Back in 2003, while researching Centauri Dreams, I interviewed physicist Geoffrey Landis at Glenn Research Center in Cleveland. At that time, Landis’ office was packed with Mars images, apropos for a man who had done so much work on rover technology. I asked him whether, after all this study, Mars had taken on the aspect of a real place to him, like Cleveland. Not surprisingly, he said that it had, and he credited 3-D images from Mars Pathfinder for that. Wearing glasses, Landis said, “It was as if you were standing on Mars. You could see ups and downs, ridges and valleys. That changed the view of Mars from another planet to a place you could go out and walk around.”
We’re a long way from 3-D close-ups, but I suspect some astronomers are starting to feel that way about Beta Pictoris, a young star some 63 light years away in the southern constellation Pictor that first drew attention to itself because of excess infrared radiation. A warm circumstellar disk was surely the cause, and indeed, such a disk was imaged by ground-based telescopes in 1984, revealing itself to be nearly edge-on to Earth. Since those observations, the Hubble space telescope found what seemed to be a warp in the disk, one studied extensively by Sara Heap at GSFC.
Is the warp evidence of more than one disk? The image below makes a strong case, showing in visible light a distinct secondary disk tilted about four degrees from the main one. It took the use of a coronagraph with Hubble’s Advanced Camera for Surveys to get this image, blocking out light from the star itself to reveal the disk structure (Beta Pictoris puts out a lot of light; in fact, it’s nine times more luminous than the Sun). The secondary disk is visible out to 24 billion miles from the star.
Image: This Hubble Space Telescope view of Beta Pictoris clearly shows a primary dust disk and a much fainter secondary dust disk. The secondary disk extends at least 24 billion miles from the star and is tilted roughly 4 to 5 degrees from the primary disk. The secondary disk is circumstantial evidence for the existence of a planet in a similarly inclined orbit. Credit: NASA, ESA, D. Golimowski (Johns Hopkins University), D. Ardila (IPAC), J. Krist (JPL), M. Clampin (GSFC), H. Ford (JHU), and G. Illingworth (UCO/Lick) and the ACS Science Team.
What’s causing this unusual disk situation? An unseen planet is the most likely culprit, a gas giant perhaps 20 times the mass of Jupiter. In orbit within the secondary disk, the planet would be tapping the primary disk for its building materials. And that leads to an interesting thought: what if Beta Pictoris is not so unusual? Perhaps planetary systems often form this way, from two or even more disks whose interactions are fertile ground for planetesimals. Listen to what David Golimowski (Johns Hopkins) has to say about this; he led the team that made this find:
“The Hubble observation shows that it is not simply a warp but two concentrations of dust in two separate disks. The finding suggests that planetary systems could be forming in two different planes. We know this can happen because the planets in our solar system are typically inclined to Earth’s orbit by several degrees. Perhaps stars forming more than one dust disk may be the norm in the formative years of a star system.”
And that several degree spread around Beta Pictoris isn’t really so different from the several degree separation of planets in our own Solar System’s plane. There is even some evidence from work at the Keck II Observatory in Hawaii of a possible third disk, one the size of our own Solar System that Golimowski’s team couldn’t see because it was covered by the coronagraph in Hubble’s Advanced Camera for Surveys. Whatever the case, Beta Pictoris, that unique laboratory for the study of planet formation, is becoming more and more of a ‘place’ all the time, one that will be visited by astronomers for years to come as it slowly gives up its secrets.
The paper is Golimowski, Ardila, Krist et al., “Hubble Space Telescope ACS Multiband Coronagraphic Imaging of the Debris Disk Around ? Pictoris,” Astronomical Journal 131:3109-3130 (June, 2006).
Physicist and highly-excellent science fiction writer Geoffrey Landis!
Yes, and we should give a nod not only to Geoff’s wonderful Mars Crossing (talk about seeng Mars as a place!), but also his fine collection Impact Parameter: And Other Quantum Realities. How he finds time to keep both careers going is beyond me…
Astrophysics, abstract
astro-ph/0703616
From: Ilaria Pascucci [view email]
Date: Fri, 23 Mar 2007 16:49:50 GMT (85kb)
Detection of [Ne II] Emission from Young Circumstellar Disks
Authors: I. Pascucci, D. Hollenbach, J. Najita, J. Muzerolle, U. Gorti, G. J. Herczeg, L. A. Hillenbrand, J. S. Kim, J. M. Carpenter, M. R. Meyer, E. E. Mamajek, J. Bouwman
Comments: accepted for publication to The Astrophysical Journal
We report the detection of [Ne II] emission at 12.81 micron in four out of the six optically thick dust disks observed as part of the FEPS Spitzer Legacy program. In addition, we detect a H I(7-6) emission line at 12.37 micron from the source RXJ1852.3-3700. Detections of [Ne II] lines are favored by low mid-infrared excess emission. Both stellar X-rays and extreme UV (EUV) photons can sufficiently ionize the disk surface to reproduce the observed line fluxes, suggesting that emission from Ne+ originates in the hot disk atmosphere. On the other hand, the H I(7-6) line is not associated with the gas in the disk surface and magnetospheric accretion flows can account only for at most ~30% of the observed flux. We conclude that accretion shock regions and/or the stellar corona could contribute to most of the H I(7-6)emission. Finally, we discuss the observations necessary to identify whether stellar X-rays or EUV photons are the dominant ionization mechanism for Ne atoms.
Because the observed [Ne II] emission probes very small amounts of gas in the disk surface (~10^{-6} Jupiter masses) we suggest using this gas line to determine the presence or absence of gas in more evolved circumstellar disks.
http://arxiv.org/abs/astro-ph/0703616
Astrophysics, abstract
astro-ph/0703608
From: Mark Wyatt [view email]
Date: Fri, 23 Mar 2007 10:55:40 GMT (430kb)
Steady-state evolution of debris disks around A stars
Authors: M. C. Wyatt, R. Smith, K. Y. L. Su, G. H. Rieke, J. S. Greaves, C. A. Beichman, G. Bryden
Comments: Accepted by ApJ
In this paper a simple analytical model for the steady-state evolution of debris disks due to collisions is confronted with Spitzer observations of main sequence A stars. All stars are assumed to have planetesimal belts with a distribution of initial masses and radii. In the model disk mass is constant until the largest planetesimals reach collisional equilibrium whereupon the mass falls off oc 1/t. We find that the detection statistics and trends seen at both 24 and 70um can be fitted well by the model. While there is no need to invoke stochastic evolution or delayed stirring to explain the statistics, a moderate rate of stochastic events is not ruled out. Potentially anomalous systems are identified by a high dust luminosity compared with the maximum permissible in the model (HD3003, HD38678, HD115892, HD172555). Their planetesimals may have unusual properties (high strength or low eccentricity) or this dust could be transient. While transient phenomena are also favored for a few systems in the literature, the overall success of our model, which assumes planetesimals in all belts have the same strength, eccentricity and maximum size, suggests a large degree of uniformity in the outcome of planet formation. The distribution of planetesimal belt radii, once corrected for detection bias, follows N(r) oc r^{-0.8+-0.3} for 3-120AU. Since the inner edge is often attributed to an unseen planet, this provides a unique constraint on the planetary systems of A stars. It is also shown that P-R drag may sculpt the inner edges of A star disks close to the Spitzer detection threshold (HD2262, HD19356, HD106591, HD115892). This model can be readily applied to the interpretation of future surveys, and predictions are made for the upcoming SCUBA-2 survey, including that >17% of A stars should be detectable at 850um.
http://arxiv.org/abs/astro-ph/0703608
Evidence for Early Circumstellar Disk Evolution in NGC 2068/71
Authors: K.M. Flaherty (1), J. Muzerolle (1) ((1) Steward Observatory, University of Arizona)
(Submitted on 10 Dec 2007)
Abstract: We study the disk and accretion properties of young stars in the NGC 2068 and NGC 2071 clusters. Using low-resolution optical spectra, we define a membership sample and determine an age for the region of ~2 Myr. Using high-resolution spectra of the H-alpha line we study the accretion activity of these likely members and also examine the disk properties of the likely members using IRAC and MIPS mid-infrared photometry. A substantial fraction (79%) of the 67 members have an infrared excess while all of the stars with significant infrared excess show evidence for active accretion. We find three populations of evolved disks (IRAC-weak, MIPS-weak and transition disks) all of which show decreased accretion activity in addition to the evidence for evolution in the dust disk.
Comments: 44 pages (20 pages of text), 16 figures, 5 tables
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0712.1601v1 [astro-ph]
Submission history
From: Kevin Flaherty [view email]
[v1] Mon, 10 Dec 2007 21:09:42 GMT (233kb)
http://arxiv.org/abs/0712.1601
A simple determination of some characteristics of the beta Pictoris system
Authors: V.Celebonovic
(Submitted on 1 May 2008)
Abstract: The aim of this contribution is to determine the lower limit of the electron number density,the Debye temperature and the specific heat of solid grains present in the protoplanetary grains around beta Pictoris. The calculation has been performed using the Salpeter criterion,slightly modified a couple of years ago. The results obtained are physically reasonable and could be helpful in determining the chemical composition of solid particles in this system.
Comments: plain LaTeX,4 pages,presented at the 23rd SPIG Conference,Kopaonik (Serbia),August 2006
Subjects: Astrophysics (astro-ph)
Journal reference: Contrib.papers of the 23 SPIG,ed.by N.S.Simonovic et al.,p.587,IOP,Belgrade (2006)
Cite as: arXiv:0805.0146v1 [astro-ph]
Submission history
From: Vladan Celebonovic [view email]
[v1] Thu, 1 May 2008 20:12:56 GMT (6kb)
http://arxiv.org/abs/0805.0146
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