New constraints on the millimetre emission of six debris discs

Abstract

The presence of dusty debris around main sequence stars denotes the existence of planetary systems. Such debris disks are often identified by the presence of excess continuum emission at infrared and (sub-)millimetre wavelengths, with measurements at longer wavelengths tracing larger and cooler dust grains. The exponent of the slope of the disk emission at submillimetre wavelengths, 'q', defines the size distribution of dust grains in the disk. This size distribution is a function of the rigid strength of the dust producing parent planetesimals. As part of the survey 'PLAnetesimals around TYpical Pre-main seqUence Stars' (PLATYPUS) we observed six debris disks at 9-mm using the Australian Telescope Compact Array. We obtain marginal (∼ 3-σ) detections of three targets: HD 105, HD 61005, and HD 131835. Upper limits for the three remaining disks, HD 20807, HD 109573, and HD 109085, provide further constraint of the (sub-)millimetre slope of their spectral enery distributions. The values of q (or their limits) derived from our observations are all smaller than the oft-assumed steady state collisional cascade model (q = 3.5), but lie well within the theoretically expected range for debris disks q ∼ 3 to 4. The measured q values for our targets are all < 3.3, consistent with both collisional modelling results and theoretical predictions for parent planetesimal bodies being 'rubble piles' held together loosely by their self-gravity.

Debris Disks: Structure, Composition, and Variability

“…The inferred range for q in debris disks is in good agreement with the predictions of steady-state collisional models, both from the classical Dohnanyi (1969) model and from more modern models (Marshall et al 2017, and references therein). On the other hand, estimates for the power law index associated with zodiacal dust are markedly shallower, q ≈ 2-2.5, and studies of cometary dust also generally yield similar power law indices (see Figure 5).…”

“…Similarly, spectral analyses of the mid-infrared silicate feature suggest minimum sizes ranging from ≈ 2 to ≈ 20 µm (Mittal et al 2015). Finally, the observation that the actual size of debris disks as measured in thermal emission maps is larger than the prediction from pure blackbody emission by a factor of up to 10 times (e.g., indicates minimum grain sizes in the 1-10 µm range (Pawellek & Figure 5 Inferred grain size power law index for debris disks based on the (sub)mm spectral index (orange histogram; Matthews et al 2007Donaldson et al 2013;Marshall et al 2014Marshall et al , 2017Pawellek et al 2014;) and the spectral shape of the mid-infrared silicate features (blue histogram; Mittal et al 2015). The median uncertainty associated with both methods is represented by the colored errorbars.…”

The Big Sibling of AU Mic: A Cold Dust-rich Debris Disk around CP−72 2713 in the β Pic Moving Group

“…The low β value indicates an excess spectrum that is almost identical to a pure blackbody; therefore, changing the λ 0 parameter has no effect on our results. We note that though most debris disks show considerably steeper millimeter slopes (Gáspár et al 2012;MacGregor et al 2016;Marshall et al 2017), interestingly, the excess SED of AU Mic can also be well reproduced by a blackbody (Matthews et al 2015). For the fractional luminosity, we obtained f d =1.1×10 −3 .…”

Rapid grain growth in post-AGB disc systems from far-infrared and sub-millimetre photometry

“…In protoplanetary discs, the presence of gas damps the relative velocities of grains, and so collisions result in grain growth, while the lack of gas in debris discs allows a collisional cascade to proceed unimpeded, resulting in the production of a large population of small grains. We select two samples: the Taurus protoplanetary disc sample of Andrews & Williams (2005), and a sample of field debris discs (MacGregor et al 2016;Marshall et al 2017). In order to make a distribution comparable to the histogram shown in Fig.…”

“…Williams & Cieza 2011;Wyatt et al 2015;Matthews et al 2014), are typically populated by dust grains at least up to mm sizes, as probed by the spectral index in the far-infrared (FIR) and (sub-)mm wavelength ranges (e.g. Roccatagliata et al 2009;Gáspár et al 2012;MacGregor et al 2016;Marshall et al 2017). In protoplanetary discs (PPDs), the existence of these large dust grains is believed to be linked to grain-growth processes which can take place in such dense, long-lived (typical lifetimes are several megayears, e.g.…”