Sunday, August 20, 2017

In situ accretion of gaseous envelopes on to planetary cores embedded in evolving protoplanetary discs

In situ accretion of gaseous envelopes on to planetary cores embedded in evolving protoplanetary discs

Authors:


Coleman et al

Abstract:
The core accretion hypothesis posits that planets with significant gaseous envelopes accreted them from their protoplanetary discs after the formation of rocky/icy cores. Observations indicate that such exoplanets exist at a broad range of orbital radii, but it is not known whether they accreted their envelopes in situ, or originated elsewhere and migrated to their current locations. We consider the evolution of solid cores embedded in evolving viscous discs that undergo gaseous envelope accretion in situ with orbital radii in the range 0.1−10au. Additionally, we determine the long-term evolution of the planets that had no runaway gas accretion phase after disc dispersal. We find: (i) Planets with 5M⊕ cores never undergo runaway accretion. The most massive envelope contained 2.8M⊕ with the planet orbiting at 10au. (ii) Accretion is more efficient onto 10M⊕ and 15M⊕ cores. For orbital radii ap≥0.5au, 15M⊕ cores always experienced runaway gas accretion. For ap≥5au, all but one of the 10M⊕ cores experienced runaway gas accretion. No planets experienced runaway growth at ap=0.1au. (iii) We find that, after disc dispersal, planets with significant gaseous envelopes cool and contract on Gyr time-scales, the contraction time being sensitive to the opacity assumed. Our results indicate that Hot Jupiters with core masses ≲15M⊕ at ≲0.1au likely accreted their gaseous envelopes at larger distances and migrated inwards. Consistently with the known exoplanet population, Super-Earths and mini-Neptunes at small radii during the disc lifetime, accrete only modest gaseous envelopes.

Saturday, August 19, 2017

Binary Star Formation and the Outflows from their Disks

Binary Star Formation and the Outflows from their Discs

Authors:


Kuruwita et al

Abstract:
We carry out magnetohydrodynamical simulations with FLASH of the formation of a single, a tight binary (a∼2.5 AU) and a wide binary star (a∼45 AU). We study the outflows and jets from these systems to understand the contributions the circumstellar and circumbinary discs have on the efficiency and morphology of the outflow. In the single star and tight binary case we obtain a single pair of jets launched from the system, while in the wide binary case two pairs of jets are observed. This implies that in the tight binary case the contribution of the circumbinary disc on the outflow is greater than that in the wide binary case. We also find that the single star case is the most efficient at transporting mass, linear and angular momentum from the system, while the wide binary case is less efficient (∼50%,∼33%,∼42% of the respective quantities in the single star case). The tight binary's efficiency falls between the other two cases (∼71%,∼66%,∼87% of the respective quantities in the single star case). By studying the magnetic field structure we deduce that the outflows in the single star and tight binary star case are magnetocentrifugally driven, whereas in the wide binary star case the outflows are driven by a magnetic pressure gradient.

HD far infrared emission as a measure of protoplanetary disk mass

HD far infrared emission as a measure of protoplanetary disk mass

Authors:


Trapman et al

Abstract:
Protoplanetary disks around young stars are the sites of planet formation. While the dust mass can be estimated using standard methods, determining the gas mass - and thus the amount of material available to form giant planets - has proven to be very difficult. Hydrogen deuteride (HD) is a promising alternative to the commonly-used gas mass tracer, CO. We aim to examine the robustness of HD as tracer of the disk gas mass, specifically the effect of gas mass on the HD FIR emission and its sensitivity to the vertical structure. Deuterium chemistry reactions relevant for HD were implemented in the thermochemical code DALI and models were run for a range of disk masses and vertical structures. The HD J=1-0 line intensity depends directly on the gas mass through a sublinear power law relation with a slope of ~0.8. Assuming no prior knowledge about the vertical structure of a disk and using only the HD 1-0 flux, gas masses can be estimated to within a factor of 2 for low mass disks (Mdisk less than 10−3 M⊙). For more massive disks, this uncertainty increases to more than an order of magnitude. Adding the HD 2-1 line or independent information about the vertical structure can reduce this uncertainty to a factor of ~3 for all disk masses. For TW Hya, using the radial and vertical structure from Kama et al. 2016b the observations constrain the gas mass to 6⋅10−3 M⊙ less than Mdisk less than 9⋅10−3 M⊙. Future observations require a 5σ sensitivity of 1.8⋅10−20 W m−2 (2.5⋅10−20 W m−2) and a spectral resolving power R greater than 300 (1000) to detect HD 1-0 (HD 2-1) for all disk masses above 10−5 M⊙ with a line-to-continuum ratio greater than 0.01. These results show that HD can be used as an independent gas mass tracer with a relatively low uncertainty and should be considered as an important science goal for future FIR missions.

Increased H2CO production in the outer disk around HD 163296

Increased H2CO production in the outer disk around HD 163296

Authors:


Hallam et al

Abstract:
It is known that an embedded massive planet will open a gap in a protoplanetary disc via angular momentum exchange with the disc material. The resulting surface density profile of the disc is investigated for one dimensional and two dimensional disc models and, in agreement with previous work, it is found that one dimensional gaps are significantly deeper than their two dimensional counterparts for the same initial conditions. We find, by applying one dimensional torque density distributions to two dimensional discs containing no planet, that the excitement of the Rossby wave instability and the formation of Rossby vortices play a critical role in setting the equilibrium depth of the gap. Being a two dimensional instability, this is absent from one dimensional simulations and does not limit the equilibrium gap depth there. We find similar gap depths between two dimensional gaps formed by torque density distributions, in which the Rossby wave instability is present, and two dimensional planet gaps, in which no Rossby wave instability is present. This can be understood if the planet gap is maintained at marginal stability, even when there is no obvious Rossby wave instability present. Further investigation shows the final equilibrium gap depth is very sensitive to the form of the applied torque density distribution, and using improved one dimensional approximations from three dimensional simulations can go even further to reducing the discrepancy between one and two dimensional models, especially for lower mass planets. This behaviour is found to be consistent across discs with varying parameters.

Friday, August 18, 2017

The Viewing Geometry of Brown Dwarfs Influences Their Observed Colours and Variability Properties

The Viewing Geometry of Brown Dwarfs Influences Their Observed Colours and Variability Properties

Authors:


Vos et al

Abstract:
In this paper we study the full sample of known Spitzer [3.6 μm] and J-band variable brown dwarfs. We calculate the rotational velocities, vsini, of 16 variable brown dwarfs using archival Keck NIRSPEC data and compute the inclination angles of 19 variable brown dwarfs.

The results obtained show that all objects in the sample with mid-IR variability detections are inclined at an angle >20∘, while all objects in the sample displaying J-band variability have an inclination angle >35∘. J-band variability appears to be more affected by inclination than \textit{Spitzer} [3.6 μm] variability, and is strongly attenuated at lower inclinations. Since J-band observations probe deeper into the atmosphere than mid-IR observations, this effect may be due to the increased atmospheric path length of J-band flux at lower inclinations.

We find a statistically significant correlation between the colour anomaly and inclination of our sample, where field objects viewed equator-on appear redder than objects viewed at lower inclinations. Considering the full sample of known variable L, T and Y spectral type objects in the literature, we find that the variability properties of the two bands display notably different trends, due to both intrinsic differences between bands and the sensitivity of ground-based versus space-based searches. However, in both bands we find that variability amplitude may reach a maximum at ∼7−9 hr periods. Finally, we find a strong correlation between colour anomaly and variability amplitude for both the J-band and mid-IR variability detections, where redder objects display higher variability amplitudes.

HD 202206: A Circumbinary Brown Dwarf System

HD 202206: A Circumbinary Brown Dwarf System 
Authors:

Benedict et al

Abstract:
Using Hubble Space Telescope Fine Guidance Sensor astrometry and previously published radial velocity measures, we explore the exoplanetary system HD 202206. Our modeling results in a parallax, ${\pi }_{\mathrm{abs}}=21.96\pm 0.12$ milliseconds of arc, a mass for HD 202206 B of ${{ \mathcal M }}_{B}={0.089}_{-0.006}^{+0.007}\,{{ \mathcal M }}_{\odot }$, and a mass for HD 202206 c of ${{ \mathcal M }}_{c}={17.9}_{-1.8}^{+2.9}\,{{ \mathcal M }}_{\mathrm{Jup}}$. HD 202206 is a nearly face-on G + M binary orbited by a brown dwarf. The system architecture that we determine supports past assertions that stability requires a 5:1 mean motion resonance (we find a period ratio, ${P}_{c}/{P}_{B}=4.92\pm 0.04$) and coplanarity (we find a mutual inclination, ${\rm{\Phi }}=6^\circ \pm 2^\circ $).

A survey for planetary-mass brown dwarfs in the Chamaeleon I star-forming region

A survey for planetary-mass brown dwarfs in the Chamaeleon I star-forming region

Authors:

Esplin et al

Abstract:
We have performed a search for planetary-mass brown dwarfs in the Chamaeleon I star-forming region using proper motions and photometry measured from optical and infrared images from the Spitzer Space Telescope, the Hubble Space Telescope, and ground-based facilities. Through near-infrared spectroscopy at Gemini Observatory, we have confirmed six of the candidates as new late-type members of Chamaeleon I >M7.75. One of these objects, Cha J11110675-7636030, has the faintest extinction-corrected M_K among known members, which corresponds to a mass of 3-6 M_Jup according to evolutionary models. That object and two other new members have redder mid-IR colors than young photospheres at greater than M9.5, which may indicate the presence of disks. However, since those objects may be later than M9.5 and the mid-IR colors of young photospheres are ill-defined at those types, we cannot determine conclusively whether color excesses from disks are present. If Cha J11110675-7636030 does have a disk, it would be a contender for the least-massive known brown dwarf with a disk. Since the new brown dwarfs that we have found extend below our completeness limit of 6-10 M_Jup, deeper observations are needed to measure the minimum mass of the initial mass function in Chamaeleon I.