The Mass of Stirring Bodies in the AU Mic Debris Disk Inferred from Resolved Vertical Structure

Abstract

The vertical distribution of dust in debris disks is sensitive to the number and size of large planetesimals dynamically stirring the disk, and is therefore well-suited for constraining the prevalence of otherwise unobservable Uranus and Neptune analogs. Information regarding stirring bodies has previously been inferred from infrared and optical observations of debris disk vertical structure, but theoretical works predict that the small particles traced by short-wavelength observations will be 'puffed up' by radiation pressure, yielding only upper limits. The large grains that dominate the disk emission at millimeter wavelengths are much less sensitive to the effects of stellar radiation or stellar winds, and therefore trace the underlying mass distribution more directly. Here we present ALMA 1.3 mm dust continuum observations of the debris disk around the nearby M star AU Mic. The 3 au spatial resolution of the observations, combined with the favorable edge-on geometry of the system, allows us to measure the vertical thickness of the disk. We report a scale height-to-radius aspect ratio of h = 0.031 +0.005 −0.004 between radii of ∼ 23 au and ∼ 41 au. Comparing this aspect ratio to a theoretical model of size-dependent velocity distributions in the collisional cascade, we find that the perturbing bodies embedded in the local disk must be larger than about 400 km, and the largest perturbing body must be smaller than roughly 1.8 M ⊕ . These measurements rule out the presence of a gas giant or Neptune analog near the ∼ 40 au outer edge of the debris ring, but are suggestive of large planetesimals or an Earth-sized planet stirring the dust distribution.

Evidence for a high mutual inclination between the cold Jupiter and transiting super Earth orbiting π Men

“…L ppd,i ≈L dd,lt ≈L 2,lt ). If the system evolved in this way to give case II, we would expect the debris disc to be much thinner than the "re-aligned" discs described in §7.3.1, with a scale height of ≈ 0.03-0.1 similar to other imaged discs (Matrà et al 2019;Daley et al 2019). Therefore, measuring the disc scale height would help determine whether the debris disc was initially misaligned to the giant planet and later re-aligned, or always aligned to the giant planet.…”

Properties of M Dwarf Flares at Millimeter Wavelengths

“…AU Mic was observed with the ALMA 12-m array at 222 GHz (1.35 mm) three times on 2014 March 26, 2014 August 18, and 2015 June 24 (PI: Hughes, 2012.1.00198.S). The original analysis and description of these observations are presented in Daley et al (2019). For all three scheduling blocks (SBs), there were 32 − 37 antennas in the array spanning baseline lengths of 12 − 1320 m. The SB executed on 2015 June 24, which we consider in this paper, had 37 antennas with baselines between 30 − 1320 m. During this SB, AU Mic was observed in 6.5 min integrations or 'scans' alternating with a phase calibrator, J2056-3208, for a total of 33 min on-source.…”

“…However, a point source is still detected at the stellar position during quiescent periods with a flux that appears to vary on month-to year-long timescales. Daley et al (2019) fit for the quiescent stellar flux density during each of the three observations and obtain values of 390, 150, and 220 µJy for the 2014 March 26, 2014 August 18, and 2015 June 24 observations, respectively. Early ALMA observations from 2012 (PI: Wilner, 2011.0.00142.S) also detected a central point source with a flux density of 320 µJy, although no large flares were seen (MacGregor et al 2013).…”

“…For AU Mic, the data reveal a span of about 2 min of flaring activity. The on-source time in each of the three ALMA scheduling blocks for AU Mic described here and in Daley et al (2019) is 33 − 35 min, to which we add the 45 min of on-source time from an earlier ALMA observation of AU Mic described by MacGregor et al (2013) for a total monitoring time of 2.4 hours. The percentage of time AU Mic was observed in a flaring state is then about 1.4%.…”

Extrasolar Kuiper belts

“…Detailed observations of the halo of small grains seen to extend beyond a debris disk due to radiation pressure has the potential to constrain the level of stirring, with eccentricities inferred to be < 1% in one system (Thébault and Wu 2008). For some disks the vertical structure has been measured with inclinations inferred to be of order a few percent (Matrà et al 2019b;Daley et al 2019).…”