X-ray photoevaporation’s limited success in the formation of planetesimals by the streaming instability

X-ray photoevaporation’s limited success in the formation of planetesimals by the streaming instability

Authors:

Ercolano et al

Abstract:

The streaming instability is often invoked as solution to the fragmentation and drift barriers in planetesimal formation, catalysing the aggregation of dust on kyr time-scales to grow km-sized cores. However, there remains a lack of consensus on the physical mechanism(s) responsible for initiating it. One potential avenue is disc photoevaporation, wherein the preferential removal of relatively dust-free gas increases the disc metallicity. Late in the disc lifetime, photoevaporation dominates viscous accretion, creating a gradient in the depleted gas surface density near the location of the gap. This induces a local pressure maximum that collects drifting dust particles, which may then become susceptible to the streaming instability. Using a one-dimensional viscous evolution model of a disc subject to internal X-ray photoevaporation, we explore the efficacy of this process to build planetesimals. Over a range of parameters, we find that the amount of dust mass converted into planetesimals is often <1 a="" across="" an="" and="" are="" at="" au.="" be="" best="" common="" conclude="" considered="" contrast="" cores.="" debris="" discs="" driven="" far-ultra-violet="" few="" for="" formation="" in="" investigation="" large="" m="" may="" mechanism="" model="" most="" of="" our="" photoevaporation="" planetary="" rather="" recent="" relevant="" reported="" results="" similar="" spread="" tens="" than="" that="" the="" to="" we="">100 au) disc radii. The discrepancies are primarily a consequence of the different photoevaporation profiles assumed. Until observations more tightly constrain photoevaporation models, the relevance of this process to the formation of planets remains uncertain.

Radiation Hydrodynamics Simulations of Photoevaporation of Protoplanetary Disks: Metallicity Dependence

Radiation Hydrodynamics Simulations of Photoevaporation of Protoplanetary Disks: Metallicity Dependence

Authors:
Nakatani et al

Abstract:

Protoplanetary disks are thought to have lifetimes of 3−6 million years in the solar neighborhood, but recent observations suggest that the disk lifetimes are shorter in a low metallicity environment. We perform a suite of radiation hydrodynamics simulations of photoevaporation of protoplanetary disks to study the disk structure and its long-term evolution of ∼10000 years, and the metallicity dependence of mass-loss rate. Our simulations follow hydrodynamics, extreme and far ultraviolet radiative transfer, and non-equilibrium chemistry in a self-consistent manner. Dust grain temperatures are also calculated consistently by solving the radiative transfer of the stellar irradiation and grain (re-)emission. We vary the disk gas metallicity over a wide range of 10−4Z⊙≤Z≤10 Z⊙. For our fiducial model with a 0.5 M⊙ central star with solar metallicity, the time-averaged photoevaporation rate is M˙ph=1.38×10−8M⊙yr−1. The photoevaporation rate is lower with higher metallicity in the range of 10−0.5Z⊙≲Z≲10 Z⊙, because dust shielding effectively prevents far-ultra violet (FUV) photons from penetrating into and heating the dense regions of the disk. The photoevaporation rate sharply declines at even lower metallicities in 10−1Z⊙≲Z≲10−0.5Z⊙, because FUV photoelectric heating is not efficient any more to raise the gas temperature and to drive outflows. At 10−4Z⊙≤Z≲10−1Z⊙, HI photoionization heating acts as a dominant gas heating process and drives photoevaporative flows with roughly a constant rate. The typical disk lifetime is shorter at Z=0.3 Z⊙ than at Z=Z⊙, being consistent with recent observations of the extreme outer galaxy.

Hydrodynamic Photoevaporation of Protoplanetary Disks with Consistent Thermochemistry

Hydrodynamic Photoevaporation of Protoplanetary Disks with Consistent Thermochemistry

Authors:

Wang et al

Abstract:
Photoevaporation is an important dispersal mechanism for protoplanetary disks. We conduct hydrodynamic simulations coupled with ray-tracing radiative transfer and consistent thermochemistry to study photoevaporative winds driven by ultraviolet and X-ray radiation from the host star. Most models have a three-layer structure: a cold midplane, warm intermediate layer, and hot wind, the last having typical speeds ∼30 km s−1 and mass-loss rates ∼10−9 M⊙ yr−1 when driven primarily by ionizing UV radiation. Observable molecules including CO, OH and H2O re-form in the intermediate layer and survive at relatively high wind temperatures due to reactions being out of equilibrium. Mass-loss rates are sensitive to the intensity of radiation in energy bands that interact directly with hydrogen. Comparison with previous works shows that mass loss rates are also sensitive to the treatment of both the hydrodynamics and the thermochemistry. Divergent results concerning the efficiency of X-ray photoevaporation are traced in part to differing assumptions about dust and other coolants.

Compositional imprints in density-distance-time: a rocky composition for close-in low-mass exoplanets from the location of the valley of evaporation

Compositional imprints in density-distance-time: a rocky composition for close-in low-mass exoplanets from the location of the valley of evaporation

Authors:

Jin et al

Abstract:
We use a theoretical end-to-end model that includes planet formation, thermodynamic evolution, and atmospheric escape to investigate how the statistical imprints of evaporation depend on the bulk composition of the planetary cores (rocky vs. icy). We find that the typical population-wide imprints of evaporation like the location of the "evaporation valley" in the distance-radius plane and the corresponding one-dimensional bimodal distribution in planetary radii are clearly different depending on the bulk composition of close-in low-mass planetary cores. Comparison with the observed position of the valley as found recently by Fulton et al. (2017) suggests that Kepler planets in this domain have a predominately Earth-like rocky composition. Combined with the excess of period ratios outside of MMR, this suggests that low-mass Kepler planets formed inside of the iceline, but still undergoing orbital migration. The core radius becomes visible for planets losing all primordial H/He. For such planets in the "triangle of evaporation" in the distance-radius plane, the degeneracy in possible compositions is reduced. In the observed a-R diagram, we identify a trend to more volatile-rich compositions with increasing planet radius and potentially distance (R/R_earth less than 1.6 rocky; 1.6-3.0 H/He and/or ices; > 3: H/He). Moreover, we find that the mass-density distribution contains important information about planet formation and evolution. Evaporation removes close-in low-mass planets with low density in the mass-density space. This causes density and orbital distance to be anti-correlated for low-mass planets, in contrast to giant planets, where closer planets are less dense, due to inflation mechanisms. The temporal evolution of the statistical properties of the population reported here will be of particular interest for the future PLATO 2.0 mission which will be able to observe the temporal dimension.

Global Instability of Exo-Moon System Triggered by Photo-Evaporation

Global Instability of Exo-Moon System Triggered by Photo-Evaporation

Authors:

Yang et al

Abstract:

Many exoplanets have been found in orbits close to their host stars and thus they are subject to the effects of photo-evaporation. Previous studies have shown that a large portion of exoplanets detected by the Kepler mission have been significantly eroded by photo-evaporation. In this paper, we numerically study the effects of photo-evaporation on the orbital evolution of a hypothesized moon system around a planet. We find that photo-evaporation is crucial to the stability of the moon system. Photo-evaporation can erode the atmosphere of the planet thus leading to significant mass loss. As the planet loses mass, its Hill radius shrinks and its moons increase their orbital semi-major axes and eccentricities. When some moons approach their critical semi-major axes, global instability of the moon system would be triggered, which usually ends up with two, one or even zero surviving moons. Some lost moons could escape from the moon system to become a new planet orbiting the star or run away further to become a free-floating object in the Galaxy. Given the destructive role of photo-evaporation, we speculate that exo-moons are less common for close-in planets (<0.1 AU), especially those around M-type stars, because they are more X-ray luminous and thus enhancing photo-evaporation. The lessons we learn in this study may be helpful for the target selection of on-going/future exomoon searching programs.

Kepler's Ultra-Short-Period Planets Formed Water-Poor

Born Dry in the Photo-Evaporation Desert: Kepler's Ultra-Short-Period Planets Formed Water-Poor

Author:

Lopez

Abstract:

Recent surveys have uncovered an exciting new population of ultra-short-period (USP) planets with orbital periods less than a day. These planets typically have radii ≲1.5 R⊕, indicating that they likely have rocky compositions. This stands in contrast to the overall distribution of planets out to ∼100 days, which is dominated by low-density sub-Neptunes above 2 R⊕, which must have gaseous envelopes to explain their size. However, on ultra-short-period orbits, planets are bombarded by intense levels of photo-ionizing radiation and consequently gaseous sub-Neptunes are extremely vulnerable to losing their envelopes to atmospheric photo-evaporation. Using models of planet evolution, I show that the rocky USP planets can easily be produced as the evaporated remnants of sub-Neptunes with H/He envelopes and that we can therefore understand the observed dearth of USP sub-Neptunes as a natural consequence of photo-evaporation. Critically however, planets on USP orbits could often retain their envelopes if they formed with very high-metallicity water dominated envelopes. Such water-rich planets would commonly be ≳2 R⊕ today, which is inconsistent with the observed evaporation desert, indicating that most USP planets likely formed from water-poor material within the snow-line. Finally, I examine the special case of 55 Cancri e and its possible composition in the light of recent observations, and discuss the prospects for further characterizing this population with future observations.

Tracing Slow Winds from T Tauri Stars via Low Velocity Forbidden Line Emission

Tracing Slow Winds from T Tauri Stars via Low Velocity Forbidden Line Emission

Authors:

Simon et al

Abstract:

Using Keck/HIRES spectra {\Delta}v ~ 7 km/s, we analyze forbidden lines of [O I] 6300 {\AA}, [O I] 5577 {\AA} and [S II] 6731 {\AA} from 33 T Tauri stars covering a range of disk evolutionary stages. After removing a high velocity component (HVC) associated with microjets, we study the properties of the low velocity component (LVC). The LVC can be attributed to slow disk winds that could be magnetically (MHD) or thermally (photoevaporative) driven. Both of these winds play an important role in the evolution and dispersal of protoplanetary material.

LVC emission is seen in all 30 stars with detected [O I] but only in 2 out of eight with detected [S II] , so our analysis is largely based on the properties of the [O I] LVC. The LVC itself is resolved into broad (BC) and narrow (NC) kinematic components. Both components are found over a wide range of accretion rates and their luminosity is correlated with the accretion luminosity, but the NC is proportionately stronger than the BC in transition disks.

The FWHM of both the BC and NC correlates with disk inclination, consistent with Keplerian broadening from radii of 0.05 to 0.5 AU and 0.5 to 5 AU, respectively. The velocity centroids of the BC suggest formation in an MHD disk wind, with the largest blueshifts found in sources with closer to face-on orientations. The velocity centroids of the NC however, show no dependence on disk inclination. The origin of this component is less clear and the evidence for photoevaporation is not conclusive.

A Photo-evaporative gap Observed in the TW Hydrae

A photo-evaporative gap in the closest planet forming disc

Authors:

Ercolano et al

Abstract:

The dispersal of the circumstellar discs of dust and gas surrounding young low- mass stars has important implications for the formation of planetary systems. Photo- evaporation from energetic radiation from the central object is thought to drive the dispersal in the majority of discs, by creating a gap which disconnects the outer from the inner regions of the disc and then disperses the outer disc from the inside-out, while the inner disc keeps draining viscously onto the star. In this Letter we show that the disc around TW Hya, the closest protoplanetary disc to Earth, may be the first object where a photoevaporative gap has been imaged around the time at which it is being created. Indeed the detected gap in the ALMA images is consistent with the expectations of X-ray photoevaporation models, thus not requiring the presence of a planet. The photoevaporation model is also consistent with a broad range of properties of the TW Hya system, e.g. accretion rate and the location of the gap at the onset of dispersal. We show that the central, unresolved 870 {\mu}m continuum source might be produced by free free emission from the gas and/or residual dust inside the gap.

IMAGING THE PHOTOEVAPORATING DISK AND RADIO JET OF GM AUR

IMAGING THE PHOTOEVAPORATING DISK AND RADIO JET OF GM AUR

Authors:

Macías et al

Abstract:

Photoevaporation is probably the main agent for gas dispersal during the last stages of protoplanetary disk evolution. However, the overall mass-loss rate in the photoevaporative wind and its driving mechanism are still not well understood. Here we report multi-configuration Very Large Array observations at 0.7, 3, and 5 cm toward the transitional disk of GM Aur. Our radio continuum observations allow us to image and spatially resolve, for the first time, the three main components at work in this stage of the disk evolution: the disk of dust, the ionized radio jet perpendicular to it, and the photoevaporative wind arising from the disk. The mass-loss rate inferred from the flux density of the radio jet is consistent with the ratio between ejection and accretion rates found in younger objects, suggesting that transitional disks can power collimated ejections of material apparently following the same physical mechanisms as much younger protostars. Our results indicate that extreme-UV (EUV) radiation is the main ionizing mechanism of the photoevaporative wind traced by the free–free emission. The required low EUV photon luminosity of ~6 × 1040 s−1 would produce a photoevaporation rate of only ${\dot{M}}_{w,\mathrm{EUV}}\simeq 1.3\times {10}^{-10}\,{M}_{\odot }$ yr−1. Therefore, other mechanisms are required to disperse the disk in the timescale imposed by observations.

On dust entrainment in photoevaporative winds

On dust entrainment in photoevaporative winds

Authors:

Hutchison et al

Abstract:

We investigate dust entrainment by photoevaporative winds in protoplanetary discs using dusty smoothed particle hydrodynamics. We use unequal-mass particles to resolve more than five orders of magnitude in disc/outflow density and a one-fluid formulation to efficiently simulate an equivalent magnitude range in drag stopping time. We find that only micron-sized dust grains and smaller can be entrained in extreme-UV radiation-driven winds. The maximum grain size is set by dust settling in the disc rather than aerodynamic drag in the wind. More generally, there is a linear relationship between the base flow density and the maximum entrainable grain size in the wind. A pileup of micron-sized dust grains can occur in the upper atmosphere at critical radii in the disc as grains decouple from the low-density wind. Entrainment is a strong function of location in the disc, resulting in a size sorting of grains in the outflow – the largest grain being carried out between 10 and 20 au. The peak dust density for each grain size occurs at the inner edge of its own entrainment region.

Blueshifted [OI] lines from protoplanetary discs: the smoking gun of X-ray photoevaporation

Blueshifted [OI] lines from protoplanetary discs: the smoking gun of X-ray photoevaporation

Authors:

Ercolano et al

Abstract:

Photoevaporation of protoplanetary discs by high energy radiation from the central young stellar object is currently the favourite model to explain the sudden dispersal of discs from the inside out. While several theoretical works have provided a detailed pictured of this process, the direct observational validation is still lacking. Emission lines produced in these slow moving protoplanetary disc winds may bear the imprint of the wind structure and thus provide a potential diagnostic of the underlying dispersal process. In this paper we primarily focus on the collisionally excited neutral oxygen line at 6300A. We compare our models predictions to observational data and demonstrate a thermal origin for the observed blueshifted low-velocity component of this line from protoplanetary discs. Furthermore our models show that while this line is a clear tell-tale-sign of a warm, quasi-neutral disc wind, typical of X-ray photoevaporation, its strong temperature dependence makes it unsuitable to measure detailed wind quantities like mass-loss-rate.

The Smoking gun of X-ray Photoevaporation for Protoplanetary Disks

Blueshifted [OI] lines from protoplanetary discs: the smoking gun of X-ray photoevaporation.

Authors:

Ercolanno et al

Abstract:

Photoevaporation of protoplanetary discs by high energy radiation from the central young stellar object is currently the favourite model to explain the sudden dispersal of discs from the inside out. While several theoretical works have provided a detailed pictured of this process, the direct observational validation is still lacking. Emission lines produced in these slow moving protoplanetary disc winds may bear the imprint of the wind structure and thus provide a potential diagnostic of the underlying dispersal process. In this paper we primarily focus on the collisionally excited neutral oxygen line at 6300A. We compare our models predictions to observational data and demonstrate a thermal origin for the observed blueshifted low-velocity component of this line from protoplanetary discs. Furthermore our models show that while this line is a clear tell-tale-sign of a warm, quasi-neutral disc wind, typical of X-ray photoevaporation, its strong temperature dependence makes it unsuitable to measure detailed wind quantities like mass-loss-rate.

Photoevaporation and close encounters: how the environment around Cygnus OB2 affects the evolution of protoplanetary disks

Photoevaporation and close encounters: how the environment around Cygnus OB2 affects the evolution of protoplanetary disks

Authors:

Guarcello et al

Abstract:

In our Galaxy, star formation occurs in a variety of environments, with a large fraction of stars formed in clusters hosting massive stars. OB stars have an important feedback on the evolution of protoplanetary disks around nearby young stars and likely on the process of planet formation occurring in them. The nearby massive association Cygnus OB2 is an outstanding laboratory to study this feedback. It is the closest massive association to our Sun, and hosts hundreds of massive stars and thousands of low mass members. In this paper, we analyze the spatial variation of the disk fraction in Cygnus OB2 and we study its correlation with the local values of Far and Extreme ultraviolet radiation fields and the local stellar surface density. We present definitive evidence that disks are more rapidly dissipated in the regions of the association characterized by intense local UV field and large stellar density. In particular, the FUV radiation dominates disks dissipation timescales in the proximity (i.e. within 0.5 pc) of the O stars. In the rest of the association, EUV photons potentially induce a significant mass loss from the irradiated disks across the entire association, but the efficiency of this process is reduced at increasing distances from the massive stars due to absorption by the intervening intracluster material. We find that disk dissipation due to close stellar encounters is negligible in Cygnus OB2, and likely to have affected 1% or fewer of the stellar population. Disk dissipation is instead dominated by photoevaporation. We also compare our results to what has been found in other young clusters with different massive populations, concluding that massive associations like Cygnus OB2 are potentially hostile to protoplanetary disks, but that the environments where disks can safely evolve in planetary systems are likely quite common in our Galaxy.

How dusty are photoevaporative winds?

How dusty are photoevaporative winds?

Authors:

Hutchinson et al

Abstract:

Not very. We perform dusty smoothed particle hydrodrodynamic (SPH) calculations of photoevaporation in protoplanetary discs. We use unequal-mass particles to resolve more than five orders of magnitude in disc/outflow density and a one-fluid formulation to efficiently simulate an equivalent magnitude range in drag stopping time. We find that only micron sized dust grains and smaller can be entrained in photoevaporative winds. This makes it difficult to explain the dust holes seen in transition discs using photoevaporation and implies that only small grains can be transported to the outer disc by this mechanism. A pileup of micron sized dust grains can occur in the upper atmosphere at critical radii in the disc as grains decouple from the low-density wind. Entrainment is a strong function of location in the disc, resulting in a size sorting of grains in the outflow---the largest grain being carried out between 10--20AU. The peak dust density for each grain occurs at the inner edge of its entrainment region.

A Plane-Parallel Wind Solution For Testing Numerical Simulations of Photoevaporation

A Plane-Parallel Wind Solution For Testing Numerical Simulations of Photoevaporation

Authors:

Hutchinson et al

Abstract:

Here we derive a Parker-wind like solution for a stratified, plane-parallel atmosphere undergoing photoionisation. The difference compared to the standard Parker solar wind is that the sonic point is crossed only at infinity. The simplicity of the analytic solution makes it a convenient test problem for numerical simulations of photoevaporation in protoplanetary discs.

The Debris Disk Fraction for M-dwarfs in Nearby, Young, Moving Groups

The Debris Disk Fraction for M-dwarfs in Nearby, Young, Moving Groups

Author:

Binks

Abstract:

I present the first substantial work to measure the fraction of debris disks for M-dwarfs in nearby moving groups (MGs). Utilising the AllWISE IR catalog, 17 out of 151 MG members are found with an IR photometric excess indicative of disk structure. The M-dwarf debris disk fraction is ≲6 per cent in MGs younger than 40 Myr, and none are found in the groups older than 40 Myr. Simulations show, however, that debris disks around M-dwarfs are not present above a WISE W1−W4 colour of ∼2.5, making calculating the absolute disk fractions difficult. The debris disk dissipation timescale appears to be faster than for higher-mass stars, and mechanisms such as enhanced stellar wind drag and/or photoevaporation could account for the more rapid decline of disks observed amongst M-dwarfs.

External photoevaporation of protoplanetary discs in sparse stellar groups: the impact of dust growth

External photoevaporation of protoplanetary discs in sparse stellar groups: the impact of dust growth

Authors:

Facchini et al

Abstract:

We estimate the mass loss rates of photoevaporative winds launched from the outer edge of protoplanetary discs impinged by an ambient radiation field. We focus on mild/moderate environments (the number of stars in the group/cluster is N ~ 50), and explore disc sizes ranging between 20 and 250 AU. We evaluate the steady-state structures of the photoevaporative winds by coupling temperature estimates obtained with a PDR code with 1D radial hydrodynamical equations. We also consider the impact of dust dragging and grain growth on the final mass loss rates. We find that these winds are much more significant than have been appreciated hitherto when grain growth is included in the modelling: in particular, mass loss rates > 1e-8 M_sun/yr are predicted even for modest background field strengths ( ~ 30 G_0) in the case of discs that extend to R > 150 AU. Grain growth significantly affects the final mass loss rates by reducing the average cross section at FUV wavelengths, and thus allowing a much more vigorous flow. The radial profiles of observable quantities (in particular surface density, temperature and velocity patterns) indicate that these winds have characteristic features that are now potentially observable with ALMA. In particular, such discs should have extended gaseous emission that is dust depleted in the outer regions, characterised by a non-Keplerian rotation curve, and with a radially increasing temperature gradient.

The Correlation of Host Star Classification and the PhotoEvaporation of hot Jupiters

A bimodal correlation between host star chromospheric emission and the surface gravity of hot Jupiters

Authors:

Fossati et al

Abstract:

The chromospheric activity index logR'HK of stars hosting transiting hot Jupiters appears to be correlated with the planets' surface gravity. One of the possible explanations is based on the presence of condensations of planetary evaporated material located in a circumstellar cloud that absorbs the CaII H&K and MgII h&k resonance line emission flux, used to measure chromospheric activity. A larger column density in the condensations, or equivalently a stronger absorption in the chromospheric lines, is obtained when the evaporation rate of the planet is larger, which occurs for a lower gravity of the planet. We analyze here a sample of stars hosting transiting hot Jupiters tuned in order to minimize systematic effects (e.g., interstellar medium absorption). Using a mixture model, we find that the data are best fit by a two-linear-regression model. We interpret this result in terms of the Vaughan-Preston gap. We use a Monte Carlo approach to best take into account the uncertainties, finding that the two intercepts fit the observed peaks of the distribution of logR'HK for main-sequence solar-like stars. We also find that the intercepts are correlated with the slopes, as predicted by the model based on the condensations of planetary evaporated material. Our findings bring further support to this model, although we cannot firmly exclude different explanations. A precise determination of the slopes of the two linear components would allow one to estimate the average effective stellar flux powering planetary evaporation, which can then be used for theoretical population and evolution studies of close-in planets.

Constraining X Ray Driven Photoevaporation in Protoplanetary Disks Around M Dwarfs

Constraining X-ray-Induced Photoevaporation of Protoplanetary Disks Orbiting Low-Mass Stars

Authors:

Punzi et al

Abstract:

Low-mass, pre-main sequence stars possess intense high-energy radiation fields as a result of their strong stellar magnetic activity. This stellar UV and X-ray radiation may have a profound impact on the lifetimes of protoplanetary disks. We aim to constrain the X-ray-induced photoevaporation rates of protoplanetary disks orbiting low-mass stars by analyzing serendipitous XMM-Newton and Chandra X-ray observations of candidate nearby (D less than 100 pc), young (age < 100 Myr) M stars identified in the GALEX Nearby Young-Star Survey (GALNYSS).

Exoplanet Densities Determined by Impacts, not PhotoEvaporation

Stealing the Gas: Giant Impacts and the Large Diversity in Exoplanet Densities

Authors:

Inamdar et al

Abstract:

Although current sensitivity limits are such that true Solar System analogs remain challenging to detect, numerous planetary systems have been discovered that are very different from our own Solar System. The majority of systems harbor a new class of planets, bodies that are typically several times more massive than the Earth but that orbit their host stars well inside the orbit of Mercury. These planets frequently show evidence for large Hydrogen and Helium envelopes containing several percent of the planet's mass and display a large diversity in mean densities. Here we show that this wide range can be achieved by one or two late giant impacts, which are frequently needed to achieve long-term orbital stability in multiple planet systems once the gas disk has disappeared. We demonstrate using hydrodynamical simulations that a single collision between similarly sized exoplanets can easily reduce the envelope-to-core-mass ratio by a factor of two and show that this leads to a corresponding increase in the observed mean density by factors of 2-3. In addition we investigate how envelope-mass-loss depends on envelope mass, planet radius, semi-major axis, and the mass distribution inside the envelope. We propose that a small number of giant impacts may be responsible for the large observed spread in mean densities, especially for multiple-planet systems containing planets with very different densities and which have not been significantly sculpted by photo evaporation.