A new LMC K-band distance from precision measurements of nearby red clump stars

High-precision (sigma < 0.01) new JHK observations of 226 of the brightest and nearest red clump stars in the solar neighbourhood are used to determine distance moduli for the LMC. The resulting K- and H-band values of 18.47\pm0.02 and 18.49\pm0.06 imply that any correction to the K-band Cepheid PL relation due to metallicity differences between Cepheids in the LMC and in the solar neighborhood must be quite small.Comment: 7 pages, 4 figure

Arguing security: validating security requirements using structured argumentation

This paper proposes using both formal and structured informal arguments to show that an eventual realized system can satisfy its security requirements. These arguments, called 'satisfaction arguments', consist of two parts: a formal argument based upon claims about domain properties, and a set of informal arguments that justify the claims. Building on our earlier work on trust assumptions and security requirements, we show how using satisfaction arguments assists in clarifying how a system satisfies its security requirements, in the process identifying those properties of domains that are critical to the requirements

A framework for security requirements engineering

This paper presents a framework for security requirements elicitation and analysis, based upon the construction of a context for the system and satisfaction arguments for the security of the system. One starts with enumeration of security goals based on assets in the system. These goals are used to derive security requirements in the form of constraints. The system context is described using a problem-centered notation, then this context is validated against the security requirements through construction of a satisfaction argument. The satisfaction argument is in two parts: a formal argument that the system can meet its security requirements, and a structured informal argument supporting the assumptions expressed in the formal argument. The construction of the satisfaction argument may fail, revealing either that the security requirement cannot be satisfied in the context, or that the context does not contain sufficient information to develop the argument. In this case, designers and architects are asked to provide additional design information to resolve the problems

Arguing satisfaction of security requirements

This chapter presents a process for security requirements elicitation and analysis, based around the construction of a satisfaction argument for the security of a system. The process starts with the enumeration of security goals based on assets in the system, then uses these goals to derive security requirements in the form of constraints. Next, a satisfaction argument for the system is constructed, using a problem-centered representation, a formal proof to analyze properties that can be demonstrated, and structured informal argumentation of the assumptions exposed during construction of the argument. Constructing the satisfaction argument can expose missing and inconsistent assumptions about system context and behavior that effect security, and a completed argument provides assurances that a system can respect its security requirements

Are the Luminosities of RR Lyrae Stars Affected by Second Parameter Effects?

There is a serious discrepancy between the distance to the LMC derived from the Cepheid Period-Luminosity relation and that obtained by using the Galactic calibration for the luminosity of RR Lyrae stars. It is suggested that this problem might be due to the fact that second parameter effects make it inappropriate to apply Galactic calibrations to RR Lyrae variables in the Magellanic Clouds, i.e. Mv(RR) could depend on both [Fe/H] and on one or more second parameters.Comment: 10 pages as uuencoded compressed Postscript. Also available at http://www.dao.nrc.ca/DAO/SCIENCE/science.htm

Cepheid Period-Radius and Period-Luminosity Relations and the Distance to the LMC

We have used the infrared Barnes-Evans surface brightness technique to derive the radii and distances of 34 Galactic Cepheid variables. Radius and distance results obtained from both versions of the technique are in excellent agreement. The radii of 28 variables are used to determine the period-radius relation. This relation is found to have a smaller dispersion than in previous studies, and is identical to the period-radius relation found by Laney & Stobie from a completely independent method, a fact which provides persuasive evidence that the Cepheid period-radius relation is now determined at a very high confidence level. We use the accurate infrared distances to determine period-luminosity relations in the V, I, J, H and K passbands from the Galactic sample of Cepheids. We derive improved slopes of these relations from updated LMC Cepheid samples and adopt these slopes to obtain accurate absolute calibrations of the PL relation. By comparing these relations to the ones defined by the LMC Cepheids, we derive strikingly consistent and precise values for the LMC distance modulus in each of the passbands which yield a mean value of DM (LMC) = 18.46 +- 0.02. Our results show that the infrared Barnes-Evans technique is very insensitive to both Cepheid metallicity and adopted reddening, and therefore a very powerful tool to derive accurate distances to nearby galaxies by a direct application of the technique to their Cepheid variables, rather than by comparing PL relations of different galaxies, which introduces much more sensitivity to metallicity and absorption corrections which are usually difficult to determine.Comment: LaTeX, AASTeX style, 9 Figures, 10 Tables, The Astrophysical Journal in press (accepted Oct. 14, 1997). Fig. 3 replace

Theoretical Models for Classical Cepheids: IV. Mean Magnitudes and Colors and the Evaluation of Distance, Reddening and Metallicity

We discuss the metallicity effect on the theoretical visual and near-infrared PL and PLC relations of classical Cepheids, as based on nonlinear, nonlocal and time--dependent convective pulsating models at varying chemical composition. In view of the two usual methods of averaging (magnitude-weighted and intensity-weighted) observed magnitudes and colors over the full pulsation cycle, we briefly discuss the differences between static and mean quantities. We show that the behavior of the synthetic mean magnitudes and colors fully reproduces the observed trend of Galactic Cepheids, supporting the validity of the model predictions. In the second part of the paper we show how the estimate of the mean reddening and true distance modulus of a galaxy from Cepheid VK photometry depend on the adopted metal content, in the sense that larger metallicities drive the host galaxy to lower extinctions and distances. Conversely, self-consistent estimates of the Cepheid mean reddening, distance and metallicity may be derived if three-filter data are taken into account. By applying the theoretical PL and PLC relations to available BVK data of Cepheids in the Magellanic Clouds we eventually obtain Z \sim 0.008, E(B-V) \sim 0.02 mag, DM \sim 18.63 mag for LMC and Z \sim 0.004, E(B-V) \sim 0.01 mag., DM \sim 19.16 mag. for SMC. The discrepancy between such reddenings and the current values based on BVI data is briefly discussed.Comment: 16 pages, 11 postscript figures, accepted for publication on Ap

Cepheid Parallaxes and the Hubble Constant

Revised Hipparcos parallaxes for classical Cepheids are analysed together with 10 HST-based parallaxes (Benedict et al.). In a reddening-free V,I relation we find that the coefficient of logP is the same within the uncertainties in our Galaxy as in the LMC, contrary to some previous suggestions. Cepheids in the inner region of NGC4258 with near solar metallicities (Macri et al.) confirm this result. We obtain a zero-point for the reddening-free relation and apply it to Cepheids in galaxies used by Sandage et al. to calibrate the absolute magnitudes of SNIa and to derive the Hubble constant. We revise their result from 62 to 70+/-5 km/s/Mpc. The Freedman et al. 2001 value is revised from 72 to 76+/-8 km/s/Mpc. These results are insensitive to Cepheid metallicity corrections. The Cepheids in the inner region of NGC4258 yield a modulus of 29.22+/-0.03(int) compared with a maser-based modulus of 29.29+/-0.15. Distance moduli for the LMC, uncorrected for any metallicity effects, are; 18.52+/-0.03 from a reddening-free relation in V,I; 18.47+/-0.03 from a period-luminosity relation at K; 18.45+/-0.04 from a period-luminosity-colour relation in J,K. Adopting a metallicity correction in V,I from Marci et al. leads to a true LMC modulus of 18.39+/-0.05.Comment: 9 pages, 1 figure, on-line material from [email protected]. Accepted for MNRA

The Shape and Scale of Galactic Rotation from Cepheid Kinematics

A catalog of Cepheid variables is used to probe the kinematics of the Galactic disk. Radial velocities are measured for eight distant Cepheids toward l = 300; these new Cepheids provide a particularly good constraint on the distance to the Galactic center, R_0. We model the disk with both an axisymmetric rotation curve and one with a weak elliptical component, and find evidence for an ellipticity of 0.043 +/- 0.016 near the Sun. Using these models, we derive R_0 = 7.66 +/- 0.32 kpc and v_circ = 237 +/- 12 km/s. The distance to the Galactic center agrees well with recent determinations from the distribution of RR Lyrae variables, and disfavors most models with large ellipticities at the solar orbit.Comment: 36 pages, LaTeX, 10 figure

A New Method for the Determination of the Hubble Parameter

By chance, the slope of the Mv(max) versus (B-V)max relation for recent theoretical models of supernovae of Type Ia (SNe Ia) by Hoflich & Khokhlov is indistinguishable from the slope of a reddening line in the V versus B-V plane. This coincidence allows one to determine a parameter Mv*(max) for SNe Ia that is independent of both supernova detonation model and of interstellar reddening. Calibrating Mv*(max) with observations of SNe Ia by Hamuy et al. yields values of the Hubble parameter Ho in the range 55 - 60 km s-1 Mpc-1 . The discrepancy between this result, and values of Ho recently obtained from observations of Cepheids in the Virgo cluster, suggests that either (1) the Cepheid distance scale is wrong, (2) the SN Ia models of Hoflich & Khokhlov are too bright by ~ 0.75 mag near maximum light, or (3) their models are too red by \~ 0.25 mag in B-V.Comment: 12 pages and 1 figure as uuencoded compressed Postscript. Also available at http://www.dao.nrc.ca/DAO/SCIENCE/science.htm