Results 1 - 10 of 2500
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[en] Apparent radius, visual brightness, effective temperature and absolute radius for 416 B5 v-F5 v stars of the catalogue of the Geneva Observatory (Rufener, 1976) have been determined. (orig./WL)
[en] Mass loss rates derived for a number of early-to-mid B main-sequence stars have been compared with extrapolations of recently derived empirical correlations between M and stellar parameters for luminous OB stars. Although there is sufficient scatter within the sample of B star mass loss rates to obscure any correlations that may exist within this group, the order of magnitude of the rates for these stars is consistent with a simple dependence on luminosity over the range from Of to mid-B, and is not consistent with previously derived correlations involving a dependence on mass or radius. This implies that luminosity is the dominant factor in driving the mass loss through the range of spectral types from mid-B main sequence to Of. It appears that the winds in Be stars may represent a straightforward extension of the OB star wind phenomenon to lower luminosities, although the internal scatter allows the possibility that factors other than L may have some influence on M for the B main-sequence stars. There appears to be no significant difference between B and Be stars in terms of their wind characteristics
[en] The radio spectra of main-sequence stars remain largely unconstrained due to the lack of observational data to inform stellar atmosphere models. As such, the dominant emission mechanisms at long wavelengths, how they vary with spectral type, and how much they contribute to the expected brightness at a given radio wavelength are still relatively unknown for most spectral types. We present radio continuum observations of Altair, a rapidly rotating A-type star. We observed Altair with NOEMA in 2018 and 2019 at 1.34, 2.09, and 3.22 mm and with the Very Large Array in 2019 at 6.7 and 9.1 mm. In the radio spectra, we see a brightness temperature minimum at millimeter wavelengths followed by a steep rise to temperatures larger than the optical photosphere, behavior that is unexpected for A-type stars. We use these data to produce the first submillimeter to centimeter spectrum of a rapidly rotating A-type star informed by observations. We generated both PHOENIX and KINICH-PAKAL model atmospheres and determine the KINICH-PAKAL model better reproduces Altair’s radio spectrum. The synthetic spectrum shows a millimeter brightness temperature minimum followed by significant emission over that of the photosphere at centimeter wavelengths. Together, these data and models show how the radio spectrum of an A-type star can reveal the presence of a chromosphere, likely induced by rapid rotation, and that a Rayleigh Jean’s extrapolation of the stellar photosphere is not an adequate representation of a star’s radio spectrum.
[en] The analysis of a wide sample of stars including both main-sequence and evolved stars shows that whilst flux-flux correlations are the same for the whole sample, the flux-period relations show a lower dependence on period for evolved stars. A gravity dependence also enters the flux-period relation for evolved stars. The flux-flux and flux-period relations are combined with coronal parameters to express Pc, Tc and Bc in terms of the period. All increase with decreasing period, as expected if dynamo action controls activity. Some specific relations between observable quantities are found which could be tested from further observations. Exploratory calculations of the convective zone fields in midly evolved stars suggests that a larger convective zone field results which could explain the greater emission from evolved stars for a given period
[en] Multicolor (UBVRIJHKL) photometric observations of 800 selected main-sequence stars were made. From these observations, the color-temperature relation for main-sequence stars was obtained. Intrinsic colors are defined from the mean colors of each spectral type. Normal main sequence stars are shown plotted on a two color diagram, and the reddening path for a normal extinction law line is drawn. Stars belonging to O and T associations (young stars) are shown to lie below the reddening paths of early type stars and normal main-sequence stars in the (V-R, K - L) plane. (1 fig) (GHT)
[en] The abundance effects on the tri-dimensional representation for the A0-G2 stars in the Geneva system are reviewed. B2-V1 and d are not affected by the Am characteristic, but B2-V1 is too blue (blocking effect on V1) and d smaller for the Ap stars. For B2-V1 >= 0.230, a residual effect Δ(B2-V1) = 1.20(Δm2* + 0.060) obtains. d is also affected and the residual effect is Δd = -0.4Δm2 for Δm2 >= -0.060 and Δd = 1.1Δ(B2-V1)-0.024 for Δm2 < -0.060. The abundance effects on the relations between B2-V1 and the parameters of temperature in other systems are studied. (Auth.)
[en] Photoelectric photometry in BVRI colors has been obtained at the Kitt Peak National Observatory of many stars in the regions of the Hyades, Praesepe and Coma star clusters which had no previous photometry. The stars were limited mostly to the lower main sequence. Their range is approximately 0.6 to 1.5 (and to 1.7 in the Hyades) in B-V and 0.25 to 0.85 (and to 1.3 in the Hyades) in R-I. Their ranges correspond from mid G through early M in spectral class. The purpose of this project is to detect and examine the main sequences of the nearest open clusters. (Auth.)
[en] Photometric data between 0.4 and 20 μ are discussed for K stars. Evidence of IR excess in the 11 μ region has been found for five stars in a sample of fourteen. The hypothesis of the presence of circumstellar matter in K stars seems to be confirmed. (orig.)
[en] We characterize the substructure in the simulated stellar halos of Cooper et al. which were formed by the disruption of satellite galaxies within the cosmological N-body simulations of galactic halos of the Aquarius project. These stellar halos exhibit a wealth of tidal features: broad overdensities and very narrow faint streams akin to those observed around the Milky Way. The substructures are distributed anisotropically on the sky, a characteristic that should become apparent in the next generation of photometric surveys. The normalized RMS of the density of stars on the sky appears to be systematically larger for our halos compared with the value estimated for the Milky Way from main-sequence turnoff stars in the Sloan Digital Sky Survey. We show that this is likely to be due in part to contamination by faint QSOs and redder main-sequence stars, and might suggest that ∼10% of the Milky Way halo stars have formed in situ.