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Register a new userScattering-induced intensity reduction: large mass content with small grains in the inner region of the TW Hya disk
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Dust continuum observation is one of the best methods to constrain the properties of protoplanetary disks. Recent theoretical studies have suggested that the dust scattering at the millimeter wavelength potentially reduces the observed intensity, which results in an underestimate in the dust mass. We investigate whether the dust scattering indeed reduces the observed continuum intensity by comparing the ALMA archival data of the TW Hya disk at Band 3, 4, 6, 7 and 9 to models obtained by radiative transfer simulations. We find that the model with scattering by 300 ${rm mu m}$-sized grains well reproduces the observed SED of the central part of the TW Hya disk while the model without scattering is also consistent within the errors of the absolute fluxes. To explain the intensity at Band 3, the dust surface density needs to be $sim$ 10 ${rm g,cm^{-2}}$ at 10 au in the model with scattering, which is 26 times more massive than previously predicted. The model without scattering needs 2.3 times higher dust mass than the model with scattering because it needs lower temperature. At Band 7, scattering reduces the intensity by $sim$ 35% which makes the disk looks optically thin even though it is optically thick. Our study suggests the TW Hya disk is still capable of forming cores of giant planets at where the current solar system planets exist.
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We present Atacama Large Millimeter Array (ALMA) observations of TW Hya at 3.1 mm with $sim50$ milliarcsecond resolution. These new data were combined with archival high angular resolution ALMA observations at 0.87 mm, 1.3 mm, and 2.1 mm. We analyze these multi-wavelength data to infer a disk radial profile of the dust surface density, maximum particle size, and slope of the particle size distribution. Most previously known annular substructures in the disk of TW Hya are resolved at the four wavelengths. Inside the inner 3 au cavity, the 2.1 mm and 3.1 mm images show a compact source of free-free emission, likely associated with an ionized jet. Our multi-wavelength analysis of the dust emission shows that the maximum particle size in the disk of TW Hya is $>1$ mm. The inner 20 au are completely optically thick at all four bands, which results in the data tracing different disk heights at different wavelengths. Coupled with the effects of dust settling, this prevents the derivation of accurate density and grain size estimates in these regions. At $r>20$ au, we find evidence of the accumulation of large dust particle at the position of the bright rings, indicating that these are working as dust traps. The total dust mass in the disk is between 250 and 330 $M_{oplus}$, which represents a gas-to-dust mass ratio between 50 and 70. Our mass measurement is a factor of 4.5-5.9 higher than the mass that one would estimate using the typical assumptions of large demographic surveys. Our results indicate that the ring substructures in TW Hya are ideal locations to trigger the streaming instability and form new generations of planetesimals.
We present a detailed analysis of the spatially and spectrally resolved 12CO J=2-1 and J=3-2 emission lines from the TW Hya circumstellar disk, based on science verification data from the Atacama Large Millimeter/Submillimeter Array (ALMA). These lines exhibit substantial emission in their high-velocity wings (with projected velocities out to 2.1 km/s, corresponding to intrinsic orbital velocities >20 km/s) that trace molecular gas as close as 2 AU from the central star. However, we are not able to reproduce the intensity of these wings and the general spatio-kinematic pattern of the lines with simple models for the disk structure and kinematics. Using three-dimensional non-local thermodynamic equilibrium molecular excitation and radiative transfer calculations, we construct some alternative models that successfully account for these features by modifying either (1) the temperature structure of the inner disk (inside the dust-depleted disk cavity; r < 4 AU); (2) the intrinsic (Keplerian) disk velocity field; or (3) the distribution of disk inclination angles (a warp). The latter approach is particularly compelling because a representative warped disk model qualitatively reproduces the observed azimuthal modulation of optical light scattered off the disk surface. In any model scenario, the ALMA data clearly require a substantial molecular gas reservoir located inside the region where dust optical depths are known to be substantially diminished in the TW Hya disk, in agreement with previous studies based on infrared spectroscopy. The results from these updated model prescriptions are discussed in terms of their potential physical origins, which might include dynamical perturbations from a low-mass companion with an orbital separation of a few AU.
We report the detection of spiral substructure in both the gas velocity and temperature structure of the disk around TW~Hya, suggestive of planet-disk interactions with an unseen planet. Perturbations from Keplerian rotation tracing out a spiral pattern are observed in the SE of the disk, while significant azimuthal perturbations in the gas temperature are seen in the outer disk, outside 90~au, extending the full azimuth of the disk. The deviation in velocity is either $Delta v_{phi} , / , v_{rm kep} sim 0.1$ or $Delta v_{z} , / , v_{rm kep} sim 0.01$ depending on whether the perturbation is in the rotational or vertical direction, while radial perturbations can be ruled out. Deviations in the gas temperature are $pm 4$ K about the azimuthally averaged profile, equivalent to deviations of $Delta T_{rm gas} , / , T_{rm gas} sim 0.05$. Assuming all three structures can be described by an Archimedean spiral, measurements of the pitch angles of both velocity and temperature spirals show a radially decreasing trend for all three, ranging from 9$^{circ}$ at 70 au, dropping to 3$^{circ}$ at 200 au. Such low pitch-angled spirals are not readily explained through the wake of an embedded planet in the location of previously reported at 94 au, but rather require a launching mechanism which results in much more tightly wound spirals. Molecular emission tracing distinct heights in the disk is required to accurately distinguish between spiral launching mechanisms.
We present a near-infrared direct imaging search for accretion signatures of possible protoplanets around the young stellar object (YSO) TW Hya, a multi-ring disk exhibiting evidence of planet formation. The Pa$beta$ line (1.282 $mu$m) is an indication of accretion onto a protoplanet, and its intensity is much higher than that of blackbody radiation from the protoplanet. We focused on the Pa$beta$ line and performed Keck/OSIRIS spectroscopic observations. Although spectral differential imaging (SDI) reduction detected no accretion signatures, the results of the present study allowed us to set 5$sigma$ detection limits for Pa$beta$ emission of $5.8times10^{-18}$ and $1.5times10^{-18}$ erg/s/cm$^2$ at 0farcs4 and 1farcs6, respectively. We considered the mass of potential planets using theoretical simulations of circumplanetary disks and hydrogen emission. The resulting masses were $1.45pm 0.04$ M$_{rm J}$ and $2.29 ^{+0.03}_{-0.04}$ M$_{rm J}$ at 25 and 95 AU, respectively, which agree with the detection limits obtained from previous broadband imaging. The detection limits should allow the identification of protoplanets as small as $sim$1 M$_{rm J}$, which may assist in direct imaging searches around faint YSOs for which extreme adaptive optics instruments are unavailable.
We compare line emission calculated from theoretical disk models with optical to sub-millimeter wavelength observational data of the gas disk surrounding TW Hya and infer the spatial distribution of mass in the gas disk. The model disk that best matches observations has a gas mass ranging from $sim10^{-4}-10^{-5}$ms for $0.06{rm AU} <r<3.5$AU and $sim 0.06$ms for $ 3.5 {rm AU} <r<200$AU. We find that the inner dust hole ($r<3.5$AU) in the disk must be depleted of gas by $sim 1-2$ orders of magnitude compared to the extrapolated surface density distribution of the outer disk. Grain growth alone is therefore not a viable explanation for the dust hole. CO vibrational emission arises within $rsim 0.5$AU from thermal excitation of gas. [OI] 6300AA and 5577AA forbidden lines and OH mid-infrared emission are mainly due to prompt emission following UV photodissociation of OH and water at $rlesssim0.1$AU and at $rsim 4$AU. [NeII] emission is consistent with an origin in X-ray heated neutral gas at $rlesssim 10$AU, and may not require the presence of a significant EUV ($h u>13.6$eV) flux from TW Hya. H$_2$ pure rotational line emission comes primarily from $rsim 1-30$AU. [OI]63$mu$m, HCO$^+$ and CO pure rotational lines all arise from the outer disk at $rsim30-120$AU. We discuss planet formation and photoevaporation as causes for the decrease in surface density of gas and dust inside 4 AU. If a planet is present, our results suggest a planet mass $sim 4-7$M$_J$ situated at $sim 3$AU. Using our photoevaporation models and the best surface density profile match to observations, we estimate a current photoevaporative mass loss rate of $4times10^{-9}$ms yr$^{-1}$ and a remaining disk lifetime of $sim 5$ million years.
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