Small helium white dwarfs can be caused by a binary partner (NASA)

Hubble Discovers a Strange Collection of White Dwarf… Dwarfs

24 Apr , 2009 by

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A collection of very odd white dwarfs have been discovered in a local globular cluster. Twenty-four white dwarfs (18 of them are new discoveries) have been spotted. Although these degenerate stars aren’t exactly an uncommon (they are the small sparkling remnants left over after star death), this particular set are unique; they are made from helium, rather than the “standard” carbon and oxygen. And they are small, even smaller than the smallest dwarfs.

How did this dense cluster of old stars evolve? It turns out their stellar material is being stolen, stifling their development…

Helium-core white dwarfs have only about half the mass of typical white dwarfs, but they are found concentrated in the center of the cluster,” said Prof. Adrienne Cool, from San Francisco State University, in a paper to be published in the Astrophysical Journal in July. “With such low masses, the helium-core white dwarfs ought to be floating all around the cluster, according to theory. The fact that we find them only in the central regions suggests that they have heavy companions — partner stars that anchor them to the cluster center.”

The Hubble observations show 18 previously undiscovered helium-core white dwarfs (Jay Anderson / Space Telescope Science Institute)

The Hubble observations show 18 previously undiscovered helium-core white dwarfs (Jay Anderson / Space Telescope Science Institute)

Cool and co-author Rachel R. Strickler believe they are seeing a case of stellar plasma theft by companion binary stars in the NGC 6397 cluster, approximately 7,200 light years away. These binary partners not only anchor these strange-looking white dwarfs in the centre of the cluster, they also have a huge role to play during the dwarfs evolution.

Before a white dwarf emerges from a planetary nebula, the parent star will have gone through the red giant phase (a phase our Sun is expected to go through in 4-5 billion years time). If this red giant has a binary partner (which seems to be the case of the 24 white dwarfs in this study), the outer layers of the puffed-up giant will be stripped away by the partner, stifling the red giant’s evolution. As mass is lost, the giant never gets the chance to burn helium and then progressively heavier elements such as carbon and oxygen in and around its core. Helium then becomes the key component of these smaller-than-usual white dwarfs.

This is the first time that helium-core white dwarf stars have been discovered in partnerships with other white dwarfs in a globular cluster,” Cool said. “This large sample allows us to answer questions about the mass and nature of the partner stars, and the prevalence of these kinds of binaries in the globular cluster.”

Binary stars are known to affect their partners fairly radically, they are even known to slow or even stop the development of black holes, stripping the outer layers of the dying star, stifling black hole development by removing mass from the parent star. However, not all questions have been answered.

From Cool’s calculations, 5% of the stars found in NGC 6397 should end their lives as dim helium-core white dwarf stars, but after studying Hubble data, many of these tiny dwarfs are missing. “It’s possible that these helium-core white dwarfs cool so slowly that they haven’t had time to get very faint yet,” Cool said.

There remains the possibility that the oldest binaries containing helium-core white dwarfs have actually been destroyed by interactions with other stars in the cluster. Regardless, this is a fascinating area of study. To understand how these ancient stars evolve will not only aid the development of globular cluster models, but it will provide an invaluable insight to how binary stars influence their partners.

Source: EurekAlert!


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Lawrence B. Crowell
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Lawrence B. Crowell
April 25, 2009 4:52 AM

Why do helium core dwarf stars cool so slowly. Do they have a large heat capacity?

Lawrence B. Crowell

Ivan3man_At_Large
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Ivan3man_At_Large
April 25, 2009 2:20 PM

There’s more…
An Eclipsing Millisecond Pulsar with a Possible Main-Sequence Companion in NGC 6397

Ivan3man_At_Large
Member
Ivan3man_At_Large
April 25, 2009 2:28 PM

Even more…
Chandra Exposes the Core-collapsed Globular Cluster NGC 6397

cool

Ivan3man_At_Large
Member
Ivan3man_At_Large
April 25, 2009 10:06 AM
Lawrence B. Crowell: Why do helium core dwarf stars cool so slowly. Do they have a large heat capacity? Late Stages of Evolution For Low-mass Stars: […] As the star runs lower on hydrogen, the rate at which it generates energy gradually declines. Gravity pulls the outer layers inwards on the core, but the temperatures never rise high enough for any other nuclear reactions to take place. Slowly, gradually, the star becomes fainter, cooler, and smaller. Eventually, it will shrink to a cold ball about the size of the Earth: a black dwarf. Of course, this is all just speculation. Stars with such small masses take a long, long time to run through all their hydrogen fuel. A… Read more »
Anaconda
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Anaconda
April 25, 2009 11:25 AM
Analyzing the posted story and the linked report it appears that the presence of heavy companions can only be inferred: “The fact that we find them only in the central regions suggests that they have heavy companions” A reference to a “partner star” is made, but no mention of any detectable signal from the partner star. So apprently the “companions” emit no electromagnetic signal to indicate their presence. What kind of “star” fails to emit any detectable emissions? Neither the paper or the story addresses the reason for why these “heavy companions” are undetectable. What is the background theory for why helium white dwarfs would be presumed to be more spread out in the cluster? As stated: “With… Read more »
Ivan3man_At_Large
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Ivan3man_At_Large
April 25, 2009 2:13 PM

Anaconda:

Analyzing the [blah… blah… blah…]

A reference to a “partner star” is made, but no mention of any detectable signal from the partner star.

So apprently [sic] the “companions” emit no electromagnetic signal to indicate their presence.

What kind of “star” fails to emit any detectable emissions? Neither the paper or the story addresses the reason for why these “heavy companions” are undetectable.

[Blah… blah… blah… blah… blah… blah… blah… blah… blah…]

Behold… The Peculiar Millisecond Pulsars in the Globular Clusters NGC 6397 and NGC 6752

Anaconda
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Anaconda
April 25, 2009 4:07 PM
@ Ivan3Man: I appreciate the links, particlularly the full papers. But the problem is that the observationalists in the profiled paper in the post don’t mention detecting any kind of signal. The paper you link on the same cluster, NGC 6397, only talks about one pulsar and that does send out an electromagnetic signal. Presumably the authors of the profiled paper had access to the paper you link to, yet they make no reference to that paper, or any of the observations in it. So, we’re still dealing with an “inference” of undetectable “partner stars”. So, again, while I appreciate the links you provide, they shed no evidenciary light on the present observations and the conclusion of invisible… Read more »
Nereid
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Nereid
April 25, 2009 6:30 PM

I wrote a reply, Anaconda, but it disappeared (maybe because it included a URL?).

Anyway, the net is simple: the Cool et al. paper is scheduled to be published, in ApJ, in July, and it seems there is no preprint on arXiv … so no one (other than the authors of the paper) are in a position to comment further on what you write.

I did find that the presented something at the 210th AAS meeting (in May, 2007) which seems to cover much the same topic …

Jon Hanford
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Jon Hanford
April 25, 2009 8:27 PM

Thanks for the head’s up on the preprint, Nereid. My guess is they linked positions of the He dwarfs to radio emissions from the pulsars or X-ray emission from the binary systems. This would mean that EM radiation is emitted by these “invisible’ partner stars. (I would guess probably all 3 papers are referenced in the new study, but we’ll have to wait to see a copy of it.)

Lawrence B. Crowell
Member
Lawrence B. Crowell
April 26, 2009 4:28 PM

IVAN3MAN, I am pretty aware of the evolution of such stars. It just struck me as odd that these hewhite dwarf stars would cool more slowly.

Lawrence B. Crowell

Lawrence B. Crowell
Member
Lawrence B. Crowell
April 26, 2009 4:28 PM

hewhite, should read helium core

Ivan3man_At_Large
Member
Ivan3man_At_Large
April 27, 2009 12:01 PM

@ Lawrence B. Crowell,

I’m sure that you’re aware of the evolution of such stars, I was merely pre-empting the response from the “Electric Sun” nutcases! smile

Nereid
Member
Nereid
April 27, 2009 11:47 PM

Perhaps this is the arXiv preprint?

“Helium-Core White Dwarfs in the Globular Cluster NGC 6397”, with authors R. R. Strickler, A. C. Cool, J. Anderson, H. N. Cohn, P. M. Lugger, A. M. Serenelli.

In case I can’t post URLs yet, the reference is arXiv:0904.3496v1.

If anyone’s interested, we can discuss the preprint, rather than relying upon the UT story …

Nereid
Member
Nereid
April 27, 2009 11:48 PM
Jon Hanford
Member
Jon Hanford
April 28, 2009 2:23 PM

@Nereid: Thanks for the link to the preprint version of this paper. The authors state that this is “the first extended sequence of Helium White Dwarfs found in a globular cluster” They also conclude that the Helium Dwarfs near the center of post core-collapse cluster require “heavy White Dwarf companions” to occupy their present position in NGC 6397. BTW, 2 of the 3 papers cited above by IVAN3MAN are referenced in this paper.

Nereid
Member
Nereid
April 29, 2009 10:47 AM
@Anaconda: I’ve now read the preprint, and can answer your questions (well, I can have a go at answering them). But first I should say that full answers would likely be very long, unless you already have a good grasp of the relevant parts of modern astrophysics. Also, I urge you to read the preprint yourself, as Jon Hanford seems to have done, and come back with any questions you may still have after reading it. “A reference to a “partner star” is made, but no mention of any detectable signal from the partner star. So apprently the “companions” emit no electromagnetic signal to indicate their presence. What kind of “star” fails to emit any detectable emissions? Neither… Read more »
Anaconda
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Anaconda
April 29, 2009 10:50 AM

@ Nereid:

Thank you for the link and I note the additional information provided by Jon Hanford.

I allow that Hanford has identifed the operable phrases and linked them together: “the first extended sequence of Helium White Dwarfs found in a globular cluster” They also conclude that the Helium Dwarfs near the center of post core-collapse cluster require “heavy White Dwarf companions”.

But the use of the word “require” reaffirms my position that there is only an inference of “heavy white Dwarf companions”.

These supposed objects have no directly detectable electromagnetic spectrum signal.

Nereid, do you disagree my analysis?

Anaconda
Member
Anaconda
April 29, 2009 11:15 AM
@ Nereid: Nereid presents my [Anaconda’s] statement: “A reference to a “partner star” is made, but no mention of any detectable signal from the partner star. So apprently the “companions” emit no electromagnetic signal to indicate their presence. What kind of “star” fails to emit any detectable emissions? Neither the paper or the story addresses the reason for why these “heavy companions” are undetectable.” And Nereid responds: “The preprint spends quite a bit of time on just that; in a nutshell, the expected mass of the unseen partner stars is ~1 sol; if the partners were main sequence stars or neutron stars, they’d be seen in the visual waveband or radio or x-ray, so they are likely other… Read more »
Nereid
Member
Nereid
April 29, 2009 5:53 PM
@Anaconda: the direct answer to your direct question (“Nereid, do you disagree with this proposed concise answer?”) is yes, I disagree. If you are interested in focussing on the logic chain that lead Cool et al. to conclude that the 24 He WDs in NGC 6397 likely have WD companions (i.e. are WD-WD binaries), I’d be happy to walk you through that, per the preprint. Fair warning: it will, necessarily, involve quite a lot of material, and the restrictions of these UT comments will likely make that a slow process. On the other hand, if you are looking for a Yes/No answer to the question of whether the ACS instrument on the HST recorded photon detections that could,… Read more »
Anaconda
Member
Anaconda
April 29, 2009 10:40 PM
@ Nereid: Regarding your example: No, I would say those are detected observations that are measured and are not “inferences”. The relevant question is what is the analysis and interpretation on what physical element they correlate to. With all due respect, I disagree with your assessment that the authors’ conclusions are not an inference. You offer no direct observation & measurement of physically detectable signal from the “star partners”. I appreciate your offer at detail, but that is not necessary. Just state what direct observation & measurement of the large companion white dwarfs were reported by Cool, et al. If you can’t report a direct observation & measurement, then it is an inference based on circumstantial evidence, aka,… Read more »
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