Non-planet mechanisms for Non-planet mechanisms for clearing and sculpting discsclearing and sculpting discs
• James Owen (IOA -> CITA)James Owen (IOA -> CITA)
• Barbara Ercolano (IOA -> LMU)Barbara Ercolano (IOA -> LMU)
Hope to convince you:Hope to convince you:• (Xray) photoevaporation now well (Xray) photoevaporation now well
understoodunderstood
• Significant factor in disc evolutionSignificant factor in disc evolution
• (Probably) responsible for ultimate clear out (Probably) responsible for ultimate clear out of protoplanetary discsof protoplanetary discs
• En route, produces structures with properties En route, produces structures with properties overlapping those produced by planetsoverlapping those produced by planets
Are observed structures Are observed structures (gaps, holes) pure dust (gaps, holes) pure dust
phenomena?phenomena?• Sharp edge due to radiation Sharp edge due to radiation
pressure?pressure?
• Sharp edge due to photophoresis Sharp edge due to photophoresis (Krauss et al 2007)? (Krauss et al 2007)?
Probably not….
See Dominik & Dullemond 2011
Hard to suppress small dust production
Do discs clear via viscous Do discs clear via viscous accretion?accretion?
• UV excess => do accrete at rates ~ UV excess => do accrete at rates ~ M_disc/ageM_disc/age
• Far too slow decline at late times….Far too slow decline at late times….
Phenomenological description as due to action of (pseudo) viscosity e.g for
~ R ( =>
get similarity solution:
M = M_in ( 1 + t / t )
(Lynden-Bell & Pringle 1974,
Hartmann et al 1998)
-1.5
in
~ 1/R )
Need extra gas clearing Need extra gas clearing mechanismmechanism
• Planets ?Planets ?
• Photoevaporation ?Photoevaporation ?
• Clearing by MRI driven winds? Clearing by MRI driven winds?
To clear discs within ~ 5 Myr, to make observed hole/gap structures…
Suzuki et al 2010: inner hole forms very early (0.1 A.U. @ 10^5 years,
1 A.U. @ 10^6 years) Halts accretion onto
star from very early times
also….Actually destination of `wind’ unclear (doesn’t attain escape velocity….)
Key facts for understanding Key facts for understanding Xray photoevaporationXray photoevaporation
• Temperature of Xray heated gas set byTemperature of Xray heated gas set by
• In a Parker wind, sonic transition occurs at In a Parker wind, sonic transition occurs at radius whereradius where
= isothermal , spherical
max T is 1-2 x 10^ 4 K
Results of radiation Results of radiation hydrodynamical modeling of hydrodynamical modeling of
Xray photoevaporation (Owen Xray photoevaporation (Owen et al 2010, 2011b)et al 2010, 2011b)
• R => T and (beyond 10 A.U.) fixes R => T and (beyond 10 A.U.) fixes n and hence mass fluxn and hence mass flux
sonic surface where
still holds approximately
T ~ 2000 K
T ~ 10,000 K
Result:Result:
• Proportional to L_xProportional to L_x
• Independent of M_*Independent of M_*
• Doesn’t depend on properties of Doesn’t depend on properties of underlying disc!underlying disc!
Discs with inner holesDiscs with inner holes
As vary R_hole , topology of innermost As vary R_hole , topology of innermost streamline and variation of c_s and u with streamline and variation of c_s and u with
scaled distance along streamline is scaled distance along streamline is invariantinvariant
photoevaporation rate INDEPENDENT of inner
hole size (~ 10^{-8} solar mass/yr for L_X - 10^30 erg/s)
Owen et al 2011b)
Owen et al 2010
What about other radiation What about other radiation sources?sources?
• EUV? Can’t EUV? Can’t penetrate Xray windpenetrate Xray wind
• FUV? Within 100 FUV? Within 100 A.U. only heats A.U. only heats below Xray sonic below Xray sonic surface - doesn’t surface - doesn’t change mass loss change mass loss ratesrates
(But may affect structure of subsonic region:
See Gorti & Hollenbach 2004,2008,2009)
Owen et al
2011 b)
ALSO MAY BE IMPORTANT MASS LOSS MECHANISM AT > 100 A.U.
Combining Combining photoevaporation with photoevaporation with
viscous evolution:viscous evolution:Initial
75% total lifetime 76% total lifetime 77% total lifetime
78 % total lifetime
79 % total lifetime
76 % total lifetime
80%
Etc.Four stage evolution:
Stage I
Stage II
Stage III
I Viscous dominatedI Viscous dominatedII Draining inner holeII Draining inner holeIII Outer disc clearingIII Outer disc clearingIV Thermal sweepingIV Thermal sweeping Accreting, dust free (migration), <10 AU
Empty inner hole, > 10 AU
I
II
Constant L_X
Owen et al 2011a)
NEW
Stage IV: thermal sweepingStage IV: thermal sweeping• Once Xrays penetrate a radial Once Xrays penetrate a radial
distance ~ H into disc, heated gas distance ~ H into disc, heated gas evaporates vertically in `plume evaporates vertically in `plume flow’flow’
• Residual disc clears on ~ dynamical Residual disc clears on ~ dynamical time of inner rim (~ 10s of A.U.)time of inner rim (~ 10s of A.U.)
Sets in when column density at inner rim is
~ 0.5 g/cm^2
Remove few -> 10 Jupiter masses of gas
Thermal sweeping limits Thermal sweeping limits lifetime of non-accreting lifetime of non-accreting
hole stage (stage III)hole stage (stage III)• Fraction of lifetime spent with hole (stage II Fraction of lifetime spent with hole (stage II
+ III) ~ 10%+ III) ~ 10%
• Fraction of lifetime spent with `transparent Fraction of lifetime spent with `transparent accreting’ hole (stage II) ~ 5%accreting’ hole (stage II) ~ 5%
• Fraction of lifetime spent with non-Fraction of lifetime spent with non-accreting hole (stage III) ~ 5% accreting hole (stage III) ~ 5%
Which inner hole sources Which inner hole sources could be due to could be due to
photoevaporation?photoevaporation?• Systems evolve Systems evolve
as inner holes as inner holes draindrain
• Initial Mdot Initial Mdot depends on L_Xdepends on L_X
• Initial radius Initial radius depends on M_*depends on M_*
Cyan = Brown et al 09, blue = Cieza et al 10,
Black open - Ercolano et al 09, Black filled =
Espaillat et al 08,09, red= Kim et al 09, magenta -
Merin et al 10, green = Najita et al 10
These are upper limits
Around half (those in shaded region)
These can’t
Owen et al 2011b)
Evidence for Xray Evidence for Xray photoevaporationphotoevaporation
Both Xray and EUV photoevaporation Both Xray and EUV photoevaporation explain line profiles of NeII 12.8 explain line profiles of NeII 12.8 mm
Only Xray photoevaporation explains low Only Xray photoevaporation explains low velocity (~ 5 km/s) component of OI 6300 velocity (~ 5 km/s) component of OI 6300 in T Tauri stars (cf EUV models: Font et al in T Tauri stars (cf EUV models: Font et al
2004 )2004 )
Xray:Ercolano & Owen 2010
EUV: Alexander 2008
Cf observed profiles for TW Hydra, Pascucci & Sterzik 2009
…….but note lack of blueshifted OI 6300 in TW Hyda ……Pascucci et al 2011
Evidence for Xray Evidence for Xray photoevaporation?photoevaporation?
• High L_X stars lose discs earlier - impliesHigh L_X stars lose discs earlier - impliesWTTs should have higher L_X on average.WTTs should have higher L_X on average.
Well known observational correlation. Well known observational correlation. Usually argued that Xrays Usually argued that Xrays suppressed/absorbed by accretion: suppressed/absorbed by accretion: perhaps instead accretion suppressed perhaps instead accretion suppressed by Xrays…….by Xrays…….
Preibisch et al 2005,
Greogory et al 2007
Population synthesis: Owen et al 2011a
Dependence on Dependence on metallicity:metallicity:
Photoevaporation more efficient at low Z: Photoevaporation more efficient at low Z: lower dust extinction => Xrays heat to higher lower dust extinction => Xrays heat to higher
columncolumnEXPECT SHORTER DISC LIFETIMES AT LOW Z
A DISCRIMINANT FOR DISC CLEARING: PLANETS V. PHOTOEVAPORATION?
Disc lifetime **increases*Disc lifetime **increases*strongly with decreasing Z strongly with decreasing Z if it’s instead set by time if it’s instead set by time
required for planet required for planet formationformation
• A possible observational discriminant?A possible observational discriminant?
Z
Z
-5/2
-11/2
(see Ercolano & Clarke 2009)
• Recent Recent claim of claim of shorter shorter disc disc lifetimes in lifetimes in lower Z lower Z environmeenvironmentnt
…further studies at low Z may hold the key to discriminating between photoevaporation and planet formation
CONCLUSIONSCONCLUSIONSXrays can drive photoevaporative winds of Xrays can drive photoevaporative winds of
10^-8 M_sun/yr at upper end of XLF: like 10^-8 M_sun/yr at upper end of XLF: like EUV winds, these produce a RAPID clearing EUV winds, these produce a RAPID clearing
phase but the Xray wind cuts in at much phase but the Xray wind cuts in at much higher accretion rate.higher accretion rate.
Produce small holes at range of accretion Produce small holes at range of accretion rates but no accreting holes beyond ~ 20 rates but no accreting holes beyond ~ 20 A.U.; expect accreting and non-accreting A.U.; expect accreting and non-accreting
holes to have similar frequency.holes to have similar frequency. Xray photoevaporation ==> line diagnostics ([Ne II] 12.8 m and [OI] 6300 lines )
Xray photoevaporation: shorter disc lifetimes at low Z (opposite to clearing by planet formation)
Explaining higher L_x in general in discless stars (shorter lifetimes)
Predicts shorter disc lifetimes at low Z (opposite to planets!)
Pre-main sequence stars in Pre-main sequence stars in the Xraythe Xray
• Properties well characterisedProperties well characterised
• Correlation with stellar mass (but broad spread at Correlation with stellar mass (but broad spread at given mass)given mass)
• NOT CAUSED BY ACCRETION (some anti-correlation NOT CAUSED BY ACCRETION (some anti-correlation between L_X and strength of disc diagnostics)between L_X and strength of disc diagnostics)
• Previous studies of Xray-disc interaction focused Previous studies of Xray-disc interaction focused on non-thermal ionisation by hard Xrays (=> on non-thermal ionisation by hard Xrays (=> implications for MRI)implications for MRI)
Preibisch et al 2005
See also Albacete-Colombo et al 2007
Neuhauser et al 1995, Flaccomio et al 2003
Synthetic X-EUV spectrum Synthetic X-EUV spectrum of T Tauri starof T Tauri star From Ercolano et al 2009a
EUV = 10^41 /s (cf Alexander et al 2005) L_X= 2 x 10^{30} erg/s
Generated by plasma code of Kashyap
& Drake 2000 from Chandra Xray
emission measures of T Tauri stars by
Maggio et al
2007 and a low T (~ 10^4 K)
component based on emission
measures of RS CVn binaries (Sanz
Forcada et al 2002)
Coronal emission doesn’t Coronal emission doesn’t necessarily reach discnecessarily reach disc
• Effect on spectrum of different Effect on spectrum of different screening columns close to starscreening columns close to star
EUV screened out by N_H =10^20/cm^2 Xrays penetrate to N_H = 10^22/cm^2
Use strong correlation Use strong correlation between between
T in X ray heated gas and T in X ray heated gas and ionisation parameterionisation parameter
= F_X/n
(defined locally)
HOW TO SIMULATE HYDRODYNAMICALLY:
ZEUS 2D SIMULATION: T IN XRAY ZEUS 2D SIMULATION: T IN XRAY HEATED REGION SET BY HEATED REGION SET BY
Owen, Ercolano, Clarke & Alexander 2009
Mass loss rate = 1.5 x 10^{-8} M_/yr
Use MOCASSIN on converged Use MOCASSIN on converged flow structure to check flow structure to check
temperature parametrisationtemperature parametrisation
Gas T set
by dust T
Base of Xray heated region
Xray
heated region
~few 1000 K
WHEN COMBINE EITHER WHEN COMBINE EITHER EUV OR X-EUV WIND EUV OR X-EUV WIND
PROFILES WITH VISCOUS PROFILES WITH VISCOUS EVOLUTIONEVOLUTION
3 stage evolution:3 stage evolution:a)a) Normal viscous evolutionNormal viscous evolution
b)b) Creation of inner holeCreation of inner holec)c) Evaporation of outer discEvaporation of outer disc
Similarities between EUV Similarities between EUV & &
X-EUV modelsX-EUV models
Differences:Differences:
3 stage evolution
Gap opens at few A.U.
Inner hole and outer disc clearing are both fast cf first (viscous) stage
Gap opens at much higher accretion rate in X-EUV case:
Higher accretion rate on star during inner hole draining
10 x higher outer disc mass when gap opens
Observational diagnostics Observational diagnostics of X-EUV photoevaporationof X-EUV photoevaporation
Also agrees with lack of observed Also agrees with lack of observed blueshift in inner hole source GM Aur blueshift in inner hole source GM Aur
(optically thin so line symmetric)(optically thin so line symmetric)
Ercolano & Owen 2010
Are observed inner holes due to Are observed inner holes due to photoevaporation?photoevaporation?
Can explain many inner hole Can explain many inner hole sources but not very large holes sources but not very large holes
with high accretion rate with high accretion rate
Tracks produced by models with
different L_X_
Owen et al in prep.
Herschel and ALMA will improve
statistics
• Correlation between accretion rate Correlation between accretion rate andand
L_X in T Tauri starsL_X in T Tauri stars
Red = no wind
Black = wind for
L_X =2 .10^30
Blue = low L_X
(wind x 0.1)
Broad spread in L_X at given mass
(Preibisch et al 2005):
Note phase of photoevaporation starved
accretion prior to rapid
Decline : see Drake et al 2009
The observational The observational situationsituation
L_X cf average for
stars of that mass
But not always…..But not always…..
Inner hole in image
No clue from SEDIRS 48
(Geers et al 2007)
Evidence for Evidence for photoevaporation:photoevaporation:
• Best evidence to date from line Best evidence to date from line profiles of [Ne II] 12.8 micron profiles of [Ne II] 12.8 micron emission - suggests outflow at ~ 10 emission - suggests outflow at ~ 10 km/s (=> ionised gas)km/s (=> ionised gas)
Alexander 2008
Resulting Xray mass loss Resulting Xray mass loss ratesrates
10 x higher than EUV10 x higher than EUV
Mass loss peaks at around 10-20 A.U.
(cf EUV, peaks here)