Re: Why Would Aliens Hide?

Amara Graps (
Fri, 26 Nov 1999 13:04:03 +0100

Eric Watt Forste ( 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.)

The abstract follows.


Dust in the Local Interstellar Wind

Priscilla C. Frisch
Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637

Johann M. Dorschner
Astrophysikalisches Institut und Universitäts-Sternwarte, Schillergaesschen 3, D-07745 Jena, Germany

Johannes Geiss
International Space Science Institute, Bern, Switzerland

J. Mayo Greenberg
Leiden Observatory Laboratory, Postbus 9504, 2300 RA Leiden, The Netherlands

Eberhard Grün and Markus Landgraf 1
Max-Planck-Institut für Kernphysik, Heidelberg, Germany

Peter Hoppe
Max-Planck-Institut für Chemistry, Cosmochemistry Department, P.O. Box 3060, D-55020 Mainz, Germany

Anthony P. Jones
Institut d'Astrophysique Spatiale, Université Paris XI, Bâtiment 121, 91405 Orsay Cedex, France

Wolfgang Krätschmer
Max-Planck-Institut für Kernphysik, Heidelberg, Germany

Timur J. Linde 2
University of Michigan, Aerospace Engineering, Ann Arbor, MI 48109

Gregor E. Morfill
Max-Planck-Institut für extraterrestrische Physik, 85740 Garching, Germany

William Reach
Infrared Processing and Analysis Center, California Institute of Technology, Mail Stop 100-22, Pasadena, CA 91125

Jonathan D. Slavin 3
Eureka Scientific Inc., 2452 Delmer Street, Suite 100, Oakland, CA 94602-3017

Jiri Svestka
Prague Observatory, Prague, 11846 Czech Republic

Adolf N. Witt
Ritter Astrophysical Research Center, University of Toledo, Toledo, OH 43606

Gary P. Zank
Bartol Research Institute, University of Delaware, Newark, DE 19716

Received 1998 October 9; accepted 1999 May 7

 Journal:            The Astrophysical Journal, Volume 525, Issue 1, pp.
                     492-516. (ApJ Homepage)
 Publication Date:   11/1999
 Origin:             UCP

Abstract Copyright: (c) 1999: The American Astronomical Society Bibliographic Code: 1999ApJ...525..492F

The gas-to-dust mass ratios found for interstellar dust within the solar system, versus values determined astronomically for the cloud around the solar system, suggest that large and small interstellar grains have separate histories and that large interstellar grains preferentially detected by spacecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectrum and flux rate of interstellar dust within the heliosphere are combined with information about the density, composition, and relative flow speed and direction of interstellar gas in the cloud surrounding the solar system to derive an in situ value for the gas-to-dust mass ratio, R<SUB>g/d</SUB>=94<SUP>+46</SUP><SUB>-38</SUB>. This ratio is dominated by the larger near-micron-sized grains. Including an estimate for the mass of smaller grains, which do not penetrate the heliosphere owing to charged grain interactions with heliosheath and solar wind plasmas, and including estimates for the mass of the larger population of interstellar micrometeorites, the total gas-to-dust mass ratio in the cloud surrounding the solar system is half this value. Based on in situ data, interstellar dust grains in the 10<SUP>-14</SUP> to 10<SUP>-13</SUP> g mass range are underabundant in the solar system, compared to a Mathis, Rumple, & Nordsiek mass distribution scaled to the local interstellar gas density, because such small grains do not penetrate the heliosphere. The gas-to-dust mass ratios are also derived by combining spectroscopic observations of the gas-phase abundances in the nearest interstellar clouds. Measurements of interstellar absorption lines formed in the cloud around the solar system, as seen in the direction of &epsi; CMa, give
R<SUB>g/d</SUB>=427<SUP>+72</SUP><SUB>-207</SUB> for assumed solar reference abundances and R<SUB>g/d</SUB>=551<SUP>+61</SUP><SUB>-251</SUB> for assumed B star reference abundances. These values exceed the in situ value suggesting either that grain mixing or grain histories are not correctly understood or that sweptup stardust is present. Such high values for diffuse interstellar clouds are strongly supported by diffuse cloud data seen toward &lambda; Sco and 23 Ori, provided B star reference abundances apply. If solar reference abundances prevail, however, the surrounding cloud is seen to have greater than normal dust destruction compared to higher column density diffuse clouds. The cloud surrounding the solar system exhibits enhanced gas-phase abundances of refractory elements such as Fe<SUP>+</SUP> and Mg<SUP>+</SUP>, indicating the destruction of dust grains by shock fronts. The good correlation locally between Fe<SUP>+</SUP> and Mg<SUP>+</SUP> indicates that the gas-phase abundances of these elements are dominated by grain destruction, while the poor correlation between Fe<SUP>+</SUP> and H<SUP>0</SUP> indicates either variable gas ionization or the decoupling of neutral gas and dust over parsec scale lengths. These abundances, combined with grain destruction models, indicate that the nearest interstellar material has been shocked with shocks of velocity ~150 km s<SUP>-1</SUP>. If solar reference abundances are correct, the low R<SUB>g/d</SUB> value toward &lambda; Sco may indicate that at least one cloud component in this direction contains dust grains that have retained their silicate mantles and are responsible for the polarization of the light from nearby stars seen in this general region. Weak frictional coupling between gas and dust in nearby low density gas permit inhomogeneities to be present.


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