We report new Spitzer 24 m photometry of 76 main-sequence A-type stars. We combine these results with previously reported Spitzer 24 m data and 24 and 25 m photometry from the Infrared Space Observatory and the Infrared Astronomy Satellite. The result is a sample of 266 stars with mass close to 2.5 M , all detected to at least the $7 level relative to their photospheric emission. We culled ages for the entire sample from the literature and/or estimated them using the H-R diagram and isochrones; they range from 5 to 850 Myr. We identified excess thermal emission using an internally derived K À 24 (or 25) m photospheric color and then compared all stars in the sample to that color. Because we have excluded stars with strong emission lines or extended emission (associated with nearby interstellar gas), these excesses are likely to be generated by debris disks. Younger stars in the sample exhibit excess thermal emission more frequently and with higher fractional excess than do the older stars. However, as many as 50% of the younger stars do not show excess emission. The decline in the magnitude of excess emission, for those stars that show it, has a roughly t 0 /time dependence, with t 0 $ 150 Myr. If anything, stars in binary systems (including Algoltype stars) and k Boo stars show less excess emission than the other members of the sample. Our results indicate that (1) there is substantial variety among debris disks, including that a significant number of stars emerge from the protoplanetary stage of evolution with little remaining disk in the 10-60 AU region and (2) in addition, it is likely that much of the dust we detect is generated episodically by collisions of large planetesimals during the planet accretion end game, and that individual events often dominate the radiometric properties of a debris system. This latter behavior agrees generally with what we know about the evolution of the solar system, and also with theoretical models of planetary system formation.

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 these models require a r −1 surface number density profile and a total mass of 3±1.5×10 −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 years) of these small grains and the observed total mass, ∼3×10 −3 M ⊕ , set a lower limit on the dust production rate, ∼10 15 g/s. 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 subsequent collisional cascade.

A stellar wind module has been developed for the PHOENIX stellar atmosphere code for the purpose of computing non-LTE, line-blanketed, expanding atmospheric structures and detailed synthetic spectra of hot luminous stars with winds. We apply the code to observations of Deneb, for which we report the first positive detections of mm and cm emission (obtained using the SCUBA and the VLA), as well a strong upper limit on the 850µm flux (using the HHT). The slope of the radio spectrum shows that the stellar wind is partially ionized. We report a uniform-disk angular diameter measurement, θ UD = 2.40 ± 0.06 mas, from the Navy Prototype Optical Interferometer (NPOI). The measured bolometric flux and corrected NPOI angular diameter yield an effective temperature of 8600 ± 500 K. Least-squares comparisons of synthetic spectral energy distributions from 1220Å to 3.6 cm with the observations provide estimates for the effective temperature and the mass-loss rate of ≃ 8400 ± 100 K and 8 ± 3 × 10 −7 M ⊙ yr −1 , respectively. This range of mass-loss rates is consistent with that derived from high dispersion UV spectra when non-LTE metal-line blanketing is considered. We are unable achieve a reasonable fit to a typical Hα P-Cygni profile with any model parameters over a reasonable range. This is troubling because the Hα profile is the observational basis for Wind Momentum-Luminosity Relationship. introductionA-type supergiants are the brightest stars at visual wavelengths (up to M V ≃ −9) and are therefore among the brightest single stars visible in galaxies. For this reason, these stars have been of increasing interest in extragalactic astronomy where they show potential as independent distance indicators (Bresolin et al. 2001; Kudritzki et al. 1999, hereafter K99). This potential lies in the use of the Wind Momentum-Luminosity Relationship (WMLR) which is derived from Sobolev radiation-driven stellar wind theory (K99 and references therein). Testing this relationship and the theory of radiation driven winds most critically requires an accurate determination of the stellar mass-loss rates. The WMLR states thaṫ M v ∞ ∝ L 1/α /R 0.5 ⋆ , whereṀ is the mass-loss rate, v ∞ is the terminal velocity of the wind, R ⋆ is a photospheric reference radius, and α is a parameter related to the distribution of atomic line strengths for the spectral lines which drive the wind.The application of the WMLR to nearby galaxies requires a calibration of the relationship using similar stars in the Milky Way. To determine mass-loss rates for A-type supergiants, K99 modeled the hydrogen Balmer lines Hα and Hγ for six stars in the Galaxy and M31 using a unified stellar atmosphere model. In their analysis, K99 synthesized the observed line profiles by adjusting model values for the mass-loss rate, the velocity law exponent β, and a line-broadening parameter, v t , after first adopting parameters for the effective temperature, T eff , the radius, R ⋆ , the gravity, log g, and the projected stellar rotational velocity.

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