Ultra-low Amplitude Variables in the LMC -- Classical Cepheids, Pop. II Cepheids, RV Tau Stars and Binary Variables

A search for variable stars with ultra-low amplitudes (ULA), in the millimag range, has been made in the combined MACHO and OGLE data bases in the broad vicinity of the Cepheid instability strip in the HR diagram. A total of 25 singly periodic and 4 multiply periodic ULA objects has been uncovered. Our analysis does not allow us to distinguish between pulsational and ellipsoidal (binary) variability, nor between LMC and foreground objects. However, the objects are strongly clustered and appear to be associated with the pulsational instability strips of LMC Pop. I and II variables. When combined with the ULA variables of Buchler et al (2005) a total of 20 objects fall close to the classical Cepheid instability strip. However, they appear to fall on parallel period-magnitude relations that are shifted to slightly higher magnitude which would confer them a different evolutionary status. Low amplitude RV Tauri and Pop. II Cepheids have been uncovered that do not appear in the MACHO or OGLE catalogs. Interestingly, a set of binaries seem to lie on a PM relation that is essentially parallel to that of the RV Tauri/Pop. II Cepheids.Comment: 13 pages, 13 (color) figures. Astrophysical Journal (accepted for publlication

Period doubling bifurcation and high-order resonances in RR Lyrae hydrodynamical models

We investigated period doubling, a well-known phenomenon in dynamical systems, for the first time in RR Lyrae models. These studies provide theoretical background for the recent discovery of period doubling in some Blazhko RR Lyrae stars with the Kepler space telescope. Since period doubling was observed only in Blazhko-modulated stars so far, the phenomenon can help in the understanding of the modulation as well. Utilising the Florida-Budapest turbulent convective hydrodynamical code, we identified the phenomenon in radiative and convective models as well. A period-doubling cascade was also followed up to an eight-period solution confirming that the destabilisation of the limit cycle is indeed the underlying phenomenon. Floquet stability roots were calculated to investigate the possible causes and occurrences of the phenomenon. A two-dimensional diagnostic diagram was constructed to display the various resonances between the fundamental mode and the different overtones. Combining the two tools, we confirmed that the period-doubling instability is caused by a 9:2 resonance between the 9th overtone and the fundamental mode. Destabilisation of the limit cycle by a resonance of a high-order mode is possible because the overtone is a strange mode. The resonance is found to be sufficiently strong enough to shift the period of overtone with up to 10 percent. Our investigations suggest that a more complex interplay of radial (and presumably non-radial) modes could happen in RR Lyrae stars that might have connections with the Blazhko effect as well.Comment: 9 pages, 9 figures, accepted for publication in MNRA

Modulations in Multi-Periodic Blue Variables in the LMC

As shown by Mennickent, et al(2003), a subset of the blue variable stars in the Large Magellanic Cloud exhibit brightness variability of small amplitude in the period range 2.4 to 16 days as well as larger amplitude variability with periods of 140 to 600 days, with a remarkably tight relation between the long and the short periods. Our re-examination of these objects has led to the discovery of additional variability. The Fourier spectra of 11 of their 30 objects have 3 or 4 peaks above the noise level and a linear relation of the form f_a = 2(f_b - f_L) among three of the frequencies. An explanation of this relation requires an interplay between the binary motion and that of a third object. The two frequency relations together with the Fourier amplitude ratios pose a challenging modeling problem.Comment: 4 pages, 3 figures, Astrophysical Journal (in press

An international collaboration for the development of a research training course in an emergent academic discipline

Proceedings of INTED2010 Conference. 8-10 March 2010, Valencia, Spain.In professional areas such as the creative and performing arts and design, the academic model of research has not been clearly articulated. This means that often the values held in advanced professional practice run counter to the traditional models of knowledge and research that are adopted in academia. As a result, there is a problem in accounting for research in these areas in ways that will be recognised and valued by both communities. There is an ongoing debate about the best way of dealing with and reflecting these professional values in academic research. This debate has substantiated an emergent type of research that is called ‘Practice-based Research’ (PbR). PbR introduces the claim that creative practice has an instrumental role in academic research in areas such as design and urban planning. This role is different from the one of experimentation in traditional empirical research, and different from the one of practice in professional creative practice. This paper describes the development and delivery of a research methods training course in the department of spatial planning and design (Stedenbouw) at the Technical University Delft (TU Delft, Netherlands) that engages directly with these fundamental problems. The course, Research and Design Methods, has served as a testing ground for many ideas stemming from the cooperation between TU Delft and the University of Hertfordshire (UH, UK). As part of the international knowledge transfer initiative, a member of staff from TU Delft has been working at the UH for a year. One of the outcomes of this collaboration is the design and delivery of a new course at TU Delft, which tackles the relationship between academic research and planning and design, through a dialogue between different views on the activities of the urban planner and the designer. There are challenges that arise when structuring a course within an area for which the epistemological, ontological and methodological questions are still under discussion by the community. The broad aim was to offer insight into non-traditional academic research tools and methods for different areas of urban design and planning within a broader academic context. This included the analysis of different academic traditions that were relevant for urban planning and design. We define research as a systematic investigation of a subject that leads to the production of explicit knowledge, and adds to the existing body of knowledge about the subject. In the paper, we analyse the way in which research and practice are problematized in the TU Delft course and claim that PbR manifests the differences between the worldviews of academic research and professional practice, with their different aims and values. As a result, training and expertise in the professional values of creative practice is insufficient for academic research. There is therefore a need for specific research training that addresses these differences. This need for discipline specific research training has been recognized in the Bologna Process and the TU Delft course represents one such training programme.otherPeer reviewe

A pedagogical proposal in an area of epistemological uncertainty

IV Projetar 2009: PROJETO COMO INVESTIGAÇÃO: ENSINO, PESQUISA E PRÁTICA FAU-UPM SÃO PAULO BRASIL, Outubro 2009In professional areas in which there is an element of design practice, such as architecture and urbanism, the academic model of knowledge has not been clearly articulated. This means that often the values held in professional practice run counter to the traditional models of knowledge and research that are adopted. As a result there is a problem in accounting for research in these areas in ways that will be recognised and valued by both communities. There is an ongoing debate about the best way of dealing with and reflecting these values that are, from the academic viewpoint, non-traditional. The debate has substantiated an emergent type of research that is specific to areas of design practice that is called ‘Practice-based Research’ (PbR). PbR claims that design practice has an instrumental role in research in areas such as design and urban planning. This role is different from the role of experimentation in traditional empirical research, and different from the role of practice in professional design practice. This paper describes the development and delivery of a research methods training course in the department of spatial planning and design (Stedenbouw) at the Technical University Delft (TU Delft, Netherlands) that builds on research by the ‘Non-traditional Knowledge and Communication’ project (NtKC) at the University of Hertfordshire (UK). The paper will analyse the way in which research and practice are problematized in the TU Delft course. We claim that the problem of PbR manifests the differences between the worldviews of academic research and professional practice, with their differing aims and values. As a result, training and expertise in the professional values of design practice is insufficient for academic research, leading to a need for specific training as a researcher that recognises these differences. This need has been accepted at an institutional level in many universities in Europe, and the TU Delft course represents one such training programme

Thermal flickers: A semianalytical approach

In order to enhance physical insight into the nature of thermal oscillations arising from a thin helium burning shell, the behavior in its phase plane of a simple two zone model which, however, contains all the relevant physics is analyzed. This simple model very naturally reproduces thermal flickers and is relatively insensitive to all but two parameters

Automated Nonlinear Stellar Pulsation Calculations: Applications to RR Lyrae stars. The Slope of the Fundamental Blue Edge and the First RRd Model Survey

We describe a methodology that allows us to follow the pulsational behavior of an RR Lyrae model consistently and automatically along its evolutionary track throughout the whole instability strip. It is based on the powerful amplitude equation formalism, and resorts to a judicious combination of numerical hydrodynamical simulations, the analytical signal time-series analysis, and amplitude equations. A large-scale survey of the nonlinear pulsations in RR Lyr instability strip is then presented, and the mode selection mechanism is delineated throughout the relevant regions of parameter space. We obtain and examine two regions with hysteresis, where the pulsational state depends on the direction of the evolutionary tracks, namely a region with either fundamental (RRab) or first overtone (RRc) pulsations and a region with either fundamental (RRab) or double-mode (RRd) pulsations. The regions where stable double-mode (DM, or RRd) pulsations occur are very narrow and hard to find in astrophysical parameter (L, M, T_eff, X, Z) space with hydrodynamic simulations, but our systematic and efficient methodology allows us to investigate them with unprecedented detail. It is shown that by simultaneously considering the effects of mode selection and of horizontal branch evolution we can naturally solve one of the extant puzzles involving the topologies of the theoretical and observed instability strips, namely the slope of the fundamental blue edge. The importance of the interplay between mode selection and stellar evolutionary effects is also demonstrated for the properties of double-mode RR Lyr. Finally, the Petersen diagram of double-mode RR Lyr models is discussed for the first time.Comment: 13 pages, 6 figures, accepted to be published in A&

Ultra-Low Amplitude Cepheids in the Large Magellanic Cloud

The MACHO variables of LMC Field 77 that lie in the vicinity of the Cepheid instability strip are reexamined. Among the 144 variables that we identify as Cepheids we find 14 that have Fourier amplitudes <0.05 mag in the MACHO red band, of which 7 have an amplitude <0.006 mag : we dub the latter group of stars ultra-low amplitude (ULA) Cepheids. The variability of these objects is verified by a comparison of the MACHO red with the MACHO blue lightcurves and with those of the corresponding OGLE LMC stars. The occurrence of ULA Cepheids is in agreement with theory. We have also discovered 2 low amplitude variables whose periods are about a factor of 5--6 smaller than those of F Cepheids of equal apparent magnitude. We suggest that these objects are Cepheids undergoing pulsations in a surface mode and that they belong to a novel class of Strange Cepheids (or Surface Mode Cepheids) whose existence was predicted by Buchler et al. (1997).Comment: 4 pages, 5 figures, slightly revised, to appear in ApJ Letter

Strange Cepheids and RR Lyrae

Strange modes can occur in radiative classical Cepheids and RR Lyrae models. These are vibrational modes that are trapped near the surface as a result of a 'potential barrier' caused by the sharp hydrogen partial ionization region. Typically the modal number of the strange mode falls between the 7th and 12th overtone, depending on the astrophysical parameters of the equilibrium stellar models (L, M, \Teff, X, Z). Interestingly these modes can be linearly unstable outside the usual instability strip, in which case they should be observable as new kinds of variable stars, 'strange Cepheids' or 'strange RR Lyrae' stars. The present paper reexamines the linear stability properties of the strange modes by taking into account the effects of an isothermal atmosphere, and of turbulent convection. It is found that the linear vibrational instability of the strange modes is resistant to both of these effects. Nonlinear hydrodynamic calculations indicate that the pulsation amplitude of these modes is likely to saturate at the millimagnitude level. These modes should therefore be detectable albeit not without effort.Comment: 6 pages, 7 figures, submitted to Ap