Assessing statistical significance of periodogram peaks

The least-squares (or Lomb-Scargle) periodogram is a powerful tool which is used routinely in many branches of astronomy to search for periodicities in observational data. The problem of assessing statistical significance of candidate periodicities for different periodograms is considered. Based on results in extreme value theory, improved analytic estimations of false alarm probabilities are given. They include an upper limit to the false alarm probability (or a lower limit to the significance). These estimations are tested numerically in order to establish regions of their practical applicability.Comment: 7 pages, 6 figures, 1 table; To be published in MNRA

The Lick Planet Search: Detectability and Mass Thresholds

We analyse 11 years of precise radial velocities for 76 solar type stars from the Lick survey. Eight stars in this sample have previously reported planetary-mass companions, all with mass (m sin i) less than 8 Jupiter masses (MJ). For the stars without a detected companion, we place upper limits on possible companion mass. For most stars, we can exclude companions with m sin i > 0.7 MJ (a/AU)^1/2 for orbital radii a < 5 AU. We use our results to interpret the observed masses and orbital radii of planetary-mass companions. For example, we show that the finite duration of the observations makes detection of Jupiter mass companions more and more difficult for orbital radii beyond 3 AU. Thus it is possible that the majority of solar type stars harbor Jupiter-mass companions much like our own, and if so these companions should be detectable in a few years. To search for periodicities, we adopt a "floating-mean" periodogram, which improves on the traditional Lomb-Scargle periodogram by accounting for statistical fluctuations in the mean of a sampled sinusoid. We discuss in detail the normalization of the periodogram, an issue which has been of some debate in the literature.Comment: To appear in the Astrophysical Journal (50 pages, LaTeX, including 11 figures

AD Mensae: a dwarf nova in the period gap

AD Men was classified as a probable long-period dwarf nova based on its long-term variability. Recent spectroscopic data instead suggested a short-period system. With the here presented observations we aim at clarifying its nature. Time--resolved photometry and spectroscopy has been used to get information on the orbital period of this system. The light curve shows the typical flickering and a clear hump--like periodic modulation with an average amplitude of 0.3mag and a period of P=2.20(02)h. The radial velocity measurements of the Halpha emission line confirm this value as the orbital period. AD Men is thus located at the lower end of, but clearly inside, the gap of the period distribution of cataclysmic variables, making it one of only 11 dwarf novae in this important period range.Comment: 5 pages, 6 figures, accepted by A&

Limits on I-band microvariability of the Galactic Bulge Miras

We search for microvariability in a sample of 485 Mira variables with high quality I-band light curves from the second generation Optical Gravitational Lensing Experiment (OGLE-II). Rapid variations with amplitudes in the ~0.2-1.1 mag range lasting hours to days were discovered in Hipparcos data by de Laverny et al. (1998). Our search is primarily sensitive to events with time-scales of about 1 day, but retains a few percent efficiency (per object) for detecting unresolved microvariability events as short as 2 hours. We do not detect any candidate events. Assuming that the distribution of the event time profiles is identical to that from the Hipparcos light curves we derive the 95% confidence level upper limit of 0.038 per year per star for the rate of such events (1 per 26 years per average object of the ensemble). The high event rates of the order of 1 per year per star implied by the Hipparcos study in the H_P band are excluded with high confidence by the OGLE-II data in the I band. Our non-detection could still be explained by much redder spectral response of the I filter compared to the H_P band or by population differences between the bulge and the solar neighborhood. In any case, the OGLE-II I-band data provide the first limit on the rate of the postulated microvariability events in Mira stars and offer new quantitative constraints on their properties. Similar limits are obtained for other pulse shapes and a range of the assumed time-scales and size-frequency distributions.Comment: Accepted for publication in Ap

Using Ultra Long Period Cepheids to Extend the Cosmic Distance Ladder to 100 Mpc and Beyond

We examine the properties of 17 long period (80-180 days) and very luminous (median absolute magnitude of M_I= -7.93 and M_V= -7.03) Cepheids to see if they can serve as an useful distance indicator. We find that these Ultra Long Period (ULP) Cepheids have a relatively shallow Period-Luminosity (PL) relation, so in fact they are more "standard candle"-like than classical Cepheids. In the reddening-free Wesenheit index, the slope of the ULP PL relation is ~10 times less steep than the standard PL relation for the SMC Cepheids. The scatter of our sample about the W_I PL relation is 0.22 mag, approaching that of classical Cepheids and Type Ia Supernovae. We expect this scatter to decrease as bigger and more uniform samples of ULP Cepheids are obtained. We also measure a non-zero period derivative for one ULP Cepheid (SMC HV829) and use the result to probe evolutionary models and mass loss of massive stars. ULP Cepheids main advantage over classical Cepheids is that they are more luminous, and as such show great potential as stellar distance indicators to galaxies up to 100 Mpc and beyond.Comment: Accepted for Publication in ApJ. 11 pages, 8 figure

An R- and I-Band Photometric Variability Survey of the Cygnus OB2 Association

We present a catalog of photometrically variable stars discovered within two 21'.3 X 21'.3 fields centered on the Cygnus OB2 association. There have hitherto been no deep optical variability studies of Cyg OB2 despite it being replete with early-type massive stars, perhaps due to the high and variable extinction (up to A_V ~ 20) that permeates much of the region. Here we provide results of the first variability study with this combination of spatial coverage (~ 0.5 deg) and photometric depth (R ~ 21 mag). We find 121 stars to be variable in both R- and I-band, 116 of them newly discovered. Of the 121 variables, we identify 27 eclipsing binaries (EBs) and eclipsing binary candidates, 20 potential Herbig Ae/Be stars, and 52 pulsating variables. Confirming both the status and the cluster membership of the Herbig Ae/Be stars would address the uncertainty regarding the age and star formation history of Cyg OB2. We match our catalog to known variables and binaries in the region, 2MASS near-IR (NIR) data, and Chandra X-ray observations to find counterparts to new variables in other wavelengths.Comment: 34 pages, 12 figures, submitted to Ap

Discovery of a peculiar Cepheid-like star towards the northern edge of the Small Magellanic Cloud

For seven years, the EROS-2 project obtained a mass of photometric data on variable stars. We present a peculiar Cepheid-like star, in the direction of the Small Magellanic Cloud, which demonstrates unusual photometric behaviour over a short time interval. We report on data of the photometry acquired by the MARLY telescope and spectroscopy from the EFOSC instrument for this star, called EROS2 J005135-714459(sm0060n13842), which resembles the unusual Cepheid HR 7308. The light curve of our target is analysed using the Analysis of Variance method to determine a pulsational period of 5.5675 days. A fit of time-dependent Fourier coefficients is performed and a search for proper motion is conducted. The light curve exhibits a previously unobserved and spectacular change in both mean magnitude and amplitude, which has no clear theoretical explanation. Our analysis of the spectrum implies a radial velocity of 104 km s$^{-1}$ and a metallicity of -0.4$\pm$0.2 dex. In the direction of right ascension, we measure a proper motion of 17.4$\pm$6.0 mas yr$^{-1}$ using EROS astrometry, which is compatible with data from the NOMAD catalogue. The nature of EROS2 J005135-714459(sm0060n13842) remains unclear. For this star, we may have detected a non-zero proper motion for this star, which would imply that it is a foreground object. Its radial velocity, pulsational characteristics, and photometric data, however, suggest that it is instead a Cepheid-like object located in the SMC. In such a case, it would present a challenge to conventional Cepheid models.Comment: Correction of typos in the abstrac

The catalog of short periods stars from the ''Pi of the Sky'' data

Based on the data from the ''Pi of the Sky'' project we made a catalog of the variable stars with periods from 0.1 to 10 days. We used data collected during a period of two years (2004 and 2005) and classified 725 variable stars. Most of the stars in our catalog are eclipsing binaries - 464 (about 64%), while the number of pulsating stars is 125 (about 17%). Our classification is based on the shape of the light curve, as in the GCVS catalog. However, some stars in our catalog were classified as of different type than in the GCVS catalog. We have found periods for 15 stars present in the GCVS catalog with previously unknown period.Comment: New Astronomy in prin

A Study of Cepheids in M81 with the Large Binocular Telescope (Efficiently Calibrated with HST)

We identify and phase a sample of 107 Cepheids with 10<P/days<100 in M81 using the LBT and calibrate their BVI mean magnitudes with archival HST data. The use of a ground-based telescope to identify and phase the Cepheids and HST only for the final calibration reduces the demand on HST by nearly an order of magnitude and yields Period-Luminosity (PL) relations with dispersions comparable to the best LMC samples. We fit the sample using the OGLE-II LMC PL relations and are unable to find a self-consistent distance for different band combinations or radial locations within M81. We can do so after adding a radial dependence to the PL zero point that corresponds to a luminosity dependence on metallicity of g_mu=-0.56+/-0.36 mag/dex. We find marginal evidence for a shift in color as a function of metallicity, distinguishable from the effects of extinction, of g_2=+0.07+/-0.03 mag/dex. We find a distance modulus for M81, relative to the LMC, of mu(M81-LMC)=9.39+/-0.14 mag, including uncertainties due to the metallicity corrections. This corresponds to a distance to M81 of 3.6+/-0.2 Mpc, assuming a LMC distance modulus of 18.41 mag. We carry out a joint analysis of M81 and NGC4258 Cepheids and simultaneously solve for the distance of M81 relative to NGC4258 and the metallicity corrections. Given the current data, the uncertainties of such joint fits are dominated by the relative metallicities and the abundance gradients rather than by measurement errors of the Cepheid magnitudes or colors. We find mu(M81-LMC)=9.40 (-0.11/+0.15) mag, mu(N4258-LMC)=11.08 (-0.17/+0.21) mag and mu(N4258-M81)=1.68+/-0.08 mag and joint metallicity corrections of g_mu=-0.62 (-0.35/+0.31) mag/dex and g_2=0.01+/-0.01 mag/dex. Quantitative analyses of Cepheid distances must take into account both the metallicity dependencies of the Cepheids and the uncertainties in the abundance estimates. (ABRIDGED)Comment: 45 pages, 14 figures, 4 tables, appeared in The Astrophysical Journa