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# On the optical properties of resonant drag instabilities: variability of asymptotic giant branch and R Coronae Borealis stars

Steinwandel, Ulrich P. and Kaurov, Alexander A. and Hopkins, Philip F. and Squire, Jonathan (2022) On the optical properties of resonant drag instabilities: variability of asymptotic giant branch and R Coronae Borealis stars. Monthly Notices of the Royal Astronomical Society, 515 (4). pp. 4797-4809. ISSN 0035-8711. doi:10.1093/mnras/stac2035. https://resolver.caltech.edu/CaltechAUTHORS:20220908-161233774

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## Abstract

In dusty cool-star outflow or ejection events around asymptotic giant branch (AGB) or R Coronae Borealis or RCB-like stars, dust is accelerated by radiation from the star and coupled to the gas via collisional drag forces. It has recently been shown that such dust-gas mixtures are unstable to a super-class of instabilities called the resonant drag instabilities (RDIs), which promote dust clustering. We therefore consider idealized simulations of the RDIs operating on a spectrum of dust grain sizes subject to radiative acceleration (allowing for different grain optical properties), coupled to the gas with a realistic drag law, including or excluding the effects of magnetic fields and charged grains, and calculate for the first time how the RDIs could contribute to observed variability. We show that the RDIs naturally produce significant variations (spatially and temporally) ($\sim 10\!-\!20{{\ \rm per\ cent}}$ 1 σ-level) in the extinction, corresponding to $\sim 0.1\!-\!1\,$mag level in the stellar types above, on time-scales of order months to a year. The fluctuations are surprisingly robust to the assumed size of the source as they are dominated by large-scale modes, which also means their spatial structure could be resolved in some nearby systems. We also quantify how this produces variations in the line-of-sight grain size-distribution. All of these variations are similar to those observed, suggesting that the RDIs may play a key role driving observed spatial and temporal variability in dust extinction within dusty outflow/ejection events around cool stars. We further propose that the measured variations in grain sizes could directly be used to identify the presence of the RDIs in close by systems with observations.

Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1093/mnras/stac2035DOIJournal Article
https://resolver.caltech.edu/CaltechAUTHORS:20220228-183258700Related ItemDiscussion Paper
ORCID:
AuthorORCID
Steinwandel, Ulrich P.0000-0001-8867-5026
Hopkins, Philip F.0000-0003-3729-1684
Squire, Jonathan0000-0001-8479-962X
Additional Information:We thank the referee for insightful comments that greatly improved that quality of this paper. UPS is supported by the Simons Foundation through a Flatiron Research Fellowship (FRF) at the Center for Computational Astrophysics. The Flatiron Institute is supported by the Simons Foundation. UPS acknowledges computing time granted by the Leibniz Rechenzentrum (LRZ) in Garching under the project number pn72bu and computing time granted by the c2pap-cluster in Garching under the project number pr27mi. This work was initiated as part of the Kavli Summer Program in Astrophysics, hosted at the Center for Computational Astrophysics (CCA) at the Flatiron Institute in New York. We thank the Kavli Foundation and the Simons Foundation, for their support. The Flatiron Institute is supported by the Simons Foundation. UPS thanks Kristina Monsch, Benjamin Moster, Joseph O’Leary, Eve Ostriker, Tyler Parsotan, Marius Ramsoy, and Darryl Seligman for their useful comments. UPS thanks Bruce Draine for sharing his insights on interstellar dust. Support for PFH was provided by an Alfred P. Sloan Research Fellowship, NSF Collaborative Research Grant 1715847 and CAREER grant 1455342, and NASA grants NNX15AT06G, JPL 1589742, 17-ATP17-0214. JS acknowledges the support of Rutherford Discovery Fellowship RDF-U001804 and Marsden Fund grant UOO1727, which are managed through the Royal Society Te Aparangi. The authors acknowledge the computing time provided by the Caltech computing facilities on the cluster ‘Wheeler’ as well as allocations from XSEDE TG-AST130039 and PRAC NSF.1713353 supported by the NSF, and NASA HEC SMD-16-7592.
Group:TAPIR
Funders:
Funding AgencyGrant Number
Simons FoundationUNSPECIFIED
Kavli FoundationUNSPECIFIED
Alfred P. Sloan FoundationUNSPECIFIED
NSF1715847
NSF1455342
NSF1713353
NASANNX15AT06G
JPL1589742
JPL17-ATP17-0214
RDFU001804
Marsden FundUOO1727
PRACNSF.1713353
HECSMD-16-7592
XSEDETG-AST130039
NASAHEC SMD-16-7592
Issue or Number:4
DOI:10.1093/mnras/stac2035
Record Number:CaltechAUTHORS:20220908-161233774
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20220908-161233774
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:116621
Collection:CaltechAUTHORS
Deposited By: Donna Wrublewski
Deposited On:07 Sep 2022 22:19