RECENT PROGRESS IN UNDERSTANDING THE ERUPTIONS OF CLASSICAL NOVAEShara, Michael M.
doi: 10.1086/132400pmid: N/A
The physical processes generating nova eruptions are reviewed, along with the effects on nova eruptions of binary-system parameters such as the chemical composition or mass of the white dwarf and the mass accretion rate. Also considered are the possible metamorphosis from dwarf to classical novae and back again, X-ray and gamma-ray emission from novae, and the characteristics and distributions of novae in globular clusters and extragalactic systems. Limitations of the thermonuclear runaway model are discussed.
THE H II REGIONS OF NGC 6822. II. THE LUMINOSITY FUNCTION AND SIZE DISTRIBUTIONHodge, Paul; Lee, M. G.; Kennicutt, Robert C.
doi: 10.1086/132401pmid: N/A
Calibrated Hα CCD images of 145 H II regions in the Local Group galaxy NGC 6822 are used to obtain an absolute Hα luminosity function and several versions of diameter distributions. The luminosity function reaches from the most luminous H II region, which has an Hα flux of 4 x 1038^ ergs sec-1^, to the faintest limit yet obtained for an extragalactic H II region population, at a level of 5 x 1034^ ergs sec-1^. We are thus exploring here a portion of the H II region luminosity realm that has hitherto been largely unsampled, except in the solar neighborhood. Most of the H II regions are sufficiently faint to be explained by the excitation of a single early-type star, though some of the faintest are probably segments of diffuse galactic emission. Although the bright end of the luminosity function can be fit satisfactorily to a power law, as found for most spiral galaxies, the curve has a peak at a flux of approximately 1036^ ergs sec-1^, with a shallow decrease in number for fainter H II regions. This behavior results from the fact that the amount of ultraviolet energy released decreases more steeply than the number of stars increases for later-type stars. Three different definitions of sizes are used to explore the properties of the H II region size distribution. As found for other galaxies, the size distribution can be fit by an exponential; the size scale is 33 pc. The fluxes are correlated to the 2.8 power of the diameter, but there is a large intrinsic scatter in this relationship.
THE CALCIUM ABUNDANCE IN NGC 3201Rodgers, A. W.; Harding, P.
doi: 10.1086/132402pmid: N/A
Spectra of 15 horizontal-branch stars in the galactic globular cluster NGC 3201 are analyzed to determine the value of the Ca/H abundance ratio and the color excess, E(B - V). It is found that the cluster is reddened by E(B - V) = 0.31 + or - 0.03, in good agreement with a recent determination from the integrated properties by Reed et al. (1988). The Ca/H abundance ratio derived from equivalent-width measurements of Ca II K and Balmer lines, is found to be -1.37 + or - 0.25, i.e., less than one-half that of NGC 1851, a cluster with a similar apparently retrograde orbit.
LARGE AND KINEMATICALLY UNBIASED SAMPLES OF G- AND K-TYPE STARS. III. EVOLVED YOUNG DISK STARS IN THE BRIGHT STAR SAMPLEEggen, Olin J.
doi: 10.1086/132404pmid: N/A
Photometry of evolved young disk population C and K stars in the Bright Star Catalogue, south of Dec. = +30° and between 0h and 12h R.A., is calibrated for luminosity, reddening, and metallicity on the basis of results for members of the Hyades and Sirius superclusters. The photometry is mainly on the four-color, intermediate-band, and RI systems although DDO and Geneva photometry are also included. New DDO photometry is presented for 120 stars. The calibrations lead to values of P[Fe/H], E (b -y), and modulus, respectively, of -0.31 ± 0.03 (σ) dex, 0.030 and 8.0 t ± 0.16 (σ) mag for NGC 752; +0.09 ± 0.06 (σ) dex, 0.058 mag and 9.95 ± 0.29 mag for NGC 2423; +0.24 ± 0.07 (σ) dex, 0.128 mag and 8.00 ± 0.15 (σ) mag for NGC 6633; -0.27 0.02(σ) dex, 0.018 mag and 9.40 ± 0.35 (σ) mag for NGC 2287; and -0.08 dex, 0.038 mag and 8.8 mag for one star in NGC 3532. The Cm index in the DDO system is correlated with unreddened values of(b Y) Cm = 0.70 (b -y) -0.192 for solar composition. The resulting values of ΔCN, measured by ΔCm, show strong correlation with the derived metallicities, P[Fe/H]; P[Fe/H] = 3.3ΔCN for P[Fe/H]≤ +0.3 dex and 10.0 ΔCN for P[Fe/H]> +0.3dex. The existence of stars with P[Fe/H] > +0.3 dex needs spectroscopic confirmation. The spread of CN strength among cluster giants, at a given temperature, may be a function of the metal abundance of the cluster stars. This effect, plus observational error, restricts detection of CN/Fe values between 0.5 and 2 but if CN/Fe > 2 defines "CN-strong" stars, there are none in the young disk sample. Also, less than 5% of the young disk stars have CN/Fe < 0.5. The only nonvariable, peculiar (CH and CH-like) stars in the young disk population are mild Ba ii and weak C-band objects. Stars with composite spectra in the sample include HR 3279, a member of the Sirius supercluster, HR 3, not previously noted as composite, and HR 2773, the central star in the young disk cluster Cr 135, which appears to have an early-type companion with Mv near 0 mag. Although the distribution of the young disk stars in the (U,V) plane appears nonrandom, there is no evidence for undetected aggregates such as the Sirius and Hyades superclusters. The motion of the Sun with respect to these young disk objects is (U,V,W) = (-15, +14 +6) km sec-1.
HD 41596 AND HD 36650: INVESTIGATIONS OF WEAK 4077 IN TWO MARGINAL BARIUM STARSSowell, James R.
doi: 10.1086/132406pmid: N/A
HD 41596 and HD 36650 are two faint, Southern-Hemisphere marginal Ba II stars that are reported in the Michigan Spectral Catalogue to have unusually weak Sr II 4077-A line strengths compared to the Ba II 4554-A line. An analysis of higher resolution spectra confirms the classification for HD 41596, but the case for HD 36650 is not strong. Although an abundance determination is not practical with the material, the line-strength ratio of these two ground-state doublets indicates a barium to strontium abundance ratio greater than unity. This could be produced by the s-process mechanism in a single, large neutron exposure but not by an exponential distribution of exposures.
THE PHOTOMETRIC VARIABILITY OF THE CHROMOSPHERICALLY ACTIVE BINARY STAR HD 80715Strassmeier, Klaus G.; Hooten, James T.; Hall, Douglas S.; Fekel, Francis C.
doi: 10.1086/132407pmid: N/A
Differential UBVRI photometry of the double-lined BY Dra system HD 80715 (K3 V + K3 V) obtained in December 1987 is presented. The star is found to be a variable with a full amplitude of 0.06 mag in V and a period similar or equal to the orbital period of 3.804 days. The mechanism of the variability is interpreted as rotational modulation due to dark starspots. In an attempt to detect chromospheric activity, high-resolution CCD spectra were obtained at Ca II H and K and at Fe I 6430 A and Ca I 6439 A, the photospheric lines normally used for Doppler imaging. HD 80715 shows double H and K emission features at a constant flux level for each component.
ON THE INTERPRETATION OF EMISSION WINGS OF BALMER LINES IN LUMINOUS BLUE VARIABLESHubeny, Ivan; Leitherer, Claus
doi: 10.1086/132408pmid: N/A
H-alpha line profiles calculated with plane-parallel, hydrostatic non-LTE model atmospheres are discussed. In the lowest log g models the profiles show extended emission wings. Qualitatively, these wings are similar to the extended wings generated by electron scattering of line photons in the stellar wind. It is proposed that the line wings observed in luminous blue variables may be due to a combination of the non-LTE effect discussed here and the traditional scattering mechanism.