Browsing by Author "Welch, D. L."

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    HIGH-AMPLITUDE δ-SCUTIS IN THE LARGE MAGELLANIC CLOUD
    (2010) Garg, A.; Cook, K. H.; Nikolaev, S.; Huber, M. E.; Rest, A.; Becker, A. C.; Challis, P.; Clocchiatti, A.; Miknaitis, G.; Minniti, D.; Morelli, L.; Olsen, K.; Prieto, J. L.; Suntzeff, N. B.; Welch, D. L.; Wood-Vasey, W. M.
    We present 2323 high-amplitude delta-Scuti (HADS) candidates discovered in the Large Magellanic Cloud by the SuperMACHO survey (Rest et al.). Frequency analyses of these candidates reveal that several are multimode pulsators, including 119 whose largest amplitude of pulsation is in the fundamental (F) mode and 19 whose largest amplitude of pulsation is in the first overtone (FO) mode. Using Fourier decomposition of the HADS light curves, we find that the period-luminosity (PL) relation defined by the FO pulsators does not show a clear separation from the PL relation defined by the F pulsators. This differs from other instability strip pulsators such as type c RR Lyrae. We also present evidence for a larger amplitude, subluminous population of HADS similar to that observed in Fornax.
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    Light echoes reveal an unexpectedly cool η Carinae during its nineteenth-century Great Eruption
    (2012) Rest, A.; Prieto, J. L.; Walborn, N. R.; Smith, N.; Bianco, F. B.; Chornock, R.; Welch, D. L.; Howell, D. A.; Huber, M. E.; Foley, R. J.; Fong, W.; Sinnott, B.; Bond, H. E.; Smith, R. C.; Toledo, I.; Minniti, D.; Mandel, K.
    eta Carinae is one of the most massive binary stars in the Milky Way(1,2). It became the second-brightest star in our sky during its mid-nineteenth-century 'Great Eruption', but then faded from view (with only naked-eye estimates of brightness(3,4)). Its eruption is unique in that it exceeded the Eddington luminosity limit for ten years. Because it is only 2.3 kiloparsecs away, spatially resolved studies of the nebula have constrained the ejected mass and velocity, indicating that during its nineteenth-century eruption, eta Car ejected more than ten solar masses in an event that released ten per cent of the energy of a typical core-collapse supernova(5,6), without destroying the star. Here we report observations of light echoes of eta Carinae from the 1838-1858 Great Eruption. Spectra of these light echoes show only absorption lines, which are blueshifted by -210 km s(-1), in good agreement with predicted expansion speeds(6). The light-echo spectra correlate best with those of G2-to-G5 supergiants, which have effective temperatures of around 5,000 kelvin. In contrast to the class of extragalactic outbursts assumed to be analogues of the Great Eruption of eta Carinae(7-12), the effective temperature of its outburst is significantly lower than that allowed by standard opaque wind models(13). This indicates that other physical mechanisms such as an energetic blast wave may have triggered and influenced the eruption.
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    NGC 6304: a metal rich cluster with RR Lyrae?
    (2006) De Lee, Nathan; Catelan, Márcio; Layden, Andy; Pritzl, Barton; Smith, Horace; Sweigart, Allen; Welch, D. L.
    We have carried out a new search for variable stars in the metal-rich bulge globular cluster NGC 6304 ([Fe/H] = -0.59) using CCD observations obtained at CTIO. We used two data sets: one was taken on the 0.9m in May and June of 1996, and the second was taken on the 1m Yalo telescope in February and March of 2002. We have identified and obtained BVI light curves for 11 RR Lyrae stars, including 6 RRab and 5 RRc stars within the tidal radius of the cluster, and partial light curves for several long-period variables. Most of the RR Lyrae stars had been previously discovered, although not always recognized to be RR Lyrae type variables. We are able to exclude several RR Lyrae stars as probable field stars. In light of the large number of long-period RRab stars recently discovered within the metal-rich globular clusters NGC 6388 and NGC 6441, it is noteworthy that two of the possible RRab have periods greater than 0.8 days. The nature of these long-period RR Lyrae and the question of their membership will be discussed....
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    The globular cluster M 69: color-magnitude diagram and variable stars
    (2010) Escobar, M. E.; Catelan, Marcio; Zoccali, M.; Smith, H. A.; Pritzl, B. J.; Layden, A.; Gregorsok, J.; Welch, D. L.; Webb, T.
    We present BV photometry and the results of a search for stellar variability in the Galactic globular cluster M 69. The resulting color-magnitude diagram shows significant contamination by field stars. In our variability search we found 62 variable stars, 54 of which were new discoveries....