Nearby Hot Stars May Change Our View of Distant Sources

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As if it weren’t enough that quasars — distant and bright nuclei of galaxies — twinkle of their own accord due to internal processes, nature also provides another complication: these distant radio sources can also appear to twinkle because of intervening material between them and us. A new study has identified a possible source for the material getting in the way.

Unexplained Variability

Helix Nebula

A Spitzer infrared view of the Helix nebula, which contains ionized streamers of gas extending radially outward from the central star. [NASA/JPL-Caltech/Univ. of Ariz.]

Distant quasars occasionally display extreme scintillation, twinkling with variability timescales shorter than a day. This intra-day variability is much greater than we can account for with standard models of the interstellar medium lying between the quasar and us. So what could cause this extreme scattering instead?

The first clue to this mystery came from the discovery of strong variability in the radio source PKS 1322–110. In setting up follow-up observations of this object, Mark Walker (Manly Astrophysics, Australia) and collaborators noticed that, in the plane of the sky, PKS 1322–110 lies very near the bright star Spica. Could this be coincidence, or might this bright foreground star have something to do with the extreme scattering observed?

Diagram of scintillation

Diagram explaining the source of the intra-day radio source variability as intervening filaments surrounding a hot star. [M. Walker/CSIRO/Manly Astrophysics]

Swarms of Clumps

Walker and collaborators put forward a hypothesis: perhaps the ultraviolet photons of nearby hot stars ionize plasma around them, which in turn causes the extreme scattering of the distant background sources.

As a model, the authors consider the Helix Nebula, in which a hot, evolved star is surrounded by cool globules of molecular hydrogen gas. The radiation from the star hits these molecular clumps, dragging them into long radial streamers and ionizing their outer skins.

Though the molecular clumps in the Helix Nebula were thought to have formed only as the star evolved late into its lifetime, Walker and collaborators are now suggesting that all stars — regardless of spectral type or evolutionary stage — may be surrounded by swarms of tiny molecular clumps. Around stars that are hot enough, these clumps become the ionized plasma streamers that can cause interference with the light traveling to us from distant sources.

Significant Mass

To test this theory, Walker and collaborators explore observations of two distant radio quasars that have both exhibited intra-day variability over many years of observations. The team identified a hot A-type star near each of these two sources: J1819+3845 has Vega nearby, and PKS 1257–326 has Alhakim.

line-of-sight stars

Locations of stars along the line of site to two distant quasars, J1819+3845 (top panel) and PKS 1257–326 (bottom panel). Both have a nearby, hot star (blue markers) radially within 2 pc: Vega (z = 7.7 pc) and Alhakim (z = 18 pc), respectively. [Walker et al. 2017]

By modeling the systems of the sources and stars, the authors show that the size, location, orientation, and numbers of plasma concentrations necessary to explain observations are all consistent with an environment similar to that of the Helix Nebula. Walker and collaborators find that the total mass in the molecular clumps surrounding the two stars would need to be comparable to the mass of the stars themselves.

If this picture is correct, and if all stars are indeed surrounded by molecular clumps like these, then a substantial fraction of the mass of our galaxy could be contained in these clumps. Besides explaining distant quasar scintillation, this idea would therefore have a significant impact on our overall understanding of how mass in galaxies is distributed. More observations of twinkling quasars are the next step toward confirming this picture.

Citation

Mark A. Walker et al 2017 ApJ 843 15. doi:10.3847/1538-4357/aa705c

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