skip to main content

Title: An In Situ Study of Turbulence near Stellar Bow Shocks
Abstract Stellar bow shocks are observed in a variety of interstellar environments and shaped by the conditions of gas in the interstellar medium (ISM). In situ measurements of turbulent density fluctuations near stellar bow shocks are only achievable with a few observational probes, including H α -emitting bow shocks and the Voyager Interstellar Mission (VIM). In this paper, we examine density variations around the Guitar Nebula, an H α bow shock associated with PSR B2224+65, in tandem with density variations probed by VIM near the boundary of the solar wind and ISM. High-resolution Hubble Space Telescope observations of the Guitar Nebula taken between 1994 and 2006 trace density variations over scales from hundreds to thousands of au, while VIM density measurements made with the Voyager 1 Plasma Wave System constrain variations from thousands of meters to tens of au. The power spectrum of density fluctuations constrains the amplitude of the turbulence wavenumber spectrum near the Guitar Nebula to log 10 C n 2 = − 0.8 ± 0.2 m −20/3 and for the very local ISM probed by Voyager to log 10 C n 2 = − 1.57 ± 0.02 m −20/3 . Spectral amplitudes obtained from multiepoch observations of more » four other H α bow shocks also show significant enhancements from values that are considered typical for the diffuse, warm ionized medium, suggesting that density fluctuations near these bow shocks may be amplified by shock interactions with the surrounding medium or selection effects that favor H α emission from bow shocks embedded in denser media. « less
; ; ;
Award ID(s):
Publication Date:
Journal Name:
The Astrophysical Journal
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract Large-scale disturbances generated by the Sun’s dynamics first propagate through the heliosphere, influence the heliosphere’s outer boundaries, and then traverse and modify the very local interstellar medium (VLISM). The existence of shocks in the VLISM was initially suggested by Voyager observations of the 2-3 kHz radio emissions in the heliosphere. A couple of decades later, both Voyagers crossed the definitive edge of our heliosphere and became the first ever spacecraft to sample interstellar space. Since Voyager 1’s entrance into the VLISM, it sampled electron plasma oscillation events that indirectly measure the medium’s density, increasing as it moves further away from the heliopause. Some of the observed electron oscillation events in the VLISM were associated with the local heliospheric shock waves. The observed VLISM shocks were very different than heliospheric shocks. They were very weak and broad, and the usual dissipation via wave-particle interactions could not explain their structure. Estimates of the dissipation associated with the collisionality show that collisions can determine the VLISM shock structure. According to theory and models, the existence of a bow shock or wave in front of our heliosphere is still an open question as there are no direct observations yet. This paper reviews the outstandingmore »observations recently made by the Voyager 1 and 2 spacecraft, and our current understanding of the properties of shocks/waves in the VLISM. We present some of the most exciting open questions related to the VLISM and shock waves that should be addressed in the future.« less
  2. Abstract Interstellar pickup ions are an ubiquitous and thermodynamically important component of the solar wind plasma in the heliosphere. These PUIs are born from the ionization of the interstellar neutral gas, consisting of hydrogen, helium, and trace amounts of heavier elements, in the solar wind as the heliosphere moves through the local interstellar medium. As cold interstellar neutral atoms become ionized, they form an energetic ring beam distribution comoving with the solar wind. Subsequent scattering in pitch angle by intrinsic and self-generated turbulence and their advection with the radially expanding solar wind leads to the formation of a filled-shell PUI distribution, whose density and pressure relative to the thermal solar wind ions grows with distance from the Sun. This paper reviews the history of in situ measurements of interstellar PUIs in the heliosphere. Starting with the first detection in the 1980s, interstellar PUIs were identified by their highly nonthermal distribution with a cutoff at twice the solar wind speed. Measurements of the PUI distribution shell cutoff and the He focusing cone, a downwind region of increased density formed by the solar gravity, have helped characterize the properties of the interstellar gas from near-Earth vantage points. The preferential heating of interstellar PUIsmore »compared to the core solar wind has become evident in the existence of suprathermal PUI tails, the nonadiabatic cooling index of the PUI distribution, and PUIs’ mediation of interplanetary shocks. Unlike the Voyager and Pioneer spacecraft, New Horizon’s Solar Wind Around Pluto (SWAP) instrument is taking the only direct measurements of interstellar PUIs in the outer heliosphere, currently out to $\sim47~\text{au}$ ∼ 47 au from the Sun or halfway to the heliospheric termination shock.« less
  3. Abstract

    Tight binary or multiple-star systems can interact through mass transfer and follow vastly different evolutionary pathways than single stars. The star TYC 2597-735-1 is a candidate for a recent stellar merger remnant resulting from a coalescence of a low-mass companion with a primary star a few thousand years ago. This violent event is evident in a conical outflow (“Blue Ring Nebula”) emitting in UV light and surrounded by leading shock filaments observed in Hαand UV emission. From Chandra data, we report the detection of X-ray emission from the location of TYC 2597-735-1 with a luminositylog(LX/Lbol)=5.5. Together with a previously reported period of ~14 days, this indicates ongoing stellar activity and the presence of strong magnetic fields on TYC 2597-735-1. Supported by stellar evolution models of merger remnants, we interpret the inferred stellar magnetic field as dynamo action associated with a newly formed convection zone in the atmosphere of TYC 2597-735-1, though internal shocks at the base of an accretion-powered jet cannot be ruled out. We speculate that this object will evolve into an FK Com–type source, i.e., a class of rapidly spinning magnetically active stars for which a merger origin has beenmore »proposed but for which no relic accretion or large-scale nebula remains visible. We also detect likely X-ray emission from two small regions close to the outer shock fronts in the Blue Ring Nebula, which may arise from inhomogeneities either in the circumstellar medium or in the mass and velocity distribution in the merger-driven outflow.

    « less
  4. Voyager 1 (V1) has been exploring the heliospheric boundary layer in the very local interstellar medium (VLISM) since August 2012. This study presents a broadband multi-scale analysis of VLSIM magnetic turbulence between 124 and 144 au from the Sun, as observed by V1 during the period from 2013.36 to 2019.0. We use high resolution 48-s data and show the existence of physically relevant fluctuations on scales as small as the ion inertial length in the thermal plasma. In the fine-scale regime below $\sim 10^{-3}$ au, an evidence is provided of the intermittent turbulence cascade which retains a significant level of magnetic compressibility. Observed fluctuations are compatible with the presence of filamentary structures and sawtooth-like waveforms of mixed compressible/transverse nature. A striking example of small-scale enhanced turbulence (wavelengths in the range of $\sim 1-10^3$ ion inertial lengths) is observed in front of the shock wave that overtook V1 on DOY 237, 2014 at 140 au from the Sun. This event starts on DOY 178, 2014, and suggests the presence of an ion foreshock. Besides, small-scale intermittency has been growing smoothly since 2018.5. Our analysis suggests that local processes are contributing to the production of turbulence in this regime. We identified themore »range of scales where V1 measurements may be affected by the contribution from pickup ions. On larger scales, coherent wave trains with the correlation time scale in the range of $15-100$ days dominate the spectrum of fluctuations. The spectral analysis is suggestive of a Burgers-like ($f^{-2}$) turbulence phenomenology induced by solar activity. Analysis of Coulomb collisional scales shows that the heliospheric boundary layer is not featureless at scales below the mean free path of $\sim 1$ au.« less
  5. Abstract Here we describe a new study of the supernova remnants (SNRs) and SNR candidates in nearby face-on spiral galaxy M83, based primarily on MUSE integral field spectroscopy. Our revised catalog of SNR candidates in M83 has 366 objects, 81 of which are reported here for the first time. Of these, 229 lie within the MUSE observation region, 160 of which have spectra with [S ii ]:H α ratios exceeding 0.4, the value generally accepted as confirmation that an emission nebula is shock-heated. Combined with 51 SNR candidates outside the MUSE region with high [S ii ]:H α ratios, there are 211 spectroscopically confirmed SNRs in M83, the largest number of confirmed SNRs in any external galaxy. MUSE’s combination of relatively high spectral resolution and broad wavelength coverage has allowed us to explore two other properties of SNRs that could serve as the basis of future SNR searches. Specifically, most of the objects identified as SNRs on the basis of [S ii ]:H α ratios exhibit more velocity broadening and lower ratios of [S iii ]:[S ii ] emission than H ii regions. A search for nebulae with the very broad emission lines expected from young, rapidly expanding remnants revealedmore »none, except for the previously identified B12-174a. The SNRs identified in M83 are, with few exceptions, middle-aged interstellar medium (ISM) dominated ones. Smaller-diameter candidates show a larger range of velocity broadening and a larger range of gas densities than the larger-diameter objects, as expected if the SNRs expanding into denser gas brighten and then fade from view at smaller diameters than those expanding into a more tenuous ISM.« less