arXiv subjects

Melodie M. Kao

Publications and source records attributed to Melodie M. Kao.

17 recordsLinked to original sources

arXiv2026

The MegaWave Radio Surveyor

Several Decadal-level questions in astrophysics, exoplanets, astrobiology, and cosmology can be addressed only at low radio frequencies inaccessible from Earth. The MegaWave Radio Surveyor would open this largely-unexplored region of the electromagnetic spectrum with a space-based interferometer to (1)~Track the space weather of other stars; (2)~Detect magnetically-generated emission from exoplanets to probe their interiors and assess magnetic shielding of their atmospheres; (3)~Probe the Universe's evolution during the Dark Ages via the highly-redshifted HI hyperfine line; and (4)~Assess the role of cosmic rays and magnetic fields in the cosmic web. An Astrophysics Strategic Technology & Research Accelerator (ASTRA) Initiative concept, the MegaWave Radio Surveyor's science objectives respond to the Pathways to Discovery Decadal Survey and three other National Academies studies, and it would serve as a Formative Era mission in the Enduring Quests, Daring Visions roadmap. Developments in U.S. space industries enable this observatory to be realized. The MegaWave Radio Surveyor would offer a versatile, scalable, and resilient architecture capable of sensitive and simultaneous observations below 45~MHz and unprecedented angular resolution at these frequencies. The concept builds upon NASA's Sun Radio Interferometer Space Experiment (SunRISE), Star-Planet Activity Research CubeSat (SPARCS), and Lunar Surface Electromagnetics Experiment (LuSEE-Night). The MegaWave Radio Surveyor could leverage multiple elements of the Artemis program, such as access to and beyond cislunar space and communications, and there are opportunities to infuse new autonomy/AI modes for mission operations. By opening one of the last windows in the electromagnetic spectrum and pioneering space interferometry at unprecedented scales, the MegaWave Radio Surveyor would establish a transformational capability.

astro-ph.IM↗

arXiv2026

On the Detectability of Volcanic Exo-Ios That May Fuel Auroras on Super-Jupiters

Studies suggest Jupiter's aurorae are supplied with plasma from volcanic outflows on the planet's innermost moon, Io. Repeating bursts of radio emission thought to trace massively scaled-up analogs of Jupiter's aurorae have been detected around nearly a dozen isolated substellar worlds, yet the source of the electrons fueling the aurorae remains unknown. Volcanism from tidally heated exosatellites may provide the plasma that fuel the aurora on these worlds. We assess whether transit observations provide a viable means of detecting exosatellites around aurorally active substellar worlds, thereby enabling future tests of this hypothesis. Specifically, we analyze JWST near- and mid-infrared light curves of SIMP 0136+0933, a $12.7 M_J$ "super-Jupiter", known to exhibit auroral emission. We demonstrate the capability to detect exosatellites in the SIMP 0136+0933 system with satellite-to-host mass ratios comparable to those of Jupiter's Galilean moons, achieving detection success rates of 66% for Io-to-Jupiter mass ratio satellites and 93% for Ganymede-to-Jupiter mass ratio satellites. Although the existing light curve is sufficient to demonstrate that this technique is capable of detecting transiting exosatellites, the available archival data are too short in duration to place meaningful constraints on the presence of a transiting satellite in this system. We conclude that JWST light curves spanning $\sim$1.5 days for 4-12 known aurorally active super-Jupiters would be sufficient to yield evidence for or against this hypothesis. A small target sample may suffice, as short satellite periods boost transit probabilities and aurorally active worlds may be preferentially observed near edge-on inclinations.

astro-ph.EP↗

arXiv2026

Building a Roadmap for Hubble Science into the 2030s: Revealing Atmospheric Structure and Evolution in Substellar Worlds Using HST

Substellar objects occupy a unique place in our universe, bridging the gap between the smallest stars and the largest planets, and serving as powerful laboratories for understanding extrasolar atmospheric physics without the contaminating glare of a host star. Previous studies into the atmospheric structure of these objects have revealed clouds, disequilibrium chemistry, thermal inversions, and auroral processes which each contribute to wavelength-dependent brightness variations. HST remains uniquely positioned to address key open questions in the field, such as resolving the vertical atmospheric structure, long term evolution of the atmosphere, and detection of UV aurora in the upper atmosphere, primarily in conjunction with other facilities that probe wavelength regimes that cannot be reached with instruments on HST. We advocate for three large scale initiatives and argue that the study of the atmospheres of substellar worlds directly prepares the community for atmospheric characterization with the Habitable Worlds Observatory.

astro-ph.IM↗

arXiv2024

Exoplanet Magnetic Fields

Planetary magnetic fields are important indicators of planetary processes and evolution, from a planet's outer core to its surface (if it possesses one) to its atmosphere and near-space environment. Magnetic fields are most directly measured in situ, and determining whether distant planetary objects possess magnetic fields can be challenging. At present we have no unambiguous measurements of magnetic fields on exoplanets. Nevertheless, it would be surprising if at least some exoplanets did not generate a magnetic field, like many planetary bodies in the solar system. This chapter provides an overview of the current understanding of exoplanetary magnetic fields and their consequences. In the next section we review the current understanding of planetary dynamo generation as it applies to solar system objects and discuss the implications for exoplanetary magnetic field generation. Following this, we describe seven methods for determining the existence and strength of an exoplanetary magnetic field and discuss the near-term prospects for each method. We close by highlighting four main consequences of exoplanetary magnetic fields for a planet and its evolution.

astro-ph.EP↗

arXiv2024

Great Observatory for Long Wavelengths (GO-LoW) NIAC Phase I Final Report

The low-frequency sky below $\sim$15 MHz (20 m) is obscured by the Earth's ionosphere, the layer of charged particles above the neutral atmosphere. Single spacecraft have made measurements in this band, but cannot achieve high or even moderate angular resolution because a telescope's resolution ($θ$) is set by $θ= λ/D$, where $λ$ is the wavelength and $D$ is the telescope diameter. For wavelengths that range from tens of meters to kilometers, a telescope must be hundreds of meters to many kilometers in diameter for even moderate resolution. The Great Observatory for Long Wavelengths (GO-LoW) is an interferometric mega-constellation space telescope operating between 300 kHz and 15 MHz. In a departure from the traditional approach of a single, large, expensive spacecraft (e.g., HST, Chandra, JWST), GO-LoW is an interferometric Great Observatory comprising thousands of small, inexpensive, and reconfigurable nodes. A distributed constellation of sensing elements provides (1) reliability and robustness to failures, (2) longevity by allowing for growth over time and infusion of new technology via staged replacement of nodes, (3) reduced costs through leveraging mass production, and (4) formation reconfigurability to optimize the observatory for diverse science cases. A low-frequency mega-constellation revolutionizes a number of compelling science cases: high-resolution all-sky mapping, Dark Ages/Epoch of Reionization cosmology, interstellar medium mapping, solar/planetary magnetic activity, and exoplanetary magnetospheric radio emission. This report summarizes GO-LoW's concept development under NASA's NIAC Phase I program. We discuss antenna design and sensitivity, constellation architecture, including communication and launch infrastructure, interferometric correlation and a technology roadmap.

astro-ph.IM↗

arXiv2024

Binarity Enhances the Occurrence Rate of Radiation Belt Emissions in Ultracool Dwarfs

Despite a burgeoning set of ultracool dwarf ($\leq$M7) radio detections, their radio emissions remain enigmatic. Open questions include the plasma source and acceleration mechanisms for the non-auroral "quiescent" component of these objects' radio emissions, which can trace Jovian synchrotron radiation belt analogs. Ultracool dwarf binary systems can provide test beds for examining the underlying physics for these plasma processes. We extend a recently developed occurrence rate calculation framework to compare the quiescent radio occurrence rate of binary systems to single objects. This generalized and semi-analytical framework can be applied to any set of astrophysical objects conceptualized as unresolved binary systems with approximately steady-state emission or absorption. We combine data available in the literature to create samples of 179 single ultracool dwarfs (82 M dwarfs, 74 L dwarfs, and 23 T/Y dwarfs) and 27 binary ultracool dwarf systems. Using these samples, we show that quiescent radio emissions occur in $54^{+11}_{-11}$ - $65^{+11}_{-12}$ per cent of binaries where both components are ultracool dwarfs, depending on priors. We also show that binarity enhances the ultracool dwarf quiescent radio occurrence rate relative to their single counterparts. Finally, we discuss potential implications for the underlying drivers of ultracool dwarf quiescent radio emissions, including possible plasma sources.

astro-ph.SR↗

arXiv2023

The Occurrence Rate of Quiescent Radio Emission for Ultracool Dwarfs using a Generalized Semi-Analytical Bayesian Framework

We present a generalized analytical Bayesian framework for calculating the occurrence rate of steady emission (or absorption) in astrophysical objects. As a proof-of-concept, we apply this framework to non-flaring quiescent radio emission in ultracool ($\leq$ M7) dwarfs. Using simulations, we show that our framework recovers the simulated radio occurrence rate to within 1-5% for sample sizes of 10-100 objects when averaged over an ensemble of trials and simulated occurrence rates for our assumed luminosity distribution models. In contrast, existing detection rate studies may under-predict the simulated rate by 51-66% because of sensitivity limits. Using all available literature results for samples of 82 ultracool M dwarfs, 74 L dwarfs, and 23 T/Y dwarfs, we find that the maximum-likelihood quiescent radio occurrence rate is between $15^{+4}_{-4}$ - $20^{+6}_{-5}$%, depending on the luminosity prior that we assume. Comparing each spectral type, we find occurrence rates of $17^{+9}_{-7}$ - $25^{+13}_{-10}$% for M dwarfs, $10^{+5}_{-4}$ - $13^{+7}_{-5}$% for L dwarfs, and $23^{+11}_{-9}$ - $29^{+13}_{-11}$% for T/Y dwarfs. We rule out potential selection effects and speculate that age and/or rotation may account for tentative evidence that the quiescent radio occurrence rate of L dwarfs may be suppressed compared to M and T/Y dwarfs and phenomenon. Finally, we discuss how we can harness our occurrence rate framework to carefully assess the possible physics that may be contributing to observed occurrence rate trends.

astro-ph.SR↗

arXiv2023

Resolved imaging of an extrasolar radiation belt around an ultracool dwarf

Radiation belts are present in all large-scale Solar System planetary magnetospheres: Earth, Jupiter, Saturn, Uranus, and Neptune. These persistent equatorial zones of trapped high energy particles up to tens of MeV can produce bright radio emission and impact the surface chemistry of close-in moons. Recent observations confirm planet-like radio emission such as aurorae from large-scale magnetospheric current systems on very low mass stars and brown dwarfs. These objects, collectively known as ultracool dwarfs, also exhibit quiescent radio emission hypothesized to trace stellar coronal flare activity or extrasolar radiation belt analogs. Here we present high resolution imaging of the ultracool dwarf LSR J1835+3259 demonstrating that this radio emission is spatially resolved and traces a long-lived, double-lobed, and axisymmetric structure similar in morphology to the Jovian radiation belts. Up to 18 ultracool dwarf radii separate the two lobes. This structure is stably present in three observations spanning >1 year. We infer a belt-like distribution of plasma confined by the magnetic dipole of LSR J1835+3259, and we estimate ~15 MeV electron energies that are consistent with those measured in the Jovian radiation belts. Though more precise constraints require higher frequency observations, a unified picture where radio emissions in ultracool dwarfs manifest from planet-like magnetospheric phenomena has emerged. Submitted, under review.

astro-ph.EP↗

arXiv2022

Radio Emission from Binary Ultracool Dwarf Systems

Well-characterized binary systems will provide valuable opportunities to study the conditions that are necessary for the onset of both auroral and non-auroral magnetospheric radio emission in the ultracool dwarf regime. We present new detections of non-auroral "quiescent" radio emission at 4-8 GHz of the three ultracool dwarf binary systems GJ 564 BC, LP 415-20, and 2MASS J21402931+1625183. We also tentatively detect a highly circularly polarized pulse at 4-6 GHz that may indicate aurorae from GJ 564 BC. Finally, we show that the brightest binary ultracool dwarf systems may be more luminous than predictions from single-object systems.

astro-ph.SR↗

arXiv2020

On the Correlation between L Dwarf Optical and Infrared Variability and Radio Aurorae

Photometric variability attributed to cloud phenomena is common in L/T transition brown dwarfs. Recent studies show that such variability may also trace aurorae, suggesting that localized magnetic heating may contribute to observed brown dwarf photometric variability. We assess this potential correlation with a survey of 17 photometrically variable brown dwarfs using the Karl G. Jansky Very Large Array (VLA) at 4 -- 8 GHz. We detect quiescent and highly circularly polarized flaring emission from one source, 2MASS J17502484-0016151, which we attribute to auroral electron cyclotron maser emission. The detected auroral emission extends throughout the frequency band at $\sim$5 -- 25$σ$, and we do not detect evidence of a cutoff. Our detection confirms that 2MASS J17502484-0016151 hosts a magnetic field strength of $\geq$2.9 kG, similar to those of other radio-bright ultracool dwarfs. We show that H$α$ emission continues to be an accurate tracer of auroral activity in brown dwarfs. Supplementing our study with data from the literature, we calculate the occurrence rates of quiescent emission in L dwarfs with low- and high-amplitude variability and conclude that high amplitude O/IR variability does not trace radio magnetic activity in L dwarfs.

astro-ph.SR↗

arXiv2019

Magnetism in the Brown Dwarf Regime

A suite of discoveries in the last two decades demonstrate that we are now at a point where incorporating magnetic behavior is key for advancing our ability to characterize substellar and planetary systems. The next decade heralds the exciting maturation of the now-burgeoning field of brown dwarf magnetism, and investing now in brown dwarf magnetism will provide a key platform for exploring exoplanetary magnetism and habitability beyond the solar system. We anticipate significant discoveries including: the nature of substellar and planetary magnetic dynamos, the characterization of exo-aurora physics and brown dwarf magnetospheric environments, and the role of satellites in manifestations of substellar magnetic activity. These efforts will require significant new observational capabilities at radio and near infrared wavelengths, dedicated long-term monitoring programs, and committed support for the theoretical modeling efforts underpinning the physical processes of the magnetic phenomena

astro-ph.SR↗

arXiv2017

A Panchromatic View of Brown Dwarf Aurorae

Stellar coronal activity has been shown to persist into the low-mass star regime, down to late M-dwarf spectral types. However, there is now an accumulation of evidence suggesting that at the end of the main sequence there is a transition in the nature of the magnetic activity from chromospheric and coronal to planet-like and auroral, from local impulsive heating via flares and MHD wave dissipation to energy dissipation from strong large-scale magnetospheric current systems. We examine this transition and the prevalence of auroral activity in brown dwarfs through a compilation of multi-wavelength surveys of magnetic activity, including radio, X-ray, and optical. We compile the results of those surveys and place their conclusions in the context of auroral emission as the consequence of large-scale magnetospheric current systems that accelerate energetic electron beams and drive the particles to impact the cool atmospheric gas. We explore the different manifestation of auroral phenomena in brown dwarf atmospheres, like H$α$, and define their distinguishing characteristics. We conclude that large amplitude photometric variability in the near infrared is most likely a consequence of clouds in brown dwarf atmospheres, but that auroral activity may be responsible for long-lived stable surface features. We report a connection between auroral H$α$ emission and quiescent radio emission in ECMI pulsing brown dwarfs, suggesting a potential underlying physical connection between the quiescent and auroral emissions. We also discuss the electrodynamic engines powering brown dwarf aurorae and the possible role of satellites around these systems to both power the aurorae and seed the magnetosphere with plasma.

astro-ph.SR↗

arXiv2016

A Survey for H-alpha Emission from Late L dwarfs and T dwarfs

Recently, studies of brown dwarfs have demonstrated that they possess strong magnetic fields and have the potential to produce radio and optical auroral emissions powered by magnetospheric currents. This emission provides the only window on magnetic fields in the coolest brown dwarfs and identifying additional benchmark objects is key to constraining dynamo theory in this regime. To this end, we conducted a new red optical (6300 - 9700 Angstrom) survey with the Keck telescopes looking for H-alpha emission from a sample of late L dwarfs and T dwarfs. Our survey gathered optical spectra for 29 targets, 18 of which did not have previous optical spectra in the literature, greatly expanding the number of moderate resolution (R~2000) spectra available at these spectral types. Combining our sample with previous surveys, we confirm an H-alpha detection rate of 9.2 (+3.5/-2.1) % for L and T dwarfs in the optical spectral range of L4 - T8. This detection rate is consistent with the recently measured detection rate for auroral radio emission from Kao et al. (2016), suggesting that geometrical selection effects due to the beaming of the radio emission are small or absent. We also provide the first detection of H-alpha emission from 2MASS 0036+1821, previously notable as the only electron cyclotron maser radio source without a confirmed detection of H-alpha emission. Finally, we also establish optical standards for spectral types T3 and T4, filling in the previous gap between T2 and T5.

astro-ph.SR↗

arXiv2015

Auroral radio emission from late L and T dwarfs: A new constraint on dynamo theory in the substellar regime

We have observed 6 late-L and T dwarfs with the Karl G. Jansky Very Large Array (VLA) to investigate the presence of highly circularly polarized radio emission, associated with large-scale auroral currents. Previous surveys encompassing ~60 L6 or later targets in this spectral range have yielded only one detection. Our sample includes the previously detected T6.5 dwarf 2MASS 10475385+2124234 as well as 5 new targets selected for the presence of H-alpha emission or optical/infrared photometric variability, which are possible manifestations of auroral activity. We detect 2MASS 10475385+2124234, as well as 4 of the 5 targets in our biased sample, including the strong IR variable SIMP J01365662+0933473 and bright H-alpha emitter 2MASS 12373919+6526148, reinforcing the possibility that activity at these disparate wavelengths is related. The radio emission frequency corresponds to a precise determination of the lower-bound magnetic field strength near the surface of each dwarf and this new sample provides robust constraints on dynamo theory in the low mass brown dwarf regime. Magnetic fields >2.5 kG are confirmed for 5/6 targets. Our results provide tentative evidence that the dynamo operating in this mass regime may be inconsistent with predicted values from a recently proposed model. Further observations at higher radio frequencies are essential for verifying this assertion.

astro-ph.SR↗

arXiv2014

Precious Metals in SDSS Quasar Spectra II: Tracking the Evolution of Strong, 0.4 < z < 2.3 MgII Absorbers with Thousands of Systems

We have performed an analysis of over 34,000 MgII doublets at 0.36 < z < 2.29 in Sloan Digital Sky Survey (SDSS) Data-Release 7 quasar spectra; the catalog, advanced data products, and tools for analysis are publicly available. The catalog was divided into 14 small redshift bins with roughly 2500 doublets in each, and from Monte-Carlo simulations, we estimate 50% completeness at rest equivalent width W_r ~ 0.8 Angstrom. The equivalent-width frequency distribution is described well by an exponential model at all redshifts, and the distribution becomes flatter with increasing redshift, i.e., there are more strong systems relative to weak ones. Direct comparison with previous SDSS MgII surveys reveal that we recover at least 70% of the doublets in these other catalogs, in addition to detecting thousands of new systems. We discuss how these surveys come by their different results, which qualitatively agree but, due to the very small uncertainties, differ by a statistically significant amount. The estimated physical cross-section of MgII-absorbing galaxy halos increased three-fold, approximately, from z = 0.4 --> 2.3, while the W_r >= 1 Angstrom absorber line density grew, dN_MgII/dX, by roughly 45%. Finally, we explore the different evolution of various absorber populations - damped Lyman-alpha absorbers, Lyman-limit systems, strong CIV absorbers, and strong and weaker MgII systems - across cosmic time (0 < z < 6).

astro-ph.CO↗

arXiv2013

Precious Metals in SDSS Quasar Spectra I: Tracking the Evolution of Strong, 1.5 < z < 4.5 CIV Absorbers with Thousands of Systems

We have vastly increased the CIV statistics at intermediate redshift by surveying the thousands of quasars in the Sloan Digital Sky Survey Data-Release 7. We visually verified over 16,000 CIV systems with 1.46 < z < 4.55---a sample size that renders Poisson error negligible. Detailed Monte Carlo simulations show we are approximately 50% complete down to rest equivalent widths W_r ~ 0.6 Å. We analyzed the sample as a whole and in ten small redshift bins with approximately 1500 doublets each. The equivalent width frequency distributions f(W_r) were well modeled by an exponential, with little evolution in shape. In contrast with previous studies that modeled the frequency distribution as a single power law, the fitted exponential gives a finite mass density for the CIV ions. The co-moving line density dN_CIV/dX evolved smoothly with redshift, increasing by a factor of 2.37+/-0.09 from z = 4.55 to 1.96, then plateauing at dN_CIV/dX ~ 0.34 for z = 1.96 to 1.46. Comparing our SDSS sample with z < 1 (ultraviolet) and z > 5 (infrared) surveys, we see an approximately 10-fold increase in dN_CIV/dX over z ~ 6 --> 0, for W_r >= 0.6 Å. This suggests a monotonic and significant increase in the enrichment of gas outside galaxies over the 12 Gyr lifetime of the universe.

astro-ph.CO↗

arXiv2012

Extremely metal-poor gas at a redshift of 7

In typical astrophysical environments, the abundance of heavy elements ranges from 0.001 to 2 times the solar concentration. Lower abundances have been seen in select stars in the Milky Way's halo and in two quasar absorption systems at redshift z=3. These are widely interpreted as relics from the early universe, when all gas possessed a primordial chemistry. Before now there have been no direct abundance measurements from the first Gyr after the Big Bang, when the earliest stars began synthesizing elements. Here we report observations of hydrogen and heavy element absorption in a quasar spectrum at z=7.04, when the universe was just 772 Myr old (5.6% its present age). We detect a large column of neutral hydrogen but no corresponding heavy elements, limiting the chemical abundance to less than 1/10,000 the solar level if the gas is in a gravitationally bound protogalaxy, or less than 1/1,000 solar if it is diffuse and unbound. If the absorption is truly intergalactic, it would imply that the universe was neither ionized by starlight nor chemically enriched in this neighborhood at z~7. If it is gravitationally bound, the inferred abundance is too low to promote efficient cooling, and the system would be a viable site to form the predicted but as-yet unobserved massive population III stars in the early universe.

astro-ph.CO↗