Abstracts Submitted by the SIM Science Study Teams to the June 2009 AAS Meeting, Pasadena, CA
Eric B. Ford, Thomas J. Loredo
Detecting and Measuring Interacting Exoplanetary Systems with SIM Lite
Of the approximately 300 known extrasolar planetary systems, 30 are known to harbor multiple planets. Further, analysis of residuals for systems with long-term radial velocity (RV) observations suggests that over 30% of systems with one known planet harbor at least one additional giant planet with a longer orbital period, and this frequency may rise as sensitivity to lower mass planets improves. About a third of the known multiple systems appear to have planets in or near low-order mean-motion resonances (MMRs); the dynamics of such systems can help constrain models of planet migration. Most current exoplanet data analysis methodology relies on Keplerian orbit models for planet detection and measurement. However, the motions of interacting planets are not precisely Keplerian, raising questions about the suitability and sensitivity of current methods for studying interacting systems. We investigate the ability of Keplerian orbit models to detect and accurately measure interacting planets in astrometry data expected from SIM Lite, supplemented by long-term RV data, including planets with 2:1 MMRs. We will present initial results for our analyses of interacting multiple planet systems.
When a planet with radius Rp transits in front of its parent star with radius R*, the flux of the star decreases by a fractional amount ~r^2= (Rp/R*)^2, while the stellar photocenter shifts by ~r2 theta*, where theta* is the angular radius of the star. For the nearest transiting planets, this shift is of order microarcseconds, and so is within the reach of SIM. Measurement of the astrometric shift during transit yields the angular radius of the star, which when combined with the stellar density determined from the photometric light curve, and the stellar parallax, yields the radius and mass of the star. This astrometric shift also allows one to determine the (three-dimensional) direction of the planet orbit normal, which is useful for a number of applications. I will discuss the feasibility of measuring the astrometric signature of transiting planets with SIM, paying particular attention to the practical aspects of making these measurements for the known transiting planet systems.
Dawn M. Gelino, T. E. Harrison, F. Benedict, D. Ciardi, D. Hoard, S. Howell, B. McArthur, S. Wachter
Determining Photocenter Contamination in Interacting Binaries with SIM-Lite
We are investigating how SIM-Lite can best be utilized to attain accurate masses for the primary and secondary stars in interacting binary systems. As most of these systems are too distant for ground-based astrometric measurements of their reflex motions, SIM-Lite will allow for astrometric measurements for a wide variety of interacting binary star systems. Difficulties in making these measurements arise due to the effects of multiple luminosity components (accretion disks, streams, hotspots, etc.) on the apparent photocenter of the system and its apparent motion. Photocenter contamination is an issue that affects nearly all interacting binaries. This work will allow us to quantify the effect of photocenter contamination, extract true orbital parameters, and determine masses of the interacting binary stellar components.
Jay Holberg
A Novel SIM-Based Technique for the Precise Determination of Absolute Stellar Fluxes
A novel use of SIM parallaxes to provide a 'geometrically-based' determination of absolute stellar fluxes is examined. The method relies on the use of accurate model-based fluxes for precisely characterized DA (pure-hydrogen) white dwarfs which are directly normalized to observed SIM parallaxes rather than to a traditional empirical Vega-based photometric flux scale. It has already been demonstrated that parallaxes (and absolute magnitudes) derived from broad-band photometry for DA white dwarfs are consistent with currently existing trigonometric parallaxes for these stars at the 1 percent level. This study investigates the logical extension of our technique: the direct calibration of absolute stellar flux scales below the 1% level using precise parallax data.
Wei-Chun Jao
Parallax Observations of Local Supergiants
Local supergiants in the galaxy have been watched by human beings for thousands of years. They are major components in the constellations, but we have not yet measured accurate distances to them. The SIM Lite, the only next generation astrometric mission, can offers unique opportunities to measure the first-ever meaningful parallaxes to a few micro-arcsecond precision for all these bright stars. We present analysis of these supergiants about their radii, binarities, reference fields and metallicities, so that we could select observing targets for the SIM Lite mission.
Adam Kraus
The Kinematics of Young Stars with SIM
Star clusters are the primary sites of star formation, and cluster evolution establishes the environment within which star and planet formation occurs. The primordial internal kinematics of young clusters directly constrain their initial conditions and early evolution, but the expected stellar velocities have been difficult to measure due to past observational limits. I will describe the prospects and challenges for direct measurement of primordial stellar kinematics by space-based astrometry missions like SIM.
Jianyang Li, M. J. Kuchner, R. Allen, S. S. Sheppard
Physical Properties of Outer Solar System Objects with SIM
The radii, albedos, shapes and rotations of Kuiper Belt objects (KBOs) and Centaurs can tell us important information about the formation and evolution of the Solar System. To date these fundamental properties are unknown for most of the outer Solar System objects because current observing facilities are not adequate to obtain the required observations. For the few objects where this information is obtainable the results are usually only good to about the 20% level and are often highly model-dependent. The Space Interferometry Mission (SIM Lite) provides us with the unique potential to spatially resolve many outer Solar System objects that have small angular size. Observing these objects with SIM will allow direct measurement of their sizes and their rotations and therefore tomographically reconstructed 3-D shapes. Here we present the basic concepts of applying SIM visibility science to measure the fundamental properties of KBOs and Centaurs. From simulations with realistic scattering models of planetary surfaces, it is concluded that, in general, SIM can achieve comparable to much better accuracy than the current widely used telescopes and techniques, no matter the observing geometry or photometric and thermal properties of the objects. We will demonstrate that the current measurements of some interesting objects can be improved by SIM. We will discuss the general procedure of SIM observations for this kind of measurements, and make recommendations on the configuration of SIM and the best approach of the measurements.
Noel Richardson1, D. Gies, S. Ridgway, T. Boyajian, J. Aufdenberg, M.
Ireland, G. Schaefer
Angular Diameters, Temperatures, And Luminosities Of Massive Stars: Prospects For Sim-lite
O and B stars are among the brightest stars observable in galaxies, and are often considered signs of recent star formation or used for distance estimates. However, the fundamental properties of these stars (temperature and luminosity) are poorly understood because we do not have accurate distances and diameters of nearby O and B stars. SIM Lite will be able to provide parallaxes for these bright stars accurate to 1% at a distance of 2.6 kpc. Long Baseline Optical Interferometry, from instruments such as CHARA/PAVO, can yield temperatures and luminosities accurate to 5% for these stars once the distance is known from SIM. Here we present an initial observing list for the CHARA array and the PAVO (R band) beam combiner, as well as spectral energy distributions for the sample. These SEDs will provide a direct comparison for angular diameter measurements of hot stars that will be measured with the CHARA array in the next year.
Jason P. Aufdenberg, S. T. Ridgway
Prospects for Fundamental Stellar Parameters of the Brightest Stars from SIM and Ground-Based Interferometry
Uncertainties in the fundamental parameters of the nearest and best-studied luminous stars stem in part from uncertainties in their measured annual parallaxes. There are 4806 stars with V < 6, unobservable by the GAIA mission due to brightness, which also have Hipparcos parallaxes with greater than 1% uncertainties. Among these stars are many with measured angular diameters from optical long-baseline interferometry. SIM, promising 1% distances to 2500 pc, will provide the parallaxes needed to convert these stars' angular diameters into a physical radii with uncertainties as low as 1%. The error box of these single stars in the H-R diagram is then limited by the absolute calibration of the photometry or, for very hot stars, the inability to observe the spectral energy distribution into the far ultraviolet. We summarize the prospects for constraining the fundamental parameters of stars that only SIM can observe.
Stephen T. Ridgway, S. B. Howell
Are Post-algols The Progenitors Of Cataclysmic Variables? - An Application For SIM Astrometry
While stellar evolution is generally well understood, many binaries present masses, orbits, and pairings that defy explanation. For example, low mass donor stars in cataclysmic variable mass transfer systems can show evidence for CNO processing even though they are, at present, a factor of two or more too low in mass to have turned on the CNO cycle. Observations of post-Algol binaries are wrought with uncertainty as low gravity shells surrounding the stars can masquerade as atmospheres presenting an incorrect spectral type and luminosity class. Masses of the component stars in the post-Algols are often not known to within factors of two. SIM can measure the reflex motion of a binary and help us to disentangle the true orbit. With SIM observations, and supporting spectroscopy and ground-based O/IR imaging interferometry, the astrometric signatures of post-Algols including gas streams, orbital inclinations and shells can be explored. For numerous bright systems, the semi-major axis motions will have values typically of >100 marcsec.
Alexandre Gallenne
Cepheids at high resolution: pulsation, distances and circumstellar envelopes
Although the period-luminosity relation is just 100 years old, Cepheids are still surprising stars in many respects. The recent availability of long-baseline infrared interferometers with hectometric baselines allowed us to measure their angular diameters with sub-percent accuracy. Through a variant of the classical Baade-Wesselink method, such interferometric measurements, complemented by spectroscopy, give us their distance from the comparison of the angular size and radius variation amplitudes. This method, although quasi-geometric, presents some particular difficulties that we will briefly discuss. SIM-Lite will provide high-accuracy parallaxes to ~40 nearby Cepheids. When combined with high-resolution spectroscopy and interferometry, this will allow us to calibrate the Baade-Wesselink method, and more specifically the projection factor that is currently limiting its accuracy. In the course of these interferometric observations, we discovered compact circumstellar envelopes around several nearby Cepheids in the near-infrared. In particular, we discovered in 2006 a compact envelope around the long period Cepheid L Car. Another long-period Cepheid, RS Pup, presents a large (~2') circumstellar nebula scattering the Cepheid light in the visible. These envelopes are particularly interesting for two reasons: their presence could impact the Cepheid distance scale, and they could imply stellar mass loss. We present a brief comparative view of these two Cepheid envelopes. SIM observations will provide dynamical masses for binary systems containing a Cepheid, an essential input for stellar evolution models and our understanding of mass-loss mechanisms.
Saurav Dhital, Andrew A. West, Keivan G. Stassun, & Joshua Pepper
We present a sample of wide (a > 500 AU) low-mass (K5-M6) binaries that will be ideal for targeting with the Space Interferometry Mission 'Lite'(SIM) to search for both gas giant and terrestrial planets. With only a handful of planet-hosting binaries known, there is a need for systematic searches around multiple star systems if we are to understand the effects of one or more stellar companions on the formation of planets. SIM-Lite, with its sub-microarcsecond astrometry, will be in an ideal position to explore and constrain the parameter space of planet-hosting systems. The presented binaries are a sub-sample from the SLoWPoKES catalog identified from the astrometric, photometric, and kinematic data in the Sloan Digital Sky Survey, Data Release 7. Based on their masses, distances, and possible magnetic activity, we vetted the catalog for systems that are ideal for detection of terrestrial planets by SIM-lite. We also explore the possibility of using SIM-lite astrometry to calculate orbits of these very long-period binary systems.
John P. Subasavage, Jr.
SIM's Search for Planets Orbiting Nearby White Dwarfs - Update
I propose to use the Space Interferometry Mission (SIM) to observe a sample (~25-50) of nearby white dwarfs in hopes of detecting planetary companions with masses in the 10 Earth mass range on average. Because of the nature of white dwarfs' spectral signatures (a few broad, if any, absorption lines), current radial velocity planet hunting techniques are not viable. Astrometry is currently the only technique capable of detecting low mass planets around white dwarfs and SIM would be the best suited astrometric instrument to do so once launched. As part of a SIM Science Study, I present a detailed evaluation of the star fields in the vicinity of nearby white dwarfs within 20 pc and with V < 15. Employing these criteria, there are a total of ~60 white dwarfs with accuate trigonometric parallaxes and photometry. This effort will aid in the selection of white dwarfs to be targeted for planet searches using SIM by maximizing planetary sensitivities while minimizing total mission time spent on these observations.
Tanner, A., Law, N., Plavchan, P., Catanzarite, J.
Detecting Terrestrial Mass Planets Around M-dwarfs: Is SIM’s performance competitive?
The exoplanet task force report emphasized M dwarfs as high priority targets for future planet finding efforts due to their large RV and astrometric signature and dominance in the local stellar population. Indeed, some of the least massive planets have been detected around M dwarfs and its only a matter of time before a bona fide terrestrial mass planet is detected around an M dwarf through more precise RV measurements (0.1 cm/s) or M dwarf specific transit surveys. Here, we present some results from our SIM Science Study to determine the effect of intrinsic stellar jitter on our ability to detect terrestrial planets around low-mass stars with ?as astrometry. The study has two primary goals: 1) To assess the astrophysical limits of ultra-precision astrometric measurements of M-dwarfs compared to those of other detection methods and 2) To assess the scientific impact of an M-dwarf SIM GO survey to look for terrestrial planets. To achieve these goals, we have completed the task of designing a SIM GO target sample that considered the strengths and weaknesses of making optical astrometric measurements of M dwarfs. In the future we will determine the effect of difference sources of intrinsic jitter (i.e. sunspots, granulation, flares) on astrometric and radial velocity measurements through realistic jitter models and high precision photometry, estimate the expected yield of the GO program through Monte Carlo models that utilize the info gained from the jitter models, and compare these results to a reasonably designed radial velocity survey of the same targets.
John A. Tomsick, Matthew Muterspaugh
How Well Can SIM Lite Measure the Parameters of Neutron Star and Black Hole Binaries?
With microarcsecond astrometry, we know that the sky will come alive for the Space Interferometry Mission as it uncovers stellar motions that have previously been hidden from view. In addition to SIM's ability to provide parallax and proper motion measurements for any star in our Galaxy brighter than a V-magnitude of 20, SIM will also have the capability to study orbital motions for many different types of systems, including X-ray binaries. This presentation focuses on the advances that SIM Lite will allow in the study of neutron star and black holes. In particular, we will discuss our program to simulate SIM Lite observations to determine how accurately this mission will be able to measure the masses of compact objects in X-ray binaries. Such measurements are important for placing constraints on the neutron star equation of state as well as improving our understanding of stellar evolution and compact object formation.
Return to main page.
Last Updated:
March 13, 2009