NASA telescopes help solve ancient supernova mystery

Transcription

1 NASA telescopes help solve ancient supernova mystery RCW 86: A Type Ia Supernova in a Wind-Blown Bubble Williams, Brian J., el. al. ApJ 741, 96 (2011) Jeng-Lun (Alan) Chiu Institute of Astronomy, NTHU 2011/10/31 HEAG NTHU [Picture]:

2 RCW 86 (G ;MSH 14-63) Location: (RA, Dec) = (14h 43m 04s, ) (in Circinus) Distance: ~ kpc away from Earth (Sollerman et al. 2003; Rosado et al. 1996) (~85 light year at ~8,000 light-years away) Angular Size: ~45 arcmin (larger than the moon [29-34 arcmin] as seen from Earth) A Galactic SNR proposed to be the remains of SN 185 A.D. (adopting a distance of 2.5 kpc in Williams work) The "guest star, SN 185, was observed by Chinese astronomers in the Book of Later Han in the year 185 AD and remained visible in the night sky for eight months. This is believed to have been the first supernova recorded by humankind since 1960s. The gaseous shell RCW 86 is suspected as being the supernova remnant of this event. The star's spherical remains are larger than expected with 2000-year expansion. Recent X-ray studies show a good match for the expected age. Differing modern interpretations of the Chinese records of the guest star have led to quite different suggestions for the astronomical mechanism behind the event, from a core-collapse supernova to a distant, slow-moving comet (Chin & Huang 1994) with correspondingly wide-ranging estimates of its apparent visual magnitude ( 8 to +4). The recent Chandra results suggest that it was most likely a Type Ia supernova (a type with consistent absolute magnitude), similar therefore to Tycho's star (which had apparent magnitude 4 at a similar distance).

3 News A mystery that began nearly 2,000 years ago, when Chinese astronomers witnessed what would turn out to be an exploding star in the sky, has been solved: New infrared observations from NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer (WISE) reveal how the first supernova ever recorded occurred and how its shattered remains ultimately spread out to great distances. Brian J. Williams, el. al. found that the stellar explosion took place in a hollowed-out cavity, allowing material expelled by the star to travel much faster and farther than it would have otherwise (i.e. two to three times bigger than expectation). Arguments: 1). Spitzer and WISE allowed temperature measurement of the dust making up the RCW 86 remnant at about -200 C calculated how much gas must be present within the remnant to heat the dust to those temperatures a low-density environment for much of the life of the remnant, essentially a cavity. The observations show for the first time that a white dwarf can create a cavity around it before blowing up in a Type Ia event: A cavity the ejected material would have traveled unimpeded by gas and dust and spread out quickly after explosion big remains of RCW 86. Was RCW 86 from a core-collapse (CC) supernova, where massive stars blow material away from them before they blow up, carving out holes around them? 2). X-ray data from Chandra and XMM-Newton indicated that the object consisted of high amounts of iron a telltale sign of a Type Ia blast. Together with the infrared observations, a picture of a Type Ia explosion into a cavity emerged.

4 [ A filament in the NW stretching 7 ] Significant correlation between IR, X-ray, and optical images a tight coupling between IR-emitting dust & X-ray-emitting gas. angular resolution 7 angular resolution 20 A few faint filaments are observed that correspond to features seen at 24μm. These filaments also appear as radiative filaments in the [S ii] image (Smith 1997), making it likely that the IRAC emission comes from lines (most likely [Ar ii] at 7μm and [Fe ii] at 5.4μm). It is possible that emission from polycyclic aromatic hydrocarbons (PAHs) heated immediately behind the shock front. Figure 1. [Top left]: MIPS 24 μm image, with NW extraction region indicated a green ellipse. [Top right]: MIPS 70 μm image. [Bottom right]: XMM-Newton EPIC image: ( kev in red, kev in green, and kev in blue). [Bottom left]: NOAO/CTIO star-subtracted optical image (Smith 97): Hα in red and [S ii] in green. Areas appearing yellow contain both non-radiative and radiative shocks. Cyan rectangle (lower left panel) marks the Spitzer/IRS spectral extraction region Figure 2. [Top left]: IRAC three-color image of the SW region: 8 μm in red, 5.8 μm in green, and 4.5 μm in blue. Yellow regions highlight structures also seen at 24 μm. [Top right]: the MIPS 24 μm image with IRAC regions overlaid. [Bottom right]: Optical Hα and [S ii] image, as in Fig 1, with the region highlighting a purely non-radiative filament, used for the IR analysis of the SW region. [Bottom left]: the EPIC-MOS2 X-ray image, with colors as in Figure 1. Generally speaking, emission from younger SNRs expanding into a low-density medium (characterized by non-radiative shocks in the optical) will be dominated by continuum (Williams et al. 2011), while older shocks or those encountering molecular clouds will become radiative and show strong mid-ir line emission (Hewitt et al. 2009).

5 The colors of this image, which reflect the temperature of the dust, separate the emission that is associated with the SNR from that of the background. This is particularly important in the east, where the thin red filaments trace the blast wave. Figure 3. Spitzer and WISE mosaic of the entire remnant, with Spitzer 24μm emission in red, WISE 12μm emission in green, and WISE 4.6μm emission in blue. The 22μm image from WISE, not shown, looks virtually identical to the Spitzer 24μm image. We display the MIPS 24μm image here because of the sharper resolution and better signal-to-noise ratio. Only the brightest sections of the NW and SW are visible at 12μm (note the slightly yellowish colors of these regions).

6 EPIC-MOS 1 and 2 spectra, in black and red, respectively, of the NW region, overlaid with a model of thermal and nonthermal emission. An additional thermal model to account for the Fe Kα line at 6.4 kev is included as well. Data are 3σ binned for plotting purposes only. Figure 4. (The shock speed of this filament is km/s (Ghavamian 1999).) Visible in the spectra is a continuum extending to high energies, featureless except for an Fe Kα line at 6.4 kev. We account for these components with an srcut model for the synchrotron emission (Reynolds & Keohane 1999) and an Fe-only vpshock model with kt = 5 kev and τi = 10 9 cm 3 s. The nonthermal continuum in the NW does not show the same morphology as the low-energy thermal emission. Thermal X-rays in that region are well contained to the thin filament seen in Figure 1, but the synchrotron emission appears much more diffuse. Similar discrepancies were observed in the SW, leading R02 to conclude that the synchrotron emission is associated with reverse shocks driven into the ejecta, and not the forward shock. Figure 5. EPIC-MOS image of the NW filament, with kev emission in red, kev emission in green, and kev nonthermal emission shown in blue. The low-energy bands highlight thermal emission, while the high-energy band shows the location of the nonthermal synchrotron radiation. Images have been smoothed with a two-pixel Gaussian to highlight extended emission.

7 The density profile from a 1D simulation (a model with n 0,I = cm 3 and R = 10.8 pc. The current shock speed in this model is 740 km/s) Figure 6. Density profile of the wind-blown bubble model, as described in the text, in one dimension. Identified in the image are the forward shock (FS), the contact discontinuity separating the shocked ISM material from the shocked bubble material (CD1), the contact discontinuity separating the shocked bubble material from the shocked ejecta (CD2), and the reverse shock (RS). The collision of the blast wave with a shell of density contrast 250 leads to density structures from transmitted and reflected shocks, which can also be seen in the figure. Pressure, normalized to 1 at CD2, is shown by a dashed line.

8 The extraction region in Figure 1 begins at a radial distance of 93%of the forward shock radius. SIM. Significant amounts of shocked ejecta at a radius of >93% of the forward shock radius. One thing the 2D (spherically-symmetric) hydro model cannot account for is the extremely high shock speed reported for the NE limb, found fromchandra proper motions to be 6000±2800 km/s. This symmetry must be broken to explain shock speeds that differ by an order of magnitude from one side of the remnant to the other. D: distance from the center of explosion to any point on the wall of the shell R: the fixed radius of the shell wall (in this case, 12 pc). θ: polar angle between the center of explosion and a point on the shell wall.

9 At a distance of 2.5 kpc, the diameter of the remnant is 25 pc, requiring an average shock speed of >7000 km/s. Measured shock speeds vary greatly within the remnant. In the regions of interest considered in this paper (where the shell is detected at both 24 and 70μm), shock speeds from optical spectroscopy are km/s, while shock speeds in the eastern limb from the proper motion of X-ray-emitting filaments have been reported as high as 6000 (±2800) km/s. Figure 9. Density structure in the off-center explosion model, as seen in the plane of the sky, shown at the current age of the remnant. The center of the explosion is marked by a black star, and the center of the bubble is marked by a yellow star. The distance between the two (R0 in the text) is 7 pc. At θ = 0, measured from the center of explosion, the forward shock has reached a radius of 13.5 pc with respect to the center of the bubble, while at θ = π/2, the shock is at 12.7 pc, again with respect to the center of the bubble. On the far side (θ = π), the forward shock is still in the low-density bubble and has not yet reached the wall. The lack of an instability at the contact discontinuity is the result of the low angular resolution used in this model.

10 This is the first time that this type of cavity has been seen around a white dwarf system prior to explosion. Scientists say the results may have significant implications for theories of whitedwarf binary systems and Type Ia supernovae. X-ray images from the XMM and Chandra are combined to form the blue and green colors in the image, showing the interstellar gas that has been heated to millions of degrees by the passage of the shock wave from the supernova. Infrared data from Spitzer and WISE are shown in yellow and red, revealing dust radiating at a temperature of several hundred degrees below zero, warm by comparison to normal dust in our Milky Way galaxy.

11 Thank You References: Brian J. Williams, William P. Blair, John M. Blondin, Kazimierz J. Borkowski, Parviz Ghavamian, Knox S. Long, John C. Raymond, Stephen P. Reynolds, Jeonghee Rho and P. Frank Winkler, ApJ 741 (2): (2011) Links: