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CALTECH GE 133 - X-ray Properties of Young Stars

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X-ray Properties of Young Stars and Stellar ClustersEric Feigelson and Leisa TownsleyPennsylvania State UniversityManuel G¨udelPaul Scherrer InstituteKeivan StassunVanderbilt UniversityAlthough the environments of star and planet formation are thermodynamically cold,substantial X-ray emission from 10 − 100 MK plasmas is present. In low mass pre-mainsequence stars, X-rays are produced by violent magnetic reconnection flares. In high mass Ostars, they are produced by wind shocks on both stellar and parsec scales. The recent ChandraOrion Ultradeep Project, XMM-Newton Extended Survey of Taurus, and Chandra studies ofmore distant high-mass star forming regions reveal a wealth of X-ray phenomenology andastrophysics. X-ray flares mostly resemble solar-like magnetic activity from multipolar surfacefields, although extreme flares may arise in field lines extending to the protoplanetary disk.Accretion plays a secondary role. Fluorescent iron line emission and absorption in inclined disksdemonstrate that X-rays can efficiently illuminate disk material. The consequent ionization ofdisk gas and irradiation of disk solids addresses a variety of important astrophysical issues ofdisk dynamics, planet formation, and meteoritics. New observations of massive star formingenvironments such as M 17, the Carina Nebula and 30 Doradus show remarkably complexX-ray morphologies including the low-mass stellar population, diffuse X-ray flows from blisterHII regions, and inhomogeneous superbubbles. X-ray astronomy is thus providing qualitativelynew insights into star and planet formation.1. INTRODUCTIONStar and planet formation is generally viewed as a hy-drodynamic process involving gravitational collapse of in-terstellar material at low temperatures, 10–100 K in molec-ular cloud cores and 100–1500 K in protoplanetary disks. Ifthermodynamical equilibrium holds, this material should beneutral except in localized HII regions where the bolomet-ric ultraviolet emission from massive O star photoionizationis present. However, stars have turned out to be sourcesof intense X-rays at almost every stage of early formationand evolution, from low-mass brown dwarfs to massive Ostars, to an extent that the stellar environment is ionized andheated (beyond effects due to ultraviolet radiation) out toconsiderable distances and thus made accessible to mag-netic fields.X-ray observations reveal the presence of highly-ionizedplasma with temperatures of 107− 108K. In lower-massstars, the X-ray emission is reminiscent of X-rays observedon the Sun, particularly the plasma explosively heated andconfined in magnetic loops following magnetic reconnec-tion events. X-ray flares with luminosities orders of magni-tude more powerful than seen in the contemporary Sun arefrequently seen in young stars. Evidence for an impulsivephase is seen in radio bursts and in U band enhancementspreceding X-ray flares, thought to be due to the bombard-ment of the stellar surface by electron beams. Thus, youngstars prolifically accelerate particles to relativistic energies.In rich young stellar clusters, X-rays are also produced byshocks in O star winds, on both small (< 102R?) andlarge (parsec) scales. If the region has been producing richclusters for a sufficiently long time, the resulting supernovaremnants will dominate the X-ray properties.X-ray studies with the Chandra and XMM-Newton spaceobservatories are propelling advances of our knowledge andunderstanding of high energy processes during the earli-est phases of stellar evolution. In the nearest young starsand clusters (d < 500 pc), they provide detailed informa-tion about magnetic reconnection processes. In the moredistant and richer regions, the X-ray images are amaz-ingly complex with diffuse plasma surrounding hundredsof stars exhibiting a wide range of absorptions. We con-centrate here on results from three recent large surveys:the Chandra Orion Ultradeep Project (COUP) based on anearly-continuous 13-day observation of the Orion Nebularegion in 2003, the XMM-Newton Extended Survey of Tau-rus (XEST) that maps ∼ 5 square degrees of the TaurusMolecular Cloud (TMC), and an on-going Chandra surveyof high mass star formation regions across the Galactic disk.Because the XEST study is discussed in specific detail in aclosely related chapter (G¨udel et al., this volume) togetherwith optical and infrared surveys, we present only selectedXEST results. This volume has another closely related1chapter: Bally et al. discuss X-ray emission from high-velocity protostellar Herbig-Haro outflows. The reader in-terested in earlier X-ray studies is referred to reviews byFeigelson and Montmerle (1999), Glassgold et al. (2000)in Protostars and Planets IV, Favata and Micela (2003),Paerels and Kahn (2003), and G¨udel (2004).The COUP is particularly valuable in establishing a com-prehensive observational basis for describing the physicalcharacteristics of flaring phenomena and elucidating themechanisms of X-ray production. The central portion ofthe COUP image, showing the PMS population around thebright Trapezium stars and the embedded OMC-1 popula-tions, is shown in Plate 1 (Getman et al., 2005a). X-raysare detected from nearly all known optical members exceptfor many of the bolometrically fainter M stars and browndwarfs. Conversely, 1315 of 1616 COUP sources (81%)have clear cluster member counterparts and '75 (5%) arenew obscured cloud members; most of the remaining X-raysources are extragalactic background sources seen throughthe cloud (Getman et al., 2005b).X-ray emission and flaring is thus ubiquitous in PMSstars across the Initial Mass Function (IMF). The X-ray luminosity function (XLF) is broad, spanning 28 <log Lx[erg/s] < 32 (0.5 − 8 keV), with a peak aroundlog Lx[erg/s] ∼ 29 (Feigelson et al., 2005). For compari-son, the contemporarySun emits 26 < log Lx[erg/s] < 27,with flares up to 1028erg/s, in the same spectral band. Re-sults from the more distributed star formation clouds sur-veyed by XEST reveal a very similar X-ray population asin the rich cluster of the Orion Nebula, although confinedto stars with masses mostly below 2M(see the chapterby G¨udel et al.), although there is some evidence the XLFis not identical in all regions (Section 4.1). There is noevidence for an X-ray-quiet, non-flaring PMS population.The empirical findings generate discussion on a varietyof astrophysical implications including: the nature of mag-netic fields in young stellar systems; the role of accretionin


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