Figure 5 contains the a vs. (b-y)0 diagram for the stars not contained in the sample of normal stars defined in 2.3 and displayed in Fig. 2. Not included, however, are here the normal double stars which are shown in Fig. 7. Different symbols denote the stellar types represented here including CP1, CP2, CP3, CP4, Del, Boo, emission line, supergiant stars and the Barium object. The reference line of normal stars obtained in Fig. 2 and the positive and negative 3 lines have been drawn in order to visualize the more or less deviating photometric behaviour of the different groups discussed in Sect. 3.5.
First, we discuss stars which lie above the normality line by at least 3 (see Sect. 2.3) dividing them in three groups according to their published spectral peculiarity:
It is remarkable that Michigan peculiarity coincides in 4 cases with peculiar , the BS Catalogue in 2.
HR 1754 has been classified as B7p-He weak Silicon star (=CP4) by Abt & Cardona (1983). We have additional -photometry for this star and its brighter companion HR 1753 obtained by one of us (H.M.M.) at the 1m ESO-telescope on La Silla from Dec. 28, 1988 to Jan. 1, 1989 on five nights. The mean values of both stars, 0.020 and -0.002mag, resp., agree perfectly with our present results and are supported by derived from Rufener (1988). Thus HR 1754 belongs to the magnetic branch of CP4 stars (see Maitzen 1981 and 1984) which is also corroborated by the photometric variation with rotational period P=2.287 days (Catalano et al. 1991).
HR 1973 with the second largest (=0.023mag) in this group has rather high m1-values in the literature (0.129, 0.152, 0.163mag) considering its early spectral type and negative b-y. Calculating from Rufener (1988) we find support for photometric CP2-behaviour. One should investigate the spectroscopic characteristics of this object in more detail in order to find the reason for its undetected peculiarity (spectral line variability may hamper detection of peculiarity, see Maitzen & Lebzelter 1993).
HR 2424. It was measured already by Maitzen & Vogt (1983) yielding exactly the same result: . Since Buscombe (1980) gives B9pSi and Hauck & North (1982) obtain also photometric peculiarity in the Geneva system this star can be safely assumed to be of CP2 type.
HR 3203. This star appears as supergiant in both catalogues and would therefore not belong to the CP2/4 class. However, its Geneva colours contradict supergiant luminosity (log g=3.4) and support peculiarity through . As it is not an infrequent case that Michigan classifies high luminosity where other sources have low luminosity (in agreement with photometry) we tend to attribute peculiarity to this star which should of course be substantiated by new spectroscopy.
In the second part it is to be discussed which objects apparently are CP2 according to some spectroscopic sources, but lack photometric peculiarity (i.e. less than 3) in our measurements. 9 objects of this kind (3 of which are more than 2 above the normality line) may be grouped the following way:
HR 1541 = HD 30612 is B8 II/III (p Si) according to Michigan I. In very good accord with the present value (0.010) Maitzen (1980) obtained two -values, 0.012 and 0.013, just at the threshold limit of peculiarity in his photometry. Contrary to many other cases where giant luminosity erroneously obtains instead of Silicon peculiarity, it results here from existing Stroemgren-Crawford photometry (see Hauck & Mermilliod 1980) together with peculiarity both in spectroscopy and photometry.
HR 2761 = HD 56455 has been classified as Silicon star both in the BS and the Michigan II catalogues. Support for the CP2-type came from different studies of photometric variability (for references see Catalano & Leone 1993) exhibiting a period of about 2 days and decreasing amplitude from the near ultraviolet to the yellow region. Taking Geneva photometry (Waelkens 1985), more precisely the -index we find that this star varies between -0.006 (= normal star) and +0.014 (=peculiar star). Obviously our mag corresponds to an intermediate phase.
HR 2971 = HD 61966 has been detected in the Michigan survey (MS II), but the type given indicates the marginal nature of its peculiarity: B9 IV/V (p Si). Our and the Geneva coincide in photometric non-peculiarity. One should reinvestigate this star by spectroscopy in order to have a closer look on possibly existing traces of peculiarity.
HR 3001 = NGC 2451-W233 has only one peculiarity classification: B9VpSi in Feinstein (1966). Hiltner et al. (1969) classify B7III, as well as Levato & Malaroda (1975), and Michigan III gives B8IV. Our nonpeculiar mag is corroborated by Maitzen & Catalano (1986) who obtained mag, and also the Geneva system peculiarity parameter (Hauck & North 1993) points to a normal star. There is only one source which might indicate peculiarity, i.e. Nissen (1974) who found He-weakness at the 3 limit for HR 3001 and a surface gravity typical for luminosity class V. Taking all available information together it can be assumed that this star is He-weak, but very probably belonging to the nonmagnetic branch of CP4-stars.
HR 3151 = HD 66255 is a supergiant according to Michigan II (B9 Ib), while Jaschek & Jaschek (1959) classify A0p Si using relatively high dispersion (42 A/mm). Brandi & Clariá (1973) reclassify this star and note that "Silicon is very weak''. Hauck & North (1982) obtained which is non-peculiar and matches well our low mag. Both Geneva and Stroemgren photometry indicate giant, not supergiant luminosity. This star is variable with a typical CP2-period length (P=6.81780 days, Catalano & Leone 1993). On the other hand its longitudinal magnetic field as measured by Bohlender et al. (1993) is only 1 below zero. We conclude that the majority of observational evidence is not in favour of a CP2-star, but that it deserves spectroscopic reassessment.
Using for the average scatter 4.74 mmags about the normality line for all normal stars (Sect. 2.3) our -photometry yields 30 peculiarity candidates which are above the +3 line in Fig. 2. But in Sect. 2.3 we also stated that the scatter of the hotter stars is slightly smaller than the average scatter and that for them we therefore have to use 13.5 mmags as 3 level. This way we obtain three further peculiar stars (HR 1100, HR 2424 and HR 3413). No star of the cooler division mentioned in Sect. 2.3 is lost from the list of photometric peculiar objects by applying the larger 3 threshold determined for them. Thus, we arrive finally at 33 objects which have peculiar positive -values (=deviations above the normality line).
One of them is a CP1 (Am) star (HR 1730) and 2 are A and F supergiants (HR 2785 and 2874) according to spectroscopic sources. The degree of impurity for the photometric detection of magnetic peculiar stars in the -system is therefore in our sample 3 out of 33 objects (9.1%).
Statistically this impurity contribution may be considered to be counterbalanced by those stars of our sample, which have been classified as peculiar by spectroscopy, but escaped detection in our photometric system. From the discussion in the preceding section four stars emerged with spectroscopic peculiarity classifications which can be regarded as well established: HR 1217 (at the cool end of the CP2 domain), and the hot stars HR 280, 1541 and 2761 which were previously shown to be photometrically peculiar. With some probability also HR 2875 could join this group exhibiting a -value (12 mmags) which is above the 2 level. The 4 (maybe 5) non-detections of CP2/4 stars by our actual photometry are nearly compensated statistically by the impurity fraction mentioned before. Hence, altogether, we arrive at 34 (maybe 35) magnetically peculiar stars from the photometric side.
The situation with spectroscopy is as follows: Our sample contains 34 stars with CP2 classification. In addition, there are 10 Helium abnormal stars which we denote as CP4. Therefore, both He-weak and He-strong stars appear here under this label. Both groups seem to have a magnetic and a non-magnetic branch. From all those CP4 stars only a fraction of the He-weak subgroup has been shown by Maitzen (1984) to exhibit peculiarity in as members of the magnetic branch. In the present study only two CP4-objects show up as such, and we therefore arrive at 36 CP2/4 stars to be considered as number of magnetically peculiar stars based on spectroscopy. Again applying the concept of balance between impurity and misses we notice:
Three stars (HR 2863, 3001 and 3151) with controversial peculiarity classifications and HR 2971 which may be only very marginally peculiar, are photometrically normal and seem to be poorly related to the magnetic CP-stars if at all and should therefore be considered as impurity fraction of spectroscopic detection. On the other hand there are 2 very probable misses of peculiarity (HR 1973 and 3203) by spectroscopic classification.
Considering this balance we should retain 34 well established CP2/4 magnetic stars after consideration of the spectroscopic detection technique.
Summarizing we find 34 objects which can be considered as reliable members of the group of magnetic peculiar stars in the present sample.
Comparing the raw numbers of both techniques we notice a slight preponderance of spectroscopic detections, which even becomes stronger if we include also the cases of controversial peculiarity classifications. But this difference can be safely explained by the fact that photometry (i.e. measurements of the 5200 feature) is free of subjective assessments while spectroscopic classification is a highly complex human pattern recognition process in which e.g. expectation behaviour may favour (in some cases) the detection of peculiarity.
Considering b-y-values we obtain a rather similar picture: 9% are in the hottest group (), 6% in the following bin with negative b-y values, while only 3% are peculiar in the range .
A comment is due on the shape of the histograms presented: While our sample of CP1-stars shows a very similar distribution for B-V and b-y, the resemblance for our CP2-stars is less striking. There is a slight preponderance of negative b-y values (3 objects) compared to negative B-V values which is to be expected from the differential blueing effect on the Stroemgren index b-y in comparison to B-V since b is less affected by line absorption than B and y is more influenced by the 5200 absorption feature than V. On the other hand it is astonishing that the negative values of B-V assemble with great majority in the hotter bin. But we have also to take into account that the relatively low number of CP2-stars and the corresponding need for big bin sizes are able to contribute to the actual picture.
Introducing spectroscopic results into this discussion we have to mention Abt (1979) who compared the number of Silicon stars brighter than V=6mag in Osawa's (1965) list with the number of B5-A0IV and V stars in the BS catalogue of 1964 in the same magnitude and declination ranges and derived 6.5 %.
Despite the relatively large error this value (which is rather close to our result) is clearly smaller than the average frequency of CP2 stars with negative B-V (10%) as derived from the study of Wolff (1968). Again it may be argued that this is the result of surveying a much larger volume than that used for the sample of Wolff. But there may be another or an additional explanation for this discrepancy: Abt (1979) obtains 5.4% for the frequency of cool field CP2-stars. Even with the rather large error this result (i.e. nearly equal frequencies of hot and cool peculiar stars) can hardly be reconciled with the findings of Young & Martin (1973) and North (1993) - the latter obtained a frequency of about 10% for the Silicon stars and about 4% for the cool CP2 stars, both in open clusters and the field. A rather straightforward explanation for this difference is the discrepant definition of the borderline between hot and cool Ap-stars which caused an increase of cool CP2 stars at the expense of the hot CP2 stars in the case of Abt (1979). The reversed situation seems to apply for the photometric definition of the borderline. Setting it to zero in both indices B-V and b-y we do transfer a number of objects recognized spectroscopically as cool Ap-stars to the domain of hot stars, just because of their bluer colors.
Confirmation for this argument comes from a comparison of the hot/cool CP2-star ratios of our sample. With the photometric criterion (negative/positive colour indices) we obtain 4:1, while with the spectroscopic division (Silicon and He wk/all other CP2 stars) the ratio is close to 3:1. Since the ratio for the normal stars with analogue criteria is roughly 2:1, the frequency ratios are 2 for spectroscopy and 1.5 for photometry.
With these considerations it seems that the discrepancy between spectroscopic and photometric frequencies has been removed to a large degree, especially if one takes into account that the the identification of Silicon stars suffers from the ambiguous interpretation of enhanced Silicon line strengths which may also be caused by higher luminosity of a normal star.
These stars are contained in Fig. 5 together with our main targets, the magnetic peculiar stars. By different symbols the following groups are represented there:
The following conclusions can be drawn:
This work was supported by the Austrian Fonds zur Förderung der wissenschaftlichen Forschung under project number PHY-4715. We are indebted to ESO and its staff for providing excellent support and services.
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