Introduction

The strength of a line is characterized by its equivalent width (see figure 4). The continuous background – or continuous spectrum – being taken as unity, the equivalent width is the surface enclosed by the line profile. The abscissa can be expressed either in terms of wavelength or on a frequency (or wavenumber) scale. The equivalent width is given in the same units as the abscissa and can thus be expressed in angstrom units (1 Å = 10−8 cm), or in reciprocal centimeters. Habitually one speaks of a strong line as a line having an equivalent width (W> 1 Å and of a weak line as one having W<0.1 Å.

The difficulties in measuring equivalent widths usually come from the tracing of the continuum. It can happen that there are too many lines in the region that one is studying, so that the region is so crowded that the background becomes invisible. This happens in all late type stars because of the number of lines present in their spectra. The larger the plate factor is (in Å mm−1), the more the lines are compressed and one cannot any longer see a stretch of unperturbed continuum. A partial solution is to use lower plate factors, which may allow one to find (narrow) windows of the true continuum.

In early type stars fewer lines are present and therefore one might think that the true continuum is easy to trace; but among these stars one finds a large number of rapid rotators. If rotation is very fast it draws out the line profiles, increasing superposition of neighboring lines and again the continuous spectrum is perturbed.

This chapter contains two sections, which deal with metals and rare earth elements respectively. The behavior of both groups of elements presents certain general characteristics, which would be completely lost if they were described in the sections on individual elements.

The behavior of metals

The term ‘metals’ is used by astronomers in an ill-defined way. It may refer to ‘all elements other than He’ or to ‘elements with Z>8’ or to ‘elements with Z> 21’. One should always try to ascertain what a given author means when he uses the word ‘metals’. If the term ‘metals’ is ill-defined then the same happens with ‘metal abundances’. The abundances of metals are usually defined with respect to hydrogen, in the sense N(m)/N(H), normalized with respect to the same ratio in the sun. More often one uses the decimal logarithm of this quantity in order to have small numbers without exponents. So for instance [Fe/H] = −0.6 means that the abundance of iron with respect to hydrogen is four times less than in the sun. Usually this is abbreviated to −0.6 dex.

Another possible definition, due to Kudritzki (1987), is to consider the ratio N(m)/N(H)+4N(He). This has the advantage that the denominator remains constant if the star is in its hydrogen burning phase

The two preceding definitions are clear and unambiguous. However, very often the discussions based upon them become ambiguous because authors speak of ‘metals’ when in reality they are referring to the abundance of one individual element.

This element was discovered by W. Ramsay and W. Travers in London in 1898. Its name comes from the Greek xenos (stranger).

lonization energies

Xel 12.1 eV, Xell 21.2 eV, Xelll 32.1 eV.

Behavior in stars

No Xe lines have been observed in the sun.

Xell was found by Bidelman (1962b, 1966) in two Ap stars of the Hg-Mn subgroup. Adelman (1987) measured W(4415)=0.009 for Xell and Adelman (1992) found W(4603)=0.016. Jaschek and Brandi (1972) found Xe in one star of the Cr-Eu-Sr subgroup. See also Andersen et al. (1984).

Isotopes

Xe has six stable isotopes, namely Xe 129, 130, 131, 132, 134 and 136, which occur in the solar system with 27%, 4%, 21%, 27%, 10% and 9% abundances respectively. There exist also 25 short-lived isotopes and isomers.

Origin

Xe can be produced by the r, s and p processes. Xe130 is an s process product, Xe 129, 131, 134 and 136 are pure r products and Xe132 can be produced by either the r process or the s process.

This element was discovered by M. H. Klaproth in Berlin in 1789. Its name alludes to the planet Uranus, discovered by Herschel in 1781, which in turn alludes to the Greek god Urania.

lonization energies

UI 6.1 eV, UII 14.7 eV.

U is the heaviest stable element. It has not been observed in the sun.

Behavior in non-normal stars

UII was discovered independently by Guthrie (1969) and by Jaschek and Malaroda (1970) through the presence of 3859 in Ap stars of the Sr-Cr-Eu subgroup. The two groups each found this line in a different Ap star. It was also observed by Cowley et al (1974) in other stars of this subgroup. Typically W(3859)=0.030.

A detailed study of U and its isotopes was made by Cowley et al. (1977).

Isotopes

The longest lived isotope of U is U238, with a half life of 4.5 x109 years. It is followed by U235 with a half life of 7.0 X108 years and by 13 isotopes and isomers with shorter half lives. In the solar system U238 makes up 99.3% of all U. The isotopes of U can be used for radioactive dating.

Origin

The U isotopes are produced by the actinide-producing r process.

Most of the terminology used by astronomers for spectral lines follows the definitions of the physicists. Let us recall briefly the meaning of some terms that are used in this book. For more details, the reader can consult any book on spectroscopy, like Atomic Spectra and Atomic Structure by Herzberg (1945) or Structure and Spectra of Atoms by Richards and Scott (1976).

An atom can have all its electrons, in which case it is said to be in the neutral state. If it has lost one electron it is said to be singly ionized and if it has lost n electrons it is said to be n times ionized. Degrees of ionization are indicated by Roman numerals – Nal is neutral sodium, Nail singly ionized sodium and so forth. If one wants to refer to any ionization stage of an atom then one speaks of species.

A spectral line is the result of the transition of an electron between two energy levels. Among the different energy levels available for an electron, some transitions are permitted by the selection rules. Among these one calls fundamental, resonance or ultimate lines those connecting a level to the lowest energy level. Usually these lines are the most intense ones.

Forbidden lines are formed by transitions between levels that are not allowed by the selection rules. Forbidden lines are indicated by the element enclosed in a square bracket – for example [Fell]. If the bracket appears only on the right-hand side – for example OIV] – this corresponds to a (forbidden) intersystem line.

In the first chapter of part two we discuss the behavior of molecules in stars; in chapter two, we consider groups of elements and in chapter three, stellar chromospheres and coronas are discussed.

Chapter 1

In this chapter we provide a general description of the behavior of molecules in stars and in circumstellar envelopes. After a general introduction, the behavior of each molecule is described in detail, as we did for the elemental atoms. At the end of the chapter we provide a general summary of the behavior of molecules in stars, as we did for the individual atoms in stars.

Chapter 2

This chapter contains sections on the behavior of metals and on the behavior of rare earths.

Chapter 3

This chapter provides a summary of our knowledge on stellar chromospheres and coronas.

No detailed description of the content of chapters two and three is necessary.

This part contains auxiliary tables, which facilitate reading of the book.

1. 1 the periodic table of chemical elements

2. 2 a table of chemical elements ordered by name

3. 3 a table of chemical elements ordered by formula (chemical symbol)

4. 4 a table of chemical elements ordered by atomic number

5. 5 a table of cosmic abundances of the elements.

6. 6 a table of surface gravity as a function of spectral type and luminosity class

7. 7 a table of effective temperatures as a function of spectral type and luminosity class.

This element was discovered by S. Tennant in London in 1803. The name comes from the Greek osme (smell).

lonization energies

OsI 8.7 eV, OsII 16.6 eV, OsIII 24.9 eV.

Absorption lines of OsI

The equivalent width of OsI 3232(3) in the sun is 0.018. OsI is present in M 2 III stars (Davis 1947).

Behavior in non-normal stars

Osl and OsII lines were identified by Guthrie (1969) in one Ap star of the Cr- Eu-Sr subgroup W(4608, OsII)=0.038 and W(4421, OsI)=0.040. Brandi and Jaschek (1970) and Cowley (1987) later identified this element in other Ap stars.

Isotopes

Os has seven stable isotopes, Os 184, 186, 187, 188, 189, 190 and 192. These occur in the solar system with frequencies 0.02%, 2%, 2%, 13%, 16%, 26% and 41% respectively. There also exist 12 unstable isotopes and isomers.

Origin

Os 189,190 and 192 are pure r process products. Os188 can be produced by both the r process and the s process. Os186 and Os187 are pure s process products and Os184 is produced only by the p process.

This element was discovered independently by J. Priestley in Leeds, England in 1774 and by C. Scheele in Uppsala, Sweden in 1771. The name comes from the Greek oxy genes (acid forming).

In this part of the book we shall provide short summaries (but no detailed treatment) on different topics, which are necessary for a better understanding of the book. We have grouped these matters in five chapters, namely

1. 1 Terminology of spectral lines

2. 2 The selection of stars

3. 3 Line identification

4. 4 Equivalent widths

5. 5 Abundances

We have added also chapter 6, which contains some general thoughts on the matters covered in this book, which constitutes the epilogue.

The purpose of this book is to provide an outline of our knowledge about the behavior of the chemical elements in stars. As every observational spectroscopist knows, one is often confronted with essentially simple questions of the following kinds. What is the behavior of a given element in a given group of stars, for example, europium in metallic line stars or in S-type stars? Are the neutral lines of this element visible, are they strengthened or weakened with regard to those of normal dwarfs? Questions like these are often difficult to answer even for specialists and we have thus thought that it would be useful to collect the available information and to present it in such a way as to be useful for others.

We have reviewed the literature for both normal and non-normal stars, in the classical wavelength region (3800–4800 Å) as well as in the ultraviolet and the infrared (when available) for both absorption and emission lines. We have tried to stick as closely as possible to observations and to refrain from interpretation; this means for instance that we quote equivalent widths rather than abundances, whenever possible. The separation of observations from interpretation is especially useful in fields that are in a constant state of flux. This alludes for instance to interpretations of observed abundances in terms of the thermonuclear processes going on in the stars, or to interpretations involving physical processes like diffusion in stellar atmospheres or mechanisms for heating of the corona.

This element was discovered by C. Mossander in Stockholm in 1842. The name comes from the city of Ytterby in Sweden. This city is also referred to in the names of the elements yttrium and terbium.

lonization energies

ErI 6.1 eV, ErII 11.9 eV, ErIII 22.7 eV, ErIV 42.6 eV.

Absorption lines of ErII and ErIII

ErII 3616 is seen in one FOIb star (W=0.015) according to Reynolds et al (1988).

Behavior in non-normal stars

ErII lines are strengthened in the spectra of some Ap stars of the Cr-Eu-Sr subgroup. Aikman et al. (1979) also observed ErIII in the spectra of stars with strong ErII lines, and this was confirmed by Cowley and Greenberg (1987). The ErIII line at 4000 has W= 0.040. ErII and ErIII lines were also detected in the spectrum of one Bp star of the Si subgroup (Cowley and Crosswhite 1978).

ErII lines are also seen in at least one Am star (van t'Veer-Menneret etal 1988) with W[4009)=0052.

ErII lines are enhanced in at least one Ba star (Lambert 1985) and in at least one S-type star (Bidelman 1953).

Isotopes

Er has six stable isotopes and ten unstable isotopes and isomers. The stable ones are Er 162, 164, 166, 167, 168 and 170. In the solar system Er166 represents 33% and Er 167, 168 and 170 respectively 23%, 27% and 14%.

Origin

Er is made by several processes, Er162 by the p process, Er167 and Er170 by the r process and the others can be made by two processes, namely Er164 by the p or the s and Er166 and Er168 by either the r or the s process.

This element was discovered by W. Noddack, I. Tacke and O. Berg in 1925 in Berlin. Its name comes from the Rhine river, called Rhenium in Latin.

lonization energies

Rel 7.9 eV, Rell 13.1 eV, Re III 26.0 eV.

Isotopes

Re has two stable isotopes, Re185 and Re187, which occur with frequencies 37% and 63% respectively in the solar system. There exist also 18 short-lived isotopes and isomers. Re187 has a half life of 4.5 x1010 years and can be used for radioactive dating.

Origin

Re185 can be produced by the r process and the s process and Re187 only by the r process.

This element was discovered by W. Ramsay and M. W. Travers in 1898 in London. Its name comes from the Greek kryptos (hidden).

lonization energies

KrI 14.0 eV, KrII 24.4 eV, KrIII 36.9 eV.

Absorption lines of KrI

No Kr line is seen in the solar spectrum.

Absorption lines of KrII

Following Bidelman's (1960) discovery of KrII in 3 Cen A, this element was observed in some other hot Bp stars. W(4355)=0.025 according to Hardorp (1966).

Emission lines of KrIII

Lines of [KrIII] are seen in the spectrum of at least one recurrent nova (Joy and Swings 1945).

Isotopes

Kr occurs in the form of six stable isotopes and 17 short-lived isotopes and isomers. The stable isotopes are Kr 78, 80, 82, 83, 84 and 86. In the solar system they represent 0.3%, 2%, 12%, 11%, 57% and 17% abundances respectively. Among the unstable isotopes, Kr81 has a half life of 2x 105 years.

Origin

Kr 83, 84 and 86 can be produced by both the r and the s process, Kr82 is a pure s process product, Kr78 a pure p product and Kr80 can be produced by either the s or the p process.