The Optical Evolution of the Tycho Supernova Remnant over Three Decades

TL;DR

This work presents eight epochs of Hα imaging of Tycho's SNR from 1986 to 2016, uncovering a more extensive optical filament network and faint peripheral emission. By defining linear, perpendicular regions across 52 filaments and measuring their shifts over 30 years, the authors derive proper motions for 46 filaments with high precision, finding expansion indices $m$ ranging roughly from $0.31$ to $0.61$ and an overall tendency toward Sedov-like expansion near the rim. The optical proper motions are compared with prior measurements and multi-wavelength data, revealing generally coherent expansion with some filaments decelerating, especially along the eastern rim, consistent with local density variations or cavity walls. The study highlights the potential to obtain a geometric distance to Tycho by combining measured proper motions with shock velocities from Balmer-line widths, reporting a preliminary distance of $d \approx 2.67 \pm 0.18$ kpc when combining multiple filaments, while acknowledging model uncertainties in Balmer-based velocity determinations. Public data products (MOATS and Dataverse) enable future spectroscopic follow-up across many filaments to further refine the distance and deepen understanding of the remnant's 3D kinematics and ISM interactions.

Abstract

We report a series of images of the Tycho supernova remnant at eight epochs extending over thirty years: 1986-2016. In addition to our Hα images, we have obtained matched continuum images which we subtract to reveal faint emission, including a far more extensive network of optical knots and filaments than reported previously. The deepest images also show an extremely faint, fairly diffuse arc of emission surrounding much of the circumference of Tycho to the southeast and south, coinciding with the rim of the radio/X-ray shell. We have measured proper motions for 46 filaments, including many fainter ones near the Tycho outer rim. Our measurements are generally consistent with previous ones by Kamper and vandenBergh (1978), but ours have far greater precision. Most optical filaments at the shell rim have expansion indices reasonably consistent with the Sedov value (0.40), while the interior filaments have somewhat smaller values, as expected. From the combination of proper motions of filaments at the shell rim and shock velocity at the same positions, one should be able to calculate the distance to Tycho by simple geometry. Determination of the shock velocity from broad Balmer-line profiles is subject to model uncertainties, but the availability of dozens of such filaments with a range of conditions offers the possibility to substantially improve the distance determination for Tycho.

Figures (14)

• Figure 1: The "new star" of 1572, which appears to be almost blasting Cassiopeia out of her chair (from Tycho's posthumously published Progymnasmata, 1602). (This figure does not appear in the Ap J).
• Figure 2: Left: Tycho's SNR in H$\alpha$ at epoch 2009, after continuum subtraction. North is up; east is to the left. Right: The same image as at left but with a different display scale: now the familiar portion of the optical remnant appears significantly saturated to bring out extremely faint H$\alpha$ emission in the S and SE.
• Figure 3: Difference image between those from epochs 2016 and 1990. At the later epoch the brightest filaments appear as black, while at the earlier one they are white. The expansion should be evident.
• Figure 4: A sequence of images of coherent H$\alpha$ filaments Az19.1 and Az20.0 in the northeast quadrant of Tycho's SNR. The bar is in the identical position in each frame. Each frame is 20 square. The sequence proceeds right-to-left.
• Figure 5: A sequence of images (proceeding right-to-left) of the brightest H$\alpha$ filament in the northwest quadrant of Tycho's SNR, comprising filaments Az334.4, Az336.9, and Az339.4. The red bar is in the same position in each frame. Each frame is 40 square.