exploring the diffusion induced nova scenario

Exploring the diffusion induced nova scenario

Marcelo Miguel Miller Bertolami

Leandro Gabriel Althaus(FCAGLP-UNLP/IALP CONICET, Argentina)

Exploring the diffusion induced nova scenario17th European White Dwarf Workshop. Tubingen, August 16-20,

2010


2010

Publicity: La Plata White Dwarf Models(LPCODE)

Updated physics, realistic pre-white dwarf evolution, time dependent diffusion, colors and much more!

Tracks available at http://www.fcaglp.unlp.edu.ar/evolgroup/tracks.html 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. !


2010

Diffusion Induced Novas (a.k.a. Self Induced Novas)

Diffusion triggered CNO flash at Teff~40000 K

●Iben & MacDonald (1986)


2010

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-flashIts surface abundance is mildly He-enriched with NH/NHe ~1Diffusion induced novas occur when ΔMbuffer < ΔMH /10The star luminosity brightens 10 mag, light curve look like very slow nova Diffusion induced novas are expected to be recurrentH-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

•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

Objectives

●Diffusion induced novas might lead to very thin H-envelopes as those predicted by some asteroseismological studies of DAV (see Castanheira & Kepler 2009).


2010

Evolutionary Scenario:


2010

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.

•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


2010

Modeling (some key points): The White Dwarf phase


2010

Results: A diffusion induced nova


2010

Results: A diffusion induced nova :


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Results: Predicted Properties: Timescales, abundances, etc.

Main Facts: -White dwarf age at the moment of CNO-flash (1):106 - 108 years-Expansion times (2): 2 - 10 years-Reheating time: from ~50 to ~400 years depending onMass 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

~~


2010

Results: Expected Lightcurves

Bolometric corrections from Flower (1996)

Brightening speedFrom ~1 to ~4 mag/yr


2010

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.

-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

The occurrence (or not) of diffusion induced novas seems to be more complicated!


2010

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

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)

CNO-flash

CNO-flash


2010

Differences between Diffusion induced novas and Born Again Stars

Diffusion induced novas Very late thermal pulses


2010

-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

Differences between Diffusion induced novas and Born Again Stars


2010

-RCrB-like episodes after outburstin diffusion induced novas???, I thought you needed a lot of carbon but…

Born Again Stars experience RCrB-like events after outburst

(once back on the AGB)

V605 Aql V4334 Sgr

RCrB-like

RCrB-like

But born again stars (VLTP) are very carbon rich (see Asplund et al. 1999)


2010

There were RCrB-like episodes in MV Sgr A carbon deficient RCrB star (C~0.0005, N~0.0009 by mass fraction).

Our diffusion induced novas have

With C>O and a more Nitrogen

May be Diffusion Induced Novas experienceRCrB-like events after all...

…If so…


2010

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 novaAnd measure possible radio emission form the central object


2010

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?


2010

Speculation, Possible Observational Counterparts: CK Vul?

CK Vul (Hajduk et a. 2007)

D.I.N. (fast) V.L.T.P

Last time experienced High

Mass Loss

(no old PN)

105 -106 yr ago

106 -107 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)

Equal number of UV photons if reheating with ~10000Lsun

T~10000K?

(within correct reheating timescale)

Ejected Mass ~10-4 Msun ~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)

The star becomes a yellow giant a few years after the CNO-flashIts surface abundance is mildly He-enriched with NH/NHe ~1Diffusion induced novas occur when ΔMbuffer < ΔMH /10The star luminosity brightens 10 mag, light curve look like very slow nova Diffusion induced novas are expected to be recurrentH-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


2010

Preliminary Conclusions:

TrueTrueFalseTrueFalseFalse

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?

●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.

But before finishing the presentation…


2010

Asteroseismologists!

Don’t miss the following Posters!

New DAV chemical profiles:

Evidence of differencial rotation in hot pre-white dwarfs? :


2010


2010

Initial-Final Mass Relation


2010

Castanheira & Kepler (2009): Thin H-envelopes


2010

Speculation, Possible Observational Counterparts: NVS 11479

Probably a fast born again star and not a diffusion induced nova


2010

Speculation, Possible Observational Counterparts: NVS 11479


2010

Modeling (some key points):

exploring the diffusion induced nova scenario

Documents

chemical diffusion

conditions diffusion

realistic white dwarf

time dependent diffusion

slow nova diffusion

self induced novasdiffusion

agb evolution

realistic enoughiben