# White Dwarf Merger (and the Implications)

The recent news about an unusual supernova in Hercules some 300 million light years away has a wider significance than might first appear. Supernovae are important for more than their role in seeding the cosmos with heavy metals forged in their stellar furnaces. They’re also widely used cosmological markers. Type Ia supernovae, thought to be well understood, typically occur in a band of brightness that makes them ‘standard candles,’ useful in calculating cosmic distances.

It was work on Type Ia supernovae, in fact, that led to the discovery of the universe’s accelerating expansion. And what the latest find implies is that, contrary to earlier thinking, this kind of supernova may be more varied than previously thought. The new find — supernova 2006gz — appears to result from the collision of two white dwarfs that had been in orbit around each other. The evidence: a strong spectral signature of unburned carbon and clear signs of compressed layers of silicon.

Both spectral signatures show up in computer models of merging white dwarfs, but consider what a jump this is from the standard Type Ia scenario. In it, a white dwarf draws gas from a companion star until it undergoes catastrophic fusion and explodes. All of which is different from Type II supernovae, which are massive stars that undergo core collapse before exploding. Of the two, Type Ia are the most common, their spectra usually distinctive in lacking hydrogen.

Image: This artist’s conception shows two white dwarf stars spiraling in toward each other until they collide. A collision like this is believed to have spawned supernova 2006gz. Credit: NASA/Dana Berry, Sky Works Digital.

What SN 2006gz is telling us is that a double white dwarf explosion has to be factored into the analysis when looking at Type Ia events. Malcolm Hicken, a Harvard graduate student and first author of the paper on this work, notes the implication:

“Supernova 2006gz stands out from normal Type Ia objects and wouldn’t be included in cosmology studies. But we have to be careful not to mistake a double white dwarf explosion for a single white dwarf blast. SN 2006gz was easy to recognize, but there may be less clear-cut cases.”

Indeed. And if we do mistake a white dwarf merger for a standard Type Ia supernova, we may be adding an error into any calculations involving its use as a standard candle. One suspects such events are relatively rare, but these findings are useful in adding another factor to watch for as we calibrate our measurements of what is happening at cosmological distances. The paper is Hicken et al., “The Luminous and Carbon-rich Supernova 2006gz: A Double Degenerate Merger?” Astrophysical Journal 669 (November 1, 2007), pp. L17-L20 (abstract).

Comments on this entry are closed.

• AGeek November 2, 2007, 14:30

At least one guy has been saying for some time (and getting plenty of of flak for it) that core mergers could be a problem for the “standard” cosmological interpretation of SNe Ia: http://arxiv.org/abs/astro-ph/0608386

• Adam November 2, 2007, 22:38

Hi Paul

Laughlin and Adams predict that white-dwarf mergers will be the only source of supernovae in the deep future of the Universe, once regular star formation has ground to a dusty halt.

http://xxx.lanl.gov/abs/astro-ph/9701131v1

…the basis for their excellent “Five Ages of the Cosmos”.

Their paper with Peter Bodenheimer, “The End of the Main Sequence”, is worth a look too…

http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v482n1/35131/35131.html

…in the ten years since I haven’t seen too much change on red dwarf evolution, though the surface temperatures they use are somewhat lower than current models.

• ljk November 19, 2007, 0:53

Finding Planets Around White Dwarf Remnants of Massive Stars

Authors: Andrew Gould, Mukremin Kilic

(Submitted on 15 Nov 2007)

Abstract: Planet frequency shows a strong positive correlation with host mass from the hydrogen-burning limit to M = 2Msun. No search has yet been conducted for planets of higher-mass hosts because all existing techniques are insensitive to these planets. We show that infrared observations of the white-dwarf remnants of massive stars 3Msun less than M less than 7Msun would be sensitive to these planets for reasons that are closely connected to the insensitivity of other methods.

We identify 49 reasonably bright, young, massive white dwarfs from the Palomar-Green survey and discuss methods for detecting planets and for distinguishing between planet and disk explanations for any excess flux observed. The young, bright, massive white dwarf sample could be expanded by a factor 4-5 by surveying the remainder of the sky for bright UV-excess objects.

Comments: Submitted to ApJ Letters

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0711.2508v1 [astro-ph]

Submission history

From: Mukremin Kilic [view email]

[v1] Thu, 15 Nov 2007 21:43:58 GMT (53kb)

http://arxiv.org/abs/0711.2508

• ljk November 21, 2007, 16:16

ASTRONOMERS DISCOVER STARS WITH CARBON ATMOSPHERES

November 21, 2007

Astronomers have discovered white dwarf stars with pure carbon atmospheres.

These stars possibly evolved in a sequence astronomers didn’t know before.

They may have evolved from stars that are not quite massive enough to
explode as supernovae but are just on the borderline. All but the most
massive two or three percent of stars eventually die as white dwarfs rather
than explode as supernovae.

When a star burns helium, it leaves “ashes” of carbon and oxygen. When its
nuclear fuel is exhausted, the star then dies as a white dwarf, which is an
extremely dense object that packs the mass of our sun into an object about
the size of Earth. Astronomers believe that most white dwarf stars have a
core made of carbon and oxygen which is hidden from view by a surrounding
atmosphere of hydrogen or helium.

They didn’t expect stars with carbon atmospheres.

“We’ve found stars with no detectable traces of helium and hydrogen in their
atmospheres,” said University of Arizona Steward Observatory astronomer
Patrick Dufour. “We might actually be observing directly a bare stellar
core. We possibly have a window on what used to be the star’s nuclear
furnace and are seeing the ashes of the nuclear reaction that once took
place.”

Dufour, UA astronomy Professor James Liebert and their colleagues at the
Université de Montréal and Paris Observatory published the results in the
Nov. 22 issue of Nature.

The stars were discovered among 10,000 new white dwarf stars found in the
Sloan Digital Sky Survey. The survey, known as the SDSS, found about four
times as many white dwarf stars previously known.

Liebert identified a few dozens of the newfound white dwarfs as “DQ” white
dwarfs in 2003. When observed in optical light, DQ stars appear to be mostly
helium and carbon. Astronomers believe that convection in the helium zone
dredges up carbon from the star’s carbon-oxygen core.

Dufour developed a model to analyze the atmospheres of DQ stars as part of
his doctoral research at the Université de Montréal. His model simulated
cool DQ stars, stars at temperatures between 5,000 degrees and 12,000
degrees Kelvin. For reference, our sun’s surface temperature is around 5,780
degrees Kelvin.

When Dufour joined Steward Observatory in January, he updated his code to
analyze hotter stars, stars as hot as 24,000 degrees Kelvin.

“When I first started modeling the atmospheres of these hotter DQ stars, my
first thought was that these are helium-rich stars with traces of carbon,
just like the cooler ones,” Dufour said. “But as I started analyzing the
stars with the higher temperature model, I realized that even if I increased
the carbon abundance, the model still didn’t agree with the SDSS data,”
Dufour said.

In May 2007, “out of pure desperation, I decided to try modeling a
pure-carbon atmosphere. It worked,” Dufour said. “I found that if I
calculated a pure carbon atmosphere model, it reproduces the spectra exactly
as observed. No one had calculated a pure carbon atmosphere
model before. No one believed that it existed. We were surprised and
excited.”

Dufour and his colleagues have identified eight carbon-dominated atmosphere
white dwarf stars among about 200 DQ stars they’ve checked in the Sloan data
so far.

The great mystery is why these carbon-atmosphere stars are found only
between about 18,000 degrees and 23,000 degrees Kelvin. “These stars are too
hot to be explained by the standard convective dredge-up scenario, so there
must be another explanation,” Dufour said.

Dufour and Liebert say they these stars might have evolved from a star like
the unique, much hotter star called H1504+65 that Pennsylvania State
University astronomer John A. Nousek, Liebert and others reported in 1986.
If so, carbon-atmosphere stars represent a previously unknown sequence of
stellar evolution.

H1504+65 is a very massive star at 200,000 degrees Kelvin.

Astronomers currently believe this star somehow violently expelled all its
hydrogen and all but a very small trace of its helium, leaving an
essentially bare stellar nucleus with a surface of 50 percent carbon and 50
percent oxygen.

“We think that when a star like H1504+65 cools, it eventually becomes like
the pure-carbon stars,” Dufour said. As the massive star cools, gravity
separates carbon, oxygen and trace helium. Above 25,000 degrees Kelvin, the
trace helium rises to the top, forming a thin layer above the much more
massive carbon envelope, effectively disguising the star as a
helium-atmosphere white dwarf, Dufour and Liebert said.

But between 18,000 and 23,000 degrees Kelvin, convection in the carbon zone
probably dilutes the thin helium layer. At these temperatures, oxygen, which
is heavier than carbon, has probably sunk too deep to be dredged to the
surface.

Dufour and his colleagues say that models of stars nine to 11 solar masses
might explain their peculiar carbon stars.

Astronomers predicted in 1999 that stars nine or 10 times as massive as our
sun would become white dwarfs with oxygen-magnesium-neon cores and mostly
carbon-oxygen atmospheres. More massive stars explode as supernovae.

But scientists aren’t sure where the dividing line is, whether stars eight,
nine, 10 or 11 times as massive as our sun are required to create
supernovae.

“We don’t know if these carbon atmosphere stars are the result of nine-or-10
solar mass star evolution, which is a key question,” Liebert said.

The UA astronomers plan making new observations of the carbon atmosphere
stars at the 6.5-meter MMT Observatory on Mount Hopkins, Ariz., in December
to better pinpoint their masses. The observations could help define the mass
limit for stars dying as white dwarfs or dying as supernovae, Dufour said.

SOURCE CONTACTS:

Patrick Dufour (520-621-5505; pdfour@as.arizona.edu)
James Liebert (520-621-4513; jliebert@as.arizona.edu)

PIO Source:

Lori Stiles
1-520-626-4402
lstiles@u.arizona.edu

• ljk November 24, 2007, 0:49

Rare White dwarf stars with carbon atmospheres

Authors: P. Dufour, James Liebert, G. Fontaine, N. Behara

(Submitted on 20 Nov 2007)

Abstract: White dwarfs represent the endpoint of stellar evolution for stars with initial masses between approximately 0.07 msun and 8-10 msun, where msun is the mass of the Sun (more massive stars end their life as either black holes or neutron stars). The theory of stellar evolution predicts that the majority of white dwarfs have a core made of carbon and oxygen, which itself is surrounded by a helium layer and, for ~80 per cent of known white dwarfs, by an additional hydrogen layer. All white dwarfs therefore have been traditionally found to belong to one of two categories: those with a hydrogen-rich atmosphere (the DA spectral type) and those with a helium-rich atmosphere (the non-DAs).

Here we report the discovery of several white dwarfs with atmospheres primarily composed of carbon, with little or no trace of hydrogen or helium. Our analysis shows that the atmospheric parameters found for these stars do not fit satisfactorily in any of the currently known theories of post-asymptotic giant branch evolution, although these objects might be the cooler counterpart of the unique and extensively studied PG1159 star H1504+65. These stars, together with H1504+65, might accordingly form a new evolutionary sequence that follow the asymptotic giant branch.

Comments: 7 pages, 1 figure, to appear in Nov 22nd 2007 edition of Nature

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0711.3227v1 [astro-ph]

Submission history

From: Patrick Dufour [view email]

[v1] Tue, 20 Nov 2007 21:44:05 GMT (19kb)

http://arxiv.org/abs/0711.3227

• ljk November 24, 2007, 1:08

Hot DQ White Dwarf Stars: A New Challenge to Stellar Evolution

Authors: P. Dufour, J. Liebert, G. Fontaine, N. Behara

(Submitted on 21 Nov 2007)

Abstract: We report the discovery of a new class of hydrogen-deficient stars: white dwarfs with an atmosphere primarily composed of carbon, with little or no trace of hydrogen or helium. Our analysis shows that the atmospheric parameters found for these stars do not fit satisfactorily in any of the currently known theories of post-asymptotic giant branch (AGB) evolution, although these objects might be the cooler counter-part of the unique and extensively studied PG 1159 star H1504+65. These stars, together with H1504+65, might thus form a new evolutionary post-AGB sequence.

Comments: To appear in proceedings of “Hydrogen-Deficient Stars” conference, held in Tuebingen, Germany, Sept. 17-21, 2007. 4 pages, 1 figure

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0711.3458v1 [astro-ph]

Submission history

From: Patrick Dufour [view email]

[v1] Wed, 21 Nov 2007 20:49:44 GMT (59kb)

http://arxiv.org/abs/0711.3458

• ljk December 14, 2007, 12:46

The Future is Now: the Formation of Single Low Mass White Dwarfs in the Solar Neighborhood

Authors: Mukremin Kilic, K. Z. Stanek, M. H. Pinsonneault (Ohio State)

(Submitted on 21 Jun 2007 (v1), last revised 13 Dec 2007 (this version, v2))

Abstract: Low mass helium-core white dwarfs (M less than 0.45 Msun) can be produced from interacting binary systems, and traditionally all of them have been attributed to this channel. However, a low mass white dwarf could also result from a single star that experiences severe mass loss on the first ascent giant branch.

A large population of low mass He-core white dwarfs has been discovered in the old metal-rich cluster NGC 6791. There is therefore a mechanism in clusters to produce low mass white dwarfs without requiring binary star interactions, and we search for evidence of a similar population in field white dwarfs.

We argue that there is a significant field population (of order half of the detected systems) that arises from old metal rich stars which truncate their evolution prior to the helium flash from severe mass loss. There is a consistent absence of evidence for nearby companions in a large fraction of low mass white dwarfs. The number of old metal-rich field dwarfs is also comparable with the apparently single low mass white dwarf population, and our revised estimate for the space density of low mass white dwarfs produced from binary interactions is also compatible with theoretical expectations.

This indicates that this channel of stellar evolution, hitherto thought hypothetical only, has been in operation in our own Galaxy for many billions of years.

One strong implication of our model is that single low mass white dwarfs should be good targets for planet searches because they are likely to arise from metal-rich progenitors. We also discuss other observational tests and implications, including the potential impact on SN Ia rates and the frequency of planetary nebulae.

Comments: ApJ published version

Subjects: Astrophysics (astro-ph)

Journal reference: ApJ, 2007, 671, 761

Cite as: arXiv:0706.3045v2 [astro-ph]

Submission history

From: Mukremin Kilic [view email]

[v1] Thu, 21 Jun 2007 16:35:59 GMT (19kb)

[v2] Thu, 13 Dec 2007 19:36:41 GMT (19kb)

http://arxiv.org/abs/0706.3045

• ljk January 2, 2008, 14:48

PRESS RELEASE: 08-02

WHITE DWARF PULSES LIKE A PULSAR

GREENBELT, Md. – New observations from Suzaku, a
joint Japanese Aerospace Exploration Agency (JAXA)
and NASA X-ray observatory, have challenged scientists’
conventional understanding of white dwarfs. Observers
had believed white dwarfs were inert stellar corpses that
slowly cool and fade away, but the new data tell a completely
different story.

At least one white dwarf, known as AE Aquarii, emits
pulses of high-energy (hard) X-rays as it whirls around
on its axis. “We’re seeing behavior like the pulsar in the
Crab Nebula, but we’re seeing it in a white dwarf,” says
Koji Mukai of NASA Goddard Space Flight Center in
Greenbelt, Md. The Crab Nebula is the shattered remnant
of a massive star that ended its life in a supernova explosion.

“This is the first time such pulsar-like behavior has ever
been observed in a white dwarf.” Mukai is co-author of a
paper presented at a Suzaku science conference in San
Diego, Calif., in December.

White dwarfs and pulsars represent distinct classes of
compact objects that are born in the wake of stellar death.
A white dwarf forms when a star similar in mass to our sun
runs out of nuclear fuel. As the outer layers puff off into
space, the core gravitationally contracts into a sphere about
the size of Earth, but with roughly the mass of our sun. The
white dwarf starts off scorching hot from the star’s residual
heat. But with nothing to sustain nuclear reactions, it slowly
cools over billions of years, eventually fading to near invisibility
as a black dwarf.

A pulsar is a type of neutron star, a collapsed core of an
extremely massive star that exploded in a supernova.
Whereas white dwarfs have incredibly high densities by
earthly standards, neutron stars are even denser,
cramming roughly 1.3 solar masses into a city-sized
sphere. Pulsars give off radio and X-ray pulsations in
lighthouse-like beams.

http://www.nasa.gov/topics/universe/features/whitedwarf_pulsar.html

• James M. Essig January 3, 2008, 2:21

Hi ljk;

I like the story about the white dwarf that acts like a pulsar and the statement within the above referenced NASA article about how apparently both white dwarfs and pulsars should be able to send out charged particles at nearly the speed of light via interaction with the stars very intense proximate magnetic fields.

I can imagine a space craft slowly descending above the polar emmination of the stellar body’s magnetic field wherein the polarity of the stars magnetic field with respect to the star facing end of a space craft would be of a -.- or a +,+ configuration. If the craft board magnetic field where numerically adjustable in a fine enough manner and rapidly enough, perhaps the craft could indeed descend relatively close to the polar region of the stellar surface and then suddenly switch on an extremely intense electromagnet that produces a magnetic field of the same polarity and subsequently cause the craft to be rocketed away from the star with terminal velocity approaching C – e wherein e can be made arbitrarilly small for all practicle purposes by adjusting and increasing that magnetic flux density of the electromagnet produced magnetic field.

Note that as I sure you are aware, the magnetic field strength in proximity to a neutron star can approach 10 EXP 10 Tesla or perhaps greater according to some sources and so some mechanism may be required to prevent the loss of the craft and loss of human life due to the potentially greatly elongated dipole configurations of the atoms that compose the craft and its contents.

I could think of one type of material that might be strong enough to produce a ship board electromagnet such that the craft would not be pulled by star’s gravity into the surface of the star and that material is potentially carbon nanotubes, some varieties of which may make nearly perfect conductors. Some forms of carbon nanotubes have a tensile strength on the order of 60 times greater than high strength steels and so such electromagnets on board the craft might produce magnetic fields of sufficient strenght but yet not tear the electromagnet coils to pieces by the coils own self magnetic interaction by the electromagnets magnetic field. Note that magnetic fields generated in the laboratory by continuous current electromagnets that produce field strengths on the order of 40 or 50 Tesla need to utilize materials with tensile strengths on the order of 300,000 PSI or greater in the constuction of the electrical current carrying windings in order for the coils not to be torn apart by self magnetic interaction. The use of nearly perfectly conducting carbon nanotubes of extreme strength should have tensile strengths on the rough order of 10 million PSI thus potentially permitting much higher field strength continuously operating electromagnets. Ultimately, if superconducting macroscopic quantities of solid nuetronium can somehow be produced, the potential magnetic field strengths producable by electromagnets made of such a material could be several to many orders of magnitude higher than those producable by our best continuosly operating electromagnets. In addition , the superconducting nuetronium might be imployable as a magnetic field screen or shield to prevent some or all of the ships atoms and those of the ships contents from taking on such elongated magnetic dipole configurations such that the ship and or its crew would be destroyed or otherwise rendered non-functional.

Perhaps the magnetic fields around white dwarfs are not quite as intense as they are around nuetron stars thus permitting less shielding. Also, since the gravitational flux density is not nearly as high in proximity to a white dwarf as it is near a neutron star, less intense ship board magnets might work in lowering the ship to near the surface of the star and less intense magnets would be needed to overcome the stars gravity while rocketing away from the star. Also, because of the extended geometry of the white dwarfs magnetic fields which emminate from the much larger diameter body of the white dwarf, perhaps more useful and managable levels of ship board magnetic field strength could be utilized to permit accellerations over a much longer pathlength but with much greater gentleness relative to that for a neutron star.

Also note that the craft would have to have a means to be shielded from or to avoid the very intense radiofrequency beams that are emmitted from the neutron star or white dwarf pulsars so as not to be fried by this low frequency electromagnetic radiation.

Now some blackholes might have proximate magnetic field flux densities considerably higher that that of neutrons stars but approaching that blackhole closely enough, although perhaps possibly doable by some type of very advanced ETI technology, might be problematic in terms of the risk of comming to close to the blackholes event horizon when using propulsion methods simmilar to the ones described above.

Thanks;

Jim

• george scaglione January 3, 2008, 9:27

jim,very good idea ! like it. only problem? a little outside nasa’s curent budget! and i don’t think technologically we will be able to do it anytime soon!!!! but as i always say buddy- keep plugging away!! i will. thank you george

• forrest noble January 3, 2008, 16:20

Hey Guys, White-Dwarf stars are relatively dim compared to other stars–but they still are stars. You wouldn’t want to get too close, it seems to me, without some presently unknown type of heat shielding.

According to my theory, the intensity of a magnetic field is a function of the relative internal motion of the matter within the star. A rapidly spinning star like a pulsar is spinning relative to its surrounding gravitational field. The resulting high speeds of the surface material relative to the internal stellar material results in huge electrical currents which, accordingly, would be the source of the intensity and range of the stellar magnetic field. Some pulsars spin at incredible rates of nearly one revolution per millisecond. The Sun rotates very slowly in comparison– one rotation in about 4 weeks, but it still has a very large magnetic field– but it probably pales compared to a pulsar.

all for now, your friend forrest

• James M. Essig January 3, 2008, 21:13

Hi Forrest and George;

Thanks for the responses.

George;

Definately not doable any time soon. First of all, one would have to locate and travel the light years distance to the pulsar or white dwarf to be utilized. If one were going to travel such distances, why not simply use a different type of interstellar propulsion method.

Forrest;

I can imagine that the electromagnetic flux density near any white dwarf that would be visible would be very high near its surface. The star would be radiating all of its energy from a small surface area. The surface area of a white dwarf is roughly 4 orders of magnitude smaller than that of the sun so some serious currently undeveloped type of sheilding would be required.

As for a pulsar, a rate of one revolution per milliseond, assumming its diameter is 10 miles, would put is equatorial surface rotation velocity at about 1/3 C. Talk about an increadible rotor. And this is real, not some sort of conjectured technological artifact. The energy and power of natural systems within just the observable universe never ceases to inspire a sense of awe within me. There is just so much energy waiting to be tapped out there some how for manned interstellar propulsion craft. Even the energy sources that we know about are tremendous. Any unseen energy sources or forms of energy we have not yet discovered or even mathematically and/or theoretically formulated have got to be staggering.

Thanks for the wise commentary.

• george scaglione January 4, 2008, 9:41

good to hear from you guys!!!! but LOL what are we going to do??just take our conversation and move it over to a new location where we will still be the only 3 talking!! hahahaha!!!!!! almost nuts.ps i was just over in the usual spot where once again for about the 75th time i invited others to join in! if this just keeps up the way it is then i think maybe we should just keep going the way we have been and hope for the best.pps i recognize and respect that we have some very good and talented people here…maybe i am wrong for trying to “bug” them into joining our conversations! very respectfully to one and all your friend george

• ljk January 8, 2008, 0:53

Spectra of accretion discs around white dwarfs

Authors: Irit Idan, Jean-Pierre Lasota, Jean-Marie Hameury, Giora Shaviv

(Submitted on 7 Jan 2008 (v1), last revised 7 Jan 2008 (this version, v2))

Abstract: We present spectra of accretion discs around white-dwarfs calculated with an improved and updated version of the Shaviv & Wehrse (1991) model. The new version includes line opacities and convective energy transport and can be used to calculate spectra of hot discs in bright systems (nova–like variables or dwarf novae in outburst) as well as spectra of cold accretion discs in quiescent dwarf novae.

Comments: 10 pages. Talk presented at “Jean-Pierre Lasota, X-ray binaries, accretion disks and compact stars” (October 2007); Abramowicz, M. Ed., New Astron. Rev., in press

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.1051v2 [astro-ph]

Submission history

From: Irit Idan [view email]

[v1] Mon, 7 Jan 2008 17:29:47 GMT (269kb)
[v2] Mon, 7 Jan 2008 22:02:09 GMT (269kb)

http://arxiv.org/abs/0801.1051

• ljk January 18, 2008, 13:03

Eccentric double white dwarfs as LISA sources in globular clusters

Authors: B. Willems (1), V. Kalogera (1), A. Vecchio (1,2), N. Ivanova (3), F.A. Rasio (1), J.M. Fregeau (1), K. Belczynski (4) ((1) Northwestern U., (2) U. of Birmingham, (3) U. of Toronto, (4) New Mexico State U.)

(Submitted on 29 May 2007 (v1), last revised 17 Jan 2008 (this version, v3))

Abstract: We consider the formation of double white dwarfs (DWDs) through dynamical interactions in globular clusters. Such interactions can give rise to eccentric DWDs, in contrast to the exclusively circular population expected to form in the Galactic disk.

We show that for a 5-year Laser Interferometer Space Antenna (LISA) mission and distances as far as the Large Magellanic Cloud, multiple harmonics from eccentric DWDs can be detected at a signal-to-noise ratio higher than 8 for at least a handful of eccentric DWDs, given their formation rate and typical lifetimes estimated from current cluster simulations.

Consequently the association of eccentricity with stellar-mass LISA sources does not uniquely involve neutron stars, as is usually assumed. Due to the difficulty of detecting (eccentric) DWDs with present and planned electromagnetic observatories, LISA could provide unique dynamical identifications of these systems in globular clusters.

Comments: Published in ApJ 665, L59

Subjects: Astrophysics (astro-ph); General Relativity and Quantum Cosmology (gr-qc)

Cite as: arXiv:0705.4287v3 [astro-ph]

Submission history

From: Bart Willems [view email]

[v1] Tue, 29 May 2007 20:06:52 GMT (79kb)

[v2] Mon, 18 Jun 2007 17:46:07 GMT (79kb)

[v3] Thu, 17 Jan 2008 18:48:04 GMT (78kb)

http://arxiv.org/abs/0705.4287

• ljk January 21, 2008, 11:25

The “DODO” survey I: limits on ultra-cool substellar and planetary-mass companions to van Maanen’s star (vMa 2)

Authors: M.R. Burleigh (1), F.J. Clarke (2), E. Hogan (1), C.S. Brinkworth (3), P. Bergeron (4), P. Dufour (5), P.D. Dobbie (6), A.J. Levan (7), S.T. Hodgkin (8), D.W. Hoard (3), S. Wachter (3) ((1) Department of Physics and Astronomy, University of Leicester, UK; (2) Department of Astrophysics, University of Oxford, UK; (3) Spitzer Science Center, USA; (4) Départment de Physique, Université de Montréal, Canada; (5) Department of Astronomy and Steward Observatory, University of Arizona, USA; (6) Anglo-Australian Observatory, Australia; (7) Department of Physics, University of Warwick, UK; (8) Institute of Astronomy, University of Cambridge, UK)

(Submitted on 18 Jan 2008)

Abstract: We report limits in the planetary-mass regime for companions around the nearest single white dwarf to the Sun, van Maanen’s star (vMa 2), from deep J-band imaging with Gemini North and Spitzer IRAC mid-IR photometry. We find no resolved common proper motion companions to vMa 2 at separations from 3″ – 45″, at a limiting magnitude of J~23. Assuming a total age for the system of 4.1 +/-1 Gyr, and utilising the latest evolutionary models for substellar objects, this limit is equivalent to companion masses greater than 7 +/-1 Mjup (T~300K). Taking into account the likely orbital evolution of very low mass companions in the post-main sequence phase, these J-band observations effectively survey orbits around the white dwarf progenitor from 3 – 50AU. There is no flux excess detected in any of the complimentary Spitzer IRAC mid-IR filters. We fit a DZ white dwarf model atmosphere to the optical BVRI, 2MASS JHK and IRAC photometry. The best solution gives T=6030 +/- 240K, log g=8.10 +/-0.04 and, hence, M= 0.633 +/-0.022Msun. We then place a 3sigma upper limit of 10 +/-2 Mjup on the mass of any unresolved companion in the 4.5 micron band.

Comments: Accepted for publication in MNRAS Letters

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.2917v1 [astro-ph]

Submission history

From: Matthew R. Burleigh [view email]

[v1] Fri, 18 Jan 2008 15:53:19 GMT (47kb)

http://arxiv.org/abs/0801.2917

• ljk January 22, 2008, 13:29

Limits on Planets Around White Dwarf Stars

Authors: F. Mullally, D. E. Winget, Steven Degennaro, Elizabeth Jeffery, S. E. Thompson, Dean Chandler

(Submitted on 20 Jan 2008)

Abstract: We present limits on planetary companions to pulsating white dwarf stars. A subset of these stars exhibit extreme stability in the period and phase of some of their pulsation modes; a planet can be detected around such a star by searching for periodic variations in the arrival time of these pulsations.

We present limits on companions greater than a few Jupiter masses around a sample of 15 white dwarf stars as part of an on-going survey. One star shows a variation in arrival time consistent with a 2 M_J planet in a 4.5 year orbit. We discuss other possible explanations for the observed signal and conclude that a planet is the most plausible explanation based on the data available.

Comments: 19 pages, accepted for publication in ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.3104v1 [astro-ph]

Submission history

From: Fergal Mullally [view email]

[v1] Sun, 20 Jan 2008 20:06:03 GMT (187kb)

http://arxiv.org/abs/0801.3104

• ljk January 31, 2008, 12:00

C_2 in Peculiar DQ White Dwarfs

Authors: Patrick B. Hall (1), Aaron J. Maxwell (1) ((1) York University, Toronto, Canada)

(Submitted on 30 Jan 2008)

Abstract: White dwarfs (WDs) with carbon absorption features in their optical spectra are known as DQ WDs. The subclass of peculiar DQ WDs are cool objects (T_eff less than 6000 K) which show molecular absorption bands that have centroid wavelengths ~100-300 Angstroms shortward of the bandheads of the C_2 Swan bands. These “peculiar DQ bands” have been attributed to a hydrocarbon such as C_2H. We point out that C_2H does not show strong absorption bands with wavelengths matching those of the peculiar DQ bands and neither does any other simple molecule or ion likely to be present in a cool WD atmosphere.

The most straightforward explanation for the peculiar DQ bands is that they are pressure-shifted Swan bands of C_2. While current models of WD atmospheres suggest that, in general, peculiar DQ WDs do not have higher photospheric pressures than normal DQ WDs do, that finding requires confirmation by improved models of WD atmospheres and of the behavior of C_2 at high pressures and temperatures. If it is eventually shown that the peculiar DQ bands cannot be explained as pressure-shifted Swan bands, the only explanation remaining would seem to be that they arise from highly rotationally excited C_2 (J_peak greater than 45). In either case, the absorption band profiles can in principle be used to constrain the pressure and the rotational temperature of C_2 in the line-forming regions of normal and peculiar DQ WD atmospheres, which will be useful for comparison with models. Finally, we note that progress in understanding magnetic DQ WDs may require models which simultaneously consider magnetic fields, high pressures and rotational excitation of C_2.

Comments: ApJ in press. 8 pages emulateapj style, 1 figure

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0801.4586v1 [astro-ph]

Submission history

From: Patrick B. Hall [view email]

[v1] Wed, 30 Jan 2008 00:06:38 GMT (28kb)

http://arxiv.org/abs/0801.4586

• ljk March 12, 2008, 10:32

Whole Earth Telescope observations of the hot helium atmosphere pulsating white dwarf EC 20058-5234

Authors: WET Collaboration: D.J. Sullivan, T.S. Metcalfe, D. O’Donoghue, D.E. Winget, D. Kilkenny, F. van Wyk, A. Kanaan, S.O. Kepler, A. Nitta, S.D. Kawaler, M.H. Montgomery, R.E. Nather, M.S. O’Brien, A. Bischoff-Kim, M. Wood, X.J. Jiang, E.M. Leibowitz, P. Ibbetson, S. Zola, J. Krzesinski, G. Pajdosz, G. Vauclair, N. Dolez, M. Chevreton

(Submitted on 11 Mar 2008)

Abstract: We present the analysis of a total of 177h of high-quality optical time-series photometry of the helium atmosphere pulsating white dwarf (DBV) EC 20058-5234. The bulk of the observations (135h) were obtained during a WET campaign (XCOV15) in July 1997 that featured coordinated observing from 4 southern observatory sites over an 8-day period. The remaining data (42h) were obtained in June 2004 at Mt John Observatory in NZ over a one-week observing period.

This work significantly extends the discovery observations of this low-amplitude (few percent) pulsator by increasing the number of detected frequencies from 8 to 18, and employs a simulation procedure to confirm the reality of these frequencies to a high level of significance (1 in 1000). The nature of the observed pulsation spectrum precludes identification of unique pulsation mode properties using any clearly discernable trends. However, we have used a global modelling procedure employing genetic algorithm techniques to identify the n, l values of 8 pulsation modes, and thereby obtain asteroseismic measurements of several model parameters, including the stellar mass (0.55 M_sun) and T_eff (~28200 K). These values are consistent with those derived from published spectral fitting: T_eff ~ 28400 K and log g ~ 7.86.

We also present persuasive evidence from apparent rotational mode splitting for two of the modes that indicates this compact object is a relatively rapid rotator with a period of 2h. In direct analogy with the corresponding properties of the hydrogen (DAV) atmosphere pulsators, the stable low-amplitude pulsation behaviour of EC 20058 is entirely consistent with its inferred effective temperature, which indicates it is close to the blue edge of the DBV instability strip. (abridged)

Comments: 19 pages, 8 figures, 5 tables, MNRAS accepted

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0803.1638v1 [astro-ph]

Submission history

From: Travis S. Metcalfe [view email]

[v1] Tue, 11 Mar 2008 17:24:43 GMT (501kb)

http://arxiv.org/abs/0803.1638

• ljk March 18, 2008, 0:09

Might Carbon-Atmosphere White Dwarfs Harbour a New Type of Pulsating Star?

Authors: G. Fontaine, P. Brassard, P. Dufour

(Submitted on 14 Mar 2008)

Abstract: In the light of the recent and unexpected discovery of a brand new type of white dwarfs, those with carbon-dominated atmospheres, we examine the asteroseismological potential of such stars. The motivation behind this is based on the observation that past models of carbon-atmosphere white dwarfs have partially ionized outer layers that bear strong resemblance with those responsible for mode excitation in models of pulsating DB (helium-atmosphere) and pulsating DA (hydrogen-atmosphere) white dwarfs.

Our exciting main result is that, given the right location in parameter space, some carbon-atmosphere white dwarfs are predicted to show pulsational instability against gravity modes. We are eagerly waiting the results of observational searches for luminosity variations in these stars.

Comments: 4-page letter + 4 figures

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0803.2255v1 [astro-ph]

Submission history

From: Gilles Fontaine [view email]

[v1] Fri, 14 Mar 2008 21:45:49 GMT (74kb)

http://arxiv.org/abs/0803.2255

• ljk March 19, 2008, 18:42

SDSS J142625.71+575218.3, a Prototype for a New Class of Variable White Dwarf

Authors: M. H. Montgomery (1), Kurtis A. Williams (1 and 2), D. E. Winget (1), Patrick Dufour (3), Steven Degennaro (1), James Liebert (3) ((1) Univ. of Texas at Austin, (2) NSF Astronomy & Astrophysics Postdoctoral Fellow, (3) Univ. of Arizona)

(Submitted on 18 Mar 2008)

Abstract: We present the results of a search for pulsations in six of the recently discovered carbon-atmosphere white dwarf (“hot DQ”) stars. Based on our theoretical calculations, the star SDSS J142625.71+575218.3 is the only object expected to pulsate. We observe this star to be variable, with significant power at 417.7 s and 208.8 s (first harmonic), making it a strong candidate as the first member of a new class of pulsating white dwarf stars, the DQVs.

Its folded pulse shape, however, is quite different from that of other white dwarf variables, and shows similarities with that of the cataclysmic variable AM CVn, raising the possibility that this star may be a carbon-transferring analog of AM CVn stars. In either case, these observations represent the discovery of a new and exciting class of object.

Comments: Accepted for publication in the ApJ Letters. 4 pages, 4 figures, Figure 4 reduced in resolution

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0803.2646v1 [astro-ph]

Submission history

From: Kurtis A. Williams [view email]

[v1] Tue, 18 Mar 2008 19:48:10 GMT (140kb)

http://arxiv.org/abs/0803.2646

• ljk April 3, 2008, 13:44

Spitzer IRAC Observations of White Dwarfs. II. Massive Planetary and Cold Brown Dwarf Companions to Young and Old Degenerates

Authors: J. Farihi, E. E. Becklin, B. Zuckerman

(Submitted on 1 Apr 2008)

Abstract: This paper presents a sensitive and comprehensive IRAC 3-8 $\mu$m photometric survey of white dwarfs for companions in the planetary mass regime with temperatures cooler than the known T dwarfs. The search focuses on descendants of intermediate mass stars with $M\ga3$ $M_{\odot}$ whose inner, few hundred AU regions cannot be probed effectively for massive planets and brown dwarfs by any alternative existing method.

Furthermore, examination for mid-infrared excess explores an extensive range of orbital semimajor axes, including the intermediate 5-50 AU range poorly covered and incompletely accessible by other techniques at main sequence or evolved stars.

Three samples of white dwarfs are chosen which together represent relatively young as well as older populations of stars: 9 open cluster white dwarfs, 22 high mass field white dwarfs, and 17 metal-rich field white dwarfs. In particular, these targets include: 7 Hyads and 4 field white dwarfs of similar age; 1 Pleiad and 19 field white dwarfs of similar age; van Maanen 2 and 16 similarly metal-rich white dwarfs with ages between 1 and 7 Gyr (abridged).

Comments: 41 pages, accepted to ApJ

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0804.0237v1 [astro-ph]

Submission history

From: Jay Farihi [view email]

[v1] Tue, 1 Apr 2008 20:03:24 GMT (358kb)

http://arxiv.org/abs/0804.0237

• ljk April 28, 2008, 13:23

Near-Infrared Constraints on the Presence of Warm Dust at Metal-Rich, Helium Atmosphere White Dwarfs

Authors: Mukremin Kilic, J. Farihi, Atsuko Nitta, S. K. Leggett

(Submitted on 21 Apr 2008)

Abstract: Here, we present near-infrared spectroscopic observations of 15 helium atmosphere, metal-rich white dwarfs obtained at the NASA Infrared Telescope Facility. While a connection has been demonstrated between the most highly polluted, hydrogen atmosphere white dwarfs and the presence of warm circumstellar dust and gas, their frequency at the helium atmosphere variety is poorly constrained.

None of our targets show excess near-infrared radiation consistent with warm orbiting material. Adding these near-infrared constraints to previous near- and mid-infrared observations, the frequency of warm circumstellar material at metal-bearing white dwarfs is at least 20% for hydrogen-dominated photospheres, but could be less than 5% for those effectively composed of helium alone.

The lower occurrence of dust disks around helium atmosphere white dwarfs is consistent with Myr timescales for photospheric metals in massive convection zones. Analyzing the mass distribution of 10 white dwarfs with warm circumstellar material, we search for similar trends between the frequency of disks and the predicted frequency of massive planets around intermediate mass stars, but find the probability that disk-bearing white dwarfs are more massive than average is not significant.

Comments: AJ, in press

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0804.3398v1 [astro-ph]

Submission history

From: Mukremin Kilic [view email]

[v1] Mon, 21 Apr 2008 20:04:35 GMT (109kb)

http://arxiv.org/abs/0804.3398

• ljk May 1, 2008, 9:20

McDONALD OBSERVATORY ASTRONOMERS DISCOVER NEW TYPE OF
PULSATING WHITE DWARF STAR

AUSTIN, Texas — University of Texas at Austin astronomers
Michael H. Montgomery and Kurtis A. Williams, along with
graduate student Steven DeGennaro, have predicted and
confirmed the existence of a new type of variable star with
the help of the 2.1-meter Otto Struve Telescope at McDonald
Observatory. The discovery will be announced in today¹s
issue of Astrophysical Journal Letters.

Called a “pulsating carbon white dwarf,” this is the first
new class of variable white dwarf star discovered in more
than 25 years. Because the overwhelming majority of stars in
the universe — including the Sun — will end their lives as
white dwarfs, studying the pulsations (i.e., variations in
light output) of these newly discovered examples gives
astronomers a window on an important endpoint in the lives
of most stars.

A white dwarf star is the leftover remnant of a Sun-like
star that has burned all of the nuclear fuel in its core. It
is extremely dense, packing half to 1.5 times the Sun¹s mass
into a volume about the size of Earth. Until recently, there
have been two main types of white dwarfs known: those that
have an outer layer of hydrogen (about 80 percent), and
about those with an outer layer of helium (about 20
percent), whose hydrogen shells have somehow been stripped
away.

Last year, University of Arizona astronomers Patrick Dufour
and James Liebert discovered a third type of white dwarf
star, still more rare. For reasons that are not understood,
these “hot carbon white dwarfs” have had both their hydrogen
and helium shells stripped off, leaving their carbon layer
exposed. Astronomers suspect these could be among the most
massive white dwarfs of all, and are the remnants of stars
slightly too small to end their lives in a supernova
explosion.

After these new carbon white dwarfs were announced,
Montgomery calculated that pulsations in these stars were
possible. Pulsating stars are of interest to astronomers
because the changes in their light output can reveal what
goes on in their interiors — similar to the way geologists
study seismic waves from earthquakes to understand what goes
on in Earth¹s interior. In fact, this type of star-study is
called “asteroseismology.”

So, Montgomery and Williams’ team began a systematic study
of carbon white dwarfs with the Struve Telescope at McDonald
Observatory, looking for pulsators. DeGennaro discovered
that a star about 800 light-years away in the constellation
Ursa Major, called SDSS J142625.71+575218.3, fits the bill.
Its light intensity varies regularly by nearly two percent
about every eight minutes.

“The discovery that one of these stars is pulsating is
remarkably important,” said National Science Foundation
astronomer Michael Briley. “This will allow us to probe
the white dwarf’s interior, which in turn should help us
solve the riddle of where the carbon white dwarfs come
from and what happens to their hydrogen and helium.” The
research was funded by NSF and the Delaware Asteroseismic
Research Center.

The star lies about ten degrees east northeast of Mizar, the
middle star in the handle of the Big Dipper. This white
dwarf has about the same mass as our Sun, but its diameter
is smaller than Earth’s. The star has a temperature of
35,000 degrees Fahrenheit (19,500 C), and is only 1/600th as
bright as the Sun.

None of the other stars in their sample were found to
pulsate. Given the masses and temperatures of the stars in
their sample, SDSS J142625.71+575218.3 is the only one
expected to pulsate based on Montgomery¹s calculations.

The astronomers speculate that the pulsations are caused by
changes in the star’s carbon outer envelope as the star
cools down from its formation as a hot white dwarf. The
ionized carbon atoms in the star’s outer layers return to a
neutral state, triggering the pulsations.

There is a chance that the star’s variations might have
another cause. Further study is needed, the astronomers say.
Either way, studying these stars will shed light on the
unknown process that strips away their surface layers of
hydrogen and helium to lay bare their carbon interiors.

For high-resolution images related to this release, see:

http://mcdonaldobservatory.org/news/releases/2008/0501.html

• ljk September 4, 2009, 10:08

September 3, 2009

White Dwarf “Close” to Exploding as Supernova

Written by Nancy Atkinson

ESA’s XMM-Newton orbiting X-ray telescope has uncovered the first close-up of a white dwarf star that could explode into a type Ia supernova within a few million years. That’s relatively soon in cosmic time frames, and although this white dwarf that is orbiting its companion star HD 49798, is far enough away to pose no danger to Earth, it is close enough to become an extraordinarily spectacular celestial sight.

Calculations suggest that it will blaze initially with the intensity of the full Moon and be so bright that it will be seen in the daytime sky with the naked eye. But don’t worry, it will be awhile!

Astronomers have been on the trail of this mysterious object since 1997, when they discovered that something was giving off X-rays near the bright star HD 49798. Now, thanks to XMM-Newton’s superior sensitivity, the mysterious object has been tracked along its orbit. The observation has shown it to be a white dwarf, the dead heart of a star, shining X-rays into space.

Sandro Mereghetti, INAF–IASF Milano, Italy, and collaborators also discovered that this is no ordinary white dwarf. They measured its mass and found it to be more than twice what they were expecting. Most white dwarfs pack 0.6 solar masses into an object the size of Earth.

This particular white dwarf contains at least double that mass but has a diameter just half that of Earth. It also rotates once every 13 seconds, the fastest of any known white dwarf.