Transcript High Energy Processes in Young Stellar Objects

High Energy Processes
in Young Stellar Objects
Ji Wang
Department of Astronomy
University of Florida
Young Stellar Objects
• History
– Collapsing rotating cloud
• Laplace 1796
– T Tauri Star
• Alfred Joy 1945
• Features (interior, surface,
• luminosity)
White & Hillenbrand 2004
Observation
• Outflows
– P cygni line profile
• Disk
– By millimeter interferometer and HST
• Accretion
– Evelope-disk-star
• Magnetic activity
– Flare
Stages of YSOs
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Class 0 (10^4 yrs)
– Massive cold evelope
– Disk, outflow formed
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Class I (10^5 yrs)
– Disk appears
– Larger outflowing angle
– Lower mass loss rate
•
Class II, CTT (10^6 yrs)
– Outflow exists in young stage
– Strong wind
•
Class III, WTT (10^6 yrs)
– Simple Blackbody SED
•
Post T Tauri
– Long missing from YSO sample
– Emerging from X-ray surveys
– Distinguished by lithium abundance
van Dishoeck and Blake 1998, ARAA 36, 317
Stages of YSOs
Feigelson 1999
Origins of X-ray
• Magnetic field rooted at star surface are resposible for
flaring
– Enhanced solar-type magnetic-activity
– a-w dynamo
• Successfully explains butterfly diagram
• But fails when comes to fully convective situation
– Alternatives
• Release of gravitational energy (Lamzin 1996)
• Colliding CTT star winds (Zhekov 1994)
• Star-disk interaction
– Successfully explains generation of wind and collimated outflows
– Challenged by by growing argument over field structure far
larger than those seen on the Sun
– Dipole magnetic geometry and steady-state assumption are too
simple to be ture
Possible magnetic geometries in
YSOs
Feigelson 1999
Five possibilities
• Solar type multipolar fields with both footprints rooted in stellar
photosphere
– a-w type dynamo applicable
– Fossil fields inherited from parent molecular cloud
• Field line connecting the star to circumstellar disk at corotation
radius
– Supported by rotation spin down
– Magnetically funneled accretion
– Collimated outflows in CTT stars
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Field lines above corotation radius
Magnetic loop with both feet in the disk
Binary YSOs but less important
Reconnection and displacement of footprint are more likely to occur
in YSOs due to more possible magnetic configuration offered by
star-star, star-disk, disk-disk interaction
Tracer of magnetic activity
• X-ray emission
– Continuous emission, optically thin thermal bresmsstrahlung with
associated ionized metal emisson lines from multitemperature
plasmas with 1<Tx<100MK (Montmerle 1991)
– Powerful flares (T~10^6-10^8 K)
– Plasma density and magnetic strength can be inferred
• Optical studies
– Zeeman effect
– Star spot
– Chromosphere
• Gyrosymchrotron radio-continuum
– Highly variable
– Produced by mildly relativistic electrons with energy around 1
MeV spiraling in ~1 G fields (Dulk 1985)
X-ray properties of T Tauri stars
• CTT stars and WTT stars are similar in the context of Xray properties because it demostrate little or no
dependence on disk interaction
• X-ray T Tauri Stars are variable (days), sometimes show
high amplitude flare with time scale of hours
• Spectra can be modeled as soft component (Tx~2-5 MK)
and hard component (Tx~15-30 MK) or higher
temperature component
• Relations between X-ray luminosity and stellar properties
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Lx/Lbol~10^-3
Lx~rotation indicator
Fx~Stellar surface temperature
X-ray luminosity mass correlation
Effects on circumstellar
environment
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Ionization
Heating
Modification of gas chemistry
Changing grain composition
Ionization and heating
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Ionization-coupling gas and magnetic field
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Low ionization
Regulating accretion and wind
Coupling disk and outflow
Affecting planetary formation
Compared with UV photons
– Extended region of low ionization zone
– Have potential effect even in deeply embedded environment
– Dominating cosmic ray ionization ~4000AU (Krolic & Kallman 1983)
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Ionization mechanism
– Photoionization~1 KeV
– Compton ionization~20 KeV
– Cross section~E^-2.5 and Z^3
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Continuous heating source in Molecular Cloud
– Recombination time 10 yrs
– Flare recurrence time considerably less than 10 yrs
– Dominant source within 0.1 Pc in MC
Conclusion
• High energy processes are prevalent in low mass YSOs
• If X-ray emission begins in the earliest Class 0 phase,
then YSO ionization may crucially affect the gravatational
collapse of star formation
• X-ray emission is prevalent in the Class I-II phases, Xray ionization is quite likely to play a central role in the
astrophysics and evolution of the circumstellar disk
– Introduction of MHD turbulence and viscosity, thereby regulating
accretion onto the star
– Magnetically coupling star to disk, and disk to outflow
Referrences
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Feigelson, E. D., & Montmerle, T. 1999, ARA&A, 37, 363
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Lamzin SA, Bisnovatyi-Kogan GS, Errica L,Giovannelli F, Katysheva NA, Rossi C, Vittone AA. 1996. Astro. Astrophys. 306:877-91
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Zhekov AS, Palla F, Myasnikov AV. 1994. MNRAS 271:667-75
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Monterle T, Feigelson ED, Bouvier J, Andre P. 1993. In Levy & Lunine 1993, 689-717
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Dulk GA. 1985. Annu. Rev. Astron. Astrophys. 23:169-224
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Krolik JH, Kallman TR. 1983. Astrophys. J. 267:610-24
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Chandra Orion Ultradeep Project census of X-ray stars in the BN-KL and OMC-1S regions
N. Grosso, E. D. Feigelson, K. V. Getman, L. Townsley, P. Broos, E. Flaccomio, M. J. McCaughrean, G. Micela, S. Sciortino, J. Bally, N.
Smith, A. A. Muench, G. Garmire, F. Palla
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X-ray Emission from Orion Nebula Cluster Stars with Circumstellar Disks and Jets
Joel H. Kastner, Geoffrey Franz, Nicolas Grosso, John Bally, Mark J. McCaughrean, Konstantin Getman, Eric D. Feigelson, Norbert S.
Schulz
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Bright X-ray flares in Orion young stars from COUP: evidence for star-disk magnetic elds?
F. Favata, E. Flaccomio, F. Reale, G. Micela, S. Sciortino, H. Shang, K. G. Stassun, E. D. Feigelson
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The Origin of T Tauri X-ray Emission: New Insights from the Chandra Orion Ultradeep Project
Thomas Preibisch, Yong -Cheol Kim, Fabio Favata, Eric D. Feigelson, Ettore Flaccomio, Konstantin Getman, Giusi Micela, Salvatore
Sciortino, Keivan Stassun, Beate Stelzer, Hans Zinnecker
McCaughrean 2005
OMC 1-South cloud core
http://www.astro.psu.edu/coup/Protostars.html
Grosso et al 2005
Kastner et al 2005
Favata et al 2005
Preibisch et al 2005
Hartmann 1997
Thank you