The Vega debris disk: A surprise from spitzer

We present high spatial resolution mid- and far-infrared images of the Vega debris disk obtained with the Multiband Imaging Photometer for Spitzer (MIPS). The disk is well resolved, and its angular size is much larger than found previously. The radius of the disk is at least 43'' (330 AU), 70'' (543 AU), and 105'' (815 AU) in extent at 24, 70, and 160 μm, respectively. The disk images are circular, smooth, and without clumpiness at all three wavelengths. The radial surface brightness profiles follow radial power laws of r-3 or r-4 and imply an inner boundary at a radius of 11'' ± 2'' (86 AU). Assuming an amalgam of amorphous silicate and carbonaceous grains, the disk can be modeled as an axially symmetric and geometrically thin disk, viewed face-on, with the surface particle number density following an inverse radial power law. The disk radiometric properties are consistent with a range of models using grains of sizes ~1 to ~50 μm. The exact minimum and maximum grain size limits depend on the adopted grain composition. However, all of these models require an r-1 surface number density profile and a total mass of × 10-3 M⊕ in grains. We find that a ring, containing grains larger than 180 μm and at radii of 86-200 AU from the star, can reproduce the observed 850 μm flux, while its emission does not violate the observed MIPS profiles. This ring could be associated with a population of larger asteroidal bodies analogous to our own Kuiper Belt. Cascades of collisions starting with encounters among these large bodies in the ring produce the small debris that is blown outward by radiation pressure to much larger distances, where we detect its thermal emission. The relatively short lifetime (<1000 yr) of these small grains and the observed total mass, ~3 × 10-3 M⊕, set a lower limit on the dust production rate, ~1015 g s-1. This rate would require a very massive asteroidal reservoir for the dust to be produced in a steady state throughout Vega's life. Instead, we suggest that the disk we imaged is ephemeral and that we are witnessing the aftermath of a large and relatively recent collisional event, and a subsequent collisional cascade.

[1]  N. Ostroff,et al.  Millimeter-Wave Aperture Synthesis Imaging of Vega: Evidence for a Ring Arc at 95 AU , 2001, astro-ph/0109424.

[2]  M. Holman,et al.  Structure in the Dusty Debris around Vega , 2002 .

[3]  Erick T. Young,et al.  Reduction Algorithms for the Multiband Imaging Photometer for Spitzer , 2004, SPIE Astronomical Telescopes + Instrumentation.

[4]  V. Il’in,et al.  Radiation pressure on non-spherical dust grains in envelopes of late-type giants , 1998 .

[5]  H. Walker,et al.  Infrared mapping of the dust disc around Vega , 1998 .

[6]  E. Grün,et al.  Infrared Observations Of Dust Emission From Comet Hale-Bopp , 1997 .

[7]  Paul S. Smith,et al.  Reduction Algorithms for the Multiband Imaging Photometer for Spitzer , 2005, astro-ph/0502079.

[8]  J. Burns,et al.  Radiation forces on small particles in the solar system , 1979 .

[9]  M. Barlow,et al.  Candidate Main-Sequence Stars with Debris Disks: A New Sample of Vega-like Sources , 1998 .

[10]  Frederick J. Vrba,et al.  Herbig Ae/Be Stars: Intermediate-Mass Stars Surrounded by Massive Circumstellar Accretion Disks , 1992 .

[11]  F. J. Low,et al.  DISCOVERY OF A SHELL AROUND ALPHA-LYRAE , 1984 .

[12]  Saul J. Adelman,et al.  VEGA : A RAPIDLY ROTATING POLE-ON STAR , 1994 .

[13]  J. S. Dohnanyi.  Collisional model of asteroids and their debris , 1969 .

[14]  E. Wright,et al.  The Spitzer Space Telescope Mission , 2004, astro-ph/0406223.

[15]  David E. Trilling,et al.  Decay of Planetary Debris Disks , 2005 .

[16]  S. Dermott,et al.  A 100,000-year periodicity in the accretion rate of interplanetary dust , 1998, Science.

[17]  T. Mukai,et al.  Radiation Pressure and the Poynting-Robertson Effect for Fluffy Dust Particles , 2002 .

[18]  M. Joy,et al.  On the far-infrared excess of Vega , 1984, Nature.

[19]  A. Christou.  E. Grun, B. A. S. Gustafson, S. F. Dermott, and H. Fechtig (eds.), Interplanetary Dust. , 2002 .

[20]  Ari Laor,et al.  Spectroscopic constraints on the properties of dust in active galactic nuclei , 1993 .

[21]  B. Zuckerman,et al.  Submillimetre images of dusty debris around nearby stars , 1998, Nature.

[22]  J. Lunine,et al.  Protostars and planets III , 1993 .

[23]  B. Zuckerman,et al.  Submillimeter studies of main-sequence stars , 1993 .

[24]  M. Franx,et al.  Hubble Space Telescope ACS Coronagraphic Imaging of the Circumstellar Disk around HD 141569A , 2003 .

[25]  M. Clampin,et al.  Dust around Main-Sequence Stars: Nature or Nurture by the Interstellar Medium? , 1997 .

[26]  Berkeley,et al.  Infrared Views of the TW Hydra Disk , 2001, astro-ph/0110342.

[27]  Paul S. Smith,et al.  The Multiband Imaging Photometer for Spitzer (MIPS) , 2004 .

[28]  Ages of Late Spectral Type Vega-like Stars. , 2000, The Astrophysical journal.

[29]  Massimo Marengo,et al.  First Look at the Fomalhaut Debris Disk with the Spitzer Space Telescope , 2004 .

[30]  P. Artymowicz.  Radiation Pressure Forces on Particles in the Beta Pictoris System , 1988 .

[31]  COLLISIONAL CASCADES IN PLANETESIMAL DISKS. II. EMBEDDED PLANETS , 2003, astro-ph/0309540.

[32]  E. Krugel.  The Physics of Interstellar Dust , 2002 .

[33]  J. Pollack,et al.  Composition and radiative properties of grains in molecular clouds and accretion disks , 1994 .

[34]  L. Colangeli,et al.  Optical constants of cosmic carbon analogue grains — I. Simulation of clustering by a modified continuous distribution of ellipsoids , 1996 .

[35]  Mullard Space Science Laboratory,et al.  A Dust Ring around epsilon Eridani: Analog to the Young Solar System , 1998, astro-ph/9808224.

[36]  V. Mannings,et al.  A High-Resolution Study of Gas and Dust around Young Intermediate-Mass Stars: Evidence for Circumstellar Disks in Herbig Ae Systems , 1997 .