evolving disks around post-agb stars
Post on 09-Feb-2022
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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
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
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
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