A comprehensive census of the stellar content of the OB associations within 1 kpc from the Sun is presented, based on Hipparcos positions, proper motions, and parallaxes. It is a key part of a long-term project to study the formation, structure, and evolution of nearby young stellar groups and related star-forming regions. OB associations are unbound moving groups, which can be detected kinematically because of their small internal velocity dispersion. The nearby associations have a large extent on the sky, which traditionally has limited astrometric membership determination to bright stars (V 6 mag), with spectral types earlier than ~B5. The Hipparcos measurements allow a major improvement in this situation. Moving groups are identified in the Hipparcos Catalog by combining de Bruijne's refurbished convergent point method with the Spaghetti method of Hoogerwerf & Aguilar. Astrometric members are listed for 12 young stellar groups, out to a distance of ~650 pc. These are the three subgroups Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as Vel OB2, Tr 10, Col 121, Per OB2, α Persei (Per OB3), Cas–Tau, Lac OB1, Cep OB2, and a new group in Cepheus, designated as Cep OB6. The selection procedure corrects the list of previously known astrometric and photometric B- and A-type members in these groups and identifies many new members, including a significant number of F stars, as well as evolved stars, e.g., the Wolf-Rayet stars γ2 Vel (WR 11) in Vel OB2 and EZ CMa (WR 6) in Col 121, and the classical Cepheid δ Cep in Cep OB6. Membership probabilities are given for all selected stars. Monte Carlo simulations are used to estimate the expected number of interloper field stars. In the nearest associations, notably in Sco OB2, the later-type members include T Tauri objects and other stars in the final pre–main-sequence phase. This provides a firm link between the classical high-mass stellar content and ongoing low-mass star formation. Detailed studies of these 12 groups, and their relation to the surrounding interstellar medium, will be presented elsewhere. Astrometric evidence for moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, Cam OB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive. OB associations do exist in many of these regions, but they are either at distances beyond ~500 pc where the Hipparcos parallaxes are of limited use, or they have unfavorable kinematics, so that the group proper motion does not distinguish it from the field stars in the Galactic disk. The mean distances of the well-established groups are systematically smaller than the pre-Hipparcos photometric estimates. While part of this may be caused by the improved membership lists, a recalibration of the upper main sequence in the Hertzsprung-Russell diagram may be called for. The mean motions display a systematic pattern, which is discussed in relation to the Gould Belt. Six of the 12 detected moving groups do not appear in the classical list of nearby OB associations. This is sometimes caused by the absence of O stars, but in other cases a previously known open cluster turns out to be (part of) an extended OB association. The number of unbound young stellar groups in the solar neighborhood may be significantly larger than thought previously.

/pdf/a-hipparcos-census-of-the-nearby-ob-associations-44xw838axn.pdf

A Hipparcos Census of the Nearby OB Associations

While the strong anticorrelation between chromospheric activity and age has led to the common use of the Ca II H
and K emission index (R'_(HK) = L_(HK)/L_(bol)) as an empirical age estimator for solar-type dwarfs, existing activity-age relations
produce implausible ages at both high and low activity levels.We have compiled R'_(HK)
HK data from the literature
for young stellar clusters, richly populating for the first time the young end of the activity-age relation. Combining the
cluster activity data with modern cluster age estimates and analyzing the color dependence of the chromospheric activity
age index,we derive an improved activity-age calibration for F7-K2 dwarfs (0:5 mag < B - V < 0.9 mag). We
also present a more fundamentally motivated activity-age calibration that relies on conversion of R'_(HK) values through
the Rossby number to rotation periods and then makes use of improved gyrochronology relations. We demonstrate that
our new activity-age calibration has typical age precision of ~0.2 dex for normal solar-type dwarfs aged between the
Hyades and the Sun (~0.6-4.5 Gyr). Inferring ages through activity-rotation-age relations accounts for some color-dependent
effects and systematically improves the age estimates (albeit only slightly). We demonstrate that coronal
activity as measured through the fractional X-ray luminosity (R_X = L_X/L_(bol)) has nearly the same age- and rotation inferring
capability as chromospheric activity measured through R'_(HK). As a first application of our calibrations, we
provide new activity-derived age estimates for a volume-limited sample of the 108 solar-type field dwarfs within
16 pc.

/pdf/improved-age-estimation-for-solar-type-dwarfs-using-activity-3a3h4zjus7.pdf

Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics

▪ Abstract Abundance variations within globular clusters (GCs), and of GC stars with respect to field stars, are important diagnostics of a variety of physical phenomena, related to the evolution of individual stars, mass transfer in binary systems, and chemical evolution in high density environments. The broad astrophysical implications of GCs as building blocks of our knowledge of the Universe make a full understanding of their history and evolution basic in a variety of astrophysical fields. We review the current status of the research in this field, comparing the abundances in GCs with those obtained for field stars, discussing in depth the evidence for H-burning at high temperatures in GC stars, describing the process of self-enrichment in GCs with particular reference to the case of the most massive Galactic GC (ω Cen), and discussing various classes of cluster stars with abundance anomalies. Whereas the overall pattern might appear very complex at first sight, exciting new scenarios are opening whe...

/pdf/abundance-variations-within-globular-clusters-26f3ox3sbw.pdf

Abundance Variations within Globular Clusters

We report on the results of the Sun in Time multiwavelength program (X-rays to UV) of solar analogs with ages covering ~0.1-7 Gyr. The chief science goals are to study the solar magnetic dynamo and to determine the radiative and magnetic properties of the Sun during its evolution across the main sequence. The present paper focuses on the latter goal, which has the ultimate purpose of providing the spectral irradiance evolution of solar-type stars to be used in the study and modeling of planetary atmospheres. The results from the Sun in Time program suggest that the coronal X-ray-EUV emissions of the young main-sequence Sun were ~100-1000 times stronger than those of the present Sun. Similarly, the transition region and chromospheric FUV-UV emissions of the young Sun are expected to be 20-60 and 10-20 times stronger, respectively, than at present. When we consider the integrated high-energy emission from 1 to 1200 A, the resulting relationship indicates that about 2.5 Gyr ago the solar high-energy flux was about 2.5 times the present value and about 3.5 Gyr ago was about 6 times the present value (when life supposedly arose on Earth). The strong radiation emissions inferred should have had major influences on the thermal structure, photochemistry, and photoionization of planetary atmospheres and have played an important role in the development of primitive life in the solar system. Some examples of the application of the Sun in Time results on exoplanets and on early solar system planets are discussed.

https://iopscience.iop.org/article/10.1086/427977/pdf

Evolution of the Solar Activity over Time and Effects on Planetary Atmospheres. I. High-Energy Irradiances (1-1700 Å)

An unsupervised software robot that automatically and robustly reduces and analyzes CCD observations of photometric standard stars is described. The robot measures extinction coefficients and other photometric parameters in real time and, more carefully, on the next day. It also reduces and analyzes data from an all-sky 10 μm camera to detect clouds; photometric data taken during cloudy periods are automatically rejected. The robot reports its findings to observers and data analysts via the World Wide Web. It can be used to assess photometricity and to build data on site conditions. The robot's automated and uniform site monitoring represents a minimum standard for any observing site with queue scheduling, a public data archive, or likely participation in any future National Virtual Observatory.

/pdf/a-photometricity-and-extinction-monitor-at-the-apache-point-4xhpqk47zw.pdf

A Photometricity and Extinction Monitor at the Apache Point Observatory

Utilizing Hipparcos parallaxes, original radial velocities and recent literature values, new Ca ii H and K emission measurements, literature-based abundance estimates, and updated photometry (including recent resolved measurements of close doubles), we revisit the Ursa Major moving group membership status of some 220 stars to produce a final clean list of nearly 60 assured members, based on kinematic and photometric criteria. Scatter in the velocity dispersions and H-R diagram is correlated with trial activity-based membership assignments, indicating the usefulness of criteria based on photometric and chromospheric emission to examine membership. Closer inspection, however, shows that activity is considerably more robust at excluding membership, failing to do so only for � 15% of objects, perhaps considerably less. Our UMa members demonstrate nonzero vertex deviation in the Bottlinger diagram, behavior seen in older and recent studies of nearby young disk stars and perhaps related to Galactic spiral structure. Comparison of isochrones and our final UMa group members indicates an age of 500 � 100 Myr, some 200 Myr older than the canonically quoted UMa age. Our UMa kinematic=photometric members’ mean chromospheric emission levels, rotational velocities, and scatter therein are indistinguishable from values in the Hyades and smaller than those evinced by members of the younger Pleiades and M34 clusters, suggesting these characteristics decline rapidly with age over 200–500 Myr. None of our UMa members demonstrate inordinately low absolute values of chromospheric emission, but several may show residual fluxes a factor of � 2 below a Hyades-defined lower envelope. If one defines a Maunder-like minimum in a relative sense, then the UMa results may suggest that solar-type stars spend 10% of their entire main-sequence lives in periods of precipitously low activity, which is consistent with estimates from older field stars. As related asides, we note six evolved stars (among our UMa nonmembers) with distinctive kinematics that lie along a 2 Gyr isochrone and appear to be late-type counterparts to disk F stars defining intermediate-age star streams in previous studies, identify a small number of potentially very young but isolated field stars, note that active stars (whether UMa members or not) in our sample lie very close to the solar composition zero-age main sequence, unlike Hipparcos-based positions in the H-R diagram of Pleiades dwarfs, and argue that some extant transformations of activity indices are not adequate for cool dwarfs, for which Ca ii infrared triplet emission seems to be a better proxy than H� -based values for Ca ii H and K indices.

/pdf/stellar-kinematic-groups-ii-a-reexamination-of-the-2bdm5jm64e.pdf

Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group

High-dispersion (2.4 A/mm), ultrahigh signal-to-noise ratio (3000:1) Reticon spectra of Vega revealed two distinct types of profiles. The strong lines exhibit classical rotational profiles with enhanced wings, but the weak lines have distinctly different, flat-bottomed profiles. Using ATLAS9 model atmopheres and SYNTHE synthetic spectra, Vega has been modeled as a rapidly rotating, pole-on star with a gradient in temperature and gravity over the photosphere. By fitting to the flat-bottomed line profiles of Fe 1 lambda 4528 and Ti 2 lambda 4529, we find least-squares fit values of V sin i = 21.8 plus or minus 0.2 km/sec polar T(sub eff) = 9695 plus or minus 25 K, polar log(base 10)g = 3.75 plus or minus 0.02 dex, V(sub eq) = 245 plus or minus 15 km/sec, and inclination 5 deg .1 plus or minus 0 deg .3. The variations in T(sub eff) and log(base 10)g over the photosphere total 390 K and 0.08 dex, respectively. Assuming V sin i = 21.8 km/sec, an independent fit to the observed continuous flux from 1200 to 10,500 A produced a similar set of values with polar T(sub eff) = 9595 plus or minus 20 K, polar log(base 10)g = 3.80 plus or minus 0.03 dex, and inclination 6 deg .0 plus or minus 0 deg .7.

Vega : a rapidly rotating pole-on star

Astrophysical applications of powerful new databases

The variability of the Hg ii � 3984 line in the primary of the binary starAnd was discovered through the examination of high-dispersion spectra with signal-to-noise ratios greater than 500. This first definitively identified spectrum variation in any mercury-manganese star is not due to the orbital motion of the compan- ion. Rather, the variation is produced by the combination of the 2.38236 day period of rotation of the pri- mary that we determined and a nonuniform surface distribution of mercury that is concentrated in its equatorial region. If the surface mercury distribution exhibits long-term stability, then it is likely that a weak magnetic field operates in its atmosphere, but if changes are observed in the line profile over a period of a few years, then these would constitute direct evidence for diffusion. Subject headings: binaries: general — binaries: spectroscopic — stars: chemically peculiar — stars: individual (� Andromedae)

/pdf/the-variability-of-the-hg-ii-l3984-line-of-the-mercury-2d77h7mlqz.pdf

The Variability of the Hg II λ3984 Line of the Mercury-Manganese Star α Andromedae

The formation of long-lasting structures at the surfaces of stars is commonly ascribed to the action of strong magnetic fields. This paradigm is supported by observations of evolving cool spots i ...

/pdf/weather-in-stellar-atmosphere-revealed-by-the-dynamics-of-1nap8sfd3w.pdf

Saul J. Adelman

Papers

Stellar Kinematic Groups. II. A Reexamination of the Membership, Activity, and Age of the Ursa Major Group

Vega : a rapidly rotating pole-on star

Astrophysical applications of powerful new databases

The Variability of the Hg II λ3984 Line of the Mercury-Manganese Star α Andromedae

Weather in stellar atmosphere revealed by the dynamics of mercury clouds in α Andromedae