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Explosions on Accreting White Dwarfs
Until the final explosion of an accreting WD as a Type Ia
supernovae, the brightest manifestations of mass transfer are
the thermonuclear ignitions of accreted Hydrogen and
Helium. Observing these flashes provides a census of the
numbers of such binaries in distant galaxies, allowing for
direct comparisons to the Type Ia rates.
Chris Deloye (UCSB=>Northwestern), Gijs Nelemans (U. Nijmegen),
Evan Scannapieco (KITP=>ASU), Ken Shen (UCSB), Dean Townsley
(UCSB=>U. Chicago) & Nevin Weinberg (KITP=>UCB)
Townsley and L. B., 2004, Ap. J., 600, 390 (Theoretical overview)
Townsley and L.B., 2005, Ap. J., 628, 395 (Classical Novae)
Scannapieco and L.B., 2005, Ap. J., 629, L85 (Type Ia SN Rates)
L.B., Townsley, Deloye & Nelemans 2006, Ap. J., 640, 466 (AM CVn)
Shen and L. B. 2007, Ap J, 660, 1444 (Stable H/He Burning)
L.B., Shen, Weinberg & Nelemans 2007, Ap J., 662, L95 (Faint .Ia SN)
Accreting White Dwarfs
Donor star can be H/He or pure He
0.1-1% of white dwarfs are in
binaries where accretion occurs,
releasing gravitational energy
Whereas the nuclear fusion of
H=>He or He=>C releases
White Dwarf of Carbon/Oxygen
Piro ‘05
This contrast is further enhanced
when the white dwarf stores fuel
for > 1000 years and burns it
rapidly, making these binaries
detectable in distant galaxies
during thermonuclear events.
Some numbers
for starters
M87 in Virgo
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
In 10^11 solar masses of old stars
(e.g. Elliptical galaxy), two WDs are
made per year. The observed rates for
thermonuclear events are:
• 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!
Predicted rates are:
Helium novae (Eddington-limited) every ~250 years, one large He
explosion every ~5,000 years, and WD-WD mergers every 200 years
Accreting White Dwarfs
To fill the tidal radius:
Giving the relation:
The mass transfer rate is
set by angular momentum
losses, typically from
gravitational wave
emission.
Hydrogen Burning is Usually Unstable
Townsley & Bildsten 2005
Accumulated mass
Supersoft
Sources: Burn H
Stably (van den
Heuvel et al
1992), or weakly
unstable
Cataclysmic
Variables: undergo
unstable burning,
leading to Classical
Novae. Whether
the mass stays or
goes is uncertain
Nova Rates => Population Density
• The calculated novae recurrence rate as a function of orbital
period allows for a census (Townsley & L.B. 2005) of the CV
population from the observed nova rate of 20 per year in a
10^11 L_sun, K galaxy (Williams & Shafter 2004), yielding a
CV birthrate
• Mass specific CV birthrate matches the Ia rate in Elliptical
galaxies..first time a relative rate comparison is made
• However, the WD masses in CVs are too low to ignite the
C/O in the core, unlikely that they are Type Ia progenitors
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 degenerate Helium WD.
• Giving an orbital period-donor mass relation,
and donor masses ranging from 0.006-0.12
M_sun, 6 Jupiter mass pure He objects!!!
RXJ0806 5.35 min
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
Optical Spectra are Disk + WD!
The accretion of the helium star exposes material
that has completed hydrogen burning
Marsh, Horne and Rosen 1991
• Emission lines
from the accretion
disk
• N/C>100, and
N/O=50 due to
CNO burning
• NO Hydrogen,
H/He<10^-5
• The underlying,
nearly featureless
continuum is from
the reheated white
dwarf (Bildsten et
al. ‘06)
The fate of less than 1 in 2000 white
dwarfs in our galactic disk. But
GP
COM
none yet seen in other galaxies.
These are the brightest Sources for Space-Based
Gravitational Wave Detectors, such as LISA!
The mass transfer rate and evolution of the binary is driven by loss of
angular momentum via gravitational radiation (Faulkner et al ‘72)
at the rate
And the orbital period increases as the donor loses mass
Bildsten et al. ‘06
10
20
30
40
50
Orbital Period (Minutes)
60
70
Eclipses in SDSS J0926+3624
Anderson et al (2005) found this object with 28.3 minute orbit, data
below is from Marsh et al 2006 from ULTRACAM on WHT
g’=19.3
QuickTime™ and a
TIFF (LZW) decompressor
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Marsh et al 2006
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Eclipse of the hot white dwarf, the bright spot, and
part of the accretion disk. Inclination is 83.1+_0.1 deg.
WD mass =0.84+_0.05, and donor mass = 0.03
Helium Burning
He burning is thermally stable when donor mass is 0.2-0.27, with
orbital periods of 2.5-3.5 minutes (Tutukov & Yungelson ‘96). As the
accretion rate drops, the burning becomes unstable, and flashes
commence.
Unstable Helium Burning of Interest
10
20
30
40
50
Orbital Period (Minutes)
CE 315
GP Com
CR Boo, KL Dra
V803 Cen
CP Eri
AM CVn
HP Lib
RXJ0806?
V407 Vul
ES Cet
Thick= Cold Donors
Thin= Hotter Donors
60
70
For a degenerate (or semi degenerate) donor (dashed lines) there will
be many He novae at early times, followed by one last explosive
flash with helium masses of 0.03-0.1 solar masses. .
L. B., Shen, Weinberg & Nelemans ‘07
Helium Donor Mass
For a Helium burning
star donor (dotted line;
Savonije et al 86; Ergma
& Fedorova ‘90),
Helium ignition masses
>0.2 naturally occur on
0.6 WDs and were
studied as double
detonations (Nomoto
‘82, Livne ‘90, Woosley
et al ‘86, Woosley &
Weaver ‘94).
L. B., Shen, Weinberg & Nelemans ‘07
• Early in the evolution, many flashes with masses less than 0.01
• The last flash has the largest mass (0.03 in these cases), and occurs at
1e-8--1e-7 Msun/year (depending on the accreting WD mass)
• Last flash mass is sensitive to the binary evolution and ignition mass
under changing M_dot . . the 0.8 case nearly ignited at 0.08..
• After last flash, the accreted Helium accumulates, so all AM CVns
with P_orb> 10 minutes are building He envelopes (Bildsten et al ‘06)
Path to Helium Shell Detonations
The 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, likely
triggering a detonation of the
helium shell. This condition
sets a minimum shell mass.
Binary Evolution Naturally Yields Detonations
L. B., Shen, Weinberg & Nelemans ‘07
The 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 >0.9,
likely outcome is detonation
• For lower WD masses, the
outcome may be less
violent, further work needed
Must understand the dynamic outcome and nucleosynthetic
yields from these low pressure detonations, a new regime.
Nevin Weinberg ‘07
Example
Shock goes down (blue arrow) into
the C/O and the He detonation
(red arrow) moves outward. The
shocked C/O under the layer is not
ignited. Underlying WD remains
Yields at this point in time
(0.24 seconds) are 0.012 M_sun
of 56Ni, 0.0071 of 48Cr, and
0.0076 of 52Fe.
Thermonuclear Supernova Lightcurves
• Type Ia result from burning a solar mass of C/O to ~0.6 solar masses
of 56Ni (rest burned to Si, Ca, Fe) and ejected at v=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 see Pinto & Eastman 2000) showed that the peak
in the lightcurve 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 from Type Ia SN via the
peak luminosity, yielding 0.1-1.0 solar masses.
.Ia Supernovae*
L. B., Shen, Weinberg & Nelemans ‘07
• The small helium
ignition masses (0.020.1) only detonate
helium, which leaves the
WD at 10,000 km/sec,
leading to rapid rise
times.
• The radioactive decays
of the fresh 48Cr (1.3
days), 52Fe (0.5 d) and
56Ni (8.8 days) will
provide power on this
rapid timescale!!
*Thanks to Chris Stubbs for the name
.Ia Lightcurves courtesy of Daniel Kasen (UCSC)
56Ni Balls
(M_tot,M_Ni)
56Ni/Si Balls
Delta M_15(B)>3, and sometimes 4 (typical Ia’s have 2 at most)
.Ia Supernovae Rates
• Roelofs et al ‘07 space density implies an AM CVn
birthrate
• If every AM CVn gives a .Ia, their rate would be 2-7 % of
the Type Ia rate in an Elliptical Galaxy.
• .Ia Discoveries would reveal distant AM CVns, and may
well have been missed in SN surveys due to rapid decline.
• Volume rate in the nearby universe says that upcoming
optical transient surveys with rapid cadences should find
many. . . Daily survey 1/2 sky (LSST) to V=24 would give
~1000 .Ia’s per year.
Topical Because of New Surveys!
Pan-Starrs1 (2008)
Sloan Digital Sky Survey
(NOW!)
Medium deep survey gets 10 per
year at -17, and 1 per year at -15
Current survey will find 7 per year
at -17, and 0.5 per year at -15. Total
duration = 9 months..
LSST (2014)
Large Synoptic Survey Telescope (LSST) is a proposed ground-based
8.4-meter, 10 square-degree-field telescope that will provide digital
imaging of faint astronomical objects across the entire sky every night.
Cerra Pachon, Chile.
Daily survey 1/2 sky would give up to 1000 .Ia’s per year.
“Super” luminous 1991T
Bolometric LCs
Subluminous
1991bg
Contardo et al. ‘00, A&A, 359, 876
Type Ia Supernovae Dependence on
Galaxy Type and Cosmic Rates
There are observed trends in Ia properties with galaxy type (no
evidence yet for metallicity effects):
1. Brightest (e.g. 1991T) events occur preferentially in young stellar
environments (hence mostly spiral and irregular galaxies)
2. Sub-luminous (and peculiar, eg. 1991bg) Ia’s dramatically prefer
old stellar populations . . (Elliptical and S0 Galaxies)
3. Rates track BOTH the stellar mass and the star formation rate
These are likely the result of old and young stellar populations and
motivated our (Scannapieco & LB, 2005, ApJ, 629, L85) simple
explanation for the observed cosmic Ia rate.
SN Rate Dependence
on Galaxy Type
• Infrared luminosity used to
determine the stellar mass
• Part of the Ia rate tracks the
Star formation and is 1/3 the
Core Collapse rate
• Ia Data can be “fit” with
one term that depends on
mass (confirmed in clusters:
Sharon et al ‘06) and another
that is 40% of the core
collapse rate
• Roughly one Ia every 400
years for 1 solar mass per
year of star formation.
Rates by Mass
Mannucci et al ‘05
Assume that the Ia rate tracks the stellar mass and star formation
rate as measured by Mannucci et al., then measure the constants
from local galaxies to get (Scannapieco & L.B. ‘05)
• This example shows
the outcome when the
SFR drops exponentially
on a 2 Gyr timescale
QuickTime™ and a
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• 80% of the Ia SN over
a galaxy’s lifetime come
from the prompt
contribution (see Oemler
& Tinsley 1979).
• Fe production from Ia’s
is 3X that from CC
Star Formation Rate
Iron Abundance in Galaxy Clusters
QuickTime™ and a
TIFF (Uncompressed) decompressor
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Scannapieco & Bildsten 2005
Canada-France-Hawaii Telescope SuperNova
Legacy Survey (SNLS)
125 Ia Sne, 0.2<z<0.75
Sullivan et al 2006
• Galaxies identified
from the CFHT survey.
All Ia’s are
spectroscopically
confirmed
• For the clear
counterparts (some are
ambiguous), the
galaxies were classified
via colors as vigorous
star formers, starforming, and passive.
•When SNLS is done,
this list should be ~500
Scalings with Star Formation Rate
1 every
300 years
in 10^11
Sullivan et al ‘06
Confirmation of the mass specific rate of Mannucci et al for passive
galaxies, and confirmation of the Ia rate dependence on
star formation rate.
CFHT Supernovae Legacy Survey (SNLS)
Star Forming Galaxies
Red=Passive
Star Forming-Passive
Sullivan et al 2006
Fainter
The number of faint (small stretch) Ia’s in spirals is
consistent with the old stellar population in the spiral galaxy.
The two populations are distinct, but overlapping in their
56Ni production levels.
Ia Rate vs. Redshift
Sullivan et al ‘06
Blue arrow
shows the
expected
local Ia
rate just
from the
local Klight
density
Their normalization with the SFR is 3 times smaller than
Scannapieco and Bildsten ‘05, giving a different evolution with z.
Should be resolved, but will take time. . . . .
CFHT Legacy Survey: Just Fit the Data!
Neill et al. 2007 astroph-0701161
Conclusions
• Classical Novae ignition mass calculations allow us to say
that ejected masses are similar to ignition masses
• .Ia SN expected from AM CVn binaries (which came
from double WDs) and should be found in abundance in
upcoming surveys.
• The observed Ia rate depends on both the mass of a galaxy
and the star formation rate, allowing for 1. Improved
understanding of the Fe abundance in galaxy clusters, 2.
Ia rate as a function of redshift, 3. Clear evidence for
multiple progenitors
Clearly more to learn about thermonuclear events on
accreting white dwarfs, especially in the upcoming
age of all-sky surveys
Las Cumbres Observatory
Global Telescope Network
A private scientific observatory based in Santa Barbara that owns and
operates 2 m Faulkes telescopes in Maui and Australia, and which will:
• Construct and operate ~6 sites at multiple longitudes with 3x1 meter
telescopes for continuous coverage of transients, transits, etc..
• Each telescope will have multiple instruments, and be refreshed
• Construct and operate many (>30) 0.4 meter telescopes around the
globe primarily for education and outreach.
• Remote operations, data management, and instrument construction
based in Santa Barbara.
• Strong connections to UC-Santa Barbara. Tim Brown is LCOGT
Scientific Director and Adjunct Professor of Physics at UCSB.
• The whole facility (both the rings and the smaller telescopes) will be
made available to the international scientific community.
• Fully funded for 25 years of operation. Go to www.lcogt.net