Can Evolved Stars Sustain Planets?
Asymptotic Gaint Branch (AGB) is a region In the Hertzsprung — Russel diagram that is populated by evolved cool luminous stars. All-stars lying in this region are known as AGBs and the stage after this is known as post-AGBs. This post-AGB stage is the least understood stage in the evolution cycles of low and intermediate-mass stars.
By now, it has been well established that all the post-AGBs have a common property: Spectral Energy Distribution (SED). The current research in this field has established that these SEDs are due to the presence of stable circumbinary gas and dust in keplerian rotation.
This system did not undergo as per the behavior predicted from the common envelope evolution model. Instead, it was observed that the common envelope was either very rapidly expelled or somehow avoided.
This very rapid expulsion or avoidance of the common envelope leads to the formation of the disk of gas and dust with keplerian rotation with its property resembling to that of protoplanetary disks around young stars.
In another study, the observed depletion patterns were attributed to the accretion of already depleted matter from the disk. It is also shown that the extremely low mass of the post-AGBs makes its composition vulnerable to being dominated by accreted matter. Jets with wide opening angles are found in almost all cases where there is enough spectroscopic monitoring data available.
Despite all this, the evolution of disks around post-AGB binaries is not well studied, contrary to the protoplanetary disks which have been studied extensively both theoretically as well as observationally.
How were the Stars selected?
- Firstly, they selected evolved objects with spectral types which are typical for an AGB star.
- In the selected stars, their SED should point to the presence of hot dust in the system.
- They complemented this section with evolved objects whose photospheres are adapted with a chemical analogy called depletion.
- They then added spectroscopic binaries from the sample of optically bright post-AGB stars for which the hot dust was not visible.
They built on the previous surveys of post-AGB stars with hot dust which were previously available, further omitting M-type stars. Another study had a conclusion that RV Tauri stars are mainly disks sources and owing to this conclusion, they too were added to the list of stars to be studied. The list consisted of 85 galactic sources altogether.
During their analysis, they found that stars surrounded by transition disks show chemical depletion. They also found a population of stars, that were extremely depleted. The depletion pattern observed in post-AGBs was thought to be due to the accretion of gas. Assuming that this process of gas-dust separation takes place at the inner rim, it implied that all targets accrete material with a similar composition and the observed depletion is due to the quantity of accreted material from the circumbinary disk.
A natural explanation would be the presence of a third component in the system that carves the disk hole in the dust by acting as a filter, trapping the dust in the outer disk but still letting the depleted gas be accreted onto the central stars.
This led to the conclusion that Gaint planets would be the most likely explanation for the depletion of transition disk.
The main finding of the study is that there is a mechanism separating the dust and gas within the circumbinary disk, which has been attempted to be explained by the presence of giant planets. These types of planets produce a cavity in between the dust and the gas. If it is further found to be true, this would provide the first presence of planets in these disks.
Another finding is that there is a population of “extreme” targets that are strongly depleted and do not seem linked to the disk type directly. If a planet scenario is confirmed, then it would be a laboratory for scientists to further study second-generation planet formation and further test the planet formation scenarios.