Differential disk evolution

Not only are mixed systems of accreting and non-accreting components abundant in all our samples as well as studies of other regions like Taurus (see e.g. Monin et al. 2007), but they also exhibit a remarkable binary separation distribution. In Fig. 5.2 the separation distributions in the four categories CC (both components accreting), WW (no accreting components), and CW and WC (mixed systems with accreting primary and secondary, respectively) are presented for the ONC, Cha I (both from this thesis), and Taurus (McCabe et al. 2006).

Apparently, mixed systems exist at binary projected separations from below 50 AU up to several hundred AU. The significant paucity of mixed pairs with separations smaller than 200 AU that was seen in ONC binaries (see Sect. 3.3.2) is not observed in binaries of Cha I and Taurus where several mixed systems with separations <200 AU are found. However, a systematic absence of wide CC pairs is seen in all three samples. While the distributions of mixed pairs span the whole range of separations up to 1000 AU, CC binaries are consistently found with separations below ∼350 AU with a KS probability of only 3% that CC pairs of the combined distributions are drawn from the same parent distribution as mixed pairs. This points to a synchronization of accretion in close systems that apparently cannot be supported in wider binaries. Possible mechanisms must either sustain accretion preferably in both components of close binaries, e.g., through simultaneous feeding from a common circumbinary envelope or disk, or initial conditions were more equal in closer binaries through a more similar distribution of, e.g., angular momentum or mass or both. 4_{Kozai migration is an interplay of the Kozai mechanism, forcing a planet around a binary component}

2 4 6 2 4 6 2 4 6 2 4

6 ONC Cha I Taurus

Figure 5.2: Binary accretion type as a function of the projected binary separation in three different

star forming regions. Blue outline: Orion Nebula Cluster (Chapter 3); Solid orange: Chamaeleon I

(Chapter4); Black hatched: Taurus-Auriga (McCabe et al. 2006). The four categories CC, WW, CW,

and WC refer to pairs of classical and weak-line T Tauri stars, and mixed systems with an accreting primary or secondary component, respectively. The sample of ONC binaries is limited to separations

&100 AU, while the other two samples reach∼25 AU.

The largest disk-like circumbinary structure that has been observed is found around UY Aur. The inner edge of the circumbinary material is about 500 AU from the center of mass of the ∼190 AU wide (deprojected semi-major axis) binary system (Close et al. 1998). The fact that the outer disk edge extends has a diameter of ∼1400 AU in near-IR scattered-light images (1200–1300 at a distance of 140 pc Close et al. 1998) and even more than twice this extent in radio observations of 13CO (Duvert et al. 1998), indicates that spatially large reservoirs can exist to possibly feed the individual circumstellar disks even of relatively wide binaries (the details of the feeding mechanism were explored in a few numerical studies, e.g. Artymowicz & Lubow 1996;G¨unther & Kley 2002). While in loose associations like Taurus accretion synchronization can in principle be due to circumbinary material like observed around UY Aur, this model is not able to explain the synchronization of disks in binaries of dense clusters like the ONC, where dynamical interactions limit the size of disks to small outer radii of mostly <400 AU (see the discussion in Sect. 3.3.2).

The data confirm the preference for mixed systems to have an accreting primary (17×CW vs. 7×WC in the combined sample of ONC, Cha I, and Taurus), as it was al- ready observed in earlier studies (Monin et al. 2007). This imbalance is likely to be real, since the majority of these systems were found in high-spatial resolution surveys of the three regions without preference for or against accretors. Accretion can be detected with

the same sensitivity around both components of a binary (see Sect. 4.3.2). The stronger truncation and/or external illumination of the secondary disk apparently leads to an, on average, shorter disk lifetime relative to the primary star disks.

If interpreted as an age sequence from very young (Taurus, ONC; ∼1 Myr) to slightly older (Cha I;∼2–3 Myr), the three distributions suggest the following course of differential disk evolution: While no WW binaries were found at the youngest ages, CC have disap- peared by the age of Cha I. This confirms the assumed general evolution from accreting to accretion-free binaries over the timespan of a few Myrs. The existence of mixed systems in all populations is consistent with either all or a fraction of all binaries evolving from a CC state through an intermediate mixed state and subsequently to WW. It cannot be excluded that a fraction of binary systems forms in a mixed state and remains in this state for a comparably long time. Though, with the background of the strong preference for CC systems in the younger populations this interpretation seems rather unlikely.

If the picture of a typical binary evolution from CC to WW is true, then Taurus is apparently in a younger (binary) evolutionary state than the ONC, since it has a higher fraction of CC binaries. A possible reason for a lower fraction of CC binaries in the ONC than Taurus may be stellar encounters. Simulations for the ONC and its highest density region, the Trapezium cluster, were able to show that dynamical interactions can reduce disk frequency by 5–20% over the lifetime of the ONC of ∼1 Myr (Olczak et al. 2006). The relative dynamical youth of Taurus is also suggested by its higher binary frequency. Assuming that intially all stars are born as binaries, Marks & Kroupa (2012) show that the stellar density at a cluster’s birth is the most important parameter to determine the binary frequency observed at a later age, which simultaneously explains the low binary frequency of the dense ONC and the more than twice as high binary frequency of the loose Taurus association.