Core Accretion Research Articles

SpitzerObservations of NGC 2362: Primordial Disks at 5 Myr

We present results from a mid-infrared imaging survey of the ~5 Myr old cluster NGC 2362 carried out with the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope. The archival mid-infrared data were merged with extant Hα emission data, optical and near-infrared photometry, and moderate-resolution optical spectroscopy to identify the remnant disk-bearing population of the cluster and to estimate the fraction of stars that still retain primordial circumstellar disks. The principal sample of 232 suspected cluster members with masses ranging from ~10 to 0.3 M_⊙ (B2-M5 spectral types) was drawn from known Hα emission stars, X-ray-detected stars from a single 100 ks archival Chandra observation, and established lithium-rich stars. A second sample of 153 stars over a similar mass range whose membership status was based on optical photometry alone was also examined. Measured fluxes in the optical and infrared passbands were fitted with synthetic, low-resolution spectra created using the NextGen atmospheric models, permitting the detection of infrared excesses relative to predicted stellar photospheric fluxes. Using the measured slope of the stellar spectral energy distribution through the four IRAC channels to characterize disk emission for the 195 out of 232 activity/lithium-selected stars and the 105 out of 153 photometric membership candidates having complete IRAC photometry, we derive an upper limit for the primordial, optically thick disk fraction of NGC 2362 of ~7% ± 2%, with another ~12% ± 3% of suspected members exhibiting infrared excesses indicative of weak or optically thin disk emission. The presence of circumstellar disks among candidate members of NGC 2362 is strongly mass-dependent, such that no stars more massive than ~1.2 M_⊙ exhibit significant infrared excess shortward of 8 μm. An upper limit for the fraction of stars hosting primordial, optically thick disks peaks near 10.7% ± 4% for stars with masses between 1.05 and 0.6 M_⊙, but the Spitzer IRAC survey is sensitivity-limited below ~0.3 M_⊙. From Hα emission-line strengths, an upper limit for the accretion fraction of the cluster is estimated at ~5%, with most suspected accretors associated with primordial, optically thick disks identified with Spitzer. The presence of primordial disk-bearing stars in NGC 2362, some of which are suspected of still experiencing gaseous accretion, may imply that even within dense cluster environments, sufficient numbers of inner disks survive to ages consistent with core accretion models of giant planet formation to account for the observed frequency of exoplanets within 5 AU of all FGKM-type stars.

The formation of HD 149026 b

Abstract Today, many extrasolar planets have been detected. Some of them exhibit properties quite different from the planets in our Solar system and they have eluded attempts to explain their formation. One such case is HD 149026 b. It was discovered by Sato et al. A transit-determined orbital inclination results in a total mass of 114 M⊕. The unusually small radius can be explained by a condensible element core with an inferred mass of 67 M⊕ for the best-fitting theoretical model. In the core accretion model, giant planets are assumed to form around a growing core of condensible materials. With increasing core mass, the amount of gravitationally bound envelope mass increases. This continues up to the so-called critical core mass – the largest core allowing a hydrostatic envelope. For larger cores, the lack of static solutions forces a dynamic evolution of the protoplanet, accreting large amounts of gas or ejecting the envelope in the process. This would prevent the formation of HD 149026 b. By studying all possible hydrostatic equilibria we could show that HD 149026 b can remain hydrostatic up to the inferred heavy core. This is possible if it is formed in situ in a relatively low-pressure nebula. This formation process is confirmed by fluid-dynamic calculations using the environmental conditions as determined by the hydrostatic models. We present a quantitative in situ formation scenario for the massive core planet HD 149026 b. Furthermore, we predict a wide range of possible core masses for close-in planets like HD 149026 b. This is different from migration, where typical critical core masses should be expected.

Formation of Titan in Saturn's subnebula: constraints from Huygens probe measurements

We present an evolutionary turbulent model of the Saturn's subnebula consistent with recent core accretion formation models of Saturn. Our calculations are similar to those conducted in the case of the Jovian subnebula, and take into account the vertical structure of the disk, as well as the evolution of its surface density, as given by an α-disk model. Using the thermodynamic conditions of our model, we calculate the evolution of the CO 2 :CO:CH 4 and N 2 :NH 3 molar mixing ratios in the subnebula. We thus show that the carbon and nitrogen homogeneous gas-phase chemistry is inhibited in the subnebula. We also consider the role played by Fischer-Tropsch catalysis in the gas-phase conversions of CO and CO 2 into CH 4 . We demonstrate that, even if a catalytically active zone is likely to exist in the early Saturn's subnebula, it does not alter the composition of volatiles ultimately trapped in the forming solids. We study two different formation scenarios of Titan. In each scenario, we provide observational tests that are compared with measurements made by the Huygens probe. In the first scenario, Titan is formed in a late and cold subnebula from planetesimals produced in Saturn's feeding zone that have been preserved from vaporization. In the second scenario, Titan is formed in a balmy and early subnebula. We show that the first scenario predicts a CO:CH 4 molar mixing ratio orders of magnitude larger than the observed one in the atmosphere of Titan, and requires strong variations of water abundance in the solar nebula on short lengthscales, whose origin is not explained. On the contrary, the second scenario does not require such large variations of the abundance of water, and predicts abundances of volatile species in Titan similar to the observed ones.

The Protostar in the Massive Infrared Dark Cloud IRDC 18223-3

At the onset of high-mass star formation, accreting protostars are deeply embedded in massive cores made of gas and dust. Their spectral energy distribution is still dominated by the cold dust and rises steeply from near-to far-infrared wavelengths. The young massive star-forming region IRDC18223-3 is a prototypical Infrared-Dark-Cloud with a compact mm continuum core that shows no protostellar emission below 8mum. However, based on outflow tracers, early star formation activity was previously inferred for this region. Here, we present recent Spitzer observations from the MIPSGAL survey that identify the central protostellar object for the first time at 24 and 70mum. Combining the mid- to far-infrared data with previous mm continuum observations and the upper limits below 8mum, one can infer physical properties of the central source. At least two components with constant gas mass M and dust temperature T are necessary: one cold component (~15K and ~576M_sun) that contains most of the mass and luminosity, and one warmer component (>=51K and >=0.01M_sun) to explain the 24mum data. The integrated luminosity of ~177L_sun can be used to constrain additional parameters of the embedded protostar from the turbulent core accretion model for massive star formation. The data of IRDC18223-3 are consistent with a massive gas core harboring a low-mass protostellar seed of still less than half a solar mass with high accretion rates of the order 10^-4M_sun/yr. In the framework of this model, the embedded protostar is destined to become a massive star at the end of its formation processes.

Overcoming Migration during Giant Planet Formation

In the core accretion model, gas giant formation is a race between growth and migration; for a core to become a jovian planet, it must accrete its envelope before it spirals into the host star. We use a multizone numerical model to extend our previous investigation of the "window of opportunity" for gas giant formation within a disk. When the collision cross-section enhancement due to core atmospheres is taken into account, we find that a broad range of protoplanetary disks posses such a window.

Atmospheric mass deposition by captured planetesimals

We examine the deposition of heavy elements in the envelope of a protoplanet growing according to the core accretion scenario of Pollack et al. [Pollack, J.B., Hubickyj, O., Bodenheimer, P., Lissauer, J.J., Podolak, M., Greenzweig, Y., 1996. Icarus 124, 62–85]. We use their atmospheric models and the deposition rates that they computed, and we calculate the amount of heavy elements that can be dissolved in the envelope. For planetesimals composed of a mixture of water, CHON, and rock, we find that almost all of the water is dissolved in the atmosphere. A substantial amount of CHON is also dissolved but it remains sequestered at high temperatures near the core. Some fraction of the rock is also dissolved in the very high temperature region near the core envelope boundary. If this dissolved material can be mixed upward later in the planet's evolution, the resulting structure would be much closer to that determined by matching the moments of Jupiter's gravitational field.

On the Origin of HD 149026b

The high density of the recently discovered close-in extrasolar planet HD 149026b suggests the presence of a huge core in its interior, which challenges planet formation theory. We first derive constraints on the total mass of heavy elements in the planet and find its preferred value is 50-80 M? . We then explore the possibility of the formation of HD 149026b through subcritical core accretion as envisioned for Uranus and Neptune, and find the subcritical accretion scenario is very unlikely in the case of HD 149026b for at least two reasons: (1) subcritical planets are such that the ratio of their core mass to their total mass is above ~0.7, in contradiction with constraints for all but the most extreme interior models of HD 149026b and (2) high accretion rates and large isolation mass required for the formation of a subcritical >50 M? core are possible only at specific orbital distances in a disk with a surface density of dust equal to at least 30 times that of the minimum-mass solar nebula. These facts point toward two main routes for the formation of HD 149026b: (i) gas accretion limited by a slow viscous inflow in an evaporating disk or (ii) a significant modification of the planetary composition after gas accretion ended. Illustrating the second route, we show that collision between two gas giants leads to a substantial loss of the gas component and thus may make the planet highly enriched in heavy elements. Alternatively, the planet may be supplied with heavy elements by planetesimals through secular perturbations. In both the giant impact and the secular perturbation scenarios, we expect an outer giant planet to be present. Observational studies by imaging, astrometry, and long-term interferometry of this system are needed to better narrow down the ensemble of possibilities.

Planetesimal formation via fragmentation in sel-gravitating protoplanetary discs

Abstract An unsolved issue in the standard core accretion model for gaseous planet formation is how kilometre-sized planetesimals form from, initially, micron-sized dust grains. Solid growth beyond metre sizes can be difficult both because the sticking efficiency becomes very small, and because these particles should rapidly migrate into the central star. We consider here how metre-sized particles evolve in sel-gravitating accretion discs using simulations in which the gravitational influence of the solid particles is also included. Metre-sized particles become strongly concentrated in the spiral structures present in the disc and, if the solid to gas density ratio is sufficiently high, can fragment because of their own sel-gravity to form planetesimals directly. This result suggests that planetesimal formation may occur very early in the star formation process while discs are still massive enough to be sel-gravitating. The dependence of this process on the surface density of the solids is also consistent with the observation that extrasolar planets are preferentially found around high-metallicity stars.

Meteoritical and dynamical constraints on the growth mechanisms and formation times of asteroids and Jupiter

Thermal models and radiometric ages for meteorites show that the peak temperatures inside their parent bodies were closely linked to their accretion times. Most iron meteorites come from bodies that accreted <0.5 Myr after CAIs formed and were melted by 26Al and 60Fe, probably inside 2 AU. Rare carbon-rich differentiated meteorites like ureilites probably also come from bodies that formed <1 Myr after CAIs, but in the outer part of the asteroid belt. Chondrite groups accreted intermittently from diverse batches of chondrules and other materials over a 4 Myr period starting 1 Myr after CAI formation when planetary embryos may already have formed at ∼1 AU. Meteorite evidence precludes accretion of late-forming chondrites on the surface of early-formed bodies; instead chondritic and non-chondritic meteorites probably formed in separate planetesimals. Maximum metamorphic temperatures in chondrite groups are correlated with mean chondrule age, as expected if 26Al and 60Fe were the predominant heat sources. Because late-forming bodies could not accrete close to large, early-formed bodies, planetesimal formation may have spread across the nebula from regions where the differentiated bodies formed. Dynamical models suggest that the asteroids could not have accreted in the main belt if Jupiter formed before the asteroids. Therefore Jupiter probably reached its current mass >3–5 Myr after CAIs formed. This precludes formation of Jupiter via a gravitational instability <1 Myr after the solar nebula formed, and strongly favors core accretion. Jupiter probably formed too late to make chondrules by generating shocks directly, or indirectly by scattering Ceres-sized bodies across the belt. Nevertheless, shocks formed by gravitational instabilities or Ceres-sized bodies scattered by planetary embryos may have produced some chondrules. The minimum lifetime for the solar nebula of 3–5 Myr inferred from the total spread of CAI and chondrule ages may exceed the median lifetime of 3 Myr for protoplanetary disks, but is well within the 1–10 Myr observed range. Shorter formation times for extrasolar planets may help to explain their unusual orbits compared to those of solar giant planets.

Formation of giant planets around stars with various masses

We examine the predictions of the core accretion - gas capture model concerning the efficiency of planet formation around stars with various masses. First, we follow the evolution of gas and solids from the moment when all solids are in the form of small grains to the stage when most of them are in the form of planetesimals. We show that the surface density of the planetesimal swarm tends to be higher around less massive stars. Then, we derive the minimum surface density of the planetesimal swarm required for the formation of a giant planet both in a numerical and in an approximate analytical approach. We combine these results by calculating a set of representative disk models characterized by different masses, sizes, and metallicities, and by estimating their capability of forming giant planets. Our results show that the set of protoplanetary disks capable of giant planet formation is larger for less massive stars. Provided that the distribution of initial disk parameters does not depend too strongly on the mass of the central star, we predict that the percentage of stars with giant planets should increase with decreasing stellar mass. Furthermore, we identify the radial redistribution of solids during the formation of planetesimal swarms as the key element in explaining these effects.

Atmospheres of Protoplanetary Cores: Critical Mass for Nucleated Instability

We systematically study quasi-static atmospheres of accreting protoplanetary cores for different opacity behaviors and realistic planetesimal accretion rates in various parts of the protoplanetary nebula. We demonstrate that there are two important classes of atmospheres: (1) those having an outer convective zone that smoothly merges with the surrounding nebular gas, and (2) those possessing an almost isothermal outer radiative region that effectively decouples the atmospheric interior from the nebula. The type of atmosphere accumulating around a given core depends on the core mass, nebular parameters, and accretion luminosity of the core. Cores in the inner parts of the protoplanetary disk (within roughly 0.3 AU from the Sun) have large luminosities resulting in atmospheres of the first type, while cores in the giant planet region (beyond several AU) have small accretion luminosities and always accumulate massive atmospheres of the second type. The critical core mass needed for the nucleated instability to commence is found to vary considerably as a function of distance from the Sun. This mass is 5-20 M⊕ at 0.1-1 AU, which is too large to permit the formation of hot Jupiters by nucleated instability around the cores that have grown in situ. In the region of giant planets the critical core mass depends on the gas opacity and planetesimal accretion rate but is insensitive to the nebular temperature or density provided that the opacity in the outer radiative region does not depend on the gas density (e.g., dust opacity). The critical mass in the region of giant planets can be as high as 20-60 M⊕ (for an opacity of 0.1 cm2 g-1) if planetesimal accretion is fast enough for protoplanetary cores to form prior to the nebular gas dissipation.

Formation and structure of the three Neptune-mass planets system around HD 69830

Since the discovery of the first giant planet outside the solar system in 1995 (Mayor & Queloz 1995, Nature, 378, 355), more than 180 extrasolar planets have been discovered. With improving detection capabilities, a new class of planets with masses 5-20 times larger than the Earth, at close distance from their parent star is rapidly emerging. Recently, the first system of three Neptune-mass planets has been discovered around the solar type star HD 69830 (Lovis et al. 2006, Nature, 441, 305). Here, we present and discuss a possible formation scenario for this planetary system based on a consistent coupling between the extended core accretion model and evolutionary models (Alibert et al. 2005a, AA Baraffe et al. 2004, AA Baraffe et al. 2006, A&A, 450, 1221). We show that the innermost planet formed from an embryo having started inside the iceline is composed essentially of a rocky core surrounded by a tiny gaseous envelope. The two outermost planets started their formation beyond the iceline and, as a consequence, accrete a substantial amount of water ice during their formation. We calculate the present day thermodynamical conditions inside these two latter planets and show that they are made of a rocky core surrounded by a shell of fluid water and a gaseous envelope.

Rapid Formation of Super-Earths around M Dwarf Stars

While the recent microlensing discoveries of super-Earths orbiting two M dwarf stars have been taken as support for the core accretion mechanism of giant planet formation, we show here that these planets could also have been formed by the competing mechanism of disk instability, coupled with photoevaporative loss of their gaseous envelopes by a strong external source of UV radiation, i.e., an O star. M dwarfs that form in regions of future high-mass star formation would then be expected to have super-Earths orbiting at distances of several AU and beyond, while those that form in regions of low-mass star formation would be expected to have gas giants at those distances. Given that most stars are born in the former rather than in the latter regions, M dwarfs should have significantly more super-Earths than gas giants, as seems to be indicated by the microlensing surveys.

Silicon and Nickel Enrichment in Planet Host Stars: Observations and Implications for the Core Accretion Theory of Planet Formation

We present evidence that stars with planets exhibit statistically significant silicon and nickel enrichment over the general metal-rich population. We also present simulations that predict silicon enhancement of planet hosts within the context of the core accretion hypothesis for giant planet formation. Because silicon and oxygen are both α-elements, [Si/Fe] traces [O/Fe], so the silicon enhancement in planet hosts predicts that these stars are oxygen-rich as well. We present new numerical simulations of planet formation by core accretion that establish the timescale on which a Jovian planet reaches rapid gas accretion, trga, as a function of solid surface density σsolid: (trga/1 Myr) = (σsolid/25.0 g cm-2)-1.44. This relation enables us to construct Monte Carlo simulations that predict the fraction of star-disk systems that form planets as a function of [Fe/H], [Si/Fe], disk mass, outer disk radius, and disk lifetime. Our simulations reproduce both the known planet-metallicity correlation and the planet-silicon correlation reported in this paper. The simulations predict that 15% of solar-type stars form Jupiter-mass planets, in agreement with 12% predicted from extrapolation of the observed planet frequency-semimajor axis distribution. Although a simple interpretation of core accretion predicts that the planet-silicon correlation should be much stronger than the planet-nickel correlation, we observe the same degree of silicon and nickel enhancement in planet hosts. If this result persists once more planets have been discovered, it might indicate a complexity in the chemistry of planet formation beyond the simple accumulation of solids in the core accretion theory.

Birth and fate of hot-Neptune planets

This paper presents a consistent description of the formation and the subsequent evolution of gaseous planets, with special attention to short-period, low-mass hot-Neptune planets characteristic of $\mu$ Ara-like systems. We show that core accretion including migration and disk evolution and subsequent evolution taking into account irradiation and evaporation provide a viable formation mechanism for this type of strongly irradiated light planets. At an orbital distance $a \simeq$ 0.1 AU, this revised core accretion model leads to the formation of planets with total masses ranging from $\sim$ 14 $\mearth$ (0.044 $\mjup$) to $\sim$ 400 $\mearth$ (1.25 $\mjup$). The newly born planets have a dense core of $\sim$ 6 $\mearth$, independent of the total mass, and heavy element enrichments in the envelope, $M_{\rm Z,env}/M_{\rm env} $, varying from 10% to 80% from the largest to the smallest planets. We examine the dependence of the evolution of the born planet on the evaporation rate due to the incident XUV stellar flux. In order to reach a $\mu$ Ara-like mass ($\sim$ 14 $\mearth$) after $\sim $ 1 Gyr, the initial planet mass must range from 166 $\mearth$ ($\sim$ 0.52 $\mjup$) to about 20 $\mearth$, for evaporation rates varying by 2 orders of magnitude, corresponding to 90% to 20% mass loss during evolution. The presence of a core and heavy elements in the envelope affects appreciably the structure and the evolution of the planet and yields $\sim 8%-9%$ difference in radius compared to coreless objects of solar composition for Saturn-mass planets. These combinations of evaporation rates and internal compositions translate into different detection probabilities, and thus different statistical distributions for hot-Neptunes and hot-Jupiters.

Formation of Giant Planets by Concurrent Accretion of Solids and Gas inside an Anticyclonic Vortex

We study the formation of a giant gas planet by the core accretion and gas capture process, with numerical simulations, under the assumption that the planetary core forms in the center of an anticyclonic vortex. The presence of the vortex concentrates centimeter- to meter-sized particles from the surrounding disk and speeds up the core formation process. Assuming that a planet of Jupiter mass is forming at 5 AU from the star, the vortex enhancement results in considerably shorter formation times than are found in standard core-accretion gas-capture simulations. Also, formation of a gas giant is possible in a disk with mass comparable to that of the minimum mass solar nebula.

Rapid Formation of Gas Giant Planets around M Dwarf Stars

Extrasolar planet surveys have begun to detect gas giant planets in orbit around M dwarf stars. While the frequency of gas giant planets around M dwarfs so far appears to be lower than that around G dwarfs, it is clearly not zero. Previous work has shown that the core accretion mechanism does not seem to be able to form gas giant planets around M dwarfs, because the time required for core formation scales with the orbital period, which lengthens for lower mass stars, resulting in failed (gas-poor) cores unless the gaseous protoplanetary disk survives for >10 Myr. Disk instability, on the other hand, is rapid enough (~103 yr) that it should be able to form gas giant protoplanets around even low-mass stars well before the gaseous disk disappears. A new suite of three-dimensional, radiative, gravitational hydrodynamical models is presented that calculates the evolution of initially marginally gravitationally unstable disks with masses of 0.021-0.065 M☉ orbiting around stars with masses of 0.1 and 0.5 M☉, respectively. The models show that gas giant planets are indeed likely to form by the disk instability mechanism in orbit around M dwarf stars, the opposite of the prediction for formation by the core accretion mechanism. This difference offers another observational test for discriminating between these two theoretical end members for giant planet formation. Ongoing and future extrasolar planet searches around M dwarfs by spectroscopy, microlensing, photometry, and astrometry offer the opportunity to help decide between the dominance of the two mechanisms.

On the Formation of Gas Giant Planets on Wide Orbits

A new suite of three dimensional radiative, gravitational hydrodynamical models is used to show that gas giant planets are unlikely to form by the disk instability mechanism at distances of ~100 AU to ~200 AU from young stars. A similar result seems to hold for the core accretion mechanism. These results appear to be consistent with the paucity of detections of gas giant planets on wide orbits by infrared imaging surveys, and also imply that if the object orbiting GQ Lupus is a gas giant planet, it most likely did not form at a separation of ~100 AU. Instead, a wide planet around GQ Lup must have undergone a close encounter with a third body that tossed the planet outward to its present distance from its protostar. If it exists, the third body may be detectable by NASA's Space Interferometry Mission.

Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing

In the favoured core-accretion model of formation of planetary systems, solid planetesimals accumulate to build up planetary cores, which then accrete nebular gas if they are sufficiently massive. Around M-dwarf stars (the most common stars in our Galaxy), this model favours the formation of Earth-mass (M(o)) to Neptune-mass planets with orbital radii of 1 to 10 astronomical units (au), which is consistent with the small number of gas giant planets known to orbit M-dwarf host stars. More than 170 extrasolar planets have been discovered with a wide range of masses and orbital periods, but planets of Neptune's mass or less have not hitherto been detected at separations of more than 0.15 au from normal stars. Here we report the discovery of a 5.5(+5.5)(-2.7) M(o) planetary companion at a separation of 2.6+1.5-0.6 au from a 0.22+0.21-0.11 M(o) M-dwarf star, where M(o) refers to a solar mass. (We propose to name it OGLE-2005-BLG-390Lb, indicating a planetary mass companion to the lens star of the microlensing event.) The mass is lower than that of GJ876d (ref. 5), although the error bars overlap. Our detection suggests that such cool, sub-Neptune-mass planets may be more common than gas giant planets, as predicted by the core accretion theory.

The Effects of Metallicity and Grain Size on Gravitational Instabilities in Protoplanetary Disks

Observational studies show that the probability of finding gas giant planets around a star increases with the star's metallicity. Our latest simulations of disks undergoing gravitational instabilities (GIs) with realistic radiative cooling indicate that protoplanetary disks with lower metallicity generally cool faster and thus show stronger overall GI activity. More importantly, the global cooling times in our simulations are too long for disk fragmentation to occur, and the disks do not fragment into dense protoplanetary clumps. Our results suggest that direct gas giant planet formation via disk instabilities is unlikely to be the mechanism that produced most observed planets. Nevertheless, GIs may still play an important role in a hybrid scenario, compatible with the observed metallicity trend, where structure created by GIs accelerates planet formation by core accretion.