Search

The spectra of brown dwarfs are key to exploring the chemistry and physics that take place in their atmospheres. Late-T dwarf spectra are particularly diagnostic, due to their relatively cloud-free atmospheres and deep molecular bands. With the use of powerful atmospheric retrieval tools applied to the spectra of these objects, direct constraints on molecular/atomic abundances, gravity, and vertical thermal profiles can be obtained, enabling a broad exploration of the chemical/physical mechanisms operating in their atmospheres. We present a uniform retrieval analysis on low-resolution Infrared Telescope Facility SpeX near-infrared spectra for a sample of 50 T dwarfs, including new observations as part of a recent volume-limited survey. This analysis more than quadruples the sample of T dwarfs with retrieved temperature profiles and abundances (H2O, CH4, NH3, K, and subsequent C/O and metallicities). We are generally able to constrain the effective temperatures to within 50 K, the volume mixing ratios for major species to within 0.25 dex, the atmospheric metallicities [M/H] to within 0.2, and the C/O ratios to within 0.2. We compare our retrieved constraints on the thermal structures, chemistry, and gravities of these objects with predictions from self-consistent radiative-convective equilibrium models and find, in general, though with substantial scatter, consistency with solar composition chemistry and the thermal profiles of the neighboring stellar FGK population. Objects with notable discrepancies between the two modeling techniques and potential mechanisms for their differences, be they related to the modeling approach or physically motivated, are discussed more thoroughly in the text. [ABSTRACT FROM AUTHOR]

We present the third discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) program, the COCONUTS-3 system, composed of the young M5 primary star UCAC4 374âˆ'046899 and the very red L6 dwarf WISEA J081322.19âˆ'152203.2. These two objects have a projected separation of 61 ′ ′ (1891 au) and are physically associated given their common proper motions and estimated distances. The primary star, COCONUTS-3A, has a mass of 0.123 ± 0.006 M ⊙, and we estimate its age as 100 Myr to 1 Gyr based on its stellar activity (via H α and X-ray emission), kinematics, and spectrophotometric properties. We derive its bulk metallicity as 0.21 ± 0.07 dex using empirical calibrations established by older and higher-gravity M dwarfs and find that this [Fe/H] could be slightly underestimated according to PHOENIX models given COCONUTS-3A’s younger age. The companion, COCONUTS-3B, has a near-infrared spectral type of L6 ± 1 int-g, and we infer physical properties of T eff = 1362 âˆ' 73 + 48 K, log (g) = 4.96 âˆ' 0.34 + 0.15 dex, R = 1.03 âˆ' 0.06 + 0.12 R Jup, and M = 39 âˆ' 18 + 11 M Jup using its bolometric luminosity, its host star’s age, and hot-start evolution models. We construct cloudy atmospheric model spectra at the evolution-based physical parameters and compare them to COCONUTS-3B’s spectrophotometry. We find that this companion possesses ample condensate clouds in its photosphere (f sed = 1) with the dataâ€"model discrepancies likely due to the models using an older version of the opacity database. Compared to field-age L6 dwarfs, COCONUTS-3B has fainter absolute magnitudes and a 120 K cooler T eff. Also, the J âˆ' K color of this companion is among the reddest for ultracool benchmarks with ages older than a few hundred megayears. COCONUTS-3 likely formed in the same fashion as stellar binaries given the companion-to-host mass ratio of 0.3 and represents a valuable benchmark to quantify the systematics of substellar model atmospheres. [ABSTRACT FROM AUTHOR]

Over the course of the third observing run of the LIGOâ€"Virgoâ€"KAGRA Collaboration, several gravitational-wave (GW) neutron starâ€"black hole (NSBH) candidates have been announced. By assuming that these candidates are real signals with astrophysical origins, we analyze the population properties of the mass and spin distributions for GW NSBH mergers. We find that the primary BH mass distribution of NSBH systems, whose shape is consistent with that inferred from the GW binary BH (BBH) primaries, can be well described as a power law with an index of α = 4.8 âˆ' 2.8 + 4.5 plus a high-mass Gaussian component peaking at ∼ 33 âˆ' 9 + 14 M ⊙ . The NS mass spectrum could be shaped as a nearly flat distribution between ∼1.0 and 2.1 M ⊙. The constrained NS maximum mass agrees with that inferred from NSs in our Galaxy. If GW190814 and GW200210 are NSBH mergers, the posterior results of the NS maximum mass would be always larger than ∼2.5 M ⊙ and significantly deviate from that inferred in Galactic NSs. The effective inspiral spin and effective precession spin of GW NSBH mergers are measured to potentially have near-zero distributions. The negligible spins for GW NSBH mergers imply that most events in the universe should be plunging events, which support the standard isolated formation channel of NSBH binaries. More NSBH mergers to be discovered in the fourth observing run would help to more precisely model the population properties of cosmological NSBH mergers. [ABSTRACT FROM AUTHOR]

We present detailed simulations of the kilonova and gamma-ray burst (GRB) afterglow and kilonova luminosity function from black hole–neutron star (BH–NS) mergers, and discuss the detectability of an electromagnetic (EM) counterpart in connection with gravitational wave (GW) detections, GW-triggered target-of-opportunity observations, and time-domain blind searches. The predicted absolute magnitude of BH–NS kilonovae at 0.5 days after the merger falls in the range [−10, −15.5]. The simulated luminosity function contains potential information on the viewing-angle distribution of the anisotropic kilonova emission. We simulate the GW detection rates, detectable distances, and signal duration for future networks of 2nd/2.5th/3rd generation GW detectors. BH–NSs tend to produce brighter kilonovae and afterglows if the BH has a higher aligned spin, and a less massive NS with a stiffer equation of state. The detectability of kilonovae is especially sensitive to the BH spin. If BHs typically have low spins, the BH–NS EM counterparts are hard to discover. For 2nd generation GW detector networks, a limiting magnitude of mlimit ∼ 23–24 mag is required to detect kilonovae even if high BH spin is assumed. Thus, a plausible explanation for the lack of BH–NS-associated kilonova detection during LIGO/Virgo O3 is that either there is no EM counterpart (plunging events) or the current follow-ups are too shallow. These observations still have the chance to detect the on-axis jet afterglow associated with a short GRB or an orphan afterglow. Follow-up observations can detect possible associated short GRB afterglows, from which kilonova signatures may be studied. For time-domain observations, a high-cadence search in redder filters is recommended to detect more BH–NS-associated kilonovae and afterglows. [ABSTRACT FROM AUTHOR]

We present a forward-modeling framework using the Bayesian inference tool Starfish and cloudless Sonora-Bobcat model atmospheres to analyze low-resolution (R ≈ 80–250) near-infrared (1.0–2.5 μm) spectra of T dwarfs. Our approach infers effective temperatures, surface gravities, metallicities, radii, and masses, and by accounting for uncertainties from model interpolation and correlated residuals due to instrumental effects and modeling systematics, produces more realistic parameter posteriors than traditional (χ2-based) spectral-fitting analyses. We validate our framework by fitting the model atmospheres themselves and finding negligible offsets between derived and input parameters. We apply our methodology to three well-known benchmark late-T dwarfs, HD 3651B, GJ 570D, and Ross 458C, using both solar and nonsolar metallicity atmospheric models. We also derive these benchmarks' physical properties using their bolometric luminosities, their primary stars' ages and metallicities, and Sonora-Bobcat evolutionary models. Assuming the evolutionary-based parameters are more robust, we find our atmospheric-based, forward-modeling analysis produces two outcomes. For HD 3615B and GJ 570D, spectral fits provide accurate Teff and R but underestimated (by ≈1.2 dex) and Z (by ≈0.35 dex), likely due to the systematics from modeling the potassium line profiles. For Ross 458C, spectral fits provide accurate and Z but overestimated Teff (by ≈120 K) and underestimated R (by ≈1.6×), likely because our model atmospheres lack clouds, reduced vertical temperature gradients, or disequilibrium processes. Finally, the spectroscopically inferred masses of these benchmarks are all considerably underestimated. [ABSTRACT FROM AUTHOR]

We present a search for new planetary-mass members of nearby young moving groups (YMGs) using astrometry for 694 T and Y dwarfs, including 447 objects with parallaxes, mostly produced by recent large parallax programs from UKIRT and Spitzer. Using the BANYAN Σ and LACEwING algorithms, we identify 30 new candidate YMG members, with spectral types of T0–T9 and distances of 10–43 pc. Some candidates have unusually red colors and/or faint absolute magnitudes compared to field dwarfs with similar spectral types, providing supporting evidence for their youth, including four early-T dwarfs. We establish one of these, the variable T1.5 dwarf 2MASS J21392676+0220226, as a new planetary-mass member (MJup) of the Carina-Near group (200 ± 50 Myr) based on its full six-dimensional kinematics, including a new parallax measurement from CFHT. The high-amplitude variability of this object is suggestive of a young age, given the coexistence of variability and youth seen in previously known YMG T dwarfs. Our four latest-type (T8–T9) YMG candidates, WISE J031624.35+430709.1, ULAS J130217.21+130851.2, WISEPC J225540.74–311841.8, and WISE J233226.49–432510.6, if confirmed, will be the first free-floating planets (≈2–6 MJup) whose ages and luminosities are compatible with both hot-start and cold-start evolutionary models, and thus overlap with the properties of the directly imaged planet 51 Eri b. Several of our early/mid-T candidates have peculiar near-infrared spectra, indicative of heterogenous photospheres or unresolved binarity. Radial velocity measurements needed for final membership assessment for most of our candidates await upcoming 20–30 m class telescopes. In addition, we compile all 15 known T7–Y1 benchmarks and derive a homogeneous set of their effective temperatures, surface gravities, radii, and masses. [ABSTRACT FROM AUTHOR]

The LIGO/Virgo and Fermi collaborations recently reported a possible joint detection of a subthreshold gravitational-wave (GW) event and a subthreshold gamma-ray burst (GRB), GBM-190816, that occurred 1.57 s after the merger. We perform an independent analysis of the publicly available data and investigate the physical implications of this potential association. By carefully studying the following properties of GBM-190816 using Fermi/GBM data, including signal-to-noise ratio, duration, f-parameter, spectral properties, energetic properties, and its compliance with some GRB statistical correlations, we confirm that this event is likely a typical short GRB. Assuming its association with the subthreshold GW event, the inferred luminosity is erg s−1. Based on the available information of the subthreshold GW event, we infer the mass ratio q of the compact binary as (90% confidence interval) according to the reported range of luminosity distance. If the heavier compact object has a mass >3 solar masses, q can be further constrained to. The leading physical scenario invokes an NS–BH merger system with the NS tidally disrupted. Within this scenario, we constrain the physical properties of such a system (including mass ratio q, the spin parameters, and the observer's viewing angle) to produce a GRB. The GW data may also allow an NS–BH system with no tidal disruption of the NS (the plunge events) or a BH–BH merger. We apply the charged compact binary coalescence theory (for both a constant charge and an increasing charge for the merging members) to derive the model parameters to account for GBM-190816 and found that the required parameters are extreme. Finally, we argue that the fact that the observed GW–GRB delay timescale is comparable to that of GW170817/GRB 170817A suggests that the GW–GRB time delay of these two cases is mainly defined by the timescale for the jet to propagate to the energy dissipation/GRB emission site. [ABSTRACT FROM AUTHOR]

Given its relative proximity (∼430 pc), compact size (<20′), young age (∼0.5 Myr), and rich number of young stellar objects, the Serpens-South star-forming region is a promising site for studying young substellar objects, yet the low-mass members of this region remain largely undiscovered. In this article we report on a deep photometric survey using a custom 1.45 μm filter (W band), as well as standard J and H near-IR filters, in order to identify candidate low-mass young brown dwarfs in the Serpens-South region. We constructed a reddening-insensitive index (Q) by combining J, H and W-band photometry for survey objects, in order to identify candidate low-mass members of Serpens based on the strength of the water-absorption feature at 1.45 μm in the atmospheres of mid-M and later objects. We then conducted spectroscopic follow-up to confirm youth and spectral type for our candidates. This is the first survey to identify the very low-mass and coolest members of Serpens-South. We identify four low-mass candidate Serpens members, which all display IR excess emission, indicating the likely presence of circumstellar disks around them. One of the four candidate low-mass members in our list, SERP182918-020245, exhibits Paβ and Brγ emission features, confirming its youth and ongoing magnetospheric accretion. Our new candidate members have spectral types >M4 and are the coolest and lowest mass candidate members yet identified in Serpens-South. [ABSTRACT FROM AUTHOR]

We present the first discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) program, a large-scale survey for wide-orbit planetary and substellar companions. We have discovered a comoving system COCONUTS-1, composed of a hydrogen-dominated white dwarf (PSO J058.9855+45.4184; d = 31.5 pc) and a T4 companion (PSO J058.9869+45.4296) at a 40.″6 (1280 au) projected separation. We derive physical properties for COCONUTS-1B from (1) its near-infrared spectrum using cloudless Sonora atmospheric models, and (2) its luminosity and the white dwarf’s age ( Gyr) using Sonora evolutionary models. The two methods give consistent temperatures and radii, but atmospheric models infer a lower surface gravity and therefore an unphysically young age. Assuming evolutionary model parameters ( K, dex, ), we find that cloudless model atmospheres have brighter Y- and J-band fluxes than the data, suggesting that condensate clouds have not fully dispersed around 1300 K. The W2 flux (4.6 μm) of COCONUTS-1B is fainter than models, suggesting non-equilibrium mixing of CO. To investigate the gravity dependence of the L/T transition, we compile all 60 known L6−T6 benchmarks and derive a homogeneous set of temperatures, surface gravities, and masses. As is well known, young, low-gravity late-L dwarfs have significantly fainter, redder near-infrared photometry and ≈200–300 K cooler temperatures than old, high-gravity objects. Our sample now reveals such gravity dependence becomes weaker for T dwarfs, with young objects having comparable near-infrared photometry and ≈100 K cooler temperatures compared to old objects. Finally, we find that young objects have a larger amplitude J-band brightening than old objects, and also brighten at H band as they cross the L/T transition. [ABSTRACT FROM AUTHOR]

We are conducting a proper-motion survey for young brown dwarfs in the Taurus-Auriga molecular cloud based on the Pan-STARRS1 3π Survey. Our search uses multi-band photometry and astrometry to select candidates, and is wider (370 deg2) and deeper (down to ≈3 MJup) than previous searches. We present here our search methods and spectroscopic follow-up of our high-priority candidates. Since extinction complicates spectral classification, we have developed a new approach using low-resolution (R ≈ 100) near-infrared spectra to quantify reddening-free spectral types, extinctions, and gravity classifications for mid-M to late-L ultracool dwarfs (≲100–3 MJup in Taurus). We have discovered 25 low-gravity (vl-g) and the first 11 intermediate-gravity (int-g) substellar (M6–L1) members of Taurus, constituting the largest single increase of Taurus brown dwarfs to date. We have also discovered 1 new Pleiades member and 13 new members of the Perseus OB2 association, including a candidate very wide separation (58 kau) binary. We homogeneously reclassify the spectral types and extinctions of all previously known Taurus brown dwarfs. Altogether our discoveries have thus far increased the substellar census in Taurus by ≈40% and added three more L-type members (≲5–10 MJup). Most notably, our discoveries reveal an older (>10 Myr) low-mass population in Taurus, in accord with recent studies of the higher-mass stellar members. The mass function appears to differ between the younger and older Taurus populations, possibly due to incompleteness of the older stellar members or different star formation processes. [ABSTRACT FROM AUTHOR]

Neutron stars may sustain a non-axisymmetric deformation due to magnetic distortion and are potential sources of continuous gravitational waves (GWs) for ground-based interferometric detectors. With decades of searches using available GW detectors, no evidence of a GW signal from any pulsar has been observed. Progressively stringent upper limits of ellipticity have been placed on Galactic pulsars. In this work, we use the ellipticity inferred from the putative millisecond magnetars in short gamma-ray bursts (SGRBs) to estimate their detectability by current and future GW detectors. For ∼1 ms magnetars inferred from the SGRB data, the detection horizon is ∼30 Mpc and ∼600 Mpc for the advanced LIGO (aLIGO) and Einstein Telescope (ET), respectively. Using the ellipticity of SGRB millisecond magnetars as calibration, we estimate the ellipticity and GW strain of Galactic pulsars and magnetars assuming that the ellipticity is magnetic-distortion-induced. We find that the results are consistent with the null detection results of Galactic pulsars and magnetars with the aLIGO O1. We further predict that the GW signals from these pulsars/magnetars may not be detectable by the currently designed aLIGO detector. The ET detector may be able to detect some relatively low-frequency signals (<50 Hz) from some of these pulsars. Limited by its design sensitivity, the eLISA detector seems to not be suitable for detecting the signals from Galactic pulsars and magnetars. [ABSTRACT FROM AUTHOR]

We present the discovery of eight young M7–L2 dwarfs in the Taurus star-forming region and the Scorpius–Centaurus OB Association, serendipitously found during a wide-field search for L/T transition dwarfs using Pan-STARRS1 (optical) and WISE (mid-infrared) photometry. We identify PSO J060.3200+25.9644 (near-infrared spectral type L1) and PSO J077.1033+24.3809 (L2) as new members of Taurus based on their vl-g gravity classifications, the consistency of their photometry and proper motions with previously known Taurus objects, and the low probability of contamination by field objects. PSO J077.1033+24.3809 is the coolest substellar member of Taurus found to date. Both Taurus objects are among the lowest-mass free-floating objects ever discovered, with estimated masses ≈6 , and provide further evidence that isolated planetary-mass objects can form as part of normal star formation processes. PSO J060.3200+25.9644 (a.k.a. DANCe J040116.80+255752.2) was previously identified as a likely member of the Pleiades (age ) based on photometry and astrometry, but its vl-g gravity classification and near-infrared photometry imply a much younger age and thus point to Taurus membership. We have also discovered six M7–L1 dwarfs in outlying regions of Scorpius–Centaurus with photometry, proper motions, and low-gravity spectral signatures consistent with membership. These objects have estimated masses ≈15–36 . The M7 dwarf, PSO J237.1470−23.1489, shows excess mid-infrared flux implying the presence of a circumstellar disk. Finally, we present catalogs of Pan-STARRS1 proper motions for low-mass members of Taurus and Upper Scorpius with median precisions of ≈3 mas yr−1, including 67 objects with no previous proper motion and 359 measurements that improve on literature values. [ABSTRACT FROM AUTHOR]