M. Jura

A Spitzer Study of Dusty Disks in the Scorpius-Centaurus OB Association

We have obtained Spitzer Space Telescope MIPS observations of 40 F- and G-type common proper motion members of the Scorpius-Centaurus OB association with ages between 5 and 20 Myr at 24 and 70 ?m. We report the detection of 14 objects that possess 24 ?m fluxes ?30% larger than their predicted photospheres, tentatively corresponding to a disk fraction of ?35%, including seven objects that also possess 70 ?m excesses ?100 times larger than their predicted photospheres. The 24 ?m plus 70 ?m excess sources possess high fractional infrared luminosities, LIR/L* = 7 ? 10-4 to 3 ? 10-3; either they possess optically thin, dusty ? Pictoris-like disks or compact, opaque HD 98800-like disks.

First Look at the Fomalhaut Debris Disk with the Spitzer Space Telescope

We present Spitzer Space Telescope early release observations of Fomalhaut, a nearby A-type star with dusty circumstellar debris. The disk is spatially resolved at 24, 70, and 160 � m using the Multiband Imaging Photometer for Spitzer (MIPS). While the disk orientation and outer radius are comparable to values measured in the submillimeter, the disk inner radius cannot be precisely defined: the central hole in the submillimeter ring is at least partially filled with emission from warm dust, seen inSpitzerInfrared Spectrograph (IRS) 17.5‐34 � m spectra and MIPS 24 � m images. The disk surface brightness becomes increasingly asymmetric toward shorter wavelengths, with the south-southeast ansa always brighter than the north-northwest one. This asymmetry may reflect perturbations on the disk by an unseen interior planet. Subject headingg circumstellar matter — infrared: stars — planetary systems — stars: individual (Fomalhaut)

Where Are the M Dwarf Disks Older Than 10 Million Years

We present 11.7 μm observations of nine late-type dwarfs obtained at the Keck I 10 m telescope in 2002 December and 2003 April. Our targets were selected for their youth or apparent IRAS 12 μm excess. For all nine sources, excess infrared emission is not detected. We find that stellar wind drag can dominate the circumstellar grain removal and plausibly explain the dearth of M dwarf systems older than 10 Myr with currently detected infrared excesses. We predict that M dwarfs possess fractional infrared excesses on the order of LIR/L* ~ 10-6 and that this may be detectable with future efforts.

Spitzer IRS Spectroscopy of IRAS-discovered Debris Disks*

We have obtained Spitzer Space Telescope Infrared Spectrograph (IRS) 5.5-35 μm spectra of 59 main-sequence stars that possess IRAS 60 μm excess. The spectra of five objects possess spectral features that are well-modeled using micron-sized grains and silicates with crystalline mass fractions 0%-80%, consistent with T Tauri and Herbig AeBe stars. With the exception of η Crv, these objects are young with ages ≤50 Myr. Our fits require the presence of a cool blackbody continuum, Tgr = 80-200 K, in addition to hot, amorphous, and crystalline silicates, Tgr = 290-600 K, suggesting that multiple parent body belts are present in some debris disks, analogous to the asteroid and Kuiper belts in our solar system. The spectra for the majority of objects are featureless, suggesting that the emitting grains probably have radii a > 10 μm. We have modeled the excess continua using a continuous disk with a uniform surface density distribution, expected if Poynting-Robertson and stellar wind drag are the dominant grain removal processes, and using a single-temperature blackbody, expected if the dust is located in a narrow ring around the star. The IRS spectra of many objects are better modeled with a single-temperature blackbody, suggesting that the disks possess inner holes. The distribution of grain temperatures, based on our blackbody fits, peaks at Tgr = 110-120 K. Since the timescale for ice sublimation of micron-sized grains with Tgr > 110 K is a fraction of a Myr, the lack of warmer material may be explained if the grains are icy. If planets dynamically clear the central portions of debris disks, then the frequency of planets around other stars is probably high. We estimate that the majority of debris disk systems possess parent body masses, MPB < 1 M⊕. The low inferred parent body masses suggest that planet formation is an efficient process.

Mid-Infrared Spectra of Dust Debris around Main-Sequence Stars*

We report spectra obtained with the Spitzer Space Telescope in the λ = 14-35 μm range of 19 nearby main-sequence stars with infrared excesses. The six stars with strong dust emission show no recognizable spectral features, suggesting that the bulk of the emitting particles have diameters larger than 10 μm. If the observed dust results from collisional grinding of larger solids, we infer minimum masses of the parent body population between 0.004 and 0.06 M⊕. We estimate grain production rates of ~1010 g s-1 around λ Boo and HR 1570; selective accretion of this matter may help explain their peculiar surface abundances. There appear to be inner truncations in the dust clouds at 48, 11, 52, and 54 AU around HR 333, HR 506, HR 1082, and HR 3927, respectively.

The Mid-Infrared Emitting Dust Around AB Aurigae

Using the Keck I telescope, we have obtained 11.7 and 18.7 μm images of the circumstellar dust emission from AB Aur, a Herbig Ae star. We find that AB Aur is probably resolved at 18.7 μm with an angular diameter of 12 at a surface brightness of 3.5 Jy arcsec-2. Most of the dust mass detected at millimeter wavelengths does not contribute to the 18.7 μm emission, which is plausibly explained if the system possesses a relatively cold, massive disk. We find that models with an optically thick, geometrically thin disk surrounded by an optically thin spherical envelope fit the data somewhat better than flared disk models.

The Massive Disk around OH 231.8+4.2

We have obtained 11.7 and 17.9 μm images at the Keck I telescope of the circumstellar dust emission from OH 231.8+4.2, an evolved mass-losing red giant with a well-studied bipolar outflow. We detect both a central unresolved point source with a diameter of less than 05 producing Fν(17.9 μm) = 60 Jy and emission extended more than 1 away from the star, which is aligned with the bipolar outflow seen on larger scales. We find that the unresolved central source can be explained by an opaque, flared disk with an outer radius of ~5 × 1015 cm and an outer temperature of ~130 K. One possible model to explain this flaring is that the material in the disk is orbiting the central star and not simply undergoing a radial expansion.