In the core-accretion formation scenario of gas giants, most of the gas accreting onto a planet is processed through an accretion shock. In this series of papers we study this shock because it is key ...in setting the structure of the forming planet and thus its postformation luminosity, with dramatic observational consequences. We perform one-dimensional gray radiation-hydrodynamical simulations with nonequilibrium (two-temperature) radiation transport and up-to-date opacities. We survey the parameter space of accretion rate, planet mass, and planet radius and obtain postshock temperatures, pressures, and entropies, as well as global radiation efficiencies. We find that the shock temperature is usually given by the "free-streaming" limit. At low temperatures the dust opacity can make the shock hotter but not significantly so. We corroborate this with an original semianalytical derivation of . We also estimate the change in luminosity between the shock and the nebula. Neither nor the luminosity profile depend directly on the optical depth between the shock and the nebula. Rather, depends on the immediate preshock opacity, and the luminosity change on the equation of state. We find quite high immediate postshock entropies ( -20 ), which makes it seem unlikely that the shock can cool the planet. The global radiation efficiencies are high ( ), but the remainder of the total incoming energy, which is brought into the planet, exceeds the internal luminosity of classical cold starts by orders of magnitude. Overall, these findings suggest that warm or hot starts are more plausible.
ABSTRACT We calculate the evolution of gas giant planets during the runaway gas accretion phase of formation, to understand how the luminosity of young giant planets depends on the accretion ...conditions. We construct steady-state envelope models, and run time-dependent simulations of accreting planets with the code Modules for Experiments in Stellar Astrophysics. We show that the evolution of the internal entropy depends on the contrast between the internal adiabat and the entropy of the accreted material, parametrized by the shock temperature T0 and pressure P0. At low temperatures ( - , depending on model parameters), the accreted material has a lower entropy than the interior. The convection zone extends to the surface and can drive a high luminosity, leading to rapid cooling and cold starts. For higher temperatures, the accreted material has a higher entropy than the interior, giving a radiative zone that stalls cooling. For , the surface-interior entropy contrast cannot be accommodated by the radiative envelope, and the accreted matter accumulates with high entropy, forming a hot start. The final state of the planet depends on the shock temperature, accretion rate, and starting entropy at the onset of runaway accretion. Cold starts with require low accretion rates and starting entropy, and the temperature of the accreting material needs to be maintained close to the nebula temperature. If instead the temperature is near the value required to radiate the accretion luminosity, , as suggested by previous work on radiative shocks in the context of star formation, gas giant planets form in a hot start with .
Abstract
Accreting planets have been detected through their hydrogen-line emission, specifically H
α
. To interpret this, stellar-regime empirical correlations between the H
α
luminosity
L
H
α
and ...the accretion luminosity
L
acc
or accretion rate
M
̇
have been extrapolated to planetary masses, however without validation. We present a theoretical
L
acc
–
L
H
α
relationship applicable to a shock at the surface of a planet. We consider wide ranges of accretion rates and masses and use detailed spectrally resolved, nonequilibrium models of the postshock cooling. The new relationship gives a markedly higher
L
acc
for a given
L
H
α
than fits to young stellar objects, because Ly
α
, which is not observable, carries a large fraction of
L
acc
. Specifically, an
L
H
α
measurement needs 10 to 100 times higher
L
acc
and
M
̇
than previously predicted, which may explain the rarity of planetary H
α
detections. We also compare the
M
̇
–
L
H
α
relationships coming from the planet-surface shock or implied by accretion-funnel emission. Both can contribute simultaneously to an observed H
α
signal, but at low (high)
M
̇
the planetary-surface shock (heated funnel) dominates. Only the shock produces Gaussian line wings. Finally, we discuss accretion contexts in which different emission scenarios may apply, putting recent literature models in perspective, and also present
L
acc
–
L
line
relationships for several other hydrogen lines.
The key aspect determining the postformation luminosity of gas giants has long been considered to be the energetics of the accretion shock at the surface of the planet. We use one-dimensional ...radiation-hydrodynamical simulations to study the radiative loss efficiency and to obtain postshock temperatures and pressures and thus entropies. The efficiency is defined as the fraction of the total incoming energy flux that escapes the system (roughly the Hill sphere), taking into account the energy recycling that occurs ahead of the shock in a radiative precursor. We focus in this paper on a constant equation of state (EOS) to isolate the shock physics but use constant and tabulated opacities. While robust quantitative results will have to await a self-consistent treatment including hydrogen dissociation and ionization, the results presented here show the correct qualitative behavior and can be understood from semianalytical calculations. The shock is found to be isothermal and supercritical for a range of conditions relevant to the core accretion formation scenario (CA), with Mach numbers . Across the shock, the entropy decreases significantly by a few times . While nearly 100% of the incoming kinetic energy is converted to radiation locally, the efficiencies are found to be as low as roughly 40%, implying that a significant fraction of the total accretion energy is brought into the planet. However, for realistic parameter combinations in the CA scenario, we find that a nonzero fraction of the luminosity always escapes the Hill sphere. This luminosity could explain, at least in part, recent observations in the young LkCa 15 and HD 100546 systems.
We present new methodological features and physical ingredients included in the one-dimensional radiative transfer code HELIOS, improving the hemispheric two-stream formalism. We conduct a thorough ...intercomparison survey with several established forward models, including COOLTLUSTY and PHOENIX, and find satisfactory consistency with their results. Then, we explore the impact of (i) different groups of opacity sources, (ii) a stellar path length adjustment, and (iii) a scattering correction on self-consistently calculated atmospheric temperatures and planetary emission spectra. First, we observe that temperature-pressure (T-P) profiles are very sensitive to the opacities included, with metal oxides, hydrides, and alkali atoms (and ionized hydrogen) playing an important role in the absorption of shortwave radiation (in very hot surroundings). Moreover, if these species are sufficiently abundant, they are likely to induce nonmonotonic T-P profiles. Second, without the stellar path length adjustment, the incoming stellar flux is significantly underestimated for zenith angles above 80°, which somewhat affects the upper atmospheric temperatures and the planetary emission. Third, the scattering correction improves the accuracy of the computation of the reflected stellar light by ∼10%. We use HELIOS to calculate a grid of cloud-free atmospheres in radiative-convective equilibrium for self-luminous planets for a range of effective temperatures, surface gravities, metallicities, and C/O ratios to be used by planetary evolution studies. Furthermore, we calculate dayside temperatures and secondary eclipse spectra for a sample of exoplanets for varying chemistry and heat redistribution. These results may be used to make predictions on the feasibility of atmospheric characterizations with future observations.
Abstract
HR 8799 hosts four directly imaged giant planets, but none has a mass measured from first principles. We present the first dynamical mass measurement in this planetary system, finding that ...the innermost planet HR 8799 e has a mass of
9.6
−
1.8
+
1.9
M
Jup
. This mass results from combining the well-characterized orbits of all four planets with a new astrometric acceleration detection (5
σ
) from the Gaia EDR3 version of the Hipparcos-Gaia Catalog of Accelerations. We find with 95% confidence that HR 8799 e is below 13
M
Jup
, the deuterium-fusing mass limit. We derive a hot-start cooling age of
42
−
16
+
24
Myr for HR 8799 e that agrees well with its hypothesized membership in the Columba association but is also consistent with an alternative suggested membership in the
β
Pictoris moving group. We exclude the presence of any additional ≳5 −
M
Jup
planets interior to HR 8799 e with semimajor axes between ≈3–16 au. We provide proper motion anomalies and a matrix equation to solve for the mass of any of the planets of HR 8799 using only mass ratios between the planets.
Abstract
Surveys have looked for H
α
emission from accreting gas giants but found very few objects. Analyses of the detections and nondetections have assumed that the entire gas flow feeding the ...planet is in radial freefall. However, hydrodynamical simulations suggest that this is far from reality. We calculate the H
α
emission from multidimensional accretion onto a gas giant, following the gas flow from Hill sphere scales down to the circumplanetary disk (CPD) and the planetary surface. We perform azimuthally symmetric radiation hydrodynamics simulations around the planet and use modern tabulated gas and dust opacities. Crucially, contrasting with most previous simulations, we do not smooth the gravitational potential but do follow the flow down to the planetary surface, where grid cells are 0.01 Jupiter radii small. We find that roughly only 1% of the net gas inflow into the Hill sphere directly reaches the planet. As expected for ballistic infall trajectories, most of the gas falls at too large a distance on the CPD to generate H
α
. Including radiation transport removes the high-velocity subsurface flow previously seen in hydrodynamics-only simulations, so that only the free planet surface and the inner regions of the CPD emit substantial H
α
. Unless magnetospheric accretion, which we neglect here, additionally produces H
α
, the corresponding H
α
production efficiency is much smaller than usually assumed, which needs to be taken into account when analyzing (non)detection statistics.
Context.
Stars form as an end product of the gravitational collapse of cold, dense gas in magnetized molecular clouds. This fundamentally multi-scale scenario occurs via the formation of two ...quasi-hydrostatic Larson cores and involves complex physical processes, which require a robust, self-consistent numerical treatment.
Aims.
The primary aim of this study is to understand the formation and evolution of the second hydrostatic Larson core and the dependence of its properties on the initial cloud core mass.
Methods.
We used the PLUTO code to perform high-resolution, one- and two-dimensional radiation hydrodynamic (RHD) core collapse simulations. We include self-gravity and use a grey flux-limited diffusion approximation for the radiative transfer. Additionally, we use for the gas equation of state density- and temperature-dependent thermodynamic quantities (heat capacity, mean molecular weight, etc.) to account for effects such as dissociation of molecular hydrogen, ionisation of atomic hydrogen and helium, and molecular vibrations and rotations. Properties of the second core are investigated using one-dimensional studies spanning a wide range of initial cloud core masses from 0.5
M
⊙
to 100
M
⊙
. Furthermore, we expand to two-dimensional (2D) collapse simulations for a selected few cases of 1
M
⊙
, 5
M
⊙
, 10
M
⊙
, and 20
M
⊙
. We follow the evolution of the second core for ≥100 years after its formation, for each of these non-rotating cases.
Results.
Our results indicate a dependence of several second core properties on the initial cloud core mass. Molecular cloud cores with a higher initial mass collapse faster to form bigger and more massive second cores. The high-mass second cores can accrete at a much faster rate of ≈10
−2
M
⊙
yr
−1
compared to the low-mass second cores, which have accretion rates as low as 10
−5
M
⊙
yr
−1
. For the first time, owing to a resolution that has not been achieved before, our 2D non-rotating collapse studies indicate that convection is generated in the outer layers of the second core, which is formed due to the gravitational collapse of a 1
M
⊙
cloud core. Additionally, we find large-scale oscillations of the second accretion shock front triggered by the standing accretion shock instability, which has not been seen before in early evolutionary stages of stars. We predict that the physics within the second core would not be significantly influenced by the effects of magnetic fields or an initial cloud rotation.
Conclusions.
In our 2D RHD simulations, we find convection being driven from the accretion shock towards the interior of the second Larson core. This supports an interesting possibility that dynamo-driven magnetic fields may be generated during the very early phases of low-mass star formation.
Context.
Intrinsic H
α
emission can be advantageously used to detect substellar companions because it improves contrasts in direct imaging. Characterising this emission from accreting exoplanets ...allows for the testing of planet formation theories.
Aims.
We characterise the young circumbinary planetary mass companion 2MASS J01033563-5515561 (AB)b (Delorme 1 (AB)b) through medium-resolution spectroscopy.
Methods.
We used the new narrow-field mode for the MUSE integral-field spectrograph, located on the ESO Very Large Telescope, during science verification time to obtain optical spectra of Delorme 1 (AB)b.
Results.
We report the discovery of very strong H
α
and H
β
emission, accompanied by He
I
emission. This is consistent with an active accretion scenario. We provide accretion rate estimates obtained from several independent methods and find a likely mass of 12−15
M
Jup
for Delorme 1 (AB)b. This is also consistent with previous estimates.
Conclusions.
Signs of active accretion in the Delorme 1 system might indicate a younger age than the ∼30−40 Myr expected from a likely membership in Tucana-Horologium (THA). Previous works have also shown the central binary to be overluminous, which gives further indication of a younger age. However, recent discoveries of active discs in relatively old (∼40 Myr), very low-mass systems suggests that ongoing accretion in Delorme 1 (AB)b might not require in and of itself that the system is younger than the age implied by its THA membership.
Abstract TWA 27B (2M1207b) is the first directly imaged planetary-mass ( M p ≈ 5 M J ) companion and was observed at 0.9–5.3 μ m with JWST/NIRSpec. To understand the accretion properties of TWA ...27B, we search for continuum-subtracted near-infrared helium and hydrogen emission lines and measure their widths and luminosities. We detect the He i triplet at 4.3 σ and all Paschen-series lines covered by NIRSpec (Pa α , Pa β , Pa γ , Pa δ ) at 4 σ –5 σ . The three brightest Brackett-series lines (Br α , Br β , Br γ ) as well as Pf γ and Pf δ are tentative detections at 2 σ –3 σ . We provide upper limits on the other hydrogen lines, including on H α through Hubble Space Telescope archival data. Three lines can be reliably deconvolved to reveal an intrinsic width Δ v intrsc = (67 ± 9) km s −1 , which is 60% of the surface freefall velocity. The line luminosities seem significantly too high to be due to chromospheric activity. Converting line luminosities to an accretion rate yields M ̇ ≈ 5 × 10 − 9 M J yr − 1 when using scaling relationships for planetary masses, and M ̇ ≈ 0.1 × 10 − 9 M J yr − 1 with extrapolated stellar scalings. Several of these lines represent the first detections at an accretor of such low mass. The weak accretion rate implies that formation is likely over. This analysis shows that JWST can be used to measure low line-emitting mass accretion rates onto planetary-mass objects, motivates deeper searches for the mass reservoir feeding TWA 27B, and hints that other young directly imaged objects might—hitherto unbeknownst—also be accreting.
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