arXiv subjects

Lex Kaper

Publications and source records attributed to Lex Kaper.

58 records · Page 4Linked to original sources

arXiv2001

High-mass X-ray binaries and OB-runaway stars

High-mass X-ray binaries (HMXBs) represent an important phase in the evolution of massive binary systems. HMXBs provide unique diagnostics to test massive-star evolution, to probe the physics of radiation-driven winds, to study the process of mass accretion, and to measure fundamental parameters of compact objects. As a consequence of the supernova explosion that produced the neutron star (or black hole) in these systems, HMXBs have high space velocities and thus are runaways. Alternatively, OB-runaway stars can be ejected from a cluster through dynamical interactions. Observations obtained with the Hipparcos satellite indicate that both scenarios are at work. Only for a minority of the OB runaways (and HMXBs) a wind bow shock has been detected. This might be explained by the varying local conditions of the interstellar medium.

astro-ph↗

arXiv2000

On the origin of the difference between the runaway velocities of the OB-supergiant X-ray Binaries and the Be/X-ray Binaries

The recent finding by Chevalier & Ilovaisky (1998) that OB-supergiant X-ray binaries have relatively large runaway velocities whereas Be/X-ray binaries have low runaway velocities, provides confirmation of the current models for the formation of these two types of systems. These predict a difference in runaway velocity of an order of magnitude. This difference basically results from the variation of the fractional helium core mass as a function of stellar mass, in combination with the conservation of orbital angular momentum during the mass transfer phase that preceded the formation of the compact object in the system. This combination results into: (i) Systematically narrower pre-supernova orbits in the OB-supergiant systems than in the Be-systems, and (ii) A larger fractional amount of mass ejected in the supernovae in high-mass systems relative to systems of lower mass. Regardless of possible kick velocities imparted to neutron stars at birth, this combination leads to a considerable difference in average runaway velocity between these two groups. The observed low runaway velocities of the Be/X-ray binaries confirm that in most cases not more than 1 to 2Msun was ejected in the supernovae that produced their neutron stars. This, in combination with the --on average-- large orbital eccentricities of these systems, indicates that their neutron stars must have received a velocity kick in the range 60 - 250 km/s at birth.

astro-ph↗

arXiv1997

Winds in OB-type stars

The International Ultraviolet Explorer satellite has made a tremendous contribution to the study of hot-star winds. Its long lifetime has resulted in the collection of ultraviolet spectra for a large sample of OB stars. Its unique monitoring capability has enabled detailed time-series analyses to investigate the stellar-wind variability for individual objects. IUE has also been a major driver for the development of the radiation-driven-wind theory; the synergy between theory and observations is one of the main reasons for the large progress that has been made in our understanding of hot-star winds and their impact on the atmospheres and evolution of massive stars.

astro-ph↗

arXiv1996

Discovery of a bow shock around Vela X-1

We report the discovery of a symmetric bow shock around the well-known high-mass X-ray binary (HMXB) Vela X-1. Wind bow shocks are a ubiquitous phenomenon around OB-runaway stars, but now such a structure is found around a HMXB. The presence of a bow shock indicates that the system has a high (supersonic) velocity with respect to the interstellar medium. From the symmetry of the bow shock, the direction of motion and, moreover, the origin and age of the system can be derived. Our observation supports Blaauw's scenario for the formation of an OB-runaway star by the supernova explosion of the binary companion.

astro-ph↗