Thomas J. Moffett

Magellanic Cloud Cepheids: Abundances

We have determined parameters and abundances for three SMC and nine LMC Cepheids that have not been previously studied spectroscopically for metallicity, as well as three SMC and one LMC Cepheids observed previously by Luck & Lambert. This work nearly doubles the number of Cepheids with spectroscopic metallicities in the SMC and triples the number in the LMC. For Galactic reference stars we have determined the abundances of 11 supergiants of spectral type F0 to K1. These abundances are used herein to examine the dispersion in cosmic [Fe/H] values found in LMC Cepheids by Luck & Lambert, and in later papers will be used to correct the distance moduli obtained by us for these Cepheids using the visual surface brightness technique. The Magellanic Cloud Cepheids tend to be longer-period Cepheids (the period range examined was from 37 to 133 days), and thus have physical gravities (derived from the period-luminosity relation and an inferred mass) that are 1.0 dex and lower. Spectroscopic gravities (derived by forcing the total Fe content, as determined from Fe I and Fe II, to be equal) are even lower. Fortunately, the [Fe/H] content as determined from Fe I is not strongly dependent on gravity and is thus well determined. We combine our abundances with other published high-resolution spectroscopic results, deriving a mean [Fe/H] for the SMC of -0.68 (σ = 0.13, n = 25). The total range seen in the [Fe/H] ratios derived herein is -0.84 to -0.65. This result is in agreement with previous studies showing a small intrinsic range in the metallicities in the SMC. For the LMC, from the Cepheids studied here we obtain -0.30 (σ = 0.12, n = 10), with a range of -0.55 to -0.19. Combining all available data, we obtain a mean [Fe/H] of -0.34 (σ = 0.15, n = 32). From this analysis, there is little evidence for a significant metallicity dispersion in the LMC.

A survey of chromospherically active stars

Photometric and/or spectroscopic observations have been obtained of 52 late-type stars which are suspected or known to be chromospherically active. Although not all types of observations were obtained for each star, these observations include all-sky BVRI Johnson photometry, ultraviolet spectrograms, low-dispersion blue-wavelength spectrograms, and high-dispersion red-wavelength spectrograms. From the spectroscopic observations v sin is, radial velocities, and the appearance of the Ca II H and K emission lines have been determined as well as the H-alpha line. The photometric observations indicate that chromospherically active stars have V - R and V - I color excesses. Such excesses will affect the surface fluxes determined with the surface brightness-color relationship. On the other hand all-sky BVRI photometry appears to be an excellent way to identify chromospherically active stars. A small group of moderately rapidly rotating, possibly single G8-K2 giants has been found. These stars have very modest chromospheric activity and so are not FK comae stars. A number of other unusual chromospherically active stars are identified.

The Cepheid period-luminosity relation from independent distances of 100 galactic variables

We have determined distances to 100 galactic classical Cepheids on a homogeneous system with an improved version of the surface brightness technique, using the extensive northern Cepheid observations of Moffett and Barnes, and the southern Cepheid observations of Gieren. The period-luminosity relations in the V bandpass obtained for the northern and southern Cepheid samples are found consistent, and the absolute magnitudes have been combined into a two-hemisphere PL(V) relation which is M V =−1.371−2.986 log P. ±0.095±0.094 This is the currently most precise Cepheid PL(V) relation from the surface brightness technique

Calibrating the Cepheid Period-Radius Relation with Galactic and Magellanic Cloud Cepheids

We have determined the radii of 16 long-period Cepheid variables in both Magellanic Clouds using the visual surface brightness technique and the excellent light curves of these variables given by Moffett et al. Combining these new radii with infrared radii of shorter period Galactic Cepheids, we provide a new calibration of the period-radius (PR) relation which covers the entire period spectrum of Cepheid variables (up to 130 days). We find evidence that the Cepheid period-radius relation is independent of the galaxy environment (e.g., universal) and linear up to the largest pulsation periods. Furthermore, the intrinsic dispersion of the relation is small enough to permit successful use of the Cepheid PR relation in the determination of reasonably accurate pulsation parallaxes to nearby galaxies.

BV(RI)C Photometry of Cepheids in the Magellanic Clouds

ABSTRACT We present V(RI)C data for 13 Cepheids in the Large Magellanic Cloud and eight in the Small Magellanic Cloud. The total number of new measures is 55 in each wavelength band. The median uncertainty in the photometry is ±0.03 mag. These results supplement a larger photometric program presented in the second paper in this series which contained 1000 measures (±0.01 mag) in each wavelength band on 22 variables with periods in the range 8–133 days.

CLASSICAL CEPHEID LIGHT CURVES REVISITED.

Fourier decompositions are made of the B, V, R, and I classical Cepheid observations of Moffett and Barnes. The Fourier diagrams for V agree very well with the corresponding plots of Simon and Lee. The B, R, and I diagrams resemble the plots for V, but with small, systematic shifts in the phases 21 and 31. The 2.5-day star, DT Cyg, is confirmed as a likely overtone pulsator by its anomalous position in the Fourier plots. Finally, the authors study the Fourier phase quantity 41, and introduce phase-phase diagrams to look at the Hertzsprung progression. Discontinuities in these diagrams support the idea that the long-period and short-period Cepheids may reach their limit cycles in different ways.

RADIAL VELOCITIES OF STANDARD STARS.

Radial-velocity observations obtained over a five-year period with the McDonald Observatory photoelectric radial-velocity spectrometer are here reported for those stars not known to be velocity variable. There are 259 stars having 1650 observations included in the spectral type range F0 to M0 and brighter than V = 6.5 mag. For the best-observed 134 stars, the standard error of the mean velocity is typically better than ±0.9 km s-1. Sixteen stars are shown to be constant to a sufficient level to warrant standard-star status. Six possible spectroscopic binaries are found.

A distance to the Cepheic HV 829 in the Small Magellanic Cloud

We present a distance determination to the Cepheid variable HV 829 in the Small Magellanic Cloud using the surface brightness technique. The distance so obtained is independent of any other astrophysical distance scale; in particular, it is independent of the Cepheid PL (and PLC) relations. It is also independent of any assumptions concerning reddening in the SMC. The distance obtained to HV 829 is 61±6 kpc, which corresponds to a distance modulus of 18.9±0.2 mag. Because HV 829 may not lie at the centroid distance for the SMC, this may not be representative of the mean distance to the SMC, but it does provide additional evidence for the long distance scale for the SMC

GALACTIC CLASSICAL CEPHEIDS : GAMMA VELOCITIES.

The γ velocities for 61 galactic classical Cepheids have been determined from recent homogeneous radial-velocity measurements combined with older published velocities. The results confirm those of Caldwell and Coulson (1987), who used a largely different data set and an independent analysis. This makes it unlikely that the net blueshift of 3±1 km s-1 found by Caldwell and Coulson for galactic Cepheids is a result of approximations used in determining the γ velocities. The Cepheids Y Sct and Z Lac are found to be binary star candidates.

Photoelectric observations of lunar occultations

Photoelectric observations of 13 lunar occultation disappearances during a March 1979 Hyades passage are reported. Three double stars (SAO 93925 = Fin 342 = 70 Tau, SAO 93961 = ADS 3248, and SAO 95419 = Kui 24) and one possible double (SAO 93950 = 75 Tau) were detected. A brief description is given of the Purdue University pulse-counting photometer as modified for occultation observations.