The importance of the remnant’s mass for
VLTP born again times
(published last week in MNRAS 380, 763-770, 2007)
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 times
Marcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas
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:
Simulations with standard Mixing Length Theory (MLT) approach:
• Iben & MacDonald (1995),
• Herwig et al. (1999),
• Herwig (2001),
• Lawlor & MacDonald (2002),
• Miller Bertolami et al. (2006),
0.60
M☼
0.604
M☼
0.535
M☼
0.56 - 0.61 M☼
0.589
M☼
●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
→
→
→
→
→
~17 years
~350 years
Differences up
~21 years
~4.5 - 8.5 years to a factor 70!
~5 - 10 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 to
understand 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 times
Marcelo 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 times
Marcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas
The simulations
● We performed simulations of the VLTP episode for 10 different remnant
masses and followed their expansion from the white dwarf configuration to their new
stage as born again AGB stars:
Remnant
Mass [M☼]
Initial
Born again
Mass [M☼] 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
-
Some Ratios:
12C/13C ~ 6
12C/14N ~ 12
(by mass)
Fast
Born Again
Slow
Born Again
Typical timescales of He-shell burning
Driven expansions (as can be seen in
Prof. Schönberner’s plot)
The importance of the remnant’s mass for VLTP born again times
Marcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas
Is this result physical or due to numerics?
The energetics (a rough estimation)
● We analyze the energetics of the VLTP estimating:
The energy that can be liberated
by the violent proton burning:
EH-burning=Qper proton burned × Np
● 12C+p→13N+γ→13C+e++ν
e
● 12C+p→13N+γ→13C+e++ν
e
:
Qper proton burned = 3.4573 MeV
and
working at the same rate:
Qper proton burned = 5.504 MeV
13C+p→14N+γ,
The energy needed to expand the
layers above the shell at which this
energy is liberated:
Eexpansion=|Ei+Eg|
Eexpansion
solid line
EH-burning
shaded zone
The importance of the remnant’s mass for VLTP born again times
Marcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas
The energetics (a rough estimation)
EH-burning
shaded zone
Eexpansion
solid line
This are the
Note:
sequences that
Almost all previous simulations
displayed
were confined
to a narrow long
range
around the born
“transition
mass”times!
again
(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”)
E
< Etake
But…
do not
the transition mass
H-burning
expansion
at face value!
Hydrogen driven expansion
should NOT be possible
Thus the dichotomy found in numerical
simulations seems to be physical
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
●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!)
●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
The importance of the remnant’s mass for VLTP born again times
reheating
reported
by Hajduk et al. (2005)…
Marcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas
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 times
Marcelo 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 times
Marcelo Miguel Miller Bertolami, Facultad de Ciencias Astronómicas y Geofísicas
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