Context. Massive stars like company. However, low-mass companions have remained extremely difficult to detect at angular separations (ρ) smaller than 1″ (approx. 1000–3000 au, considering the typical distance to nearby massive stars) given the large brightness contrast between the companion and the central star. Constraints on the low-mass end of the companions mass-function for massive stars are needed, however, for helping, for example, to distinguish among the various scenarios that describe the formation of massive stars. Aims. With the aim of obtaining a statistically significant constraint on the presence of low-mass companions beyond the typical detection limit of current surveys (Δmag ≲ 5 at ρ ≲ 1″), we initiated a survey of O and Wolf-Rayet stars in the Carina region using the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) coronagraphic instrument on the Very Large Telescope (VLT). In this, the first paper of the series, we aim to introduce the survey, to present the methodology and to demonstrate the capability of SPHERE for massive stars using the multiple system QZ Car. Methods. We obtained VLT-SPHERE snapshot observations in the IRDIFS_EXT mode, which combines the IFS and IRDIS sub-systems and simultaneously provides us four-dimensional (4D) data cubes in two different fields-of-view: 1.73″ × 1.73″ for IFS (39 spectral channels across the YJH bands) and 12″ × 12″ for IRDIS (two spectral channels across the K band). Angular- and spectral-differential imaging techniques as well as PSF-fitting were applied to detect and measure the relative flux of the companions in each spectral channel. The latter were then flux-calibrated using theoretical SED models of the central object and compared to a grid of ATLAS9 atmosphere model and (pre-)main-sequence evolutionary tracks, providing a first estimate of the physical properties of the detected companions. Results. Detection limits of 9 mag at ρ > 200 mas for IFS, and as faint as 13 mag at ρ > 1.″8 for IRDIS (corresponding to sub-solar masses for potential companions), can be reached in snapshot observations of only a few minutes integration times, allowing us to detect 19 sources around the QZ Car system. All but two are reported here for the first time. With near-IR magnitude contrasts in the range of 4 to 7.5 mag, the three brightest sources (Ab, Ad, and E) are most likely to be physically bound. They have masses in the range of 2 to 12 M⊙ and are potentially co-eval with QZ Car central system. The remaining sources have flux contrast of 1.5 × 105 to 9.5 × 106 (ΔK ≈ 11 to 13 mag). Their presence can be explained by the local source density and they are, thus, likely to be chance alignments. If they were members of the Carina nebula, they would be sub-solar-mass pre-main sequence stars. Conclusions. Based on this proof of concept, we show that the VLT/SPHERE allows us to reach the sub-solar mass regime of the companion mass function. It paves the way for this type of observation with a large sample of massive stars to provide novel constraints on the multiplicity of massive stars in a region of the parameter space that has remained inaccessible so far.
Direct imaging of exoplanets and circumstellar disks at optical and infrared wavelengths requires reaching high contrasts at short angular separations. This can only be achieved through the synergy of advanced instrumentation, such as adaptive optics and coronagraphy, with a relevant combination of observing strategy and post-processing algorithms to model and subtract residual starlight. In this context, VIP is a Python package providing the tools to reduce, post-process and analyze high-contrast imaging datasets, enabling the detection and characterization of directly imaged exoplanets, circumstellar disks, and stellar environments.
Context. Spectroscopic multiplicity surveys of O stars in young clusters and OB associations have revealed that a large portion (∼70%) of these massive stars (Mi > 15 M⊙) belong to close and short-period binaries (with a physical separation of less than a few astronomical units). Follow-up VLT(I) high-angular-resolution observations led to the detection of wider companions (up to d ∼ 500 au), increasing the average companion fraction to > 2. Despite the recent and significant progress, the formation mechanisms leading to such close massive multiple systems remain to be elucidated. As a result, young massive close binaries (or higher-order multiple systems) are unique laboratories for determining the pairing mechanism of high-mass stars. Aims. We present the first VLTI/GRAVITY observations of six young O stars in the M17 star-forming region (≲1 Myr) and two additional foreground stars. VLTI/GRAVITY provides the K-band high-angular-resolution observations needed to explore the close environment of young O-type stars, and, as such, offers an excellent opportunity to characterise the multiplicity properties of the immediate outcome of the massive star formation process. Methods. From the interferometric model fitting of visibility amplitudes and closure phases, we search for companions and measure their positions and flux ratios. Combining the resulting magnitude difference with atmosphere models and evolutionary tracks, we further constrain the masses of the individual components. Results. All six high-mass stars are in multiple systems, leading to a multiplicity fraction of 100% and yielding a 68% confidence interval of 94–100%. We detect a total of nine companions with separations of up to 120 au. Including previously identified spectroscopic companions, the companion fraction of the young O stars in our sample reaches 2.3 ± 0.6. The derived masses span a wide range, from 2.5 to 50 M⊙, with a great tendency towards high-mass companions. However, we do not find a significant correlation between the mass of the companions and their separation. Conclusions. While based on a modest sample, our results clearly indicate that the origin of the high degree of multiplicity is rooted in the star formation mechanism of the sample stars. No clear evidence for one of the competing concepts of massive star formation (core accretion or competitive accretion) could be found. However, given that we find all of the companions within ∼120 au, our results are compatible with migration as a scenario for the formation of close massive binaries.
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