arXiv subjects

Rainer Kuschnig

Publications and source records attributed to Rainer Kuschnig.

47 records · Page 3Linked to original sources

arXiv2008

The Very Low Albedo of an Extrasolar Planet: MOST Spacebased Photometry of HD 209458

Measuring the albedo of an extrasolar planet provides insights into its atmospheric composition and its global thermal properties, including heat dissipation and weather patterns. Such a measurement requires very precise photometry of a transiting system sampling fully many phases of the secondary eclipse. Spacebased optical photometry of the transiting system HD 209458 from the MOST (Microvariablity and Oscillations of STars) satellite, spanning 14 and 44 days in 2004 and 2005 respectively, allows us to set a sensitive limit on the optical eclipse of the hot exosolar giant planet in this system. Our best fit to the observations yields a flux ratio of the planet and star of 7 $\pm$ 9 ppm (parts per million), which corresponds to a geometric albedo through the MOST bandpass (400-700 nm) of $A_g$ = 0.038 $\pm$ 0.045. This gives a 1$σ$ upper limit of 0.08 for the geometric albedo and a 3$σ$ upper limit of 0.17. HD 209458b is significantly less reflective than Jupiter (for which $A_g$ would be about 0.5). This low geometric albedo rules out the presence of bright reflective clouds in this exoplanet's atmosphere. We determine refined parameters for the star and exoplanet in the HD 209458 system based on a model fit to the MOST light curve.

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arXiv2008

Towards the Albedo of an Exoplanet: MOST Satellite Observations of Bright Transiting Exoplanetary Systems

The Canadian MOST satellite is a unique platform for observations of bright transiting exoplanetary systems. Providing nearly continuous photometric observations for up to 8 weeks, MOST can produce important observational data to help us learn about the properties of exosolar planets. We review our current observations of HD 209458, HD 189733 with implications towards the albedo and our progress towards detecting reflected light from an exoplanet.

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arXiv2008

MOST detects variability on tau Bootis possibly induced by its planetary companion

(abridged) There is considerable interest in the possible interaction between parent stars and giant planetary companions in 51 Peg-type systems. We demonstrate from MOST satellite photometry and Ca II K line emission that there has been a persistent, variable region on the surface of tau Boo A which tracked its giant planetary companion for some 440 planetary revolutions and lies ~68deg (phi=0.8) in advance of the sub-planetary point. The light curves are folded on a range of periods centered on the planetary orbital period and phase dependent variability is quantified by Fourier methods and by the mean absolute deviation (MAD) of the folded data for both the photometry and the Ca II K line reversals. The region varies in brightness on the time scale of a rotation by ~1 mmag. In 2004 it resembled a dark spot of variable depth, while in 2005 it varied between bright and dark. Over the 123 planetary orbits spanned by the photometry the variable region detected in 2004 and in 2005 are synchronised to the planetary orbital period within 0.0015 d. The Ca II K line in 2001, 2002 and 2003 also shows enhanced K-line variability centered on phi=0.8, extending coverage to some 440 planetary revolutions. The apparently constant rotation period of the variable region and its rapid variation make an explanation in terms of conventional star spots unlikely. The lack of complementary variability at phi=0.3 and the detection of the variable region so far in advance of the sub-planetary point excludes tidal excitation, but the combined photometric and Ca II K line reversal results make a good case for an active region induced magnetically on the surface of tau Boo A by its planetary companion.

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arXiv2007

Looking for Super-Earths in the HD 189733 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 & 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 one 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 RE (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.

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arXiv2007

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.

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arXiv2007

MOST photometry and DDO spectroscopy of the eclipsing (white dwarf + red dwarf) binary V471 Tau

The Hyades K2V+WD system 471 Tau is a prototype post-common envelope system and a likely cataclysmic binary progenitor. We present 10 days of nearly continuous optical photometry by the MOST (Microvariability & Oscillations of STars) satellite and partly simultaneous optical spectroscopy from DDO (David Dunlap Observatory) of the binary. The photometric data indicate that the spot coverage of the K dwarf component was less than observed in the past, suggesting that we monitored the star close to a minimum in its activity cycle. Despite the low spot activity, we still detected seven flare-like events whose estimated energies are among the highest ever observed in V471 Tau and whose times of occurrence do not correlate with the binary orbital phase. A detailed O-C analysis of the times of eclipse over the last ~35 years reveals timing variations which could be explained in several ways, including perturbations by an as-yet-undetected third body in the system or by a small orbital eccentricity inducing slow apsidal motion. The DDO spectra result in improved determinations of the K dwarf projected rotation velocity, V_K sin i = 92 km s^-1, and the orbital amplitude, K_K=150.5 km s^-1. The spectra also allow us to measure changes in H_alpha emission strength and radial velocity (RV) variations. We measure a larger H_alpha velocity amplitude than found previously suggesting that the source of the emission in V471 Tau was less concentrated around the sub-white-dwarf point on the K star than had been observed in previous studies.

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arXiv2007

Discovery of the strongly eccentric, short-period binary nature of the B-type system HD 313926 by the MOST satellite

The MOST photometric space mission discovered an eclipsing binary among its guide stars in June 2006 which combines a relatively large eccentricity e = 0.20 with an orbital period of only 2.27 days. HD 313926 appears to consist of two early-type stars of spectral type B3 - B7. It has a largest eccentricity among known early-type binaries with periods less than 3.5 d. Despite the large components indicated by its spectral type and light curve model, and its short period, the orbit of HD 313926 has not yet circularised so it is probably very young, even compared to other young B stars.

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arXiv2007

The Differential Rotation of Kappa1 Ceti as Observed by MOST

We first reported evidence for differential rotation of Kappa1 Ceti in Paper I. In this paper we demonstrate that the differential rotation pattern closely matches that for the Sun. This result is based on additional MOST (Microvariability & Oscillations of STars) observations in 2004 and 2005, to complement the 2003 observations discussed in Paper I. Using StarSpotz, a program developed specifically to analyze MOST photometry, we have solved for k, the differential rotation coefficient, and P_{EQ}, the equatorial rotation period using the light curves from all three years. The spots range in latitude from 10 to 75 degrees and k = 0.090^{+0.006}_{-0.005} -- less than the solar value but consistent with the younger age of the star. k is also well constrained by the independent spectroscopic estimate of vsini. We demonstrate independently that the pattern of differential rotation with latitude in fact conforms to solar. Details are given of the parallel tempering formalism used in finding the most robust solution which gives P_{EQ} = 8.77^{+0.03}_{-0.04} days -- smaller than that usually adopted, implying an age < 750 My. Our values of P_{EQ} and k can explain the range of rotation periods determined by others by spots or activity at a variety of latitudes. Historically, Ca II activity seems to occur consistently between latitudes 50 and 60 degrees which might indicate a permanent magnetic feature. Knowledge of k and P_{EQ} are key to understanding the dynamo mechanism and rotation structure in the convective zone as well assessing age for solar-type stars. We recently published values of k and P_{EQ} for epsilon Eri based on MOST photometry and expect to analyze MOST light curves for several more spotted, solar-type stars.

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arXiv2007

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 about 2 Earth radii in the HD 209458 system, based on nearly uninterrupted broadband optical photometry obtained with the MOST (Microvariability and Oscillations of Stars) satellite, spanning 14 days in 2004 and 44 days in 2005. We have searched these data for limb-darkened transits at periods other than that of the known giant planet, from about 0.5 days to 2 weeks. Monte Carlo statistical tests of the data with synthetic transits inserted allow us to rule out additional close-in exoplanets with sizes ranging from about 0.20-0.36 RJ (Jupiter radii), or 2.2-4.0 RE (Earth radii) on orbits whose planes are near that of HD 209458b. These null results constrain theories that invoke lower mass planets in orbits similar to HD 209458b to explain its anomalously large radius, and those that predict "hot Earths" due to the inward migration of HD 209458b.

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arXiv2004

Differential rotation of the active G5V star Kappa1 Ceti: Photometry from the MOST satellite

About 30.5 days of nearly uninterrupted broadband photometry of the solar-type star Kappa1 Ceti, obtained with the MOST (Microvariability & Oscillations of STars) satellite, shows evidence for two large starspots with different rotation periods of 8.9 and approximately 9.3 days (DeltaOmega/Omega~4%). Ground based measurements in 2002 and 2003 of Ca II H & K emission reveal variations in chromospheric activity with a period of about 9.3 days. The data were obtained during the MOST commissioning phase. When the data are combined with historical observations, they indicate that the 9.3-day spot has been stable in its period for over 30 years. The photometry, with a sampling rate of approximately once per minute, was also used to search for acoustic (p-mode) oscillations in the star. We detect no clear evidence for p-modes in the Kappa1 Ceti photometry, with a noise level around 7-9 mu_mag at frequencies in the range 0.5-4 mHz (3-sigma detection limit of 21 - 27 mu_mag). There were no flares or planetary transits during the 30.5 days of MOST monitoring with light amplitudes greater than 2 mmag (durations greater than 200 minutes) and 3 mmag (2-200 min durations). While this rules out any close-in planets of >=0.5 Jupiter diameters with an orbital inclination close to 90 deg, the scatter in differential radial velocities permit a close giant planet in a more highly inclined orbit.

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arXiv2003

Scattered Light from Close-in Extrasolar Planets: Prospects of Detection with the MOST Satellite

The ultra-precise photometric space satellite MOST (Microvariability and Oscillations of STars) will provide the first opportunity to measure the albedos and scattered light curves from known short-period extrasolar planets. Due to the changing phases of an extrasolar planet as it orbits its parent star, the combined light of the planet-star system will vary on the order of tens of micromagnitudes. The amplitude and shape of the resulting light curve is sensitive to the planet's radius and orbital inclination, as well as the composition and size distribution of the scattering particles in the planet's atmosphere. To predict the capabilities of MOST and other planned space missions, we have constructed a series of models of such light curves, improving upon earlier work by incorporating more realistic details such as: limb darkening of the star, intrinsic granulation noise in the star itself, tidal distortion and back-heating, higher angular resolution of the light scattering from the planet, and exploration of the significance of the angular size of the star as seen from the planet. We use photometric performance simulations of the MOST satellite, with the light curve models as inputs, for one of the mission's primary targets, $τ$ Boötis. These simulations demonstrate that, even adopting a very conservative signal detection limit of 4.2 $μ$mag in amplitude (not power), we will be able to either detect the $τ$ Boötis planet light curve or put severe constraints on possible extrasolar planet atmospheric models.

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