Constraints on the Surface Magnetic Fields and Age of a Cool Hypergiant: XMM-Newton X-ray Observations of VY CMa

Abstract

The complex circumstellar ejecta of highly evolved, cool hypergiants are indicative of multiple, asymmetric mass loss events. To explore whether such episodic, non-isotropic mass loss may be driven by surface magnetic activity, we have observed the archetypical cool hypergiant VY CMa with the XMM-Newton X-ray satellite observatory. The hypergiant itself is not detected in these observations. From the upper limit on the X-ray flux from VY CMa at the time of our observations ($F_{X,UL}\approx 8\times 10^{-14}{\mathrm{erg}\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, corresponding to $\log L_X/L_{bol}\le -8$), we estimate an average surface magnetic field strength $fB\le 2\times 10^{-3}$~G (where $f$ is the filling factor of magnetically active surface regions). These X-ray results for VY CMa represent the most stringent constraints to date on the magnetic field strength near the surface of a hypergiant. VY CMa's mass loss is episodic and may have been in a state of low surface magnetic activity during the XMM observations. The XMM observations also yield detections of more than 100 X-ray sources within $\sim$15${}^{\prime }$ of VY CMa, roughly 50 of which have near-infrared counterparts. Analysis of X-ray hardness ratios and IR colors indicates that some of these field sources may be young, late-type stars associated with VY CMa, its adjacent molecular cloud complex, and the young cluster NGC 2362. Further study of the VY CMa field is warranted, given the potential to ascertain the evolutionary timescale of this enigmatic, massive star.

arXiv commentAccepted by ApJ; 9 pages; cool figures; source lists; magnetic fields

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