We combine Hubble Space Telescope (HST) G102 & G141 NIR grism spectroscopy with HST/WFC3-UVIS, HST/WFC3-IR and Spitzer/IRAC[3.6mu m] photometry to assemble a sample of massive (log(M_star/M_sun) ~ 11) and quenched galaxies at z~1.5. Our sample of 41 galaxies is the largest with G102+G141 NIR spectroscopy for quenched sources at these redshifts. In contrast to the local Universe, z~1.5 quenched galaxies in the high-mass range have a wide range of stellar population properties. We find their SEDs are well fitted with exponentially decreasing SFHs, and short star-formation time-scales (tau<100Myr). Quenched galaxies also show a wide distribution in ages, between 1-4Gyr. In the (u-r)_0-versus-mass space quenched galaxies have a large spread in rest-frame color at a given mass. Most quenched galaxies populate the z~1.5 red-sequence (RS), but an important fraction of them (32%) have substantially bluer colors. Although with a large spread, we find that the quenched galaxies ON the RS have older median ages (3.1Gyr) than the quenched galaxies OFF the RS (1.5Gyr). We also show that a rejuvenated SED cannot reproduce the observed stacked spectra of (the bluer) quenched galaxies OFF the RS. We derive the upper limit on the fraction of massive galaxies ON the RS at z~1.5 to be <43%. We speculate that the young quenched galaxies OFF the RS are in a transition phase between vigorous star formation at z>2 and the z~1.5 RS. According to their estimated ages, the time required for quenched galaxies OFF the RS to join their counterparts ON the z~1.5 RS is of the order of ~1Gyr.
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