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  • 1
    Electronic Resource
    Electronic Resource
    Dynamo-based scheme for forecasting the magnitude of solar activity cycles (1991)
    Springer
    Solar physics 132 (1991), S. 1-40 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract In this paper we present a general framework for forecasting the smoothed maximum level of solar activity in a given cycle, based on a simple understanding of the solar dynamo. This type of forecasting requires knowledge of the Sun's polar magnetic field strength at the preceeding activity minimum. Because direct measurements of this quantity are difficult to obtain, we evaluate the quality of a number of proxy indicators already used by other authors which are physically related to the Sun's polar field. We subject these indicators to a rigorous statistical analysis, and specify in detail the analysis technique for each indicator in order to simplify and systematize reanalysis for future use. We find that several of these proxies are in fact poorly correlated or uncorrelated with solar activity, and thus are of little value for predicting activity maxima. We also present a scheme in which the predictions of the individual proxies are combined via an appropriately weighted mean to produce a compound prediction. We then apply the scheme to the current cycle 22, and estimate a maximum smoothed International sunspot number of 171 ± 26, which can be expressed alternatively as a smoothed 2800 MHz radio flux (F 10.7) of 211 ± 23 × (10−22 Wm−2Hz−1), or as a smoothed sunspot area of 2660 ± 430 millionths of a solar disk. Once the actual maximum for cycle 22 has been established, we will have both additional statistics for all the proxy indicators, and a clearer indication of how accurately the present scheme can predict solar activity levels.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00159127
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    Paper (German National Licenses)
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  • 2
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    The pulsating variable star population in DDO210 (2015)
    Oxford University Press
    Details
    Publication Date: 2015-11-21
    Description: We have probed the pulsating variable star content of the isolated Local Group dwarf galaxy, DDO210 (Aquarius), using archival Advanced Camera for Surveys/ Hubble Space Telescope imaging in the F 475 W and F 814 W passbands. We find a total of 32 RR Lyrae stars (24 ab-type; 8 c-type) and 75 Cepheid variables. The mean periods of the ab-type and c-type RR Lyrae stars are calculated to be 〈P ab 〉 = 0.609 ± 0.011 and 〈P c 〉 = 0.359 ± 0.025 d, respectively. The light-curve properties of the fundamental mode RR Lyrae stars yield a mean metallicity of 〈[Fe/H]〉 = –1.63 ± 0.11 dex for this ancient population, consistent with a recent synthetic colour–magnitude diagram analysis. We find this galaxy to be Oosterhoff-intermediate and lacking in high-amplitude, short-period ab-type RR Lyrae, consistent with behaviour recently observed for many dwarf spheroidals and ultrafaint dwarfs in the Local Group. We find a distance modulus of μ = 25.07 ± 0.12 as determined by the RR Lyrae stars, slightly larger but agreeing with recent distance estimates from the red giant branch tip. We also find a sizable population of Cepheid variables in this galaxy. We provide evidence in favour of most if not all of these stars being short-period classical Cepheids. Assuming all of these stars to be classical Cepheids, we find that most of these Cepheids are ~300 Myr old, with the youngest Cepheids being offset from the older Cepheids and the centre of the galaxy. We conclude that this may have resulted from a migration of star formation in DDO210.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    Panchromatic Hubble Andromeda Treasury XIII: The Cepheid period-luminosity relation in M31 (2015)
    Oxford University Press
    Details
    Publication Date: 2015-05-31
    Description: Using Hubble Space Telescope Advanced Camera for Surveys ( HST /ACS) and Wide Field Camera 3 (WFC3) observations from the Panchromatic Hubble Andromeda Treasury (PHAT), we present new period–luminosity (P-L) relations for Cepheid variables in M31. Cepheids from several ground-based studies are identified in the PHAT photometry to derive new P-L and Wesenheit P-L relations in the near infrared and visual filters. We derive a distance modulus to M31 of 24.51 ± 0.08 in the IR bands and 24.32 ± 0.09 in the visual bands, including the first P-L relations in the F 475 W and F 814 W filters for M31. Our derived visual and IR distance moduli disagree at slightly more than a 1 level. Differences in the P-L relations between ground-based and HST observations are investigated for a subset of Cepheids. We find a significant discrepancy between ground-based and HST P-L relations with the same Cepheids, suggesting adverse effects from photometric contamination in ground-based Cepheid observations. Additionally, a statistically significant radial trend in the P-L relation is found which does not appear to be explained by metallicity.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    A far-ultraviolet variable with an 18-minute period in the globular cluster NGC 1851 (2016)
    Oxford University Press
    Details
    Publication Date: 2016-06-24
    Description: We present the detection of a variable star with an 18.05 min period in far-ultraviolet (FUV) images of the globular cluster NGC 1851 taken with the Hubble Space Telescope ( HST ). A candidate optical counterpart lies on the red horizontal branch or the asymptotic giant branch star of the cluster, but it is statistically possible that this is a chance superposition. This interpretation is supported by optical spectroscopy obtained with HST /Space Telescope Imaging Spectrograph (STIS): the spectrum contains none of the strong emission lines that would be expected if the object was a symbiotic star (i.e. a compact accretor fed by a giant donor). We therefore consider two other possibilities for the nature of FUV variable: (i) an intermediate polar (i.e. a compact binary containing an accreting magnetic white dwarf), or (ii) an AM CVn star (i.e. an interacting double-degenerate system). In the intermediate polar scenario, the object is expected to be an X-ray source. However, no X-rays are detected at its location in ~=65 ksec of Chandra imaging, which limits the X-ray luminosity to L X ≤ 10 32  erg s –1 . We therefore favour the AM CVn interpretation, but a FUV spectrum is needed to distinguish conclusively between the two possibilities. If the object is an AM CVn binary, it would be the first such system known in any globular cluster.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Evidence for temporal evolution in the M33 disc as traced by its star clusters (2015)
    Oxford University Press
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    Publication Date: 2015-06-27
    Description: We present precision radial velocities and stellar population parameters for 77 star clusters in the Local Group galaxy M33. Our Gran Telescopio de Canarias and William Herschel Telescope observations sample both young, massive clusters and known/candidate globular clusters (GCs), spanning ages ~10 6 –10 10  yr, and metallicities, [M/H] ~ –1.7 to solar. The cluster system exhibits an age–metallicity relation; the youngest clusters are the most metal rich. When compared to H i data, clusters with [M/H] ~ –1.0 and younger than ~4 Gyr are clearly identified as a disc population. The clusters show evidence for strong time evolution in the disc radial metallicity gradient (d[M/H]d t /d R  = 0.03 dex kpc –1  Gyr –1 ). The oldest clusters have stronger, more negative gradients than the youngest clusters in M33. The clusters also show a clear age–velocity dispersion relation. The line-of-sight velocity dispersions of the clusters increases with age similar to Milky Way open clusters and stars. The general shape of the relation is reproduced by disc heating simulations, and the similarity between the relations in M33 and the Milky Way suggests that heating by substructure and cooling of the interstellar medium both play a role in shaping this relation. We identify 12 ‘classical’ GCs, six of which are newly identified GC candidates. The GCs are more metal rich than Milky Way halo clusters, and show weak rotation. The inner ( R  〈 4.5 kpc) GCs exhibit a steep radial metallicity gradient (d[M/H]/d R = –0.29 ± 0.11 dex kpc –1 ) and an exponential-like surface density profile. We argue that these inner GCs are thick disc rather than halo objects.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    The Hubble Space Telescope UV Legacy Survey of galactic globular clusters - II. The seven stellar populations of NGC 7089 (M2) (2014)
    Oxford University Press
    Details
    Publication Date: 2014-12-21
    Description: We present high-precision multiband photometry for the globular cluster (GC) M2. We combine the analysis of the photometric data obtained from the Hubble Space Telescope UV Legacy Survey of Galactic GCs GO-13297, with chemical abundances by Yong et al., and compare the photometry with models in order to analyse the multiple stellar sequences we identified in the colour–magnitude diagram. We find three main stellar components, composed of metal-poor, metal-intermediate, and metal-rich stars (hereafter referred to as population A, B, and C, respectively). The components A and B include stars with different s -process element abundances. They host six sub-populations with different light-element abundances, and exhibit an internal variation in helium up to Y  ~ 0.07 dex. In contrast with M22, another cluster characterized by the presence of populations with different metallicities, M2 contains a third stellar component, C, which shows neither evidence for sub-populations nor an internal spread in light-elements. Population C does not exhibit the typical photometric signatures that are associated with abundance variations of light elements produced by hydrogen burning at hot temperatures. We compare M2 with other GCs with intrinsic heavy-element variations and conclude that M2 resembles M22, but it includes an additional stellar component that makes it more similar to the central region of the Sagittarius galaxy, which hosts a GC (M54) and the nucleus of the Sagittarius galaxy itself.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    The Hubble Space Telescope UV Legacy Survey of galactic globular clusters - II. The seven stellar populations of NGC 7089 (M2) (2014)
    Oxford University Press
    Details
    Publication Date: 2014-12-29
    Description: We present high-precision multiband photometry for the globular cluster (GC) M2. We combine the analysis of the photometric data obtained from the Hubble Space Telescope UV Legacy Survey of Galactic GCs GO-13297, with chemical abundances by Yong et al., and compare the photometry with models in order to analyse the multiple stellar sequences we identified in the colour–magnitude diagram. We find three main stellar components, composed of metal-poor, metal-intermediate, and metal-rich stars (hereafter referred to as population A, B, and C, respectively). The components A and B include stars with different s -process element abundances. They host six sub-populations with different light-element abundances, and exhibit an internal variation in helium up to Y  ~ 0.07 dex. In contrast with M22, another cluster characterized by the presence of populations with different metallicities, M2 contains a third stellar component, C, which shows neither evidence for sub-populations nor an internal spread in light-elements. Population C does not exhibit the typical photometric signatures that are associated with abundance variations of light elements produced by hydrogen burning at hot temperatures. We compare M2 with other GCs with intrinsic heavy-element variations and conclude that M2 resembles M22, but it includes an additional stellar component that makes it more similar to the central region of the Sagittarius galaxy, which hosts a GC (M54) and the nucleus of the Sagittarius galaxy itself.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    The void in the Sculptor group spiral galaxy NGC 247 (2014)
    Oxford University Press
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    Publication Date: 2014-08-09
    Description: The dwarf galaxy NGC 247, located in the Sculptor Filament, displays an apparent void on the north side of its spiral disc. The existence of the void in the disc of this dwarf galaxy has been known for some time, but the exact nature and cause of this strange feature has remained unclear. We investigate the properties of the void in the disc of NGC 247 using photometry of archival Hubble Space Telescope data to analyse the stars in and around this region. Based on a grid of isochrones from log( t ) = 6.8 to 10.0, we assign ages using nearest-neighbour interpolation. Examination of the spatial variation of these ages across the galaxy reveals an age difference between stars located inside the void region and stars located outside this region. We speculate that the void in NGC 247 's stellar disc may be due to a recent interaction with a nearly dark subhalo that collided with the disc and could account for the long-lived nature of the void.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    The properties of the Large Magellanic Cloud based on OGLE-III photometry of RR Lyrae stars (2013)
    Oxford University Press
    Details
    Publication Date: 2013-04-13
    Description: We present results from a study of ab-type RR Lyrae variables in the Large Magellanic Cloud (LMC) using the extensive data set from phase III of the Optical Gravitational Lensing Experiment (OGLE; Udalski et al.). The metallicities of the RR Lyraes, determined from the periods and amplitudes of their light curves, reveal a statistically significant radial abundance gradient that is approximately one-half of what is seen in the discs of the Milky Way and M33. The RR Lyrae abundance gradient agrees with that of the old and metal-poor LMC globular clusters. The reddenings of the OGLE RR Lyraes have been calculated using their minimum light colours and reveal a mean value of E ( V  – I ) = 0.12 ± 0.05, where the quoted uncertainty represents the standard deviation of the mean. The distribution of RR Lyrae extinctions across the face of the LMC is well correlated with the distribution and emission intensity of CO clouds based on recent millimetre wave surveys. In addition, we find that the old LMC globulars tend to be located in regions of low extinction. This underscores the need to survey the higher extinction regions with the specific aim of increasing the sample of old LMC globular clusters. Finally, we examine the distance distribution of the RR Lyraes in order to probe the structure of the LMC and investigate the possibility that some of the RR Lyraes may reside in a kinematically hot halo population. In addition to calculating a mean LMC distance of ( m  – M ) o  = 18.55 ± 0.10 mag, we conclude that some fraction of the RR Lyraes in our sample could be members of the LMC halo.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    Research Note: A new galaxy near the Local Group in Draco (2001)
    EDP Sciences
    Details
    Publication Date: 2001-11-01
    Print ISSN: 0004-6361
    Electronic ISSN: 1432-0746
    Topics: Physics
    Published by EDP Sciences
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