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Stellar Astrophysics

SIM will investigate the mass content of the Galaxy --- from huge stars to barely glimmering brown dwarfs, and from hot white dwarfs to exotic black holes. It will target various samples of the Galactic population to determine and relate the fundamental characteristics of mass, luminosity, age, composition, and multiplicity -- attributes that together yield an extensive understanding of the whole population of stars. Samples will include distant clusters that span a factor of 5000 in age, and commonplace stars and substellar objects that lurk near the Sun.

Mass is the most fundamental characteristic of a star. It governs a star's entire evolution -- determining which fuels it will burn, what color it will be, and how long it will live. It is crucial to our understanding of stellar astrophysics that we determine stellar masses to high accuracy. To put our knowledge of stellar evolution on a firmer footing, SIM will measure the distances and orbital properties of ~ 200 stars precisely enough to determine the masses of single and binary stars to an accuracy of 1%. This, in turn, will allow challenges to stellar astrophysics models more rigorous than ever before. SIM will (1) define the mass-luminosity relation for main sequence stars in five fundamental clusters so that effects of age and metallicity can be mapped (Trapezium, TW Hydrae, Pleiades, Hyades, and M67), and (2) determine accurate masses for representative examples of nearly every type of star, stellar descendant, or brown dwarf in the Galaxy.

Mass information is required to calibrate the evolutionary tracks of pre-main sequence stars that serve as a chronometer ordering the events that occur during the evolution of young stars and planetary systems. Accurate masses are also needed for stars that have extrasolar planets.

For the study of post-Asymptotic Giant Branch (post-AGB) stars, wide angle measurements will determine the parallax, while narrow angle measurements will be made to determine to the orbital inclinations of several iron-deficient post-AGB stars (in binary systems), whose peculiar abundances and infrared excesses are evidence that they are accreting gas depleted of dust from a circumbinary disk. Measurement of the orbital inclination, companion mass, and parallax will provide critical constraints.

SIM will help our understanding of transition of spherically symmetric Asymptotic Giant Branch (AGB) stars to asymmetric planetary nebulae (PNe). Models based on interacting winds have had considerable success explaining PNe morphologies. However, it is still unknown whether the asymmetries observed in PNe are present in the progenitor AGB stars themselves, and whether these asymmetries could be produced by mechanisms such as non-radial pulsations or unseen companions. SIM observations of a number of AGB stars shown to have asymmetric envelopes, through radio observations of circumstellar masers, will allow a test of these theories.

SIM can probe the Galactic distribution of X-ray binaries through parallaxes and proper motions by measuring about 50 X-ray binaries. These measurements will also eliminate the uncertainties in the luminosities of individual sources, which is currently up to a full order of magnitude. This will enable more detailed comparisons of X-ray observations to physical models such as advection-dominated accretion flows.

Precise observations of several X-ray binaries will be made to determine their orbits. The main goal of these narrow-angle SIM observations is to measure compact object masses in black hole and neutron star X-ray binaries. These objects are faint and will make use of SIM's ability to make accurate observations of faint objects.

A complete census of the stellar population of the Galaxy will be made by SIM including both ordinary stars and "dark" stars. Ordinary stars, burning their nuclear fuel and shining, can perhaps best be studied with traditional astronomical techniques, but dark stars, by which we include old brown dwarfs, black holes, old white dwarfs, neutron stars, and perhaps exotic objects such as mirror matter stars or primordial black holes, can only be studied by their gravitational effects. Traditionally, these objects have been probed in binaries, and thus selected in a way that may or may not be representative of their respective field populations.

The only way to examine the field population of these dark stars is through microlensing, the deflection of light from a visible star in the background by an object (dark or not) in the foreground. When lensed, there are two images of the background star. Although these images cannot be resolved when the lens has only a stellar-size mass, the lensing effect can be detected in two ways: photometrically, by measuring the magnification of the source by the lens, and astrometrically, by measuring the shift in the centroid of the two images. The centroid of light of the two blended images will be shifted slightly from the true position of the source towards the brighter image during the lensing event, typically by about 100 mas. With its 4 mas precision, SIM will be able to accurately measure this shift.

From the difference in the event as seen by these SIM and groundbased telescopes, one can reconstruct more details of the microlensing event. Since SIM will be in a trailing solar orbit, moving away from the Earth at 0.1 AU/yr, SIM photometry (a byproduct of its astrometric observations) can be used to determine the second parameter that is necessary to compute the lens' mass.

An alternative mass determination relies on proper motion measurements. The proper motion of nearby stars will cause them to pass in front of a more distant star, close enough to allow a measurable deflection of its light. By measuring this deflection with SIM, the mass of the nearby star can be measured to 1% accuracy.

SIM will probe the physics of chromospherically active stars. In these close 'active' binary systems there are interactions between the stars that are known to generate radio emission. The emission mechanism(s) responsible for generating radio emission in chromospherically active stars is not yet understood. Is it thermal, relativistic synchrotron, or gyro-synchrotron? No consensus has yet developed concerning the physics of the formation and evolution of the radio emission associated with the active binary star systems. Also, for most active binaries, the location of the radio emission with respect to the binary components is unknown; e.g. is the radio emitting region centered on one of the stars, is it located in the intra-binary region, or does it surround both stars? This uncertainty can be attributed, in part, to inadequacies in the radio/optical frame link. This limitation will be eliminated by the SIM frame tie (see Reference Frames).

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