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Hipparcos, Gaia, and RVs reveal that the radio emitting F star HD 220242 has an M dwarf companion, a likely source of the radio emission

Megan Delamer, Guðmundur Stefánsson, Suvrath Mahadevan, Caleb I. Cañas, Harish K. Vedantham, Arvind F. Gupta, Joseph R. Callingham, Juan Bautista Climent Oliver, William Cochran, Rachel B. Fernandes, Evan Fitzmaurice, Jose Carlos Guirado, Michael Hartmann, Artie P. Hatzes, Elise Koo, Jessica E. Libby-Roberts, Joe P. Ninan, Miguel Pérez-Torres, Paul Robertson, Arpita Roy, Christian Schwab

Abstract

The detection of circularly polarized, low frequency radio emission offers the tantalizing possibility of the observation of interactions between stars and their possible substellar companions, as well as direct emission from exoplanets. Additional follow up of systems with radio emission is key to understanding the true origin of the emission, since multiple astrophysical mechanisms can plausibly lead to such signals. While nineteen M dwarfs were detected by LOFAR in circular polarization as part of the V-LoTSS survey, HD~220242 is the only F star to have a circularly polarized low frequency radio detection in the same survey. We conducted radial velocity follow up with the Habitable-zone Planet Finder and combined these observations with additional archival RVs and \textit{Hipparcos}-\textit{Gaia} proper motion accelerations to determine that HD~220242 has a stellar companion with P=16.79$\pm$0.04\,yrs and a mass of $0.619\pm0.014$\,M$_\odot$. We use Spectral Energy Distribution fitting and lack of any UV excess to rule out a co-evolved white dwarf companion and confirm that the companion is an M dwarf star. Given that F stars lack the coronal properties to produce such coherent emission, and the companion mass and lack of UV excess are consistent with an M dwarf, the radio emission is most plausibly associated with the companion.

Hipparcos, Gaia, and RVs reveal that the radio emitting F star HD 220242 has an M dwarf companion, a likely source of the radio emission

Abstract

The detection of circularly polarized, low frequency radio emission offers the tantalizing possibility of the observation of interactions between stars and their possible substellar companions, as well as direct emission from exoplanets. Additional follow up of systems with radio emission is key to understanding the true origin of the emission, since multiple astrophysical mechanisms can plausibly lead to such signals. While nineteen M dwarfs were detected by LOFAR in circular polarization as part of the V-LoTSS survey, HD~220242 is the only F star to have a circularly polarized low frequency radio detection in the same survey. We conducted radial velocity follow up with the Habitable-zone Planet Finder and combined these observations with additional archival RVs and \textit{Hipparcos}-\textit{Gaia} proper motion accelerations to determine that HD~220242 has a stellar companion with P=16.790.04\,yrs and a mass of \,M. We use Spectral Energy Distribution fitting and lack of any UV excess to rule out a co-evolved white dwarf companion and confirm that the companion is an M dwarf star. Given that F stars lack the coronal properties to produce such coherent emission, and the companion mass and lack of UV excess are consistent with an M dwarf, the radio emission is most plausibly associated with the companion.

Paper Structure

This paper contains 6 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Observations
  3. Spectroscopic Data
  4. High-Contrast Imaging
  5. Analysis
  6. SED Fitting

Figures (2)

  • Figure 1: The 5$\sigma$ contrast curves for HD 220242 obtained from speckle imaging with NESSI in the 562 and 832 nm filters. The insets are the NESSI speckle images centered on HD 220242A in each filter. The grey dashed line indicates the location of the companion at the time of the observations, interior to where limits can be placed. However, we find no evidence of any other bright companions.
  • Figure 2: The available HD 220242 photometry (aqua and green circles) plotted alongside the synthetic photometry derived from convolving the best-fitting atmospheric model with the bandpass of each instrument, which are plotted as purple diamonds. Photometric points in green were not used in the fit, as ARIADNE uses photometric points redder than W2 solely to visualize potential IR excess; they are included only for completeness. The BTSettl-AGSS2009 model (grey line) with characteristics most similar to those found in the ARIADNE fit is included to guide the eye, but is not expected to be identical to the photometry. Below: The residuals between the synthetic and observed photometry.