More AB Aur
Lauren Biddle, who you may know from a recent AAVSO collaboration, is lead author of a paper looking at AB Aur b, a giant planet forming in the depths of the star's protoplanetary disc. Giant planets such as this grow by accreting gas from the disc, but little is known about the timescale and mechanisms involved in the planet assembly process because few accreting protoplanets have been discovered.
Recent visible and IR imaging revealed a potential accreting protoplanet within the transition disk around AB Aur. Additional imaging in Hα probed for accretion and found agreement between the line-to-continuum flux ratio of the star and companion, raising the possibility that the emission source could be a compact disk feature seen in scattered starlight.
The paper presents new deep Keck/NIRC2 high-contrast imaging of AB Aur to characterize emission in Paβ, an accretion tracer less subject to extinction. They do not detect significant emission at the expected location of the companion, nor from other any other source in the system, suggesting that if AB Aur b is a protoplanet, it is not heavily accreting or accretion is stochastic and was weak during the observations.
and on the same lines...
The model of disc fragmentation due to gravitational instabilities offers an alternate formation mechanism for gas giant planets, especially those on wide orbits. The goal of a recent paper was to determine the 3D structure of disc-instability protoplanets and to examine how this relates to the thermal physics of the fragmentation process.
They set up routines to model the fragmentation of gravitationally unstable discs and followed the evolution of the protoplanets formed through the first and second-hydrostatic core phases. They found that the 3D structure of disc-instability protoplanets is affected by the disc environment and the formation history of each protoplanet (e.g., interactions with spiral arms, mergers).
The large majority of the protoplanets that form in the simulations are oblate spheroids rather than spherical, and they accrete faster from their poles, so it seems that these 3D structures are expected to affect their observed properties and should be taken into account when interpreting observations of protoplanets embedded in their parent discs.
Two fave stars studied
The accretion and ejection of mass in PMS stars are key processes in stellar evolution as they shape the stellar angular momentum transport necessary for the stars’ stability. Magnetospheric accretion onto classical T Tauri stars and similar has been widely studied in the single-star case. This process cannot be directly transferred to PMS binary systems, as tidal and gravitation effects, and/or accretion from a circumbinary disc (with variable separation of the components in the case of eccentric orbits) are in place. DQ Tau and AK Sco, 2 such stars, were recently studied to see how such accretion can be applied to these systems. Both systems showed these signs, despite
their slightly different configurations.
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AF Leporis
Dynamical masses of young planets aged between 10 and 200 Myr detected in imaging play a crucial role in shaping models of giant planet formation. Regrettably, only a few such objects possess these characteristics. Furthermore, the evolutionary pattern of young sub-stellar companions in near-infrared colour-magnitude diagrams might diverge from free-floating objects, possibly due to differing formation processes.
The recent identification of a giant planet around AF Lep, part of the β Pic moving group (BPMG), encouraged a re-examination of these points. The study considered updated dynamical masses and luminosities for the sub-stellar objects in the BPMG. In addition, they compared the properties of sub-stellar companions and free-floating objects in the BPMG and other young associations, remapping the positions of the objects in the colour-magnitude diagram into a dustiness-temperature plane.
It was found that cold-start evolutionary models do not reproduce the mass-luminosity relation for sub-stellar companions in the BPMG, which aligns rather closely with predictions from ’hot start’ scenarios and is consistent with recent planet formation models. Good agreement was had with masses from photometry and the remapping approach compared to actual dynamical masses; also a strong suggestion that the near-infrared colour-magnitude diagram for young companions is different from that of free-floating objects belonging to the same young associations.
If confirmed by further data, this last result would imply that cloud settling - which likely causes the transition between L and T spectral types - occurs at a lower effective temperature in young companions than in free-floating objects. This might tentatively be explained with a different chemical composition.
Now back to YSOs!
During the PMS stage of a star, significant amounts of stellar mass are accreted during episodic accretion events, such as FUOR-type outbursts. A recent study which included our old friend Dirk Froebrich, presented a near-infrared spectroscopic follow-up study of 33 high-amplitude (most with ∆M > 4 mag) variable sources discovered by the VVV survey.
Based on the spectral features, 25 sources are classified as eruptive YSOs, including 15 newly identified FUors, six with long-lasting but EXor-like bursts of magnetospheric accretion and four displaying outflow-dominated spectra.
By examining the photometric behaviours of eruptive YSOs, they found that most FUor-type outbursts have higher amplitudes, faster eruptive timescales and bluer infrared colours than the other outburst types. In addition, they identified seven PMS variables apparently associated with deep dipping events and an eruptive star with deep AlO absorption bands resembling those seen in the V838 Mon stellar merger. |
