lars bildsten kavli institute for theoretical physics university of california santa barbara
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Putting Helium Layers onto Massive White Dwarfs. Lars Bildsten Kavli Institute for Theoretical Physics University of California Santa Barbara. - PowerPoint PPT PresentationTRANSCRIPT
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Lars BildstenKavli Institute for Theoretical
PhysicsUniversity of California Santa
Barbara
Putting Helium Layers onto Massive
White Dwarfs
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The newfound prevalence of double white dwarfs with one being a low-mass (M<0.20M) helium white dwarf has re-motivated our exploration of possible explosive outcomes, as has the discovery of the first Helium burning star in a tight binary.
Jared Brooks (UCSB), David Kaplan (UWM), Kevin Moore (UCSB=>UCSC), Bill Paxton (KITP), Ken Shen (UCB), Justin Steinfadt (UCSB=>ATT), &
Dean Townsley (U Alabama)
Recent Collaborators:
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Disclaimer from my 2010 Leiden Talk
“I was asked by the organizers to give a pedagogical talk…. so my referencing of all the vast literature is not thorough. Please don’t be offended if I don’t reference YOUR most excellent paper.”
Trust me, I read your paper and I loved it!!
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Double White Dwarfs + WD/SdB Stars Galore
Kaplan, L.B. & Steinfadt ‘12
• These stay bright due to a stably burning H envelope (Panei et al ’07)• Many found to be with WDs in tight orbits (Badenes, Brown, Kilic, Mullally, Steinfadt. . • Many will reach contact in 10 Gyr• Note lack of binaries with Mt>1.4
SDSS revealed a large population of Helium WDs
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Helium Burning Stars + WD Binaries
• A long discussed channel has been helium burning donors. These must come into contact in <100 Myr so that the Helium star is still burning and has a large radius.
• Early work found one (KPD 1930+2752; Geier et al. ’07) but at Porb=136 minutes, too long.
• Now have one good system CD-30 11223 (Vennes et al. 2013; Geier et al. 2013) with Porb=70 minutes and M1=0.74, M2=0.47. Will come into contact prior to burnout of the He burning star!
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Gravitational Waves Drive the Objects into Contact: Direct Impact destabilizes many of
the Double WDs
Marsh, Nelemans & Steeghs ‘04
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Outcomes for Unstable Mass Transfer
• One distinct possibility is formation of an R Cor Bor Star, with a stably burning He shell on the WD core
• Another option is that the He detonates and creates a thermonuclear transient. . .
• I am not discussing these cases here . . . but, yes, I read your paper!!
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Three Pieces when Stably Accreting • Resulting accretion rate sets He shell masses
– He Burning stars made from Common Envelope evolution are large and fill RL at 20-40 minutes
– Low mass He WDs that fill RL “sooner” than expected
• Unstable helium shell burning outcomes– Defines a He Shell mass above which dynamical
burning can occur.
• Outcome from dynamical burning, potentially detonations that, themselves, can shock the C/O WD = > Type Ia ?
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MESA is open source: anyone (over 600 users!) can download the source code, compile it, and run it for their own research or education purposes.
Bill Paxton, Father of MESA
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Second “Instrument Paper” just Appeared
http://mesa.sourceforge.net
Third MESA Summer School at UCSB August 11-15, 2014
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Stable and Unstable Helium BurningIben & Tutukov 1989
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Mass Transfer for Roche Lobe Filling Low Mass Helium WDs
Kap
lan,
Bil
dste
n &
Ste
infa
dt ‘
12
MWD=0.8
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Mass Transfer for RL Filling He Burning Stars
Brooks et al, in preparation
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Helium Donor Mass
Iben & Tutukov ‘89 He Ignition Mass
MESA calculations of accumulated masses.
Evolution in timeEvolution in time
Mass Transfer Rates and Helium Mass for Ignition
• The higher accretion rate for the Helium WD donors lead to many flashes• He burning star donors (dashed line) have a much lower accretion rate and lead to thicker shell accumulation.
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Path to Dynamical Helium ShellsThe radial expansion of the convective region allows the pressure at the base to drop. For low shell masses, this quenches burning. For a massive shell, however, the heating timescale set by nuclear reactions:
will become comparable to lthe dynamical time,
Allowing for a more dynamic outcome than just mass loss in a wind or RL overflow that would be a nova.. (V445 Puppis?)
Shen & LB ‘09
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MESA Calculations for He Accretion
from a WD Donor• Series of weak He flashes: basically He Novae• Mass loss occurs due to Roche Lobe overflow during the Helium flashes • Final flash has a minimum heating time of 10 seconds (Bildsten et al. ‘07; Shen & LB ‘09) and becomes dynamical.
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Onset of Rapid Burning => ? ?
• From bottom to top, three different accretion rate, 1.0x10-7 M/year. 6x10-8 M/year. 3x10-8
M/year.
• Lower accretion rates far more likely to become dynamical..
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Burns Incompletely and Moves Slow (0.028 M He on a 1.0 MWD)
Dis
tanc
e (1
08 cm
)
Dis
tanc
e (1
08 cm
)
Distance (108 cm)Distance (108 cm)
Townsley, Moore and LB, 2012
Propagated at 9000 km/sec. Made mostly 44Ti, 48Cr, little 52Fe and 56Ni
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Open Questions• He burning star channel gives He masses likely
large enough to trigger detonations. Does this ignite the C/O or remain a .Ia like outcome (especially if there is an O/Ne WD)?
• What about rotation? I don’t think it’s a big issue, as B fields drive accreted material into solid body rotation long before ignition; but see Yoon and Langer
• More to follow on the diversity of outcomes of the He Detonations. .
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MESA He Flash CalculationAccretion onto a 1.0 M at 3.7x10-8 M/year. Accumulated He was 0.093M whereas convective shell has 0.022 M