Multiwavelength Observations of the Runaway Binary HD 15137

📝 Abstract

HD 15137 is an intriguing runaway O-type binary system that offers a rare opportunity to explore the mechanism by which it was ejected from the open cluster of its birth. Here we present recent blue optical spectra of HD 15137 and derive a new orbital solution for the spectroscopic binary and physical parameters of the O star primary. We also present the first XMM-Newton observations of the system. Fits of the EPIC spectra indicate soft, thermal X-ray emission consistent with an isolated O star. Upper limits on the undetected hard X-ray emission place limits on the emission from a proposed compact companion in the system, and we rule out a quiescent neutron star in the propellor regime or a weakly accreting neutron star. An unevolved secondary companion is also not detected in our optical spectra of the binary, and it is difficult to conclude that a gravitational interaction could have ejected this runaway binary with a low mass optical star. HD 15137 may contain an elusive neutron star in the ejector regime or a quiescent black hole with conditions unfavorable for accretion at the time of our observations.

💡 Analysis

HD 15137 is an intriguing runaway O-type binary system that offers a rare opportunity to explore the mechanism by which it was ejected from the open cluster of its birth. Here we present recent blue optical spectra of HD 15137 and derive a new orbital solution for the spectroscopic binary and physical parameters of the O star primary. We also present the first XMM-Newton observations of the system. Fits of the EPIC spectra indicate soft, thermal X-ray emission consistent with an isolated O star. Upper limits on the undetected hard X-ray emission place limits on the emission from a proposed compact companion in the system, and we rule out a quiescent neutron star in the propellor regime or a weakly accreting neutron star. An unevolved secondary companion is also not detected in our optical spectra of the binary, and it is difficult to conclude that a gravitational interaction could have ejected this runaway binary with a low mass optical star. HD 15137 may contain an elusive neutron star in the ejector regime or a quiescent black hole with conditions unfavorable for accretion at the time of our observations.

📄 Content

Multiwavelength Observations of the Runaway Binary HD 15137 1 M. Virginia McSwain2 Department of Physics, Lehigh University, Bethlehem, PA 18015 mcswain@lehigh.edu Micha¨el De Becker Institut d’Astrophysique et G´eophysique, Universit´e de Li`ege, FNRS, Belgium debecker@astro.ulg.ac.be Mallory S. E. Roberts Eureka Scientific, Inc., Oakland, CA 94602-3017 malloryr@gmail.com Tabetha S. Boyajian2,3, Douglas R. Gies Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30302-4106 tabetha@chara.gsu.edu, gies@chara.gsu.edu Erika D. Grundstrom2 Physics and Astronomy Department, Vanderbilt University, Nashville, TN 37235 erika.grundstrom@vanderbilt.edu Christina Aragona2, Amber N. Marsh2, Rachael M. Roettenbacher2 Department of Physics, Lehigh University, Bethlehem, PA 18015 cha206@lehigh.edu, anm506@lehigh.edu, rmr207@lehigh.edu ABSTRACT HD 15137 is an intriguing runaway O-type binary system that offers a rare opportunity to explore the mechanism by which it was ejected from the open cluster of its birth. Here we present recent blue optical spectra of HD 15137 and derive a new orbital solution for the spectroscopic binary and physical parameters of the O star primary. We also present the first XMM-Newton observations of the system. Fits of the EPIC spectra indicate soft, thermal X-ray emission consistent with an isolated O star. Upper limits on the undetected hard X-ray emission place limits on the emission from a proposed compact companion in the system, and we rule out a quiescent neutron star in the propellor regime or a weakly accreting neutron star. An unevolved secondary companion is also not detected in our optical spectra of the binary, and it is difficult to conclude that a gravitational interaction could have ejected this runaway binary with a low mass optical star. HD 15137 may contain an elusive neutron star in the ejector regime or a quiescent black hole with conditions unfavorable for accretion at the time of our observations. Subject headings: stars: individual(HD 15137), binaries: spectroscopic, X-rays: binaries 1 arXiv:0912.5133v1 [astro-ph.SR] 28 Dec 2009 1. Introduction While most O- and B-type stars are believed to form in open clusters and stellar associations, some are observed at high galactic latitudes and with large peculiar space velocities (Gies & Bolton 1986). These stars were likely ejected from the clusters of their birth, and there are two accepted mechanisms to explain the origin of their run- away velocities. Close multi-body interactions in a dense cluster environment may cause one or more stars to be scattered out of the region (Poveda et al. 1967), and about 10% are expected to be ejected as binary pairs (Leonard & Duncan 1990). An alternative mechanism involves a su- pernova explosion within a close binary, ejecting the secondary due to the conservation of momen- tum (Zwicky 1957). The resulting neutron star (NS) or black hole (BH) may remain bound to the secondary if not enough mass is lost during the explosion or if a retrograde kick velocity occurs, and a binary fraction of 20 −40% is predicted for these runaways (Portegies Zwart 2000). The ob- served fraction of binaries among runaways seems consistent with either scenario (5 −26%; Mason et al. 1998). While both processes do occur in nature, it is not clear which process is dominant. Distinguishing between the dynamical ejection and binary supernova scenarios for an isolated run- away star can be nearly impossible. However, bi- nary runaway systems offer a significant advantage to cluster ejection studies since multiwavelength observations can pinpoint NS companions pro- duced in supernovae. Optical spectroscopic stud- ies alone offer circumstantial evidence for com- pact companions, primarily by high eccentricity and low mass function determined from the orbit, but also due to nitrogen and/or helium enrichment and fast rotational velocity that suggest prior mass transfer (Hoogerwerf et al. 2001). A few runaway massive stars are predicted to be associated with radio pulsars (Portegies Zwart 2000), but so far 2Visiting Astronomer, Kitt Peak National Observatory. KPNO is operated by AURA, Inc. under contract to the National Science Foundation. 3Hubble Fellow 1Based partly on observations collected at the Observa- toire de Haute-Provence (France) and on observations ob- tained with XMM-Newton, an ESA science mission with in- struments and contributions directly funded by ESA Mem- ber States and NASA. none have been found (Philp et al. 1996; Sayer et al. 1996). X-ray emission is a better diagnostic since stellar wind interactions with a NS produce a measurable X-ray excess above the normal, in- trinsic emission from shocks in the stellar winds (Sana et al. 2006; Popov et al. 2000; Lamers et al. 1976). HD 15137 is a runaway binary that offers the rare opportunity to diagnose its cluster ejection mechanism. Its spectral type is O9.5 III(n). Boy- ajian et al. (2005) found that this runaway single- line spectroscopic

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