of discs in binary star systems
This work aimed to gain insight into the typical lifetime of discs around binary stars. I did this by determining the multiplicity of disc-bearing stars in Upper Scorpius (US) and Upper Centaurus-Lupus (UCL). These regions were selected because they are significantly older than the typical protoplanetary disc lifetime determined from previous works (Haisch et al. 2001; Mamajek 2009), with ages of 11 Myr (Pecaut et al. 2012) and 17 Myr (Mamajek et al. 2002), respectively. The motivation is that if there were any systematic variation in disc lifetime around binary stars, compared
5.2 Summary of observational work: survivability of discs in binary
star systems 137
to single stars, this would be amplified at older ages. For a given population of disc- bearing stars, if the binary fraction was greater than that given by the field star multiplicity then this would suggest that discs around binary stars are longer lived. This is because the discs around single stars would have dispersed faster, increasing the binary fraction of disc-bearing stars. However, if the reverse was found, i.e. the binary fraction is much less than that of the field stars, then it would have suggested discs around binary stars are short lived.
I used infrared data from the WISE space satellite to search for IR excesses around objects in Upper-Scorpius and Upper Centaurus-Lupus. I then used the WiFeS spectrograph to search for radial velocity (RV) variation that would be caused by a binary star system. Due to the baseline of radial velocity measurements, the survey is restricted to the detection of binaries with periods up to 20 yr. The method employed to calculate radial velocities used cross-correlation with template spectra. This method also restricted the observable targets to spectral types later than F because sufficient spectral lines are needed to be able to cross-correlate with template spectra. Despite these restrictions I was able to observe 55 Upper Scorpius and 28 Upper Centaurus-Lupus members.
The median number of observations per target was four, which is not enough data to resolve a full orbit. The only targets that I could conclusively say were binary stars were the double-lined spectroscopic binaries RIK-96 and UCAC4-161328427 (see Figure 4.6). These two objects have very large IR excesses which may be indicative of these close binaries hosting large circumbinary discs. To properly determine the disc mass sub-millimetre observations would be necessary. For the other targets Bayesian statistics had to be incorporated to determine the likelihood of being a binary or a single star. From this Bayesian analysis we determined four more objects that were very likely to be binaries (UCAC4-445278523, UCAC4-447414452, UCAC4- 1253626396 and UCAC4-450968247). We also identified 5 objects with Bayes factors between 1 and 10 meaning the likelihood of being a binary is greater than being a single star, but it is not at a level where we can be certain of their binarity. From the Bayes factors obtained for each object we determined the multiplicity of disc-bearing stars in Upper Scorpius and Upper Centaurus-Lupus to be 0.06+0−0..0702 and 0.13+0−0..0603, respectively. These values are consistent with our expected binary
fraction of 0.12+0−0..0201 to within one sigma (see Figure 4.8). This expectation value was calculated from the binary fraction per spectral type given from Raghavan et al. (2010). These results suggest that overall, the lifetime of discs do not differ between single and binary star systems.
Previous works looking at multiplicity and protoplanetary discs have mostly used direct imaging and this is one of the first using radial velocity to probe a parameter space not accessible by photometry: closer binaries with periods <20 yr. Previous work, including Cieza et al. (2009); Kraus et al. (2012); Harris et al. (2012) and Cox et al. (2017), have been limited to binary separations of a few AU, maybe down to 5 AU, like Kraus et al. (2016). These surveys predominantly concerned the frequency of circumstellar discs in binary star systems, however, Harris et al. (2012) and Cox et al. (2017) do note some circumbinary discs found in their surveys. For the 1−2 Myr Taurus-Auriga star forming region, Harris et al. (2012) find a multiplicity of disc-bearing stars to be 31%−36%, while Kraus et al. (2016), looking specifically at < 40 AU binaries, find a raw multiplicity of 22%. Looking at the 2− 5 Myr (Wilking et al. 2008) star forming region ρ Ophiuchus, Cox et al. (2017) find a raw multiplicity of disc-bearing objects to also be around one third (33%). These studies conclude that most (about 2/3) of binaries disperse their discs within the first 2−3 Myr however they have not investigated the survivability of the remaining discs which is what my observational study looked at. Previous studies have consistently concluded that there is a deficit of disc-bearing binaries for separations < 40 AU, however, this does not equate to a deficit at much smaller separations or shorter orbital periods, because these studies are unable to resolve down to these separations. Identifying whether a disc deficit persists to the shortest orbital periods is important to understanding the formation of close binaries and should make up the basis of future work.
Many works have looked at characterising the multiplicity of young stars, but have often ignored the presence of discs. Kounkel et al. (2016) looked at determining the multiplicity of stars in the 1−2 Myr Orion Nebula Cloud (ONC) and 1.5−3 Myr NGC 2264 for separations up to 10 AU. They find multiplicities of 5.3±1.2% and 5.8±1.1% for ONC and NGC 2264 respectively. My study and the study of Kounkel et al. (2016) explore the parameter space of close binaries. Given the multiplicity