Molecular Gas Clumps from the Destruction of Icy Bodies in the β Pictoris Debris Disk

Abstract

Abstract:Many stars are surrounded by disks of dusty debris formed in the collisions of asteroids, comets and dwarf planets. But is gas also released in such events? Observations at submm wavelengths of the archetypal debris disk around β Pictoris show that 0.3% of a Moon mass of carbon monoxide orbits in its debris belt. The gas distribution is highly asymmetric, with 30% found in a single clump 85AU from the star, in a plane closely aligned with the orbit of the inner planet, β Pic b. This gas clump delineates a region of enhanced collisions, either from a mean motion resonance with an unseen giant planet, or from the remnants of a collision of Mars--mass planets. Main Text:Debris disks are the end product of collisional cascades of km--sized bodies orbiting stars (including the Sun), and are normally thought to contain negligible gas. At a distance of 19.44 parsec (1) and age of 20 million years (2), β Pictoris is one of the closest, brightest and youngest examples. Its edge--on disk was the first to be imaged in scattered light, showing the distribution of micron--sized dust grains (3). Various subsequent observations have provided evidence of infalling comets within a few Astronomical Units (AU) of the star (4), a massive planet at ~10 AU (5), as well as atomic gas extending out to ~300AU (6); it is still unclear how these features are linked. By observing such debris disks at mm wavelengths, it is possible to trace the mm--sized dust and, by inference, the parent bodies of the collisional cascade, known as planetesimals (7).We observed β Pic using the Atacama Large Millimeter/submillimeter Array (ALMA) at a wavelength of 870µm in both the continuum and the J=3--2 12 CO line with a resolution of 12AU (8). The continuum image (Fig.1A) shows maxima in the surface brightness ~60AU either side of the star. At separations from 30--80AU the continuum is, on average, 15% brighter to the southwest. These results indicate that the mm grains lie in a broad slightly asymmetric belt nearly co--located with the disk of sub--micron reflecting dust (9). The total flux of 60±6mJy corresponds to a dust mass of 4.7±0.5x10 23 kg (6.4M Moon ), if we assume a standard dust mass opacity (0.15m 2 kg --1 at 850µm) and temperature of 85K (10).The planet β Pic b was close to maximum SW elongation at the time of the observations, with a projected separation of 8.7AU (11). Its location is coincident with a dip in the continuum surface brightness at a significance level of 4σ, suggesting that it may influence the innermost dust distribution.ALMA also detected the disk in the 12 CO J=3--2 transition (Fig.1B), with a clear velocity gradient along the major axis, illustrated in the position--velocity (PV) diagram (Fig.2). This shows the characteristic distribution of a broad belt of orbiting gas, with inner and outer radii of 50 and 160AU, and a peak around 85AU. No gas 3 emission is seen inside 50AU. The CO distribution is, on average, a factor of 2 brighter to the southwest, and a similar asymmetry was seen in the mid--infrared emission...

A self-consistent model for the evolution of the gas produced in the debris disc of β Pictoris

“…This translates as O/C ∼ 2.5 and O/H ∼ 1 (see Table 4). Using the total CO mass derived by Dent et al (2014) and the photodissociation timescales for H2O and CO, one finds that the total H2O mass in the gas phase is ∼ 2 × 10 −9 M⊕. This does not change our other results as we checked that the temperature (which fixes the viscosity) does not vary assuming this new oxygen (and hydrogen) content.…”

“…We model the subsequent evolution of this gaseous component using standard accretion disc physics. The modelled gas sits in a dust disc, which extends from ∼ 50 to ∼ 150AU (Augereau et al 2001;Dent et al 2014) and might affect the thermal state of the gas. Throughout this paper, we choose to use our general model on β Pic so that we take the gas injection location to be R0 = 85 AU, where the bulk of CO is located.…”

“…• We use the ALMA J=3-2 12 CO (observed at 867.5µm, Dent et al 2014) to fix two of our free parameters. The resolution was ∼ 12AU (using 27 antennas with projected baseline lengths from 15 to 380m) and the spectral resolution 0.85km/s.…”

Resolved Co Gas Interior to the Dust Rings of the Hd 141569 Disk

“…similar position in the CO(3-2) data, with a total flux of 0.11 Jy km s −1 integrated over the four CO(3-2) channels that overlap with the single CO(1-0) channel. The low CO(3-2)/CO (1-0) flux ratio implies an excitation temperature of only 5 K. This cold, compact feature may be a result of asymmetries in the disk structure, similar to what has been seen in the CO gas of the debris disk around β Pic (Dent et al 2014). Further data are needed to confirm the nature of this residual feature.…”

“…Radio surveys revealed that HD 21997, a 30 Myr old A star, is surrounded by a broad disk of CO with a mass of M CO ∼5×10 −2 M ⊕ (Kóspál et al 2013), while 49 Ceti also has a large ring of CO emission, with a mass estimated at M CO ∼10 −3 M ⊕ (Hughes et al 2008a). ALMA observations of β Pic find CO gas confined to a belt, with clear asymmetries, and a mass of only 2×10 −5 M ⊕ (Dent et al 2014). Recently HD 131835 was discovered to have CO emission consistent with M CO ∼4×10 −4 M ⊕ (Moór et al 2015).…”