Binary evolution predicts a plethora of double WD binaries, some close enough to come into contact. I will explain our theoretical work on the outcomes when stable transfer of Helium results, predicting a new kind of faint and fast “.Ia” Supernova.

Faint Supernovae from Double White Dwarfs

Dan Kasen (UCB), Kevin Moore (UCSB), Gijs Nelemans (U. Nijmegen), Bill Paxton (KITP), Evan

Scannapieco (ASU), Ken Shen (UCSB=>UCB), Justin Steinfadt (UCSB) and Nevin Weinberg (UCB)

PN image from HSTRing Nebulae (M 57)

Young White Dwarf

Stars with < 6-8 M make 0.5-1.0 M Carbon/Oxygen white dwarfs, and likely 1.0-1.2 M Oxygen /Neon

Kalirai et al ‘07

Kalirai et al ‘07

1.05 M

500 100 50Stellar Lifetime (Myr)

Making Helium White Dwarfs• Wind mass loss near the tip of the Red Giant Branch creates isolated He WDs of M>0.4 M (D’Cruz et al ‘96, Hansen ‘05; Kalirai et al. ‘07) • Binaries tight enough so that the <2.5 M fills the Roche lobe on the red giant branch reveals a He core 0.15-0.48 M • Most of the first known, lower mass, He WDs were in binaries, often with M stars (Marsh et al…) de Kool ‘92; de Kool & Ritter ‘93; Iben & Tutukov ’93. Politano ’96; Nelemans et al. 2001

Binary White Dwarfs

Extremely Low-Mass He WDs

Brown et al. 2010 (posted Monday night!)

• SDSS is revealing a large population of <0.2M He WDs (Kilic, Brown, . . )

• These stay bright longer due to a stably burning H envelope (Panei et al ‘07)

• Expectation that they are in binaries. Many found to be with WDs in tight orbits (Badenes, Kilic, Mullally, . . ) to reach contact in 10 Gyr

ZZ Ceti Searches: Do they Pulsate?

Steinfadt, Bildsten & Arras ‘10

A New Object. . . . From Faulkes North (LCOGT)

NLTT 11748

First measurement of lowmass He WD Radius=0.039-0.043 R

Many Double White Dwarfs that Will Come into Contact

Stability at Contact: Open Issue

NLTT 11748

WD-WD Mergers: R Cor Bor stars

• Variable stars at Mv=-4 to -5. No hydrogen present, He+Carbon. Lines due to C2 and CN. Variability due to dust formation episodes.

• MACHO found many, rough numbers for our galaxy are 3000 (Alcock et al. 2001). OGLE, EROS as well

• Current hypothesis is He+C/O mergers, followed by burning for 100,000 years=> birthrate is 3 in 100 years in our galaxy. . . .

Piro ‘05

Stably Accreting White Dwarfs

Donor star of pure He

White Dwarf of Carbon/OxygenOr Oxygen / Neon

GP COMThe fate of ~1 in 2000 white dwarfs in our galactic disk are AM CVn binaries

These are VERY LOUD sources for Space-Based Gravitational Wave Detectors (e.g. LISA)

AM CVn Binaries: Pure Helium Accretors!

• Found by Humason and Zwicky (‘47) as faint blue stars, spectra by Greenstein & Matthews (‘57) only showed helium lines.• Later work found 17 minutes orbital period• The accretor is a C/O or O/Ne WD, where the donor is a Helium WD.

• Giving an orbital period-donor mass relation,

and donor masses ranging from 0.006-0.12M, interesting for stellar structure (Deloye et al ’05. ’07)

Object Porb

RXJ0806 5.35

V407 Vul 9.49

ES Cet 10.3

AM CVn 17.1

HP Lib 18.4

CR Boo 24.5

KL Dra 25.0

V803 Cen 26.9

SDSSJ0926 28.3

CP Eri 28.4

2003aw 33.9

SDSSJ1240 37.4

SDSSJ1411 46.0

GP Com 46.5

SDSSJ1552 56.7

CE 315 65.1

+ > 15 more!

Helium Burning

10 20 30 40 50 60 70 Orbital Period (Minutes)

GP

Com

CP

Eri

ES

Cet

AM

CV

n

CE

315

HP

Lib

RX

J080

6?

CR

Boo

, KL

Dra

V40

7 V

ul

V80

3 C

en

He burning is thermally stable when donor mass is 0.20-0.27M, with Porb =2.5-3.5 minutes (Tutukov & Yungelson ‘96). As the accretion rate drops, the burning becomes unstable, and flashes commence.

Thick= Cold DonorsThin= Hotter Donors

Unstable Helium Burning of Interest

September 2, 2010 Santa Barbara, California

Jesusita Fire, May 2009Photo: K. Paxton

MESA Overall Philosophy

MESA is open source: anyone can download the source code, compile it, and run it for their own research or education purposes.

It is meant to engage the broader community of astrophysicists in related fields and encourage contributions in the form of testing, finding and fixing bugs, adding new capabilities, and, generally, sharing experience with the MESA community.

http://mesa.sourceforge.net/

Helium Donor Mass

Many He novae at early times, followed by one last flash with helium mass of 0.03-0.1M, likely large enough to trigger an explosion (L.B, Shen, Weinberg & Nelemans ‘07, Shen and LB ’09)

Iben & Tutukov ‘89 He Ignition Mass

These MESA calculations of accumulated masses are preliminary !

Evolution in timeEvolution in time

Preliminary MESA

Calculations for an AM CVn scenario

• Series of weak He flashes: basically He Novae (e.g. V445 Puppis)• Mass loss occurs due to Roche Lobe overflow that • Final flash has a minimum heating time of 10 seconds!

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

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 less than the dynamical time,

So that the heat cannot escape during the burn, potentially triggering a detonation of the helium shell. This condition sets a minimum shell mass.

Shen & LB ‘09

L. B., Shen, Weinberg & Nelemans ‘07

Evolution Naturally Yields Dynamical He ShellThe intersection of the ignition masses with that of the donor yields the hatched region, most of which lie above the dynamical event line

• For WD masses > 0.9M, likely outcome is dynamical

• For lower WD masses, the outcome may be less violent.

Must understand the dynamic outcome and nucleosynthetic yields from these low pressure burns, a new regime.

sample

Shock (blue arrow) goes into the C/O and a He detonation (red arrow) moves outward. The shocked C/O under the layer is not ignited. Underlying WD remains unless converging shocks detonate it (see Livne & Glasner; Fink, Ropke & Hillebrandt)

Yields are 0.012 M of 56Ni, 0.0071M of 48Cr, and 0.0076M of 52Fe. Much He unburned.

Shen et al. ’10

Sample Detonation

Thermonuclear Supernova Lightcurves• Type Ia result from burning 1.3M of C/O to ~ 0.6M of 56Ni (rest burned to Si, Ca, Fe) and ejected at 10,000 km/sec. • This matter would cool by adiabatic expansion, but instead is heated by the radioactive decay chain 56Ni 56Co 56Fe• Arnett (1982) (also Pinto & Eastman 2000) showed that the peak in the light-curve occurs when the radiation diffusion time through the envelope equals the time since explosion, giving

• The luminosity at peak is set by the instantaneous radioactive decay heating rate can measure the 56Ni mass via the peak luminosity, yielding 0.10-1.1 M for Type Ia Supernovae

• The 0.02-0.10M ignition masses only burns the helium, which leaves the WD at 10,000 km/sec, leading to brief events

• The radioactive decays of the freshly synthesized 48Cr (1.3 d), 52Fe (0.5 d) and 56Ni (8.8 d) provide power on this short timescale!!

L. B., Shen, Weinberg & Nelemans ’07Shen et al 2010

.Ia Supernovae

x10

Faint and Fast Events as a Challenge to the Observers!

Shen et al ‘10

M87 in Virgo

Some numbers:

• 20 Classical Novae (Hydrogen fuel) per year, implying a white dwarf/main sequence contact binary birthrate (Townsley & LB 2005) of one every 400 years.

• One Type Ia Supernovae every 250 years, or one in 500 WDs explode!

Two WDs are made per year in a 1011 M

elliptical galaxy. The observed rates for thermonuclear events are:

Helium novae (Eddington-limited) every ~250 years, one large He explosion every ~5,000 years, and WD-WD mergers of all kinds every 200 years.

Predicted rates are:

2002bj: Poznanski et al. ‘09

Helium, Carbon and maybe V!

Sloan Digital Sky Survey (Dilday et al 2008)

Completed survey (V=22.5, 260 deg2)

Surveys, Surveys, Surveys!

ROTSE (V=18, 200 deg2)

SkyMapper (‘10; V=19, 1000 deg2

every 3-4 d)

Pan-Starrs1 (‘09)

Medium deep survey (V=24, 50 deg2)

Palomar Transient Factory• A 100 Mega-pixel CCD camera on the 48 inch Schmidt Telescope at Palomar (near San Diego) that:

-- scans 10% of the sky every week-- finds nearly 1000 transients per year that are tracked by small telescopes for photometry and larger (3-10m) for spectroscopy -- Is creating a deep sky image in 2 bands (g and R) that will have lasting value for galactic science

Survey Volumes and Expectations

Bildsten ‘09Bildsten ‘10

• The boxes plot the volume rate * duration for Type Ia (30 d), Type IIp (100 d) and .Ia (5 d)• Densities rough for LSNe, ‘Super-Chandra’ and Faint Ia (bg)• Lines show the 1 event per “exposure” line for

• ROTSE (green)• SKYM (blue-dotted)• SNLS (dashed)• SDSS (long-dashed)• PS1 (blue-dashed)• PTF (blue-solid) • DES (red)• LSST (heavy-black)

PTF 10bhp=2010X

• Peaks at 1042

• Decay time is 5 days• Velocities of 10,000 km/second• Spectra shows Ca, C, Ti, Fe• Maybe He, Na and Al?

Kasliwal et al. ‘10

Comparisons to Predictions

Comparison to .Ia models from Shen et al. (2010) imply an ejected mass of ~0.06-0.09M with roughly 1/2 being 56Ni, and the rest mostly Helium. . .

Fink shock plotsF

i nk,

Hil

lebr

andt

and

Rop

ke 2

007

Fink shock plotsF

i nk,

Hil

lebr

andt

and

Rop

ke 2

007

Where we Stand!

Bildsten et al. 2007

Many remaining theoretical issues:

• What are the final He ignition masses?

• How does a transverse detonation propagate, especially in the differentiated He layer?

•Does the C/O core ignite? If so, what does it make?

•What’s the event rate for either case?

Kasliwal 2010

WHY NOW?

• Openness: should be open to any researcher, both to advance the pace of scientific discovery, but also to share the load of updating physics, fine-tuning, and further development.

• Modularity: should provide independent, reusable modules.

• Wide Applicability: should be capable of calculating the evolution of stars in a wide range of environments, enabling multi-problem, multi-object physics validation.

Stellar evolution calculations remain a basic tool of broad impact for astrophysics. New observations constantly test the models, even those living in 1D. The continued demand requires the construction of a general, modern stellar evolution code that combines the following advantages:

WHY NOW? (continued)

• Modern techniques: should employ modern numerical approaches, including high-order interpolation schemes, advanced AMR, simultaneous operator solution; should support well defined interfaces for related applications (e.g. atmospheres, winds)

• Microphysics: should allow for up-to-date, wide-ranging, flexible and modular micro-physics.

• Performance: should parallelize on present and future shared-memory, multi-core/thread and possibly hybrid architectures, so that performance continues to grow within the new computational paradigm.

What Can it Do?Run time=2hr, 22 minutes on mac with gfortran, 4 threads

MESA Code of Conduct• That all publications and presentations (research, educational, or

outreach) deriving from the use of MESA acknowledge the Paxton et al. (2010) publication and MESA website.

• That user modifications and additions are given back to the community.

• That users alert the MESA Council about their publications, either pre-release or at the time of publication.

• That users make available in a timely fashion (e.g., online at the MESA website) all information needed for others to recreate their MESA results -- ``open know how'' to match ``open source.'’

• That users agree to help others learn MESA, giving back as the project progresses.