1. Exploring the diffusion induced nova scenario
Marcelo Miguel Miller Bertolami
Leandro Gabriel Althaus
(FCAGLP-UNLP/IALP CONICET, Argentina)
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

2. Publicity: La Plata White Dwarf Models (LPCODE)
Updated physics, realistic pre-white dwarf evolution, time dependent diffusion,
colors and much more!
Tracks available at in two different flavors:
H-rich: Renedo et al. 2010, ApJ, 717, 183
H-deficient: Althaus et al. 2009, ApJ, 704, 1605
See poster by Renedo et al. !
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

3. Diffusion Induced Novas (a.k.a. Self Induced Novas)
●Iben & MacDonald (1986)
Diffusion triggered CNO flash at Teff~40000 K
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

4. Objectives Motivation
●Although mentioned in several reviews and articles (D’Antona & Mazzitelli 1990, Iben 1995, Hansen 2003, Lawlor & MacDonald 2006 Kalirai et al 2007, Althaus et al. 2010), first and only simulation of
the event dates from 25 years ago.
(later: white dwarf models were criticized for not being realistic enough)
●Iben & MacDonald made several predictions/speculations:
The star becomes a yellow giant a few years after the CNO-flash
Its surface abundance is mildly He-enriched with NH/NHe ~1
Diffusion induced novas occur when ΔMbuffer < ΔMH /10
The star luminosity brightens 10 mag, light curve look like very slow nova
Diffusion induced novas are expected to be recurrent
H-burning and strong winds during the giant phases will lead to H-depletion of the star, leading to the formation of non-DA white dwarfs
●Diffusion induced novas might lead to very thin H-envelopes as those predicted by some asteroseismological studies of DAV (see Castanheira & Kepler 2009).
Objectives
Find if diffusion induced novas occur in realistic white dwarf models.
If so, find under which conditions diffusion induced novas occur (thin He-buffers).
Find if diffusion induced novas can lead to DAs with thin H-envelopes
Make predictions on its surface abundances and expansion times (light curves)
Compare with Iben & MacDonald predictions/speculations
Think/speculate about possible observational counterparts
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

5. Evolutionary Scenario:
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

6. Modeling (some key points): Pre-white dwarfs evolution
In order to produce realistic white dwarf models we computed “cradle to grave” evolutionary sequences following the evolution from the ZAMS through the TP-AGB phase
Standard AGB mass loss prescriptions were adopted during the AGB evolution and during the final He-shell flash (exception: some tuning of mass loss to obtain desired type of final flash)
No extramixing (overshooting) included at the base of the convective envelope and the He-shell flash convective zone. No (strong) third dredge up is present in low mass AGB stars in agreement with the Initial Final Mass Function (Salaris et al. 2009) and observations of carbon stars (Guandalini et al. 2007)
We followed 16 nuclear species with a detailed nuclear network.
Nuclear burning and convective mixing are solved simultaneously.
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

7. Modeling (some key points): The White Dwarf phase
Time-dependent diffusion was considered. In particular we consider gravitational settling, thermal and chemical diffusion (no radiative levitation). This is done within the multicomponent gas treatment of Burgers (1969) and solved like in Iben & MacDonald (1985):
Equations of state: Segretain (1994, high density regime), Magni & Mazzitelli (1979, updated 1993, low density regime)
Conductive opacities: Cassisi et al. (2007)
Radiative opacities: Iglesias & Rogers (1996) + Alexander & Ferguson (1994) consistent with the metal content of each layer predicted by time-dependent diffusion
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

8. Results: A diffusion induced nova
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

9. Results: A diffusion induced nova :
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

10. Results: Predicted Properties: Timescales, abundances, etc.
Main Facts:
-White dwarf age at the moment of CNO-flash (1):
years
-Expansion times (2): years
-Reheating time: from ~50 to ~400 years depending on
Mass loss at low temperatures
-NH/NHe ~1 ; Significant N-abundance
-Sequences of higher mass MWD > 0.65 Msun or higher metallicity Z>0.001, DID NOT experience CNO flashes ~ ~
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

11. Results: Expected Lightcurves
Brightening speed
From ~1 to ~4 mag/yr
Bolometric corrections from Flower (1996)
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

12. Results:
-Diffusion induced novas are not recurrent. Evolution during the second white dwarf stage is much faster. There is no enough time for diffusion to act.
The occurrence (or not) of diffusion induced novas seems to be more complicated!
-Our simulations do not follow the criterion: ΔMbuffer < ΔMH /10.
For our ~0.54 Msun sequences the critical value is ΔMbuffer ~ ΔMH /5
And for our ~0.62 Msun sequences it was ΔMbuffer ~ ΔMH /30
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

13. Results:
-Diffusion induced novas do not reduce the thickness of the H-envelope by many orders of magnitude. DAs with thin H-envelopes are not produced this way
CNO-flash
CNO-flash
Digression (observation for asteroseismologists):
Late Thermal Pulses in stars that DO experience third dredge up are a much better way of getting thin H-envelopes for DAV (Althaus et al. 2005, A&A, 440, L1)
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

14. Differences between Diffusion induced novas and Born Again Stars
Very late thermal pulses
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

15. Differences between Diffusion induced novas and Born Again Stars
-Diffusion induced novas are brighter (~2 to ~3 mag) than fast born again stars during outburst, and ~1.5 mag brighter at maximum.
-Diffusion induced novas are also hotter/bluer during outburst
-RCrB-like episodes after outburst???... Let’s see
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

16. Born Again Stars experience RCrB-like events after outburst
(once back on the AGB)
RCrB-like
RCrB-like
V605 Aql
V4334 Sgr
But born again stars (VLTP) are very carbon rich (see Asplund et al. 1999)
-RCrB-like episodes after outburstin diffusion induced novas???,
I thought you needed a lot of carbon but…
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

17. There were RCrB-like episodes in MV Sgr
A carbon deficient RCrB star (C~0.0005, N~ by mass fraction).
Our diffusion induced novas have
With C>O and a more Nitrogen
May be Diffusion Induced Novas experience
RCrB-like events after all...
…If so…
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

18. Speculation: Possible Observational Counterparts:
-Very Slow Novas? (Iben & Mac Donald 1986), which takes us to…
-Enigmatic object CK Vul???
(see Hajduk et al. 2007, 3 years ago in this very same room!)
-Anthelms-Hevelius nova (CK Vul).
Erupted between 1669 and 1670 and probably experienced RCrB like episodes before fading from view
-Shara & Moffat recovered a nebula asociated with the 1669 event
-Hajduk confirmed the link between the nebula and the nova
And measure possible radio emission form the central object
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

19. Speculation, Possible Observational Counterparts: CK Vul?
The enigmatic Hevelius nova CK Vul escapes all present explanations.
Many scenarios have been proposed to explain the object:
-Very slow nova (Shara et al. 1985)
-Hibernating nova (Shara et al. 1986)
-Born Again Star (Harrison et al. 1996, Evans et al. 2002)
-Merger of main sequence stars (Kato 2003)
-”Gentle(?) Supernova”; “Light Nova”, ”Accretion Induced Born Again” (Hajduk et al. 2007)
The born again scenario seems to be the best one,
(Hajduk et al 2007, Evans et al. 2002) but still (very)
far from the observations…
-Why not a diffusion induced nova?
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

20. Speculation, Possible Observational Counterparts: CK Vul?
(Hajduk et a. 2007)
D.I.N.
(fast) V.L.T.P
Last time experienced High Mass Loss
(no old PN)
yr ago yr
~104 yr
(old PN?)
H-abundance
H-rich (probably)
H-rich
H-deficient
RCrB-like events
Yes? ?
yes
Luminosity
~1Lsun from UV photons assuming central object with T=70000K
Equal number of UV photons if reheating with ~10000Lsun T~10000K?
(within correct reheating timescale)
Ejected Mass
~10-4 Msun
From ~10-4 Msun to ~10-2 Msun
Maximum Brightness
Mv ≈ -8
(but < Mv > ≈ -6.8)
Mv ≈ -5.5
Mv ≈ -4
Eruption time
Rose more than two magnitudes before maximum in less than a yr
1 to 4 magnitudes per year
Less than 1 magnitudes
per year
So neither of the scenarios correctly account for the observations but the diffusion induced nova seems to come a bit closer than the best scenario available (fast VLTP)
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

21. Preliminary Conclusions:
●An AGB final thermal pulse or a late thermal pulse seem to provide the correct scenario to form white dwarfs with thin He-buffers.
●Diffusion induced novas DO occur in realistic white dwarf models. Together wih a thin He-buffer, low mass and metallicity seem to be necessary conditions.
●Diffusion induced novas do not produce DAs with thin H-layers as suggested by asteroseismology of some DAVs.
True
False
The star becomes a yellow giant a few years after the CNO-flash
Its surface abundance is mildly He-enriched with NH/NHe ~1
Diffusion induced novas occur when ΔMbuffer < ΔMH /10
The star luminosity brightens 10 mag, light curve look like very slow nova
Diffusion induced novas are expected to be recurrent
H-burning and strong winds during the giant phases will lead to H-depletion of the star, leading to the formation of non-DA white dwarfs
Main Open Questions:
●How frequent do low mass stars depart from the AGB an experience an AFTP/LTP?
●Anthelme-Hevelius nova CK Vul might have been a diffusion induced nova?
But before finishing the presentation…
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

22. Don’t miss the following Posters!
Asteroseismologists!
Don’t miss the following Posters!
New DAV chemical profiles:
Evidence of differencial rotation in hot pre-white dwarfs? :
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

23. Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

24. Initial-Final Mass Relation
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

25. Castanheira & Kepler (2009): Thin H-envelopes
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

26. Speculation, Possible Observational Counterparts: NVS 11479
Probably a fast born again star and not a diffusion induced nova
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

27. Speculation, Possible Observational Counterparts: NVS 11479
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

28. Modeling (some key points):
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010

29. Modeling (some key points):
Exploring the diffusion induced nova scenario
17th European White Dwarf Workshop. Tubingen, August 16-20, 2010