A la recherche du neutrino perdu

Article for general public (2005)

Les secrets du pendule

Article for general public (2005)

Les Belges vibrent pour les étoiles

Article for general public (2005)

Frédéric Chaberlot, La Voie Lactée: histoire des conceptions et des modèles de notre Galaxie des temps anciens aux années 1930, Bibliographie critique

in Archives Internationales d'Histoire des Sciences (2005), (154), 302-302

A period and a prediction for the Of ? p spectrum alternator HD 191612

in Astrophysical Journal (2004), 617(1, Part 2), 61-64

The observational picture of the enigmatic O-type spectrum variable HD 191612 has been sharpened substantially. A symmetrical, low-amplitude light curve with a period near 540 days has recently been ... [more ▼]

The observational picture of the enigmatic O-type spectrum variable HD 191612 has been sharpened substantially. A symmetrical, low-amplitude light curve with a period near 540 days has recently been reported from Hipparcos photometry. This period satisfies all of the spectroscopy since at least 1982, including extensive new observations during 2003 and 2004, and it has predicted the next transition during 2004 September - October. Measurements of the Ha equivalent width reveal a sharp emission peak in the phase diagram, in contrast to the apparently sinusoidal light curve. The He II absorption-line strength is essentially constant, while He I varies strongly, possibly filled in by emission in the O6 state, thus producing the apparent spectral type variations. The O8 state appears to be the "normal" one. Two intermediate O7 observations have been obtained, which fall at the expected phases, but these are the only modern observations of the transitions so far. The period is too long for rotation or pulsation; although there is no direct evidence as yet for a companion, a model in which tidally induced oscillations drive an enhanced wind near periastron of an eccentric orbit appears promising. Further observations during the now predictable transitions may provide a critical test. Ultraviolet and X-ray observations during both states will likely also prove illuminating. [less ▲]

Mystery Star 1054

Article for general public (2004)

It all began like any ordinary day. But suddenly, the sky changed with the appearance of a new star. Men and women raised their heads, shaking with fear. The sky should not change, they cried---this must ... [more ▼]

It all began like any ordinary day. But suddenly, the sky changed with the appearance of a new star. Men and women raised their heads, shaking with fear. The sky should not change, they cried---this must be a sign of the gods! We are doomed: this guest in our sky can only bring plague and disaster. [less ▲]

An x-ray investigation of the NGC 346 field in the small Magellanic Cloud. III. XMM-Newton data

in Astrophysical Journal (2004), 608(1, Part 1), 208-219

We present new XMM-Newton results on the field around the NGC 346 star cluster in the SMC. This continues and extends previously published work on Chandra observations of the same field. The two XMM ... [more ▼]

We present new XMM-Newton results on the field around the NGC 346 star cluster in the SMC. This continues and extends previously published work on Chandra observations of the same field. The two XMM-Newton observations were obtained, respectively, 6 months before and 6 months after the previously published Chandra data. Of the 51 X-ray sources detected with XMM-Newton, 29 were already detected with Chandra. Comparing the properties of these X-ray sources in each of our three data sets has enabled us to investigate their variability on timescales of a year. Changes in the flux levels and/or spectral properties were observed for 21 of these sources. In addition, we discovered long-term variations in the X-ray properties of the peculiar system HD 5980, a luminous blue variable star that is likely to be a colliding wind binary system, which displayed the largest luminosity during the first XMM-Newton observation. [less ▲]

WR 20a: A massive cornerstone binary system comprising two extreme early-type stars

in Astronomy and Astrophysics (2004), 420(2), 9-13

We analyse spectroscopic observations of WR20a revealing that this star is a massive early-type binary system with a most probable orbital period of similar to3.675 days. Our spectra indicate that both ... [more ▼]

We analyse spectroscopic observations of WR20a revealing that this star is a massive early-type binary system with a most probable orbital period of similar to3.675 days. Our spectra indicate that both components are most likely of WN6ha or 031f*/WN6ha spectral type. The orbital solution for a period of 3.675 days yields extremely large minimum masses of 70.7 +/- 4.0 and 68.8 +/- 3.8 M. for the two stars. These properties make WR 20a a cornerstone system for the study of massive star evolution. [less ▲]

XMM-Newton observations of the giant H II region N 11 in the LMC

in Astronomy and Astrophysics (2004), 418(3), 841-855

Using the sensitive XMM-Newton observatory, we have observed the giant H II region N11 in the LMC for similar to30 ks. We have detected several large areas of soft diffuse X-ray emission along with 37 ... [more ▼]

Using the sensitive XMM-Newton observatory, we have observed the giant H II region N11 in the LMC for similar to30 ks. We have detected several large areas of soft diffuse X-ray emission along with 37 point sources. One of the most interesting results is the possible association of a faint X-ray source with BSDL 188, a small extended object of uncertain nature, The OB associations in the field-of-view (LH9, LH10 and LH13) are all detected with XMM-Newton, but they appear very different from one another. The diffuse soft X-ray emission associated with LH9 peaks near HD 32228, a dense cluster of massive stars. The combined emission of all individual massive stars of LH9 and of the superbubble they have created is not sufficient to explain the high level of emission observed: hidden SNRs, colliding-wind binaries and the numerous pre-main sequence stars of the cluster are most likely the cause of this discrepancy. The superbubble may also be leaking some hot gas in the ISM since faint, soft emission can be observed to the south of the cluster. The X-ray emission from LH10 consists of three pointlike sources and a soft extended emission of low intensity. The two brightest point sources are clearly associated with the fastest expanding bubbles blown by hot stars in the SW part of the cluster. The total X-ray emission from LH10 is rather soft, although it presents a higher temperature than the other soft emissions of the field. The discrepancy between the combined emission of the stars and the observed luminosity is here less severe than for LH9 and could be explained in terms of hot gas filling the wind-blown bubbles. On the other hand, the case of LH13 is different: it does not harbour any extended emission and its X-ray emission could most probably be explained by the Sk -66degrees41 cluster alone. Finally, our XMM-Newton observation included simultaneous observations with the OM camera that provide us with unique UV photometry of more than 6000 sources and enable the discovery of the UV emission from the SNR N11L. [less ▲]

Probing the interactions of early-type stars with their surroundings through X-ray and optical observations

Doctoral thesis (2004)

This work is devoted to the study of the interactions of massive stars with their environment, either through their intense radiation fields or through their powerful stellar winds. First, we have studied ... [more ▼]

This work is devoted to the study of the interactions of massive stars with their environment, either through their intense radiation fields or through their powerful stellar winds. First, we have studied the close neighbourhood of the peculiar Of?p star HD108, i.e. its stellar wind and associated emissions. We have notably uncovered the important variations of the emission lines seen in the visible domain. At the same time, in contrast, the X-ray emission remained stable. To better understand the X-rays emitted by the star, we have developed a program for modelling the ionisation of the stellar wind, that enabled us to determine the absorption of this ionised wind in the X-ray domain. Our X-ray observations have then permitted us to discard several models proposed to explain the nature of the system, and we have discussed the remaining possibilities in the light of the behaviour of the two other Of?p stars, the possible ``twins" of HD108. The wind modelling program was also used to study the properties, in the X-ray range, of the Wolf-Rayet star WR40: combined to the observations taken by XMM-Newton, we have shown that the non-detection of X-rays from this star can be explained by the very large opacity of its wind. The study of the ionisation of the surrounding gas by Wolf-Rayet stars was extended through the analysis of HeII nebulae. For the first time, high quality images of the highly excited regions surrounding Wolf-Rayet stars were taken and analysed, enabling to derive the extreme UV flux of these stars, and thus their temperature, from ground-based observations. In addition to the ionisation by massive stars, the impact of their stellar winds was also considered in this work, especially the wind-blown structures called ``bubbles''. These bubbles arise from the action of the winds of massive stars, in isolation or in clusters, onto the interstellar medium. We first focused on the bubbles blown by isolated stars, and we started by the study of the high energy properties of the bubble blown by WR40. Thanks to XMM-Newton data, we showed that even the most recent theoretical models cannot account for the lack of X-ray emission of this bubble. To better understand the discrepancies between the theory and the observations of wind-blown bubbles, we then turned to the most simple structures, the ones blown by single main sequence massive stars. Several bubbles have been discovered in N11B, N180B, and N44, and their properties agree better with theoretical expectations than in the case of WR bubbles and superbubbles. Before our study, only a few interstellar bubbles were known: we showed that a simple morphological search was insufficient, and that kinematic studies were essential. In fact, the low-velocity expansion of these bubbles implies a weak compression of the gas, that can inhibit the morphological detection. Finally, we investigated the properties of the interaction of massive star clusters with their surroundings in the high-energy range. The clusters of N11 were observed in X-rays and UV thanks to the XMM-Newton satellite: their differences or similarities appear clearly at these energies, and we note that the diffuse X-ray emission always exceeded the expected level. Moreover, a unique dataset composed of Chandra and XMM-Newton observations enabled us to study the largest star formation region of the Small Magellanic Cloud, NGC346. Notably, the X-ray emissions from the NGC346 cluster itself and from the peculiar WR binary system HD5980 were discovered and analysed in details. Our work underlines the complementarity between the visible, UV and X-ray domains, and the necessity to have observatories on the ground as well as in space. It also illustrates the important contribution of multiwavelength datasets to the study of massive stars. These observations indeed play a crucial role in the determination of the properties of early-type stars but they also provide critical tests for theoretical models (present or future). [less ▲]