Mapping High-velocity H-alpha and Lyman-alpha Emission from Supernova 1987A

We present new {\it Hubble Space Telescope} images of high-velocity H-$\alpha$ and Lyman-$\alpha$ emission in the outer debris of SN~1987A. The H-$\alpha$ images are dominated by emission from hydrogen atoms crossing the reverse shock. For the first time we observe emission from the reverse shock surface well above and below the equatorial ring, suggesting a bipolar or conical structure perpendicular to the ring plane. Using the H$\alpha$ imaging, we measure the mass flux of hydrogen atoms crossing the reverse shock front, in the velocity intervals ($-$7,500~$<$~$V_{obs}$~$<$~$-$2,800 km s$^{-1}$) and (1,000~$<$~$V_{obs}$~$<$~7,500 km s$^{-1}$), $\dot{M_{H}}$ = 1.2~$\times$~10$^{-3}$ M$_{\odot}$ yr$^{-1}$. We also present the first Lyman-$\alpha$ imaging of the whole remnant and new $Chandra$ X-ray observations. Comparing the spatial distribution of the Lyman-$\alpha$ and X-ray emission, we observe that the majority of the high-velocity Lyman-$\alpha$ emission originates interior to the equatorial ring. The observed Lyman-$\alpha$/H-$\alpha$ photon ratio, $\langle$$R(L\alpha / H\alpha)$$\rangle$ $\approx$~17, is significantly higher than the theoretically predicted ratio of $\approx$ 5 for neutral atoms crossing the reverse shock front. We attribute this excess to Lyman-$\alpha$ emission produced by X-ray heating of the outer debris. The spatial orientation of the Lyman-$\alpha$ and X-ray emission suggests that X-ray heating of the outer debris is the dominant Lyman-$\alpha$ production mechanism in SN 1987A at this phase in its evolution.Comment: 6 pages, 5 figures. ApJL - accepte

Effects of a Local Interstellar Magnetic Field on Voyager 1 and 2 Observations

We show that that an interstellar magnetic field can produce a north/south asymmetry in solar wind termination shock. Using Voyager 1 and 2 measurements, we suggest that the angle $\alpha$ between the interstellar wind velocity and magnetic field is $30^{\circ} < \alpha < 60^{\circ}$. The distortion of the shock is such that termination shock particles could stream outward along the spiral interplanetary magnetic field connecting Voyager 1 to the shock when the spacecraft was within $\sim 2~AU$ of the shock. The shock distortion is larger in the southern hemisphere, and Voyager 2 could be connected to the shock when it is within $\sim 5~AU$ of the shock, but with particles from the shock streaming inward along the field. Tighter constraints on the interstellar magnetic field should be possible when Voyager 2 crosses the shock in the next several years.Comment: 12 pages, 5 figure

The symbiotic star CH Cygni. II. The broad Ly alpha emission line explained by shocks

Context. In 1985, at the end of the active phase 1977-1986, a broad (4000 km/s) Ly alpha line appeared in the symbiotic system CH Cygni that had never been observed previously. Aims. In this work we investigate the origin of this anomalous broad Ly alpha line. Methods. We suggest a new interpretation of the broad Ly alpha based on the theory of charge transfer reactions between ambient hydrogen atoms and post-shock protons at a strong shock front. Results. We have found that the broad Ly alpha line originated from the blast wave created by the outburst, while the contemporary optical and UV lines arose from the nebula downstream of the expanding shock in the colliding wind scenario.Comment: 5 pages, 2 figures, accepted for publication in A&A on 7th April 200

Evolution of the Reverse Shock Emission from SNR 1987A

We present new (2004 July) G750L and G140L Space Telescope Imaging Spectrograph (STIS) data of the H-alpha and Ly-alpha emission from supernova remnant (SNR) 1987A. With the aid of earlier data, from Oct 1997 to Oct 2002, we track the local evolution of Ly-alpha emission and both the local and global evolution of H-alpha emission. In addition to emission which we can clearly attribute to the surface of the reverse shock, we also measure comparable emission, in both H-alpha and Ly-alpha, which appears to emerge from supernova debris interior to the surface. New observations taken through slits positioned slightly eastward and westward of a central slit show a departure from cylindrical symmetry in the H-alpha surface emission. Using a combination of old and new observations, we construct a light curve of the total H-alpha flux, F, from the reverse shock, which has increased by a factor ~ 4 over about 8 years. However, due to large systematic uncertainties, we are unable to discern between the two limiting behaviours of the flux - F ~ t (self-similar expansion) and F ~ t^5 (halting of the reverse shock). Such a determination is relevant to the question of whether the reverse shock emission will vanish in less than about 7 years (Smith et al. 2005). Future deep, low- or moderate-resolution spectra are essential for accomplishing this task.Comment: 28 pages, 12 figures. Accepted by Ap

The Transition Zone in Balmer-Dominated Shocks

We examine the structure of the post-shock region in supernova remnants (SNRs). The ``shock transition zone'' is set up by charge transfer and ionization events between atoms and ions, and has a width $\sim 10^{15}$ cm$^{-2}$ $n^{-1}_0$, where $n_0$ is the total pre-shock density (including both atoms and ions). For Balmer-dominated SNRs with shock velocity $v_s \gtrsim 1000$ km s$^{-1}$, the Rankine-Hugoniot conditions for ion velocity and temperature are obeyed instantly, leaving the full width at half-maximum (FWHM) of the broad H$\alpha$ line versus $v_s$ relation intact. However, the spatial variation in the post-shock densities is relevant to the problem of Ly$\alpha$ resonant scattering in young, core-collapse SNRs. Both two- (pre-shock atoms and ions) and three-component (pre-shock atoms, broad neutrals and ions) models are considered. We compute the spatial emissivities of the broad ($\xi_b$) and narrow ($\xi_n$) H$\alpha$ lines; a calculation of these emissivities in SN 1006 is in general agreement with the computed ones of Raymond et al. (2007). The (dimensionless) spatial shift, $\Theta_{\rm{shift}}$, between the centroids of $\xi_b$ and $\xi_n$ is unique for a given shock velocity and $f_{\rm{ion}}$, the pre-shock ion fraction. Measurements of $\Theta_{\rm{shift}}$ can be used to constrain $n_0$.Comment: 25 pages, 8 figures. Accepted by Astrophysical Journa

Resolved shock structure of the Balmer-dominated filaments in Tycho's supernova remnant: Cosmic-ray precursor?

We report on the results from H{\alpha} imaging observations of the eastern limb of Tycho's supernova remnant (SN1572) using the Wide Field Planetary Camera 2 on the Hubble Space Telescope. We resolve the detailed structure of the fast, collisionless shock wave into a delicate structure of nearly edge-on filaments. We find a gradual increase of H{\alpha} intensity just ahead of the shock front, which we interpret as emission from the thin (~1") shock precursor. We find that a significant amount of the H{\alpha} emission comes from the precursor and that this could affect the amount of temperature equilibration derived from the observed flux ratio of the broad and narrow H{\alpha} components. The observed H{\alpha} emission profiles are fit using simple precursor models, and we discuss the relevant parameters. We suggest that the precursor is likely due to cosmic rays and discuss the efficiency of cosmic-ray acceleration at this position.Comment: Prepared with emulateapj.sty (5 pages, 4 figures). Published in ApJL

The Cone-Like H-alpha Nebula in NGC 4945: A Galactic Superwind Bow Shock ?

We find that a non-axisymmetric bow shock model, with an appropriate choice of parameters, could fit the line splitting velocity field of the cone-like H$\alpha$ nebula in NGC 4945 better than a canonical cone model. Meanwhile, the bow shock model could also reproduce the morphology of the H$\alpha$ nebula. The bow shock results from the interaction of the galactic superwinds with a giant HII region. It is implied that the starburst ring or disk around the galactic nucleus should be generating strong winds, and the bright H$\alpha$ knot northwest of the nucleus be suffering an anisotropic mass loss process.Comment: 14 pages, aasms4.sty, 3 figures not included (available upon request) To appear in ApJ Letters. email chy, [email protected]