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Resolved Spectroscopy of a Brown Dwarf Binary at the T Dwarf/Y Dwarf Transition

We report resolved near-infrared imaging and spectroscopic observations of the T8.5 binary WISEP J045853.90+643452.6AB obtained with Keck/NIRC2, Keck/OSIRIS and the Keck Laser Guide Star Adaptive Optics system. These data confirm common proper and radial motion for the two components, and we see the first indications of orbital motion (mostly radial) for this system. H-band spectroscopy identifies both components as very late-type brown dwarfs with strong H2O and CH4 absorption. The spectrum of WISE J0458+6434B also exhibits a compelling signature of NH3 absorption over 1.52-1.54 micron when compared to the T9 dwarf UGPS J072227.51-054031.2. Comparison to T8-Y0 spectral standards and H-band spectral indices indicate classifications of T8.5 and T9.5 for these two components, approaching the boundary between the T dwarf and Y dwarf spectral classes.


INTRODUCTION
The lowest-temperature brown dwarfs-the L, T and Y dwarfs-span an astrophysically-rich parameter space of mass, temperature and atmospheric chemistry that bridges the lowest-mass stars and the warmest exoplanets (Kirkpatrick 2005 and references therein). These sources also probe the efficiency of low-mass star formation and the chemical enrichment history of the Galaxy (Burgasser 2004;Metchev et al. 2008;Bate 2009). Very low-temperature brown dwarfs down to effective temperatures (T eff s) of ∼300-500 K are now being identified (e.g., Lucas et al. 2010;Luhman et al. 2011;Liu et al. 2011;Cushing et al. 2011;Kirkpatrick et al. 2011), primarily in deep, wide-field infrared surveys such as the United Kingdom Infrared Telescope Deep Sky Survey (UKIDSS; Lawrence et al. 2007); the Canada-France Brown Dwarf Survey (CFBDS; Delorme et al. 2008b) and, most recently, the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010). These discoveries probe previously unexplored realms of gas and condensate atmospheric chemistry Lodders & Fegley 2006), and encompass the newly identified Y dwarf spectral class .
While changes in spectral morphology are largely tied to changes in temperature, the transitions between late-type spectral classes also trace significant changes in atmospheric chemistry and dynamics. The M dwarf to L dwarf transition is accompanied by the formation of mineral condensates (Tsuji et al. 1996;Cooper et al. 2003;Helling et al. 2008), which deplete the photosphere of metal oxide gases (TiO, VO) and lead to more reddened optical and near-infrared spectral energy distributions. In addition, magnetic coro- nal and chromospheric emission drops precipitously across this M/L transition (Gizis et al. 2000;West et al. 2004), the result of both increasingly neutral atmospheres (Gelino et al. 2002;Mohanty et al. 2002) and reduced magnetic field energy with reduced mass (Christensen et al. 2009). The L dwarf to T dwarf transition, signaled by the emergence of CH 4 absorption at near-infrared wavelengths, is coincident with the removal of mineral condensate clouds from the photosphere (Ackerman & Marley 2001;Burgasser et al. 2002b;Knapp et al. 2004;Tsuji 2005) and the increased influence of non-equilibrium chemistry on observed molecular abundances (Noll et al. 1997;Griffith & Yelle 1999;Saumon et al. 2006;Yamamura et al. 2010). Given the very recent discovery of Y dwarfs, transitional behaviors have yet to be explored at these spectral types, although atmospheric models predict the formation of salt and water condensates that may significantly shift photospheric chemistry and spectral appearance (e.g., Lodders 1999;Burrows et al. 2003;Leggett et al. 2007).
Ideal probes of such spectral transitions are physical binaries whose components straddle the corresponding classes. These presumably coeval systems share a common age and bulk chemical composition, and reside at a common distance, making relative comparisons less dependent on these parameters. For example, evidence for a rapid dissipation of clouds at the L dwarf/T dwarf transition includes the apparent brightening of early T dwarfs at 1 µm as observed among L/T transition binaries (Burgasser et al. 2006b;Liu et al. 2006;Looper et al. 2008), and the higher apparent multiplicity rate of these systems (Burgasser 2007). Physical parameters such as age, mass and radii have also been measured for low-mass binaries; these serve as critical empirical constraints for evolutionary models (e.g., Lane et al. 2001;Zapatero Osorio et al. 2004;Stassun et al. 2006;Dupuy et al. 2009;Konopacky et al. 2010). Finally, companions to low-luminosity sources are frequently lowtemperature extrema. The prototypes for both the L dwarf (GD 165B; Becklin & Zuckerman 1988) and T dwarf (Gliese 229B; Nakajima et al. 1995) spectral classes were identified as companions to low-luminosity stars, as were two of the coldest brown dwarfs currently known, WD 0806-661B (Luhman et al. 2011) and CF-BDSIR J1458+1013B (Liu et al. 2011).
One of the first very late-type T dwarfs to be uncovered by the WISE survey was WISEP J045853.90+643452.6 (hereafter WISE J0458+6434; Mainzer et al. 2011), a T8.5 dwarf at an estimated distance of 10 pc from the Sun. Subsequent adaptive optics (AO) imaging observations by Gelino et al. (2011) revealed this source to be a binary with a separation of 0. ′′ 5 and a relative brightness of ∆J = 0.98±0.08. Given the late spectral type of the composite spectrum and the significant magnitude difference between the components, WISE J0458+6434B was estimated to have a T9 spectral type and T eff ≈ 500 K. However, uncertainties in the distance of this system, the unknown spectral energy distribution of the secondary, and the very recent empirical definition of the Y dwarf spectral class ) means that WISE J0458+6434AB could potentially straddle the T dwarf/Y dwarf transition.
In this article, we present resolved near-infrared imaging and spectroscopy of the WISE J0458+6434 system obtained with the Keck II NIRC2 camera, OH-Suppressing InfraRed Integral field Spectrograph (OSIRIS; Larkin et al. 2006) and Laser Guide Star Adaptive Optics system (LGSAO; Wizinowich et al. 2006;van Dam et al. 2006). These data confirm common proper motion for the system and the first evidence for orbital motion. We use the H-band OSIRIS spectra to classify the components, and identify tentative evidence for near-infrared NH 3 absorption in WISE J0458+6434B. In Section 2 we describe our observations and data reduction methods. In Section 3 we analyze our astrometric measurements, and provide an initial qualitative characterization of the binary orbit. In Section 4 we analyze the spectral data, deriving classifications using both spectral templates and H-band indices, and examining evidence for NH 3 absorption. In Section 5 we discuss our results, focusing on the absolute magnitudes of WISE J0458+6434AB in the context of the T dwarf/Y dwarf transition.

NIRC2 Imaging
New high-resolution LGSAO imaging observations of WISE J0458+6434 were obtained with NIRC2 on the 10 m Keck II Telescope on 2011 August 29 (UT). Conditions were clear with seeing of 0. ′′ 5. Data were obtained with the narrow plate scale mode of NIRC2 (9.963±0.011 mas pixel −1 ; Pravdo et al. 2006) at an orientation of 0 • and airmass ≈1.5. We observed the target in the J-, Hand K s -band filters using a 3-point dither pattern that avoided the noisy, lower left quadrant of the focal plane array. Individual exposure times were 120 s in J and K s and 300 s in H, for total integrations of 360 s, 900 s and 360 s in J, H and K s , respectively. The sodium LGS provided the wave front reference source for AO correction, while tip-tilt aberrations and quasi-static changes were measured by monitoring the R = 15.6 field star USNO-B1.0 1545-0122611 (Monet et al. 2003) located ρ = 27. ′′ 7 from WISE J0458+6434.
Images were reduced using custom IDL 4 scripts. Sky background and dark current were removed from each image by pairwise subtracting it with the following image in the dither sequence, then dividing by a normalized dome flat to correct for pixel-to-pixel sensitivity variations. The calibrated image frames were registered to the peak of the primary's point spread function (PSF), then median-combined to produce the final mosaics shown in Figure 1.
The quality of these data are significantly improved over the discovery observations from Gelino et al. (2011), with components that are well-separated in the field-ofview (FOV). Relative fluxes were computed using aperture photometry on the coadded images. Given the wide separation of the components, no PSF subtraction or other specialized procedures were required for the photometry. The radial separation and position angle of the fainter component with respect to the brighter component was measured on each of the nine individual exposures. The values listed in Table 1 report the average and standard deviation of those nine measurements. Relative J-and H-band flux measurements are in excellent agreement with those reported in Gelino et al. (2011), but are of far greater precision. In contrast, there is a significant shift in the relative separation of the two components, with ∆α = 54±14 mas and ∆δ = 27±16 mas. Notably, this shift is largely in a radial direction. We discuss the astrometry in further detail in Section 3.

OSIRIS Spectroscopy
Resolved spectroscopy of WISE J0458+6434 was obtained with OSIRIS and LGSAO on 2011 February 3 (UT), in clear conditions with seeing of 0. ′′ 8-1. ′′ 0. The 50 mas-scale camera and H bb filter were employed, providing 1.47-1.80 µm spectroscopy at an average resolution of 3800 and dispersion of 2.1Å pixel −1 over a 0. ′′ 8×3. ′′ 2 FOV. The source was first acquired in the imaging camera, then offset to the spectrograph array. The image rotator was set to 0 • . Eight exposures of 600 s each were obtained over an airmass range of 1.40-1.52 using a two-step dither pattern with steps of 1. ′′ 0 along the long axis of the FOV. For LGSAO tip-tilt correction, we employed the same star as that used for the NIRC2 imaging. We also obtained four 30s observations of the A0 V star BD +64 489 (V = 9.43) in natural guide star AO mode at an airmass of 1.72 for telluric correction and flux calibration purposes.
Data were reduced with the OSIRIS data reduction pipeline (Krabbe et al. 2004), version 2.3. We first subtracted from the source and calibrator images mediancombined sets of dark frames with identical exposure times. We then used the pipeline to adjust bias levels, remove detector artifacts and cosmic rays, extract and wavelength-calibrate the position-dependent spectra (using the most current rectification files as of February 2011), assemble 3D data cubes and correct for dispersion. Figure 1 displays a 1 ′′ ×1 ′′ section of this cube coadded over the wavelength range 1.56-1.60 µm. The two components are cleanly resolved, separated by ∼10 spaxels (0. ′′ 5) along a NW-SE axis. The full width at half maximum of the PSF was measured to be 150 mas (3 spaxels) over this wavelength range for both components, slightly elongated in the direction of the tip-tilt star. Relative astrometry from these data were determined from each of the reduced data cubes, using a median stack over the spectral dimension 1.56-1.60 µm. The IDL routine cntrd was used to measure the peak pixel positions of the primary and secondary in each image, and pixel separations along each axis were converted to angular separations in Right Ascension and declination. Mean measurements and standard deviations are listed in Table 1. The offsets are intermediate between those of the two NIRC2 observations, consistent with a continuous reduction in separation over nearly constant position angle.
Spectra for both components of WISE J0458+6434 and the calibrator star were extracted directly from the data cube via aperture photometry in each image plane, using a 1.5-spaxel (75 mas) aperture for WISE J0458+6434 and a 3-spaxel (150 mas) aperture for BD +64 489, with 10-20 spaxel (0. ′′ 5-1. ′′ 0) sky annuli. The individual spectra for all three sources were scaled and combined using the xcombspec routine in SpeXtool (Cushing et al. 2004). Flux calibration and telluric correction of the WISE J0458+6434AB spectra were performed using the xtellcor general routine in SpeXtool, assuming a 20 nm Gaussian kernel to model the widths of the A0V H I lines, and a Kurucz model spectrum of Vega (Vacca et al. 2003). Figure 2 displays the reduced spectra of the WISE J0458+6434 components. Both are unambiguously late-type T dwarfs based on the presence of strong H 2 O absorption up to 1.55 µm and strong CH 4 absorption beyond 1.59 µm. These bands constrain the Hband flux to a discrete 1.50-1.62 µm peak, which is narrower in the spectrum of WISE J0458+6434B. Un- binned peak signal-to-noise (S/N) is ∼40 in the spectrum of WISE J0458+6434A and ∼15 in the spectrum of WISE J0458+6434B.

ASTROMETRIC ANALYSIS
As noted above, astrometry from NIRC2 and OSIRIS observations of WISE J0458+6434 indicate a monotonic decrease in the separation of this pair along a near constant position angle of 320 • , with the separations in Right Ascension from the two NIRC2 observations differing by nearby 4σ. To assess how this relative motion relates to systemic common motion, and hence physical association, we compared our measurements against predicted separations anchored to our 2011 August 29 (UT) astrometry, considering three limiting scenarios: (1) the secondary is a background source with negligible parallactic or proper motion, (2) the secondary is at the same distance as the primary, but with negligible proper mo-tion, and (3) the primary and secondary maintain constant separation at all times. Figure 3 shows that none of these scenarios are fully consistent with the data, although the last provides the closest match. Allowing relative motions and distances to vary freely, we found that our measured astrometry (with 3σ uncertainty allowance) constrains the relative proper motion to be less than 77 mas yr −1 , or a mere 4 km s −1 at a distance of 10 pc. For comparison, the circular orbital speed of a 0.05 M ⊙ brown dwarf in an equal-mass binary separated by 5 AU is ∼3 km s −1 . We also measured the relative radial velocity (∆RV) of the two components by cross-correlating our OSIRIS spectra over the 1.58-1.65 µm range, properly accounting for noise through Monte Carlo simulation. We find ∆RV = 3±15 km s −1 , consistent with zero to within 1/5 of a spectral resolution element.
These stringent limits on the relative motion of the  WISE J0458+6434 components indicate that the two sources comprise a common proper motion, physicallybound binary. As such, the small but significant astrometric shifts observed must arise from orbital motion. Figure 4 provides a visualization of our three separation measurements projected onto the plane of the sky. The near radial motion and marginal indication of an inward acceleration (ρ = −1.6±1.3 km s −1 yr −1 ) suggests an eccentric and/or nearly edge-on orbit that is moving toward alignment. In the most optimistic case-if WISE J0458+6434 was observed at or near maximum elongation in the first NIRC2 epoch-these observations Note. -Angular separation (ρ) and position angle (θ) are measured from the brighter primary to the fainter secondary. a From Gelino et al. (2011). constrain the orbital semimajor axis to the range 5/(1+e) a 5/(1−e) AU, where e is the orbital eccentricity and a distance of 10 pc is assumed. Unfortunately, more stringent constraints on this system's orbit are not possible with these few measurements clumped in one region of separation space. Over the next decade it should be possible to disentangle the eccentricity and inclination of the orbit, and provide more robust constraints on the period and total system mass.

Component Spectral Classifications
The strong H 2 O and CH 4 absorption in the spectra of WISE J0458+6434AB are consistent with very late T and possibly Y spectral types. To robustly determine the classifications, we first compared the spectra to the standards 2MASS J04151954−0935066 (T8; hereafter 2MASS J0415−0935; Burgasser et al. 2002aBurgasser et al. , 2004, UGPS J072227.51−054031.2 (T9; hereafter UGPS J0722−0540; Lucas et al. 2010;Cushing et al. 2011), and WISEP J173835.52+273258.9 (Y0; hereafter WISE J1738+2732; Cushing et al. 2011). The latter two sources were defined by Cushing et al. (2011) to extend the T dwarf near-infrared classification scheme of Burgasser et al. (2006a) across the T dwarf/Y dwarf transition. Figure 5 shows these comparisons, with all of the spectra smoothed to a common resolution of λ/∆λ = 100 and normalized at 1.58 µm. WISE J0458+6434A is a reasonably fair match to 2MASS J0415−0935, albeit with slightly deeper absorption on the blue side of the 1.58 µm peak and a somewhat narrower peak. It has weaker 1.6 µm CH 4 absorption than UGPS J0722−0540, indicating that it is not as late as T9. WISE J0458+6434B, on the other hand, has stronger H 2 O and CH 4 absorption than 2MASS J0415−0935 and  References.
We also computed H-band spectral classification indices H 2 O-H and CH 4 -H (Burgasser et al. 2006a) and NH 3 -H (Delorme et al. 2008a) for both components of WISE J0458+6434. The last index was shown by Cushing et al. (2011) to exhibit a distinct break at the T/Y transition. Values are listed in Table 2  . Combining the index types and direct spectral comparisons, we infer subtypes of T8.5 and T9.5 for WISE J0458+6434A and B.

Ammonia in the Spectrum of WISE J0458+6434B
Close examination of the spectral comparisons in Figure 5 indicates additional absorption in the spectrum of WISE J0458+6434B over the 1.52-1.54 µm region compared to both 2MASS J0415−0935 and UGPS J0722−0540. This feature is similar to those noted by Cushing et al. (2011) in the spectra of the Y0 dwarfs WISEPC J140518.40+553421.5 and WISE J1738+2732, which were tentatively attributed to NH 3 absorption. We examined whether the excess absorption in WISE J0458+6434B could also be due to NH 3 by com-  Overplotted are wavelength-dependent absorption coefficients for H 2 O (blue), CH 4 (green) and NH 3 (red), based on scattering cross-sections from Freedman et al. (2008) at T = 500 K and P = 1 bar, and assuming nonequlibrium abundances from Saumon et al. (2006). Note the region longward of 1.52 µm where NH 3 opacity is predicted to dominate over H 2 O. (Right) Relative flux between UGPS J0722−0540 and WISE J0458+6434B (black line) on a logarithmic scale, compared to the relative absorption coefficient between NH 3 versus H 2 O and CH 4 combined (red line). The peak in relative flux is near the peak in relative absorption.
paring its spectrum and that of UGPS J0722−0540 to absorption coefficients (α[λ]) for H 2 O, CH 4 and NH 3 . Absorption coefficients were computed by multiplying scattering cross-sections per molecule (σ[λ]), as tabulated by Freedman et al. (2008) for T = 500 K and P = 1 bar, with molecular number densities (n i ) assuming fractional abundances (f i ) based on the non-equilibrium chemistry calculations of Saumon et al. (2006): Here, n = 1.4 × 10 19 cm −3 is the total number density of gas particles assuming an ideal gas. From Saumon et al. (2006), we adopt log 10 f H2O = −3.1, log 10 f CH4 = −3.3 and log 10 f N H3 = −4.9. Note that non-equilibrium chemistry reduces NH 3 abundances by a factor of 10. Figure 6 compares the smoothed absorption coefficients for these molecules to equivalently smoothed spectra of UGPS J0722−0540 and WISE J0458+6434B. We find that NH 3 absorption exceeds that of H 2 O beyond 1.52 µm, around the region where we also see excess absorption in the spectrum of WISE J0458+6434B. The right panel of Figure 6 shows another view of this, comparing the relative absorption between UGPS J0722−0540 and WISE J0458+6434B to the differential absorption between NH 3 , H 2 O and CH 4 . There is relatively close spectral alignment between excess NH 3 opacity and excess absorption in the spectrum of WISE J0458+6434B, suggesting NH 3 as the primary absorber at these wavelengths. However, this conclusion carries a few caveats. First, the photospheres of brown dwarfs are not homogeneous, and absorption contributing to this spectral region is integrated over a pathlength that spans a range of temperatures and pressures that do not necessary coincide with the values used here to calculate molecular opacities. Second, the opacities of absorbers other than NH 3 are also likely to differ between these two sources, which may add additional structure to the relative intensities. Finally, while the excess absorption in WISE J0458+6434B is statistically significant, the precise location of the relative intensity peak is not robust due to the large uncertainties in the spectrum of WISE J0458+6434B at these (low intensity) wavelengths. Hence, while evidence for NH 3 absorption in the near-infrared spectrum of WISE J0458+6434B is compelling, confirmation of this will require higher signal-tonoise data and more detailed modeling of the spectrum.

DISCUSSION
One of the benefits of studying a physical binary is that its components share a common distance. In the case of WISE J0458+6434AB, this allows us to bootstrap existing absolute magnitude/spectral type trends out to the end of the T dwarf sequence. Figure 7 shows absolute MKO J and H magnitudes for 27 unresolved T6-Y0 dwarfs with reported parallax distance measurements 5 . For sources with types later than T8, we adopt the revised classifications listed in Cushing et al. (2011). To delineate trends in absolute magnitudes from T6 to T9, we generated a series of second-order polynomial fits to these data accounting for uncertainties through Monte Carlo sampling 6 . Figure 7 shows the resulting mean trends and 1σ deviations in M J and M H as a function of spectral type, which were used to estimate the absolute magnitudes for WISE J0458+6434A and B (Table 3) Kirkpatrick et al. (2011) and Liu et al. (2011). We excluded two sources from this sample: the T6.5 SDSSp J134646.45−003150.4, for which existing photometry is blended with an unassociated background source (Burgasser et al., in prep.); and the T6.5p ULAS J115038.79+094942, which has a poorly constrained parallax (σπ/π = 0.45; Marocco et al. 2010). 6 We assumed classification uncertainties of 0.5 subtypes and absolute magnitude uncertainties as indicated. We then performed 1000 fits to simulated data, offsetting the original spectral types and absolute magnitudes of each source by random draws from a normal distribution with mean zero and standard deviation equal to the uncertainty.  Note. -Component brightnesses are based on the new NIRC2 relative photometry reported here, combined light photometry from Mainzer et al. (2011), and the 2MASS-MKO filter corrections given in Gelino et al. (2011). Absolute magnitudes are based on the relations shown in Figure 7.

Spectral
statistically consistent distance estimates of 10.5±1.8 pc and 11.2±2.2 pc for the A and B components, respectively. These are also formally consistent with the systemic spectrophotometric estimates of Mainzer et al. (2011, 9.0±1.9 pc) and Gelino et al. (2011, 10.5±1.4 pc). The agreement between these distances further supports the identification of this system as a physical binary; or, assuming that, justifies the extrapolation of existing absolute magnitude/spectral type trends to subtype T9.5.
Currently, the only Y dwarf with a parallax distance measurement is WISEP J154151.65−225025.2 (hereafter WISE J1541−2250), classified Y0 by Cushing et al. (2011). Kirkpatrick et al. (2011) report a preliminary parallax of 351±108 mas for this source, corresponding to a distance of 2.8±0.9 pc and absolute magnitudes M J = 23.9±0.8 and M H = 23.7±0.8. These values make WISE J1541−2250 roughly 100 times fainter than WISE J0458+6434B in the near-infrared, despite the seemingly minor change in spectral morphology (cf. comparison of WISE J0458+6434B to the Y0 standard WISE J1738+2732 in Figure 5). This jump in brightness, also discussed by Cushing et al. (2011), suggests a dramatic redistribution of radiation out of the near-infrared band, as the corresponding difference in T eff , roughly 500 K to 350 K, reduces the luminosity by only a factor of 4, assuming equivalent radii. A greater fraction of flux is certainly being emitted at mid-infrared wavelengths, due to the redward shift in the blackbody peak and increased near-infrared absorption from H 2 O, CH 4 and collision-induced H 2 with lower T eff . Additional absorption may also arise from water ice clouds that are expected to form at these temperatures, as scattering opacity from 10-100 µm-sized ice grains would efficiently suppress near-infrared emission just as mineral grains are observed to do so in the L dwarfs (Ackerman & Marley 2001). While an improved parallax measurement for WISE J1541−2250 and other newly-discovered Y dwarfs are needed to confirm this dramatic drop in near-infrared flux, these early indications suggest that the T/Y transition, like other late spectral class transitions, may trace significant changes in atmospheric chemistry as well as declining temperatures.
The authors would like to thank Keck observing assistants Terry Stickel and Cynthia Wilburn, and instrument scientists Scott Dahm and Hien Tran, for their assistance in the observations reported here. We also acknowledge guidance on OSIRIS data reduction from Breann Sitarski and Shelley Wright, and Richard Freedman for providing scattering cross section spectra for various molecules. We thank our referee for her/his helpful and prompt review. This work was supported by a NASA Keck PI Data Award, administered by the NASA Exoplanet Science Institute. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France; the M, L, and T dwarf compendium housed at http://dwarfarchives.org; and the SpeX Prism Spectral Libraries at http://www.browndwarfs.org/spexprism. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.