[extropy-chat] Dark matter and ET
Amara Graps
amara at amara.com
Fri Jul 15 06:28:07 UTC 2005
> http://arxiv.org/abs/astro-ph/0506110
>Galactic Gradients, Postbiological Evolution and the Apparent Failure
>of SETI
>Authors:
>Milan M. Cirkovic,
>Robert J. Bradbury
Also, note the topic here on the list 11/23/99-11/26/99,
'Why would Aliens Hide'. The archives are not online, so I will repost
what I have in my archive.
-------------------------------------------------
Re: Why Would Aliens Hide?
Eric Watt Forste (arkuat at idiom.com)
Tue, 23 Nov 1999 15:11:33 -0800
Robert Bradbury writes:
> On Sat, 20 Nov 1999 Spudboy100 at aol.com wrote:
> > Unless we gain positve evidence, Fermi and Tipler are right.
> > We have met 'intelligent-life' and they is us.
>
> Distinctly possible, but saying it doesn't explain *why* that is
> the case.
Perhaps Universe is so young that we are the first kids on the block.
I have been looking around for data on what a chemist would call the
reaction rate of the production of metals in this and other galaxies.
Lighter metals such as carbon and oxygen are produced in medium-size
stars and distributed by planetary nebula ejection and by white-dwarf
novas. The heaviest elements are made and distributed only in
supernovas. There are several different kinds of events, and each has
a different characteristic rate, and the rate for each event varies
from galaxy to galaxy. As a result, I haven't been able to find any
good numbers yet. I did find a textbook of cosmochemistry that looked
promising, but if the numbers I'm looking for were in there, they were
deeply buried in mathematics that I have yet to do the homework for.
Another way to estimate these rates is to look for a metallicity
gradient in redshift. How much richer in metal are nearby (older)
galaxies with respect to distant (younger) galaxies? I haven't yet
done any research in this direction. I don't even know whether it is
more difficult to measure metallicity from distant-galaxy spectra than
from individual star spectra.
In the absence of this information, I'm comfortable with the
assumption that most planetary systems formed before 4.6 gigayears ago
(when ours formed) were below the metallicity threshold required for
the spontaneous development of self-reproducing molecules.
If any astrophysicists or cosmochemists on the list know where to get
to the data that bear on this question (Amara?), I'd love to learn
more about it.
--
arkuat
--
Re: Why Would Aliens Hide?
Robert J. Bradbury (bradbury at www.aeiveos.com)
Wed, 24 Nov 1999 01:14:53 -0800 (PST)
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[ Next ]In reply to: Eric Watt Forste Next in thread: Liam. A. Chu
On Tue, 23 Nov 1999, Eric Watt Forste wrote:
> Perhaps Universe is so young that we are the first kids on the
> block.
Doubtful, Kardashev points out in several of his articles that the age
of the universe is 2, perhaps 3 times the age of our solar system.
>
> ... Lighter metals such as carbon and oxygen are produced in
> medium-size stars and distributed by planetary nebula ejection and
> by white-dwarf novas. The heaviest elements are made and distributed
> only in supernovas. There are several different kinds of events,
> and each has a different characteristic rate, and the rate for each
> event varies from galaxy to galaxy.
The heavy metals get produced very rapidly, within a few hundred
million years of the formation of the galaxies (high H gas cloud
density leads to rapid formation of stars with M > 10 M_sun that go
supernova after very short lives).
>From Astronomy Today (1997):
Star Class Mass Luminosity Lifetime
(M_sun) (L_sun) (years)
Rigel B8Ia 10 44000 20*10^6
Sirus A1V 2.3 23 1000*10^6
A. Centuri G2V 1.1 1.4 7000*10^6
Sun G2V 1.0 1.0 10000*10^6
P. Centuri M5V 0.1 0.6 1000000*10^6
So, the big stars burn very fast. Supernovas, I believe also throw out
a fair amount of C & O. I'm unsure of the relative contributions of
"life elements" by novas vs. SN but am fairly sure that you get
sufficient materials within the first couple of billion years to form
planetary systems that can support life. A more serious constraint
might be the gradual decline in nearby SN that would fatally damage
proto-genetic material. But we have discussed that extensively. And of
course Deinococcus radiodurans does show that life can be radition
tolerant.
> I did find a textbook of
> cosmochemistry that looked promising, but if the numbers I'm looking
> for were in there, they were deeply buried in mathematics that I
> have yet to do the homework for.
The best book, IMO, is "Nucleosynthesis and Chemical Evolution of
Galaxies" by Bernard E. J. Pagel, and you are right that the numbers
are probably buried in the math and a collection of astronomy papers.
>
> Another way to estimate these rates is to look for a metallicity
> gradient in redshift. How much richer in metal are nearby (older)
> galaxies with respect to distant (younger) galaxies? I haven't yet
> done any research in this direction. I don't even know whether it
> is more difficult to measure metallicity from distant-galaxy spectra
> than from individual star spectra.
It is more difficult, because the lines will be weaker and you will
need longer observation times. You probably also have an interesting
problem of an assortment of star ages in a galactic "point source" and
the line doppler shifting caused by stars rotating toward you and away
from you (in side on) galaxies. You may be limited to picking a
handful of stars of a specific spectral class and measuring the
average metal abundances in them. But since stars in our galaxy have
some really wide ranges of metal abundances (probably depending on
local space region history) extracting an overall accumulation of
metal profile for a galaxy seems really difficult.
This type of work has been done in globular clusters that seem to be
old (because the metal content is low). But this fails to consider
whether the metal content is low because they might have been mined.
If you need "rare" elements and are planning to be around for
trillions of years, it might make sense to construct "star-processing"
stations. You then run simulations of galactic orbital dynamics and
locate good sling-shot candidates whare a minor orbital tweek will
send the star out of the galaxy to one of the "star-processing"
stations (aka globular cluster). The stars we now see there are the
leftovers.
>
> In the absence of this information, I'm comfortable with the
> assumption that most planetary systems formed before 4.6 gigayears
> ago (when ours formed) were below the metallicity threshold required
> for the spontaneous development of self-reproducing molecules.
Bad bet, IMO. I'd push the threshold for planetary systems back to
8-10 Gyr. Keeping in mind that *if* star mining can occur on a large
scale, then the stellar age estimates based on metal accumulation will
be *underestimates*.
The problems the biotech people are going to have with the right and
left wing luddites are going to be small compared to the problems the
transhumanists are going to have getting the astrophysicists to
consider a "life-filled" universe.
Robert
--
To: extropians at extropy.org
From: Amara Graps <amara at amara.com>
Subject: Re: Why Would Aliens Hide?
Eric Watt Forste (arkuat at idiom.com) Tue, 23 Nov 1999 writes:
I'm not a cosmochemist, but I have some words that might help.
>I have been looking around for data on what a chemist would call the
>reaction rate of the production of metals in this and other
>galaxies.
>...
>Another way to estimate these rates is to look for a metallicity
>gradient in redshift. How much richer in metal are nearby (older)
>galaxies with respect to distant (younger) galaxies?
I did a quick search on the Web when I saw your question (I didn't
have time to try the NASA ADS, but I recommend that for you, using
some keywords like "metallicity production rate galaxies" or some
combination of it in the keywords field), and I found a paper:
"Metals and Dust in High Redshift Galaxies"
by Pettini, M., King,D., Smith, L., Lipman, K., Hunstead, R.
They report from observations of 30 distant QSOs that the epoch of
chemical enrichment in galaxies may have begun at z~2.5-3, which
corresponds to a look-back time of 14 Gyr (using expansion constants
H_0 = 50 km/s/Mpc, q_0 = 0.01), and that at z~2, the typical
metallicity of the universe was 1/15 of the solar value.
>Lighter metals such as carbon and oxygen are produced in medium-size
>stars and distributed by planetary nebula ejection and by
>white-dwarf novas. The heaviest elements are made and distributed
>only in supernovas. There are several different kinds of events, and
>each has a different characteristic rate, and the rate for each
>event varies from galaxy to galaxy.
That's what I used to think too ... but every once in a while I bump
into research where there seems to be some open questions about
whether stars can really produce all of the dust that we observe. In
a talk that I heard today at my institute, the conclusion was that
stars cannot produce all of the dust that we see in galactic disks.
And some years back, I found an interesting table in Gehrz's chapter
(pg 447) of the IAU #135 Interstellar Dust book: "Types of Dust
Grains in Stellar Outflows", that lays out dust production/dust
deficit:
Stellar Type Input to Interstellar Medium, Relative to all Stars
M Stars (Miras) 35%
RLOH/IR stars 32%
Carbon stars 20%
Supernovae 8%
M supergiants 4%
Wolf-Rayet stars 0.5%
Planetary Nebulae 0.2%
Novae 0.1%
RV Tauri stars 0.02%
O,B stars 0
Gehrz concludes in his last section titled: "The Ecology of Stardust in
the Galaxy" that:
1. M stars, RLOH/IR stars and M supergiants are the primary sources
of silicates, while carbon stars, WR stars and novae produce most of
the carbon and SiC. Novae, supernovae, and WR stars may be
responsible for most of the grains with chemical anomalies.
2. The current star formation rate implies that star formation is
depleting the interstellar medium (ISM) gas by some 3 to 10 solar
masses per year.
3. There is a deficit in stardust production/grain destruction.
Supernovae shock waves destroy ISM grains on very short time-scales
(Seab, 1987, _Interstellar Processes_, Hollenbach and Thronson ed.
Reidel) processing 10-30 solar masses per year and destroying
0.1-0.3 solar masses per year in dust. Gehrz estimates that
0.01-0.08 solar masses per year of dust is returned to the ISM by
stars. He feels that grain growth in dark clouds is an attractive
mechanism to make up the dust deficit.
(I pulled the above from my essay: "Cosmic Dust and its Evolution"
http://www.amara.com/ftpstuff/dustevolve.txt It might answer some of
your questions too.)
In the talk I heard today, I saw a similar production/deficit list
of dust sources in our galaxy from a paper (Jones, 1997, I don't
know the title but NASA ADS should help you), that indicates a
factor of 10 deficit in the production rate. Known sources of dust
input dust into our galaxy, but when observational measurements are
made of amounts of dust present in our galaxy, these known sources
of dust are not enough (I believe that I have this argument correct.
Please go check Jones' paper, though, to be sure.)
Some other open questions. (This was presented in the talk given
today too)
The interstellar grains that we've detected in-situ with the Ulysses
dust detector instrument indicates more interstellar dust, and
bigger grains than from model predictions for our local
insterstellar medium neighborhood.
Other work using ISOPHOT infrared data of our galaxy are pointing to
the fact that we need a new population of large grains to explain
what we see in our galaxy, and that we need more gas, and that we
need larger grains (which qualitatively agrees with the above
in-situ observations).
So to recap we see more dust than what one would expect from current
models (I'm not an expert on interstellar medium models, so I can't
give any details about the models.)
I hope that this answers some of your questions (and raises new ones
:-) )
Amara
P.S.
I see a couple of sections in a book that might help you:
"Chemical-Composition Gradients in the Disk" and
"Chemical-Composition Gradients in the Spheroidal Component", in the
book: _Galactic Astronomy_ by Mihalas and Binney, but this book is a
little bit old (1981), and probably Robert's suggestion for a
book is better.
Date: Fri, 26 Nov 1999 13:04:03 +0100
From: Amara Graps <Amara.Graps at mpi-hd.mpg.de>
To: extropians at extropy.org
Subject: Re: Why Would Aliens Hide?
Eric Watt Forste (arkuat at idiom.com) Wed, 24 Nov 1999 writes:
>How do current elemental abundances observed in the interstellar
>medium of the galaxy compare to these pristine Solar system
>abundances? Has the interstellar medium been significantly enriched in
>the 4.6 Gyrs since the formation of the Sun? Where are the extra
>metals coming from if our models of stellar evolution and observations
>of actual stars cannot account for them?
I've had some more time to think about this.
I think the direction of research is that, the picture for what we
_used to think_ (say 10 years ago) for dust producing stars/etc is
changing. For example, Supernovae used to _not_ be thought of as a
heavy dust producer, but it looks like they produce a lot more dust
than we thought. (At least that's what I got out of the lecture a
couple of days ago.)
Also, there is plenty of evidence now that supernovae had an important
role in the formation of our solar system because of the physical
properties (shock fronts etc.) of the "bubbles" in our local
interstellar cloud that surround our solar system, and also because of
measurements of minerals in presolar grains of meterorites that could
only be formed by r,s,p processes in late-evolution stars.
>Have there been other
>processes of metal formation going on, or are our models of stellar
>evolution going to require considerably more adjustment before they
>synch up with the cosmochemical details we observe?
The models of stellar evolution are probably OK, maybe some tweaking
in the concepts of grain formation..
And we know that dust cannot simply condense out of the gas in the
interstellar medium because the density and temperatures are not
right.
The ISO observations of the amount of dust in our Galaxy being off by
about a factor 100 from what the scientists count (currently) as dust
sources is a problem.
The other observations that are off by a factor 10, may not be so
worrisome, because the discrepancy is possibly in the error bars, and
a factor of 10 off in astronomy is sometimes OK ;-)
I should direct you to a newly-published paper (1 Nov 1999, ApJ) that
gives a large overview of this topic. "
"Dust in the Local Interstellar Wind" by P. Frisch and a large list of
authors. She is an expert on our local interstellar medium and the
relationships between our local bubble and our Solar System.
In particular, look at:
Section 6.2: "Isotopic Compositions and Stellar Sources"
Section 6.4: "Presolar Silicates and GEMS"
And this Table 4: (I hope that this table doesn't get too mangled in
the email translation)
Table 4 Types of Presolar Grains in Primitive Meteorites
Abundance Size Isotopic
Mineral (PPM) (micron) Signature Stellar Sources
Diamond... 1400 0.002 Xe-HL Type
II supernovae
SiC 14 0.1-20 Enhancementsa
C-rich AGB stars
mainstream... in 13C, 14N, 22Ne,
heavy trace elements
Graphite... 10 0.8-12 Enhancements Type
II supernovae,
in 12C, 18O,
(Wolf-Rayet stars)
extinct 44Ti
Corundum... 0.3 0.3-5 Enhancements Red
giant, AGB stars
in 17O, Depletion
in 18O
SiC X 0.1 0.5-10 Enhancements Type
II supernovae
grains... in 12C, 15N, 28Si
Extinct 26Al, 44Ti
Silicon 0.002 ~1 Enhancements Type
II supernovae
nitride... in 12C, 15N, 28Si
Extinct 26Al
a Enhancement values are given relative to the solar system isotopic
composition.
(I just discovered this paper this morning, and it's a long and detailed
paper, so I won't summarize it here.)
Amara
--
********************************************************************
Amara Graps, PhD email: amara at amara.com
Computational Physics vita: ftp://ftp.amara.com/pub/resume.txt
Multiplex Answers URL: http://www.amara.com/
********************************************************************
"Oh you damned observers, you always find extra things."
-- Fred Hoyle [quoted by Richard Ellis at IAU Symposium 183]
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