Spectral Analyses of 4 Lacertae and ν Cephei

Supergiants are the most luminous stars in galaxies that are currently forming many stars. Reliable abundance determinations, for the most part, reflect the values of the interstellar material out of which they formed. Thus, they can be used to trace the chemical history of their parent galaxies. This study derived the stellar atmospheric parameters and elemental abundances of the early spectral type supergiants 4 Lac (B9 Iab) (HR 8541, HD 212593) and ν Cep (A2 Ia) (HR 8334, HD 207260) from Reticon and CCD exposures obtained with the long camera of the 1.22 m telescope of the Dominion Astrophysical Observatory in Victoria, British Columbia. They cover 3830–4950 Å with signal‐to‐noise ratios of at least 200 and a 2 pixel resolution of 0.072 Å. The spectra were measured with the graphics spectroscopic analysis program REDUCE (G. Hill & W. A. Fisher 1986, Publ. Dominion Astrophys. Obs., 16, 193). The tailored analyses were performed using the LTE plane‐parallel model atmospheres code ATLAS9 (R. L. Kurucz 1993, Kurucz CD‐ROM 13 [Cambridge: Smithsonian Astrophys. Obs.]). They are similar to those performed by K. A. Venn (1995, ApJS, 99, 659) and B. Albayrak (2000, A&A, 364, 237), who studied supergiants with similar and cooler effective temperatures. The study of 4 Lac extends this type of analysis to a slightly hotter effective temperature.

For weak features, the lines were measured using profiles whose width was found from medium‐strength unblended lines to aid the process of trying to deconvolve blended lines. For each spectrogram, I identified the cleanest spectral features and found the radial velocity using the laboratory wavelengths. The remaining lines were identified with A Multiplet Table of Astrophysical Interest (C. E. Moore 1945, Princeton: Princeton Univ. Obs.) and more recent references.

The adopted values T_eff = 10,250 K and log g = 1.85 for 4 Lac and T_eff = 8400 K and log g = 1.10 for ν Cep were determined using the wings of Hβ, and Fe i/ii and Si ii/iii ionization equilibria for 4 Lac and the wings of Hβ and Fe i/ii and Cr i/ii ionization equilibria for ν Cep. A microturbulence of 2.8 km s^-1 for 4 Lac was found from Fe ii lines using WIDTH9 (R. L. Kurucz 1993, Kurucz CD‐ROM 13 [Cambridge: Smithsonian Astrophys. Obs.]). For ν Cep, microturbulences of 6.9, 6.8, and 7.0 km s ^-1, respectively, were found from Cr ii, Ti ii, and Fe ii lines. I used 7.0 km s ^-1 for the derivation of the other atomic species of ν Cep.

I used SYNTHE (R. L. Kurucz & E. H. Avrett 1981, SAO Special Report 391) to calculate the synthetic spectra. The rotational and macroturbulent velocities are, respectively, 14 ± 2 and 15 ± 2 km s ^-1 for 4 Lac and 26 ± 2 and 12 ± 2 km s ^-1 for ν Cep. By varying the input macroturbulence, I was able to match the line profiles. To optimize the fit, I made slight adjustments to the rotational velocities obtained initially from measuring medium‐strength clean unblended lines.

The oscillator strengths are from critical compilations published by NIST, the recommendations of NIST atomic physicists concerning more recently published material, and the compilation of R. L. Kurucz & B. Bell (1995, Kurucz CD‐ROM 23 [Cambridge: Smithsonian Astrophys. Obs.]). Line‐broadening constants, which affect only strong lines, are also from this source. For the He i lines, I used SYNSPEC (I. Hubeny, T. Lanz, & C. S. Jeffrey, 1994, Daresbury Lab. New. Anal. Astron. Spectra, 20, 30) to calculate the theoretical line profiles that were suitably broadened for comparison with the observations.

Both stars show He, C, N, O, and light‐element abundances, which are solar or overabundant, while iron‐group and heavy‐element abundances are generally solar or underabundant. 4 Lac is helium overabundant by 0.20 dex relative to solar, while ν Cep is He rich by 0.18 dex. In 4 Lac, the carbon and nitrogen surface abundances seem to indicate evidence of CNO core‐processed material being present in the photosphere. This suggests that 4 Lac has evolved through a red supergiant branch. Similarly, the effective temperature and surface gravity of 4 Lac, along with the solar‐metallicity evolutionary tracks of G. Schaller et al. (1992, A&AS, 96, 269), put it on the 12 M_⊙ track, which contains the blue and red giant branches. After the star had evolved to the red giant branch, it returned to the blue giant branch and is now a blue supergiant. The CNO abundances of ν Cep are inconsistent with the mixing of the CNO‐cycled products into the surface layers. These abundances agree with a model in which the star initiated helium core ignition without visiting the red giant branch. Similarly, the effective temperature and surface gravity of ν Cep put it on the 20 M_⊙ track. This star is a blue supergiant.