Debris Disk Evolution around A Stars

The Astrophysical Journal (Impact Factor: 5.99). 12/2008; 653(1):675. DOI: 10.1086/508649
Source: arXiv


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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Available from: James Muzerolle, Jun 10, 2014
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    • "The debris disc around Vega was the first debris disc discovered in this way (Aumann et al. 1984) and after that more than 100 discs have been subsequently discovered. Observations from recent surveys indicate that at least 15 per cent of FGK stars and 32 per cent of A stars have a detectable amount of circumstellar debris (Bonsor et al. (2014), Bryden et al. (2006), Moro-Martín et al. (2007), Hillenbrand et al. (2008),Greaves et al. (2009) and Su et al. (2006)). "
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    ABSTRACT: Dust in debris discs is constantly replenished by collisions between larger objects. In this paper, we investigate a method to detect these collisions. We generate models based on recent results on the Fomalhaut debris disc, where we simulate a background star transiting behind the disc, due to the proper motion of Fomalhaut. By simulating the expanding dust clouds caused by the collisions in the debris disc, we investigate whether it is possible to observe changes in the brightness of the background star. We conclude that in the case of the Fomalhaut debris disc, changes in the optical depth can be observed, with values of the optical depth ranging from $10^{-0.5}$ for the densest dust clouds to $10^{-8}$ for the most diffuse clouds with respect to the background optical depth of $\sim1.2\times10^{-3}$.
    Preview · Article · Feb 2014 · Monthly Notices of the Royal Astronomical Society
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    • "Debris disks are characterized by a lower gas/dust ratio than that of protoplanetary disks and transitional disks, though the total mass of dust in debris disks is lower than that of protoplanetary disks (Wyatt 2008). Around 900 debris disks have been detected to date and an investigation of 160 A-type stars bySu et al. (2006)showed that 32 ± 5 % possessed 24 µm infrared excess indicative of a debris disk. "
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    ABSTRACT: The late stages of planetary formation are not well understood. It is known that a protostar at some point looses the majority of its circumstellar gas. How, when and to what degree gas can remain is not well known. Young main-sequence stars are sometimes observed to have debris disks; circumstellar dust. A small sample of these debris disks has also been found to possess circumstellar gas. Current theories suggest that the gas and dust is secondary, i.e. reproduced after being largely cleared from the system. We seek to extend the number of known debris disks with circumstellar gas and to measure the amount of gas in the disks. By finding a larger sample of disks with co-existing circumstellar gas and dust we can better estimate the frequency of gas in debris disks, and detailed analysis of the composition of the gas could give important clues to its origin. Gas can also be used to probe the conditions in the disk, such as temperature and radiative conditions. We have used high-resolution echelle spectro-scopy to observe 22 mostly nearby A-and B-stars of Vega-excess type (i.e. main-sequence stars with known infrared excess) and look for line absorption from gas in the optical regime. A Monte-Carlo approach was used to assess the statistical significant of line detection during visual inspection. Based on non-detection we set 95% upper limit constraints on the line-of-sight column density, ranging from 6.2 x 10 9 to 5.9 x 10 10 particles per cm 2 in the case of the Na I line. Statistically investigating the disk orientations of our sample we conclude that with 95% certainty, at least one of our disks should be viewed edge-on ± 11º. On this basis we calculate upper limit column densities in the equatorial plane of our stars for three different scale heights, assuming cylindrical symmetry and a Gaussian vertical distribution of gas. We find that equatorial column densities for the most edge-on disk in the sample could be 1.2 to 9.1 times higher than line-of-sight values, depending on scale height. With a simple gas distribution model we also make upper limit estimates of the total gas mass, based on our upper limit equatorial column densities. Assuming solar elemental abundances and a disk scale height of 0.1, we get a total upper mass limit of 4.2 x 10-4 M⊕, while a � Pictoris-like composition with scale height 0.1 yields a total mass of 3.4 x 10-4 M⊕. We conclude that reproduction of lost gas in young debris disk systems is rare, and confirm that such gas, if produced, is produced in low amounts.
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    ABSTRACT: The dust disks observed around mature stars are evidence that plantesimals are present in these systems on spatial scales that are similar to that of the asteroids and the Kuiper belt objects (KBOs) in the solar system. These dust disks (a.k.a. "debris disks") present a wide range of sizes, morphologies, and properties. It is inferred that their dust mass declines with time as the dust-producing planetesimals get depleted, and that this decline can be punctuated by large spikes that are produced as a result of individual collisional events. The lack of solid-state features indicate that, generally, the dust in these disks have sizes >~10 æm, but exceptionally, strong silicate features in some disks suggest the presence of large quantities of small grains, thought to be the result of recent collisions. Spatially resolved observations of debris disks show a diversity of structural features, such as inner cavities, warps, offsets, brightness asymmetries, spirals, rings, and clumps. There is growing evidence that, in some cases, these structures are the result of the dynamical perturbations of a massive planet. Our solar system also harbors a debris disk and some of its properties resemble those of extrasolar debris disks. From the cratering record, we can infer that its dust mass has decayed with time, and that there was at least one major "spike" in the past during the late heavy bombardment. This offers a unique opportunity to use extrasolar debris disks to shed some light in how the solar system might have looked in the past. Similarly, our knowledge of the solar system is influencing our understanding of the types of processes that might be at play in the extrasolar debris disks.
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