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Ringing the Stellar Bell

56 light years from Earth, the star Iota Horologii is a member of the ‘Hyades stream,’ a number of stars moving in a similar direction with respect to the rest of the galaxy. It’s also an exoplanet host star, known to have a planet twice the mass of Jupiter in a 320-day orbit. The two factors — the position of the star within the stream and the planet that accompanies it — play into an unusual application of asteroseismology, the study of the sound waves that move through a star.

The star Iota Horologii

I want to note this work particularly because it has a bearing on planet formation, about which the more we learn the better as we continue the hunt for exoplanets. But let’s pause on asteroseismology itself. You may recall that using this technique for studying the interiors of stars is one of the purposes of the COROT mission, the other being the detection of planetary transits. Asteroseismology is invariably explained with musical metaphors, likening the sound moving within a star to the ringing of a bell.

Just how the bell rings can tell us much about the star. In the case of Iota Horologii, the team studying the star (led by Sylvie Vauclair of the University of Toulouse) was able to pick out the signature of 25 separate waves, as if the ringing bell had produced 25 ‘notes.’ Out of that data come measurements that are quite precise. Iota Horologii is now believed to have a mass 1.25 times that of the Sun, with a temperature of 6150 K and an age of 625 million years.

Image: Using HARPS on ESO’s 3.6-m telescope at La Silla, astronomers were able to study in great detail the star Iota Horologii, known to harbour a giant planet, and make a very precise portrait of it: its temperature is 6150 K, its mass is 1.25 times that of the Sun, and its age is 625 million years. Moreover, the star is found to be more metal-rich than the Sun by about 50%. This means the star must have drifted from the Hyades cluster where it formed. Credit: Digital Sky Survey/VirGO.

And note this: The star is about fifty percent more metal-rich than the Sun. That’s useful information because it tell us that Iota Horologii has the same metal abundance and age as the Hyades cluster, located 151 light years away. Note the distance — the star has drifted about 130 light years from its apparent birthplace, although it continues to move in the general direction of the cluster. The paper on this work makes the claim that Iota Horologii is one of the 15 percent of stars in the Hyades stream that share this origin, the others being simply “…field-like stars sharing the Hyades galactic velocities because of galactic dynamical effects.”

The high metal content of this star is significant in its own right. Metals in astrophysical terms are elements higher than hydrogen and helium, and the question of how they play into planetary development remains a thorny one. If you take an average of the stars known to have exoplanets, they turn out to be far more laden with metals than our own Sun. Is this because the clouds that gave birth to these stars already contained high levels of metals, or because planets and planetesimals accrete around these stars early on, increasing their metallicity?

This is a testable issue, for if the metals come from the original cloud, then they extend throughout the star, whereas if they’re the result of accretion, only the star’s outer layers should show high metal levels. You can see how asteroseismology ties in to all this. The paper summarizes the result, noting that this is the first time these methods have been able to deliver unambiguous results, despite studies of the star Mu Arae, which was observed with the same HARPS spectrograph:

In the case of ι Hor, the question may be approached in a different way. Asteroseismology leads to the conclusion that this star was evaporated from the primordial Hyades cluster, sharing the same age, helium abundance and overmetallicity. This is one proof that the origin of this overmetallicity is primordial, from the original cloud. This result has important consequences for the formation and subsequent evolution of galactic clusters and the theories of exoplanets formation and migration.

Or as Vauclair herself puts it in this ESO news release:

“The chicken and egg question of whether the star got planets because it is metal-rich, or whether it is metal-rich because it made planets that were swallowed up is at least answered in one case.”

What we need, of course, are many, many cases as we continue to investigate how planets form. As that work continues, expect asteroseismology to play a significant role. The idea that we can use a spectrograph to study the motions on stellar surfaces and translate that information into solid data about the interior of stars — age, helium abundance, gravity, mass — is striking, and in this case led to an identification of origins that feeds into broader exoplanet questions. As the paper summarizes the issue:

This work shows how powerful asteroseismology can be in deriving the characteristic parameters of a star with the help of spectroscopic analysis, but finally obtaining much more precise results than with spectroscopy alone.

The paper is Vauclair et al., “The exoplanet-host star iota Horologii: an evaporated member of the primordial Hyades cluster,” accepted by Astronomy and Astrophysics and available online.

Comments on this entry are closed.

  • andy April 16, 2008, 14:31

    Seems like planet formation can proceed quite nicely in open clusters: there’s a planet around Epsilon Tauri in the Hyades proper, plus planets around giant stars in the clusters NGC 2423 and NGC 4349, plus there is evidence for terrestrial planet formation around HD 23514 in the Pleiades. There are a handful of old and metal rich open clusters listed in this paper on SETI target selection, of which the nearest are Ruprecht 46 at 750 parsecs (though this is apparently a region of anomalously high star density rather than a true cluster) and Messier 67 at 900 parsecs away. Unfortunately this is far enough away that a Sunlike star in M67 would have an apparent magnitude of about 14-15, which is rather faint for radial velocity studies. Giant stars on the other hand would certainly be bright enough for RV.

  • Administrator April 16, 2008, 14:54

    andy, help me on this — how do we distinguish between anomalously high star density and a true cluster?

  • andy April 16, 2008, 15:19

    In a true cluster the stars have the same age and metallicity, so will show evidence of a turn-off in the main sequence (because the more massive stars will have evolved).

  • Administrator April 16, 2008, 15:53

    Helpful indeed. Thanks, andy.

  • James M. Essig April 17, 2008, 0:20

    Hi Folks;

    It is interesting to note how the presence of heavy metals and even the presence of organic compounds in interstellar gas clouds can effect the size of the stars that can form from these clouds.

    The concept that Cold Dark Matter within the confines of the cores of certain stars may be able to moderate the absolute rate and relative rate of the various fusion sequences within stars is very interesting also.



  • Hans Bausewein April 19, 2008, 14:36

    Reading this I was wondering where our solar system came from. Has it started with the stars we see today?
    (not so likely, I guess: many bright stars are probably much younger)

    Have some been identified as originating from the same cloud?

    Would be nice be able to point to them in the night sky.


  • Administrator April 19, 2008, 15:02

    Hans, here’s a link to a 2006 paper by Leslie Looney and collaborators that studies the Sun’s origins in a large open cluster:


    The influence of a nearby supernova is discussed here as well. From the abstract:

    “In one scenario, a collection of low-mass stars, including our sun, formed in a group or cluster with an intermediate- to high-mass star that ended its life as a supernova while our sun was still a protostar, a starless core, or perhaps a diffuse cloud. Using recent observations of protostars to estimate the size of the protosolar nebula constrains the distance of the supernova at 0.02 to 1.6 pc.”

    The stars in the surrounding cluster would have dissipated over the last five billion years, though I imagine there’s lots of work to be done in studying the cluster’s spread.

  • Hans Bausewein April 19, 2008, 17:34

    Thanks for the link.

    If it was a small cluster or an even more isolated cloud, it will be very difficult to find other members. Five billion years is a long time.

  • Hans Bausewein April 20, 2008, 3:54

    Much closer to home:

    “Solar flares set the Sun quaking”:


    Good that the only star that can be studied in detail can be compared to many others.


  • ljk April 28, 2008, 12:35

    Radio Emission from Exoplanets

    Authors: Samuel J. George, Ian R. Stevens

    (Submitted on 24 Apr 2008)

    Abstract: We present results from new low frequency observations of two extrasolar planetary systems (Epsilon Eridani and HD128311) taken at 150 MHz with the Giant Metrewave Radio Telescope (GMRT). We do not detect either system, but are able to place tight upper limits on their low frequency radio emission.

    Comments: 3 pages, 1 figure, From Planets to Dark Energy: the Modern Radio Universe, POS(MRU) 098

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0804.3927v1 [astro-ph]

    Submission history

    From: Samuel George [view email]

    [v1] Thu, 24 Apr 2008 13:49:49 GMT (98kb)


  • ljk May 12, 2008, 14:01

    L dwarfs in the Hyades

    Authors: E. Hogan (1), R. F. Jameson (1), S. L. Casewell (1), S. L. Osbourne (1), N. C. Hambly (2) ((1) Department of Physics and Astronomy, University of Leicester, (2) Scottish Universities’ Physics Alliance (SUPA), Institute for Astronomy, School of Physics, University of Edinburgh)

    (Submitted on 8 May 2008)

    Abstract: We present the results of a proper motion survey of the Hyades to search for brown dwarfs, based on UKIDSS and 2MASS data. This survey covers ~275 square degrees to depth of K~15 mag, equivalent to a mass of 0.05 solar masses assuming a cluster age of 625 Myr. The discovery of 12 L dwarf Hyades members is reported. These members are also brown dwarfs, with masses between 0.05 < M < 0.075 solar masses. A high proportion of these L dwarfs appear to be photometric binaries.

    Comments: 7 pages, 4 figures, 2 tables. Accepted for publication in MNRAS

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0805.1189v1 [astro-ph]

    Submission history

    From: Emma Hogan [view email]

    [v1] Thu, 8 May 2008 17:21:35 GMT (49kb)