Ca I lines (see for instance 4226, the resonance line), appear at about A 0 and grow steadily toward later types. At K 5 the line has an equivalent width of about 10 Åand is still growing in intensity toward later types. It thus constitutes an important temperature indicator for spectral classification.

Before type K a slight positive luminosity effect exists. For stars cooler than K 2 the luminosity effect is negative. It becomes more pronounced in type (the 6439 line). For detailed comments see Burwell (1930) and Keenan and McNeil (1976). This behavior closely parallels that of the NaI line.

The CaI lines 4435 and 4455(4) also show a negative luminosity effect for late type stars (Adorns and Pease 1914).

Emission lines of Ca I

4226 (M.2) appears in emission in T Tau stars (Joy 1945).

Absorption lines of CaII

The values for 8542 from A 7 to G 2 are from Jaschek and Jaschek (unpublished) and those for the later typestars from Zhou Xu (1991).

The most important CaII lines in the classic region are theso-called Fraunhofer H and K lines, which correspond to M.1 (resonance lines), 3968 and 3933. In late type stars (types K, M, C and S) in the infrared region the dominating feature is the triplet 8498,8542 and 8662(2). The triplet lines have been used to determine physical parameters like gravity and metal abundance of late type stars (see for instance Diaz et al. (1989)).

CaII lines are seen faintly at mid-B-type. They increase steadily toward later types, with a maximum at about K 0 (for example 8542). A positive luminosity effect exists.

In late type giants and supergiants there very often exist circumstellar components in these lines, which can only be seen at high dispersion. The velocity and strength of these lines can be used to establish the mass loss of the stars (see for instance Sanner (1976)). The strength of the CAII line can also be measured photoelectrically (Lockwood 1968) which can provide a rapid assessment of the Ca abundance.

Emission fines of CaII

CaII emissions originate in the chromospheres of late type stars (see also the discussion on chromospheres) and are frequently used to identify mass motion and the presence of circumstellar material. The emissions appear in the enter of the deep absorption lines 3933 and 3968 (see figure). The emission itself may have a central reversal and be shifted with respect to the center of the absorption line. The emission peaks may also have different intensities. CaII emissions are a fairly common phenomenon. Broadly speaking they are rare in F-type, become more frequent in G-type and are very common in K- and IVI-type stars. Bidelman (1954) published a very useful catalog of Call emission objects.

Profiles of Call emissions.

Abscissa: wavelength in ångström units; ordinate: absolute flux. The two profiles correspond to the extreme profiles observed within the period covered by the observations. From Rebolo et al. (1989). (Courtesy of AA SuppL.)

The separation of the emission peaks (located within the broad absorption line) is related to the absolute magnitude of stars, in the form

This is the so-called Wilson-Bappu effect.

The CaII emission line intensity has been used extensively by Wilson and co-workers studying chromospheres (for details see Jaschek and Jaschek (1987a)). Among their most important results we may quote two, namely the demonstration of the existence of stellar cycles, analogous to the solar 11-year cycle and the demonstration of the (slow) rotation of late type stars.

The most recent catalog of CaII measurements is the one by Duncan et al. (1991). Another useful older catalog is the one by Glebocki et al. (1980), now replaced by Strassmeier et al. (1990), who have produced a catalog of chromospherically active stars.

T Tau stars frequently exhibit CaII in emission (Sun et al. 1985) both in the photographic and in the infrared. CaII also appears in emission in compact infrared sources (McGregor et al. 1984).

CaII emissions appear in classical Cepheids (of periods larger than about four days) after minimum light and persist typically for 0.4 of the period (Kraft 1960).

CaII emission is a characteristic phenomenon of long period variables, which is strongly phase-dependent. Whereas at maximum light the CaII H and K lines are seen in absorption, emission lines appear 40 days after maximum and persist during a major part of the cycle. The lines of the infrared triplet (M.2) on the other hand are present in emission around the maximum (Merrill 1960).

CaII lines appear in emission in the `principal spectrum' phase of novae (Warner 1989) and in supernovae (Fillipenko 1988). In the latter one sees also (CaII) 7290 in the emission spectrum.

CaII infrared emissions are seen in Oe stars (Andrillat et al. 1982). In early Be stars the infrared CaII triplet is frequently seen in emission (Polidan and Peters 1976), although in stars later than B 6 the phenomenon becomes rare. In general the behavior of the CaII emissions parallels that of the Paschen emissions (Andrillat et al. 1990). In B[e] stars the triplet is strongly emitted (Jaschek et al. 1992).

Strong CaII infrared emissions are also seen in cataclysmic variables (Persson 1988).

It should be added that in some Be stars (but also in S-type stars), the infrared Ca triplet and the H and K lines do not always behave in parallel - one may be found in emission, whereas the other is in the absorption (Bretz 1966).

Forbidden lines of higher ionization stages

Lines of CaXII (3328), CaXIII and CaXV, with ionization potentials between 589 and 814 eV are seen in the spectrum of the solar corona (Zirin 1988) (see also the discussion on coronae). CaV, VI, VII and XIII forbidden lines have been observed in the recurrent nova RS Oph (Joy and Swings 1945). Forbidden Ca V fines are present in the C star RX Pup (Swings and Strove 1941b).

Behavior in non-normal stars

Ca is weak in Ap stars of the Cr-Fu-Sr subgroup (Adelman 1973b).

CaII is very weak in Am stars, which is easily visible and has led many authors to classify Am stars according to the CaII line strength only. Since Ca is weak, the spectral type corresponding to these lines is always earlier than the other spectral types adjudicated on the basis of the hydrogen or metallic lines. The W values of CaII are smaller by factors about 3-4 than in normal stars of the same temperature.

Ca lines are also weak in delta Del stars (Kurtz 1976).

CaII is weak also in lambda Boo stars. Typically W values are smaller by factors 1.5-3 than in normal stars of the same temperature (Venn and Lambert 1990).

Ca lines are weakened in F-type HB stars by factors up to six with respect to normal stars of the same colours (Adelman and Hill 1987). Adelman and Philip (1992a) later extended this result to all HB stars.

CaII lines are present in the spectra of supernovae. Ca II and (CaII) emission lines are prominent in the nebular phase of supernovae of class II (Branch 1990).

The infrared Ca lines can be used to separate RV Tau stars from normal supergiants (Mantegazza 1991).

CaII lines exist in the spectra of many, if not all, DZ degenerates (Sinn et al. 1990), W(3933) up to 73 Å. CaII lines were also found in DB and DA stars (Sinn et al. 1988), here their presence was not expected, since DB stars had been defined as possessing only He Lines and DA stars as showing only hydrogen lines. (For more information see Jaschek and Jaschek (1987a)). On the other hand, Ca I is usually weak in DZ degenerates W < 2.5 Å (Sion et al. 1990).

Ca is overabundant with respect to iron in metal-weak stars (Luck and Bond 1985, Magain 1989) by a factor of me order of two and this seems also to be true for globular cluster stars (Wheeler et al. 1989, Francois 1991).

Ca I lines are weakened in the spectra of CH stars (Yamashita 1967).

Forbidden lines of CaV, CaVI and CaVII are sometimes observed in the spectra of symbiotic objects (Freitas Pacheco and Costa 1992).

Isotopes

Ca has 14 isotopes. Among these six are stable, namely Ca 40, 42, 43, 44, 46 and 48. In the solar system most (97% is in the form of Ca⁴⁰. Of the unstable isotopes, Ca^4l has a half life of 8 × 10⁴ years.

Origin

Ca⁴⁰ is produced by explosive synthesis and other stable isotopes by this process or others, namely Ca⁴² by oxygen burning, Ca⁴³ by carbon burning or the s process, Ca⁴⁴ by the s process and Ca⁴⁶ by carbon or neon burning.

Published in "The Behavior of Chemical Elements in Stars", Carlos Jaschek and Mercedes Jaschek, 1995, Cambridge University Press.