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[MUSIC]
音乐。
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Good morning, good afternoon, good
evening, whatever time,
早上好，中午好，晚上好，啥时候都好吧，

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your enthusiasm for exoplanets is bringing
you back, welcome.
你对外星的热爱又把你带到这里，欢迎。

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In this week, during this week, we will
continue the description
在本周的时间里，我们会继续说明

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of the properties of the exoplanetary
system that have been detected.
已经探测到的外星系统的性质。

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Of course the idea is, always to bring
constraints for planet formations, making
意思是，要一直考虑外星形态存在的限制，

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the link between initial conditions and
the
在初始条件和观察到的

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observed end state of the formation
process.
经过演化进程的状态间寻找联系。

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Now we start with the properties of the
central mass in the system.
我们先从系统中的中心质量开始。

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What do we expect?
我们希望什么？

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We assume that protoplanetary discs have
masses that
我们假设初始的时候圆盘形状态是有质量的，

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are scaling with the mass of the central
star.
并且用中央的恒星作度量。

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So you form bigger stars in bigger discs.
所以可以认为越大的恒星，周围的圆盘物质越大。

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Makes sense and as at least from the core
operation scenario you need more
这可以说的过去，并且至少需要更多物质去构建核心，

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solid material to build up the core and
then gather an equation of the core.
以聚合成相等的核心？

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it seems that you should be able to do it
more efficiently in a big disk.
看起来似乎圆盘越大，构建核心效率越高。

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So for more massive planets in more
massive discs,
同样盘子越大，行星也越大，

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so have more massive planets around more
massive stars.
同样盘子越大，行星也越大，

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And actually, that's the same for the
gravitational instability approach.
事实上，这和重力不稳定性接近是一样的。

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Because to get instability, you need a
massive disc.
因为要变得不稳定，就需要一个巨大的盘子。
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So you will, with this process, form
massive planets.
所以，照着这个进程，形成巨大的行星。

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It's even probably impossible to form
重力不稳定的圆型尘埃中形成小质量行星几乎

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small mass planets with gravitational
instability.
是不可能的。

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So, the prediction we can make is that
more
因此，我们可以预言，越大质量的恒星附近更有可能找到大质量行星

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massive planets are probably found around
more massive stars.
越大质量的恒星附近更有可能找到大质量行星

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So, what do we know from the observational
point of view.
如此，我们根据观察都发现了哪些观点。

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There have been several projects targeting
low mass
目前已经有一些项目把小质量恒星

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stars and high mass stars to search for
planet and try to answer this question.
和大质量恒星作为研究目标，以尝试回答这些问题。

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And maybe the first one to mention is the
one around low mass stars and dwarfs.
首先被提及的会是在低质量恒星和矮星附近的目标。

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And I will start with [INAUDIBLE]
velocities that's programmed here,
我将会从径向速度法开始，

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that is described that has been conducted
by Xavier [UNKNOWN]
描述的是由Xavier指导的

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from The [INAUDIBLE] Observatory in
collaboration with our team in
来自于位于Geneva的合作的天文台，

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Geneva and we have fellow [INAUDIBLE] M
dwarfs from the sun.
我们跟踪M矮星

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And from these observations, we could
infer frequencies
并且由这些观察，我们可以推断在那些恒星附近

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of planets of different types around those
stars.
不同类型行星出现的频数。

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For example, we have seen that giant
planets are very rare at the level
例如，我们发现巨大行星在1%~2%等级里是非常罕见的，

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of a percent or 2 if we go out to hundred
day period.
如果周期大于100Days。

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So big planets are rare around M dwarfs
from the radial velocity survey.
从径向速度法获取结果说明大行星在M矮星附近是很稀有的。

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On the contrary, super Earths and Neptune
are very
相反，超级地球和海王星却很多，

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numerous, at the level of 50%, plus minus
20.
在50%±20%的水平，

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So that's the same type of numbers
这和我们寻找类太阳的恒星，

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than what we find for solar type stars.
相比也是有同样的数量级。

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So it's easy to form small mass planets
around small star.
所以，更容易在小质量恒星附近形成行星。

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These findings can be compared with
这些发现可以和开普勒卫星

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the results obtained with the Kepler
satellite.
为所公认的结果相比较。

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In the Kepler field, we have something
like
在开普勒的领域，

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4,000 stars with a temperature smaller
than 4000 degrees.
像小于4000°的恒星有4000个。

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And those are defining our low mass stars.
并且那些都被定义为小质量恒星。

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Courtney Dressing and David Charbonneau
from The Center for Astrophysics in Boston
天文物理学的两个波士顿的科学家，

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had a look at the sample of M stars in
Kepler and derived the fraction of
在M恒星发现了一个例子，由此推断平均每个恒星有小部分行星

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planet per star on average around M dwarfs
in the Kepler sample.
附近有M矮星。

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The results, well one of the interesting
results is that there is
关于结果，其中一个有趣的结果是，

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almost no giant planet transit in front of
M dwarfs.
几乎没有巨大行星在M矮星前发生凌日现象。

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There is only one case known and
已知只有一个案例并且

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that has confirmed by ground-based radial
velocities.
通过了基于地面的径向速度法确认。

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But most of the planets are of small size
because from the
但是，大部分行星都很小，

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transit, from the Kepler data, we measure
the size of the planet.
因为，通过凌日，通过开普勒的数据，我们可以测量行星的大小。

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And so the findings of Dressing and
Charbonneau is that if we restrict the
所以D和C的发现

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sample of planets to be smaller than 4
earth's radii, so
如果我们限制为小于4倍地球半径，

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smaller than Neptune basically, then there
is an average of .9 planet
那么经本上小于海王星，那么平均每个M矮星会有一个0.9个行星。

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per M dwarf in the sample and if we only
consider even smaller planets,
如果我们只考虑更小的行星，

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earth size planet still within the 50 day
period, then
像地球大小的行星周期仍然小于50Days，

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we have an average of .5 planet per star
[INAUDIBLE] M dwarf.
那么平均一个M矮星会有0.5个行星。

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So very clearly the the formation
因此，很明显，

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of planet around M dwarfs is favoring the
M矮星附近，

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low size planets.
更容易形成小行星。

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Now if we have a look at the other side of
the spectrum of the sun trial star mass,
现在，如果我们看一下太阳频谱的另外一边，

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we need to go to stars with 2-3 solar
我们需要到2到3倍质量的恒星，

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masses to see the difference with the
solar type stars.
才能看到与类太阳恒星的区别。

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And so on the main sequence, we go up to
the AB dwarfs,
所以按照主星序，我们看看AB矮星，

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but those are hot, high-rotation stars for
which the spectral
但是那些外星很热，转速很高，对于那些光谱很

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lines are very broadened and so it's very
difficult to detect planet around those.
宽，并且很难在他们周围探测到行星。

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So a better approach is to look at G,
所以，更好的方法是观察巨大的，

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giants, that are evolved version of these
A-type star.
那些是进化了的A型恒星。

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And here in the diagram obtained from the
Exoplanet Encyclopedia in Paris,
在这里图表中，可以从外星百科全书中得到，

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it's just a number of planet in function
of


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the mass of the central star, and we see
that for high masses,

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2-3 solar masses, there are detection,
there are several tens of them.

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So, we do find planets around giant stars.

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If we look at the same sample, but now in
a diagram showing the planet

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mass versus the star mass, we see that
most of them are massive planets.

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Of course, there is a detection bias

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here because for giant stars, because of
intrinsic

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variation of the star, it's very difficult

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with radial velocities to detect small
mass planets.

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So there is a bias, but we clearly see

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that there are a lot fo massive planets
around giants.

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So how now, can we compare?

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Because we are completely biased for these

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giant stars, we have no small planet
detections.

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So we cannot compare directly the
frequencies with other type of stars.

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So what we can do is to consider the total
mass on the, the form of planets,

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the normal planetary mass in the system
normalized by the number of stars

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that is in the given bin of star mass.

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So, we can roughly make 3 big bins.

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We have the small mass stars, the M
dwarfs.

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We have the solar type stars, and we have

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the more massive stars, the evolved stars,
the giants.

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And in each bin, we put the total mass of
the

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planets and we normalize by the number of
stars in each bin.

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And you obtain this figure that was made

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by my colleague [UNKNOWN], a few years
back.

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And that very clearly shows how the
content under the form of planet, the, the

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material under the form of planet is

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increasing with the mass of the primary
star.

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So the mass of planetary material scales
with the mass of the star.

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And again, if we come back as you already
have seen this slide

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before in the course, that's the detection
of very massive

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planet in the outer regions of systems of
massive stars.

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They are A Stars and the detection have
been made by direct imaging.

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Of course if they are massive and far from

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the star, it's easier to detect the planet
through imaging.

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But if those planets would exist around
smaller mass stars, it

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would even be easier to detect them and we
do not.

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So it seems that these massive stars have,
are

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favorable place for the formation of these
big planets outside.

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And there is even another case where the
central star is a B9,

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it's compound [INAUDIBLE], and that there
is a companion of the level of

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12-13 Jupiter masses at 55 AU from the
central star.

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So, are those system very common or not?

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That's a very important question because
planet formation scenario

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based on the core [UNKNOWN] have
difficulties to form them.

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So, you can form maybe the planet in the
inner region of the

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system and have the planet move

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outside by migration, by planet-planet
interaction, all,

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all type of processes that can happened,
but if most of those stars

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have these planets, then it's starting to
be difficult to explain it this way.

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So, the statistics about the massive
planet in the outer region

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of massive stars will be an interesting
question to be addressed by the

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new generation of planet imager that are
being installed these days on the

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large telescope G pie on Gemini, and a
sphere on the VAT.

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And maybe in the next version of the book,
we will have results on these aspects.

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Thank you.

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[MUSIC]