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We combine the CfA3 supernova Type Ia (SN Ia) sample with samples from theliterature to calculate improved constraints on the dark energy equation ofstate parameter, w. The CfA3 sample is added to the Union set of Kowalski etal. (2008) to form the Constitution set and, combined with a BAO prior,produces 1+w=0.013 +0.066/-0.068 (0.11 syst), consistent with the cosmologicalconstant. The CfA3 addition makes the cosmologically-useful sample of nearby SNIa between 2.6 and 2.9 times larger than before, reducing the statisticaluncertainty to the point where systematics play the largest role. We use fourlight curve fitters to test for systematic differences: SALT, SALT2, MLCS2k2(R_V=3.1), and MLCS2k2 (R_V=1.7). SALT produces high-redshift Hubble residualswith systematic trends versus color and larger scatter than MLCS2k2. MLCS2k2overestimates the intrinsic luminosity of SN Ia with 0.7 < Delta < 1.2. MLCS2k2with R_V=3.1 overestimates host-galaxy extinction while R_V=1.7 does not. Ourinvestigation is consistent with no Hubble bubble. We also find that, afterlight-curve correction, SN Ia in Scd/Sd/Irr hosts are intrinsically fainterthan those in E/S0 hosts by 2 sigma, suggesting that they may come fromdifferent populations. We also find that SN Ia in Scd/Sd/Irr hosts have lowscatter (0.1 mag) and reddening. Current systematic errors can be reduced byimproving SN Ia photometric accuracy, by including the CfA3 sample to retrainlight-curve fitters, by combining optical SN Ia photometry with near-infraredphotometry to understand host-galaxy extinction, and by determining ifdifferent environments give rise to different intrinsic SN Ia luminosity aftercorrection for light-curve shape and color.

Improved Dark Energy Constraints from ~100 New CfA Supernova Type Ia Light Curves