Optical Signatures of Circumstellar Interaction in Type IIP Supernovae

We propose new diagnostics for circumstellar interaction in Type IIP supernovae by the detection of high velocity (HV) absorption features in Halpha and He I 10830 A lines during the photospheric stage. To demonstrate the method, we compute the ionization and excitation of H and He in supernova ejecta taking into account time-dependent effects and X-ray irradiation. We find that the interaction with a typical red supergiant wind should result in the enhanced excitation of the outer layers of unshocked ejecta and the emergence of corresponding HV absorption, i.e. a depression in the blue absorption wing of Halpha and a pronounced absorption of He I 10830 A at a radial velocity of about -10,000 km/s. We identify HV absorption in Halpha and He I 10830 A lines of SN 1999em and in Halpha of SN 2004dj as being due to this effect. The derived mass loss rate is close to 10^{-6} Msun/yr for both supernovae, assuming a wind velocity 10 km/s. We argue that, in addition to the HV absorption formed in the unshocked ejecta, spectra of SN 2004dj and SN 1999em show a HV notch feature that is formed in the cool dense shell (CDS) modified by the Rayleigh-Taylor instability. The CDS results from both shock breakout and radiative cooling of gas that has passed through the reverse shock wave. The notch becomes dominant in the HV absorption during the late photospheric phase, ~60 d. The wind density deduced from the velocity of the CDS is consistent with the wind density found from the HV absorption produced by unshocked ejecta.Comment: 38 pages, 12 figures, ApJ, in pres

The spectacular evolution of Supernova 1996al over 15 years: a low energy explosion of a stripped massive star in a highly structured environment

Spectrophotometry of SN 1996al carried out throughout 15 years is presented. The early photometry suggests that SN 1996al is a Linear type-II supernova, with an absolute peak of Mv ~ -18.2 mag. Early spectra present broad, asymmetric Balmer emissions, with super-imposed narrow lines with P-Cygni profile, and He I features with asymmetric, broad emission components. The analysis of the line profiles shows that the H and He broad components form in the same region of the ejecta. By day +142, the Halpha profile dramatically changes: the narrow P-Cygni profile disappears, and the Halpha is fitted by three emission components, that will be detected over the remaining 15 yrs of the SN monitoring campaign. Instead, the He I emissions become progressively narrower and symmetric. A sudden increase in flux of all He I lines is observed between 300 and 600 days. Models show that the supernova luminosity is sustained by the interaction of low mass (~1.15 Msun) ejecta, expelled in a low kinetic energy (~ 1.6 x 10^50 erg) explosion, with highly asymmetric circumstellar medium. The detection of Halpha emission in pre-explosion archive images suggests that the progenitor was most likely a massive star (~25 Msun ZAMS) that had lost a large fraction of its hydrogen envelope before explosion, and was hence embedded in a H-rich cocoon. The low-mass ejecta and modest kinetic energy of the explosion are explained with massive fallback of material into the compact remnant, a 7-8 Msun black hole.Comment: 27 pages, 23 figures, Accepted for publication in MNRA

Puzzle of [Ar II] 7 mcm line broad component of SN 1987A

We explore the origin of the broad component of the [Ar II] 7 mcm line emission related to the ejecta excitation by the neutron star of SN 1987A. We argue that the line broad wings are emitted at the tmperature of $\sim300$ K. The flux excess in the red wing of [Ar II] line is reproduced by the line photons scattering off the optically thin uniform dust component with the grain size of 1 - 2 mcm and the total mass of \mbox{(several)}\times10^{-3}\,M{\odot}. The dusty opaque clumps containing almost all the dust of SN~1987A have a low occultation optical depth and line photon scattering on dusty clumps do not contribute noticeably in the red wing. The additional heating might be related to ionization losses of relativistic protons

Origin of post-maximum bump in luminous Type Ic supernova 2019stc

ABSTRACT We address the issue of the post-maximum bump observed in the light curve of some superluminous supernovae. We rule out the popular mechanism of a circumstellar interaction suggested for the bump explanation. Instead, we propose that the post-maximum bump is caused by the magnetar dipole field enhancement several months after the explosion. The modelling of SN 2019stc light curve based on the thin shell approximation implies that at the age of ∼90 d, the initial dipole magnetic field should be amplified by a factor of 2.8 to account for the post-maximum bump. The specific mechanism for the field amplification of the newborn magnetar on the time-scale of several months has yet to be identified.</jats:p

North Polar Spur: Gaseous plume(s) from star-forming regions ∼3–5 kpc from the Galactic Center?

We argue that the North Polar Spur (NPS) and many less prominent structures are formed by gaseous metal-rich plumes associated with star-forming regions (SFRs). The SFRs located at the tangent to the 3−5 kpc rings might be particularly relevant to the NPS. A multi-temperature mixture of gaseous components and cosmic rays rises above the Galactic disk under the action of their initial momentum and buoyancy. Eventually, the plume velocity becomes equal to that of the ambient gas, which rotates with different angular speeds than the stars in the disk. As a result, the plumes acquire characteristic bent shapes. An ad hoc model of plumes’ trajectories shows an interesting resemblance to the morphology of structures seen in the radio continuum and X-rays