Heating and Ionization of Protoplanetary

Gaseous Disk Atmospheres

TIARA Workshop Presentation byAl Glassgold

December 5, 2005

Heating and Ionization of Protoplanetary

Gaseous Disk Atmospheres

A. The Role of Heating and Ionization

B. Stellar X-Rays and FUV

C. Mechanical Heating

D. Modeling Results

A. INTRODUCTION

The Role of Heating and Ionization

The Objective: Gas-Phase Diagnostics

• Gas is the main constituent of young disks. Much less is known about it than the dust.

• The gas is the reservoir for flows out of the disk, i.e., accretion onto the star, winds from the inner disk including evaporation -- and building giant planets.

• The gas affects the migration of massive bodies.

• Detailed observations are still in the future.

Importance of Heating & Ionization • Together with the dynamics, the processes that heat and

ionize the gas determine the physical properties of the gas (n, T, xe).

• They in turn determine the abundances (and vice versa) and the spectral signatures: heating and ionization affect the diagnostics.

• The results of modeling physical properties are to be compared with observations in an iterative process that eventually leads to an understanding of the evolution of disk gas.

Direct Effects of the Ions Electrons -- excitation & ionization,

with a special role for secondary electrons

Ions -- coupling to neutrals, including momentum transfer (ambipolar diffusion)

-- ion-molecule chemistry

These processes also heat the gas.

• Calculating the ionization is difficult:

disk chemistry is poorly understood, e.g., the role of grains in adsorption, desorption, & surface reactions.

Sources of Heating & Ionization

• Stellar and interstellar radiation:

FUV, X-rays, Cosmic Rays

• Dissipation of mechanical flow energy

in winds, accretion, instabilities, etc.

The ionizing radiations are usually external to the disk; mechanical heating is either external or internal.

• YSOs are strong emitters of X-rays & FUV.

This will be the focus today.

X-ray & FUV Radiation:Comparison of Physical Effects

Property X-rays FUV

Energy keV 12 eV

Absorption inner shells valence shell

Secondary electrons

27 per keV none

Ionization H, H2, He etc.

Max (xe) = 1

C, S, etc

Max (xe) = 10-3

Absorption 1022 cm-2 (1 keV) 5x1020 cm-2

Heating Efficiency

> 50% 3-10 %

B. STELLAR X-Rays and FUV

The Observational Situation

Observation of YSO X-rays • Known since 1970s (UHURU, EINSTEIN) • Extensively studied in the 1990s by:

ROSAT – 0.1-2.5 keV, 2” ASCA – 0.4-10 keV, 30”• Last 5-6 years: Chandra and XMM-Newton

Chandra is especially suitable for young clusters:• sensitive to 0.1 -10 keV X-rays• angular resolution of 0.5” approaches HST

• Chandra Orion Ultradeep Project (COUP): 10-day exposure of the Orion Nebula Cluster

(ApJS, 160 [October] 2005)

COUP X-Ray Spectrum of a Sun-like YSO

0.5 5.0E (keV)

YSO X-ray Emission

Count rate for 567

3. Hard X-rays penetrate large column densities

absorption cross ~ E-2.65: soft X-rays absorbed

1. LX / Lbol = 10-4 – 10-3

4. Variable on all timescales; flares every few days

2. Median luminosity - log LX ~ 30 Median peak luminosity of flares - log LX ~ 31

“hard X-rays”

1.0

5 d 1 d

Peak X-ray Luminosity:0.1 Lsun

From COUP sample of sun-like stars Wolk et al. (ApJS 160, 2005)

XMM-Newton Low Resolution X-ray Spectrum of TW Hya

(Stelzer & Schmitt, A&A, 418, 617, 2004)

keV7.1 keV,27.0 keV,17.0

s erg104.1 130

kTkTkT

LX

hard component

soft components

Ne lines

TW Hya nearby (56 pc) CTTSwith face-on disk.

X-Ray Effects on the YSO Environs BASIC PROCESSES1. Absorption by heavy ion K, L shell electrons2. Quick de-excitation via Auger process plus fluorescence3. Energy degradation of fast electrons in collisions thationize and excite H and He, producing secondary electrons that provide most of ionization.

• The X-ray ionization rate for a sun-like YSO at 1AU is, ignoring attenuation, ζ ≈ 10-9 - 10-8 s-1 -- 8 dex > galactic cosmic ray ionization.

• The unshielded stellar FUV ionization rates (e.g., C, S, etc.) are even larger.

• Shielding makes all the difference

“Typical” TTS UV SpectrumBergin et al. ApJ 591, L159, 4003

heavy solid line – BP Tau (HST)

light line – TW Hya x 3.5 (FUSE)

(flux at 100 AU)

X-ray vs. FUV Ionization TW Hya is a good example: the X-ray & FUV

(91.2 - 110 nm) luminosities are similar,

L(X-rays) = 1.4x1030 ergs s-1

L(FUV) = 9.0x1029 ergs s-1.

But rates, ignoring shielding, are dissimilar, e.g., at 1 AU,

GFUV(CO) = 2x10-4 s-1

ζX(CO) = 8x10-8 s-1.

The difference arises mainly from the energy dependence of the absorption cross section in going from 10 eV to 1 keV. But the smaller

absorption column cancels this advantage.

C. MECHANICAL HEATING

Preliminary Ideas

Mechanical Heating the Disk Surface(GNI04: ApJ 615, 974, 2004)

• A. Wind-disk Interaction – suggested by (Carr et al. 1993). Order of magnitude estimate by GNI04 supports this idea, but a numerical simulation is needed to help pin down the depth of the turbulent mixing layer, etc.

• B. MRI Turbulence – dissipation in surface or mid-plane (then propagated to the surface by MHD waves). Supported by simulations (Miller & Stone 2000). GNI used the formula,

• Where alpha_h is a phenemenolgical parameter

parameter gicalphemomenol a with h

2h

, 4

9

c

Mechanical Heating and Ionization of the Mid-Plane

(Inutsuka & Sano, ApJ 628, L155, 2005)

Mechanically Ionizing the Dead Zone

Inutsuka & Sano advance two mechanisms:• The electric fields generated by the MRI produce

19-MeV electrons that can ionize hydrogen • The turbulence of the MRI mix electrons from the

upper regions of the disk to the midplane

Are these processes really effective?

Order of magnitude estimates suggest that more detailed calculations are needed.

D. MODELING RESULTS

With X-rays and FUV, but not both (yet)

Simple X-Ray Chemistry For Protoplanetary Disk Atmospheres

GNI (ApJ 615, 972, 2004) • Dust growth & depletion:Gas to dust ratio = 100Dust size > 0.01 microns• D’Alessio et al.(1999) dust model for CTTSMdot = 10-8 Msun per yr • Chemistry focus: H2, CO, & H2O (+ intermediaries)ionic & neutral reactions25 species, 115 reactions

accretion heating

X-rays

Chemical Transitions

Gas Temperature Inversion

1 AU

1021 cm-2

FUV Heating of Small Particles(Nomura & Millar, A&A, 438, 923, 2005)

• Self-consistent hydrostatic model of TTTS disk atmosphere for both gas and dust, using simplified gas chemistry and Draine & Weingartner (2001) dust model. • The FUV radiation field of TW Hya at 100nm is 3 timessmaller than Bergin et al. (2003). See Kamp & Dullemond (2004) for similar results for an active TTS.

The temperature inversionis smaller by a factor of several than produced by X-rays.

FAR UV & X-ray Chemistry(Markwick et al. A&A 385, 632, 2002)

• Active TTS: assume Tgas = Tdust ; low, schematic X-ray ionization rate; FUV unspecified.

• Adapt chemistry of Willacy et. al. (1998): adsorption & desorption can occur on grains according to their temperature; otherwise the chemistry is gas phase.

• High ionization rates enhance the abundance of ions (e.g., HCO+) & radicals (e.g. CN).

• High abundances of simple organics, e.g., CH4, is about as abundant as CO in the inner disk.

Sample Results of Markwick et al.

CS NH3

HCO+HCS+

midplane

surface

2 4Radius (AU)

8

midplane

surfacesurface

surface

Gibb (2004) et al. failed to detect the predicted CH4 in the NIR .Lahuis et al. (2005) have detected gaseous CO, HCN, & C2H2 in absorption towards one edge-on disk, with ratios in agreement with Markwick et al.

Summary of Disk Chemistry

• A comprehensive disk chemistry still needs

to be developed.

• Both X-rays & FUV are required for the thermal & chemical treatment of disk gas.

• Observed X-ray & FUV luminosities & spectral distributions should be used.

• Dust and PAHS are crucial for X-ray & FUV radiation transfer, heating, & surface chemistry.

CONCLUSIONS• Detailed observations are still in future.

• Need good diagnostics, and not just for surface regions.

• Modeling requires a sound thermal-chemical basis.

• Heating & ionization are fundamental.

• Encouraging results (with special facilities):

NIR CO & UV H2 for inner disk

Sub-mm CO for outer disk