Explaining the "Brown-Dwarf desert"

[Widdekind]

Exoplanets are not Brown Dwarfs

 

1. Star Formation

 

Stars form within Giant Molecular Clouds (GMCs). These dark clouds block the light from background stars, thereby becoming black patches in the night sky*. Now,

Dark clouds spin slowly. Molecules of gas & dust inside the clouds bump around, just as they do in Earth's atmosphere.

Deep inside these clouds, regions called

Cores

form

.

Cores

spin, just like the dark clouds that surround them.

A

Core

is the densest part of a cloud

, but still far less dense than air

**

.

And, it is within these Cores, or "Star Factories", that "most stars are born, within groups of hundreds or thousands of stars"***.

*

Ray Jayawardhana.

Star Factories

, pg. 5. William Herschel, and his sister Caroline

(

18th Century AD

)

, called these dark nebulae "holes in the heavens".

**

ibid

, pg. 18.

***

ibid

., pg. 24.

Now, newborn stars are often surrounded by Proto-Planetary Disks of gas & dust:

The dust in a

Disk

absorbs light from the star inside the

Disk

. The dust warms a little, and the warm dust glows at

Infrared

wavelengths.

Astronomers

can detect the

Infrared Emissions

that come from the dust. A star w/ a

Disk

looks brighter in

Infrared

light than a similar star w/o a

Disk...

 

In the

Star Factories

that have been observed, at least half of all the

one-million-year-old

stars emit Infrared light we can measure. That may mean that

about half of all young stars have

Disks

*

.

Each of these Disks "has more than ten times the mass" of our own Sun's Planetary System** -- to wit, over 5000 Earth-masses (M_Earth) of material, as compared to some 500 M_Earth of material, mostly made up by Jupiter (318 M_Earth) & Saturn (96 M_Earth), in our own Planetary System. Thus, "young Disks have huge amounts of gas & dust" (~1 Myr), while the Disks of adolescent stars have "much less gas & dust" (~10 Myr), and the Disks of mature stars have "only a small amount of gas & dust" (~100 Myr)***. This is probably b/c "young stars often produce strong [stellar] Winds", which gradually drive away Disk materials#. Indeed, our own Sun's Proto-Planetary Disk probably originally contained some 6000 M_Earth of gas & dust##.

*

ibid

., pp. 27-28.

**

ibid

., pg. 29.

***

ibid

., pp. 43,32.

#

ibid

., pg. 46 ;

cf

.

http://sciencenow.sciencemag.org/cgi/content/full/2009/514/1?etoc

##

C.A.Scharf.

Extrasolar Planets & Astrobiology

, pp. 101-102. B/c "the planets represent material retained in the

Inner Solar System

following the heating up & eventual ignition of the proto-Sun", the observed amount of material making up our planets is "biased towards metals, since lighter elements will be lost". Therefore, this figure is deduced by adding back enough

Hydrogen & Helium

gas, to bring the

Solar System

back up to primordial

Solar Element Abundance Ratios

. Thus,

Planetary Formation

in our

Solar System

was probably only about 10% efficient.

B/c of the particular physics of Core Collapses, in GMCs, only stellar-mass objects (M > 0.063 M_Sun) can be created*.

*

Scharf,

ibid

., pp. 50-52.

Cf. Jeans Mass

.

2. Big Planets are not small Stars

 

Based upon observations of companion objects to Solar-type stars, "there are very few objects in the 'Brown Dwarf desert' between 0.01 and 0.08 solar masses". Indeed, objects possessing "a mass of the order of 0.08 times that of the Sun (about 80 times the mass of Jupiter)" generate enough internal heat & pressure to fuse Hydrogen into Helium, and become Hydrogen-burning Red Dwarfs. But, "if a body has less than thirteen times the mass of Jupiter (about 0.01 of the mass of the Sun)", then it cannot generate enough internal heat & pressure even to fuse Heavy Hydrogen (Deuterium), and "will therefore remain a planet". So,

This clear separation of stars

[M > 0.08 M

_Sun

]

and planets

[M < 0.01 M

_Sun

]

reminds us that the two types of bodiy are formed in different ways -- stars via gravitational collapse [in GMC

Cores

], and planets by accretion of matter onto a rocky embryo w/in a

[Proto-Planetary] Disk

of gas & dust

*

.

Note that the biggest observed Exoplanets (M ~ 13 M_Jupiter) contain around 4000 M_Earth of material. Thus, the biggest possible planets gobble up essentially all of their parent stars' Disk materials (~5000 M_Earth of material; see above). This clear constraint surely explains the observed lack of supra-planet, sub-stellar mass objects (to wit, Brown Dwarfs).

*

F.Casoli & T.Encrenaz.

The New Worlds

, pp. 61-63.

Edited May 17, 2009 by Widdekind

[Piltdown man]

Are you saying that there are no brown dwarfs, or that there are very few. Is it the case that what were thought to be brown dwarfs, were also calculated to be at least 18 billion years old, and hence required a fair degree of "jiggery-pokery" in order to accomodate the lower age that the universe is thought to be?

Edited May 30, 2009 by Piltdown man
word omission

[Martin]

... Is it the case that what were thought to be brown dwarfs, were also calculated to be at least 18 billion years old, and hence required a fair degree of "jiggery-pokery" in order to accomodate the lower age that the universe is thought to be?

 

Piltdown, that is a strange question. Would you please give us a link to a website where you saw information like that?

 

Right now, I don't see how to put what you say into contact with reality. It sounds like you have read something on some website and then garbled it. If I could read what the source actually said, I would have a better chance of making sense of it.

[Piltdown man]

Why, when I type a reply, does this wretched site cut me off?

[Phi for All]

Why, when I type a reply, does this wretched site cut me off?

Wretched?! It's all about love.

 

To paraphrase Malcolm Reynolds, the first rule of science forums is love. You can learn all the math in the 'verse, but you post on a forum that you don't love, she'll shake you off just as sure as the turn of the worlds. Love keeps her on the web when she ought to crash. Tells you she's hurting before she keels. Makes her a home.

[Piltdown man]

Forgive me. You are quite right, love conquers all. But when, after registering, and labouriously single finger typing on a keyboard that seems to have mobile letters, I submit reply and it tells me that I cannot do that, please refresh page, which I don't know how to do, when I return all is lost, I confess to a mild sense of frustration. This also happened several times on your sister site. To answer Martin, I have no idea where I read that, but it was some years back, (20 plus), when arguments were still raging re the age of the universe. This question was not meant to be argumentative, but a genuine "do they still exist, or did they never?"

[Martin]

... Is it the case that what were thought to be brown dwarfs, were also calculated to be at least 18 billion years old,..?

 

To answer Martin, I have no idea where I read that, but it was some years back, (20 plus), when arguments were still raging re the age of the universe. This question was not meant to be argumentative, but a genuine "do they still exist, or did they never?"

 

According to my info the first confirmed brown dwarf was reported in 1995 and they come in all ages, like stars do.

http://astro.berkeley.edu/~basri/bdwarfs/index.html

 

Simply by googling "brown dwarf" I found an article about one estimated to be about 1 billion years old.

 

I never heard anything about brown dwarves having to be 18 or more billion years old so hearing something like that makes me curious.

 

The standard mass range for brown dwarf is between 13 and 75 Jupiters.

Less than 13 Jupiters and she cannot even fuse deuterium (D) and lithium (Li). Above 75 and she can fuse hydrogen and is considered to be a normal star. (The sun is roughly 1000 Jupiters.)

 

In between 13 and 75, she is not a planet because she can fuse D and Li.

So there is some slow faint fusion going on in the core, which is just hot and dense enough to fuse these low percentage constituents.

 

In our galaxy, the smaller stars, less than half solar mass, are by far the most numerous. As objects, the brown dwarves are essentially just too-small stars or too-large planets---presumably formed by the same processes. They must be quite numerous as well. There are hundreds of billions of stars (most of them less than half the mass of the sun) in our galaxy. It is harder to pin down the number of brown dwarves because they are too dim to be seen at distances of more than a few hundred lightyears.*

=====================

 

Advice: base your questions on fresh information. Assumptions remembered from 20 years back can lead to confused/confusing questions. If you want to ask a question, figure out what assumptions it is based on and in case of doubt google to find a source. This may result in your asking a better question.

======================

 

Your question: a genuine "do they [brown dwarves] still exist, or did they never?"

 

Answer: There must be billions of brown dwarves in our galaxy alone. It astonishes me that someone could ask such a question. Of course they still exist, why would they go away? How could it be the case that they "never" did exist when if fact we see them.

 

*this paper reports one of the most distant detected so far

http://arxiv.org/abs/0804.1477

at around 100 parsecs or 300 lightyears.

the things are so dim it is difficult to find them outside a severely restricted radius

---

Merged post follows:

Consecutive posts merged

To paraphrase Malcolm Reynolds, the first rule of science forums is love. You can learn all the math in the 'verse, but you post on a forum that you don't love, she'll shake you off just as sure as the turn of the worlds. Love keeps her on the web when she ought to crash. Tells you she's hurting before she keels. Makes her a home.

 

Beautifully put!

 

But this brown dwarf business is turning out to be quite interesting.

I've found David Darling's encyclopedia to be reliable in the past. Quite good on astronomy topics, when I've consulted it. Here is his brown dwarf page:

http://www.daviddarling.info/encyclopedia/B/browndwarf.html

 

==sample excerpt==

...A surprisingly high proportion of brown dwarfs have been found as companions to low-mass (red dwarf or other brown dwarf) stars, and, within these systems, the separation between the two components is typically very small, averaging about 4 AU. This goes against the prediction by some theorists that most very low-mass stars and brown dwarfs are solo objects, wandering though space alone after being ejected out of their stellar nurseries during the star formation process. Very few brown dwarf companions of larger, Sun-like have been found inside 5 AU, a deficiency that has been dubbed the "brown dwarf desert;" however, there is no such desert associated with low-mass stars. The observations to date strongly support the idea that low-mass binaries form in a process similar to that of more massive binaries, and that the percentage of binary systems is similar for bodies spanning the range from one solar mass to as little as 0.05 solar mass.

==endquote==

 

Low mass stars much more numerous than Sun-like ones, so in terms of overall numbers the so-called "desert" cannot be very significant. The

SciAm article on B.D. an estimate of some 100 billion in our galaxy. However still a puzzle why Sun-like stars would tend not to develop B.D. companions, while lower-mass stars do. The April 2000 SciAm article is here:

http://astro.berkeley.edu/~basri/bdwarfs/SciAm-book.pdf

==excerpt==

 

More important, the Canary Islands group conducted the first useful assessment of the number of brown dwarfs in the Pleiades by counting the most likely candidates in a small surveyed area and then extrapolating the

tally for the entire cluster. Their results indicated comparable numbers of stars and brown dwarfs in the Pleiades. If true in

general, this would mean that our galaxy alone contains about 100 billion brown dwarfs...

==endquote==

 

I would conjecture a possible answer to the puzzle of why massive bright hot stars like the Sun (about 2X average mass) do not tend to have B.D. companions, though less massive, more average size stars have them. the conjecture is simply that hot stars have sunlight pressure and a stellar "wind" like the Solar wind and this drives away a large part of the proto-star disk.

Less massive stars have a cooler dimmer light, maybe a weaker stellar wind as well. So more of the crud from the stellar nursery can stay around and co-agulate. This kind of speculation is not my dish, maybe someone else can think of a better explanation. Widdekind does us a favor by raising an interesting topic (as usual) but we shouldn't automatically assume that he has got it right.

Edited June 2, 2009 by Martin
Consecutive posts merged.

[Piltdown man]

Thank you Martin. I consider myself admonished. I will now try to find a new memory and someold books.

[Martin]

Thank you Martin. ...I will now try to find a new memory...

 

You are most cordially welcome! You obviously have an excellent memory so it will be great when it is stocked with a lot of the new astrophysics and cosmology that has come out since 1998! There has been quite a revolution in cosmology in the past ten years.

 

Thanks are due to Widdekind for starting this thread. He has a style which is very good at motivating discussion. Stimulating, sometimes provocative. A curiously good match for the group here.

 

It certainly caused me to delve into brown dwarf lore! So thanks Widdekind.

[rpf_81]

Brown dwarfs, objects that are less massive than stars but larger than planets, just got more elusive, based on a study of 233 nearby multiple-star systems by NASA's Hubble Space Telescope. Hubble found only two brown dwarfs as companions to normal stars. This means the so-called "brown dwarf desert" (the absence of brown dwarfs around solar-type stars) extends to the smallest stars in the universe.

[Martin]

Brown dwarfs, objects that are less massive than stars but larger than planets, just got more elusive, based on a study of 233 nearby multiple-star systems by NASA's Hubble Space Telescope. Hubble found only two brown dwarfs as companions to normal stars. This means the so-called "brown dwarf desert" (the absence of brown dwarfs around solar-type stars) extends to the smallest stars in the universe.

 

That is exciting and to me quite unexpected! Could you give us a link to source? I went to the "Science Hub" page that you have in your signature and saw that it was full of interesting news, but I didn't see the brown dwarf study.

You must have it handy, so please share it with us.

[Widdekind]

I think this is the link:

http://www.nasa.gov/mission_pages/hubble/news/brown_dwarfs.html

Note that the Brown Dwarf Desert applies only to "companions to normal stars":

This means the so-called "Brown Dwarf Desert" (the absence of brown dwarfs around solar-type stars) extends to the smallest stars in the universe...

 

"We still did not find brown dwarfs around small red stars whose mass is only slightly above the hydrogen burning limit. Especially when we consider the fact that brown dwarfs binaries do exist, the fact that there are very few binaries whose components lie on different sides of the Hydrogen Burning Limit is significant," says Dieterich

Indeed, according to the article Unlikely Suns, Improbable Planets in the June 2009 AD issue of Scientific American, Brown Dwarf sub-stars are as common as true stars, in the Sun's vicinity. Thus, although the nearest Star System is the Alpha Centauri Trinary at 4.3 light-years, Brown Dwarf sub-Star Systems could be closer still. And, Brown Dwarfs could also likely have planets.

 

So, it seems that Brown Dwarfs are quite common "on their own", but not as Stellar Companions.

[Widdekind]

http://astro.berkeley.edu/~basri/bdwarfs/