2022ApJ...937..121M

We present our velocity measurements of 59 clumpy, metal-rich ejecta knots in the supernova remnant (SNR) of SN 1572 (Tycho). We use our 450 ks Chandra High Energy Transmission Grating Spectrometer observation to measure the Doppler shift of the He-like Si Kα line-center wavelength emitted from these knots to find their line-of-sight (radial) velocities (v_r). We find v_r up to ∼5500 km s^–1, with roughly consistent speeds between blueshifted and redshifted ejecta knots. We also measure the proper motions (PMs) for our sample based on archival Chandra Advanced CCD Imaging Spectrometer data taken from 2003, 2009, and 2015. We estimate PMs up to 0.″35 yr^–1, which corresponds to a transverse velocity of about 5800 km s^–1 for the distance of 3.5 kpc to Tycho. Our v_r and transverse velocity measurements imply space velocities of ∼1900-6000 km s^–1 for the ejecta knots in Tycho. We estimate a new expansion center of R.A.(J2000) = 00^h25^m18^s.725 ± 1^.ˢ157 and decl.(J2000) = +64°08'02.″5 ± 11.″2 from our PM measurements, consistent to within ∼13″ of the geometric center. The distribution of space velocities throughout the remnant suggests that the southeast quadrant generally expands faster than the rest of the SNR. We find that blueshifted knots are projected more in the northern shell, while redshifted knots are more in the southern shell. The previously estimated reverse shock position is consistent with most of our estimated ejecta distribution; however, some ejecta show deviations from the 1D picture of the reverse shock.