Debris Disk Evolution around A Stars

The Astrophysical Journal (Impact Factor: 6.73). 12/2008; 653(1):675. DOI: 10.1086/508649
Source: arXiv

ABSTRACT We report 24 and/or 70 μm measurements of ~160 A-type main-sequence stars using the Multiband Imaging Photometer for Spitzer (MIPS). Their ages range from 5 to 850 Myr, based on estimates from the literature (cluster or moving group associations) or from the H-R diagram and isochrones. The thermal infrared excess is identified by comparing the deviation (~3% and ~15% at the 1 σ level at 24 and 70 μm, respectively) between the measurements and the synthetic Kurucz photospheric predictions. Stars showing excess infrared emission due to strong emission lines or extended nebulosity seen at 24 μm are excluded from our sample; therefore, the remaining infrared excesses are likely to arise from circumstellar debris disks. At the 3 σ confidence level, the excess rate at 24 and 70 μm is 32% and ≥33% (with an uncertainty of 5%), considerably higher than what has been found for old solar analogs and M dwarfs. Our measurements place constraints on the fractional dust luminosities and temperatures in the disks. We find that older stars tend to have lower fractional dust luminosity than younger ones. While the fractional luminosity from the excess infrared emission follows a general 1/t relationship, the values at a given stellar age vary by at least 2 orders of magnitude. We also find that (1) older stars possess a narrow range of temperature distribution peaking at colder temperatures, and (2) the disk emission at 70 μm persists longer than that at 24 μm. Both results suggest that the debris disk clearing process is more effective in the inner regions.

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    ABSTRACT: We describe a search for infra-red excess emission from dusty circumstellar material around 180,000 stars observed by the Kepler and WISE missions. This study is motivated by i) the potential to find bright warm disks around planet host stars, ii) a need to characterise the distribution of rare warm disks, and iii) the possible identification of candidates for discovering transiting dust concentrations. We find about 8,000 stars that have excess emission, mostly at 12um. The positions of these stars correlate with the 100um background level so most of the flux measurements associated with these excesses are spurious. We identify 271 stars with plausible excesses by making a 5MJy/sr cut in the IRAS 100um emission. The number counts of these excesses, at both 12 and 22um, have the same distribution as extra-Galactic number counts. Thus, although some excesses may be circumstellar, most can be explained as chance alignments with background galaxies. The one exception is a 22um excess associated with a relatively nearby A-type star that we were able to confirm because the disk occurrence rate is independent of stellar distance. Despite our low detection rate, these results place valuable upper limits on the distribution of large mid-infrared excesses; e.g. fewer than 1:1000 stars have 12um excesses (F_ obs/F_star) larger than a factor of five. In contrast to previous studies, we find no evidence for disks around 1790 stars with candidate planets (we attribute one significant 12um excess to a background galaxy), and no evidence that the disk distribution around planet hosts is different to the bulk population. Higher resolution imaging of stars with excesses is the best way to rule out galaxy confusion and identify more reliable disk candidates among Kepler stars. A similar survey to ours that focusses on nearby stars would be well suited to finding the distribution of rare warm disks.
    Monthly Notices of the Royal Astronomical Society 07/2012; 426(1). · 5.52 Impact Factor
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    ABSTRACT: We present an analysis of debris disc data around Solar-type stars (spectral types F0–K5) using the steady-state analytical model of Wyatt et al. Models are fitted to published data from the FEPS project and various GTO programmes obtained with MIPS on the Spitzer Space Telescope at 24 and 70 μm, and compared to a previously published analysis of debris discs around A stars using the same evolutionary model. We find that the model reproduces most features found in the data sets, noting that the model disc parameters for solar-type stars are different to those of A stars. Although this could mean that discs around Solar-type stars have properties different from their counterparts around earlier-type stars, it is also possible that the properties of discs around stars of different spectral types appear more different than they are because the blackbody disc radius underestimates the true disc radius by a factor Xr which varies with spectral type. We use results from realistic grain modelling to quantify this effect for solar-type stars and for A stars. Our results imply that planetesimals around solar-type stars are on average larger than around A stars by a factor of a few but that the mass of the discs are lower for discs around FGK stars, as expected. We also suggest that discrepancies between the evolutionary time-scales of 24-μm statistics predicted by our model and that observed in previous surveys could be explained by the presence of two-component discs in the samples of those surveys, or by transient events being responsible for the 24-μm emission of cold discs beyond a few Myr. Further study of the prevalence of two-component discs, and of constraints on Xr, and increasing the size of the sample of detected discs, are important for making progress on interpreting the evolution of discs around solar-type stars.
    Monthly Notices of the Royal Astronomical Society 06/2011; 414(3):2486 - 2497. · 5.52 Impact Factor
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    ABSTRACT: Dust in debris disks is generated by collisions among planetesimals. The existence of these planetesimals is a consequence of the planet formation process, but the relationship between debris disks and planets has not been clearly established. Here we analyze Spitzer/MIPS 24 and 70 micrometer data for 150 planet-bearing stars, and compare the incidence of debris disks around these stars with a sample of 118 stars around which planets have been searched for, but not found. Together they comprise the largest sample ever assembled to deal with this question. The use of survival analysis techniques allows us to account for the large number of non-detections at 70 micrometer. We discovered 10 new debris disks around stars with planets and one around a star without known planets. We found that the incidence of debris disks is marginally higher among stars with planets, than among those without, and that the brightness of the average debris disk is not significantly different in the two samples. We conclude that the presence of a planet that has been detected via current radial velocity techniques is not a good predictor of the presence of a debris disk detected at infrared wavelengths. Comment: Accepted for publication in the Astrophysical Journal Letters, 20 pages, 2 figures, 3 tables (Table 2 is available in machine readable form in the online journal)
    The Astrophysical Journal 06/2009; · 6.73 Impact Factor

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