Debris disks are extrasolar analogs to our own Kuiper Belt and they are detected around at least 17% of nearby Sun-like stars. The morphology and dynamics of a disk encode information about its ...history, as well as that of any exoplanets within the system. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to obtain 1.3 mm observations of the debris disk around the nearby F5V star HD 170773. We image the face-on ring and determine its fundamental parameters by forward-modeling the interferometric visibilities through a Markov Chain Monte Carlo approach. Using a symmetric Gaussian surface density profile, we find a 71 4 au wide belt with a radius of au, a relatively large radius compared with most other millimeter-resolved belts around late A/early F type stars. This makes HD 170773 part of a group of four disks around A and F stars with radii larger than expected from the recently reported planetesimal belt radius-stellar luminosity relation. Two of these systems are known to host directly imaged giant planets, which may point to a connection between large belts and the presence of long-period giant planets. We also set upper limits on the presence of CO and CN gas in the system, which imply that the exocomets that constitute this belt have CO and HCN ice mass fractions of <77% and <3%, respectively. This is consistent with solar system comets and other exocometary belts.
ABSTRACT
Despite being >10 Myr, there are ∼10 debris discs with as much CO gas as in protoplanetary discs. Such discs have been assumed to be ‘hybrid’, i.e. with secondary dust but primordial gas. ...Here, we show that both the dust and gas in such systems could instead be secondary, with the high CO content caused by accumulation of neutral carbon (C0) that shields CO from photodissociating; i.e. these could be ‘shielded secondary discs’. New ALMA observations are presented of HD131835 that detect ∼3 × 10−3 M⊕ of C0, the majority 40–200 au from the star, in sufficient quantity to shield the previously detected CO. A simple semi-analytic model for the evolution of CO, C, and O originating in a volatile-rich planetesimal belt shows how CO shielding becomes important when the viscous evolution is slow (low α parameter) and/or the CO production rate is high. Shielding by C0 may also cause the CO content to reach levels at which CO self-shields, and the gas disc may become massive enough to affect the dust evolution. Application to the HD 131835 observations shows these can be explained if α ∼ 10−3; an inner cavity in C0 and CO may also mean the system has yet to reach steady state. Application to other debris discs with high CO content finds general agreement for α = 10−3 to 0.1. The shielded secondary nature of these gas discs can be tested by searching for C0, as well as CN, N2, and CH+, which are also expected to be shielded by C0.
Abstract
This paper uses observations of dusty debris discs, including a growing number of gas detections in these systems, to test our understanding of the origin and evolution of this gaseous ...component. It is assumed that all debris discs with icy planetesimals create second generation CO, C and O gas at some level, and the aim of this paper is to predict that level and assess its observability. We present a new semi-analytical equivalent of the numerical model of Kral et al. allowing application to large numbers of systems. That model assumes CO is produced from volatile-rich solid bodies at a rate that can be predicted from the debris discs fractional luminosity. CO photodissociates rapidly into C and O that then evolve by viscous spreading. This model provides a good qualitative explanation of all current observations, with a few exceptional systems that likely have primordial gas. The radial location of the debris and stellar luminosity explain some non-detections, e.g. close-in debris (like HD 172555) is too warm to retain CO, while high stellar luminosities (like η Tel) result in short CO lifetimes. We list the most promising targets for gas detections, predicting >15 CO detections and >30 C i detections with ALMA, and tens of C ii and O i detections with future far-IR missions. We find that CO, C i, C ii and O i gas should be modelled in non-LTE for most stars, and that CO, C i and O i lines will be optically thick for the most gas-rich systems. Finally, we find that radiation pressure, which can blow out C i around early-type stars, can be suppressed by self-shielding.
ABSTRACT
Only 20 per cent of old field stars have detectable debris discs, leaving open the question of what disc, if any, is present around the remaining 80 per cent. Young moving groups allow to ...probe this population, since discs are expected to have been brighter early on. This paper considers the population of F stars in the 23 Myr-old β Pictoris moving group (BPMG) where we find that 9/12 targets possess discs. We also analyse archival ALMA data to derive radii for four of the discs, presenting the first image of the 63 au radius disc of HD 164249. Comparing the BPMG results to disc samples from ∼45-Myr and ∼150-Myr-old moving groups, and to discs found around field stars, we find that the disc incidence rate in young moving groups is comparable to that of the BPMG and significantly higher than that of field stars. The BPMG discs tend to be smaller than those around field stars. However, this difference is not statistically significant due to the small number of targets. Yet, by analysing the fractional luminosity versus disc radius parameter space, we find that the fractional luminosities in the populations considered drop by two orders of magnitude within the first 100 Myr. This is much faster than expected by collisional evolution, implying a decay equivalent to 1/age2. We attribute this depletion to embedded planets, which would be around 170 Mearth to cause a depletion on the appropriate time-scale. However, we cannot rule out that different birth environments of nearby young clusters result in brighter debris discs than the progenitors of field stars that likely formed in a more dense environment.
Models of terrestrial planet formation predict that the final stages of planetary assembly--lasting tens of millions of years beyond the dispersal of young protoplanetary disks--are dominated by ...planetary collisions. It is through these giant impacts that planets like the young Earth grow to their final mass and achieve long-term stable orbital configurations.sup.1. A key prediction is that these impacts produce debris. So far, the most compelling observational evidence for post-impact debris comes from the planetary system around the nearby 23-million-year-old A-type star HD 172555. This system shows large amounts of fine dust with an unusually steep size distribution and atypical dust composition, previously attributed to either a hypervelocity impact.sup.2,3 or a massive asteroid belt.sup.4. Here we report the spectrally resolved detection of a carbon monoxide gas ring co-orbiting with dusty debris around HD 172555 between about six and nine astronomical units--a region analogous to the outer terrestrial planet region of our Solar System. Taken together, the dust and carbon monoxide detections favour a giant impact between large, volatile-rich bodies. This suggests that planetary-scale collisions, analogous to the Moon-forming impact, can release large amounts of gas as well as debris, and that this gas is observable, providing a window into the composition of young planets.
ABSTRACT
Gas detection around main-sequence stars is becoming more common with around 20 systems showing the presence of CO. However, more detections are needed, especially around later spectral type ...stars to better understand the origin of this gas and refine our models. To do so, we carried out a survey of 10 stars with predicted high likelihoods of secondary CO detection using ALMA in band 6. We looked for continuum emission of mm-dust as well as gas emission (CO and CN transitions). The continuum emission was detected in 9/10 systems for which we derived the discs’ dust masses and geometrical properties, providing the first mm-wave detection of the disc around HD 106906, the first mm-wave radius for HD 114082, 117214, HD 15745, HD 191089, and the first radius at all for HD 121191. A crucial finding of our paper is that we detect CO for the first time around the young 10–16 Myr old G1V star HD 129590, similar to our early Sun. The gas seems colocated with its planetesimal belt and its total mass is likely in the range of (2–10) × 10−5 M⊕. This first gas detection around a G-type main-sequence star raises questions as to whether gas may have been released in the Solar system as well in its youth, which could potentially have affected planet formation. We also detected CO gas around HD 121191 at a higher signal-to-noise ratio than previously and find that the CO lies much closer-in than the planetesimals in the system, which could be evidence for the previously suspected CO viscous spreading owing to shielding preventing its photodissociation. Finally, we make estimates for the CO content in planetesimals and the HCN/CO outgassing rate (from CN upper limits), which we find are below the level seen in Solar system comets in some systems.
A Complete ALMA Map of the Fomalhaut Debris Disk MacGregor, Meredith A.; Matrà, Luca; Kalas, Paul ...
Astrophysical journal/The Astrophysical journal,
06/2017, Letnik:
842, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We present ALMA mosaic observations at 1.3 mm (223 GHz) of the Fomalhaut system with a sensitivity of 14 Jy/beam. These observations provide the first millimeter map of the continuum dust emission ...from the complete outer debris disk with uniform sensitivity, enabling the first conclusive detection of apocenter glow. We adopt an MCMC modeling approach that accounts for the eccentric orbital parameters of a collection of particles within the disk. The outer belt is radially confined with an inner edge of 136.3 0.9 au and width of 13.5 1.8 au. We determine a best-fit eccentricity of 0.12 0.01. Assuming a size distribution power-law index of q = 3.46 0.09, we constrain the dust absorptivity power-law index β to be 0.9 < β < 1.5. The geometry of the disk is robustly constrained with inclination 65 6 0 3, position angle 337 9 0 3, and argument of periastron 22 5 4 3. Our observations do not confirm any of the azimuthal features found in previous imaging studies of the disk with Hubble Space Telescope, SCUBA, and ALMA. However, we cannot rule out structures ≤10 au in size or that only affect smaller grains. The central star is clearly detected with a flux density of 0.75 0.02 mJy, significantly lower than predicted by current photospheric models. We discuss the implications of these observations for the directly imaged Fomalhaut b and the inner dust belt detected at infrared wavelengths.
Abstract
The young A0V star HR 4796A is host to a bright and narrow ring of dust, thought to originate in collisions between planetesimals within a belt analogous to the Solar system’s ...Edgeworth–Kuiper belt. Here we present high spatial resolution 880 μm continuum images from the Atacama Large Millimeter Array. The 80 au radius dust ring is resolved radially with a characteristic width of 10 au, consistent with the narrow profile seen in scattered light. Our modelling consistently finds that the disc is also vertically resolved with a similar extent. However, this extent is less than the beam size, and a disc
that is dynamically very cold (i.e. vertically thin) provides a better theoretical explanation for the narrow scattered light profile, so we remain cautious about this conclusion. We do not detect 12CO J=3–2 emission, concluding that unless the disc is dynamically cold the CO+CO2 ice content of the planetesimals is of order a few per cent or less. We consider the range of semi-major axes and masses of an interior planet supposed to cause the ring’s eccentricity, finding that such a planet should be more massive than Neptune and orbit beyond 40 au. Independent of our ALMA observations, we note a conflict between mid-IR pericentre-glow and scattered light imaging interpretations, concluding that models where the spatial dust density and grain size vary around the ring should be explored.
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