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Spitzer Mid-IR Spectra of Dust Debris Around A and Late B Type Stars: Asteroid Belt Analogs and Power-Law Dust Distributions
Using the Spitzer/Infrared Spectrograph (IRS) low-resolution modulescovering wavelengths from 5 to 35 ?m, we observed 52 main-sequence Aand late B type stars previously seen using Spitzer/Multiband ImagingPhotometer (MIPS) to have excess infrared emission at 24 ?m abovethat expected from the stellar photosphere. The mid-IR excess isconfirmed in all cases but two. While prominent spectral features arenot evident in any of the spectra, we observed a striking diversity inthe overall shape of the spectral energy distributions. Most of the IRSexcess spectra are consistent with single-temperature blackbodyemission, suggestive of dust located at a single orbital radius—anarrow ring. Assuming the excess emission originates from a populationof large blackbody grains, dust temperatures range from 70 to 324 K,with a median of 190 K corresponding to a distance of 10 AU. Thirteenstars however, have dust emission that follows a power-law distribution,F ? = F 0??, withexponent ? ranging from 1.0 to 2.9. The warm dust in these systemsmust span a greater range of orbital locations—an extended disk.All of the stars have also been observed with Spitzer/MIPS at 70 ?m,with 27 of the 50 excess sources detected (signal-to-noise ratio >3). Most 70 ?m fluxes are suggestive of a cooler, Kuiper Belt-likecomponent that may be completely independent of the asteroid belt-likewarm emission detected at the IRS wavelengths. Fourteen of 37 sourceswith blackbody-like fits are detected at 70 ?m. The 13 objects withIRS excess emission fit by a power-law disk model, however, are alldetected at 70 ?m (four above, three on, and six below theextrapolated power law), suggesting that the mid-IR IRS emission andfar-IR 70 ?m emission may be related for these sources. Overall, theobserved blackbody and power-law thermal profiles reveal debrisdistributed in a wide variety of radial structures that do not appear tobe correlated with spectral type or stellar age. An additional 43fainter A and late B type stars without 70 ?m photometry were alsoobserved with Spitzer/IRS; results are summarized in Appendix B.
| Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
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