The distance to NGC 6397 by M-subdwarf main-sequence fitting

Recent years have seen a substantial improvement both in photometry of low luminosity stars in globular clusters and in modelling the stellar atmospheres of late-type dwarfs. We build on these observational and theoretical advances in undertaking the first determination of the distance to a globular cluster by main-sequence fitting using stars on the lower main sequence. The calibrating stars are extreme M subdwarfs, as classified by Gizis (1997), with parallaxes measured to a precision of better than 10%. Matching against King et al's (1998) deep (V, (V-I)) photometry of NGC 6397, and adopting E_{B-V}=0.18 mag, we derive a true distance modulus of 12.13 +- 0.15 mag for the cluster. This compares with (m-M)_0=12.24 +- 0.1 derived through conventional main-sequence fitting in the (V, (B-V)) plane. Allowing for intrinsic differences due to chemical composition, we derive a relative distance modulus of delta (m-M)_0=2.58 mag between NGC 6397 and the fiducial metal-poor cluster M92. We extend this calibration to other metal-poor clusters, and examine the resulting RR Lyrae (M_V, [Fe/H]) relation.Comment: 19 pages, AASTeX, to appear in the December 1998 A

Younger and brighter - New distances to globular clusters based on Hipparcos parallax measurements of local subdwarfs

We have used new parallax measurements, obtained by the Hipparcos satellite, of fifteen nearby, metal-poor stars to re-define the subdwarf main-sequence. All of these stars have parallaxes determined to an accuracy of at least 12 %. Comparing these measurements against previous ground-based data for nine stars reveals a systematic offset of 5 %, in the sense that the Hipparcos parallaxes are smaller (i.e. the inferred distances are larger). The availability of the Hipparcos observations expands the local subdwarf sample to the extent that we can separate the stars by abundance into intermediate ([Fe/H] ~ -1.4) and extreme ([Fe/H] ~ -2) subsets. Main-sequence fitting techniques are then used to match stars of the appropriate abundance range to the colour-magnitude diagrams of the seven globular clusters M5, NGC 6752, M13, M15, M92, M30 and M68. We derive respective distance moduli of 14.45, 13.17, 14.48, 15.38, 14.93, 14.95 and 15.29 magnitudes, with formal uncertainties of +/- 0.1 magnitude. The metal-poor systems M68, M15 and M30 have moderate foreground reddening, and varying E(B-V) by +/- 0.02 magnitudes can change the derived distances by up to +/- 7 %. With the exception of NGC 6752, however, our derived distances exceed previous estimates, particularly in the case of the four [Fe/H] ~ -2.1 globulars, where our distance moduli are ~ 0.3 magnitudes higher than the current standard values. We discuss briefly how these findings affect the RR Lyrae distance scale, isochrone-based estimates of the age of globular clusters and our picture of the early stages of star formation in the Galaxy. We note that our results go some way towards reconciling the apparent contradiction between the cluster ages and recent determinations of the Hubble constant.Comment: to appear in AJ; 47 pages, including 13 embedded, postscript figures and 3 tables; uses AAS LaTeX style files (not included

Requirements Engineering as Creative Problem Solving: A Research Agenda for Idea Finding

This vision paper frames requirements engineering as a creative problem solving process. Its purpose is to enable requirements researchers and practitioners to recruit relevant theories, models, techniques and tools from creative problem solving to understand and support requirements processes more effectively. It uses 4 drivers to motivate the case for requirements engineering as a creative problem solving process. It then maps established requirements activities onto one of the longest-established creative problem solving processes, and uses these mappings to locate opportunities for the application of creative problem solving in requirements engineering. The second half of the paper describes selected creativity theories, techniques, software tools and training that can be adopted to improve requirements engineering research and practice. The focus is on support for problem and idea finding - two creative problem solving processes that our investigation revealed are poorly supported in requirements engineering. The paper ends with a research agenda to incorporate creative processes, techniques, training and tools in requirements projects

FUSE, STIS, and Keck spectroscopic analysis of the UV-bright star vZ 1128 in M3 (NGC 5272)

We present a spectral analysis of the UV-bright star vZ 1128 in M3 based on observations with the Far Ultraviolet Spectroscopic Explorer (FUSE), the Space Telescope Imaging Spectrograph (STIS), and the Keck HIRES echelle spectrograph. By fitting the H I, He I, and He II lines in the Keck spectrum with non-LTE H-He models, we obtain Teff = 36,600 K, log g = 3.95, and log N(He)/N(H) = -0.84. The star's FUSE and STIS spectra show photospheric absorption from C, N, O, Al, Si, P, S, Fe, and Ni. No stellar features from elements beyond the iron peak are observed. Both components of the N V 1240 doublet exhibit P~Cygni profiles, indicating a weak stellar wind, but no other wind features are seen. The star's photospheric abundances appear to have changed little since it left the red giant branch (RGB). Its C, N, O, Al, Si, Fe, and Ni abundances are consistent with published values for the red-giant stars in M3, and the relative abundances of C, N, and O follow the trends seen on the cluster RGB. In particular, its low C abundance suggests that the star left the asymptotic giant branch before the onset of third dredge-up.Comment: 15 pages, 10 figures, to be published in MNRA