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Title: On the Absence of Non-thermal X-Ray Emission around Runaway O Stars

Abstract

Theoretical models predict that the compressed interstellar medium around runaway O stars can produce high-energy non-thermal diffuse emission, in particular, non-thermal X-ray and γ -ray emission. So far, detection of non-thermal X-ray emission was claimed for only one runaway star, AE Aur. We present a search for non-thermal diffuse X-ray emission from bow shocks using archived XMM-Newton observations for a clean sample of six well-determined runaway O stars. We find that none of these objects present diffuse X-ray emission associated with their bow shocks, similarly to previous X-ray studies toward ζ Oph and BD+43°3654. We carefully investigated multi-wavelength observations of AE Aur and could not confirm previous findings of non-thermal X-rays. We conclude that so far there is no clear evidence of non-thermal extended emission in bow shocks around runaway O stars.

Authors:
 [1];  [2];  [3]
  1. Institute of Astronomy and Astrophysics, Academia Sinica (ASIAA), Taipei 10617, Taiwan (China)
  2. Institute for Physics and Astronomy, University of Potsdam, D-14476 Potsdam (Germany)
  3. Department of Physics and Astronomy, East Tennessee State University, Johnson City, TN 37614 (United States)
Publication Date:
OSTI Identifier:
22654503
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 838; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; AUGER ELECTRON SPECTROSCOPY; DETECTION; EMISSION; GAMMA RADIATION; MASS TRANSFER; SHOCK WAVES; SIMULATION; STARS; STELLAR WINDS; WAVELENGTHS; X RADIATION

Citation Formats

Toalá, J. A., Oskinova, L. M., and Ignace, R. On the Absence of Non-thermal X-Ray Emission around Runaway O Stars. United States: N. p., 2017. Web. doi:10.3847/2041-8213/AA667C.
Toalá, J. A., Oskinova, L. M., & Ignace, R. On the Absence of Non-thermal X-Ray Emission around Runaway O Stars. United States. doi:10.3847/2041-8213/AA667C.
Toalá, J. A., Oskinova, L. M., and Ignace, R. Sat . "On the Absence of Non-thermal X-Ray Emission around Runaway O Stars". United States. doi:10.3847/2041-8213/AA667C.
@article{osti_22654503,
title = {On the Absence of Non-thermal X-Ray Emission around Runaway O Stars},
author = {Toalá, J. A. and Oskinova, L. M. and Ignace, R.},
abstractNote = {Theoretical models predict that the compressed interstellar medium around runaway O stars can produce high-energy non-thermal diffuse emission, in particular, non-thermal X-ray and γ -ray emission. So far, detection of non-thermal X-ray emission was claimed for only one runaway star, AE Aur. We present a search for non-thermal diffuse X-ray emission from bow shocks using archived XMM-Newton observations for a clean sample of six well-determined runaway O stars. We find that none of these objects present diffuse X-ray emission associated with their bow shocks, similarly to previous X-ray studies toward ζ Oph and BD+43°3654. We carefully investigated multi-wavelength observations of AE Aur and could not confirm previous findings of non-thermal X-rays. We conclude that so far there is no clear evidence of non-thermal extended emission in bow shocks around runaway O stars.},
doi = {10.3847/2041-8213/AA667C},
journal = {Astrophysical Journal Letters},
number = 2,
volume = 838,
place = {United States},
year = {Sat Apr 01 00:00:00 EDT 2017},
month = {Sat Apr 01 00:00:00 EDT 2017}
}
  • Non-thermal radiation has been predicted within bow shocks around runaway stars by recent theoretical works. We present X-ray observations toward the runaway stars ζ Oph by Chandra and Suzaku and of BD+43°3654 by XMM-Newton to search for the presence of non-thermal X-ray emission. We found no evidence of non-thermal emission spatially coincident with the bow shocks; nonetheless, diffuse emission was detected in the vicinity of ζ Oph. After a careful analysis of its spectral characteristics, we conclude that this emission has a thermal nature with a plasma temperature of T ≈ 2 × 10{sup 6} K. The cometary shape ofmore » this emission seems to be in line with recent predictions of radiation-hydrodynamic models of runaway stars. The case of BD+43°3654 is puzzling, as non-thermal emission has been reported in a previous work for this source.« less
  • Runaway stars produce shocks when passing through interstellar medium at supersonic velocities. Bow shocks have been detected in the mid-infrared for several high-mass runaway stars and in radio waves for one star. Theoretical models predict the production of high-energy photons by non-thermal radiative processes in a number sufficiently large to be detected in X-rays. To date, no stellar bow shock has been detected at such energies. We present the first detection of X-ray emission from a bow shock produced by a runaway star. The star is AE Aur, which was likely expelled from its birthplace due to the encounter ofmore » two massive binary systems and now is passing through the dense nebula IC 405. The X-ray emission from the bow shock is detected at 30'' northeast of the star, coinciding with an enhancement in the density of the nebula. From the analysis of the observed X-ray spectrum of the source and our theoretical emission model, we confirm that the X-ray emission is produced mainly by inverse Compton upscattering of infrared photons from dust in the shock front.« less
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  • X-ray observations of a sample of 11 4C radio galaxies in clusters with z< or approx. =0.1 were performed with the IPC (resolution FWHMapprox.1'.5) on the Einstein Observatory. The sample galaxies lie in the directions of non-Abell Zwicky clusters; these clusters are generally poor, ranging in effective Abell richness class from < or approx. =0 to 1. In 10 out of the 11 observed fields 1--3 keV emission was detected. In nearly every case, the X-ray emission is centered on the radio galaxy; 5 out of 10 of the X-ray sources are resolved with HWHM of 70--300 kpc. The X-raymore » luminosities are (1--60) x 10/sup 42/ ergs s/sup -1/ (H/sub 0/ = 75 km s/sup -1/ Mpc/sup -1/), generally less than the richer Abell clusters and undetectable with previous satellites. Examinations of the optical fields and preliminary velocity data suggest that most of the radio sources are associated with dominant galaxies which are at rest at the gravitational centers of the clusters. Internal relativistic particle and magnetic field pressures of the relaxed outer radio structures are on the same order as the thermal pressure produced by the X-ray emitting gas. We argue that this class of radio source is confined (and possibly triggered) by gas accumulated at the cluster centers. The X-ray emission is a cosmic signpost indicating the presence of large quantities of gas around the central radio galaxies. Recent radio observations indicate that the probability of galaxy radio emission is much higher for cluster galaxies than for very isolated galaxies; these new X-ray observations of poor clusters combined with previous data on Abell clusters suggest that the presence of cluster gas is the important factor in determining the link between cluster environment and radio emission.« less
  • Extreme ultraviolet (EUV; 13.6 eV <h{nu} {approx}< 100 eV) and X-rays in the 0.1-2 keV band can heat the surfaces of disks around young, low-mass stars to thousands of degrees and ionize species with ionization potentials greater than 13.6 eV. Shocks generated by protostellar winds can also heat and ionize the same species close to the star/disk system. These processes produce diagnostic lines (e.g., [Ne II] 12.8 {mu}m and [O I] 6300 A) that we model as functions of key parameters such as EUV luminosity and spectral shape, X-ray luminosity and spectral shape, and wind mass loss rate and shockmore » speed. Comparing our models with observations, we conclude that either internal shocks in the winds or X-rays incident on the disk surfaces often produce the observed [Ne II] line, although there are cases where EUV may dominate. Shocks created by the oblique interaction of winds with disks are unlikely [Ne II] sources because these shocks are too weak to ionize Ne. Even if [Ne II] is mainly produced by X-rays or internal wind shocks, the neon observations typically place upper limits of {approx}<10{sup 42} s{sup -1} on the EUV photon luminosity of these young low-mass stars. The observed [O I] 6300 A line has both a low velocity component (LVC) and a high velocity component. The latter likely arises in internal wind shocks. For the former we find that X-rays likely produce more [O I] luminosity than either the EUV layer, the transition layer between the EUV and X-ray layer, or the shear layer where the protostellar wind shocks and entrains disk material in a radial flow across the surface of the disk. Our soft X-ray models produce [O I] LVCs with luminosities up to 10{sup -4} L{sub sun}, but may not be able to explain the most luminous LVCs.« less