[Paper Review] Multitechnique testing of the viscous decretion disk model I. The stable and tenuous disk of the late-type Be star $\beta$ CMi

Robert Klement, A. C. Carciofi+11 more|arXiv (Cornell University)|Oct 5, 2015

Astrophysics and Star Formation Studies60 references17 citations

TL;DR

This study tests the viscous decretion disk (VDD) model on the late-type Be star β CMi using the largest multitechnique, multiwavelength dataset to date. It finds that the inner disk requires a steeper density falloff (n = 3.5), while the outer disk favors a shallower profile (n = 3.0), indicating possible outer truncation by an unseen binary companion, with radio observations constraining the disk extent at 35+10−5 stellar radii and polarimetry indicating a near-critical rotation rate (W ≳ 0.98).

ABSTRACT

The viscous decretion disk (VDD) model is able to explain most of the currently observable properties of the circumstellar disks of Be stars. However, more stringent tests, focusing on reproducing multitechnique observations of individual targets via physical modeling, are needed to study the predictions of the VDD model under specific circumstances. In the case of nearby, bright Be star $\beta$ CMi, these circumstances are a very stable low-density disk and a late-type (B8Ve) central star. The aim is to test the VDD model thoroughly, exploiting the full diagnostic potential of individual types of observations, in particular, to constrain the poorly known structure of the outer disk if possible, and to test truncation effects caused by a possible binary companion using radio observations. We use the Monte Carlo radiative transfer code HDUST to produce model observables, which we compare with a very large set of multitechnique and multiwavelength observations that include ultraviolet and optical spectra, photometry covering the interval between optical and radio wavelengths, optical polarimetry, and optical and near-IR (spectro)interferometry. Due to the absence of large scale variability, data from different epochs can be combined into a single dataset. A parametric VDD model with radial density exponent of $n$ = 3.5, which is the canonical value for isothermal flaring disks, is found to explain observables typically formed in the inner disk, while observables originating in the more extended parts favor a shallower, $n$ = 3.0, density falloff. Modeling of radio observations allowed for the first determination of the physical extent of a Be disk (35$^{+10}_{-5}$ stellar radii), which might be caused by a binary companion. Finally, polarization data allowed for an indirect measurement of the rotation rate of the star, which was found to be $W \gtrsim 0.98$, i.e., very close to critical.

Motivation & Objective

• To rigorously test the viscous decretion disk (VDD) model on a single, well-studied Be star using diverse observational techniques.
• To constrain the radial density profile of the circumstellar disk, especially in its outer regions where data are sparse.
• To investigate potential disk truncation mechanisms, particularly due to a binary companion.
• To determine the rotation rate of the central star using polarimetric diagnostics.
• To assess the physical extent of the Be disk using radio continuum observations for the first time.

Proposed method

• Utilized the HDUST Monte Carlo radiative transfer code to simulate observables under the VDD model.
• Fitted the model to a comprehensive dataset spanning UV to radio wavelengths, including SED, spectroscopy, polarimetry, and interferometry.
• Employed multiwavelength interferometry (optical, near-IR, mid-IR) to probe disk geometry and kinematics.
• Analyzed radio continuum data to estimate the outer disk radius and test for truncation.
• Used Paschen continuum polarimetry to infer the stellar rotation rate (W).
• Compared model predictions with observations across multiple techniques to break degeneracies in disk structure.

Experimental results

Research questions

• RQ1 Does the VDD model accurately reproduce the full multiwavelength and multitechnique dataset of β CMi?
• RQ2 What is the radial density profile of the disk, and does it vary between inner and outer regions?
• RQ3 Is the outer disk truncated, and if so, what mechanism—such as a binary companion—could be responsible?
• RQ4 What is the rotation rate of the central B8Ve star, and can it be constrained independently via polarimetry?
• RQ5 What is the physical extent of the circumstellar disk, and can radio observations provide a direct measurement?

Key findings

• The inner disk requires a radial density exponent of n = 3.5, consistent with isothermal flaring disks, while the outer disk favors a shallower profile with n = 3.0.
• The discrepancy in required density profiles cannot be explained by nonisothermal effects, suggesting a physical cause such as tidal truncation.
• Radio observations constrain the disk outer radius at 35+10−5 stellar radii, indicating a physical truncation scale.
• The observed positive polarimetric slope in the Paschen continuum requires a stellar rotation rate W ≳ 0.98, indicating near-critical rotation.
• The inclination angle was precisely estimated at i = 43+3°−2° using AMBER spectrointerferometry.
• The combination of spectroscopic, photometric, and interferometric data supports a binary companion as the most likely cause of disk truncation, with a late-type or subdwarf companion favored.