Evolving disks around post-AGB stars

● Perhaps 10% of central stars of planetary nebulae show compact dust disks

● What are these disks, how do they form, and ho do they evolve?

● Do they survive into the white dwarf phase?

Disk in the Helix nebula● Discovered Su et al. 2007

● Dust excess, interpreted as remnant debris disk

Red Rectangle

● Post-AGB star

● Binary period 318 days

● Waters et al. 2008: discovery of oxygen-rich dust disk ~ 2 10^-3 Msun

Post-AGB dust disks

● Commonly seen around binary post-AGB stars

● Period ~ 1 year

● Circumbinary disk

● Accretion of depleted gas

● Not seen around post-common-envelope systems

● Less common in planetary nebulae

● High crystalline fractions

● Inner edge at sublimation temperature

● Large grains compared to non-disk nebulae (mm: de Ruyter 2006, Sahai et al. 2011)

● Always oxygen-rich

● Dust masses ~10^-3 Msun (Sahai et al. 2011)

Formation

● Captured from the AGB wind

● Angular momentum transfer from the binary companion

(limits mass to 0.1-0.01 Msun)

● Dust in disk formed in the AGB wind● Proven by depletion of refractory elements in the star

● Disk formed over ~10^5 - 10^6 yr● From crystalline fraction and composition

Ant Nebula

● Bipolar PN

● Compact non-stellar core

● Strong IR source in core

● Observed with the VLTI

● Combining VLT 8-m unit telescope

● Effective resolution 10 mas

● Mid-infrared (10 micon) observations

Interferometry results

Ant nebula disk

● PN disk with strong similarities to post-AGB disks● But lower mass

● Inner edge at sublimation temperature

● Aligned with nebula minor axis

● Star likely a binary● Based on morphology (Soker's theorem)

Chesneau, Lykou et al

CPD -56 8032

M2-29 Bulge PN

H-alpha Dust

Eclipse of the century

M2-29

● Decline caused by eclipse behind a circum-binary disk

● Secondary eclipse?● P=17 yr

● Fast jitter during eclipse● 23-day low-mass

companion

Hajduk et al.

M 2-29 gas+dust disk● Compact source seen in

dust and in gas emission lines

● Ha, OIII, OI

● HST shows diameter < 250 AU

● Density n_e ~ 6 10^5

● Mass (gas) 10^-4 Msun

● Line widths decrease from 36 km/s in [OIII] to a few km/s in [OI]

● Consistent with rotational velocities, decreasing with distance

Disk evaporation

● Current mass 10^-4Msun

● Current age 5 10^3 yr

● Ionized gas: thermal velocities exceed rotational velocities

● Disk evaporation caused by ionization

Gesicki et al. 2010

● (Disk) wind Mdot ~ 10^-8 Msun/yr

● Disk evaporates over a time scale of ~10^4 yr

● Comparable to age of the disk

● Wind appears to be decreasing with time

Disk evolution

● Initial gas mass ~ 10^-2 – 10^-1 Msol

● decreases rapidly after ionization

● Dust mass decreases from 10^-3 to ~10^-7 Msun

● Remnant disk stabilizes when star enters the white dwarf cooling track

Dust evolution

● Grains in AGB winds have sizes of ~1 micron ● (Norris et al. 2012)

● Dust in post-AGB disks grow to mm size (and beyond?)● Waters et al. 1998 speculated that planets could form

in them

● Large particles left behind when disk evaporates● If planets / planetsimals: orbits may be unstable; high

collision rate

● Dust disk can contract as star fades● Reducing sublimation radius

PNe: Post-AGB disks or debris disks?

● Bilikova et al 2012: 8 out of 72 PNe have dust disks

● If post-AGB disks● Likely wide binaries

● Full range of dust sizes

● Dust mass decreasing with evolution

● If main sequence debris disks● No direct relation to PN morphology

● Dust content may increase with time

White dwarfs

● Do debris disks around white dwarfs derive from pre-existing systems or from post-AGB disks?

● Perhaps both● Single stars: pre-existing planetary systems

● Binaries (p = 0.1-10 yr) : post-AGB disks

● ~10% of white dwarfs may have had post-AGB disks

Conclusions

● Disks form in binary systems● Capture 1-10% of ejected gas and dust

● Oxygen-rich

● Grain growth to at least cm size

● Disks extend from 10-100 AU● Plus inner gaseous accretion disk

● Disks evaporate once ionization starts● Leaving a ~10^-7 solar mass dust disk behind

● Remnant disks may contribute to white dwarf debris disks

Midsomer Murders

WRITTEN IN THE STARSSeries 15, Episode 4

As darkness covers Midsomer Stanton during a total eclipse of the sun, amateur astronomer Jeremy Harper is killed by a blow to the head with a meteorite...

Debris disks are dangerous..