CONSTRAINTS ON THE SURFACE MAGNETIC FIELDS AND AGE OF A COOL HYPERGIANT:XMM-NEWTONX-RAY OBSERVATIONS OF VY CMa

Abstract

The complex circumstellar ejecta of highly evolved, cool hypergiants are indicative of multiple, asymmetric massloss 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,U L ≈ 8 × 10 −14 erg cmwe estimate an average surface magnetic field strength f B 2 × 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, however, and the hypergiant 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 ∼15 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.

The post-common-envelope X-ray binary nucleus of the planetary nebula NGC 2392

“…Previous spectral fitting of the X-ray spectrum of the central star of NGC 2392 involved fitting optically thin thermal plasmas (Guerrero 2012;Montez et al 2015). The orbital parameters of the newly discovered binary (Section 3) provide some additional insights into the mechanisms behind the X-ray emission.…”

“…Although binary interactions offer the leading explanation for many phenomena (De Marco 2009;Jones & Boffin 2017), there are some phenomena that remain particularly enigmatic. The Chandra detection of relatively hard X-ray emission with peak energies > 0.5 keV in some central stars is one such phenomenon (Guerrero et al 2001, Guerrero, Chu, & Gruendl 2011Guerrero 2012Guerrero , 2015Kastner et al 2012;Freeman et al 2014;Montez et al 2015;Montez & Kastner 2013;Montez 2017). A wide range of explanations have been proposed (see Guerrero 2012 andMontez et al 2015 for comprehensive discussions) and include processes that can operate from central stars on their own (shocks in the stellar wind, emission from the atmosphere, and accretion of nebular or debris disk material) or in a binary system (colliding winds, accretion onto a companion, and coronal activity from late-type companions).…”

“…The Chandra detection of relatively hard X-ray emission with peak energies > 0.5 keV in some central stars is one such phenomenon (Guerrero et al 2001, Guerrero, Chu, & Gruendl 2011Guerrero 2012Guerrero , 2015Kastner et al 2012;Freeman et al 2014;Montez et al 2015;Montez & Kastner 2013;Montez 2017). A wide range of explanations have been proposed (see Guerrero 2012 andMontez et al 2015 for comprehensive discussions) and include processes that can operate from central stars on their own (shocks in the stellar wind, emission from the atmosphere, and accretion of nebular or debris disk material) or in a binary system (colliding winds, accretion onto a companion, and coronal activity from late-type companions). Some detections may be explained by coronal activity only when the presence of late-type stellar companions was already independently established (Montez et al 2010(Montez et al , 2015.…”

“…A wide range of explanations have been proposed (see Guerrero 2012 andMontez et al 2015 for comprehensive discussions) and include processes that can operate from central stars on their own (shocks in the stellar wind, emission from the atmosphere, and accretion of nebular or debris disk material) or in a binary system (colliding winds, accretion onto a companion, and coronal activity from late-type companions). Some detections may be explained by coronal activity only when the presence of late-type stellar companions was already independently established (Montez et al 2010(Montez et al , 2015. However, in those sources without a proven late-type companion, it is very difficult to determine which of the several possible mechanisms may produce the relatively hard X-ray emission.…”

“…The hardest X-ray emission of any central star belongs to NGC 2392 a (PN G197.8+17.3, Figure 1) which exhibits a spectrum peaking above 1 keV and a hard energy tail up to 5 keV (Guerrero 2012;Montez et al 2015). Guerrero (2012) favoured accretion onto a compact companion to explain the X-ray spectrum.…”