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Puzzling Stars in Omega Centauri

Globular clusters are vast cities of tens of thousands of stars, traditionally thought to have been formed from a single interstellar cloud at roughly the same time. But Omega Centauri is different. As viewed by Hubble, this southern cluster (15,000 light years away in the direction of the constellation Centaurus) contains two separate stellar populations. Its blue stars, about one quarter of the total, are well outnumbered by a second hydrogen-burning population of redder stars.

Now the European Southern Observatory’s Very Large Telescope has collected data that show the blue stars, contrary to expectation, are metal-rich when compared to their red counterparts, meaning they include elements heavier than hydrogen and helium. Astronomers call elements heavier than helium ‘metals’ — the Sun, for example, is made up of 70 percent hydrogen and 28 percent helium, with the remaining two percent being classed as metals.

Omega Centauri clusterCurrent theories of star formation suggest that as metallicity increases, stars become redder. According to the ESO team, the only way to explain the oddity of metal-rich blue stars is to assume that they have a far higher helium content than their red counterparts. That would make them the most helium-rich stars ever found, and raises a major question: with the present abundance of helium in the Milky Way at 28 percent, how did this globular cluster produce stars with a 39 percent abundance?

Image: Omega Centauri, largest of the 160 globular clusters in the Mily Way. In the center of such a cluster, stars are packed more than 10,000 times more closely than in the neighborhood of our Sun. Credit: P. Seitzer (U. Michigan).

ESO’s Luigi Bedin has a possible solution:

“The scenario we presently favour is one in which the high helium content originates from material ejected during the supernovae explosions of massive stars. It is possible that the total mass of Omega Centauri was just right to allow the material expelled by high-mass supernovae to escape, while the matter from explosions of stars with about 10-12 times the mass of the Sun was retained.”

Omega Centauri, then, would have seen two generations of stars. The first produced the redder stars, whose most massive members exploded as supernovae within tens of millions of years. The second population of blue stars then formed from this helium-rich environment. But the real issue raised by Omega Centauri is broader: why did this globular cluster produce extremely helium-rich stars, whereas all other known clusters did not?

Centauri Dreams take: another suggestion that globular cluster star formation is poorly understood comes from a Hubble study that scanned the globular star cluster 47 Tucanae (located 15,000 light-years away in the southern constellation Tucana) using the transit method to search for planets. As reported in 2000, the team expected to find about 17 hot Jupiter-class planets, but they found none at all. Is the deficiency of heavier elements found in most globular clusters an indication that planets can only form in a metal-rich universe? And if so, what does this imply about Omega Centauri?

For more on the Omega Centauri conundrum, see Piotto, G., Villanova, S. Begin, L. et al., “Metallicities on the Double Main Sequence of omega Centauri Imply Large Helium Enhancement,” now available on the ArXiv site and also in the March 10 issue of the Astrophysical Journal, Vol. 621, p. 777. ESO’s press release is here.

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  • ljk January 21, 2008, 10:52

    Gemini and Hubble Space Telescope Evidence for an Intermediate Mass Black Hole in omega Centauri

    Authors: Eva Noyola (1,2), Karl Gebhardt (2), Marcel Bergmann (3) ((1) MPE, (2) UT Austin, (3) Gemini)

    (Submitted on 17 Jan 2008)

    Abstract: The globular cluster omega Centauri is one of the largest and most massive members of the galactic system. However, its classification as a globular cluster has been challenged making it a candidate for being the stripped core of an accreted dwarf galaxy; this together with the fact that it has one of the largest velocity dispersions for star clusters in our galaxy makes it an interesting candidate for harboring an intermediate mass black hole.

    We measure the surface brightness profile from integrated light on an HST}/ACS image of the center, and find a central power-law cusp of logarithmic slope -0.08. We also analyze Gemini GMOS-IFU kinematic data for a 5×5 arcsec field centered on the nucleus of the cluster, as well as for a field 14 arcsecaway. We detect a clear rise in the velocity dispersion from 18.6 km/s at 14 arcsec to 23 km/s in the center. A rise in the velocity dispersion could be due to a central black hole, a central concentration of stellar remnants, or a central orbital structure that is radially biased.

    We discuss each of these possibilities. An isotropic, spherical dynamical model implies a black hole mass of 4.0^{+0.75}_{-1.0} times 10^4 M_sun, and excludes the no black hole case at greater than 99% significance. We have also run flattened, orbit-based models and find similar results. While our preferred model is the existence of a central black hole, detailed numerical simulations are required to confidently rule out the other possibilities.

    Comments: 8 pages, 9 figures, ApJ accepted

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.2782v1 [astro-ph]

    Submission history

    From: Eva Noyola [view email]

    [v1] Thu, 17 Jan 2008 22:16:12 GMT (224kb)

    http://arxiv.org/abs/0801.2782

  • ljk March 17, 2008, 22:23

    MAD@VLT: Deep into the Madding Crowd of Omega Centauri

    Authors: G. Bono, A. Calamida, C.E. Corsi (OAR/INAF), P.B. Stetson (HIA/NRC), E. Marchetti, P. Amico (ESO), P.G. Prada Moroni (Univ. Pisa), I. Ferraro, G. Iannicola (INAF), M. Monelli (IAC), R. Buonanno (Univ. Rome), F. Caputo, M. Dall’Ora (INAF), S. Degl’Innocenti (Univ. Pisa), S. D’Odorico (ESO), L.M. Freyhammer (Univ. Lancashire), D. Koester (Univ. of Kiel), M. Nonino, A.M. Piersimoni, L. Pulone (INAF), M. Romaniello (ESO)

    (Submitted on 14 Mar 2008)

    Abstract: We present deep and accurate Near-Infrared (NIR) photometry of the Galactic Globular Cluster (GC) Omega Cen. Data were collected using the Multi-Conjugate Adaptive Optics Demonstrator (MAD) on VLT (ESO). The unprecedented quality of the images provided the opportunity to perform accurate photometry in the central crowded regions. Preliminary results indicate that the spread in age among the different stellar populations in Omega Cen is limited.

    Comments: 6 pages, 3 figures, to appear in the Springer Astrophysics and Space Science Proceedings, “Science with the VLT in the ELT era”, ed. A. Moorwood

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0803.2207v1 [astro-ph]

    Submission history

    From: Annalisa Calamida [view email]

    [v1] Fri, 14 Mar 2008 17:03:48 GMT (283kb)

    http://arxiv.org/abs/0803.2207

  • ljk April 2, 2008, 8:54

    Black hole found in Omega Centauri

    Omega Centauri has been known to be an unusual globular cluster
    for a long time.

    A new result obtained by Hubble and the Gemini Observatory
    reveals that the globular cluster may have a rare intermediate-mass
    black hole hidden in its centre, implying that is likely not a globular
    cluster at all, but a dwarf galaxy stripped of its outer stars.

    More at:

    http://www.esa.int/esaSC/SEMPGM5QGEF_index_0.html

  • ljk April 11, 2008, 13:16

    Testing the AGB Scenario as the Origin of the Extreme-Helium Population in omega Centauri

    Authors: Ena Choi, Sukyoung K. Yi

    (Submitted on 10 Apr 2008)

    Abstract: The most massive Galactic globular cluster, Omega centauri, appears to have multiple populations. Its bluest main sequence and extended horizontal branch stars are suggested to have the common origin, that is, an extremely high helium abundance of Y ~ 0.4. The high helium abundance is most often attributed to asymptotic giant branch (AGB) stars.

    In this study we test the AGB hypothesis. We simulate the maximum-AGB models where the impact of AGB stars is maximised by assuming that supernova explosions do not affect the chemical evolution of the proto cloud. We compare the enrichment history of helium, metals, carbon and nitrogen to the observed values. Even under the most generous condition, the maximum-AGB models fail to reproduce the large values of helium Y ~ 0.4 and helium enrichment parameter Delta Y / Delta Z ~ 70 which were deduced from the colour-magnitude diagram fits. They also fail to reproduce the C and N contents of the blue population spectroscopically determined.

    We conclude that the AGB scenario with the canonical stellar evolution theory cannot explain the observational constraints and that the self chemical enrichment does not provide a viable solution. Alternative processes are desperately called for.

    Comments: 6 pages, 5 figures, Accepted by MNRAS

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0804.1598v1 [astro-ph]

    Submission history

    From: Ena Choi [view email]

    [v1] Thu, 10 Apr 2008 01:44:12 GMT (59kb)

    http://arxiv.org/abs/0804.1598

  • ljk May 7, 2008, 22:39

    A Spectroscopic Survey for Omega Centauri Members at and beyond the Cluster Tidal Radius

    Authors: G. S. Da Costa, M. G. Coleman

    (Submitted on 7 May 2008)

    Abstract: We have used the two-degree field (2dF) multi-fibre spectrograph of the Anglo-Australian Telescope to search for candidate members of the unusual globular cluster omega Centauri at and beyond the cluster tidal radius. Velocities with an accuracy of ~10 kms-1 were obtained for 4105 stars selected to lie in the vicinity of the lower giant branch in the cluster colour-magnitude diagram and which cover an area on the sky of ~2.4×3.9 deg2 centered on the cluster. Within the velocity interval 190-270 kms-1, the cluster member candidates have a steeply declining surface density distribution consistent with the adopted tidal radius of 57`.

    For stars in the sample beyond the tidal radius, an analysis of line-strengths from the spectra, as well as radial velocities, identifies only six stars as possible candidates for extra-tidal association with the cluster. If all six of these stars are indeed related to the cluster, then a maximum of 0.7 +/- 0.2 per cent of the total cluster mass is contained in the region between one and two tidal radii. Given this limit, we conclude that there is no compelling evidence for any significant extra-tidal population in omega Cen. The effects of tidal shocks on the outer parts of the cluster are consistent with this limit.

    Theories for the origin of omega Cen frequently suggest that the cluster is the former nucleus of a tidally stripped dwarf galaxy. Our results require that the stripping process must have been largely complete at early epochs, consistent with dynamical models of the process. The stripped former dwarf galaxy stars are therefore now widely distributed around the Galaxy.

    Comments: 29 pages, 9 figures, accepted for publication in AJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.0823v1 [astro-ph]

    Submission history

    From: G. S. Da Costa [view email]

    [v1] Wed, 7 May 2008 01:31:09 GMT (2160kb,D)

    http://arxiv.org/abs/0805.0823

  • ljk April 12, 2009, 23:35

    Mixed Populations in Globular Clusters: Et Tu, 47 Tuc?

    Authors: J. Anderson, G. Piotto, I. R. King, L. R. Bedin, P. Guhathakurta

    (Submitted on 10 Apr 2009)

    Abstract: We exploit the large number of archival HST images of 47 Tuc to examine its subgiant branch (SGB) and main sequence (MS) for signs of multiple populations.

    In the cluster core, we find that the cluster’s SGB exhibits a clear spread in luminosity, with at least two distinct components: a brighter one with a spread that is real but not bimodal, and a second one about 0.05 mag fainter, containing about 10% of the stars.

    In a less crowded field 6 arcminutes from the center, we find that the MS is broadened much more than can be accounted for by photometric errors, and that this broadening increases at fainter magnitudes.

    Comments: 11 pages, 3 figures

    Subjects: Galaxy Astrophysics (astro-ph.GA)

    Cite as: arXiv:0904.1626v1 [astro-ph.GA]

    Submission history

    From: Jay Anderson [view email]

    [v1] Fri, 10 Apr 2009 01:48:43 GMT (320kb)

    http://arxiv.org/abs/0904.1626