# ROME III. The Arecibo Search for Auroral Emissions and Star-Planet
Interactions at 5 GHz

Matthew Route Department of Astronomy and Astrophysics, the Pennsylvania State
University, 525 Davey Laboratory, University Park, PA 16802  Center for
Exoplanets and Habitable Worlds, the Pennsylvania State University, 525 Davey
Laboratory, University Park, PA 16802  Northrop Grumman Electronic Systems,
6120 Longbow Drive, Boulder, CO 80301  Research Computing, Purdue University,
155 S. Grant St., West Lafayette, IN 47907, USA  Alex Wolszczan Department of
Astronomy and Astrophysics, the Pennsylvania State University, 525 Davey
Laboratory, University Park, PA 16802  Center for Exoplanets and Habitable
Worlds, the Pennsylvania State University, 525 Davey Laboratory, University
Park, PA 16802 Matthew Route<EMAIL_ADDRESS>
###### Abstract

Despite many attempts to detect radio emissions from magnetized exoplanets, a
reproducible, unambiguous detection remains elusive. On the other hand, the
search for periodic radio emissions from exoplanet host stars that may be
induced by star-planet interactions in, for example, hot Jupiter systems, has
only recently begun. In this third paper of the ROME (Radio Observations of
Magnetized Exoplanets) series, we present the results of a targeted radio
survey of 17 nearby systems that host exoplanet, brown dwarf, and low-mass
stellar companions conducted with the Arecibo radio telescope at $\sim$5 GHz.
This GHz-frequency survey has the greatest frequency coverage of any to date,
while providing mJy-level sensitivity over $<$1 s integration times. Neither
auroral radio emissions from the substellar targets, nor exoplanet-induced
stellar radio bursts were detected. These results are considered within the
context of observed brown dwarf radio emissions, which support magnetospheric
phenomenon similar to that found at Jupiter. We also describe the orbital
phase coverage of our data for systems that may feature star-planet
interactions, and briefly examine our results within the context of other
searches for star-planet interactions within the same systems. Finally, as
several of our targets are within their respective systems’ habitable zones,
we consider the implications of our survey on the search for technosignatures
within those systems.

Star-planet interactions; Magnetospheric radio emissions; Exoplanets;
Planetary magnetospheres; Stellar activity; Non-thermal radiation sources

††software: IDL, MATLAB

##  1 Introduction

While searching for radio emissions from the Crab Nebula (M1), Burke &
Franklin (1955) serendipitously discovered radio emission from Jupiter. These
emissions occurred at frequencies $\leq$38 MHz, and were found to be highly
elliptically polarized (Barrow & Morrow, 1968). Several decades of both remote
and _in situ_ observations of the Jovian magnetosphere have revealed that
these early radio signatures were just one component of a legion of radio
emissions caused by several processes and emission mechanisms. From 74 MHz to
$\gtrsim$20 GHz, Jupiter emits decimetric (DIM) synchrotron radiation from its
magnetic poles and magnetic equator. These emissions are $\sim$20% linearly
polarized with circular polarization fractions that vary $\pm$0.5% with
rotational phase. The first radio signals detected from Jupiter were
components of the decametric (DAM) emission, which ranges from 4 to 40 MHz.
These emissions are $\sim$100% circularly polarized with helicity and cutoff
frequency of emission dependent upon which magnetic pole is observed. Some of
the DAM activity is modulated by Io’s and Ganymede’s orbits. All DAM
emissions, independent of source, are generated by the electron cyclotron
maser (ECM) acting in the polar regions of the planet. The ECM also causes
hectometric (HOM) emission, which is observed from $\sim$400 kHz to $\sim$4
MHz, and also is strongly circularly polarized. From $\sim$20 kHz to 400 kHz,
sporadic broadband (bKOM) and narrowband (nKOM) kilometric radiation are
observed, which are generally linked to the propagation of ECM emission
through the Io plasma torus. Within the 3.5 to 23 kHz frequency window,
quasiperiodic emissions have been detected that appear as enhancements of
continuum radio emissions. These continuum emissions span $\sim$100 Hz to 25
kHz and are emitted in the ordinary mode. Among these components, the most
powerful radio emissions are the DAM, HOM, and rapid S-bursts caused by
coherent radiation processes; they are $\sim$105 more luminous than the
weakest emission, incoherent DIM (Zarka et al., 2004; Treumann, 2006; de Pater
& Lissauer, 2015). It is therefore prudent that any search for auroral radio
emissions from exoplanets should focus on detecting the signatures of ECM
emission processes within their magnetospheres.

Beyond the Solar System, Yantis et al. (1977) sought to detect the first
extrasolar planets via their Jovian-like radio emissions. Although their
survey of 22 star systems failed to detect any candidates, it nevertheless
served as a template for numerous subsequent survey efforts at radio
frequencies (for a review see, for example, Lazio (2018)). These efforts
sought to directly detect auroral radio emissions from the exoplanets
following the examples of Jupiter and the other magnetized planets in the
Solar System (e.g., Zarka (1998)).

In order to identify the most promising targets for surveys, numerous papers
sought to estimate the radio flux density and frequency of exoplanetary radio
emissions (e.g., Lazio et al. (2004); Griessmeier et al. (2007); Ignace et al.
(2010); Reiners & Christensen (2010); Nichols (2012); Zaghoo & Collins
(2018)). Important ingredients in estimating the radio flux density are
properties of the stellar wind (density, velocity, and interplanetary magnetic
field), while exoplanet age, mass, and rotation rate are important
determinants of the magnetic field strength (e.g., Christensen (2010)). The
type of interaction that occurs between host stars and exoplanets are
determined by whether the planet is strongly magnetized, or
weakly/unmagnetized, leading to kinetic, magnetic, and unipolar interactions
(e.g., Zarka (2007)). The first two types lead to the stellar-wind induced
auroral emissions at Jupiter, while the last is exemplified by the Jupiter-Io
system. Within the star-planet interaction context, it is possible that
exoplanets orbiting close to their host stars, also known as hot Jupiters, may
induce increased activity in their chromospheres, transition regions, and
coronae via unipolar induction.

Cuntz et al. (2000) hypothesized that tidal and magnetic interactions between
host stars and exoplanets with semimajor axes $d\lesssim 0.1$ AU would
increase stellar activity in several ways. First, tidal bulges would cause
expansion and contraction of the outer layers of the stellar atmosphere,
increasing turbulent and flow velocities, and thereby increasing the energy
available to generate acoustic and magnetic waves. In addition, increased
turbulence may enhance dynamo activity within the convective zone, such as the
subsurface $\alpha$-effect or turbulent dynamos. It is also possible that
$\alpha$-enhancement may be transmitted to the tachocline at the bottom of the
convective zone, leading to an overall enhancement of stellar activity
throughout the stellar atmosphere. Independent of tidal effects, the star-
planet interaction between exoplanet and host star may be magnetic, featuring
stellar active regions that interact with exoplanet magnetospheres, enhancing
processes such as coronal magnetic reconnection. Magnetic and tidal effects
should be distinguishable by the presence of either one or two enhancements in
activity per stellar rotation period, respectively. Note that both tidal and
magnetic effects should be most pronounced in the corona and transition
region, and to a lesser extent the chromosphere, due to their comparatively
lower densities and proximity to the interacting substellar companion (Saar &
Cuntz, 2001).

Some observational support for this scenario comes from a chromospheric
enhancement monitoring campaign conducted by Shkolnik et al. (2005). Several
data sets of Ca II K residuals collected over two years from HD 179949 and
$\upsilon$ And may show enhanced activity at the $\sim$1% level, phased with
the orbital period of the closest orbiting planet. However, the degree of
modulation varies from epoch to epoch, with 25% of the epochs showing
modulation of $\lesssim$0.005%. In addition, while planetary orbital phases of
$\phi_{orb}$=0 and potentially $\phi_{orb}$=0.5 might be expected to produce
enhanced activity in the above paradigm, Shkolnik et al. (2008) suggested that
a more complicated energy transport model would be required to explain the
enhanced activity found at $\phi_{orb}=$0.8 to 0.95 in the case of HD 179949
and $\phi_{orb}=$0.53 for $\upsilon$ And. These works suggested that several
additional exoplanets may interact with their host stars, such as HD 189733b.
On the other hand, other research programs have failed to find a statistically
significant correlation between exoplanets with small semimajor axes and
chromospheric activity enhancement (e.g., Canto Martins et al. (2011)).

Rubenstein & Schaefer (2000) hypothesized that as-yet undetected planetary
companions could cause the emission of superflares ($E\sim$1033-1038 ergs)
from F and G main-sequence stars. They argued that the properties of flares
emitted from $\kappa$ Ceti and $\pi^{1}$ UMa were similar to those emitted by
RS Canum Venaticorum (RS CVn) stars. They suggested that open field lines from
the host star were anchored at the exoplanetary companion. The magnetic field
strength would be amplified by a wrapping of magnetic field lines as the
exoplanet orbits the star, and subsequent interactions of magnetic loops
between the star and planet would result in magnetic reconnection events that
would trigger superflares. They further hypothesized that these flares would
be luminous at X-ray to radio wavelengths, in analogy to RS CVn and solar
flares.

Therefore, a useful lens through which to search for and observe stellar
activity is at radio wavelengths, which provide powerful diagnostics of
coronal flaring and shock processes that may be induced or enhanced by star-
planet interactions. In addition to manifesting another aspect of coronal
activity, radio emissions provide information on plasma density, local
magnetic field, particle acceleration, and wave propagation processes for a
number of stars (e.g., Benz & Güdel (2010)). The best studied amongst these is
obviously the Sun, which, due to its proximity, enables for the study of
magnetic activity at great sensitivity with high spatial and temporal
resolution. Against a background of thermal bremsstrahlung emission, both
incoherent gyrosynchrotron and coherent emissions processes contribute. At mm
to cm wavelengths, gyrosynchrotron dominates, while at dm to m wavelengths,
coherent plasma radiation dominates (Bastian et al., 1998). Another coherent
emission mechanism, ECM, may cause solar radio emissions with narrowband
components, such as the fine structure within Type IV bursts and solar
microwave spikes. The frequencies of emission of these two phenomena range
from $\sim$50 MHz to 10 GHz for the former and $\sim$100 MHz to 5 GHz for the
latter (Treumann, 2006; Pick & Vilmer, 2008). Dulk (1987) summarized how these
four radio emission mechanisms - thermal bremsstrahlung, gyrosynchrotron,
plasma, and electron cyclotron maser - differ in terms of their brightness
temperatures, $T_{B}$, circular polarization fractions, and timescales of
variability in addition to their operation in differing frequency domains.
Observations at 5-10 GHz are especially promising, as microwave bursts
generated by gyrosynchrotron emission tend to peak within this frequency
window, and several types of ECM-generated activity are also present.

Given these currents in research within the community, we conducted a survey
of nearby star systems with known substellar companions at Arecibo Observatory
(AO) in 2010-2011. A detailed account of the analysis and interpretation of
one exoplanet system that we surveyed, HD 189733, has recently been presented
in ROME I (Route, 2019) and ROME II (Route & Looney, 2019). In Section 2, we
describe our target selection methodology, instrumentation, and observations.
In Sections 3 and 4, we present the results of our survey, and contextualize
these results both in terms of ultracool dwarf (UCD) radio activity, and star-
planet interactions. In the latter section, we also discuss the implications
of our results on the search for direct emissions from exoplanets and
potential civilizations within their habitable zones (HZs). Finally, we
conclude by reviewing the significance of this survey and providing
suggestions for future work in Section 5. Together, these results and
considerations indicate that the direct detection of exoplanet auroral
emissions, and their indirect detection and characterization via star-planet
interactions remains an admirable, although difficult, goal.

##  2 Target Selection and Observations

###  2.1 Target Selection

Survey targets were chosen based on two classes of factors: the physical
properties of the systems and their location in the sky (Table 1). We targeted
stars with substellar companions thought to be exoplanets at the time that
were $<$50 pc from the Sun. This was necessary because even for the most
optimistic estimates of their emitted auroral radio flux (e.g., Lazio et al.
(2004)), they would still be marginally detectable if observed at the cutoff
frequency of their emission. Similarly, stellar coronal flare luminosities
follow a power-law distribution (e.g., Güdel et al. (2003)), so that
increasingly distant sources require stronger, and rarer, star-planet induced
flares for detection.

We further constrained our sources by mass and semi-major axis: over three
quarters of our targets are $<$0.5 au from their host stars and over half of
our targeted systems have an orbiting companion $>$3 MJ. Exoplanets close to
their parent stars were preferred because they are hypothesized to have
greater radio luminosity due to the greater dissipated kinetic or magnetic
power from the stellar wind impinging on their magnetospheres (Zarka, 2007).
Eight of our chosen planetary systems are within the semimajor axis range
$a\lesssim 0.1$ au considered by Cuntz & Shkolnik (2002) to be especially
promising for the detection of enhanced activity caused by star-planet
interactions, including 51 Peg b and HD 209458b which both appeared in their
study.

We selected larger exoplanet masses because theoretical models suggest that
these planets have greater thermal flux available to power convection-driven
magnetic dynamos, resulting in larger mean magnetic field strengths
(Christensen et al., 2009; Reiners & Christensen, 2010). We note that in many
cases, the quantity $M~{}sin~{}i$, as opposed to the true mass, is measured
for substellar companions, resulting in an uncertain classification as
exoplanets based on their minimum potential masses. However, given geometric
effects due the ambiguity in the orbital inclination angle, $i$, the true mass
of a companion is typically $\sim$15% higher111 “Exoplanets Data Explorer $|$
Table,” available at http://exoplanets.org/table (e.g., Reffert & Quirrenbach
(2011)). We therefore targeted HD 38529b (now HD 38529c) and HD 114762 Ab as
candidates that potentially straddle the exoplanet/brown dwarf boundary,
despite having reported masses of 12.7 and 11 MJ, respectively (Latham et al.,
1989; Fischer et al., 2003). In these latter cases, their radio emission may
be quite strong and at $\sim$4-5 GHz frequencies as it is for the $\sim$900 K,
T6.5 brown dwarf 2MASS J10475385+212434 (J1047+21; Route & Wolszczan (2012)).

Finally, the single, fixed-dish nature of the Arecibo radio telescope required
our targets to have declinations of 0∘ to +38∘.

###  2.2 Instrumentation and Observations

The exoplanet targets comprised approximately half of the objects surveyed
during AO program A2471, conducted from 2010 January 8 to 2011 September 6;
the other half were UCDs of spectral types $\geq$M7, with results presented in
Route & Wolszczan (2012, 2013). Each exoplanetary system was observed for a
total of 40 mins to 2.5 hrs, sometimes across multiple epochs, although no
single epoch was longer than 2 hrs (Table 2). The $\sim$2 hr observing session
limitation represents the maximum time for a target to transit the field of
view of the fixed dish of the Arecibo radio telescope. During these observing
sessions, 20-sec calibration on-off scans using a local oscillator were
interlaced between 10-minute target scans.

Although the 305 m dish is fixed, the William E. Gordon radio telescope at AO
offered several advantages relative to other facilities, including its
exquisite sensitivity, accurate polarization measurements over short time
scales, and large, $\sim$1 GHz bandpass, which was exemplary in 2010-2011. The
observing system consisted of the C-band receiver222“C-Band,” available at
http://www.naic.edu/$\sim$astro/RXstatus/Cband/Cband.shtml and the recently
commissioned Mock spectrometer333“The ‘Mock’ Spectrometer,”
http://www.naic.edu/%7Eastro/guide/node9.html. The system temperature ranged
from 25–32 K, with antenna gains of 5.5–9.0 K Jy-1. The half-power beam width
varied from 0.97 $\times$ 1.09 arcmin (azimuth $\times$ zenith angle) at 4500
MHz central frequency to 0.79 $\times$ 0.92 arcmin at 5400 MHz (Chris Salter,
personal communication). Full Stokes parameters were computed by the Mock
spectrometer array based on the dual-linear polarization input signals from
the receiver. This array consists of seven field-programmable gate array
(FGPA) Fast Fourier Transform (FFT) Mock spectrometers, which were
individually tuned to central frequencies of 4325, 4466, 4608, 4750, 4892,
5034, and 5176 MHz, resulting in a $\sim$1 GHz simultaneous bandpass
stretching from 4239 to 5262 MHz. Although each 8192-chanel spectrometer
permits a 172 MHz bandpass, in practice edge effects reduce the usable
bandpass to $\sim$140 MHz (Salter, 2009). The spectrometer data were sampled
in 0.1 s intervals.

Prior to analysis, data were resampled at 83.6 kHz and 0.9 s frequency and
temporal resolutions, respectively, to improve the signal-to-noise ratio.
Radio frequency interference (RFI) was reduced through an iterative
statistical process detailed in (Route, 2013). The 1$\sigma$ instrumental
sensitivity was $\sim$0.1-1.2 mJy, with per-scan sensitivity distribution
properties described in (Route, 2017b). Data analysis consisted of the
creation of dynamic spectra (time-frequency spectrograms) and time series
graphs of flux density in each polarization across all spectrometers. Since
one of the hallmarks of the two most likely types of radio emission, ECM and
gyrosynchrotron, is significant circular polarization, we searched Stokes V
dynamic spectra for $\gtrsim$10% bursts of emission that did not have
accompanying Stokes U and V counterparts. AO is insensitive to quiescent radio
emission due to its local calibration procedure and confusion limitations,
thus, only circularly polarized radio bursts of several minutes’ duration
could be detected. However, this may not be a major limitation of our
observing methodology since both Jupiter and Uranus emit bursts of radio
emission in addition to their smooth components (Zarka, 1998).

##  3 Results

###  3.1 Direct Search for Radio Emission from Companion Objects

No radio bursts of any type were detected from any system hosting exoplanet or
brown dwarf companions. We reiterate that this survey was sensitive only to
$\gtrsim$10% circularly polarized bursts of radio emission of several minutes
duration, and this survey was entirely insensitive to quiescent and/or non-
polarized emission from the systems observed. Due to the angular resolution of
the beam, this result means that for each targeted system, no radio emission
was directly detected from exoplanets within the system, nor was host star
radio emission induced by tidal or magnetic effects from the closest
exoplanets detected.

However, we can place upper limits on circularly polarized radio emission from
the systems studied by computing the standard deviation of the frequency-
integrated time series of the cleanest Mock spectrometer, centered at 4466
MHz. For complete results of the frequency-dependent sensitivity of
observations on a system-by-system basis, see Route (2013). Table 3 lists the
3$\sigma$ upper limits on the detection of radio emission from the exoplanet
systems observed. A comparison of these detection limits to several types of
flares is depicted in Figure 1, and Figure 2 displays the detection limits
from this survey compared to other surveys of UCDs. We note that these upper
limits provide tighter constraints on target radio emission than in the sister
surveys of UCD radio activity conducted with the same instrumental set up and
analysis software, due to the quieter radio environment around AO in 2010-2011
(Route, 2013; Route & Wolszczan, 2013, 2016b).

####  3.1.1 Companion Bolometric Luminosities

It is common in the substellar community to report the radio emission from
targets as a fraction of their bolometric lumniosities. There are several ways
to obtain these luminosities. High-contrast near infrared imaging and
spectroscopy enable a direct measurement of the bolometric luminosities of the
members of the HR 8799 system (Marois et al., 2010; Brandt et al., 2021). The
revised determination of the mass of HD 114762 Ab enables us to estimate its
spectral type (M5) and, therefore, its bolometric luminosity (Reid & Hawley,
2005; Kiefer et al., 2021). For a limited number of transiting exoplanets (55
Cnc e, HD 189733b), we leverage direct measurements of their radii and
effective temperatures to determine their bolometric luminosities via the
Stefan-Boltzmann equation.

However, in most cases, the radii and effective temperatures of the companion
objects are unknown. For many substellar companions, the radii are estimated
from the mass-radius relationships found in Figures 1 and 2 of Chabrier et al.
(2009) for companions with M$\geq$1 MJ, and in Figure 4 of Seager et al.
(2007) for companions of sub-Jupiter mass. When direct measurements of
effective temperatures exist (HD 10697b, 55 Cnc e, HD 189733b) we have
averaged their day and nightside temperatures. Next, when available, we use
effective temperatures reported in the literature. For the remaining cases
($\epsilon$ Tau b; GJ 176b; HD 38529b; HD 50544b; 55 Cnc b, c, and d; HD
102195b; 70 Vir b; HD 178011 Bb; HD 195019b) we can crudely estimate the
equilibrium effective temperatures by following the prescription in Marcy &
Butler (1996):

$T_{eff}\approx T_{\ast}(R_{\ast}/2a)^{1/2}(1-A)^{1/4},$ (1)

where $T_{\ast}$ and $R_{\ast}$ are the effective temperature and radius of
the host star, respectively, $a$ is the companion semimajor axis, and $A$ is
the Bond albedo of the companion, which is assumed to be $\sim$0.35 as it is
for Jupiter. The measured and computed bolometric luminosity for each
companion is given in Table 4, and its computed $\nu~{}L_{\nu}/L_{bol}$ is
listed in Table 3.

An alternative method would use evolutionary models for cool brown dwarfs and
giant planets (M$\geq$0.5 MJ) (e.g., Baraffe et al. (2003)). However, given
that both isolated and irradiated models appear to systematically
underestimate the bolometric luminosities of exoplanets (e.g., HD 209458b) we
have opted not to use them here. Nevertheless, we note that were these models
to be used, for system ages of $\sim$5 Gyr, the modeled bolometric
luminosities differ by 0.0 to -6.4 dex. Thus, in general, evolutionary models
systematically shift nearly all points upward in $\nu~{}L_{\nu}/L_{bol}$ in
Figure 3.

###  3.2 Orbital Phase Coverage of Companions for Star-Planet Interactions

We next evaluate the constraints placed on the occurrence of star-planet
interactions within the systems we targeted. Borrowing from nomenclature for
hot Jupiters, we first divide our sample into targets with hot exoplanets
($P<$10 d), warm exoplanets (10$\leq~{}P<$200 d), and longer period targets
(e.g., Dawson & Johnson (2018)). For “hot” and “warm” exoplanets, we compute
the orbital phase coverage of our observations using

$T_{0}=T_{t}+P~{}E_{Orb},$ (2)

where T0 represents the beginning or end of the observation, Tt represents the
measured (or estimated) midpoint of transit (or periapse), $P$ is the
exoplanet orbital period, and Eorb is the orbital cycle. Orbital phase 0.0
denotes when transit or periapse would occur.

The results for “hot” targets are reported in Table 5. Even among the
shortest-period exoplanets, our orbital phase coverage is quite modest since
many targets were observed for $\sim$1 hr, and only three targeted systems for
$>$1.5 hr. The peak orbital phase coverage ($\Delta\phi_{orb}$=0.08) occurs
for 55 Cnc e, although the coverage and timing of the observations for HD
189733 is also significant (Section 4.1.7). Among “warm” targets, the maximum
coverage is $\Delta\phi_{orb}$=0.004 for 55 Cnc b and HD 195019b. More
importantly, however, even the warm Jupiter closest to its host star, HD
195019b ($a$=21.62 R∗), is too far away to induce enhanced stellar activity
through magnetic and tidal star-planet interactions. In summary, despite the
excellent sensitivity of AO, the modest orbital phase coverage only loosely
constrains the incidence of star-planet interactions within the observed
systems.

##  4 Discussion

###  4.1 Companion Objects of Special Interest

####  4.1.1 HD 114762 Ab

In Section 2.1, we noted that the measurement of $M~{}sin~{}i$ via the radial
velocity method leaves an ambiguity in the true mass of the object, which can
lead to a corresponding underestimate of its internal energy to power a
convective dynamo. This would then result in an underestimate of the magnetic
field strength of the substellar object, and accompanying searches at too low
of frequencies. While we did not detect radio emissions from any exoplanet
candidate targeted by our survey, we highlight that several of our targets
were subsequently reclassified as brown dwarfs or low-mass stars.

Given the updated mass estimate for HD 114762 Ab from Kiefer et al. (2021) and
the mass-spectral type relationship depicted in Figure 10 of Chabrier &
Baraffe (2000), we estimate that this companion is actually an M5 dwarf, as
opposed to a $\sim$11 MJ giant planet as originally thought (Latham et al.,
1989). Although we did not detect any high brightness temperature, highly-
circularly polarized ECM-generated radio flares from this system, these flares
are common among M dwarfs. They have been observed in variety of M dwarf
containing systems, including at 4.85 GHz from the M3.5 dwarf AD Leo (Stepanov
et al., 2001) and potentially from the M5 companion in the “white dwarf
pulsar” cataclysmic variable system AR Sco at 1-10 GHz (Lyutikov et al.,
2020).

####  4.1.2 HD 38529 and HD 106252b

Based on their radiometric Bode’s law, Lazio et al. (2004) estimated that if
HD 38529c and HD 106252b were exoplanets, they would be sources of
$\Phi_{radio}\sim$13 $\mu$Jy and $\Phi_{radio}\sim$32 $\mu$Jy radio emissions
extending to cutoff frequencies of $\nu\sim$1.6 GHz and $\nu\sim$576 MHz,
respectively. These estimates were based on the ability of their respective
magnetospheres to dissipate incident stellar wind power. However, the revised
mass estimates for HD 38529c and HD 106252b (Table 1) clearly place them in
the brown dwarf domain (Fritz Benedict et al., 2010; Reffert & Quirrenbach,
2011). Using these revised masses together with 5 Gyr isochrone models
(Baraffe et al., 2003) and the effective temperatures from Cushing et al.
(2011), we estimate that HD 38529c and HD 106252b have spectral types of Y0
and T9, respectively. The 2010 observations of these two objects, therefore,
constitute the first, although inadvertent, search for radio emission from
potential Y dwarfs. Since the latest spectral type and coolest effective
temperature for which radio emission has been discovered is from the
$T_{eff}\sim$900 K J1047+21, which was a source of $\Phi_{radio}\sim$3 mJy
radio emissions at $\nu\gtrsim$4 GHz (Route & Wolszczan, 2012), these systems
may also be sources of radio emission at $\sim$GHz frequencies and mJy flux
densities. Unfortunately, both systems are $\sim$4$\times$ further away than
J1047+21, so that while the instrumentation and techniques to search for radio
emissions remain the same for all three objects, the upper limits for
detectable radio emission from HD 38529c and HD 106252b are nearly an order-
of-magnitude higher.

The HD 38529 system also contains a warm Jupiter, HD 38529b, which Lazio et
al. (2004) predicted could be a source of $\Phi_{radio}\sim$6 mJy radio
emission up to a cutoff frequency of $\nu\sim$16 MHz.

####  4.1.3 HD 46375

Lazio et al. (2004) estimated that HD 46375b may be a source of
$\Phi_{radio}\sim$80 mJy auroral radio emission up to a cutoff frequency of
$\nu\sim$0.6 MHz, which is below the frequency threshold at which radio
emissions can penetrate the terrestrial ionosophere. Shkolnik et al. (2005)
searched for periodic, star-planet induced chromospheric variability in the Ca
II H & K lines (3968 and 3933 $\AA$) among HD 46375 and several other program
stars. No change was detected to a level of 0.001. Similarly, we did not
detect exoplanet auroral emission or any sign of star-planet interactions in
this system, although our observations only occur at $\phi_{orb}\sim$0.68.
This orbital phase is somewhat removed from $\phi_{orb}\sim$0.5, which theory
indicates may experience augmented activity due to tidal star-planet
interactions.

####  4.1.4 70 Vir

Lazio et al. (2004) anticipated that 70 Vir b would be a $\Phi_{radio}\sim$2
mJy source of auroral radio emissions at $\nu\sim$546 MHz, based on its
ability to dissipate incident stellar wind power. Saar & Cuntz (2001) searched
for star-planet induced chromospheric activity in the Ca II infrared triplet
(IRT; 8662 $\AA$) among several exoplanet-hosting systems, including 70 Vir.
They found no variability down to $\sim$3.5%, and we found no evidence for
coronal activity enhancement. We also did not directly detect radio emissions
from any exoplanet auroral activity.

####  4.1.5 51 Peg

Lazio et al. (2004) and Reiners & Christensen (2010) estimated that 51 Peg b
may emit $\nu\sim$1.2 MHz, or $\nu\sim$7 MHz, radio waves up to a level of
$\Phi_{radio}\sim$251 mJy, or $\Phi_{radio}\sim$0.8 mJy, respectively. Both
cutoff frequencies, however, are below the terrestrial ionospheric frequency
threshold. Saar & Cuntz (2001) found no sign of star-planet interactions in
the Ca II IRT down to $\sim$3.2%. Shkolnik et al. (2003) reported significant
nightly activity for 51 Peg which they suggested might be cyclical and phase
shifted based on $\sim$10 nights of Ca II H & K observations. Subsequent
observations found no variability down to 0.001 (Shkolnik et al., 2005). While
our observations are agnostic with respect to this prediction of auroral
emissions, we found no evidence for star-planet induced coronal activity at
$\phi_{orb}\sim$0.22, nor at $\phi_{orb}\sim$0.43, near the anticipated tidal
activity enhancement zone.

####  4.1.6 HD 209458

Lazio et al. (2004) used their radiometric Bode’s law to predict that HD
209458b’s auroral radio emission might peak at $\nu\sim$2.9 MHz with a flux
density of $\Phi_{radio}\sim$25 mJy. As with other predictions where the
cutoff frequency $\nu<$10 MHz, such emission would be undetectable from the
ground due to reflection from the terrestrial ionosphere. As with 51 Peg b,
preliminary monitoring of chromospheric Ca II H & K activity suggested nightly
activity which could be periodic in nature, but was later found to lack
variability down to the level of 0.001 (Shkolnik et al., 2003, 2005).
Alternatively, Jensen et al. (2012) found an anomalous shifting of the broad
component relative to the narrow component of the H$\alpha$ line in
transmission spectra during transit and secondary eclipse. They suggested that
stellar chromospheric activity enhanced by star-planet interactions may alter
the H$\alpha$ line shape, although this phenomenon may also point to a data
reduction artifact. Our radio observations do not find evidence of auroral
activity, nor star-planet induced coronal activity within this system at
$\phi_{orb}\sim$0.01 nor $\phi_{orb}\sim$0.87. Note that enhanced activity
near $\phi_{orb}=$0.0 would be anticipated in both tidal and magnetic
interaction cases.

####  4.1.7 HD 189733

The proximity of the HD 189733 system to the Earth, the brightness of the
primary star, and fortuitous alignment of the hot Jupiter companion that
result in a transit every 2.219 d have made the system a particularly inviting
target for study. With a chromospheric activity index, S-value=0.525, HD
189733A is among the 10% most active K dwarfs in the the California and
Carnegie Planet Search Project. Transmission spectroscopy of the frequent
transits have resulted in the discovery of the exoplanet atmosphere’s
composition, thermal properties, and evaporation. Multiwavelength observations
of the system have revealed a wealth of stellar activity from optical to X-ray
wavelengths, including spots and flares at all rotational and exoplanetary
orbital phases. Compellingly, several research groups announced the discovery
of enhanced chromospheric, transition region, and coronal activity that may be
phased with exoplanet orbital phases $\phi_{orb}\sim$0.5–1.0, that may be
caused by magnetic and/or tidal star-planet interactions. In one case, it was
furthermore suggested that the primary star may be actively accreting
evaporated material from HD 189733b (see ROME I and references therein).

In ROME I, we analyzed AO radio observations to search for enhanced coronal
activity associated with such star-planet interactions. We also analyzed
_Microvariability and Oscillations of Stars (MOST)_ , Automated Photoelectric
Telescope (APT), and Wise photometry to search for periodicities suggestive of
stellar hemispheres that contain more darkened starspots than average, that
may point to enhanced photospheric activity associated with these
interactions. We also compiled all observations and reported instances of
stellar spotting and flaring at radio through X-ray wavelengths to attempt to
discover enhancements phased to the orbital period of the exoplanet. Our
results could find no statistically significant enhancement, including at the
orbital phases that other research groups thought might suggest star-planet
interactions. In ROME II, we again leveraged _MOST_ , APT, and Wise photometry
to search for photometric bright features that may be related to accretion
structures, using the T Tauri paradigm as a template. After computing plasma
parameters such as density, temperature, accretion rate, and flare structure
sizes, we determined an accretion rate of $\dot{M}\sim 10^{9}$ to 1011 g s-1,
which would result in undetectably low activity at all wavelengths. Since the
publication of ROME II, new flare observations and several reanalyses of
existing optical and UV data sets have been presented, which we will address
in a forthcoming publication (e.g., Bourrier et al. (2020) and references
therein).

However, for the purposes of this paper, we will remain focused on research
that may directly influence the ability of HD 189733A or its exoplanet
companion to produce radio emission. Based on their convective dynamo model,
Reiners & Christensen (2010) estimated that HD 189733b would have a detectable
radio flux density of $\Phi_{radio}\sim$57 mJy up to an ECM cutoff frequency
$\nu\sim$39 MHz. Zaghoo & Collins (2018) used a revised interior conductivity
profile to estimate peak ECM emission at $\nu\sim$15 MHz with a radio flux
density of $\Phi_{radio}\sim$20 mJy. Kavanagh et al. (2019) leveraged the 3D
magnetohydrodynamic modeling code BATS-R-US to investigate the prospects for
the detection of radio emissions from HD 189733b. They used the magnetic field
topology maps of HD 189733A derived from several epochs of ZDI observations
from Fares et al. (2017) as inputs. They found that an exoplanetary polar
magnetic field $B\sim$10 G would be required to create ECM emission at
$\nu\sim 25$ MHz that is higher than the plasma frequency of the stellar wind
environment and thus could escape to an observer with $\Phi_{radio}\sim$50-130
mJy. Our radio observations do not shed any light on these predictions as no
exoplanet auroral activity was observed. We also find no evidence for star-
planet induced coronal activity within this system at
$\phi_{orb}\sim$0.57–0.61 (ROME I).

####  4.1.8 55 Cnc

The 55 Cnc binary system consists of a K0 primary, an M4.5 secondary, and five
exoplanets that circle the primary. The closest orbiting exoplanet, 55 Cnc e,
has a sub-Neptune mass and may be close enough to its host star to cause star-
planet interactions, although its magnetosphere would likely present a very
small cross section for magnetospheric interactions. 55 Cnc f orbits within
its host star’s HZ, which will be discussed in more detail in Section 4.3.
Lazio et al. (2004) estimated that 55 Cnc b, c, and d would be sources of
radio emission up to cutoff frequencies of $\nu\sim$17.6 MHz, 1.7 MHz, and 242
MHz, with flux densities of $\Phi_{radio}\sim$80 mJy, 40 mJy, and 80 $\mu$Jy,
respectively. Unfortunately, no auroral emissions, nor star-planet induced
coronal activity were observed in this system.

####  4.1.9 HD 195019b and HD 178911 Bb

Lazio et al. (2004) suggested that the two warm Jupiters, HD 195019b and HD
178911 Bb may be sources of $\Phi_{radio}\sim$16 mJy, and $\sim$0.63 mJy
auroral radio emissions up to cutoff frequencies of $\nu\sim$184 MHz, and
$\sim$504 MHz, respectively. Unfortunately, we did not detect any signatures
of exoplanet auroral emissions or star-planet interactions in these systems.

####  4.1.10 HD 10697 and HD 50554

Leveraging their radiometric Bode’s law, Lazio et al. (2004) predicted that HD
10697b and HD 50554b would produce auroral radio emissions at $\nu\sim$545 MHz
and 333 MHz with flux densities of $\Phi_{radio}\sim$80 and 63 $\mu$Jy,
respectively. Although these exoplanets have relatively high masses and
estimated cutoff frequencies, their $a>$2 au semimajor axes place them at a
significant disadvantage in terms of dissipating incident solar wind power
compared to our other targets. No auroral emissions were detected in these
systems.

###  4.2 Reasons for Non-Detections

There are several potential reasons for the non-detection of radio emissions
from the targeted systems, which must be assessed in light of both intrinsic
exoplanet magnetospheric radio emissions, and star-planet induced coronal
radio emissions. These reasons include sensitivity, observing frequency,
observation durations, and viewing geometry.

####  4.2.1 Sensitivity

Although this survey is the most sensitive to date, in many cases, the
exoplanet auroral emissions would need to be $\sim$106 greater than those of
Jupiter in order to be detectable (Table 3). By comparison, Zarka (2007)
estimated that hot Jupiters might produce radio emissions 103 to 106 times
more intense than those emitted by Jupiter based on their empirically derived
radio-magnetic Bode’s law. Somewhat less optimistically, Lazio et al. (2004)
suggested that greater stellar wind loading among hot Jupiters would scale-up
emissions analogous to Jovian peak power emissions by 102. Our effort, then,
is at the edge of viability with respect to these models.

Within the context of star-planet interactions, we note that ECM-induced
stellar flaring would be detectable in nearly all cases (Figure 1), as was
found in our study comparing the activity of HD 189733A to active M dwarf ECM
flares (ROME I). Similar to the findings of that work, only the most luminous
gyrosynchrotron flares might be visible from some targets within our survey.
Unfortunately, the anticipated radio luminosity of flares (as derived from the
X-ray emissions and the Güdel-Benz relationship) from isolated, active stars
such as the K2V $\epsilon$ Eri, does not exceed the detection thresholds for
flares from our most sensitive observations. On the other hand, it is possible
that the radio counterparts of the extremely luminous superflares observed in
the optical may be visible from our survey (Schaefer et al., 2000). As
mentioned in Section 1, it has been hypothesized that such flares may be
powered by the magnetospheric interaction of close-in giant exoplanets.
However, generally speaking, it is non-trivial to estimate the radio
luminosity of flares based on their optical emissions alone (e.g., Priest
(2014)). If such flares have radio luminosities comparable to their optical
emission, $L\sim$5$\times$1031 ergs s-1 would be readily detectable by our
survey.

####  4.2.2 Observing Frequency and Magnetic Field Strengths

Another important facet to consider is the frequency coverage of our survey.
Since our survey was primarily sensitive to ECM emissions generated either by
the direct exoplanet auroral emissions or exoplanet-induced stellar flaring,
we will consider only this emission mechanism. The cutoff frequency of ECM
radio emissions is related to the local cyclotron frequency, $\nu_{c}$, by
$\nu_{c}$[MHz] $=2.8nB$ [Gauss], where $n$ is the harmonic number, and $B$ is
the local magnetic field strength. Therefore, since we surveyed targets within
the frequency range 4250 MHz to 5250 MHz, this corresponds to emission from
magnetic fields of strength $B\sim$1.5 to 1.9 kG for emission at the
fundamental frequency, and $B\sim$760 to 940 G at the second harmonic.
Theoretical models indicate that both modes are possible, although which mode
of emission dominates is dependent upon the local plasma conditions (Treumann,
2006). We note that Trigilio et al. (2011) has found evidence for near-
simultaneous radio emission in both modes from the magnetic chemically
peculiar star CU Virginis. At first blush, the magnetic field ranges that
correspond to the frequency range of our survey appear to be too high to be
scientifically useful.

For the most part, radio surveys of exoplanets have sought emission at much
lower frequencies. Only a handful of targets have traditionally been
hypothesized to have $\sim$GHz radio emissions, such as HD 114762 Ab, HD
38529c (before their updated mass determinations), HD 43197b, PSR B1620-26 b,
and WASP-77A b (Lazio et al., 2004; Sirothia et al., 2014). However, given the
$Msini$ mass ambiguity, if, for example, the mass of an object such as HD
38529c was underestimated by a factor of 1.8, this would correspond to an
increase in its emission frequency of $\sim$2.7, placing it within our survey
range. Indeed, after the completion of our survey it was confirmed that HD
38529c, HD 106252b, and HD 114762 Ab were more massive than first thought,
with the first two targets declared to be brown dwarfs and the last we
estimate to be an M5 dwarf.

The primary component in the determination of the frequency of intrinsic radio
emissions from substellar companions is their magnetic field strengths as
governed by the internal energy available to power their convective magnetic
dynamos. This internal energy can be estimated based on parameters such as
mass, radius, age, rotation rate, and internal composition and conductivity.
Figure 1 of Reiners & Christensen (2010) depicts the magnetic field
evolutionary tracks for stars and substellar objects of various masses given
the geodynamo scaling law devised by Christensen et al. (2009). From these
models, the maximum magnetic field strengths found in $\sim$1 Gyr M dwarfs,
brown dwarfs, and exoplanets, should be approximately 4 kG, 3 kG, and 200 G,
respectively. However, there is some evidence that these models may, in fact,
underestimate the magnetic dynamo strengths that can be achieved in these
objects. Following our detection of a $B\sim$1.7 kG magnetic field from the
$\sim$900 K, T6.5 brown dwarf J1047+21, additional observations at 10 GHz
(Williams & Berger, 2015) and 15.75 GHz (Kao et al., 2018) have suggested that
this brown dwarf may host magnetic fields as strong as 5.6 kG, assuming ECM
radiation at the fundamental frequency. Furthermore, the latter paper suggests
that the T2.5 brown dwarf SIMP 01365662+0933473 may actually be a $\sim$200
Myr substellar object of only $\sim$13 MJ, yet with a magnetic field strength
$B\sim$3.2 kG.

Confirmation of these characteristics would imply that brown dwarfs and
exoplanets maintain much stronger magnetic fields than anticipated, and will
provide useful constraints to revise magnetic dynamo models. We reserve a more
thorough discussion of dynamo scaling laws as applied to exoplanets and brown
dwarfs for a later paper in the ROME series. However, these considerations
indicate that it is not unreasonable to survey exoplanets at higher
frequencies than traditionally assumed, given enough sensitivity. On the other
hand, it is important to note the dynamo models of Christensen et al. (2009)
and Reiners & Christensen (2010) require rapid rotation. Since the hot
Jupiters surveyed may be tidally locked, the convective motions of their
internal dynamos may be drastically reduced, resulting in diminished global
dipolar magnetic fields (e.g., Sánchez-Lavega (2004)). Alternatively, Yadav &
Thorngren (2017) argued that the surprisingly large radii of hot Jupiters
indicate that the stellar deposition of heat into their interiors should
enhance convective dynamo activity, leading to inflated interiors and stronger
magnetic fields. It is also possible that the dense stellar wind environment
in which these hot Jupiters orbit their host stars may preclude the escape of
any ECM radio emissions (e.g., Weber et al. (2017); Kavanagh et al. (2019)).
Together, these considerations demonstrate that the detection of radio
emissions from substellar objects will provide valuable information about
myriad system properties and parameters.

Since another focus of our effort was to search for stellar flaring induced by
star-planet interactions, we should also consider the frequency ranges that
correspond to coronal magnetic fields. While the mean solar photospheric
magnetic field has strength of $B\sim$3-10 G, within active regions this field
can reach $B\sim$100 G, and within sunspot umbrae, field strengths as high as
$B\sim$6 kG have been measured (Priest, 2014). Within the solar corona,
magnetic fields as high as $B\sim$2.5 kG have been measured above photospheric
activity regions. Gyroresonance, gyrosynchrotron, and ECM emissions all
contribute to solar coronal radio emissions that range in frequency from
$\nu=$1 to 20 GHz (Aschwanden, 2005). Among these, only activity caused by the
latter two processes would be detectable at great distances. Thus, the
frequency range of our survey was well-suited for stellar coronal magnetic
fields associated with both intrinsic and star-planet induced activity.

However, our survey failed to detect any direct radio emissions from
substellar companion auroral activity, or indirect emissions from induced
stellar coronal activity at the frequencies surveyed. Thus, in addition to
sensitivity, it is likely that most of our observed targets, with the
exceptions of HD 114762 Ab, HD 38529c, HD 196252b, and HR 8799 b, c, d, and e
were most likely observed at too high of frequencies to detect companion
auroral activity. On the other hand, given the frequencies at which solar
radio emissions have been detected and the strength of the magnetic fields
associated with stellar active regions, the frequencies at which we searched
for star-planet induced stellar radio emissions were entirely plausible.

####  4.2.3 Observation Durations

Since our survey searched for companion auroral emission and radio emissions
linked to stellar flares that are induced by star-planet interactions, we must
consider both contexts when evaluating the temporal coverage of our survey
targets. In the first case, companion auroral radio emission generated by the
ECM mechanism is beamed into a hollow cone with large opening angles relative
to the magnetic field, that is modulated at the rotation period of the object
(Zarka, 2007). As described in Table 2, all companion-hosting systems were
observed for 0.6 to 7.2 ks. We consider exoplanet rotation periods ranging
from $\sim$10 hrs as is the case for Jupiter and Saturn, to 3-4 d for spin-
orbit synchronized, sidereal rotation (Zarka, 2007; de Pater & Lissauer,
2015). Thus, in the best case scenario of a distant giant exoplanet that has
not undergone significant orbital evolution, our observations only cover
$\sim$10-20% of rotational phase for magnetic activity (e.g., $\epsilon$ Tau).
The rotational phase coverage of our two brown dwarf targets, HD 38529c and HD
106252b, is probably significantly better. Magnetically active UCDs have a
mean rotation period of 1.22 hrs, although their rotation periods range from
perhaps as little as 0.288 hrs (WISEPC J112254.73+255021.5; J1122+25) to
$\sim$50 hrs (SDSS J151643.01+305344.4) (Route, 2017a, b). Since HD 38529c and
HD 106252b were both observed for 3.6 ks, if they rotate at the mean UCD
rotation period, then their rotational phase coverage is likely $\sim$80%,
although given the spread in rotation periods, it could be as low as $\sim$2%.
A similar result applies to the $\sim$M5 HD 114762 Ab.

In the second case of star-planet interactions, both host star rotation
periods and hot Jupiter orbital periods are of interest (as is their beat
period), as it is necessary to untangle the effects of intrinsic stellar
activity from activity induced by star-planet interactions. However, hot
Jupiter orbital periods are shorter than the rotational periods of their host
stars, making the orbital phase coverage the timescale of interest. As
described in Section 3.2, the maximum orbital phase coverage for our hot
Jupiter targets is listed in Table 5, with $\Delta\phi_{orb}\leq$0.08. Thus,
in order to detect star-planet induced activity and untangle its effects from
intrinsic stellar activity, we would likely need nearly complete orbital phase
coverage over multiple orbits. As ROME I and ROME II have pointed out, even
for as well of a studied case as HD 189733, it still remains unclear whether
star-planet interactions occur in that system.

In summary, the duration of our observations provide rotational phase coverage
of $\sim$2 to 80% for exoplanet or brown dwarf auroral radio emissions and
$\leq$8% orbital phase coverage of hot Jupiters surveyed for star-planet
interactions.

####  4.2.4 Viewing Geometry

Based on an examination of the Solar System planets, we may assume that the
magnetic and rotation axes of exoplanet and brown dwarf companions are roughly
aligned, and that they are generally aligned with their orbital axes in low-
obliquity orbits. Since ECM-induced auroral emissions are beamed into a hollow
cone at angles nearly perpendicular to the magnetic axis of the exoplanet or
brown dwarf, the optimal viewing angle for these emissions is at high
inclination to the orbital axis. In this scenario, transiting and near-
transiting systems, such as HD 189733 and the other targets in Table 5, are
optimal targets to search for auroral radio emissions. On the other hand,
directly-imaged systems, such as the companions of HR 8799, are only likely to
yield radio emissions if the exoplanet magnetic axes are significantly tilted
with respect to their orbital axes.

Within the context of the search for radio emissions from star-planet
interactions, substellar companions may induce enhanced photospheric,
chromospheric, and coronal emissions, including radio emissions, either once
per orbit due to magnetic effects, or twice per orbit due to tidal effects.
While these effects are thought to occur at particular orbital phases, there
are otherwise no additional known geometric effects to consider that may
influence their detection.

Therefore, while viewing geometry may enhance or hinder the prospects for the
detection of radio emissions on a case by case basis, it should not adversely
affect the detection of radio emission from every target. We only suspect it
is a contributing factor in the failure to detect emissions from systems
viewed at low viewing angles with respect to their companions’ orbital axes,
such as HR 8799 b, c, d, and e.

####  4.2.5 Significance

Numerous attempts have been made to detect the auroral emissions from
exoplanets at radio wavelengths since the work of Yantis et al. (1977). It is
beyond the scope of this paper to discuss in detail the merits of these
efforts. Instead, we will briefly assess how our survey instrumentation and
strategy compare to previous efforts, before examining how this survey,
together with surveys of low-mass stars and brown dwarfs, may guide future
detection efforts.

Our survey at AO has several distinguishing features relative to previous
work. AO provided exquisite sensitivity at short timescales, enabling us to
achieve 3$\sigma$ detection limits of $\sim$1 mJy in 0.9 s integrations. This
compares very favorably with many previous large surveys that achieved
detection limits of 3$\sigma\sim$ 10 to 100 mJy in $\sim$1 ks integrations at,
for instance, the Murchison Wide Array (MWA), the Ukrainian T-shaped Radio
Telescope second modification (UTR-2), and the Very Large Array (VLA). We also
simultaneously observed over a $\sim$1 GHz bandpass, as opposed to many
earlier efforts that were limited due to existing technology to bandwidths
$\Delta\nu\leq$50 MHz. Finally, our effort was spectropolarimetric in nature,
yielding dynamic spectra in all four Stokes parameters for every target.
Earlier efforts often only measured total intensity (Stokes I) from targets,
whereas our survey focused on circularly polarized (Stokes V) emission and
used linear polarization (Stokes Q and U) as an RFI mitigation strategy. Newer
facilities with more recent technology have continued to improve survey
characteristics in the same general directions as our survey (e.g., Turner et
al. (2021)).

In Route & Wolszczan (2016b), we discussed trends in $\nu~{}L_{\nu}$ and
$\nu~{}L_{\nu}$/$L_{bol}$ among UCDs, a class of objects that consists of
spectral types $\geq$M7, including fully-convective low-mass stars of spectral
type M, and brown dwarfs of types L, T, and presumably, Y. Although these
objects differ in terms of their near infrared and optical spectroscopy, they
display similarities in their magnetism as manifest by X-ray, H$\alpha$, and
radio activity (e.g., McLean et al. (2012)). We noted in our earlier work that
our detection of flaring radio emission from two late T brown dwarfs, J1047+21
and J1122+25 (Route & Wolszczan, 2016a), helped to pin down how these trends
might evolve toward exoplanet-mass substellar objects. Figures 2 and 3 update
the plots (Figures 3 and 4; Route & Wolszczan (2016b)) of detected radio
emissions from objects near the stellar/substellar boundary that are pertinent
to this discussion, and include a side-by-side comparison of efforts to survey
UCDs with our current exoplanet survey efforts.

Jupiter’s decametric emission is 4-5 orders of magnitude below the UCD flaring
radio emission trend line in $\nu~{}L_{\nu}$. Our $\sim$M5 dwarf HD 114762 Ab
detection limit is somewhat high compared to the trend in UCD radio emissions,
but acceptable given its earlier spectral type and other similar detections at
M7-M9. In Route & Wolszczan (2016b) we mentioned that radio emission
($\nu~{}L_{\nu}$) either appears to generally remain constant from M7 to T7,
or perhaps declines from L2 to T3, followed by a minor rise from T3 to
$\sim$T6. Yet at some spectral type $\gtrsim$T7, greatly reduced flaring and
quiescent radio emission from UCDs and giant exoplanets must occur in order
for the trend at $\sim$T6 to connect with Jovian emissions. Unfortunately, our
non-detection of emissions from HD 106252b ($\sim$T9) and HD 38529c ($\sim$Y0)
does not shed any light on this matter. The upper limits on their emissions
exceed the UCD trend line and the emissions from J1047+21 and J1122+25 (Figure
2), but are consistent with expectations of their emitted radio power as a
fraction of their total luminosity according to the trends in Figure 3.

We can speculate about how radio emission declines precipitously from spectral
types $\gtrsim$T7, and smoothly connects the activity from J1047+21 and
J1122+25 with the Jovian DAM. Compared with this hypothetical trend, our
detection limits for even the most massive exoplanets surveyed, such as those
of the HR 8799 system and 70 Vir b, are all likely $\gtrsim$3 orders of
magnitude above the trend line in Figure 2. Similarly, in terms of radio
luminosity as a fraction of bolometric luminosity (Figure 3), for the best
case scenario for detection, 51 Peg b, our detection limit was still $\sim$2
orders of magnitude too large to be successful. Nevertheless, consideration of
a hypothetical $\nu~{}L_{\nu}$ luminosity trend linking UCD radio activity
with that from giant exoplanets suggests that a $\gtrsim$3 order-of-magnitude
increase in sensitivity may be required to detect exoplanet emissions. This
estimate is somewhat greater than that of Lazio et al. (2004), but consistent
with the range suggested by Zarka (2007).

On the other hand, an appraisal of the search for enhanced coronal activity
from star-planet interactions is more optimistic. Figure 1 demonstrates that
an approximately single order of magnitude increase in sensitivity should make
stellar flares of the nearest active stars, such as HD 189733A, accessible. An
improvement in sensitivity of $\sim$102 would also enable the characterization
of coronal activity from older, less active stars such as the Sun. However,
given the difficulty of untangling star-planet induced flaring from intrinsic
coronal activity (i.e. ROME I), searches for this phenomenon will require
repeated observations over multiple epochs that occur close together in time.

###  4.3 Implications for Direct Emissions from Exoplanets within Habitable
Zones

Two of our targeted systems host exoplanets (or exomoons) that are within
their host stars’ HZs for the entire duration of their orbits: HD 10697b and
55 Cnc f. Several others spend a fraction of their orbit within the optimistic
HZ, such as HD 38529c (57%), HD 50554b (26%), HD 106252b (20%)444“Habitable
Zone Gallery,” available at http://www.hzgallery.org/table.html (Kane &
Gelino, 2012; Kopparapu et al., 2014). Both HD 10697b and 55 Cnc f are
significantly more massive than the Earth, so that any life within those
systems would likely reside on orbiting exomoons. Perhaps such exomoons could
generate powerful radio emissions akin to the Io-controlled DAM, and their
presence could be inferred through the modulation of exoplanetary radio
emissions by the exomoons’ orbital periods.

It may be of interest to various communities how our radio survey of
exoplanets at AO compares to efforts by SETI that have searched for
technosignatures, such as those conducted via Project Phoenix at multiple
facilities and more recently, those at the Allen Telescope Array (e.g., Harp
et al. (2016)). The exquisitively sensitive Mock Spectrometer at AO could only
detect signals as narrow in frequency as 20.9 kHz over a wide bandpass
(resampled to 83.6 kHz to improve signal-to-noise characteristics). This
compares favorably with remote observations of astrophysical phenomenon, such
the Jovian nKOM radiation, with bandwidths $\Delta\nu\sim$ 100 kHz, but
unfavorably with _in situ_ measurements of the terrestrial auroral kilometric
radiation (AKR) which has instantaneous bandwidths $\Delta\nu<$ 1 kHz
(Treumann, 2006; de Pater & Lissauer, 2015). By comparison, previous searches
for technosignatures have required bandwidths of $\Delta\nu<$ 1 Hz to
$\Delta\nu\sim$ 100 Hz. Although our principal aims in this study were to
search for auroral exoplanetary radio emissions and coronal emissions from
star-planet interactions, the results in Table 3 also place upper limits on
the detectability of technosignatures within surveyed habitable systems across
the bandpass ranging from $\sim$4255 to 5330 MHz. Using the narrowest
frequency resolution, together with the least and most sensitive observations
of our survey, those of the 51 Peg and 55 Cnc systems, respectively, we
achieve a 3$\sigma$ threshold signal level of 43 $\times$10-26 W m-2 and 21
$\times$10-26 W m-2, respectively.

##  5 Conclusion

In 2010-2011, we initiated a survey of exoplanets and brown dwarfs in a quest
to detect and characterize their radio emissions, and thereby gain insight
into their magnetic activity, magnetic fields, and surrounding plasma
environments. The results of the UCD portion of the survey were presented
previously (Route & Wolszczan, 2012, 2013). In this third installment in the
Radio Observations of Magnetized Exoplanets (ROME) series, we take a step back
from analyzing the details of a particularly interesting exoplanetary system,
HD 189733A/B/b (ROME I and II), and present the results of the entire
exoplanet portion of the survey conducted at AO. This survey sought two types
of radio emission. The first type are those powered by stellar wind
interactions with exoplanet magnetospheres, leading to the generation of
Jupiter-like auroral radio emissions powered by the electron cyclotron maser
(ECM). The second type are those generated by tidal or magnetic star-planet
interactions with close-in exoplanets that may induce enhanced coronal,
transition region, chromospheric, and photospheric activity in their host
stars. Such activity may generate powerful ECM or gyrosynchotron flares that
are phased to the exoplanet orbit.

Our survey leveraged the best technology to date: the exquisitely sensitive
305-m Arecibo radio telescope. With this telescope, we targeted 17 apparently
exoplanet-hosting systems, some with multiple exoplanets. We targeted systems
with massive exoplanets because their greater mass and reservoir of internal
energy may drive more powerful convective dynamos that may generate stronger
magnetic fields than generally has been anticipated. Eight of our systems also
feature an exoplanet within 0.1 au of its host star to maximize the
probability of creating star-planet interactions. Our survey was conducted at
4.2-5.2 GHz, in order to search for stronger magnetic fields and because this
frequency range is optimal for the detection of solar radio emissions. We
leveraged the Mock Spectrometers to construct dynamic spectra in all four
Stokes parameters. Both ECM and gyrosynchrotron activity are characterized by
circularly polarized emissions; thus the selection of events based on circular
polarization fractions $\gtrsim$10% also acts as an RFI mitigation strategy.
All targets were observed for $\lesssim$2 hrs, the maximum amount of time it
takes for them to transit the fixed dish.

Our survey did not detect any signatures of exoplanet auroral emissions, or
stellar radio flares. Our 3$\sigma$ sensitivity for targeted systems ranged
from 0.98–2.1 mJy, resulting in $\nu~{}L_{\nu}$ upper limits of
5.0$\times$1023 ergs s-1 to 1.5$\times$1025 ergs s-1. These luminosity
detection thresholds correspond to modest ECM flares, such as those that
appear on the M3.5 AD Leo, and powerful gyrosynchrotron flares, such as from
the K4+K7.5 BY Dra AB system. It has been hypothesized that extremely luminous
optical superflares from FGK stars might be caused by star-planet
interactions, which have luminosities $L\sim$5$\times$1031 ergs s-1. Although
our survey was sensitive to radio emissions of equal energy output to these
optical superflares, it is unclear whether the _radio_ counterparts to such
flares would be detectable with current instrumentation.

Several of the targeted systems we surveyed are now known to host low-mass
star or brown dwarf companions. These systems include HD 114762 Ab ($\sim$M5),
HD 106252b ($\sim$T9), and HD 38529c ($\sim$Y0). Similar objects are known to
be powerful radio emitters, especially in the $\sim$5 GHz frequency window,
yet no emissions were detected. In each of these cases, the object was likely
observed for a significant fraction of its rotation period, suggesting that
they are either magnetically inactive, or sporadic radio emitters, as is not
uncommon among UCDs (spectral types $\geq$M7).

Based on empirically-derived radiometric Bode’s laws, several of our targets
were thought to be especially promising for the generation of high-frequency
radio emissions, including HD 38529c, HD 106252b, 70 Vir b, 55 Cnc d, HD
178911 Bb, HD 10697b, and HD 50554b. However, no signs of auroral radio
emissions were detected. Several of our targets, including HD 46375, 70 Vir,
51 Peg, HD 209458 and HD 189733 have previously been investigated for signs of
chromospheric activity enhancement that may be due to star-planet
interactions. In many of these cases, subsequent work demonstrated that they
do not seem to exhibit increased magnetic activity phased with the orbital or
beat periods of their hot Jupiter companions. Similarly, our search for
enhanced coronal activity did not reveal any promising signs of activity.

There are several potential reasons for why this search failed. First, if we
construct a trend line connecting the radio activity from the coolest, latest
type brown dwarfs to that of Jupiter, we find that instrumental sensitivity
must increase by $\gtrsim$103 in order to detect auroral radio emissions. More
optimistically, star-planet enhanced coronal activity likely require a
$\gtrsim$101 increase in sensitivity to be detected at radio wavelengths.
Second, although it is likely that the frequency range that we observed
targets at was too high, it remains unknown what are the cutoff frequencies
(local cyclotron frequencies) of giant exoplanets. However, the frequency
range with which we searched for enhanced coronal emissions due to star-planet
interactions was appropriate. Third, since giant exoplanet rotation periods
are likely $\gtrsim$ 10 hrs, our rotational phase coverage of their auroral
activity varied from 2–80%. In the search for star-planet interactions, our
orbital phase coverage was $\leq$8% for close-in, hot exoplanet targets.

This diagnosis of our survey points the way to improving the productivity of
future searches for exoplanet auroral emissions and enhanced coronal activity
due to star-planet interactions. First, great improvements in sensitivity are
required to search for both types of emission, although only an increase in
sensitivity of $\sim$10–100 is required to enable the search for enhanced
coronal activity from star-planet interactions on the nearest stars. For the
detection of exoplanet auroral emissions, the situation is rather more
pessimistic. Second, future instrumentation should seek to simultaneously
observe as wide of a bandpass as possible given the great uncertainty in
exoplanet magnetic field strengths, and therefore, the frequency of their
cyclotron emissions. Such an advance will also improve the ability to detect
flares from star-planet interactions. Third, future surveys should leverage
full Stokes parameters to distinguish and characterize among various types of
astrophysical emission, and as an RFI mitigation strategy. Fourth, future work
will require longer, and more frequent, observations of planet-hosting
systems. High cadence observations will enable researchers to untangle the
effects of intrinsic stellar activity from differentially rotating stars, from
enhanced activity induced by tidal and magnetic star-planet interactions, and
from variable and sporadic exoplanet auroral radio emissions.

##  6 Acknowledgments

MR would like to acknowledge support from the Center for Exoplanets and
Habitable Worlds and the Zaccheus Daniel Fellowship. The Center for Exoplanets
and Habitable Worlds is supported by the Pennsylvania State University, and
the Eberly College of Science. Data storage and analysis support has been made
possible with the Theodore Dunham, Jr. Grants for Research in Astronomy. At
the time of the observations that are the subject of this publication, the
Arecibo Observatory was operated by SRI International under a cooperative
agreement with the National Science Foundation (AST-1100968), and in alliance
with Ana G. Méndez-Universidad Metropolitana, and the Universities Space
Research Association.

This research has made use of NASA’s Astrophysics Data System and the SIMBAD
database, operated at CDS, Strasbourg, France. Guidance on properties and
publications related to objects of interest were obtained from the catalog
located at exoplanet.eu.

Arecibo

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Table 1: Survey Target Properties

Name | R.A. | Dec. | Host Star | Dist. | Semimajor | Period | Mass | Discovery | Properties
---|---|---|---|---|---|---|---|---|---
| (hh0mm0ss) | (0° 0′ 0″) | Type | (pc) | Axis (AU) | (d) | (MJ) | Ref(s) | Refs
HD 10697 baaAlso known as 109 Psc b. | 01 44 55 | +20 04 59 | G5 IV | 32.6 | 2.12 | 1072.3 | 6.837 | 1 |  1,2
$\epsilon$ Tau bbbAlso known as HD 28305 b. | 04 28 37 | +19 10 50 | K0 III | 47.5 | 1.90 | 594.9 | 7.34 | 3 |  3,4,4
GJ 176 bccAlso known as HD 285968 b. | 04 42 56 | +18 57 29 | M2.5 Ve | 9.4 | 0.066 | 8.7836 | 0.0264 | 5 |  5,5
HD 38529 b | 05 46 34 | +01 10 05 | G4 IV | 42.4 | 0.131 | 14.31 | 0.90 | 6 |  6,7,7
c |  |  |  |  | 3.697 | 1236.14 | 17.6ddWith a mass of 0.017$M_{\odot}$, 5 Gyr isochrones yield $T_{eff}\sim$430 K and an estimated spectral type of Y0 (Baraffe et al., 2003; Cushing et al., 2011). | 8 |  6,7,7
HD 46375 b | 06 33 12 | +05 27 46 | K1 IV | 33.4 | 0.0398 | 3.024 | 0.226 | 9 |  10,10
HD 50554 b | 06 54 42 | +24 14 44 | F8 V | 31.03 | 2.28 | 1224 | 4.46 | 11 |  10,10
55 Cnc e | 08 52 37 | +28 20 02 | K0 V | 12.59 | 0.0154 | 0.7365 | 0.027 | 12 |  13,14,13
b |  |  |  |  | 0.1134 | 14.6516 | 0.8036 | 12 |  13,14,14
c |  |  |  |  | 0.2373 | 44.3989 | 0.1611 | 12 |  13,14,14
f |  |  |  |  | 0.7708 | 259.88 | 0.1503 | 12 |  13,14,14
d |  |  |  |  | 5.450 | 5574.2 | 3.12 | 12 |  13,14,14
GJ 436 b | 11 42 11 | +26 42 23 | M2.5 V | 10.23 | 0.02887 | 2.644 | 0.0737 | 15 |  10,16,16
HD 102195 b | 11 45 42 | +02 49 17 | G8 V | 29 | 0.049 | 4.114 | 0.46 | 17 |  17,18,18
HD 106252 b | 12 13 29 | +10 02 29 | G0 V | 37.4 | 2.6 | 1531 | 30.6ee5 Gyr isochrones yield $T_{eff}\sim$610 K and an estimated spectral type of T9 for an object of mass 0.029$M_{\odot}$ (Baraffe et al., 2003; Cushing et al., 2011). | 11 |  10,19,19
HD 114762 Ab | 13 12 19 | +17 31 01 | F9 V | 38.7 | 0.361 | 83.915 | 147ffFigure 10 in Chabrier & Baraffe (2000) indicates spectral type M5 roughly corresponds to a 0.140$M_{\odot}$ object that is 5 Gyr old of solar metallicity. | 20 |  20,21,21
70 Vir b | 13 28 26 | +13 47 12 | G4 V | 22 | 0.481 | 116.69 | 7.4 | 22 |  22,23,23
HD 178911 Bb | 19 09 03 | +34 35 59 | G5 | 46.73 | 0.339 | 71.484 | 7.03 | 24 |  25,25
HD 189733 b | 20 00 43 | +22 42 39 | K2 V | 19.78 | 0.0313 | 2.219 | 1.13 | 26 |  27,26,28
HD 195019 b | 20 28 17 | +18 46 12 | G3 IV/V | 37.4 | 0.1388 | 18.2 | 3.69 | 29 |  10,10
HD 209458 b | 22 03 10 | +18 53 04 | G0 V | 47 | 0.0467 | 3.5247 | 0.714 | 30,31 |  30,31,16,16
51 Peg b | 22 57 27 | +20 46 07 | G2 IV | 15.36 | 0.053 | 4.231 | 0.46 | 32 |  10,33

Note. — Exoplanet, brown dwarf, and low-mass stellar companion hosting system
properties. In the properties references column (column 10), bold, underlined,
and _italicized_ numerals denote distance from Earth, semimajor axis, and
orbital period references, respectively. The final number in the column in
normal font provides the companion object’s mass reference(s). References. (1)
Vogt et al. (2000); (2) Simpson et al. (2010); (3) Sato et al. (2007); (4)
Kunitomo et al. (2011); (5) Forveille et al. (2009); (6) Fischer et al.
(2001); (7) Fritz Benedict et al. (2010); (8) Fischer et al. (2003); (9) Marcy
et al. (2000); (10) Butler et al. (2006); (11) Perrier et al. (2003); (12)
Fischer et al. (2008); (13) Crida et al. (2018); (14) Bourrier et al. (2018);
(15) Butler et al. (2004); (16) Southworth (2010); (17) Ge et al. (2006); (18)
Guilluy et al. (2019); (19) Reffert & Quirrenbach (2011); (20) Latham et al.
(1989); (21) Kiefer et al. (2021); (22) Marcy & Butler (1996); (23) Kane et
al. (2015); (24) Zucker et al. (2002); (25) Wittenmyer et al. (2009); (26)
Bouchy et al. (2005); (27) Gaia Collaboration (2018); (28) Boisse et al.
(2009); (29) Fischer et al. (1999); (30) Henry et al. (2000); (31) Charbonneau
et al. (2000); (32) Mayor & Queloz (1995); (33) Martins et al. (2015).

Table 2: Observations List

Name | UT Date | File Date | Scan Range | Number | Time on
---|---|---|---|---|---
| (yyyymmdd) | (yyyymmdd) |  | of Scans | Source (ks)
HD 10697 | 2010 12 21 | 2010 12 20 | 00000-01200 | 4 | 2.4
$\epsilon$ Tau | 2010 01 09 | 2010 01 08 | 00000-02000 | 7 | 4.2
| 2010 12 18 | 2010 12 17 | 10100-10700* | 3 | 1.8
| 2010 12 18 | 2010 12 18 | 00000-00400 | 1 | 0.6
| 2010 12 21 | 2010 12 20 | 02500-03600 | 4 | 2.4
GJ 176 | 2010 01 10 | 2010 01 09 | 01800-03500 | 6 | 3.6
| 2010 12 19 | 2010 12 18 | 12500-13100* | 3 | 1.8
| 2010 12 19 | 2010 12 19 | 00000-00700 | 2 | 1.2
HD 38529 | 2010 01 09 | 2010 01 08 | 02100-03800 | 6 | 3.6
HD 46375 | 2010 01 10 | 2010 01 09 | 03600-04800* | 5 | 3.0
| 2010 01 10 | 2010 01 10 | 00000-00400 | 1 | 0.6
HD 50554 | 2010 01 09 | 2010 01 08 | 03900-04200* | 2 | 1.2
| 2010 01 09 | 2010 01 09 | 00000-00400 | 1 | 0.6
| 2010 12 18 | 2010 12 18 | 00500-02800 | 8 | 4.8
| 2010 12 19 | 2010 12 19 | 00800-01900 | 4 | 2.4
55 Cnc | 2011 01 02 | 2011 01 02 | 02700-05000 | 8 | 4.8
GJ 436 | 2010 01 06 | 2010 01 06 | 01800-03500 | 6 | 3.6
HD 102195 | 2010 01 07 | 2010 01 07 | 00000-01700 | 6 | 3.6
HD 106252 | 2010 01 07 | 2010 01 07 | 01800-03500 | 6 | 3.6
HD 114762 | 2010 01 07 | 2010 01 07 | 03600-03800 | 1 | 0.6
| 2011 01 02 | 2011 01 02 | 08700-10500 | 7 | 4.2
70 Vir | 2010 01 06 | 2010 01 06 | 03600-05600 | 7 | 4.2
HD 178911 B | 2011 07 20 | 2011 07 19 | 06200-07100* | 4 | 2.4
| 2011 07 20 | 2011 07 20 | 00000-01000 | 3 | 1.8
HD 189733 | 2011 09 07 | 2011 09 06 | 00000-03500 | 12 | 7.2
HD 195019 | 2011 07 20 | 2011 07 20 | 01100-04000 | 10 | 6.0
HD 209458 | 2011 07 20 | 2011 07 20 | 04100-05800 | 6 | 3.6
| 2011 09 07 | 2011 09 06 | 03600-05500 | 7 | 4.2
51 Peg | 2011 07 19 | 2011 07 18 | 04100-06100 | 7 | 4.2
| 2011 09 07 | 2011 09 06 | 05700 | 1 | 0.6
| 2011 09 07 | 2011 09 07 | 00000-01600 | 5 | 3.0

Note. — Asterisks in the scan range column denote an observing session that
directly continued onto the following day.

Table 3: Survey Detection Results

Object | Mass | Detected Flux |  $\nu$Lν |  $\nu$Lν |  $\nu$Lν aaLuminosity in terms of the average power output of Jupiter’s ECM-induced DAM during maximum solar activity (as opposed to average power or peak power), or $L_{J,rad}=8.2\times$1017 erg s-1 (Zarka et al., 2004). | Lbol |  $\nu$Lν/Lbol
---|---|---|---|---|---|---|---
| (MJ) | (mJy) | (ergs s-1) | (log L⊙) | (log LJ,rad) | (log L⊙) | (log L⊙)
HD 10697 b | 6.837 |  $<$1.305 |  $<$7.413$\times$1024 |  $<$-8.714 |  $<$6.956 | -7.293 |  $<$-1.421
$\epsilon$ Tau b | 7.34 |  $<$1.101 |  $<$1.382$\times$1025 |  $<$-8.461 |  $<$7.209 | -6.010 |  $<$-2.451
GJ 176 b | 0.0264 |  $<$1.065 |  $<$5.030$\times$1023 |  $<$-9.883 |  $<$5.788 | -8.039 |  $<$-1.844
HD 38529 b | 0.90 |  $<$1.425 |  $<$1.369$\times$1025 |  $<$-8.448 |  $<$7.223 | -6.934 |  $<$-1.514
c | 17.6 |  $<$1.425 |  $<$1.369$\times$1025 |  $<$-8.448 |  $<$7.223 | -3.117 |  $<$-5.331
HD 46375 b | 0.226 |  $<$1.134 |  $<$6.761$\times$1024 |  $<$-8.754 |  $<$6.916 | -4.441 |  $<$-4.313
HD 50554 b | 4.46 |  $<$1.038 |  $<$5.342$\times$1024 |  $<$-8.857 |  $<$6.814 | -8.000 |  $<$-0.857
55 Cnc e | 0.027 |  $<$0.984 |  $<$8.336$\times$1023 |  $<$-9.663 |  $<$6.007 | -5.307 |  $<$-4.356
b | 0.8036 |  $<$0.984 |  $<$8.336$\times$1023 |  $<$-9.663 |  $<$6.007 | -7.741 |  $<$-1.922
c | 0.1611 |  $<$0.984 |  $<$8.336$\times$1023 |  $<$-9.663 |  $<$6.007 | -7.278 |  $<$-2.385
f | 0.1503 |  $<$0.984 |  $<$8.336$\times$1023 |  $<$-9.663 |  $<$6.007 | -7.998 |  $<$-1.665
d | 3.12 |  $<$0.984 |  $<$8.336$\times$1023 |  $<$-9.663 |  $<$6.007 | -9.172 |  $<$-0.491
GJ 436 b | 0.0737 |  $<$1.047 |  $<$5.856$\times$1023 |  $<$-9.817 |  $<$5.854 | -6.616 |  $<$-3.201
HD 102195 b | 0.46 |  $<$1.302 |  $<$5.852$\times$1024 |  $<$-8.817 |  $<$6.854 | -7.178 |  $<$-1.639
HD 106252 b | 30.6 |  $<$1.194 |  $<$8.926$\times$1024 |  $<$-8.634 |  $<$7.037 | -3.068 |  $<$-5.566
HD 114762 Ab | 147 |  $<$1.140 |  $<$9.125$\times$1024 |  $<$-8.624 |  $<$7.046 | -2.750 |  $<$-5.874
70 Vir b | 7.4 |  $<$1.047 |  $<$2.708$\times$1024 |  $<$-9.152 |  $<$6.519 | -6.333 |  $<$-2.819
HD 178911 Bb | 7.03 |  $<$1.287 |  $<$1.502$\times$1025 |  $<$-8.408 |  $<$7.263 | -6.410 |  $<$-1.998
HD 189733 bbbRadio emission flux density upper limit presented in Route (2019), with luminosity updated due to revised distance to HD 189733 (Gaia Collaboration, 2018). | 1.13 |  $<$1.158 |  $<$2.421$\times$1024 |  $<$-9.200 |  $<$6.470 | -4.763 |  $<$-4.437
HD 195019 b | 3.69 |  $<$1.203 |  $<$8.944$\times$1024 |  $<$-8.630 |  $<$7.040 | -5.422 |  $<$-3.208
HD 209458 b | 0.714 |  $<$1.155 |  $<$1.364$\times$1025 |  $<$-8.450 |  $<$7.221 | -4.106 |  $<$-4.344
51 Peg b | 0.46 |  $<$2.067 |  $<$2.606$\times$1024 |  $<$-9.168 |  $<$6.502 | -4.097 |  $<$-5.071
HR 8799ccRadio emission flux density upper limits for all HR 8799 exoplanets were reported in relation to our first UCD survey (Route & Wolszczan, 2013). e | 9.6 |  $<$1.044 |  $<$8.662$\times$1024 |  $<$-8.647 |  $<$7.024 | -4.520 |  $<$-4.127
d | 7.2 |  $<$1.044 |  $<$8.662$\times$1024 |  $<$-8.647 |  $<$7.024 | -4.7 |  $<$-3.947
c | 7.2 |  $<$1.044 |  $<$8.662$\times$1024 |  $<$-8.647 |  $<$7.024 | -4.7 |  $<$-3.947
b | 5.8 |  $<$1.044 |  $<$8.662$\times$1024 |  $<$-8.647 |  $<$7.024 | -5.1 |  $<$-3.547

Note. — Detection limits and radio luminosities of target systems.

Table 4: Companion Bolometric Luminosities

Companion | Radius | Teff | Lbol | Refs | Stellar Radius | Stellar Teff | Stellar Refs
---|---|---|---|---|---|---|---
| (RJ) | (K) | (ergs s-1) |  | (R⊙) | (K) |
HD 10697 b | 1.152* | 255$\dagger$ | 1.954$\times$1026* |  1,2 | - | - | -
$\epsilon$ Tau b | 1.012* | 570* | 3.754$\times$1027* | 3 | 13.7 | 4901 | 4
GJ 176 b | 0.183* | 417* | 3.507$\times$1025* | 5 | 0.453 | 3679 | 6
HD 38529 b | 0.100* | 1064* | 4.470$\times$1026* | 3 | 2.44 | 5697 | 7
c | 0.955* | 3100* | 2.933$\times$1030* |  3,8 | - | - | -
HD 46375 b | 1.020 | 1400* | 1.392$\times$1029* |  9,10 | - | - | -
HD 50554 b | 1.035* | 179* | 3.839$\times$1025* | 3 | 1.11 | 5929 |  11, 12
55 Cnc e | 0.178 | 2037$\dagger$ | 1.892$\times$1028* |  13,14 | - | - | -
b | 0.102* | 661* | 6.972$\times$1025* | 3 | 0.98 | 5196 |  13,15
c | 0.365* | 457* | 2.025$\times$1026* | 5 | 0.98 | 5196 |  13,15
f | 0.365* | 302* | 3.858$\times$1025* |  5,2 | - | - | -
d | 1.035* | 95* | 2.585$\times$1024* | 3 | 0.98 | 5196 |  13,15
GJ 436 b | 0.365 | 669* | 9.293$\times$1026 | 16 | - | - | -
HD 102195 b | 0.096* | 947* | 2.551$\times$1026* | 3 | 0.837 | 5291 | 17
HD 106252 b | 0.891* | 3300* | 3.281$\times$1030* |  3,8 | - | - | -
HD 114762 Ab | - | - | 6.827$\times$1030* | _18_ | - | - | -
70 Vir b | 1.012 | 473* | 1.784$\times$1027* | 3 | 1.94 | 5439 | 19
HD 178911 Bb | 1.023* | 450* | 1.495$\times$1027* | 3 | 1.14 | 5668 |  20,21
HD 189733 b | 1.154 | 1093$\dagger$ | 6.619$\times$1028 |  22,23 | - | - | -
HD 195019 b | 1.035* | 790* | 1.454$\times$1028* | 3 | 1.38 | 5788 |  20,12
HD 209458 b | 1.380 | 1459* | 3.005$\times$1029 | 16 | - | - | -
51 Peg b | 1.9 | 1250* | 3.069$\times$1029* |  24,25 | - | - | -
HR 8799 e | - | - | 1.159$\times$1029 | _26_ | - | - | -
d | - | - | 7.660$\times$1028 | _27_ | - | - | -
c | - | - | 7.660$\times$1028 | _27_ | - | - | -
b | - | - | 3.049$\times$1028 | _27_ | - | - | -

Note. — Measured and inferred bolometric luminosities of the surveyed
companion low-mass stars, brown dwarfs, and exoplanets. For companion
properties (columns 2-4), bold, underlined, and _italicized_ numerals denote
radius, temperature, and luminosity references (column 5), respectively.
Similarly, for stellar properties (columns 6-7), bold and underlined numerals
denote radius and temperature references, respectively, while relevant semi-
major axes can be found in Table 1. Asterisks (*) denote values that are
either estimated within the given references, or computed based upon the
relevant stellar and orbital properties (see text for more details). Dagger
symbols ($\dagger$) denote companion Teffs computed by averaging measured
minimum and maximum temperatures. Below the horizontal bar are the measured
luminosities of the hot, young exoplanets of the HR 8799 system for
comparison.

References. (1) Luhn et al. (2019); (2) Kane & Gelino (2012); (3) Chabrier et
al. (2009); (4) Sato et al. (2007); (5) Seager et al. (2007); (6) von Braun et
al. (2014); (7) Fritz Benedict et al. (2010); (8) Chabrier & Baraffe (2000);
(9) Wang & Ford (2011); (10) Marcy et al. (2000); (11) Baines et al. (2008);
(12) Butler et al. (2006); (13) Crida et al. (2018); (14) Demory et al.
(2016); (15) von Braun et al. (2011); (16) Southworth (2010); (17) Guilluy et
al. (2019); (18) Reid & Hawley (2005); (19) Kane et al. (2015); (20) Fischer &
Valenti (2005); (21) Wittenmyer et al. (2009); (22) Bakos et al. (2006); (23)
Knutson et al. (2007); (24) Martins et al. (2015); (25) Marcy et al. (1997);
(26) Brandt et al. (2021); (27) Marois et al. (2010).

Table 5: Orbital Phase Coverage of Radio Observations

Object | Period | Tt aaTts are measured or estimated ($\dagger$), depending upon the reference. Asterisks (*) denote values that are in BJD. | Obs Start | Obs End | Orbital Phases | Orbital Refs
---|---|---|---|---|---|---
| (d) | (d) | (d) | (d) | Monitored |
GJ 176 b | 8.7836 | 54399.790* | 55206.590 | 55206.634 | 0.853-0.858 | 1
|  |  | 55549.655 | 55549.691 | 0.910-0.915 |
HD 46375 b | 3.023573 | 51071.357$\dagger$ | 55206.639 | 55206.683 | 0.681-0.695 | 2
55 Cnc e | 0.7365474 | 57063.210* | 55563.752 | 55563.810 | 0.207-0.287 | 3
GJ 436 b | 2.64389524 | 51549.560* | 55202.865 | 55202.909 | 0.789-0.806 |  4,2
HD 102195 b | 4.1139 | 53731.701$\dagger$ | 55203.855 | 55203.898 | 0.849-0.859 |  5,2
HD 189733 bbbOrbital phase coverage from Route (2019). | 2.218575 | 53629.389* | 55811.509 | 55811.598 | 0.568-0.608 |  6,7
HD 209458 b | 3.52474859 | 52854.830* | 55762.785 | 55762.829 | 0.011-0.023 |  4,2
|  |  | 55811.605 | 55811.660 | 0.861-0.877 |
51 Peg b | 4.230785 | 50001.886$\dagger$ | 55761.784 | 55761.835 | 0.425-0.438 | 2
|  |  | 55811.665 | 55811.709 | 0.215-0.226 |

Note. — Orbital phase coverage of the AO radio observations for systems with
exoplanet targets that have orbital periods $<$10 d. Orbital periods are
reported to maximum precision in the reference. In the orbital references
column, bold and underlined text denote orbital period and transit time (Tt)
references, respectively. Transit times, observation start, and observation
end dates (columns 3-5) are reported in HJD (or BJD) -2,4000,000.

References. (1) Forveille et al. (2009); (2) Butler et al. (2006); (3)
Bourrier et al. (2018); (4) Southworth (2010); (5) Guilluy et al. (2019); (6)
Bouchy et al. (2005); (7) Fares et al. (2017).

Figure 1: Survey $\nu L_{\nu}$ radio luminosity upper limits compared to other
stellar flare luminosities. For each object, the spectral type(s) are given in
parentheses. The flare radio luminosity ranges for HD 189733A, $\epsilon$ Eri,
and the interacting binary BY Dra AB are calculated from their X-ray flare
luminosity ranges via the Güdel-Benz relationship (Benz & Güdel, 2010; Route,
2019). While these emissions are generated by incoherent, gyrosynchrotron
flaring, flares observed at AD Leo are created by the ECM process, and due to
their coherence and beaming, are more readily detectable (Stepanov et al.,
2001). We establish a maximum luminosity for the “Superflares” category using
the 15-day $B$ and $V$ band flare observations of the F8 IV star 5 Serpentis,
although it is non-trivial to relate the optical properties of flares to those
at radio wavelengths (Schaefer et al., 2000; Priest, 2014). The dashed and
dotted lines mark the detection thresholds of our most and least sensitive
observations, those at GJ 176 and HD 178911B, respectively, relative to these
flare energies. Figure 2: Logarithmic plots of $\nu L_{\nu}$ radio luminosity
upper limits versus spectral type (left panel) or exoplanet mass (right panel)
of surveyed brown dwarf and low-mass stellar companions, as compared to
previously detected radio-emitting UCDs (Burgasser et al., 2015; Route &
Wolszczan, 2016b; Route, 2017a; Williams & Berger, 2015; Williams et al.,
2014, 2015, 2017; Zic et al., 2019; Richey-Yowell et al., 2020). The two AO
discoveries of the coolest known radio-flaring brown dwarfs near the brown
dwarf/ exoplanet boundary are marked by filled stars (Route & Wolszczan, 2012,
2016a). HD 114762 Ab appears at upper left with an estimated spectral type of
M5. HD 106252b ($\sim$T9) and HD 38529c ($\sim$Y0) appear in the left panel at
upper right. Our earlier upper limits to the radio emission from the four
young exoplanets of the HR 8799 system are graphed based on their estimated
masses (Table 1), as opposed to their spectral types (Oppenheimer et al.,
2013; Route & Wolszczan, 2013). Two surveyed exoplanets are within the
habitable zones of their host stars: clockwise from upper left are HD 10697b
and 55 Cnc f (Kane & Gelino, 2012). The Jovian auroral decametric radio
emission as measured by _Cassini_ anchors the right plot in the lower right
corner (Zarka et al., 2004). Figure 3: Logarithmic plots of radio luminosities
as a fraction of bolometric luminosities versus spectral type (left panel) or
exoplanet mass (right panel) of surveyed brown dwarf and low-mass stellar
companions, as compared to previously detected radio-emitting UCDs (Burgasser
et al., 2015; Route & Wolszczan, 2016b; Route, 2017a; Williams & Berger, 2015;
Williams et al., 2014, 2015, 2017; Zic et al., 2019; Richey-Yowell et al.,
2020). The two radio-flaring, late T-type brown dwarfs, J1047+21 and J1122+25,
near the brown dwarf/ exoplanet boundary discovered by AO are denoted by
filled stars. The upper limit for HD 114762 Ab (spectral type $\sim$M5) is in
the left panel near the left-hand y-axis. Upper limits for HD 106252b
($\sim$T9) and HD 38529c ($\sim$Y0) appear in the left panel at right. The
upper limits to the radio emission from the four young exoplanets of the HR
8799 system (Route & Wolszczan, 2013) are graphed based on their properties in
Tables 1 and 3, as opposed to their brown dwarf-like luminosities and spectral
properties. 51 Peg b has the most similar limit on radio emission as a
fraction of its bolometric luminosity ($\nu~{}L_{\nu}/L_{bol}$) to that of
Jupiter and appears approximately two orders of magnitude above it. Two
surveyed exoplanets are within the habitable zones of their host stars:
clockwise from one o’clock are HD 10697b and 55 Cnc f (Kane & Gelino, 2012).
The Jovian auroral decametric radio emission relative to its cloud-top
bolometric luminosity is represented by the square at lower right in the right
panel (Zarka et al., 2004; Guillot, 2005).