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Planet Formation in the Pleiades

I’ve always enjoyed Lynette Cook’s work. As you can see in the image below, this space artist captures the drama of celestial events by drawing on recent findings. Like Chesley Bonestell, Cook can take you to an exotic place and leave you staring, but her focus is tighter, homing in on exoplanets as filtered through ongoing work at observatories worldwide. The wonders she’ll have to work with as we find more and more such worlds can only be imagined. The dazzling collision below is her take on what may be happening as rocky planets form around HD 23514.

The star’s designation doesn’t jump out but its location does, the oft-studied Pleiades star cluster. Joseph Rhee (UCLA) and collaborators have been working infrared wavelengths using the Gemini North Telescope (Mauna Kea) and space-based infrared instruments, measuring the hot dust around this 100-million year old star. HD 23514 is Sun-like enough to add to the intrigue of this exercise, and it’s orbited by hundreds of thousands of times more dust than our Sun. Evidence for catastrophic collisions in an evolving planetary system? Perhaps.

Planet formation in the Pleiades

Whatever is happening seems to originate in a zone between 1/4 to 2 AU from the star, the region between Mercury and Mars in our own Solar System. Benjamin Zuckerman (UCLA) thinks the sheer amount of hot dust is the result of a fairly recent collision between large rocky bodies, perhaps reminiscent of the ‘Big Whack’ that produced the Moon four billion years ago. Says Zuckerman: “Indeed, the collision that generated the Moon sent a comparable mass of debris into interplanetary orbits as is now observed in HD 23514.”

Image: Artist’s rendering of what the environment around HD 23514 might look like as two Earth-sized bodies collide. Artwork by Lynette Cook for Gemini Observatory.

This isn’t the first time Zuckerman has worked on terrestrial planet formation around Sun-like stars. It was back in 2005 that his team reported on BD+20 307, some 300 light years away in the direction of the constellation Aries. That one sports fully one million times the amount of dust that is orbiting our Sun, a step up even from HD 23514. The conclusion: Most young Sun-like stars are probably building terrestrial-style planets through recurring violent collisions. Indeed, comments Rhee, “This is the first clear evidence for planet formation in the Pleiades, and the results we are presenting strongly suggest that terrestrial planets like those in our solar system are quite common.”

Much younger stars in the 10 million year old range or younger are far more likely to have this much dust around them than stars as old as HD 23514, making it and BD+20 307 useful examples of how planet formation may occur around such ‘adolescent’ stars. The paper is Rhee et al., “Warm dust in the terrestrial planet zone of a sun-like Pleiad: collisions between planetary embryos?” accepted for publication in The Astrophysical Journal and available online. The paper on BD+20 307 is Song et al., “Extreme collisions between planetesimals as the origin of warm dust around a Sun-like star,” Nature 436 (21 July 2005), 363-365 (doi: 10.1038/nature03853). Abstract available.

Comments on this entry are closed.

  • Rick November 17, 2007, 15:15

    Great article with more information than is in the other releases that every astroblog (including mine) made mention of. I enjoy this blog quite a bit; keep up the good work!

    The Pleiades is a long-time favorite of amateur astronomers and my personal favorite open cluster. Great to see some exoplanetary action happening in something visible with the naked eye.

  • Administrator November 17, 2007, 15:40

    Very kind of you, Rick, and nice to make the acquaintance of Deep Sky Blog, which I’ve just linked to in our weblogs section.

  • Adam November 18, 2007, 18:53

    Hi Paul

    All that dust won’t last – the Poynting-Robertson effect will dump it into the star, while some will be gravitationally sling-shot out of the system. Makes me wonder just how much gets shared between star systems, especially when they’re relatively close together like in the Pleiades – even more so when stars are newly minted and very close to each other.

  • Administrator November 18, 2007, 20:49

    You’re right about that dust not lasting, Adam, but what’s unusual in this case is that it’s lasted as long as it has. Very few 100 million year old stars show dust in this amount, making these quite an interesting study.

  • ljk November 26, 2007, 17:34

    Growing and moving low-mass planets in non-isothermal disks

    Authors: S.-J. Paardekooper, G. Mellema

    (Submitted on 22 Nov 2007)

    Abstract: We study the interaction of a low-mass planet with a protoplanetary disk with a realistic treatment of the energy balance by doing radiation-hydrodynamical simulations. We look at accretion and migration rates and compare them to isothermal studies. We used a three-dimensional version of the hydrodynamical method RODEO, together with radiative transport in the flux-limited diffusion approach. The accretion rate, as well as the torque on the planet, depend critically on the ability of the disk to cool efficiently. For densities appropriate to 5 AU in the solar nebula, the accretion rate drops by more than an order of magnitude compared to isothermal models, while at the same time the torque on the planet is positive, indicating outward migration. It is necessary to lower the density by a factor of 2 to recover inward migration and more than 2 orders of magnitude to recover the usual Type I migration. The torque appears to be proportional to the radial entropy gradient in the unperturbed disk. These findings are critical for the survival of protoplanets, and they should ultimately find their way into population synthesis models.

    Comments: Accepted for publication in Astronomy and Astrophysics

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0711.3601v1 [astro-ph]

    Submission history

    From: Sijme-Jan Paardekooper [view email]

    [v1] Thu, 22 Nov 2007 15:35:52 GMT (2792kb)


  • ljk December 11, 2007, 11:15

    On the age heterogeneity of the Pleiades, Hyades and Sirius moving groups

    Authors: Benoit Famaey, Arnaud Siebert, Alain Jorissen

    (Submitted on 10 Dec 2007)

    Abstract: We investigate the nature of the classical low-velocity structures in the local velocity field, i.e. the Pleiades, Hyades and Sirius moving groups. After using a wavelet transform to locate them in velocity space, we study their relation with the open clusters kinematically associated with them. By directly comparing the location of moving group stars in parallax space to the isochrones of the embedded clusters, we check whether, within the observational errors on the parallax, all moving group stars could originate from the on-going evaporation of the associated cluster.

    We conclude that, in each moving group, the fraction of stars making up the velocity-space overdensity superimposed on the background is higher than the fraction of stars compatible with the isochrone of the associated cluster. These observations thus favour a dynamical (resonant) origin for the Pleiades, Hyades and Sirius moving groups.

    Comments: 8 pages, 8 figures, accepted for publication in A&A

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0712.1470v1 [astro-ph]

    Submission history

    From: Famaey Benoit [view email]

    [v1] Mon, 10 Dec 2007 13:47:43 GMT (80kb)


  • ljk January 10, 2008, 14:50

    The Distribution of Stellar Mass in the Pleiades

    Authors: Joseph M. Converse, Steven W. Stahler

    (Submitted on 8 Jan 2008)

    Abstract: As part of an effort to understand the origin of open clusters, we present a statistical analysis of the currently observed Pleiades. Starting with a photometric catalog of the cluster, we employ a maximum likelihood technique to determine the mass distribution of its members, including single stars and both components of binary systems. We find that the overall binary fraction for unresolved pairs is 68%. Extrapolating to include resolved systems, this fraction climbs to about 76%, significantly higher than the accepted field-star result. Both figures are sensitive to the cluster age, for which we have used the currently favored value of 125 Myr. The primary and secondary masses within binaries are correlated, in the sense that their ratios are closer to unity than under the hypothesis of random pairing. We map out the spatial variation of the cluster’s projected and three-dimensional mass and number densities. Finally, we revisit the issue of mass segregation in the Pleiades. We find unambiguous evidence of segregation, and introduce a new method for quantifying it.

    Comments: 41 pages, 14 figures To Be Published in The Astrophysical Journal

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0801.1329v1 [astro-ph]

    Submission history

    From: Joseph Converse [view email]

    [v1] Tue, 8 Jan 2008 22:33:58 GMT (390kb)


  • ljk February 22, 2008, 11:16

    Chemical composition of A and F dwarfs members of the Pleiades open cluster

    Authors: M. Gebran, R. Monier

    (Submitted on 21 Feb 2008)

    Abstract: Abundances of 18 chemical elements have been derived for 16 A (normal and chemically peculiar CP) and 5 F dwarfs members of the Pleiades open cluster in order to set constraints on evolutionary models. The abundances, rotational velocities and microturbulent velocities were derived by iteratively adjusting synthetic spectra to observations at high resolution (R~42000 and R~75000) and high signal-to-noise (S/N) ratios. The abundances obtained do not exhibit any clear correlation with the effective temperature nor the projected rotational velocity. Interestingly, A stars exhibit larger star-to-star variations in C, Sc, Ti, V, Cr, Mn, Sr, Y, Zr and Ba than F stars. F stars exhibit solar abundances for almost all the elements. In A stars, the abundances of Si, Ti and Cr are found to be correlated with that of Fe, the [X/Fe] ratios being solar for these three elements.

    The derived abundances have been compared to the predictions of published evolutionary models at the age of Pleiades (100 Myr). For the F stars, the predicted slight underabundances of light elements and overabundances of Cr, Fe and Ni are indeed confirmed by our findings. For A stars, the predicted overabundances in iron peak elements are confirmed in a few stars only. The large scatter of the abundances in A stars, already found in the Hyades, Coma Berenices and the UMa group and in field stars appears to be a characteristic property of dwarf A stars. The occurence of hydrodynamical processes competing with radiative diffusion in the radiative zones of the A dwarfs might account for the found scatter in abundances.

    Comments: 7 pages, 3 figures, accepted in A&A

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.3148v1 [astro-ph]

    Submission history

    From: Marwan Gebran [view email]

    [v1] Thu, 21 Feb 2008 16:04:04 GMT (54kb)


  • ljk June 10, 2008, 13:45

    Dynamics of Planetary Systems in Star Clusters

    Authors: R. Spurzem, M. Giersz, D. C. Heggie, D. N. C. Lin

    (Submitted on 28 Dec 2006 (v1), last revised 5 Jun 2008 (this version, v2))

    Abstract: At least 10-15% of nearby sun-like stars have known Jupiter-mass planets. In contrast, very few planets are found in mature open and globular clusters such as the Hyades and 47 Tuc.

    We explore here the possibility that this dichotomy is due to the post-formation disruption of planetary systems associated with the stellar encounters in long-lived clusters. One supporting piece of evidence for this scenario is the discovery of freely floating low-mass objects in star forming regions.

    We use two independent numerical approaches, a hybrid Monte Carlo and a direct N-body method, to simulate the impact of the encounters. We show that the results of numerical simulations are in reasonable agreement with analytical determinations in the adiabatic and impulsive limits. They indicate that distant stellar encounters generally do not significantly modify the compact and nearly circular orbits.

    However, moderately close stellar encounters, which are likely to occur in dense clusters, can excite planets’ orbital eccentricity and induce dynamical instability in systems which are closely packed with multiple planets. The disruption of planetary systems occurs primarily through occasional nearly parabolic, non adiabatic encounters, though eccentricity of the planets evolves through repeated hyperbolic adiabatic encounters which accumulate small-amplitude changes. The detached planets are generally retained by the potential of their host clusters as free floaters in young stellar clusters such as $\sigma$ Orionis.

    We compute effective cross sections for the dissolution of planetary systems and show that, for all initial eccentricities, dissolution occurs on time scales which are longer than the dispersion of small stellar associations, but shorter than the age of typical open and globular clusters.

    Comments: 57 pages, 14 figures, 4 tables, major revision by authors, now submitted to ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:astro-ph/0612757v2

    Submission history

    From: Mirek Giersz [view email]

    [v1] Thu, 28 Dec 2006 13:00:11 GMT (873kb)

    [v2] Thu, 5 Jun 2008 23:19:28 GMT (916kb)


  • ljk September 4, 2009, 11:44

    The Metallicity of the Pleiades

    Authors: David R. Soderblom, Tanmoy Laskar, Jeff A. Valenti, John R. Stauffer, Luisa M. Rebull

    (Submitted on 2 Sep 2009)

    Abstract: We have measured the abundances of Fe, Si, Ni, Ti, and Na in 20 Pleiads with \teff values near solar and with low \vsini using high-resolution, high signal-to-noise echelle spectra. We have validated our procedures by also analyzing 10 field stars of a range of temperatures and metallicities that were observed by \citet{Vale05}.

    Our result for the Pleiades is [Fe/H] = $+0.03\pm0.02\pm0.05$ (statistical and systematic). The average of published measurements for the Pleiades is $+0.042\pm0.021$.

    Comments: accepted by Astron. J. for 2009-11

    Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:0909.0511v1 [astro-ph.SR]

    Submission history

    From: David R. Soderblom [view email]

    [v1] Wed, 2 Sep 2009 20:06:02 GMT (31kb)