The Planetary Accretion Shock: I. Framework for Radiation-hydrodynamical Simulations and First Results

The Planetary Accretion Shock: I. Framework for Radiation-hydrodynamical Simulations and First Results

Authors:

Marleau et al

Abstract:
The key aspect determining the post-formation luminosity of gas giants has long been considered to be the energetics of the accretion shock at the planetary surface. We use 1D radiation-hydrodynamical simulations to study the radiative loss efficiency and to obtain post-shock temperatures and pressures and thus entropies. The efficiency is defined as the fraction of the total incoming energy flux which escapes the system (roughly the Hill sphere), taking into account the energy recycling which occurs ahead of the shock in a radiative precursor. We focus here on a constant equation of state to isolate the shock physics but use constant and tabulated opacities. While robust quantitative results will require a self-consistent treatment including hydrogen dissocation and ionization, the results show the correct qualitative behavior and can be understood semi-analytically. The shock is found to be isothermal and supercritical for a range of conditions relevant to core accretion (CA), with Mach numbers greater than ca. 3. Across the shock, the entropy decreases significantly, by a few entropy units (k_B/baryon). While nearly 100 percent of the incoming kinetic energy is converted to radiation locally, the efficiencies are found to be as low as roughly 40 percent, implying that a meaningful fraction of the total accretion energy is brought into the planet. For realistic parameter combinations in the CA scenario, a non-zero fraction of the luminosity always escapes the system. This luminosity could explain, at least in part, recent observations in the LkCa 15 and HD 100546 systems.

Volatile carbon locking and release in protoplanetary disks

Volatile carbon locking and release in protoplanetary disks. A study of TW Hya and HD 100546

Authors:

Kama et al

Abstract:

The composition of planetary solids and gases is largely rooted in the processing of volatile elements in protoplanetary disks. To shed light on the key processes, we carry out a comparative analysis of the gas-phase carbon abundance in two systems with a similar age and disk mass, but different central stars: HD 100546 and TW Hya. We combine our recent detections of C0 in these disks with observations of other carbon reservoirs (CO, C+, C2H) and gas mass and warm gas tracers (HD, O0), as well as spatially resolved ALMA observations and the spectral energy distribution. The disks are modelled with the DALI 2D physical-chemical code. Stellar abundances for HD 100546 are derived from archival spectra. Upper limits on HD emission from HD 100546 place an upper limit on the total disk mass of ≤0.1M⊙. The gas-phase carbon abundance in the atmosphere of this warm Herbig disk is at most moderately depleted compared to the interstellar medium, with [C]/[H]gas=(0.1−1.5)×10−4. HD 100546 itself is a λBo\"{o}tis star, with solar abundances of C and O but a strong depletion of rock-forming elements. In the gas of the T Tauri disk TW Hya, both C and O are strongly underabundant, with [C]/[H]gas=(0.2−5.0)×10−6 and C/O greater than 1. We discuss evidence that the gas-phase C and O abundances are high in the warm inner regions of both disks. Our analytical model, including vertical mixing and a grain size distribution, reproduces the observed [C]/[H]gas in the outer disk of TW Hya and allows to make predictions for other systems.

Water ice at the surface of HD 100546 disk

Water ice at the surface of HD 100546 disk

Authors:

Honda et al

Abstract:

We made near infrared multicolor imaging observations of a disk around Herbig Be star HD100546 using Gemini/NICI. K (2.2\,μm), H2O ice (3.06\,μm), and L'(3.8\,μm) disk images were obtained and we found the 3.1\,μm absorption feature in the scattered light spectrum, likely due to water ice grains at the disk surface. We compared the observed depth of the ice absorption feature with the disk model based on \cite{Oka2012} including water ice photodesorption effect by stellar UV photons. The observed absorption depth can be explained by the both disk models with/without photodesorption effect within the measurement accuracy, but slightly favors the model with photodesorption effects, implying that the UV photons play an important role on the survival/destruction of ice grains at the Herbig Ae/Be disk surface. Further improvement on the accuracy of the observations of the water ice absorption depth is needed to constrain the disk models.

Hunting for the Exoplanets in HD 100546's Protoplanetary Disk

The SPHERE view of the planet-forming disk around HD100546

Authors:

Garufi et al

Abstract:

We image with unprecedented spatial resolution and sensitivity disk features that could be potential signs of planet-disk interaction. Two companion candidates have been claimed in the disk around the young Herbig Ae/Be star HD100546. Thus, this object serves as an excellent target for our investigation of the natal environment of giant planets. We exploit the power of extreme adaptive optics operating in conjunction with the new high-contrast imager SPHERE to image HD100546 in scattered light. We obtain the first polarized light observations of this source in the visible (with resolution as fine as 2 AU) and new H and K band total intensity images that we analyze with the Pynpoint package. The disk shows a complex azimuthal morphology, where multiple scattering of photons most likely plays an important role. High brightness contrasts and arm-like structures are ubiquitous in the disk. A double-wing structure (partly due to ADI processing) resembles a morphology newly observed in inclined disks. Given the cavity size in the visible (11 AU), the CO emission associated to the planet candidate 'c' might arise from within the circumstellar disk. We find an extended emission in the K band at the expected location of 'b'. The surrounding large-scale region is the brightest in scattered light. There is no sign of any disk gap associated to 'b'.

HD 100546b is a SuperJovian and HD 100546c is a SuperJovian at 13 AU

Resolving the HD 100546 Protoplanetary System with the Gemini Planet Imager: Evidence for Multiple Forming, Accreting Planets

Authors:

Currie et al

Abstract:

We report Gemini Planet Imager H band high-contrast imaging/integral field spectroscopy and polarimetry of the HD 100546, a 10 Myr-old early-type star recently confirmed to host a thermal infrared bright (super)jovian protoplanet at wide separation, HD 100546 b. We resolve the inner disk cavity in polarized light, recover the thermal-infrared (IR) bright arm, and identify one additional spiral arm. We easily recover HD 100546 b and show that much of its emission originates an unresolved, point source. HD 100546 b likely has extremely red infrared colors compared to field brown dwarfs, qualitatively similar to young cloudy superjovian planets, however, these colors may instead indicate that HD 100546 b is still accreting material from a circumplanetary disk. Additionally, we identify a second point source-like peak at rproj ∼ 13 AU, located just interior to or at inner disk wall consistent with being a 10--20 MJ candidate second protoplanet-- "HD 100546 c" -- and lying within a weakly polarized region of the disk but along an extension of the thermal IR bright spiral arm. Alternatively, it is equally plausible that this feature is a weakly polarized but locally bright region of the inner disk wall. Astrometric monitoring of this feature over the next 2 years and emission line measurements could confirm its status as a protoplanet, rotating disk hot spot that is possibly a signpost of a protoplanet, or a stationary emission source from within the disk.

Herbig Ae/Be star HD 100546's Disk is Being fed by its Gas Giant Exoplanets

High-resolution Br-gamma spectro-interferometry of the transitional Herbig Ae/Be star HD 100546: a Keplerian gaseous disc inside the inner rim

Authors:

Mendigutía et al

Abstract:

We present spatially and spectrally resolved Br-gamma emission around the planet-hosting, transitional Herbig Ae/Be star HD 100546. Aiming to gain insight into the physical origin of the line in possible relation to accretion processes, we carried out Br-gamma spectro-interferometry using AMBER/VLTI from three different baselines achieving spatial and spectral resolutions of 2-4 mas and 12000. The Br-gamma visibility is larger than that of the continuum for all baselines. Differential phases reveal a shift between the photocentre of the Br-gamma line -displaced 0.6 mas (0.06 au at 100 pc) NE from the star- and that of the K-band continuum emission -displaced 0.3 mas NE from the star. The photocentres of the redshifted and blueshifted components of the Br-gamma line are located NW and SE from the photocentre of the peak line emission, respectively. Moreover, the photocentre of the fastest velocity bins within the spectral line tends to be closer to that of the peak emission than the photocentre of the slowest velocity bins. Our results are consistent with a Br-gamma emitting region inside the dust inner rim (less than 0.25 au) and extending very close to the central star, with a Keplerian, disc-like structure rotating counter-clockwise, and most probably flared (25 deg). Even though the main contribution to the Br-gamma line does not come from gas magnetically channelled on to the star, accretion on to HD 100546 could be magnetospheric, implying a mass accretion rate of a few 10^-7 Msun/yr. This value indicates that the observed gas has to be replenished on time-scales of a few months to years, perhaps by planet-induced flows from the outer to the inner disc as has been reported for similar systems.

Resolving the HD 100546's Disk Structure

Resolving structure of the disk around HD100546 at 7 mm with ATCA

Authors:

Wright et al

Abstract:

There is much evidence that planet formation is occurring in the disk around the Herbig Be star HD100546. To learn more about the processes occurring in this disk we conducted high resolution imaging at 43/45 GHz with the Australia Telescope Compact Array (ATCA). Multiple array configurations were used, providing a best spatial resolution of ∼ 0.15 arcsec, or 15 AU at HD100546's distance of ∼ 100 pc. Significant structure is revealed, but its precise form is dependent on the u−v plane sampling used for the image reconstruction. At a resolution of ≤ 30 AU we detected an inner gap in the disk with a radius of ∼ 25 AU and a position angle approximately along the known disk major axis. With different weighting, and an achieved resolution of ∼ 15 AU, emission appears at the centre and the disk takes on the shape of an incomplete ring, much like a horseshoe, again with a gap radius of ∼ 25 AU. The position angle of the disk major axis and its inclination from face-on are determined to be 140∘±5∘ and 40∘±5∘ respectively. The ∼ 25 AU gap radius is confirmed by a null in the real part of the binned visibilities at 320±10 kλ, whilst the non-axisymmetric nature is also confirmed through significant structure in the imaginary component. The emission mechanism at the central peak is most likely to be free-free emission from a stellar or disk wind. Overall our data support the picture of at least one, but probably several, giant planets orbiting HD100546 within 25 AU.

Are the gas Giants in HD 100546's Protoplanetary Disk Forming Sequentially?

Sequential planet formation in the HD 100546 protoplanetary disk?

Authors:

Pinilla et al

Abstract:

Context.

The disk around the Herbig Ae star, HD 100546, shows structures that suggest the presence of two companions in the disk at ∼10 and ∼70 AU. The outer companion seems to be in the act of formation.

Aims.

Our aims are to provide constraints on the age of the planets in HD 100546 and to explore the potential evidence for sequential planet formation in transition disks such as HD 100546.

Methods.

We compare the recent resolved continuum observations of the disk around HD 100546 with the results of dust evolution simulations using an analytical prescription for the shapes of gaps carved by massive planets.

Results.

An inner pressure bump must have been present since early in the disk lifetime to have good agreement between the dust evolution models and the continuum observations of HD 100546. This pressure bump may have resulted from the presence of a very massive planet (∼20MJup), which formed early in the inner disk (r∼10 AU). If only this single planet exists, the disk is likely to be old, comparable to the stellar age (∼5-10 Myr). Another possible explanation is an additional massive planet in the outer disk (r∼70 AU): either a low-mass outer planet (≲5MJup) injected at early times, or a higher mass outer planet (≳15MJup) formed very recently, traps the right amount of dust in pressure bumps to reproduce the observations. In the latter case, the disk could be much younger (∼3.0 Myr).

Conclusions.

In the case in which two massive companions are embedded in the disk around HD 100546, as suggested in the literature, the outer companion could be at least ≳2.5 Myr younger than the inner companion.

The Sign Posts for Detecting Gas Giants Forming in Protoplanetary Disks

Planet formation signposts: observability of circumplanetary disks via gas kinematics

Authors:

Perez et al

Abstract:

The identification of on-going planet formation requires the finest angular resolutions and deepest sensitivities in observations inspired by state-of-the-art numerical simulations. Hydrodynamic simulations of planet-disk interactions predict the formation of circumplanetary disks (CPDs) around accreting planetary cores. These CPDs have eluded unequivocal detection -their identification requires predictions in CPD tracers. In this work, we aim to assess the observability of embedded CPDs with ALMA as features imprinted in the gas kinematics. We use 3D Smooth Particle Hydrodynamic (SPH) simulations of CPDs around 1 and 5 M_Jup planets at large stellocentric radii, in locally isothermal and adiabatic disks. The simulations are then connected with 3D radiative transfer for predictions in CO isotopologues. Observability is assessed by corrupting with realistic long baseline phase noise extracted from the recent HL Tau ALMA data. We find that the presence of a CPD produces distinct signposts: 1) compact emission separated in velocity from the overall circumstellar disk's Keplerian pattern, 2) a strong impact on the velocity pattern when the Doppler shifted line emission sweeps across the CPD location, and 3) a local increase in the velocity dispersion. We test our predictions with a simulation tailored for HD 100546 -which has a reported protoplanet candidate. We find that the CPDs are detectable in all 3 signposts with ALMA Cycle 3 capabilities for both 1 and 5 M_Jup protoplanets, when embedded in an isothermal disk.

The Apparent Asymmetry of HD 100546's Protoplanetary Disk

On the asymmetry of the OH ro-vibrational lines in HD 100546

Authors:

Fedele et al

Abstract:

We present multi-epoch high-spectral resolution observations with VLT/CRIRES of the OH doublet 2Π3/2 P4.5 (1+,1-) (2.934 μm) towards the protoplanetary disk around HD 100546. The OH doublet is detected at all epochs and is spectrally resolved while nearby H2O lines remains undetected. The OH line velocity profile is different in the three datasets: in the first epoch (April 2012, PA=26∘) the OH lines are symmetric and line broadening is consistent with the gas being in Keplerian rotation around the star. No OH emission is detected within a radius of 8-11 au from the star: the line emitting region is similar in size and extent to that of the CO ro-vibrational lines. In the other two epochs (March 2013 and April 2014, PA=90∘ and 10∘, respectively) the OH lines appear asymmetric and fainter compared to April 2012. We investigate the origin of these line asymmetries which were taken by previous authors as evidence for tidal interaction between an (unseen) massive planet and the disk. We show that the observed asymmetries can be fully explained by a misalignment of the slit of order 0\farcs04-0\farcs20 with respect to the stellar position. The disk is spatially resolved and the slit misalignment is likely caused by the extended dust emission which is brighter than the stellar photosphere at near-infrared wavelengths which is the wavelength used for the pointing. This can cause the photo-center of HD 100546 to be mis-aligned with the stellar position at near-infrared wavelengths.

HD 100546b: a Protoplanet SuperJovian Gas Giant

Recovery of the Candidate Protoplanet HD 100546 b with Gemini/NICI and Detection of Additional (Planet-Induced?) Disk Structure at Small Separations

Authors:

Currie et al

Abstract:

We report the first independent, second-epoch (re-)detection of a directly-imaged protoplanet candidate. Using L′ high-contrast imaging of HD 100546 taken with the Near-Infrared Coronagraph and Imager (NICI) on Gemini South, we recover `HD 100546 b' with a position and brightness consistent with the original VLT/NaCo detection from Quanz et al, although data obtained after 2013 will be required to decisively demonstrate common proper motion. HD 100546 b may be spatially resolved, up to ≈ 12-13 AU in diameter, and is embedded in a finger of thermal IR bright, polarized emission extending inwards to at least 0.3". Standard hot-start models imply a mass of ≈ 15 MJ. But if HD 100546 b is newly formed or made visible by a circumplanetary disk, both of which are plausible, its mass is significantly lower (e.g. 1--7 MJ). Additionally, we discover a thermal IR-bright disk feature, possibly a spiral density wave, at roughly the same angular separation as HD 100546 b but 90 degrees away. Our interpretation of this feature as a spiral arm is not decisive, but modeling analyses using spiral density wave theory implies a wave launching point exterior to ≈ 0.45" embedded within the visible disk structure: plausibly evidence for a second, hitherto unseen wide-separation planet. With one confirmed protoplanet candidate and evidence for 1--2 others, HD 100546 is an important evolutionary precursor to intermediate-mass stars with multiple super-jovian planets at moderate/wide separations like HR 8799.

More Evidence for a Gas Giant Forming in HD 100546's Protoplanetary Disk

NIR SPECTROSCOPY OF THE HAeBe STAR HD 100546. III. FURTHER EVIDENCE OF AN ORBITING COMPANION?

Authors:

Brittain et al

Abstract:

We report high-resolution NIR spectroscopy of CO and OH emission from the Herbig Be star HD 100546. We discuss how our results bear striking resemblance to several theoretically predicted signposts of giant planet formation. The properties of the CO and OH emission lines are consistent with our earlier interpretation that these diagnostics provide indirect evidence for a companion that orbits the star close to the disk wall (at ~13 AU). The asymmetry of the OH spectral line profiles and their lack of time variability are consistent with emission from gas in an eccentric orbit at the disk wall that is approximately stationary in the inertial frame. The time variable spectroastrometric properties of the CO v = 1-0 emission line point to an orbiting source of CO emission with an emitting area similar to that expected for a circumplanetary disk (~0.1 AU2) assuming the CO emission is optically thick. We also consider a counterhypothesis to this interpretation, namely that the variable CO emission arises from a bright spot on the disk wall. We conclude with a brief suggestion of further work that can distinguish between these scenarios.

HD 100546 may Have two Exoplanets in its Protoplanetary Disk

ALMA hints at the presence of two companions in the disk around HD 100546

Authors:

Walsh et al

Abstract:

HD 100546 is a well-studied Herbig Be star-disk system that likely hosts a close-in companion with compelling observational evidence for an embedded protoplanet at 68 AU. We present ALMA observations of the HD 100546 disk which resolve, for the first time, the gas and dust structure at (sub)mm wavelengths. The CO emission (at 345.795 GHz) originates from an extensive molecular disk (390 AU in radius) whereas the continuum emission is more compact (230 AU in radius) suggesting radial drift of the mm-sized grains. The CO emission is similar in extent to scattered light images indicating well-mixed gas and um-sized grains in the disk atmosphere. Assuming an azimuthally-symmetric disk, the continuum visibilities at long baselines (greater than 100 klambda) are reproduced by a compact ring with a width of 21 AU centered at 26 AU. An outer component is required to fit the short baselines: assuming a flat brightness distribution, the best-fit model is a ring with a width of 75 AU centered at 190 AU. The influence of a companion and protoplanet on the dust evolution is investigated. The companion at 10 AU facilitates the accumulation of mm-sized grains within a compact ring, ~20-30 AU, by ~10 Myr. The injection of a protoplanet at 1 Myr hastens the ring formation (~1.2 Myr) and also triggers the development of an outer ring (~100-200 AU). These observations provide additional evidence for the presence of a close-in companion and hint at dynamical clearing by a protoplanet at 68 AU.

Scattered Light Observations of HD 100546's Circumstellar Disk

HD100546 Multi-Epoch Scattered-Light Observations

Authors:

Avenhaus et al

Abstract:

We present H, Ks and L filter polarimetric differential imaging (PDI) data for the transitional disk around HD100546 obtained in 2013, together with an improved re-reduction of previously published 2006 data. We reveal the disk in polarized scattered light in all three filters, achieving an inner working angle of 0.1 arcsec. Additional, short-exposure observations in the H and Ks filter probe the surrounding of the star down to about 0.03 (about 3 AU). HD100546 is fascinating because of its variety of sub-structures possibly related to forming planets in the disk, and PDI is currently the best technique to image them in the near-IR. Our key results are: (1) For the first time ever, we detect a disk in L-band PDI data. (2) We constrain the outer radius of the inner hole to 14pm2 AU and its eccentricity to less than 0.133. (3) We detect a dark lane in the front side of the disk, which is likely an effect of the scattering angle and the scattering function of the grains. (4) We find a spiral arm in the northeast which has no obvious connection to spiral arms seen before by other authors further out in the disk, but winds in the same direction (clockwise). (5) The two bright scattering peaks along the semi-major axis are asymmetric, with the southeastern one being significantly brighter. This could be related to the inner companion candidate that is close to the brighter side of the disk at the time of the observations. (6) The scattering color is close to grey between H and Ks filter, but the scattering in L filter is significantly weaker. (7) We measure the position angle of the disk to be 138pm3 deg, consistent with previous observations. (8) We derive the dust scattering function in the H and Ks filter between 35 and 130 deg at two different radii (30-50 and 80-110 AU) and show that our results are consistent with a disk that is more strongly flared in the outer regions.

Protoplanetary Disk Around HD 100546 Imaged

Resolved images of the protoplanetary disk around HD 100546 with ALMA

Authors:

Pineda et al

Abstract:

The disk around the Herbig Ae/Be star HD 100546 has been extensively studied and it is one of the systems for which there are observational indications of ongoing and/or recent planet formation. However, up until now no resolved image of the millimeter dust emission or the gas has been published. We present the first resolved images of the disk around HD 100546 obtained in Band 7 with the ALMA observatory. The CO (3-2) image reveals a gas disk that extends out to 350 au radius at the 3-sigma level. Surprisingly, the 870um dust continuum emission is compact (radius less than 60 au) and asymmetric. The dust emission is well matched by a truncated disk with outer radius of ≈50 au. The lack of millimeter-sized particles outside the 60 au is consistent with radial drift of particles of this size. The protoplanet candidate, identified in previous high-contrast NACO/VLT L' observations, could be related to the sharp outer edge of the millimeter-sized particles. Future higher angular resolution ALMA observations are needed to determine the detailed properties of the millimeter emission and the gas kinematics in the inner region (less than 2arcsec). Such observations could also reveal the presence of a planet through the detection of circumplanetary disk material.