X-ray and optical spectroscopy of the massive young open cluster IC1805

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 IC1805 open cluster is very interesting as it hosts the O4If$^+$ star HD15570 thought to be in an evolutionary stage intermediate between a normal O-star and a Wolf-Rayet star. 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. We have observed IC1805 with XMM-Newton and further collected optical spectroscopy of some of the O-star members of the cluster. The optical spectra allow us to revisit the orbital solutions of BD+60$^{\circ}$ 497 and HD15558, and provide the first evidence of binarity for BD+60$^{\circ}$ 498. X-ray emission from colliding winds does not appear to play an important role among the O-stars of IC1805. 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$^{\circ}$ 513, and to a lesser extent the O4If$^+$ star HD15570 appear somewhat underluminous. Whilst the underluminosity of HD15570 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.Comment: Accepted for publication in Astronomy & Astrophysic

X-ray emission from interacting massive binaries: a review of 15 years of progress

Previous generations of X-ray observatories revealed a group of massive binaries that were relatively bright X-ray emitters. This was attributed to emission of shock-heated plasma in the wind-wind interaction zone located between the stars. With the advent of the current generation of X-ray observatories, the phenomenon could be studied in much more detail. In this review, we highlight the progress that has been achieved in our understanding of the phenomenon over the last 15 years, both on theoretical and observational grounds. All these studies have paved the way for future investigations using the next generation of X-ray satellites that will provide crucial information on the X-ray emission formed in the innermost part of the wind-wind interaction.Comment: Accepted for publication in a special issue of Advances in Space Research on X-ray Emission from Hot Stars and their Wind

X-ray emission of massive stars and their winds

Most types of massive stars display X-ray emission that is affected by the properties of their stellar winds. Single non-magnetic OB stars have an X-ray luminosity that scales with their bolometric luminosity and their emission is thought to arise from a distribution of wind-embedded shocks. The lack of significant short-term stochastic variability indicates that the winds consist of a large number of independent fragments. Detailed variability studies unveiled a connection between the photosphere and the wind: well-studied O-type stars exhibit a ~ 10% modulation of their emission on timescales consistent with the rotation period, and a few early B-type pulsators display ~ 10% modulations of their X-ray flux with the pulsation period. Unlike OB stars, their evolved descendants (WR and LBV stars) lack a well-defined relation between their X-ray and bolometric luminosities, and several subcategories of objects remain undetected. These properties most likely stem from the combined effects of wind optical depth and wind velocity. Magnetic OB stars display an enhanced X-ray emission frequently modulated by the rotation of the star. These properties are well explained by the magnetically confined wind shock model and an oblique magnetic rotator configuration. Some massive binaries display phase-dependent excess emission arising from the collision between the winds of the binary components. Yet, the majority of the massive binaries do not show such an emission, probably as a consequence of radiative cooling of the shock-heated plasma. Finally, a growing subset of the Be stars, the so-called gamma Cas stars, feature an unusually hard and strong thermal X-ray emission that varies over a wide range of timescales. Several scenarios have been proposed to explain these properties, but the origin of the phenomenon remains currently one of the major unsolved puzzles in stellar X-ray astrophysics.Comment: Invited chapter of the Handbook of X-ray and Gamma-ray Astrophysic

Fe xxv line profiles in colliding wind binaries

Strong wind-wind collisions in massive binaries generate a very hot plasma that frequently produces a moderately strong iron line. The morphology of this line depends upon the properties of the wind interaction zone and its orientation with respect to the line of sight. As the binary components revolve around their common centre of mass, the line profiles are thus expected to vary. With the advent of the next generation of X-ray observatories (Astro-H, Athena) that will offer high-resolution spectroscopy above 6 keV, it will become possible to exploit these changes as the most sensitive probe of the inner parts of the colliding wind interaction. Using a simple prescription of the wind-wind interaction in an early-type binary, we have generated synthetic line profiles for a number of configurations and orbital phases. These profiles can help constrain the properties of the stellar winds in such binary systems.Comment: Accepted for publication in New Astronom

Quest for the tertiary component in Cyg OB2 #5

The Cyg OB2 #5 system is thought to consist of a short-period (6.6 d) eclipsing massive binary orbited by an OB-star orbiting with a period of ~6.7 yr; these stars in turn are orbited by a distant early B-star with a period of thousands of years. However, while the inner binary has been studied many times, information is missing on the other stars, in particular the third star whose presence was indirectly postulated from recurrent modulations in the radio domain. Besides, to this date, the X-ray light curve could not be fully interpreted, for example in the framework of colliding-wind emission linked to one of the systems. We obtained new optical and X-ray observations of Cyg OB2 #5, which we combined to archival data. We performed a thorough and homogeneous investigation of all available data, notably revisiting the times of primary minimum in photometry. In the X-ray domain, XMM-Newton provides scattered exposures over ~5000 d whilst Swift provides a nearly continuous monitoring for the last couple of years. Although the X-ray light curve reveals clear variability, no significant period can be found hence the high-energy emission cannot be explained solely in terms of colliding winds varying along either the short or intermediate orbits. The optical data reveal for the first time clear signs of reflex motion. The photometry indicates the presence of a 2366 d (i.e. 6.5 yr) period while the associated radial velocity changes are detected at the 3 sigma level in the systemic velocity of the He II 4686 emission line. With the revised period, the radio light curve is interpreted consistently in terms of a wind interaction between the inner binary and the tertiary star. From these optical and radio data, we derive constraints on the physical properties of the tertiary star and its orbit.Comment: Accepted for publication in Astronomy & Astrophysic

X-ray properties of the young open clusters HM1 and IC2944/2948

Using XMM data, we study for the first time the X-ray emission of HM1 and IC2944/2948. Low-mass, pre-main-sequence objects with an age of a few Myr are detected, as well as a few background or foreground objects. Most massive stars in both clusters display the usual high-energy properties of that type of objects, though with log(Lx/Lbol) apparently lower in HM1 than in IC2944/2948. Compared with studies of other clusters, it seems that a low signal-to-noise ratio at soft energies, due to the high extinction, may be the main cause of this difference. In HM1, the two Wolf-Rayet stars show contrasting behaviors: WR89 is extremely bright, but much softer than WR87. It remains to be seen whether wind-wind collisions or magnetically confined winds can explain these emissions. In IC2944/2948, the X-ray sources concentrate around HD101205; a group of massive stars to the north of this object is isolated, suggesting that there exist two subclusters in the field-of-view.Comment: 29 pages in total with 10 figures (12 pages paper + supplementary material), accepted by A&

Similar but different: the varied landscape of Onfp/Oef stars variability

The Oef category gathers rapidly rotating and evolved O-stars displaying a centrally reversed He II 4686 emission line. The origin of the variability of their photospheric and wind spectral lines is debated, with rotational modulation or pulsations as the main contenders. To shed new light on this question, we analysed high-quality and high-cadence TESS photometric time series for five Oef stars. We also collected a new time series of spectra for one target (lambda Cep) which had been the subject of specific debates in the last years. These observations reveal the variety of Oef behaviours. While space-based photometric data reveal substantial red noise components in all targets, only zeta Pup seems to display a long-lived periodicity. In our sample, stars exhibit a dominant signal at low frequencies but it appears relatively short-lived. This is reminiscent of rotational modulations by transient photospheric spots, though this scenario is challenged by the case of HD 14442, whose 1.230 d$^{-1}$ signal significantly exceeds the critical rotational frequency. In parallel, no evidence of persistent p-mode non-radial pulsations is found in either photometry or spectroscopy of the stars, only temporary excitation of g-mode pulsations could offer an alternative explanation for the dominant signals. Finally, the revised luminosities of the stars using GAIA-DR2 show that they are not all supergiants as zeta Pup. The question then arises whether the Oef peculiarity denotes a homogeneous class of objects after all.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Societ

The Multiwavelength Picture of Star Formation in the Very Young Open Cluster NGC6383

We review the properties of the very young (2 Myr) open cluster NGC6383. The cluster is dominated by the massive binary HD159176 (O7V + O7V). The distance to NGC6383 is consistently found to be 1.3 +- 0.1 kpc and the average reddening is determined to be E(B-V) = 0.32 +- 0.02. Several pre-main sequence candidates have been identified using different criteria relying on the detection of emission lines, infrared excesses, photometric variability and X-ray emission.Comment: 12 pages, 5 figures, to appear in the Handbook of Star Forming Regions, Vol. II (The Southern Sky), ed. Bo Reipurt

TESS lightcurves of gamma-Cas stars

gamma-Cas stars constitute a subgroup of Be stars showing unusually hard and bright X-ray emission. In search for additional peculiarities, we analyzed the TESS lightcurves of 15 gamma-Cas analogs. Their periodograms display broad frequency groups and/or narrow isolated peaks, often superimposed over red noise. The detected signals appear at low frequencies, with few cases of significant signals beyond 5/d (and all of them are faint). The signal amplitudes, and sometimes the frequency content, change with time, even in the absence of outburst events. On the basis of their optical photometric variability, gamma-Cas stars reveal no distinctive behaviour and thus appear similar to Be stars in general.Comment: accepted by MNRAS - the arxiv version has figures in low-resolutio

The Massive Binary System 9 Sgr Revisited: New Insights into Disentangling Methods

Disentangling techniques are often needed to obtain the spectra of the individual components of binary or multiple systems. A thorough analysis of the shift-and-add algorithm of Marchenko, Moffat, & Eenens (1998) reveals that in many cases the line fluxes are poorly reproduced and spurious wings appear. The causes of these discrepancies are discussed and a new disentangling package, QER20, is presented which significantly reduces these errors and vastly increases the performance. When applied to the massive binary 9 Sgr, our new code yields line fluxes which are notably different from those previously published and lead us to revise the spectral classification to slightly earlier subtypes: O3V((f +)) for the primary and O5V((f)) for the secondary. We show that with the MME98 algorithm the classification of massive stars in binaries can be off by several subtypes whilst there are no such errors when the QER20 package is used.Comment: Accepted for publication in Astronomical Notes, 12 pages, 10 figure