Lynne A. Hillenbrand

CUNY Graduate Center, New York City, New York, United States

Are you Lynne A. Hillenbrand?

Claim your profile

Publications (330)1087.12 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The classical T Tauri star AA Tau presented photometric variability attributed to an inner disk warp, caused by the interaction between the inner disk and an inclined magnetosphere. Previous studies of NGC 2264 have shown that similar photometric behavior is common among CTTS. The goal of this work is to investigate the main causes of the observed photometric variability of CTTS in NGC 2264 that present AA Tau-like light curves, and verify if an inner disk warp could be responsible for their variability. We investigate veiling variability in their spectra and u-r color variations and estimate parameters of the inner disk warp using an occultation model proposed for AA Tau. We compare infrared and optical light curves to analyze the dust responsible for the occultations. AA Tau-like variability is transient on a timescale of a few years. We ascribe it to stable accretion regimes and aperiodic variability to unstable accretion regimes and show that a transition, and even coexistence, between the two is common. We find evidence of hot spots associated with occultations, indicating that the occulting structures could be located at the base of accretion columns. We find average values of warp maximum height of 0.23 times its radial location, consistent with AA Tau, with variations of on average 11% between rotation cycles. We show that extinction laws in the inner disk indicate the presence of grains larger than interstellar grains. The inner disk warp scenario is consistent with observations for all but one periodic star in our sample. AA Tau-like systems comprise 14% of CTTS observed in NGC 2264, though this increases to 35% among systems of mass 0.7M_sun<M<2.0M_sun. Assuming random inclinations, we estimate that nearly all systems in this mass range likely possess an inner disk warp, possibly because of a change in magnetic field configurations among stars of lower mass.
    Astronomy and Astrophysics 02/2015; 577. DOI:10.1051/0004-6361/201425475 · 4.48 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We identify nine young stellar objects (YSOs) in the NGC 2264 star-forming region with optical {\em CoRoT} light curves exhibiting short-duration, shallow, periodic flux dips. All of these stars have infrared (IR) excesses that are consistent with their having inner disk walls near the Keplerian co-rotation radius. The repeating photometric dips have FWHM generally less than one day, depths almost always less than 15%, and periods (3<P<11 days) consistent with dust near the Keplerian co-rotation period. The flux dips vary considerably in their depth from epoch to epoch, but usually persist for several weeks and, in two cases, were present in data collected on successive years. For several of these stars, we also measure the photospheric rotation period and find that the rotation and dip periods are the same, as predicted by standard "disk-locking" models. We attribute these flux dips to clumps of material in or near the inner disk wall, passing through our line of sight to the stellar photosphere. In some cases, these dips are also present in simultaneous {\em Spitzer} IRAC light curves at 3.6 and 4.5 microns. We characterize the properties of these dips, and compare the stars with light curves exhibiting this behavior to other classes of YSO in NGC 2264. A number of physical mechanisms could locally increase the dust scale height near the inner disk wall, and we discuss several of those mechanisms; the most plausible mechanisms are either a disk warp due to interaction with the stellar magnetic field or dust entrained in funnel-flow accretion columns arising near the inner disk wall.
    The Astronomical Journal 01/2015; 149(4). DOI:10.1088/0004-6256/149/4/130 · 4.05 Impact Factor
  • Source
    Trevor J. David, Lynne A. Hillenbrand
    [Show abstract] [Hide abstract]
    ABSTRACT: Age determination is undertaken for nearby early-type (BAF) stars, which constitute attractive targets for high-contrast debris disk and planet imaging surveys. Our analysis sequence consists of: acquisition of uvby{\beta} photometry from catalogs, correction for the effects of extinction, interpolation of the photometry onto model atmosphere grids from which atmospheric parameters are determined, and finally, comparison to the theoretical isochrones from pre-main sequence through post-main sequence stellar evolution models, accounting for the effects of stellar rotation. We calibrate and validate our methods at the atmospheric parameter stage by comparing our results to fundamentally determined Teff and log g values. We validate and test our methods at the evolutionary model stage by comparing our results on ages to the accepted ages of several benchmark open clusters (IC 2602, {\alpha} Persei, Pleiades, Hyades). Finally, we apply our methods to estimate stellar ages for 3493 field stars, including several with directly imaged exoplanet candidates.
    The Astrophysical Journal 01/2015; 804(2). DOI:10.1088/0004-637X/804/2/146 · 6.28 Impact Factor
  • Source
    Krzysztof Findeisen, Ann Marie Cody, Lynne Hillenbrand
    [Show abstract] [Hide abstract]
    ABSTRACT: Aperiodic variability is a characteristic feature of young stars, massive stars, and active galactic nuclei. With the recent proliferation of time domain surveys, it is increasingly essential to develop methods to quantify and analyze aperiodic variability. We develop three timescale metrics that have been little used in astronomy -- Δm-Δt plots, peak-finding, and Gaussian process regression -- and present simulations comparing their effectiveness across a range of aperiodic light curve shapes, characteristic timescales, observing cadences, and signal to noise ratios. We find that Gaussian process regression is easily confused by noise and by irregular sampling, even when the model being fit reflects the process underlying the light curve, but that Δm-Δt plots and peak-finding can coarsely characterize timescales across a broad region of parameter space. We make public the software we used for our simulations, both in the spirit of open research and to allow others to carry out analogous simulations for their own observing programs.
    The Astrophysical Journal 01/2015; 798(2):89. DOI:10.1088/0004-637X/798/2/89 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: HD 19467 B is presently the only directly imaged T dwarf companion known to induce a measurable Doppler acceleration around a solar type star. We present spectroscopy measurements of this important benchmark object taken with the Project 1640 integral field unit at Palomar Observatory. Our high-contrast R~30 observations obtained simultaneously across the $JH$ bands confirm the cold nature of the companion as reported from the discovery article and determine its spectral type for the first time. Fitting the measured spectral energy distribution to SpeX/IRTF T dwarf standards and synthetic spectra from BT-Settl atmospheric models, we find that HD 19467 B is a T5.5+/-1 dwarf with effective temperature Teff=$978^{+20}_{-43}$ K. Our observations reveal significant methane absorption affirming its substellar nature. HD 19467 B shows promise to become the first T dwarf that simultaneously reveals its mass, age, and metallicity independent from the spectrum of light that it emits.
    12/2014; 798(2). DOI:10.1088/2041-8205/798/2/L43
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The growth of luminous structures and the building blocks of life in the Universe began as primordial gas was processed in stars and mixed at galactic scales. The mechanisms responsible for this development are not well understood and have changed over the intervening 13 billion years. To follow the evolution of matter over cosmic time, it is necessary to study the strongest (resonance) transitions of the most abundant species in the Universe. Most of them are in the ultraviolet (UV; 950A-3000A) spectral range that is unobservable from the ground. A versatile space observatory with UV sensitivity a factor of 50-100 greater than existing facilities will revolutionize our understanding of the Universe. Habitable planets grow in protostellar discs under ultraviolet irradiation, a by-product of the star-disk interaction that drives the physical and chemical evolution of discs and young planetary systems. The electronic transitions of the most abundant molecules are pumped by the UV field, providing unique diagnostics of the planet-forming environment that cannot be accessed from the ground. Earth's atmosphere is in constant interaction with the interplanetary medium and the solar UV radiation field. A 50-100 times improvement in sensitivity would enable the observation of the key atmospheric ingredients of Earth-like exoplanets (carbon, oxygen, ozone), provide crucial input for models of biologically active worlds outside the solar system, and provide the phenomenological baseline to understand the Earth atmosphere in context. In this white paper, we outline the key science that such a facility would make possible and outline the instrumentation to be implemented.
    Astrophysics and Space Science 11/2014; 354(1):229-246. DOI:10.1007/s10509-014-1942-7 · 2.40 Impact Factor
  • Source
    Ann Marie Cody, Lynne A. Hillenbrand
    [Show abstract] [Hide abstract]
    ABSTRACT: In 2005, Palla & Baraffe proposed that brown dwarfs (BDs) and very low mass stars (VLMSs; <0.1 solar masses) may be unstable to radial oscillations during the pre-main-sequence deuterium burning phase. With associated periods of 1-4 hours, this potentially new class of pulsation offers unprecedented opportunities to probe the interiors and evolution of low-mass objects in the 1-15 million year age range. Following up on reports of short-period variability in young clusters, we designed a high-cadence photometric monitoring campaign to search for deuterium-burning pulsation among a sample of 348 BDs and VLMSs in the four young clusters $\sigma$ Orionis, Chamaeleon I, IC 348, and Upper Scorpius. In the resulting light curves we achieved sensitivity to periodic signals of amplitude several millimagnitudes, on timescales from 15 minutes to two weeks. Despite the exquisite data quality, we failed to detect any periodicities below seven hours. We conclude that D-burning pulsations are not able to grow to observable amplitudes in the early pre-main sequence. In spite of the non-detection, we did uncover a rich set of variability behavior- both periodic and aperiodic- on day to week timescales. We present new compilations of variable sources from our sample, as well as three new candidate cluster members in Chamaeleon I.
    The Astrophysical Journal 10/2014; 796(2). DOI:10.1088/0004-637X/796/2/129 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present an analysis of the orbital motion of the four sub-stellar objects orbiting HR8799. Our study relies on the published astrometric history of this system augmented with an epoch obtained with the Project 1640 coronagraph + Integral Field Spectrograph (IFS) installed at the Palomar Hale telescope. We first focus on the intricacies associated with astrometric estimation using the combination of an Extreme Adaptive Optics system (PALM-3000), a coronagraph and an IFS. We introduce two new algorithms. The first one retrieves the stellar focal plane position when the star is occulted by a coronagraphic stop. The second one yields precise astrometric and spectro-photometric estimates of faint point sources even when they are initially buried in the speckle noise. The second part of our paper is devoted to studying orbital motion in this system. In order to complement the orbital architectures discussed in the literature, we determine an ensemble of likely Keplerian orbits for HR8799bcde, using a Bayesian analysis with maximally vague priors regarding the overall configuration of the system. While the astrometric history is currently too scarce to formally rule out coplanarity, HR8799d appears to be misaligned with respect to the most likely planes of HR8799bce orbits. This misalignment is sufficient to question the strictly coplanar assumption made by various authors when identifying a Laplace resonance as a potential architecture. Finally, we establish a high likelihood that HR8799de have dynamical masses below 13 M_Jup using a loose dynamical survival argument based on geometric close encounters. We illustrate how future dynamical analyses will further constrain dynamical masses in the entire system.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The YSOVAR (Young Stellar Object VARiability) Spitzer Space Telescope observing program obtained the first extensive mid-infrared (3.6 & 4.5 um) time-series photometry of the Orion Nebula Cluster plus smaller footprints in eleven other star-forming cores (AFGL490, NGC1333, MonR2, GGD 12-15, NGC2264, L1688, Serpens Main, Serpens South, IRAS 20050+2720, IC1396A, and Ceph C). There are ~29,000 unique objects with light curves in either or both IRAC channels in the YSOVAR data set. We present the data collection and reduction for the Spitzer and ancillary data, and define the "standard sample" on which we calculate statistics, consisting of fast cadence data, with epochs about twice per day for ~40d. We also define a "standard sample of members", consisting of all the IR-selected members and X-ray selected members. We characterize the standard sample in terms of other properties, such as spectral energy distribution shape. We use three mechanisms to identify variables in the fast cadence data--the Stetson index, a chi^2 fit to a flat light curve, and significant periodicity. We also identified variables on the longest timescales possible of ~6 years, by comparing measurements taken early in the Spitzer mission with the mean from our YSOVAR campaign. The fraction of members in each cluster that are variable on these longest timescales is a function of the ratio of Class I/total members in each cluster, such that clusters with a higher fraction of Class I objects also have a higher fraction of long-term variables. For objects with a YSOVAR-determined period and a [3.6]-[8] color, we find that a star with a longer period is more likely than those with shorter periods to have an IR excess. We do not find any evidence for variability that causes [3.6]-[4.5] excesses to appear or vanish within our data; out of members and field objects combined, at most 0.02% may have transient IR excesses.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report initial results from our long term search using precision radial velocities for planetary-mass companions located within a few AU of stars younger than the Sun. Based on a sample of >150 stars, we define a floor in the radial velocity scatter, sigma_RV, as a function of the chromospheric activity level R'_{HK}. This lower bound to the jitter, which increases with increasing stellar activity, sets the minimum planet mass that could be detected. Adopting a median activity-age relationship reveals the astrophysical limits to planet masses discernable via radial velocity monitoring, as a function of stellar age. Considering solar-mass primaries having the mean jitter-activity level, when they are younger than 100 / 300 / 1000 Myr, the stochastic jitter component in radial velocity measurements restricts detectable companion masses to > 0.3 / 0.2 / 0.1 M_Jupiter. These numbers require a large number -- several tens -- of radial velocity observations taken over a time frame longer than the orbital period. Lower companion mass limits can be achieved for stars with less than the mean jitter and/or with an increased number of observations.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The emission from young stellar objects (YSOs) in the mid-IR is dominated by the inner rim of their circumstellar disks. We present an IR-monitoring survey of about 800 objects in the direction of the Lynds 1688 (L1688) star forming region over four visibility windows spanning 1.6 years using the \emph{Spitzer} space telescope in its warm mission phase. Among all lightcurves, 57 sources are cluster members identified based on their spectral-energy distribution and X-ray emission. Almost all cluster members show significant variability. The amplitude of the variability is larger in more embedded YSOs. Ten out of 57 cluster members have periodic variations in the lightcurves with periods typically between three and seven days, but even for those sources, significant variability in addition to the periodic signal can be seen. No period is stable over 1.6 years. Non-periodic lightcurves often still show a preferred timescale of variability which is longer for more embedded sources. About half of all sources exhibit redder colors in a fainter state. This is compatible with time-variable absorption towards the YSO. The other half becomes bluer when fainter. These colors can only be explained with significant changes in the structure of the inner disk. No relation between mid-IR variability and stellar effective temperature or X-ray spectrum is found.
    The Astronomical Journal 08/2014; 148(6). DOI:10.1088/0004-6256/148/6/122 · 4.05 Impact Factor
  • Source
    J. A. Eisner, L. A. Hillenbrand, Jordan M. Stone
    [Show abstract] [Hide abstract]
    ABSTRACT: We present Keck Interferometer observations of T Tauri and Herbig Ae/Be stars with a spatial resolution of a few milliarcseconds and a spectral resolution of ~2000. Our observations span the K-band, and include the Br gamma transition of Hydrogen and the v=2-0 and v=3-1 transitions of carbon monoxide. For several targets we also present data from Keck/NIRSPEC that provide higher spectral resolution, but a seeing-limited spatial resolution, of the same spectral features. We analyze the Br gamma emission in the context of both disk and infall/outflow models, and conclude that the Br gamma emission traces gas at very small stellocentric radii, consistent with the magnetospheric scale. However some Br gamma-emitting gas also seems to be located at radii of >0.1 AU, perhaps tracing the inner regions of magnetically launched outflows. CO emission is detected from several objects, and we generate disk models that reproduce both the KI and NIRSPEC data well. We infer the CO spatial distribution to be coincident with the distribution of continuum emission in most cases. Furthermore the Br gamma emission in these objects is roughly coincident with both the CO and continuum emission. We present potential explanations for the spatial coincidence of continuum, Br gamma, and CO overtone emission, and explore the implications for the low occurrence rate of CO overtone emission in young stars. Finally, we provide additional discussion of V1685 Cyg, which is unusual among our sample in showing large differences in emitting region size and spatial position as a function of wavelength.
    Monthly Notices of the Royal Astronomical Society 06/2014; 443(3). DOI:10.1093/mnras/stu1281 · 5.23 Impact Factor
  • Source
    Gregory J. Herczeg, Lynne A. Hillenbrand
    [Show abstract] [Hide abstract]
    ABSTRACT: Measurements of masses and ages of young stars from their location in the HR diagram are limited by not only the typical observational uncertainties that apply to field stars, but also by large systematic uncertainties related to circumstellar phenomena. In this paper, we analyze flux calibrated optical spectra to measure accurate spectral types and extinctions of 283 nearby T Tauri stars. The primary advances in this paper are (1) the incorporation of a simplistic accretion continuum in optical spectral type and extinction measurements calculated over the full optical wavelength range and (2) the uniform analysis of a large sample of stars. Comparisons between the non-accreting TTS photospheric templates and stellar photosphere models are used to derive conversions from spectral type to temperature. Differences between spectral types can be subtle and difficult to discern, especially when accounting for accretion and extinction. The spectral types measured here are mostly consistent with spectral types measured over the past decade. However, our new spectral types are 1-2 subclasses later than literature spectral types for the original members of the TWA and are discrepant with literature values for some well known Taurus CTTSs. Our extinction measurements are consistent with other optical extinction measurements but are typically 1 mag lower than nIR measurements, likely the result of methodological differences and the presence of nIR excesses in most CTTSs. As an illustration of the impact of accretion, SpT, and extinction uncertainties on the HR diagrams of young clusters, we find that the resulting luminosity spread of stars in the TWA is 15-30%. The luminosity spread in the TWA and previously measured for binary stars in Taurus suggests that for a majority of stars, protostellar accretion rates are not large enough to significantly alter the subsequent evolution.
    The Astrophysical Journal 03/2014; 786(2). DOI:10.1088/0004-637X/786/2/97 · 6.28 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present the Coordinated Synoptic Investigation of NGC 2264, a continuous 30-day multi-wavelength photometric monitoring campaign on more than 1000 young cluster members using 16 telescopes. The unprecedented combination of multi-wavelength, high-precision, high-cadence, and long-duration data opens a new window into the time domain behavior of young stellar objects. Here we provide an overview of the observations, focusing on results from Spitzer and CoRoT. The highlight of this work is detailed analysis of 162 classical T Tauri stars for which we can probe optical and mid-infrared flux variations to 1% amplitudes and sub-hour timescales. We present a morphological variability census and then use metrics of periodicity, stochasticity, and symmetry to statistically separate the light curves into seven distinct classes, which we suggest represent different physical processes and geometric effects. We provide distributions of the characteristic timescales and amplitudes, and assess the fractional representation within each class. The largest category (>20%) are optical "dippers" having discrete fading events lasting ~1-5 days. The degree of correlation between the optical and infrared light curves is positive but weak; notably, the independently assigned optical and infrared morphology classes tend to be different for the same object. Assessment of flux variation behavior with respect to (circum)stellar properties reveals correlations of variability parameters with H$\alpha$ emission and with effective temperature. Overall, our results point to multiple origins of young star variability, including circumstellar obscuration events, hot spots on the star and/or disk, accretion bursts, and rapid structural changes in the inner disk.
    The Astronomical Journal 01/2014; 147(4):82. DOI:10.1088/0004-6256/147/4/82 · 4.05 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Based on more than four weeks of continuous high cadence photometric monitoring of several hundred members of the young cluster NGC 2264 with two space telescopes, NASA's Spitzer and the CNES CoRoT (Convection, Rotation, and planetary Transits), we provide high quality, multi-wavelength light curves for young stellar objects (YSOs) whose optical variability is dominated by short duration flux bursts, which we infer are due to enhanced mass accretion rates. These light curves show many brief -- several hour to one day -- brightenings at optical and near-infrared (IR) wavelengths with amplitudes generally in the range 5-50% of the quiescent value. Typically, a dozen or more of these bursts occur in a thirty day period. We demonstrate that stars exhibiting this type of variability have large ultraviolet (UV) excesses and dominate the portion of the u-g vs. g-r color-color diagram with the largest UV excesses. These stars also have large Halpha equivalent widths, and either centrally peaked, lumpy Halpha emission profiles or profiles with blue-shifted absorption dips associated with disk or stellar winds. Light curves of this type have been predicted for stars whose accretion is dominated by Rayleigh-Taylor instabilities at the boundary between their magnetosphere and inner circumstellar disk, or where magneto-rotational instabilities modulate the accretion rate from the inner disk. Amongst the stars with the largest UV excesses or largest Halpha equivalent widths, light curves with this type of variability greatly outnumber light curves with relatively smooth sinusoidal variations associated with long-lived hot spots. We provide quantitative statistics for the average duration and strength of the accretion bursts and for the fraction of the accretion luminosity associated with these bursts.
    The Astronomical Journal 01/2014; 147(4). DOI:10.1088/0004-6256/147/4/83 · 4.05 Impact Factor
  • Krzysztof Findeisen, Lynne Hillenbrand, Ann Marie Cody
    [Show abstract] [Hide abstract]
    ABSTRACT: Ongoing time domain surveys such as PTF, CRTS, and Pan-STARRS1, as well as upcoming surveys such as LSST, promise to revolutionize optical astronomy by providing a comprehensive picture of the variability properties of everything from local flare stars to distant quasars. Time domain surveys have already proven a boon for studies of young stars, whose variability is frequently aperiodic and may have time scales of days to decades, depending on the physics underlying the variability. I present an overview of the PTF-NAN (North America Nebula) survey, which allows us, for the first time, to simultaneously resolve day-scale variability and to monitor changes in photometric behavior in young stars over several years, without large data gaps and without any assumptions about periodicity. I describe preliminary results of the survey, including a search for episodic stellar behavior, a study of the most robust methods for identifying the characteristic time scale(s) of an aperiodic signal, and a characterization of the full range of amplitudes and time scales represented in optical variability of young stars.
    American Astronomical Society, AAS Meeting #223; 01/2014
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Interstellar reddening corrections are necessary to reconstruct the intrinsic spectral energy distributions (SEDs) of accreting protostellar systems. The stellar SED determines the heating and chemical processes that can occur in circumstellar disks. Measurement of neutral hydrogen absorption against broad Lyman-$\alpha$ emission profiles in young stars can be used to obtain the total H I column density (N(H I)) along the line of sight. We measure N(H I) with new and archival ultraviolet observations from the Hubble Space Telescope ($HST$) of 31 classical T Tauri and Herbig Ae/Be stars. The H I column densities range from log$_{10}$(N(H I)) $\approx 19.6 - 21.1$, with corresponding visual extinctions of A$_{V}$ $= 0.02 - 0.72$ mag, assuming an R$_{V}$ of 3.1. We find that the majority of the H I absorption along the line of sight likely comes from interstellar rather than circumstellar material. Extinctions derived from new $HST$ blue-optical spectral analyses, previous IR and optical measurements, and new X-ray column densities on average overestimate the interstellar extinction toward young stars compared to the N(H I) values by $\sim 0.6$ mag. We discuss possible explanations for this discrepancy in the context of a protoplanetary disk geometry.
    The Astrophysical Journal 12/2013; 780(2). DOI:10.1088/0004-637X/780/2/150 · 6.28 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We describe the status of the PALM-3000 adaptive optics facility instrument for the Hale telescope at Palomar Observatory. Since first light in June 2011, PALM-3000 has made significant advances in both performance and sensitivity. Using Strehl ratio as our performance metric, we present results in 64x64 and 32x32 wavefront sensor pupil sampling modes on a range of guide stars from V ~ 3 to 12. We describe our automated reconstructor pipeline tool, which incorporates pupil illumination and an optimal-estimator Baysian approach which serve to boost faint guide star performance. We conclude by presenting initial high-contrast circumstellar disk results from the PHARO vector vortex coronagraph and exoplanet spectra from the P1640 integral field spectrograph.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Determining the sequence of events in the formation of stars and planetary systems and their time-scales is essential for understanding those processes, yet establishing ages is fundamentally difficult because we lack direct indicators. In this review we discuss the age challenge for young stars, specifically those less than ~100 Myr old. Most age determination methods that we discuss are primarily applicable to groups of stars but can be used to estimate the age of individual objects. A reliable age scale is established above 20 Myr from measurement of the Lithium Depletion Boundary (LDB) in young clusters, and consistency is shown between these ages and those from the upper main sequence and the main sequence turn-off -- if modest core convection and rotation is included in the models of higher-mass stars. Other available methods for age estimation include the kinematics of young groups, placing stars in Hertzsprung-Russell diagrams, pulsations and seismology, surface gravity measurement, rotation and activity, and lithium abundance. We review each of these methods and present known strengths and weaknesses. Below ~20 Myr, both model-dependent and observational uncertainties grow, the situation is confused by the possibility of age spreads, and no reliable absolute ages yet exist. The lack of absolute age calibration below 20 Myr should be borne in mind when considering the lifetimes of protostellar phases and circumstellar material.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Spectropolarimetric observations combined with tomographic imaging techniques have revealed that all pre-main sequence (PMS) stars host multipolar magnetic fields, ranging from strong and globally axisymmetric with ~>kilo-Gauss dipole components, to complex and non-axisymmetric with weak dipole components (<~0.1 kG). Many host dominantly octupolar large-scale fields. We argue that the large-scale magnetic properties of a PMS star are related to its location in the Hertzsprung-Russell diagram. This conference paper is a synopsis of Gregory et al. (2012), updated to include the latest results from magnetic mapping studies of PMS stars.
    Proceedings of the International Astronomical Union 09/2013; 9(S302). DOI:10.1017/S1743921314001677

Publication Stats

9k Citations
1,087.12 Total Impact Points

Institutions

  • 2013
    • CUNY Graduate Center
      New York City, New York, United States
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2000–2013
    • California Institute of Technology
      • • Jet Propulsion Laboratory
      • • Department of Astronomy
      • • Division of Physics, Mathematics, and Astronomy
      Pasadena, California, United States
    • Centro de Investigaciones de Astronomia (CIDA)
      Mérida, Estado Mérida, Venezuela
  • 2012
    • NSF
      Ann Arbor, Michigan, United States
    • Massachusetts Institute of Technology
      • Department of Physics
      Cambridge, Massachusetts, United States
    • University of Exeter
      Exeter, England, United Kingdom
  • 2010–2012
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
    • University of Toronto
      • Dunlap Institute for Astronomy and Astrophysics
      Toronto, Ontario, Canada
    • Rutgers, The State University of New Jersey
      • Department Physics and Astronomy
      New Brunswick, New Jersey, United States
  • 1995–2012
    • University of California, Berkeley
      • Department of Astronomy
      Berkeley, California, United States
  • 2011
    • Las Cumbres Observatory Global Telescope Network
      Goleta, California, United States
    • California State University, Los Angeles
      • Department of Physics and Astronomy
      Los Angeles, California, United States
  • 2008
    • The Washington Institute
      Washington, Washington, D.C., United States
  • 1993
    • University of Massachusetts Amherst
      • Department of Astronomy
      Amherst Center, Massachusetts, United States