Stars, Neutron stars, and SIM Lite

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Stars are the fundamental building blocks of every galaxy and the two most fundamental aspects of stars are luminosity and mass. Both are necessary for understanding the physics of how they work as evidenced by the two most useful tools in astronomy, the Hertzsprung-Russell diagram and the Mass-Luminosity diagram.

First, to know the luminosities of stars we need to know their distances, but presently we have accurate distances only to a limited number of stars. Small samples have an additional problem in that they contain few rare stars. In fact, the uncertainty in the luminosity of rare OB stars, which are the dominant source of dynamic and radiative forces in star forming regions can be as high as 50%!!!

In recent years, with theories linking core collapse supernovae as a source of gamma ray bursts, the necessity of understanding the physics of OB stars has become even more imperative. To understand these phenomena requires a thorough understanding of the evolution of OB stars which requires knowledge of their masses, their luminosities, their mass loss rates, and their metallicities. This can be achieved by conducting a census of a large sample of OB stars which, in turn, can only be accomplished by surveying a large volume space: something which lies squarely in the domain of SIM Lite.

At the other end of stellar evolution lie super-giants and planetary nebulae. On the high mass end, super-giants, with their high luminosity, are distance indicators in the nearby Universe. Yet like OB stars they are rare and far away and so have poorly determined absolute luminosities. SIM Lite can help calibrate the wind-momentum luminosity relation and the flux weighted gravity-luminosity relation of super-giants.

Characteristic regions of the Hertzsprung-Russell diagram to be explored effectively by SIM Lite (blue) and Gaia (yellow), with error bars representative of current knowledge in the SIM Lite regions. Supergiants have been plotted using Hipparcos data for stars brighter than V = 6 with luminosity classes I or I/II. The mean parallax error is 37 percent for these stars.

Second, to learn masses of stars requires the study of binary systems. The main uncertainty in deriving masses from binaries is the uncertainty in the inclination of the system. The masses determined from the physical shift in position that SIM Lite will monitor are not subject to the same uncertainty in inclination that limit the spectroscopic techniques that are presently used. In addition the greater sensitivity of SIM Lite will allow the masses of a large number of stars in binary systems to be known and those large number statistics will allow the determination of the masses of rarer types of stars for which there is significant uncertainty.

One subsample of those rare stars are neutron stars. Neutron stars are by their very nature a laboratory of exotic physics. The great debate about the interiors of neutron stars has surrounded the Equation of State (EOS). Is the EOS soft where the neutrons transmute to kaons in the core or is the EOS hard where the center is a quark-gluon plasma? Fortunately, each of those EOSs results in a different final mass for the neutron star. Based on radio measurements of neutron star masses in binaries the dividing line seems to be at 1.5 solar masses as no neutron star seems to be above this line. SIM Lite will expand the radio sample and add in x-ray binaries where even one definitive measurement of a greater than 1.5 solar mass neutron star will rule out the soft EOS. Successively higher masses rule out harder EOSs. This is a discovery space to be uniquely exploited by SIM Lite.

For more information on SIM Lite’s stellar mass determinations click here. For more information on SIM Lite’s neutron star investigations click here.