# A White Dwarf with Supernova Possibilities

I try to run interesting astronomical art wherever I can find it, but the image that accompanies this ESA news release on the discovery of an interesting white dwarf just doesn’t cut it. So use your imagination as I describe the results of a study using data from ESA’s XMM-Newton X-ray telescope, which have given us something we’ve long lacked — highly accurate mass information for an accreting white dwarf in a binary system, one that is growing close to the point of becoming a supernova.

Something in the vicinity of HD 49798 has been known to be giving off X-rays since 1997, but it has taken XMM-Newton to nail the culprit. The white dwarf near the larger star is twice as massive as expected, cramming about 1.3 solar masses into an object with a diameter about half that of our planet. Rotating every thirteen seconds, this object boasts the fastest white dwarf rotation known.

Why the larger mass? We’re looking at a white dwarf that is pulling gaseous material out of its companion star. The process points to an interesting future: When a white dwarf in this situation reaches 1.4 solar masses, it will doubtless explode, becoming what is known as a type Ia supernova. Such events are the ‘standard candles’ that astronomers use to study the expansion of the universe, and here we see a type Ia supernova in the process of developing.

Sandro Mereghetti (INAF–IASF Milan) describes the find in terms of a useful historical analogy:

“This is the Rosetta stone of white dwarfs in binary systems. Our precise determination of the masses of the two stars is crucial. We can now study it further and try to reconstruct its past, so that we can calculate its future.”

Not that we need wait up for this one. A supernova near HD 49798 shouldn’t occur for a few million years, and even when it does, it poses no danger to Earth. That will be an interesting event, visible in broad daylight with the naked eye, but for today’s astronomical purposes, this white dwarf tells us much about the supernovae so critical in our understanding of a universe whose expansion seems to be accelerating.

The paper is Mereghetti et al., “An ultra massive fast-spinning white dwarf in a peculiar binary system,” Science Vol. 325, No. 5945 (4 September 2009), pp. 1222-1223 (abstract).

Comments on this entry are closed.

• ljk September 8, 2009, 12:49

Speaking of space art, check out this new article in The Space Review:

http://www.thespacereview.com/article/1460/1

• Adam September 8, 2009, 15:37

Well that tells where not to buy real estate in the Galaxy. White dwarfs are fascinating objects – quantum mechanics holds them up against gravitational collapse. If their constiuents suddenly turned into bosons, say via supersymmetry, they’d collapse and release massive amounts of gamma-rays. A binary pair of white dwarfs provides a gravity machine that can boost payloads up to ~0.01 c for essentially free, so they may well be a sought for resource in an industrialised Galaxy.

• Parmanello September 8, 2009, 15:50

How can the object be the size of the earth but also half the diameter?

• Administrator September 8, 2009, 16:21

Good point, Parmanello. I’ve adjusted the original entry to correct my mistake.

• Ronald September 9, 2009, 5:58

Rather off-topic (in fact awfully off-topic, but I don’t know where else to put it), but at least nova related and maybe also to Adam’s advice with regard to real estate prospecting:

The nearby and well-known solar type star Delta Pavonis (GJ 780) is usually considered a G5 or G8 star in the process of ‘moving off the main sequence’ into subgiant stage, i.e. class V/IV or IV. Its mass is estimated at about solar (0.98 – 1.02) and its luminosity is 1.18 solar (I have read luminosities up to 1.27 sol), confirming its move toward subgiant. So far nothing surprising.

However, the age is estimated by various sources at about 6 – 7 gy, sometimes a bit lower.
This I find puzzling: how is it possible that a G5/G8, solar mass star is already in the process of moving off te main sequence into subgiant stage (i.e. not just the within-main-sequence gradual brightening of an aging star, as we have discussed in various threads), something which I would not expect before, say, 10 gy of age.

Can anybody explain this? Is Delta Pavonis really becoming a subgiant already and could this also have implications for other solar mass stars with regard to possibly strong early brightening and habitable lifespan?

• ljk September 9, 2009, 11:59

Accreting White Dwarfs as Supersoft X-ray Sources

Authors: Mariko Kato (Keio Univ.)

(Submitted on 8 Sep 2009)

Abstract: I review various phenomena associated with mass-accreting white dwarfs (WDs) in the view of supersoft X-ray sources. When the mass-accretion rate is low (\dot M_{acc} < a few \times 10^{-7} M_\sun yr^{-1}), hydrogen nuclear burning is unstable and nova outbursts occur.

A nova is a transient supersoft X-ray source (SSS) in its later phase which timescale depends strongly on the WD mass. The X-ray turn on/off time is a good indicator of the WD mass. At an intermediate mass-accretion rate an accreting WD becomes a persistent SSS with steady hydrogen burning. For a higher mass-accretion rate, the WD undergoes "accretion wind evolution" in which the WD accretes matter from the equatorial plane and loses mass by optically thick winds from the other directions.

Two SSS, namely RX J 0513-69 and V Sge, are corresponding objects to this accretion wind evolution. We can specify mass increasing WDs from light-curve analysis based on the optically thick wind theory using multiwavelength observational data including optical, IR, and supersoft X-rays.

Mass estimates of individual objects give important information for the binary evolution scenario of type Ia supernovae.

Comments: 6 pages including 10 figures, conference proceedings of "Supersoft X-ray Sources – New Developments" (18th – 20th May 2009, European Space Astronomy Centre (ESAC), Villafranca del Castillo, Madrid, Spain)

Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Cite as: arXiv:0909.1497v1 [astro-ph.SR]

Submission history

From: Izumi Hachisu [view email]

[v1] Tue, 8 Sep 2009 14:19:53 GMT (176kb)

http://arxiv.org/abs/0909.1497

• andy September 9, 2009, 13:51

Ronald: doing a quick search on VizieR, the majority of age estimates for Delta Pavonis seem to be around 9 Gyr… evolutionary tracks suggest that stars of solar mass and twice solar metallicity run out of core hydrogen around 7-8 Gyr…

• Ronald September 9, 2009, 15:33

andy; thanks.

The pubs that I consulted on age, such as “Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics” by Mamajek and Hillenbrand, suggest an age toward 7 gy, but I realize that age estimation is often difficult.
The combination of age with (very) high metallicity may indeed be the answer, thanks once again.

• Adam September 9, 2009, 15:54

Hi Ron & andy

Thanks for the succinct reply to Ron, andy. Interesting too. I knew metallicity had an effect, but didn’t realise it was quite so dramatic. I was going to say that the sub-giant stage tends to push a star red-wards, thus why it’s now G-5/G-8, when it might’ve been brighter.

• andy September 9, 2009, 16:38

Slight typo in my previous… should have stated 7-10 Gyr.

Using the Geneva grids, for solar-mass stars, the last non-zero core hydrogen value is at 9.42 Gyr, first zero core hydrogen at 9.84 Gyr.

For twice solar metallicity, last non-zero core hydrogen is at 7.41 Gyr, first zero core hydrogen is at 9.51 Gyr. (Unfortunate lack of data points between the two)

• ljk September 14, 2009, 12:52

Limits on Unresolved Planetary Companions to White Dwarf Remnants of 14 Intermediate-Mass Stars

Authors: Mukremin Kilic, Andrew Gould, Detlev Koester

(Submitted on 10 Sep 2009)

Abstract: We present Spitzer IRAC photometry of white dwarf remnants of 14 stars with M = 3-5 Msol. We do not detect mid-infrared excess around any of our targets. By demanding a 3 sigma photometric excess at 4.5 micron for unresolved companions, we rule out planetary mass companions down to 5, 7, or 10 M_J for 13 of our targets based on the Burrows et al. (2003) substellar cooling models.

Combined with previous IRAC observations of white dwarf remnants of intermediate-mass stars, we rule out \geq 10 M_J companions around 40 white dwarfs and \geq 5 M_J companions around 26 white dwarfs.

Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Cite as: arXiv:0909.2038v1 [astro-ph.SR]

Submission history

From: Mukremin Kilic [view email]

[v1] Thu, 10 Sep 2009 20:02:13 GMT (66kb)

http://arxiv.org/abs/0909.2038

• Ronald September 15, 2009, 17:00

andy:

“For twice solar metallicity, last non-zero core hydrogen is at 7.41 Gyr, first zero core hydrogen is at 9.51 Gyr.”

Shouldn’t those two dates be swapped around?

Do you know the relationship (correllation) between metallicity and those parameters?

• andy September 19, 2009, 8:16

Ronald: nope – the last entry with core hydrogen should come before the first entry with zero core hydrogen.

On the other hand it seems that the evolutionary tracks I used didn’t have good enough resolution to get the right conclusion… according to this paper, decreasing metallicity results in decreased main sequence lifetimes. This seems to be borne out by the results I get using the Padova isochrones (which explicitly mark Xc=0) and the Yonsei-Yale isochrones (which don’t list Xc at all, but if I use the increase of the M_core column for the end of the main sequence I get results that bear this out).

Incidentally, for Delta Pavonis, RECONS lists a mass of 1.1 times solar, which given the 240% metallicity from Wikipedia and using the Yonsei-Yale interpolator code, gives an end of main sequence at roughly 7.3 Gyr.

• Ronald September 19, 2009, 9:20

@andy: thanks!

• ljk October 9, 2009, 22:06

The White Dwarfs within 20 Parsecs of the Sun: Kinematics and Statistics

Authors: Edward M. Sion, Jay B. Holberg, Terry D. Oswalt, George P. McCook, Richard Wasatonic

(Submitted on 7 Oct 2009 (v1), last revised 8 Oct 2009 (this version, v2))

Abstract: We present the kinematical properties, distribution of spectroscopic subtypes, stellar population subcomponents of the white dwarfs within 20 pc of the sun. We find no convincing evidence of halo white dwarfs in the total 20 pc sample of 129 white dwarfs nor is there convincing evidence of genuine thick disk subcomponent members within 20 parsecs.

Virtually the entire 20 pc sample likely belongs to the thin disk. The total DA to non-DA ratio of the 20 pc sample is 1.6, a manifestation of deepening envelope convection which transforms DA stars with sufficiently thin H surface layers into non-DAs.

The addition of 5 new stars to the 20 pc sample yields a revised local space density of white dwarfs of $4.9\pm0.5 \times 10^{-3}$ M$_{\sun}$/yr and a corresponding mass density of $3.3\pm0.3 \times 10^{-3}$ M$_{\sun}$/pc$^{3}$.

We find that at least 15% of the white dwarfs within 20 parsecs of the sun (the DAZ and DZ stars) have photospheric metals that possibly originate from accretion of circumstellar material (debris disks) around them.

If this interpretation is correct, this suggests the possibility that the same percentage have planets or asteroid-like bodies orbiting them.

Comments: Accepted for publication in The Astronomical Journal

Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Galaxy Astrophysics (astro-ph.GA)

Cite as: arXiv:0910.1288v2 [astro-ph.SR]

Submission history

From: Edward Sion [view email]

[v1] Wed, 7 Oct 2009 15:30:28 GMT (67kb)

[v2] Thu, 8 Oct 2009 20:45:29 GMT (67kb)

http://arxiv.org/abs/0910.1288