Where can a Trappist-1 planetary system be produced? Haworth, Thomas J; Facchini, Stefano; Clarke, Cathie J ...
Monthly notices of the Royal Astronomical Society,
04/2018, Letnik:
475, Številka:
4
Journal Article
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Abstract
We study the evolution of protoplanetary discs that would have been precursors of a Trappist-1-like system under the action of accretion and external photoevaporation in different radiation ...environments. Dust grains swiftly grow above the critical size below which they are entrained in the photoevaporative wind, so although gas is continually depleted, dust is resilient to photoevaporation after only a short time. This means that the ratio of the mass in solids (dust plus planetary) to the mass in gas rises steadily over time. Dust is still stripped early on, and the initial disc mass required to produce the observed 4 M⊕ of Trappist-1 planets is high. For example, assuming a Fatuzzo & Adams distribution of UV fields, typical initial disc masses have to be >30 per cent the stellar (which are still Toomre Q stable) for the majority of similar mass M dwarfs to be viable hosts of the Trappist-1 planets. Even in the case of the lowest UV environments observed, there is a strong loss of dust due to photoevaporation at early times from the weakly bound outer regions of the disc. This minimum level of dust loss is a factor of 2 higher than that which would be lost by accretion on to the star during 10 Myr of evolution. Consequently, even in these least irradiated environments, discs that are viable Trappist-1 precursors need to be initially massive (>10 per cent of the stellar mass).
We present high-resolution ALMA Band 6 and 7 observations of the tidally disrupted protoplanetary disks of the RW Aurigae binary. Our observations reveal tidal streams in addition to the previously ...observed tidal arm around RW Aur A. The observed configuration of tidal streams surrounding RW Aur A and B is incompatible with a single star-disk tidal encounter, suggesting that the RW Aurigae system has undergone multiple flyby interactions. We also resolve the circumstellar disks around RW Aur A and B, with CO radii of 58 au and 38 au consistent with tidal truncation, and 2.5 times smaller dust emission radii. The disks appear misaligned by 12° or 57°. Using new photometric observations from the American Association of Variable Star Observers (AAVSO) and the All Sky Automated Survey for SuperNovae (ASAS-SN) archives, we have also identified an additional dimming event of the primary that began in late 2017 and is currently ongoing. With over a century of photometric observations, we are beginning to explore the same spatial scales as ALMA.
RW Aurigae (RW Aur) is a binary star system with a long molecular arm trailing the primary star. Cabrit et al. noted the resemblance between this extended structure and the tidal arm stripped from ...the primary star in the simulations of star–disc encounters by Clarke & Pringle. In this paper, we use new hydrodynamical models and synthetic observations to fit many of the parameters of RW Aur. Using hydrodynamic models, we find that the morphological appearance of RW Aur can be indeed explained by a tidal encounter with the secondary star. We reproduce all the major morphological and kinematic features of the system. Using radiative transfer calculations, we find that synthetic CO and dust continuum observations of our hydrodynamic models agree well with observations. We reproduce all the main features of the line profiles, from emission fluxes to the optical depth of the different components of the system. The agreement between observations and simulations thus lends strong support to the hypothesis of a tidal encounter scenario. Finally, we propose a possible solution for the origin of the dimming of the primary star observed in 2010/2011 by Rodriguez et al.
Abstract
We model the radiatively driven flow from IM Lup – a large protoplanetary disc expected to be irradiated by only a weak external radiation field (at least 104 times lower than the ...ultraviolet field irradiating the Orion Nebula Cluster proplyds). We find that material at large radii (>400 au) in this disc is sufficiently weakly gravitationally bound that significant mass-loss can be induced. Given the estimated values of the disc mass and accretion rate, the viscous time-scale is long (∼10 Myr) so the main evolutionary behaviour for the first Myr of the disc's lifetime is truncation of the disc by photoevaporation, with only modest changes effected by viscosity. We also produce approximate synthetic observations of our models, finding substantial emission from the flow that can explain the CO halo observed about IM Lup out to ≥1000 au. Solutions that are consistent with the extent of the observed CO emission generally imply that IM Lup is still in the process of having its disc outer radius truncated. We conclude that IM Lup is subject to substantial external photoevaporation, which raises the more general possibility that external irradiation of the largest discs can be of significant importance even in low mass star forming regions.
Planet-forming disc evolution is not independent of the star formation and feedback process in giant molecular clouds. In particular, OB stars emit UV radiation that heats and disperses discs in a ...process called ‘external photoevaporation’. This process is understood to be the dominant environmental influence acting on planet-forming discs in typical star-forming regions. Our best studied discs are nearby, in sparse stellar groups where external photoevaporation is less effective. However, the majority of discs are expected to reside in much stronger UV environments. Understanding external photoevaporation is therefore key to understanding how most discs evolve, and hence, how most planets form. Here, we review our theoretical and observational understanding of external photoevaporation. We also lay out key developments for the future to address existing unknowns and establish the full role of external photoevaporation in the disc evolution and planet formation process.
ABSTRACT
We couple star cluster formation and feedback simulations of a Carina-like star-forming region with 1D disc evolutionary models to study the impact of external photoevaporation on disc ...populations in massive star-forming regions. To investigate the effect of shielding of young stellar objects by star-forming material, we track the far-ultraviolet (FUV) field history at each star in the cluster with two methods: (i) Monte Carlo radiative transfer accounting for the shielding of stars from the FUV by the star-forming cloud and (ii) geometric dilution of the radiation from other stars, which ignores shielding effects. We found that significant shielding only occurs for a small fraction of discs and offers protection from external photoevaporation for <0.5 Myr. However, this initial protection can prevent significant early gas/dust mass-loss and disc radius reduction due to external photoevaporation. Particularly, shielding for 0.5 Myr is sufficient for much of the solid reservoir to evolve to larger sizes where it will not be entrained in an external wind. Shielding is therefore potentially significant for terrestrial planet formation in retaining the solid mass budget, but the continued stripping of gas when shielding ends could still impact migration and the gas reservoir for giant planet atmospheres. Our models highlight issues with treating all discs in a cluster with a single characteristic age, since shielded objects are typically only the youngest. Our model predicts that the majority of discs in a 2 Myr Carina-like environment are subject to strong external photoevaporation.
We investigate the effect of including diffuse field radiation when modelling the radiatively driven implosion of a Bonnor-Ebert sphere (BES). Radiation-hydrodynamical calculations are performed by ...using operator splitting to combine Monte Carlo photoionization with grid-based Eulerian hydrodynamics that includes self-gravity. It is found that the diffuse field has a significant effect on the nature of radiatively driven collapse which is strongly coupled to the strength of the driving shock that is established before impacting the BES. This can result in either slower or more rapid star formation than expected using the on-the-spot approximation depending on the distance of the BES from the source object. As well as directly compressing the BES, stronger shocks increase the thickness and density in the shell of accumulated material, which leads to short, strong, photoevaporative ejections that reinforce the compression whenever it slows. This happens particularly effectively when the diffuse field is included as rocket motion is induced over a larger area of the shell surface. The formation and evolution of 'elephant trunks' via instability is also found to vary significantly when the diffuse field is included. Since the perturbations that seed instabilities are smeared out elephant trunks form less readily and, once formed, are exposed to enhanced thermal compression.
ABSTRACT
We demonstrate the utility of C i as a tracer of photoevaporative winds that are being driven from discs by their ambient UV environment. Commonly observed CO lines only trace these winds in ...relatively weak UV environments and are otherwise dissociated in the wind at the intermediate to high UV fields that most young stars experience. However, C i traces unsubtle kinematic signatures of a wind in intermediate UV environments (∼1000 G0) and can be used to place constraints on the kinematics and temperature of the wind. In C i position–velocity (PV) diagrams external photoevaporation results in velocities that are faster than those from Keplerian rotation alone, as well as emission from quadrants of PV space in which there would be no Keplerian emission. This is independent of viewing angle because the wind has components that are perpendicular to the azimuthal rotation of the disc. At intermediate viewing angles (∼30–60°) moment 1 maps also exhibit a twisted morphology over large scales (unlike other processes that result in twists, which are typically towards the inner disc). C i is readily observable with ALMA, which means that it is now possible to identify and characterize the effect of external photoevaporation on planet-forming discs in intermediate UV environments.
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