Near-Infrared Study of Galactic Supernova Remnants

Abstract

The Galactic supernova remnants (SNRs) play a crucial role in the evolution of our Galaxy, since it is responsible for interstellar turbulence, large galactic structures, and various types of heavy elements. I present near-infrared (NIR) imaging and spectroscopic studies for the Galactic SNRs in this thesis which is composed of four individual parts. The first two parts are concerned with [Fe II] 1.64 um and H2 2.12 um emission line studies for the ``evolved'' SNRs interacting with their surrounding medium, and the last two parts deal with NIR spectroscopic studies for one of the youngest Galactic SNR (Cassiopeia A) where the imprints of SN explosion remain.

In the first part, I report the detection of [Fe II] 1.64 um and H2 2.12 um line features around the Galactic SNRs using UWIFE and UWISH2 surveys that cover the first galactic quadrant of 7 deg < l < 62 deg and

b

< 1.3 deg. Among the 79 Galactic SNRs fully covered by both surveys, I detect a total of 19 [Fe II]-emitting and 19 H2-emitting SNRs which is corresponding to the overall detection rate of ~24%. Eleven SNRs show both emission features. The detection rate of [Fe II] is highest at the Galactic longitude of 40 deg—50 deg, whereas that of H2 peaks at l=30 deg—40 deg. The different peaks may be due to different SN populations along the Galactic longitude. The total [Fe II] flux we estimated is at least a few times fainter than the expected [Fe II] flux from the SN rate of our Galaxy, implying either that there are many SNRs whose [Fe II] lines are undetected in the survey, or that there are many missing SNRs that have not been known so far. Five out of the 11 SNRs emitting both emission lines clearly show ``[Fe II]-H2 reversal

'' the H2 emission lines are detected far beyond the [Fe II]/radio boundary. From NIR spectra for the extended H2 emission lines, I find that the H2 lines may arise from collisionally excited H2 gas, but their exciting sources remain to be explored.

In the second part, I report the results of high resolution NIR spectroscopy of the five Galactic SNRs (G11.2-0.3, KES 73, W44, 3C 396, W49B), showing the extended H2 filaments far beyond the strong NIR [Fe II] lines and radio continuum in the border of the remnants. H2 1--0 S(1) 2.12 um and 1--0 S(0) 2.22 um lines are clearly detected in all slits. Their radial velocities are well consistent with the systematic velocity of the remnants, and these imply that they are indeed physically associated with the remnants. They also show the broad line width (> 10 km/s) as well as the flux ratios analogous to thermal excitation, which imply that the extended H2 filaments are collisionally excited by shock. I suggest three possibilities of the origin of the shock: (1) magnetic precursor ahead of the supernova (SN) shock, (2) projection effect of the SN shock, and (3) slow non-dissociative C-shock produced by pre-SN wind. I discuss about the origin of the extended H2 emission features detected around each SNR.

In the third part, I report the results of broadband (0.95--2.46 um) NIR spectroscopic observations of the Cassiopeia A SNR. Using a clump-finding algorithm in two-dimensional dispersed images, I identify 63 `knots' from eight slit positions and derive their spectroscopic properties. All of the knots emit [Fe II] lines together with other ionic forbidden lines of heavy elements, and some of them also emit H and He lines. I identify 46 emission line features in total from the 63 knots and measure their fluxes and radial velocities. The results of our analyses of the emission line features based on principal component analysis show that the knots can be classified into three groups: (1) He-rich, (2) S-rich, and (3) Fe-rich knots. The He-rich knots have relatively small, < 200 km/s, line-of-sight speeds and radiate strong He I and [Fe II] lines resembling closely optical quasi-stationary flocculi of circumstellar medium, while the S-rich knots show strong lines from O-burning material with large radial velocities up to ~ 2000 km/s indicating that they are SN ejecta material known as fast-moving knots. The Fe-rich knots also have large radial velocities but show no lines from O-burning material. I discuss the origin of the Fe-rich knots and conclude that they are most likely ``pure'' Fe ejecta synthesized in the innermost region during the SN explosion. The comparison of [Fe II] images with other waveband images shows that these dense Fe ejecta are mainly distributed along the southwestern shell just outside the unshocked 44Ti in the interior, supporting the presence of unshocked Fe associated with 44Ti.

In the last part, I present the results of extinction measurements toward the main ejecta shell of the Cassiopeia A SNR using the flux ratios between the two NIR [Fe II] lines at 1.26 and 1.64 \micron. I find a clear correlation between the NIR extinction (E(J-H)) and the radial velocity of ejecta knots, showing that redshifted knots are systematically more obscured than blueshifted ones. This internal ``self-extinction'' strongly indicates that a large amount of SN dust resides inside and around the main ejecta shell. At one location in the southern part of the shell, I measure E(J-H) by the SN dust of 0.23+-0.05 mag. By analyzing the spectral energy distribution of thermal dust emission at that location, I show that there are warm (~100 K) and cool (~ 40 K) SN dust components and that the latter is responsible for the observed E(J-H). I investigate the possible grain species and size of each component and find that the warm SN dust needs to be silicate grains such as MgSiO3, Mg2SiO4, and SiO2, whereas the cool dust could be either small (< 0.01 um) Fe or large (> 0.1 um) Si grains. I suggest that the warm and cool dust components in Cassiopeia A represent grain species produced in diffuse SN ejecta and in dense ejecta clumps, respectively.