SIM Science Studies Call Accepted Proposal Abstracts
Abstracts of accepted SIM proposals
Eric Ford The combination of SIM and long-term high-precision radial velocity (RV) observations will provide a unique tool to precisely measure planet masses and orbital elements, enabling precision dynamical modeling. Since some (but not all) planet formation models predict many low-mass planets may be found in mean motion resonances (MMRs), measuring the frequency of such planets will test planet formation models. For technical reasons, detecting and characterizing such planetary systems may be significantly more challenging than non-resonant planetary system. We propose to explore the sensitivity of SIM+RV for detecting planets in or near mean motion resonances. We will study how the number and time span of observations affects the detection probability and the precision of orbital elements for resonant planetary systems. We will identify when it is essential to include mutual planetary interactions and pay particular attention to identifying what types of planetary systems and observing strategies would be able to distinguish systems _in resonance_ from those _near resonance_. Our results will help to inform the design of SIM planet searches and could lay the foundation for a future SIM observing proposal to determine the frequency of resonant planetary systems as a function of mass and orbital period. Ultimately, we aim to improve the capability of SIM observations to test, and perhaps distinguish, between models of planet formation, migration, and eccentricity excitation.
Jonathan Tan We propose to carry out a detailed study of how high precision astrometric measurements by SIM of stars involved in dynamical ejection events from star clusters can constrain theories of massive star and star cluster formation. Our study focuses on the Orion Nebula Cluster (ONC) and has two distinct parts. First, we will investigate the rich scientific potential associated with an accurate measurement of the distance and proper motion of Theta 1 Orionis C, which is the most massive star in the cluster and was recently involved (about 4000 years ago) in the ejection of a now embedded B star: the Becklin-Neugebauer (BN) star. The motion of the BN star has taken it close to a massive protostar, known as source I, where it appears to have influenced the accretion and outflow activity, most likely by a tidal interaction with the accretion disk. An accurate proper motion measurement of Theta 1 Orionis C will constrain BN's initial motion, allowing us to search for deflections caused by the gravitational potential of the massive protostar. Second, we will search the Hipparcos catalog for candidate runaway stars, i.e. that have been dynamically ejected from the cluster over the course of the last several Myr. SIM observations of these stars will be needed to confirm their origin from the ONC. The results of this study will constrain the star cluster formation timescale and the statistics of the population of ejected stars.
Bernard Gaudi When a planet with radius R_p transits in front of its parent star with radius R_*, the flux of the star decreases by a fractional amount ~r^2= (R_p/R_*)^2, while the stellar photocenter shifts by ~r^2 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 propose to perform an in-depth study of the astrometric signature of transiting planets as applied to SIM, and in particular fully explore the feasibility of detecting this signature, considering all practical aspects including mission scheduling and pointing constraints. In addition, I will consider the astrometric signature of eclipsing binaries. I am requesting funds to cover one month of my summer salary, as well as publication and travel costs associated with disseminating my findings.
Wei-Chun Jao We propose to use the Space Interferometer Mission (SIM) to measure accurate parallaxes of supergiants in the near spiral arms of the Galaxy. All selected targets have V < 6, so that no other astrometry effort in this era --- neither Pan-STARRS, LSST, nor Gaia --- can observe them because of those projects' bright magnitude cutoffs. SIM offers unique opportunities to measure the first-ever meaningful parallaxes to a few microarcsecond precision for a large sample of supergiants. The improved parallaxes will provide accurate supergiant luminosities so the supergiants can be placed on the HR diagram and will permit the eventual derivation of their stellar radii and mass loss rates at dramatic moments in the evolution of massive stars. In addition, hidden among the supergiant sample are luminous representatives of object classes that can potentially be used as reliable extragalactic distance estimators. Because so few supergiant distances are currently known, this work will undoubtedly yield fundamental breakthroughs in stellar astrophysics, and will likely lead to new insights that cannot yet be anticipated.
John Subasavage Once launched, SIM will be the most precise astrometric instrument ever developed. These capabilities are vital to exoplanetary studies, in particular, for low-mass, Earthlike planets. I propose to use SIM to observe a sample ~25 of nearby white dwarfs in hopes of detecting planetary companions with masses in the 10 Earth mass range. 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. Planetary detections around white dwarfs would better enable us to probe planetary formation theory as well as planetary evolution theory in conjunction with stellar evolution. Because astrometric signatures are inversely related to distance, the closer the system, the larger the signature (all else being equal). Because most stars will eventually end their lives as white dwarfs, these objects are plentiful and on average, closer to the Sun than more rare objects. Thus, a number of white dwarfs are close enough to the Sun to permit low mass planetary signature detections. Given that white dwarfs are the remnants of main-sequence dwarfs with spectral classes from B to K (thus far), we could better understand planetary formation over a broader range of objects than those currently investigated using radial velocity techniques (F, G, and K stars primarily). One advantage of white dwarfs is that they have lost a significant amount of mass during their evolution so that an astrometric signature is amplified when compared to an identical system around the more massive progenitor.
Rob Olling The Space Interferometry Mission (SIM) can provide data with high enough quality to determine luminosity-independent distances to the nearest spiral galaxies by employing the "Rotational Parallax" technique [RP; Peterson & Shao (1997); Olling & Peterson (2000); Olling (2007, hereafter O2007)]. Since proper motion is defined as velocity over distance, the distance follows from proper-motion and radial velocity observations. An accuracy of around 1% is possible for M31 and M33 using about 200 stars per galaxy. Due its large random internal motions, the LMC is not a SIM-RP target. Such 1% error is ~8 [3] times better than the systematic error on H_0 attained by HST/WMAP [extra-galactic water masers]. In our review of methods that can potentially yield extra-galactic distances at the 1% level (O2007), we find that the RP method is the most accurate distance indicator because: 1) it is a 100% geometric method (e.g., eclipsing binaries also rely on astrophysics to derive distances), and 2) it samples a large part of the stellar disk so that non-axisymmetric motions can be determined accurately (e.g., in contrast to nuclear water masers that sample just three lines of sight). Hu (2005) and O2007 showed that knowledge of the Hubble constant to better than 1% is crucial for constraining the Equation of State of Dark Energy, in combination with PLANCK data. Accurate RP distances facilitate detailed comparisons between almost all "standard candles" between various zero-points (MW, M 31, M 33, the LMC and NGC 4258). Successful cross-checks are crucial if we are to believe galaxy distances (and H0) at the 1% level. The RP technique may be complicated by non-circular motions which could be due to, for example, spiral structure. However, the initial analyses suggest that these effects can be diagnosed and remedied. Because SIM will provide 5D phase space information, the RP galaxies (+ Milky Way) will be the galaxy-dynamics laboratories for decades to come. Currently, there is no Rotational Parallax SIM project, while the Key Project on proper motions of nearby galaxies will observe just a few stars per galaxy (Shaya et al. 2003). We propose extensive theoretical analyses of the RP technique, focused on non-circular motions. In addition, we will investigate observational aspects such as the trades between the number of stars, the accuracy per star and mission time. We will also specify the requirements of a locally defined astrometric grid.
Jay Holberg We propose a novel use of SIM parallaxes to provide a geometrically-based determination of absolute stellar fluxes. Our 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 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% level. This study will investigate the logical extension of our technique: the direct calibration of absolute stellar flux scales below the 1% level using precise parallax data.
Marc Kuchner We propose to study and plan a SIM survey of giant KBOs and Centaurs, their dynamical relatives. This survey will measure precise sizes and shapes of these newly discovered primordial objects, constraining their compositions, material strengths, and other properties in a way no other technique can. We will use Hapke models of rotating bodies combined with light curve data and thermal measurements to model the visibilities of these targets and select an optimal observing strategy. We will investigate non-sidereal tracking and the use of the co-linear guide interferometer baseline to do visibility science.
William Hartkopf Massive stars play an essential role in enriching the interstellar medium with material later recycled into stars and planets. However, the most basic information about these stars --- their mass --- is not well-determined. Major reasons for this include high multiplicity rates, crowded fields, and interstellar extinction, all leading to poorly known distances. Runaway O stars can provide a potential "clean sample" of single O stars which reduces some of these problems, allowing much more accurate calibration of the spectroscopic distance scale. We propose to verify the runaway nature of the current tentative list of these objects and to augment that list from current catalogs of O stars and Wolf-Rayets, using new proper motion information from the upcoming "Third USNO CCD Astrograph Catalog" (UCAC3). The new runaway sample will provide an observing list for SIM parallax determinations of these important objects. SIM has distinct advantages over Gaia in its ability to provide these new parallaxes.
Stephen Ridgway This proposal addresses several interesting and important astrophysical questions concerning stars. SIM astrometry will be used in combination with precision ground-based measurements, particularly optical interferometry, and supporting modeling. We will extend an on-going study of Cepheid stars, with emphasis on resolution of possible biases in the use of the P-L relation, aiming for a confidence level of better than 1%. We will determine the radii, Teff, luminosity, and in some cases masses, of massive stars with sufficient accuracy to validate models of their structure and evolution with dramatically improved discrimination. We will determine the orbits of post-Algol systems, to test the hypothesis that they are the precursors to Cataclysmic Variable stars and the wide variety of evolved objects that they produce. We will measure the radii of nearby stars to support asteroseismological studies of the stellar interiors. For all measurements here proposed for SIM, GAIA will not provide a realistic alternative, owing to brightness of the targets, expected errors, and/or required observational cadence.
Angelle Tanner In the past few years, there have been public claims that SIM is unnecessary as a terrestrial planet search tool since radial velocity studies will be able to reach sensitivities of 10 cm/s. This is adequate to detect terrestrial planets in the habitable zones of M and K dwarfs. However, it has not been demonstrated that the RV technique will be sensitive to terrestrial planets at these separations under the different sources of stellar jitter inherent to M dwarfs - granulation, star spots, flares and p-mode oscillations. Therefore, we have designed a study to investigate the astrophysical jitter inherent to potential SIM M dwarf targets using space (i.e. CoRot, HST, Spitzer, MOST, etc) and ground-based, ultra-precise photometric data. The goal of the study will be to present a thorough comparison of the sensitivity to terrestrial planets using either SIM or 10 cm/s radial velocity measurements with realistic noise sources. Since the exoplanet taskforce has recently placed M dwarfs as high priority targets, the results of this study can be used to guide near-term planet search programs as well as promote SIM.
John Tomsick The vast improvement that SIM will provide for astrometry will allow for the measurement of orbital motions of many types of binary systems. Some of the most interesting cases are the binaries for which one component is a compact object. This proposal focuses on the advances that SIM will allow in the study of neutron stars and black holes. In particular, we are proposing to perform simulations to determine how accurately SIM will be able to measure the orbital parameters of X-ray binaries, including compact object masses. In the neutron star case, a direct dynamical mass measurement will be possible, and SIM is critical for measuring the parameters, such as binary inclination and source distance, that are the most difficult to determine with current techniques. We have experience with performing realistic SIM simulations for X-ray binaries and for planetary studies, and we expect that our work will lead to improved computer code for analyzing SIM data, optimizing SIM observing strategies, and choosing the best reference stars as well as targets.
Guillem Anglada-Escude According to current plans, SIM/NASA mission with be launched just after the end of operations of Gaia/ESA mission. This is a new situation which enables long term astrometric projects that could not be achieved by either of mission alone, or will increase the science cases of SIM targets with a much smaller effort than originally assumed. This SIM Science Study study will be the first to analyze in detail this new situation and try to explore the benefits that can be obtained by both communities (NASA and ESA) by combining both datasets. A few particular sciences cases will be analyzed in great detail to prove with examples the capabilities of long term astrometric coverage. Before any attempt of combination of both datasets, several issues must be addressed, such as the reference frame used, and the precise coordinate definition of the observable quantities in both missions.
Dawn Gelino We propose to investigate how SIM can best be utilized to attain accurate masses for the primary and secondary stars in interacting binary systems. Currently, there are two SIM key programs to measure the masses of black holes and neutron stars in binary systems. Accurate and precise orbital solutions are needed across the full mass spectrum of interacting binaries in order to fully understand the secular evolution of binary systems. A more complete picture of stellar evolution requires the inclusion of lower mass degenerate stars. These interacting binaries are complex, including a degenerate primary star, a main sequence or giant secondary star, and accretion material flowing from the secondary to the primary. We propose to investigate many systems in each interacting binary class spanning the entire range of primary component masses in order to study the effects of multiple luminosity components on the apparent photocenter of the system and its apparent motion. This work will allow us to quantify the effect of photocenter determination, extract true orbital parameters, and determine masses of the interacting binary stellar components. Photocenter contamination is an issue that affects all interacting binaries, including those sources already selected by SIM for study. Our work will benefit the entire SIM community.
Keivan Stassun A critical piece of SIM exoplanet science will be to determine the frequency and nature of planets in binary star systems. Among the most scientifically interesting of these will be wide, low-mass binaries, in which planetary orbits about one or both stars are stable and where the detection of planets in the habitable zone is most feasible. We are assembling the largest catalog to date of wide (typical orbital separations ~3000 AU), low-mass (typical spectral type ~M0) binaries. The binaries in our sample are in a range of brightnesses easily amenable to study with SIM. Importantly, our sample includes a broad diversity of stellar sub-populations that is of considerable interest for determining the frequency of planets in different regimes of parameter space: stellar mass ratios, metallicity, age, activity, and dynamical history. Finally, to explore the value-added stellar science made possible with our sample, we will study the extent to which multiple observations of these binaries with SIM's exquisite astrometric precision will permit the determination of orbital parameters and dynamical stellar masses with which to test stellar evolutionary models.
Adam Kraus Star clusters are the primary sites of star formation, and cluster evolution establishes the environment within which star and planet formation occur. The internal kinematics of young clusters directly constrain the initial conditions and early evolution, but the typical proper motion dispersions (<1-2 km/s; <1 mas/yr) are impossible to measure from the ground. SIM's unprecedented astrometric capabilities represent a transformative opportunity for studying the primordial kinematics of young clusters, reaching precisions of 5-10 m/s for the nearest star-forming populations. We propose to study the requirements and expected results for a kinematic survey of nearby young clusters and associations. The first stage of our study will use existing RV surveys and new statistical methods to estimate the velocity dispersion as a function of angular scale for these target populations. We will then use results from the literature, including our numerous high-resolution imaging surveys, to screen unsuitable candidate targets like binary systems and spatially resolved edge-on disks. Finally, we will estimate the total mission time required to study each association and will recommend a final set of targets that maximize the scientific return from this unprecedented survey.
Ruth C. Peterson We propose to investigate how to determine 1) the distance to NGC 6791, from SIM-based parallax measurements of stars we have identified as single; 2) the masses of one or two subgiants we have identified in binaries, from SIM-based astrometric determinations of the orbit of the primary and the radial-velocity displacement of primary and secondary; and 3) the frequency and mass-ratio distribution of its substantial population of binaries, from existing photometry and future radial-velocity measurements. To better constrain membership and binarity of stars in the NGC 6791 field, we will take advantage of our ongoing decade-long program that has determined radial velocities good to 0.2 km/s for all 88 red stars with V < 14.7 and monitored their variability. The goal is to ultimately provide from SIM observations an improved parallax distance for NGC 6791, and the masses of stars in binary systems consisting of one subgiant or giant and one near-main-sequence star, which will stringently constrain calculations of single-star evolution at high metallicity. Independent of SIM observations, we also propose to establish 4) how to determine reddening, temperature, metallicity, and binarity simultaneously at high metallicity from panchromatic color information. We plan to do this empirically by constructing color-color diagrams from existing photometry in a multitude of bandpasses for the cluster. We will then attempt to model each diagram theoretically, by extending calculations of fluxes and colors for solar-metallicity and metal-rich stars across the range of temperatures from the giant branch to the main sequence. By itself this will yield color conversions from the observational CMD colors to the physical stellar parameters of temperature and metallicity, also of critical importance to age and metallicity determinations based on comparing cluster color-magnitude diagrams (CMD's) to theoretical isochrones. Applied directly to NGC 6791 photometry, it will yield constraints on the frequency and mass ratio distribution of cluster binaries.
Rob Olling SIM astrometry in combination with HIPPARCOS data and other astrometric observations from the 20th century can uncover giant extra-solar planets with masses exceeding Jupiter's with orbital periods up to about 200 years. At a mission time of under 2.5 hour per star, SIM could survey 500-1,000 nearby stars to determine the frequency of very-long period massive extra-solar planets. Detection of planetary companions can be achieved for systems with periods up to about 1,000 years. Current theories of planet formation predict that migration moves the outer cutoff of giant planets from the ~200 yr to the ~600 yr regime. Our proposed survey will probe giant planets and light brown dwarfs with these periods and can help constrain planetary migration models. We propose to study how well period- and mass estimation will work in the long-period regime, with a major focus on the effects of elliptical orbits. In addition to the HIPPARCOS data, astrometric catalogs dating back as far as 90 years are expected to signifcantly constrain the presence of high-mass companions such as brown dwarfs. We will also investigate alternate observing strategies that could reduce the required mission time by a factor up to about four.
Manoj Kaplinghat Dwarf Spheroidal (dSph) satellite galaxies in the Local Group provide ideal laboratories for deciphering the nature of Dark Matter and testing theories of hierarchical structure formation on small scales. Theoretically, their status as the most dark matter dominated galaxies in the universe enables the determination of their dark matter density structure without the intrinsic uncertainties usually associated with baryonic mass contributions. Observationally, their proximity allows for detailed studies of their dark matter density structure via proper-motion studies with SIM PlanetQuest. Moreover, the intrinsically high phase-space densities of these small galaxies make them ideal candidates for constraining the particle properties of dark matter. We propose to develop methods to use proper-motion measurements to constrain fundamental properties of the dark matter particle. In the standard model, the dark matter is a cold thermal relic, and is born with a high primordial phase space density, which allows the dark matter to collapse into halos with very steep density cusps in their centers. Observations of dark-matter dominated galaxies suggest that dark matter halos may have shallow density slopes in their centers, which is more suggestive of 'warm' dark matter models. However, there are several potential systematic problems with interpreting these observations associated with uncertain baryon physics. We propose to develop methods to constrain the central densities of dwarf spheroidal galaxies using proper motion observations. While line-of-sight motions alone are unable to place constraints on the log-slope, proper motions will provide a definitive measurement of the log slope and a direct way to connect the dynamic properties of stars in local dwarf galaxies to the microphysical properties of dark matter. We will identify the best Milky Way satellite candidates for this purpose and develop the theoretical machinary necessary to connect measured log-slopes to constraints on the primordial phase-space-density of dark matter, and to the microphysical properties of the dark matter particle. Return to main page.
Last Updated:
22 July 2008 |