Abstract
Gravitational microlensing provides a new technique for studying the surfaces of distant stars. Microlensing events are detected in real time and can be followed up with precision photometry and spectroscopy. This method is particularly adequate for studying red giants in the Galactic bulge. Recently we developed an efficient method capable of computing the lensing effect for thousands of frequencies in a high-resolution stellar spectrum. Here we demonstrate the effects of microlensing on synthesized optical spectra of red giant model atmospheres. We show that different properties of the stellar surface can be recovered from time-dependent photometry and spectroscopy of a point-mass microlensing event with a small impact parameter. In this study we concentrate on center-to-limb variation of spectral features. Measuring such variations can reveal the depth structure of the atmosphere of the source star.
Highlights
Stars populating the red giant branch spend a substantial fraction (≥ 5%) of their entire lifetime there
Despite significant progress in computing extended spherically-symmetric models (e.g., Scholz & Tsuji 1984; Plez, Brett, & Nordlund 1992; to name a few), it is still difficult to use these improvements in massive grids of models, which continue to be computed as plane-parallel (Houdashelt et al 2000)
Stellar evolution models depend on a number of parameters which describe poorly understood physics, such as convection near the surface of the star
Summary
Stars populating the red giant branch spend a substantial fraction (≥ 5%) of their entire lifetime there. Gravitational microlensing offers an accessible, immediate, and inexpensive means for imaging at least large stars, such as red giants It offers, by its nature, access to stellar populations in the Galactic bulge and the Magellanic Clouds, which are well beyond the reach of any interferometer. In order to optimize observations of spectral effects in actual events, the most practical approach is to theoretically predict the lensing effect on the entire synthesized optical spectrum of the source in microlensing alerts similar to M95-30 This way one can determine the most sensitive spectral features before the event peaks. In this work we use the method to explore point-mass microlensing effects on both low and high resolution optical spectra, synthesized directly from up-to-date red giant model atmosphere calculations.
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