Search

The ages of the most common stars - low-mass (cool) stars like the Sun, and smaller - are difficult to derive because traditional dating methods use stellar properties that either change little as the stars age or are hard to measure. The rotation rates of all cool stars decrease substantially with time as the stars steadily lose their angular momenta. If properly calibrated, rotation therefore can act as a reliable determinant of their ages based on the method of gyrochronology. To calibrate gyrochronology, the relationship between rotation period and age must be determined for cool stars of different masses, which is best accomplished with rotation period measurements for stars in clusters with well-known ages. Hitherto, such measurements have been possible only in clusters with ages of less than about one billion years, and gyrochronology ages for older stars have been inferred from model predictions. Here we report rotation period measurements for 30 cool stars in the 2.5-billion-year-old cluster NGC 6819. The periods reveal a well-defined relationship between rotation period and stellar mass at the cluster age, suggesting that ages with a precision of order 10 per cent can be derived for large numbers of cool Galactic field stars., Comment: 24 pages, 12 figures, 1 table, published in Nature January 2015

The observable properties of star clusters hold vital information concerning the star formation process. In this introductory chapter we review the properties of open clusters within the Galaxy. Starting with the most basic properties of open clusters, such as age, mass and metallicity, we then move onto a discussion of how these properties vary depending on the number density, cluster mass and spatial distribution across the Galaxy. In addition to these observables, we also describe methods of selecting members of open clusters through the use of proper motion and radial velocity kinematic information, and demonstrate their usage in highlighting dynamical evolutionary processes, for example, mass segregation. Finally, we end this chapter with a discussion of unbound stellar populations, namely OB associations, and their relation to molecular clouds within the Galaxy.

Theory of Star Formation and Dynamical Evolution of Stellar Systems.- Kinematics of Star Clusters and Associations.- From whence the Field.

The previous chapters on star-forming regions have focussed on the spatial distributions of gas and stars. This chapter therefore focusses on the internal kinematics of the young stars within the star-forming regions. Our discussion covers the kinematics of objects in OB associations and young star clusters as well as star-forming regions, detailing not only their use in assigning membership, but also in highlighting the kinematic differences between these different environments.

Multiple stars represent approximately 50 % of the stellar population, and as a result, have implications for the dynamics of young star clusters and associations. This chapter reviews several aspects of multiple star systems, namely the field solar-type multiple population, the field OB star multiple population, and finally the open cluster solar-type multiple population. We discuss each in terms of observed distributions and how these vary depending on their environment. Finally, we close with some thoughts concerning the significance of multiple star systems to the dynamics of young star clusters and associations, specifically how these can be used to further constrain star formation processes and whether these systems can be used as a tracer of identifying the origin of the field population.

The dynamical evolution of young star clusters and associations is an evolving interplay of the gravitational potential, as determined by the spatial distribution of volume densities and the motions of both the gas and stars. In this chapter we briefly discuss several nearby star-forming regions which are relevant to our discussion on the dynamics of these young systems, concentrating on those which have been intensively investigated, namely the Taurus-Auriga star-forming region, the Orion Molecular Cloud and the statistical properties of young embedded clusters.

In the previous chapters we have discussed the products of the star formation process. In the remaining chapters we focus on an overview of star-forming regions; however, in this chapter we take an in-depth look at one young region in particular, the \(\lambda \) Orionis association. Our discussion begins by placing the \(\lambda \) Ori region in context through investigating the local environment, namely the interstellar dust and molecular gas. We then move on to a census of the stellar population of the \(\lambda \) Ori region, briefly introducing the high-mass stars, before providing a more comprehensive description of the low-mass stars, specifically in terms of the history of their identification and what information we can extract from them concerning the star formation history of the region. We conclude by using this analysis to create an almost complete picture of how the region formed and subsequently evolved to its current state of dissolving into the Galactic field.

We present the barium surface abundance of 12 blue stragglers (BSs) and 18 main-sequence (MS) stars in the intermediate-age open cluster NGC 6819 (2.5 Gyr) based on spectra obtained from the Hydra Multi-object Spectrograph on the WIYN 3.5 m telescope. For the MS stars we find [Fe/H] = $+$0.05 $\pm$ 0.04 and [Ba/Fe] = $-$0.01 $\pm$ 0.10. The majority of the BS stars are consistent with these values. We identify five BSs with significant barium enhancement. These stars most likely formed through mass transfer from an asymptotic giant branch star that polluted the surface of the BS with the nucleosynthesis products generated during thermal pulsations. This conclusion aligns with the results from the substantial work done on the BSs in old open cluster NGC 188 that identifies mass transfer as the dominant mechanism for BS formation in that open cluster. However, four of the BSs with enhanced barium show no radial-velocity evidence for a companion. The one star that is in a binary is a double-lined system, meaning the companion is not a white dwarf and not the remnant of a prior AGB star. In this paper we attempt to develop a consistent scenario to explain the origin of these five BSs., Comment: 11 pages, 6 figures

We present a catalogue of near-infrared photometry of young stars associated with the Ophiuchus molecular cloud, based on observations made with the Hubble Space Telescope NICMOS-3 camera at 1.1 $\mu$m and 1.6 $\mu$m. Our survey covers 0.02 square degrees centered on the dense molecular cores in Lynds 1688. We detect 165 sources at 1.6 $\mu$m and 65 sources at 1.1 $\mu$m, within our estimated completeness limits of 21.0 mag and 21.5 mag, respectively. An analysis of the cloud extinction, based on existing molecular line maps, suggests that most of the sources lying within the 40 $\rm{A_V}$ extinction contour of the cloud are probable cloud members. Approximately half (58/108) of these sources are previously unpublished. The faint embedded sources revealed by these observations are spatially concentrated in three regions of high stellar space density (N$> 10^4$ stars pc$^{-3}$). While the spatial distribution of these sources reflects that of the brighter, well--known population of young stars in Ophiuchus, it is distinctly different from the distribution of cool concentrations seen in the submillimeter. Seven new brown dwarf candidates are identified, based on their infrared colors and their projected locations on high column-density regions of the molecular cloud. Eight new candidate binary and five new candidate triple systems, having separations between 0\farcs2 to 10\arcsec (29 to 1450 AU) are reported. The spatial resolution and sensitivity of these observations reveal five apparent disk/envelope systems seen via scattered light, and four nebulous objects with complex morphologies.

We use differential CCD photometry to search for variability in BVI among 990 stars projected in and around the old open cluster M 67. In a previous paper we reported results for 22 cluster members that are optical counterparts to X-ray sources; this study focuses on the other stars in our observations. A variety of sampling rates were employed, allowing variability on time scales ranging from \sim 0.3 hours to \sim 20 days to be studied. Among the brightest sources studied, detection of variability as small as sigma approx 10 mmag is achieved (with > 3 sigma confidence); for the typical star observed, sensitivity to variability at levels sigma approx 20 mmag is achieved. The study is unbiased for stars with 12.5 < B < 18.5, 12.5 < V < 18.5, and 12 < I < 18 within a radius of about 10 arcmin from the cluster centre. In addition, stars with 10 < BVI < 12.5 were monitored in a few small regions in the cluster. We present photometry for all 990 sources studied, and report the variability characteristics of those stars found to be variable at a statistically significant level. Among the variables, we highlight several sources that merit future study, including stars located on the cluster binary sequence, stars on the giant branch, blue stragglers, and a newly discovered W UMa system., 12 pages, including 6 figures and 5 tables. Tables 1 and 3 only available in electronic version of paper. Accepted by A&A

TWA 3A is the most recent addition to a small group of young binary systems that both actively accrete from a circumbinary disk and have spectroscopic orbital solutions. As such, it provides a unique opportunity to test binary accretion theory in a well-constrained setting. To examine TWA 3A's time-variable accretion behavior, we have conducted a two-year, optical photometric monitoring campaign, obtaining dense orbital phase coverage (~20 observations per orbit) for ~15 orbital periods. From U-band measurements we derive the time-dependent binary mass accretion rate, finding bursts of accretion near each periastron passage. On average, these enhanced accretion events evolve over orbital phases 0.85 to 1.05, reaching their peak at periastron. The specific accretion rate increases above the quiescent value by a factor of ~4 on average but the peak can be as high as an order of magnitude in a given orbit. The phase dependence and amplitude of TWA 3A accretion is in good agreement with numerical simulations of binary accretion with similar orbital parameters. In these simulations, periastron accretion bursts are fueled by periodic streams of material from the circumbinary disk that are driven by the binary orbit. We find that TWA 3A's average accretion behavior is remarkably similar to DQ Tau, another T Tauri binary with similar orbital parameters, but with significantly less variability from orbit to orbit. This is only the second clear case of orbital-phase-dependent accretion in a T Tauri binary., 6 pages, 4 figures

The old (7 Gyr) open cluster NGC 188 has yielded a wealth of astrophysical insight into its rich blue straggler population. Specifically, the NGC 188 blue stragglers are characterised by: A binary frequency of 80 % for orbital periods less than 104 days; Typical orbital periods around 1,000 days; Typical secondary star masses of 0.5 M⊙; At least some white dwarf companion stars; Modestly rapid rotation; A bimodal radial spatial distribution; Dynamical masses greater than standard stellar evolution masses (based on short-period binaries); Underluminosity for dynamical masses (short-period binaries).

The open cluster NGC 6791 has been the focus of much recent study due to its intriguing combination of old age and high metallicity ($\sim$8 Gyr, [Fe$/$H]=$+$0.30), as well as its location within the Kepler field. As part of the WIYN Open Cluster Study, we present precise ($\sigma=0.38$ km s$^{-1}$) radial velocities for proper-motion candidate members of NGC 6791 from Platais et al. Our survey, extending down to $g^\prime\sim16.8$, is comprised of the evolved cluster population, including blue stragglers, giants, and horizontal branch stars. Of the 280 proper-motion-selected stars above our magnitude limit, 93% have at least one radial-velocity measurement and 79% have three measurements over the course of at least 200 days, sufficient for secure radial-velocity-determined membership of non-velocity-variable stars. The Platais et al. proper-motion catalog includes twelve anomalous horizontal branch candidates blueward of the red clump, of which we find only four to be cluster members. Three fall slightly blueward of the red clump and the fourth is consistent with being a blue straggler. The cleaned color-magnitude diagram shows a richly populated red giant branch and a blue straggler population. Half of the blue stragglers are in binaries. From our radial-velocity measurement distribution we find the cluster's radial-velocity dispersion to be $\sigma_c=0.62\pm0.10$ km s$^{-1}$. This corresponds to a dynamical mass of $\sim$4600 $M_\odot$., Comment: Accepted to The Astrophysical Journal July 28th, 2014. 12 pages, 8 figures, 5 tables

What is assessment? Why do it? Why do it in a particular way? This document addresses these important questions and provides a practical "how-to" guide for doing assessment. Assessment drives student learning; it is thus imperative that instructors conduct assessment in a manner that is well aligned with the instructor’s goals for the course. This requires (a) that course goals be formalized, and (b) that the instructor have knowledge of various classroom assessment techniques and the kinds of course goals to which each of these assessment techniques is best suited. We briefly present several Classroom Assessment Techniques (CATs) that can be used to help instructors evaluate the extent to which course goals are being achieved, to help guide students toward desired learning outcomes, and to improve student self-evaluation of understanding. In addition, we outline a practical, generalized model for course development with which we demonstrate how to do assessment. For an on-line, user-friendly guide and resource to classroom assessment in college science courses, the reader is invited to visit the Field- Tested Learning Assessment Guide (FLAG) developed by the National Institute for Science Education (http://www.wcer.wisc.edu/nise/cl1/flag).

The λ Ori star-forming region presents a snapshot of a moderate-mass giant molecular cloud 1–2 Myr after cloud disruption by OB stars, with the OB stars, the low-mass stellar population, remnant molecular clouds, and the dispersed gas all still present. We have used optical photometry and multiobject spectroscopy for lithium absorption to identify 266 pre–main-sequence stars in 8 deg2 of the region. We also present new Stromgren photometry for the massive stars, from which we derive a distance of 450 pc and a turnoff age of 6–7 Myr. Using these parameters and pre–main-sequence evolutionary models, we map the star formation history of the low-mass stars. We find that low-mass star formation started throughout the region at about the same time as the birth of the massive stars, and thereafter the birth rate accelerated. Within the last 1–2 Myr star formation ceased in the center of the star-forming region, near the concentration of OB stars, while it continues in dark clouds 20 pc away. We suggest that a supernova 1–2 Myr ago destroyed the molecular cloud core from which the OB stars formed, but it did not terminate star formation in more distant reaches of the giant molecular cloud. We find no secure evidence for triggered or sequential star formation in the outer molecular clouds. The global star formation of the λ Ori region has generated the field initial mass function, but local star formation in subregions shows large deviations from the expected ratio of high- to low-mass stars.

Let me begin by taking a very broad historical perspective on the study of pre-main-sequence binaries. You have seen reference several times at this meeting to that first paper on T Tauri binaries by Joy & Van Biesbroeck (1944). Following that paper there followed a long period that might be called the Decades of Ignorance. I refer here to a period of time in the study of star formation where we largely ignored the fact that we knew that most stars were binaries. The high frequency of binaries among stars is something that we've known for a very long time, well over a century. Indeed, looking back, it is remarkable that the paper in 1982 by Mel Dyck and his collaborators identifying T Tauri as a binary caused such a stir. This paper is sometimes cited as the birth of modern research into young binaries. But how many of our colleagues in the realm of main-sequence binaries had already told us over and over again that most of these young stars had to be binaries? Of course, the reason this important paper caused much ado was in part because T Tauri is the very prototype star of star-formation study.

Observations show that mass accretion can occur at the surfaces of stars in young binaries of all separations. Given that resonant torques have been expected to clear gaps and dam the inward flow of material from circumbinary disks, this result begs the question of how mass accretion is supplied in binary systems. In this paper we address this question in the context of close binaries. Spectral energy distributions indicate that dynamical clearing is not complete for close binaries having massive circumbinary disks. High-resolution infrared spectra of CO rotational and ro-vibrational lines have also detected small amounts of gas which can be located in the region of dynamical clearing. Thus we have signatures of matter from the circumbinary environment through the predicted gap region to the stellar surfaces. We suggest that accretion streams may supply both dynamically cleared regions and circumstellar disks.

This triennial report (1996-1999) reviews the subject from a somewhat personal angle, mostly related to binary star formation and young binary star populations – a subject whose time had come in the early 1990s and is now in full swing.Many astronomers have searched for binary systems among main-sequence stars, and two large-scale surveys published in 1991 and 1992 have already become classics. Well before they became famous for finding extrasolar planets (see below), observing teams led by Michel Mayor (Geneva Observatory) and Geoffrey Marcy (San Francisco State Univ., now Univ. of Calif, at Berkeley) spent many years searching for low-mass stellar companions of nearby stars. The late Antoine Duquennoy and Mayor surveyed all solar-type dwarfs (spectral types F7 through G9) within 20 pc of the Sun, while Debra Fischer and Marcy studied stars with somewhat lower mass (M dwarfs) slightly nearer to the Sun.

We have found 72 premain-sequence (PMS) stars near the center of the j Orionis star-forming region by spectroscopically testing a magnitude-limited sample for the presence of lithium j6708 absorption, a diagnostic of youth. All of these stars show large lithium equivalent widths and radial velocities consis- tent with Orion membership, but only two were discovered previously via Ha or X-ray surveys. Com- parison with PMS evolutionary tracks show that the low-mass star formation did not begin prior to the initiation of high-mass star formation 5¨7 Myr ago. However, the subsequent detailed star formation history is model dependent. BaraUe et al. isochrones imply that high- and low-mass stars began to form together 5¨7 Myr ago, with the low-mass stellar formation ceasing abruptly 1 Myr ago. On the other hand, DAntona & Mazzitelli isochrones indicate a narrow spread of PMS ages, which suggests a burst of low-mass star formation 1¨2 Myr ago. Furthermore, kinematic arguments require that the parent molecular cloud gravitationally bound the stars together until recently, but at present the requisite gas mass is not visible. This leads us to conjecture that both the high- and low-mass stars were in a tightly bound cluster until a supernova blast about 1 Myr ago disrupted the parent cloud. This supernova also impacted on the PMS formation process by either (1) ceasing formation through removal of the gas supply or (2) triggering star births via cloud compression, depending on choice of stellar evolution models. Finally, we —nd that despite their youth, only four of the 72 PMS stars have T Taurilike Ha emission, suggesting the absence of accretion disks. We conjecture that this may be the result of photo- evaporation of the disks while the low-mass stars were in much closer proximity to the OB stars prior to becoming gravitationally unbound.

We report rotation periods for 254 stars in an area 40' × 80' centered on the Orion Nebula. We show that these stars are likely members of the young (~106 yr) Orion OBIc/d association. The rotation period distribution we determine, which is sensitive to periods 0.1 < P < 8 days, shows a sharp cutoff for periods P < 0.5 days, corresponding to breakup velocity for these stars. Above 0.5 days the distribution is consistent with a uniform distribution; we do not find evidence for a gap of periods at 4–5 days. We find signatures of active accretion among stars at all periods; active accretion does not occur preferentially among slow rotators in our sample. We find no correlation between rotation period and near-IR signatures of circumstellar disks. In addition, we show that the distribution of v sin i among stars in our sample bears striking resemblance to that of low-mass Pleiades stars. We discuss the implications of our findings for the evolution of stellar angular momentum during the pre–main-sequence phase. We argue that all stars in our sample must still deplete angular momentum by factors of roughly 5–10, if they are to preserve their v sin i distribution over approximately the next 100 Myr. We consider in detail whether our findings are consistent with disk-regulated stellar rotation. We do not find observational evidence that magnetic disk-locking is the dominant mechanism in angular momentum evolution during the pre–main-sequence phase.

Complete uvby light curves of the detached Herbig Be eclipsing binary TY Coronae Australis are presented. A total of 1789 photometric measurements in each of the four colours were obtained in 1989 and in 1992-1994. A detailed analysis of the y light curve obtained in 1992-1993 is published separately (Casey et al. 1997). The reflection nebula around the system contributes about 30% of the light in all four passbands. Here we present and discuss the non-eclipse-related photometric variability of the system. We suggest that these variations are the result of variable obscuration, possibly linked to dust shells physically associated to the system.

We analyze new photometric data for the Herbig Be eclipsing binary TY CrA, which securely reveal the secondary eclipse, ~0.03 mag deep in y. From the light-curve solution and our previous spectroscopic data, absolute dimensions of the primary and secondary stars are derived. The masses are found to be M1 = 3.16 ± 0.02 M☉ and M2 = 1.64 ± 0.01 M☉, the radii are R1 = 1.80 ± 0.10 R☉ and R2 = 2.08 ± 0.14 R☉, the luminosities are L1 = 67 ± 12 L☉ and L2 = 2.4 ± 0.8 L☉, and the effective temperatures are T1 = 12,000 ± 500 K and T2 = 4900 ± 400 K. Here the uncertainties represent high-confidence limits, not standard deviations. The secondary star is a pre–main-sequence star located at the base of the Hayashi tracks. As such, it is the least evolved star with a dynamically measured mass. Given higher effective temperatures for the primary (e.g., 12,500 K), the solar-composition 1.64 M☉ evolutionary tracks of Swenson et al., Claret, and D'Antona & Mazzitelli are all consistent with the properties of the TY CrA secondary and suggest an age of order 3 Myr. The radius and projected rotational velocity of the secondary star are consistent with synchronous rotation. The primary star is located near the zero-age main sequence, which, for solar compositions, is consistent with an age of 3 Myr. However, the primary star is not well represented by any of the 3.16 M☉ evolutionary models, which predict somewhat higher effective temperatures than observed.

Based on a sample of 45 main-sequence solar-type spectroscopic binaries we find the tidal circularization cutoff in the open cluster NGC 188 (age 7 Gyr) to be well-defined at a period of 15.0 days. When compared to shorter tidal circularization cutoff periods in the younger clusters M67 and the Hyades, this result establishes that tidal circularization processes are effective in solar-type main-sequence binaries for timescales longer than 1 Gyr. This result is in contrast to the theoretical prediction by Zahn & Bouchet that tidal circularization due to equilibrium tides is ineffective for late-type main sequence stars, and that tidal circularization cutoff periods are instead set only by pre-main-sequence tidal circularization. The NGC 188 cutoff period is also longer than can be explained by present theories of tidal circularization due to dynamical tides.

We report the discovery that the pre-main sequence object LkCa3 in the Taurus-Auriga star-forming region is a hierarchical quadruple system of M stars. It was previously known to be a close (~0.5 arc sec) visual pair, with one component being a moderately eccentric 12.94-day single-lined spectroscopic binary. A re-analysis of archival optical spectra complemented with new near-infrared spectroscopy shows both visual components to be double-lined, the second one having a period of 4.06 days and a circular orbit. In addition to the orbital elements, we determine optical and near-infrared flux ratios, effective temperatures, and projected rotational velocities for all four stars. Using existing photometric monitoring observations of the system that had previously revealed the rotational period of the primary in the longer-period binary, we detect also the rotational signal of the primary in the 4.06-day binary, which is synchronized with the orbital motion. With only the assumption of coevality, a comparison of all of these constraints with current stellar evolution models from the Dartmouth series points to an age of 1.4 Myr and a distance of 133 pc, consistent with previous estimates for the region and suggesting the system is on the near side of the Taurus complex. Similar comparisons of the properties of LkCa3 and of the well-known quadruple pre-main sequence system GG Tau with the widely used models from the Lyon series for a mixing length parameter of alpha_ML = 1.0 strongly favor the Dartmouth models., Comment: Accepted for publication in ApJ; 15 pages including figures and tables, in emulateapj format; minor clarifications added

We report orbital parameters for two low-mass, pre-main sequence, double-lined spectroscopic binaries VSB 111 and VSB 126. These systems were originally identified as single-lined on the basis of visible-light observations. We obtained high-resolution, infrared spectra with the 10-m Keck II telescope, detected absorption lines of the secondary stars, and measured radial velocities of both components in the systems. The visible light spectra were obtained on the 1.5-m Wyeth reflector at the Oak Ridge Observatory, the 1.5-m Tillinghast reflector at the F. L. Whipple Observatory, and the 4.5-m equivalent Multiple Mirror Telescope. The combination of our visible and infrared observations of VSB 111 leads to a period of 902.1+/-0.9 days, an eccentricity of 0.788+/-0.008, and a mass ratio of 0.52+/-0.05. VSB 126 has a period of 12.9244+/-0.0002 days, an eccentricity of 0.18+/-0.02, and a mass ratio of 0.29+/-0.02. Visible-light photometry, using the 0.8-m telescope at Lowell Observatory, provided rotation periods for the primary stars in both systems, 3.74+/-0.02 days for VSB 111 and 5.71+/-0.07 days for VSB 126. Both binaries are located in the young, active star-forming cluster NGC 2264 at a distance of ~800 pc. The difference in the center-of-mass velocities of the two systems is consistent with the radial velocity gradient seen across NGC 2264. To test the evolutionary models for accuracy and consistency, we compare the stellar properties derived from several sets of theoretical calculations for pre-main sequence evolution with our dynamical results.

We present the discovery of the totally eclipsing long-period (P = 771.8 d) binary system WOCS 23009 in the old open cluster NGC 6819 that contains both an evolved star near central hydrogen exhaustion and a low-mass (0.45 Msun) star. This system was previously known to be a single-lined spectroscopic binary, but the discovery of an eclipse near apastron using data from the Kepler space telescope makes it clear that the system has an inclination that is very close to 90 degrees. Although the secondary star has not been identified in spectra, the mass of the primary star can be constrained using other eclipsing binaries in the cluster. The combination of total eclipses and a mass constraint for the primary star allows us to determine a reliable mass for the secondary star and radii for both stars, and to constrain the cluster age. Unlike well-measured stars of similar mass in field binaries, the low-mass secondary is not significantly inflated in radius compared to model predictions. The primary star characteristics, in combination with cluster photometry and masses from other cluster binaries, indicates a best age of 2.62+/-0.25 Gyr, although stellar model physics may introduce systematic uncertainties at the ~10% level. We find preliminary evidence that the asteroseismic predictions for red giant masses in this cluster are systematically too high by as much as 8%., 38 pages,11 figures, accepted for ApJ

(Abridged) Following on from a recently completed radial-velocity survey of the old (7 Gyr) open cluster NGC 188 in which we study in detail the solar-type hard binaries and blue stragglers of the cluster, here we investigate the dynamical evolution of NGC 188 through a sophisticated N-body model. We employ the observed binary properties of the young (150 Myr) open cluster M35, where possible, to guide our choices for parameters of the initial binary population. At 7 Gyr the main-sequence solar-type hard-binary population in the model matches that of NGC 188 in both binary frequency and distributions of orbital parameters. This agreement between the model and observations is in a large part due to the similarities between the NGC 188 and M35 solar-type binaries. Indeed, among the 7 Gyr main-sequence binaries in the model, only those with P>1000 days show potentially observable evidence for modifications by dynamical encounters. This emphasizes the importance of defining accurate initial conditions for star cluster models, which we propose is best accomplished through comparisons with observations of young open clusters like M35. Furthermore, this suggests that observations of the present-day binaries in even old open clusters can provide valuable information on their primordial binary populations. However, despite the model's successes at matching the true cluster, the model underproduces blue stragglers and produces an overabundance of long-period circular main-sequence--white-dwarf binaries as compared to NGC 188. We conclude that improvements in the physics of mass transfer and common envelope may in fact solve both discrepancies with the observations. This project highlights the unique accessibility of open clusters to both comprehensive observational surveys and full-scale N-body simulations, and underscores the importance of open clusters to the study of star cluster dynamics., 25 pages, 19 figures, accepted for publication in AJ (minor changes from previous submitted version)

The young, low-mass, triple system NTTS 155808-2219 (ScoPMS 20) was previously identified as a ~17-day period single-lined spectroscopic binary with a tertiary component at 0.21 arcseconds. Using high-resolution infrared spectra, acquired with NIRSPEC on Keck II, both with and without adaptive optics, we measured radial velocities of all three components. Reanalysis of the single-lined visible light observations, made from 1987 to 1993, also yielded radial velocity detections of the three stars. Combining visible light and infrared data to compute the orbital solution produces orbital parameters consistent with the single-lined solution and a mass ratio of q = 0.78 +/- 0.01 for the SB. We discuss the consistency between our results and previously published data on this system, our radial-velocity analysis with both observed and synthetic templates, and the possibility that this system is eclipsing, providing a potential method for the determination of the stars' absolute masses. Over the ~20 year baseline of our observations, we have measured the acceleration of the SB's center-of-mass in its orbit with the tertiary. Long-term, adaptive optics imaging of the tertiary will eventually yield dynamical data useful for component mass estimates., Comment: 6 Tables, 8 Figures, updated to match published text

The observational study of pre-main-sequence (PMS) binary stars is in many ways a very young field; most PMS binaries known today were discovered in the past decade. Nonetheless, T Tauri stars have been under study for more than a half century, and the serendipitous discovery of visual pairs has always been a by-product of their observation (e.g. Joy & Van Biesbroeck 1944). The acceleration of discovery in recent years has at least two stimuli, one technical and one sociological. First, the frequency of binaries among main-sequence solar-type stars peaks at semimajor axes of order 50 AU, projecting to less than 0.5" at the distance of the nearest star-forming regions. The requisite high­ angular-resolution techniques (and near-infrared detectors) have only recently permitted the major surveys for PMS binaries now coming to fruition. Second, and equally important, the formation and early evolution of binaries has attracted increasing attention from those studying star formation. This can perhaps be attributed to both a growing confidence in our general picture for single-star formation (e.g. Shu et a1 1987) and recognition that however correct our theories of single-star formation may be, the usual product of a star-formation event is a multiple-star system. Until recently, the primary observational constraint on the mechanisms of binary formation and early evolution have been provided by main-sequence (MS) binaries in the solar vicinity, acting as a surrogate for the zero-age-main­ sequence (ZAMS) binary population. As the ultimate product of the binary formation process, ZAMS binaries do supply essential constraints. However

In open star clusters, where all members formed at about the same time, blue straggler stars are typically observed to be brighter and bluer than hydrogen-burning main-sequence stars, and therefore should already have evolved into giant stars and stellar remnants. Correlations between blue straggler frequency and cluster binary fraction, core mass, and radial position suggest that mass transfer or mergers in binary stars dominates the production of blue stragglers in open clusters. Analytic models, detailed observations, and sophisticated N-body simulations, however, argue in favor of stellar collisions. Here we report that the blue stragglers in long-period binaries in the old (7 Gyr) open cluster NGC 188 have companions with masses of about half a solar mass, with a surprisingly narrow mass distribution. This conclusively rules out a collisional origin, as the collision hypothesis predicts a companion-mass distribution with significantly higher masses. Mergers in hierarchical triple stars are marginally permitted by the data, but the observations do not favor this hypothesis. The data are closely consistent with a mass transfer origin for the long-period blue straggler binaries in NGC 188, in which the companions would be white dwarfs of about half a solar mass., 14 pages, 3 figures, Nature in press

(abridged) We present an in-depth study of the hard-binary population of the old (7 Gyr) open cluster NGC 188. The main-sequence solar-type hard binaries in NGC 188 are nearly indistinguishable from similar binaries in the Galactic field. We find a global solar-type main-sequence hard-binary frequency in NGC 188 of 29 +/- 3 % for binaries with periods less than 10^4 days. For main-sequence hard binaries in the cluster we observe a log-period distribution that rises towards our detection limit, a roughly Gaussian eccentricity distribution centered on e = 0.35 (for binaries with periods longer than the circularization period), and a secondary-mass distribution that rises towards lower-mass companions. Importantly, the NGC 188 blue straggler binaries show significantly different characteristics than the solar-type main sequence binaries in NGC 188. We observe a blue straggler hard-binary frequency of 76 +/- 19 %, three times that of the main sequence. The blue straggler binary eccentricity - log period distribution is distinct from that of the main sequence at the 99% confidence level, with the majority of the blue straggler binaries having periods of order 1000 days and lower eccentricities. The secondary-mass distribution for these long-period blue straggler binaries is narrow and peaked with a mean value of about 0.5 Msun. Predictions for mass-transfer products are most closely consistent with the binary properties of these NGC 188 blue stragglers, which comprise two-thirds of the blue straggler population. Additionally we compare the NGC 188 binaries to those evolved within the sophisticated Hurley et al. (2005) N-body open cluster simulation. We find that additional simulations with initial conditions that are better motivated by observations are necessary to properly investigate the dynamical evolution of a rich binary population in open clusters like NGC 188., Comment: 23 pages, 22 figures, accepted for publication in AJ

We present results from a 5-month photometric survey for stellar rotation periods combined with a 4-year radial-velocity survey for membership and binarity in the 220Myr open cluster M34. We report surface rotation periods for 120 stars, 83 of which are late-type cluster members. A comparison to previous work serves to illustrate the importance of high cadence long baseline photometric observations and membership information. The new M34 periods are less biased against slow rotation and cleaned for non-members. The rotation periods of the cluster members span more than an order of magnitude from 0.5 day up to 11.5 days, and trace two distinct rotational sequences - fast (C) and moderate-to-slow (I) - in the color-period diagram. The sequences represent two different states in the rotational evolution of the late-type cluster members. We use the color-period diagrams for M34 and for younger and older clusters to estimate the timescale for the transition from the C to the I sequence and find ~, 47 pages (12pt, preprint), 14 figures, 2 tables, Accepted for publication in ApJ, format of RA coordinates in Table 2 corrected in latest version

We present 5201 radial-velocity measurements of 1144 stars, as part of an ongoing study of the young (150 Myr) open cluster M35 (NGC 2168). We have observed M35 since 1997, using the Hydra Multi-Object Spectrograph on the WIYN 3.5m telescope. Our stellar sample covers main-sequence stars over a magnitude range of 13.0=3 measurements, we derive radial-velocity membership probabilities and identify radial-velocity variables, finding 360 cluster members, 55 of which show significant radial- velocity variability. Using these cluster members, we construct a color-magnitude diagram for our stellar sample cleaned of field star contamination. We also compare the spatial distribution of the single and binary cluster members, finding no evidence for mass segregation in our stellar sample. Accounting for measurement precision, we place an upper limit on the radial-velocity dispersion of the cluster of 0.81 +/- 0.08 km/s. After correcting for undetected binaries, we derive a true radial-velocity dispersion of 0.65 +/- 0.10 km/s., Comment: accepted for publication in AJ

Recent extensive radial‐velocity surveys of old open clusters have revealed rich spectroscopic binary populations among the blue stragglers. The WIYN Open Cluster Survey of NGC 188 in particular has shown among the blue stragglers: i) a high binary frequency of 76±19% (as compared to 23±3% for the solar‐type main sequence stars); ii) a concentration of orbital periods around P∼1000 days, with a range of eccentricities that includes e∼0; iii) at least 40% rotating faster (vsin i∼10–50 km/s) than old main‐sequence stars of similar surface temperature; iv) a bi‐modal spatial distribution with distinct core and halo populations; v) underluminosity for their masses.The very high binary frequency points definitively to an origin in multiple‐star systems for most of the NGC188 blue stragglers. In fact, triple systems may play a crucial role in the formation of blue stragglers, perhaps through Kozai processes or through enhancing the rates of dynamical encounters for very hard binaries Finally, if the internal st...

It has long been recognized that tidal interaction between the components of close binaries will tend to circularize their orbits (e.g., Darwin 1879; Zahn 1977). In fact observations show an abundance of circular orbits in binaries with periods of days. However, the dissipation mechanism required for circularization, the timescales involved, and the dependence on period, mass, evolutionary state, and initial eccentricity of the system remain subjects of debate.Circularization theories such as Zahn (1977) state that circularization in binaries where at least one component possesses a convective envelope is much more efficient than in binaries where both stars have radiative envelopes. Giuiricin et al. (1984; GMM) studied the period-eccentricity distribution of O-, B- and A-type binaries and argued that the observations were consistent with Zahn’s theory for radiative envelopes. But recently, Tassoul (1988) has presented an alternative circularization theory with similarly efficient circularization in radiative- and convective-envelope binaries. In addition, period-eccentricity distributions for many samples of main-sequence convective-envelope binaries are now available. Here we reconsider the observed short-period eccentricity distribution of radiative-envelope binaries in light of these new observations and theory. Specifically, we investigate the period-eccentricity distribution of a sample of binaries with A-type primary stars.

The interaction of disks and young binary stars is discussed, with particular emphasis on disk structure, the existence of accretion at the stellar surfaces and binary orbital evolution.

The pre-main sequence (PMS) binary frequency is a fundamental datum in the study of binary formation. It reflects on numerous basic issues, such as:• The formation process. Binary stars are the primary branch of the star-formation process, and thus their frequency is an essential challenge to star-formation theories. (Indeed, the infrequency of single-star formation is likely as significant as the binary frequency.)• The epoch of binary formation. Assessing whether the binary population exists in total by the pre-main sequence phase sets an upper limit on the binary formation timescale.• Early period evolution. The frequency distribution as a function of period of PMS binaries, when compared to the distribution at the zero-age main sequence, can shed light on early orbital evolution.• The interaction of binaries with disks. The formation and consequent dynamical evolution of a binary with semi-major axis less than typical disk radii must substantially modify disk structures and accretion flows. Thus the binary frequency might differ between PMS stars with and without associated disks.

'Blue stragglers' are massive stars in clusters — where all the stars are pretty much the same age — that at their luminosity, should have evolved to become giants and white dwarfs. They are thought to be normal main-sequence stars that have gained mass through either transfer between binary companions or direct collision and merger between two stars. A study of the highly crowded star cluster M 30 suggests that both of these mechanisms are at work during the dramatic phase of the cluster core collapse. Two distinct parallel sequences of blue stragglers are present in M 30, a 'bluer' population arising from direct stellar collisions and a 'redder' one from the evolution of close binaries. Observations of the 21 blue stragglers in the old open cluster NGC 188 show that 16 (76%) are currently in binary systems, a frequency three times that found among normal solar-type main-sequence stars. Most of the NGC 188 blue stragglers are rotating faster than normal main-sequence stars of the same surface temperatures. In News & Views, Melvyn Davies reflects on what these two studies say about the origin of blue stragglers. 'Blue straggler' stars lie on or near the main sequence of star clusters and are sufficiently massive that they should have evolved into white dwarfs long ago. Statistical evidence indicates that in globular star clusters the blue stragglers probably form from binary stars. Here, 76 per cent of the blue stragglers in the open cluster NGC 188 are found to be currently in binary systems, a three times higher frequency than that among normal solar-type main-sequence stars. Blue straggler stars lie on or near the main sequences of star clusters (all members of which formed around the same time), but typically are more luminous than the turn-off stars and therefore long ago should have evolved off the main sequence to become giants and white dwarfs. They are thought to derive from normal main-sequence stars that have undergone a recent increase in mass. Statistical evidence indicates that in globular star clusters the blue stragglers probably form from binary stars1. The specific formation processes, such as mass transfer, mergers or stellar collisions during dynamical encounters of binary stars, remain unresolved. Here we report that 16 of the 21 blue stragglers (76 per cent) in the old (7-Gyr; ref. 2) open cluster NGC 188 are currently in binary systems, a frequency three times that found among normal solar-type main-sequence stars. These blue straggler binaries have a remarkable period–eccentricity distribution, with all but three having orbital periods of ∼1,000 days. Moreover, these stars are rotating faster than normal main-sequence stars of the same surface temperatures. These data show that most, and possibly all, blue stragglers derive from multiple-star systems, and indicate that the several formation processes operate simultaneously. We suggest that rapid rotation of blue stragglers may place upper limits on their ages.

We present the current results from our ongoing radial-velocity survey of the intermediate-age (2.4 Gyr) open cluster NGC 6819. Using both newly observed and other available photometry and astrometry we define a primary target sample of 1454 stars that includes main-sequence, subgiant, giant, and blue straggler stars, spanning a magnitude range of 11= 3 measurements, providing the first comprehensive membership study of the cluster core that includes stars from the giant branch through the upper main sequence. We identify 480 cluster members. Additionally, we identify velocity-variable systems, all of which are likely hard binaries that dynamically power the cluster. Using our single cluster members, we find a cluster average radial velocity of 2.34 +/- 0.05 km/s . We use our kinematic cluster members to construct a cleaned color- magnitude diagram from which we identify rich giant, subgiant, and blue straggler populations and a well-defined red clump. The cluster displays a morphology near the cluster turnoff clearly indicative of core convective overshoot. Finally, we discuss a few stars of note, one of which is a short-period red-clump binary that we suggest may be the product of a dynamical encounter., 11 pages, accepted for publication in AJ

We present a large multi-epoch high resolution spectroscopic investigation for the search of binary candidates in the Galactic Globular Cluster (GGC) M4. The aim of our work is the identification of the binary candidates, and the determination of the binary fraction and of the binary radial distribution. We present a large multi-epoch high resolution spectroscopic investigation for the search of binary candidates in the Galactic Globular Cluster (GGC) M4. The aim of our work is the identification of the binary candidates, and the determination of the binary fraction and of the binary radial distribution. The average radial velocity of the observed cluster members is $70.29+/-0.07(+/-0.3)(+/-0.1)km/s. The search for variations in radial velocities among the stars with multi-epoch observations yielded 57 binary star candidates. Our radial velocity measurement accuracy allowed us to identify at a 3sigma level binaries with radial velocity variations larger than ~0.3km/s for the target stars with V15. We identified 4 binary star candidates out of 97 observed targets inside the core radius, and 53 candidates out of 2372 observed stars outside the core radius. Accounting for the incompleteness affecting our survey, the lower limit for the total binary fraction is f=3.0+/-0.3%. The lower limit for the binary fraction in the cluster core is f=5.1+/-2.3%, while outside the core it decreases to f=3.0+/-0.4%. Similarly, we found f=4.5+/-0.4% and f=1.8+/-0.6% for the binary fraction inside and outside the half mass radius., 13 pages, 12 figures, Accepted for publication in A&A

We present time-series radial-velocity and photometric observations of a solar-type double-lined eclipsing binary star (V12) near the turnoff in the old open cluster NGC188. We use these data to determine the spectroscopic orbit and the photometric elements for V12. From our analysis we determine accurate masses (Mp=1.103+-0.007Msun, Ms=1.081+-0.007Msun) and radii (R =1.424+-0.019Rsun, Rs=1.373+-0.019Rsun) for the primary (p) and secondary (s) binary components. We adopt a reddening of E_(B-V)=0.087 for NGC188, and derive component effective temperatures of 5900+-100K and 5875+-100K, respectively, for the primary and secondary stars. From their absolute dimensions, the two components of V12 yield identical distance moduli of Vo-MV=11.24+-0.09, corresponding to 1770+-75pc. We determine an age of 6.2+-0.2Gyr for V12 and NGC188, from a comparison with theoretical isochrones in the mass-radius diagram. This age is independent of distance, reddening, and color-temperature transformations. We use isochrones from Victoria-Regina (VRSS) and Yonsei-Yale (Y2) with [Fe/H]=-0.1 and [Fe/H]=0.0. From the solar metallicity isochrones, an age of 6.4Gyr provides the best fit to the binary components for both sets of models. For the isochrones with [Fe/H]=-0.1, ages of 6.0Gyr and 5.9Gyr provide the best fits for the VRSS and Y2 models, respectively. We use the distance and age estimates for V12, together with best estimates for the metallicity and reddening of NGC188, to investigate the locations of the corresponding VRSS and Y2 isochrones relative to cluster members in the color-magnitude diagram. Plausible changes in model metallicity and distance to better match the isochrones to the cluster sequences, result in a range of ages for NGC188 that is more than 3 times that resulting from our analysis of V12., Comment: 36 pages, 10 figures (1 color), 9 tables

We present the JHKs light curves for the double-lined eclipsing binary 2MASS J05352184-0546085, in which both components are brown dwarfs. We analyze these light curves with the published Ic-band light curve and radial velocities to provide refined measurements of the system's physical parameters. The component masses and radii are here determined with an accuracy of ~6.5% and ~1.5%, respectively. We confirm the previous surprising finding that the primary brown dwarf has a cooler effective temperature than its companion. Next, we perform a detailed study of the variations in the out-of-eclipse phases of the light curves to ascertain the properties of any inhomogeneities on the surfaces of the brown dwarfs. Our analysis reveals two low-amplitude periodic signals, one attributable to the rotation of the primary (with a period of 3.293+/-0.001 d) and the other to that of the secondary (14.05+/-0.05 d). Finally, we explore the effects on the derived physical parameters of the system when spots are included in the modeling. The observed low-amplitude rotational modulations are well fit by cool spots covering a small fraction of their surfaces. To mimic the observed ~200 K suppression of the primary's temperature, our model requires that the primary possess a very large spot coverage fraction of ~65%. Altogether, a spot configuration in which the primary is heavily spotted while the secondary is lightly spotted can explain the apparent temperature reversal and can bring the temperatures of the brown dwarfs into agreement with the predictions of theoretical models., Comment: Accepted for publication in The Astrophysical Journal; 13 pages, 8 figures, 6 tables. Updated parameter uncertainties

We present new Spitzer IRAC/PU/MIPS photometry from 3.6 to 24 um, and new Gemini GMOS photometry at 0.48 um, of the young brown dwarf eclipsing binary 2MASS J05352184-0546085, located in the Orion Nebula Cluster. No excess disk emission is detected: The measured fluxes at lambda < 8 um are within 1 sigma (< 0.1 mJy) of a bare photosphere, and the 3 sigma upper limit at 16 um is a mere 0.04 mJy above the bare photospheric level. Together with the known properties of the system, this implies the absence of optically thick disks around the individual components. It also implies that if any circumbinary disk is present, it must either be optically thin and extremely tenuous (10^{-10} Msun) if it extends in to within ~0.1 AU of the binary (the approximate tidal truncation radius), or it must be optically thick with a large inner hole, > 0.6-10 AU in radius depending on degree of flaring. The consequence in all cases is that disk accretion is likely to be negligible or absent. This supports the recent proposal that the strong Halpha emission in the primary (more massive) brown dwarf results from chromospheric activity, and thereby bolsters the hypothesis that the surprising Teff inversion observed between the components is due to strong magnetic fields on the primary. Our data also set constraints on the Teff of the components independent of spectral type, and thereby on models of the aforementioned magnetic field effects. We discuss the consequences for the derived fundamental properties of young brown dwarfs and very low-mass stars in general. Specifically, if very active isolated young brown dwarfs and very low-mass stars suffer the same activity/field related effects as the 2M0535-05 primary, the low-mass stellar/substellar IMF currently derived from standard evolutionary tracks may be substantially in error., 19 pages, 5 figures, accepted by ApJ

We present the results of a 5 month photometric time-series survey for stellar rotation over a 40'x40' field on the 150 Myr open cluster M35. We report rotation periods for 441 stars and determine their cluster membership and binarity based on a decade-long radial-velocity survey, proper-motion measurements, and multi-band photometric observations. We find that 310 of the stars with measured rotation periods are late-type members of M35. Their distribution of rotation periods span more than two orders of magnitude from ~0.1-15 days, not constrained by the sampling frequency and the time-span of the survey. With an age between the zero-age main-sequence and the Hyades, and with ~6 times more rotation periods than measured in the Pleiades, M35 permit detailed studies of early rotational evolution of late-type stars. Nearly 80% of the 310 rotators lie on two distinct sequences in the color-period plane, defining clear relations between stellar rotation period and color (mass). The M35 color-period diagram enables us to determine timescales for the transition between the two rotational states for G and K dwarfs, respectively. These timescales are inversely related to the mass of the convective envelope, and offer constraints on the rates of internal and external angular momentum transport and of the evolution of stellar dynamos. A comparison to the Hyades, confirm the Skumanich (1972) spindown-dependence for G dwarfs on one rotational state, but suggest that K dwarfs spin down more slowly. The locations of the rotational sequences in the M35 color-period diagram support the use of rotational isochrones to determine ages for coeval stellar populations. We use such gyrochronology to determine "gyro-ages" for M35. We use the M35 data to evaluate new color dependencies for the rotational isochrones., 73 pages, 16 figures, Accepted for publication in ApJ. Replacement reflect minor changes suggested by referee

The mass and chemical composition of a star are the primary determinants of its basic physical properties--radius, temperature, luminosity--and how those properties evolve with time. Thus, two stars born at the same time, from the same natal material, and with the same mass are 'identical twins,' and as such might be expected to possess identical physical attributes. We have discovered in the Orion Nebula a pair of stellar twins in a newborn binary star system. Each star in the binary has a mass of 0.41 +/- 0.01 solar masses, identical to within 2 percent. Here we report that these twin stars have surface temperatures that differ by ~300K (~10%), and luminosities that differ by ~50%, both at high confidence level. Preliminary results indicate that the stars' radii also differ, by 5-10%. These surprising dissimilarities suggest that one of the twins may have been delayed by several hundred thousand years in its formation relative to its sibling. Such a delay could only have been detected in a very young, definitively equal-mass binary system3 such as that reported here. Our findings reveal cosmic limits on the age synchronisation of young binary stars, often used as tests for the age calibrations of star-formation models., Comment: Published in Nature, 19 June 2008

We present preliminary results from an observational campaign aimed at the study of the binary fraction and binary radial distribution in Galactic globular clusters. In particular, we concentrate on the ongoing observational campaign for the search of spectroscopic binaries.

12 pages, 7 figures.-- ArXiv pre-print available at: http://arxiv.org/abs/0810.1541, Spectroscopic and eclipsing binary systems offer the best means for determining accurate physical properties of stars, including their masses and radii. The data available for low-mass stars have yielded firm evidence that stellar structure models predict smaller radii and higher effective temperatures than observed, but the number of systems with detailed analyses is still small. In this paper, we present a complete reanalysis of one of such eclipsing systems, CM Dra, composed of two dM4.5 stars. New and existing light curves as well as a radial velocity curve are modeled to measure the physical properties of both components. The masses and radii determined for the components of CM Dra are M1 = 0.2310 ± 0.0009 Msun, M2 = 0.2141 ± 0.0010 Msun, R1 = 0.2534 ± 0.0019 Rsun, and R2 = 0.2396 ± 0.0015 Rsun. With relative uncertainties well below the 1% level, these values constitute the most accurate properties to date for fully convective stars. This makes CM Dra a valuable benchmark for testing theoretical models. In comparing our measurements with theory, we confirm the discrepancies previously reported for other low-mass eclipsing binaries. These discrepancies seem likely to be due to the effects of magnetic activity. We find that the orbit of this system is slightly eccentric, and we have made use of eclipse timings spanning three decades to infer the apsidal motion and other related properties., JCM, IR and CJ acknowledge support from the Spanish Ministerio de Educación y Ciencia via grants AYA2006-15623-C02-01 and AYA2006-15623-C02-02. GT acknowledges partial support for this work from NSF grant AST-0708229 and NASA’s MASSIF SIM Key Project (BLF57-04). EFG acknowledges support for the FCAPT research under NSF/RUI grant AST05-07536. DC and FTOD acknowledge support for the Sleuth Observatory work by the National Aeronautics and Space Administration under grant NNG05GJ29G issued through the Origins of Solar Systems Program. FTOD also acknowledges partial support for this work provided through the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA. The investigation of MW was supported by the Grant Agency of the Czech Republic under grant No. 205/06/0217.