The Close Companion Mass-Ratio Distribution of Intermediate-Mass Stars

Abstract

Binary stars and higher-order multiple systems are an ubiquitous outcome of star formation, especially as the system mass increases. The companion mass-ratio distribution is a unique probe into the conditions of the collapsing cloud core and circumstellar disk(s) of the binary fragments. Inside a ∼ 1000 AU the disks from the two forming stars can interact, and additionally companions can form directly through disk fragmentation. We should therefore expect the mass-ratio distribution of close companions (a 100 AU) to differ from that of wide companions. This prediction is difficult to test using traditional methods, especially with intermediate-mass primary stars, for a variety of observational reasons. We present the results of a survey searching for companions to A-and B-type stars using the direct spectral detection method, which is sensitive to late-type companions within ∼ 1 of the primary and which has no inner working angle. We estimate the temperatures and surface gravity of most of the 341 sample stars, and derive their masses and ages. We additionally estimate the temperatures and masses of the 64 companions we find, 23 of which are new detections. We find that the mass-ratio distribution for our sample has a maximum near q ∼ 0.3. Our mass-ratio distribution has a very different form than in previous work, where it is usually well-described by a power law, and indicates that close companions to intermediate-mass stars experience significantly different accretion histories or formation mechanisms than wide companions.

Mind Your Ps and Qs: The Interrelation between Period (P) and Mass-ratio (Q) Distributions of Binary Stars

“…Given the median distance d≈100 pc to the 341 systems in their sample, the binaries have separations a200 au. In total, Gullikson et al (2016a) find 64 companions within a200au of the 341 primaries. Gullikson et al (2016a) also estimate their detection efficiencies as a function of mass ratio q (see their Figure 6).…”

“…In total, Gullikson et al (2016a) find 64 companions within a200au of the 341 primaries. Gullikson et al (2016a) also estimate their detection efficiencies as a function of mass ratio q (see their Figure 6). The completeness estimates peak at ≈90% near q≈0.3.…”

“…Gullikson et al (2016a) obtained large signal-to-noise ratio (S/N ∼ 100-1000), high-resolution (R=80,000) spectra of 341 bright (V<6 mag) A-type and B-type MS primaries with moderate to large rotation velocities v rot sin i>80 km s . By exploiting the differences in spectral absorption widths between the rapidly rotating primaries and low-mass companions (M 2 1.5 M e , T eff 6500 K) that quickly spin down to v rot sin i≈15 km s −1 , Gullikson et al (2016a) can distinguish the two components in a single-epoch spectrum. Technically, Gullikson et al (2016a) cross-correlate the observed spectrum with model spectra of cooler companions in order to measure the spectral type of the secondary.…”

“…By exploiting the differences in spectral absorption widths between the rapidly rotating primaries and low-mass companions (M 2 1.5 M e , T eff 6500 K) that quickly spin down to v rot sin i≈15 km s −1 , Gullikson et al (2016a) can distinguish the two components in a single-epoch spectrum. Technically, Gullikson et al (2016a) cross-correlate the observed spectrum with model spectra of cooler companions in order to measure the spectral type of the secondary. Although the orbital periods cannot be measured with a single-epoch spectrum, this technique is sensitive to companions with projected separations  r  2 that lie within the slit or optical fiber.…”

“…Utilizing their estimated detection efficiencies, Gullikson et al (2016a) recover the intrinsic mass-ratio distribution of A-type and B-type binaries with a200 au. They find that the mass-ratio distribution peaks at q=0.3.…”