First Direct Measurement of Brown Dwarf Mass Limit: 6.7% Sun

[CygnusX-1]

by Ken Croswell

New observations indicate that objects born with a mass just 6.7 per cent that of the Sun can shine for trillions of years rather than fizzle out as failed stars known as brown dwarfs.

Link: New Scientist

 

[stefan r]

Why call them "failed star"? They succeeded in avoiding the hot fusion disaster. They are successfully storing hydrogen for future galactic development instead wasting it.

 

[Bandersnatch]

storing hydrogen for future galactic development

What do you mean by this? Storing implies retrieval. How's that ever going to happen?

 

[mfb]

They could collide with another star, but that is a very unlikely process.

What is a "hot fusion disaster"?

 

[|Glitch|]

Is this not something we already knew? Brown dwarfs are objects that fuse deuterium and/or lithium and range anywhere from 13 to 80 Jupiter masses. This 6.7% of the Sun's mass is just another way of saying 70.2 Jupiter masses. Which is slightly less massive than originally estimated. If the object is massive enough to begin hydrogen fusion then it has become a star. Brown dwarfs are not "failed stars" any more than planets are "failed stars." They simply lack the mass to begin the hydrogen fusion process. An object with just over 70 Jupiter masses that is fusing hydrogen is also known as a Red Dwarf star, and they do indeed fuse hydrogen at their core for well over a trillion years. Brown Dwarfs, on the other hand, will fuse all of their deuterium and/or lithium in less than a billion years. But we knew this as well. I'm struggling to discover any new revelations in this article.

 

[nikkkom]

Is this not something we already knew? Brown dwarfs are objects that fuse deuterium and/or lithium and range anywhere from 13 to 80 Jupiter masses. This 6.7% of the Sun's mass is just another way of saying 70.2 Jupiter masses. Which is slightly less massive than originally estimated.

I take it the exact boundary was not something we already knew.

 

[mfb]

Which is slightly less massive than originally estimated.

And now with a more precise estimate. That is the point.

 

[|Glitch|]

I take it the exact boundary was not something we already knew.

That is true. It is only estimated that somewhere between 13 and 14 Jupiter masses deuterium begins to fuse, and Brown Dwarf is only able to fuse lithium once it became greater than 65 Jupiter masses. The upper end for a Brown dwarf was estimated to be between 75 and 80 Jupiter masses before hydrogen fusion began. According to the article, these new observations suggest hydrogen fusion can began with only 70.2 Jupiter masses (6.7% of the sun's mass).

The mass of the object is really immaterial. It is the actual processes that is occurring that makes the difference. Regardless of its mass, if an object is not fusing hydrogen, the object cannot be a star. Regardless of its mass, if an object is not fusing deuterium then the object cannot be a Brown dwarf. The mass range is just our best estimate as to the ranges of these various processes.

 

[stefan r]

What do you mean by this? Storing implies retrieval. How's that ever going to happen?

I do not need to know. If no one can retrieve the hydrogen then the brown dwarf succeed in avoiding retrieval too. I just wanted to know why it is labeled "failed star". Why is proton fusion considered a success?

 

[Drakkith]

I do not need to know. If no one can retrieve the hydrogen then the brown dwarf succeed in avoiding retrieval too. I just wanted to know why it is labeled "failed star". Why is proton fusion considered a success?

It's just a figure of speech used to describe the fact that they are just under the mass limit necessary to be stars.

 

[snorkack]

What is the central density of a 70 Jupiter mass brown dwarf?

 

[stefan r]

What is the central density of a 70 Jupiter mass brown dwarf?

will depend on elements it is made of and the temperature.

 

[snorkack]

will depend on elements it is made of and the temperature.

Is it a property of brown dwarfs, shared with planets and white dwarfs, that their interior is degenerate and its density depends weakly on temperature?

 

[CygnusX-1]

The new work is noteworthy for two reasons:

1. It's the first DIRECT MEASUREMENT of this number. Previous work over the past 50+ years has relied on theoretical calculations. In contrast, the new work measures actual masses of 37 red and brown dwarfs--enough to see the boundary between the two.

2. The number comes in slightly below previous (theoretical) estimates, although the discrepancy is probably not that great, considering the uncertainties in both the theoretical calculations and the new observations.

Bottom line: It's nice to finally have an actual MEASUREMENT of this number.

 

[stefan r]

Is it a property of brown dwarfs, shared with planets and white dwarfs, that their interior is degenerate and its density depends weakly on temperature?

Yes. but gas law is still there adding pressure. Someone should calculate for pure hydrogen.

When stars collapse from gas clouds they are hot enough to stop contracting for awhile. Should be true for brown dwarfs and planets too. The gas law still applies in degenerate matter but it is tiny.

If you are O.K. with estimates than your 70 Jupiter dwarf has around 70 Jupiter dwarf density.

 

[snorkack]

Yes. but gas law is still there adding pressure. Someone should calculate for pure hydrogen.

When stars collapse from gas clouds they are hot enough to stop contracting for awhile. Should be true for brown dwarfs and planets too. The gas law still applies in degenerate matter but it is tiny.

How so?
Heat compressed ice. At warming to about 0 degrees, the pressure drops to 6 mbar for a wide range of initial pressures of compressed ice.
While most other condensed substances do increase pressure on heating, it is not clear that the pressure should be a linear sum of ideal gas pressure to the condensed matter pressure.

 

[stefan r]

How so?
Heat compressed ice. At warming to about 0 degrees, the pressure drops to 6 mbar for a wide range of initial pressures of compressed ice.
While most other condensed substances do increase pressure on heating, it is not clear that the pressure should be a linear sum of ideal gas pressure to the condensed matter pressure.

Can ice exist with degenerate matter?

0 degrees is not likely to exist.

 

[snorkack]

Do brown dwarfs contain any oxygen nuclei?

 

[mfb]

Everything apart from the very first stars does. Why do you ask?

 

[stefan r]

Do brown dwarfs contain any oxygen nuclei?

Everything apart from the very first stars does. Why do you ask?

The very old ones would still have some oxygen from inter-stellar gas.

 

[snorkack]

Everything apart from the very first stars does. Why do you ask?

Do ice, rock and iron in brown dwarfs remain evenly dissolved in hydrogen and helium, or precipitate into the centre?

 

[stefan r]

Do ice, rock and iron in brown dwarfs remain evenly dissolved in hydrogen and helium, or precipitate into the centre?

Jupiter is still an open research field. We certainly have no samples from inside of a gas giant. The Juno spacecraft is supposed to collect some data to help determine if there is a rocky core. "Rocky" is not very detailed. Telescope measurements of brown dwarves will be hundreds of thousands or many million times dimmer. We currently have no way to get a probe to a brown dwarf within the lifetime of grandchildren.

I am fairly confident the oxygen would precipitate.

 

[qraal]

Hi
Red Dwarf Stars are fully convective, so 'heavy elements' are mixed evenly. Brown dwarfs... it's harder to tell. If sorting by atomic mass was the only thing happening, then maybe there'd be a core of the z fraction. But there's a lot of heat production and heat flow happening, so a core is unlikely.

 

[snorkack]

Jupiter is still an open research field. We certainly have no samples from inside of a gas giant. The Juno spacecraft is supposed to collect some data to help determine if there is a rocky core. "Rocky" is not very detailed.

When powerful shock waves are emitted from atmosphere of a gas giant, such as by explosions of comet fragments, precisely how do the sound waves propagate through gas giant core to the far side? Are they only refracted by smooth and continuous profile of sound speed in hot compressed hydrogen, or do they encounter any composition discontinuities?

 

[qraal]

What is the central density of a 70 Jupiter mass brown dwarf?

Very high? They can be modeled as n=1.5 Polytropes, which means their central density is ~6 times their average density. In solar units a brown dwarf's radius is roughly its mass i.e. a 0.07 Solar mass brown dwarf would have a radius of ~0.07 solar radii. So it'd roughly have a central density of 6 x (0.07/(0.07)^3) x average solar density ~1,730 g/cc. The Sun only manages 150 g/cc, so it's pretty dense.

A useful review can be found here: https://www.hindawi.com/journals/aa/2016/5743272/

 

[stefan r]

Very high? They can be modeled as n=1.5 Polytropes, which means their central density is ~6 times their average density. In solar units a brown dwarf's radius is roughly its mass i.e. a 0.07 Solar mass brown dwarf would have a radius of ~0.07 solar radii. So it'd roughly have a central density of 6 x (0.07/(0.07)^3) x average solar density ~1,730 g/cc. The Sun only manages 150 g/cc, so it's pretty dense.

A useful review can be found here: https://www.hindawi.com/journals/aa/2016/5743272/

Equation 13 in your article has radius proportional to cube root of solar mass. That is consistent with non-degenerate objects. An iron rich object with 0.001 Earth mass should have radius around 0.1. Electron degeneracy pressure mangles that relationship (article). The brown dwarfs end up about the size of Jupiter

0.64–1.13R_J with an average radius of 0.83R_J.

 

[snorkack]

For comparison:
Under low pressure, in diprotium molecule ground state, the distance between nearest neighbours is about 74 pm.
Under the same conditions, the distance between nearest neighbour molecules has been quoted as 330 or 390 pm (which of them is right?) and density is something like 0,083 g/cc.
Correct?