Burton F. Jones

The evolution of the lithium abundances of solar-type stars. III - The Pleiades

New measurements of lithium in more than 100 Pleiades F, G, and K dwarfs are reported. Abundances are determined from spectrum synthesis fits to the data as well as from use of new covers of growth from the Li 6708-A feature. It is argued that most Late-F and early-G dwarfs in the Pleiades are consistent with the tight N(Li) vs mass relation seen in the Hyades in the same mass range. Most Li-rich stars have abundances at or near the primordial level for Population I, and none exceed that level by a significant amount. At any given color the stars that rotate fast have the most Li and have the strongest chromospheric activity. Ways in which an apparent spread in N(Li) could arise from an intrinsically tight n(Li)-mass relation are considered, and it is concluded that the spread is probably real and is not an artifact of line formation conditions or inhomogeneous atmospheres on the stars.

Rotation and chromospheric emission among F, G, and K dwarfs of the Pleiades

We report and discuss high-resolution echelle spectra of more than 100 F, G, and K dwarfs in the Pleiades. Comparisons of the profiles of Hα and the Ca II infrared triplet to chromospherically inactive field stars allow us to measure chromospheric activity in these stars, and we have determined v sin i values to a limit of 7 km s −1 . We have combined these new observations with published data and have conducted a census to assess the state of our knowledge of Pleiades solar-type stars. After allowing for a patch of heavy reddening in the southwest part of the cluster, consistent dereddened colors are determined from the available photometry and temperatures are derived. Rotation is fully resolved in the Pleiades for stars more massive than the Sun, but for less massive stars there are still too many upper limits to v sin i to dertermine the underlying, intrinsic distribution of rotation

CONFIRMATION OF ERRORS IN HIPPARCOS PARALLAXES FROM HUBBLE SPACE TELESCOPE FINE GUIDANCE SENSOR ASTROMETRY OF THE PLEIADES

We present absolute trigonometric parallaxes and relative proper motions for three members of the Pleiades, obtained with the Hubble Space Telescopes Fine Guidance Sensor 1r, a white-light interferometer. We estimate spectral types and luminosity classes of the stars comprising the astrometric reference frame from R ? 2000 spectra, VJHK photometry, and reduced proper motions. From these we derive estimates of absolute parallaxes and introduce them into our model as observations with error. We constrain the three cluster members to have a 1 ? dispersion in distance less than 6.4 pc and find an average ?abs = 7.43 ? 0.17 ? 0.20 mas, where the second error is systematic due to member placement within the cluster. This parallax corresponds to a distance of 134.6 ? 3.1 pc or a distance modulus of m - M = 5.65 ? 0.05 for these three Pleiades stars, presuming a central location. This result agrees with three other independent determinations of the Pleiades distance. Presuming that the cluster depth systematic error can be significantly reduced because of the random placement of these many members within the cluster, these four independent measures yield a best-estimate Pleiades distance of ?abs = 7.49 ? 0.07 mas, corresponding to a distance of 133.5 ? 1.2 pc or a distance modulus of m - M = 5.63 ? 0.02. This resolves the dispute between the main-sequence fitting and the Hipparcos distance moduli in favor of main-sequence fitting.

The Evolution of the Lithium Abundances of Solar-Type Stars. VIII. M67 (NGC 2682)

We have used the HIRES spectrograph on the Keck 10 m telescope to obtain echelle spectra of 25 solar-type stars in the old open cluster M67 (NGC 2682) and have used these spectra to derive lithium abundances. This cluster, which has approximately the same age and composition as the Sun, shows a pattern of Li depletion different from either the Pleiades or the Hyades. At M 1 M⊙, M67 is more Li depleted than the Hyades. For ~1 M⊙ stars, M67 shows a significant spread in Li, with about two-thirds of the stars being Li-rich and the remainder Li-poor. The data are consistent with M67 having the same initial Li abundance as the initial solar abundance. This is further evidence that Li depletion in solar-type stars occurs during their main-sequence lifetimes, although at a slower rate than pre–main-sequence depletion. These M67 observations also indicate that a 1 M⊙ star at the Suns age can have an Li abundance within a range of ~1 dex, and that the Sun is not necessarily representative of all stars of its mass, age, and composition.

The evolution of angular momentum among zero-age main-sequence solar-type stars

We consider a survey of rotation among F, G, and K dwarfs of the Pleiades in the context of other young clusters (Alpha Persei and the Hyades) and pre-main-sequence (PMS) stars (in Taurus-Auriga and Orion) in order to examine how the angular momentum of a star like the sun evolves during its early life on the main sequence. The rotation of PMS stars can be evolved into distributions like those seen in the young clusters if there is only modest, rotation-independent angular momentum loss prior to the ZAMS. Even then, the ultrafast rotators (UFRs, or ZAMS G and K dwarfs with v sin i equal to or greater than 30 km/s) must owe their extra angular momentum to their conditions of formation and to different angular momentum loss rates above a threshold velocity, for it is unlikely that these stars had angular momentum added as they neared the ZAMS, nor can a spread in ages within a cluster account for the range of rotation seen. Only a fraction of solar-type stars are thus capable of becoming UFRs, and it is not a phase that all stars experience. Simple scaling relations (like the Skumanich relation) applied to the observed surface rotation rates of young solar-type stars cannot reproduce the way in which the Pleiades evolve into the Hyades. We argue that invoking internal differential rotation in these ZAMS stars can explain several aspects of the observations and thus can provide a consistent picture of ZAMS angular momentum evolution.

Rotational Velocities of Low-Mass Stars in the Pleiades and Hyades

We have obtained high-resolution spectra of 89 M dwarf members of the Pleiades and Hyades and have derived radial velocities, Hα equivalent widths, and spectroscopic rotational velocities for these stars. Typical masses of the newly observed Pleiades and Hyades stars are ~0.4 M⊙ and ~0.2 M⊙, respectively. We combine our new observations with previously published data to explore the rotational evolution of young stars with M ≤ 0.4 M⊙. The average rotation rate in the Hyades (age 600 Myr) is about 0.4 times that in the Pleiades (110 Myr), and the mean equivalent widths of Hα are also lower. As found in previous studies, the correlation between rotation and chromospheric activity is identical in both clusters, implying that the lower activity in the Hyades is a result of the lower rotation rates. We show that a simple scaling of the Pleiades rotational distribution for M ≤ 0.4 M⊙, corrected for the effects of structural evolution, matches that of the Hyades if the average angular momentum loss from the Pleiades to the Hyades age is factor of ≈6. This suggests that the distribution of initial angular momenta and disk-locking lifetimes for the lowest mass stars was similar in both clusters. We argue that this result provides further evidence for a saturation of the angular momentum loss rate at high rotational velocities.

Why Are the K Dwarfs in the Pleiades So Blue

The K dwarfs in the Pleiades fall nearly 1 mag below a main-sequence isochrone when plotted in a color- magnitude diagram utilizing V magnitude as the luminosity index and BV as the color index. This peculiarity has been known for 40 years but has gone unexplained and mostly ignored. When compared to Praesepe members, the Pleiades K dwarfs again are subluminous (or blue) in a color-magnitude diagram using BV as the color index. However, using VI as the color index, stars in the two clusters are coincident to MV � 10; using VK as the color index, Pleiades late K and M stars fall above the main-sequence locus defined by Praesepe members. We believe that the anomalous spectral energy distributions for the Pleiades K dwarfs, as compared to older clusters, are a consequence of rapid stellar rotation and may be primarily due to spottedness. If so, the required areal filling factor for the cool component has to be very large (� 50%). Weak- lined T Tauri stars have similar color anomalies, and we suspect that this is a common feature of all very young K dwarfs (spectral type >K3). The peculiar spectral energy distribution needs to be considered in deriving accurate pre-main-sequence isochrone-fitting ages for clusters like the Pleiades, since the age derived will depend on the temperature index used.

The evolution of the lithium abundances of solar-type stars. II - The Ursa Major Group

We draw upon a recent study of the membership of the Ursa Major Group (UMaG) to examine lithium among 0.3 Gyr old solar-type stars. For most G and K dwarfs, Li confirms the conclusions about membership in UMaG reached on the basis of kinematics and chromospheric activity. G and K dwarfs in UMaG have less Li than comparable stars in the Pleiades. This indicates that G and K dwarfs undergo Li depletion while they are on the main sequence, in addition to any pre-main-sequence depletion they may have experienced. Moreover, the Li abundances of the Pleiades K dwarfs cannot be attributed to main-sequence depletion alone, demonstrating that pre-main-sequence depletion of Li also takes place. The suns Li abundance implies that the main-sequence mechanism becomes less effective with age. The hottest stars in UMaG have Li abundances like those of hot stars in the Pleiades and Hyades and in T Tauris, and the two genuine UMaG members with temperatures near Boesgaards Li chasm have Li abundances consistent with that chasm developing fully by 0.3 Gyr for stars with UMaGs metallicity. We see differences in the abundance of Li between UMaG members of the same spectral types, indicating that a real spread in the lithium abundance exists within this group.

The evolution of the lithium abundances of solar-type stars. IV: Praesepe

Echelle observations are presented of lithium in 63 F and G dwarfs of the Praesepe cluster. For stars earlier than about G0V, Praesepe follows the same trends seen in the Hyades, which has approximately the same age and composition. Stars in Praesepe later than about G5V have more Li than their Hyades counterparts, possibly because Praesepe is slightly younger than the Hyades or has slightly lower metallicity. Significant differences in the abundance of Li are seen among stars of the same color, and, as in the Hyades, there is a tendency for the deviant stars to be binaries to the extent that duplicity in Praesepe is known. There are also stars with much less Li than most cluster members yet which appear to be true members of Praesepe. The close binary KW 181 has a normal Li abundance, despite the fact that similar close binaries in the Hyades are Li rich.

HD 98800: A unique stellar system of post-T tauri stars

HD 98800 is a system of four stars, and it has a large infrared excess that is thought to be due to a dust disk within the system. In this paper we present new astrometric observations made with Hipparcos, as well as photometry from Hubble Space Telescope WFPC2 images. Combining these observations and reanalyzing previous work allow us to estimate the age and masses of the stars in the system. Uncertainty in these ages and masses results from uncertainty in the temperatures of the stars and any reddening they may have. We find that HD 98800 is most probably about 10 Myr old, although it may be as young as 5 Myr or as old as 20 Myr old. The stars in HD 98800 appear to have metallicities that are about solar. An age of 10 Myr means that HD 98800 is a member of the post T Tauri class of objects, and we argue that the stars in HD 98800 can help us understand why post T Tauris have been so elusive, HD 98800 may have formed in the Centaurus star-forming region, but it is extraordinary in being so young and yet so far from where it was born.