Using simultaneous mass accretion and external photoevaporation rates for d203-504 to constrain disc evolution processes

TL;DR

The paper addresses how protoplanetary disc evolution is governed by angular momentum transport and dispersal processes by exploiting the first system, d203-504, with simultaneous measurements of the accretion rate $\dot{M}_{\rm acc}$ and the external photoevaporation rate $\dot{M}_{\rm E,FUV}$. It combines 1D disc evolution with either viscous transport ($\alpha_{\nu}$) or MHD wind losses ($\alpha_{\rm DW}$), plus internal and external photoevaporation computed via the FRIED framework, to reproduce the observed disc radius $r_d$, gas mass, and the two rates. The study finds that both viscous and MHD wind–driven discs can match the data, but MHD wind discs constrain $\alpha_{\rm DW}$ to roughly $2\times10^{-3}$–$10^{-2}$ (for shielding scenarios) and $\alpha_{\nu}$ to $\sim 3\times10^{-4}$–$2\times10^{-3}$ (no shielding, when viable), with a preferred initial disc mass near $0.05$–$0.09\,M_\odot$ and initial radii around tens of au. Importantly, the results imply a short irradiation age after exposure to the external UV field, offering a resolution to the proplyd lifetime problem and highlighting the diagnostic value of combining accretion and external photoevaporation measurements for constraining disc evolution physics and wind microphysics, including PAH-to-gas considerations that affect mass loss rates. The approach demonstrates how targeted observations of a single system can yield tight constraints on disc evolution parameters and motivate broader surveys in Orion and similar environments.

Abstract

We cannot understand planet formation without understanding disc evolutionary processes. However, there is currently ambiguity about how protoplanetary discs transport angular momentum (e.g. via viscosity or winds) and the relative contributions and interplay of different dispersal mechanisms. A key difficulty is that for any given system only a handful of disc parameters are usually available to constrain theoretical models. Recent observations of the d203-504 disc in Orion, have yielded values of the stellar accretion rate, external photoevaporative mass loss rate, stellar mass and the disc size and mass. In particular, having the combination of accretion rate and external photoevaporative rate is new. Using this unique combination of observables, we run a suite of disc evolution simulations to constrain which scenarios can match the observed values. We explore both viscous and MHD wind-driven discs, finding that they best match observations when the angular momentum transport $α$ parameter is $3\times10^{-4}\leqα_ν\leq2\times10^{-3}$ for viscous discs, and $2\times10^{-3}\leqα_{\rm DW}\leq10^{-2}$ for MHD wind-driven discs, consistent with other estimates in the literature. As well constraining the disc properties and evolution, the d203-504 disc allows us to define a new irradiation age, since in order to match observations, it was required that the disc had only just appeared in the extreme UV environment it is currently exposed to (a known issue for proplyds referred to as the proplyd lifetime problem). This indicates that it is either very young, i.e. <0.1 Myr, or it has been shielded until recently, which would have protected the planet forming reservoir and helped facilitate planet growth despite it now residing in a harsh UV environment.

Figures (10)

• Figure 1: Temporal evolution of the disc radius (top-left), disc mass (bottom-left), accretion rate (top-right) and the external photoevaporation rate (bottom-right) for the simulation that best fits the observed values. The disc had a shielding time $t_{\rm sh}=0$ Myr, the initial scale radius $r_{\rm C}=50\, {\rm au}$, the initial disc mass $M_{\rm d}=0.057\, {\rm M}_{\odot}$, and $\alpha=3\times10^{-3}$. The dashed horizontal line denotes the observed values for d203-504, whilst the dotted vertical line shows the time at which the simulation best fits the observations.
• Figure 2: Contour plot showing the disc mass for at the time of best fit to the observed data for d203-504. The white contours highlight regions where the differences in log space between the simulations and observations are less than 0.041 (solid), 0.079 (dashed), 0.176 (dot-dashed) and 0.3 (dotted). These values correspond to differences of factors: 1.1, 1.2, 1.5 and 2 respectively. The red lines shows the same as the white lines, except the observed accretion rates are not compared with the simulations, highlighting the importance of knowing the accretion rate for determining compatible values of $\alpha$.
• Figure 3: Same as Fig. \ref{['fig:contour_wind_50_0']}, but for different shielding times: 0.1 Myr (left-hand panel), 0.3 Myr (middle panel), and 0.5 Myr (right-hand panel).
• Figure 4: Contours showing the locations of simulations that best fit observations for different $\alpha_{\rm DW}$ and disc masses. Different colours show different initial scale radii of 20$\, {\rm au}$ (blue), 30$\, {\rm au}$ (red), 40$\, {\rm au}$ (yellow) and 50$\, {\rm au}$ (green). Solid contours show regions where the simulation matched observations to within a factor 1.1 whilst dashed-dotted contours show for differences of factor 1.5. The different panels differentiate between different shielding times of: 0 Myr (top-left), 0.1 Myr (top-right), 0.3 Myr (bottom-left) and 0.5 Myr (bottom-right).
• Figure 5: Cumulative distribution function showing the time in a disc lifetime that best fits observed values. Time since irradiation is determined relative to the shielding time. Blue lines shows cumulative distribution functions for best fit instances with differences of up to factor 1.5, whilst red shows for differences up to factor 1.1, and yellow lines show for differences of up to factor 1.05.