Toward Complete Statistics of Massive Binary Stars: Penultimate Results from the Cygnus OB2 Radial Velocity Survey (1406.6655v1)

Abstract: We analyze orbital solutions for 48 massive multiple-star systems in the Cygnus OB2 Association, 23 of which are newly presented here, to find that the observed distribution of orbital periods is approximately uniform in log P for P<45 d, but it is not scale-free. Inflections in the cumulative distribution near 6 d, 14, d, and 45 d, suggest key physical scales of ~0.2, ~0.4, and ~1 A.U. where yet-to-be-identified phenomena create distinct features. No single power law provides a statistically compelling prescription, but if features are ignored, a power law with exponent beta = -0.22 provides a crude approximation over P=1.4 -- 2000 d, as does a piece-wise linear function with a break near 45 d. The cumulative period distribution flattens at P > 45 d, even after correction for completeness, indicating either a lower binary fraction or a shift toward low-mass companions. A high degree of similarity (91% likelihood) between the Cyg OB2 period distribution and that of other surveys suggests that the binary properties at P<25 d are determined by local physics of disk/clump fragmentation and are relatively insensitive to environmental and evolutionary factors. Fully 30% of the unbiased parent sample is a binary with period P < 45 d. Completeness corrections imply a binary fraction near 55% for P < 5000 d. The observed distribution of mass ratios 0.2<q<1 is consistent with uniform, while the observed distribution of eccentricities 0.1<e<0.6 is consistent with uniform plus an excess of e ~ 0 systems. We identify six stars, all supergiants, that exhibit aperiodic velocity variations of ~30 km/s attributed to atmospheric fluctuations.

Analysis of Massive Binary Stars in the Cygnus OB2 Association: Insights from Radial Velocity Data

This research paper explores the comprehensive analysis of massive binary star systems within the Cygnus OB2 Association, presenting orbital solutions for 48 systems, including 23 newly identified binaries. The paper contributes valuable insights into the distribution of orbital periods, eccentricities, and mass ratios, which are crucial for understanding the formation and evolution of massive stars.

Key Findings and Analysis

Orbital Period Distribution: The period distribution is observed to be approximately uniform in $\log~P$ for $P<$45 days, but notable deviations are identified. Inflections in the cumulative distribution at phases corresponding to $\simeq$0.2, $\simeq$0.4, and $\simeq$1 A.U. highlight potential physical phenomena that influence the binary system formation. The uniform distribution breaks down beyond 45 days, suggesting either a reduced binary fraction or a shift toward low-mass companions.

Eccentricity and Mass Ratio: The eccentricity distribution is approximately uniform for $0.1 < e < 0.6$, with observed systems showing an excess around $e\simeq0$. The mass ratio distribution for $0.2 < q < 1$ is consistent with uniformity, highlighting a regularity in mass distribution across these binary systems.

Implications for Stellar Physics: A high degree of similarity (91% likelihood) is found between the Cygnus OB2 period distribution and that of other surveys, indicating that short-period binary properties are strongly determined by local physics, such as disk fragmentation, rather than external environmental and evolutionary influences. This reinforces the notion that massive star formation processes are relatively homogeneous across different regions of the galaxy.

Binary Fraction Completion: After completeness corrections, a binary fraction near 55% is derived for $P<$5000 days, suggesting a robust occurrence of binaries among massive stars. This ratio may escalate to around 70% when extrapolated to much longer periods, up to $10^5$ days.

Long Period Systems: The survey identifies several long-period systems, such as MT91~234 and MT91~485, which challenge detection due to limited observational data and high eccentricities. These systems provide crucial insights into the configuration dynamics of such expansive stellar arrangements.

Practical and Theoretical Implications

The findings present significant implications for understanding the dynamics of massive star evolution and supernova progenitors. The catalog of binaries provides a foundational dataset for modeling stellar evolution scenarios, including the potential development of Type Ibc supernovae through common envelope phases and the production of gravitational waves by compact object binaries. Moreover, the information on period distribution and mass ratios will aid in refining models of massive star formation that account for binary interactions and resultant stellar configurations.

Future Directions in Astrophysical Research

The paper bolsters this association's binary census, yet the anticipation of discovering further binary systems persists, especially those with longer periods or higher eccentricities that have eluded detection. Future work will likely focus on improved observational techniques to capture more data on these systems, facilitating a more comprehensive understanding of massive binary formation. Moreover, integrating multi-method approaches, such as high-resolution imaging and spectropolarimetric data, will enhance insights into the binary interaction processes and atmospheric dynamics of massive stars.

In conclusion, the Cygnus OB2 Radial Velocity Survey forms a significant milestone in advancing the empirical basis for stellar modeling, offering crucial data for ongoing and future investigations into the complexities of massive star systems.