Stars and Interstellar Matter

The intermediate stages between interstellar matter and the formation of protostars are seen in very dense and very cool clouds of interstellar matter. 

The dense clouds mentioned in the first paragraph cannot be investigated by means of the X 21 cm line, or any optical transition, but only by means of molecular lines in the radio frequency range. This is why interstellar molecules are so important in modern astrophysics. Molecular lines contain much information on the physical state of these dense and cool clouds. Moreover, the abundance of interstellar molecules itself can shed light on the physical conditions, including the radiation field in dense clouds, once we more fully understand 

The determination and interpretation of element abundances plays an ever-increasing role in our effort to understand the evolution of the Universe. Molecular lines may not contribute to the determination of element abundances but they already play an important role in the determination of isotopic element abundances, and these in turn can be used to discriminate between various possible thermonuclear reaction chains. 

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Fig. 3. Distribution of stars and interstellar matter in the Galaxy. The lower part of the diagram is a cross section perpendicular to the galactic plane. Globular clusters are the oldest stellar systems and must therefore have been formed in the early evolutionary stages of the Galaxy. The mass of the stars, however, forms a flat layer with a nuclear bulge in the center. The interstellar matter forms an even flatter layer which widens up towards the edges of the Galaxy.

The concern of cosmochemistry is the investigation of extraterrestrial matter (sun, moon, planets, stars and interstellar matter) and their chemical changes. Meteorites are an object of special interest in cosmochemistry, because of the nuclear reactions induced by high-energy protons in cosmic radiation ( (p) up to about 10 GeV) and by other particles, such as a particles and various heavy ions. Measurement of the radionuclides produced in meteorites by cosmic radiation gives information about the intensity of this radiation in interstellar space and about the age and the history of meteorites. 

Figure 17.2 shows the relative abundance of the elements of the universe and of the earth. The abundances are approximate, as a consequence of die difficulties in their assessment and limitations of experimental techniques. The abundances in the universe (based on spectral measurements on stars and interstellar matter) are used as a refinement of data obtained for the solar system. Stellar light is divided in spectral classes depending on the surface temperature of the star, see Fig. 17.1. The various classes (Harvard Spectral Classification) show lines of the elements as listed below in approximately decreasing intensity ...

The space between the stars. The interstellar matter that occupies this space constitutes several percent of the Galaxy s total mass and It Is horn this matter that new stars are formed. The matter Is primarily hydrogen, in which a number of other molecules and radicals have been detected, together with small solid dust grains. On average the density of matter in interstellar space Is about 10 hydrogen atoms per cubic metre, but the gas Is not uniformly distributed, being clumped into interstellar clouds of various sizes and densities. 

Sometimes a star explodes in a supernova cast mg debris into interstellar space This debris includes the elements formed during the life of the star and these elements find their way into new stars formed when a cloud of matter collapses in on itself Our own sun is believed to be a second generation star one formed not only from hydrogen and helium but containing the elements formed in earlier stars as well... 

The dark clouds were responsible for the discovery of ISM, as they absorb the light from stars which lies behind these clouds of interstellar matter. It is difficult to obtain reliable information on the dust particles. They are probably about 0.1 pm in diameter, consisting of a silicate nucleus and an envelope of compounds containing the elements C, O and N, which, with H and He, are the main elements present in interstellar space. There are only two sources of information for more exact characterisation of the dust particles ... 

It is very useful to complement the compositional analysis of stars by a like analysis of the interstellar medium. This can be done by making use of absorption lines which the latter removes from the UV spectrum of hot, bright stars (Fig. 8.8). Measured abundances only concern gases lying between the source star and the observer. Matter contained in dust grains escapes detection. 

If all the matter in our galaxy, which is about evenly divided between the stars and the regions between them, were uniformly distributed, its average density would be about 6 X 10 24 g/cm3. The solid component of the interstellar medium composes about 5% of its mass. But despite its low average density—about 1.5 X 10-26 g/cm3—interstellar dust has an important effect on the distribution of electromagnetic radiation in our galaxy. 

Within some meteorites are also found minuscule presolar grains, providing an opportunity to analyze directly the chemistry of interstellar matter. Some of these tiny grains are pure samples of the matter ejected from dying stars and provide constraints on our understanding of how elements were forged inside stars before the Sun s birth. Once formed, these... 

Observations of isotopic abundances provides information on the nucleosynthesis operating in the compact core of stars and supernova explosions and on the chemical evolution of the Galaxy. The CNO nuclides in late-type stars are affected by freshly synthesized core material brought up by dredge-up events. On the other hand, the Si isotopes are involved in later phases of nuclear burning, a narrow span of the red giant lifetime before planetary nebulae or supernovae. Therefore relative abundances of Si isotopes we observe remain unchanged from those of interstellar matter from which a star was formed. 

This difficulty for CS 776 can be assessed quantitatively. Le Bertre (1990) derives a mass loss rate from IRC-20131 of 4.5 x 10 7 MQ a-1. Scaled to the outflow velocity of the material of 26 km s-1 (Zuckerman Dyck 1989) instead of the assumed value of 15 km s 1 and using Le Bertre s distance of 1.3 kpc instead of their estimate of 1.43 kpc, the re-computed mass loss rate from Claussen et al. (1987) is 6.6 x 10 Mq a-1 in reasonable agreement with the rate estimated by Le Bertre (1990). Because the separation of the A star companion from the carbon star CS 776 is 1.81", the projected separation between the two stars is 3.6 x 1016 cm. Therefore, according to equation (3), the column density of circumstellar hydrogen between us and the A star companion to CS 776 is 1.5 x 1019 cm-2. However, the total extinction toward this companion is Av = 1.71 mag (Le Bertre 1990) which, for a standard interstellar dust to gas ratio, corresponds to a hydrogen column density of 3x 1021 cm-2 (Spitzer 1978). This column density is consistent with the expected concentration of interstellar matter within the plane of the Milky Way. Thus, towards the companion to CS 776, there appears to be about 100 times more interstellar than circumstellar matter. Therefore, unless the diffuse bands are extremely strong in the circumstellar matter around CS 776, it seems quite likely that the bulk of the diffuse bands in the spectrum result from interstellar matter. 

To top off the action, dust grains condense within the cooling surface gas as it leaves the star as a dense wind. They are sprayed into the interstellar matter. The mainstream SiC grains have this origin. They record the unusual isotopes that were present in the surface gas when it cooled. These and other presolar grains are found within meteorites. They have inspired a new astronomy of AGB stars based on the isotopic analysis of the grains. They record isotopic information about the star with far greater precision than can be obtained by other astronomical means. 

The interstellar medium is the medium that fills the space between the stars. This space is far from empty. It includes magnetic fields, gas composed of atoms and ions at several different temperatures and densities, cosmic rays, and dust particles. The material content of the ISM changes with time owing to the formation of new stars from it and the ejection of matter from stars into it. The latter include the new nuclei thathave just been assembled by nucleosynthesis in the stars. The state of this medium is turbulent, driven by the shock waves from exploding supernovae. Dust comprises about one percent of the mass of the interstellar matter. It is measured by its infrared radiation and by its obscuration and reddening of starlight. 

The visible matter of galaxies is concentrated in mainly three components stars, interstellar matter, and stellar remnants. Since the early days of galaxy formation there is a vivid exchange of matter between the stellar component and the interstellar matter. Stars are formed in local concentrations of the ISM, the molecular clouds they live for a certain period of time while burning their nuclear fuels and they die... 

In effect, stars return between about 50% and 90% of their initial mass by winds or by explosive mass ejection. Figure 2.3 gives an overview of the relative contributions of the different stellar types to the total mass replenishment. The mass returned by the stars becomes part of the ISM and serves as raw material for the formation of the next stellar generations. In this way part of the baryonic matter in a galaxy is continuously cycled between stars and the interstellar matter. Only the very-low-mass stars (initial masses < 0.8 M0) are not involved in this matter cycle because they have lifetimes exceeding the present age of the Universe and have not yet evolved very much. Some fraction of the matter therefore accumulates in very-low-mass stars and in stellar remnants, but at least part of this loss from the matter cycle... 

The density and temperature distribution of interstellar matter, contrary to its elemental composition, is strongly inhomogeneous. At least three different phases exist (e.g. Tielens 2005) (i) extended low-density bubbles of hot ionized gas (hot interstellar medium or HIM, mass fraction 0.003, volume fraction 0.5), resulting from series of SN explosions in mass-rich stellar clusters (ii) cold and dense clouds of neutral gas (cold and neutral interstellar medium or CNM, mass fraction 0.3, volume fraction 0.01), resulting from sweeping up of warm gas and (iii) a warm, either ionized or neutral, medium in between (warm interstellar medium or WIM, mass fraction 0.5, volume fraction 0.5). The essential properties of the three phases are indicated in Fig. 2.4. The coolest and most massive of the clouds are the molecular clouds (MC, mass fraction 0.2, volume fraction 0.0005), a separate component, that are the places of star formation, where new stars are formed as stellar clusters with total masses between about 200 and several 106 M0. 

The interstellar matter continuously cycles through these phases and the molecular clouds HIM WIM CNM MC —HIM, WIM, and stars. The molecular clouds live typically for 20 Myr (Blitz el al. 2007) and convert about 25% of their mass into stars (10% to 30% according to Lada Lada 2003). Since they contain 20% of total mass, 4 x 5 = 20 cycles are required for 100% conversion of interstellar matter into stars. Since the cycle time between stars and the ISM is about... 

Cameron (1973) speculated that grains from stellar sources survive in the interstellar medium, become incorporated into bodies of the Solar System, and may be found in meteorites, because some meteorites represent nearly unprocessed material from the time of Solar System formation. These grains may be identified by unusual isotopic abundance ratios of some elements, since material from nuclear burning zones is mixed at the end of the life of stars into the matter from which dust is formed. Indeed, these presolar dust grains3 were found in the late 1980s in meteorites (and later also in other types of primitive Solar System matter) and they contain rich information on their formation conditions and on nucleosynthetic processes in stars (see Section 2.2). By identifying such grains in primitive Solar System matter it is possible to study the nature and composition of at least some components of the interstellar dust mixture in the laboratory. 

The average lifetimes of dust grains in the ISM of about 0.5 Gyr have to be compared with a turnaround time of about 2.5 Gyr for the matter cycle between stars and the ISM, which would result in a small depletion S 0.8 of the refractory elements in the ISM into dust, if depletion of the refractory elements in the returned mass from stars was strong and if no accretion of refractory elements onto dust occurred in the ISM. This clearly contradicts the high observed depletion in the ISM. Hence, most of the interstellar dust is formed in the ISM and is not stardust (Draine 1995 Zhukovska et al. 2008). The most likely place for dust growth in the ISM is in the dense molecular clouds (Draine 1990), but the processes responsible for growth are presently unknown. 

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