Stellar Astrophysics

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Recent reports are listed below, along with the originally accepted key projects and recent community science studies.

Key Projects

Determining the Mass-Luminosity Relation for Stars of Various Ages, Metallicities and Evolutionary States Determine to an accuracy of one percent the mass of 100 main sequence stars and a special sample of 100 additional field stars. The improved mass-luminosity relation derived from this work would impact many fields of astrophysics and could be one of the major accomplishments of the SIM mission. Dr. Todd J. Henry Georgia State University, Atlanta http://www.chara.gsu.edu/~thenry/

Summary      Full proposal (PDF)      SIM book Theme III

Stellar, Remnant, Planetary, and Dark-Object Masses from Astrometric Micro-lensing A novel technique of micro-lensing will be used to make exceptionally precise measurements of the masses of stars and a variety of other astronomical sources. Micro-lensing involves changes to a star's appearance that occur due to gravity from a nearby object. Dr. Andrew P. Gould Ohio State University, Columbus

Summary      Full proposal (PDF)      SIM book Theme III

Anchoring the Population II Distances and Ages of Globular Clusters This program will make observations to determine the ages and distances of globular clusters which are needed to determine the age of the universe. Dr. Brian C. Chaboyer Dartmouth College Hanover, N.H. http://www.dartmouth.edu/~chaboyer/

Summary      Full proposal (PDF)      SIM book Theme III

Education and Public Outreach Scientist Dr. Guy P. Worthey Washington State University Pullman, Washington

Summary      Full proposal (PDF)      SIM book Theme III

Data Scientist Dr. Andreas Quirrenbach Leiden University The Netherlands

Summary      Full proposal (PDF)      SIM book Theme III

Community SIM Science Studies

Precision Stellar Astrophysics

Stephen Ridgway (NOAO)

This proposal addresses several interesting and important astrophysical questions concerning stars. SIM Lite astrometry will be used in combination with precision ground-based measurements, particularly optical interferometry, and supporting modeling. We will extend an ongoing 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 percent. 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 Lite, Gaia will not provide a realistic alternative, owing to brightness of the targets, expected errors, and/or required observational cadence.

Dynamical Processes in Massive Star and Star Cluster Formation

Jonathan Tan (U. Florida)

We propose to carry out a detailed study of how high-precision astrometric measurements by SIM Lite 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 Lite 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 time scale and the statistics of the population of ejected stars.

Parallax Observations of Local Supergiants

Wei-Chun Jao (Georgia State U.)

We propose to use SIM Lite 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 — not the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), the Large Synoptic Survey Telescope (LSST), nor Gaia — can observe them because of those projects’ bright-magnitude cutoffs. SIM Lite offers unique opportunities to measure the first-ever meaningful parallaxes to a few ?as 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.

A Novel Technique for the Precise Determination of Absolute Stellar Fluxes

Jay Holberg (U. Arizona)

We propose a novel use of SIM Lite 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 that are directly normalized to observed SIM Lite parallaxes rather than to a traditional Vega-based photometric flux scale. It has already been demonstrated that parallaxes (and absolute magnitudes) derived from broadband photometry for DA white dwarfs are consistent with currently existing trigonometric parallaxes for these stars at the 1 percent level. This study will investigate the logical extension of our technique: the direct calibration of absolute stellar flux scales below the 1 percent level using precise parallax data.

How Accurately Can SIM Measure Parameters of Neutron Star and Black Hole
Binaries?

John Tomsick (UC Berkeley)

The vast improvement that SIM Lite 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 Lite will allow in the study of neutron stars and black holes. In particular, we are proposing to perform simulations to determine how accurately SIM Lite 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 Lite 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 Lite simulations for X-ray binaries and for planetary studies, and we expect that our work will lead to improved computer code for analyzing SIM Lite data, optimizing SIM Lite observing strategies, and choosing the best reference stars as well as targets.

The Dynamical Legacy of Star Formation

Adam Kraus (Caltech)

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 to 2 km/s; <1 mas/yr) are impossible to measure from the ground. SIM Lite’s unprecedented astrometric capabilities represent a transformative opportunity for studying the primordial kinematics of young clusters, reaching precisions of 5 to 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.

NGC6791: SIM Plans for Binaries, Colors, and Parallaxes

Ruth C. Peterson (UCO/Lick Observatory)

We propose to investigate how to determine (1) the distance to the open star cluster NGC6791 from SIM Lite–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 Lite–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 NGC6791 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 Lite observations an improved parallax distance for NGC6791, 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 Lite 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 color-magnitude diagram (CMD) colors to the physical stellar parameters of temperature and metallicity, also of critical importance to age and metallicity determinations based on comparing cluster CMDs to theoretical isochrones. Applied directly to NGC6791 photometry, it will yield constraints on the frequency and mass ratio distribution of cluster binaries.