ESO: How Much Mass Makes a Black Hole?

[bystander]

How Much Mass Makes a Black Hole?
European Southern Observatory | eso1034 | 18 Aug 2010

Artist’s impression: Magnetar in Westerlund 1 [i](Credit: ESO/L. Calçada)[/i]

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Star Cluster Westerlund 1 [i](Credit: ESO/MPG/WFI)[/i]

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Using ESO’s Very Large Telescope, European astronomers have for the first time demonstrated that a magnetar — an unusual type of neutron star — was formed from a star with at least 40 times as much mass as the Sun. The result presents great challenges to current theories of how stars evolve, as a star as massive as this was expected to become a black hole, not a magnetar. This now raises a fundamental question: just how massive does a star really have to be to become a black hole?

To reach their conclusions, the astronomers looked in detail at the extraordinary star cluster Westerlund 1^[1], located 16 000 light-years away in the southern constellation of Ara (the Altar). From previous studies (eso0510), the astronomers knew that Westerlund 1 was the closest super star cluster known, containing hundreds of very massive stars, some shining with a brilliance of almost one million suns and some two thousand times the diameter of the Sun (as large as the orbit of Saturn).
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Westerlund 1 is a fantastic stellar zoo, with a diverse and exotic population of stars. The stars in the cluster share one thing: they all have the same age, estimated at between 3.5 and 5 million years, as the cluster was formed in a single star-formation event.

A magnetar (eso0831) is a type of neutron star with an incredibly strong magnetic field — a million billion times stronger than that of the Earth, which is formed when certain stars undergo supernova explosions. The Westerlund 1 cluster hosts one of the few magnetars known in the Milky Way. Thanks to its home in the cluster, the astronomers were able to make the remarkable deduction that this magnetar must have formed from a star at least 40 times as massive as the Sun.

2. The open cluster Westerlund 1 was discovered in 1961 from Australia by Swedish astronomer Bengt Westerlund, who later moved from there to become ESO Director in Chile (1970–74). This cluster is behind a huge interstellar cloud of gas and dust, which blocks most of its visible light. The dimming factor is more than 100 000, and this is why it has taken so long to uncover the true nature of this particular cluster.

A VLT/FLAMES survey for massive binaries in Westerlund 1:

• II. Dynamical constraints on magnetar progenitor masses from the eclipsing binary W13 - BW Ritchie et al
  Astronomy and Astrophysics (accepted 14 Jun 2010) DOI: 10.1051/0004-6361/201014834
  arXiv.org > astro-ph > arXiv:1008.2840 > 17 Aug 2010

A VLT/FLAMES survey for massive binaries in Westerlund 1:

• I. First observations of luminous evolved stars - BW Ritchie et al
  Astronomy and Astrophysics 207(3) (Dec 2009) DOI: 10.1051/0004-6361/200912686
  arXiv.org > astro-ph > arXiv:0909.3815 > 21 Sep 2009 (v1), 03 Oct 2009 (v2)

A Neutron Star with a Massive Progenitor in Westerlund 1 - MP Muno et al

• Astrophysical Journal Letters 636(1) L41 (Jan 2006) DOI: 10.1086/499776
  arXiv.org > astro-ph > arXiv:astro-ph/0509408 > 14 Sep 2005 (v1), 26 Jan 2006 (v3)

[bystander]

The Mystery of the Absent Black Hole
Science NOW | Astronomy | 18 Aug 2010

Where's the black hole? That's what astronomers are asking as they gaze upon the burned-out remnant of a stellar explosion some 16,000 light-years away in the southern constellation Ara. The defunct star once held at least twice the mass necessary to create a black hole when it exploded as a supernova, yet somehow only an extremely magnetic, asteroid-sized object known as a magnetar remains.

When it comes to stars, astronomers like to say that mass is destiny. Relatively small objects like our sun can live up to 10 billion years before briefly expanding into red giants and then slowly dying as white dwarfs. Bigger stars with masses at least five times greater than the sun's have much shorter life spans—sometimes only a few hundred thousand years—and die spectacularly in huge explosions called supernovae. Current theory also suggests that if the doomed star holds less than 20 times the sun's mass, its explosive death should produce a small but extremely dense remnant called a neutron star. And if its mass tips the scale at 20 or more suns, the supernova should produce a black hole.

That's what makes the object astronomers have found so unusual. Like all magnetars, CXOU J164710.2-455216 is a rare kind of neutron star that for as-yet-unexplained reasons possesses the most powerful magnetic field in the universe. Yet according to detailed measurements of the relative motions of the surrounding stars, the team reports in an upcoming issue of Astronomy & Astrophysics, that like every neighboring star, the mass of the magnetar's progenitor must have been at least 40 times greater than the sun's. And thus it shouldn't be a magnetar at all, but a black hole.

[neufer]

<<When, in a supernova, a star collapses to a neutron star, its magnetic field increases dramatically in strength. Halving a linear dimension increases the magnetic field fourfold. Duncan and Thompson calculated that the magnetic field of a neutron star, normally an already enormous 10⁸ teslas could, through the dynamo mechanism, grow even larger, to more than 10¹¹ teslas (or 10¹⁵ gauss). The result is a magnetar. The supernova might lose 10% of its mass in the explosion. In order for such large stars (10 to 30 solar masses) not to collapse directly into a black hole, they have to shed a larger proportion of their mass— perhaps another 80%. It is estimated that about one in ten supernova explosions results in a magnetar rather than a more standard neutron star or pulsar.

Magnetars are primarily characterized by their extremely powerful magnetic field, which can often reach the order of ten gigateslas. These magnetic fields are hundreds of thousands of times stronger than any man-made magnet, and quadrillions of times more powerful than the field surrounding Earth. As of 2010, they are the most magnetic objects ever detected in the universe. A magnetic field of 10 gigateslas is enormous relative to magnetic fields typically encountered on Earth. Earth has a geomagnetic field of 30–60 microteslas, and a neodymium based rare earth magnet has a field of about 1 tesla, with a magnetic energy density of 4.0×10⁵ J/m3. A 10 gigatesla field, by contrast, has an energy density of 4.0×10²⁵ J/m3, with an E/c² mass density >10,000 times that of lead. The magnetic field of a magnetar would be lethal even at a distance of 1000 km, tearing tissues due to the diamagnetism of water. At a distance halfway to the moon, a magnetar could strip information from all credit cards on Earth.

As described in the February 2003 Scientific American cover story, remarkable things happen within a magnetic field of magnetar strength. "X-ray photons readily split in two or merge together. The vacuum itself is polarized, becoming strongly birefringent, like a calcite crystal. Atoms are deformed into long cylinders thinner than the quantum-relativistic wavelength of an electron." In a field of about 10⁵ teslas atomic orbitals deform into rod shapes. At 10¹⁰ teslas, a hydrogen atom becomes a spindle 200 times narrower than its normal diameter.>>