A pair of brown dwarfs found around a star, that they shouldn't be around
Yousuf Khan
"Several million years ago, BD +20 2457 was on the "main sequence," the
stage in stellar evolution where the star produced light by burning its
hydrogen fuel, much like our sun does now. Except this star, three times
the mass of the sun, was much hotter and more luminous.
"The intense radiation of this star would have heated up and evaporated
anything that was still forming around it," Wolszczan said. "The fact
that these dwarfs are still here means that they had to accumulate a lot
of material very quickly and be fully formed by the time the star
'switched on.' "
A star like BD +20 2457 takes about 10 million years to form and enter
the main sequence. As a rough estimate, in order keep up with their
parent star, the dwarfs would have to accrue as much mass as the Earth's
moon every year."
http://www.physorg.com/news180627887.html
BradGuth
A molecular dark matter cloud of 1e15/m3 plus gravity seeds.
Imagine the terrific molecular cloud of <12.5e6 Ms that formed the
nearby Sirius star/solar system of <12.5 Ms.
~ BG
Andrew Usher
> Brown dwarf pair mystifies astronomers
> http://www.physorg.com/news180627887.html
This seems to be garbled. According to the only paper I can find
( http://arxiv.org/abs/0906.1804v1 ), the 'brown dwarfs' are not a
pair, but orbit the star independently. They are very likely planets
and are included as such in exoplanet database. Therefore, there is no
mystery unless some new discovery about the system has been made.
Andrew Usher
Yousuf Khan
Yes, the brown dwarfs aren't orbiting each other, they are independently
orbiting their star. What the mystery is that there was enough material
in the dust disk to build not just one of these brown dwarfs, but two of
them! Plus, how were they able to accumulate this material themselves
quickly enough, before the local star was able to hog it away from them?
Yousuf Khan
Andrew Usher
> > This seems to be garbled. According to the only paper I can find
> > (http://arxiv.org/abs/0906.1804v1), the 'brown dwarfs' are not a
> > pair, but orbit the star independently. They are very likely planets
> > and are included as such in exoplanet database. Therefore, there is no
> > mystery unless some new discovery about the system has been made.
>
> Yes, the brown dwarfs aren't orbiting each other, they are independently
> orbiting their star. What the mystery is that there was enough material
> in the dust disk to build not just one of these brown dwarfs, but two of
> them!
It is believed that the maximum mass of giant planets scales with that
of the star, and this parent star is about 3 Msun; so even though it's
pretty extreme it's not totally out of line with other extrasolar
planets detected.
> Plus, how were they able to accumulate this material themselves
> quickly enough, before the local star was able to hog it away from them?
Only gas, not rocky matter, can be drawn into the star. Therefore, the
logical assumption is that these planets formed from very large rocky
cores, that were able to accrete lots of gas before it spiraled in.
This would suggest that the star should have a very high metallicity.
The paper says under the section for 'BD +20 2457' that 'BD +14 4559'
has [Fe/H] ~ -1.0 - if we assume that's an error for 'BD +20 2457' (as
is probable) it would indicate a metallicity almost too low to form
planets according to the normal theories. I don't know what to make of
this; but the paper that value comes from finds that planet-hosting
giants have _lower_ metallicity than non-planet-hosts, contrary to
what's observed for dwarfs, so maybe it's a little dubious - is there
some reason spec. metallicity estimates should be worse for giants?
Andrew Usher
Yousuf Khan
> On Dec 28, 10:52 am, Yousuf Khan <bbb...@yahoo.com> wrote:
>
>>> This seems to be garbled. According to the only paper I can find
>>> (http://arxiv.org/abs/0906.1804v1), the 'brown dwarfs' are not a
>>> pair, but orbit the star independently. They are very likely planets
>>> and are included as such in exoplanet database. Therefore, there is no
>>> mystery unless some new discovery about the system has been made.
>> Yes, the brown dwarfs aren't orbiting each other, they are independently
>> orbiting their star. What the mystery is that there was enough material
>> in the dust disk to build not just one of these brown dwarfs, but two of
>> them!
>
> It is believed that the maximum mass of giant planets scales with that
> of the star, and this parent star is about 3 Msun; so even though it's
> pretty extreme it's not totally out of line with other extrasolar
> planets detected.
Have they found others with two brown dwarfs in them before? I know
they've found others with multiple gas-giant planets, but I can't recall
one with two brown dwarfs. I mean the dust disk that must've formed
these brown dwarfs must've been colossal, at least 34 Jupiter masses, if
we just count the two brown dwarfs. It was probably much larger than
that originally.
>> Plus, how were they able to accumulate this material themselves
>> quickly enough, before the local star was able to hog it away from them?
>
> Only gas, not rocky matter, can be drawn into the star. Therefore, the
> logical assumption is that these planets formed from very large rocky
> cores, that were able to accrete lots of gas before it spiraled in.
The mystery is about the accumulation model employed by these brown
dwarfs. Did they use the same method that gas giant planets use, or did
they use the method used by stars, to accumulate this much mass so
quickly? Because their local star is so close and so massive, during the
original formation process, much of the dust should've headed towards
the star rather than these brown dwarfs, thus preventing them from
becoming brown dwarfs in the first place.
> This would suggest that the star should have a very high metallicity.
> The paper says under the section for 'BD +20 2457' that 'BD +14 4559'
> has [Fe/H] ~ -1.0 - if we assume that's an error for 'BD +20 2457' (as
> is probable) it would indicate a metallicity almost too low to form
On page 17 of that paper, they have a table 1 which lists the Fe/H
ratios for each of the systems studied. The values are listed under the
proper columns there. What you read is probably just a typo, or more
likely a copy'n'paste error where they forgot to change one system name
from the previous system name.
> planets according to the normal theories. I don't know what to make of
> this; but the paper that value comes from finds that planet-hosting
> giants have _lower_ metallicity than non-planet-hosts, contrary to
> what's observed for dwarfs, so maybe it's a little dubious - is there
> some reason spec. metallicity estimates should be worse for giants?
That's entirely reasonable, as a previous study had found that stars
(like our Sun) which have low lithium counts are more likely to have
planets.
http://tinyurl.com/yb8ok64
or,
http://groups.google.com/group/sci.astro/tree/browse_frm/thread/7a7ed3ab23c4d2fc/a371213ace43b6fd?_done=%2Fgroup%2Fsci.astro%2Fbrowse_frm%2Fthread%2F7a7ed3ab23c4d2fc%2Fa371213ace43b6fd%3Fq%3Dbbbl67%2BOR%2Byjkhan%2Blithium%26&q=bbbl67+OR+yjkhan+lithium
Yousuf Khan
Andrew Usher
> > It is believed that the maximum mass of giant planets scales with that
> > of the star, and this parent star is about 3 Msun; so even though it's
> > pretty extreme it's not totally out of line with other extrasolar
> > planets detected.
>
> Have they found others with two brown dwarfs in them before? I know
> they've found others with multiple gas-giant planets, but I can't recall
> one with two brown dwarfs. I mean the dust disk that must've formed
> these brown dwarfs must've been colossal, at least 34 Jupiter masses, if
> we just count the two brown dwarfs. It was probably much larger than
> that originally.
Yes, I think this is the highest total mass yet observed.
> The mystery is about the accumulation model employed by these brown
> dwarfs. Did they use the same method that gas giant planets use, or did
> they use the method used by stars, to accumulate this much mass so
> quickly? Because their local star is so close and so massive, during the
> original formation process, much of the dust should've headed towards
> the star rather than these brown dwarfs, thus preventing them from
> becoming brown dwarfs in the first place.
Conservation of angular momentum doesn't allow dust to just fall into
the star. Nonetheless, these may indeed by an example of 'disc
instability' or the more star-like formation, which has been
hypothesised as the source of giant planets; the star's low
metallicity makes this especially likely.
> > This would suggest that the star should have a very high metallicity.
> > The paper says under the section for 'BD +20 2457' that 'BD +14 4559'
> > has [Fe/H] ~ -1.0 - if we assume that's an error for 'BD +20 2457' (as
> > is probable) it would indicate a metallicity almost too low to form
>
> On page 17 of that paper, they have a table 1 which lists the Fe/H
> ratios for each of the systems studied. The values are listed under the
> proper columns there. What you read is probably just a typo, or more
> likely a copy'n'paste error where they forgot to change one system name
> from the previous system name.
OK, got it. As I said, that ratio makes it unlikely that very large
solid cores could have built up, so - as your article says - it's
remarkable that they accreted so much gas so quickly.
> > planets according to the normal theories. I don't know what to make of
> > this; but the paper that value comes from finds that planet-hosting
> > giants have _lower_ metallicity than non-planet-hosts, contrary to
> > what's observed for dwarfs, so maybe it's a little dubious - is there
> > some reason spec. metallicity estimates should be worse for giants?
>
> That's entirely reasonable, as a previous study had found that stars
> (like our Sun) which have low lithium counts are more likely to have
> planets.
As that article makes clear, lithium doesn't behave like the ordinary
metals (carbon and heavier); instead it must be that the existence of
planets has somehow disturbed the outer layers of the star to allow
more lithium to burn up.
My question, too, was why we observe _different_ behavior in giants
compared to dwarfs.
Andrew Usher
Yousuf Khan
> Yousuf Khan wrote:
>>> this; but the paper that value comes from finds that planet-hosting
>>> giants have _lower_ metallicity than non-planet-hosts, contrary to
>>> what's observed for dwarfs, so maybe it's a little dubious - is there
>>> some reason spec. metallicity estimates should be worse for giants?
>> That's entirely reasonable, as a previous study had found that stars
>> (like our Sun) which have low lithium counts are more likely to have
>> planets.
>
> As that article makes clear, lithium doesn't behave like the ordinary
> metals (carbon and heavier); instead it must be that the existence of
> planets has somehow disturbed the outer layers of the star to allow
> more lithium to burn up.
>
> My question, too, was why we observe _different_ behavior in giants
> compared to dwarfs.
Sorry, I'm not clear about what "different" behaviour we're talking
about? You said that the size of the most massive planets are related to
how massive their stars are. So a really massive star may produce things
as big as brown dwarf stars, but something smaller would produce smaller
gas giant planets. Isn't that just expected?
Actually it's looking like our Sun is a bit of an odd-ball. Shouldn't we
have something much larger than Jupiter as our largest planet, according
to our star's size?
Yousuf Khan
Andrew Usher
> > My question, too, was why we observe _different_ behavior in giants
> > compared to dwarfs.
>
> Sorry, I'm not clear about what "different" behaviour we're talking
> about?
That in dwarfs (main sequence stars) the existence of planets is
positively correlated to metallicity, while (according to that paper)
the opposite holds in giants. Since all giants evolved from dwarfs,
and their planetary systems should have already been present, this is
paradoxical.
> Actually it's looking like our Sun is a bit of an odd-ball. Shouldn't we
> have something much larger than Jupiter as our largest planet, according
> to our star's size?
Not really. There's large variation, and it doesn't only depend on the
star's mass. Remember, too,
that our detections are biased toward the most massive; there are no
doubt planetary systems with no giants, but we don't see them.
Andrew Usher
Yousuf Khan
> Yousuf Khan wrote:
>
>>> My question, too, was why we observe _different_ behavior in giants
>>> compared to dwarfs.
>> Sorry, I'm not clear about what "different" behaviour we're talking
>> about?
>
> That in dwarfs (main sequence stars) the existence of planets is
> positively correlated to metallicity, while (according to that paper)
> the opposite holds in giants. Since all giants evolved from dwarfs,
> and their planetary systems should have already been present, this is
> paradoxical.
Are you talking about metallicity in general, or specifically about
lithium metallicity? If it's lithium, then the other study seems to
conclude that planet formation requires lower lithium levels for all
stars. So maybe general metallicity has nothing to do with planet
formation, just only lithium metallicity?
>> Actually it's looking like our Sun is a bit of an odd-ball. Shouldn't we
>> have something much larger than Jupiter as our largest planet, according
>> to our star's size?
>
> Not really. There's large variation, and it doesn't only depend on the
> star's mass. Remember, too,
> that our detections are biased toward the most massive; there are no
> doubt planetary systems with no giants, but we don't see them.
The reason I bring it up is because it looks like maybe our solar system
is a bit of a desert, it's not blessed with an overabundance of
materials compared to other systems. This in itself might not be a bad
thing, having too many things floating around in our solar system might
mean more chances of colliding with things that could kill us. Also
we're just now starting to see super-Earths which might be even better
places for life to grow than our Earth, because they might be 100%
covered with ocean, thus they have much more water. You can't help but
thinking we've been deprived and we didn't even know it.
Yousuf Khan
Andrew Usher
> > That in dwarfs (main sequence stars) the existence of planets is
> > positively correlated to metallicity, while (according to that paper)
> > the opposite holds in giants. Since all giants evolved from dwarfs,
> > and their planetary systems should have already been present, this is
> > paradoxical.
>
> Are you talking about metallicity in general, or specifically about
> lithium metallicity?
Maybe you need to read the paper again. My paper does not say anything
about lithium. It says exactly what I related above.
> If it's lithium, then the other study seems to
> conclude that planet formation requires lower lithium levels for all
> stars. So maybe general metallicity has nothing to do with planet
> formation, just only lithium metallicity?
No. The paper on lithium says that planet formation must somehow cause
conditions that destroy lithium inside the star. All stars are born
with essentially the same amount of lithium.
Andrew Usher