view Abstract Citations (10) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Secondary Variation of δ Scuti Sterne, T. E. Abstract It is found by harmonic analysis of Fath's photoelectric measures of ô Scuti that its magnitude contains a periodic component, superimposed upon the fundamental varia- tion, and whose period differs from that (o~I~3~) of the fundamental variation and all its submultiples. Since all the photoelectric measures have been made at nearly the same part of the Julian day, there is a major indeterminacy in the value of the second- ary periodicity. Two of the possible values are o~I57388 and o~r86876, each of which is, in turn, uncertain by a small amount because of the seasonal distribution of the ob- servations. Corresponding to the former value, the (semi-)amplitude of the secondary variation is found by least squares to be O'~oI77 ± o.ooi~; corresponding to the latter value, the amplitude is o~oI87 ± 0.0013. The secondary period does not have the value, 0~2OI 20, suggested by Fath. Further observations, made from different longi- tudes, are needed to remove the major indeterminacy. The radial velocity is found, from a preliminary examination, to contain, in all prob- abifity, a periodic secondary variation having the same period-at present indeterminate -as that of the secondary variation in magnitude. The latter variation cannot be at- tributed, therefore, to a variation of the comparison star employed by Fath. The periodic variation in the range, reported by Fath, can be explained by a sec- ondary variation in the magnitude, having a period o~I57388 or 04186876. A periodic term in the ephemeris of the dates of median increasing magnitude is predicted on the basis of the secondary variation in magnitude; and such a term is found, with almost exactly the predicted amplitude and phase. It is suggested (a) that all (or nearly all) real "sine terms" in ephemerides of short-period Cepheids may arise from secondary variations having periods different from the fundamental periods and their submultiples, and (b) that such periodic secondary variations may be fairly com- mon among short-period Cepheids. From the absolute magnitude and spectral type a value is obtained for the mean den- sity. The fundamental period could be explained as the lowest mode of radial oscillation of an Eddington model having this mean density and a value of `y equal to about 1.54. Such a model could not, however, have the period o~I86876 for any other of its possible radial modes. Even if the preceding value of the mean density is rejected, the pair of periods 0~I9377 and 04186876 cannot be reconciled with the pulsations of any of the stellar models (which include Eddington's and cover a wide range of degrees of central condensation) considered by the author (MN., ~7, 582, 1937); and they appear to be inconsistent with the pulsation theory. The period 04157388 and the fundamental pe- riod are probably not inconsistent with the pulsation theory, provided that a suitable choice can be made of the stellar model Publication: The Astrophysical Journal Pub Date: March 1938 DOI: 10.1086/143913 Bibcode: 1938ApJ....87..133S full text sources ADS | data products SIMBAD (1) Related Materials (1) Erratum: 1938ApJ....88..208.