An upper boundary in the mass-metallicity plane of exo-Neptunes

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An upper boundary in the mass-metallicity plane of exo-Neptunes

Post by Shellface on 4th May 2016, 7:56 pm

(not quite sure where to put this)

An upper boundary in the mass-metallicity plane of exo-Neptunes

With the progress of detection techniques, the number of low-mass and small-size exoplanets is increasing rapidly. However their characteristics and formation mechanisms are not yet fully understood. The metallicity of the host star is a critical parameter in such processes and can impact the occurence (sic) rate or physical properties of these planets. While a frequency-metallicity correlation has been found for giant planets, this is still an ongoing debate for their smaller counterparts. Using the published parameters of a sample of 157 exoplanets lighter than 40 Mearth, we explore the mass-metallicity space of Neptunes and Super-Earths. We show the existence of a maximal mass that increases with metallicity, that also depends on the period of these planets. This seems to favor in situ formation or alternatively a metallicity-driven migration mechanism. It also suggests that the frequency of Neptunes (between 10 and 40 Mearth) is, like giant planets, correlated with the host star metallicity, whereas no correlation is found for Super-Earths (<10 Mearth).
As can be seen from their figure 1, there is a lack of planets intermediate in mass to Saturn and Neptune around metal-poor stars, irrespective of observation biases.

Interpreting this is not easy, moreso because their sample is stripped of information pertaining to planet frequency per star, but the authors are able to draw some key conclusions.

Additionally, they make the following observation:
We know at least 18 planets more massive than 40 M with [Fe/H] < -0.4 (source : exoplanets.org), when no Neptunes are found
in that metallicity range, cf Section 4. Moreover the orbital periods of these planets are very different, from 2.96 days (WASP-98 b) to 956 days (HD 181720 b). There is therefore a turnover in the distribution that coincides with the planet desert that separates Neptunian and Jovian planets, between 40 and 60 M. This implies that the formation mechanism of giant planets is significantly different to that of low mass planets and reinforces the role of metallicity in planetary formation.
This indicates that there is a physical distinction, stemming from planet formation, to be made between gas giants and ice giants. But how do planets with higher gas content form more than those with lower gas content within this range?

Something that caught my eye while reading this is the following:
We also included 44 planets from Mayor et al. (2011) as they are already present in the exoplanet.eu database, updated by the recent resubmitted version (private communication).
If we are truly blessed, this means that the HARPS team are going to get around to publishing the Mayor et al. systems properly in the near future. Tch, it's only been 5 years! Get on with it, HARPS team!

Anyway. From the planet table, one can pick out many changes for HARPS stars compared to Mayor et al. (2011). I list the ones I noticed below:

  • HD 10180 b is not listed.
  • HD 134606 is listed with 5 planets, up from 3.
  • HD 189567 is listed with 3 planets, up from 1.
  • HD 20781 is listed with 4 planets, up from 2.
  • HD 45184 is listed with 2 planets, up from 1.
  • HD 93385 is listed with 3 planets, up from 2.
  • ditto for HD 96700.

A few differences are, from personal knowledge, likely in error:

  • The new additions are not listed consistently with those present in Mayor et al. (e.g HD 20781 b -> HD 20781 d).
  • HD 192310 b and Gliese 785 b are the same, yet they are listed separately. HD 192310 c is also missing.
  • HD 134060 b is listed with a very different period.
  • HD 215152 c has a greatly increased mass.
  • Mu Arae c is inexplicably listed twice as both c and d, which is surely the best way to resolve its naming dispute. From a decade ago.

These do not particularly change the main results, seeing as they are only concerned with the highest-mass planets in the sample.

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Re: An upper boundary in the mass-metallicity plane of exo-Neptunes

Post by Lazarus on 5th May 2016, 3:43 pm

Good news on Mayor et al. (2011), was wondering what was happening with that one (NASA Exoplanet Catalog doesn't even include these planets in their list yet).

Wonder if the full set of orbital elements will be published this time: as I recall, both the argument of periastron ω and orbital phase information (time of periastron/conjunction/whatever) was missing the last time around...
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Re: An upper boundary in the mass-metallicity plane of exo-Neptunes

Post by Shellface on 5th May 2016, 5:16 pm

Wonder if the full set of orbital elements will be published this time.
Certainly, yes. As I understand it, the planets were presented in Mayor et al. (2011) for clarity's sake, seeing as the paper was about the statistics of HARPS planets, rather than to actually talk about the systems. The team appear to have intended to properly announce them immediately (they refer to several papers slated for that year in table 3), but seem to have halted for… some reason. I can say that the systems were perfectly presentable in the 2011 dataset, and only ~2 need significant revisions, but the wait continues.

We have seen one (technically two, including a CORALIE paper) of the intended papers published, namely Díaz et al. (2015) (listed as Ségransan [et al.] (2011) in Mayor et al.), but those listed under Queloz, Udry and Benz have yet to appear. Many of the relevant systems are extremely interesting, so it is strange to see the HARPS team wait on them for so long.

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