L-type Brown Dwarfs Research Articles

Uniform Forward-modeling Analysis of Ultracool Dwarfs. III. Late-M and L Dwarfs in Young Moving Groups, the Pleiades, and the Hyades

We present a uniform forward-modeling analysis of 90 late-M and L dwarfs in nearby young (∼10–200 Myr) moving groups, the Pleiades, and the Hyades using low-resolution (R ≈ 150) near-infrared (0.9–2.4 μm) spectra and the BT-Settl model atmospheres. We derive the objects’ effective temperatures, surface gravities, radii, and masses by comparing our spectra to the models using a Bayesian framework with nested sampling and calculate the same parameters using evolutionary models. Assuming the evolutionary-based parameters are more robust, our spectroscopically inferred parameters from BT-Settl exhibit two types of systematic behavior for objects near the M-L spectral type boundary. Several objects are clustered around T eff ≈ 1800 K and logg≈5.5 dex, implying impossibly large masses (150–1400 M Jup), while others are clustered around T eff ≳ 3000 K and logg≲3.0 dex, implying unphysically low masses and unreasonably young ages. We find the fitted BT-Settl model spectra tend to overpredict the peak J- and H-band flux for objects located near the M-L boundary, suggesting the dust content included in the model atmospheres is insufficient to match the observations. By adding an interstellar medium–like reddening law to the BT-Settl model spectra, we find the fits between models and observed spectra are greatly improved, with the largest reddening coefficients occurring at the M-L transition. This work delivers a systematic examination of the BT-Settl model atmospheres and constitutes the largest spectral analysis of benchmark late-M- and L-type brown dwarfs to date.

Measurement of stellar and substellar winds using white dwarf hosts

ABSTRACT White dwarfs stars are known to be polluted by their active planetary systems, but little attention has been paid to the accretion of wind from low-mass companions. The capture of stellar or substellar wind by white dwarfs is one of few methods available to astronomers which can assess mass-loss rates from unevolved stars and brown dwarfs, and the only known method to extract their chemical compositions. In this work, four white dwarfs with closely orbiting, L-type brown dwarf companions are studied to place limits on the accretion of a substellar wind, with one case of a detection, and at an extremely non-solar abundance mNa/mCa > 900. The mass-loss rates and upper limits are tied to accretion in the white dwarfs, based on limiting cases for how the wind is captured, and compared with known cases of wind pollution from close M dwarf companions, which manifest in solar proportions between all elements detected. For wind captured in a Bondi–Hoyle flow, mass-loss limits $\dot{M}\lesssim 5\times 10^{-17}$ ${\rm M}_\odot \, {\rm yr}^{-1}$ are established for three L dwarfs, while for M dwarfs polluting their hosts, winds in the range 10−13−10−16 ${\rm M}_\odot \, {\rm yr}^{-1}$ are found. The latter compares well with the $\dot{M}\sim 10^{-13} {\!-\!} 10^{-15}$ ${\rm M}_\odot \, {\rm yr}^{-1}$ estimates obtained for nearby, isolated M dwarfs using Ly$\alpha$ to probe their astropsheres. These results demonstrate that white dwarfs are highly sensitive stellar and substellar wind detectors, where further work on the actual captured wind flow is needed.

Evidence for chromium hydride in the atmosphere of hot Jupiter WASP-31b

Context. The characterisation of exoplanet atmospheres has shown a wide diversity of compositions. Hot Jupiters have the appropriate temperatures to host metallic compounds, which should be detectable through transmission spectroscopy. Aims. We aim to detect exotic species in the transmission spectra of hot Jupiters, specifically WASP-31b, by testing a variety of chemical species to explain the spectrum. Methods. We conduct a re-analysis of publicly available transmission data of WASP-31b using the Bayesian retrieval framework TAUREX II. We retrieve various combinations of the opacities of 25 atomic and molecular species to determine the minimum set that is needed to fit the observed spectrum. Results. We report evidence for the spectroscopic signatures of chromium hydride (CrH), H2O, and K in WASP-31b. Compared to a flat model without any signatures, a CrH-only model is preferred with a statistical significance of ~3.9σ. A model consisting of both CrH and H2O is found with ~2.6 and ~3σ confidence over a CrH-only model and an H2O-only model, respectively. Furthermore, weak evidence for the addition of K is found at ~2.2σ over the H2O+CrH model, although the fidelity of the data point associated with this signature was questioned in earlier studies. Finally, the inclusion of collision-induced absorption and a Rayleigh scattering slope (indicating the presence of aerosols) is found with ~3.5σ confidence over the flat model. This analysis presents the first evidence for signatures of CrH in a hot Jupiter atmosphere. At a retrieved temperature of 1481−355+264 K, the atmosphere of WASP-31b is hot enough to host gaseous Cr-bearing species, and the retrieved abundances agree well with predictions from thermal equilibrium chemistry. Furthermore, the retrieved abundance of CrH agrees with the abundance in an L-type brown dwarf atmosphere. However, additional retrievals using VLT FORS2 data lead to a non-detection of CrH. Future observations with James Webb Space Telescope have the potential to confirm the detection and/or discover other CrH features.

WISEA J041451.67–585456.7 and WISEA J181006.18–101000.5: The First Extreme T-type Subdwarfs?

Abstract 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.

Complexity of magnetic fields on red dwarfs

Context. Magnetic fields in cool stars can be investigated by measuring Zeeman line broadening and polarization in atomic and molecular lines. Similar to the Sun, these fields are complex and height-dependent. Many molecular lines dominating M-dwarf spectra (e.g., FeH, CaH, MgH, and TiO) are temperature- and Zeeman-sensitive and form at different atmospheric heights, which makes them excellent probes of magnetic fields on M dwarfs. Aims. Our goal is to analyze the complexity of magnetic fields in M dwarfs. We investigate how magnetic fields vary with the stellar temperature and how “surface” inhomogeneities are distributed in height – the dimension that is usually neglected in stellar magnetic studies. Methods. We have determined effective temperatures of the photosphere and of magnetic features, magnetic field strengths and filling factors for nine M dwarfs (M1–M7). Our χ2 analysis is based on a comparison of observed and synthetic intensity and circular polarization profiles. Stokes profiles were calculated by solving polarized radiative transfer equations. Results. Properties of magnetic structures depend on the analyzed atomic or molecular species and their formation heights. Two types of magnetic features similar to those on the Sun have been found: a cooler (starspots) and a hotter (network) one. The magnetic field strength in both starspots and network is within 3–6 kG, on average it is 5 kG. These fields occupy a large fraction of M dwarf atmospheres at all heights, up to 100%. The plasma β is less than one, implying highly magnetized stars. Conclusions. A combination of molecular and atomic species and a simultaneous analysis of intensity and circular polarization spectra have allowed us to better decipher the complexity of magnetic fields on M dwarfs, including their dependence on the atmospheric height. This work provides an opportunity to investigate a larger sample of M dwarfs and L-type brown dwarfs.

Cloud Atlas: Discovery of Rotational Spectral Modulations in a Low-mass, L-type Brown Dwarf Companion to a Star

Abstract Observations of rotational modulations of brown dwarfs and giant exoplanets allow the characterization of condensate cloud properties. As of now, rotational spectral modulations have only been seen in three L-type brown dwarfs. We report here the discovery of rotational spectral modulations in LP261-75B, an L6-type intermediate surface gravity companion to an M4.5 star. As a part of the Cloud Atlas Treasury program, we acquired time-resolved Wide Field Camera 3 grism spectroscopy (1.1–1.69 μm) of LP261-75B. We find gray spectral variations with the relative amplitude displaying only a weak wavelength dependence and no evidence for lower-amplitude modulations in the 1.4 μm water band than in the adjacent continuum. The likely rotational modulation period is 4.78 ± 0.95 hr, although the rotational phase is not well sampled. The minimum relative amplitude in the white light curve measured over the whole wavelength range is 2.41% ± 0.14%. We report an unusual light curve, which seems to have three peaks approximately evenly distributed in rotational phase. The spectral modulations suggests that the upper atmosphere cloud properties in LP261-75B are similar to two other mid-L dwarfs of typical infrared colors, but differ from that of the extremely red L-dwarf WISE0047.

Minor Contribution of Quasars to Ionizing Photon Budget at z ∼ 6: Update on Quasar Luminosity Function at the Faint End with Subaru/Suprime-Cam

Abstract We constrain the quasar contribution to the cosmic reionization based on our deep optical survey of z ∼ 6 quasars down to z R = 24.15 using Subaru/Suprime-Cam in three UKIDSS-DXS fields covering 6.5 deg2. In Kashikawa et al. (2015), we select 17 quasar candidates and report our initial discovery of two low-luminosity quasars ( ) from seven targets, one of which might be a Lyα-emitting galaxy. From an additional optical spectroscopy, none of the four candidates out of the remaining 10 turn out to be genuine quasars. Moreover, the deeper optical photometry provided by the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) shows that, unlike the two already-known quasars, the i − z and z − y colors of the last six candidates are consistent with M- or L-type brown dwarfs. Therefore, the quasar luminosity function (QLF) measurement in the previous paper is confirmed. Compiling the QLF measurements from the literature over a wide magnitude range, including an extremely faint AGN candidate from Parsa et al. (2017), to fit them with a double power law, we find that the best-fit faint-end slope is ( ) and characteristic magnitude is ( ) in the case of two (one) quasar detection. Our result suggests that, if the QLF is integrated down to , quasars produce ∼1%–12% of the ionizing photons required to fully ionize the universe at z ∼ 6 with a confidence level, assuming that the escape fraction is and the intergalactic medium clumpy factor is C = 3. Even when the systematic uncertainties are taken into account, our result supports the scenario that quasars are the minor contributors of the reionization.

KEPLERMONITORING OF AN L DWARF I. THE PHOTOMETRIC PERIOD AND WHITE LIGHT FLARES

We report on the results of fifteen 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 $16.35^{+0.36}_{-0.34}$ 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 hours and an amplitude of 1.5%, with a consistent phase throughout the year. We infer a radius of $0.92 \pm 0.07 R_J$ and $\sin i > 0.57$ from the observed period, luminosity ($10^{-3.67 \pm 0.03} L_\odot$), effective temperature ($2300 \pm 75$K), and $v \sin i$ ($11.2 \pm 2.2$ km/s). 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 $\gtrsim 2%$ 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 $\sim 1.6 \times 10^{32}$ ergs in white light emission. Using the Kepler data, we identify similar flares and estimate that white light flares with optical/ultraviolet energies of $10^{31}$ ergs or more occur on W1906+40 as often as 1-2 times per month.

The prevalence of dust on the exoplanet HD 189733b from Hubble and Spitzer observations

The hot Jupiter HD189733b is the most extensively observed exoplanet. Its atmosphere has been detected and characterised in transmission and eclipse spectroscopy, and its phase curve measured at several wavelengths. This paper brings together results of our campaign to obtain the complete transmission spectrum of the atmosphere of this planet from UV to IR with HST, using STIS, ACS and WFC3. We provide a new tabulation of the transmission spectrum across the entire visible and IR range. The radius ratio in each wavelength band was rederived to ensure a consistent treatment of the bulk transit parameters and stellar limb-darkening. Special care was taken to correct for, and derive realistic estimates of the uncertainties due to, both occulted and unocculted star spots. The combined spectrum is very different from the predictions of cloud-free models: it is dominated by Rayleigh scattering over the whole visible and near infrared range, the only detected features being narrow Na and K lines. We interpret this as the signature of a haze of condensate grains extending over at least 5 scale heights. We show that a dust-dominated atmosphere could also explain several puzzling features of the emission spectrum and phase curves, including the large amplitude of the phase curve at 3.6um, the small hot-spot longitude shift and the hot mid-infrared emission spectrum. We discuss possible compositions and derive some first-order estimates for the properties of the putative condensate haze/clouds. We finish by speculating that the dichotomy between the two observationally defined classes of hot Jupiter atmospheres, of which HD189733b and HD209458b are the prototypes, might not be whether they possess a temperature inversion, but whether they are clear or dusty. We also consider the possibility of a continuum of cloud properties between hot Jupiters, young Jupiters and L-type brown dwarfs.

Near-infrared low-resolution spectroscopy of Pleiades L-type brown dwarfs

The fundamental properties of brown dwarfs evolve with age. Models describing the evolution of luminosities and effective temperatures, among other physical parameters, can be empirically constrained using brown dwarfs of various masses in star clusters of well determined age and metallicity. We aim to carry out a spectroscopic and photometric characterization of low-mass brown dwarfs of the ~120 Myr old Pleiades open cluster. We obtained low-resolution near-infrared spectra of the J=17.4-18.8 mag candidate L-type brown dwarfs PLIZ 28 and 35, BRB 17, 21, 23, and 29, which are Pleiades members by photometry and proper motion. We also obtained spectra of the well-known J=15.4-16.1 mag late M-type cluster members PPl 1, Teide 1, and Calar 3. We find that the former six objects have early- to mid-L spectral types and confirm previously reported M-types for the other three objects. The spectra of the L0-type BRB 17 and PLIZ 28 present a triangular H-band continuum shape, indicating that this peculiar spectral feature persists until at least the age of the Pleiades. We add to our sample 36 reported M5-L0-type cluster members, collecting their I_c - and UKIDSS ZYJHK-band photometry. We confirm a possible interleaving of the Pleiades and field L-type sequences in the JHK absolute magnitude versus spectral type diagrams, and quantify marginally redder Pleiades J-K colours, by 0.12+-0.20 mag, possibly related to both reddening and youth. Using field dwarf bolometric correction - and effective temperature - spectral type relations, we obtain the Hertzsprung-Russell diagram of the Pleiades sample. Theoretical models reproduce well the spectral sequence at M5.5-9, but appear to overestimate the luminosity or underestimate the effective temperature at L0-5. For the Pleiades early- to mid L-type brown dwarfs, we estimate theoretical masses to be in the range 0.025-0.035 M_Sol.

Brown Dwarf Model Atmospheres Based on Multi-Dimensional Radiation Hydrodynamics

AbstractThe atmospheres of Brown Dwarfs (BDs) are the site of molecular opacities and cloud formation, and control their cooling rate, radius and brightness evolution. Brown dwarfs evolve from stellar-like properties (magnetic activity, spots, flares, mass loss) to planet-like properties (electron degeneracy of the interior, cloud formation, dynamical molecular transport) while retaining, due to their fully convective interior, larger rotational velocities (≤ 30 km/s i.e. P < 4 hrs versus 11 hrs for Jupiter). Model atmospheres treating all this complexity are therefore essential to understand the evolution properties, and to interpret the observations of these objects. While the pure gas-phase based NextGen model atmospheres (Allard et al. 1997, Hauschildt et al. 1999) have allowed the understanding of the several populations of Very Low Mass Stars (VLMs), the AMES-Dusty models (Allard et al. 2001) based on equilibrium chemistry have reproduced some near-IR photometric properties of M and L-type brown dwarfs, and played a key role in the determination of the mass of brown dwarfs and Planetary Mass Objects (PMOs) in the eld and in young stellar clusters. In this paper, we present a new model atmosphere grid for VLMs, BDs, PMOs named BT-Settl, which includes a cloud model and dynamical molecular transport based on mixing information from 2D Radiation Hydrodynamic (RHD) simulations (Freytag et al. 2009). We also present the status of our 3D RHD simulations including rotation (Coriolis forces) of a cube on the surface of a brown dwarf. The BT-Settl model atmosphere grid will be available shortly via the Phoenix web simulator (http://phoenix.ens-lyon.fr/simulator/).

A search for substellar members in the Praesepe and $\mathsf{\sigma}$ Orionis clusters

We have conducted deep photometric searches for substellar members of the Praesepe (0.5-1 Gyr) and σ Orionis (3 Myr) star clusters using the Sloan i' and z' broad-band filters, with the 3.5-m and the 5-m Hale telescopes on the Calar Alto and Palomar Observatories. The total area surveyed was 1177 arcmin 2 and 1122 arcmin 2 towards the central regions of Praesepe and σ Orionis, respectively. The 5-σ detection limit of our survey is measured at i' = 24.5 and z' = 24 mag, which according to state-of-the-art evolutionary models corresponds to masses of 50-55 M jup (Praesepe) and 6 M Jup (σ Orionis), i.e., well within the substellar regime. Besides recovering previously known cluster members reported in the literature, we have identified new photometric candidates in both clusters whose masses expand the full range covered by our study. In σ Orionis, follow-up near-infrared photometry has allowed us to confirm the likely cluster membership of three newly discovered planetary-mass objects. The substellar mass function of σ Orionis, which is complete from the star-brown dwarf borderline down to 7 M Jup , rises smoothly with a slope of a = 0.6 +0.5 -0.1 (dN/dM M -α ). One of the faintest Praesepe candidates for which we have obtained follow-up near-infrared JHK s photometry closely fits the expected optical and infrared photometric sequence of the cluster. From its colors, we have estimated its spectral type to be between L4 and L6. If confirmed as a true Praesepe member, it would become the first L-type brown dwarf (50-60 M Jup ) identified in an intermediate-age star cluster. Our derivation of the Praesepe mass function, which is based on state-of-the-art evolutionary models, depends strongly on the cluster age. For the youngest possible ages (500-700 Myr), our results suggest that there is a deficit of Praesepe brown dwarfs in the central regions of the cluster, while the similarity between the Praesepe and σ Orionis mass functions increases qualitatively for models older than 800 Myr.

Emission spectra of TiH and TiD near 938 nm

High resolution, near infrared emission spectra of TiH and TiD have been recorded with a Fourier transform spectrometer. The TiH and TiD molecules were made in a titanium hollow cathode lamp operated with a mixture of neon and hydrogen or deuterium gases. A heavily perturbed band system near 938 nm is assigned as a Φ4–X 4Φ electronic transition. Line positions and approximate spectroscopic constants are reported for the 0–0 band of the new transition. The Φ4–X 4Φ TiH transition might potentially be observed in late M-type stars and in L-type brown dwarfs.

Middle-Infrared Observations of DENIS J0255-4700

The first infrared photometry of an L-type brown dwarf, DENIS J0255-4700, includes N-band and narrow-band 8.8-μm detections, with upper limits in narrow-band 10.3 and 11.7 μm. Model-independent blackbody fits of existing data yield Teff = 1250 – 1750 K, with models favoring the lower end of that range. Dusty atmospheric models by Allard, Burrows and Marley which match the near-infrared photometry are not completely consistent with our mid-infrared photometry.

Searching for weather in brown dwarfs

We have used an innovative tunable filter technique to carry out a search for the variability signatures of meteorological processes in the atmospheres of two nearby brown dwarfs. We find no evidence for variability in the L-type brown dwarf DENIS-P J1228-1547 in observations spanning approximately half a rotation period (3 h), and evidence for variability in the M-type brown dwarf LP 944-20 in multiple observations spanning approximately one third of a rotation period (1.5 h).

The First L-Type Brown Dwarf in the Pleiades

We have obtained low-resolution optical spectra of three faint brown dwarf candidates in the Pleiades open cluster. The objects observed are Roque 12 (IC=18.5), Roque 5 (IC=19.7), and Roque 25 (IC=21.2). The spectrum of Roque 25 does not show the strong TiO band heads that characterize the optical spectra of M-type stars, but molecular bands of CaH, CrH, and VO are clearly present. We classify Roque 25 as an early L-type brown dwarf. Using current theoretical evolutionary tracks we estimate that the transition from M type to L type in the Pleiades (age ~120 Myr) takes place at Teff ~2200 K or M ~0.04 M. Roque 25 is a benchmark brown dwarf in the Pleiades because it is the first known one that belongs to the L-type class. It provides evidence that the initial mass function (IMF) extends down to about 0.035 M☉ and serves as a guide for future deep searches for even less massive young brown dwarfs.