Lynne A. Hillenbrand

Bio: Lynne A. Hillenbrand is an academic researcher from California Institute of Technology. The author has contributed to research in topics: Stars & T Tauri star. The author has an hindex of 104, co-authored 562 publications receiving 37665 citations. Previous affiliations of Lynne A. Hillenbrand include University of California, Berkeley & University of Massachusetts Amherst.

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

Abstract: 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.

A Preliminary Study of the Orion Nebula Cluster Structure and Dynamics

Abstract: We use optical and near-infrared star counts to explore the structure and dynamics of the Orion Nebula Cluster (ONC). This very young (<1 Myr) cluster is not circularly symmetric in projection but is elongated north-south in a manner similar to the molecular gas distribution in the region, suggesting that the stellar system may still reflect the geometry of the protocluster cloud. Azimuthally averaged stellar source counts compare well with simple spherically symmetric, single-mass King cluster models. The model fits suggest that the inner Trapezium region should be regarded as the core of the ONC, not as a distinct entity as sometimes advocated. We estimate that the core radius of the cluster is 0.16-0.21 pc and that the central stellar density approaches 2 × 10^4 stars pc^(-3). Adopting the stellar velocity dispersion from published proper-motion studies, virial equilibrium would require a total mass within about 2 pc of the Trapezium of ~4500 M_☉, slightly more than twice the mass of the known stellar population and comparable to the estimated mass in molecular gas projected onto the same region of the sky. If ≳ 20% of the remaining molecular gas is converted into stars, thus adding to the binding mass, given that the present stellar population alone has a total energy close to zero, the ONC is likely to produce a gravitationally bound cluster. The ONC also exhibits mass segregation, with the most massive (Trapezium) stars clearly concentrated toward the center of the cluster and some evidence for the degree of central concentration to decrease with decreasing mass down to 1-2 M_☉, as would be expected for general mass segregation. Given the extreme youth of the stars compared with the estimated range of collisional relaxation times, the mass segregation is unlikely to be the result of cluster relaxation. Instead, we suggest that the mass segregation reflects a preference for higher mass stars to form in dense, central cluster regions.

Herbig Ae/Be Stars: Intermediate-Mass Stars Surrounded by Massive Circumstellar Accretion Disks

Abstract: The proposition that Herbig Ae/Be stars are young intermediate mass stars surrounded by optically thick accretion disks is explored. From a study of 47 such objects, a subset of 30 stars is identified whose spectral energy distributions can be interpreted convincingly in terms of pre-main sequence stars surrounded by massive optically thick circumstellar accretion disks. Constraints on the physical properties of the disks, such as size, mass, accretion rate, lifetime, and radial structure are derived from the photometric data.

Phd by thesis

Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

On the variation of the initial mass function

Abstract: A universal initial mass function (IMF) is not intuitive, but so far no convincing evidence for a variable IMF exists. The detection of systematic variations of the IMF with star-forming conditions would be the Rosetta Stone for star formation. In this contribution an average or Galactic-field IMF is defined, stressing that there is evidence for a change in the power-law index at only two masses: near 0.5 M⊙ and near 0.08 M⊙. Using this supposed universal IMF, the uncertainty inherent in any observational estimate of the IMF is investigated by studying the scatter introduced by Poisson noise and the dynamical evolution of star clusters. It is found that this apparent scatter reproduces quite well the observed scatter in power-law index determinations, thus defining the fundamental limit within which any true variation becomes undetectable. The absence of evidence for a variable IMF means that any true variation of the IMF in well-studied populations must be smaller than this scatter. Determinations of the power-law indices α are subject to systematic errors arising mostly from unresolved binaries. The systematic bias is quantified here, with the result that the single-star IMFs for young star clusters are systematically steeper by Δα≈0.5 between 0.1 and 1 M⊙ than the Galactic-field IMF, which is populated by, on average, about 5-Gyr-old stars. The MFs in globular clusters appear to be, on average, systematically flatter than the Galactic-field IMF (Piotto & Zoccali; Paresce & De Marchi), and the recent detection of ancient white-dwarf candidates in the Galactic halo and the absence of associated low-mass stars (Ibata et al.; Mendez & Minniti) suggest a radically different IMF for this ancient population. Star formation in higher metallicity environments thus appears to produce relatively more low-mass stars. While still tentative, this is an interesting trend, being consistent with a systematic variation of the IMF as expected from theoretical arguments.

Kepler Planet-Detection Mission: Introduction and First Results

Abstract: The Kepler mission was designed to determine the frequency of Earth-sized planets in and near the habitable zone of Sun-like stars. The habitable zone is the region where planetary temperatures are suitable for water to exist on a planet’s surface. During the first 6 weeks of observations, Kepler monitored 156,000 stars, and five new exoplanets with sizes between 0.37 and 1.6 Jupiter radii and orbital periods from 3.2 to 4.9 days were discovered. The density of the Neptune-sized Kepler-4b is similar to that of Neptune and GJ 436b, even though the irradiation level is 800,000 times higher. Kepler-7b is one of the lowest-density planets (~0.17 gram per cubic centimeter) yet detected. Kepler-5b, -6b, and -8b confirm the existence of planets with densities lower than those predicted for gas giant planets.

Embedded Clusters in Molecular Clouds

Abstract: ▪ Abstract Stellar clusters are born embedded within giant molecular clouds (GMCs) and during their formation and early evolution are often only visible at infrared wavelengths, being heavily obscured by dust. Over the past 15 years advances in infrared detection capabilities have enabled the first systematic studies of embedded clusters in galactic molecular clouds. In this article we review the current state of empirical knowledge concerning these extremely young protocluster systems. From a survey of the literature we compile the first extensive catalog of galactic embedded clusters. We use the catalog to construct the mass function and estimate the birthrate for embedded clusters within ∼2 kpc of the sun. We find that the embedded cluster birthrate exceeds that of visible open clusters by an order of magnitude or more indicating a high infant mortality rate for protocluster systems. Less than 4–7% of embedded clusters survive emergence from molecular clouds to become bound clusters of Pleiades age. Th...

Star Formation in the Milky Way and Nearby Galaxies

Abstract: We review progress over the past decade in observations of large-scale star formation, with a focus on the interface between extragalactic and Galactic studies. Methods of measuring gas contents and star-formation rates are discussed, and updated prescriptions for calculating star-formation rates are provided. We review relations between star formation and gas on scales ranging from entire galaxies to individual molecular clouds.