Abstract
The study of the multiplicity of massive stars gives hints of their formation processes and their evolution path. Optical interferometry is mandatory to fulfill our knowledge of their multiplicity by probing the separation gap between 1 and 50 mas. We demonstrated the capability of the new interferometric instrument MIRC-X, located at the CHARA array, to study a large sample of more than 120 (H < 7.5) O-type stars. We observed 29 O-type star systems, including a couple of systems in average atmospheric conditions around a magnitude of H = 7.5. Out of these 29 systems, we detected 18 companions in 16 different systems, resulting in a multiplicity fraction fm = 16 / 29 = 0.55, and a companion fraction of fc = 18 / 29 = 0.62. We observed for the first time 11 of these detected companions. This study concludes that a large survey on more than 120 Northern O-type stars is possible with MIRC-X.
Keywords: (stars:) binaries: general, stars: formation, stars: statistics, infrared: stars
1. Introduction
Massive stars are key components of the evolution of their host galaxy. They are the main producers of heavy elements, and the momentum and kinetic energy involved in their death have an influence on a large part of their galaxy (Zinnecker and Yorke, 2007). They are also at the origin of the compact objects that produce the gravitational waves (GW) that we can currently detect (Abbott et al., 2016).
However, their short lifetime (a few million years) and their rapid formation process (105 years) make the observation of their early ages difficult. Indeed, their lifetime makes them rare, and so to observe a large number of massive stars (> 100), one needs to look for them at significant distances, typically 1 to 3 kpc. In consequence, the formation process of massive stars is still actively discussed. The outcomes of the different formation models predict different multiplicity parameters. Hence, the study of the multiplicity of massive stars, after their formation process, should provide relevant constraints on these formation models.
To probe the full range of orbital separations of systems situated at a typical distance of 2 kpc, one needs to use different observational techniques. For separations between 1 and 50 mas, the only technique we can use is Optical Long Baseline Interferometry (OLBI). But until recently, this technique was limited by its sensitivity and could only be applied to a modest sample of massive stars.
In this proceeding, we present the results of the pilot survey performed on 29 systems, with the goal to demonstrate the feasibility of a large survey of more than 100 O-type stars with the CHARA/MIRC-X instrument (Kraus et al., 2018), which requires a limiting magnitude of H =7.5.
Figure 1.
Left: Histogram of the H-band magnitude of our observed sample in green, the original sample planned for this pilot survey in orange and the planned sample for the large survey in blue. Right: Histogram of detected companions as a function of the estimated mass ratio q, when taking into account only the first detected companions (blue), and when adding the second companions detected (orange).
2. Observations
We built our sample using the Galactic O-star Catalog (GOSC Maíz Apellániz et al., 2013). We selected every O-star observable by CHARA and within the limiting magnitude of MIRC-X, which is H = 6.5 at the time of this pilot study. Our input sample is therefore magnitude limited. During our observation campaign, we could obtain good quality data on 29 O-type star systems, with the histogram of their magnitude displayed in Figure 1 (left). We can see that we observed a couple of systems with magnitude around H = 7.5, in normal seeing conditions, demonstrating the feasibility of observing a large sample of more than 100 O-type stars.
3. Statistics on detected companions
Among the 29 systems observed with good quality data, we confirm the detection of 18 companions for 16 multiple systems. Out of these 18 companions, 11 are detected for the first time. This gives us a multiplicity fraction fm = 16/29 = 0.55 ± 0.09, and a companion fraction of fc = 18/29 = 0.62 ± 0.13.
Figure 1 (right) displays the distribution of the estimated mass ratio of the detected companions. In this figure, we can see that the distribution seems to be bi-modal, with a lack of companions between q = 0.4 and 0.6, and favoring a lower mass ratio. However, our sample is too small to perform any statistical study accurately enough to conclude on massive star formation models.
4. Conclusion
This pilot survey shows that a large survey of more than 100 O-type stars is possible with CHARA/MIRC-X, and our sample is too small to perform an accurate statistical study, showing that the large survey is required. More detail on this pilot study and the implication for further study can be found in Lanthermann et al. 2022 (in preparation).
