An Empirical Measurement of the Initial-Final Mass Relation with Gaia White Dwarfs

We use data from Gaia DR2 to constrain the initial-final mass relation (IFMR) for field stars with initial masses $0.9 \lesssim m_{\rm in}/M_{\odot} \lesssim 8$. Precise parallaxes reveal unprecedented substructure in the white dwarf (WD) cooling sequence on the color-magnitude diagram (CMD). We develop a generative model to predict the WD CMD as a function of the initial mass function (IMF), stellar age distribution, and a flexibly parameterized IFMR. We use the model to fit the Gaia CMD of 6400 bright WDs within 100 pc, for which the completeness is well-understood. The resulting best-fit IFMR reproduces much of the observed substructure in the CMD even when including only DA and DB models and assuming a constant star formation history. In our model, the observed bimodality in the CMD is a consequence of flattening in the IFMR at $3 \lesssim m_{\rm in}/M_{\odot} \lesssim 6$, which produces a secondary peak in the WD mass distribution at $0.75 \lesssim m_{\rm WD}/M_{\odot} \lesssim 0.85$. Such an IFMR is broadly consistent with weaker constraints obtained from star clusters in previous work. We demonstrate that if the observed bimodality in the Gaia CMD stems from the IFMR, a visibly bimodal CMD is only predicted for mixed-age stellar populations: in single-age clusters, more massive white dwarfs reach the bottom of the cooling sequence before the first lower-mass white dwarfs appear. This may explain why bimodal cooling sequences have thus far evaded detection in cluster CMDs.