arXiv subjects

Emily L. Rice

Publications and source records attributed to Emily L. Rice.

At least 19 recordsLinked to original sources

arXiv2022

Let the Great World Spin: Revealing the Stormy, Turbulent Nature of Young Giant Exoplanet Analogs with the Spitzer Space Telescope

We present a survey for photometric variability in young, low-mass brown dwarfs with the Spitzer Space Telescope. The 23 objects in our sample show robust signatures of youth and share properties with directly-imaged exoplanets. We present three new young objects: 2MASS J03492367$+$0635078, 2MASS J09512690 $-$8023553 and 2MASS J07180871$-$6415310. We detect variability in 13 young objects, and find that young brown dwarfs are highly likely to display variability across the L2--T4 spectral type range. In contrast, the field dwarf variability occurrence rate drops for spectral types $>$L9. We examine the variability amplitudes of young objects and find an enhancement in maximum amplitudes compared to field dwarfs. We speculate that the observed range of amplitudes within a spectral type may be influenced by secondary effects such as viewing inclination and/or rotation period. We combine our new rotation periods with the literature to investigate the effects of mass on angular momentum evolution. While high mass brown dwarfs ($>30 M_{\mathrm{Jup}}$) spin up over time, the same trend is not apparent for lower mass objects ($<30 M_{\mathrm{Jup}}$), likely due to the small number of measured periods for old, low-mass objects. The rotation periods of companion brown dwarfs and planetary-mass objects are consistent with those of isolated objects with similar ages and masses, suggesting similar angular momentum histories. Within the AB Doradus group, we find a high variability occurrence rate and evidence for common angular momentum evolution. The results are encouraging for future variability searches in directly-imaged exoplanets with facilities such as the James Webb Space Telescope and 30-meter telescopes.

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arXiv2021

Evaluating Rotation Periods of M Dwarfs Across the Ages

In this work we examine M dwarf rotation rates at a range of ages to establish benchmarks for Mdwarf gyrochronology. This work includes a sample of 713 spectroscopically-classified M0-M8 dwarfs with new rotation rates measured from K2 light curves. We analyzed data and recover rotation rates for 179 of these objects. We add these to rotation rates for members of clusters with known ages (5-700 Myr), as well as objects assumed to have field ages ($>$1 Gyr). We use Gaia DR2 parallax and (G-GRP) photometry to create color-magnitude diagrams to compare objects across samples. We use color-period plots to analyze the period distributions across age, as well as incorporate Halpha equivalent width and tangential velocity where possible to further comment on age dependence. We find that the age of transition from rapid to slow rotation in clusters, which we define as an elbow in the period-color plots, depends on spectral type. Later spectral types transition at older ages: M4 for Praesepe at 700 Myr, one of the oldest clusters for which M dwarf rotation rates have been measured. The transition from active to inactive Halpha equivalent width also occurs at this elbow, as objects transition from rapid rotation to the slowly rotating sequence. Redder or smaller stars remain active at older ages. Finally, using Gaia kinematics we find evidence for rotation stalling for late Ms in the field sample, suggesting the transition happens much later for mid to late-type M dwarfs.

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arXiv2019

Radial Velocities, Space Motions, and Nearby Young Moving Group Memberships of Eleven Candidate Young Brown Dwarfs

We present new radial velocity (RV) measurements for 11 candidate young very-low-mass stars and brown dwarfs, with spectral types from M7 to L7. Candidate young objects were identified by features indicative of low surface gravity in their optical and/or near-infrared spectra. RV measurements are derived from high resolution (R=$λ$/$Δλ$=20,000) $J$ band spectra taken with NIRSPEC at the Keck Observatory. We combine RVs with proper motions and trigonometric distances to calculate three-dimensional space positions and motions and to evaluate membership probabilities for nearby young moving groups (NYMGs). We propose 2MASS J00452143+1634446 (L2$β$, $J$=13.06) as an RV standard given the precision and stability of measurements from three different studies. We test the precision and accuracy of our RV measurements as a function of spectral type of the comparison object, finding that RV results are essentially indistinguishable even with differences of $\pm$5 spectral subtypes. We also investigate the strengths of gravity-sensitive K~{\sc i} lines at 1.24--1.25 $μ$m and evaluate their consistency with other age indicators. We confirm or re-confirm four brown dwarf members of NYMGs -- 2MASS J00452143+1634446, WISE J00470038+6803543, 2MASS J01174748$-$3403258, and 2MASS J19355595$-$2846343 -- and their previous age estimates. We identify one new brown dwarf member of the Carina-Near moving group, 2MASS J21543454$-$1055308. The remaining objects do not appear to be members of any known NYMGs, despite their spectral signatures of youth. These results add to the growing number of very-low-mass objects exhibiting signatures of youth that lack likely membership in a known NYMG, thereby compounding the mystery regarding local, low-density star formation.

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arXiv2018

GPI Spectroscopy of the Mass, Age, and Metallicity Benchmark Brown Dwarf HD 4747 B

The physical properties of brown dwarf companions found to orbit nearby, solar-type stars can be benchmarked against independent measures of their mass, age, chemical composition, and other parameters, offering insights into the evolution of substellar objects. The TRENDS high-contrast imaging survey has recently discovered a (mass/age/metallicity) benchmark brown dwarf orbiting the nearby (d=18.69+/-0.19 pc), G8V/K0V star HD 4747. We have acquired follow-up spectroscopic measurements of HD 4747 B using the Gemini Planet Imager to study its spectral type, effective temperature, surface gravity, and cloud properties. Observations obtained in the H-band and K1-band recover the companion and reveal that it is near the L/T transition (T1+/-2). Fitting atmospheric models to the companion spectrum, we find strong evidence for the presence of clouds. However, spectral models cannot satisfactorily fit the complete data set: while the shape of the spectrum can be well-matched in individual filters, a joint fit across the full passband results in discrepancies that are a consequence of the inherent color of the brown dwarf. We also find a $2σ$ tension in the companion mass, age, and surface gravity when comparing to evolutionary models. These results highlight the importance of using benchmark objects to study "secondary effects" such as metallicity, non-equilibrium chemistry, cloud parameters, electron conduction, non-adiabatic cooling, and other subtleties affecting emergent spectra. As a new L/T transition benchmark, HD 4747 B warrants further investigation into the modeling of cloud physics using higher resolution spectroscopy across a broader range of wavelengths, polarimetric observations, and continued Doppler radial velocity and astrometric monitoring.

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arXiv2017

Surface Gravities for 228 M, L, and T Dwarfs in the NIRSPEC Brown Dwarf Spectroscopic Survey

We combine 131 new medium-resolution (R~2000) J-band spectra of M, L, and T dwarfs from the Keck NIRSPEC Brown Dwarf Spectroscopic Survey (BDSS) with 97 previously published BDSS spectra to study surface-gravity-sensitive indices for 228 low-mass stars and brown dwarfs spanning spectral types M5-T9. Specifically, we use an established set of spectral indices to determine surface gravity classifications for all M6-L7 objects in our sample by measuring equivalent widths (EW) of the K I lines at 1.1692, 1.1778, 1.2529 um, and the 1.2 um FeHJ absorption index. Our results are consistent with previous surface gravity measurements, showing a distinct double peak - at ~L5 and T5 - in K I EW as a function of spectral type. We analyze K I EWs of 73 objects of known ages and find a linear trend between log(Age) and EW. From this relationship, we assign age ranges to the very low gravity, intermediate gravity, and field gravity designations for spectral types M6-L0. Interestingly, the ages probed by these designations remain broad, change with spectral type, and depend on the gravity sensitive index used. Gravity designations are useful indicators of the possibility of youth, but current datasets cannot be used to provide a precise age estimate.

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arXiv2017

LACEwING: A New Moving Group Analysis Code

We present a new nearby young moving group (NYMG) kinematic membership analysis code, LocAting Constituent mEmbers In Nearby Groups (LACEwING), a new Catalog of Suspected Nearby Young Stars, a new list of bona fide members of moving groups, and a kinematic traceback code. LACEwING is a convergence-style algorithm with carefully vetted membership statistics based on a large numerical simulation of the Solar Neighborhood. Given spatial and kinematic information on stars, LACEwING calculates membership probabilities in 13 NYMGs and three open clusters within 100 pc. In addition to describing the inputs, methods, and products of the code, we provide comparisons of LACEwING to other popular kinematic moving group membership identification codes. As a proof of concept, we use LACEwING to reconsider the membership of 930 stellar systems in the Solar Neighborhood (within 100 pc) that have reported measurable lithium equivalent widths. We quantify the evidence in support of a population of young stars not attached to any NYMGs, which is a possible sign of new as-yet-undiscovered groups or of a field population of young stars.

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arXiv2016

Young Stars with SALT

We present a spectroscopic and kinematic analysis of 79 nearby M dwarfs in 77 systems. All are low-proper-motion southern hemisphere objects and were identified in a nearby star survey with a demonstrated sensitivity to young stars. Using low-resolution optical spectroscopy from the Red Side Spectrograph (RSS) on the South African Large Telescope (SALT), we have determined radial velocities, H-alpha, Lithium 6708Å, and Potassium 7699Å~equivalent widths linked to age and activity, and spectral types for all our targets. Combined with astrometric information from literature sources, we identify 44 young stars. Eighteen are previously known members of moving groups within 100 parsecs of the Sun. Twelve are new members, including one member of the TW Hydra moving group, one member of the 32 Orionis moving group, nine members of Tucana-Horologium, one member of Argus, and two new members of AB Doradus. We also find fourteen young star systems that are not members of any known groups. The remaining 33 star systems do not appear to be young. This appears to be evidence of a new population of nearby young stars not related to the known nearby young moving groups.

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arXiv2016

Discovery of a Substellar Companion to the Nearby Debris Disk Host HR 2562

We present the discovery of a brown dwarf companion to the debris disk host star HR 2562. This object, discovered with the Gemini Planet Imager (GPI), has a projected separation of 20.3$\pm$0.3 au (0.618$\pm$0.004") from the star. With the high astrometric precision afforded by GPI, we have confirmed common proper motion of HR 2562B with the star with only a month time baseline between observations to more than $5σ$. Spectral data in $J$, $H$, and $K$ bands show morphological similarity to L/T transition objects. We assign a spectral type of L7$\pm$3 to HR 2562B, and derive a luminosity of $\log$(L$_{\rm bol}$/L$_{\odot}$)=-4.62$\pm$0.12, corresponding to a mass of 30$\pm$15 M$_{\rm Jup}$ from evolutionary models at an estimated age of the system of 300-900 Myr. Although the uncertainty in the age of the host star is significant, the spectra and photometry exhibit several indications of youth for HR 2562B. The source has a position angle consistent with an orbit in the same plane as the debris disk recently resolved with Herschel. Additionally, it appears to be interior to the debris disk. Though the extent of the inner hole is currently too uncertain to place limits on the mass of HR 2562B, future observations of the disk with higher spatial resolution may be able to provide mass constraints. This is the first brown dwarf-mass object found to reside in the inner hole of a debris disk, offering the opportunity to search for evidence of formation above the deuterium burning limit in a circumstellar disk.

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arXiv2016

Population Properties of Brown Dwarf Analogs to Exoplanets

We present a kinematic analysis of 152 low surface gravity M7-L8 dwarfs by adding 8 parallaxes, 38 radial velocities, and 19 proper motions. We find 39 objects to be high-likelihood or bona fide members of nearby moving groups, 92 objects to be ambiguous members and 21 objects that are non-members. We find that gravity classification and photometric color separate 5-150 Myr sources from > 3 Gyr field objects, but they do not correlate one-to-one with the narrower 5 -150 Myr age range. The absolute magnitudes of low-gravity sources from J band through W3 show a flux redistribution when compared to equivalent field sources that is correlated with spectral subtype. Clouds, which are a far more dominant opacity source for L dwarfs, are the likely cause. On CMDs, the latest-type low-gravity L dwarfs drive the elbow of the L/T transition up to 1 mag redder and 1 mag fainter than field dwarfs at M_J but are consistent with or brighter than the elbow at M_W1 and M_W2. Furthermore, there is an indication on CMD's (such as M_J versus (J-W2) of increasingly redder sequences separated by gravity classification. Examining bolometric luminosities for planets and low-gravity objects, we confirm that young M dwarfs are overluminous while young L dwarfs are normal compared to the field. This translates into warmer M dwarf temperatures compared to the field sequence while lower temperatures for L dwarfs.

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arXiv2016

Characterization of the Companion to $μ$ Her

$μ$ Her is a nearby quadruple system with a G-subgiant primary and several low mass companions arranged in a 2+2 architecture. While the BC components have been well characterized, the Ab component has been detected astrometrically and with direct imaging but there has been some confusion over its nature, in particular whether the companion is stellar or substellar. Using near-infrared spectroscopy we are able to estimate the spectral type of the companion as a M4$\pm$1V star. In addition, we have measured the astrometry of the system for over a decade. We combined the astrometry with archival radial velocity measurements to compute an orbit of the system. From the combined orbit, we are able to compute the mass sum of the system. Using the estimated mass of the primary, we estimate the mass of the secondary as 0.32 M_sun, which agrees with the estimated spectral type. Our computed orbit is preliminary due to the incomplete orbital phase coverage, but it should be sufficient to predict ephemerides over the next decade.

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arXiv2015

Fundamental Parameters and Spectral Energy Distributions of Young and Field Age Objects with Masses Spanning the Stellar to Planetary Regime

We combine optical, near-infrared and mid-infrared spectra and photometry to construct expanded spectral energy distributions (SEDs) for 145 field age (\textgreater 500 Myr) and 53 young (lower age estimate \textless 500 Myr) ultracool dwarfs (M6-T9). This range of spectral types includes very low mass stars, brown dwarfs, and planetary mass objects, providing fundamental parameters across both the hydrogen and deuterium burning minimum masses for the largest sample assembled to date. A subsample of 29 objects have well constrained ages as probable members of a nearby young moving group (NYMG). We use 182 parallaxes and 16 kinematic distances to determine precise bolometric luminosities ($L_\text{bol}$) and radius estimates from evolutionary models give semi-empirical effective temperatures ($T_\text{eff}$) for the full range of young and field age late-M, L and T dwarfs. We construct age-sensitive relationships of luminosity, temperature and absolute magnitude as functions of spectral type and absolute magnitude to disentangle the effects of degenerate physical parameters such as $T_\text{eff}$, surface gravity, and clouds on spectral morphology. We report bolometric corrections in $J$ for both field age and young objects and find differences of up to a magnitude for late-L dwarfs. Our correction in $Ks$ shows a larger dispersion but not necessarily a different relationship for young and field age sequences. We also characterize the NIR-MIR reddening of low gravity L dwarfs and identify a systematically cooler $T_\text{eff}$ of up to 300K from field age objects of the same spectral type and 400K cooler from field age objects of the same $M_H$ magnitude.

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arXiv2015

Know the Star, Know the Planet. V. Characterization of the Stellar Companion to the Exoplanet Host HD 177830

HD 177830 is an evolved K0IV star with two known exoplanets. In addition to the planetary companions it has a late-type stellar companion discovered with adaptive optics imagery. We observed the binary star system with the PHARO near-IR camera and the Project 1640 coronagraph. Using the Project 1640 coronagraph and integral field spectrograph we extracted a spectrum of the stellar companion. This allowed us to determine that the spectral type of the stellar companion is a M4$\pm$1V. We used both instruments to measure the astrometry of the binary system. Combining these data with published data, we determined that the binary star has a likely period of approximately 800 years with a semi-major axis of 100-200 AU. This implies that the stellar companion has had little or no impact on the dynamics of the exoplanets. The astrometry of the system should continue to be monitored, but due to the slow nature of the system, observations can be made once every 5-10 years.

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arXiv2015

A New Method for Characterizing Very-Low-Mass Companions with Low Resolution Near-Infrared Spectroscopy

We present a new and computationally efficient method for characterizing very low mass companions using low resolution ($R\sim$30) near-infrared ($YJH$) spectra from high contrast imaging campaigns with integral field spectrograph (IFS) units. We conduct a detailed quantitative comparison of the efficacy of this method through tests on simulated data comparable in spectral coverage and resolution to the currently operating direct imaging systems around the world. In particular, we simulate Project 1640 data as an example of the use, accuracy, and precision of this technique. We present results from comparing simulated spectra of M, L, and T dwarfs with a large and finely-sampled grid of synthetic spectra using Markov Chain Monte Carlo techniques. We determine the precision and accuracy of effective temperature and surface gravity inferred from fits to PHOENIX dusty and cond, which we find reproduce the low-resolution spectra of all objects within the adopted flux uncertainties. Uncertainties in effective temperature decrease from $\pm$100-500 K for M dwarfs to as small as $\pm$30 K for some L and T spectral types. Surface gravity is constrained to within 0.2-0.4 dex for mid-L through T dwarfs, but uncertainties are as large as 1.0 dex or more for M dwarfs. Results for effective temperature from low-resolution $YJH$ spectra generally match predictions from published spectral type-temperature relationships except for L-T transition objects and young objects. Single-band spectra (i.e., narrower wavelength coverage) result in larger uncertainties and often discrepant results, suggesting that high contrast IFS observing campaigns can compensate for low spectral resolution by expanding the wavelength coverage for reliable characterization of detected companions. [Abstract truncated]

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arXiv2014

Direct Spectrum of the Benchmark T dwarf HD 19467 B

HD 19467 B is presently the only directly imaged T dwarf companion known to induce a measurable Doppler acceleration around a solar type star. We present spectroscopy measurements of this important benchmark object taken with the Project 1640 integral field unit at Palomar Observatory. Our high-contrast R~30 observations obtained simultaneously across the $JH$ bands confirm the cold nature of the companion as reported from the discovery article and determine its spectral type for the first time. Fitting the measured spectral energy distribution to SpeX/IRTF T dwarf standards and synthetic spectra from BT-Settl atmospheric models, we find that HD 19467 B is a T5.5+/-1 dwarf with effective temperature Teff=$978^{+20}_{-43}$ K. Our observations reveal significant methane absorption affirming its substellar nature. HD 19467 B shows promise to become the first T dwarf that simultaneously reveals its mass, age, and metallicity independent from the spectrum of light that it emits.

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arXiv2014

The Solar Neighborhood. XXXIII. Parallax Results from the CTIOPI 0.9m Program: Trigonometric Parallaxes of Nearby Low-Mass Active and Young Systems

We present basic observational data and association membership analysis for 45 young and active low-mass stellar systems from the ongoing RECONS photometry and astrometry program at the Cerro Tololo Inter-American Observatory. Most of these systems have saturated X-ray emission (log(Lx/Lbol) > -3.5) based on X-ray fluxes from the ROSAT All-Sky Survey, and many are significantly more luminous than main-sequence stars of comparable color. We present parallaxes and proper motions, Johnson-Kron-Cousins VRI photometry, and multiplicity observations from the CTIOPI program on the CTIO 0.9m telescope. To this we add low-resolution optical spectroscopy and line measurements from the CTIO 1.5m telescope, and interferometric binary measurements from the Hubble Space Telescope Fine Guidance Sensors. We also incorporate data from published sources: JHKs photometry from the 2MASS point source catalog; X-ray data from the ROSAT All-Sky Survey; and radial velocities from literature sources. Within the sample of 45 systems, we identify 21 candidate low-mass pre-main-sequence members of nearby associations, including members of beta Pictoris, TW Hydrae, Argus, AB Doradus, two ambiguous 30 Myr old systems, and one object that may be a member of the Ursa Major moving group. Of the 21 candidate young systems, 14 are newly identified as a result of this work, and six of those are within 25 parsecs of the Sun.

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arXiv2013

The Kappa Andromedae System: New Constraints on the Companion Mass, System Age & Further Multiplicity

Kappa Andromedae is a B9IVn star at 52 pc for which a faint substellar companion separated by 55 AU was recently announced. In this work, we present the first spectrum of the companion, "kappa And B," using the Project 1640 high-contrast imaging platform. Comparison of our low-resolution YJH-band spectra to empirical brown dwarf spectra suggests an early-L spectral type. Fitting synthetic spectra from PHOENIX model atmospheres to our observed spectrum allows us to constrain the effective temperature to ~2000K, as well as place constraints on the companion surface gravity. Further, we use previously reported log(g) and effective temperature measurements of the host star to argue that the kappa And system has an isochronal age of 220 +/- 100 Myr, older than the 30 Myr age reported previously. This interpretation of an older age is corroborated by the photometric properties of kappa And B, which appear to be marginally inconsistent with other 10-100 Myr low-gravity L-dwarfs for the spectral type range we derive. In addition, we use Keck aperture masking interferometry combined with published radial velocity measurements to rule out the existence of any tight stellar companions to kappa And A that might be responsible for the system's overluminosity. Further, we show that luminosity enhancements due to a nearly "pole-on" viewing angle coupled with extremely rapid rotation is unlikely. Kappa And A is thus consistent with its slightly evolved luminosity class (IV) and we propose here that kappa And, with a revised age of 220 +/- 100 Myr, is an interloper to the 30 Myr Columba association with which it was previously associated. The photometric and spectroscopic evidence for kappa And B combined with our re-assesment of the system age implies a substellar companion mass of 50^{+16}_{-13} Jupiter Masses, consistent with a brown dwarf rather than a planetary mass companion.

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arXiv2013

Young Brown Dwarfs as Giant Exoplanet Analogs

Young brown dwarfs and directly-imaged exoplanets have enticingly similar photometric and spectroscopic characteristics, indicating that their cool, low gravity atmospheres should be studied in concert. Similarities between the peculiar shaped H band, near and mid-IR photometry as well as location on color magnitude diagrams provide important clues about how to extract physical properties of planets from current brown dwarf observations. In this proceeding we discuss systems newly assigned to 10-150 Myr nearby moving groups, highlight the diversity of this uniform age-calibrated brown dwarf sample, and reflect on their implication for understanding current and future planetary data.

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arXiv2012

2MASSJ035523.51+113337.4: A Young, Dusty, Nearby, Isolated Brown Dwarf Resembling A Giant Exoplanet

We present parallax and proper motion measurements, near-infrared spectra, and WISE photometry for the low surface gravity L5gamma dwarf 2MASSJ035523.37+113343.7 (2M0355). We use these data to evaluate photometric, spectral, and kinematic signatures of youth as 2M0355 is the reddest isolated L dwarf yet classified. We confirm its low-gravity spectral morphology and find a strong resemblance to the sharp triangular shaped $H$-band spectrum of the 10 Myr planetary-mass object 2M1207b. We find that 2M0355 is underluminous compared to a normal field L5 dwarf in the optical and MKO J,H, and K bands and transitions to being overluminous from 3-12 microns, indicating that enhanced photospheric dust shifts flux to longer wavelengths for young, low-gravity objects, creating a red spectral energy distribution. Investigating the near-infrared color magnitude diagram for brown dwarfs confirms that 2M0355 is redder and underluminous compared to the known brown dwarf population, similar to the peculiarities of directly imaged exoplanets 2M1207b and HR8799bcd. We calculate UVW space velocities and find that the motion of 2M0355 is consistent with young disk objects (< 2-3 Gyr) and it shows a high likelihood of membership in the AB Doradus association.

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