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Dimitar Sasselov

Publications and source records attributed to Dimitar Sasselov.

At least 91 records · Page 5Linked to original sources
arXiv2011

A Closely-Packed System of Low-Mass, Low-Density Planets Transiting Kepler-11

When an extrasolar planet passes in front of its star (transits), its radius can be measured from the decrease in starlight and its orbital period from the time between transits. Multiple planets transiting the same star reveal more: period ratios determine stability and dynamics, mutual gravitational interactions reflect planet masses and orbital shapes, and the fraction of transiting planets observed as multiples has implications for the planarity of planetary systems. But few stars have more than one known transiting planet, and none has more than three. Here we report Kepler spacecraft observations of a single Sun-like star that reveal six transiting planets, five with orbital periods between 10 and 47 days plus a sixth one with a longer period. The five inner planets are among the smallest whose masses and sizes have both been measured, and these measurements imply substantial envelopes of light gases. The degree of coplanarity and proximity of the planetary orbits imply energy dissipation near the end of planet formation.

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arXiv2010

The gamma-ray binary LS 5039: mass and orbit constraints from MOST observations

The results of a coordinated space-based photometric and ground-based spectroscopic observing campaign on the enigmatic gamma-ray binary LS 5039 are reported. Sixteen days of observations from the MOST satellite have been combined with high-resolution optical echelle spectroscopy from the 2.3m ANU Telescope in Siding Spring, Australia. These observations were used to measure the orbital parameters of the binary and to study the properties of stellar wind from the O primary. We found that any broad-band optical photometric variability at the orbital period is below the 2 mmag level, supporting the scenario that the orbital eccentricity of the system is near the 0.24 +/- 0.08 value implied by our spectroscopy, which is lower than values previously obtained by other workers. The low amplitude optical variability also implies the component masses are at the higher end of estimates based on the primary's O6.5V((f)) spectral type with a primary mass of ~26 solar masses and a mass for the compact star of at least 1.8 solar masses. The mass loss rate from the O primary was determined to be 3.7E-7 to 4.8E-7 solar masses per year.

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arXiv2010

Analysis of the MOST light curve of the heavily spotted K2IV component of the single-line spectroscopic binary II Pegasi

Continuous photometric observations of the visible component of the single-line, K2IV spectroscopic binary II Peg carried out by the MOST satellite during 31 consecutive days in 2008 have been analyzed. On top of spot-induced brightness modulation, eleven flares were detected of three distinct types characterized by different values of rise, decay and duration times. The flares showed a preference for occurrence at rotation phases when the most spotted hemisphere is directed to the observer, confirming previous similar reports. An attempt to detect a grazing primary minimum caused by the secondary component transiting in front of the visible star gave a negative result. The brightness variability caused by spots has been interpreted within a cold spot model. An assumption of differential rotation of the primary component gave a better fit to the light curve than a solid-body rotation model.

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arXiv2010

Analysis of variability of TW Hya as observed by MOST and ASAS in 2009

As a continuation of our previous studies in 2007 and 2008, new photometric observations of the T Tauri star TW Hya obtained by the MOST satellite and the ASAS project over 40 days in 2009 with temporal resolution of 0.2 days are presented. A wavelet analysis of the combined MOST-ASAS data provides a rich picture of coherent, intermittent, variable-period oscillations, similarly as discovered in the 2008 data. The periods (1.3 - 10 days) and systematic period shortening on time scales of weeks can be interpreted within the model of magneto-rotationally controlled accretion processes in the inner accretion disk around the star. Within this model and depending on the assumed visibility of plasma parcels causing the oscillations, the observed shortest-period oscillation period may indicate the stellar rotation period of 1.3 or 2.6 d, synchronized with the disk at 4.5 or 7.1 solar radii, respectively.

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arXiv2010

Water/Icy Super-Earths: Giant Impacts and Maximum Water Content

Water-rich super-Earth exoplanets are expected to be common. We explore the effect of late giant impacts on the final bulk abundance of water in such planets. We present the results from smoothed particle hydrodynamics simulations of impacts between differentiated water(ice)-rock planets with masses between 0.5 and 5 M_Earth and projectile to target mass ratios from 1:1 to 1:4. We find that giant impacts between bodies of similar composition never decrease the bulk density of the target planet. If the commonly assumed maximum water fraction of 75wt% for bodies forming beyond the snow line is correct, giant impacts between similar composition bodies cannot serve as a mechanism for increasing the water fraction. Target planets either accrete materials in the same proportion, leaving the water fraction unchanged, or lose material from the water mantle, decreasing the water fraction. The criteria for catastrophic disruption of water-rock planets are similar to those found in previous work on super-Earths of terrestrial composition. Changes in bulk composition for giant impacts onto differentiated bodies of any composition (water-rock or rock-iron) are described by the same equations. These general laws can be incorporated into future N-body calculations of planet formation to track changes in composition from giant impacts.

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arXiv2010

Minimum Radii of Super-Earths: Constraints from Giant Impacts

The detailed interior structure models of super-Earth planets show that there is degeneracy in the possible bulk compositions of a super-Earth at a given mass and radius, determined via radial velocity and transit measurements, respectively. In addition, the upper and lower envelopes in the mass--radius relationship, corresponding to pure ice planets and pure iron planets, respectively, are not astrophysically well motivated with regard to the physical processes involved in planet formation. Here we apply the results of numerical simulations of giant impacts to constrain the lower bound in the mass--radius diagram that could arise from collisional mantle stripping of differentiated rocky/iron planets. We provide a very conservative estimate for the minimum radius boundary for the entire mass range of large terrestrial planets. This envelope is a readily testable prediction for the population of planets to be discovered by the Kepler mission.

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arXiv2010

Kepler Observations of Transiting Hot Compact Objects

Kepler photometry has revealed two unusual transiting companions orbiting an early A-star and a late B-star. In both cases the occultation of the companion is deeper than the transit. The occultation and transit with follow-up optical spectroscopy reveal a 9400 K early A-star, KOI-74 (KIC 6889235), with a companion in a 5.2 day orbit with a radius of 0.08 Rsun and a 10000 K late B-star KOI-81 (KIC 8823868) that has a companion in a 24 day orbit with a radius of 0.2 Rsun. We infer a temperature of 12250 K for KOI-74b and 13500 K for KOI-81b. We present 43 days of high duty cycle, 30 minute cadence photometry, with models demonstrating the intriguing properties of these object, and speculate on their nature.

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arXiv2010

Kepler Mission Design, Realized Photometric Performance, and Early Science

The Kepler Mission, launched on Mar 6, 2009 was designed with the explicit capability to detect Earth-size planets in the habitable zone of solar-like stars using the transit photometry method. Results from just forty-three days of data along with ground-based follow-up observations have identified five new transiting planets with measurements of their masses, radii, and orbital periods. Many aspects of stellar astrophysics also benefit from the unique, precise, extended and nearly continuous data set for a large number and variety of stars. Early results for classical variables and eclipsing stars show great promise. To fully understand the methodology, processes and eventually the results from the mission, we present the underlying rationale that ultimately led to the flight and ground system designs used to achieve the exquisite photometric performance. As an example of the initial photometric results, we present variability measurements that can be used to distinguish dwarf stars from red giants.

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arXiv2010

Kepler-4b: 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.

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arXiv2009

Direct Calculation of the Turbulent Dissipation Efficiency in Anelastic Convection

The current understanding of the turbulent dissipation in stellar convective zones is based on the assumption that the turbulence follows Kolmogorov scaling. This assumption is valid for some cases in which the time frequency of the external shear is high (e.g., solar p modes). However, for many cases of astrophysical interest (e.g., binary orbits, stellar pulsations, etc.), the timescales of interest lie outside the regime of applicability of Kolmogorov scaling. We present direct calculations of the dissipation efficiency of the turbulent convective flow in this regime, using simulations of anelastic convection with external forcing. We show that the effects of the turbulent flow are well represented by an effective viscosity coefficient, we provide the values of the effective viscosity as a function of the perturbation frequency and compare our results to the perturbative method for finding the effective viscosity of Penev et al. that can be applied to actual simulations of the surface convective zones of stars.

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arXiv2009

Dissipation Efficiency in Turbulent Convective Zones in Low Mass Stars

We extend the analysis of Penev et al. (2007) to calculate effective viscosities for the surface convective zones of three main sequence stars of 0.775Msun, 0.85Msun and the present day Sun. In addition we also pay careful attention to all normalization factors and assumptions in order to derive actual numerical prescriptions for the effective viscosity as a function of the period and direction of the external shear. Our results are applicable for periods that are too long to correspond to eddies that fall within the inertial subrange of Kolmogorov scaling, but no larger than the convective turnover time, when the assumptions of the calculation break down. We find linear scaling of effective viscosity with period and magnitudes at least three times larger than the Zahn (1966, 1989) prescription.

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arXiv2009

Collisional Stripping and Disruption of Super-Earths

The final stage of planet formation is dominated by collisions between planetary embryos. The dynamics of this stage determine the orbital configuration and the mass and composition of planets in the system. In the solar system, late giant impacts have been proposed for Mercury, Earth, Mars, and Pluto. In the case of Mercury, this giant impact may have significantly altered the bulk composition of the planet. Here we present the results of smoothed particle hydrodynamics simulations of high-velocity (up to ~5 v_esc) collisions between 1 and 10 M_Earth planets of initially terrestrial composition to investigate the end stages of formation of extrasolar super-Earths. As found in previous simulations of collisions between smaller bodies, when collision energies exceed simple merging, giant impacts are divided into two regimes: (1) disruption and (2) hit-and-run (a grazing inelastic collision and projectile escape). Disruption occurs when the impact parameter is near zero, when the projectile mass is small compared to the target, or at extremely high velocities. In the disruption regime, we derive the criteria for catastrophic disruption (when half the total colliding mass is lost), the transition energy between accretion and erosion, and a scaling law for the change in bulk composition (iron-to-silicate ratio) resulting from collisional stripping of a mantle.

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arXiv2009

First Results From New 3D Spectral Simulations Of Anelastic Turbulent Convection

We have adapted the anelastic spectral code of Barranco & Marcus (2006) to simulate a turbulent convective layer with the intention of studying the effectiveness of turbulent eddies in dissipating external shear (e.g. tides). We derive the anelastic equations, show the time integration scheme we use to evolve these equations and present the tests we ran to confirm that our code does what we expect. Further we apply a perturbative approach to find an approximate scaling of the effective eddy viscosity with frequency, and find that it is in general agreement with an estimate obtained by applying the same procedure to a realistic simulation of the upper layers of the solar convective zone.

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arXiv2008

Photometric variability of the T Tauri star TW Hya on time scales of hours to years

MOST (Microvariability & Oscillations of STars) and ASAS (All Sky Automated Survey) observations have been used to characterize photometric variability of TW Hya on time scales from a fraction of a day to 7.5 weeks and from a few days to 8 years, respectively. The two data sets have very different uncertainties and temporal coverage properties and cannot be directly combined, nevertheless, they suggests a global variability spectrum with "flicker noise" properties, i.e. with amplitudes a ~ 1/sqrt(f), over >4 decades in frequency, in the range f = 0.0003 to 10 cycles per day (c/d). A 3.7 d period is clearly present in the continuous 11 day, 0.07 d time resolution, observations by MOST in 2007. Brightness extrema coincide with zero-velocity crossings in periodic (3.56 d) radial velocity variability detected in contemporaneous spectroscopic observations of Setiawan et al. (2008) and interpreted as caused by a planet. The 3.56/3.7 d periodicity was entirely absent in the second, four times longer MOST run in 2008, casting doubt on the planetary explanation. Instead, a spectrum of unstable single periods within the range of 2 - 9 days was observed; the tendency of the periods to progressively shorten was well traced using the wavelet analysis. The evolving periodicities and the overall flicker-noise characteristics of the TW Hya variability suggest a combination of several mechanisms, with the dominant ones probably related to the accretion processes from the disk around the star.

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arXiv2008

MOST satellite photometry of stars in the M67 field: Eclipsing binaries, blue stragglers and delta-Scuti variables

We present two series of MOST (Microvariability & Oscillations of STars) space-based photometry, covering nearly continuously 10 days in 2004 and 30 days in 2007, of selected variable stars in the upper Main Sequence of the old open cluster M67. New high-precision light curves were obtained for the blue-straggler binary/triple systems AH Cnc, ES Cnc and EV Cnc. The precision and phase coverage of ES Cnc and EV Cnc is by far superior to any previous observations. The light curve of ES Cnc is modelled in detail, assuming two dark photospheric spots and Roche geometry. An analysis of the light curve of AH Cnc indicates a low mass ratio (q about 0.13) and a high inclination angle for this system. Two new long-period eclipsing binaries, GSC 814-323 and HD75638 (non-members of M67) were discovered. We also present ground-based DDO spectroscopy of ES Cnc and of the newly found eclipsing binaries. Especially interesting is HD75638, a member of a visual binary, which must itself be a triple or a higher-multiplicity system. New light curves of two delta Scuti pulsators, EX Cnc and EW Cnc, have been analyzed leading to detection of 26 and 8 pulsation frequencies of high temporal stability.

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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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