Chemo-spectrophotometric evolution of spiral galaxies – IV. Star formation efficiency and effective ages of spirals     

S. Boissier  A. Boselli  N. Prantzos  G. Gavazzi 《Monthly notices of the Royal Astronomical Society》2001,321(4):733-742

We study the star formation history of normal spirals by using a large and homogeneous data sample of local galaxies. For our analysis we utilize detailed models of chemical and spectrophotometric galactic evolution, calibrated on the Milky Way disc. We find that star formation efficiency is independent of galactic mass, while massive discs have, on average, lower gas fractions and are redder than their low-mass counterparts; put together, these findings convincingly suggest that massive spirals are older than low-mass ones. We evaluate the effective ages of the galaxies of our sample and we find that massive spirals must be several Gyr older than low-mass ones. We also show that these galaxies (having rotational velocities in the 80–400 km s⁻¹ range) cannot have suffered extensive mass losses, i.e. they cannot have lost during their lifetime an amount of mass much larger than their current content of gas+stars.  相似文献   

Cygnus X-3 with ISO: investigating the wind     

R. N. Ogley  †  S. J. Bell Burnell  R. P. Fender 《Monthly notices of the Royal Astronomical Society》2001,322(1):177-186

We have observed the energetic binary Cygnus X-3 in both quiescent and flaring states between 4 and 16 μm using the ISO satellite. We find that the quiescent source shows the thermal free–free spectrum typical of a hot, fast stellar wind, such as from a massive helium star. The quiescent mass-loss rate arising from a spherically symmetric, non-accelerating wind is found to be in the range (0.4–2.9)×10⁻⁴ M_⊙ yr⁻¹, consistent with other infrared and radio observations, but considerably larger than the 10⁻⁵ M_⊙ yr⁻¹ deduced from both the orbital change and the X-ray column density. There is rapid, large-amplitude flaring at 4.5 and 11.5 μm at the same time as enhanced radio and X-ray activity, with the infrared spectrum apparently becoming flatter in the flaring state. We believe that non-thermal processes are operating, perhaps along with enhanced thermal emission.  相似文献   

Comparison of atmospheric aerosol backscattering and air mass back trajectories     

J. M. Vaughan  R. H. Maryon  N. J. Geddes 《Meteorology and Atmospheric Physics》2002,79(1-2):33-46

Summary Atmospheric backscattering from aerosol particulates has been measured over the Atlantic at 10.6 μm wavelength with an airborne, coherent heterodyne, lidar, and corresponding air mass back trajectories have been calculated. These back trajectories (usually extended up to 10 days prior to the backscatter measurement) have shown very diverse origins for the air parcels at different altitudes. In many cases it has been possible to attribute the observed levels of scattering to these origins over oceanic, arctic, continental, industrial etc. regions. This is illustrated by 6 flight records: out of Ascension Island in the South Atlantic; over the Azores in the mid North Atlantic; over the UK and the North Sea; and in the Arctic along 71° North. In each of these regions the profiles of backscatter versus altitude show highly variable features; remarkably different origins for air masses at different altitudes are evident from the corresponding back trajectory analyses. It is thus possible for the first time to present probable explanations for the different levels of scattering observed at different altitudes. Received July 14, 2000 Revised May 14, 2001  相似文献   

Variable stars in the field of open cluster NGC 6819     

R. A.Street  KeithHorne  T. A.Lister  A.Penny  Y.Tsapras  A.Quirrenbach  N.Safizadeh  J.Cooke  D.Mitchell  A.Collier Cameron 《Monthly notices of the Royal Astronomical Society》2002,330(3):737-754

We report on the discovery of 25 variable stars plus 13 suspected variables found in the field of the open cluster NGC 6819. The stars were identified from time-series photometric data obtained on the Isaac Newton Telescope, La Palma, during two observing runs covering the 19 nights between 1999 June  22–30  and 1999 July  22–31  . The variables found include 12 eclipsing binaries with an additional three suspected, nine BY Draconis systems, plus four variables of other types, including one star believed to be a Cepheid. Three of the 15 eclipsing binaries are believed to be cluster members. Details of a further 10 suspected variable stars are also included.  相似文献   

Common Proper Motion Search for Faint Companions Around Early-Type Field Stars – Progress Report     

Valentin D. Ivanov  G. Chauvin  C. Foellmi  M. Hartung  N. Huélamo  C. Melo  D. Nürnberger  M. Sterzik 《Astrophysics and Space Science》2006,304(1-4):247-249

The multiplicity of early-type stars is still not well established. The derived binary fraction is different for individual star forming regions, suggesting a connection with the age and the environment conditions. The few studies that have investigated this connection do not provide conclusive results. To fill in this gap, we started the first detailed adaptive-optic-assisted imaging survey of early-type field stars to derive their multiplicity in a homogeneous way. The sample has been extracted from the Hipparcos Catalog and consists of 341 BA-type stars within ∼300 pc from the Sun. We report the current status of the survey and describe a Monte-Carlo simulation that estimates the completeness of our companion detection.  相似文献   

Umbral Three-Minute Oscillations and Running Penumbral Waves     

N. I. Kobanov  D. Y. Kolobov  D. V. Makarchik 《Solar physics》2006,238(2):231-244

We present results of investigations into chromospheric velocity oscillations in sunspots, carried out at the Sayan Solar Observatory. It is shown that the “chevron” structures in the space-time diagrams demonstrate wavetrain properties. Such structures are indicators of a propagating wave process and they are typical of many sunspots. In the authors’ opinion, three-minute umbral oscillations are not the source of running penumbral waves (RPW). It is very likely that umbral oscillations and RPW initially propagate along different magnetic field lines. We explain the decrease in RPW propagation velocity and frequency in the outer penumbra, as compared with the inner, by the combined action of different frequency modes. To better reveal the properties of these modes, frequency filtering was used. Our measurements of the RPW (five-minute mode) wavelength and RPW propagation velocity in different sunspots vary from 12^″ to 30^″ and from 28 to 60 – 70 km s⁻¹ correspondingly.  相似文献   

Systematic uncertainties in the analysis of star cluster parameters based on broad-band imaging observations     

R. de Grijs  P. Anders  H. J. G. L. M. Lamers  N. Bastian  U. Fritze-v. Alvensleben  G. Parmentier †  M. E. Sharina  S. Yi 《Monthly notices of the Royal Astronomical Society》2005,359(3):874-894

  相似文献   

Alkali elemental and potassium isotopic compositions of Semarkona chondrules     

Conel M. O'D  Jeffrey N. GROSSMAN 《Meteoritics & planetary science》2005,40(4):541-556

Abstract— We report measurements of K isotope ratios in 28 Semarkona chondrules with a wide range of petrologic types and bulk compositions as well as the compositions of CPX‐mesostasis pairs in 17 type I Semarkona chondrules, including two chondrules with radial alkali zonation and 19 type II chondrules. Despite the wide range in K/Al ratios, no systematic variations in K isotopic compositions were found. Semarkona chondrules do not record a simple history of Rayleigh‐type loss of K. Experimentally determined evaporation rates suggest that considerable alkali evaporation would have occurred during chondrule formation. Nevertheless, based on Na CPX‐mesostasis distribution coefficients, the alkali contents of the cores of most chondrules in Semarkona were probably established at the time of final crystallization. However, Na CPX‐mesostasis distribution coefficients also show that alkali zonation in type I Semarkona chondrules was produced by entry of alkalis after solidification, probably during parent body alteration. This alkali metasomatism may have gone to completion in some chondrules. Our preferred explanation for the lack of systematic isotopic enrichments, even in alkali depleted type I chondrule cores, is that they exchanged with the ambient gas as they cooled.  相似文献   

Report of the International Astronomical Union Division I Working Group on Precession and the Ecliptic   总被引：1，自引：0，他引：1  

J. L. Hilton  N. Capitaine  J. Chapront  J. M. Ferrandiz  A. Fienga  T. Fukushima  J. Getino  P. Mathews  J.-L. Simon  M. Soffel  J. Vondrak  P. Wallace  J. Williams 《Celestial Mechanics and Dynamical Astronomy》2006,94(3):351-367

The IAU Working Group on Precession and the Equinox looked at several solutions for replacing the precession part of the IAU 2000A precession–nutation model, which is not consistent with dynamical theory. These comparisons show that the (Capitaine et al., Astron. Astrophys., 412, 2003a) precession theory, P03, is both consistent with dynamical theory and the solution most compatible with the IAU 2000A nutation model. Thus, the working group recommends the adoption of the P03 precession theory for use with the IAU 2000A nutation. The two greatest sources of uncertainty in the precession theory are the rate of change of the Earth’s dynamical flattening, ΔJ₂, and the precession rates (i.e. the constants of integration used in deriving the precession). The combined uncertainties limit the accuracy in the precession theory to approximately 2 mas cent⁻². Given that there are difficulties with the traditional angles used to parameterize the precession, z_A, ζ_A, and θ_A, the working group has decided that the choice of parameters should be left to the user. We provide a consistent set of parameters that may be used with either the traditional rotation matrix, or those rotation matrices described in (Capitaine et al., Astron. Astrophys., 412, 2003a) and (Fukushima Astron. J., 126, 2003). We recommend that the ecliptic pole be explicitly defined by the mean orbital angular momentum vector of the Earth–Moon barycenter in the Barycentric Celestial Reference System (BCRS), and explicitly state that this definition is being used to avoid confusion with previous definitions of the ecliptic. Finally, we recommend that the terms precession of the equator and precession of the ecliptic replace the terms lunisolar precession and planetary precession, respectively.  相似文献   

Observations of hydrogen deficient binary Upsilon sagittarii     

N. Kameswara Rao  V. R. Venugopal 《Journal of Astrophysics and Astronomy》1985,6(2):101-112

The absolute magnitudeM _v of the hydrogen deficient binary υ Sgr has been estimated as -4.8 ± 1.0 from the distribution of the interstellar reddening, polarization and interstellar lines of the surrounding stars. From the ANS observations obtained at the time of the secondary eclipse, it appears that the hotter secondary is surrounded by a disc with colours of a B8-B9 star. The λ 1550 CIv absorption line arising in the stellar wind does not show any change in strength during the secondary minimum. The upper limit to the mass-loss rate from the high temperature wind is estimated as ≤ 5 × 10^-7 M⊙ yr^-1 from the 2 cm and 6 cm radio observations. Based on observations obtained with the Astronomical Netherlands Satellite and VLA. The National Radio Astronomy Observatory’s Very Large Array at Socorro, New Mexico is operated by Associated Universities Inc. under contract with the National Science Foundation.  相似文献