Abstract
We report multiepoch radial velocities, rotational velocities, and atmospheric parameters for 37 T-type brown dwarfs observed with Keck/NIRSPEC. Using a Markov Chain Monte Carlo ...forward-modeling method, we achieve median precisions of 0.5 and 0.9 km s
−1
for radial and rotational velocities, respectively. All of the T dwarfs in our sample are thin-disk brown dwarfs. We confirm previously reported moving group associations for four T dwarfs. However, the lack of spectral indicators of youth in two of these sources suggests that these are chance alignments. We confirm two previously unresolved binary candidates, the T0+T4.5 2MASS J11061197+2754225 and the L7+T3.5 2MASS J21265916+7617440, with orbital periods of 4 and 12 yr, respectively. We find a kinematic age of 3.5 ± 0.3 Gyr for local T dwarfs, consistent with nearby late M dwarfs (4.1 ± 0.3 Gyr). Removal of thick-disk L dwarfs in the local ultracool dwarf sample gives a similar age for L dwarfs (4.2 ± 0.3 Gyr), largely resolving the local L dwarf age anomaly. The kinematic ages of local late M, L, and T dwarfs can be accurately reproduced with population simulations incorporating standard assumptions of the mass function, star formation rate, and brown dwarf evolutionary models. A kinematic dispersion break is found at the L4–L6 subtypes, likely reflecting the terminus of the stellar main sequence. We provide a compilation of precise radial velocities for 172 late M, L, and T dwarfs within ∼20 pc of the Sun.
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 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 among 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 ≤ −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.
THE GALEX NEARBY YOUNG-STAR SURVEY Rodriguez, David R.; Zuckerman, B.; Kastner, Joel H. ...
The Astrophysical journal,
09/2013, Letnik:
774, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We describe a method that exploits data from the Galaxy Evolution Explorer (GALEX) ultraviolet and Wide-field Infrared Survey Explorer and Two Micron All Sky Survey infrared source catalogs, combined ...with proper motions and empirical pre-main sequence isochrones, to identify candidate nearby, young, low-mass stars. Applying our method across the full GALEX-covered sky, we identify 2031 mostly M-type stars that, for an assumed age of 10 (100) Myr, all lie within ~150 (~90) pc of Earth. The distribution of M spectral subclasses among these ~2000 candidate young stars peaks sharply in the range M3-M4; these subtypes constitute 50% of the sample, consistent with studies of the M star population in the immediate solar neighborhood. We focus on a subset of 58 of these candidate young M stars in the vicinity of the Tucana-Horologium association. Only 20 of these 58 candidates were detected in the ROSAT All-Sky X-ray Survey-reflecting the greater sensitivity of GALEX for the purposes of identifying active nearby, young stars, particularly for stars of type M4 and later. Based on statistical analysis of the kinematics and/or spectroscopic followup of these 58 M stars, we find that 50% (29 stars) indeed have properties consistent with Tuc-Hor membership, while 12 are potential new members of the Columba association, and 2 may be AB Dor moving group members. Hence, ~75% of our initial subsample of 58 candidates are likely members of young (age ~ 10-40 Myr) stellar moving groups within 100 pc, verifying that the stellar color- and kinematics-based selection algorithms described here can be used to efficiently isolate nearby, young, low-mass objects from among the field star population. Future studies will focus on characterizing additional subsamples selected from among this list of candidate nearby, young M stars.
Abstract
In this work, we calibrate the relationship between H
α
emission and M dwarf ages. We compile a sample of 892 M dwarfs with H
α
equivalent width (H
α
EW) measurements from the literature ...that are either comoving with a white dwarf of known age (21 stars) or in a known young association (871 stars). In this sample we identify 7 M dwarfs that are new candidate members of known associations. By dividing the stars into active and inactive categories according to their H
α
EW and spectral type (SpT), we find that the fraction of active dwarfs decreases with increasing age, and the form of the decline depends on SpT. Using the compiled sample of age calibrators, we find that H
α
EW and fractional H
α
luminosity (
L
H
α
/
L
bol
) decrease with increasing age. H
α
EW for SpT ≤ M7 decreases gradually up until ∼1 Gyr. For older ages, we found only two early M dwarfs that are both inactive and seem to continue the gradual decrease. We also found 14 mid-type M dwarfs, out of which 11 are inactive and present a significant decrease in H
α
EW, suggesting that the magnetic activity decreases rapidly after ∼1 Gyr. We fit
L
H
α
/
L
bol
versus age with a broken power law and find an index of
for ages ≲776 Myr. The index becomes much steeper at older ages, but a lack of field age-calibrators (≫1 Gyr) leaves this part of the relation far less constrained. Finally, from repeated independent measurements for the same stars, we find that 94% of them have a level of H
α
EW variability ≤5 Å at young ages (<1 Gyr).
ABSTRACT We present a new radial velocity measurement that, together with a trigonometric parallax, proper motion and signs of low gravity from the literature, confirms that SDSS J111010.01+011613.1 ...is a new T5.5 bona fide member of AB Doradus. Fitting 6000 Folded-port InfraRed Echellette spectroscopy in the 1.20-1.33 m region to BT-Settl atmosphere models yielded a radial velocity of 7.5 3.8 km s−1. At such a young age (110-130 Myr), current evolution models predict a mass of ∼10-12 , thus placing SDSS J1110+0116 well into the planetary-mass regime. We compare the fundamental properties of SDSS J1110+0116 with a sequence of seven recently identified M8-T5 brown dwarf bona fide or high-confidence candidate members of AB Doradus. We also note that its near-infrared J-K color is redder than field T5-T6 brown dwarfs, however its absolute J-band magnitude is similar to them. SDSS J1110+0116 is one of the few age-calibrated T dwarfs known to date, as well as one of the coolest bona fide members of a young moving group.
ABSTRACT Radial velocity measurements are presented for 85 late M- and L-type very low-mass stars and brown dwarfs obtained with the Magellan Echellette spectrograph. Targets primarily have distances ...within 20 pc of the Sun, with more distant sources selected for their unusual spectral energy distributions. We achieved precisions of 2-3 km s−1, and combined these with astrometric and spectrophotometric data to calculate UVW velocities. Most are members of the thin disk of the Galaxy, and velocity dispersions indicate a mean age of 5.2 0.2 Gyr for sources within 20 pc. We find signficantly different kinematic ages between late-M dwarfs (4.0 0.2 Gyr) and L dwarfs (6.5 0.4 Gyr) in our sample that are contrary to predictions from prior simulations. This difference appears to be driven by a dispersed population of unusually blue L dwarfs which may be more prevalent in our local volume-limited sample than in deeper magnitude-limited surveys. The L dwarfs exhibit an asymmetric U velocity distribution with a net inward flow, similar to gradients recently detected in local stellar samples. Simulations incorporating brown dwarf evolution and Galactic orbital dynamics are unable to reproduce the velocity asymmetry, suggesting non-axisymmetric perturbations or two distinct L dwarf populations. We also find the L dwarfs to have a kinematic age-activity correlation similar to more massive stars. We identify several sources with low surface gravities, and two new substellar candidate members of nearby young moving groups: the astrometric binary DENIS J08230313-4912012AB, a low-probability member of the β Pictoris Moving Group; and 2MASS J15104786-2818174, a moderate-probability member of the 30-50 Myr Argus Association.
We present medium-resolution optical (lambda/ Delta lambda ~ 4000) and near-infrared (lambda/ Delta lambda ~ 8000) spectral data for components of the newly discovered WISE J104915.57-531906.1 AB ...(Luhman 16AB) brown dwarf binary. Compared to the strength of K I line absorption in equivalent spectral subtype brown dwarfs, Luhman 16A is weaker while Luhman 16B is stronger. Analyzing the spectral region around each doublet in distance scaled flux units and comparing the two sources, we confirm the J-band flux reversal and find that Luhman 16B has a brighter continuum in the 1.17 mu m and 1.25 mu m regions than Luhman 16A. We calculate bolometric luminosities from observed data supplemented with best fit models for longer wavelengths and find the components are consistent within la with resultant T sub(effs) of 1310 + or - 30 K and 1280 + or - 75 K for Luhman 16AB respectively.
Abstract
We present the discovery of 34 comoving systems containing an ultracool dwarf found by means of the NOIRLab Source Catalog (NSC) DR2. NSC’s angular resolution of ∼ 1″ allows for the ...detection of small separation binaries with significant proper motions. We used the catalog’s accurate proper motion measurements to identify the companions by cross-matching a previously compiled list of brown dwarf candidates with NSC DR2. The comoving pairs consist of either a very low-mass star and an ultracool companion, or a white dwarf and an ultracool companion. The estimated spectral types of the primaries are in the K and M dwarf regimes, those of the secondaries in the M, L, and T dwarf regimes. We calculated angular separations between ∼2″ and ∼ 56″, parallactic distances between ∼43 and ∼261 pc, and projected physical separations between ∼169 and ∼8487 au. The lowest measured total proper motion is 97 mas yr
−1
, with the highest 314 mas yr
−1
. Tangential velocities range from ∼23 to ∼187 km s
−1
. We also determined comoving probabilities, estimated mass ratios, and calculated binding energies for each system. We found no indication of possible binarity for any component of the 34 systems in the published literature. The discovered systems can contribute to the further study of the formation and evolution of low-mass systems as well as to the characterization of cool substellar objects.
We present the discovery of CWISE J052306.42−015355.4, which was found as a faint, significant proper-motion object (0(\farcs)52 ± 0(\farcs)08 per yr) using machine-learning tools on the unWISE ...re-processing of time series images from the Wide-field Infrared Survey Explorer. Using the CatWISE2020 W1 and W2 magnitudes along with a J-band detection from the VISTA Hemisphere Survey, the location of CWISE J052306.42−015355.4 on the W1 − W2 versus J − W2 diagram best matches that of other known, or suspected, extreme T subdwarfs. As there is currently very little knowledge concerning extreme T subdwarfs we estimate a rough distance of ≤68 pc, which results in a tangential velocity of ≤167 km/s, both of which are tentative. A measured parallax is greatly needed to test these values. We also estimate a metallicity of −1.5 < M/H < −0.5 using theoretical predictions.
Abstract
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 2 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.
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