TY - JOUR
T1 - A far-ultraviolet variable with an 18-minute period in the globular cluster NGC 1851
AU - Zurek, D. R.
AU - Knigge, C.
AU - Maccarone, T. J.
AU - Pooley, D.
AU - Dieball, A.
AU - Long, K. S.
AU - Shara, M.
AU - Sarajedini, A.
N1 - Funding Information:
Support for program GO-10184 and GO-13394 was provided by NASA through a grant from Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. All of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. We thank Jeno Sokoloski for many helpful conversations. We are also grateful to Retha Pretorius and Koji Mukai for helpful discussions regarding IPs and for providing distances for some of the systems shown in Fig. 7.
Publisher Copyright:
© 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.
PY - 2016/8/21
Y1 - 2016/8/21
N2 - We present the detection of a variable star with an 18.05 min 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 spectroscopy obtained with HST/Space Telescope Imaging Spectrograph (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 ≃65 ksec of Chandra imaging, which limits the X-ray luminosity to LX ≤ 1032 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.
AB - We present the detection of a variable star with an 18.05 min 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 spectroscopy obtained with HST/Space Telescope Imaging Spectrograph (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 ≃65 ksec of Chandra imaging, which limits the X-ray luminosity to LX ≤ 1032 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.
KW - Binaries: close
KW - Globular clusters: individual: NGC 1851
KW - Novae, cataclysmic variables
KW - Ultraviolet: stars
UR - http://www.scopus.com/inward/record.url?scp=84980372012&partnerID=8YFLogxK
U2 - 10.1093/mnras/stw1190
DO - 10.1093/mnras/stw1190
M3 - Article
AN - SCOPUS:84980372012
VL - 460
SP - 3660
EP - 3668
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
SN - 0035-8711
IS - 4
ER -