The Complete Census of 70 μm–Bright Debris Disks within “The Formation and Evolution of Planetary Systems”SpitzerLegacy Survey of Sun‐like Stars

Abstract

We report detection with the Spitzer Space Telescope of cool dust surrounding solar type stars. The observations were performed as part of the Legacy Science Program, "Formation and Evolution of Planetary Systems" (F EP S). From the overall F EP S sample of 328 stars having ages ∼0.003-3 Gyr we have selected sources with 70 µm flux densities indicating excess in their spectral energy distributions above expected photospheric emission. Six strong excess sources are likely primordial circumstellar disks, remnants of the star formation process. Another 25 sources having ≥ 3σ excesses are associated with dusty debris disks, generated by collisions within planetesimal belts that are possibly stirred by existing planets. We draw attention to six additional sources with ≥ 2σ excesses which require confirmation as debris disks. In our analysis, most (>80%) of the debris disks identified via 70 µm excesses have ≥ 3σ excesses at 33 µm as well, while only a minority (<40%) have ≥ 3σ excesses at 24 µm.The rising spectral energy distributions towards -and perhaps beyond -70 µm imply dust temperatures T dust <45-85 K for debris in equilibrium with the stellar radiation field. We infer bulk properties such as characteristic temperature, location, fractional luminosity, and mass of the dust from fitted single temperature blackbody models. For >1/3 of the debris sources we find that multiple temperature components are suggested, implying a spatial distribution of dust extending over many tens of AU. Because the disks are dominated by collisional processes, the parent body (planetesimal) belts may be extended as well. Preliminary assessment of the statistics of cold debris around sun-like stars shows that ∼10% of F EP S targets with masses between 0.6 and 1.8 M ⊙ and ages between 30 Myr and 3 Gyr exhibit 70 µm emission in excess of the expected photospheric flux density. We find that fractional excess amplitudes appear higher for younger stars and that there may be a trend in 70 µm excess frequency with stellar mass.The F EP S program utilized all three Spitzer science instruments -IRAC, IRS, and MIPS -to observe 328 solar-type stars. Meyer et al. (2006) provides a description of the F EP S observing strategy. Among the F EP S sample are 15 previously suspected (based on IRAS or ISO literature) debris or long-lived primordial disk systems, only 11 of which are in fact confirmed by Spitzer. Ten of these 15 were observed by F EP S for the purpose of probing primordial gas disk dissipation (e.g. Pascucci et al 2006Pascucci et al , 2007 while the others were either serendipitously on our lists or discovered as excess sources after the F EP S program was submitted. The sources selected ab initio because they were claimed to exhibit infrared excess emission can not be included in statistical analyses of F EP S Spitzer data for debris characteristics as a function of e.g. stellar age, stellar mass, stellar metallicity, stellar rotation, etc. However, we do include them in this paper which presents disk detections and simple dust models....

EXPLORATIONS BEYOND THE SNOW LINE:SPITZER/IRS SPECTRA OF DEBRIS DISKS AROUND SOLAR-TYPE STARS

“…The separation between the 32 μm and 70 μm distributions in Figure 11, for example, can be translated to a representative dust temperature of ∼65 K. In reality, a range of temperatures are present and, as is found in Section 5.2.1, the dust in each system is often not well fit by a single emission temperature, but rather by a distribution. This is also apparent from the overall statistics, as evidenced by the inability of a single blackbody to fit the trends seen in Figure 11; the separation between the 24 and 70 μm curves is consistent with 75 K dust, while the separation between the 10 and 70 μm curves corresponds to dust temperatures >100 K. A similar trend is found by Hillenbrand et al (2008), who find that >1/3 of their surveyed debris disks have evidence for multiple dust temperatures based on their colors at MIPS and IRS wavelengths.…”

“…Hillenbrand et al (2008) find a similar trend, with >80% of their stars with MIPS 70 μm excesses also possessing IRS 33 μm excesses, and no reported stars possessing an IRS excess with no corresponding MIPS 70 μm excess. This implies that there may be a lower limit to debris disk temperatures, with a corresponding upper limit on disk sizes.…”

“…Using our criteria of >3σ above the photosphere (or >2σ with a known 70 μm excess), Hillenbrand et al (2008) find excesses in the 30-34 μm band for 22 out of the sample of 328 stars, although not all stars in the sample have reported IRS spectra. Carpenter et al (2008) measure excesses using colors rather than comparison with Kurucz models, and find 71 out of 314 stars (22.6% ± 2.7%) with excesses in the long-wavelength IRS band, and 2 out of 314 (0.6% ± 0.5%) in the short-wavelength IRS band.…”

“…The FEPS survey (Carpenter et al 2008;Hillenbrand et al 2008) used Spitzer to observe nearby sun-like stars with ages between 3 Myr and 3 Gyr. Using our criteria of >3σ above the photosphere (or >2σ with a known 70 μm excess), Hillenbrand et al (2008) find excesses in the 30-34 μm band for 22 out of the sample of 328 stars, although not all stars in the sample have reported IRS spectra.…”

A search for debris disks in theHerschel-ATLAS

“…Our search finds a much lower fraction of candidate debris disks than previous Spitzer and SCUBA studies (e.g. Carpenter et al 2009;Hillenbrand et al 2008;Wyatt 2008), which typically find a 7−14% disk detection fraction. However we note that direct comparisons between these detection fractions should not be made as H-ATLAS is flux-limited rather than volume-limited and we do not reach the exquisite photospheric signal-to-noise levels of the Spitzer studies (Carpenter et al 2009).…”