«J. Astrophys. Astr. (1989) 10, 183–196 Giant CP Stars? L. O. Lodén Astronomiska Observatoriet, Box 515, S-75120 Uppsala, Sweden A. Sundman ...»
J. Astrophys. Astr. (1989) 10, 183–196
Giant CP Stars?
L. O. Lodén Astronomiska Observatoriet, Box 515, S-75120 Uppsala, Sweden
A. Sundman Stockholms Observatorium, S-13336 Saltsjöbaden, Sweden
Received 1988 August 31; revised 1988 December 28; accepted 1989 February 2
Abstract. This study is part of an investigation of the possibility of using
chemically peculiar (CP) stars to map local galactic structure. Correct
luminosities of these stars are therefore crucial. CP stars are generally regarded as main-sequence or near-main-sequence objects. However, some CP stars have been classified as giants.
A selection of stars, classified in literature as CP giants, are compared to normal stars in the same effective temperature interval and to ordinary ‘non giant’ CP stars. We find no clear confirmation of a higher luminosity for ‘CP giants’, than for CP stars in general. In addition, CP characteristics seem to be individual properties not repeated in a component star or other cluster members.
Key words: spectral classification—stars, chemically peculiar
1. Introduction Recognition of special characteristics in a class of objects generally opens up several ways of fruitful research. In the case of chemically peculiar stars one expects that revealing the mechanisms behind this peculiarity will make significant contribution to the understanding of stellar structure. Viewed from a different angle one may neglect the physical background to the phenomenon and ask if the CP stars, which are fairly numerous and also intrinsically bright, might be a class of objects suitable for investigations of local galactic structure—in this connection the structure within about 2kpc.
This implies studies of the occurrence of CP stars in certain directions in the Milky Way, in special regions or in clusters. Investigations in this line have been performed before, a few of them with more or less convincing results, others without (e.g. Abt 1979; Hartoog 1976; Maitzen 1981; North & Cramer 1981; Young & Martin 1973).
The first problem one encounters is the vague border line between peculiar and ‘normal’ stars (e.g. Durrant 1970; Megessier 1971; Lodén & Sundman 1987) and the confusing variety of different types of peculiarity, An extensive survey of the present state of the classification of CP stars has recently been presented by Faraggiana (1987).
In a previous paper (Lodén & Sundman 1987) we introduced the concept ‘degree of peculiarity’ as a complement to ‘type of peculiarity’ and it was shown that if one avoids extreme cases of peculiarity, CP and normal stars can be pretty well classified by the same temperature criteria. We are here concerned with stars in the effective temperature interval 7500 Κ to 15000 K, with a strong concentration around 10000 K.
In the present paper we will pay attention to possible luminosity effects in CP star spectra. The general consensus thus far has been that CP stars are main-sequence or near-main-sequence objects (Abt et al. 1968; Bonsack 1976; Hyland 1967; Preston 1970; Wolff 1975; Baschek & Oke 1965; Strittmatter & Sargent 1966). This would indeed be a favourable situation for galactic structure studies, keeping errors in luminosity estimates down. The CP stars, although enigmatic in many respects, could prove to be reliable distance indicators.
However, in the Michigan Catalogue (Houk 1978,1982; Houk & Cowley 1975) one also finds objects classified as CP and of high luminosity. A classical case is also the very complicated star 3 Pup (Crespin & Swensson 1952; Swings 1950) as well as υ Sgr (Greenstein 1940; Greenstein Sc Adams 1947; Greenstein & Merrill 1946; Morgan 1934).
Do we face here a real extension of the CP phenomenon to the giant branches, or do luminosity and CP criteria mix in an unfavourable manner to cause misclassification?
It is clear that quite a few spectral lines characteristic for chemical peculiarity are also sensitive to luminosity.
We have made a selection of CP stars classified as giants (Table 2). This selection is conditioned by observational circumstances. These ‘giant CP’ stars have been compared to normal stars in the same spectral interval (Table 4) and to ordinary ‘non giant CP’ stars (Table 3). The comparison includes the two colour diagram (Fig. 1), average colour excess, Balmer line intensities (Fig. 2) and the appearance of some other spectral parameters (Table 5).
There are, however, some basic problems concerning selection effects and restrictions on observable quantities which will first be considered.
2. Recognizing a CP star
It is a well known fact that the number of detected CP stars will increase with a certain power of the dispersion. This implies that, statistically, the fraction of detected CP stars will decrease with the distance from the observer. With increasing distance a larger fraction of the peculiar stars will go unnoticed and be treated as normal ones. With rough estimates of absolute magnitude of various types of CP stars we may give the following statistics for the distances to the detected CP stars up to now: 25 per cent of all known CP stars are situated within 100 pc, 57 per cent within 200 pc, 75 per cent within 300 pc, 87 per cent within 400 pc and 94 per cent within 500 pc. Practically no ‘traditional’ CP star has been detected beyond 700 pc.
The conditions are further complicated by the fact that different types of peculiarity require different minimum dispersion for detection. It would be highly desirable to find a peculiarity criterion that could be applicable also to stars for which the realistic maximum dispersion is not larger than say 50 Å mm -1. For these comparatively distant stars we would like to estimate a degree of peculiarity, hopefully also classify a type of peculiarity, and then it is important to know the risk of misclassification.
As indicated in a previous paper (Lodén & Sundman 1987) there is no clear border line between ‘normal’ and ‘peculiar’ stars. Minor differences in the transition sequence could then be characterized as effects of individuality rather than stages of peculiarity.
Under idealized conditions one should not be able to find two identical stars in the whole Milky Way or anywhere else if the scrutiny was detailed enough. This has Giant C P Stars? 185 already been pointed out by several authors. According to Abt & Moyd (1973) for instance, “there are probably no slowly-rotating late Α-type stars that one could reasonably classify as normal”.
Even with the modest dispersion that is realistic for our classification principles it is difficult to keep the number of classification boxes at a practical level. Although there are a few distinct types of peculiar stars, there can also be found a multitude of combination and transitional types as well as more or less unique ones. To a certain extent, this opulence could be a consequence of multiplicity and differences in rotational attitude—may be in the orientation of the magnetic field—but very probably there is also really a palpable variety of the relevant physical and chemical parameters in stellar atmospheres.
Accepted main types of CP stars are (essentially according to Bonsack 1981):
It is very difficult, however, to satisfy the real need with only these groups or ‘boxes’ and even if we extend the number of groups considerably, there will always be stars which do not fit particularly well into any group defined beforehand. Already at a superficial glance at the CP stars known today one obtains a very confusing picture, illustrated in Table 1 which is by no means complete.
It should also be mentioned that peculiarity classifications from different sources are in many cases rather discordant. A particularly embarrassing circumstance is the fact that about eight per cent of the stars classified as Am stars by certain authors have been classified as Ap stars of various types by one or more other authors. Spectrum variability due to stellar rotation certainly contributes to this confusion but it cannot be excluded that also long-term variations, with or without recurrence, may be found.
3. The observations
Observations have been carried out at the European Southern Observatory in Chile, essentially in 1985 May (photometry) and 1987 February (spectroscopy). In a few cases a comparison object from an earlier observing run has been added in order to complete the list.
The photoelectric UBV and Hβ photometry has been performed by means of the 50 cm ESO telescope, and for connection to the Johnson system a selection from the ESO Standard star list has been used according to the description given in previous papers (i.e. Lodén 1979). The corresponding spectroscopic material has been secured with the coudé spectrograph at the 1.5 m telescope. The dispersion is 20 Å mm –1 and the plates have subsequently been measured in the Grant Machine at the ESO Headquarters in Garching.
186 L. Ο. Lodén & A. Sundman Table 1. Relative frequency of various types of CP stars, essentially according to Bertaud-Floquet (1974). The fact that there are different classifications of one and the same star makes the sum of the frequencies exceed 100 per cent.
For conventional theoretical investigations of stellar structure the apparent equivalent width is the most suitable quantity representing the intensity of a spectral line. For reasonably strong lines, like Balmer lines, the Ca II Κ line and occasionally a few more there is no essential technical problem to measure this equivalent width even on spectra of rather modest dispersion. For weaker lines the alternative nearest at hand is the depth(1– R) where R is the measured residual intensity of the apparent line centre, although a series of complications have to be considered when it is applied. In cases of a completely ‘natural’ line, i.e. a line that appears in the spectrum of a star with ν sin i = 0 obtained with infinitely narrow slits, the depth should be a fairly good intensity parameter at least for lines of low and intermediate intensity. This is no realistic case, however, and the depth of such lines is particularly sensitive to ν sin i.
Strong lines are less sensitive and besides, their equivalent width, which is in a first approximation rather independent of υ sin i, can easily be determined. When the slit width of the spectrum microphotometer increases, the measured line depth gradually turns into a more or less unique function of the equivalent width. Hence, with a careful selection of the slit width with respect to the typical width of the relevant lines the measured depth will again be a good parameter for many purposes.
Typical half width for most of the lines, except the Κ line and strong Balmer lines, in this effective temperature range appear between 0.6 and 1.0 Å and the effective slit width is about 0.3 Å. A study of the present material together with material of the same quality from earlier investigations show that up to a depth of 0.25 the correlation between depth and equivalent width is practically linear.
Giant C P Stars? 187
5. The observational result
The ‘CP giants’ are presented in Table 2. Column 6 shows the number of photometric observations. When “—” appears in this column the photometry is obtained from Buscombe (1974–1984). Columns 7 and 8 show the spectral type and CP characteristic given in the Michigan catalogue, column 9 gives an indication of the suspected photometric variability (0 = no variability, 1 = variability possible, 2 = variability probable, 3 = variability evident, — = material insufficient for conclusion). Column 10 shows the number of spectral plates for each star.
Table 3 gives a sample selection of comparison stars classified as main sequence CP stars and Table 4 a corresponding selection of ‘normal’ stars of various luminosity classes. In applicable cases the column division is identical in Tables 2–4. The last column in Tables 3 and 4 give the references for spectral type and CP characteristics.
Figs la–d show two-colour diagrams for the objects in Tables 2–4 in which the stars classified as giants are indicated by large symbols and the corresponding main
Table 2. Giant CP stars.
* The star has one or more components.
$ Spectrum variable.
Period 3.38 days according to North (1984).
Variable, but period uncertain according to Manfroid & Mathys (1986).
Period 5 days according to Catalano & Renson (1984).
Period 2.9 days according to Catalano & Renson (1984).
According to Michigan catalogue.
188 L. Ο. Lodén & A. Sundman
Non-giant CP comparison stars.Table 3.
* Borrowed from an earlier investigation (Lodén and Sundman 1987).
*UBV colours taken from Buscombe (1974–84).
Variable, with exceptionally large amplitude in U compared to other wavelengths according to Waelkens (1985).
Period 2.8 days according to Catalano and Renson (1984).
See North (1987) for detailed discussion.
Reference in last column only concerns spectral type and CP characteristics: (1) Michigan Catalog, (2) Bertaud-Floquet (1974), (3) Lodén-Sundman (1987).
In case of two references, the first one concerns spectral type and the second one CP characteristics.
sequence stars by small ones. In these diagrams there is no convincing separation between giants and dwarfs—at least not as far as the majority is concerned. In order to obtain a more quantitative conception one may compare the relative colour excesses,
assuming that all stars are main sequence objects. One finds:
*The star has one or more components.
**The star is a component of a primary object in this investigation. Luminosity classification derived from magnitude of primary.
# Member of NGC 6475 and borrowed from an earlier investigation (Lodén 1984) &This investigation. Other references as in Table 3.