Red variables in the OGLE-II database. I. Pulsations and period-luminosity relations below the tip of the Red Giant Branch of the LMC

We present period-luminosity relations for more than 23,000 red giants in the Large Magellanic Cloud observed by the OGLE-II microlensing project. The OGLE period values were combined with the 2MASS single-epoch JHK photometric data. For the brighter stars we find agreement with previous results (four different sequences corresponding to different modes of pulsation in AGB stars). We also discovered two distinct and well-separated sequences below the tip of the Red Giant Branch. They consist of almost 10,000 short-period (15 d <P< 50 d), low-amplitude (A_I<0.04 mag) red variable stars, for which we propose that a significant fraction is likely to be on the Red Giant Branch, showing radial pulsations in the second and third overtone modes. The excitation mechanism could be either Mira-like pulsation or solar-like oscillations driven by convection.Comment: 5 pages, 4 figures; accepted for publication in MNRAS (Pink Pages); proof corrections adde

Mass and orbit constraints of the gamma-ray binary LS 5039

We present the results of space-based photometric and ground-based spectroscopic observing campaigns on the gamma-ray binary LS 5039. The new orbital and physical parameters of the system are similar to former results, except we found a lower eccentricity. Our MOST-data show that any broad-band optical photometric variability at the orbital period is below the 2 mmag level. Light curve simulations support the lower value of eccentricity and imply that the mass of the compact object is higher than 1.8 solar masses.Comment: 2 pages, 1 figure (with 2 panels); to be published in the Proceedings: From Interacting Binaries to Exoplanets: Essential Modeling Tools, IAU Symposium 282 (18-22 July, 2011, Tatranska Lomnica, Slovakia

Oscillations in Arcturus from WIRE photometry

Observations of the red giant Arcturus (Alpha Boo) obtained with the star tracker on the Wide Field Infrared Explorer (WIRE) satellite during a baseline of 19 successive days in 2000 July-August are analysed. The amplitude spectrum has a significant excess of power at low-frequencies. The highest peak is at about 4.1 micro-Hz (2.8 d), which is in agreement with previous ground-based radial velocity studies. The variability of Arcturus can be explained by sound waves, but it is not clear whether these are coherent p-mode oscillations or a single mode with a short life-time.Comment: 6 pages, 1 Latex file, 4 .eps figures, 2 .sty files, ApJL, 591, L151 See erratum (astro-ph/0308424

Discovery of the spectroscopic binary nature of six southern Cepheids

We present the analysis of photometric and spectroscopic data of six bright Galactic Cepheids: GH Carinae, V419 Centauri, V898 Centauri, AD Puppis, AY Sagittarii, and ST Velorum. Based on new radial velocity data (in some cases supplemented with earlier data available in the literature), these Cepheids have been found to be members in spectroscopic binary systems. V898 Cen turned out to have one of the largest orbital radial velocity amplitude (> 40 km/s) among the known binary Cepheids. The data are insufficient to determine the orbital periods nor other orbital elements for these new spectroscopic binaries. These discoveries corroborate the statement on the high frequency of occurrence of binaries among the classical Cepheids, a fact to be taken into account when calibrating the period-luminosity relationship for Cepheids. We have also compiled all available photometric data that revealed that the pulsation period of AD Pup, the longest period Cepheid in this sample, is continuously increasing with Delta P = 0.004567 d/century, likely to be caused by stellar evolution. The wave-like pattern superimposed on the parabolic O-C graph of AD Pup may well be caused by the light-time effect in the binary system. ST Vel also pulsates with a continuously increasing period. The other four Cepheids are characterised with stable pulsation periods in the last half century.Comment: accepted by the MNRAS, 11 pages, 16 figures, 18 tables, a part of the data can be downloaded from the online version of this articl

Scattering quantum random-walk search with errors

We analyze the realization of a quantum-walk search algorithm in a passive, linear optical network. The specific model enables us to consider the effect of realistic sources of noise and losses on the search efficiency. Photon loss uniform in all directions is shown to lead to the rescaling of search time. Deviation from directional uniformity leads to the enhancement of the search efficiency compared to uniform loss with the same average. In certain cases even increasing loss in some of the directions can improve search efficiency. We show that while we approach the classical limit of the general search algorithm by introducing random phase fluctuations, its utility for searching is lost. Using numerical methods, we found that for static phase errors the averaged search efficiency displays a damped oscillatory behaviour that asymptotically tends to a non-zero value.Comment: 10 pages, 10 figures. Two figures added for clarity, also made improvements to the tex

Impact of constrained rewiring on network structure and node dynamics

In this paper, we study an adaptive spatial network. We consider a susceptible-infected-susceptible (SIS) epidemic on the network, with a link or contact rewiring process constrained by spatial proximity. In particular, we assume that susceptible nodes break links with infected nodes independently of distance and reconnect at random to susceptible nodes available within a given radius. By systematically manipulating this radius we investigate the impact of rewiring on the structure of the network and characteristics of the epidemic.We adopt a step-by-step approach whereby we first study the impact of rewiring on the network structure in the absence of an epidemic, then with nodes assigned a disease status but without disease dynamics, and finally running network and epidemic dynamics simultaneously. In the case of no labeling and no epidemic dynamics, we provide both analytic and semianalytic formulas for the value of clustering achieved in the network. Our results also show that the rewiring radius and the network’s initial structure have a pronounced effect on the endemic equilibrium, with increasingly large rewiring radiuses yielding smaller disease prevalence

Photometric biases due to stellar blending: implications for measuring distances, constraining binarity and detecting exoplanetary transits

We investigate blending, binarity and photometric biases in crowded-field CCD imaging. For this, we consider random blend losses, which correspond to the total number of stars left undetected in unresolved blends. We present a simple formula to estimate blend losses, which can be converted to apparent magnitude biases using the luminosity function of the analyzed sample. Because of the used assumptions, our results give lower limits of the total bias and we show that in some cases even these limits point toward significant limitations in measuring apparent brightnesses of ``standard candle'' stars, thus distances to nearby galaxies. A special application is presented for the OGLE-II BVI maps of the Large Magellanic Cloud. We find a previously neglected systematic bias up to 0.2-0.3 mag for faint stars (V~18.0-19.0) in the OGLE-II sample, which affects LMC distance measurements using RR Lyrae and red clump stars. We also consider the effects of intrinsic stellar correlations, i.e. binarity, via calculating two-point correlation functions for stellar fields around seven recently exploded classical novae. In two cases, for V1494 Aql and V705 Cas, the reported close optical companions seem to be physically correlated with the cataclysmic systems. Finally, we find significant blend frequencies up to 50-60% in the samples of wide-field exoplanetary surveys, which suggests that blending calculations are highly advisable to be included into the regular reduction procedure.Comment: 10 pages, 7 figures, accepted for publication in MNRA

Discovery of the spectroscopic binary nature of three bright southern Cepheids

We present an analysis of spectroscopic radial velocity and photometric data of three bright Galactic Cepheids: LR Trianguli Australis (LR TrA), RZ Velorum (RZ Vel), and BG Velorum (BG Vel). Based on new radial velocity data, these Cepheids have been found to be members of spectroscopic binary systems. The ratio of the peak-to-peak radial velocity amplitude to photometric amplitude indicates the presence of a companion for LR TrA and BG Vel. IUE spectra indicate that the companions of RZ Vel and BG Vel cannot be hot stars. The analysis of all available photometric data revealed that the pulsation period of RZ Vel and BG Vel varies monotonically, due to stellar evolution. Moreover, the longest period Cepheid in this sample, RZ Vel, shows period fluctuations superimposed on the monotonic period increase. The light-time effect interpretation of the observed pattern needs long-term photometric monitoring of this Cepheid. The pulsation period of LR TrA has remained constant since the discovery of its brightness variation. Using statistical data, it is also shown that a large number of spectroscopic binaries still remain to be discovered among bright classical Cepheids.Comment: 9 pages, 14 figure