Re: Why Would Aliens Hide?

Amara Graps (
Wed, 24 Nov 1999 21:20:02 +0100

Eric Watt Forste ( 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

>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" 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 :-) )


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.

Amara Graps               | Max-Planck-Institut fuer Kernphysik
Interplanetary Dust Group | Saupfercheckweg 1
+49-6221-516-543          | 69117 Heidelberg, GERMANY *

"Never fight an inanimate object." - P. J. O'Rourke