arXiv subjects

Adam J. Burgasser

Publications and source records attributed to Adam J. Burgasser.

At least 145 records · Page 8Linked to original sources

arXiv2017

2MASS J11151597+1937266: A Young, Dusty, Isolated, Planetary-Mass Object with a Potential Wide Stellar Companion

We present 2MASS J11151597+1937266, a recently identified low-surface gravity L dwarf, classified as an L2$γ$ based on Sloan Digital Sky Survey optical spectroscopy. We confirm this spectral type with near-infrared spectroscopy, which provides further evidence that 2MASS J11151597+1937266 is a low-surface gravity L dwarf. This object also shows significant excess mid-infrared flux, indicative of circumstellar material; and its strong H$α$ emission (EW$_{\mathrm{H}α}=560\pm82$ Å) is an indicator of enhanced magnetic activity or weak accretion. Comparison of its spectral energy distribution to model photospheres yields an effective temperature of $1724^{+184}_{-38}$ K. We also provide a revised distance estimate of $37\pm6$ pc using a spectral type-luminosity relationship for low-surface gravity objects. The 3-dimensional galactic velocities and positions of 2MASS J11151597+1937266 do not match any known young association or moving group. Assuming a probable age in the range of 5-45 Myr, the model-dependent estimated mass of this object is between 7-21 $M_\mathrm{Jup}$, making it a potentially isolated planetary-mass object. We also identify a candidate co-moving, young stellar companion, 2MASS J11131089+2110086.

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arXiv2017

Stellar parameters for TRAPPIST-1

TRAPPIST-1 is an ultracool dwarf star transited by seven Earth-sized planets, for which thorough characterization of atmospheric properties, surface conditions encompassing habitability and internal compositions is possible with current and next generation telescopes. Accurate modeling of the star is essential to achieve this goal. We aim to obtain updated stellar parameters for TRAPPIST- 1 based on new measurements and evolutionary models, compared to those used in discovery studies. We present a new measurement for the parallax of TRAPPIST-1, 82.4 $\pm$ 0.8 mas, based on 188 epochs of observations with the TRAPPIST and Liverpool Telescopes from 2013 to 2016. This revised parallax yields an updated luminosity of $L_*=(5.22\pm0.19)\times 10^{-4} L_{\odot}$, very close to the previous estimate but almost twice more precise. We next present an updated estimate for TRAPPIST-1 stellar mass, based on two approaches: mass from stellar evolution modeling, and empirical mass derived from dynamical masses of equivalently classified ultracool dwarfs in astrometric binaries. We combine them through a Monte-Carlo approach to derive a semi-empirical estimate for the mass of TRAPPIST-1. We also derive estimate for the radius by combining this mass with stellar density inferred from transits, as well as estimate for the effective temperature from our revised luminosity and radius. Our final results are $M_*=0.089 \pm 0.006 M_{\odot}$, $R_* = 0.121 \pm 0.003 R_{\odot}$, and $T_{\rm eff} =$ 2516 $\pm$ 41 K. Considering the degree to which TRAPPIST-1 system will be scrutinized in coming years, these revised and more precise stellar parameters should be considered when assessing the properties of TRAPPIST-1 planets.

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arXiv2017

A Candidate Wide Brown Dwarf Binary in the Argus Association: 2MASS J14504216-7841413 and 2MASS J14504113-7841383

[not part of Research Note] We report the discovery of a widely-separated low-mass binary as a candidate member of the $\sim$40 Myr Argus Association. Resolved imaging and astrometry with 2MASS and LDSS-3 reveal a common proper motion pair of red sources separated by 4.23''$\pm$0.11'', with the secondary roughly one magnitude fainter at $i$, $z$ and $J$. Resolved spectroscopy indicates component types of M8pec and M9pec, the peculiarities arising from weak Na I and strong VO absorption characteristic of low gravity sources. With its small proper motion and estimated 75$\pm$25 pc distance, the BANYAN II tool indicates a membership probability of 93% in Argus, which would be consistent with a pair of brown dwarfs of mass $\sim$0.04 M$_{\odot}$ separated by $\sim$300 AU.

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arXiv2017

Identification of WISE J000100.45+065259.6 as an M8.5+T5 Spectral Binary Candidate

[not part of RNAAS note] We report the discovery of WISE J000100.45+065259.6 as a very low mass star/brown dwarf spectral binary candidate, on the basis of low resolution near-infrared spectroscopy obtained with IRTF/SpeX. Decomposition of the spectrum indicates component types of M8.5+T5 with a predicted $Δ{J}$ = 3.5. As the majority of confirmed spectral binary candidates to date are very closely-separated systems ($ρ$ $\lesssim$ 3 AU; $P$ $\lesssim$ 15~yr), this source may provide mass measurements across the hydrogen burning limit within the decade.

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arXiv2017

Cloud Atlas: Discovery of Rotational Spectral Modulations in a Low-mass, L-type Brown Dwarf Companion to a Star

Observations of rotational modulations of brown dwarfs and giant exoplanets allow the characterization of condensate cloud properties. As of now rotational spectral modulations have only been seen in three L-type brown dwarfs. We report here the discovery of rotational spectral modulations in LP261-75B, an L6-type intermediate surface gravity companion to an M4.5 star. As a part of the Cloud Atlas Treasury program we acquired time-resolved Wide Field Camera 3 grism spectroscopy (1.1--1.69~$μ$m) of LP261-75B. We find gray spectral variations with the relative amplitude displaying only a weak wavelength dependence and no evidence for lower-amplitude modulations in the 1.4~$μ$m water band than in the adjacent continuum. The likely rotational modulation period is 4.78$\pm$0.95 h, although the rotational phase is not well sampled. The minimum relative amplitude in the white light curve measured over the whole wavelength range is 2.41$\pm$0.14\%. We report an unusual light curve with seemingly three peaks approximately evenly distributed in rotational phase. The spectral modulations suggests that the upper atmosphere cloud properties in {LP261-75B} are similar to two other mid-L dwarfs of typical infrared colors, but differ from that of the extremely red L-dwarf WISE0047.

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arXiv2017

The SpeX Prism Library Analysis Toolkit (SPLAT): A Data Curation Model

I describe our team's development of the SpeX Prism Library Analysis Toolkit (SPLAT), a combined spectral data repository for over 2500 low-resolution spectra of very low mass dwarfs (late M, L and T dwarfs), and Python-based analysis toolkit. SPLAT was constructed through a collaborative, student-centered, research-based model with high school, undergraduate and graduate students and regional K-12 science teachers. The toolkit enables spectral index analysis, classification, spectrophotometry, atmosphere model comparison, population synthesis, and other analyses. I summarize the current components of this code, sample applications, and future development plans.

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arXiv2017

Spectral Variability of Two Rapidly Rotating Brown Dwarfs: 2MASS J08354256-0819237 and 2MASS J18212815+1414010

L dwarfs exhibit low-level, rotationally-modulated photometric variability generally associated with heterogeneous, cloud-covered atmospheres. The spectral character of these variations yields insight into the particle sizes and vertical structure of the clouds. Here we present the results of a high precision, ground-based, near-infrared, spectral monitoring study of two mid-type L dwarfs that have variability reported in the literature, 2MASS J08354256-0819237 and 2MASS J18212815+1414010, using the SpeX instrument on the Infrared Telescope Facility. By simultaneously observing a nearby reference star, we achieve <0.15% per-band sensitivity in relative brightness changes across the 0.9--2.4um bandwidth. We find that 2MASS J0835-0819 exhibits marginal (< ~0.5% per band) variability with no clear spectral dependence, while 2MASS J1821+1414 varies by up to +/-1.5% at 0.9 um, with the variability amplitude declining toward longer wavelengths. The latter result extends the variability trend observed in prior HST/WFC3 spectral monitoring of 2MASS J1821+1414, and we show that the full 0.9-2.4 um variability amplitude spectrum can be reproduced by Mie extinction from dust particles with a log-normal particle size distribution with a median radius of 0.24 um. We do not detect statistically significant phase variations with wavelength. The different variability behavior of 2MASS J0835-0819 and 2MASS J1821+1414 suggests dependencies on viewing angle and/or overall cloud content, underlying factors that can be examined through a broader survey.

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arXiv2017

K2 Ultracool Dwarfs Survey II: The White Light Flare Rate of Young Brown Dwarfs

We use Kepler K2 Campaign 4 short-cadence (one-minute) photometry to measure white light flares in the young, moving group brown dwarfs 2MASS J03350208+2342356 (2M0335+23) and 2MASS J03552337+1133437 (2M0355+11), and report on long-cadence (thirty-minute) photometry of a superflare in the Pleiades M8 brown dwarf CFHT-PL-17. The rotation period (5.24 hr) and projected rotational velocity ($45$ km s$^{-1}$) confirm 2M0335+23 is inflated ($R \ge 0.20 R_\odot$) as predicted for a $0.06M_\odot$, 26-Myr old brown dwarf $β$Pic moving group member. We detect 22 white light flares on 2M0335+23. The flare frequency distribution follows a power-law distribution with slope $-α= -1.8 \pm 0.2$ over the range $10^{31}$ to $10^{33}$ erg. This slope is similar to that observed in the Sun and warmer flare stars, and is consistent with lower energy flares in previous work on M6-M8 very-low-mass stars; taken the two datasets together, the flare frequency distribution for ultracool dwarfs is a power law over 4.3 orders of magnitude. The superflare ($2.6\times10^{34}$ erg) on CFHT-PL-17 shows higher energy flares are possible. We detect no flares down to a limit of $2 \times 10^{30}$ erg in the nearby L$5γ$ AB Dor Moving Group brown dwarf 2M0355+11, consistent with the view that fast magnetic reconnection is suppressed in cool atmospheres. We discuss two multi-peaked flares observed in 2M0335+23, and argue that these complex flares can be understood as sympathetic flares, in which a fast-mode MHD waves similar to EUV waves in the Sun trigger magnetic reconnection in different active regions.

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arXiv2017

Sloan Digital Sky Survey IV: Mapping the Milky Way, Nearby Galaxies, and the Distant Universe

We describe the Sloan Digital Sky Survey IV (SDSS-IV), a project encompassing three major spectroscopic programs. The Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2) is observing hundreds of thousands of Milky Way stars at high resolution and high signal-to-noise ratio in the near-infrared. The Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey is obtaining spatially-resolved spectroscopy for thousands of nearby galaxies (median redshift of z = 0.03). The extended Baryon Oscillation Spectroscopic Survey (eBOSS) is mapping the galaxy, quasar, and neutral gas distributions between redshifts z = 0.6 and 3.5 to constrain cosmology using baryon acoustic oscillations, redshift space distortions, and the shape of the power spectrum. Within eBOSS, we are conducting two major subprograms: the SPectroscopic IDentification of eROSITA Sources (SPIDERS), investigating X-ray AGN and galaxies in X-ray clusters, and the Time Domain Spectroscopic Survey (TDSS), obtaining spectra of variable sources. All programs use the 2.5-meter Sloan Foundation Telescope at Apache Point Observatory; observations there began in Summer 2014. APOGEE-2 also operates a second near-infrared spectrograph at the 2.5-meter du Pont Telescope at Las Campanas Observatory, with observations beginning in early 2017. Observations at both facilities are scheduled to continue through 2020. In keeping with previous SDSS policy, SDSS-IV provides regularly scheduled public data releases; the first one, Data Release 13, was made available in July 2016.

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arXiv2017

On the Age of the TRAPPIST-1 System

The nearby (d = 12 pc) M8 dwarf star TRAPPIST-1 (2MASS J23062928-0502285) hosts a compact system of at least seven exoplanets with sizes similar to Earth. Given its importance for testing planet formation and evolution theories, and for assessing the prospects for habitability among Earth-size exoplanets orbiting the most common type of star in the Galaxy, we present a comprehensive assessment of the age of this system. We collate empirical age constraints based on the color-absolute magnitude diagram, average density, lithium absorption, surface gravity features, metallicity, kinematics, rotation, and magnetic activity; and conclude that TRAPPIST-1 is a transitional thin/thick disk star with an age of 7.6$\pm$2.2 Gyr. The star's color-magnitude position is consistent with it being slightly metal-rich ([Fe/H] $\simeq$ +0.06), in line with its previously reported near-infrared spectroscopic metallicity; and it has a radius (R = 0.121$\pm$0.003 R$_{\odot}$) that is larger by 8-14% compared to solar-metallicity evolutionary models. We discuss some implications of the old age of this system with regard to the stability and habitability of its planets.

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arXiv2017

SIMP J013656.5+093347 is Likely a Planetary-Mass Object in the Carina-Near Moving Group

We report the discovery that the nearby ($\sim$ 6 pc) photometrically variable T2.5 dwarf SIMP~J013656.5+093347 is a likely member of the $\sim$ 200 Myr-old Carina-Near moving group with a probability of $>$ 99.9% based on its full kinematics. Our $v\sin i$ measurement of $50.9 \pm 0.8$ km s$^{-1}$ combined with the known rotation period inferred from variability measurements provide a lower limit of $1.01 \pm 0.02$ $R_{\mathrm{Jup}}$ on the radius of SIMP0136+0933, an independent verification that it must be younger than $\sim$ 950 Myr according to evolution models. We estimate a field interloper probability of 0.2% based on the density of field T0-T5 dwarfs. At the age of Carina-Near, SIMP0136+0933 has an estimated mass of $12.7 \pm 1.0$ $M_{\mathrm{Jup}}$ and is predicted to have burned roughly half of its original deuterium. SIMP0136+0933 is the closest known young moving group member to the Sun, and is one of only a few known young T dwarfs, making it an important benchmark for understanding the atmospheres of young planetary-mass objects.

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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

K2 Ultracool Dwarfs Survey I: Photometry of an L Dwarf Superflare

We report on K2 Campaign 8 measurements of a huge white light flare on the L1 dwarf SDSSp J005406.55-003101.8 (EPIC 220186653). The source is a typical L1 dwarf at a distance of $\sim50$ pc, probably an old hydrogen-burning star rather than a young brown dwarf. In the long (30-minute) cadence photometry, the flare peak is 21 times the flux of the stellar photosphere in the broad optical Kepler filter, which we estimate corresponds to $ΔV \approx -7.1$. The total equivalent duration of the flare is 15.4 hr. We estimate the total bolometric energy of the flare was $4 \times 10^{33}$ erg, more powerful than the previously reported Kepler white light flares for the L1 dwarf WISEP J190648.47+401106.8, but weaker than the $ΔV = -11$ L0 dwarf superflare ASASSN-16ae. The initial (impulsive) cooling phase is too rapid to resolve with our 30-minute cadence data, but after one hour the gradual cooling phase has an exponential time constant of 1.8 hours. We use template fitting to estimate that the full-time-width-at-half-amplitude of the light curve is $<10$ minutes and that the true flare maximum reached $\sim70$ times the stellar photosphere, or $ΔV \approx -8$. This flare is comparable to the most powerful Kepler flares observed on the active M4 dwarf GJ 1243.

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arXiv2016

BANYAN. IX. The Initial Mass Function and Planetary-Mass Object Space Density of the TW Hya Association

A determination of the initial mass function (IMF) of the current, incomplete census of the 10 Myr-old TW Hya association (TWA) is presented. This census is built from a literature compilation supplemented with new spectra and 17 new radial velocities, as well as a re-analysis of Hipparcos data that confirmed HR 4334 (A2Vn) as a member. Though the dominant uncertainty in the IMF remains census incompleteness, a detailed statistical treatment is carried out to make the IMF determination independent of binning, while accounting for small number statistics. The currently known high-likelihood members are fitted by a log-normal distribution with a central mass of $0.21^{+0.11}_{-0.06}$ $M_{\odot}$ and a characteristic width of $0.8^{+0.2}_{-0.1}$ dex in the 12 $M_{\rm Jup}$-2 $M_{\odot}$ range, whereas a Salpeter power law with $α= 2.2^{+1.1}_{-0.5}$ best describes the IMF slope in the $0.1-2$ $M_{\odot}$ range. This characteristic width is higher than other young associations, which may be due to incompleteness in the current census of low-mass TWA stars. A tentative overpopulation of isolated planetary-mass members similar to 2MASS J11472421-2040204 and 2MASS J11193254-1137466 is identified: this indicates that there might be as many as $10^{+13}_{-5}$ similar members of TWA with hot-start model-dependent masses estimated at $\sim$ 5-7 $M_{\rm Jup}$, most of which would be too faint to be detected in 2MASS. Our new radial velocity measurements corroborate the membership of 2MASS J11472421-2040204, and secure TWA 28 (M8.5$γ$), TWA 29 (M9.5$γ$) and TWA 33 (M4.5e) as members. The discovery of 2MASS J09553336-0208403, a young L7-type interloper unrelated to TWA, is also presented.

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arXiv2016

The Late-Type Extension to MoVeRS (LaTE-MoVeRS): Proper motion verified low-mass stars and brown dwarfs from SDSS, 2MASS, and WISE

We present the Late-Type Extension to the Motion Verified Red Stars (LaTE-MoVeRS) catalog, containing 46,463 photometric late-type ($>$ M5) dwarfs within the Sloan Digital Sky Survey (SDSS) footprint. Proper motions were computed for objects combining astrometry from the SDSS Data Release 12 (DR12), the Two-Micron All-Sky Survey (2MASS) Point Source Catalog (PSC), and the \emph{Wide-field Infrared Survey Explorer} (\emph{WISE}) AllWISE datasets. LaTE-MoVeRS objects were required to have significant proper motion ($μ_\mathrm{tot} \geqslant 2σ_{μ_\mathrm{tot}}$). Using the LaTE-MoVeRS sample and \emph{Gaia} Data Release 1, we estimate \emph{Gaia} will be $\sim$64\% complete for very-low-mass objects ($>$ M5) in comparison to the combined SDSS+2MASS+\emph{WISE} dataset ($i < 21.3$). We computed photometric distances and estimated stellar effective temperatures for the LaTE-MoVeRS catalog. The majority of the dwarfs in the sample have distances $< 150$ pc and $T_\mathrm{eff} < 3000$ K. Thirteen objects were identified within LaTE-MoVeRS with estimated photometric distances within 25 pc that have not been previously identified as nearby objects. We also identified one new object with a large amount of excess mid-infrared flux that has not been previously identified (2MASS J11151597$+$1937266). This object appears to be an L2$γ$ at $\sim$50 pc showing spectroscopic signs of a flaring event (e.g., strong Hydrogen Balmer emission lines). This object does not exhibit kinematics similar to any known kinematic association. The LaTE-MoVeRS catalog is available through SDSS CasJobs and VizieR.

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arXiv2016

A Brown Dwarf Census from the SIMP Survey

We have conducted a near-infrared (NIR) proper motion survey, the Sondage Infrarouge de Mouvement Propre, in order to discover field ultracool dwarfs (UCD) in the solar neighborhood. The survey was conducted by imaging $\sim28\%$ of the sky with the Caméra PAnoramique Proche-InfraRouge both in the southern hemisphere at the Cerro Tololo Inter-American Observatory 1.5 m telescope, and in the northern hemisphere at the Observatoire du Mont-Mégantic 1.6 m telescope and comparing the source positions from these observations with the Two Micron All-Sky Survey Point Source Catalog (2MASS PSC). Additional color criteria were used to further discriminate unwanted astrophysical sources. We present the results of an NIR spectroscopic follow-up of 169 M, L, and T dwarfs. Among the sources discovered are 2 young field brown dwarfs, 6 unusually red M and L dwarfs, 25 unusually blue M and L dwarfs, 2 candidate unresolved L+T binaries, and 24 peculiar UCDs. Additionally, we add 9 L/T transition dwarfs (L6--T4.5) to the already known objects.

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arXiv2016

Cloud Atlas: Discovery of Patchy Clouds and High-amplitude Rotational Modulations In a Young, Extremely Red L-type Brown Dwarf

Condensate clouds fundamentally impact the atmospheric structure and spectra of exoplanets and brown dwarfs but the connections between surface gravity, cloud structure, dust in the upper atmosphere, and the red colors of some brown dwarfs remain poorly understood. Rotational modulations enable the study of different clouds in the same atmosphere, thereby providing a method to isolate the effects of clouds. Here we present the discovery of high peak-to-peak amplitude (8%) rotational modulations in a low-gravity, extremely red (J-Ks=2.55) L6 dwarf WISEP J004701.06+680352.1 (W0047). Using the Hubble Space Telescope (HST) time-resolved grism spectroscopy we find a best-fit rotational period (13.20$\pm$0.14 hours) with a larger amplitude at 1.1 micron than at 1.7 micron. This is the third largest near-infrared variability amplitude measured in a brown dwarf, demonstrating that large-amplitude variations are not limited to the L/T transition but are present in some extremely red L-type dwarfs. We report a tentative trend between the wavelength dependence of relative amplitude, possibly proxy for small dust grains lofted in the upper atmosphere, and the likelihood of large-amplitude variability. By assuming forsterite as haze particle, we successfully explain the wavelength dependent amplitude with submicron-sized haze particles sizes of around 0.4 μm. W0047 links the earlier spectral and later spectral type brown dwarfs in which rotational modulations have been observed, the large amplitude variations in this object make this a benchmark brown dwarf for the study of cloud properties close to the L/T transition.

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arXiv2016

WISEP J060738.65+242953.4: A Nearby. Pole-On L8 Brown Dwarf with Radio Emission

We present a simultaneous, multi-wavelength campaign targeting the nearby (7.2 pc) L8/L9 (optical/near-infrared) dwarf WISEP J060738.65+242953.4 in the mid-infrared, radio, and optical. Spitzer Space Telescope observations show no variability at the 0.2% level over 10 hours each in the 3.6 and 4.5 micron bands. Kepler K2 monitoring over 36 days in Campaign 0 rules out stable periodic signals in the optical with amplitudes great than 1.5% and periods between 1.5 hours and 2 days. Non-simultaneous Gemini optical spectroscopy detects lithium, constraining this L dwarf to be less than ~2 Gyr old, but no Balmer emission is observed. The low measured projected rotation velocity (v sin i < 6 km/s) and lack of variability are very unusual compared to other brown dwarfs, and we argue that this substellar object is likely viewed pole-on. We detect quiescent (non-bursting) radio emission with the VLA. Amongst radio detected L and T dwarfs, it has the lowest observed L_nu and the lowest v sin i. We discuss the implications of a pole-on detection for various proposed radio emission scenarios.

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