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Abundant Circumstellar Silica Dust and SiO Gas Created by a Giant Hypervelocity Collision in the ~12 Myr HD172555 System
The fine dust detected by infrared (IR) emission around the nearbyβ Pic analog star HD172555 is very peculiar. The dust mineralogy iscomposed primarily of highly refractory, nonequilibrium materials, withapproximately three quarters of the Si atoms in silica (SiO2)species. Tektite and obsidian lab thermal emission spectra(nonequilibrium glassy silicas found in impact and magmatic systems) arerequired to fit the data. The best-fit model size distribution for theobserved fine dust is dn/da = a -3.95±0.10.While IR photometry of the system has stayed stable since the 1983 IRASmission, this steep a size distribution, with abundant micron-sizedparticles, argues for a fresh source of material within the last0.1 Myr. The location of the dust with respect to the star is at5.8 ± 0.6 AU (equivalent to 1.9 ± 0.2 AU from the Sun),within the terrestrial planet formation region but at the outer edge ofany possible terrestrial habitability zone. The mass of fine dust is 4× 1019-2 × 1020 kg, equivalent toa 150-200 km radius asteroid. Significant emission featurescentered at 4 and 8 μm due to fluorescing SiO gas are alsofound. Roughly 1022 kg of SiO gas, formed by vaporizingsilicate rock, is also present in the system, and a separate populationof very large, cool grains, massing 1021-1022 kgand equivalent to the largest sized asteroid currently found in thesolar system's main asteroid belt, dominates the solid circumstellarmaterial by mass. The makeup of the observed dust and gas, and the notedlack of a dense circumstellar gas disk, strong stellar X-ray activity,and an extended disk of β meteoroids argues that the source of theobserved circumstellar materials is a giant hypervelocity(>10 km s-1) impact between large rockyplanetesimals, similar to the ones which formed the Moon and whichstripped the surface crustal material off of Mercury's surface.
| Characterization of Dusty Debris Disks: The IRAS and Hipparcos Catalogs
Dusty debris disks around main-sequence stars are signposts for theexistence of planetesimals and exoplanets. From cross-correlatingHipparcos stars with the IRAS catalogs, we identify 146 stars within 120pc of Earth that show excess emission at 60 μm. This search tookspecial precautions to avoid false positives. Our sample is reasonablywell distributed from late B to early K-type stars, but it contains veryfew later type stars. Even though IRAS flew more than 20 years ago andmany astronomers have cross-correlated its catalogs with stellarcatalogs, we were still able to newly identify debris disks at as manyas 33 main-sequence stars; of these, 32 are within 100 pc of Earth. Thepower of an all-sky survey satellite like IRAS is evident when comparingour 33 new debris disks with the total of only 22 dusty debris diskstars first detected with the more sensitive, but pointed, satelliteISO. Our investigation focuses on the mass, dimensions, and evolution ofdusty debris disks.
| Nearby Debris Disk Systems with High Fractional Luminosity Reconsidered
By searching the IRAS and ISO databases, we compiled a list of 60 debrisdisks that exhibit the highest fractional luminosity values(fd>10-4) in the vicinity of the Sun (d<120pc). Eleven out of these 60 systems are new discoveries. Special carewas taken to exclude bogus disks from the sample. We computed thefractional luminosity values using available IRAS, ISO, and Spitzer dataand analyzed the Galactic space velocities of the objects. The resultsrevealed that stars with disks of high fractional luminosity oftenbelong to young stellar kinematic groups, providing an opportunity toobtain improved age estimates for these systems. We found thatpractically all disks with fd>5×10-4 areyounger than 100 Myr. The distribution of the disks in the fractionalluminosity versus age diagram indicates that (1) the number of oldsystems with high fd is lower than was claimed before, (2)there exist many relatively young disks of moderate fractionalluminosity, and (3) comparing the observations with a currenttheoretical model of debris disk evolution, a general good agreementcould be found.
| On the Variability of A0-A2 Luminosity Class III-V Stars
We study the Hipparcos Satellite photometry of A0-A2 luminosity classIII-V stars. Most are not particularly variable. A few stars for whichfurther study is desirable are identified.
| Four-colour and H beta photometry of southern B stars at high galactic latitudes
Four-color and H beta photometry has been obtained for 105 early-type HDstars with galactic latitudes less than -45 deg. They are mostly late Bstars of luminosity class V to III. Two new Am stars and several Ap orBp stars are detected photometrically. Absolute magnitudes aredetermined from the photometry and from MK types where available. Theinterstellar reddening of many of the more distant stars is very small,suggesting either the existence of undetected peculiar stars in thesample or that there are areas of effectively zero reddening at highsouthern galactic latitudes.
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