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We use deep Hubble Space Telescope imaging to study the resolved stellarpopulations in BST1047+1156, a gas-rich, ultradiffuse dwarf galaxy found in theintragroup environment of the Leo I galaxy group. While our imaging reachesapproximately two magnitudes below the tip of the red giant branch at the Leo Idistance of 11 Mpc, we find no evidence for an old red giant sequence thatwould signal an extended star formation history for the object. Instead, weclearly detect the red and blue helium burning sequences of its stellarpopulations, as well as the fainter blue main sequence, all indicative of arecent burst of star formation having taken place over the past 50--250 Myr.Comparing to isochrones for young metal-poor stellar populations, we infer thispost-starburst population to be moderately metal poor, with metallicity [M/H]in the range -1 to -1.5. The combination of a young, moderately metal-poor poststarburst population and no old stars motivates a scenario in which BST1047 wasrecently formed during a weak burst of star formation in gas that was tidallystripped from the outskirts of the neighboring massive spiral M96. BST1047'sextremely diffuse nature, lack of ongoing star formation, and disturbed HImorphology all argue that it is a transitory object, a "failing tidal dwarf" inthe process of being disrupted by interactions within the Leo I group. Finally,in the environment surrounding BST1047, our imaging also reveals the old,metal-poor ([M/H]=-1.3 +/- 0.2) stellar halo of M96 at a projected radius of 50kpc.

BST1047+1156: A (Failing) Ultradiffuse Tidal Dwarf in the Leo I Group