Nami Mowlavi

Cepheids in open clusters: an 8D all-sky census

Cepheids in open clusters (cluster Cepheids: CCs) are of great importance as zeropoint calibrators of the Galactic Cepheid period-luminosity relationship (PLR). We perform an 8-dimensional all-sky census that aims to identify new bona-fide CCs and provide a ranking of membership confidence for known CC candidates according to membership probabilities. The probabilities are computed for combinations of known Galactic open clusters and classical Cepheid candidates, based on spatial, kinematic, and population-specific membership constraints. Data employed in this analysis are taken largely from published literature and supplemented by a year-round observing program on both hemispheres dedicated to determining systemic radial velocities of Cepheids. In total, we find 23 bona-fide CCs, 5 of which are candidates identified for the first time, including an overtone-Cepheid member in NGC129. We discuss a subset of CC candidates in detail, some of which have been previously mentioned in the literature. Our results indicate unlikely membership for 7 Cepheids that have been previously discussed in terms of cluster membership. We furthermore revisit the Galactic PLR using our bona fide CC sample and obtain a result consistent with the recent calibration by Turner (2010). However, our calibration remains limited mainly by cluster uncertainties and the small number of long-period calibrators. In the near future, Gaia will enable our study to be carried out in much greater detail and accuracy, thanks to data homogeneity and greater levels of completeness.

REVEALING δ CEPHEI’S SECRET COMPANION AND INTRIGUING PAST

Classical Cepheid variable stars are crucial calibrators of the cosmic distance scale thanks to a relation between their pulsation periods and luminosities. Their archetype, {delta} Cephei, is an important calibrator for this relation. In this paper, we show that {delta} Cephei is a spectroscopic binary based on newly-obtained high-precision radial velocities. We combine these new data with literature data to determine the orbit, which has period 2201 days, semi-amplitude 1.5 km/s, and high eccentricity (e = 0.647). We re-analyze Hipparcos intermediate astrometric data to measure {delta} Cepheis parallax (

Long period variables in the Large Magellanic Cloud from the EROS-2 survey

varpi = 4.09 pm 0.16

Stellar variability in open clusters - I. A new class of variable stars in NGC 3766

mas) and find tentative evidence for an orbital signature, although we cannot claim detection. We estimate that Gaia will fully determine the astrometric orbit. Using the available information from spectroscopy, velocimetry, astrometry, and Geneva stellar evolution models (

Searching transients in large-scale surveys - A method based on the Abbe value

M_{delta Cep} ~ 5.0 - 5.25 M_odot

The variable Universe Through the Eyes of Gaia

), we constrain the companion mass to within

Time series data mining for the Gaia variability analysis.

0.2 < M_2 < 1.2 M_odot

Gaia and variable stars

. We discuss the potential of ongoing and previous interactions between the companion and {delta} Cephei near pericenter passage, informing reported observations of circumstellar material and bow-shock. The orbit may have undergone significant changes due to a Kozai-Lidov mechanism driven by the outer (visual and astrometric) companion HD 213307. Our discovery of {delta} Cepheis nature as a spectroscopic binary exposes a hidden companion and reveals a rich and dynamical history of the archetype of classical Cepheid variables.

Populations of Variable Stars in Open Clusters

Context. The EROS-2 survey has produced a database of millions of time series from stars monitored for more than six years, allowing to classify some of their sources into different variable star types. Among these, Long Period Variables (LPVs), known to follow sequences in the period-luminosity diagram, include long secondary period variables whose variability origin is still a matter of debate. nAims.We use the 856 864 variable stars available from the Large Magellanic Cloud (LMC) in the EROS-2 database to detect, classify and characterize LPVs. nMethods. Our method to extract LPVs is based on the statistical Abbe test. It investigates the regularity of the light curve with respect to the survey duration in order to extract candidates with long-term variability. The period search is done by Deeming, Lomb-Scargle and generalized Lomb-Scargle methods, combined with Fourier series fit. Color-magnitude, period-magnitude and period-amplitude diagrams are used to characterize our candidates. nResults. We present a catalog of 43 551 LPV candidates for the Large Magellanic Cloud. For each of them, we provide up to five periods, mean magnitude in EROS-2, 2MASS and Spitzer bands, BE-RE color, RE amplitude and spectral type.We use infrared data to make the distinction between RGB, O-rich, C-rich and extreme AGB stars. Properties of our LPV candidates are investigated by analyzing period-luminosity and period-amplitude diagrams.

Models for Pop I stars: implications for age determinations

Aims. We analyze the population of periodic variable stars in the open cluster NGC 3766 based on a 7-year multi-band monitoring campaign conducted on the 1.2 m Swiss Euler telescope at La Silla, Chili. nMethods. The data reduction, light curve cleaning and period search procedures, combined with the long observation time line, allow us to detect variability amplitudes down to the milli-magnitude level. The variability properties are complemented with the positions in the color-magnitude and color-color diagrams to classify periodic variable stars into distinct variability types. nResults. We find a large population (36 stars) of new variable stars between the red edge of slowly pulsating B (SPB) stars and the blue edge of delta Sct stars, a region in the Hertzsprung-Russell (HR) diagram where no pulsation is predicted to occur based on standard stellar models. The bulk of their periods ranges from 0.1 to 0.7 d, with amplitudes between 1 and 4 mmag for the majority of them. About 20% of stars in that region of the HR diagram are found to be variable, but the number of members of this new group is expected to be higher, with amplitudes below our milli-magnitude detection limit. nThe properties of this new group of variable stars are summarized, and arguments set forth in favor of a pulsation origin of the variability, with g-modes sustained by stellar rotation. Potential members of this new class of low-amplitude periodic (most probably pulsating) A and late-B variables in the literature are discussed. nWe additionally identify 16 eclipsing binary, 13 SPB, 14 delta Sct and 12 gamma Dor candidates, as well as 72 fainter periodic variables. All are new discoveries. nConclusions. We encourage to search for the existence of this new class of variables in other young open clusters, especially in those hosting a rich population of Be stars.