HYDEF07: A Summary on the Fly
especially the Friday morning talks
James Liebert
University of Arizona
Tucson, AZ USA
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Greenstein and Sargent (1974 ApJS) study of sdB - sdO - hot white dwarf stars
Anneila Sargent, PhD Caltech, Distinguished Professor, Caltech
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First Big Theme:
White Dwarf Mergers: and
How they result in different types
Of HYDEF stars
There’s even motivation from
the white dwarf mass distribution
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Liebert, Bergeron, Holberg
348 DA White Dwarfs
From the
Palomar Green Survey
A simple histogram
of the numbers from
a magnitude limited sample
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3% (<0.5)
72% (0.5-0.8)
25%(>0.8)
Mass Distribution Corrected to Fixed Volume of Space
I suspect that most hot, high mass white dwarfs found at high Galactic latitude
Formed from mergers, and are old.
If the cooling time is fairly short (eg. a few 108 years)
and it came from a 5-8M progenitor (nuclear lifetime < or about 108 years)
then the white dwarf should be associated with a cluster or association
in Gould’s Belt or the like -- and some massive white dwarfs ARE associated
with Gould’s Belt -- for example a one solar mass WD in the Pleiades -(see the Appendix of Liebert, Bergeron, and Holberg 2006 ApJS
For a plot of white dwarfs that might be associated with Gould’s belt.)
But a lot of them in the Palomar Green Survey (blat > 30 degrees)
and in SDSS are usually pretty much alone.
So, I think we need many of them to form from mergers of two white dwarfs
of lower mass.
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Close binary white dwarf mergers with Mtot < MCh
may occur frequently, as predicted in the 1980s by
Tutukov, Iben, Yungelson
“40% of white dwarfs are, or were, in close binaries.”
Well then, there should be three kinds:
-- Podsiadlowski talk
1. He + He (pretty low Mtot)
2. He + CO (Mtot ~ 1.0 M)
3. CO + CO (Mtot ~ 1.2 M or more)
disregarding the likely rare ONeMg WDs
A lot of folks at this meeting think that
some of these merging WDs
can be progenitors of certain classes of H-deficient evolved stars.
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RCrB Stars
theory:
B. Gustafsson, Y. Pavlenko (atmospheres and abundances)
A. Rosenbush (dust shells)
H. Saio (radial and non-radial pulsations)
observations:
N. K. Rao (aspects)
G. Pandey (Fluorine)
J. Clayton (broad review)
R Corona Borealis
one of first variable stars found -- during term of first U.S. President
George Washington (1795)
6th magnitude usually
prototype of class -- though in wrong hemisphere compared with most others
amorphous carbon produces featureless mid-IR, no PAHs because there is no H
smooth pulsations in L (3.6µ) band
-- Jeff Clayton
but what are they?
“Born again” late helium flashers (no, not Baptist fundamentalists)
or
(2) Merger products of double degenerates (no, not a pair of immoral,
sinful people)
(1)
Consensus seems to be building in favor of (2)
Why C/He ratio cannot be measured
-- N. Kameswara Rao
But believed to be only 1 %
-- J. Clayton
Most Daunting Mystery:
The C I line problem
-- B. Gustafsson
C I line strengths hardly depend on abundance
because line and cont both scale with n(C)
Fit to spectral energy distribution is OK
Even in the UV -- gets good Teff estimate
But lines, 5000-7000Å, are only 1/3 as strong
As models predict
Only solution: flatten T gradient where lines form
But why should the atmosphere do this?
(1) RCrB may be result of He + CO WD merger
-- many people here
May evolve (ironically) to very hot, carbon-poor O(He) stars
(yes, I realize C/He is only 0.01 -- still makes lots of dust!)
but how is Li produced in 4 of them? (Clayton)
High luminosity Extreme Helium Stars may also have come from
He + CO mergers -- S. Jeffery
Or C-rich He-sdB stars could be the progeny (Ahmad)
Or He-sdB could come from post+sdB merging with WD (Podsiadlowski)
And then.. On to the hottest DO, KPD0005+5106
(Werner poster, Rauch)
(2) He + He WD mergers may evolve to lower luminosity EHe Stars
Low Baade masses estimated for pulsators -- eg. 0.59M V652 Her-consistent with this idea (Jeffery)
Or He+He to He-sdO (Napiwotzki) since they are seldom binary (Heber)
Or maybe the merger evolves to C-poor He-sdB stars (Ahmad)
(3) What about CO + CO WDs?
Probably carbon ignites and burns stably (or in flashes)
Get ONeMg WD of very high mass
-- Bildsten, private remarks
Or even, if M > Mch , then maybe
BOOM!
or just an unstable ONeMg WD core collapse
A word about the physics and modeling of the WD+WD merger.
As complicated a 3D, SPH, hydrodynamics simulation problem as there is.
Demands high spatial resolution. Wide variance in relevent physical scales.
Sharp composition discontinuities.
Shocks and feedback. How to do this?
Probably a detailed nuclear reaction network with neutrino cooling
to be followed in high time resolution.
Calculate very time-dependent abundances.
Get the angular momentum conservation right.
Do it all together with stellar evolution. (Fryer)
I am glad Chris Fryer is optimistic. I look forward to the results.
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Second Theme:
Most interesting HYDEF types
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[Wolf Rayet] evolutionary story, abundances, and modelling the winds,
seem interesting, and not in a bad state
-- observations and abundances (Crowther, Todt)
WCL “iron forest” amazing! Not just Lyman  in high-z QSOs
(Todt and collaborators)
-- wind models (Graefner)
WCL - WCE - WO - PG1159 Stars - DO white dwarfs
Usually substantial, similar abundances of He, C, less O
N, Ne an order of magnitude less
Mass loss (wind) declines along sequence
Sorry, but I didn’t understand where the WELS (Marcolino),
the Weak Emission Line Stars with otherwise similar properties,
fit into the sequence
Longmore 4 (Bond, after Werner and collaborators) has outbursts
which change its spectrum from PG1159 to Wce
Or, WELS are simply low mass objects (they never have PN)
- O. De Marco
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On to PG1159 Abundances (Werner, Reiff, Ziegler)
And pulsations (Quirion, Bond, Solheim)
Different mixes of He, C, O, even H
With small amounts of N, Ne
but
No “iron forest” in PG1159 Stars (Reiff)
Fe not even detected!
Ni is solar, but should arguably have been more
PG1159 stars occupy a chunk of the hot HRD
from log g ~ 5+ to nearly 8
200,000K (if you count H1504) down to ~75,000K
and they can pulsate over most of this range
..or not, as the case may be
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Three post-AGB - post-merger - post HB Sequences
1. H-dominated Atmosphere:
H-rich PNN - sdO - DA white dwarf
2. He-dominated Atmosphere: He or HeCO-dominated PNN/PG1159 - DO white dwarf
RCrB? O(He) Star - KPD0005 hot DO - DO white dwarf
3. C(O?)-dominated Atmosphere: H1504+65 - C-dominated hot DQ white dwarf
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Third Theme: Very Late Thermal Pulses (VLTP) and asking
How many HDEF evolved stars, PNN, WDs form from fast VLTP (V605 Aqu, Sakarai’s)
And slow LTP (FG Sge) events?
Answer will be determined empirically by time-domain (repeat) observations of
Nearby galaxies (PanSTARRS, SkyMapper, LSST)
--note: these don’t happen in our Galaxy often enough to do statistics
and we have a bad view sitting inside it -- Galactic plane extinction
We should make predictions -- create a “benchmark” to be tested
How?
HDEF evolved stars appear to be about 15% of total
6:1 Ratio H/HDEF PNN and Hot White Dwarfs (DA/DB+DO)
If 2 white dwarfs form per year per 1011 M (visible matter) galaxy,
Then upper limit is 0.3 about VLTP events per year, if 100% of HDEFS
came from them. Of course only a too-be-determined fraction, f, do.
So
Feed in realistic mass distributioln of white dwarfs from local disk
(assume extragalactic populations have similar distribution -though they may not!)
Estimate what WD masses are appropriate for rapid VLTP brightening
(Miller Bertolami). In principle, this would yield the event rate -- 0.3f per year.
Also, how does frequency of TPs depend on AGB and post-AGB core mass?
(How often are they just slow LTP events, which may also produce HYDEFS?)
Should say a little bit about the physics of the VLTP
No time to review V605 Aql and Sakarai’s
HYDEF post-AGB stars and the VLTP
--Herwig
No evolutionary model is correct until
(1) You do 3D
(2) A hydrodynamical treatment of convection is employed
Even AGB Calculations with H, He shell sources are
Hard enough
Not one, but two convective layers
--the helium shell when it pulses
--the hydrogen envelope
The Schwarzschild criterion simply does not
bound the mixing -- there is always overshooting.
Two convection zones!
“Double, Double, Toil and Trouble.”
--MacBeth
Fourth Theme: Common Envelope Evolution and Consequences
(Diehl)
Thermal pulses on AGB trigger radius peaks which may “capture” companion.
It spirals in, removing outer envelope.
The lower the companion mass, the less efficient it is in shedding envelope
And the farther in it spirals.
Did 0.25 and 0.05M (substellar) calculations.
This calculation needs to be extended to lower masses.
Maxted, Napiwotzki et al. (SPY project) made an interesting
discovery last year, published in Nature: a double-lined white dwarf /
brown dwarf binary. The brown dwarf was measured to be
0.055M at orbital period ~ 2 hours. They estimate Roche lobe
overflow will occur at about P ~ 75 minutes, creating a
short period cataclysmic variable.
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Now.. It was pointed out by a few people at this meeting that
few planetary nebula are spherical -- most are bipolar, elliptical.
It has been argued that a close companion, perhaps even a
Planet, could provide the polar axis for the expelled gas.
It has been suggested (Solheim talk, Soker) that a 72.5-minute
stable periodicity observed in the pulsations of NGC246
could be due to a (obviously close) substellar companion .
(Its just too fast to be rotation.)
(Note this appears a bit snug not to trigger Roche lobe overflow,
but maybe its just real close.)
However, the Maxted et al. brown dwarf which survived common
envelope evolution is rare…
It is well known that there is a BROWN DWARF DESERT or
lack of brown dwarf comanions to stars at separations
Measurable by the radial velocity surveys. Mayor, Marcy,
Queloz, Butler & others have found very few brown dwarfs.
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But Jovian mass companions is what they have found in
considerable numbers.
But would a Jovian or several Jovian mass planet
survive the common envelope?
Back in the 1980s, Livio and Soker published a
study saying “NO!”
They found a rough dividing line near 10 Jupiter masses = 0.01 M
below which the planet is evaporated away in the envelope, and
above which it should actually grow in mass by accretion, as it
spirals in.
If a planetary nebula could speak, maybe NGC246 would say
“But I have a planet in a 72 minute orbit.”
A planet might fit inside the Roche lobe better than a brown dwarf.
Now Diehl and Fryer -- and others -- need to reinvestigate
this important question.
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A Very Big Issue:
The Origin of those Cosmology Probes..
The SN Ia ‘s
-- If I have learned some about this recently,
its because of Lars’ KITP program
You can download the talks and slides still
from the web page!
What kind of accreting white dwarf?
(or could it be a merger of double degenerates?)
Pardon my sarcasm:
Do we want our cosmology model to rely on systems
the Universe -- but not we -- know how to make?
Doesn’t stop theorists from telling us how the peak luminosities
And light curves depend on redshift.
If you took a vote, the community might favor this
scenario:*
Steady nuclear burning on WD at VERY HIGH rate
--needs subgiant or giant donor
--associated with super-soft X-ray sources
•Maybe this sort of thing should not be decided by vote.
(I vote for Pluto!)
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The companion had better be big, to have a big Mdot.
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Issues/Problems with the scenario:
1. Mdot had better not “get out of the box”
..or you’ll get nova outbursts, and lose the accreted mass
..or lose it in a common envelope phase
2. A subgiant / giant donor may have a whole
lot of hydrogen still left when the WD blows..but
SNIa’s show no H (or He) in the spectrum
Studied by Y. Marietta and A. Burrows -- the shock just blows past, frees the
Envelope, but does not carry it along. Only late observations could
reveal the narrow H lines from the original envelope (if there was one).
3. The catalogued Galactic and LMC S.S.Sources are an
odd, heterogeneous collection of different kinds of objects
-- are there enough of them?
Rosanne Di Stefano: an emphatic No!
Not enough super-soft sources in M31 and M33 by up to 2 orders of magnitude
From Ken Nomoto
Bildsten and Fryer showed versions of this.
Problems with double degenerate merger scenario:
 Not enough of them? -- only one with close enough
Separation, and combined mass (barely) > Mch
In SPY Project
plus
 One sdB + white dwarf binary KPD 1930+2752
(Geier et al. 2006, Leicester white dwarf workshop,
astro-ph/0612532)
How much mass is lost in the potentially-powerful wind
as the massive remnant of the secondary
tries to stuff itself on the primary,
yielding lots of gravitational energy
Note, however:
No one has proven that if the merger has Mtot < Mch
that a detonation doesn’t result,
On the other hand, you have heard the case this morning
(Podsiadlowski, Bildsten)
that a merger at Mch could result in just a collapse with little explosion.
But “Maybe the Details can Save Us” (Fryer)
And.. how do you account for three apparent
“super Chandrasekhar” mass SNIa’s ?
1.2 M of 56Ni, instead of (often considerably less than 1M
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Then there’s the old edge-lit detonation idea:
Detonation of helium at inner edge of massive helium envelope
Triggers carbon core to explode with Mtot < Mch
Out of favor these days
-- Gets too much production of intermediate mass nucleons
-- Predicted spectra do not match observed
-- Bildsten et al 2007: you get halfway there
the helium detonates but not the carbon
an underluminous SN 1a - like explosion
or a
.1a “supernova” (Bildsten)
But, if this were a possible SN1a
There may be lots of potential systems
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I want to reiterate Lars’ (also Paolo Mazzali’s) point:
SNIa’s pop out both “soon and late”
We probably need at least 2 progenitor classes
(Branch normal and Branch davidian)
One pretty young (a few x 108 to 109 years)
One that is old -- in red and dead galaxies
(less luminous, fewer)
But we have yet to zero in on one progenitor class!!!
KITP punch line: “There are no progenitors.”
We may need them all!
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I am running out of time !!!
much I’m just not going to get to.
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Got to get this one in..
CK Vul -- ??? M. Hajduk and collaborators
Did good job of outlining the facets of the mystery
the slowest nova?
hibernating nova?
a very late thermal pulse object?
3rd magnitude star - twice in last millenium
Now there is scarcely anything to be seen
reddened nebulosity -- H-deficient knots
Yet radio emission means an ongoing ionization source
(since recombination time ~1 year!)
What to make of this? I haven’t a clue.
Maybe it will rise to 3rd magnitude again, and tell us.
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AM CVn binaries
Found by SDSS researchers
Anderson et al.
2005 AJ, 130, 2230
This one eclipses
Tom Marsh gave a nice review.
Much more to be done on these,
(And their compact X-ray binary
Counterparts -- Nelemans).
May they go on aggravating
the gravity wave people, as
Unwanted “background”.
Now we learn that
They might even
EXPLODE!
.1A “Supernovae”
(Bildsten, Nelemans & Co, )
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The Central Region of the Galaxy
Is a Special Place
It is where most Wolf Rayet stars are,
I think. The “Arches cluster” has
quite a few.
Some 300 known
--Hamann
They can shed their dust like a “pinwheel”
Strong winds, but not necessarily X-ray
Emitters.
Wind Models: Graefener
but are the winds driven by “strange mode” pulsations
(Glatzel)
or radiation pressure, as they are near Ledd
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“Special” Wolf-Rayet Stars -- Type 1c Hypernova -Collapse to Massive Black Hole -Bipolar Jets -- Gamma Ray Burst
“Spin” Problem -- Wolf Rayet phase will lose the star’s angular momentum.
May need to have a close companion to spin it up.
Or a binary merger (Fryer) -- “exotic”
-- Langer
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