Asteroseismology Using The Galactic Bulge Time Domain Survey
Wide-Field Science – Regular
Marc Pinsonneault / Ohio State University, PI
Overview: Evolved red giant stars are detected as high-amplitude non-radial oscillators in time-domain space missions. Precise, regularly sampled, long-duration photometric observations can be used to characterize their oscillation frequency pattern. Masses, radii, and ages have been measured for tens of thousands of stars observed by Kepler, K2 and TESS by combining this asteroseismic data with stellar metallicity and Teff. We propose to quantify the unique asteroseismic capability of the Nancy Grace Roman Space Telescope by: 1) generating a detailed model of asteroseismic detectability with Roman light curves; 2) simulating the expected population of bulge red giants that would be detectable; 3) curating a target list with existing ancillary spectrophotometric data; and 4) exploring the potential of Roman astrometry and photometry to greatly expand the sample size, precision, and accuracy. Roman asteroseismology will constrain the properties of an important Galactic population and will be important for interpreting the results of the Roman Galactic Bulge Time Domain Survey (GBTDS).
Background: Roman is especially well-suited for asteroseismology of core He-burning, or red clump (RC), stars in the Galactic bulge because of its high spatial resolution, IR passband, aperture, and observing cadence. Asteroseismology requires the ability to detect oscillations and key stellar properties (Teff and metallicity) to infer mass and age. If L and Teff are known, R can be inferred, reducing the information needed for masses. Our project therefore begins with detectability and proceeds to catalog stars with Teff and metallicity (of order 110,000). We then explore the prospect of using Roman astrometry and photometry to provide R and Teff for numerous fainter stars (of order 420,000).
Methods: Prior work has established the feasibility of asteroseismology in Roman. Saturation will be important for RC stars in the Galactic bulge. We will develop an improved asteroseismic detection model, including the effect of saturation on variability and detection probabilities as a function of magnitude and intrinsic luminosity. We will then generate a mock RC catalogue to predict detections once Roman is launched. It will also serve as a reference for testing how different color and magnitude cuts, and different survey footprints, affect the predicted yields.
Based on the mock catalog, we will then assemble an asteroseismic target list (ATL) based on real data in the GBTDS footprint. The target list will consist of RC stars with Gaia and 2MASS photometry whose solar-like oscillations we determine to be detectable, and we will use it to motivate spectroscopic survey follow-up. We will also provide target lists with metallicities and temperatures from low-resolution optical Gaia BP/RP spectra and high-resolution infrared APOGEE spectra. We will use this data to iteratively calibrate the mock catalog parameters and infer the selection function for the ATL.
Roman astrometry is precise but needs to be tied to the Gaia system to place it on an absolute scale. We will combine an improved astrometric and photometric model with data from our mock catalog to infer the density of Gaia calibrators and make predictions for uncertainties in Roman RC parallaxes and radii, crucial for extending asteroseismology to fainter targets.
Future impact: The catalogs and codes will be resources for the Roman centers and the microlensing PIT. The detectability and astrometry models that we develop are applicable to other science cases, especially those involving saturated stars. Our projections for what Roman will achieve for asteroseismology will motivate broader science cases for Roman, such as insights into the ages and chemical properties of the bulge stellar population from which stellar exoplanet hosts are sampled. Population-level discoveries will likewise revolutionize our understanding of the primordial bulge from a Galactic archaeology standpoint.