TW Hya Research Articles

Dust Unveils the Formation of a Mini-Neptune Planet in a Protoplanetary Ring

Abstract Rings and radial gaps are ubiquitous in protoplanetary disks, yet their possible connection to planet formation is currently subject to intense debates. In principle, giant planet formation leads to wide gaps that separate the gas and dust mass reservoir in the outer disk, while lower mass planets lead to shallow gaps that are manifested mainly on the dust component. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the star HD 169142, host to a prominent disk with deep, wide gaps that sever the disk into inner and outer regions. The new ALMA high-resolution images allow for the outer ring to be resolved as three narrow rings. The HD 169142 disk thus hosts both the wide gap trait of transition disks and a narrow-ring system similar to those observed in the TW Hya and HL Tau systems. The mass reservoir beyond a deep gap can thus host ring systems. The observed rings are narrow in radial extent (width/radius of 1.5/57.3, 1.8/64.2, and 3.4/76.0 in au) and have asymmetric mutual separations: the first and middle ring are separated by 7 au while the middle and outermost ring are distanced by ∼12 au. Using hydrodynamical modeling we found that a simple explanation, involving a single migrating low-mass planet (10 M ⊕), entirely accounts for such an apparently complex phenomenon. Inward migration of the planet naturally explains the ring’s asymmetric mutual separation. The isolation of HD 169142's outer rings thus allows a proof of concept to interpret the detailed architecture of the outer region of protoplanetary disks with low-mass planet formation of mini-Neptune’s size, i.e., as in the protosolar nebula.

Discovery of An au-scale Excess in Millimeter Emission from the Protoplanetary Disk around TW Hya

Abstract We report the detection of an excess in dust continuum emission at 233 GHz (1.3 mm in wavelength) in the protoplanetary disk (PPD) around TW Hya revealed through high-sensitivity observations at ∼3 au resolution with the Atacama Large Millimeter/submillimeter Array. The sensitivity of the 233 GHz image has been improved by a factor of 3 with regard to that of our previous cycle 3 observations. The overall structure is mostly axisymmetric, and there are apparent gaps at 25 and 41 au as previously reported. The most remarkable new finding is a few astronomical-unit-scale excess emission in the southwest part of the PPD. The excess emission is located at 52 au from the disk center and is 1.5 times brighter than the surrounding PPD at a significance of 12σ. We performed a visibility fitting to the extracted emission after subtracting the axisymmetric PPD emission and found that the inferred size and the total flux density of the excess emission are 4.4 × 1.0 au and 250 μJy, respectively. The dust mass of the excess emission corresponds to 0.03 M ⊕ if a dust temperature of 18 K is assumed. Because the excess emission can also be marginally identified in the Band 7 image at almost the same position, the feature is unlikely to be a background source. The excess emission can be explained by a dust clump accumulated in a small elongated vortex or a massive circumplanetary disk around a Neptune-mass-forming planet.

The Anomalously Low (Sub)Millimeter Spectral Indices of Some Protoplanetary Disks May Be Explained By Dust Self-scattering

Abstract Previous (sub)millimeter observations have found that the spectral indices of dust emission from some young stellar objects are lower than that of the blackbody emission in the Rayleigh–Jeans limit (i.e., 2.0). In particular, the recent Atacama Large Millimeter Array observations have spatially resolved that the innermost regions of the protoplanetary disks TW Hya and HD 163296 present anomalously low (i.e., <2.0) millimeter spectral indices. In some previous works, such anomalously low millimeter spectral indices were considered unphysical and were attributed to measurement errors. The present work clarifies that if the albedo is high and is increasing with frequency, it is possible to reproduce such anomalously low spectral indices when the emission source is optically thick. In addition, to yield lower than 2.0 spectral index at (sub)millimeter bands, the required dust maximum grain size a max is on the order of 10–100 μm, which is consistent with the previously derived a max values based on multiwavelength dust polarimetric observations. In light of this, measuring the Stokes I spectral index may also serve as an auxiliary approach for assessing whether the observed dust polarization is mainly due to dust scattering or to the aligned dust grains.

Line Ratios Reveal N2H+ Emission Originates above the Midplane in TW Hydrae

Abstract Line ratios for different transitions of the same molecule have long been used as a probe of gas temperature. Here we use ALMA observations of the N2H+ J = 1–0 and J = 4–3 lines in the protoplanetary disk around TW Hya to derive the temperature at which these lines emit. We find an averaged temperature of 39 K with a 1σ uncertainty of 2 K for the radial range 0.″8–2″, which is significantly warmer than the expected midplane temperature beyond 0.″5 in this disk. We conclude that the N2H+ emission in TW Hya is not emitting from near the midplane, but rather from higher in the disk, in a region likely bounded by processes such as photodissociation or chemical reprocessing of CO and N2 rather than freeze-out.

Spontaneous ring formation in wind-emitting accretion discs

Rings and gaps have been observed in a wide range of proto-planetary discs, from young systems like HLTau to older discs like TW Hydra. Recent disc simulations have shown that magnetohydrodynamic (MHD) turbulence (in both the ideal or non-ideal regime) can lead to the formation of rings and be an alternative to the embedded planets scenario. In this paper, we have investigated the way in which these ring form in this context and seek a generic formation process, taking into account the various dissipative regimes and magnetisations probed by the past simulations. We identify the existence of a linear and secular instability, driven by MHD winds, and giving birth to rings of gas that have a width larger than the disc scale height. We show that the linear theory is able to make reliable predictions regarding the growth rates, the contrast and spacing between ring and gap, by comparing these predictions to a series of 2D (axisymmetric) and 3D MHD numerical simulations. In addition, we demonstrate that these rings can act as dust traps provided that the disc is sufficiently magnetised, with plasma beta lower than 104. Given its robustness, the process identified in this paper could have important implications, not only for proto-planetary discs but also for a wide range of accreting systems threaded by large-scale magnetic fields.

Complex organic molecules tracing the comet-forming zones in protoplanetary disks

AbstractResolved emission from gas-phase methanol can reveal the abundance and distribution of the comet-forming ice reservoir in protoplanetary disks. ALMA Cycle 4 observations of four transitions of gas-phase methanol in TW Hya allow the first model-independent determination of the rotational temperature of methanol in a prototoplanetary disk. The data confirm that the methanol is rotationally cold (Trot < 50 K), and well constrain the column density to 2 × 1012 cm−2. Astrochemical models will constrain the chemical origin of methanol in TW Hya.

Tracing the physical conditions of planet formation with molecular excitation

AbstractUnderstanding the physical structure of the planet formation environment, the protoplanetary disk, is essential for the interpretation of high resolution observations of the dust and future observations of the magnetic field structure. Observations of multiple transitions of molecular species offers a unique view of the underlying physical structure through excitation analyses. Here we describe a new method to extract high-resolution spectra from low-resolution observations, then provide two case studies of how molecular excitation analyses were used to constrain the physical structure in TW Hya, the closest protoplanetary disk to Earth.

Stringent limits on the magnetic field strength in the disc of TW Hya

Despite their importance in the star formation process, measurements of magnetic field strength in proto-planetary discs remain rare. While linear polarisation of dust and molecular lines can give insight into the magnetic field structure, only observations of the circular polarisation produced by Zeeman splitting provide a direct measurement of magnetic field strenghts. One of the most promising probes of magnetic field strengths is the paramagnetic radical CN. Here we present the first Atacama Large Millimeter/submillimeter Array (ALMA) observations of the Zeeman splitting of CN in the disc of TW Hya. The observations indicate an excellent polarisation performance of ALMA, but fail to detect significant polarisation. An analysis of eight individual CN hyperfine components as well as a stacking analysis of the strongest (non-blended) hyperfine components yields the most stringent limits obtained so far on the magnetic field strength in a proto-planetary disc. We find that the vertical component of the magnetic field |Bz| < 0.8 mG (1σ limit). We also provide a 1σ toroidal field strength limit of <30 mG. These limits rule out some of the earlier accretion disc models, but remain consistent with the most recent detailed models with efficient advection. We detect marginal linear polarisation from the dust continuum, but the almost purely toroidal geometry of the polarisation vectors implies that his is due to radiatively aligned grains.

Upper limits on CH3OH in the HD 163296 protoplanetary disk

Context. Methanol (CH3OH) is at the root of organic ice chemistry in protoplanetary disks. Its connection to prebiotic chemistry and its role in the chemical environment of the disk midplane make it an important target for disk chemistry studies. However, its weak emission has made detections difficult. To date, gas-phase CH3OH is detected in only one Class II disk, TW Hya. Aims. We aim to constrain the methanol content of the HD 163296 protoplanetary disk. Methods. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to search for a total of four CH3OH emission lines in bands six and seven toward the disk around the young Herbig Ae star HD 163296. The disk-averaged column density of methanol and its related species formaldehyde (H2CO) were estimated assuming optically thin emission in local thermodynamic equilibrium. We compared these results to the gas-phase column densities of the TW Hya disk. Results. No targeted methanol lines were detected with Keplerian masking in the image plane nor with matched filter analysis in the uv plane individually nor after line stacking. The 3σ disk-integrated intensity upper limits are <51 mJy km s−1 for the band six lines and <26 mJy km s−1 for the band seven lines. The band seven lines provide the strictest 3σ upper limit on disk-averaged column density with Navg < 5.0 × 1011 cm−2. The methanol-to-formaldehyde ratio is CH3OH∕H2CO<0.24 in the HD 163296 disk compared to a ratio of 1.27 in the TW Hya disk. Conclusions. The HD 163296 protoplanetary disk is less abundant in methanol with respect to formaldehyde compared to the disk around TW Hya. Differences in the stellar irradiation in this Herbig Ae disk as compared to that of a disk around a T Tauri star likely influence the gaseous methanol and formaldehyde content. Possible reasons for the lower HD 163296 methanol-to-formaldehyde ratio include: a higher than expected gas-phase formation of H2CO in the HD 163296 disk, uncertainties in the grain surface formation efficiency of CH3OH and H2CO, and differences in the disk structure and/or CH3OH and H2CO desorption processes that drive the release of the molecules from ice mantles back into the gas phase. These results provide observational evidence that the gas-phase chemical complexity found in disks may be strongly influenced by the spectral type of the host star.

The Synthetic ALMA Multiband Analysis of the Dust Properties of the TW Hya Protoplanetary Disk

Abstract Analyzing multiband observations of dust continuum emission is one of the useful tools to constrain dust properties that help us to understand the physical properties of the protoplanetary disks. We perform a synthetic ALMA multiband analysis to find the best ALMA band set for constraining the dust properties of the TW Hya disk. We find that the Band [10, 6, 3] set is the best set among the possible combinations of ALMA Bands [3, 4, 5, 6, 7, 8, 9, 10]. We also find two conditions for the good ALMA band sets providing narrow constraint ranges on dust properties: (1) Band 9 or 10 is included in the band set, and (2) there are enough frequency intervals between the bands. These are related to the conditions that give good constraints on dust properties: the combination of optically thick and thin bands is required, and large β (β is the power-law index of dust opacity, κ ν ∝ ν β ) and low dust temperature are preferable. To examine our synthetic analysis results, we apply the multiband analysis to ALMA archival data of the TW Hya disk at Bands 4, 6, 7, and 9. The Band [9, 6, 4] set provides the dust properties close to the model profile, while the Band [7, 6, 4] set gives the dust properties deviating from the model at all radii with too broad a constraint range to specify the accurate values of dust temperature, optical depth, and β. Since these features are expected by the synthetic multiband analysis, we confirm that the synthetic multiband analysis is consistent with the results derived from real data.

Scattered light shadows in warped protoplanetary discs

3D hydrodynamic numerical simulations have demonstrated that the structure of a protoplanetary disc may be strongly affected by a planet orbiting in a plane that is misaligned to the disc. When the planet is able to open a gap, the disc is separated into an inner, precessing disc and an outer disc with a warp. In this work, we compute infrared scattered light images to investigate the observational consequences of such an arrangement. We find that an inner disc misaligned by less than a degree to the outer disc is indeed able to cast a shadow at larger radii. In our simulations, a planet of |${\gtrsim }6\, M_{\rm J}$| inclined by ≳2° is enough to warp the disc and cast a shadow with a depth of |${\gtrsim } 10{{\ \rm per\ cent}}$| of the average flux at that radius. We also demonstrate that the warp in the outer disc can cause a variation in the azimuthal brightness profile at large radii. Importantly, this latter effect is a function of the distance from the star and is most prominent in the outer disc. We apply our model to the TW Hya system, where a misaligned, precessing inner disc has been invoked to explain a recently observed shadow in the outer disc. Consideration of the observational constraints suggests that an inner disc precessing due to a misaligned planet is an unlikely explanation for the features found in TW Hya.

A search for accreting young companions embedded in circumstellar disks

Context. In recent years, our understanding of giant planet formation progressed substantially. There have even been detections of a few young protoplanet candidates still embedded in the circumstellar disks of their host stars. The exact physics that describes the accretion of material from the circumstellar disk onto the suspected circumplanetary disk and eventually onto the young, forming planet is still an open question. Aims. We seek to detect and quantify observables related to accretion processes occurring locally in circumstellar disks, which could be attributed to young forming planets. We focus on objects known to host protoplanet candidates and/or disk structures thought to be the result of interactions with planets. Methods. We analyzed observations of six young stars (age 3.5–10 Myr) and their surrounding environments with the SPHERE/ZIMPOL instrument on the Very Large Telescope (VLT) in the Hα filter (656 nm) and a nearby continuum filter (644.9 nm). We applied several point spread function (PSF) subtraction techniques to reach the highest possible contrast near the primary star, specifically investigating regions where forming companions were claimed or have been suggested based on observed disk morphology. Results. We redetect the known accreting M-star companion HD142527 B with the highest published signal to noise to date in both Hα and the continuum. We derive new astrometry (r=62.8−2.7+2.1 mas and PA=(98.7±1.8)°) and photometry (ΔN_Ha = 6.3−0.3+0.2 mag, ΔB_Ha = 6.7 ± 0.2 mag and ΔCnt_Ha = 7.3−0.2+0.3 mag) for the companion in agreement with previous studies, and estimate its mass accretion rate (Ṁ ≈ 1−2 × 10−10 M⊙yr−1). A faint point-like source around HD135344 B (SAO206462) is also investigated, but a second deeper observation is required to reveal its nature. No other companions are detected. In the framework of our assumptions we estimate detection limits at the locations of companion candidates around HD100546, HD169142, and MWC 758 and calculate that processes involving Hα fluxes larger than ~ 8 × 10−14–10−15 erg s−1 cm−2 (Ṁ > 10−10−10−12 M⊙yr−1) can be excluded. Furthermore, flux upper limits of ~10−14−10−15 erg s−1 cm−2 (Ṁ < 10−11–10−12 M⊙yr−1) are estimated within the gaps identified in the disks surrounding HD135344 B and TW Hya. The derived luminosity limits exclude Hα signatures at levels similar to those previously detected for the accreting planet candidate LkCa15 b.

Dust Pileup at the Dead-zone Inner Edge and Implications for the Disk Shadow

Abstract We perform simulations of the dust and gas disk evolution to investigate the observational features of a dust pileup at the dead-zone inner edge. We show that the total mass of accumulated dust particles is sensitive to the turbulence strength in the dead zone, α dead, because of the combined effect of turbulence-induced particle fragmentation (which suppresses particle radial drift) and turbulent diffusion. For a typical critical fragmentation velocity of silicate dust particles of 1 m s−1, the stress-to-pressure ratio α dead needs to be lower than 3 × 10−4 for dust trapping to operate. The obtained dust distribution is postprocessed using the radiative transfer code RADMC-3D to simulate infrared scattered-light images of the inner part of protoplanetary disks with a dust pileup. We find that a dust pileup at the dead-zone inner edge, if present, casts a shadow extending out to ∼10 au. In the shadowed region the temperature significantly drops, which in some cases yields even multiple water snow lines. We also find that even without a dust pileup at the dead-zone inner edge, the disk surface can become thermally unstable, and the excited waves can naturally produce shadows and ring-like structures in observed images. This mechanism might account for the ring-like structures seen in the scattered-light images of some disks, such as the TW Hya disk.

Direct imaging of molten protoplanets in nearby young stellar associations

During their formation and early evolution, rocky planets undergo multiple global melting events due to accretionary collisions with other protoplanets. The detection and characterization of their post-collision afterglows (magma oceans) can yield important clues about the origin and evolution of the solar and extrasolar planet population. Here, we quantitatively assess the observational prospects to detect the radiative signature of forming planets covered by such collision-induced magma oceans in nearby young stellar associations with future direct imaging facilities. We have compared performance estimates for near- and mid-infrared instruments to be installed at ESO’s Extremely Large Telescope (ELT), and a potential space-based mission called Large Interferometer for Exoplanets (LIFE). We modelled the frequency and timing of energetic collisions usingN-body models of planet formation for different stellar types, and determine the cooling of the resulting magma oceans with an insulating atmosphere. We find that the probability of detecting at least one magma ocean planet depends on the observing duration and the distribution of atmospheric properties among rocky protoplanets. However, the prospects for detection significantly increase for young and close stellar targets, which show the highest frequencies of giant impacts. For intensive reconnaissance with aKband (2.2μm) ELT filter or a 5.6μm LIFE filter, theβPictoris, Columba, TW Hydrae, and Tucana-Horologium associations represent promising candidates for detecting a molten protoplanet. Our results motivate the exploration of magma ocean planets using the ELT and underline the importance of space-based direct imaging facilities to investigate and characterize planet formation and evolution in the solar vicinity. Direct imaging of magma oceans will advance our understanding of the early interior, surface and atmospheric properties of terrestrial worlds.

ALMA pinpoints the formation site of an extrasolar planet around a young star TW hydrae

ALMA pinpoints the formation site of an extrasolar planet around a young star TW hydrae

Super-Earths in the TW Hya disc

ABSTRACT We test the hypothesis that the sub-millimetre thermal emission and scattered light gaps seen in recent observations of TW Hya are caused by planet–disc interactions. We perform global three-dimensional dusty smoothed particle hydrodynamics simulations, comparing synthetic observations of our models with dust thermal emission, CO emission, and scattered light observations. We find that the dust gaps observed at 24 au and 41 au can be explained by two super-Earths (∼ 4 M⊕). A planet of approximately Saturn-mass can explain the CO emission and the depth and width of the gap seen in scattered light at 94 au. Our model produces a prominent spiral arm while there are only hints of this in the data. To avoid runaway growth and migration of the planets we require a disc mass of ${\lesssim } 10^{-2}\, \mathrm{M}_{\odot }{}$ in agreement with CO observations but 10–100 times lower than the estimate from HD line emission.

Probing the protoplanetary disk gas surface density distribution with 13CO emission

Context. How protoplanetary disks evolve is still an unsolved problem where different processes may be involved. Depending on the process, the disk gas surface density distribution Σgas may be very different and this could have diverse implications for planet formation. Together with the total disk mass, it is key to constrain Σgas as function of disk radius R from observational measurements. Aims. In this work we investigate whether spatially resolved observations of rarer CO isotopologues, such as 13CO, may be good tracers of the gas surface density distribution in disks. Methods. Physical-chemical disk models with different input Σgas(R) were run, taking into account CO freeze-out and isotope-selective photodissociation. The input disk surface density profiles were compared with the simulated 13CO intensity radial profiles to check whether and where the two follow each other. Results. For each combination of disk parameters, there is always an intermediate region in the disk where the slope of the 13CO radial emission profile and Σgas(R) coincide. In the inner part of the disk, the line radial profile underestimates Σgas, as 13CO emission becomes optically thick. The same happens at large radii where the column densities become too low and 13CO is not able to efficiently self-shield. Moreover, the disk becomes too cold and a considerable fraction of 13CO is frozen out, thus it does not contribute to the line emission. If the gas surface density profile is a simple power-law of the radius, the input power-law index can be retrieved within a ~20% uncertainty if one choses the proper radial range. If instead Σgas(R) follows the self-similar solution for a viscously evolving disk, retrieving the input power-law index becomes challenging, in particular for small disks. Nevertheless, we find that the power-law index γ can be in any case reliably fitted at a given line intensity contour around 6 K km s−1, and this produces a practical method to constrain the slope of Σgas(R). Application of such a method is shown in the case study of the TW Hya disk. Conclusions. Spatially resolved 13CO line radial profiles are promising to probe the disk surface density distribution, as they directly trace Σgas(R) profile at radii well resolvable by ALMA. There, chemical processes like freeze-out and isotope-selective photodissociation do not affect the emission, and, assuming that the volatile carbon does not change with radius, no chemical model is needed when interpreting the observations.

Multiple Disk Gaps and Rings Generated by a Single Super-Earth. II. Spacings, Depths, and Number of Gaps, with Application to Real Systems

Abstract The Atacama Large Millimeter/submillimeter Array (ALMA) has found multiple dust gaps and rings in a number of protoplanetary disks in continuum emission at millimeter wavelengths. The origin of such structures is under debate. Recently, we documented how one super-Earth planet can open multiple (up to five) dust gaps in a disk with low viscosity (α ≲ 10−4). In this paper, we examine how the positions, depths, and total number of gaps opened by one planet depend on input parameters, and apply our results to real systems. Gap locations (equivalently, spacings) are the easiest metric to use when making comparisons between theory and observations, as positions can be robustly measured. We fit the locations of gaps empirically as functions of planet mass and disk aspect ratio. We find that the locations of the double gaps in HL Tau and TW Hya, and of all three gaps in HD 163296, are consistent with being opened by a sub-Saturn mass planet. This scenario predicts the locations of other gaps in HL Tau and TW Hya, some of which appear consistent with current observations. We also show how the Rossby wave instability may develop at the edges of several gaps and result in multiple dusty vortices, all caused by one planet. A planet as low in mass as Mars may produce multiple dust gaps in the terrestrial planet-forming region.

A Bayesian Framework for Exoplanet Direct Detection and Non-detection

Abstract Rigorously quantifying the information in high-contrast imaging data is important for informing follow-up strategies to confirm the substellar nature of a point source, constraining theoretical models of planet–disk interactions, and deriving planet occurrence rates. However, within the exoplanet direct imaging community, non-detections have almost exclusively been defined using a frequentist detection threshold (i.e., contrast curve) and associated completeness. This can lead to conceptual inconsistencies when included in a Bayesian framework. A Bayesian upper limit is such that the true value of a parameter lies below this limit with a certain probability. The associated probability is the integral of the posterior distribution with the upper limit as the upper bound. In summary, a frequentist upper limit is a statement about the detectability of planets while a Bayesian upper limit is a statement about the probability of a parameter to lie in an interval given the data. The latter is therefore better suited for rejecting hypotheses or theoretical models based on their predictions. In this work we emphasize that Bayesian statistics and upper limits are more easily interpreted and typically more constraining than the frequentist approach. We illustrate the use of Bayesian analysis in two different cases: (1) with a known planet location where we also propose to use model comparison to constrain the astrophysical nature of the point source and (2) gap-carving planets in TW Hya. To finish, we also mention the problem of combining radial velocity and direct imaging observations.

Dynamical ages of the young local associations with Gaia (Corrigendum)

The Young Local Associations constitute an excellent sample for the study of a variety of astrophysical topics, especially the star formation process in low-density environments. Data from the Gaia mission allows us to undertake studies of the YLAs with unprecedented accuracy. We determine the dynamical age and place of birth of a set of associations in a uniform and dynamically consistent manner. There are nine YLAs in our sample $\epsilon$ Chamaeleontis, TW Hydrae, $\beta$ Pictoris, Octans, Tucana-Horologium, Columba, Carina, Argus and AB Doradus. We designed a method for deriving the dynamical age of the YLAs based on the orbital integration. It involves a strategy to account for the effect of observational errors and we tested it using mock YLAs. Finally, we applied it to our set of nine YLAs with astrometry from the first Gaia data release and complementary on-ground radial velocities from the literature. Our orbital analysis yields a first estimate of the dynamical age of 3$^{+9}_{-0}$ Myr, 13$^{+7}_{-0}$ Myr and 5$^{+23}_{-0}$ Myr for $\epsilon$ Cha, $\beta$ Pict and Tuc-Hor, respectively. For four other associations (Oct, Col, Car and Arg), we provide a lower limit for the dynamical age. Our rigorous error treatment indicates that TW Hya and AB Dor deserve further study. Our dynamical ages are compatible spectroscopic and isochrone fitting ages obtained elsewhere. From the orbital analysis, we suggest a scenario with two episodes of star formation: one ~40 Myr ago in the first quadrant that gave birth to $\epsilon$ Cha, TW Hya and $\beta$ Pic, and another 5-15 Myr ago close to the Sun that formed Tuc-Hor, Col, and Car. Future Gaia data will provide the necessary accuracy to improve the present results, especially for the controversial age determinations, and additional evidence for the proposed scenario, once a complete census of YLAs and better membership can be obtained.