Non-planet mechanisms for clearing and sculpting discs

1
Non-planet mechanisms for clearing and sculpting
discs

• James Owen (IOA -gt CITA)
• Barbara Ercolano (IOA -gt LMU)

2
Hope to convince you

• (Xray) photoevaporation now well understood
• Significant factor in disc evolution
• (Probably) responsible for ultimate clear out of
  protoplanetary discs
• En route, produces structures with properties
  overlapping those produced by planets

?
3
Are observed structures (gaps, holes) pure dust
phenomena?
Probably not.

• Sharp edge due to radiation pressure?
• Sharp edge due to photophoresis (Krauss et al
  2007)?

See Dominik Dullemond 2011
Hard to suppress small dust production
4
Do discs clear via viscous accretion?

• UV excess gt do accrete at rates M_disc/age
• Far too slow decline at late times.

?
(Lynden-Bell Pringle 1974, Hartmann et al
1998)
Phenomenological description as due to action of
(pseudo) viscosity e.g for R (
gt get similarity solution
M M_in ( 1 t /
t )
?
? 1/R )
-1.5
?
in
5
Need extra gas clearing mechanism
To clear discs within 5 Myr, to make observed
hole/gap structures

• Planets ?
• Photoevaporation ?
• Clearing by MRI driven winds?

Suzuki et al 2010 inner hole forms very early
(0.1 A.U. _at_ 105 years,

1 A.U. _at_ 106 years)
? Halts accretion onto star from very early times
also.
Actually destination of wind unclear (doesnt
attain escape velocity.)
6
Key facts for understanding Xray photoevaporation

• Temperature of Xray heated gas set by
• In a Parker wind, sonic transition occurs at
  radius where

???? max T is 1-2 x 10 4 K
?
? isothermal , spherical
7
Results of radiation hydrodynamical modeling of
Xray photoevaporation (Owen et al 2010, 2011b)
T 2000 K ?
???????????? sonic surface where
still holds approximately
? ? ? T 10,000 K

• R gt T and (beyond 10 A.U.) fixes n
  and hence mass flux

8
Result

• Proportional to L_x
• Independent of M_
• Doesnt depend on properties of underlying disc!

9
Discs with inner holes
Owen et al 2010

• As vary R_hole , topology of innermost streamline
  and variation of c_s and u with scaled distance
  along streamline is invariant

???? photoevaporation rate INDEPENDENT of inner
hole size ( 10-8 solar mass/yr for L_X -
1030 erg/s)
Owen et al 2011b)
10
What about other radiation sources?

• EUV? Cant penetrate Xray wind
• FUV? Within 100 A.U. only heats below Xray sonic
  surface - doesnt change mass loss rates

? ? ? ? ?
? ?

Owen et al 2011 b)
(But may affect structure of subsonic region See
Gorti Hollenbach 2004,2008,2009)
ALSO MAY BE IMPORTANT MASS LOSS MECHANISM AT gt
100 A.U.
11
Combining photoevaporation with viscous evolution
?
Constant L_X
Owen et al 2011a)
I II
Initial ?
75 total lifetime ?
Stage I
76 total lifetime ?
77 total lifetime ?
78 total lifetime ?
Stage III
79 total lifetime ?
Etc.
Four stage evolution
Stage II

• I Viscous dominated
• II Draining inner hole
• III Outer disc clearing
• IV Thermal sweeping

? ? ? ?

• ?
• 76
• total
• lifetime

? 80
?Accreting, dust free (migration), lt10 AU
? Empty inner hole, gt 10 AU
? NEW
12
Stage IV thermal sweeping

• Once Xrays penetrate a radial distance H into
  disc, heated gas evaporates vertically in plume
  flow
• Residual disc clears on dynamical time of inner
  rim ( 10s of A.U.)

Sets in when column density at inner rim is 0.5
g/cm2 Remove few -gt 10 Jupiter masses of gas
13
Thermal sweeping limits lifetime of non-accreting
hole stage (stage III)

• Fraction of lifetime spent with hole (stage II
  III) 10
• Fraction of lifetime spent with transparent
  accreting hole (stage II) 5
• Fraction of lifetime spent with non-accreting
  hole (stage III) 5

14
Which inner hole sources could be due to
photoevaporation?
????????????
Around half (those in shaded region)

• Systems evolve ? as inner holes drain
• Initial Mdot depends on L_X
• Initial radius depends on M_

Owen et al 2011b)
? These cant
? These are upper limits
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
15
Evidence for Xray photoevaporation
XrayErcolano Owen 2010 EUV Alexander 2008

• Both Xray and EUV photoevaporation explain line
  profiles of NeII 12.8 ?m
• Only Xray photoevaporation explains low velocity
  ( 5 km/s) component of OI 6300 in T Tauri stars
  (cf EUV models Font et al 2004 )

?
Cf observed profiles for TW Hydra, Pascucci
Sterzik 2009
?
.but note lack of blueshifted OI 6300 in TW
Hyda Pascucci et al 2011
16
Evidence for Xray photoevaporation?

• High L_X stars lose discs earlier - implies
• WTTs should have higher L_X on average.
• Well known observational correlation. Usually
  argued that Xrays suppressed/absorbed by
  accretion perhaps instead accretion suppressed
  by Xrays.

Preibisch et al 2005, Greogory et al 2007
Population synthesis Owen et al 2011a
?
17
Dependence on metallicity
A DISCRIMINANT FOR DISC CLEARING PLANETS V.
PHOTOEVAPORATION?

• Photoevaporation more efficient at low Z lower
  dust extinction gt Xrays heat to higher column

EXPECT SHORTER DISC LIFETIMES AT LOW Z
18
Disc lifetime increasesstrongly with
decreasing Z if its instead set by time
required for planet formation
-5/2
Z
? ?
(see Ercolano Clarke 2009)
-11/2
Z

• A possible observational discriminant?

19

• Recent claim of shorter disc lifetimes in lower Z
  environment

further studies at low Z may hold the key to
discriminating between photoevaporation and
planet formation
20
CONCLUSIONS

• Xrays can drive photoevaporative winds of 10-8
  M_sun/yr at upper end of XLF like EUV winds,
  these produce a RAPID clearing phase but the Xray
  wind cuts in at much higher accretion rate.
• Produce small holes at range of accretion rates
  but no accreting holes beyond 20 A.U. expect
  accreting and non-accreting holes to have similar
  frequency.

? ? ? ?
Xray photoevaporation gt 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!)
?
21
(No Transcript)
22
Pre-main sequence stars in the Xray

• Properties well characterised
• Correlation with stellar mass (but broad spread
  at given mass)
• NOT CAUSED BY ACCRETION (some anti-correlation
  between L_X and strength of disc diagnostics)
• Previous studies of Xray-disc interaction focused
  on non-thermal ionisation by hard Xrays (gt
  implications for MRI)

Preibisch et al 2005
See also Albacete-Colombo et al 2007
Neuhauser et al 1995, Flaccomio et al 2003
23
Synthetic X-EUV spectrum of T Tauri star
From Ercolano et al 2009a
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 ( 104 K)
component based on emission measures of RS CVn
binaries (Sanz Forcada et al 2002)
?

• 
  
  ?

EUV 1041 /s (cf Alexander et al 2005)
L_X 2 x 1030 erg/s
24
Coronal emission doesnt necessarily reach disc

• Effect on spectrum of different screening columns
  close to star

EUV screened out by N_H 1020/cm2
Xrays penetrate to N_H 1022/cm2
25
Use strong correlation between T in X ray heated
gas and ionisation parameter
HOW TO SIMULATE HYDRODYNAMICALLY

• F_X/n
• (defined locally)

26
ZEUS 2D SIMULATION T IN XRAY HEATED REGION SET
BY ?

• Owen, Ercolano, Clarke Alexander 2009

Mass loss rate 1.5 x 10-8 M_?/yr
27
Gas T set by dust T
Base of Xray heated region
?
Xray heated region few 1000 K
?
?

• Use MOCASSIN on converged flow structure to check
  temperature parametrisation

?
28
WHEN COMBINE EITHER EUV OR X-EUV WIND PROFILES
WITH VISCOUS EVOLUTION

• 3 stage evolution
• Normal viscous evolution
• Creation of inner hole
• Evaporation of outer disc

29
Similarities between EUV X-EUV models
3 stage evolution Gap opens at few A.U.
Inner hole and outer disc clearing are both
fast cf first (viscous) stage

• Differences

• 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

30
Observational diagnostics of X-EUV
photoevaporation
Ercolano Owen 2010

• Also agrees with lack of observed blueshift in
  inner hole source GM Aur (optically thin so line
  symmetric)

31
Are observed inner holes due to
photoevaporation?Can explain many inner
hole sources but not very large holes with high
accretion rate
Tracks produced by models with different L_X_
?
Owen et al in prep.
? ? ?

Herschel and ALMA will improve statistics
32

• Correlation between accretion rate and
• L_X in T Tauri stars

?
Broad spread in L_X at given mass (Preibisch et
al 2005)
Red no wind Black wind for L_X 2 .1030 Blue
low L_X (wind x 0.1)
Note phase of photoevaporation starved accretion
prior to rapid Decline see Drake et al 2009
? ?
33
The observational situation
L_X cf average for stars of that mass
34
But not always..
No clue from SED

• IRS 48
• (Geers et al 2007)

Inner hole in image
35
Evidence for photoevaporation

• Best evidence to date from line profiles of Ne
  II 12.8 micron emission - suggests outflow at
  10 km/s (gt ionised gas)

Alexander 2008
36
(No Transcript)
37
Resulting Xray mass loss rates10 x higher than
EUV
Mass loss peaks at around 10-20 A.U.
?
(cf EUV, peaks here)