We present the discovery of CWISE J050626.96\(+\)073842.4 (CWISE J0506\(+\)0738), an L/T transition dwarf with extremely red near-infrared colors discovered through the Backyard Worlds: Planet 9 ...citizen science project. Photometry from UKIRT and CatWISE give a \((J-K)_{\rm MKO}\) color of 2.97\(\pm\)0.03 mag and a \(J_{\rm MKO}-\)W2 color of 4.93\(\pm\)0.02 mag, making CWISE J0506\(+\)0738 the reddest known free-floating L/T dwarf in both colors. We confirm the extremely red nature of CWISE J0506\(+\)0738 using Keck/NIRES near-infrared spectroscopy and establish that it is a low-gravity late-type L/T transition dwarf. The spectrum of CWISE J0506\(+\)0738 shows possible signatures of CH\(_4\) absorption in its atmosphere, suggesting a colder effective temperature than other known, young, red L dwarfs. We assign a preliminary spectral type for this source of L8\(\gamma\)-T0\(\gamma\). We tentatively find that CWISE J0506\(+\)0738 is variable at 3-5 \(\mu\)m based on multi-epoch WISE photometry. Proper motions derived from follow-up UKIRT observations combined with a radial velocity from our Keck/NIRES spectrum and a photometric distance estimate indicate a strong membership probability in the \(\beta\) Pic moving group. A future parallax measurement will help to establish a more definitive moving group membership for this unusual object.
In an effort to identify nearby and unusual cold objects in the solar neighborhood, we searched for previously unidentified moving objects using CatWISE2020 proper motion data combined with machine ...learning methods. We paired the motion candidates with their counterparts in 2MASS, UHS, and VHS. Then we searched for white dwarf, brown dwarf, and subdwarf outliers on the resulting color-color diagrams. This resulted in the discovery of 16 new dwarfs including two nearby M dwarfs (< 30 pc), a possible young L dwarf, a high motion early T dwarf and 3 later T dwarfs. This research represents a step forward in completing the census of the Sun's neighbors.
Through the Backyard Worlds: Planet 9 citizen science project we discovered a late-type L dwarf co-moving with the young K0 star BD+60 1417 at a projected separation of 37" or 1662 AU. The secondary ...- CWISER J124332.12+600126.2 (W1243) - is detected in both the CatWISE2020 and 2MASS reject tables. The photometric distance and CatWISE proper motion both match that of the primary within ~1sigma and our estimates for chance alignment yield a zero probability. Follow-up near infrared spectroscopy reveals W1243 to be a very red 2MASS color(J-Ks=2.72), low-surface gravity source that we classify as L6 - L8gamma. Its spectral morphology strongly resembles that of confirmed late-type L dwarfs in 10 - 150 Myr moving groups as well as that of planetary mass companions. The position on near- and mid-infrared color-magnitude diagrams indicates the source is redder and fainter than the field sequence, a telltale sign of an object with thick clouds and a complex atmosphere. For the primary we obtained new optical spectroscopy and analyzed all available literature information for youth indicators. We conclude that the Li I abundance, its loci on color-magnitude and color-color diagrams, and the rotation rate revealed in multiple TESS sectors are all consistent with an age of 50 - 150 Myr. Using our re-evaluated age of the primary, the Gaia parallax along with the photometry and spectrum for W1243 we find a Teff=1303+/-31 K, logg=4.3+/-0.17 cm s-2, and a mass of 15+/-5 MJup. We find a physical separation of ~1662 AU and a mass ratio of ~0.01 for this system. Placing it in context with the diverse collection of binary stars, brown dwarf and planetary companions, the BD+60 1417 system falls in a sparsely sampled area where the formation pathway is difficult to assess.
Through the Backyard Worlds: Planet 9 citizen science project, we have identified a wide-separation (\(\sim\)10', \(\sim\)9900 au projected) substellar companion to the nearby (\(\sim\)17.5 pc), ...mid-M dwarf Ross 19. We have developed a new formalism for determining chance alignment probabilities based on the BANYAN \(\Sigma\) tool, and find a 100% probability that this is a physically associated pair. Through a detailed examination of Ross 19A, we find that the system is metal-poor (Fe/H=\(-\)0.40\(\pm\)0.12) with an age of 7.2\(^{+3.8}_{-3.6}\) Gyr. Combining new and existing photometry and astrometry, we find that Ross 19B is one of the coldest known wide-separation companions, with a spectral type on the T/Y boundary, an effective temperature of 500\(^{+115}_{-100}\) K, and a mass in the range 15-40 \(M_{\rm Jup}\). This new, extremely cold benchmark companion is a compelling target for detailed characterization with future spectroscopic observations using facilities such as the Hubble Space Telescope or James Webb Space Telescope.
Schneider et al. (2020) presented the discovery of WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5, which appear to be the first examples of extreme T-type subdwarfs (esdTs; metallicity <= -1 ...dex, T_eff <= 1400 K). Here we present new discoveries and follow-up of three T-type subdwarf candidates, with an eye toward expanding the sample of such objects with very low metallicity and extraordinarily high kinematics, properties that suggest membership in the Galactic halo. Keck/NIRES near-infrared spectroscopy of WISEA J155349.96+693355.2, a fast-moving object discovered by the Backyard Worlds: Planet 9 citizen science project, confirms that it is a mid-T subdwarf. With H_W2 = 22.3 mag, WISEA J155349.96+693355.2 has the largest W2 reduced proper motion among all spectroscopically confirmed L and T subdwarfs, suggesting that it may be kinematically extreme. Nevertheless, our modeling of the WISEA J155349.96+693355.2 near-infrared spectrum indicates that its metallicity is only mildly subsolar. In analyzing the J155349.96+693355.2 spectrum, we present a new grid of low-temperature, low-metallicity model atmosphere spectra. We also present the discoveries of two new esdT candidates, CWISE J073844.52-664334.6 and CWISE J221706.28-145437.6, based on their large motions and colors similar to those of the two known esdT objects. Finding more esdT examples is a critical step toward mapping out the spectral sequence and observational properties of this newly identified population.
We present the discoveries of WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5, two low-temperature (1200\(-\)1400 K), high proper motion T-type subdwarfs. Both objects were discovered via ...their high proper motion (\(>\)0.5 arcsec yr\(^{-1}\)); WISEA J181006.18-101000.5 as part of the NEOWISE proper motion survey and WISEA J041451.67-585456.7 as part of the citizen science project Backyard Worlds; Planet 9. We have confirmed both as brown dwarfs with follow-up near-infrared spectroscopy. Their spectra and near-infrared colors are unique amongst known brown dwarfs, with some colors consistent with L-type brown dwarfs and other colors resembling those of the latest-type T dwarfs. While no forward model consistently reproduces the features seen in their near-infrared spectra, the closest matches suggest very low metallicities (Fe/H \(\leq\) -1), making these objects likely the first examples of extreme subdwarfs of the T spectral class (esdT). WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5 are found to be part of a small population of objects that occupy the "substellar transition zone," and have the lowest masses and effective temperatures of all objects in this group.
We present Spitzer follow-up imaging of 95 candidate extremely cold brown dwarfs discovered by the Backyard Worlds: Planet 9 citizen science project, which uses visually perceived motion in ...multi-epoch WISE images to identify previously unrecognized substellar neighbors to the Sun. We measure Spitzer 3.6-4.5 color to phototype our brown dwarf candidates, with an emphasis on pinpointing the coldest and closest Y dwarfs within our sample. The combination of WISE and Spitzer astrometry provides quantitative confirmation of the transverse motion of 75 of our discoveries. Nine of our motion-confirmed objects have best-fit linear motions larger than 1"/yr; our fastest-moving discovery is WISEA J155349.96+693355.2 (total motion ~2.15"/yr), a possible T type subdwarf. We also report a newly discovered wide-separation (~400 AU) T8 comoving companion to the white dwarf LSPM J0055+5948 (the fourth such system to be found), plus a candidate late T companion to the white dwarf LSR J0002+6357 at 5.5' projected separation (~8,700 AU if associated). Among our motion-confirmed targets, five have Spitzer colors most consistent with spectral type Y. Four of these five have exceptionally red Spitzer colors suggesting types of Y1 or later, adding considerably to the small sample of known objects in this especially valuable low-temperature regime. Our Y dwarf candidates begin bridging the gap between the bulk of the Y dwarf population and the coldest known brown dwarf.
Infrared excesses due to dusty disks have been observed orbiting white dwarfs with effective temperatures between 7200 K and 25000 K, suggesting that the rate of tidal disruption of minor bodies ...massive enough to create a coherent disk declines sharply beyond 1~Gyr after white dwarf formation. We report the discovery that the candidate white dwarf LSPM J0207+3331, via the Backyard Worlds: Planet 9 citizen science project and Keck Observatory follow-up spectroscopy, is hydrogen-dominated with a luminous compact disk (L\(_{\rm IR}\)/L\(_{\star}\)=14%) and an effective temperature nearly 1000K cooler than any known white dwarf with an infrared excess. The discovery of this object places the latest time for large scale tidal disruption events to occur at \(\sim\)3 Gyr past the formation of the host white dwarf, making new demands of dynamical models for planetesimal perturbation and disruption around post main sequence planetary systems. Curiously, the mid-IR photometry of the disk cannot be fully explained by a geometrically thin, optically thick dust disk as seen for other dusty white dwarfs, but requires a second ring of dust near the white dwarf's Roche radius. In the process of confirming this discovery, we found that careful measurements of WISE source positions can reveal when infrared excesses for white dwarfs are co-moving with their hosts, helping distinguish them from confusion noise.
Using a sample of 361 nearby brown dwarfs, we have searched for 4.6\(\mu\)m variability indicative of large-scale rotational modulations or large-scale long-term changes on timescales of over 10 ...years. Our findings show no statistically significant variability in \textit{Spitzer} ch2 or \textit{WISE} W2 photometry. For \textit{Spitzer} the ch2 1\(\sigma\) limits are \(\sim\)8 mmag for objects at 11.5 mag and \(\sim\)22 mmag for objects at 16 mag. This corresponds to no variability above 4.5\(\%\) at 11.5 mag and 12.5\(\%\) at 16 mag. We conclude that highly variable brown dwarfs, at least two previously published examples of which have been shown to have 4.6\(\mu\)m variability above 80 mmag, are very rare. While analyzing the data, we also developed a new technique for identifying brown dwarfs binary candidates in \textit{Spitzer} data. We find that known binaries have IRAC ch2 PRF (point response function) flux measurements that are consistently dimmer than aperture flux measurements. We have identified 59 objects that exhibit such PRF versus apertures flux differences and are thus excellent binary brown dwarf candidates.
Using a sample of 361 nearby brown dwarfs, we have searched for 4.6$\mu$m
variability indicative of large-scale rotational modulations or large-scale
long-term changes on timescales of over 10 years. ...Our findings show no
statistically significant variability in \textit{Spitzer} ch2 or \textit{WISE}
W2 photometry. For \textit{Spitzer} the ch2 1$\sigma$ limits are $\sim$8 mmag
for objects at 11.5 mag and $\sim$22 mmag for objects at 16 mag. This
corresponds to no variability above 4.5$\%$ at 11.5 mag and 12.5$\%$ at 16 mag.
We conclude that highly variable brown dwarfs, at least two previously
published examples of which have been shown to have 4.6$\mu$m variability above
80 mmag, are very rare. While analyzing the data, we also developed a new
technique for identifying brown dwarfs binary candidates in \textit{Spitzer}
data. We find that known binaries have IRAC ch2 PRF (point response function)
flux measurements that are consistently dimmer than aperture flux measurements.
We have identified 59 objects that exhibit such PRF versus apertures flux
differences and are thus excellent binary brown dwarf candidates.
