The dispersion of all spectrograms illustrated is ~ 125 A/mm, unless otherwise noted.
We may obtain spectral types earlier than O4-O5 by discovering stellar spectra showing a higher level of excitation than the stars of similar luminosity which are MK standards at O4-O5: we may also obtain earlier types by expanding the scale of classification for the O stars by moving the MK standards toward earlier types. The first procedure is clear; the second tends to distort the scale of the MK system.
An important problem for the future is the spectrographic identification of the ZAMS for the O7-B2 stars. The brightest members of the Orion Nebula cluster show abnormally broad absorption wings for the Balmer series of H (see Ann. Rev. Astronomy and Astrophysics, Vol. 11, Illustration on page 35, 1973). This phenomenon is not shown by the main-sequence stars illustrated in Plate 1. A question to be answered is: Are the luminosities of the stars in Plate 1 higher than those of similar spectral type in the Orion Nebula cluster (the Trapezium cluster)?
The scale of the print of Pup is somewhat more extended in the ultraviolet near N IV 3479-85; the position of this absorption feature actually matches that of the same line in Cep.
An interesting point is raised with regard to the strong violet-shifted He I 3888 absorption in the two WN stars: Are these He I 3888 components found in similar strength and velocity in the WC stars?
Especial care must be taken in the use of N IV 3479-85 (absorption) as a luminosity criterion at O6. This feature comes to a maximum in main-sequence stars at O4 V (see Plate 1); at this type, it is of similar strength to that in the O6 If standard. It is crucial to the use of this feature that there be no systematic shift in the relative positions of the standard If and class V sequences.
The use of the Botto-Hack criterion for the determination of luminosity classes for the O stars depends sensitively on the dispersion and width of the spectrograms used.
The supression of the higher members of the diffuse triplets of He I which lie over the Balmer continuum is probably already present at B2 V. The effect is very pronounced at type B3 V.
The relationship of the Orion "helium stars" to the ZAMS is an interesting problem. In Ori (ft) the He I sharp triplet at 4121 is much narrower than the He I diffuse singlets at 4009 and 4144. He I 4121 is similar in width in the B2 V standard Sco (ft) and in Ori (ft), but the diffuse singlets and triplets are considerably narrower in the former. Can this be considered as indicating a lower spectroscopic luminosity for Ori (ft) than for Sco (ft)?
It is possible that the weakening of the higher members of the diffuse triplets in Ori (ft), as compared to HR 1890, can be explained in terms of the sudden decrease in intensity of these lines on passing from B2 V to B3 V (see Plate 4). There is some evidence that Ori (ft) is very slightly later in spectral type than HR 1890 (~ B2.5 Vh).
The discriminating characteristic for these five stars is the presence in each spectrum of very broad lines of He I, coupled with a much narrower stellar Ca II K-line. The hydrogen lines, in each case, indicate a luminosity class of III; that is, the stars are giants, definitely evolved stars, of the order of 1 1/2 magnitudes above the ZAMS at the spectral type at which they have been classified here. L.M. Hobbs has established that the stellar K-lines in the first four stars do not contain any substantial interstellar contribution, by use of an echelle grating spectrograph giving a dispersion of 0.59 A/mm, attached to the 2.7 m reflector of the McDonald Observatory (P.A.S.P., 90, 301, 1978).
Each of the first four stars is well-suited to act as a fundamental standard: Tau as MK-78 standard for B7 III; 20 Tau as standard for a B7 giant which shows abnormally strong Fe II lines, together with the narrow K, broad He I characteristic; Scl as a spectacular example of the presence of disparate features like strong C II 4267, Fe II, and probably faint Sr II - together with the sharp K, diffuse He I characteristic; µ Lep as one of the classical mercury-manganese stars with the same K, He I appearance.
The principal standard used in the classification of 20 Tau and a Scl was Tau. In the case of 20 Tau, the comparison with Tau is close, except for a slight weakening in the He I lines and strengthening in the Fe II lines in the former. The comparison of Tau with Scl is also good, except for the well-known enhancement of C II 4267 and strengthening of Fe II in Scl, and a definite weakening of the Balmer lines in the latter.
The spectrum of µ Lep resembles closely that of a number of the Hg-Mn stars; in particular, we find that the narrow K, diffuse He I structure is present in And and Cnc, both of which are classical members of the mercury-manganese group.
The high optical quality of the low-dispersion spectrographs designed by A.B. Meinel makes practicable extremely sensitive discrimination of the extent of the wings of the Balmer lines for stars in the neighborhood of class A0. This has made possible the splitting of MK box A0 V into two: A0 Va and A0 Vb. Stars of the former class turn out to be, in general, main-sequence non-cluster stars (such as the fundamental standard Lyrae) and main-sequence stars in evolved clusters. Spectral type A0 Vb exhibits markedly broader wings on the H lines, for stars of similar rotational line-broadening; examples are found among clusters having main-sequence turn-offs at B3 and earlier. The star NGC 2516 #29 is located at the extreme edge of the cluster, and does not seem to be a cluster member, since its cluster luminosity would be MV ~ 0; the spectrum does not resemble that of a white dwarf, since many members of the Balmer series are observed. Its true nature remains to be determined.
It appears feasible at the present time to split spectral types B9 V and B9.5 V into luminosity classes Va and Vb. The star HD 19805 (HL 167 in the Per cluster) is a good candidate for the standard B9.5 Vb; it is illustrated on Plate 13, as of type B9.5 V.
The discovery by Babcock that HR 2534 is a very rapid spectrum variable (pronounced changes in 24 hours) makes it of importance to obtain a series of low dispersion spectrograms for investigating the behavior of the line patterns with the spectral variations. If these line patterns show great changes in appearance, it may be possible to discover other examples of this uniquely interesting star from low-dispersion plates - or even with objective prism cameras.
The exceedingly strong shell absorptions of Ti II at 3685 and 3759-61 in 17 Lep are from the lower level a2F, which is metastable and has an E P of 0.6 volt.
These same absorptions have been observed as shell lines in rapidly rotating A stars by Abt and Moyd (Ap. J., 182, 814, 1973).
HR. 4369 = HD 98088 is a spectroscopic binary whose orbit was determined by Abt. It is also a magnetic variable (Babcock: Ap. J. Suppl., 30, 1958). Abt et al. (Ap. J., 153, 177, 1968) have measured the radial velocity of the secondary; the authors state that "... a secondary component of the K-line can be found", but only measured H and the D-lines. The authors reject a value of MV of 0.5 mag. because secondary components would have been observed for many more lines with such a small difference in brightness between the two stars. They adopt a MV of 1.2 mag. and derive a type of A8.5 V for the secondary.
The width of the wide, shallow K-line in HR 4369 is equal to - or greater than - that in the F0 III standard Leo. If this strong line were identifiable with the spectrum of the secondary star, it seems likely that many secondary components of strong neutral metals would also have been observed. For this reason, we feel that the diffuse K-line in HR 4369 is probably similar in nature to that observed in so many Sr II stars.
The announcement of a Scl as a member of the peculiar manganese group by Morgan (Ap. J., 73, 109, 1931) was an error, and is withdrawn herewith.