arXiv subjects

Adam J. Burgasser

Publications and source records attributed to Adam J. Burgasser.

At least 91 records · Page 5Linked to original sources

arXiv2023

A 1.55 R$_{\oplus}$ habitable-zone planet hosted by TOI-715, an M4 star near the ecliptic South Pole

A new generation of observatories is enabling detailed study of exoplanetary atmospheres and the diversity of alien climates, allowing us to seek evidence for extraterrestrial biological and geological processes. Now is therefore the time to identify the most unique planets to be characterised with these instruments. In this context, we report on the discovery and validation of TOI-715 b, a $R_{\rm b}=1.55\pm 0.06\rm R_{\oplus}$ planet orbiting its nearby ($42$ pc) M4 host (TOI-715/TIC 271971130) with a period $P_{\rm b} = 19.288004_{-0.000024}^{+0.000027}$ days. TOI-715 b was first identified by TESS and validated using ground-based photometry, high-resolution imaging and statistical validation. The planet's orbital period combined with the stellar effective temperature $T_{\rm eff}=3075\pm75~\rm K$ give this planet an instellation $S_{\rm b} = 0.67_{-0.20}^{+0.15}~\rm S_\oplus$, placing it within the most conservative definitions of the habitable zone for rocky planets. TOI-715 b's radius falls exactly between two measured locations of the M-dwarf radius valley; characterising its mass and composition will help understand the true nature of the radius valley for low-mass stars. We demonstrate TOI-715 b is amenable for characterisation using precise radial velocities and transmission spectroscopy. Additionally, we reveal a second candidate planet in the system, TIC 271971130.02, with a potential orbital period of $P_{02} = 25.60712_{-0.00036}^{+0.00031}$ days and a radius of $R_{02} = 1.066\pm0.092\,\rm R_{\oplus}$, just inside the outer boundary of the habitable zone, and near a 4:3 orbital period commensurability. Should this second planet be confirmed, it would represent the smallest habitable zone planet discovered by TESS to date.

astro-ph.EP↗

arXiv2023

JWST/NIRCam discovery of the first Y+Y brown dwarf binary: WISE J033605.05$-$014350.4

We report the discovery of the first brown dwarf binary system with a Y dwarf primary, WISE J033605.05$-$014350.4, observed with NIRCam on JWST with the F150W and F480M filters. We employed an empirical point spread function binary model to identify the companion, located at a projected separation of 84 milliarcseconds, position angle of 295 degrees, and with contrast of 2.8 and 1.8 magnitudes in F150W and F480M, respectively. At a distance of 10$\,$pc based on its Spitzer parallax, and assuming a random inclination distribution, the physical separation is approximately 1$\,$au. Evolutionary models predict for that an age of 1-5 Gyr, the companion mass is about 4-12.5 Jupiter masses around the 7.5-20 Jupiter mass primary, corresponding to a companion-to-host mass fraction of $q=0.61\pm0.05$. Under the assumption of a Keplerian orbit the period for this extreme binary is in the range of 5-9 years. The system joins a small but growing sample of ultracool dwarf binaries with effective temperatures of a few hundreds of Kelvin. Brown dwarf binaries lie at the nexus of importance for understanding the formation mechanisms of these elusive objects, as they allow us to investigate whether the companions formed as stars or as planets in a disk around the primary.

astro-ph.SR↗

arXiv2023

Discovery of the Exceptionally Short Period Ultracool Dwarf Binary LP 413-53AB

We report the detection of large-amplitude, rapid radial velocity (RV) variations and line-splitting in high-resolution Keck/NIRSPEC spectra of the M9 dwarf LP 413-53. We attribute these features to binary motion. Analyzing data spanning 15 years, we infer a preliminary orbital period of 0.7106156$\pm$0.0000002 day, an eccentricity of 0.0088$\pm$0.0017, a primary RV semi-amplitude of 23.70$\pm$0.05 km s$^{-1}$, and a secondary RV semi-amplitude of 28.41$\pm$0.06 km s$^{-1}$, implying a system mass ratio $M_\mathrm{secondary}$/$M_\mathrm{primary}$ = 0.8340$\pm$0.0017. These measurements identify LP 413-53 as the shortest-period ultracool binary discovered to date, and one of the smallest separation main sequence binaries known. The position and velocity of the system rule out previously reported membership in the Hyades Moving Group, and indicate that this is likely a pair of evolved (age $\gtrsim$ 1 Gyr), very-low-mass stars. Assuming masses consistent with evolved late-M and L dwarfs, we estimate an orbital separation of 0.0081$-$0.0084 au or 17$-$19 stellar radii, and an orbital inclination angle of 24$^{\circ}$, making it unlikely that this system exhibits eclipse events. The larger radii of these stars at young ages would have put them near contact at the system's current separation, and we speculate that this system has undergone dynamical evolution, either through orbital angular momentum loss or ejection of a third component followed by tidal circularization. While further observations are needed to fully constrain the orbital and physical parameters of LP 413-53, this ultra-short-period UCD binary system serves as a new testbed for formation and dynamical evolution models of very-low-mass multiples.

astro-ph.SR↗

arXiv2023

Redder than Red: Discovery of an Exceptionally Red L/T Transition Dwarf

We present the discovery of CWISE J050626.96$+$073842.4 (CWISE J0506$+$0738), an L/T transition dwarf with extremely red near-infrared colors discovered through the Backyard Worlds: Planet 9 citizen science project. Photometry from UKIRT and CatWISE give a $(J-K)_{\rm MKO}$ color of 2.97$\pm$0.03 mag and a $J_{\rm MKO}-$W2 color of 4.93$\pm$0.02 mag, making CWISE J0506$+$0738 the reddest known free-floating L/T dwarf in both colors. We confirm the extremely red nature of CWISE J0506$+$0738 using Keck/NIRES near-infrared spectroscopy and establish that it is a low-gravity late-type L/T transition dwarf. The spectrum of CWISE J0506$+$0738 shows possible signatures of CH$_4$ absorption in its atmosphere, suggesting a colder effective temperature than other known, young, red L dwarfs. We assign a preliminary spectral type for this source of L8$γ$-T0$γ$. We tentatively find that CWISE J0506$+$0738 is variable at 3-5 $μ$m based on multi-epoch WISE photometry. Proper motions derived from follow-up UKIRT observations combined with a radial velocity from our Keck/NIRES spectrum and a photometric distance estimate indicate a strong membership probability in the $β$ Pic moving group. A future parallax measurement will help to establish a more definitive moving group membership for this unusual object.

astro-ph.SR↗

arXiv2022

Characterization of Population III Stars with Stellar Atmosphere and Evolutionary Modeling and Predictions of their Observability with the James Webb Space Telescope

Population III stars were the first stars to form after the Big Bang, and are believed to have made the earliest contribution to the metal content of the universe beyond the products of the Big Bang Nucleosynthesis. These stars are theorized to have had extremely short lifespans, and therefore would only be observable at high redshifts ($z \geq 3-17$) and faint apparent magnitudes ($m_{AB} \gtrsim 40$). The direct detection of Population III stars therefore remains elusive. However, the recently launched James Webb Space Telescope (JWST) may be capable of detecting stars in the relevant magnitude range in the event of favorable gravitational lensing. Theoretical models are required to interpret these future observations. In this study, new evolutionary models and non-equilibrium model atmospheres were used to characterize the observable properties of zero-age main sequence Population III stars. The calculated models cover a wide range of possible Population III stellar masses, from the minimum mass predicted by star formation studies to the maximum mass capable of maintaining hydrostatic equilibrium. Synthetic photometry and theoretical color-magnitude diagrams were calculated for the bands of the Near-Infrared Camera (NIRCam) on JWST. The final results are compared to the scales of known lensing events and JWST magnitude limits. The purpose of this study is to calculate the observable parameters of Population III stars in the most optimal JWST bands in order to provide a theoretical foundation for anticipated future observations of this stellar population.

astro-ph.SR↗

arXiv2022

Beyond the Local Volume. II. Population Scaleheights and Ages of Ultracool Dwarfs in Deep HST/WFC3 Parallel Fields

Ultracool dwarfs represent a significant proportion of stars in the Milky Way,and deep samples of these sources have the potential to constrain the formation history and evolution of low-mass objects in the Galaxy. Until recently, spectral samples have been limited to the local volume (d<100 pc). Here, we analyze a sample of 164 spectroscopically-characterized ultracool dwarfs identified by Aganze et al. (2022) in the Hubble Space Telescope WFC3 Infrared Spectroscopic Parallel (WISP) Survey and 3D-HST. We model the observed luminosity function using population simulations to place constraints on scaleheights, vertical velocity dispersions and population ages as a function of spectral type. Our star counts are consistent with a power-law mass function and constant star formation history for ultracool dwarfs, with vertical scaleheights 249$_{-61}^{+48}$ pc for late M dwarfs, 153$_{-30}^{+56}$ pc for L dwarfs, and 175$_{-56}^{+149}$ pc for T dwarfs. Using spatial and velocity dispersion relations, these scaleheights correspond to disk population ages of 3.6$_{-1.0}^{+0.8}$ for late M dwarfs, 2.1$_{-0.5}^{+0.9}$ Gyr for L dwarfs, and 2.4$_{-0.8}^{+2.4}$ Gyr for T dwarfs, which are consistent with prior simulations that predict that L-type dwarfs are on average a younger and less dispersed population. There is an additional 1-2 Gyr systematic uncertainty on these ages due to variances in age-velocity relations. We use our population simulations to predict the UCD yield in the JWST PASSAGES survey, a similar and deeper survey to WISPS and 3D-HST, and find that it will produce a comparably-sized UCD sample, albeit dominated by thick disk and halo sources.

astro-ph.GA↗

arXiv2022

Discovery of 34 low-mass comoving systems using NOIRLab Source Catalog DR2

We present the discovery of 34 comoving systems containing an ultra-cool dwarf found by means of the NOIRLab Source Catalog (NSC) DR2. NSC's angular resolution of $\sim$1" allows for the detection of small separation binaries with significant proper motions. We used the catalog's accurate proper motion measurements to identify the companions by cross-matching a previously compiled list of brown dwarf candidates with NSC DR2. The comoving pairs consist of either a very low-mass star and an ultra-cool companion, or a white dwarf and an ultra-cool companion. The estimated spectral types of the primaries are in the K and M dwarf regimes, those of the secondaries in the M, L and T dwarf regimes. We calculated angular separations between $\sim$2 and $\sim$56", parallactic distances between $\sim$43 and $\sim$261 pc and projected physical separations between $\sim$169 and $\sim$8487 AU. The lowest measured total proper motion is 97 mas yr$^{-1}$, the highest 314 mas yr$^{-1}$. Tangential velocities range from $\sim$23 to $\sim$187 km s$^{-1}$. We also determined comoving probabilities, estimated mass ratios and calculated binding energies for each system. We found no indication of possible binarity for any component of the 34 systems in the published literature. The discovered systems can contribute to the further study of the formation and evolution of low-mass systems as well as to the characterization of cool substellar objects.

astro-ph.SR↗

arXiv2022

Retrieval Study of Brown Dwarfs Across the L-T Sequence

A large suite of 228 atmospheric retrievals is performed on a curated sample of 19 brown dwarfs spanning the L0 to T8 spectral types using the open-source Helios-r2 retrieval code, which implements the method of short characteristics for radiative transfer and a finite-element description of the temperature-pressure profile. Surprisingly, we find that cloud-free and cloudy (both gray and non-gray) models are equally consistent with the archival SpeX data from the perspective of Bayesian model comparison. Only upper limits for cloud properties are inferred if log-uniform priors are assumed, but the cloud optical depth becomes constrained if a uniform prior is used. Water is detected in all 19 objects and methane is detected in all of the T dwarfs, but no obvious trend exists across effective temperature. As carbon monoxide is only detected in a handful of objects, the inferred carbon-to-oxygen ratios are unreliable. The retrieved radius generally decreases with effective temperature, but the values inferred for some T dwarfs are implausibly low and may indicate missing physics or chemistry in the models. For the early L dwarfs, the retrieved surface gravity depends on whether the gray or non-gray cloud model is preferred. Future data are necessary for constraining cloud properties and the vertical variation of chemical abundances, the latter of which is needed for distinguishing between the chemical instability versus traditional cloud interpretation of the L-T transition.

astro-ph.EP↗

arXiv2022

The HST large programme on $ω$ Centauri -- V. Exploring the Ultracool Dwarf Population with Stellar Atmosphere and Evolutionary Modelling

Brown dwarfs can serve as both clocks and chemical tracers of the evolutionary history of the Milky Way due to their continuous cooling and high sensitivity of spectra to composition. We focus on brown dwarfs in globular clusters that host some of the oldest coeval populations in the galaxy. Currently, no brown dwarfs in globular clusters have been confirmed, but they are expected to be uncovered with advanced observational facilities such as JWST. In this paper we present a new set of stellar models specifically designed to investigate low-mass stars and brown dwarfs in $ω$ Centauri -- the largest known globular cluster. The parameters of our models were derived from iterative fits to HST photometry of the Main Sequence members of the cluster. Despite the complex distribution of abundances and the presence of multiple Main Sequences in $ω$ Centauri, we find that the modal colour-magnitude distribution can be represented by a single stellar population with parameters determined in this study. The observed luminosity function is well-represented by two distinct stellar populations having solar and enhanced helium mass fractions and a common initial mass function, in agreement with previous studies. Our analysis confirms that the abundances of individual chemical elements play a key role in determining the physical properties of low-mass cluster members. We use our models to draw predictions of brown dwarf colours and magnitudes in anticipated JWST NIRCam data, confirming that the beginning of the substellar sequence should be detected in $ω$ Centauri in forthcoming observations.

astro-ph.SR↗

arXiv2022

Investigating the Nature of the Luminous Ambiguous Nuclear Transient ASASSN-17jz

We present observations of the extremely luminous but ambiguous nuclear transient (ANT) ASASSN-17jz, spanning roughly 1200 days of the object's evolution. ASASSN-17jz was discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN) in the galaxy SDSS J171955.84+414049.4 on UT 2017 July 27 at a redshift of $z=0.1641$. The transient peaked at an absolute $B$-band magnitude of $M_{B,{\rm peak}}=-22.81$, corresponding to a bolometric luminosity of $L_{\rm bol,peak}=8.3\times10^{44}$~erg~s$^{-1}$, and exhibited late-time ultraviolet emission that was still ongoing in our latest observations. Integrating the full light curve gives a total emitted energy of $E_{\rm tot}=(1.36\pm0.08)\times10^{52}$~erg, with $(0.80\pm0.02)\times10^{52}$~erg of this emitted within 200 days of peak light. This late-time ultraviolet emission is accompanied by increasing X-ray emission that becomes softer as it brightens. ASASSN-17jz exhibited a large number of spectral emission lines most commonly seen in active galactic nuclei (AGNs) with little evidence of evolution. It also showed transient Balmer features which became fainter and broader over time, and are still being detected $>1000$ days after peak brightness. We consider various physical scenarios for the origin of the transient, including supernovae (SNe), tidal disruption events (TDEs), AGN outbursts, and ANTs. We find that the most likely explanation is that ASASSN-17jz was an SN~IIn occurring in or near the disk of an existing AGN, and that the late-time emission is caused by the AGN transitioning to a more active state.

astro-ph.HE↗

arXiv2022

CWISE J014611.20-050850.0AB: The Widest Known Brown Dwarf Binary in the Field

While stars are often found in binary systems, brown dwarf binaries are much rarer. Brown dwarf--brown dwarf pairs are typically difficult to resolve because they often have very small separations. Using brown dwarfs discovered with data from the Wide-field Infrared Survey Explorer (WISE) via the Backyard Worlds: Planet 9 citizen science project, we inspected other, higher resolution, sky surveys for overlooked cold companions. During this process we discovered the brown dwarf binary system CWISE J0146$-$0508AB, which we find has a very small chance alignment probability based on the similar proper motions of the components of the system. Using follow-up near-infrared spectroscopy with Keck/NIRES, we determined component spectral types of L4 and L8 (blue), making CWISE J0146$-$0508AB one of only a few benchmark systems with a blue L dwarf. At an estimated distance of $\sim$40 pc, CWISE J0146$-$0508AB has a projected separation of $\sim$129 AU, making it the widest separation brown dwarf pair found to date. We find that such a wide separation for a brown dwarf binary may imply formation in a low-density star-forming region.

astro-ph.SR↗

arXiv2021

A Wide Planetary Mass Companion Discovered Through the Citizen Science Project Backyard Worlds: Planet 9

Through the Backyard Worlds: Planet 9 citizen science project we discovered a late-type L dwarf co-moving with the young K0 star BD+60 1417 at a projected separation of 37" or 1662 AU. The secondary - CWISER J124332.12+600126.2 (W1243) - is detected in both the CatWISE2020 and 2MASS reject tables. The photometric distance and CatWISE proper motion both match that of the primary within ~1sigma and our estimates for chance alignment yield a zero probability. Follow-up near infrared spectroscopy reveals W1243 to be a very red 2MASS color(J-Ks=2.72), low-surface gravity source that we classify as L6 - L8gamma. Its spectral morphology strongly resembles that of confirmed late-type L dwarfs in 10 - 150 Myr moving groups as well as that of planetary mass companions. The position on near- and mid-infrared color-magnitude diagrams indicates the source is redder and fainter than the field sequence, a telltale sign of an object with thick clouds and a complex atmosphere. For the primary we obtained new optical spectroscopy and analyzed all available literature information for youth indicators. We conclude that the Li I abundance, its loci on color-magnitude and color-color diagrams, and the rotation rate revealed in multiple TESS sectors are all consistent with an age of 50 - 150 Myr. Using our re-evaluated age of the primary, the Gaia parallax along with the photometry and spectrum for W1243 we find a Teff=1303+/-31 K, logg=4.3+/-0.17 cm s-2, and a mass of 15+/-5 MJup. We find a physical separation of ~1662 AU and a mass ratio of ~0.01 for this system. Placing it in context with the diverse collection of binary stars, brown dwarf and planetary companions, the BD+60 1417 system falls in a sparsely sampled area where the formation pathway is difficult to assess.

astro-ph.SR↗

arXiv2021

Ammonia-methane ratios from H-band near-infrared spectra of late-T and Y dwarfs

Gas cells containing ammonia and methane at atmospheric pressure were custom-made in our chemical laboratory. Low-resolution near-infrared spectroscopy of these gas cells were collected in our optical laboratory. They are compared with simulated spectra using the high-resolution transmission molecular absorption database (HITRAN) for temperatures of 300 K and 500 K, and with near-infrared spectra of late-T and Y dwarfs, Jupiter and Saturn. We selected for this investigation the spectral region between 1.5 and 1.7 microns (H-band) because it is covered by the Euclid red grism, it is particularly sensitive to the relative proportions of ammonia and methane opacity, and it is free from strong contributions of other abundant molecules such as water vapour. The laboratory spectra showed that the ammonia and methane features that are present in the simulations using the HITRAN database are incomplete. Using our laboratory spectra, we propose a modified version of the NH3-H spectral ratio with expanded integration limits that increases the amplitude of variation of the index with respect to spectral type. Combinations of our laboratory spectra were used to find the best fits to the observed spectra with the relative absorption ratio of ammonia to methane as a free parameter. A relationship was found between the T$\rm_{eff}$ and the ratio of ammonia to methane from spectral classes T5 to Y2 (1100 K -- 350 K), in qualitative fairly good agreement with theoretical predictions for high gravity objects and temperatures from 1100 K to 500 K. The ammonia to methane ratios in late-T and Y dwarfs are similar to that of Jupiter, suggesting a similar chemical composition. Simulations of the spectroscopic performance of Euclid suggest that it will yield T$\rm_{eff}$ values and ratios of ammonia to methane for over 10$^3$ late T dwarfs in the whole wide survey.

astro-ph.SR↗

arXiv2021

An Early-Time Optical and Ultraviolet Excess in the type-Ic SN 2020oi

We present photometric and spectroscopic observations of Supernova 2020oi (SN 2020oi), a nearby ($\sim$17 Mpc) type-Ic supernova (SN Ic) within the grand-design spiral M100. We undertake a comprehensive analysis to characterize the evolution of SN 2020oi and constrain its progenitor system. We detect flux in excess of the fireball rise model $δt \approx 2.5$ days from the date of explosion in multi-band optical and UV photometry from the Las Cumbres Observatory and the Neil Gehrels Swift Observatory, respectively. The derived SN bolometric luminosity is consistent with an explosion with $M_{\rm ej} = 0.81 \pm 0.03 M_{\odot}$, $E_{k}= 0.79 \pm 0.09 \times 10^{51} \rm{erg} \rm{s}^{-1}$, and $M_{\rm Ni56} = 0.08 \pm 0.02 M_{\odot}$. Inspection of the event's decline reveals the highest $Δm_{15,\rm{bol}}$ reported for a stripped-envelope event to date. Modeling of optical spectra near event peak indicates a partially mixed ejecta comparable in composition to the ejecta observed in SN 1994I, while the earliest spectrum shows signatures of a possible interaction with material of a distinct composition surrounding the SN progenitor. Further, Hubble Space Telescope (HST) pre-explosion imaging reveals a stellar cluster coincident with the event. From the cluster photometry, we derive the mass and age of the SN progenitor using stellar evolution models implemented in the BPASS library. Our results indicate that SN 2020oi occurred in a binary system from a progenitor of mass $M_{\rm ZAMS} \approx 9.5 \pm 1.0 M_{\odot}$, corresponding to an age of $27 \pm 7$ Myr. SN 2020oi is the dimmest SN Ic event to date for which an early-time flux excess has been observed, and the first in which an early excess is unlikely to be associated with shock-cooling.

astro-ph.HE↗

arXiv2021

Final Targeting Strategy for the SDSS-IV APOGEE-2N Survey

APOGEE-2 is a dual-hemisphere, near-infrared (NIR), spectroscopic survey with the goal of producing a chemo-dynamical mapping of the Milky Way Galaxy. The targeting for APOGEE-2 is complex and has evolved with time. In this paper, we present the updates and additions to the initial targeting strategy for APOGEE-2N presented in Zasowski et al. (2017). These modifications come in two implementation modes: (i) "Ancillary Science Programs" competitively awarded to SDSS-IV PIs through proposal calls in 2015 and 2017 for the pursuit of new scientific avenues outside the main survey, and (ii) an effective 1.5-year expansion of the survey, known as the Bright Time Extension, made possible through accrued efficiency gains over the first years of the APOGEE-2N project. For the 23 distinct ancillary programs, we provide descriptions of the scientific aims, target selection, and how to identify these targets within the APOGEE-2 sample. The Bright Time Extension permitted changes to the main survey strategy, the inclusion of new programs in response to scientific discoveries or to exploit major new datasets not available at the outset of the survey design, and expansions of existing programs to enhance their scientific success and reach. After describing the motivations, implementation, and assessment of these programs, we also leave a summary of lessons learned from nearly a decade of APOGEE-1 and APOGEE-2 survey operations. A companion paper, Santana et al. (submitted), provides a complementary presentation of targeting modifications relevant to APOGEE-2 operations in the Southern Hemisphere.

astro-ph.GA↗

arXiv2021

Ross 19B: An Extremely Cold Companion Discovered via the Backyard Worlds: Planet 9 Citizen Science Project

Through the Backyard Worlds: Planet 9 citizen science project, we have identified a wide-separation ($\sim$10', $\sim$9900 au projected) substellar companion to the nearby ($\sim$17.5 pc), mid-M dwarf Ross 19. We have developed a new formalism for determining chance alignment probabilities based on the BANYAN $Σ$ tool, and find a 100% probability that this is a physically associated pair. Through a detailed examination of Ross 19A, we find that the system is metal-poor ([Fe/H]=$-$0.40$\pm$0.12) with an age of 7.2$^{+3.8}_{-3.6}$ Gyr. Combining new and existing photometry and astrometry, we find that Ross 19B is one of the coldest known wide-separation companions, with a spectral type on the T/Y boundary, an effective temperature of 500$^{+115}_{-100}$ K, and a mass in the range 15-40 $M_{\rm Jup}$. This new, extremely cold benchmark companion is a compelling target for detailed characterization with future spectroscopic observations using facilities such as the Hubble Space Telescope or James Webb Space Telescope.

astro-ph.EP↗

arXiv2021

The Brown Dwarf Kinematics Project (BDKP). V. Radial and Rotational Velocities of T Dwarfs from Keck/NIRSPEC High-Resolution Spectroscopy

We report multi-epoch radial velocities, rotational velocities, and atmospheric parameters for 37 T-type brown dwarfs observed with Keck/NIRSPEC. Using a Markov Chain Monte Carlo forward-modeling method, we achieve median precisions of 0.5 km s$^{-1}$ and 0.9 km s$^{-1}$ for radial and rotational velocities, respectively. All of the T dwarfs in our sample are thin disk brown dwarfs. We confirm previously reported moving group associations for four T dwarfs. However, the lack of spectral indicators of youth in two of these sources suggests that these are chance alignments. We confirm two previously un-resolved binary candidates, the T0+T4.5 2MASS J11061197+2754225 and the L7+T3.5 2MASS J21265916+7617440, with orbital periods of 4 yr and 12 yr, respectively. We find a kinematic age of 3.5$\pm$0.3 Gyr for local T dwarfs, consistent with nearby late-M dwarfs (4.1$\pm$0.3 Gyr). Removal of thick disk L dwarfs in the local ultracool dwarf sample gives a similar age for L dwarfs (4.2$\pm$0.3 Gyr), largely resolving the local L dwarf age anomaly. The kinematic ages of local late-M, L, and T dwarfs can be accurately reproduced with population simulations incorporating standard assumptions of the mass function, star formation rate, and brown dwarf evolutionary models. A kinematic dispersion break is found at the L4$-$L6 subtypes, likely reflecting the terminus of the stellar Main Sequence. We provide a compilation of precise radial velocities for 172 late-M, L, and T dwarfs within $\sim$20 pc of the Sun.

astro-ph.SR↗

arXiv2021

New Candidate Extreme T Subdwarfs from the Backyard Worlds: Planet 9 Citizen Science Project

Schneider et al. (2020) presented the discovery of WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5, which appear to be the first examples of extreme T-type subdwarfs (esdTs; metallicity <= -1 dex, T_eff <= 1400 K). Here we present new discoveries and follow-up of three T-type subdwarf candidates, with an eye toward expanding the sample of such objects with very low metallicity and extraordinarily high kinematics, properties that suggest membership in the Galactic halo. Keck/NIRES near-infrared spectroscopy of WISEA J155349.96+693355.2, a fast-moving object discovered by the Backyard Worlds: Planet 9 citizen science project, confirms that it is a mid-T subdwarf. With H_W2 = 22.3 mag, WISEA J155349.96+693355.2 has the largest W2 reduced proper motion among all spectroscopically confirmed L and T subdwarfs, suggesting that it may be kinematically extreme. Nevertheless, our modeling of the WISEA J155349.96+693355.2 near-infrared spectrum indicates that its metallicity is only mildly subsolar. In analyzing the J155349.96+693355.2 spectrum, we present a new grid of low-temperature, low-metallicity model atmosphere spectra. We also present the discoveries of two new esdT candidates, CWISE J073844.52-664334.6 and CWISE J221706.28-145437.6, based on their large motions and colors similar to those of the two known esdT objects. Finding more esdT examples is a critical step toward mapping out the spectral sequence and observational properties of this newly identified population.

astro-ph.SR↗