A multi-epoch XMM-Newton campaign on the core of the massive Cygnus OB2 association

Mahy

Damerdji

et al. 2012

A&A

Context. Nonthermal radio emission in massive stars is expected to arise in wind-wind collisions occurring inside a binary system. One such case, the O-type star Cyg OB2 #9, was proven to be a binary only four years ago, but the orbital parameters remained uncertain. The periastron passage of 2011 was the first one to be observable under good conditions since the discovery of binarity. Aims. In this context, we have organized a large monitoring campaign to refine the orbital solution and to study the wind-wind collision.Methods. This paper presents the analysis of optical spectroscopic data, as well as of a dedicated X-ray monitoring performed with Swift and XMM-Newton. Results. In light of our refined orbital solution, Cyg OB2 #9 appears as a massive O+O binary with a long period and high eccentricity; its components (O5-5.5I for the primary and O3-4III for the secondary) have similar masses and similar luminosities. The new data also provide the first evidence that a wind-wind collision is present in the system. In the optical domain, the broad Hα line varies, displaying enhanced absorption and emission components at periastron. X-ray observations yield the unambiguous signature of an adiabatic collision, because as the stars approach periastron, the X-ray luminosity closely follows the 1/D variation expected in that case. The X-ray spectrum appears, however, slightly softer at periastron, which is probably related to winds colliding at slightly lower speeds at that time. Conclusions. It is the first time that such a variation has been detected in O+O systems, and the first case where the wind-wind collision is found to remain adiabatic even at periastron passage.

Section: The Spectrasupporting

confidence: 50%

Section: Xmm-newtonmentioning

confidence: 54%

Section: Swiftmentioning

confidence: 99%

See 1 more Smart Citation

The 2.35 year itch of Cygnus OB2 #9

Mahy

Damerdji

et al. 2012

A&A

“…For the X-ray brightest PMS candidates, we found f X 0.5 × 10 −13 erg cm −2 s −1 (Table 5) which then translates into L X 3.5 × 10 31 erg s −1 . Whilst this is a relatively high value, it is not unusual for PMS stars in young open clusters, especially during flares (Sana et al 2007;Rauw 2011). We note also that our cross-correlation with catalogues from the literature (see Table A.1) resulted in the association of six X-ray sources with low-mass YSOs identified with WISE (Straižys et al 2013).…”

Section: Other Bright X-ray Sources In the Field Of Viewsupporting

confidence: 57%

X-ray and optical spectroscopy of the massive young open cluster IC 1805

Rauw

2016

A&A

Self Cite

Context. Very young open clusters are ideal places to study the X-ray properties of a homogeneous population of early-type stars. In this respect, the IC 1805 open cluster is very interesting as it hosts the O4 If + star HD 15570 thought to be in an evolutionary stage intermediate between a normal O-star and a Wolf-Rayet star. Aims. Such a star could provide a test for theoretical models aiming at explaining the empirical scaling relation between the X-ray and bolometric luminosities of O-type stars. Methods. We have observed IC 1805 with XMM-Newton and further collected optical spectroscopy of some of the O-star members of the cluster. Results. The optical spectra allow us to revisit the orbital solutions of BD+60• 497 and HD 15558, and provide the first evidence of binarity for BD+60• 498. X-ray emission from colliding winds does not appear to play an important role among the O-stars of IC 1805. Notably, the X-ray fluxes do not vary significantly between archival X-ray observations and our XMM-Newton pointing. The very fast rotator BD+60• 513, and to a lesser extent the O4 If + star HD 15570 appear somewhat underluminous. Whilst the underluminosity of HD 15570 is only marginally significant, its amplitude is found to be compatible with theoretical expectations based on its stellar and wind properties. A number of other X-ray sources are detected in the field, and the brightest objects, many of which are likely low-mass pre-main sequence stars, are analyzed in detail.

“…5.2.) There is furthermore some discussion about the distance to the Cyg OB2 association (e.g., Rauw 2011). A larger distance (1.7-2.0 kpc instead of our assumed 1.45 kpc) would explain the discrepancy equally well.…”

Section: Free-free Contribution Stellar Windsmentioning

confidence: 71%

The 2.35 year itch of Cygnus OB2 #9

Blomme

Volpi

et al. 2013

A&A

Context. Cyg OB2 #9 is one of a small set of non-thermal radio emitting massive O-star binaries. The non-thermal radiation is due to synchrotron emission in the colliding-wind region. Cyg OB2 #9 has only recently been discovered to be a binary system, and a multi-wavelength campaign was organized to study its 2011 periastron passage. Aims. We want to better determine the parameters of this system and model the wind-wind collision. This will lead to a better understanding of the Fermi mechanism that accelerates electrons up to relativistic speeds in shocks and its occurrence in collidingwind binaries. We report here on the results of the radio observations obtained in the monitoring campaign and present a simple model to interpret the data. Methods. We used the Expanded Very Large Array (EVLA) radio interferometer to obtain 6 cm and 20 cm continuum fluxes during the Cyg OB2 #9 periastron passage in 2011. We introduce a simple model to solve the radiative transfer in the stellar winds and the colliding-wind region, and thus determine the expected behaviour of the radio light curve. Results. The observed radio light curve shows a steep drop in flux sometime before periastron. The fluxes drop to a level that is comparable to the expected free-free emission from the stellar winds, suggesting that the non-thermal emitting region is completely hidden at that time. After periastron passage, the fluxes slowly increase. We use the asymmetry of the light curve to show that the primary has the stronger wind. This is somewhat unexpected if we use the astrophysical parameters based on theoretical calibrations. But it becomes entirely feasible if we take into account that a given spectral type-luminosity class combination covers a range of astrophysical parameters. The colliding-wind region also contributes to the free-free emission, which can help explain the high values of the spectral index seen after periastron passage. Combining our data with older Very Large Array (VLA) data allows us to derive a period P = 860.0 ± 3.7 days for this system. With this period, we update the orbital parameters that were derived in the first paper of this series. Conclusions. A simple model introduced to explain only the radio data already allows some constraints to be put on the parameters of this binary system. Future, more sophisticated, modelling that will also include optical, X-ray, and interferometric information will provide even better constraints.