Discovery of the Remarkably Red L/T Transition Object VHS J183135.58-551355.9

We present the discovery of VHS J183135.58$-$551355.9 (hereafter VHS J1831$-$5513), an L/T transition dwarf identified as a result of its unusually red near-infrared colors ($J-K_{\rm S}=3.633\pm0.277$ mag; $J-W2=6.249\pm0.245$ mag) from the VISTA Hemisphere Survey and CatWISE2020 surveys. We obtain low resolution near-infrared spectroscopy of VHS J1831$-$5513 using Magellan/FIRE to confirm its extremely red nature and assess features sensitive to surface gravity (i.e., youth). Its near-infrared spectrum shows multiple CH$_{\rm 4}$ absorption features, indicating an exceptionally low effective temperature for its spectral type. Based on proper motion measurements from CatWISE2020 and a photometric distance derived from its $K_{\rm S}$-band magnitude, we find that VHS J1831$-$5513 is a likely ($\sim$85$\%$ probability) kinematic member of the $\beta$ Pictoris moving group. Future radial velocity and trigonometric parallax measurements will clarify such membership. Follow-up mid-infrared or higher resolution near-infrared spectroscopy of this object will allow for further investigation as to the cause(s) of its redness, such as youth, clouds, and viewing geometry.

1. INTRODUCTION At young ages, brown dwarfs have not yet contracted to their final equilibrium radii (Burrows et al. 1997) and have low masses for their effective temperatures.They therefore have lower surface gravities and lower pressure atmospheres than their field-age counterparts.This low pressure reduces collision-induced absorption by H 2 (Linsky 1969;Borysow et al. 1997) and it is hypothesized that it causes an excess of clouds and dust in their photospheres (Cushing et al. 2008;Faherty et al. 2016), shifting emergent flux to longer wavelengths (e.g., Faherty et al. 2012).Consequently, the nearinfrared colors of young brown dwarfs are notably redder Corresponding author: T. Bickle tombickleastro@outlook.com than those of field age objects of the same spectral type (Faherty et al. 2016;Liu et al. 2016).
Young, red L and T dwarfs can act as valuable analogs for young, giant exoplanets, possessing similar effective temperatures, masses, and atmospheric properties (Liu et al. 2013;Faherty et al. 2016).The planets HR8799bcde (Marois et al. 2008) and planetary-mass companions VHS 1256−1257 b (Gauza et al. 2015), 2M1207b (Chauvin et al. 2004) and BD+60 1417b (Faherty et al. 2021) are apt examples of this, all exhibiting near-infrared spectra consistent with red, lowgravity, late-L dwarfs.Crucially, young free-floating substellar objects can be studied without the interference of light from a host star, providing convenient and effective laboratories for tests of planetary theory (Faherty et al. 2016).This convenience does, however, come at a cost; whereas companions can have properties (e.g., age and metallicity) inferred from their host star under the assumption of common formation, the properties of free-floating brown dwarfs are notoriously difficult to determine.An age-mass-temperature degeneracy exists for brown dwarfs (Burrows et al. 1997), making it challenging to pinpoint these three key properties for isolated brown dwarfs.However, if a brown dwarf can be linked through kinematics and age diagnostics to a known moving group with a well constrained age, this degeneracy inherent in brown dwarfs can be broken, and properties consistent throughout that group's membership, such as age (e.g., Bell et al. 2015) and metallicity (e.g., Viana Almeida et al. 2009), may be attributed to the object.When the age of an object is inferred, evolutionary models can be used to estimate physical properties such as mass and effective temperature (e.g., Filippazzo et al. 2015;Suárez et al. 2021).Such objects are vital benchmarks for studying brown dwarf evolution, and at young ages and low masses, can provide important empirical constraints to the initial mass function (Gagné et al. 2017;Kirkpatrick et al. 2021Kirkpatrick et al. , 2024)).
While targeted searches using the Two Micron All-Sky Survey (2MASS; Skrutskie et al. 2006) have discovered unusually red, nearby L dwarfs (Kellogg et al. 2015;Schneider et al. 2017), the detection limit of 2MASS (J ≈ 17 mag) means that many low mass, red L/T dwarfs are missed.This restriction is the primary reason that the number of spectroscopically confirmed free-floating L/T dwarfs with extremely red colors (J − K > 2.2 mag) remains low (∼15).The advent of a new generation of surveys, such as the VISTA Hemisphere Survey (VHS; McMahon et al. 2013), the UKIDSS Large Area Survey (UKIDSS LAS; Lawrence et al. 2007) and UKIRT Hemisphere Survey (UHS; Dye et al. 2018), which probe significantly deeper than 2MASS (5σ J-band depths of 20.2, 19.9 and 19.6 mag respectively), has brought the potential for the discovery of objects with fainter Jband magnitudes than previously possible (e.g., Schneider et al. 2023b).Consequently, these surveys are ideal hunting grounds for young, red L and T dwarfs, the J-bands of which are inherently suppressed.Reaching fainter Jband magnitudes also means that redder objects than previously known have become detectable.Indeed, until Schneider et al. (2023b)'s discovery of the reddest known freefloating L/T dwarf CWISE J050626.96+073842.4 (CWISE J0506+0738; (J − K) MKO =2.974±0.039mag) using UHS, it had been approximately a decade since the discovery of the previously reddest known objects PSO J318.5338−22.8603((J − K) MKO =2.74±0.04mag; Liu et al. 2013) and ULAS J222711−004547 ((J − K) MKO =2.79±0.06mag; Marocco et al. 2014).The discovery of such extraordinarily red objects is important as it allows us to explore parameters that influence the appearance of brown dwarfs such as youth, metallicity, clouds, and viewing inclination (e.g., Looper et al. 2008;Vos et al. 2017;Suárez & Metchev 2022;Suárez et al. 2023).It is also vital for understanding the differences between free-floating planetary mass objects and directly imaged exoplanets.Directly imaged exoplanets occupy a unique color-magnitude sequence (e.g., Gratton et al. 2024), and the discovery of extremely red, faint objects such as CWISE J0506+0738 (a strong candidate member of the β Pictoris Moving Group; Schneider et al. 2023b), serves to narrow the color-magnitude gap.
In this work, we present VHS J1831−5513, an L/T transition dwarf with anomalously red near-infrared colors.At J − K S = 3.633 ± 0.277 mag, it is potentially the reddest known free-floating substellar object, more so than the current reddest spectroscopically confirmed object in the literature, CWISE J0506+0738, by ∼0.66 mag.In Section 2 we discuss the discovery of VHS J1831−5513.In Section 3, we present Magellan/FIRE spectroscopic follow-up data and in Section 4, we analyze and discuss those data.In Section 5, we evaluate young moving group membership, estimate physical properties, and investigate the prospects for variability in VHS J1831−5513.

IDENTIFICATION OF VHS J1831-5513
The unique color space occupied by young L dwarfs makes it possible to perform targeted searches using a photometric approach; a particularly valuable property given their typically low tangential velocities (Faherty et al. 2009(Faherty et al. , 2012)), which makes their discovery through proper motionbased searches (e.g., Backyard Worlds; Kuchner et al. 2017;Humphreys et al. 2020) challenging.Utilizing this principle, we performed a half-sky search of CatWISE2020 (Cat-WISE; Marocco et al. 2021) and VHS for objects with extremely red (J − K S > 2 mag, J −W 2 > 4 mag) near-infrared colors (Bickle et al., in prep).VHS J1831−5513 was discovered during a secondary search, which used the CatWISE reject table instead of the main CatWISE catalog, in order to collect any spuriously rejected candidates.VHS J1831−5513 was rejected by CatWISE due to the overlap of a latent (a.k.a.charge persistence; Eisenhardt et al. 2020) artifact in WISE.We used WiseView1 (Caselden et al. 2018), a tool designed to aid the visualisation of motion in WISE time-resolved coadds, to vet VHS J1831−5513, and confirmed the presence of an overlapping latent.We therefore checked the unTimely catalog (Meisner et al. 2023;Kiwy 2022) to investigate if its CatWISE photometry was adversely affected.This showed that both the W1 and W2 bands were affected by ∼ 0.1 − 0.2 mag in the scan direction where the latent was present, but both to a similar extent, so while the CatWISE W1-W2 color is not completely unaffected, it is approximately accurate, with an offset from the average unTimely W1-W2 color in the epochs where the latent is absent of only 0.012 mag.We also visually inspected the higher-resolution VHS imagery to confirm there were no obvious contaminants or resolved companions affecting its near-infrared colors.Once these checks were complete, and we were satisfied that VHS J1831−5513 was a genuine extremely red candidate, it was added to our list for follow-up spectroscopy.
3. OBSERVATIONS We used the 6.5m Magellan Baade telescope and the Folded-port InfraRed Echellette (FIRE; Simcoe et al. 2013) spectrograph to obtain near-infrared spectra for VHS J1831−5513.Observations were made on 2023 June 30 under clear conditions.We operated FIRE in the low resolution prism mode using the 0. ′′ 8 slit (resolution λ/∆λ ∼100) covering the full 0.8−2.5 µm wavelength range.The source was observed in an ABBA pattern with 16 individual exposures of 120 seconds each, using the Sample Up the Ramp (SUTR) read mode.Immediately after the science image we obtained a Neon, Neon, Argon lamp and the A0 star HD 153842 for telluric, wavelength and flux calibration.All data were reduced using a custom version of the FIREHOSE (Gagné et al. 2015) package.The pipeline was modified such that boxcar extractions were performed on A-B images to avoid large variations in the slope of the resulting spectra, and subroutines from the SpeXtool v1 package (Cushing et al. 2004;Vacca et al. 2003) were used to combine, mask, and inspect the resulting spectra.
The final reduced spectrum is shown in Figure 1, in which prominent absorption features and telluric bands are labeled.We caution that due to the nonlinear response of the detector, the slope after the ∼2.3 µm CO edge may be unreliable as response decreases rapidly towards the edge of the detector.

ANALYSIS 4.1. Spectral Type
The left panel of Figure 2 compares the near-infrared spectrum of VHS J1831−5513 to several field-age L/T transi-tion spectral standards (Burgasser et al. 2006;Kirkpatrick et al. 2010;Cruz et al. 2018).The extremely red spectral slope and low gravity features (e.g., steep H-band slope) in the near-infrared spectrum of VHS J1831−5513 leads to poor fits to the spectral standards over the 1-2.4µm range as a whole, as is the case for many other young, red objects (e.g., Faherty et al. 2013;Liu et al. 2013;Schneider et al. 2016Schneider et al. , 2023b)).The best fit over the J-band is the L9 standard 2MASS J02550357−4700509 (Kirkpatrick et al. 2010).Although progress is being made in the spectral classification of young L and T dwarfs (e.g., Piscarreta et al. 2024), there is not yet a unified approach tailored to young objects, nor a complete set of young L/T transition spectral standards.The diversity seen in the nearinfrared slopes and molecular absorption features within the known young populations remains a challenge for the development of a uniform classification technique of young objects.The right panel of Figure 2 (Miles et al. 2023).The two objects have several likely CH 4 absorption features in common, as highlighted in the left column of Figure 3.The wavelengths of these features coincide with CH 4 absorption bands that begin to form in model spectra of low gravity objects at effective temperatures ≲ 1500 K and become more prominent as the temperature decreases.The right column of Figure 3 shows Sonora Bobcat (Marley et al. 2021) models at solar metallicity and a constant surface gravity (log(g)= 3.5), with temperatures ranging from 1100K to 1500K.Highlighted are the locations of the absorption features that occur in the spectra of both VHS J1831−5513 and VHS 1256−1257 b.
Unlike CWISE J0506+0738 and VHS 1256−1257 b, VHS J1831−5513 displays a prominent absorption feature at the peak of the J-band, centered at ∼1.3 µm.This feature is not reproduced by the Sonora models and, to our knowledge, no feature centered at this wavelength has previously been noted in other L dwarfs.In T dwarfs, a 1.3 µm CH 4 band is responsible for the gradually increasing downward slope on the red side of the J-band peak, starting in early T dwarfs and progressing through later subtypes.It eventually merges with the 1.4 µm H 2 O band to produce the characteristic, thin   2010), and SDSS J120747.17+024424.8 (T0) and SDSS J083717.21−000018.0 (T1) from Burgasser et al. (2006).Right: VHS J1831−5513 compared to the young, planetary-mass objects WISEA J1147−2040 (Schneider et al. 2016), PSO J318.5−22 (Liu et al. 2013) and CWISE J0506+0738 (Schneider et al. 2023b).All spectra are normalized between 1.27 and 1.29 µm.J-band peak of late-T dwarfs (Burgasser et al. 2002).Given the presence of likely CH 4 absorption elsewhere in the spectrum of VHS J1831−5513, the observed 1.3 µm feature may also be the result of CH 4 absorption.Given that such absorption has only been seen in T dwarfs, this would be suggestive of VHS J1831−5513 being narrowly on the T side of the L/T transition.However, we caution that CH 4 is not known to exhibit any sharp features in this area, and this would therefore be a very unusual presentation for CH 4 absorption.There is a known telluric O 2 feature close to the wavelength in question (though not at the exact wavelength) which may cause contamination (Kausch et al. 2015 and references therein).We therefore inspected the raw science and telluric data to check if the feature was the result of poor telluric correction, but no obvious cause was found.The feature also does not appear in any of the other spectra taken with the instrument on the night.For now, we are uncertain of the feature's cause, and characterize it as an unknown absorber.Future acquisition of higher resolution, higher signal-to-noise (S/N) spectra will allow this feature to be investigated further using the spectral inversion (retrieval) technique, which has been effectively utilized in recent studies to constrain cloud properties and elemental abundances of substellar objects (e.g., Burningham et al. 2021;Calamari et al. 2022;Vos et al. 2023;Hood et al. 2023;Adams et al. 2023;Whiteford et al. 2023).
Considering the L9 standard being the best J-band fit, the deep 1.1 µm H 2 O band, and the Hand K-band CH 4 absorption features, we assigned a preliminary spectral type of L8-T0 (v.red), prior to gravity assessment.

Gravity
Although the extremely red near-infrared colors of VHS J1831−5513 are a potential indicator of youth, a small population of field gravity objects are also known to possess red colors (e.g., Marocco et al. 2014), possibly due to an excess of sub-micron sized dust grains in their photospheres (Hiranaka et al. 2016).A near equator-on inclination angle has also been found to be a cause of red colors among field objects (Vos et al. 2017;Guerra Toro et al. 2022;Suárez et al. 2023), indicating that red colors in brown dwarfs are more complex diagnostically than previously thought.Hence, other spectral diagnostics must be performed to confirm youth.
Youth in brown dwarfs and planetary-mass objects can also be inferred through evidence of low surface gravity, as young objects are still contracting (Burrows et al. 1997) and have larger radii than equivalent temperature field objects.Intermediate and low surface gravity objects, suffixed β and γ respectively (Kirkpatrick 2005; or INT-G and VL-G per Allers & Liu 2013 indices), can be discerned from field gravity objects by utilizing morphological differences in their optical and near-infrared spectra.Gravity-sensitive indices are well established for early-to-mid type L dwarfs (Lucas et al. 2001;McGovern et al. 2004;Kirkpatrick et al. 2006;Cruz et al. 2009;Allers & Liu 2013;Martin et al. 2017;Cruz et al. 2018).Some rely on the weakening of atomic lines in the spectra of lower gravity objects.Others measure the shape of spectral regions, the morphologies of which are particularly affected by the strength of H 2 collision-induced absorption (CIA), which is reduced in lower gravity objects.However, most gravity-sensitive indices do not function well for late type L dwarfs (>L5).
One index that may function for objects as late in type as VHS J1831−5513 is the H-slope index (Schneider et al. 2023b).Low gravity (young) objects have more peaked Hbands caused by reduced H 2 CIA in their low-pressure photospheres.H-slope aims to quantify this by using a linear least-squares fit between 1.45-1.64µm to measure the slope of the blue side of the H-band, where the unit is normalized flux/wavelength.Schneider et al. (2023b) found that for L7-T0 dwarfs, typical field gravity objects have H-slope values ranging from 2-4, and low gravity objects have values between 3-5.VHS J1831−5513 has an H-slope value of 5.068, considerably higher than those of field objects, and even slightly exceeding the top end of the young population.This result is extremely similar to that of the spectrum of CWISE J0506+0738, which returns an H-slope value of 5.0432 .
The H 2 (K) index (Canty et al. 2013) has also been shown to differentiate low gravity and field gravity objects at late-L subtypes (Schneider et al. 2014).Like the H-cont and Hslope indices, it utilizes the reduced H 2 CIA that occurs in lower gravity objects, but in the K-band.To do this, it measures the slope between 2.17 µm and 2.24 µm by calculating the ratio of the median fluxes in 0.02 µm ranges centered at those wavelengths in the unsmoothed spectrum of an object.For L7-T0 spectral types, typical index scores for field gravity objects are 1.05 ⩽H 2 (K)⩽ 1.2, whereas low gravity objects score much lower than their field gravity counterparts of the same spectral type (Schneider et al. 2014).VHS J1831−5513 returns a score of H 2 (K)=1.045,over 1σ below the field sequence at L7, and significantly further below for later spectral types (∼3σ below for L8), where the field sequence sees a rapid increase (see fig. 10 of Schneider et al. 2014).
We caution that as most of these indices predate the discovery that inclination angle is correlated with cloud opacity (Vos et al. 2017;Guerra Toro et al. 2022;Suárez et al. 2023), they are not designed to account for this, and as such, it is possible that the viewing geometry of VHS J1831−5513 may interfere with the gravity assessment they provide.The existence of such contamination and extent to which this may impact the index scores is yet to be fully tested.
Considering the extremely red near-infrared slope, the H-slope and H 2 (K) indicators, and the probable kinematic match to the ∼22 Myr β Pictoris Moving Group (Zuckerman et al. 2001;Shkolnik et al. 2017) discussed later, we conclude that VHS J1831−5513 has low surface gravity, and tentatively classify it as L8γ-T0γ.Both a higher S/N spectrum and the future development of gravity-sensitive indices applicable to L/T transition objects will contribute to a higher confidence gravity assessment for VHS J1831−5513.

Moving Group Membership
Young moving groups typically retain similar 3D velocities for a few hundred Myr, and their population encompasses the whole range of masses from planetary-mass to stars (Zuckerman & Song 2004;Torres et al. 2008;Malo et al. 2013;Gagné et al. 2014;Faherty et al. 2016).Young moving groups are coeval, so objects that are found to be a member of a group can be assigned the age of that group and become key benchmarks for its population.
To determine if VHS J1831−5513 is a member of a known young moving group, we used the BANYAN Σ algorithm (Gagné et al. 2018c), which uses Bayes' theorem to select the best-matching XYZUVW model among 27 young associations and the distribution of nearby field stars, while optionally marginalizing over missing radial velocities or distances with an exact, analytical solution to the marginalization integrals.BANYAN Σ requires minimum inputs of position and proper motions, and can optionally use distance and heliocentric radial velocity with respective uncertainties.If distance and/or radial velocity are not provided, it calculates and provides optimal values for these, assuming membership in each of the 27 groups.
The long time baseline of CatWISE (12 epochs over 8 years) means that it can provide proper motion measurements with reasonable confidence (± ∼10 mas yr −1 for a W1 = 14 mag source; Marocco et al. 2021).We therefore use the CatWISE proper motion data for the BANYAN Σ input.However, CatWISE has known systematic astrometric offsets3 compared to Gaia DR2.We therefore applied the offsets for the CatWISE tile in which VHS J1831−5513 is located, and used the corrected proper motion values for our BANYAN Σ input.When only provided with the position and proper motions of VHS J1831−5513, with respective uncertainties, BANYAN Σ gives a 79.7% membership probability for the ∼22 Myr β Pictoris Moving Group (BPMG; Zuckerman et al. 2001;Shkolnik et al. 2017), an 8.6% probability for the ∼45 Myr Argus Association (ARG; Torres et al. 2008;Zuckerman 2019), a 2% probability for the ∼130 Myr AB Doradus Moving Group (ABDMG; Zuckerman et al. 2004;Gagné et al. 2018b), and a 9.8% probability of VHS J1831−5513 not being a member of any known group.
VHS J1831−5513 is too faint at optical wavelengths to be detected by the Gaia mission (Gaia Collaboration et al. 2023), and in surveys where it is detected, insufficient astrometric data of adequate precision to determine a parallax exist.We instead calculated a photometric distance for VHS J1831−5513.As has been noted in previous works, ob-jects with unusual near-infrared colors do not conform well to standard absolute magnitude relations (Faherty et al. 2013(Faherty et al. , 2016;;Filippazzo et al. 2015;Liu et al. 2016).In investigating this, Schneider et al. (2023b) compared the parallactic distances of several very red, free-floating objects to their J, K, W1, and W2 photometric distances, and showed that for those objects, the K-band photometric distance provided the best match on average to the measured parallax.We therefore used the VHS K s -band magnitude to calculate a photometric distance for VHS J1831−5513 using the absolute magnitudespectral type relations in Dupuy & Liu (2012).Using a spectral type of L9±1, we determine a K s -band photometric distance for VHS J1831−5513 of 55.4±5.5 pc.This distance agrees well with the optimal kinematic distances for BPMG (51.5±5.1 pc) and ARG (54.8±5.8 pc) membership provided by BANYAN Σ. Adding this photometric distance to the existing position and proper motion parameters, BANYAN Σ gives an 85.2% membership probablity for BPMG, 9.7% for ARG, 0.4% for ABDMG and 4.6% for no group.While promising, this is not a conclusive indicator of BPMG membership, as a probability of > 90% is typically used as a cutoff to select high probability group members (Gagné et al. 2015).Regardless, the indications of low surface gravity in its spectrum and the fairly low precision proper motion measurements from CatWISE for VHS J1831−5513 merit further investigation of a ∼ 85% membership probability.A summary of the BANYAN Σ results are provided in Table 2.
In light of its positive spectral indicators of youth and its best BANYAN Σ kinematic match, we find that VHS J1831−5513 is most likely a member of the BPMG.A higher precision proper motion measurement, as well as future radial velocity and trigonometric parallax measurements are required to confirm or refute this membership.

Physical Properties
Under the assumption that it is a member of the BPMG, certain physical properties of VHS J1831−5513 can be inferred.Viana Almeida et al. (2009) found that the BPMG has approximately solar metallicity, so we assume solar metallicity for our analysis.Firstly, we estimated the bolometric luminosity of VHS J1831−5513.To do this, we used the Sanghi et al. (2023) empirical bolometric correction-spectral type polynomial relations for young objects, in combination with the absolute VHS K S -band magnitude based on our previously calculated photometric distance.We estimate a bolometric luminosity for VHS J1831−5513 of log(L bol /L ⊙ ) = −4.57± 0.09.Note that several assumptions are made when determining this value, such as our photometric distance being accurate and the Sanghi et al. (2023) relations being applicable to such an unusually red object.A more accurate bolometric luminosity can be determined with a measured parallax and broader wavelength coverage.
To test this value, we also derive a bolometric correction for VHS 1256-1257b using the bolometric luminosity from Miles et al. (2023) and photometry from the Ultracoolsheet (Best et al. 2024 and references therein), and apply it to VHS J1831−5513.VHS 1256-1257b is known to be
red and young, and the broad wavelength coverage provided by the JWST observations of this object allows for a precise determination of its L bol value.This yields a value of log(L bol /L ⊙ ) = −4.51± 0.09 for VHS J1831−5513.
We used the Sonora Bobcat (Marley et al. 2021) and Diamondback (Morley et al. 2024) evolutionary models at solar metallicity to estimate the mass and radius of VHS J1831−5513 using the age of BPMG (22 ± 6 Myr;Shkolnik et al. 2017) and the bolometric luminosity derived from the Sanghi et al. (2023) relations.Using the Sonora Bobcat models, assuming BPMG membership, we infer a radius of 1.32 +0.03 −0.02 R Jup and a mass of 7 ± 1 M Jup .Using the calculated bolometric luminosity, derived radius, and the Stefan-Boltzmann law, we estimate an effective temperature for VHS J1831−5513 of T eff = 1130 ± 60 K. From the Sonora Diamondback models, taking the same age and bolometric luminosity, we infer a radius of 1.42 +0.04 −0.03 R Jup , a mass of 6.5 ± 1.5 M Jup and an effective temperature of T eff = 1085 ± 60 K.
While both models suggest a similar mass and effective temperature for VHS J1831−5513, the radius is notably different.The Diamondback values are preferred, as the model incorporates silicate clouds, as well as gravity-dependent cloud clearing, which appears to occur at the L/T transition (Marley et al. 2010).In both cases, the mass is well below the deuterium burning limit of ∼13 M Jup (Spiegel et al. 2011 and references therein) and the effective temperature is ∼150-200 K cooler than an L9 at field age (Kirkpatrick et al. 2021), which is consistent with previous findings that young objects are cooler than field age objects of equivalent spectral type (Filippazzo et al. 2015;Faherty et al. 2016;Liu et al. 2016;Suárez et al. 2021).An insufficient number of L/T transition objects are currently known in BPMG to infer the temperature at which the L/T transition occurs in the group.However, the temperature at which the L/T transition occurs decreases with age, and Gagné et al. (2018a) found that the L/T transition for ABDMG is ∼1150 K, suggesting that if VHS J1831−5513 is a true member of BPMG, the L/T transition may occur at a similar temperature in the two groups, despite the ∼100 Myr age difference.
VHS J1831−5513 is notably ∼100 K cooler than PSO J318.5−22 (Miles et al. 2018) which displays no corresponding near-infrared CH 4 absorption features.It is how-ever, an extremely similar effective temperature to CWISE J0506+0738 (T eff = 1140 ± 80 K; Schneider et al. 2023b), which exhibits likely CH 4 absorption at both the Hand Kband peaks.This temperature similarly bolsters the empirical temperature constraint provided by CWISE J0506+0738 for the introduction of CH 4 absorption into the near infrared spectra of young L/T transition objects.
BANYAN Σ returns a non-zero probability of VHS J1831−5513 being a member of ABDMG, which has very similar UVW and distance values among its membership to that of BPMG.These similarities are known to confuse membership assignment for some objects between the two groups (Gagné et al. 2018c).In case of the unlikely event that VHS J1831−5513 is an ABDMG member, we repeated the analysis using the age of the ABDMG (133 +15 −20 Myr; Gagné et al. 2018b).Assuming ABDMG membership, the Marley et al. (2021) models suggest a radius of 1.23 +0.13  −0.11 R Jup , an effective temperature of 1170 ± 50 K and a mass of 12 ± 0.2 M Jup for VHS J1831−5513.This would still place VHS J1831−5513 below the deuterium burning limit, solidifying its status as a planetary-mass object.We reiterate that the scenario where VHS J1831−5513 is a member of ABDMG is unlikely, and BPMG is the strong favorite for membership.

Prospects for Variability
Young L and T dwarfs are known to exhibit stronger photometric and spectrophotometric variability than field age objects, driven by rotational modulations of condensate clouds in their atmospheres (Biller et al. 2015(Biller et al. , 2018;;Metchev et al. 2015;Lew et al. 2016;Schneider et al. 2018;Vos et al. 2018Vos et al. , 2019Vos et al. , 2020Vos et al. , 2022;;Eriksson et al. 2019;Miles-Páez et al. 2019;Bowler et al. 2020;Manjavacas et al. 2021).Furthermore, the L/T transition has been shown to have a higher variability occurrence rate than other spectral types (Radigan 2014).Indeed, Liu et al. (2024) determined that the variability rate of young objects at the L/T transition is 64 +23 −22 %, compared to 31 +12 −9 % outside the L/T transition.Variable L dwarfs are more likely to have cloudier atmospheres (Suárez & Metchev 2022) and redder colors (Ashraf et al. 2022) than non-variable counterparts.Additionally, both variability amplitude and infrared colors have been found to be correlated with inclination angle, with equator-on objects having higher amplitudes and redder colors than those which are viewed pole-on (Vos et al. 2017).Near-infrared colors also depend on cloud thickness, where redder L dwarfs have higher cloud opacity than bluer objects (Suárez & Metchev 2022).Suárez et al. (2023) further demonstrated a correlation between infrared color, cloud opacity, and viewing inclination for L dwarfs, finding that equator-on objects exhibit more clouds and, therefore, are redder than objects viewed close to pole-on.This indicates that equatorial latitudes are cloudier than polar latitudes and the color diversity of brown dwarfs is strongly influenced by their viewing geometry.
The VHS photometry of VHS J1831−5513 indicates a color of J − K S = 3.633 ± 0.277 mag.However, visual inspection and comparison of its Magellan/FIRE spectrum showed that its spectrum is not as red as CWISE J0506+0738 (see Figure 2), which has a color of (J − K) MKO =2.974±0.039mag (Schneider et al. 2023b).We therefore extracted synthetic photometry from the spectrum of VHS J1831−5513 using a custom code with filter profiles from SVO (Rodrigo et al. 2012;Rodrigo & Solano 2020).This gives a color of (J − K S ) synth = 2.914 ± 0.008 mag, which is discrepant from the VHS color by 0.719 mag (3.07σ).The photometric and synthetic colors of VHS J1831−5513 are compared to the photometric colors of CWISE J0506+0738 and other L and T dwarfs in Figure 4.
The VHS Jand K S -band exposures were taken ∼6 minutes apart, so can be treated as near-simultaneous; the shortest known variability period of a brown dwarf is ∼1.08 hr (Tannock et al. 2021;Vos et al. 2022), and young brown dwarfs are known to be slow rotators (Schneider et al. 2018;Vos et al. 2022).Thus, such a short time difference should be insufficient to cause a significant enough photometric change to explain the observed color discrepancy in VHS J1831−5513.
While we cannot definitively rule out that the sole cause of the color discrepancy is the low S/N of the VHS J-band detection (S/N≈4.6),we note that VHS J1831−5513 possesses exceptionally red near-infrared colors, which is suggestive of an extreme atmosphere, potentially viewed near equatoron.Complex clouds, disequilibrium chemistry, and powerful atmospheric dynamics may be expected in such a young, red atmosphere (Showman & Kaspi 2013;Suárez & Metchev 2023;Suárez et al. 2023), and can affect the emergent flux observed from the object.It is therefore possible that clouddriven variability in VHS J1831−5513 may contribute to the observed (J −K S ) color discrepancy.The ∼9 yr difference between the VHS images being taken and the Magellan/FIRE spectrum being collected is easily sufficient for variability to have played a role in causing the discrepancy between the two.
Pressure plays a key role in the behavior of clouds within the atmospheres of substellar objects (Buenzli et al. 2012;Yang et al. 2016).With the Jand K S -bands probing different atmospheric depths, they may exhibit different variability amplitudes, as has been observed in other L/T transition objects, which show higher amplitudes in the J-band than the K S -band (Artigau et al. 2009;Radigan et al. 2012;Vos et al. 2019).VHS 1256−1257 b, an object similar in nature to VHS J1831−5513, is highly variable in the J-band, with both Bowler et al. (2020) and Zhou et al. (2020) finding a 1.27 µm variability amplitude of ∼25%, and Zhou et al. (2022) finding a broader J-band variation of up to 38% on a ∼1 year timescale, the highest ever observed in a substellar object.In the same vein, phase shifts between Jand K S light curves can also affect the J − K S color of L/T transition dwarfs (e.g., McCarthy et al. 2024;Plummer et al. 2024).Future timeresolved monitoring will provide insight into whether differing variability amplitudes and/or a phase shift between the Jand K S -bands of VHS J1831−5513 exist, and if so, to what extent they impact its J − K S color.
The correlation between near infrared color, viewing inclination, and cloud opacity in Suárez et al. (2023) is helpful to understand the appearance of objects like VHS J1831−5513.This may indicate that its extremely red near-infrared colors and potentially the CH 4 features in its Magellan/FIRE spectrum, are explained, at least partially, by unusually thick clouds as a consequence of the object being viewed equatoron.We note, however, that the Suárez et al. ( 2023) study focused on L3-L7 dwarfs and did not take into account objects as late in type as VHS J1831−5513.L7 marks the peak for J − K colors on the field sequence as objects at later types rapidly get bluer as condensates sink below the photosphere.While the trends between near-infrared color and inclination in Vos et al. (2017) and Vos et al. (2020) are observed into the L/T transition, it is an evolutionary stage in which marked changes, both chemical and physical, occur in the atmospheres of brown dwarfs, so further investigation is needed to determine if the trends identified in Suárez et al. (2023) are applicable to the L/T transition.
The only multi-epoch survey to detect VHS J1831−5513 is the WISE mission.However, because of the latent artifact contamination described in Section 2, we were unable to confidently explore the variability properties of VHS J1831−5513 using the WISE data.Nonetheless, VHS 1831−5513 is highly likely to exhibit detectable variability given its red infrared colors, young age, possible near equator-on inclination angle and cloudy atmosphere, and its position at the L/T transition (Radigan 2014;Vos et al. 2017Vos et al. , 2019;;Suárez & Metchev 2022), making it a promising target for future variability studies.

CONCLUSION
We have presented the discovery of VHS J1831−5513, an extremely red, young L/T transition object, identified through a search of the CatWISE reject and VHS catalogs.We obtained Magellan/FIRE near-infrared spectroscopy of VHS J1831−5513, which shows that it is an ∼L8γ-T0γ dwarf with likely CH 4 absorption at the Hand K-band peaks.The presence of CH 4 in the atmosphere of VHS J1831−5513 indicates a T eff at the L/T transition.We find that its spectral morphology is similar to those of other young, red late-type L dwarfs, including the planetary mass companion VHS 1256−1257 b, with which it shares a number of individual CH 4 absorption features.VHS J1831−5513 also exhibits a 1.3 µm absorption feature, the cause of which is unclear.This feature must be investigated further, potentially at higher resolution, in order to confidently assert its cause.
Using the BANYAN Σ Bayesian algorithm, we determine that VHS J1831−5513 is a likely member of the ∼22 Myr BPMG based on its kinematics.This membership assignment is supported by its low gravity score in pressuresensitive indices and its red near-infrared slope.Future trigonometric parallax, radial velocity, and higher precision proper motion measurements will help to test this membership.Using the age of BPMG, we inferred physical properties for VHS J1831−5513.Its estimated mass of 6.5 ± 1.5 M Jup places it among the lowest mass free-floating BPMG members known (alongside CWISE J0506+0738 at 7 ± 2 M Jup ; Schneider et al. 2023b), contributing to empirical constraints on the group's initial mass function.Its estimated effective temperature of T eff = 1085 ± 60 K is well below that of field-age objects of similar spectral types, consistent with previous findings for other young brown dwarfs.Given the spectroscopic evidence of the presence of CH 4 in its atmosphere, this temperature estimate also places empirical constraints on the temperature at which CH 4 begins to appear in the near-infrared spectra of young L/T transition dwarfs.
The extremely red near-infrared colors and potentially near equator-on inclination angle of VHS J1831−5513 make it highly likely to be extremely cloudy.This, in combination with its young age, and position at the L/T transition, suggests that VHS J1831−5513 is highly likely to be variable, and is therefore an excellent prospective target for future photometric and spectrophotometric monitoring.A significant (∼3.1σ)J − K S color discrepancy noticed between its Magellan/FIRE spectrum and VHS photometry may also indicate that it has a significant difference between its Jand K S -band variability amplitudes or a potential phase shift between its Jand K S light curves, making it a yet more intriguing target for variability monitoring.
The discoveries of VHS J1831−5513 in this work and CWISE J0506+0738 in Schneider et al. (2023b) hint at the existence of a population of exceptionally red L/T transition objects which, until the introductions of newer near-infrared surveys with deeper detection thresholds, were simply too faint to be detected in the J-band by the previous generation of near-infrared survey instruments (e.g., 2MASS).Future targeted searches of these catalogs are warranted to discover more examples of such extraordinarily red objects.This also highlights the importance of the upcoming Euclid mission (Laureijs et al. 2011), which has an expected 5σ point source J-band depth of ∼24.5 mag (Euclid Collaboration et al. 2022), and so will be able to detect not only intrinsically fainter red objects, but red objects at further distances, expanding the census of known red objects and enabling a better understanding of their space density, a key component for exploring the mass functions of nearby associations.
VHS J1831−5513 is an excellent prospective JWST target to facilitate atmospheric retrieval studies with high resolution data.This would also represent an excellent opportunity to probe the L/T transition at young ages; an area not yet fully understood, primarily due to the paucity of known young L/T transition objects.

ACKNOWLEDGMENTS
The Backyard Worlds: Planet 9 team would like to thank the many Zooniverse volunteers who have participated in this project.We would also like to thank the Zooniverse web development team for their work creating and maintaining the Zooniverse platform and the Project Builder tools.This research was supported by NASA grant 2017-ADAP17-0067.This material is supported by the National Science Foundation under grant No. 2007068, 2009136, and 2009177.JF acknowledges the Heising Simons Foundation as well as NSF award #1909776, and NASA Award #80NSSC22K0142.J. M. V. acknowledges support from a Royal Society -Sci- compares VHS J1831−5513 to the young, red objects WISEA J114724.10−204021.3 (WISEA J1147−2040; L7γ; Schneider et al. 2016), PSO J318.5338−22.8603(PSO J318.5−22;L7 VL-G; Liu et al. 2013) and CWISE J050626.96+073842.4 (L8-T0γ; Schneider et al. 2023b).These comparisons highlight the extremely red near-infrared slope of VHS J1831−5513, which is noticeably redder than many members of the young planetary-mass population, and is matched only by CWISE J0506+0738, the reddest free-floating L/T dwarf currently known.Like CWISE J0506+0738, the spectrum of VHS J1831−5513 displays likely CH 4 absorption features at the Hband peak and on the red side of the K-band peak.These features also occur in the spectrum of another young, planetarymass L dwarf, VHS 1256−1257 b.The left column of Figure 3 compares the Hand K-bands of the Magellan/FIRE spectrum of VHS J1831−5513 and the James Webb Space Telescope (JWST) spectrum of VHS 1256−1257 b

Figure 1 .
Figure 1.Magellan/FIRE spectrum of VHS J1831−5513 (black), Gaussian smoothed using a width of 15 pixels.Key molecular absorption features are labeled.Original resolution data are plotted in grey and flux uncertainty is shown in blue.

Figure 3 .
Figure 3. Left column: Comparison between the H-(top) and K-(bottom) bands of VHS J1831−5513 (black -smoothed; grey -original resolution) and the young, planetary-mass companion VHS 1256-1257 b (red; Miles et al. 2023).CH4 absorption features that appear in both objects are marked by blue bands.Right column: Sonora Bobcat (Marley et al. 2021) models at solar metallicity and log(g) = 3.5, with a range of effective temperatures.The blue bands represent the locations of the absorption features in the corresponding band of the left column.

Figure 4 .
Figure 4. Color-color diagrams comparing VHS J1831−5513 (blue square) to L and T dwarfs with photometry from Schneider et al. (2023a) (yellow to red circles) and CWISE J0506+0738 (green diamond; Schneider et al. 2023b).VHS J1831−5513 is plotted using its VHS and CatWISE photometry, and represents a significant outlier from even the reddest known objects.The blue lines in the left and right plots indicate the synthetic J − KS color calculated from the Magellan/FIRE spectrum of VHS J1831−5513, which is still an outlier from the bulk of the brown dwarf population, but is similar to that of CWISE J0506+0738, the reddest known L/T transition dwarf.