Debris Disk Evolution around A Stars

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 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 two 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.

The Gemini NICI Planet-Finding Campaign: The Frequency of Giant Planets around Young B and A Stars

“…Stars with an independent age measurement from membership in a moving group (as detailed in Table 1) are plotted twice, once as an empty green circle with error bars indicating the results of the Bayesian analysis and again as a filled green circle indicating the age of the moving group, which we adopt as the final age for that star. There is good agreement between our work and Su et al (2006) for stars that Su et al (2006) flag as older than 100 Myr, but below that age our Bayesian analysis finds systematically older ages as expected. For the moving group stars, since the Bayesian analysis of photometry can only place a star in the first third of its main-sequence lifetime, additional information is needed to age-date young stars with greater accuracy.…”

“…For the ages derived by our Bayesian analysis, we find good agreement for stars which Su et al (2006) determine to be older than 100 Myr. Stars that Su et al (2006) find to be younger than 100 Myr show a systematic disagreement, with the median of our Bayesian age distribution significantly older than the single age found by Su et al (2006). This is as we would expect, since most movement across the color-magnitude diagram happens in the final two-thirds of a star's main-sequence lifetime, so older stars can be age-dated more definitively while younger stars can be anywhere in the first third of their main-sequence lifetimes.…”

“…Figure 10 compares our ages to stars that we have in common with Su et al (2006). For the ages derived by our Bayesian analysis, we find good agreement for stars which Su et al (2006) determine to be older than 100 Myr.…”

Insights into Planet Formation from Debris Disks

“…It was later shown that both the scatter seen in the observations and the decay in emission levels could be explained by the steady state erosion of extrasolar Kuiper belts (Wyatt et al 2007b); the level observed simply reflects the initial mass in the belt and its distance from the star, which was far enough that the emission was generally cold and so also explained the observations at 70 µm toward the same stars (Su et al 2006). The same issue affected the interpretation of Sun-like stars, since it was found that the emission spectrum for the majority of these stars was rising toward longer wavelengths implying that the 24 µm emission arises in a belt far enough from the star (> 10 au) to be explained by steady state processes (Carpenter et al 2009).…”

ALMA and Herschel observations of the prototype dusty and polluted white dwarf G29-38

“…Also in Figure 4 are plotted model extrapolations for dusty A-type stars from Su et al (2006), as their current dust properties would appear if the host stars were evolved into cool white dwarfs with parameters similar to G29-38. To begin, the inferred orbital radii of the dust components on the main sequence were expanded by a factor r wd /rms = Mms/M wd ≈ 3.5, appropriate for G29-38 and a representative value for A stars based on the initial-to-final mass relations (e.g.…”