Evolution of Gas in Disks

Joan NajitaNational Optical Astronomy Observatory

Steve Strom John Carr

Al Glassgold

Evolution of Gas in Disks: Outline

Why do we care? Observational tools to study gas in disks

» In situ probes of gas in disks» Stellar accretion rates

Using gas to probe the evolutionary status of

» Transitional T Tauri stars» Weak T Tauri stars

Constrains planet formation processes and outcomes What are total (gas) disk masses? (>0.1 M*?) Lifetime of gas in the giant planet region? (MJ, 10Myr?)

» Constrains mode of giant planet formation

Role of Gas in Planet Formation

Gravitational Instability (rapid, large masses)

Core Accretion(slow, modest mass)

Role of Gas in Planet Formation

Lifetime of gas in the terrestrial planet region?

» Residual gas affects terrestrial planet Mp, e,

habitability via gas drag, resonances (Kominami & Ida; Agnor &

Ward)

(1 g/cm2 at 1 AU? cf. 1500 g/cm2 MMSN)

Copyright Lynette Cook, used with permission

How do Gas Disks Evolve?

Theoretical Expectation» Accretion (inner) + spreading» Photoevaporation (outer) » Planet formation

10 AUHollenbach et al. 2000

Copyright Lynette Cook, used with permission

How do Gas Disks Evolve? Observationally

» Dust excess and stellar accretion decline with age

» Gas?

Disk dissipation or grain growth?Debris production? How does scale with Mdot?

Haisch et al. 2001

IR E

xce

ss F

ract

ion

Age (Myr)

log (

Acc

reti

on r

ate

)

log (Age/yr)

Muzerolle et al. 2005

How do Gas Disks Evolve?

Gas drag: rapid inspiral of dust in outer disk?» Grain growth to mm-sizes observed » Dissipates solids in 1Myr; long-lived gaseous disk?

106

106Dust

Gas

S

urf

ace

den

sity

Distance (AU)

Takeuchi & Lin 2005

Tools: Probes of Gaseous Disks

Terrestrial Planet Region:• NIR CO, OH, H2O • UV H2

Kuiper Belt and Beyond:• Millimeter transitions;• NIR H2 ro-vib;• Optical atomic lines

Copyright Lynette Cook, used with permission

Copyright Lynette Cook, used with permissionGiant Planet Region:• Mid-IR atomic and molecular transitions

Kuiper Belt and Beyond: Millimeter Molecular

TransitionsStrengths CO: abundant, low ncrit

Disk sizes (> 100 AU) Disk rotation, M*, i

Challenges Warm surface layer emission + midplane condensation; disk masses from dust Probes mainly large radii

Simon et al. 2000

Aikawa et al. 2002

Millimeter Molecular SurveysCO Surveys Zuckerman et al. (1995): dissipation < 1 Myr

» Depletion a concern More recent studies: WTTS, AeBe stars

» 1 WTTS detected; limited sensitivity» Gas can survive (to 7Myr, >20 AU), mass uncertain(e.g., Duvert et al. 2000; Thi et al. 2001; Dent et al. 2005)

Current Status/Future? Diagnostics e.g., HCO+ probe higher densities, smaller radii ALMA sensitivity + angular res. will probe < 30 AU Models needed to derive mass

Greaves et al. 2004

HCO+ 4-3

Giant Planet Region: MIR Transitions

Strengths Atomic and molecular lines (e.g. H2) may be detectable Probes warm 100K) gas H2 in gas phase, dominates mass

Challenges Models needed to convert warm H2 mass to total Depend on assumed disk structure High res spectra reduce ambiguity

Gorti & Hollenbach 2004

Richter et al. 2005AB AurTEXES/IRTF17m H2

Giant Planet Region: MIR Transitions

Surveys ISO: MJ gas in 20 Myr systems

» low ncrit, extended emission? Unconfirmed by Spitzer or from ground

» high angular + spectral res. (Thi et al.; Richter et al., Sheret et al., Sako et al.)

Challenges Narrow width (r > 10 AU) Weaker emission from small r req. e.g., TEXES/Gemini, TMT

Chen et al. (2004)

Pic: 17m Non-detectionSpitzer

M(warm H2) < 11 ME

CO v=1-0, 2-1, 3-2, 13CO lines detected.

Najita et al.,Brittain et al., Blake & Boogert

Strengths Common 100% CTTS Probes Rin to 1-2 AU 70 km/s FWHM Surprisingly warm gas 1000K gas cf. <400K dust Wide range of column density 10-4 -- 1 g/cm2

4.7m CO Emission from CTTS

Terrestrial Planet Region: CO v=1 Emission

Terrestrial Planet Region: Models

Tg

Td

X-rays

Accretion =1

Gas-Dust =0.1

R = 1 AU

H- 3-body

Neutral reactions

C/CO

H/H2

H2O

CO emission from warm, mid-z region ( > 0.001 g cm-2) Heated by accretion and X-rays

Glassgold et al. (2004)

TW Hya

UX Tau A: H EW=4AAge ~ 8 Myr

V836 Tau: H EW=9AMdot=4x10-10

Age ~ 3 Myr

TW Hya: Mdot=4x10-10-2x10-9

Age ~ 8 Myr

Gas in optically thin inner disks

Najita et al; Rettig et al., Blake et al.

CO Emission from Weak/Transitional TTS

Terrestrial Planet Region: CO v=1 Emission

Challenges Emission strength correlates with accretion Structure in stellar photosphere Models needed to infer total column densities

K-L

Mass

Acc

reti

on

Rate

ClassicalTTS

Carr, Najita 2005TW Hya4.6m CO

Model stellar atmosphere

**

**

*

Indirect Tool: Stellar Accretion

For steady accretion:~ Mdot /

Strengths Given Mdot & , infer Independent measure

Challenges Is relation valid? What is ? Measuring low Mdot

One approach Use measured to determine Is a constant (with r and from source to source)?

Muzerolle et al. 2005

Stellar Accretion Rates

~ Mdot /

If = constant… Wide range in at any age

» ~ 100 g/cm2 at 1 AU for Mdot = 10-8, =0.01 Long-lived gaseous disks

WTTS V836 Tau: 3 Myr, Mdot=4x10-10 or 4 g/cm2 at 1AU TTS St34: 25 Myr, Mdot=2x10-10 or 2 g/cm2 at 1AU

Dynamically significant e.g., producing Earth-like Mp, e

Muzerolle et al. 2005

Evolutionary Status: Transitional TTS

Definition Photospheric at short , excess at long 10% of TTS

Nature? Formed giant planets?Formed planetary cores? Photoevaporation + viscous dissipation?

Constrains timescales either for forming planetary cores, accreting gaseous envelopes, or dissipating disks

Median Taurus SED

TW HyaCalvet et al. 2002

Quillen et al. 2004

log

F

Clarke et al. 2001

Surf

ace

densi

ty

Distance

Case Study: TW Hya = 32g/cm2 at 20 AU (SED)Outer disk is too massive for photoevap to create inner hole Mdot=5x10-10-5x10-9

Evolutionary Status: Transitional TTS

0.1 g/cm2 at20AU,13Myr

Cores or Planets? If =0.01, =5-50 g/cm2 at 1AU

giant planet formation? If =0.0003, =100-1000 g/cm2

core formation?

Measurements of disk gas content can resolve this

Evolutionary Status: Weak TTS

Definition Weakly/non-accreting No IR excess50% of TTS Ages 0.1--10 Myr

Nature? Small initial Mdisk?

failed PF Large initial Mdisk?

successful PFRapid inspiral of dust

possible PF?

Hartmann & Kenyon 1995

Surf

ace

densi

ty

Distance (AU)

Dust

Gas

Takeuchi et al. 2005Mayer et al.

Evolutionary Status: Weak TTS

Nature? Large vs. small initial Mdisk?

» Gravitational instability is quick; WTTS < 1 Myr old?» Search for massive, distant planetary companions

Rapid inspiral of dust» Planet formation possible if cores have formed» Search for gas reservoir

Measuring disk gas content + companion search can resolve this

DustGas

Summary: Evolution of Gas in Disks

An interesting problem! Disk masses and gas dissipation

timescales: Constrain mode(s) of giant planet

formation Outcome of terrestrial planet formation Giant planet migration, etc.

Summary: Evolution of Gas in Disks

Interesting but difficult! Past decade: development of many probes

of gas in disks. - UV, optical, IR, millimeter

In situ diagnostics require - High sensitivity, high resolution observations

(e.g., Spitzer, TEXES/Gemini, ALMA, TMT)- Reliable thermal-chemical models of disks

Stellar accretion rates: dynamically significant reservoirs survive 10 Myr in some systems- How much gas and how frequently requires

calibration of Mdot with gas measurements

Summary: Evolution of Gas in Disks

Gas content probes evolutionary status Transitional TTS: constrains timescales for

»Forming planetary cores»Accreting gaseous envelopes»Photoevaporating disks

Weak TTS: »Failed »Successful »Possibly ongoing planet formation?

Constrain planet formation processes and outcomes