JWST/NIRSpec Observations of the Coldest Known Brown Dwarf

We present 1–5 μm spectroscopy of the coldest known brown dwarf, WISE J085510.83−071442.5 (WISE 0855), performed with the Near-Infrared Spectrograph (NIRSpec) on board the James Webb Space Telescope (JWST). NIRSpec has dramatically improved the measurement of the spectral energy distribution (SED) of WISE 0855 in terms of wavelength coverage, signal-to-noise ratios, and spectral resolution. We have performed preliminary modeling of the NIRSpec data using the ATMO 2020 models of cloudless atmospheres, arriving at a best-fitting model that has T eff = 285 K. That temperature is ∼20 K higher than the value derived by combining our luminosity estimate with evolutionary models (i.e., the radius in the model fit to the SED is somewhat smaller than expected from evolutionary models). Through comparisons to the model spectra, we detect absorption in the fundamental band of CO, which is consistent with an earlier detection in a ground-based spectrum and indicates the presence of vertical mixing. Although PH3 is expected in Y dwarfs that experience vertical mixing, it is not detected in WISE 0855. Previous ground-based M-band spectroscopy of WISE 0855 has been cited for evidence of H2O ice clouds, but we find that the NIRSpec data in that wavelength range are matched well by our cloudless model. Thus, clear evidence of H2O ice clouds in WISE 0855 has not been identified yet, but it may still be present in the NIRSpec data. The physical properties of WISE 0855, including the presence of H2O clouds, can be better constrained by more detailed fitting with both cloudless and cloudy models and the incorporation of unpublished 5–28 μm data from the Mid-infrared Instrument on JWST.


INTRODUCTION
Y is the coldest spectral class of brown dwarfs (T eff 500 K, Cushing et al. 2011;Kirkpatrick et al. 2021).
Because the near-infrared (IR) fluxes of brown dwarfs collapse from late T into the Y class (Kirkpatrick et al. 2011), sensitive space-based telescopes operating at mid-IR wavelengths offer the best opportunity for detecting Y dwarfs.
The coldest known brown dwarf is WISE J085510.83−071442.5 (hereafter WISE 0855).Luhman (2014a) and Kirkpatrick et al. (2014) identified it as a high proper motion object using two epochs of images from WISE.By obtaining images at two additional epochs with Spitzer and measuring its parallax, Luhman (2014b) demonstrated that WISE 0855 is a nearby brown dwarf (the fourth closest known system) and estimated a temperature of ∼250 K from its absolute magnitude at 4.5 µm.The distance of WISE 0855 has been further refined through continued astrometric monitoring with Spitzer, the reactivated WISE mission (NEOWISE, Mainzer et al. 2014), and the Hubble Space Telescope (2.278±0.012pc, Luhman & Esplin 2014, 2016;Wright et al. 2014;Kirkpatrick et al. 2019Kirkpatrick et al. , 2021)).In an effort to measure its spectral energy distribution (SED) for comparison to models of Y dwarfs, deep optical and near-IR imaging has been performed on WISE 0855 with Hubble and large ground-based telescopes (Beamin et al. 2014;Faherty et al. 2014;Kopytova et al. 2014;Luhman 2014b;Wright et al. 2014;Luhman & Esplin 2016;Schneider et al. 2016;Zapatero Osorio et al. 2016;Leggett et al. 2017).Near-IR spectroscopy of WISE 0855 is not feasible with these facilities, but through very long exposure times with Gemini Observatory, Skemer et al. (2016) and Morley et al. (2018) were able to obtain spectra within portions of the mid-IR (the L and M bands) where the atmosphere is not opaque.
JWST is the most sensitive IR telescope deployed to date and operates across a wavelength range that should encompass most of the flux emitted by Y dwarfs.Therefore, it is the ideal facility for measuring the SEDs of Y dwarfs like WISE 0855 (Beiler et al. 2023).In fact, given its status as both the coldest known brown dwarf and the fourth closest known system and the limited ability of other telescopes to observe it, WISE 0855 is one of the most appealing targets of any kind for JWST.In this paper, we present spectroscopy of WISE 0855 from 0.8-5.5 µm performed with the Near-Infrared Spectrograph on JWST (NIRSpec, Jakobsen et al. 2022).
2. OBSERVATIONS WISE 0855 was observed with NIRSpec through guaranteed time observation (GTO) program 1230 (PI: C. Alves de Oliveira) on 2023 April 17 (UT).After slewing to WISE 0855, wide aperture target acquisition (WATA) was performed, which employs the S1600A1 aperture (1.′′ 6 × 1. ′′ 6).We used a combination of filter (CLEAR), readout pattern (NRSRAPID), and subarray (SUB2048) that would produce a suitable signal-to-noise ratio (SNR) for acquisition given the SED of WISE 0855 (Luhman & Esplin 2016).Through WATA, the centroid of the target was measured, a small angle maneuver was performed to center it in S1600A1, and an offset was applied to place the target in the S200A1 fixed slit, which has an angular size of 3. ′′ 2 × 0. ′′ 2. One set of data was collected with the G395M grating, the F290LP filter, the NRSRAPID readout pattern, 840 groups per integration, one integration per nod, and three nod positions.A second data set was taken with the PRISM disperser, the CLEAR filter, the NRSRAPID readout pattern, 1083 groups per integration, three integrations per nod, and three nod positions.Both observations employed the SUBS200A1 subarray.The total exposure times with these configurations were 3931 and 15,200 s, respectively.

DATA REDUCTION
The data reduction began with the retrieval of the uncal files from the Mikulksi Archive for Space Telescopes (MAST): doi:10.17909/1fsf-6j64.We performed ramps-to-slopes processing on those files using a custom pipeline developed by the ESA NIRSpec science operations team, which adopts the same algorithms included in the official STScI pipeline (Alves de Oliveira et al. 2018;Ferruit et al. 2022), except for a correction for "snowballs" (Böker et al. 2023) and a correction for residual correlated noise that is based on unilluminated pixels in the subarray outside the spectral trace (Espinoza et al. 2023).For the PRISM data, which had multiple integrations, we adopted the median of the individual integration count rate maps at each nod position.The median count rate map for each nod was background subtracted using the averaged rate maps of the other two nods and flat fielded.Spectra were extracted for the individual nod positions and combined with outlier rejection.The same processing steps were performed for standard stars Gaia DR3 2260019315938461952 and Gaia DR3 1634280312200704768 in G395M and PRISM, respectively, which were observed during JWST commissioning through program 1128 (PI: N. Lützgendorf).The spectra of these stars and their flux models (Bohlin et al. 2014) 3 were used for an initial flux calibration of the spectra of WISE 0855.The flux calibrations were then adjusted so that they agree with photometry from Spitzer (Section 4.1).The reduced spectra have sufficient SNRs to be useful at 2.87-5.09and 5.37-5.54µm (G395M) and 0.77-5.53µm (PRISM).The gap in the G395M data is from the separation between the two detector arrays.Our reduction yielded the widest possible wavelength coverage, extending slightly beyond the nominal "science range" officially supported by the STScI pipeline, while ensuring spectrophotometric calibration.The spectral resolution is ∼1000 for G395M and ranges from ∼ 40 near 1.1 µm to ∼300 at the longest wavelengths for the PRISM disperser.

Comparison to Previous Observations
We compare the NIRSpec data to previous observations of WISE 0855 between 1-5 µm.We first consider the photometric measurements with the smallest errors, consisting of data in the following filters: the F105W, F110W, F127M, and F160W bands on Hubble's Wide Field Camera 3 (Kimble et al. 2008) measured by Luhman & Esplin (2016) and Schneider et al. (2016), the [3.6] and [4.5] bands of Spitzer measured by Luhman & Esplin (2016), and W2 from the NEOWISE-R Single Exposure Source Table (Cutri et al. 2023), which contains 18 epochs for WISE 0855 between 2014 and 2022.Each of the NEOWISE epochs consisted of ∼ 10-20 measurements that span a few days.For each epoch, we calculated the median and standard error of W2.We exclude the two epochs from the original WISE survey in 2010 since the photometry of WISE 0855 was contaminated by emission from background stars (Luhman 2014b).For bands that have multiple epochs of photometry with the exception of [3.6], we adopt the midpoint of the measurements and an uncer-3 https://www.stsci.edu/hst/instrumentation/reference-data-for-calibration-and-tools/astronomical-catalogs/calspectainty that is represented by the range spanned by those measurements and their errors.The mean, median, and midpoint are nearly identical for [4.5], and the same is true for W2 as well.The adopted values of [4.5] and W2 are 13.87±0.08 and 13.90±0.16,respectively. Since [3.6] was measured in only four of the nine [4.5] epochs from Luhman & Esplin (2016), we have calculated the weighted mean of the [3.6] − [4.5] colors among those four epochs (3.49) and applied it to the adopted measurement of [4.5] to arrive at our adopted value for [3.6].The uncertainty for [4.5] is also assigned to [3.6].
We have calculated synthetic Vega magnitudes in the aforementioned bands from the NIRSpec PRISM spectrum of WISE 0855.In addition, photometry has been derived from the G395M spectrum in the two bands that it fully covers, [3.6] and [4.5].These calculations were performed by convolving the spectra of WISE 0855 and a spectrum of Sirius (Rieke et al. 2023) with the photoncounting spectral responses of the filters (Hora et al. 2008;Jarrett et al. 2011;Krick et al. 2021) 4 , integrating the resulting fluxes in photons, and assuming that Sirius has a magnitude of −1.395 in each band.
In Figure 1, we have plotted the differences between the imaging and synthetic photometry.For most bands, the synthetic photometry falls within the range of previous measurements.The primary exception is [3.6], which is much fainter (∼0.4 mag) in both NIR-Spec spectra than in the Spitzer images.The spectra and imaging agree more closely in [4.5], which means that [3.6] − [4.5] is redder in the spectra than in the images.To further investigate this discrepancy, we have derived synthetic [3.6] and [4.5] for the one other Y dwarf with an available NIRSpec PRISM spectrum, WISE J035934.06−540154.6 (hereafter WISE 0359, Beiler et al. 2023).We find that its color from NIRSpec is 0.28 mag redder than the value measured with Spitzer (Kirkpatrick et al. 2012), resembling the result for WISE 0855.We also have compared the synthetic and imaging colors in W2− shown large enough variability to explain the discrepancies in [3.6], as discussed earlier in this section and elsewhere (Esplin et al. 2016;Brooks et al. 2023).Synthetic [3.6] photometry for T and Y dwarfs is particularly sensitive to errors in the response function since their fluxes vary significantly across the bandpass, which overlaps with the fundamental band of CH 4 .The response functions for the IRAC filters do shift slightly in wavelength with position on the detector arrays (Hora et al. 2008).
For the [3.6] band, the maximum redward shift relative to the average response function that we have adopted is ∼ 80 Å, but we find that a shift of ∼ 500 Å would be needed to account for the discrepancy in the photometry.An error in the response function could partially explain why synthetic values of [3.6] − [4.5] from model spectra for Y dwarfs are redder than observed colors (Leggett et al. 2021).
Among the bands that we have discussed, [4.5] is the best one for refining the flux calibrations of the PRISM and G395M spectra of WISE 0855 since it contains a large fraction of the flux in those data, is fully encompassed by both spectra, and has the most accurate photometry available from previous imaging.Based on their synthetic photometry, the PRISM spectrum is 0.053 mag fainter and the G395M spectrum is 0.022 mag brighter than our adopted photometry of [4.5]=13.87.We have elected to multiply the spectra by the appropriate factors to bring them into agreement with the latter.Doing so results in a modest discrepancy of ∼0.1 mag between synthetic and imaging photometry in F160W, which could be explained by variability.Skemer et al. (2016) and Morley et al. (2018) obtained M and L-band spectra, respectively, of WISE 0855 with the Gemini Near-Infrared Spectrograph (GNIRS, Elias et al. 2006).An updated reduction of the data from Skemer et al. (2016) was presented by Miles et al. (2020).The data were binned to resolutions of ∼ 250 (L) and ∼ 300 (M ), after which they had median SNRs of ∼5 (L) and 13 (M ).In Figure 2, we compare the binned spectra from Morley et al. (2018) and Miles et al. (2020) to the NIRSpec PRISM spectrum, which has a similar resolution.The errors in the NIRSpec fluxes are not plotted since most are too small to be distinguishable from the spectrum.The GNIRS and NIRSpec spectra have similar spectral slopes and show many of the same absorption features, although the strengths of some of the features differ by substantially more than 1 σ.

Spectral Features
The NIRSpec spectra of WISE 0855 are presented in Figures 3 and 4 (PRISM and G395M).The spectra are available in electronic files associated with those figures.Since the fluxes span a very large range, the spectra are shown on both linear and logarithmic flux scales.Fluxes that are consistent with zero at 1 σ are not plotted.Given that the fluxes vary significantly with wavelength, the SNRs do so as well.In the fainter half of the PRISM spectrum (<2.5 µm), the brightest 50% of pixels have a median SNR of ∼ 20 while no significant flux is detected in the strongest absorption bands.At the wavelengths of that half of the PRISM spectrum, only photometry (usually with low SNR) has been available from previous observations (Section 4.1).The brightest fluxes in the PRISM data are at 4-5 µm, where the median SNR is ∼ 90.Given their SNRs, wavelength coverage, and spectral resolution (for G395M), the NIR-Spec data have provided a dramatic improvement in the characterization of the SED of WISE 0855.
The SED of WISE 0855 is shaped by absorption from several molecular species.In the bottom panels of Figures 3 and 4, we have plotted a representation of the opacities for the dominant absorbers that are expected in Y dwarfs, consisting of H 2 O, CH 4 , NH 3 , CO, CO 2 , and PH 3 and collision-induced absorption (CIA) of H 2 and He.In those opacity diagrams, the vertical thickness of each band is proportional to the logarithm of the abundance-weighted absorption cross section for a given molecule at P=1 bar and T eff = 250 K based on the model of WISE 0855 from Leggett et al. (2021).We have summed the abundance-weighted cross sections for the CIA absorption of H 2 and He.For the model in question, the opacities of CO 2 , CO, and PH 3 are much smaller than those of the other species, so for the purposes of Figures 3 and 4, we have scaled the opacities of CO 2 , CO, and PH 3 by factors of 1000, 100, and 1000, respectively.
For comparison to WISE 0855, we have included in Figure 3 the NIRSpec PRISM spectrum of WISE 0359 (Beiler et al. 2023), which has a spectral type of Y0 (Kirkpatrick et al. 2012).The data for WISE 0359 have been scaled to roughly match the fluxes of WISE 0855 at 4-5 µm.WISE 0359 is bluer from near-to mid-IR wavelengths than WISE 0855, which reflects its earlier spectral type and higher temperature (∼450 K, Beiler et al. 2023).WISE 0359 has strong absorption from CO 2 and CO at 4-5 µm, whereas WISE 0855 lacks CO 2 and has much weaker CO (Miles et al. 2020, Section 4.5).Beiler et al. (2023) noted the detection of a relatively narrow feature at 3 µm in WISE 0359, which they attributed to the Q branch of the ν 1 band of NH 3 .That feature is detected in WISE 0855 as well.

Spectral Classification
Most known Y dwarfs have been classified as Y0 or Y1 (Kirkpatrick et al. 2021).One Y dwarf, WISEPA J182831.08+265037.8, has a spectral type of ≥Y2 (Kirkpatrick et al. 2012).WISE 0855 is the one remaining Y dwarf, which has been assigned a tentative classification of Y4 based on its colors and absolute magnitudes (Kirkpatrick et al. 2019(Kirkpatrick et al. , 2021)).NIRSpec has provided the first spectrum of WISE 0855 that is suitable for spectral classification.Those data are consistent with an effective temperature that is significantly cooler than that of other known Y dwarfs (Figure 3, Section 4.5), so Y4 remains a reasonable choice for its spectral type.The definition of classes later than Y1 will require JWST spectra of a larger sample of the coolest known Y dwarfs (e.g., JWST program 2302, PI: M. Cushing) and would be facilitated by the discovery of objects that more closely approach the temperature of WISE 0855.

Previous Comparisons to Models
Previous studies have compared photometry and spectroscopy of WISE 0855 to the predictions of atmospheric models in an attempt to characterize its atmosphere and test the models.Such models are categorized in part based on whether they assume chemical equilibrium or nonequilibrium chemistry due to vertical mixing, and whether they assume clear or cloudy atmospheres.Warmer Y dwarfs may have sulfide clouds (Morley et al. 2012) and show evidence of nonequilibrium chemistry (Cushing et al. 2011;Miles et al. 2020;Leggett et al. 2021;Beiler et al. 2023).A brown dwarf as cold as WISE 0855 is expected to have clouds of H 2 O ice (<350 K, Burrows et al. 2003;Morley et al. 2014a) and could experience nonequilibrium chemistry given its presence in the atmosphere of Jupiter (Prinn & Lewis 1975;Visscher et al. 2006).
Early studies of WISE 0855 found that its photometric SED was poorly matched by all of the models from available grids and did not favor one model prescription over another (e.g., cloudless vs. cloudy, Luhman & Esplin 2014, 2016;Schneider et al. 2016;Leggett et al. 2017).Photometric monitoring with Spitzer has measured the mid-IR variability of WISE 0855, which is similar to that of warmer Y dwarfs and is inconclusive regarding the presence of H 2 O ice clouds (Esplin et al. 2016).Skemer et al. (2016), Morley et al. (2018), andMiles et al. (2020) found that cloudy models provided a better match to their M -band spectra than cloudless models, which was cited as evidence of H 2 O clouds.However, given the untested nature of models in the temperature regime of WISE 0855, if one model prescription appears to be favored by data in a limited range of wavelengths, the same may not be true at other wavelengths or with other suites of models.Indeed, the cloudy models from those studies were discrepant in multiple bands of the photometric SED while Leggett et al. (2021) were able to use cloudless models (Phillips et al. 2020) to simultaneously reproduce the slopes of the L/M -band spectra and provide a good fit to the SED.Meanwhile, detections of PH 3 and CO would be two of the most obvious signatures of nonequilibrium chemistry.The former was not detected in the L/Mband spectra (Skemer et al. 2016;Morley et al. 2018;Miles et al. 2020), but Miles et al. (2020) reported the presence of weak CO absorption in the M -band spectrum.In their modeling of the SED of WISE 0855, Lacy & Burrows (2023) found that models with clouds and equilibrium chemistry provided the best fit.
Given that the SED of WISE 0855 has been sampled sparsely and typically with low SNRs and given the degeneracies and uncertainties in the model spectra, it has been unclear from modeling of previous data whether the atmosphere of WISE 0855 experiences H 2 O ice clouds or nonequilibrium chemistry.

Comparison of NIRSpec Data to Models
In an initial attempt at modeling the NIR-Spec data for WISE 0855, we have searched for model spectra that reproduce the PRISM data from among available grids that are defined by the following features: cloudless atmospheres with either chemical equilibrium and nonequilibrium chemistry (ATMO 2020, Tremblin et al. 2015;Phillips et al. 2020;Leggett et al. 2021;Meisner et al. 2023); cloudless atmospheres and chemical equilibrium (Sonora Bobcat, Marley et al. 2021), cloudless atmospheres and nonequilibrium chemistry (Saumon et al. 2012), partly cloudy atmospheres and chemical equilibrium (Morley et al. 2012(Morley et al. , 2014a)), and cloudy and cloudless atmospheres with chemical equilibrium and nonequilibrium chemistry (Lacy & Burrows 2023).None of the grids provided a sufficiently close match to the PRISM spectrum to be useful.
For all of the model suites that we considered, it is possible that the parameters defining the grids are too sparsely sampled and that a good fit could be achieved with custom models in which the parameters are tuned for WISE 0855.We have performed such an exercise with the ATMO 2020 models for both chemical equilibrium and nonequilibrium chemistry.As an initial model, we adopted the one for WISE 0855 from Leggett et al. (2021), which had T eff = 260 K, a surface gravity of log g=4 [cm s −2 ], an eddy diffusion coefficient of K zz = 10 8.7 , an effective adiabatic index of γ = 1.33, and solar values for metallicity and C/O.We then increased the effective temperature to improve the fit at 2-4 µm, scaled all fluxes downward to maintain agreement near 4.5-5 µm (i.e., decreased the radius), and decreased γ to further improve the fit at <3 µm.The resulting model, which we refer to as the "base model", has T eff = 285 K and γ = 1.3 while the remaining parameters are unchanged from the model in Leggett et al. (2021).
In Figure 5, we show a portion of the G395M spectrum that includes bands from PH 3 , NH 3 , and CO.The top panel compares the data to the spectrum from our base model.The prescription for nonequilibrium chemistry in that model produces mixing ratios of 7.7 × 10 −5 for NH 3 and 2 × 10 −7 for CO.The model does not account for mixing of PH 3 , and instead uses the mixing ratio of 5 × 10 −7 derived for chemical equilibrium (Phillips et al. 2020;Leggett et al. 2021).The strongest features from NH 3 and PH 3 are evident in the model spectrum at 4.2 and 4.3 µm, respectively.Additional absorption at other wavelengths from PH 3 and CO is present in the model, but it does not appear as well-defined features against the backdrop of CH 4 and H 2 O bands.
To illustrate the influence of PH 3 and CO on the model spectra, we show in Figure 5 modified versions of the base model after removing the opacities of PH 3 , removing both PH 3 and CO, and reducing the mixing ratio of PH 3 to 10 −8 .For each of these models, the pressure/temperature profile was reconverged to maintain hydrostatic and energy equilibria.The model with no PH 3 agrees well with the data near the wavelength of the strongest feature of PH 3 (4.3µm), indicating that PH 3 is not detected.The model that lacks both PH 3 and CO is significantly brighter than the data across the wavelength range of CO (4.5-4.9 µm), demonstrating that CO is clearly detected.Meanwhile, the spectrum in that range is well-matched by the base model.The mixing ratio of CO in the base model is similar to the value derived by Miles et al. (2020) from modeling of the ground-based M -band spectrum obtained by Skemer et al. (2016).In the model with a reduced mixing ratio of 10 −8 for PH 3 , the 4.3 µm feature is weak but detectable, which provides a rough upper limit on the abundance of that species.In Figure 5, the models in which PH 3 has been removed or reduced are brighter than the data at 4.3-4.5 µm, which is discussed later in the context of the PRISM data.
As discussed in Miles et al. (2020), the detection of CO in WISE 0855 indicates the presence of nonequilibrium chemistry due to vertical mixing.The degree of vertical mixing needed to account for the observed CO absorption in WISE 0855 is expected to produce strong PH 3 as well, but it is not detected.Like WISE 0855, the Y dwarf WISE 0359 exhibits absorption in CO but not PH 3 in its NIRSpec data (Beiler et al. 2023).
In Figure 6, we compare the PRISM data to the spectra produced by the base model and the modified version with PH 3 removed, which provided the best match to the G395M spectrum.When normalizing the model to the data at 4.5-5 µm as we have done, the model agrees with the data at 0.8-1.6 µm, is too faint at 1.6-4 and 5-5.6 µm, and is too bright at 4.3-4.5 µm (see also Figure 5).It is possible that the fit could be improved by adjusting the abundances of some of the dominant absorbers.For instance, the abundance of NH 3 in our model may need to be reduced based on the strength of the 4.2 µm absorption feature (Figure 5).The source of the discrepancy at 4.3-4.5 µm is unclear.We note that this wavelength range corresponds to the minimum in the opacities of the dominant absorbers (Figure 6).
The temperature for our best model for WISE 0855 (285 K) is somewhat higher than the value of 253-276 K derived by combining our luminosity estimate with the luminosities predicted by evolutionary models (Section 4.6).As an alternative illustration of the difference, the model temperature combined with our luminosity estimate correspond to a radius of 0.092 R ⊙ , which is smaller than the radii of 0.10-0.11R ⊙ that are predicted by evolutionary models for brown dwarfs at the luminosity of WISE 0855 for ages of 1-10 Gyr.
As mentioned in Section 4.1, Skemer et al. ( 2016), Morley et al. (2018), andMiles et al. (2020) found that their ground-based M -band spectrum of WISE 0855 was better matched by cloudy models than cloudless models, which was cited as evidence of H 2 O ice clouds.However, the spectrum produced by our cloudless model agrees well with the NIRSpec data in that wavelength range (Figure 5).
Future studies can improve upon our modeling of WISE 0855 by incorporating 5-28 µm data that were collected with the Mid-infrared Instrument on JWST (MIRI, Rieke et al. 2015) during the same visit as the NIRSpec observations, optimizing the abundances of the dominant absorbers in cloudless models like those from ATMO 2020, tuning the latest cloudy models to fit WISE 0855 (Lacy & Burrows 2023), and performing retrieval analysis (Burningham et al. 2017;Line et al. 2017;Zalesky et al. 2019;Rowland et al. 2023).

Luminosity Estimate
According to the model spectra for WISE 0855, the NIRSpec PRISM data (0.77-5.53 µm) should encompass roughly one third of its total flux.We have estimated the bolometric luminosity of WISE 0855 by integrating the flux in the PRISM spectrum and our 285 K model at shorter and longer wavelengths and applying the parallactic distance measured by Kirkpatrick et al. (2021), arriving at log L/L ⊙ = −7.305± 0.020.This calculation is similar to simply quoting the luminosity of our best fit model.As mentioned in Section 4.1, the NIRSpec spectra have been flux calibrated using the mean of multiple epochs of photometry at 4.5 µm with Spitzer.We have adopted the standard deviation of those measurements as the uncertainty in the flux calibration.The error in our luminosity estimate does not include the uncertainty in the model flux beyond the wavelength range of the PRISM data.It will be possible to eventually eliminate most of that uncertainty by incorporating the 5-28 µm data from MIRI.
We can estimate the effective temperature of WISE 0855 from its luminosity and the radii predicted by evolutionary models.In Figure 7, we have plotted the luminosity estimate for WISE 0855 with the luminosities as a function of temperature predicted by two sets of evolutionary models for ages of 1 and 10 Gyr, which encompass most members of the solar neighborhood.We have included the luminosity for WISE 0359 from Beiler et al. (2023) for comparison.We have shown models of cloudless atmospheres with chemical equilibrium and solar metallicity (Marley et al. 2021;Chabrier et al. 2023).Very similar luminosities are produced by models with cloudy atmospheres and nonequilibrium chemistry (Saumon et al. 2012;Morley et al. 2014a;Chabrier et al. 2023).Based on the evolutionary models, the luminosity of WISE 0855 corresponds to a temperature of 253-276 K for an age of 1-10 Gyr, which is somewhat cooler than the value implied by the model spectra (Section 4.5).As mentioned earlier, the true uncertainty in our luminosity estimate is larger than the value we have quoted and plotted in Figure 7, so the same is true for the temperature estimate.
To illustrate the expected surface gravity of WISE 0855, we have included in Figure 7 a diagram of the predicted luminosities as a function of surface gravity for 1 and 10 Gyr.The luminosity of WISE 0855 is indicative of log g ∼4 [cm s −2 ].Meanwhile, the luminosity corresponds to a mass range of 3-10 M Jup for ages of 1-10 Gyr according to the evolutionary models.

CONCLUSIONS
We have used NIRSpec on JWST to perform 1-5 µm spectroscopy on WISE 0855, which is the coldest known brown dwarf.Our results are summarized as follows: 1. We observed WISE 0855 in the fixed slit mode of NIRSpec with the PRISM and G395M dispersers (R∼40-300 and 1000), which produced useful data at 0.77-5.53µm and 2.87-5.09/5.37-5.54µm, respectively.NIRSpec has provided the first spectroscopy of WISE 0855 for most of those wavelengths.Although the brown dwarf is extremely faint at <2.5 µm, the combination of NIRSpec's sensitivity and the long exposure time for the PRISM data (4.2 hrs) has yielded relatively high SNRs in that wavelength range, with a median value of ∼20 for the brightest 50% of pixels (i.e., outside of the deepest bands).
2. We have calculated synthetic photometry from the NIRSpec spectra in several filters in which photometry has been previously measured.3. We have performed preliminary modeling of the NIRSpec data using the ATMO 2020 models of cloudless atmospheres (Tremblin et al. 2015;Phillips et al. 2020).Our best fitting model has T eff = 285 K, nonequilibrium chemistry (K zz = 10 8.7 ), an effective adiabatic index of γ = 1.3, and solar metallicity.The modeling demonstrates a clear detection of the fundamental band of CO, which is consistent with an earlier detection by Miles et al. (2020) with ground-based data and indicates the presence of vertical mixing.Previous observations of WISE 0855 and other Y dwarfs have found an absence of PH 3 that seems inconsistent with the vertical mixing implied by other species like CO (Miles et al. 2020;Beiler et al. 2023).Similarly, NIRSpec does not detect PH 3 in WISE 0855, providing an improved constraint in its mixing ratio (< 10 −8 ).
4. The temperature of our best fitting model is ∼20 K higher than the value derived when our luminosity estimate is combined with evolutionary models (i.e., the radius implied by the fit to the SED is somewhat smaller than expected based on evolutionary models).
5. Previous ground-based M -band spectroscopy of WISE 0855 (4.5-5.1 µm) has been cited for evi-dence of H 2 O ice clouds, but we find that the NIR-Spec data in that wavelength range are matched well by our cloudless model.Thus, clear evidence of H 2 O ice clouds in WISE 0855 has not been identified yet, but it may still be present in the NIR-Spec data.The physical properties of WISE 0855, including the presence of H 2 O clouds, can be better constrained with more concerted modeling efforts using both cloudless and cloudy models and the incorporation of 5-28 µm data from MIRI.
[4.5] for these Y dwarfs.The PRISM data indicate W2−[4.5]= −0.02for both WISE 0855 and WISE 0359 whereas Spitzer and WISE/NEOWISE have measured 0.03 and 0.06, respectively.It would be useful to perform similar comparisons for a larger sample of T and Y dwarfs observed by NIRSpec.The differences between synthetic and imaging values of [3.6] − [4.5] and W2−[4.5] for WISE 0855 and WISE 0359 could be caused by errors in the filter response functions or variability, although neither Y dwarf has

Figure 1 .
Figure 1.Differences between previous photometry from images of WISE 0855 and synthetic photometry measured from NIRSpec PRISM and G395M spectra.

Figure 3 .
Figure3.NIRSpec PRISM spectra of WISE 0855 (this work) and the Y0 dwarf WISE 0359(Beiler et al. 2023) on linear and logarithmic flux scales (top and middle).The spectrum of WISE 0359 has been scaled to roughly match the data for WISE 0855 at 4-5 µm.Uncertainties at ±1σ are plotted as gray and pink bands.Fluxes that are consistent with zero (F λ − σ ≤ 0) have been omitted.The bottom panel shows a representation of opacities in which the vertical thickness of each band is proportional to the logarithm of the abundance-weighted absorption cross section for a given molecule at P=1 bar and T eff = 250 K (bottom).The opacities of CO2, CO, and PH3 have has been scaled by factors of 1000, 100, and 1000, respectively, so that their strongest features are visible.

Figure 4 .
Figure 4. NIRSpec G395M spectrum of WISE 0855 on linear and logarithmic flux scales (top and middle).Uncertainties at ±1σ are plotted as gray bands.Fluxes that are consistent with zero (F λ − σ ≤ 0) have been omitted.The bottom panel shows the opacities from Figure 3.

Figure 5 .
Figure5.NIRSpec G395M spectrum of WISE 0855 compared to spectra produced by models of cloudless atmospheres(Tremblin et al. 2015;Phillips et al. 2020).The top panel shows our base model, which uses T eff = 285 K and nonequilibrium chemistry.In the remaining panels, we have modified the base model to remove PH3, remove both PH3 and CO, and reduce the mixing ratio of PH3 to 10 −8 .

Figure 6 .
Figure6.NIRSpec PRISM spectrum of WISE 0855 compared to spectra produced by models of cloudless atmospheres(Tremblin et al. 2015;Phillips et al. 2020).The top panel shows our base model for T eff = 285 K and nonequilibrium chemistry.In the bottom panel, we have modified the base model to remove PH3.

Figure 7 .
Figure 7. Luminosity estimates for WISE 0855 (this work) and WISE 0359 (Beiler et al. 2023) compared to luminosities as a function of temperature and surface gravity predicted for ages for 1 and 10 Gyr by the evolutionary models of Marley et al. (2021, solid lines) and Chabrier et al. (2023, dashed lines).