Mean Star Research Articles

A HIGH FALSE POSITIVE RATE FORKEPLERPLANETARY CANDIDATES OF GIANT STARS USING ASTERODENSITY PROFILING

Asterodensity Profiling (AP) is a relatively new technique for studying transit light curves. By comparing the mean stellar density derived from the transit light curve to that found through some independent method, AP provides information on several useful properties such as orbital eccentricity and blended light. We present an AP survey of 41 Kepler Objects of Interest (KOIs), with a single transiting candidate, for which the target star's mean stellar density has been measured using asteroseismology. The ensemble distribution of the AP measurements for the 31 dwarf stars in our sample shows excellent agreement with the spread expected if the KOIs were genuine and have realistic eccentricities. In contrast, the same test for the 10 giants in our sample reveals significant incompatibility at 4$\sigma$ confidence. Whilst extreme eccentricities could be invoked, this hypothesis requires four of the KOIs to contact their host star at periastron passage, including the recently claimed confirmation of Kepler-91b. After carefully examining several hypotheses, we conclude that the most plausible explanation is that the transiting objects orbit a different star to that measured with asteroseismology - cases we define as false-positives. Based on the AP distribution, we estimate a false positive rate for Kepler's giant stars with a single transiting object of FPR$\simeq70\pm30$%.

THE HOT-JUPITER KEPLER-17b: DISCOVERY, OBLIQUITY FROM STROBOSCOPIC STARSPOTS, AND ATMOSPHERIC CHARACTERIZATION

This paper reports the discovery and characterization of the transiting hot giant exoplanet Kepler-17b. The planet has an orbital period of 1.486 days, and radial velocity measurements from the Hobby-Eberly Telescope (HET) show a Doppler signal of 420+/-15 m.s-1. From a transit-based estimate of the host star's mean density, combined with an estimate of the stellar effective temperature T_eff=5630+/-100 K from high-resolution spectra, we infer a stellar host mass of 1.061+/-0.067 M_sun and a stellar radius of 1.019+/-0.033 R_jup. We estimate the planet mass and radius to be Mp=2.450+/-0.114 M_jup and Rp=1.312+/-0.018 R_jup and a planet density near 1.35 g.cm-3. The host star is active, with dark spots that are frequently occulted by the planet. The continuous monitoring of the star reveals a stellar rotation period of 11.89 days, 8 times the the planet's orbital period; this period ratio produces stroboscopic effects on the occulted starspots. The temporal pattern of these spot-crossing events shows that the planet's orbit is prograde and the star's obliquity is smaller than 15 deg. We detected planetary occultations of Kepler-17b with both the Kepler and Spitzer Space Telescopes. We use these observations to constrain the eccentricity, e, and find that it is consistent with a circular orbit (e<0.0011). The brightness temperatures of the planet the infrared bandpasses are T_3.6um=1880+/-100 K and T4.5um=1770+/-150 K. We measure the optical geometric albedo A_g in the Kepler bandpass and find A_g = 0.10+/-0.02. The observations are best described by atmospheric models for which most of the incident energy is re-radiated away from the day side.

Mode identification from monochromatic amplitude and phase variations for the rapidly pulsating subdwarf B star EC 20338−1925

We obtain time-series spectrophotometry observations at the VLT with the aim of partially identifying the dominant oscillation modes in the rapidly pulsating subdwarf B star EC 20338-1925 on the basis of monochromatic amplitude and phase variations. From the data gathered, we detect four previously known pulsations with periods near 147, 168, 126 and 140 s and amplitudes between 0.2 and 2.3 % of the star's mean brightness. We also determine the atmospheric parameters of EC 20338-1925 by fitting our non-LTE model atmospheres to an averaged combined spectrum. The inferred parameters are Teff = 34,153+-94 K, log g =5.966+-0.017 and log[N(He)/N(H)] = - 1.642+-0.022, where the uncertainty estimates quoted refer to the formal fitting errors. Finally, we calculate the observed monochromatic amplitudes and phases for the periodicities extracted using least-squares fitting to the light curves obtained for each wavelength bin. These observed quantities are then compared to the corresponding theoretical values computed on the basis of dedicated model atmosphere codes and also taking into account non-adiabatic effects. We find that the quality of the data is sufficient to identify the dominant pulsation at 146.9 s as a radial mode, while two of the lower amplitude periodicities must be low-degree modes with l=0-2. This is the first time that monochromatic amplitudes and phases have been used for mode identification in a subdwarf B star, and the results are highly encouraging.

KEPLER-4b: A HOT NEPTUNE-LIKE PLANET OF A G0 STAR NEAR MAIN-SEQUENCE TURNOFF

Early time-series photometry from NASA's Kepler spacecraft has revealed a planet transiting the star we term Kepler-4, at RA = 19h02m27.68s, Dec = +50:08:08.7. The planet has an orbital period of 3.213 days and shows transits with a relative depth of 0.87 x 10^{-3} and a duration of about 3.95 hours. Radial velocity measurements from the Keck HIRES spectrograph show a reflex Doppler signal of 9.3 (+1.1 -1.9) m/s, consistent with a low-eccentricity orbit with the phase expected from the transits. Various tests show no evidence for any companion star near enough to affect the light curve or the radial velocities for this system. From a transit-based estimate of the host star's mean density, combined with analysis of high-resolution spectra, we infer that the host star is near turnoff from the main sequence, with estimated mass and radius of 1.223 (+0.053 -0.091) solar masses and 1.487 (+0.071 -0.084) solar radii. We estimate the planet mass and radius to be 24.5 +/- 3.8 Earth masses and 3.99 +/- 0.21 Earth radii. The planet's density is near 1.9 g/cm^3; it is thus slightly denser and more massive than Neptune, but about the same size.

Observations and asteroseismic analysis of the rapidly pulsating hot B subdwarf PG 0911+456

The principal aim of this project is to determine the structural parameters of the rapidly pulsating subdwarf B star PG 0911+456 from asteroseismology. Our work forms part of an ongoing programme to constrain the internal characteristics of hot B subdwarfs with the long-term goal of differentiating between the various formation scenarios proposed for these objects. First comparisons of asteroseismic values with evolutionary theory look promising, however it is clear that more targets are needed for meaningful statistics to be derived. The observational pulsation periods of PG 0911+456 were extracted from rapid time-series photometry using standard Fourier analysis techniques. Supplemented by spectroscopic estimates of the star's mean atmospheric parameters, they were used as a basis for the forward modelling approach in asteroseismology. The latter culminates in the identification of one or more optimal models that can accurately reproduce the observed period spectrum. This naturally leads to an identification of the oscillations detected in terms of degree l and radial order k, and infers the structural parameters of the target. From the photometry it was possible to extract 7 independent pulsation periods in the 150-200 s range with amplitudes between 0.05 and 0.8 % of the star's mean brightness. An asteroseismic search of parameter space identified several models that matched the observed properties of PG 0911+456 well, one of which was isolated as the optimal model on the basis of spectroscopic and mode identification considerations. All the observed pulsations are identified with low-order acoustic modes with degree indices l=0,1,2 and 4, and match the computed periods with a dispersion of only ~0.26 %.

The RS CVn binary HK Lacertae: long-term photometry from Sonneberg sky-patrol plates

Long-term photographic photometry of the active long-period RS CVn binary HK Lac (HD 209813) was obtained from more than 2000 Sonneberg Sky-Patrol plates taken between 1956 and 1996. We achieve an internal accuracy of 0 07. The correspondence with contemporaneous high-precision photoelectric photometry from automatic telescopes is striking and successfully demonstrates the feasibility of our approach. Based on a Bayesian time series analysis, we improve the previously published cycle period to years, and present evidence of an additional period of years. This establishes the multi-periodicity of dynamo action in these overactive stars as compared to the Sun. The already known 6.7-years cycle turns out to be an overtone of the dominating 13.4-years cycle. Our long-term photographic photometry even allowed the detection of the star's mean rotational period of 24.35 days.

Asteroseismological Studies of Long‐Period Variable Subdwarf B Stars. I. A Multisite Campaign on PG 1627+017

We present the results of an extensive multisite campaign on the long-period variable subdwarf B star PG 1627+017. We gathered 300 hr of useful R-band and ~50 hr of simultaneous U/R differential photometry. From the R-band data we were able to extract 23 periodicities in the 4500-9000 s range with amplitudes between 0.05% and 0.5% of the star's mean brightness. The oscillations with the highest amplitudes cluster between 6300 and 7050 s and are thought to exhibit frequency splitting due to binary-synchronous stellar rotation. Interestingly, we find the observed period distribution to be extremely nonuniform, with dense frequency multiplets occurring in several narrow band passes. In order to compare the observed period spectrum to theoretical predictions, we constructed a set of newly updated and improved subdwarf B star models. We find that by invoking degree indices of l = 2, 3, and 4, nonadiabatic calculations can qualitatively reproduce the range of periodicities measured for PG 1627+017 if its atmospheric parameters are pushed to the lower end of their spectroscopic temperature uncertainties. However, the exploitation of rotational splitting and the U/R photometry, as well as the mean spacing between periodicities, indicate that at least the four highest amplitude peaks probably correspond to modes with l = 1. While this points to deficiencies in our models at the nonadiabatic level, the resulting constraints on mode identification are invaluable to first attempts at asteroseismology. Indeed, we identify only a few families of models that can closely reproduce the main periodicities observed in terms of dipole modes. This leaves us hopeful that, given a larger number of partially identified observed frequencies, asteroseismology may be achieved for long-period variable subdwarf B stars.

Stellar Active Region Evolution - II. Identification and Evolution of Variance Morphologies in CA II H+K Time Series

The relative distribution of pooled variance computed at various time scales for records of chromospheric activity has been calculated for approximately 100 stars observed at Mount Wilson Observatory. As shown in Paper I, analysis of the pooled variance provides a technique for estimating the lifetimes of stellar active regions and their influence on chromospheric time series used for determining rotation and activity cycle periods. Pooled variance diagrams may be divided into three morphological types which depend to a large extent on a star's mean level of chromospheric activity (i.e., age) and B-V color (i.e., mass), and possibly depend on star's evolutionary state.