Jason F. Rowe

VARIABLE STARS IN GALACTIC GLOBULAR CLUSTERS

Based on a search of the literature up to 2001 May, the number of known variable stars in Galactic globular clusters is approximately 3000. Of these, more than 2200 have known periods and the majority (approximately 1800) are of the RR Lyrae type. In addition to the RR Lyrae population, there are approximately 100 eclipsing binaries, 120 SX Phoenicis variables, 60 Cepheids (including Population II Cepheids, anomalous Cepheids and RV Tauri), and 120 SR/red variables. The mean period of the fundamental mode RR Lyrae variables is 0.585 days, for the overtone variables it is 0.342 days (0.349 days for the first-overtone pulsators and 0.296 days for the second-overtone pulsators) and approximately 30% are overtone pulsators. These numbers indicate that about 65% of RR Lyrae variables in Galactic globular clusters belong to Oosterhoff type I systems. The mean period of the RR Lyrae variables in the Oosterhoff type I clusters seems to be correlated with metal abundance in the sense that the periods are longer in the more metal poor clusters. Such a correlation does not exist for the Oosterhoff type II clusters. Most of the Cepheids are in clusters with blue horizontal branches.

An Upper Limit on the Albedo of HD 209458b: Direct Imaging Photometry with the MOST Satellite

We present space-based photometry of the transiting exoplanetary system HD 209458 obtained with the Microvariablity and Oscillations of Stars (MOST) satellite, spanning 14 days and covering 4 transits and 4 secondary eclipses. The HD 209458 photometry was obtained in MOSTs lower precision direct imaging mode, which is used for targets in the brightness range 6.5 ≥ V ≥ 13. We describe the photometric reduction techniques for this mode of observing, in particular the corrections for stray earthshine. We do not detect the secondary eclipse in the MOST data, to a limit in depth of 0.053 mmag (1 σ). We set a 1 σ upper limit on the planet-star flux ratio of 4.88 × 10-5 corresponding to a geometric albedo upper limit in the MOST bandpass (400-700 nm) of 0.25. The corresponding numbers at the 3 σ level are 1.34 × 10-4 and 0.68, respectively. HD 209458b is half as bright as Jupiter in the MOST bandpass. This low geometric albedo value is an important constraint for theoretical models of the HD 209458b atmosphere, in particular ruling out the presence of reflective clouds. A second MOST campaign on HD 209458 is expected to be sensitive to an exoplanet albedo as low as 0.13 (1 σ), if the star does not become more intrinsically variable in the meantime.

Stellar activity of planetary host star HD 189733

Aims. Extra-solar planet search programs require high-precision velocity measurements. They need to determine how to differentiate between radial-velocity variations due to Doppler motion and the noise induced by stellar activity. Methods. We monitored the active K2V star HD 189733 and its transiting planetary companion, which has a 2.2-day orbital period. We used the high-resolution spectograph SOPHIE mounted on the 1.93-m telescope at the Observatoire de Haute-Provence to obtain 55 spectra of HD 189733 over nearly two months. We refined the HD 189733b orbit parameters and placed limits on both the eccentricity and long-term velocity gradient. After subtracting the orbital motion of the planet, we compared the variability in spectroscopic activity indices with the evolution in the radial-velocity residuals and the shape of spectral lines. Results. The radial velocity, the spectral-line profile, and the acti vity indices measured in He I (5875.62 A), Hα (6562.81 A), and both of the Ca II H&K lines (3968.47 A and 3933.66 A, respectively) exhibit a periodicity close to the stellar-rotation pe riod and the correlations between them are consistent with a spotted stellar surface in rotation. We used these correlations to corr ect for the radialvelocity jitter due to stellar activity. This results in ach ieving high precision in measuring the orbital parameters, with a semi-amplitude ◦ .

MOST* Detects g-Modes in the Late-Type Be Star β Canis Minoris (B8 Ve)

The Microvariability and Oscillations of Stars (MOST) satellite has detected low-amplitude light variations (?m ~ 1 mmag) in the Be star ? CMi (B8 Ve). The observations lasted 41 days and the variations have typical periods ~0.3 days. We demonstrate that the dominant frequencies are consistent with prograde high-order g-modes of m = -1 excited by the Fe bump of opacity in an intermediate-mass (?3.5 M?) star with a nearly critical rotation period of 0.38 days. This is the first detection of nonradial g-mode pulsations in a Be star later than B6 leading to the possibility that pulsations are excited in all classical Be stars.

MOST SPACE-BASED PHOTOMETRY OF THE TRANSITING EXOPLANET SYSTEM HD 209458: TRANSIT TIMING TO SEARCH FOR ADDITIONAL PLANETS

We report on the measurement of transit times for the HD 209458 planetary system from photometry obtained with the MOST (Microvariability and Oscillations of Stars) space telescope. Deviations from a constant orbital period can indicate the presence of additional planets in the system that are yet undetected, potentially with masses approaching an Earth mass. The MOST data sets of HD 209458 from 2004 and 2005 represent unprecedented time coverage with nearly continuous observations spanning 14 and 43 days and monitoring three transits and 12 consecutive transits, respectively. The transit times that we obtain show no variations on three scales: (1) no long-term change in P since before 2004 at 25 ms level, (2) no trend in transit timings during the 2005 run, and (3) no individual transit timing deviations above 80 s level. Together with previously published transit times from Agol & Steffen, this allows us to place limits on the presence of additional close-in planets in the system, in some cases down to below an Earth mass. This result, along with previous radial velocity work, now eliminates the possibility that a perturbing planet could be responsible for the additional heat source needed to explain HD 209458bs anomalous low density.

MOST* Space-based Photometry of the Transiting Exoplanet System HD 189733: Precise Timing Measurements for Transits across an Active Star

We have measured transit times for HD 189733b passing in front of its bright (V = 7.67), chromospherically active, and spotted parent star. Nearly continuous broadband optical photometry of this system was obtained with the Microvariability and Oscillations of Stars (MOST) space telescope during 21 days in 2006 August, monitoring 10 consecutive transits. We have used these data to search for deviations from a constant orbital period which can indicate the presence of additional planets in the system that are as yet undetected by Doppler searches. There are no transit timing variations above the level of ±45 s, ruling out super-Earths (of masses 1-4 M⊕) in the 1:2 and 2:3 inner resonances, and planets of 20 M⊕ in the 2:1 outer resonance of the known planet. We also discuss complications in measuring transit times for a planet that transits an active star with large starspots, and how the transits can help constrain and test spot models. This has implications for the large number of such systems expected to be discovered by the COROT and Kepler missions.

MOST ⋆ photometry of the RRd Lyrae variable AQ Leo: Two radial modes, 32 combination frequencies, and beyond

Highly precise and nearly uninterrupted optical photometry of the RR Lyrae star AQ Leo was obtained with the MOST (Microvariability & Oscillations of STars) satellite over 34.4 days in February-March 2005. AQ Leo was the first known double-mode RR Lyrae pulsator (RRd star). Three decades after its discovery, MOST observations have revealed that AQ Leo oscillates with at least 42 frequencies, of which 32 are linear combinations (up to the sixth order) of the radial fundamental mode and its first overtone. Evidence for period changes of these modes is found in the data. The other intrinsic frequencies may represent an additional nonradial pulsation mode and its harmonics (plus linear combinations) which warrant theoretical modeling. The unprecedented number of frequencies detected with amplitudes down to millimag precision also presents an opportunity to test nonlinear theories of mode growth and saturation in RR Lyrae pulsators.

Looking for giant earths in the HD 209458 system : A search for transits in most space-based photometry

We have made a comprehensive transit search for exoplanets down to 1.5-2 Earth radii in the HD 189733 system, based on 21 days of nearly uninterrupted broadband optical photometry obtained with the MOST (Microvariability and Oscillations of STars) satellite in 2006. We have searched these data for realistic limb-darkened transits from exoplanets other than the known hot Jupiter, HD 189733b, with periods ranging from about 0.4 days to 1 week. Monte Carlo statistical tests of the data with synthetic transits inserted into the data set allow us to rule out additional close-in exoplanets with sizes ranging from about 0.15-0.31 RJ (Jupiter radii), or 1.7-3.5 -->R? (Earth radii), on orbits whose planes are near that of HD 189733b. These null results constrain theories that invoke lower mass hot super-Earth and hot Neptune planets in orbits similar to HD 189733b, due to the inward migration of this hot Jupiter. This work also illustrates the feasibility of discovering smaller transiting planets around chromospherically active stars.

The MACHO Project Large Magellanic Cloud Variable-Star Inventory. XIII. Fourier Parameters for the First-Overtone RR Lyrae Variables and the LMC Distance

Shapes of RR Lyrae light curves can be described in terms of Fourier coefficients that past research has linked with physical characteristics such as luminosity, mass, and temperature. Fourier coefficients have been derived for the V and R light curves of 785 overtone RR Lyrae variables in 16 MACHO fields near the bar of the LMC. In general, the Fourier phase differences 21, 31, and 41 increase and the amplitude ratio R21 decreases with increasing period. The coefficients for both the V and R magnitudes follow these patterns, but the phase differences for the R curves are on average slightly greater, and their amplitudes are about 20% smaller, than the ones for the V curves. The 31 and R21 coefficients have been compared with those of the first-overtone RR Lyrae variables in the Galactic globular clusters NGC 6441, M107, M5, M3, M2, ω Centauri, and M68. The results indicate that many of the LMC variables have properties similar to the ones in M2, M3, M5, and the Oosterhoff type I variables in ω Cen, but they are different from the Oosterhoff type II variables in ω Cen. Equations derived from hydrodynamic pulsation models have been used to calculate the luminosity and temperature for the 330 bona fide first-overtone variables. The results indicate that they have log L in the range 1.6–1.8 L⊙ and log Teff between 3.85 and 3.87. Based on these temperatures, a mean color excess E(V-R) = 0.08 mag, equivalent to E(B-V) = 0.14 mag, has been estimated for these 330 stars. The 80 M5-like variables (selected according to their location in the 31–log P plot) are used to determine an LMC distance. After correcting for the effects of extinction and crowding, a mean apparent magnitude V0 = 18.99 ± 0.02 (statistical) ±0.16 (systematic) has been estimated for these 80 stars. Combining this with a mean absolute magnitude MV = 0.56 ± 0.06 for M5-like stars derived from Baade-Wesselink analyses, main-sequence fitting, Fourier parameters, and the trigonometric parallax of RR Lyrae, we derive an LMC distance modulus μ = 18.43 ± 0.06 (statistical) ±0.16 (systematic) mag. The large systematic error arises from the difficulties of correcting for interstellar extinction and for crowding.

Detection of long-period variations in the subdwarf B star PG 0101+039 on the basis of photometry from the MOST satellite

We report the detection of three discrete pulsation frequencies in the long-period variable subdwarf B star PG 0101+039 on the basis of ~400 hr of MOST wide-band photometry. The periodicities uncovered lie at 7235, 5227, and 2650 s, respectively, and are associated with amplitudes between 0.03% and 0.06% of the mean brightness, lower than those measured in any other variable of this kind. We also find evidence for luminosity variations consistent with an ellipsoidal deformation of the subdwarf in the rotationally locked short-period binary system predicted from radial velocity measurements and evolutionary models. Our atmospheric modeling of two independent time-averaged optical spectra of PG 0101+039 yields Teff 28,300 K and log g 5.52, making it one of the hottest long-period variable subdwarf B stars known. The fact that we nevertheless detect brightness variations in the data is in conflict with predictions from current models, which place the theoretical blue edge for observable long-period instabilities at a temperature around 4000 K cooler than that of PG 0101+039.