A Simultaneous Optical and X-ray Variability Study of the Orion Nebula Cluster. II. A Common Origin in Magnetic Activity

We present a statistical analysis of simultaneous optical and X-ray light curves, spanning 600 ks, for 814 pre-main-sequence (PMS) stars in the Orion Nebula Cluster. The aim of this study is to establish the relationship, if any, between the sites of optical and X-ray variability, and thereby to elucidate the origins of X-ray production in PMS stars. In a previous paper we showed that optical and X-ray variability in PMS stars are very rarely time-correlated. Here, using time-averaged variability indicators to examine the joint occurrences of optical and X-ray variability, we confirm that the two forms of variability are not directly causally related. However, a strong and highly statistically significant correlation is found between optical variability and X-ray luminosity. As this correlation is found to be independent of accretion activity, we argue that X-ray production in PMS stars must instead be intimately connected with the presence and strength of optically variable, magnetically active surface regions (i.e. spots) on these stars. Moreover, because X-ray variability and optical variability are rarely time-correlated, we conclude that the sites of X-ray production are not exclusively co-spatial with these regions. We argue that solar-analog coronae, heated by topologically complex fields, can explain these findings.Comment: To appear in the Astrophysical Journal. 33 pages, 3 figure

Gaia Stellar Kinematics in the Head of the Orion A Cloud: Runaway Stellar Groups and Gravitational Infall

This work extends previous kinematic studies of young stars in the Head of the Orion A cloud (OMC-1/2/3/4/5). It is based on large samples of infrared, optical, and X-ray selected pre-main sequence stars with reliable radial velocities and Gaia-derived parallaxes and proper motions. Stellar kinematic groups are identified assuming they mimic the motion of their parental gas. Several groups are found to have peculiar kinematics: the NGC 1977 cluster and two stellar groups in the Extended Orion Nebula (EON) cavity are caught in the act of departing their birthplaces. The abnormal motion of NGC 1977 may have been caused by a global hierarchical cloud collapse, feedback by massive Ori OB1ab stars, supersonic turbulence, cloud-cloud collision, and/or slingshot effect; the former two models are favored by us. EON groups might have inherited anomalous motions of their parental cloudlets due to small-scale `rocket effects' from nearby OB stars. We also identify sparse stellar groups to the east and west of Orion A that are drifting from the central region, possibly a slowly expanding halo of the Orion Nebula Cluster. We confirm previously reported findings of varying line-of-sight distances to different parts of the cloud's Head with associated differences in gas velocity. Three-dimensional movies of star kinematics show contraction of the groups of stars in OMC-1 and global contraction of OMC-123 stars. Overall, the Head of Orion A region exhibits complex motions consistent with theoretical models involving hierarchical gravitational collapse in (possibly turbulent) clouds with OB stellar feedback.Comment: Accepted for publication in MNRAS. 26 pages, 13 figures. The two 3-D stellar kinematic movies, aimed as Supplementary Materials, can be found on YouTube at: https://youtu.be/B4GHCVvCYfo (`restricted' sample) and https://youtu.be/6fUu8sP0QFI (`full' sample

Multi-wavelength observing of a forming solar-like star

V2129 Oph is a 1.35 solar mass classical T Tauri star, known to possess a strong and complex magnetic field. By extrapolating from an observationally derived magnetic surface map, obtained through Zeeman-Doppler imaging, models of V2129 Oph's corona have been constructed, and used to make predictions regarding the global X-ray emission measure, the amount of modulation of X-ray emission, and the density of accretion shocks. In late June 2009 we will under take an ambitious multi-wavelength, multi-observing site, and near contemporaneous campaign, combining spectroscopic optical, nIR, UV, X-ray, spectropolarimetric and photometric monitoring. This will allow the validity of the 3D field topologies derived via field extrapolation to be determined.Comment: 4 pages, proceedings of the 3rd MSSL workshop on High Resolution X-ray Spectroscopy: towards IX

Binarity as a key factor in protoplanetary disk evolution: Spitzer disk census of the eta Chamaeleontis cluster

The formation of planets is directly linked to the evolution of the circumstellar (CS) disk from which they are born. The dissipation timescales of CS disks are, therefore, of direct astrophysical importance in evaluating the time available for planet formation. We employ Spitzer Space Telescope spectra to complete the CS disk census for the late-type members of the ~8 Myr-old eta Chamaeleontis star cluster. Of the 15 K- and M-type members, eight show excess emission. We find that the presence of a CS disk is anti-correlated with binarity, with all but one disk associated with single stars. With nine single stars in total, about 80% retain a CS disk. Of the six known or suspected close binaries the only CS disk is associated with the primary of RECX 9. No circumbinary disks have been detected. We also find that stars with disks are slow rotators with surface values of specific angular momentum j = 2-15 j_sun. All high specific angular momentum systems with j = 20-30 j_sun are confined to the primary stars of binaries. This provides novel empirical evidence for rotational disk locking and again demonstrates the much shorter disk lifetimes in close binary systems compared to single star systems. We estimate the characteristic mean disk dissipation timescale to be ~5 Myr and ~9 Myr for the binary and single star systems, respectively.Comment: Accepted by ApJ

On the future of astrostatistics: statistical foundations and statistical practice

This paper summarizes a presentation for a panel discussion on "The Future of Astrostatistics" held at the Statistical Challenges in Modern Astronomy V conference at Pennsylvania State University in June 2011. I argue that the emerging needs of astrostatistics may both motivate and benefit from fundamental developments in statistics. I highlight some recent work within statistics on fundamental topics relevant to astrostatistical practice, including the Bayesian/frequentist debate (and ideas for a synthesis), multilevel models, and multiple testing. As an important direction for future work in statistics, I emphasize that astronomers need a statistical framework that explicitly supports unfolding chains of discovery, with acquisition, cataloging, and modeling of data not seen as isolated tasks, but rather as parts of an ongoing, integrated sequence of analyses, with information and uncertainty propagating forward and backward through the chain. A prototypical example is surveying of astronomical populations, where source detection, demographic modeling, and the design of survey instruments and strategies all interact.Comment: 8 pp, 2 figures. To appear in "Statistical Challenges in Modern Astronomy V," (Lecture Notes in Statistics, Vol. 209), ed. Eric D. Feigelson and G. Jogesh Babu; publication planned for Sep 2012; see http://www.springer.com/statistics/book/978-1-4614-3519-

A probabilistic approach to emission-line galaxy classification

We invoke a Gaussian mixture model (GMM) to jointly analyse two traditional emission-line classification schemes of galaxy ionization sources: the Baldwin-Phillips-Terlevich (BPT) and $\rm W_{H\alpha}$ vs. [NII]/H$\alpha$ (WHAN) diagrams, using spectroscopic data from the Sloan Digital Sky Survey Data Release 7 and SEAGal/STARLIGHT datasets. We apply a GMM to empirically define classes of galaxies in a three-dimensional space spanned by the $\log$ [OIII]/H$\beta$, $\log$ [NII]/H$\alpha$, and $\log$ EW(H${\alpha}$), optical parameters. The best-fit GMM based on several statistical criteria suggests a solution around four Gaussian components (GCs), which are capable to explain up to 97 per cent of the data variance. Using elements of information theory, we compare each GC to their respective astronomical counterpart. GC1 and GC4 are associated with star-forming galaxies, suggesting the need to define a new starburst subgroup. GC2 is associated with BPT's Active Galaxy Nuclei (AGN) class and WHAN's weak AGN class. GC3 is associated with BPT's composite class and WHAN's strong AGN class. Conversely, there is no statistical evidence -- based on four GCs -- for the existence of a Seyfert/LINER dichotomy in our sample. Notwithstanding, the inclusion of an additional GC5 unravels it. The GC5 appears associated to the LINER and Passive galaxies on the BPT and WHAN diagrams respectively. Subtleties aside, we demonstrate the potential of our methodology to recover/unravel different objects inside the wilderness of astronomical datasets, without lacking the ability to convey physically interpretable results. The probabilistic classifications from the GMM analysis are publicly available within the COINtoolbox (https://cointoolbox.github.io/GMM\_Catalogue/).Comment: Accepted for publication in MNRA

An X-Ray Spectral Classification Algorithm with Application to Young Stellar Clusters

A large volume of low signal-to-noise, multidimensional data is available from the CCD imaging spectrometers aboard the Chandra X-Ray Observatory and the X-Ray Multimirror Mission (XMM-Newton). To make progress analyzing this data,itisessentialtodevelopmethods tosort,classify,and characterize thevastlibrary of X-rayspectrain a nonparametric fashion (complementary to current parametric model fits). We have developed a spectral classification algorithm that handles large volumes of data and operates independently of the requirement of spectral model fits.Weuseprovenmultivariatestatisticaltechniquesincludingprincipalcomponentanalysisandanensembleclassifier consistingofagglomerativehierarchicalclusteringandK-meansclusteringappliedforthefirsttimeforspectralclassification.Thealgorithmpositionsthesourcesinamultidimensionalspectralsequenceandthengroupstheorderedsources into clusters based on their spectra. These clusters appear more distinct for sources with harder observed spectra. The apparent diversity ofsource spectra isreduced toa three-dimensional locus inprincipal component space,withspectral outliers falling outside this locus. The algorithm was applied to a sample of 444 strong sources selected from the 1616 X-ray emitting sources detected in deep Chandra imaging spectroscopy of the Orion Nebula Cluster. Classes form sequencesinNH,AV,andaccretionactivityindicators,demonstratingthatthealgorithmefficientlysortstheX-raysources into a physically meaningful sequence. The algorithm also isolates important classes of very deeply embedded, active young stellar objects, and yields trends between X-ray spectral parameters and stellar parameters for the lowest mass, pre‐main-sequence stars

51 Eri and GJ 3305: A 10-15 Myr old binary star system at 30 parsecs

Following the suggestion of Zuckerman et al. (2001, ApJ, 562, L87), we consider the evidence that 51 Eri (spectral type F0) and GJ 3305 (M0), historically classified as unrelated main sequence stars in the solar neighborhood, are instead a wide physical binary system and members of the young beta Pic moving group (BPMG). The BPMG is the nearest (d < 50 pc) of several groups of young stars with ages around 10 Myr that are kinematically convergent with the Oph-Sco-Cen Association (OSCA), the nearest OB star association. Combining SAAO optical photometry, Hobby-Eberly Telescope high-resolution spectroscopy, Chandra X-ray data, and UCAC2 catalog kinematics, we confirm with high confidence that the system is indeed extremely young. GJ 3305 itself exhibits very strong magnetic activity but has rapidly depleted most of its lithium. The 51 Eri/GJ 3305 system is the westernmost known member of the OSCA, lying 110 pc from the main subgroups. The system is similar to the BPMG wide binary HD 172555/CD -64d1208 and the HD 104237 quintet, suggesting that dynamically fragile multiple systems can survive the turbulent environments of their natal giant molecular cloud complexes, while still being imparted high dispersion velocities. Nearby young systems such as these are excellent targets for evolved circumstellar disk and planetary studies, having stellar ages comparable to that of the late phases of planet formation.Comment: 27 pages, 7 figures. Accepted for publication in the Astronomical Journal. For a version with high resolution figures, see http://www.astro.psu.edu/users/edf/51Eri.pd

The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center

We examine the X-ray spectra and variability of the sample of X-ray sources with L_X = 10^{31}-10^{33} erg s^{-1} identified within the inner 9' of the Galaxy. Very few of the sources exhibit intra-day or inter-month variations. We find that the spectra of the point sources near the Galactic center are very hard between 2--8 keV, even after accounting for absorption. When modeled as power laws the median photon index is Gamma=0.7, while when modeled as thermal plasma we can only obtain lower limits to the temperature of kT>8 keV. The combined spectra of the point sources is similarly hard, with a photon index of Gamma=0.8. Strong line emission is observed from low-ionization, He-like, and H-like Fe, both in the average spectra and in the brightest individual sources. The line ratios of the highly-ionized Fe in the average spectra are consistent with emission from a plasma in thermal equilibrium. This line emission is observed whether average spectra are examined as a function of the count rate from the source, or as a function of the hardness ratios of individual sources. This suggests that the hardness of the spectra may in fact to due local absorption that partially-covers the X-ray emitting regions in the Galactic center systems. We suggest that most of these sources are intermediate polars, which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely exhibit either flares on short time scales or changes in their mean X-ray flux on long time scales, and (3) are the most numerous hard X-ray sources with comparable luminosities in the Galaxy.Comment: 27 pages, including 13 figures. To appear in ApJ, 1 October 2004, v613 issue. An electronic version of table 2 is on http://astro.ucla.edu/~mmuno/sgra/table2_electronic.txt and reduced data files for each source are available on http://www.astro.psu.edu/users/niel/galcen-xray-data/galcen-xray-data.htm