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A Close Stellar Encounter?

Astronomers have found a highly elliptical debris disk around the star HD 15115, one that seen virtually edge-on from Earth gives the appearance of a needle running straight through its star. The disk was first imaged by the Hubble Space Telescope in 2006, its unusual shape causing astronomers to request near-infrared imaging by the W.M. Keck Observatory in Hawaii. Keck’s images, in conjunction with the Hubble data, revealed the disk’s uncommon blue color.

Debris disk around HD 15115

So what’s going on around this F-class star? First of all, let’s distinguish between protoplanetary disks, which give birth to planets, and debris disks like this one, which resemble our own Kuiper Belt. The latter are made up of the remnants of planetary formation. This debris disk seems to extend some ten times further from its star than the Kuiper Belt, according to a Keck news release, though our limited knowledge of the Kuiper Belt makes me a bit wary of the statement, as the latter’s dimensions are still under active investigation.

Image: Dust orbits in a needle-shaped ring around the star HD 15115 in this Hubble Space Telescope image. An occulting mask was used to block out bright starlight. The masks can be seen in the image as the dark circle in the center and the dark bar on the left. Credit: NASA\ESA\UC Berkeley.

But a debris disk in a highly elliptical orbit is an oddity no matter what its extent. One intriguing possibility is the influence of the star HIP 12545, some ten light years away from HD 15115. A close pass at some point in the past? Perhaps, because both stars seem to be part of the Beta Pictoris Moving Group, an expanded cluster of stars thought to have a common birthplace and age that are moving together through space.

The paper on this work cites evidence for the stellar flyby idea, from which this excerpt:

This geometry is consistent with the dynamical simulation of a disk disrupted by a stellar flyby… [T]he long end of a highly perturbed disk is located in the direction of periastron. The perturber follows a parabolic trajectory such that in a later epoch it is located in the direction opposite of periastron, or in the direction of the truncated side of the disk. Periastron in these models is ∼700 AU, with an initial disk radius of ∼500 AU. Overall, the ensemble of evidence favors further consideration of HD 15115 and HIP 12545 as a possible wide-separation multiple system with a highly eccentric orbit (e > 0.95).

It is also notable that the debris disk around HD 15115 shows significantly less dust than disks observed around three other stars in this group. Paul Kalas (UC-Berkeley), lead author of the paper, thinks the missing matter is connected to the disk’s elliptical shape, saying “The missing mass is quite interesting. Perhaps the mechanism which perturbed the disk into its current asymmetric morphology also shaved away a significant fraction of the mass.” Kalas plans further work on both stars this fall, using Keck’s adaptive optics. It will be interesting to see whether HIP 12545, a M-class red dwarf, has a disk of its own.

A second hypothesis to account for the disk’s odd shape is closer to home: Disturbances caused by planets near the star. Here the obvious analogue is not only Neptune’s influence on Kuiper Belt objects but the theory that Neptune may originally have formed inside the orbit of Saturn and Uranus, only to be pushed to its present position by gravitational disturbances as the orbits of Jupiter and Saturn stabilized. Whether true of Neptune or not, profound planetary upheaval of this magnitude could also explain the disk asymmetry around HD 15115.

The paper is Kalas et al., “Discovery of extreme asymmetry in the debris disk surrounding HD 15115,” accepted by Astrophysical Journal Letters and available as a preprint. The team plan to present a more detailed analysis of dust scattering and thermal emission around this star in a future paper.

Addendum: Curious about the stellar flyby scenario and the stability of the debris disk around HD 15115, I wrote Dr. Kalas. Here is part of his response:

The stellar flyby hypothesis that we invoked as one possible explanation is extremely rare and the effects are short-lived (<1 Myr). Since 1 km/s = 1 pc / Myr, and the present separation is ~3 pc between HD 15115 and the suspected perturber, the perturber must have passed by HD 15115 by at least 3 km/s, which is not implausible, and which can be tested by future observations.

Dr. Kalas also provided this link to his earlier work on the stellar flyby model and asymmetric disk environments.

Comments on this entry are closed.

  • djlactin July 20, 2007, 7:56

    hmmm. echoes of the eliptical, off-centre fomalhaut ring story?

  • James Nicoll July 20, 2007, 11:14

    Rather off-topic but have you seen this?

    1964’s Habitable Planets for Man by Stephen Dole

    http://www.rand.org/pubs/reports/R414/

  • Administrator July 20, 2007, 13:33

    Isaac Asimov worked with Dole to produce a mass-market version of the same report. It’s quite interesting even today, well worth a look in used book stores.

  • Administrator July 20, 2007, 13:45

    djlactin, Paul Kalas noted this in his recent e-mail:

    “The planetary disturbance hypothesis is also possible. Violent gravitational interactions between two planets can insert one of the planets into a highly elliptical orbit, and then the surrounding dust disk can become lopsided by a slow perturbation called a “secular perturbation”. The secular perturbation hypothesis has been used to explain Fomalhaut’s off-center dust belt.”

    It looks as though asymmetric debris disks may not be all that uncommon — in each case we’ll have to puzzle out the options.

  • philw July 20, 2007, 14:00

    I heartily recommend Dole’s “Habitable Planets” book. I still refer to it. Informative. Thought provoking. Basics are solid though I’d kill for a 21st century update.

  • ljk August 29, 2007, 14:28

    Water Vapor Seen ‘Raining Down’ On Young Star System

    NASA’s Spitzer Space Telescope has detected enough water vapor to fill the oceans on Earth five times inside the collapsing nest of a forming star system. Astronomers say the water vapor is pouring down from the system’s natal cloud and smacking into a dusty disk where planets are thought to form.

    The observations provide the first direct look at how water, an essential ingredient for life as we know it, begins to make its way into planets, possibly even rocky ones like our own.

    “For the first time, we are seeing water being delivered to the region where planets will most likely form,” said Dan Watson of the University of Rochester, N.Y. Watson is the lead author of a paper about this “steamy” young star system, appearing in the Aug. 30 issue of Nature.

    The star system, called NGC 1333-IRAS 4B, is still growing inside a cool cocoon of gas and dust. Within this cocoon, circling around the embryonic star, is a burgeoning, warm disk of planet-forming materials. The new Spitzer data indicate that ice from the stellar embryo’s outer cocoon is falling toward the forming star and vaporizing as it hits the disk.

    “On Earth, water arrived in the form of icy asteroids and comets. Water also exists mostly as ice in the dense clouds that form stars,” said Watson. “Now we’ve seen that water, falling as ice from a young star system’s envelope to its disk, actually vaporizes on arrival. This water vapor will later freeze again into asteroids and comets.”

    Water is abundant throughout our universe. It has been detected in the form of ice or gas around various types of stars, in the space between stars, and recently Spitzer picked up the first clear signature of water vapor on a hot, gas planet outside our solar system, named HD 189733b.

    In the new Spitzer study, water also serves as an important tool for studying long-sought details of the planet formation process. By analyzing what’s happening to the water in NGC 1333-IRAS 4B, the astronomers are learning about its disk. For example, they calculated the disk’s density (at least 10 billion hydrogen molecules per cubic centimeter or 160 billion hydrogen molecules per cubic inch); its dimensions (a radius bigger than the average distance between Earth and Pluto); and its temperature (170 Kelvin, or minus 154 degrees Fahrenheit).

    “Water is easier to detect than other molecules, so we can use it as a probe to look at more brand-new disks and study their physics and chemistry,” said Watson. “This will teach us a lot about how planets form.”

    Watson and his colleagues studied 30 of the youngest known stellar embryos using Spitzer’s infrared spectrograph, an instrument that splits infrared light open into a rainbow of wavelengths, revealing “fingerprints” of molecules. Of the 30 stellar embryos, they found only one, NGC 1333-IRAS 4B, with a whopping signature of water vapor. This vapor is readily detectable by Spitzer, because as ice hits the stellar embryo’s planet-forming disk, it heats up very rapidly and glows with infrared light.

    Why did only one stellar embryo of 30 show signs of water? The astronomers say this is most likely because NGC 1333-IRAS 4B is in just the right orientation for Spitzer to view its dense core. Also, this particular watery phase of a star’s life is short-lived and hard to catch.

    “We have captured a unique phase of a young star’s evolution, when the stuff of life is moving dynamically into an environment where planets could form,” said Michael Werner, project scientist for the Spitzer mission at NASA’s Jet Propulsion Laboratory, Pasadena, Calif.

    NGC 1333-IRAS 4B is located in a pretty star-forming region approximately 1,000 light-years away in the constellation Perseus. Its central stellar embryo is still “feeding” off the material collapsing around it and growing in size. At this early stage, astronomers cannot tell how large the star will ultimately become.

    Other authors of the Nature paper include: Chris Bohac, Chat Hull, Bill Forrest, Ben Sargent, Joel Green and Kyoung Hee Kim of the University of Rochester; Elise Furlan of the University of California at Los Angeles; Joan Najita of the National Optical Astronomy Observatory; Nuria Calvet and Lee Hartmann of the University of Michigan, Ann Arbor; Paola d’Alessio of the National Autonomous University of Mexico; and Jim Houck of Cornell University, Ithaca, N.Y.

    JPL manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA. Spitzer’s infrared spectrograph was built by Cornell University. Its development was led by co-author Houck. Watson and Forrest are also members of the team that built the spectrograph.

    For graphics and more information about Spitzer, visit

    http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer .

  • ljk October 24, 2007, 9:45

    Long-Term Collisional Evolution of Debris Disks

    Authors: Torsten Löhne, Alexander V. Krivov, Jens Rodmann

    (Submitted on 23 Oct 2007)

    Abstract: We simulated the long-term collisional depletion of debris disks around solar-type (G2V) stars with our code. The numerical results were supplemented by, and interpreted through, a new analytic model. A few general scaling rules for the disk evolution are suggested. The timescale of the collisional evolution is inversely proportional to the initial disk mass and scales with radial distance as r^4.3 and with eccentricities of planetesimals as e^2.3. Further, we show that at actual ages of debris disks between 10 Myr and 10 Gyr, the decay of the dust mass and the total disk mass follow different laws. The reason is that the collisional lifetime of planetesimals is size-dependent. At any moment, there exists a transitional size, which separates larger objects that still have the “primordial” size distribution set in the growth phase from small objects whose size distribution is already set by disruptive collisions. The dust mass and its decay rate evolve as that transition affects objects of ever-larger sizes. Under standard assumptions, the dust mass, fractional luminosity, and thermal fluxes all decrease as t^xi with xi = -0.3…-0.4. Specific decay laws of the total disk mass and the dust mass, including the value of xi, largely depend on a few model parameters, such as the critical fragmentation energy as a function of size, the primordial size distribution of largest planetesimals, as well as the characteristic eccentricity and inclination of their orbits. With standard material prescriptions and a distribution of disk masses and extents, a synthetic population of disks generated with our analytic model agrees quite well with the observed Spitzer/MIPS statistics of 24 and 70 micron fluxes and colors versus age.

    Comments: 16 pages, 15 figures, accepted for publication in ApJ (23 Oct 2007), abstract shortened

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0710.4294v1 [astro-ph]

    Submission history

    From: Torsten L\”ohne [view email]

    [v1] Tue, 23 Oct 2007 19:00:26 GMT (141kb)

    http://arxiv.org/abs/0710.4294