The importance of the remnant’s mass for VLTP born again times

Marcelo Miguel Miller Bertolami

Part of the PhD thesis work (in progress) under the supervision of

L. G. Althaus and A. H. Córsico at the University of La Plata, Argentina

The importance of the remnant’s mass for VLTP born again timesMarcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas

(published last week in MNRAS 380, 763-770, 2007)

Motivation of this work●Previous works on the Very Late Thermal Pulse (VLTP) that include the violent H-burning display a wide range of born again timescales:

• Iben & MacDonald (1995), 0.60 M☼ → ~17 years• Herwig et al. (1999), 0.604 M☼ → ~350 years• Herwig (2001), 0.535 M☼ → ~21 years• Lawlor & MacDonald (2002), 0.56 - 0.61 M☼ → ~4.5 - 8.5 years• Miller Bertolami et al. (2006), 0.589 M☼ → ~5 - 10 years

Simulations with standard Mixing Length Theory (MLT) approach:

Differences up to a factor 70!

●Born again times are particularly Relevant after the discovery of V4334 Sgr by Yukio Sakurai

V4334 Sgr showed a born again timescale of a few years

Herwig (2001) suggested V4334 Sgr could be used to test convection theory and proposed that convective mixing velocities should be strongly reduced to account for the fast evolution observed in V4334 Sgr:0.604 M☼ with vMLT/3 → timescale ~195 yr 0.604 M☼ with vMLT/30 → timescale ~10 yr

Thus if born again timescales are important tounderstand convection of reactive convective fluids. An understanding of the different timescales found by different authors is needed

The importance of the remnant’s mass for VLTP born again timesMarcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas

Modeling (some key points):

• Sequences are calculated from the ZAMS through the TP-AGB until they reach the post AGB phase. • During the departure from the AGB mass loss is tuned to obtain a VLTP.• Treatment of mixing processes:

1- During the VLTP the timescales for H-burning and convective mixing are similar, one must drop the usual instantaneous mixing approach:

We adopt a scheme of diffusive convective mixing (within the MLT) coupled with nuclear burning.

2- We adopt an exponentially decaying overshooting as in Herwig et al. 1997:

With free parameter f=0.016 at all convective boundaries

(mainly because these sequences were originally created to obtain post-AGB models with PG1159-like surface abundances)

The importance of the remnant’s mass for VLTP born again timesMarcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas

● We performed simulations of the VLTP episode for 10 different remnantmasses and followed their expansion from the white dwarf configuration to their new stage as born again AGB stars:

The simulations

Remnant Mass [M☼]

Initial Mass [M☼]

Born again

timescale

0.515 1 14 yr

0.530 1 5 yr

0.542 1 5.1 yr

0.561 1.8 7.5 yr

0.565 2.2 10.8 yr

0.584 2.5 8.9 yr

0.609 3.05 157 yr

0.664 3.5 106 yr

0.741 3.75 65 yr

0.870 5.5 -

FastBorn Again

SlowBorn Again

Is this result physical or due to numerics?The importance of the remnant’s mass for VLTP born again times

Marcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas

Some Ratios:12C/13C ~ 6

12C/14N ~ 12(by mass)

Typical timescales of He-shell burningDriven expansions (as can be seen in Prof. Schönberner’s plot)

● We analyze the energetics of the VLTP estimating:

The energetics (a rough estimation)

The energy needed to expand the layers above the shell at which this energy is liberated:

Eexpansion=|Ei+Eg|

The energy that can be liberated by the violent proton burning:

EH-burning=Qper proton burned × Np

● 12C+p→13N+γ→13C+e++νe :Qper proton burned = 3.4573 MeV● 12C+p→13N+γ→13C+e++νe and 13C+p→14N+γ, working at the same rate:Qper proton burned = 5.504 MeV

Eexpansion

solid lineEH-burning

shaded zone

The importance of the remnant’s mass for VLTP born again timesMarcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas

The energetics (a rough estimation)

Eexpansion

solid line

EH-burning

shaded zone

EH-burning < Eexpansion

Hydrogen driven expansion should NOT be possible

This are the sequences that displayed long born again times!

Thus the dichotomy found in numerical simulations seems to be physical

Note:Almost all previous simulationswere confined to a narrow range around the “transition mass”(were things are expected to become sensitive to modeling and numerics)

H burning closer to surface(“D increased”)

H burning deeper into the star(“D reduced”)

The importance of the remnant’s mass for VLTP born again timesMarcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas

But… do not take the transition massat face value!

The importance of the remnant’s mass for VLTP born again times

Marcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas

The previous result, suggests that the evolution observed in V4334 Sgr could be understood as the VLTP evolution of a low mass remnant (M<0.6M☼)

In fact when we compare with observations of V4334 Sgr we find: Pre-outburst Post-outburst

●Also the drop in H abundance observed during 1996 is “qualitatively” reproduced by that model (although quantitatively wrong by… ~1 order of magnitude!).

Pre- and outburst values of V4334 Sgr agree better with standard mixing efficiency models of low mass than with higher mass models with reduced mixing efficiency. And without any (new) free parameter.

Unfortunately, our models fail to reproduce the fast reheating reported by Hajduk et al. (2005)…

●Pre- and post- outburst values of Teff, and L☼ are nicely reproduced by a model of ~0.56 M☼ if a distance of 3-4 Kpc is adopted. (in agreement with independent distance determinations!)

Reheating of V4334 Sgr

Increasing the mass loss rate can help to shorten the reheating born again times

But it is also possible that this failure is because our models are not physically sound in the high luminosity/ low temperature regime (e.g. assumed hydrostatic equilibrium is not valid in the outer layers of the star).

A better description of the outer layers of the envelope may be necessary to predict the correct photospheric behavior.

The importance of the remnant’s mass for VLTP born again timesMarcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas

Conclusions• We find a dichotomy in the VLTP born again times with low mass remnants displaying timescales of a few years and high mass remnants failing to expand due to the violent H-burning (there is no enough H!).

• When comparing simulations to real stars different remnant masses have to be considered.

• Choosing a low remnant mass allow us to obtain short born again times as those observed in V4334 Sgr and V605 Aql without the inclusion of an (ad-hoc) mixing efficiency: V4334 Sgr pre-outburst and outburst determinations can be nicely reproduced by VLTP sequences with standard mixing efficiency of ~0.56 M☼ if a distance of ~3 to 4 Kpc is assumed.

•The scatter in born again timescales of previous works most probably related to the closeness to the transition mass between two types of post VLTP evolutions.

(Main) shortcomings of the present simulations:• The fast reheating reported in V4334 Sgr can not be reproduced unless mass loss is significantly higher than reported. (poor treatment of the outer layers of the star?)

• The use of diffusive convective mixing approach may not be justified (we have to wait until multidimensional simulations of the violent H-mixing and burning became available)

The importance of the remnant’s mass for VLTP born again timesMarcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas

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