“Gas planets around solar-type stars may still form during the Class II pre-main sequence phase”Mendigutía, I. 2019, submitted.

An alternative approach

1) LOWER LIMITS OF THE DISC MASSES are derived from published accretion rates (Macc

) and ages (to) of 430 pre-main sequence (PMS), Class II accreting stars that will

lead to solar objects. Minimum gas disc masses are obtained by fitting the empirical distribution Macc

vs to, and then integrating M

disc ≥ ∫ Macc (t) dt between t

0 and the time to

reach the main-sequence (100 Myr for solar-type stars). Minimum dust disc masses are later derived assuming gas/dust = 100. This methodology leads to total disc masses that are typically a factor ≥ 10 than inferred from dust continuum emission1,5,6. 2) UPPER LIMITS OF THE MASS OF EXOPLANETARY SYSTEMS are derived from the known population of 581 exoplanets around 439 main sequence (MS) solar-type stars. The maximum gas masses of the exoplanetary systems around each star are obtained by summing up the masses of the orbiting giant planets and super-earths. The maximum solid masses of the exoplanetary systems around each star are derived by summing up the masses of the corresponding rocky planets, plus the most massive possible solid cores of the orbiting super-earths and giant planets, according to an empirically-based conversion between the total mass and the solid mass7.

New results and implications

The comparison between accretion-based disc masses and exoplanetary masses around solar-type Class II and main sequence stars leads to two main results:

1) The gas mass distributions of discs and exoplanetary systems are statistically related, pointing to a direct physical link between both. Moreover, when the fraction of accreting stars and the averages are considered, the minimum gas disc masses up to 10 Myr are larger than the maximum gas content of the exoplanetary systems.

2) When the gas disc and total planetary masses are converted into dust and solids the statistical connection breaks, and on average the minimum dust masses of the Class II discs older than 2 Myr are smaller than the maximum solid masses of exoplanetary systems.

These results open the door to the possibility that gas planets form during a long period within the Class II phase. In turn, they also suggest that the solid cores of the gas planets form during earlier Class 0/I stages (although strong dependencies on not well know gas-to-solid relations are present). This view is consistent with planet formation timescales in the solar system, in which the terrestrial planet embryos and the cores of the giant planets formed in less than 3 Myr9, but gas accreted for a longer period10.

The mismatch between disc mass estimates from accretion and from dust emission deserves careful attention. Not only this issue has a critical effect on our understanding of planet formation, but could have fundamental implications on any other conclusion based on circumstellar masses that may be systematically

larger than usually inferred from dust continuum emission.

References

Fig. 1: Distributions of maximum gas and solid masses of exoplanetary systems for solar-type MS stars (top), and minimum gas and dust disc masses for Class II, accreting PMS stars that will lead to solar objects (bottom). The distributions within the first bins are in red, and the values for our solar system in blue. A Kolmogorov-Smirnov test confirms that the distributions in the left panels are statistically related to each other, although gas disc masses are ~ 100x the gaseous mass of exoplanetary systems. In contrast, the distributions in the right panels are not statistically related.

Fig. 2: The solid triangles are the averages of the minimum gas (top) and dust (bottom) disc mass around solar-type PMS stars, weighted by the fraction of accreting stars at each stellar age8. The open triangles are the averages of the maximum gas (top) and solid (bottom) mass of the exoplanetary systems around MS stars. The values for our solar system are in blue.

STARRY conference, Leeds, UK, June 2019. The author acknowledges the Government of Comunidad Autónoma de Madrid, Spain, which has funded this work through a “Talento” Fellowship (2016-T1/TIC-1890)

3: Najita & Kenyon 2014, MNRAS, 445, 33154: Manara et al. 2018, A&A, 618, L3

7: Thorngren et al. 2016, ApJ, 831, 648: Fedele et al. 2010, A&A, 510, A72

9: Tang & Dauphas 2014, E&PScL, 390, 264 10: Desch et al. 2018, ApJS, 238, 11

1: Andrews & Williams 2007, ApJ, 671, 18002: Greaves & Rice 2010, MNRAS, 407, 1981

Gas planets around solar-type stars may still form during the Class II pre-main sequence phase

I. Mendigutía

Centro de Astrobiología (CSIC-INTA), ESA-ESAC campus, Madrid, Spain. imendigutia@cab.inta-csic.es

Are Class II protoplanetary discs massive enough to account for the observed masses of exoplanetary systems?

APPARENTLY NOT1,2,3,4, at least if disc masses are estimated from sub-mm and mm dust continuum emission and a gas/dust ratio = 100. This result suggests that planet formation may take place mainly at earlier Class 0/I embedded stages, when the mass reservoir is more abundant. BUT, there is an alternative explanation: disc masses could be significantly underestimated. The reason is that they are based on dust continuum emission that may not reflect the whole solid population in a disc, rely on a relatively uncertain value of the dust opacity, and assume a interstellar gas-to-dust ratio of 100 that could not be representative of the circumstellar environment.

5: Hartmann et al. 1998, ApJ, 495, 385 6: Mendigutía et al. 2012, A&A, 543, A59

0 2 4 6 8 10 12 14 16 18 >20Maximum gas mass (MJup)

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