HELIUM-WEAK STARS MERCEDES JASCHEK, CARLOS JASCHEK, AND MARCELO ARNAL. Observatorio Astronómico La Plata, Argentina. Received Jufy 10,1969

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1 HELIUM-WEAK STARS MERCEDES JASCHEK, CARLS JASCHEK, AND MARCEL ARNAL bservatorio Astronómico La Plata, Argentina Received Jufy 10,1969 A systematic survey was made for helium-weak stars and eight new objects were found. In recent years an increasing number of stars have been found in which the color index is too blue for the spectral type. A number of objects of this type are known (see below), but since they were discovered almost by chance, it is unknown how frequent they are in space. For the same reason it is impossible to ascertain if the properties of the objects found are characteristic of the whole group. We have undertaken, therefore, a survey program in order to cover a large sample of prospective members of the group. The program stars were selected from the Catalogue of Bright Stars (Hoffleit 1964) and consisted of all those objects for which the observed (B V) color index was bluer than the average color index corresponding to the spectral type of the object. These stars were then observed with the 32-inch La Plata reflector with the Hiltner camera giving 110 Â/mm dispersion. A few additional plates were taken with the 60-inch reflector of the Bosque Alegre Station of the Cordoba bservatory and with the 69-inch reflector of the Flagstaff Station of the Perkins bservatory; these plates have a dispersion of 42 Â/mm. The results of the observations are given in Tables I and II. Table I contains all those stars in which there is a large disagreement between color index and spectral type. The spectral types were established by comparison with suitable standards taken with the same spectrograph. If not stated otherwise in the notes, the general appearance of the spectra are normal for their type. This evidently does not preclude that when examined at higher dispersion other anomalies might appear. The color indices were taken from the Catalogue of Photometric Data in preparation for publi- 650

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4 HELIUM-WEAK STARS 653 cation at La Plata, except for HR 4519 measured by A. Feinstein at the Cerro Tololo Inter-American bservatory. The spectral types inferred from the color indices are also given (in the fifth column of Tables I and II). The average color indices were taken from Johnson (1966) and are summarized in Table III. As can be seen in Table I, the disagreement between the two spectral types is at least two tenths of a spectral class. It could be objected that the spread of color indices for a given spectral subclass (i.e., for B8 V stars) is so large that the inferred spectral type is almost meaningless. This would imply that the association between color index and spectral type is rather loose. To investigate this point the range for the (U B) indices for each spectral class was calculated. In order to deal only with homogeneous classifications, the sample of stars to be studied was selected from Slettebak s (1954, 1955, 1963) papers on stellar rotation. The UBV values were taken from the already mentioned Catalogue of Photometric Data. In this catalog if several values existed for an object they were averaged, provided that the range in (B V) was less than 0^3 and in (U B) less than (T. Now the spread is certainly enlarged by the reddening of some of the stars. Since here one is interested only in the spread in (U B), the simplest procedure is to push back all stars to the main sequence on the reddening path with an average slope for the reddening equal to The range of the (U B) color indices, with all stars moved onto the main sequence, was then obtained and is given in the column of Table III headed R(U B). The number of stars used is given in the last column. It should be added that the range is centered in all cases on the average value. From an examination of Table III it can be concluded that in no case do normal B9 stars have a color index corresponding to B7 or B6 and that as a consequence the stars of Table I show a real discrepancy. From here on, we will call helium weak only those stars in which the disagreement between the observed spectral type and the one inferred from the UBV photometry is equal to or larger than two-tenths of a spectral type. We want to emphasize strongly, however, that we call the stars helium weak only because of the lack of another name, and not because we have strong reasons to believe that there is a real underabundance of helium. For the stars of Table I we have also examined the previous classifications, as listed in the Catalogue of Stellar Spectra Classified

5 654 JASCHEK, JASCHEK, AND ARNAL TABLE III Color Indices for Dwarfs Sp b-v U-B R(U-B) N B3 B5 B6 B7 B8 B9-0*20.16 ' *06-0* *19-0* * in the MK System (Jaschek, Conde, and Sierra 1964). But either no previous classifications existed, or they were in agreement Since the possibility of periodic variations like in HD (Jaschek, Jaschek, and Kucewicz 1968) cannot be excluded, a program is under way at La Plata to detect possible changes in all stars of Table I and some from Table II. Turning now to Táble II, we have collected there all program stars which did not turn out to be helium weak in the sense of Tablé I. Thus they comprise a mixture of objects, some being mis- classifications like HR 3241, while some others show disagreements between spectral type and color index. We call minor disagreement one in which the spectral type inferred from the UBV color indices differs one-tenth in spectral class. It seems impossible to decide whether such disagreement is real or whether it is due to observa- tional errors. ' ; % Discussion The procedure applied for finding new helium-weak stars has been shown to be successful since it permitted the detection of eight new objects, which add to the 17 stars already known. The stars already known are listed in Table IV. Helium-weak stars are therefore not very exceptional. We esti- mate that our survey has covered about one-third of the whole sky, so that the eight new stars come from about 250 normal B8 and B9 stars. The minimum percentage of helium-weak stars is therefore of the order of three percent. ; : _ The distribution on the sky of the helium-weak stars is rather distorted; there is strong concentration in the first and last hours of right ascension and a gap in the middle. Some of this is due to the

6 HELIUM-WEAK STARS TABLE IV Previously Known Helium-Weak Stars Name HD Aiithor Group a Scl 57?7-, Jugaku and Sargent (1961) Sco-Cen 20 Tau Huang and Stmve (1956) Field HR Roman and porgan (1Ö50) «Per McNajnara and Larsson (1962) rion Mprg^n and Lodén (1966) rion ri B Slettebak (1963) rion Sargent and Strittmatter (1966) rion 378 Sharpless (1952). rion 37129,, Sharpless (1952) rion 3,7807 McNamara and Larsson (1962), rion 36 Lyn Searle and Sargent (1964) Field 3 Sco Garrison (1967) Sco-Cen HR Garrison (1967) Sco-Cen HR Garrison (1967) Sco-Cen Garrison (1967) Sco-Cen HR Guthrie (1965) Sco-Cen Guthrie (1965) Sco-Cen Sco-Cen incompleteness of observations around 14 h, but the absence of can- didates is too strong to be attributed only to this. The distribution resembles very much those of the Si-A 4200 stars. These stars also have weak helium lines for their color, but in view of their other peculiarities, we have excluded them from all our lists. If one selects a list of possible candidates for the helium-weak group with- out knowing accurate spectral types, it turns out that about half of them belong to the Si-A 4200 group, which means very probably that both groups are closely related. In this connection it would be high- ly desirable to observe all these stars at higher dispersion, in order to study not only the behavior of the ionized silicon but also the be- havior of all the other elements. REFERENCES Cowley, A. P., Cowley, C. R., Hiltner, W. A., Jaschek, M., and Jaschek, C. 1968, Pub.A.S.P. 80,746. Garrison, R. F. 1967, Ap.J. 147, Guthrie, B. N. G. 1965, Pub. Roy. bs. Edinburgh 5, 1. Hoffleit, D. 1964, Catalogue of Bright Stars (New Haven, Conn.: Yale University bservatory). Huang, S.-S., and Struve,. 1956, Ap.J. 123,231. Jaschek, C., Conde, H., and Sierra, A. C. de 1965, Catalogue of Stellar Spectra Classified in the MK System, La Plata Pub. Ser. Astr. 28, Part 2.

7 656 JASCHEK, JASCHEK, AND ARNAL Jaschek, M., and Jaschek, C. 1962, in Symposium on Stellar Evolution, J. Sahade, ed. (La Plata: bservatorio Astronómico). Jaschek, M., Jaschek, C., and Kucewicz, B. 1968, Nature 219, Johnson, H. L. 1966, Annual Rev. of Astr. and Astrophysics 4, 193. Jugaku, J., and Sargent, W. L. W. 1961, Pub. A.S.P. 73, 249. McNamara, D. H., and Larsson, H. J. 1962, Ap.J. 135,748. Morgan, W. W., and Lodén, K. 1966, in Vistas in Astronomy, Vol. 8, A. Beer, ea. (London: Pergamon Press), p Roman, N. G., and Morgan, W. W. 1950, Ap.J. Ill, 426. Searle, L., and Sargent, W. L. W. 1964, Ap.J. 139, 793. Sharpless, S. 1952, Ap.J 116, 251. Slettebak, A. 1954, Ap.J. 119, , Ap.J. 138, 118. Slettebak, A., and Howard, R. F. 1955, Ap.J. 121, 102. Strittmatter, P. A., and Sargent, W. L. W. 1966, Ap.J. 145, 130.