Stellar multiplicity in the Milky Way Galaxy
We present our models of the effect of binaries on high-resolution spectroscopic surveys. We want to determine how many binary stars will be observed, whether unresolved binaries will contaminate measurements of chemical abundances, and how we can use spectroscopic surveys to better constrain the population of binary stars in the Galaxy. Using a rapid binary-evolution algorithm that enables modelling of the most complex binary systems we generate a series of large binary populations in the Galactic disc and evaluate the results. As a first application we use our model to study the binary fraction in APOGEE giants. We find tentative evidence for a change in binary fraction with metallicity.
Stellar multiplicity in the Milky Way Galaxy] Stellar multiplicity in the Milky Way Galaxy E. Stonkutė et al.] E. Stonkutė, R. P. Church, S. Feltzing and J. A. Johnson 2017 \volume334 \jnameRediscovering our Galaxy \editorsC. Chiappini, I. Minchev, E. Starkenburg, & M. Valentini, eds.
Stellar multiplicity leads to many interesting astrophysical phenomena, for example, gravitational wave sources, gamma-ray bursts, and type Ia supernovae. It is known that 34% 2% of solar-type (F6–K3) stars in the Solar Neighbourhood are in binaries (Raghavan et al., 2010), but the frequency of binary stars outside the Solar Neighbourhood is uncertain. For ongoing and up-coming large spectroscopic surveys, such as RAVE (Kunder et al., 2017), APOGEE (Majewski et al., 2015), Gaia-ESO (Gilmore et al., 2012), GALAH (De Silva et al., 2015), LAMOST (Deng et al., 2012) or 4MOST (de Jong et al., 2016) it is important to identify as well as quantify the binaries to clean the survey products from potentially faulty results.
2 The effect of binaries on high-resolution spectroscopic surveys
As an application we make mock APOGEE observations of red giants and subgiants. Our stars selection function mimics the selection function of APOGEE. We selected Galactic disc stars from APOGEE DR 12 where Galactic longitude: 24 240 and latitude: 16; the signal-to-noise ratio of individual spectra 20; the effective temperature: 3500 Teff 5500, [K] and log(g) 4.0, [cgs]; and stars that have been visited more than two times: Nvists 2.
Binary and single star evolution is performed by the rapid binary-star evolution (bse) algorithm (Hurley et al., 2002). We assume that the initially more massive stars in the binary have masses between . We generate from the initial mass function of Kroupa et al. (1993). The mass of the companion is between and is drawn assuming the mass ratio distribution is flat in , where 1. Our next assumption is that our binary stars in the Galactic disc are in three metallicity bins [Fe/H]=) and have broad uniform age distribution from 0 to 10 Gyr. The distribution of binary periods, , is log-normal with = 4.8 and = 2.3, here the orbital period is in days. The distribution of the orbital eccentricity () is chosen to be dynamically relaxed (thermal).
In Fig. 1 the cumulative functions suggest that the model fits the observations well, which is consistent with most of the stars with high velocity scatter being binaries, and with the binary population being the same elsewhere in the Galaxy. The estimated initial binary fraction () for [Fe/H] = 0.0 is 36% which is consistent with Raghavan et al. (2010) for solar-type stars. There is evidence for decreasing with increasing [Fe/H] consistent with other studies (e.g. Yuan et al., 2015).
We intend to investigate to what extent we can constrain the frequency of binaries, and whether we can detect any systematic variation with metallicity in the Galaxy. The detailed results will be presented in Stonkutė, Church & Feltzing 2017 (in prep.).
Acknowledgments. E.S., R.C. and S.F were supported by the project grant “The New Milky Way” from Knut and Alice Wallenberg Foundation.
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