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A Survey For Planetary-mass Brown Dwarfs in the Taurus and Perseus Star-forming Regions

We present the initial results from a survey for planetary-mass brown dwarfs in the Taurus star-forming region. We have identified brown dwarf candidates in Taurus using proper motions and photometry from several ground- and space-based facilities. Through spectroscopy of some of the more promising candidates, we have found 18 new members of Taurus. They have spectral types ranging from mid M to early L and they include the four faintest known members in extinction-corrected K_s, which should have masses as low as ~4-5 M_Jup according to evolutionary models. Two of the coolest new members (M9.25, M9.5) have mid-IR excesses that indicate the presence of disks. Two fainter objects with types of M9-L2 and M9-L3 also have red mid-IR colors relative to photospheres at<=L0, but since the photospheric colors are poorly defined at>L0, it is unclear whether they have excesses from disks. We also have obtained spectra of candidate members of the IC 348 and NGC 1333 clusters in Perseus that were identified by Luhman et al. (2016). Eight candidates are found to be probable members, three of which are among the faintest and least-massive known members of the clusters (~5 M_Jup).


INTRODUCTION
The identification of large samples of planetary-mass brown dwarfs (< 15 M Jup ) is important for measuring the minimum mass of the initial mass function and for helping to interpret observations of directly imaged planetary companions. This is most easily done in the nearest young clusters and associations ( 10 Myr, 100-300 pc) given the sensitivities of existing telescopes and the predicted luminosities of brown dwarfs as a function of age (Burrows et al. 1997;Chabrier et al. 2000). The Taurus and Perseus star-forming regions are two of the most appealing targets for a brown dwarf survey because of their proximity (d = 140 and 300 pc; Torres et al. 2012;Schlafly et al. 2014, references therein) and the relatively large sizes of their stellar populations (N∼400 and 800, Kenyon et al. 2008;Bally et al. 2008). Luhman et al. (2016Luhman et al. ( , 2017 have provided recent summaries of previous surveys for members of Taurus and the two richest clusters in Perseus, IC 348 and NGC 1333 (see also Kraus et al. (2017) for Taurus). The census of each region contains several dozen objects with spectral types indicative of brown dwarfs (>M6), most of which were identified with optical and infrared (IR) photometry.
We have begun a new search for brown dwarfs in Taurus that improves upon previous surveys of the region in terms of both coverage and depth. This work is based on photometry and astrometry from wide-field images collected by the Two Micron All Sky Survey (2MASS, Skrutskie et al. 2006), the Spitzer Space Telescope (Werner et al. 2004), the United Kingdom Infrared Telescope Infrared Deep Sky Survey (UKIDSS, Lawrence et al. 2007), Pan-STARRS1 (PS1; Kaiser et al. 2002Kaiser et al. , 2010, the Sloan Digital Sky Survey (SDSS, York et al. 2000;Finkbeiner et al. 2004), Gaia (Perryman et al. 2001), and the Wide-field Infrared Survey Explorer (WISE, Wright et al. 2010). In addition, we have continued the survey of IC 348 and NGC 1333 from Luhman et al. (2016) by obtaining spectra of candidates for planetary-mass brown dwarfs from that study. In this paper, we apply updates to the census of previously known members of Taurus from Luhman et al. (2017) (Section 2), identify candidate members of Taurus using color-magnitude diagrams (CMDs) and proper motions (Section 3), present spectroscopic classifications of an initial sample of promising candidates (Section 4), and check the new members for mid-IR excess emission that would indicate the presence of circumstellar disks (Section 5). We then present spectroscopy for several candidate members of IC 348 and NGC 1333 from Luhman et al. (2016) (Section 6).

PREVIOUSLY KNOWN MEMBERS OF TAURUS
Before presenting our survey for brown dwarfs in Taurus, we describe our adopted list of previously known members. A recent compilation of members was presented by Luhman et al. (2017). Since that study, Kraus et al. (2017) and Best et al. (2017) have proposed additional members of Taurus. In this section, we examine those candidates to determine whether to add them to the list from Luhman et al. (2017). We also reject a few stars from the latter that do not appear to be members. Kraus et al. (2017) Kraus et al. (2017) compiled diagnostics of membership in the Taurus star-forming region for 396 candidate members that had been identified in previous surveys. They selected candidates that lacked evidence of disks in earlier studies 4 and that were within a large area extending well beyond the Taurus clouds that was defined by right ascensions (α) of 3 h 50 m -5 h 40 m and declinations (δ) of 14 • -34 • (J2000). The diagnostics consisted of positions in the Hertzsprung-Russel diagram, lithium absorption, gravity-sensitive spectral features, Hα emission, proper motions, and radial velocities. A given candidate was treated as a member of Taurus if it appeared to be a pre-main-sequence star based on the first four measurements (when available) and if its kinematics were similar to those of the previously known members. Some candidates lacked sufficient data for definitive assessments of their membership. Kraus et al. (2017) concluded that 218 of the 396 candidates were confirmed or likely members, 82 of which were absent from the compilation of members in Luhman et al. (2017). Most of those 82 stars are older (10-30 Myr) and more widely distributed than the previously known members (∼ 1 Myr), so Kraus et al. (2017) proposed that they represent an earlier generation of star formation that is related to the Taurus cloud complex. Although it was not included in their sample, Kraus et al. (2017) noted St 34 as an additional example of an intermediate-age pre-main-sequence star that appears to be co-moving with the known Taurus members, satisfying their criteria for membership.

Candidate Members from
For four stars adopted as members by Luhman et al. (2017), Kraus et al. (2017) found insufficient data for membership assessment (V410 Anon 25, V410 X-ray 4) or proper motions that appeared inconsistent with membership (V410 X-ray 5a, LH 0419+15). We treat the first two stars as members because they are too highly reddened for foreground stars (A V ∼ 25 and 19), are too bright for background dwarfs, have 2.3 µm CO bandheads that are too weak for background giants , and have proper motions that are consistent with membership (Section 3.1.1). Both of the other two stars, V410 X-ray 5a and LH 0419+15, are very young ( 10 Myr) based on the gravity-sensitive features in their spectra , references therein). Our measured proper motions for V410 X-ray 5a also support membership (Sections 3.1.1, 3.1.4). However, LH 0419+15 is unlikely to be a member based on the proper motion data from Harris et al. (1999) and Section 3.1.4 (µ α , µ δ = 0.6 ± 4.1, −5.5 ± 5.4 mas yr −1 ). It is also fainter than any other known member near its spectral type (M6-M7). Therefore, we no longer adopt LH 0419+15 as a member.
The field that we have considered for our survey of Taurus (α = 4 h -5 h 10 m , δ = 15-31 • , Section 3) is smaller than that from Kraus et al. (2017), but it still encompasses all of the Taurus clouds and the previously adopted members from Luhman et al. (2017). Our field contains 56 of the 82 proposed members from Kraus et al. (2017) that were absent from Luhman et al. (2017). One of those 56 stars, XEST 08-014, was spectroscopi-4 One of these stars, 2MASS J05080816+2427150, was classified as diskless by Esplin et al. (2014), but we find that it does show evidence of a disk (Section 5).
cally classified as a field dwarf by Luhman et al. (2009b). A second star, HBC 407, was rejected by  because its proper motion was inconsistent with membership. Among the remaining 54 candidates from Kraus et al. (2017) within our survey field, 16 have measurements of parallaxes and proper motions in the first data release of Gaia. To assess the membership of those 16 stars, we plot them in diagrams of Gaia magnitude (G) versus parallax, µ δ versus µ α , and extinctioncorrected M K versus spectral type in Figure 1. The latter diagram is based on K s from the 2MASS Point Source Catalog, the extinctions and spectral types adopted by Kraus et al. (2017), and the Gaia parallactic distances. For comparison, we also show the 16 known members from Luhman et al. (2017) that have Gaia parallaxes and proper motions. Their spectral types and extinctions are taken from Luhman et al. (2017). For a given space velocity, a star's proper motion varies with position on the sky and distance. To reduce these projection effects, we have subtracted from the proper motion of each star the motion expected for the mean space velocity of known Taurus members (Luhman et al. 2009b) and the α, δ, and parallax of the star. A diagram of the resulting offsets is included in Figure 1.
Most of the known members of Taurus in Figure 1 have parallactic distances near the value of ∼140 pc that has been previously measured for a small subset of members (Wichmann et al. 1998;Loinard et al. 2005;Torres et al. 2007Torres et al. , 2009Torres et al. , 2012. The primary exception is DR Tau, which would appear to be located behind Taurus according to Gaia (207 +14 −12 pc). In comparison, most of the candidates have closer distances of 100-120 pc, which was noticed by Kraus et al. (2017) during their examination of the same Gaia parallaxes. The diagram of M K versus spectral type in Figure 1 indicates that most of the candidates are older than the known members, appearing between the isochrones for 10 and 40 Myr from Baraffe et al. (2015). In the proper motion diagrams, most of the candidates are distinct from the known members; the two populations differ by ∼ 10 mas yr −1 . 2MASS J04244815+2643161 is the only candidate for which both its proper motion and parallax are similar to those of the known members. In addition, it is one of the two candidates that appear above the 10 Myr isochrone in Figure 1 and it is located near known members. As a result, we choose to adopt it as a member of Taurus. We note that the proper motion measurement for St 34 from Zacharias et al. (2017) (µ α , µ δ = 5.8 ± 1.4, −12.7 ± 1.3 mas yr −1 ) coincides with the group of candidates from Kraus et al. (2017) rather than the known members ( Figure 1). Hartmann et al. (2005a) proposed that St 34 is ∼30-40 pc closer than Taurus, which would also place it in the same range of distances as those candidates.
Parallaxes are not available for 38 of the 54 candidates from Kraus et al. (2017) in our survey field. For 36 of those 38 stars, proper motions can be measured with astrometry from 2MASS and Gaia or 2MASS and PS1. In Figure 2, we plot the 2MASS/Gaia motions for those 36 candidates when available, and otherwise show the 2MASS/PS1 measurements. The 38 candidates also are plotted in a diagram of extinction-corrected K s versus spectral type in Figure 2. In both diagrams, we have included the previously known members from Luhman et al. (2017). We have marked the 10 mas yr −1 radius threshold in the proper motion diagram that we apply to 2MASS/Gaia motions in our survey for new members (Section 3.1.3). That criterion is satisfied (at 1 σ) by 24 of the 36 candidates from Kraus et al. (2017) whose proper motions are shown in Figure 2. Because the errors in these proper motions are larger than those from Gaia, we are unable to determine whether the candidates in Figure 2 form a well-defined, distinct population like that found for the candidates with Gaia motions in Figure 1.
In the diagram of K s versus spectral type in Figure 2, many of the candidates are fainter than the sequence of the known members, indicating that they have older ages if they are near the distance of the Taurus clouds. Only five candidates satisfy our proper motion threshold, appear within the sequence of known members in K s versus spectral type, and exhibit evidence of youth among the diagnostics from Kraus et al. (2017). Two of them, 2MASS J05080816+2427150 and 2MASS J04355683+2352049, are near known members, and the former has red mid-IR colors that indicate the presence of a disk (Section 5), so we add them to our census of Taurus. For the other three stars that are far from members, 2MASS J04091700+1716081, 2MASS J04515424+1758280, and 2MASS J04525015+1622092, we defer an assessment of their membership until the next data release for Gaia, which should provide measurements of their proper motions and parallaxes.
In the preceding discussion, among the candidates from Kraus et al. (2017) that were absent from the compilation of known members in Luhman et al. (2017), we adopted three stars as Taurus members because they appear to have similar ages, kinematics, and distances (when available) as those known members. Most of the other candidates are older than the known members, and many have kinematics or distances that are noticeably different. In particular, the subset of candidates with Gaia parallaxes and proper motions are distinct from the known members in the Gaia data. As a result, we contend that those candidates and the known members should not be treated as constituents of the same stellar population, even if their origins are related.
Any relationship between the older candidates from Kraus et al. (2017) and the known Taurus members may be tenuous. Kraus et al. (2017) suggested that their candidates formed from either the current Taurus clouds or previously existing clouds that have dispersed. The former scenario is unlikely given that 1) the lifetimes of molecular clouds (∼1-2 Myr, Hartmann et al. 2001Hartmann et al. , 2012 are much shorter than the ages of those stars (10-40 Myr), 2) the candidates are not isotropic relative to the known members in the Gaia proper motions, and 3) the candidates are systematically offset relative to the known members in the Gaia parallaxes. Instead, it is much more likely that the candidates originated in clouds that are no longer present. Kraus et al. (2017) proposed that those earlier clouds were closely associated with the current Taurus clouds such that they together represented a long-lived star-forming complex. However, the differences in Gaia proper motions and parallaxes for candidates and known members (when available) indicate that the clouds that produced the former may have been quite far from the gas that would eventually become the Taurus clouds. For instance, the offset of ∼10 mas yr −1 in the Gaia proper motions of the two populations corresponds to a relative drift of nearly 30 • over 10 Myr. It is possible that the natal clouds of the candidates from Kraus et al. (2017) and the known Taurus members have no relationship beyond the fact that, like a number of other molecular clouds, they both formed just beyond the edge of the Local Bubble.
2.2. Candidate Members from Best et al. (2017) Best et al. (2015 recently used photometry from WISE and PS1 to search for brown dwarfs in the solar neighborhood that are near the L/T transition. They avoided areas of high extinction like the dark clouds in Taurus, but their survey did include the outskirts of Taurus, where they uncovered two late-type dwarfs, PSO J060.3+25 (2MASS J04011678+2557527) and PSO J077.1+24 (2MASS J05082480+2422518). Both objects were classified as young L dwarfs and proposed as new members of Taurus. Best et al. (2017) used near-IR spectroscopy to measure spectral types of L1 and L2 for PSO J060.3+25 and PSO J077.1+24, respectively. To obtain types that have been measured in the same system as the known latetype members of Taurus, we have classified the spectra from Best et al. (2017) with the M/L standard spectra from Luhman et al. (2017). The latter were constructed from spectra of M-type members of Taurus and other young populations ( 10 Myr) that have optical spectral types measured with the methods from Luhman (1999) and spectra of the youngest field L dwarfs that have optical types derived with the scheme from Cruz et al. (2009) and Kirkpatrick et al. (2010). When comparing each PS1 object to a standard at a given spectral type, the latter was artificially reddened to match the spectral slope of the former using the extinction law from Cardelli et al. (1989) (only positive values of extinction were allowed).
In Figure 3, we compare the data for PSO J060.3+25 and PSO J077.1+24 to standard spectra between M9 and L2 with the best-fitting reddenings. We find that the spectra of PSO J060.3+25 and PSO J077.1+24 are best matched by M9.25 with A V = 0.6 and M9.25 with no extinction, respectively. Both objects are much bluer than our standard spectra for young L dwarfs as well as the spectrum from Bowler et al. (2014) for Taurus member 2MASS J04373705+2331080, whose optical spectrum exhibits a type of L0 and little extinction (Luhman et al. 2009b). As discussed by Best et al. (2017), PSO J060.3+25 and PSO J077.1+24 are clearly young based on the gravity-sensitive features in their spectra. However, PSO J060.3+25 has an H-band continuum that is slightly less triangular and an FeH feature (0.99 µm) that is somewhat stronger than our young standards, indicating that it may be older than the objects associated with the Taurus dark clouds (> 10 Myr). PSO J077.1+24 also has stronger Na I absorption at 2.2 µm than the standards, which would suggest an older age as well, although the S/N is low near that feature. We note that our classifications are valid only if the PS1 objects have ages similar to those of known Taurus members. If they are older, then one would need to perform the classifications with older standards, which could produce slightly later spectral types than those we have derived with < 10 Myr standards given that near-IR spec-tra are bluer and the steam bands are more shallow at older ages for a given optical M/L type. PSO J060.3+25 and PSO J077.1+24 are located on the western and eastern edges of Taurus, respectively, as shown in Figure 4. PSO J060.3+25 is also on the western periphery of the Pleiades open cluster (125 Myr, Stauffer et al. 1998), and in fact was identified as a candidate member of the Pleiades by Sarro et al. (2014) and Bouy et al. (2015) based on its photometry and proper motion. Best et al. (2017) found that the proper motion of PSO J060.3+25 from Bouy et al. (2015) was consistent with membership in either Taurus or the Pleiades, and that the PS1 motion for PSO J077.1+24 was consistent with membership in Taurus, although the errors in that measurement were fairly large (∼ 12 mas yr −1 ). The positions of PSO J060.3+25 in diagrams of near-IR magnitudes versus spectral type from Best et al. (2017) coincided more closely with the Taurus sequence than the Pleiades sequence, so that study favored membership in Taurus. However, its position agrees better with the Pleiades sequence if our spectral classification of M9.25 is adopted. Indeed, both PSO J060.3+25 and PSO J077.1+24 are fainter than any known members of Taurus at M9-M9.5, as illustrated in the diagram of extinction-corrected K s versus spectral type in Figure 5. Based on their position in that diagram and their gravity-sensitive spectral features, we conclude that PSO J060.3+25 and PSO J077.1+24 could have ages of >10 Myr and may not be members of the Taurus starforming population. Late-type objects with surface gravities intermediate between those of Taurus members and field dwarfs have been identified in previous surveys of Taurus (Luhman 2006;Slesnick et al. 2006), some of which were included in the sample of proposed members from Kraus et al. (2017). We have found additional late-type objects with intermediate ages in our survey (Section 4).

Updates to the Census from Luhman et al. (2017)
To construct a list of the previously known members of Taurus, we begin with the compilation from Luhman et al. (2017). The following stars from that list are now rejected because they appear unlikely to be members: L1551-55, RXJ05072+2437, HBC 376, 2MASS J04163048+3037053, 2MASS J04215851+1520145, 2MASS J04374333+3056563, 2MASS J04080782+2807280, 2MASS J04505356+2139233, ITG 1 (2MASS J04375670+2546229), and LH 0419+15. The first seven stars are significantly fainter than known Taurus members near their spectral types. Stars that are occulted by circumstellar material and seen in scattered light can appear unusually faint, but six of those seven stars lack the mid-IR excess emission expected from a circumstellar disk in photometry from Spitzer and WISE. One of those six stars, 2MASS J04215851+1520145, was identified as a member of Taurus based on a mid-IR excess in the WISE All-Sky Source Catalog (Esplin et al. 2014), but newer data in the AllWISE Source Catalog do not exhibit an excess. The seventh star, 2MASS J04374333+3056563, has an excess only longward of 10 µm, whereas an edge-on disk should show excess emission at shorter wavelengths as well. The eighth star, 2MASS J04505356+2139233, is rejected because its Li absorption is rather weak for a Taurus member near its spectral type (Findeisen & Hillenbrand 2010) and it appears along the lower envelope of the Taurus sequence in K versus spectral type. ITG 1 was classified as a Taurus member based on its mid-IR excess emission, which suggested that it harbored a circumstellar disk and therefore was likely to be a young star Furlan et al. 2011). However, measurements of its proper motion with Spitzer and UKIDSS (µ α , µ δ = −5.3 ± 2.7, 2.5 ± 5.7 and −5.1 ± 4.7, 7.4 ± 4.7 mas yr −1 ) and from other astrometric catalogs (Roeser et al. 2010) are inconsistent with membership (Section 3.1). ITG 1 is probably in the background of Taurus based on its small proper motion, possibly a dusty evolved star. The reasons for rejecting LH 0419+15 were described in Section 2.1.
We have also included 2MASS J04390453+2333199, 2MASS J04354778+2523436, and 2MASS J04225416+2439538 in our list of members. They were classified as new members by Aberasturi et al. (2014) based on their proper motions, spectroscopic indicators of youth, and positions in CMDs. As mentioned in Section 2.1, the Gaia parallax of DR Tau differs significantly from the parallaxes of other known members. We retain it in our catalog of members for the purposes of this study, but its membership should be reassessed with the proper motion and parallax from the next data release of Gaia.
In Table 1, we list the 409 previously known members of Taurus and the 18 new members presented in this study (Section 4). Binaries that are not resolved in any of our photometric catalogs (Section 3.2) appear as a single entry.

Spitzer
A large portion of Taurus has been imaged with the Infrared Array Camera (IRAC; Fazio et al. 2004) on the Spitzer Space Telescope. Those data have been previously used to classify the circumstellar disks of the known members (Hartmann et al. 2005b;Luhman et al. 2006Luhman et al. , 2010Guieu et al. 2007;Esplin et al. 2014) and to search for new disk-bearing members (Luhman et al. , 2009aRebull et al. 2010). Because the IRAC images have been obtained at multiple epochs that span nearly a decade, they can be used to search for new members based on proper motions. These data are well-suited for detecting low-mass brown dwarfs in Taurus given that such objects are brightest at IR wavelengths and extinction is low in the IRAC bands.
IRAC contains four 256×256 arrays and four broadband filters centered at 3.6, 4.5, 5.8, and 8.0 µm, which are denoted as [3.6] We have retrieved from the Spitzer archive all [3.6] and [4.5] images for areas that were imaged at multiple epochs that spanned several years. In Table 2, we list the Astronomical Observing Requests (AORs), program identifications (PIDs), and principle investigators (PIs) for these observations in Table 2. The spatial coverages of the six largest maps are shown in Figure 6. Additional details regarding the observations from the cryogenic phase (e.g., exposure times) are compiled by Luhman et al. (2010). The one set of observations from the post-cryogenic phase consists of mosaics in which the individual images have exposure times of 10.4 s and dither steps of 100 , resulting in nine frames per position for a given band.
We measured astrometry for all sources in the IRAC images using the methods described in  and Esplin et al. (2017). In summary, we 1) measured positions, fluxes (F ν ), and signal-to-noise ratios (S/N's) using the point response function fitting routine in the Astronomical Point source EXtractor (APEX; Makovoz & Marleau 2005), 2) corrected those positions for distortion, 3) iteratively refined the relative offsets and orientations between individual frames, and 4) measured astrometry for each source that was detected in at least three frames among the [3.6] and [4.5] data. Because the astrometry was unreliable for the brightest stars that were near saturation, we rejected detections with F ν /(exposure time) > 0.73 and >0.82 Jy/s in [3.6] and [4.5], respectively.
For each IRAC source, a relative proper motion was calculated by applying a linear fit to the measurements of right ascension and declination as a function of time. As done in Esplin et al. (2017) for Chamaeleon I, we estimated the 25%, 50%, and 75% quantiles in the errors in µ α and µ δ as a function of S/N in the final epoch at [3.6] by applying local linear quantile regression with the function lprq in the R package quantreg (Koenker 2016). At S/N≥100, the median errors for both µ α and µ δ are 2.5 mas yr −1 , and 50% of those errors are between 1.9-3.7 mas yr −1 . The errors increase with lower S/N, reaching a median value of 25 mas yr −1 at S/N=3. At that S/N, 50% of the errors are between 17-38 mas yr −1 . To minimize the number of contaminants while maximizing the number of known Taurus members in our proper motion catalog, we only considered motions in which the errors in both µ α and µ δ are ≤10 mas yr −1 . We also omitted measurements with S/N<5.5 and 7.0 in [3.6] and [4.5], respectively, so that at least 25% of sources above these limits in S/N have errors less than 10 mas yr −1 . The faintest objects that satisfy these criteria have [3.6] ∼ 17 and [4.5] ∼ 16.6, which corresponds to K ∼ 18 for members of Taurus. In Table 1 To design proper motion criteria for selecting candidate members of Taurus, we began by estimating the intrinsic spread in proper motions within the Taurus population using the highly accurate proper motions that are available for 16 known members from the first data release of the Gaia mission (Gaia Collaboration et al. 2016a,b). As shown in Figure 1, those motions do exhibit a spread that is significantly larger than the errors in the measurements, and most of them are contained within a radius of 10 mas yr −1 . Therefore, for each source of proper motions employed in our survey (IRAC, UKIDSS, etc.), we identify candidate members based on proper motions that have 1 σ errors that overlap with the range of motions defined by a radius of 10 mas yr −1 from the median value of the motions of the known members. That threshold is indicated in Figure 7 for each catalog of proper motions. The IRAC proper motion of one known member, XEST 08-047, fails our criterion for selecting new candidates, but only by a small margin. Its motion from Roeser et al. (2010) is consistent with membership, so we retain it as a member. We note that Best et al. (2017) measured a motion for 2MASS J04373705+2331080 that was discrepant from the known members by 2 σ, but the motion measured with IRAC is consistent with membership.

UKIDSS
The UKIDSS survey has imaged large sections of Taurus in five bands (ZY JHK), as illustrated in Figure 8. The K-band images exhibit S/N=10 at K ∼ 17.5, which is roughly comparable to the IRAC images in terms of sensitivity to brown dwarfs in Taurus. Some areas of Taurus were observed by UKIDSS at epochs that spanned several years, which enabled the measurement of proper motions. Therefore, we have made use of the proper motions from data release 10 of UKIDSS that are available within a large area encompassing nearly all of the known members of Taurus, which we defined as α = 4 h to 5 h 10 m and δ = 15 • to 31 • (J2000). As with the IRAC proper motions, we have excluded UKIDSS motions with errors >10 mas yr −1 in µ α or µ δ . We also have omitted measurements for stars with K s < 11.5 because of saturation. UKIDSS motions that satisfy those criteria are available for 17 known Taurus members (includes new members). Those motions are presented in Table 1 and Figure 7. They have median errors of ∼5 mas yr −1 in µ α and µ δ . We examined the UKIDSS images of those members for blending with neighboring stars, which might affect the proper motion measurements, but all of them appeared as unblended point sources. One of these stars, 2MASS J04414825+2534304, has a UKIDSS motion that is discrepant from the 10 mas yr −1 radius threshold in Figure 7 by more than 1 σ, but its motions from IRAC and the combination of 2MASS and PS1 (Section 3.1.4) are consistent with membership.

2MASS and Gaia
In its first data release, the Gaia all-sky survey has provided photometry in a broad optical band (G) and singleepoch positions for stars with G 20 and measurements of parallaxes and proper motions for stars with G 12. The former set of measurements reaches Taurus members as late as ∼M9 and as faint as K ∼ 14. 2MASS offers slightly better sensitivity to members of Taurus and it was conducted more than a decade prior to Gaia. Therefore, we can use proper motions based on the combination of Gaia and 2MASS to help search for new substellar members of Taurus. We have measured proper motions for sources projected against Taurus (i.e., α = 4 h -5 h 10 m , δ = 15-31 • ) with the single-epoch positions from Gaia and the astrometry from the 2MASS Point Source Catalog for sources detected by both surveys. We have excluded measurements with errors >10 mas yr −1 in either component of the proper motion. In Figure 7, we have indicated the 10 mas yr −1 radius threshold used for identifying candidate members based on the 2MASS/Gaia motions.
For each Taurus member that was detected by both Gaia and 2MASS, we have inspected the 2MASS images to check whether the object was blended noticeably with other stars. If it did not appear as a single point source, we ignored its 2MASS/Gaia proper motion measurement. We also discarded proper motions for members that were unresolved by 2MASS but resolved as multiple objects by Gaia. The resulting 2MASS/Gaia motions for 210 known Taurus members are included in Table 1 and Figure 7. For those members, the median errors in µ α and µ δ are ∼4 mas yr −1 . Thirteen members are discrepant by > 1 σ from the 10 mas yr −1 radius criterion in Figure 7, but all of them have motions from other catalogs that are consistent with membership.

2MASS and Pan-STARRS1
The PS1 3π survey obtained images of nearly all areas of sky at δ −30 • in five bands (grizy P 1 , Tonry et al. 2012) at several epochs between 2009 and 2014 ). The first data release of PS1 has provided photometry and astrometry measured from coadditions of the multiple epochs in the 3π survey. The stacked images in the bands at the longest wavelengths are capable of detecting members of Taurus at K 15.5, corresponding to spectral types of early L. We have measured proper motions for sources projected against Taurus using the astrometry from PS1 and 2MASS. To avoid saturated detections, we only considered PS1 sources for which all bands were fainter than 15 mag. As with the other sources of proper motions in our survey, we have excluded 2MASS/PS1 measurements that have errors >10 mas yr −1 .
We list 2MASS/PS1 motions for 195 known members of Taurus in Table 1. We do not report measurements for members that are blended noticeably with other stars in the 2MASS or PS1 images. We retain measurements for members that are known to be multiple systems but that are unresolved by 2MASS or PS1. The values of µ α and µ δ in Table 1 have median errors of slightly more than ∼4 mas yr −1 . The 2MASS/PS1 motions for known members are shown in Figure 7 with the 10 mas yr −1 radius threshold for identifying new candidate members. That criterion is not satisfied by 16 known members, but most of them have motions in other catalogs that are consistent with membership. The remaining source, 2MASS J04574903+3015195, is discrepant by only slightly more than 1 σ and shows strong evidence of youth in its spectrum, so we retain it as a member for the purposes of this study.
We note that WISE also has measured astrometry across our entire survey field, but we did not include those data in our proper motion measurements since the WISE images have lower resolution than the other sources of astrometry that we have considered.

Color-magnitude Diagrams
To further refine our candidate members of Taurus selected by proper motions and to search for additional candidates that may lack such data, we have constructed CMDs from several bands of optical and IR photometry. As with the proper motion measurements, we have considered photometry for objects at α = 4 h -5 h 10 m and δ = 15 • -31 • . These data consist of G from the first data release of Gaia, JHK s from the 2MASS Point Source Catalog, ZY JHK from data release 10 of UKIDSS, W 1 and W 2 5 from the AllWISE Source Catalog, riz from data release 13 of the Sloan Digital Sky Survey (SDSS; Albareti et al. 2016), rizy P 1 from the first data release of PS1 (Flewelling et al. 2016), and [3.6] and [4.5] from all IRAC observations of Taurus prior to 2010, which were processed during the study of Luhman et al. (2010). Additional bands are available from some of these catalogs, but they are not sensitive to low-mass members of Taurus. Some of the surveys offer multiple options of photometry for a given band. We adopted the 1 radius aperture magnitudes from UKIDSS, the pointspread-function (PSF) magnitudes from SDSS, and the PSF magnitudes from the stacked images in PS1. To avoid erroneous photometry of saturated stars, we did not use any PS1 measurements brighter than 15 mag and we excluded UKIDSS data at J < 11, H < 11.5, and K s < 10.5. The Z and Y data from UKIDSS were also omitted for J < 11. For stars observed at two epochs in K by UKIDSS, we adopted the average of those measurements.
We identified all matches among the sources from the various catalogs. When merging the photometry from different sources, we treated the following filters as the same: JHK s (2MASS)/JHK(UKIDSS) and ri(SDSS)/ri P 1 . When both 2MASS and UKIDSS data were available for a star, we adopted the 2MASS measurement if its error was ≤0.06. Otherwise, we adopted the UKIDSS photometry. If a star was detected in both SDSS and PS1 for r or i, we adopted the PS1 measurement. The bands Z(UKIDSS)/z P 1 and Y (UKIDSS)/y P 1 are sufficiently different that they are used separately in our CMDs. We found that the z photometry from SDSS did not provide significant added value in identifying candidates beyond the data in Z(UKIDSS) and z P 1 , so those measurements are not included in our CMDs.
In most of our CMDs, we have selected K s (or K) for the vertical axis and have used colors that contain that band because it offers the greatest sensitivity to low-mass members of Taurus (as well as low extinction) among the available options. The one exception is a diagram of W 1 versus W 1 − W 2. As done in our previous surveys of this kind (e.g., Luhman et al. 2003), we have corrected the data in the CMDs for extinction, which reduces contamination of background stars in the areas of the diagrams inhabited by Taurus members. We have estimated the extinction for each star by dereddening its data in near-IR CMDs to the typical locus of young stars at the distance of Taurus to a locus defined by [J − K s = 1] for K s < 14.5 and [J − K s = 0.3 × K s − 3.35] for K s ≥ 14.5. For sources that lack the data necessary for either of those extinction estimates, no correction is applied to their data. For those stars, one could instead adopt the values inferred from extinction maps (e.g., Dobashi et al. 2005), but such estimates would have large uncertainties because of the low resolution of those maps, and they would be overestimated for Taurus members, most of which are not subject to the total extinction through the clouds. Since the Taurus dark clouds cover only a small fraction of our survey field (Figure 4), most stars in our CMDs exhibit negligible extinction (A J < 0.1). Near the clouds, the extinction estimates are concentrated at A J < 1 and reach as high as A J ∼ 7. When correcting the photometry for extinction, we have adopted the reddening relations from Schlafly et al. (2016), A 3.6 = 0.6 A K (Xue et al. 2016, references therein), and A H /A K = 1.55 (Indebetouw et al. 2005). The resulting extinction-corrected CMDs for Taurus are shown in Figure 9. In each CMD, we have selected a boundary that follows the lower envelope of the sequence of known members. We have identified stars as photometric candidate members if they appear above a boundary in any diagram and do not fall below a boundary in any diagram.

Observations
We have pursued spectroscopy of an initial sample of our candidate members of Taurus so that we can determine whether they are likely to be members based on their spectral types and gravity-sensitive features. During the selection of these targets, we gave higher priority to candidates that were identified with both CMDs and proper motions, appear in multiple CMDs, are located within a few degrees of known members, and have CMD positions that are indicative of later spectral types. For a given object, if the proper motion from one source supported membership but the motion from another source was inconsistent with membership, we still treated it as a viable candidate.
We obtained near-IR spectra of 25 candidates with SpeX  at the NASA Infrared Telescope Facility (IRTF). The instrument was operated in the prism mode with the 0. 8 slit (0.7-2.5 µm, R=150). We also observed nine additional candidates with the Gemini Near-Infrared Spectrograph (GNIRS; Elias et al. 2006) in the cross-dispersed mode with the 31.7 l mm −1 grating and the 0. 67 slit (0.8-2.5 µm, R=800). The dates of these observations are listed in Table 3. The SpeX data were reduced with the Spextool package (Cushing et al. 2004) and corrected for telluric absorption with the methods from Vacca et al. (2003). The GNIRS data were reduced in a similar manner with routines in IRAF. To increase the S/N of the GNIRS spectra, we binned them in 25 pixel increments, producing a resolution comparable to that of the SpeX data.

Spectral Classifications
We have used our near-IR spectra to measure spectral types for the candidates and to assess whether they are likely to be members of Taurus. Based on the photometry of these candidates, they should have spectral types of mid-M or later if they are Taurus members. Six candidates lack steam absorption bands, indicating that they are earlier than ∼M0, and thus are likely reddened background stars. For the remaining 28 candidates that do show those absorption bands, we have assessed their ages, and thus their membership in Taurus, using spectral features that are sensitive to surface gravity, such as the shape of the H-band continuum and the strength of the FeH band at 0.99 µm (Lucas et al. 2001). These objects were compared to both standards for field dwarfs (Cushing et al. 2005;Rayner et al. 2009) and standards for ages of 10 yr . Four candidates are best matched by the field dwarf standards and 24 candidates show evidence of youth. Eighteen of the young objects have gravity-sensitive features that agree closely with those of known members of Taurus , references therein) and have K s data that are consistent with other Taurus members near their types ( Figure 5), so they are treated as confirmed members. Their spectra are shown in Figure 10.
We classify six of the young objects as likely nonmembers for the following reasons. Four of the candidates exhibit less triangular H-band continua (2MASS J04252314+1735150, 2MASS J04095154+2000428) or stronger FeH absorption (2MASS J04175948+2521283, 2MASS J04263219+1800280) than members of Taurus, indicating somewhat older ages (see Figure 11). All six candidates are unusually faint for Taurus members at their spectral types, as shown in Figure 5. None of them have mid-IR excess emission that would indicate the presence of circumstellar material, so their faint positions in that diagram cannot be explained by edge-on disks. All but one of the six objects (2MASS J04095154+2000428) have proper motions that are in the outskirts of the distribution of motions for known members (Figure 7), one of which fails the criteria used for selecting candidates with UKIDSS motions. Finally, all but one of those six candidates (2MASS J04175948+2521283) are far from known members of Taurus ( Figure 4).
Our classifications of the 34 spectroscopic targets are presented in Table 3.
Several of our new objects are among the faintest known members of Taurus, as illustrated in the diagram of extinction-corrected K s versus spectral type in Figure 5. For instance, our sample includes the four faintest known members and eight of the 10 faintest ones in extinction-corrected K s . Assuming an age of 1 Myr and a K s -band bolometric correction for young late-M/early-L objects (Filippazzo et al. 2015), the faintest new members should have masses near 4-5 M Jup according to evolutionary models (Burrows et al. 1997;Chabrier et al. 2000; Baraffe et al. 2015). Luhman et al. (2010) and Esplin et al. (2014) presented the most recent compilations of mid-IR photometry from Spitzer and WISE for the known members of Taurus. They used those data to identify and classify circumstellar disks. Roughly 2/3 of the known members exhibited mid-IR excess emission that indicated the presence of a disk. In their sample of members, Esplin et al. (2014) included the new members from Luhman et al. (2017) with the exception of 2MASS J04344586+2445145, which was mistakenly omitted. In Table 4, we have compiled Spitzer and WISE data for that star, the three new members from Aberasturi et al. (2014), the three stars from Kraus et al. (2017) that we have adopted as members, and the new members from our study. One of our new members, UGCS J045210.35+303734.3, is absent from Table 4 since it was not observed by Spitzer and it was not detected by WISE. For Spitzer, we have considered the four bands of IRAC and the 24 µm band from the Multiband Imaging Photometer for Spitzer (MIPS; Rieke et al. 2004), which is denoted as [24]. The Spitzer data were measured in the manner described by Luhman et al. (2010). The WISE data are from the AllWISE Source Catalog.

MEMBERS
To determine whether the stars in Table 4 exhibit evidence of disks in their mid-IR photometry, we begin by plotting K s − W 3, K s − W 4, and K s − [24] as a function of spectral type in Figure 12 for the stars that have detections in W 3, W 4, or [24]. For comparison, we have included previously known members of Taurus within that range of spectral types and estimates for the typical intrinsic photospheric colors of young stars (K. Luhman, in preparation). Among the sources in Table 4, 2MASS J04492210+2911124 (M8.5) and 2MASS J05080816+2427150 (K5) show significant color excesses relative to photospheric colors in Figure 12. In their survey for disk-bearing members of Taurus using WISE data, Esplin et al. (2014) did not identify 2MASS J04492210+2911124 as a candidate because its photometric error in W 3 is slightly larger than the threshold that was adopted when selecting data to consider. They did select 2MASS J05080816+2427150 as a disk candidate based on W 1 − W 4, but they found that it was heavily blended with another star in the WISE Atlas Image at W 4, so that excess was ignored. However, the Atlas Images were artificially blurred, whereas the versions produced by Lang (2014) and Meisner et al. (2017) retain the intrinsic resolution of the original data. In the W 4 image of 2MASS J05080816+2427150 from the latter studies, the two stars are sufficiently resolved that we now accept the W 4 excess as reliable. As shown in Figure 12, 2MASS J05080816+2427150 also has an excess in W 3. Even the lower resolution Atlas Image indicates that the W 3 measurement should have little contamination from the neighboring star, but that excess was not noticed by Esplin et al. (2014) because W 1 − W 3 was just below their threshold for identifying candidates.
We have also examined the IRAC photometry in Table 4 for color excesses. In Figure 13, (M9-L3), are significantly redder than stellar photospheres at ≤L0, but since the photospheric colors are ill-defined at >L0 for ages of a few Myr, it is unclear whether color excesses are present. Additional data (i.e., detections at longer wavelengths) are needed to determine if these two objects harbor cir-cumstellar disks.
We comment briefly on the mid-IR colors of the previously known L0 member 2MASS J04373705+2331080. Luhman et al. (2009b) found that it is only slightly redder than young L0 dwarfs in the field, so they concluded that its colors were probably entirely photospheric rather than containing excesses from a disk. However, it does exhibit significant excesses at both [5.8] and [8.0] relative to our adopted photospheric sequences, which were estimated from the bluest known members of Taurus, Chamaeleon I, Upper Sco, IC 348, and the TW Hya association. There are two possible explanations for those red colors for 2MASS J04373705+2331080: it does have a disk, or the photospheres of young L0 objects at a given age have a large spread in their mid-IR colors. As with our new L0 members, additional photometry at longer wavelengths is necessary for determining whether 2MASS J04373705+2331080 has a disk.

SPECTROSCOPY OF PERSEUS CANDIDATES
In a recent survey for new members of IC 348 and NGC 1333 in Perseus, Luhman et al. (2016) used CMDs, proper motions, and other indicators of membership to identify candidate members of the clusters. They obtained spectra of some of those candidates to measure their spectral types and determine if they were members. We have done the same for nine additional candidates from the CMDs in Luhman et al. (2016). We have also performed spectroscopy on source 48 from Oasa et al. (2008), which is a candidate companion to a known member of NGC 1333, and [SVS76] NGC 1333 7, which is a probable member of NGC 1333 that was excluded from the census in Luhman et al. (2016) because its spectral classification was uncertain.
We obtained near-IR spectra of the 11 candidates in IC 348 and NGC 1333 with SpeX and GNIRS. The data were collected, reduced, and classified in the same manner as the SpeX and GNIRS spectra of the candidates in Taurus except for [SVS76] NGC 1333 7, which was observed with the SXD mode of SpeX (R = 750). In Table 5, we list the spectral types, extinctions derived from the spectra, membership assessments, spectrographs, and observing dates for nine of the candidates. The remaining two candidates, IC 348 IRS J03443631+3205066 and NGC 1333 IRS J03293317+3125495, are not included in Table 5 since the S/N's in their spectra (from GNIRS) were too low for classification. The spectra of LRL 595 and LRL 596 have insufficient S/N to definitively determine whether they are young, but they are better matched by young objects than field dwarfs, so we tentatively treat them as members of IC 348. As indicated in Table 5, two and six candidates in IC 348 and NGC 1333 are classified as members, respectively, which results in a total of 480 and 209 known members when combined with the members compiled by Luhman et al. (2016). We show the spectra of the eight new members in Figure 14.
To illustrate how the new members of IC 348 and NGC 1333 compare to previously known members in terms of magnitude and reddening, we have plotted diagrams of K s versus H − K s with the previously known and new members in Figure 15. Three of the new members of these clusters are among the faintest known members, and hence are contenders for the least massive known members (∼ 5 M Jup ). J03291180+3122036 is much fainter than other members of NGC 1333 near its spectral type (M6), which indicates that it may be occulted by a circumstellar disk and seen in scattered light. We cannot check whether it exhibits the mid-IR excess emission expected from a disk because it is projected against the bright extended emission that surrounds [SVS76] NGC 1333 3, which prevents a detection in images from Spitzer.

CONCLUSIONS
We have begun a survey for planetary-mass brown dwarfs in the Taurus star-forming region, and we have continued a previous survey for such objects in the IC 348 and NGC 1333 clusters in Perseus . Our results are summarized as follows.
1. Luhman et al. (2017) recently presented a compilation of known members of Taurus. We have applied revisions to that list by rejecting probable non-members and adding stars that exhibit sufficient evidence of membership from previous studies. While identifying stars to include as members, we examined the 82 candidates from Kraus et al. (2017) that were absent from the list in . We have adopted three of those candidates as members because they are similar to the known members in terms of ages, kinematics, and distances (when available). Most of the candidates from Kraus et al. (2017) that have Gaia parallaxes and proper motions are distinct from the known members in those parameters, indicating that they represent a different stellar population. We have also considered two objects that were classified as L-type members of Taurus by Best et al. (2017). We find that the spectra of both objects are best matched by young standards at M9.25. Although they are clearly young, neither of them shows strong evidence of membership, and they instead may be intermediate-age (>10 Myr) non-members.
2. We have identified candidate members of Taurus using proper motions and photometry from a variety of sources (Spitzer, 2MASS, Gaia, PS1, UKIDSS, SDSS, WISE). We have performed near-IR spectroscopy on some of the more promising candidates, confirming 18 of them as new members. They exhibit spectral types ranging from M4 to early L, and they include the four faintest known members in extinction-corrected K s . The faintest new member should have a mass of roughly 4-5 M Jup according to evolutionary models.
3. Two of the coolest new Taurus members (M9.25, M9.5) have mid-IR excess emission, which indicates the presence of circumstellar disks. Two additional members from our survey (M9-L2, M9-L3) also exhibit red mid-IR colors relative to the photospheric values at ≤L0, but it is unclear whether they have disks given the uncertainties in their spectral types and the ill-defined nature of mid-IR colors of young photospheres later than L0.
4. We have obtained near-IR spectra of candidate members of the IC 348 and NGC 1333 clusters in Perseus from Luhman et al. (2016). Eight of these candidates are classified as new members, three of which are among the faintest and least-massive known members (∼ 5 M Jup ).
This work was supported by grant AST-1208239 from the NSF. We thank William Best for providing his SpeX data for PSO J060.3+25 and PSO J077.1+24 and we thank Lee Hartmann and Eric Mamajek for their comments on the paper. The Spitzer Space Telescope       Most of the candidates have parallactic distances of 100-120 pc, placing them in the foreground of the known members. One of the known members, DR Tau, exhibits a discrepant parallax relative to the other members. Top right: extinction-corrected M K versus spectral type with model isochrones from Baraffe et al. (2015), which indicate ages of 10-40 Myr for most of the candidates. Bottom left: Gaia proper motions. Bottom right: Offsets of the proper motions relative to the values expected for the positions and parallaxes of the stars assuming the mean space velocity of Taurus members (Luhman et al. 2009b). The spread in motions due to projection effects should be reduced in these offsets. In both of the bottom diagrams, most of the candidates have motions that are distinct from those of the known Taurus members.  Figure 1) and are within the boundaries of our survey (α = 4 h -5 h 10 m , δ = 15-31 • ). Left: proper motions measured with astrometry from 2MASS/Gaia or 2MASS/PS1. For reference, we have marked the threshold that we have applied to proper motions from these catalogs for our survey (large circle). The typical errors for these data are indicated. Right: extinction-corrected Ks versus spectral type. Most of the members that are unusually faint for their spectral types are occulted by circumstellar material and seen in scattered light, or they exhibit mid-IR excess emission indicating the presence of a disk, making scattered light from an occulting disk a possibility. None of the candidates show mid-IR excesses, so those that appear below the Taurus sequence are likely older or more distant than the known members. , which were proposed as members of Taurus and were classified as L1 and L2 in that study, respectively. Those data are compared to standard spectra from M9-L2 for ages of 10 Myr (red lines, Luhman et al. 2017). If PSO J060.3+25 or PSO J077.1+24 was redder than a given standard spectrum, the latter was artificially reddened to match the slope of the former. If they are members of Taurus, both objects would have types of M9.25 according to these standards. However, we find that neither object shows strong evidence of membership in Taurus (Section 2.2).    Best et al. (2017) (crosses). The members that appear below the sequence may be seen primarily in scattered light because of an occulting circumstellar disk, which is plausible given that they exhibit mid-IR excess emission.  -Near-IR spectra of objects from our survey that have spectral features indicative of youth but that appear unlikely to be members of Taurus. These data have been dereddened to match standard young brown dwarfs  and have a resolution of R = 150. The data used to create this figure are available.  -Extinction-corrected mid-IR colors as a function of spectral type for late-type members of Taurus (filled circles). The members that have been adopted since Esplin et al. (2014) (Table 4) are plotted in red and with the errors in their colors. Two of these members exhibit significant excess emission, which indicates the presence of circumstellar disks. We mark the intrinsic photospheric colors for young objects (blue lines, K. Luhman, in preparation). -Extinction-corrected mid-IR colors versus spectral type for late-type members of Taurus (filled circles). The members that have been adopted since Esplin et al. (2014) (Table 4) are plotted in red and with the errors in their colors. Uncertainties in spectral types are also included for the new members that are later than M9. We mark the intrinsic photospheric colors for young objects (blue lines, K. Luhman, in preparation). Two of the new Taurus members with precise spectral types (at M9.25 and M9.5) show clear excesses at [5.8] and [8.0], indicating that they have circumstellar disks. Two additional new members are also redder than the photospheric sequences at ≤L0, but since they have larger spectral type errors that extend later than L0, and since the photospheric colors are ill-defined at those types, we cannot determine conclusively whether color excesses from disks are present.   Table 5).