A CHARA Array Survey of Circumstellar Disks around Nearby Be-type Stars

Abstract: We report on a high angular resolution survey of circumstellar disks around 24 northern sky Be stars. The K-band continuum survey was made using the CHARA Array long baseline interferometer (baselines of 30 to 331 m). The interferometric visibilities were corrected for the flux contribution of stellar companions in those cases where the Be star is a member of a known binary or multiple system. For those targets with good uv coverage, we used a four parameter Gaussian elliptical disk model to fit the visibilities and to determine the axial ratio, position angle, K-band photospheric flux contribution, and angular diameter of the disk major axis. For the other targets with relatively limited uv coverage, we constrained the axial ratio, inclination angle, and or disk position angle where necessary in order to resolve the degeneracy between possible model solutions. We also made fits of the ultraviolet and infrared spectral energy distributions to estimate the stellar angular diameter and infrared flux excess of each target. The mean ratio of the disk diameter (measured in K-band emission) to stellar diameter (from SED modeling) is 4.4 among the 14 cases where we reliably resolved the disk emission, a value which is generally lower than the disk size ratio measured in the higher opacity Halpha emission line. We estimated the equatorial rotational velocity from the projected rotational velocity and disk inclination for 12 stars, and most of these stars rotate close to or at the critical rotational velocity.

• The paper presents high-resolution interferometric observations that reveal a mean disk-to-star diameter ratio of approximately 4.4 in the K-band.
• Methodology involves fitting Gaussian elliptical disk models and correcting for companion stars to accurately extract disk parameters.
• The study infers near-critical equatorial rotational velocities, supporting theories that rapid rotation is key to Be star disk formation.

An Interferometric Survey of Circumstellar Disks around Be-Type Stars Using the CHARA Array

The paper entitled "A CHARA Array Survey of Circumstellar Disks around Nearby Be-type Stars" by Touhami et al. presents the findings from an extensive high angular resolution survey targeting circumstellar disks around 24 Be-type stars located in the northern sky. Utilizing the capabilities of the CHARA Array, a long-baseline optical/infrared interferometer, the authors provide significant insights into the spatial characteristics of these disks by analyzing observations in the $K$-band.

The methodology revolves around correcting interferometric visibilities, which are key measurements in characterizing spatial distributions around stars. The authors accounted for companion stars in known binary or multiple systems among the surveyed sample, ensuring accurate readings of disk visibilities. For targets with adequate $(u,v)$ coverage, the data was fitted using a Gaussian elliptical disk model to measure parameters such as the axial ratio, position angle, and angular diameter of the major axis.

In cases where visibility readings were limited, constraints were applied to eliminate model degeneracies. The authors further enhanced their analysis by fitting ultraviolet and infrared SEDs, deducing stellar diameters and infrared flux excess, thereby capturing disk properties from multiple perspectives.

The findings suggest that the mean ratio of the disk diameter (derived from $K$-band imaging) to the stellar diameter is approximately 4.4 across 14 instances where disk emission was convincingly resolved. This result contrasts with traditionally larger ratios obtained from H$\alpha$ emission line measurements, signifying that $K$-band emissions originate from a more compact region closer to the star.

Additionally, the study inferred equatorial rotational velocities for 12 stars by integrating projected rotational velocities and disk inclinations, revealing that many of these Be stars rotate near or at critical velocities. This supports theories that rapid rotation is integral to Be star phenomena.

The implications of this research are multifaceted, enhancing our understanding of disk structures and behaviors in Be stars. The moderate disk sizes in the $K$-band relative to H$\alpha$ measurements could influence theoretical models of disk opacity and density distribution. Practically, the study underscores the necessity of correcting for companions in interferometric observations and provides a pivotal dataset for future temporal studies on disk evolution.

Future directions could involve exploring detailed disk dynamics and their interactions with stellar winds, potentially leveraging multi-wavelength interferometry and spectropolarimetry to gain deeper insight into the astrophysical processes governing Be systems. The CHARA Array, with its high-resolution capabilities, remains a crucial instrument for such endeavors, underlining the socio-scientific importance of continued support for ground-based interferometric astronomy.