 if they are non-turbulent.

An Empirical Test and Calibration of HiiRegion Diagnostics

We present spectrophotometry in the 3600-9700 A region for a sample of 39 H II regions in the Galaxy and Magellanic Clouds, for which independent information is available on the spectral types and effective temperatures of the ionizing stars. The spectra have been used to evaluate nebular diagnostics of stellar temperature, metal abundance, and ionization parameter, and compare the observed behavior of the line indices with predictions of nebular photoionization models. We observe a strong degeneracy between forbidden-line sequences produced by changes in stellar Teff and metal abundance, which severely complicates the application of many forbidden-line diagnostics to extragalactic H II regions. Our data confirm however that the Edmunds and Pagel [O II]+[O III] abundance index and the Vilchez and Pagel `eta' index provide more robust diagnostics of metal abundance and stellar effective temperature, respectively. A comparison of the fractional helium ionization of the H II regions with stellar temperature confirms the reliability of the spectral type vs Teff calibration for the relevant temperature range Teff < 38000 K. We use empirical relations between the nebular hardness indices and Teff to reinvestigate the case for systematic variations in the stellar effective temperatures and the upper IMFs of massive stars in extragalactic H II regions. The data are consistent with a significant softening of the ionizing spectra (consistent with cooler stellar temperatures) with increasing metal abundance, especially for Z less than solar. However unresolved degeneracies between Z and Teff still complicate the interpretation of this result.

Rotations and Abundances of Blue Horizontal-Branch Stars in Globular Cluster M15

High-resolution optical spectra of 18 blue horizontal-branch stars in the globular cluster M15 indicate that their stellar rotation rates and photospheric compositions vary strongly as a function of effective temperature. Among the cooler stars in the sample, at Teff approximately 8500 K, metal abundances are in rough agreement with the canonical cluster metallicity, and the vsini rotations appear to have a bimodal distribution, with eight stars at vsini<15 km s-1 and two stars at vsini approximately 35 km s-1. Most of the stars at Teff>/=10,000 K, however, are slowly rotating, vsini<7 km s-1, and their iron and titanium are enhanced by a factor of 300 to solar abundance levels. Magnesium maintains a nearly constant abundance over the entire range of Teff, and helium is depleted by factors of 10-30 in three of the hotter stars. Diffusion effects in the stellar atmospheres are the most likely explanation for these large differences in composition. Our results are qualitatively very similar to those previously reported for M13 and NGC 6752, but with even larger enhancement amplitudes, presumably due to the increased efficiency of radiative levitation at lower intrinsic [Fe/H]. We also see evidence for faster stellar rotation explicitly preventing the onset of the diffusion mechanisms among a subset of the hotter stars.

A Uniform Analysis of Boron in F and G Disk Dwarfs fromHubble Space TelescopeArchival Spectra

The boron abundance is derived for 14 near-solar metallicity F and G dwarfs from the archived data set of spectra obtained with the Hubble Space Telescope Goddard High Resolution Spectrograph in the region of the B I resonance line near 2497 A. These abundances are derived in a uniform way with respect to recent work on solar boron. The synthetic spectra of the B I region were calculated by means of an improved linelist to fit the spectra of the stars spanning a range in Teff between 5650 and 6700 K and metallicity between [Fe/H] = -0.75 and +0.15. The synthetic spectra were computed considering all major sources of metal opacities and, in particular, an updated lower value of Mg I b-f opacity at 2500 A was used in the calculations. Adopting Li and Be abundances from the literature, significant B depletions (~0.3-0.5 dex) are found only for the two stars with the most extreme Li depletions (≥2.5 dex) and Be depletions (≥1.5 dex), although the trend of B versus Be indicates that mild B depletions may exist in many of the stars. These results are in concordance with the Boesgaard et al. conclusion that a slow mixing in the outer envelope is responsible for depletions of Li, Be, and B. Stars with less severe Li and Be depletions exhibit roughly solar [B/Fe] ratios. A comparison of the boron abundances from the disk stars (with undepleted boron) with metal-poor halo dwarfs reveals that B is linearly related to Fe over the entire range of metallicities from [Fe/H] ~ -2.5 to 0.1. The B/Be ratio found for stars with near-solar metallicities and no Li depletion is B/Be = 27 ± 5, equal to the solar ratio (B/Be = 23).

SPECTRAL CALIBRATION AND KINEMATIC PROPERTIES OF THE FIELD HORIZONTAL BRANCH STARS

ABSTRACT An understanding of the kinematic properties of the Galactic halo is an essential component to the ultimate understanding of the Galactic formation history. The blue field horizontal branch stars (FHB) are excellent stellar probes with which to explore these properties. The FHB stars are both intrinsically luminous, which allows kinematic exploration beyond the confines of the local solar neighborhood, and numerous, which is important for good statistical results. In the past, however, isolating a pure sample of FHB stars from their main sequence, A-star, counter-parts has proven to be a difficult and highly uncertain task leaving the FHB kinematic results open to skepticism. This dissertation explores the kinematic properties of an FHB sample identified through the use of a new spectral calibration technique. Follow up medium resolution (1-2 A), low S/N (10 < S/N < 20) spectroscopy and UBV photometry have been obtained for 1213 FHB and main sequence A-type stars (FHB/A) drawn from the candidates in the HK objective prism surveys of Beers et al. (1988, ApJS, 67, 461). This data was used to determine stellar radial velocities, metal abundance, and to separate the FHB/A candidates, by exploiting the inherent differences in surface gravity between the two classes. To determine the stellar physical parameters, an 833 point calibration grid was constructed spanning the range of Teff, log g and [Fe/H] for the program objects. The grid includes the widths of H-delta, H-gamma and the CaII K line, determined from synthetic spectra, and the Kurucz Atlas9 color relations. A calibration technique was then developed which gives varying weights to the observables in accord with their dependency on the physical parameters. Comparison to standard stars, noise-injected synthetic spectra, and globular cluster HB stars indicate an uncertainty in the derived physical parameters of sigma Teff = ± 250 K, sigma log g = ± 0.25, and sigma [Fe/H] = ± 0.30. Results from the noisy synthetic data place class misidentifications at less than 8%. A statistical analysis of the FHB star kinematics indicate the halo is composed of at least two components. The locally dominant component shows flattening in the vertical velocity dispersion and an overall velocity ellipsoid of (sigmar: sigmaphi: sigmatheta) = (124 ± 32:163 ± 35:62 ± 23) km/s. This component has a prograde systemic rotation of Vrot = 40 ± 17 km/s. The second component, which begins to dominate at |Z| > 4 kpc, and [Fe/H] < -1.6, has a more spherical velocity ellipsoid of (sigmar: sigmaphi: sigmatheta) = (86 ± 43:146 ± 54:122 ± 33) km/s. Also, the second component is in systemic counter-rotation with Vrot = -93 ± 36 km/s. It is expected that the rather large rotational velocity dispersion, which is found in both results, is in part due to mixing of the two components. There is only marginal kinematic evidence for the existence of a metal-weak thick disk in the FHB sample. The kinematics of the metal-poor ([Fe/H] < -1.0) main sequence A-stars (BMP) indicate an isotropic velocity ellipsoid of ~90 km/s and a systemic rotation of Vrot = 85 ± 18 km/s. These values are uniquely different from either the halo or thick-disk populations. There is also strong kinematic evidence for a cutoff in the BMP population at metal abundances of [Fe/H] < -2.2. The stars with metallicities below this value have a systemic rotation of Vrot ~0 km/s and are expected to be either misidentified FHB stars or "true" halo blue stragglers. The kinematic evidence from the FHB stars strongly suggests that the outer halo of the Galaxy formed from accretion of discrete fragments while the inner halo likely formed from a dissipative collapse. The BMP kinematics are consistent with an origin from one or more delayed accretion events. The progenitor fragments of the BMPs are expected to have had a range in metallicty of -1.0 > [Fe/H] > -2.2.