Dispersal Of Protoplanetary Disks Research Articles

Testing protoplanetary disc dispersal with radio emission

We consider continuum free–free radio emission from the upper atmosphere of protoplanetary discs as a probe of the ionized luminosity impinging upon the disc. Making use of previously computed hydrodynamic models of disc photoevaporation within the framework of extreme-ultraviolet (EUV) and X-ray irradiation, we use radiative transfer post-processing techniques to predict the expected free–free emission from protoplanetary discs. In general, the free–free luminosity scales roughly linearly with ionizing luminosity in both EUV- and X-ray-driven scenarios, where the emission dominates over the dust tail of the disc and is partial optically thin at cm wavelengths. We perform a test observation of GM Aur at 14–18 GHz and detect an excess of radio emission above the dust tail to a very high level of confidence. The observed flux density and spectral index are consistent with free–free emission from the ionized disc in either the EUV- or the X-ray-driven scenario. Finally, we suggest a possible route to testing the EUV- and X-ray-driven dispersal model of protoplanetary discs, by combining observed free–free flux densities with measurements of mass-accretion rates. On the point of disc dispersal one would expect to find an |$\dot{M}_*^2$| scaling with free–free flux in the case of EUV-driven disc dispersal or an Ṁ* scaling in the case of X-ray-driven disc dispersal.

The dispersal of protoplanetary discs

Protoplanetary discs are a natural consequence of the star formation process and as such are ubiquitous around low-mass stars. They are fundamental to planet formation as they hold the reservoir of material from which planets form. Their evolution and final dispersal and the timescales that regulate these process are therefore of particular interest. In this contribution I will review the observational evidence for the dispersal of discs being dominated by two timescales and for the final dispersal to occur quickly and from the inside out. I will discuss the current theoretical models, including X-ray photoevaporation, showing that the latter provides a natural explanation to the observed behaviour and review supporting and contrasting evidence. I will finally introduce a new mechanism based on the interaction between planet formation and photoevaporation that may explain a particular class of transition discs with large inner holes and high accretion rates that are problematic for photoevaporation models and planet formation models alone.

FREE-FREE EMISSION AND RADIO RECOMBINATION LINES FROM PHOTOEVAPORATING DISKS

Recent infrared observations have demonstrated that photoevaporation driven by high-energy photons from the central star contributes to the dispersal of protoplanetary disks. Here, we show that photoevaporative winds should produce a detectable free-free continuum emission given the range of stellar ionizing photons and X-ray luminosities inferred for young sun-like stars. We point out that VLA observations of the nearby disk around TWHya might have already detected this emission at centimeter wavelengths and calculate the wind electron density and mass flow rate. We also estimate the intensities of H radio recombination lines tracing the wind and discuss which ones could be detected with current instrumentation. The detection and profiles of these recombination lines would unambiguously prove our inference of free-free emission from photoevaporating disks like TWHya. In addition, radio/millimeter data can help constraining wind parameters such as temperature and electron density that are fundamental in measuring mass flow rates.

On the theory of disc photoevaporation

We discuss a hydrodynamical model for the dispersal of protoplanetary discs around young, low mass (<1.5 M_sun) stars by photoevaporation from the central object's energetic radiation, which considers the far-ultraviolet as well as the X-ray component of the radiation field. We present analytical scaling relations and derive estimates for the total mass-loss rates, as well as discussing the existence of similarity solutions for flows from primordial discs and discs with inner holes. Furthermore, we perform numerical calculations, which span a wide range of parameter space and allow us to provide accurate scalings of the mass-loss rates with the physical parameters of the systems (X-ray and FUV luminosity, stellar mass, disc mass, disc temperature and inner hole radius). The model suggest that the X-ray component dominates the photoevaporative mass-loss rates from the inner disc. The mass-loss rates have values in the range from 10e-11 to 10e-7 M_sun/yr and scale linearly with X-ray luminosity, with only a weak dependence on the other parameters explored. However, in the case of high FUV to X-ray (L_FUV/L_X>100) luminosity ratios, the FUV constricts the X-ray flow and may dominate the mass-loss. Simulations of low mass discs with inner holes demonstrate a further stage of disc clearing, which we call `thermal sweeping'. This process occurs when the mid-plane pressure drops to sufficiently low values. At this stage a bound, warm, X-ray heated region becomes sufficiently large and unstable, such that the remaining disc material is cleared on approximately dynamical time-scales. This process significantly reduces the time taken to clear the outer regions of the disc, resulting in an expected transition disc population that will be dominated by accreting objects, as indicated by recent observations.

THE ROLE OF MULTIPLICITY IN DISK EVOLUTION AND PLANET FORMATION

The past decade has seen a revolution in our understanding of protoplanetary disk evolution and planet formation in single star systems. However, the majority of solar-type stars form in binary systems, so the impact of binary companions on protoplanetary disks is an important element in our understanding of planet formation. We have compiled a combined multiplicity/disk census of Taurus-Auriga, plus a restricted sample of close binaries in other regions, in order to explore the role of multiplicity in disk evolution. Our results imply that the tidal influence of a close (<40 AU) binary companion significantly hastens the process of protoplanetary disk dispersal, as ~2/3 of all close binaries promptly disperse their disks within <1 Myr after formation. However, prompt disk dispersal only occurs for a small fraction of wide binaries and single stars, with ~80%-90% retaining their disks for at least ~2--3 Myr (but rarely for more than ~5 Myr). Our new constraints on the disk clearing timescale have significant implications for giant planet formation; most single stars have 3--5 Myr within which to form giant planets, whereas most close binary systems would have to form giant planets within <1 Myr. If core accretion is the primary mode for giant planet formation, then gas giants in close binaries should be rare. Conversely, since almost all single stars have a similar period of time within which to form gas giants, their relative rarity in RV surveys indicates either that the giant planet formation timescale is very well-matched to the disk dispersal timescale or that features beyond the disk lifetime set the likelihood of giant planet formation.

Protoplanetary disc evolution and dispersal: the implications of X-ray photoevaporation

(Abridged) We explore the role of X-ray photoevaporation in the evolution and dispersal of viscously evolving T-Tauri discs. We show that the X-ray photoevaporation wind rates scale linearly with X-ray luminosity, such that the observed range of X-ray luminosities for solar-type T-Tauri stars (10e28-10e31 erg\s) gives rise to vigorous disc winds with rates of order 10e-10-10e-7 M_sun/yr. We use the wind solutions from radiation-hydrodynamic models, coupled to a viscous evolution model to construct a population synthesis model so that we may study the physical properties of evolving discs and so-called `transition discs'. Current observations of disc lifetimes and accretion rates can be matched by our model assuming a viscosity parameter alpha = 2.5e-3. Our models confirm that X-rays play a dominant role in the evolution and dispersal of protoplanetary discs giving rise to the observed diverse population of inner hole `transition' sources which include those with massive outer discs, those with gas in their inner holes and those with detectable accretion signatures. To help understand the nature of observed transition discs we present a diagnostic diagram based on accretion rates versus inner hole sizes that demonstrate that, contrary to recent claims, many of the observed accreting and non accreting transition discs can easily be explained by X-ray photoevaporation. Finally, we confirm the conjecture of Drake et al. (2009), that accretion is suppressed by the X-rays through `photoevaporation starved accretion' and predict this effect can give rise to a negative correlation between X-ray luminosity and accretion rate, as reported in the Orion data.

Dispersal of protoplanetary disks by wind stripping

We present a model for the dispersal of protoplanetary disks by winds from either the central star or the inner disk. These winds obliquely strike the flaring disk surface and strip away disk material by entraining it in an outward radial-moving flow at the disk-wind interface. This interface lies several disk scale heights above the mid plane. The disk dispersal time depends on the velocity at which disk material flows into the mixing layer. If this velocity is ~10% of the sound speed, the disk dispersal time at ~1–10 AU is ~5 Myr for a 0.01 M ๏ disk around a solar mass star, with a spherical wind launched from the inner disk or central star with a typical mass loss rate of 10 -8 M ๏ yr -1 and terminal velocity of v w = 100 kms -1 . We conclude that wind stripping is not a dominant disk dispersal mechanism compared with viscous accretion and photoevaporation. Nevertheless, wind stripping may affect the evolution of the intermediate disk regions.

DISPERSAL OF PROTOPLANETARY DISKS BY CENTRAL WIND STRIPPING

We present a model for the dispersal of protoplanetary disks by winds from either the central star or the inner disk. These winds obliquely strike the flaring disk surface and strip away disk material by entraining it in an outward radial-moving flow at the wind-disk interface, which lies several disk scale heights above the midplane. The disk dispersal time depends on the entrainment velocity, vd = cs , at which disk material flows into this turbulent shear layer interface, where is a scale factor and cs is the local sound speed in the disk surface just below the entrainment layer. If ~ 0.1, a likely upper limit, the dispersal time at 1 AU is ~6 Myr for a disk with a surface density of 103 g cm–2, a solar mass central star, and a wind with an outflow rate and terminal velocity vw = 200kms–1. When compared with photoevaporation and viscous evolution, wind stripping can be a dominant mechanism only for the combination of low accretion rates (10–8 M ☉ yr–1) and wind outflow rates approaching these accretion rates. This case is unusual since generally outflow rates are 0.1 of accretion rates.