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Good morning, good afternoon, good evening, whatever time,
your enthusiasm for exoplanets is bringing you back, welcome.
In this week, during this week, we will continue the description
of the properties of the exoplanetary system that have been detected.
Of course the idea is, always to bring constraints for planet formations, making
the link between initial conditions and the
observed end state of the formation process.
Now we start with the properties of the central mass in the system.
What do we expect?
We assume that protoplanetary discs have masses that
are scaling with the mass of the central star.
So you form bigger stars in bigger discs.
Makes sense and as at least from the core operation scenario you need more
solid material to build up the core and then gather an equation of the core.
it seems that you should be able to do it more efficiently in a big disk.
So for more massive planets in more massive discs,
so have more massive planets around more massive stars.
And actually, that's the same for the gravitational instability approach.
Because to get instability, you need a massive disc.
So you will, with this process, form massive planets.
It's even probably impossible to form
small mass planets with gravitational instability.
So, the prediction we can make is that more
massive planets are probably found around more massive stars.
So, what do we know from the observational point of view.
There have been several projects targeting low mass
stars and high mass stars to search for planet and try to answer this question.
And maybe the first one to mention is the one around low mass stars and dwarfs.
And I will start with [INAUDIBLE] velocities that's programmed here,
that is described that has been conducted by Xavier [UNKNOWN]
from The [INAUDIBLE] Observatory in collaboration with our team in
Geneva and we have fellow [INAUDIBLE] M dwarfs from the sun.
And from these observations, we could infer frequencies
of planets of different types around those stars.
For example, we have seen that giant planets are very rare at the level
of a percent or 2 if we go out to hundred day period.
So big planets are rare around M dwarfs from the radial velocity survey.
On the contrary, super Earths and Neptune are very
numerous, at the level of 50%, plus minus 20.
So that's the same type of numbers
than what we find for solar type stars.
So it's easy to form small mass planets around small star.
These findings can be compared with
the results obtained with the Kepler satellite.
In the Kepler field, we have something like
4,000 stars with a temperature smaller than 4000 degrees.
And those are defining our low mass stars.
Courtney Dressing and David Charbonneau from The Center for Astrophysics in Boston
had a look at the sample of M stars in Kepler and derived the fraction of
planet per star on average around M dwarfs in the Kepler sample.
The results, well one of the interesting results is that there is
almost no giant planet transit in front of M dwarfs.
There is only one case known and
that has confirmed by ground-based radial velocities.
But most of the planets are of small size because from the
transit, from the Kepler data, we measure the size of the planet.
And so the findings of Dressing and Charbonneau is that if we restrict the
sample of planets to be smaller than 4 earth's radii, so
smaller than Neptune basically, then there is an average of .9 planet
per M dwarf in the sample and if we only consider even smaller planets,
earth size planet still within the 50 day period, then
we have an average of .5 planet per star [INAUDIBLE] M dwarf.
So very clearly the the formation
of planet around M dwarfs is favoring the
low size planets.
Now if we have a look at the other side of the spectrum of the sun trial star mass,
we need to go to stars with 2-3 solar
masses to see the difference with the solar type stars.
And so on the main sequence, we go up to the AB dwarfs,
but those are hot, high-rotation stars for which the spectral
lines are very broadened and so it's very difficult to detect planet around those.
So a better approach is to look at G,
giants, that are evolved version of these A-type star.
And here in the diagram obtained from the Exoplanet Encyclopedia in Paris,
it's just a number of planet in function of
the mass of the central star, and we see that for high masses,
2-3 solar masses, there are detection, there are several tens of them.
So, we do find planets around giant stars.
If we look at the same sample, but now in a diagram showing the planet
mass versus the star mass, we see that most of them are massive planets.
Of course, there is a detection bias
here because for giant stars, because of intrinsic
variation of the star, it's very difficult
with radial velocities to detect small mass planets.
So there is a bias, but we clearly see
that there are a lot fo massive planets around giants.
So how now, can we compare?
Because we are completely biased for these
giant stars, we have no small planet detections.
So we cannot compare directly the frequencies with other type of stars.
So what we can do is to consider the total mass on the, the form of planets,
the normal planetary mass in the system normalized by the number of stars
that is in the given bin of star mass.
So, we can roughly make 3 big bins.
We have the small mass stars, the M dwarfs.
We have the solar type stars, and we have
the more massive stars, the evolved stars, the giants.
And in each bin, we put the total mass of the
planets and we normalize by the number of stars in each bin.
And you obtain this figure that was made
by my colleague [UNKNOWN], a few years back.
And that very clearly shows how the content under the form of planet, the, the
material under the form of planet is
increasing with the mass of the primary star.
So the mass of planetary material scales with the mass of the star.
And again, if we come back as you already have seen this slide
before in the course, that's the detection of very massive
planet in the outer regions of systems of massive stars.
They are A Stars and the detection have been made by direct imaging.
Of course if they are massive and far from
the star, it's easier to detect the planet through imaging.
But if those planets would exist around smaller mass stars, it
would even be easier to detect them and we do not.
So it seems that these massive stars have, are
favorable place for the formation of these big planets outside.
And there is even another case where the central star is a B9,
it's compound [INAUDIBLE], and that there is a companion of the level of
12-13 Jupiter masses at 55 AU from the central star.
So, are those system very common or not?
That's a very important question because planet formation scenario
based on the core [UNKNOWN] have difficulties to form them.
So, you can form maybe the planet in the inner region of the
system and have the planet move
outside by migration, by planet-planet interaction, all,
all type of processes that can happened, but if most of those stars
have these planets, then it's starting to be difficult to explain it this way.
So, the statistics about the massive planet in the outer region
of massive stars will be an interesting question to be addressed by the
new generation of planet imager that are being installed these days on the
large telescope G pie on Gemini, and a sphere on the VAT.
And maybe in the next version of the book, we will have results on these aspects.
Thank you.
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