A far-ultraviolet variable with an 18-minute period in the globular cluster NGC 1851

Abstract

We present the detection of a variable star with an $18.05$ minute period in far-ultraviolet (FUV) images of the globular cluster NGC 1851 taken with the Hubble Space Telescope (HST). A candidate optical counterpart lies on the red horizontal branch or the asymptotic giant branch star of the cluster, but it is statistically possible that this is a chance superposition. This interpretation is supported by optical spectroscopt obtained with HST/STIS: the spectrum contains none of the strong emission lines that would be expected if the object was a symbiotic star (i.e. a compact accretor fed by a giant donor). We therefore consider two other possibilities for the nature of FUV variable: (i) an intermediate polar (i.e. a compact binary containing an accreting magnetic white dwarf), or (ii) an AM CVn star (i.e. an interacting double-degenerate system). In the intermediate polar scenario, the object is expected to be an X-ray source. However, no X-rays are detected at its location in $\simeq 65$~ksec of {\em Chandra} imaging, which limits the X-ray luminosity to $L_X⩽10^{32}$~erg~s${}^{-1}$. We therefore favour the AM CVn interpretation, but a FUV spectrum is needed to distinguish conclusively between the two possibilities. If the object is an AM CVn binary, it would be the first such system known in any globular cluster.

10 pages, 10 figures, Accepted to MNRAS 2016 May 16

References in corpus (7)

• The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851
• Dynamical Formation of Close Binaries in Globular Clusters II: Cataclysmic Variables
• Constraints on the space density of intermediate polars from the Swift-BAT survey
• Long-term Photometric Behavior of Outbursting AM CVn Systems
• On the orbital periods of the AM CVn stars HP Librae and V803 Centauri
• Interrupted Stellar Encounters in Star Clusters
• XMM-Newton observations of AM CVn binaries: V396 Hya & SDSS J1240-01

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• Identification of Faint Chandra X-ray Sources in the Core-Collapsed Globular Cluster NGC 6752
• On the absence of symbiotic stars in globular clusters
• MOCCA-SURVEY database I. Accreting white dwarf binary systems in globular clusters -- II. Cataclysmic variables -- progenitors and population at birth
• Far-Ultraviolet Observation of the Globular Cluster NGC 6397
• A Catalogue of Ultracompact Mass-Transferring White Dwarf Binaries
• Astrophysics with the Laser Interferometer Space Antenna
• Far-ultraviolet investigation into the galactic globular cluster M30 (NGC 7099): I. Photometry and radial distributions
• Far-ultraviolet investigation into the galactic globular cluster M30 (NGC 7099): II. Potential X-ray counterparts and variable sources