Abstract
We present the positions, proper motions, and near-infrared photometry for 966 known objects with spectral types later than M that were observed as part of the the UKIRT Hemisphere Survey ...(UHS). We augment the photometry and astrometry from UHS with information from Gaia DR3, Pan-STARRS DR2, and CatWISE 2020 to produce a database of homogeneous photometry and astrometry for this sample. The multi-epoch survey strategy of UHS allows us to determine the proper motions for most sources, with a median proper motion uncertainty of ∼3.6 mas yr
−1
. Our UHS proper motion measurements are generally in good agreement with those from Gaia DR3, Pan-STARRS, and CatWISE 2020, with UHS proper motions typically more precise than those from CatWISE 2020 and Pan-STARRS but not Gaia DR3. We critically analyze the publicly available spectra for 406 members of this sample and provide updated near-infrared spectral types for ∼100 objects. We determine typical colors as a function of spectral type and provide absolute magnitude versus spectral type relations for UHS
J
- and
K
-band photometry. Using newly determined proper motions, we highlight several objects of interest, such as objects with large tangential velocities, widely separated co-moving companions, and potential members of young nearby associations.
We report the discovery of a repeating photometric signal from a low-mass member of the Praesepe open cluster that we interpret as a Neptune-sized transiting planet. The star is JS 183 (HSHJ 163, ...EPIC 211916756), with Teff = 3325 100 K, M* = 0.44 0.04 M , R* = 0.44 0.03 R , and . The planet has an orbital period of 10.134588 days and a radius of RP = 0.32 0.02 RJ. Since the star is faint at V = 16.5 and J = 13.3, we are unable to obtain a measured radial velocity orbit, but we can constrain the companion mass to below about 1.7 MJ, and thus well below the planetary boundary. JS 183b (since designated as K2-95b) is the second transiting planet found with K2 that resides in a several-hundred-megayear open cluster; both planets orbit mid-M dwarf stars and are approximately Neptune sized. With a well-determined stellar density from the planetary transit, and with an independently known metallicity from its cluster membership, JS 183 provides a particularly valuable test of stellar models at the fully convective boundary. We find that JS 183 is the lowest-density transit host known at the fully convective boundary, and that its very low density is consistent with current models of stars just above the fully convective boundary but in tension with the models just below the fully convective boundary.
We present preliminary trigonometric parallaxes of 184 late-T and Y dwarfs using observations from Spitzer (143), the U.S. Naval Observatory (18), the New Technology Telescope (14), and the United ...Kingdom Infrared Telescope (9). To complete the 20 pc census of ≥T6 dwarfs, we combine these measurements with previously published trigonometric parallaxes for an additional 44 objects and spectrophotometric distance estimates for another 7. For these 235 objects, we estimate temperatures, sift into five 150 K wide Teff bins covering the range 300-1050 K, determine the completeness limit for each, and compute space densities. To anchor the high-mass end of the brown dwarf mass spectrum, we compile a list of early- to mid-L dwarfs within 20 pc. We run simulations using various functional forms of the mass function passed through two different sets of evolutionary code to compute predicted distributions in Teff. The best fit of these predictions to our L, T, and Y observations is a simple power-law model with 0.6 (where ), meaning that the slope of the field substellar mass function is in rough agreement with that found for brown dwarfs in nearby star-forming regions and young clusters. Furthermore, we find that published versions of the log-normal form do not predict the steady rise seen in the space densities from 1050 to 350 K. We also find that the low-mass cutoff to formation, if one exists, is lower than ∼5 MJup, which corroborates findings in young, nearby moving groups and implies that extremely low-mass objects have been forming over the lifetime of the Milky Way.
We report parallax measurements for 70 ultracool dwarfs (UCDs) including 11 late-M, 32 L, and 27 T dwarfs. In this sample, 14 M and L dwarfs exhibit low surface gravity features, 6 are close binary ...systems, and 2 are metal-poor subdwarfs. We combined our new measurements with 114 previously published UCD parallaxes and optical-mid-IR photometry to examine trends in spectral-type/absolute magnitude, and color-color diagrams. We report new polynomial relations between spectral type and M sub(JHK). Including resolved L/T transition binaries in the relations, we find no reason to differentiate between a "bright" (unresolved binary) and a "faint" (single source) sample across the L/T boundary. Isolating early T dwarfs, we find that the brightening of T0-T4 sources is prominent in M sub(J) where there is a 1.2-1.4 mag difference. A similar yet dampened brightening of 0.3-0.5 mag happens at M sub(H) and a plateau or dimming of -0.2 to -0.3 mag is seen in M sub(K). Comparison with evolutionary models that vary gravity, metallicity, and cloud thickness verifies that for L into T dwarfs, decreasing cloud thickness reproduces brown dwarf near-IR color-magnitude diagrams. However we find that a near constant temperature of 1200 + or -100 K along a narrow spectral subtype of T0-T4 is required to account for the brightening and color-magnitude diagram of the L-dwarf/T-dwarf transition. There is a significant population of both L and T dwarfs which are red or potentially "ultra-cloudy" compared to the models, many of which are known to be young indicating a correlation between enhanced photospheric dust and youth. For the low surface gravity or young companion L dwarfs we find that 8 out of 10 are at least 0.2-1.0mag underluminous in M sub(JH) and/or M sub(K) compared to equivalent spectral type objects. We speculate that this is a consequence of increased dust opacity and conclude that low surface gravity L dwarfs require a completely new spectral-type/absolute magnitude polynomial for analysis.
Abstract
We present 10 seasons of Sloan Digital Sky Survey
r
-band monitoring observations and five seasons of
H
-band observations of the two-image system FBQ J0951+2635 from the Kaj Strand ...Astrometric Reflector at the United States Naval Observatory, Flagstaff Station. We supplement our light curves with six seasons of monitoring data from the literature to yield a 10 + 6 season combined data set, which we analyzed with our Monte Carlo microlensing analysis routine to generate constraints on the structure of this system’s continuum emission source and the properties of the lens galaxy. Complementing our optical light curves with the five-season near-infrared light curves, we ran a joint Monte Carlo analysis to measure the size of the continuum emission region at both wavelengths, yielding log(
r
1/2
cm
−1
) =
16.24
−
0.36
+
0.33
in the
r
band and
17.04
−
0.30
+
0.26
in the
H
band at rest wavelengths of 2744 and 7254 Å, respectively, correcting for an assumed inclination angle of 60°. Modeling the accretion disk temperature profile as a power law
T
(
r
) ∝
r
−
β
, we successfully constrain the slope for FBQ J0951+2635 to
β
=
0.50
−
0.18
+
0.50
, shallower than, but nominally consistent with, the predictions of standard thin-disk theory,
β
= 0.75.
Abstract
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
)
MKO
color of 2.97 ± 0.03 mag and a
J
MKO
− W2 color of 4.93 ± 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
γ
–T0
γ
. We tentatively find that CWISE J0506+0738 is variable at 3–5
μ
m based on multiepoch 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
β
Pic moving group. A future parallax measurement will help to establish a more definitive moving group membership for this unusual object.
We present a microlensing analysis of updated light curves in three filters, the g-band, r-band, and H-band, for the gravitationally lensed quasars Q0957+561 and SBS0909+532. Both systems display ...prominent microlensing features which we analyze using our Bayesian Monte Carlo technique to constrain the quasar continuum emission region sizes in each band. We report sizes as half-light radii scaled to a 60° inclination angle. For Q0957+561 we measure , , and in g-, r-, and H-band, respectively. For SBS0909+532 we measure , , and in the g-, r-, and H-band respectively. With size measurements in three bands spanning the quasar rest frame ultraviolet to optical, we can place constraints on the scaling of accretion disk size with wavelength, . In a joint analysis of both systems we find a slope shallower than that predicted by thin disk theory, , consistent with other constraints from multi-epoch microlensing studies.
We report on the results of 15 months of monitoring the nearby field L1 dwarf WISEP J190648.47+401106.8 (W1906+40) with the Kepler mission. Supporting observations with the Karl G. Jansky Very Large ...Array and Gemini North Telescope reveal that the L dwarf is magnetically active, with quiescent radio and variable H alpha emission. A preliminary trigonometric parallax shows that W1906+40 is at a distance of 16.35 super(+0.363) sub(-0.34) pc, and all observations are consistent with W1906+40 being an old disk star just above the hydrogen-burning limit. The star shows photometric variability with a period of 8.9 hr and an amplitude of 1.5%, with a consistent phase throughout the year. We infer a radius of 0.92 + or - 0.07 R sub(J) and sin i > 0.57 from the observed period, luminosity (10 super(-3.67+ or -0.03) L sub(middot in circle)), effective temperature (2300 + or - 75 K), and v sin i (11.2 + or - 2.2 km s super(-1)). The light curve may be modeled with a single large, high latitude dark spot. Unlike many L-type brown dwarfs, there is no evidence of other variations at the gap2% level, either non-periodic or transient periodic, that mask the underlying rotation period. We suggest that the long-lived surface features may be due to starspots, but the possibility of cloud variations cannot be ruled out without further multi-wavelength observations. During the Gemini spectroscopy, we observed the most powerful flare ever seen on an L dwarf, with an estimated energy of ~1.6 x 10 super(32) erg in white light emission. Using the Kepler data, we identify similar flares and estimate that white light flares with optical/ultraviolet energies of 10 super(31) erg or more occur on W1906+40 as often as 1-2 times per month.
Through the Backyard Worlds: Planet 9 citizen science project, we have identified a wide-separation (∼10', ∼9900 au projected) substellar companion to the nearby (∼17.5 pc), mid-M dwarf Ross 19. We ...have developed a new formalism for determining chance alignment probabilities based on the BANYAN Σ 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 ± 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(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.
RELATIVE PROPER MOTIONS IN THE RHO OPHIUCHI CLUSTER Wilking, Bruce A.; Vrba, Frederick J.; Sullivan, Timothy
Astrophysical journal/The Astrophysical journal,
12/2015, Letnik:
815, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT Near-infrared images optimized for astrometry have been obtained for four fields in the high-density L 1688 cloud core over a 12 year period. The targeted regions include deeply embedded ...young stellar objects (YSOs) and very low luminosity objects too faint and/or heavily veiled for spectroscopy. Relative proper motions in R.A. and decl. were computed for 111 sources and again for a subset of 65 YSOs, resulting in a mean proper motion of (0,0) for each field. Assuming each field has the same mean proper motion, YSOs in the four fields were combined to yield estimates of the velocity dispersions in R.A. and decl. that are consistent with 1.0 km s−1. These values appear to be independent of the evolutionary state of the YSOs. The observed velocity dispersions are consistent with the dispersion in radial velocity derived for optically visible YSOs at the periphery of the cloud core and are consistent with virial equilibrium. The higher velocity dispersion of the YSOs in the plane of the sky relative to that of dense cores may be a consequence of stellar encounters due to dense cores and filaments fragmenting to form small groups of stars or the global collapse of the L 1688 cloud core. An analysis of the differential magnitudes of objects over the 12 year baseline has not only confirmed the near-infrared variability for 29 YSOs established by prior studies, but has also identified 18 new variability candidates. Four of these have not been previously identified as YSOs and may be newly identified cluster members.
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