The Three Phase ISM
The interstellar medium (ISM) is the gas and dust between the stars. Like Gaul in Caesar's time, the ISM may be divided into three parts: a cold, dense phase dominated by molecular hydrogen (H2); a warmer, less dense phase dominated by neutral hydrogen (HI); and a hot, tenuous phase dominated by ionized hydrogen (HII). The three phases are not static but rather are dynamically interconverting on timescales of tens to hundreds of millions of years. Stars form in the dense molecular clouds. The more massive of these stars tend to heat and ionize the surrounding medium. Moreover, many of these stars eventually explode as supernovae, which further heats and ionizes the molecular cloud material and converts it to hot gas and dust. In this way, the supernovae can evaporate the molecular cloud. The hot, ionized medium eventually cools to become the warm interstellar medium (ISM). The warm ISM can further cool and condense to form molecular clouds, and the cycle repeats. Interestingly, the three phases of the ISM are thought to be in a rough pressure equilibrium. This means that, although the colder phases probably contain most of the mass of the ISM, the hot phase comprises most of the volume because of its considerably greater temperature.
Radioactivity in the Average ISM
In the course of their lives, stars create new atoms. The first conclusive proof of this came in the 1950s when Paul Merrill observed technetium in the atmospheres of N-type stars. Because technetium has no stable isotope, the star must have produced the observed technetium. When stars die, they eject these new atoms into the ISM in the form of gas and dust. The new atoms will remain in the ISM until they condense again into new stars. Because stars tend to build heavier nuclei from lighter ones, particularly hydrogen and helium, the ongoing cycle of stellar life and death leads to an enrichment of heavy isotopes in the ISM with time.
Such an enrichment with time does not necessarily occur for radioactive species, however. Stars are continuously creating new radioactive atoms and expelling them into the ISM, but there they decay with a rate specific to their atomic charge and mass. Over a long period of time, a steady state abundance of such radioactive species, such as 26Al, builds up in the ISM such that the rate of production of the radioactivity by stars balances its destruction by decay.
Interestingly, it is sometimes possible to observe the decay of radioactive isotopes in our Galaxy. For example, when 26Al decays, it leaves its daugher isotope 26Mg in an excited nuclear state. This state decays to its ground by emitting a gamma ray, which may be detected by satellites orbiting the Earth. From the number of gamma rays observed per unit time and the decay rate of the radioactivity, we may infer the abundance of that isotope in the ISM. For example, one may infer that the ISM in our Galaxy contains roughly 1 solar mass of 26Al, which in turn suggests about 5 solar masses of gas are turning into stars per year in our Galaxy.
Radioactivity in the Early Solar System
The gas and dust that condensed to form our Solar System inherited radioactive nuclei from the ISM. Evidence for this lies in the primitive minerals, such as calcium-aluminum-rich inclusions (CAIs), in some meteorites. These primitive minerals were some of the first solids to form in the solar system. Some of these minerals show excesses of daughter isotopes of radioactive species that correlate with stable parent isotopes. For example, CAIs from the Allende meteorite show excesses of 26Mg that correlate with the aluminum content of the mineral. From the slope of the correlation line, one infers that the early solar system had five 26Al atoms for every 100,000 27Al atoms.
How does the concentration of 26Al in the early solar system relate to the expected abundance in the ISM? More particularly, should we compare the ratio 26Al / 27Al = 5 x 10-5 to the ratio we would find the in the average ISM? Because we expect stars to form in the dense molecular clouds, the answer is probably no. Rather, we should compare the ratio inferred from the meteorites to that expected in the dense phase of the interstellar medium. This latter ratio may differ from the ratio expected in the average ISM, a number we can, for the 26Al/27Al ratio at least, infer from the gamma-ray observations. Calculating the ratio in the three phases of the ISM, however, is not a simple matter. It requires a knowledge of the phases of the ISM, their relative masses, and the timescales on which they interconvert.
The Three Phase ISM Tool
The purpose of our Three Phase ISM Tool is to allow an internet user to calculate the abundance ratio of two species in the average ISM and in each of the ISM's three phases. To do this, we employ a model developed by Donald Clayton. In this model, stars eject fresh radioactivities into the hot ISM (Clayton's phase 3) and must then work their way, via the cool clouds (phase 2), into the cold molecular clouds (his phase 1). In this way, the concentration of radioactivities in the cold clouds is less than that in the hot ISM, and the degree of this difference depends on the time for the phases to change into each other.
The tool requires input about the two species of interest, some parameters describing the evolution of the Galaxy, and parameters describing the three phases of the ISM. Upon submission of the data, the server at Clemson University computes the results and then sends them to the user's browser.
The best way to start using the tool is to try out the tutorials. Help links in the tool also aid the user in determining the correct input. More background and other related information is available from the Papers & Links sidebar link. We hope you enjoy the tool, and, as always, we welcome feedback.
