arXiv subjects

Rachel A. Osten

Publications and source records attributed to Rachel A. Osten.

At least 37 records · Page 2Linked to original sources

arXiv2021

Discovery of an Extremely Short Duration Flare from Proxima Centauri Using Millimeter through FUV Observations

We present the discovery of an extreme flaring event from Proxima Cen by ASKAP, ALMA, HST, TESS, and the du Pont Telescope that occurred on 2019 May 1. In the millimeter and FUV, this flare is the brightest ever detected, brightening by a factor of >1000 and >14000 as seen by ALMA and HST, respectively. The millimeter and FUV continuum emission trace each other closely during the flare, suggesting that millimeter emission could serve as a proxy for FUV emission from stellar flares and become a powerful new tool to constrain the high-energy radiation environment of exoplanets. Surprisingly, optical emission associated with the event peaks at a much lower level with a time delay. The initial burst has an extremely short duration, lasting for <10 sec. Taken together with the growing sample of millimeter M dwarf flares, this event suggests that millimeter emission is actually common during stellar flares and often originates from short burst-like events.

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arXiv2020

Properties of M Dwarf Flares at Millimeter Wavelengths

We report on two millimeter flares detected by ALMA at 220 GHz from AU Mic, a nearby M dwarf. The larger flare had a duration of only $\sim35$ sec, with peak $L_{R}=2\times10^{15}$ erg s$^{-1}$ Hz$^{-1}$, and lower limit on linear polarization of $|Q/I|>0.12\pm0.04$. We examine the characteristics common to these new AU Mic events and those from Proxima Cen previously reported in MacGregor et al. (2018) - namely short durations, negative spectral indices, and significant linear polarization - to provide new diagnostics of conditions in outer stellar atmospheres and details of stellar flare particle acceleration. The event rates ($\sim20$ and $4$ events day$^{-1}$ for AU Mic and Proxima Cen, respectively) suggest that millimeter flares occur commonly but have been undetected until now. Analysis of the flare observing frequency and consideration of possible incoherent emission mechanisms confirms the presence of MeV electrons in the stellar atmosphere occurring as part of the flare process. The spectral indices point to a hard distribution of electrons. The short durations and lack of pronounced exponential decay in the light curve are consistent with formation in a simple magnetic loop, with radio emission predominating from directly precipitating electrons. We consider the possibility of both synchrotron and gyrosynchrotron emission mechanisms, although synchrotron is favored given the linear polarization signal. This would imply that the emission must be occurring in a low density environment of only modest magnetic field strength. A deeper understanding of this newly discovered and apparently common stellar flare mechanism awaits more observations with better-studied flare components at other wavelengths.

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arXiv2019

Short Duration Stellar Flares in GALEX Data

We report on a population of short duration near-ultraviolet (NUV) flares in stars observed by the Kepler and GALEX missions. We analyzed NUV light curves of 34,276 stars observed from 2009-2013 by both the GALEX (NUV) and Kepler (optical) space missions with the eventual goal of investigating multi-wavelength flares. From the GALEX data we constructed light curves with a 10 second cadence, and ultimately detected 1,904 short duration flares on 1,021 stars. The vast majority (94.5\%) of these flares have durations less than five minutes, with flare flux enhancements above the quiescent flux level ranging from 1.5 to 1700. The flaring stars are primarily solar-like, with T$_{\rm eff}$ ranging from 3,000-11,000 K and radii between 0.5-15 R$_{\odot}$. This set of flaring stars is almost entirely distinct from that of previous flare surveys of Kepler data and indicates a previously undetected collection of small flares contained within the Kepler sample. The range in flare energies spans 1.8$\times$10$^{32}$-8.9$\times$10$^{37}$ erg, with associated relative errors spanning 2-87\%. The flare frequency distribution by energy follows a power-law with index $α=1.72\pm0.05$, consistent with results of other solar and stellar flare studies at a range of wavelengths. This supports the idea that the NUV flares we observe are governed by the same physical processes present in solar and optical flares. The relationship between flare duration and associated flare energy extends results found for solar and stellar white-light flares, and suggests that these flares originate in regions with magnetic field strengths of several hundred Gauss, and length scales of order 10$^{10}$ cm.

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arXiv2019

X-ray Studies of Exoplanets: A 2020 Decadal Survey White Paper

Over the last two decades, the discovery of exoplanets has fundamentally changed our perception of the universe and humanity's place within it. Recent work indicates that a solar system's X-ray and high energy particle environment is of fundamental importance to the formation and development of the atmospheres of close-in planets such as hot Jupiters, and Earth-like planets around M stars. X-ray imaging and spectroscopy provide powerful and unique windows into the high energy flux that an exoplanet experiences, and X-ray photons also serve as proxies for potentially transfigurative coronal mass ejections. Finally, if the host star is a bright enough X-ray source, transit measurements akin to those in the optical and infrared are possible and allow for direct characterization of the upper atmospheres of exoplanets. In this brief white paper, we discuss contributions to the study of exoplanets and their environs which can be made by X-ray data of increasingly high quality that are achievable in the next 10--15 years.

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arXiv2019

High-Energy Photon and Particle Effects onExoplanet Atmospheres and Habitability

It is now recognized that energetic stellar photon and particle radiation evaporates and erodes planetary atmospheres and controls upper atmospheric chemistry. Key exoplanet host stars will be too faint at X-ray wavelengths for accurate characterization using existing generation and future slated X-ray telescopes. Observation of stellar coronal mass ejections and winds are also beyond current instrumentation. In line with theCommittee on an Exoplanet Science Strategy recognition that holistic observational approaches are needed, we point out here that a full understanding of exoplanet atmospheres, their evolution and determination of habitability requires a powerful high-resolution X-ray imaging and spectroscopic observatory. This is the only capability that can: (1) characterize by proxy the crucial, difficult to observe, EUV stellar flux, its history and its variations for planet hosting stars; (2) observe the stellar wind; (3) detect the subtle Doppler signatures of coronal mass ejections.

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arXiv2018

The Near-Ultraviolet Continuum Radiation in the Impulsive Phase of HF/GF-Type dMe Flares I: Data

We present NUV flare spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph during two moderate-amplitude U-band flares on the dM4e star GJ 1243. These spectra are some of the first accurately flux-calibrated, NUV flare spectra obtained over the impulsive phase in M dwarf flares. We observed these flares with a fleet of nine ground-based telescopes simultaneously, which provided broadband photometry and low-resolution spectra at the Balmer jump. A broadband continuum increase occurred with a signal-to-noise > 20 in the HST spectra, while numerous Fe II lines and the Mg II lines also increased but with smaller flux enhancements compared to the continuum radiation. These two events produced the most prominent Balmer line radiation and the largest Balmer jumps that have been observed to date in dMe flare spectra. A T=9000 K blackbody under-estimates the NUV continuum flare flux by a factor of two and is a poor approximation to the white-light in these types of flare events. Instead, our data suggest that the peak of the specific continuum flux density is constrained to U-band wavelengths near the Balmer series limit. A radiative-hydrodynamic simulation of a very high energy deposition rate averaged over times of impulsive heating and cooling better explains the lam>2500 Angstrom flare continuum properties. These two events sample only one end of the empirical color-color distribution for dMe flares, and more time-resolved flare spectra in the NUV, U-band, and optical from lam=2000-4200 Angstrom are needed during more impulsive and/or more energetic flares.

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arXiv2018

Science with an ngVLA: The ngVLA Science Case and Associated Science Requirements

The science case and associated science requirements for a next-generation Very Large Array (ngVLA) are described, highlighting the five key science goals developed out of a community-driven vision of the highest scientific priorities in the next decade. Building on the superb cm observing conditions and existing infrastructure of the VLA site in the U.S. Southwest, the ngVLA is envisaged to be an interferometric array with more than 10 times the sensitivity and spatial resolution of the current VLA and ALMA, operating at frequencies spanning $\sim1.2 - 116$\,GHz with extended baselines reaching across North America. The ngVLA will be optimized for observations at wavelengths between the exquisite performance of ALMA at submm wavelengths, and the future SKA-1 at decimeter to meter wavelengths, thus lending itself to be highly complementary with these facilities. The ngVLA will be the only facility in the world that can tackle a broad range of outstanding scientific questions in modern astronomy by simultaneously delivering the capability to: (1) unveil the formation of Solar System analogues; (2) probe the initial conditions for planetary systems and life with astrochemistry; (3) characterize the assembly, structure, and evolution of galaxies from the first billion years to the present; (4) use pulsars in the Galactic center as fundamental tests of gravity; and (5) understand the formation and evolution of stellar and supermassive blackholes in the era of multi-messenger astronomy.

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arXiv2018

The ngVLA's Role in Exoplanet Science: Constraining Exo-Space Weather

Radio observations are currently the only way to explore accelerated particles in cool stellar environments. We describe how a next generation VLA can contribute to the understanding of the stellar contribution to exo-space weather. This area holds both academic and popular interest, and is expected to grow in the next several decades.

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arXiv2017

Key Science Goals for the Next Generation Very Large Array (ngVLA): Report from the ngVLA Science Advisory Council

This document describes some of the fundamental astrophysical problems that require observing capabilities at millimeter- and centimeter wavelengths well beyond those of existing, or already planned, telescopes. The results summarized in this report follow a solicitation from the National Radio Astronomy Observatory to develop key science cases for a future U. S.-led radio telescope, the "next generation Very Large Array" (ngVLA). The ngVLA will have roughly 10 times the collecting area of the Jansky VLA, operate at frequencies from 1 GHz to 116 GHz with up to 20 GHz of bandwidth, possess a compact core for high surface-brightness sensitivity, and extended baselines of at least hundreds of kilometers and ultimately across the continent to provide high-resolution imaging. The ngVLA builds on the scientific and technical legacy of the Jansky VLA and ALMA, and will be designed to provide the next leap forward in our understanding of planets, galaxies, and black holes.

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arXiv2017

Quantifying the ngVLA's Contribution to Exo-Space Weather: Results of a Community Studies Report Next Generation VLA Memo #31

A forward-looking facility such as the next generation Very Large Array (ngVLA) requires forward-looking science. The ngVLA will enable stellar wind detections or robust constraints on upper limits sufficient to bridge the gap between current radio upper limits on cool stellar mass loss and current indirect estimates of stellar mass loss from astrospheric measurements. This will aid in understanding the interplay between the rotational spindown of stars and wind mass loss rates, important for an understanding not just of stellar astrophysics but the impact on planet formation and planetary dynamos. Radio observations are currently the only way to explore accelerated particles in cool stellar environments. The upgraded sensitivity of the ngVLA will enable the detection and characterization of essentially all classes of radio-active cool stars in the galaxy. This will open up new fields of study and contribute to the understanding of the stellar contribution to exo-space weather, especially with the emphasis on sensitivity above a frequency of about 10 GHz, where the transition to optically thin gyrosynchrotron emission occurs. These topics connect and extend the key science topics in the cradle of life, in an area which there is both high academic as well as popular interest, and which is expected to continue into the next several decades.

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arXiv2017

Radio Exploration of Planetary Habitability: Conference Summary

Radio Exploration of Planetary Habitability was the fifth in the series of American Astronomical Society's Topical Conference Series. Notable aspects of the conference included the interdisciplinary nature of both the topics and the intellectual breadth of the participants, the diversity of approaches to studying this topic presented by recent discoveries and of the participants themselves, the expanding meaning of the topic of "star-planet interactions," and the expectation of an increasingly statistical approach to the topic. Potential areas of future research include the actual extent to which planetary magnetic fields shield planetary atmospheres; the planetary dynamo process itself, particularly once multiple extrasolar planetary magnetic fields are confirmed; and "planet-star interactions." A major major topic of the conference concerned observational opportunities, highlighted by a number of new or upcoming, specialized observatories to observe exoplanets especially at radio wavelengths. This article summarizes these main points of the conference and expands briefly upon these potential avenues for future investigation. A future meeting on this topic, given the variety of data sets being generated over the next few years, is warranted.

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arXiv2017

Hydrogen Balmer Line Broadening in Solar and Stellar Flares

The broadening of the hydrogen lines during flares is thought to result from increased charge (electron, proton) density in the flare chromosphere. However, disagreements between theory and modeling prescriptions have precluded an accurate diagnostic of the degree of ionization and compression resulting from flare heating in the chromosphere. To resolve this issue, we have incorporated the unified theory of electric pressure broadening of the hydrogen lines into the non-LTE radiative transfer code RH. This broadening prescription produces a much more realistic spectrum of the quiescent, A0 star Vega compared to the analytic approximations used as a damping parameter in the Voigt profiles. We test recent radiative-hydrodynamic (RHD) simulations of the atmospheric response to high nonthermal electron beam fluxes with the new broadening prescription and find that the Balmer lines are over-broadened at the densest times in the simulations. Adding many simultaneously heated and cooling model loops as a "multithread" model improves the agreement with the observations. We revisit the three-component phenomenological flare model of the YZ CMi Megaflare using recent and new RHD models. The evolution of the broadening, line flux ratios, and continuum flux ratios are well-reproduced by a multithread model with high-flux nonthermal electron beam heating, an extended decay phase model, and a "hot spot" atmosphere heated by an ultrarelativistic electron beam with reasonable filling factors: 0.1%, 1%, and 0.1% of the visible stellar hemisphere, respectively. The new modeling motivates future work to understand the origin of the extended gradual phase emission.

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arXiv2016

A Very Bright, Very Hot, and Very Long Flaring Event from the M Dwarf Binary System DG CVn

On April 23, 2014, the Swift satellite responded to a hard X-ray transient detected by its Burst Alert Telescope, which turned out to be a stellar flare from a nearby, young M dwarf binary DG~CVn. We utilize observations at X-ray, UV, optical, and radio wavelengths to infer the properties of two large flares. The X-ray spectrum of the primary outburst can be described over the 0.3-100 keV bandpass by either a single very high temperature plasma or a nonthermal thick-target bremsstrahlung model, and we rule out the nonthermal model based on energetic grounds. The temperatures were the highest seen spectroscopically in a stellar flare, at T$_{X}$ of 290 MK. The first event was followed by a comparably energetic event almost a day later. We constrain the photospheric area involved in each of the two flares to be $>$10$^{20}$ cm$^{2}$, and find evidence from flux ratios in the second event of contributions to the white light flare emission in addition to the usual hot, T$\sim$10$^{4}$K blackbody emission seen in the impulsive phase of flares. The radiated energy in X-rays and white light reveal these events to be the two most energetic X-ray flares observed from an M dwarf, with X-ray radiated energies in the 0.3-10 keV bandpass of 4$\times$10$^{35}$ and 9$\times$10$^{35}$ erg, and optical flare energies at E$_{V}$ of 2.8$\times$10$^{34}$ and 5.2$\times$10$^{34}$ erg, respectively. The results presented here should be integrated into updated modelling of the astrophysical impact of large stellar flares on close-in exoplanetary atmospheres.

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arXiv2015

New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation

(abridged) The heating mechanism at high densities during M dwarf flares is poorly understood. Spectra of M dwarf flares in the optical and near-ultraviolet wavelength regimes have revealed three continuum components during the impulsive phase: 1) an energetically dominant blackbody component with a color temperature of T $\sim$ 10,000 K in the blue-optical, 2) a smaller amount of Balmer continuum emission in the near-ultraviolet at lambda $<$ 3646 Angstroms and 3) an apparent pseudo-continuum of blended high-order Balmer lines. These properties are not reproduced by models that employ a typical "solar-type" flare heating level in nonthermal electrons, and therefore our understanding of these spectra is limited to a phenomenological interpretation. We present a new 1D radiative-hydrodynamic model of an M dwarf flare from precipitating nonthermal electrons with a large energy flux of $10^{13}$ erg cm$^{-2}$ s$^{-1}$. The simulation produces bright continuum emission from a dense, hot chromospheric condensation. For the first time, the observed color temperature and Balmer jump ratio are produced self-consistently in a radiative-hydrodynamic flare model. We find that a T $\sim$ 10,000 K blackbody-like continuum component and a small Balmer jump ratio result from optically thick Balmer and Paschen recombination radiation, and thus the properties of the flux spectrum are caused by blue light escaping over a larger physical depth range compared to red and near-ultraviolet light. To model the near-ultraviolet pseudo-continuum previously attributed to overlapping Balmer lines, we include the extra Balmer continuum opacity from Landau-Zener transitions that result from merged, high order energy levels of hydrogen in a dense, partially ionized atmosphere. This reveals a new diagnostic of ambient charge density in the densest regions of the atmosphere that are heated during dMe and solar flares.

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arXiv2015

Connecting Flares and Transient Mass Loss Events in Magnetically Active Stars

We explore the ramification of associating the energetics of extreme mag- netic reconnection events with transient mass loss in a stellar analogy with solar eruptive events. We establish energy partitions relative to the total bolometric radiated flare energy for different observed components of stellar flares, and show that there is rough agreement for these values with solar flares. We apply an equipartition between the bolometric radiated flare energy and kinetic energy in an accompanying mass ejection, seen in solar eruptive events and expected from reconnection. This allows an integrated flare rate in a particular waveband to be used to estimate the amount of associated transient mass loss. This approach is supported by a good correspondence between observational flare signatures on high flaring rate stars and the Sun, which suggests a common physical origin. If the frequent and extreme flares that young solar-like stars and low-mass stars experience are accompanied by transient mass loss in the form of coronal mass ejections, then the cumulative effect of this mass loss could be large. We find that for young solar-like stars and active M dwarfs, the total mass lost due to transient magnetic eruptions could have significant impacts on disk evolution, and thus planet formation, and also exoplanet habitability.

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arXiv2013

Time-Resolved Properties and Global Trends in dMe Flares from Simultaneous Photometry and Spectra

We present a homogeneous survey of line and continuum emission from near-ultraviolet (NUV) to optical wavelengths during twenty M dwarf flares with simultaneous, high cadence photometry and spectra. These data were obtained to study the white-light continuum components at bluer and redder wavelengths than the Balmer jump. Our goals were to break the degeneracy between emission mechanisms that have been fit to broadband colors of flares and to provide constraints for radiative-hydrodynamic (RHD) flare models that seek to reproduce the white-light flare emission. The main results from the continuum analysis are the following: 1) the detection of Balmer continuum (in emission) that is present during all flares and with a wide range of relative contributions to the continuum flux at bluer wavelengths than the Balmer jump; 2) a blue continuum at flare maximum that is linearly decreasing with wavelength from λ= 4000-4800Å, matched by the spectral shape of hot, blackbody emission with typical temperatures of T_{BB}~9000-14,000 K; 3) a redder continuum apparent at wavelengths longer than Hβ (λ> 4900Å) which becomes relatively more important to the energy budget during the late gradual phase. We calculate Balmer jump flux ratios and compare to RHD model spectra. The model ratios are too large and the blue-optical (λ= 4000-4800Å) slopes are too red in both the impulsive and gradual decay phases of all twenty flares. This discrepancy implies that further work is needed to understand the heating at high column mass during dMe flares. (Abridged)

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arXiv2012

DRAFTS: A Deep, Rapid Archival Flare Transient Search in the Galactic Bulge

We utilize the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) HST/ACS dataset for a Deep Rapid Archival Flare Transient Search (DRAFTS) to constrain the flare rate toward the older stellar population in the Galactic bulge. During 7 days of monitoring 229,293 stars brighter than V=29.5, we find evidence for flaring activity in 105 stars between V=20 and V=28. We divided the sample into non-variable stars and variable stars whose light curves contain large-scale variability. The flare rate on variable stars is \sim 700 times that of non-variable stars, with a significant correlation between the amount of underlying stellar variability and peak flare amplitude. The flare energy loss rates are generally higher than those of nearby well-studied single dMe flare stars. The distribution of proper motions is consistent with the flaring stars being at the distance and age of the Galactic bulge. If they are single dwarfs, they span a range of \approx 1.0 - 0.25M\odot. A majority of the flaring stars exhibit periodic photometric modulations with P <3d. If these are tidally locked magnetically active binary systems, their fraction in the bulge is enhanced by a factor of \sim20 compared to the local value. These stars may be useful for placing constraints on the angular momentum evolution of cool close binary stars. Our results expand the type of stars studied for flares in the optical band, and suggest that future sensitive optical time-domain studies will have to contend with a larger sample of flaring stars than the M dwarf flare stars usually considered.

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arXiv2011

Simultaneous X-ray and radio observations of Young Stellar Objects in NGC 1333 and IC 348

Young Stellar Objects (YSOs) and in particular protostars are known to show a variety of high-energy processes. Observations in the X-ray and centimetric radio wavelength ranges are thought to constrain some of these processes, e.g., coronal-type magnetic activity. There is a well-known empirical correlation of radio and X-ray luminosities in active stars, the so-called Guedel-Benz relation. Previous evidence whether YSOs are compatible with this relation remains inconclusive for the earliest evolutionary stages. The main difficulty is that due to the extreme variability of these sources, simultaneous observations are essential. Until now, only few YSOs and only a handful of protostars have been observed simultaneously in the X-ray and radio range. To expand the sample, we have obtained such observations of two young clusters rich in protostars, NGC 1333 and IC 348. While the absolute sensitivity is lower for these regions than for more nearby clusters like CrA, we find that even in deep continuum observations carried out with the NRAO Very Large Array, the radio detection fraction for protostars in these clusters is much lower than the X-ray detection fraction. Very few YSOs are detected in both bands, and we find the radio and X-ray populations among YSOs to be largely distinct. We combine these new results with previous simultaneous Chandra and VLA observations of star-forming regions and find that YSOs with detections in both bands appear to be offset toward higher radio luminosities for given X-ray luminosities when compared to the Guedel-Benz relation, although even in this sensitive dataset most sources are too weak for the radio detections to provide information on the emission processes. The considerably improved sensitivity of the Expanded Very Large Array will provide a better census of the YSO radio population as well as better constraints on the emission mechanisms.

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