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Star in Tight Orbit around Black Hole

Beyond their obvious value in advancing our knowledge, astronomical discoveries can be thought of as exercises for the imagination, making us think about what we would see if we were actually near the phenomenon being observed. The view from a planet deep in a globular cluster can only be spectacular, and has been the subject of my own musings for many years. But this morning’s topic, a white dwarf star in a fantastically tight orbit around a black hole, leaves my imagination reeling. Just what would a scenario like the one playing out in the globular cluster 47 Tucanae, some 14,800 light years from Earth, look like up close?

Here we find, thanks to the space-based Chandra X-Ray Observatory and NuSTAR (Nuclear Spectroscopic Telescope Array), as well as the Australia Telescope Compact Array, a binary that consists of an apparent white dwarf orbiting a black hole every 28 minutes. The presence of large amounts of oxygen in the system gives weight to the idea that the companion is indeed a white dwarf, as an abundance of oxygen is a characteristic feature of white dwarfs.

The orbit here is so tight that the star is separated from the apparent black hole by a distance about 2.5 times the separation between the Earth and the Moon.

“This white dwarf is so close to the black hole that material is being pulled away from the star and dumped onto a disk of matter around the black hole before falling in,” says lead author Arash Bahramian (University of Alberta and Michigan State University). “Luckily for this star, we don’t think it will follow this path into oblivion, but instead will stay in orbit.”

47-Tuc-X9-cloud

Image: An artist’s impression of a white dwarf star (left) in orbit around a black hole and so close that much of its material is being pulled away. There is a hotspot where the gas from the white dwarf hits the disc of matter swirling around the black hole. The black hole itself is surrounded by a cloud of ionised gas, which contains large amounts of oxygen. Credit: X-ray: NASA/CXC/University of Alberta/A.Bahramian et al.; Illustration: NASA/CXC/M.Weiss.

The orbital determination is made from the changes of brightness observed on a regular 28 minute cycle by Chandra. And the likely outcome of this tight pairing for the white dwarf is that it will eventually become something even more unusual. Says James Miller-Jones (Curtin University, Western Australia and the International Centre for Radio Astronomy Research):

“We think the star may have been losing gas to the black hole for tens of millions of years and by now has now lost the majority of its mass. Over time, we think that the star’s orbit will get wider and wider as even more mass is lost, eventually turning into an exotic object similar to the famous diamond planet discovered a few years ago.”

Small digression: More commonly known as a ‘carbon planet,’ an exotic world like the one Miller-Jones mentions draws on work from Marc Kuchner (NASA GSFC) and Sara Seager (MIT), who investigated the possibility that carbon-rich, oxygen-poor protoplanetary disks might form such planets. Here the visualization exercise gets another workout: Layers of diamond could exist within the mantle, possibly erupting to the surface during volcanic episodes. In addition to mountains of diamond, the surface could contain liquid or frozen tar and methane.

Although 55 Cancri e is frequently cited as a possible carbon planet, pulsar planets have also been suggested as candidates, and in connection with today’s story, it’s worth pointing out that carbon planets are also speculated to be found in globular clusters, where stars have a higher carbon-to-oxygen ratio than the Sun — hence Miller-Jones’ allusion to diamond planets in the quotation above. As to the binary in the globular cluster 47 Tucanae, called X9, the origin scenarios being discussed are intriguing. From the paper:

Low mass X-ray binaries (LMXBs) are systems in which a compact object [neutron star (NS) or black hole (BH)] accretes matter from a low mass companion (typically a main sequence star) through Roche-lobe overflow or wind-fed accretion (from a red giant). It has been long noticed that the population of LMXBs per unit mass in globular clusters (GCs) is orders of magnitude higher than that of the Galactic field… This overabundance has been associated with the high stellar density in GCs. While most LMXBs in the Galactic field are formed through binary evolution of primordial binaries, it has been shown that in GCs, the dominant channel to form LMXBs is through encounters. These scenarios include tidal capture by an NS or BH, exchange of a compact object into a primordial binary, or collision of a compact object with a red giant…

Here too the visualization game stretches the imagination. A black hole encounters a red giant star, with gas from the star being ejected and the remnants of the red giant’s atmosphere becoming a companion to the black hole in the form of a white dwarf. Whether or not we are looking at the result of such an encounter in X9, or whether this is actually a white dwarf paired with a neutron star that is stripping material from its companion, is not entirely clear, though X9 lacks many features of this kind of transitional millisecond pulsar.

“We’re going to watch this binary closely in the future, since we know little about how such an extreme system should behave,” says co-author Vlad Tudor (Curtin University/ICRAR). “We’re also going to keep studying globular clusters in our galaxy to see if more evidence for very tight black hole binaries can be found.”

Before now, the closest star around a likely black hole was the system called MAXI J1659-152, where the orbital period is 2.4 hours, much longer than the 28 minutes we see in X9. There is also evidence for a similar system in the galaxy M49 (about 56 million light years away in the Virgo Cluster). That one is believed to have an orbit even tighter than X9’s, but Miller-Jones points out that there is no direct measurement of its period, making this work on X9 the first time that an orbital period this short in a system containing a likely black hole has ever been measured. An interesting exercise for the imagination indeed.

Although the gravitational waves from this system are too weak to detect with present ground-based observatories, future space-based platforms may be able to detect them.

The paper is Bahramian et al., “The ultracompact nature of the black hole candidate X-ray binary 47 Tuc X9,” Monthly Notices of the Royal Astronomical Society 14 March 2017 (abstract / preprint).

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Comments on this entry are closed.

  • galacsi March 20, 2017, 14:45

    Hi
    I don’t understand why a white dwarf losing mass should slowly escape the black hole or neutron star. IMO the black hole being heavier and heavier will shorten the orbit of the white dwarf.
    As I am not an astronomer , I must surely be wrong but I don’t see how .

    • Jeff Wright March 24, 2017, 13:25

      If you keep your speed as you lose mass–you gain distance and the orbit opens up.

  • Paul Gilster March 20, 2017, 14:54

    As I understand it, the mass of the star is steadily decreasing as material is drawn into the black hole, with the decrease in mass widening the orbit. The orbit of the star would originally have become tight until the loss of this mass caused it to widen over time.

    Miller-Jones has this to say about the phenomenon:

    “In the most likely scenario, this particular star would have first started losing mass to the suspected black hole several tens of millions of years ago when it was much closer, in an orbit with a period of just minutes. Over time, as that star has lost most of its mass, the size of the orbit would have increased, and the rate at which mass has been lost to the black hole would have decreased. The rate of mass loss would once have been a billion times higher. So yes, the star would initially have been much closer to the black hole. How close a star can get to a black hole before starting to lose mass to the black hole depends on the kind of star it is. Big, fluffy giant stars can lose gas to a black hole when they are much further away than small, compact stellar remnants like this white dwarf, whose gravity is strong enough that they are able to hold onto their mass more tightly, so need to get much closer before mass can be torn away. We also think that this star will have been gradually losing mass over tens to hundreds of millions of years; in this case it is not being torn apart in a single cataclysmic event that results in it being shredded into streams of debris, as we have seen in spectacular outbursts from the centres of some external galaxies (known as tidal disruption events). Rather, in this case, we have a steady loss of mass to the black hole over time.”

    • J. Jason Wentworth March 24, 2017, 4:57

      Indeed–a closer-to-home example is our own Moon. If the Earth had no Moon the size of ours, a tiny moonlet (like Mars’ satellites Phobos and Deimos) could never stay in such a distant geocentric orbit for very long (nor could a far smaller, artificial satellite), for two inter-related reasons that are relevant to the white dwarf star’s movement away from the black hole:

      At the Moon’s distance of roughly a quarter of a million miles, a mass which weighs 1 ton on the Earth’s surface would weigh (if it were suspended in space above the Earth, instead of orbiting it) just a few ounces! With the Earth’s gravity being so feeble at that distance, a natural or artificial satellite which had no appreciable gravity of its own would soon be pulled out of its Earth-centered orbit by the gravity of the other planets and the Sun. The mutual gravitational attraction between our much larger Moon and the Earth keeps the Moon firmly chained in its orbit, but:

      If our Moon lost mass (due to very massive impacts, insanely-high rates of export of mined lunar materials [ :-) ], etc.), it would–even without the currently-ongoing tidal “boosting” of its orbit that is due to the tidal bulge on the rapidly-rotating Earth–spiral outward away from the Earth, because its gravitational pull on the Earth would weaken as the Moon lost mass. This is happening to the white dwarf star; as it loses mass to the black hole, it is slowly spiraling outward from it.

      • Ron S March 25, 2017, 8:58

        “With the Earth’s gravity being so feeble at that distance, a natural or artificial satellite which had no appreciable gravity of its own would soon be pulled out of its Earth-centered orbit by the gravity of the other planets and the Sun.”

        Mass of an object does not prescribe the orbit. Dropped objects of any mass fall at the same rate (remember Galileo?). Any mass object can be in any orbit, provided that the “orbiting” object is quite a lot less massive than the object it is orbiting.

        The ratio of “pull” from other Earth, Sun, etc. is identical for an object orbiting the Earth. But low mass objects are more susceptible to non-symmetric interactions, such as satellites in LEO which easily drift due to inhomogeneities in the Earth’s structure passing below the satellite.

        It’s the dynamics, not mass, that are more at play in these scenarios. For example, how an object gains or losses mass determines the transfer of angular momentum (rotation and orbit) and conversion of angular momentum to heat. The scenario in the referenced paper is a bit conjectural and a brief skim of a few highlights didn’t tell me much that was definitive. Maybe it’s in there and I just didn’t spend enough time to find it.

        Is the white dwarf losing mass by tidal effects from the BH or its corona extending in the BH sphere of influence? What is the inclination of the white dwarf’s axis and orbit to the BH axis? How close to the accretion disk is the white dwarf, and do they intersect? It’s questions like these that would matter most, I believe.

        • J. Jason Wentworth March 26, 2017, 0:28

          I don’t think so, Ron. The statement that “all objects fall at the same rate” is only a first approximation, which holds true only for very small objects. Above its atmosphere, objects fall to Earth (or to any celestial body of appreciable mass) at the same rate only if their masses are so small–compared with Earth’s–that the Earth’s gravity is the only measurable one in the interaction. For example:

          If dropped from, say, 1,000 miles above the Earth, a feather and a Mack truck would fall toward the Earth’s surface at the same rate (until they encountered its atmosphere, of course). But if a 100 mile-wide asteroid and either of those other objects were dropped from the same height above the Earth (making sure that the asteroid’s center of mass was at the 1,000 mile-altitude mark), the asteroid would fall slightly faster, since it has a measurable–although still quite small–gravitational field. Moreover:

          This same phenomenon affects orbital velocity. If the objects in question are of negligible mass compared with the planet or star that they’re orbiting, they will orbit their primary at the same speed (if they’re at the same distance from the primary), because they have no measurable gravitational fields that can “pull on” the primary from their orbital height. But:

          An object that is large enough to measurably “pull on” its primary from its orbital height (our Moon does this, as the tides demonstrate) will orbit its primary more rapidly than would a tiny object at the same distance, because a large satellite’s gravitational field has a measurable ability to attract its primary (or to measurably curve space around its primary, to put it in Einsteinian terms). Even Phobos’ feeble gravity is able to raise a very small tidal bulge in Mar’s surface, which–since the tidal bulge pulls on Phobos from behind, because it orbits faster than Mars rotates–is causing Phobos’ orbit to very slowly decay. In addition:

          Because the black hole is almost certainly rotating very rapidly (and the matter pulled from the white dwarf star that’s “spiraling down the black hole’s drain” is certainly spinning, before it disappears across the back hole’s event horizon, much faster than the white dwarf is orbiting the black hole), the tidal forces from the rapidly-spiraling matter are likely boosting the white dwarf’s orbit. (The Earth’s Moon-caused tidal bulge is boosting the Moon’s orbit in the same way, because the tidal bulge in the rapidly-spinning Earth is slightly ahead of the Moon and pulls it forward [opposite the case with Mars and Phobos]).

          • Ron S March 26, 2017, 15:24

            “which holds true only for very small objects”

            Which I said, yet you did not notice.

            “has a measurable–although still quite small–gravitational field”

            This and your following text is an odd way of describing what’s going on. If the barycenter is sufficiently (depending on your required accuracy) separate from the COG of one of the interacting bodies Newton’s analytical solution for M>>m does not apply. The Moon orbits the barycenter (as does Earth) which is of course closer to the Moon than Earth’s COG.

            “the matter pulled from the white dwarf star that’s “spiraling down the black hole’s drain””

            Matter “pulled” (or simply leaving, as I already described) from the WD is interacting with the accretion disk, not the BH (which the paper notes). As I said, the dynamics are what’s important. For example, inclination of the WD orbit to the BH spin (and accretion disk), WD rotating or counter-rotating with respect to the accretion disk. From my admittedly brief skim of the paper I don’t believe they make a specific finding on these parameters.

            “The Earth’s Moon-caused tidal bulge is boosting the Moon’s orbit in the same way, because the tidal bulge in the rapidly-spinning Earth is slightly ahead of the Moon and pulls it forward”

            Not quite how I would describe it. The gravitational tides have two effects: dissipate rotational angular momentum as heat, due to Earth’s day and Moon’s orbit having different periods; transfer Earth’s rotational angular momentum (lengthens our day) to Moon’s orbital angular momentum (boosts orbit), via the dynamics you describe.

            • J. Jason Wentworth March 26, 2017, 23:41

              My apologies, Ron, for not noticing (or “registering”) all of your points–being in declining health, I have good days and bad days, and my posting wasn’t made on one of the good ones. (I much prefer contemplating and reading and writing about the subject matter on “Centauri Dreams” to turning inward and letting my maladies and pains become ‘my universe’–this blog is a wonderful antidote and preventative for that!)

          • Michael March 26, 2017, 16:45

            This loosening of the orbit may be a tool to determine the ‘properties’ of the blackhole.

  • galacsi March 20, 2017, 15:12

    If more mass is lost to space in générl than massis added to the black hole ,it can be possible.
    But if mass is accreted by the blackhole ,then its attraction will become stronger and the white dwarf orbit will decrease.
    May be the mass of the white dwarf is greater than the mass of the black hole , then I could understand why the orbit is increasing when losing mass.

    • Alexander Tolley March 20, 2017, 21:03

      Then the black hole would be orbiting the white dwarf.

      I’m pretty certain that the widening orbit about conserving angular momentum as the white dwarf lose mass.

    • Ron S March 21, 2017, 21:40

      “May be the mass of the white dwarf is greater than the mass of the black hole”

      Unlikely: https://en.wikipedia.org/wiki/Chandrasekhar_limit

  • Geoffrey Hillend March 20, 2017, 17:23

    I am not an expert on astrophysics but maybe the loss of mass of the white dwarf has something to do with the conservation of angular momentum which is transferred to the orbit of the white dwarf so it’s orbit gets larger?

    Also the common comparison to a white dwarf to a diamond planet is only an analogy. The crystal lattice of a diamond is different than lattice of carbon in a white dwarf which has a temperature of 4000 degrees at the surface and it gets hotter towards the core with an increasing pressure caused by the gravity. The crystal lattice of like that of a diamond probably can’t form in electron degeneracy or without it near the surface due to the great pressure and temperature. White dwarfs are white hot and also radiate in the ultra violet and soft x-ray black body spectrum, a high temperature remaining from the gravitational compression or contraction.

  • Michael March 21, 2017, 2:52

    If the BH is spinning faster than the orbit of the star which it most likely, then due to Space-Time been dragged around it is possible for an object to recede much like what is happening to the moon through tidal interactions.

  • Mark Zambelli March 21, 2017, 11:42

    Delving into speculation here but White dwarfs have surfaces that are around 100,000 K and it’s been my understanding that only after they’ve cooled, many millions of yrs hence, could they become crystalized carbon… so giant diamonds. If this X9 White Dwarf is losing mass it’s fate is accelerated but I seem to be missing the connection to that and this mentioning of carbon-planets that form in a completely different way… am I wrong?

  • Geoffrey Hillend March 21, 2017, 17:34

    Yes I was thinking it might be analogous to the Earth Moon system. Mark Zambelli, I know about the cooling of white dwarfs. Don’t forget about the electron degeneracy of carbon is not really the like the crystal of a diamond. The electron degeneracy is mostly in the center but it’s not near the surface but there still is a great deal of pressure there per square inch due to gravity. The crystals might be more denser than regular diamond crystals and can crystals form normally like diamonds on Earth when all the electrons in every shell are filled due to the electron degeneracy? I doubt it. Consequently, the whole white dwarf can’t be a giant diamond as some articles have claimed..

  • Geoffrey Hillend March 21, 2017, 18:05

    Maybe I’m just thinking about individual crystals and a completely cool, whole, white dwarf can be thought of a giant diamond of carbon covalently bonded. Some white dwarfs have oxygen, hydrogen and helium depending on their mass though.

  • galacsi March 26, 2017, 14:32

    Thanks . That’s a very good answer to my question.

  • ljk May 30, 2017, 11:54

    Do Stars Fall Quietly into Black Holes, or Crash into Something Utterly Unknown?

    Release No.: 2017-16

    For Release: Tuesday, May 30, 2017 – 11:00 am

    Cambridge, MA –

    Astronomers at The University of Texas at Austin and Harvard University have put a basic principle of black holes to the test, showing that matter completely vanishes when pulled in. Their results constitute another successful test for Albert Einstein’s General Theory of Relativity.

    Most scientists agree that black holes, cosmic entities of such great gravity that nothing can escape their grip, are surrounded by a so-called event horizon. Once matter or energy gets close enough to the black hole, it cannot escape — it will be pulled in. Though widely believed, the existence of event horizons has not been proved.

    “Our whole point here is to turn this idea of an event horizon into an experimental science, and find out if event horizons really do exist or not,” said Pawan Kumar, a professor of astrophysics at The University of Texas at Austin.

    Supermassive black holes are thought to lie at the heart of almost all galaxies. But some theorists suggest that there’s something else there instead — not a black hole, but an even stranger supermassive object that has somehow managed to avoid gravitational collapse to a singularity surrounded by an event horizon. The idea is based on modified theories of General Relativity, Einstein’s theory of gravity.

    While a singularity has no surface area, the noncollapsed object would have a hard surface. So material being pulled closer — a star, for instance — would not actually fall into a black hole, but hit this hard surface and be destroyed.

    Kumar, his graduate student Wenbin Lu, and Ramesh Narayan, a theorist from the Harvard-Smithsonian Center for Astrophysics, have come up with a test to determine which idea is correct.

    “Our motive is not so much to establish that there is a hard surface,” Kumar said, “but to push the boundary of knowledge and find concrete evidence that really, there is an event horizon around black holes.”

    The team figured out what a telescope would see when a star hit the hard surface of a supermassive object at the center of a nearby galaxy: The star’s gas would envelope the object, shining for months, perhaps even years.

    Full article here:

    https://www.cfa.harvard.edu/news/2017-16

    The artwork alone is worth the price of admission.