Abstracts Submitted by the SIM Science Study Teams to the January 2009 AAS Meeting, Long Beach, CAGuillem Anglada-Escude
Gaia-SIM legacy project
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.
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. This frequency may rise as sensitivity to lower mass planets improves. Most current exoplanet data analysis methodology relies on Keplerian orbit models. For systems with multiple planets the motions are not precisely Keplerian, raising questions about the suitability and sensitivity of current methods for studying interacting systems. We are investigating the ability of Keplerian orbit models to detect and accurately characterize interacting planetary systems in astrometry data expected from SIM, supplemented by long-term RV data. We generate ensembles of systems with a variety of planet masses, periods, and eccentricities and use Burlish-Stoer n-body integration to calculate the orbits. Then we simulate astrometric data as expected from SIM along with supplementary RV data, and apply a data analysis pipeline that relies on several generalized periodograms for period searching, followed by detailed Keplerian orbit fitting using Bayesian methods. Finally, we use n-body integrations to investigate the long-term orbital evolution of the resulting planetary systems. We investigate which types of planetary systems and observing strategies allow accurate identification of the dynamical state of the system. This presentation describes the data analysis pipeline, and presents initial results for example interacting systems. We are paying particular attention to planetary systems in the 2:1 and 3:1 mean-motion resonances (MMRs), as degeneracies make these difficult to detect with RV observations alone. The effects of planetary interactions can be particularly significant for such systems, helping their orbital dynamics to constrain models of planet formation and migration.
SIM-Lite will allow for astrometric measurements of the masses for a wide variety of interacting binary star systems. Nearly all interacting binaries are too distant for ground-based astrometric measurements of their reflex motions, thus other techniques must be used to estimate component masses. But for interacting binaries, the presence of on-going accretion makes it difficult to apply such techniques. SIM-Lite will be a powerful tool that will be used to directly measure the reflex motion of the components in distant, astrophysically interesting binaries. But the presence of accretion disks, streams, and ``hotspots'' in the majority of the systems of interest imposes some uncertainty in the derived astrometric orbits. We have undertaken a study to determine the best SIM-Lite observing procedure for each class of interacting binary.
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.
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.
We are about to enter an era of micro-arcsecond astrometry in the coming decade. All these high accuracy astrometry missions, like GAIA, Space Interferometry Mission (SIM), Large Synoptic Survey Telescope (LSST) , Pan-STARRS or SkyMapper can reach stars much fainter than those in the Hipparcos and Yale parallax catalog. However, because of the magnitude limit of these missions, stars brighter than V=6 will be obsoleted by all missions but SIM. SIM offers unique opportunities to measure the first-ever meaningful parallaxes to a few micro-arcsecond precision for all these bright stars, especially for those supergiants at a few Kpc away. I discuss how SIM can provide accurate supergiant luminosities and what fundamental breakthroughs will it have in stellar astrophysics.Manoj Kaplinghat, Louis Strigari
Dark Mater And Dwarf Galaxies In The Era Of SIM Planetquest
The era of high precision astrometry will allow for a study of internal proper motions of stars in the dark matter dominated Milky Way satellite galaxies. Internal proper motions of stars in these galaxies are beneficial because they will break the degeneracy between the slope of the dark matter halo profile and the velocity anisotropy, and thus reveal whether there is a dark matter core or cusp in these halos. These astrometric measurements will directly constrain the phase space density of these dark matter halos and thus strongly constrain the nature of the dark matter particle. I will discuss this idea in the context of the SIM PlanetQuest Mission, which will have the sensitivity to obtain hundreds of internal proper motions in several dSphs.
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 their initial conditions and early evolution, but the expected stellar velocities have been difficult to measure because they did not exceed past observational limits. I will describe an ongoing effort to constrain the velocity power spectra of nearby star-forming regions, and then I will discuss the prospect for future direct measurement of primordial stellar kinematics by space-based astrometry missions like SIM.
Dwarf planets and other larger Kuiper Belt objects (KBOs) contain important clues to the formation of the Solar System. Currently the radii and albedo of these objects are unknown or only good to the 20 percent level. A single visibility measurement with the Space Interferometry Mission (SIM Lite) will enable us to directly measure the angular size and albedo of these bodies. A time series of SIM observations should allow us to tomographically reconstruct their 3-D shapes as they rotate past. This basic physical information will allow us to determine other important physical properties of these objects, including temperature, density, porosity, etc, in conjunction with other visible and/or IR data collected elsewhere. Here we show the basic methods of applying SIM visibility science in observing KBOs and Centaurs. We used the best currently available observations of a short list of bright KBOs and Centaurs to generate simulated images and rotation sequences of these objects under various illumination and viewing geometries, assuming particular surface scattering models. These simulations were used to construct a practical observing sequence and demonstrate the tomographic reconstruction of the 3-D shapes of these bodies.
We present initial results from a SIM Science Study aimed at identifying extra-solar planetary systems that resemble the outer solar system. Such Solar System Giant Analogs (SOSGAs) would have periods between 12 and 165 years and are virtually impossible to characterize using conventional astrometric techniques. We present a new method based on SIM proper-motions and accelerations AND observed positions at earlier times, such as at the Hipparcos (1991) and the ACT (1907) epochs. Preliminary results indicate that Jupiter-mass objects can be characterized (mass & period determination) with periods up to 80 years. Objects at the planet/brown-dwarf mass limit can be characterized if their periods are less than about 200 years.
In this SIM Science Study, we will be examining the potential benefits of applying the rotational parallax method to galaxies (specifically to M31 and M33). The rotational parallax method combines differential proper motion measurements of stars in a galaxy plus radial velocity measurements to obtain a geometrical determination of distance. We argue that luminosity independent distance determinations are needed to provide confidence as distance indicators reach the 1% level in accuracy. With 1% distances to M31 and M33 we also obtain accurate luminosities to millions of resolved stars which will be very helpful for calibration of the more familiar distance indicators: Cepheids, RR-Lyrae, Tip of the red giant branch, Red Clump, Tully-Fisher, planetary nebulae, eclipsing binaries, etc. In addition, the SIM data will also provide unprecedented information about internal dynamics in M31 and M33 by revealing non-circular velocities in these galaxies due to spiral wave patterns, bars, or warps.
Ruth C. Peterson
At 8Gyr and three times solar metallicity, the open cluster NGC 6791 is uniquely old, massive, homogenous, metal-rich, and nearby, and can test stellar isochrones and provide a resolved template for ancient 'red-and-dead' elliptical galaxies. Largely neglected to date is the influence of its binary population, despite a 50% main-sequence binary fraction. To better understand the frequency of production of merger products such as extreme blue horizontal branch stars and blue stragglers, whose blue light can mimic that of young stars, we have undertaken radial-velocity surveys of its giants and red and blue horizontal branch stars, which we summarize here.
Long-period binaries are difficult to detect, however, because velocity amplitudes are generally a few km/s since eccentricities are significant. As part of our SIM Science Studies program, we will photometrically examine membership and binarity as well. SIM is capable of measuring the parallax and binary orbits of well-chosen giant stars, a significant goal here, but the technique can be applied all across the color-magnitude diagram (CMD).
We will begin 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.
SIM astrometry can be used in combination with precision ground-based measurements, particularly optical interferometry, and supporting modeling, to address fundamental questions in stellar physics. 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.
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 zones 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.
Once launched, the Space Interferometry Mission (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 (~25-50) 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. 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).
We study how high precision astrometric measurements by SIM and GAIA of stars involved in dynamical ejection events from star clusters can constrain theories of massive star and star cluster formation. We focus on the Orion Nebula Cluster (ONC). First, we investigate the scientific potential associated with an accurate measurement of the distance and proper motion of Theta 1 Ori C, which is the most massive star in the cluster and was recently involved (about 4000 years ago) in the ejection of a 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 Ori C will constrain BN's initial motion, allowing us to search for deflections caused by the gravitational potential of the massive protostar. Second, we 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 and GAIA 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. JCT acknowledges support from from NSF CAREER grant AST-0645412 and a grant from NASA for SIM Science Studies.
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. Here, we present our strategy for completing this study and their benefits to the exoplanet community as a whole.
With microarcsecond astrometry, we know that the sky will come alive for the Space Interferometry Mission as it uncovers stellar motions that have been previously 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 stars 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.
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31 October 2008