Hubble Discovers a Strange Collection of White Dwarf… Dwarfs

by Ian O'Neill on April 24, 2009

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

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

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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  • Nereid

    Thanks for the reply, Anaconda, I think it helps get at least the two of us closer to a base of common understanding that we can build on, to have a meaningful conversation.

    However, you still seem to be conflating “detectABLE” and “detectED”, and now also observABLE and observED.

    But first, let’s make sure we are on the same page wrt inferences, and in particular “the analysis and interpretation on what physical element they correlate to”.

    I’ll continue with the 495.9 and 500.7 nm [OIII] lines.

    What “physical element” do these correlate to? Why a pair of atomic transitions of doubly ionised oxygen (that’s what the “OIII” is shorthand for), and forbidden transitions to boot (that’s what the square brackets are shorthand for).

    Now these transitions have never been observed in any lab, here on Earth, coming from a plasma which contains oxygen in a vessel in said lab.

    So how come the lines are designated [OIII], if no one has produced them in any controlled experiment in any lab here on Earth?

    Once I understand the extent to which you are prepared to accept things from contemporary astronomy as observations, vs inferences, I can re-phrase my answers to accord with your standards.

    Now to get there we may have to take a detour, and spend some time on your simple, yet profound, question “do the assumptions reflect reality?” But not wrt the Cool et al. paper, rather wrt contemporary astronomy in general.

  • Nereid

    Anaconda, here is an example, from our recent comments, of us talking past each other:

    ===================================
    YOU: 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 white dwarfs.”
    Nereid, it appears that the direct response to my statement would be: “Yes, you are right, there are no detectable signals from the supposed companion stars, it is only an inference.”
    Nereid, do you disagree with this proposed concise answer?
    ===================================
    ME: the direct answer to your direct question (”Nereid, do you disagree with this proposed concise answer?”) is yes, I disagree.
    ===================================
    YOU: 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”.
    ===================================

    So how is this talking past each other?

    Let me explain.

    You offered only “AGREE” and “DISAGREE” as possible answers to your question, and as it is worded the only possible choice (of three – I could have said “I don’t know” or similar) is DISAGREE.

    Then you ran with my one word answer and gave it a meaning quite different than what I intended, and quite different from the concise statement you asked me to respond to.

    You see your concise statement contains the word “detectable”, so IF you assume there are partner stars to the observed He WDs, THEN light (electromagnetic radiation) from them should be detectABLE.

    Further, I can’t see how your ‘with all due respect’ comment follows, logically, from my one word answer, especially as the chain that lead us here contains your “So apprently the “companions” emit no electromagnetic signal to indicate their presence”; this is surely a conclusion too far (Cool et al. certainly don’t even hint at this extreme inference, and I think I’ve been clear that it is an invalid conclusion).

    Do you see how communication has broken down?

  • Nereid

    Anaconda, to help me understand how you parse detection from direct observation from inference, in astronomy, I’ve prepared a series of examples. If you could please indicate the extent to which there is detection, direct observation, indirect observation, and inference in each I’d appreciate it.

    Of course, my list is very terse, and so you may not grasp what an item is about; if so, please ask for clarification.

    -> DLA systems (objects detected by the absence of photons)

    -> transiting exoplanets (ditto)

    -> exoplanets discovered solely by doppler shift methods (detection vs inference)

    -> objects discovered solely by microlensing (ditto)

    -> 10 TeV gamma-ray emission by the Crab nebula (detection/observation/inference wrt IACTs)

    -> energy and source direction of UHECRs (detection/observation/inference distinctions)

    -> proper motion and parallax of Barnard’s star as determined by HIPPARCOS (ditto)

    -> CMB angular power spectrum as determined by WMAP (ditto)

    -> galactic 408 MHz emission as synchrotron radiation (ditto).

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