Abstract
We present new high-fidelity optical coronagraphic imagery of the inner ∼50 au of
Export citation and abstract BibTeX RIS
Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
1. Introduction
The mechanism(s) driving the observed variability in
In this Letter, we present new white-light optical coronagraphic imagery of the inner region of
2. HST BAR5 Coronagraphic Imagery
Coronagraphic observations of
Observations of
These data were reduced and calibrated using the same procedures and techniques as outlined in Schneider et al. (2018). Following bias, dark, and flat-field correction using the temporally nearest calibration data, we located the position of the occulted star in each image using the "X marks the spot" diffraction spike fitting method of Schneider et al. (2014). Within each orbit, all images observed with the same dither position were then median combined and cleaned of cosmic-rays. Image co-registration, and later PSF subtractions, were done with sinc-apodized bi-cubic sub-pixel interpolation using the IDL-based, IDP3 software (Stobie & Ferro 2006). The relative brightness and (x, y) position of the PSF star HD 191849 were treated, and iteratively adjusted, as free parameters to minimize the variance in difference image pixels not dominated by disk flux.
We found that while chromatic residuals were fully mitigated with our disk-obscured BAR5 observation of
3. Analysis
3.1. HST Imaging Results
We display our resultant PSF-subtracted imagery of
Download figure:
Standard image High-resolution imageTo further explore new information encoded within these high-resolution BAR5 imagery, we applied a high-pass filter to these data (panel (d), Figure 1). We fit two one-dimensional Guassians to the loop-like structure to quantify its size and projected location. These fits reveal that the loop-like structure has a projected width of 1.5 au and rises to a projected height above the midplane of 2.3 au. The centroid of these fits also imply that the loop-like structure resides at a projected stellocentric radial distance of 14.2 au from the host star.
3.2. TESS Photometry
Download figure:
Standard image High-resolution image3.3. Starspot Modeling
Extracting robust information about the surface distribution of starspots and spot complexes from traditional spot modeling is often not possible as there exists a degeneracy between the stellar inclination and spot latitude (Walkowicz et al. 2013). One can break this degeneracy in special cases, such as transiting planetary systems (Morris et al. 2017) or systems where the evolution of multiple spots can place weak constraints on their location (Davenport et al. 2015).
Walkowicz et al. (2013) showed that the fraction of time that spots are visible in a system whose stellar inclination is seen edge-on, i.e., i = 90°, is a nearly uniform ∼55% of its rotational period, that rises to 60% near polar latitudes. Figure 3 demonstrates that the smaller amplitude spot complex in
We utilize the starspot modeling software STSP developed by L. Hebb (2019, in preparation), as described in detail within Davenport et al. (2015), to model the TESS data. This software generates synthetic light curves for a star having a pre-defined number of static spots (or spot complexes), and computes spot properties (latitude, longitude, radius) from a
We further explored the possibility of a mis-aligned B field in the system via a proxy, namely by varying the star's "rotation axis" using a grid of MCMC runs from 90° (i.e., edge-on) to 30° in steps of 15°. Finding evidence of a preferred non-edge-on orientation via this modeling (given the evidence we have that the stellar rotational axis is co-aligned with the edge-on disk) serves as a proxy for the potential B field misalignment. Full 20,000 step MCMC explorations were independently run for each inclination, and the best (lowest
The resultant best-fit synthetic spot modeling light curve is shown in Figure 4 (red), along with the underlying TESS data (blue). The most likely relative latitude/longitude distribution of the two spot complexes in
Download figure:
Standard image High-resolution image4. Discussion
4.1. Origin of AU Mic's Loop-like Disk Structure
Multi-epoch ground-based near-infrared coronagraphic imagery of
4.2. Evidence of Mis-aligned B Field and Potential Ramifications
We have shown in Section 3.3 that
We consider the potential implications of
4.3. Alternate Forms of Misalignment and Potential Ramifications
Our spot modeling (Section 3.3) and the associated interpretation of these results (Section 4.2) utilized previous research and observational properties of the system that indicated co-alignment of
4.4. Using Edge-on Disks to Break Starspot Modeling Degeneracies
The advent of high-precision, high-cadence, long-duration photometric data sets from space-based missions like Kepler and TESS provide a unique opportunity to identify and characterize stellar activity arising from spot modulation across a large sample of low mass stars. We remark that our work with
We thank our referee for providing feedback that helped to improve both the clarity and content of this manuscript. This work was supported by a grant from STScI for GO-15219. We thank Suzanne Hawley, Jamie Lomax, Peter Plavchan, and Ben Tofflemire for helpful discussions of this work.