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

TL;DR

XMM-Newton X-ray observations of VY CMa set the most stringent limits to date on its surface magnetic field, suggesting episodic mass loss and low magnetic activity during the observations, with additional insights into nearby X-ray sources.

Contribution

First X-ray constraints on the magnetic field strength of a cool hypergiant, providing insights into its mass loss mechanisms and local stellar environment.

Findings

VY CMa was not detected in X-rays, implying a magnetic field strength $fB \\leq 2 \\times 10^{-3}$ G.

Over 100 X-ray sources detected near VY CMa, some likely young stars.

VY CMa's low X-ray flux suggests low surface magnetic activity during observations.

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 ($F_{X,UL}~\approx~8\times10^{-14} {\rm~erg~cm}^{-2} {\rm~s}^{-1}$, corresponding to $\log~L_X/L_{bol}\leq-8$), we estimate an average surface magnetic field strength $fB \leq 2\times10^{-3}$~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.