Radio Emission from Ultra-Cool Dwarfs

Radio Emission from Ultra-Cool Dwarfs

Author:

Williams

Abstract:

The 2001 discovery of radio emission from ultra-cool dwarfs (UCDs), the very low-mass stars and brown dwarfs with spectral types of ~M7 and later, revealed that these objects can generate and dissipate powerful magnetic fields. Radio observations provide unparalleled insight into UCD magnetism: detections extend to brown dwarfs with temperatures less than 1000 K, where no other observational probes are effective. The data reveal that UCDs can generate strong (kG) fields, sometimes with a stable dipolar structure; that they can produce and retain nonthermal plasmas with electron acceleration extending to MeV energies; and that they can drive auroral current systems resulting in significant atmospheric energy deposition and powerful, coherent radio bursts. Still to be understood are the underlying dynamo processes, the precise means by which particles are accelerated around these objects, the observed diversity of magnetic phenomenologies, and how all of these factors change as the mass of the central object approaches that of Jupiter. The answers to these questions are doubly important because UCDs are both potential exoplanet hosts, as in the TRAPPIST-1 system, and analogues of extrasolar giant planets themselves.

Exoplanets as probes of the winds of host stars: the case of the M dwarf GJ 436

Exoplanets as probes of the winds of host stars: the case of the M dwarf GJ 436

Authors:

Vidotto et al

Abstract:
Winds of cool dwarfs are difficult to observe, with only a few M dwarfs presenting observationally-derived mass-loss rates (Mdot), which span several orders of magnitude. Close-in exoplanets are conveniently positioned in the inner regions of stellar winds and can, thus, be used to probe the otherwise-unobservable local properties of their host-stars' winds. Here, we use local stellar wind characteristics observationally-derived in the studies of atmospheric evaporation of the warm-neptune GJ436 b to derive the global characteristics of the wind of its M-dwarf host. Using an isothermal wind model, we constrain the stellar wind temperature to be in the range [0.36,0.43] MK, with Mdot=[0.5,2.5] x 10^{-15} Msyn/yr. By computing the pressure balance between the stellar wind and the interstellar medium, we derive the size of the astrophere of GJ436 to be around 25 au, significantly more compact than the heliosphere. We demonstrate in this paper that transmission spectroscopy, coupled to planetary atmospheric evaporation and stellar wind models, can be a useful tool for constraining the large-scale wind structure of planet-hosting stars. Extending our approach to future planetary systems discoveries will open new perspectives for the combined characterisation of planetary exospheres and winds of cool dwarf stars.

Ultracool dwarf benchmarks with Gaia primaries

Ultracool dwarf benchmarks with Gaia primaries

Authors:

Marocco et al

Abstract:
We explore the potential of Gaia for the field of benchmark ultracool/brown dwarf companions, and present the results of an initial search for metal-rich/metal-poor systems. A simulated population of resolved ultracool dwarf companions to Gaia primary stars is generated and assessed. Of order ∼24,000 companions should be identifiable outside of the Galactic plane (|b| > 10 deg) with large-scale ground- and space-based surveys including late M, L, T, and Y types. Our simulated companion parameter space covers 0.02 ≤ M/M⊙ ≤ 0.1, 0.1 ≤ age/Gyr ≤ 14, and −2.5 ≤ [Fe/H] ≤ 0.5, with systems required to have a false alarm probability less than 10−4, based on projected separation and expected constraints on common-distance, common-proper motion, and/or common-radial velocity. Within this bulk population we identify smaller target subsets of rarer systems whose collective properties still span the full parameter space of the population, as well as systems containing primary stars that are good age calibrators. Our simulation analysis leads to a series of recommendations for candidate selection and observational follow-up that could identify ∼500 diverse Gaia benchmarks. As a test of the veracity of our methodology and simulations, our initial search uses UKIDSS and SDSS to select secondaries, with the parameters of primaries taken from Tycho-2, RAVE, LAMOST and TGAS. We identify and follow-up 13 new benchmarks. These include M8-L2 companions, with metallicity constraints ranging in quality, but robust in the range −0.39 ≤ [Fe/H] ≤ +0.36, and with projected physical separation in the range 0.6 less than s/kau less than 76. Going forward, Gaia offers a very high yield of benchmark systems, from which diverse sub-samples may be able to calibrate a range of foundational ultracool/sub-stellar theory and observation.

Hints for Small Disks around Very Low Mass Stars and Brown Dwarfs

Hints for Small Disks around Very Low Mass Stars and Brown Dwarfs

Authors:

Hendler et al

Abstract:

The properties of disks around brown dwarfs and very low mass stars (hereafter VLMOs) provide important boundary conditions on the process of planet formation and inform us about the numbers and masses of planets than can form in this regime. We use the Herschel Space Observatory PACS spectrometer to measure the continuum and [O i] 63 μm line emission toward 11 VLMOs with known disks in the Taurus and Chamaeleon I star-forming regions. We fit radiative transfer models to the spectral energy distributions of these sources. Additionally, we carry out a grid of radiative transfer models run in a regime that connects the luminosity of our sources with brighter T Tauri stars. We find that VLMO disks with sizes 1.3–78 au, smaller than typical T Tauri disks, fit well the spectral energy distributions assuming that disk geometry and dust properties are stellar mass independent. Reducing the disk size increases the disk temperature, and we show that VLMOs do not follow previously derived disk temperature–stellar luminosity relationships if the disk outer radius scales with stellar mass. Only 2 out of 11 sources are detected in [O i] despite a better sensitivity than was achieved for T Tauri stars, suggesting that VLMO disks are underluminous. Using thermochemical models, we show that smaller disks can lead to the unexpected [O i] 63 μm nondetections in our sample. The disk outer radius is an important factor in determining the gas and dust observables. Hence, spatially resolved observations with ALMA—to establish if and how disk radii scale with stellar mass—should be pursued further.

A WISE-based search for debris discs amongst M-dwarfs in nearby, young, moving groups

A WISE-based search for debris discs amongst M-dwarfs in nearby, young, moving groups

Authors:

Binks et al

Abstract:

We present a search for debris discs amongst M-dwarf members of nearby, young (5-150 Myr) moving groups (MGs) using infrared (IR) photometry, primarily from the Wide Infrared Survey Explorer (WISE). A catalogue of 100 MG M-dwarfs that have suitable WISE data is compiled and 19 of these are found to have significant IR excess emission at 22μm. Our search is likely to be complete for discs where the ratio of flux from the disc to flux from the star fd/f∗>10−3. The spectral energy distributions are supplemented with 2MASS photometry and data at longer wavelengths and fitted with simple disc models to characterise the IR excesses. There is a bimodal distribution -- twelve targets have W1−W4>3, corresponding to fd/f∗>0.02 and are likely to be gas-rich, primordial discs. The remaining seven targets have W1−W4<1 (fd/f∗≲10−3) and include three objects with previously known or suspected debris discs and four new debris disc candidates that are all members of the Beta Pic MG. All of the IR excesses are identified in stars that are likely members of MGs with age <30 Myr. The detected debris disc frequency falls from 13 to 5 per cent to <7 per cent (at 95 per cent confidence) for objects younger or older than 30 Myr respectively. This provides evidence for the evolution of debris discs on this timescale and does not support models where the maximum of debris disc emission occurs much later in lower-mass stars.

M Dwarf Stellar Companions to the Exoplanet Host Stars HD 2638 and HD 164509

Stellar Companions to the Exoplanet Host Stars HD 2638 and HD 164509

Authors:

Wittrock et al

Abstract:

An important aspect of searching for exoplanets is understanding the binarity of the host stars. It is particularly important because nearly half of the solar-like stars within our own Milky Way are part of binary or multiple systems. Moreover, the presence of two or more stars within a system can place further constraints on planetary formation, evolution, and orbital dynamics. As part of our survey of almost a hundred host stars, we obtained images at 692 nm and 880 nm bands using the Differential Speckle Survey Instrument (DSSI) at the Gemini-North Observatory. From our survey, we detect stellar companions to HD 2638 and HD 164509. The stellar companion to HD 2638 has been previously detected, but the companion to HD 164509 is a newly discovered companion. The angular separation for HD 2638 is $0.512 \pm 0.002\arcsec$ and for HD 164509 is $0.697 \pm 0.002\arcsec$. This corresponds to a projected separation of 25.6±1.9 AU and 36.5±1.9 AU, respectively. By employing stellar isochrone models, we estimate the mass of the stellar companions of HD 2638 and HD 164509 to be 0.483±0.007 $M_\sun$ and 0.416±0.007 $M_\sun$, respectively, and their effective temperatures to be 3570±8~K and 3450±7~K, respectively. These results are consistent with the detected companions being late-type M dwarfs.

Magnetic cycles in a dynamo simulation of fully convective M-star Proxima Centauri

Magnetic cycles in a dynamo simulation of fully convective M-star Proxima Centauri

Authors:

Yadav et al

Abstract:

The recent discovery of an Earth-like exoplanet around Proxima Centauri has shined a spot light on slowly rotating fully convective M-stars. When such stars rotate rapidly (period ≲20 days), they are known to generate very high levels of activity that is powered by a magnetic field much stronger than the solar magnetic field. Recent theoretical efforts are beginning to understand the dynamo process that generates such strong magnetic fields. However, the observational and theoretical landscape remains relatively uncharted for fully convective M-stars that rotate slowly. Here we present an anelastic dynamo simulation for Proxima Centauri, a representative case for slowly rotating fully connective M-stars. The rotating convection spontaneously generates strong differential rotation in the convection zone which drives coherent magnetic cycles where the axisymmetric magnetic field repeatedly changes polarity at all latitudes as time progress. The typical length of the `activity' cycle in the simulation is about nine years, in good agreement with the recently proposed activity cycle length of about seven years for Proxima Centauri. Comparing our results with earlier work, we hypothesis that the dynamo mechanism undergoes a fundamental change in nature as fully convective stars spin down with age.

Fragmentation of protoplanetary disks around M-dwarfs

Fragmentation of protoplanetary disks around M-dwarfs

Authors:

Backus et al

Abstract:

We investigate the conditions required for planet formation via gravitational instability (GI) and protoplanetary disk (PPD) fragmentation around M-dwarfs. Using a suite of 64 SPH simulations with 106 particles, the parameter space of disk mass, temperature, and radius is explored, bracketing reasonable values based on theory and observation. Our model consists of an equilibrium, gaseous, and locally isothermal disk orbiting a central star of mass M∗=Msol/3. Disks with a minimum Toomre Q of Qmin≲0.9 will fragment and form gravitationally bound clumps. Some previous literature has found Qmin

A Brown Dwarf Census from the SIMP Survey

A Brown Dwarf Census from the SIMP Survey

Authors:

Robert et al

Abstract:

We have conducted a near-infrared (NIR) proper motion survey, the Sondage Infrarouge de Mouvement Propre (SIMP), in order to discover field ultracool dwarfs (UCD) in the solar neighborhood. The survey was conducted by imaging ∼28% of the sky with the Camera PAnoramique Proche-InfraRouge (CPAPIR) both in the southern hemisphere at the Cerro Tololo Inter-American Observatory (CTIO) 1.5-m telescope, and in the northern hemisphere at the Observatoire du Mont-M\'egantic (OMM) 1.6-m telescope and comparing the source positions from these observations with the Two Micron All-Sky Survey Point Source Catalog (2MASS PSC). Additional color criteria were used to further discriminate unwanted astrophysical sources. We present the results of a NIR spectroscopic follow-up of 169 M, L and T dwarfs. Among the sources discovered are two young field brown dwarfs, six unusually red M and L dwarfs, twenty-five unusually blue M and L dwarfs, two candidate unresolved L+T binaries and twenty-four peculiar UCDs. Additionally, we add nine L/T transition dwarfs (L6-T4.5) to the already known objects.

M Dwarf AWI0005x3s has Peter Pan Protoplanetary Disk

A group of citizen scientists and professional astronomers, including Carnegie's Jonathan Gagné, joined forces to discover an unusual hunting ground for exoplanets. They found a star surrounded by the oldest known circumstellar disk--a primordial ring of gas and dust that orbits around a young star and from which planets can form as the material collides and aggregates.

Led by Steven Silverberg of University of Oklahoma, the team described a newly identified red dwarf star with a warm circumstellar disk, of the kind associated with young planetary systems. Circumstellar disks around red dwarfs like this one are rare to begin with, but this star, called AWI0005x3s, appears to have sustained its disk for an exceptionally long time. The findings are published by The Astrophysical Journal Letters.

"Most disks of this kind fade away in less than 30 million years," said Silverberg. "This particular red dwarf is a candidate member of the Carina stellar association, which would make it around 45 million years old [like the rest of the stars in that group]. It's the oldest red dwarf system with a disk we've seen in one of these associations."

link.

X-ray to Infrared Spectra of 11 M and K Stars Hosting Planets

The MUSCLES Treasury Survey III: X-ray to Infrared Spectra of 11 M and K Stars Hosting Planets

Authors:

Parke Loyd et al

Abstract:

We present a catalog of panchromatic spectral energy distributions (SEDs) for 7 M and 4 K dwarf stars that span X-ray to infrared wavelengths (5 {\AA} - 5.5 {\mu}m). These SEDs are composites of Chandra or XMM-Newton data from 5 - ~50 {\AA}, a plasma emission model from ~50 - 100 {\AA}, broadband empirical estimates from 100 - 1170 {\AA}, HST data from 1170 - 5700 {\AA}, including a reconstruction of stellar Ly{\alpha} emission at 1215.67 {\AA}, and a PHOENIX model spectrum from 5700 - 55000 {\AA}. Using these SEDs, we computed the photodissociation rates of several molecules prevalent in planetary atmospheres when exposed to each star's unattenuated flux ("unshielded" photodissociation rates) and found that rates differ among stars by over an order of magnitude for most molecules. In general, the same spectral regions drive unshielded photodissociations both for the minimally and maximally FUV active stars. However, for ozone visible flux drives dissociation for the M stars whereas NUV flux drives dissociation for the K stars. We also searched for an FUV continuum in the assembled SEDs and detected it in 5/11 stars, where it contributes around 10% of the flux in the range spanned by the continuum bands. An ultraviolet continuum shape is resolved for the star {\epsilon} Eri that shows an edge likely attributable to Si II recombination. The 11 SEDs presented in this paper, available online through the Mikulski Archive for Space Telescopes, will be valuable for vetting stellar upper-atmosphere emission models and simulating photochemistry in exoplanet atmospheres.

Trigonometric Parallaxes and Proper Motions of 134 Southern Late M, L, and T Dwarfs from the Carnegie Astrometric Planet Search Program

Trigonometric Parallaxes and Proper Motions of 134 Southern Late M, L, and T Dwarfs from the Carnegie Astrometric Planet Search Program

Authors:

Weinberger et al

Abstract:

We report trigonometric parallaxes for 134 low mass stars and brown dwarfs, of which 38 have no previously published measurement and 79 more have improved uncertainties. Our survey targeted nearby targets, so 119 are closer than 30 pc. Of the 38 stars with new parallaxes, 14 are within 20 pc and seven are likely brown dwarfs (spectral types later than L0). These parallaxes are useful for studies of kinematics, multiplicity, and spectrophotometric calibration. Two objects with new parallaxes are confirmed as young stars with membership in nearby young moving groups: LP 870-65 in AB Doradus and G 161-71 in Argus. We also report the first parallax for the planet-hosting star GJ 3470; this allows us to refine the density of its Neptune-mass planet. One T-dwarf, 2MASS J12590470-4336243, previously thought to lie within 4 pc, is found to be at 7.8 pc, and the M-type star 2MASS J01392170-3936088 joins the ranks of nearby stars as it is found to be within 10 pc. Five stars that are over-luminous and/or too red for their spectral types are identified and deserve further study as possible young stars.

OGLE-2012-BLG-0724Lb: a Saturn Class Gas Giant Orbitng a M Dwarf Star

OGLE-2012-BLG-0724Lb: A Saturn-mass Planet around an M-dwarf

Authors:

Hirao et al

Abstract:

We report the discovery of a planet by the microlensing method, OGLE-2012-BLG-0724Lb. Although the duration of the planetary signal for this event was one of the shortest seen for a planetary event, the anomaly was well covered thanks to high cadence observations taken by the survey groups OGLE and MOA. By analyzing the light curve, this planetary system is found to have a mass ratio q=(1.58±0.15)×10−3. By conducting a Bayesian analysis, we estimate that the host star is an M-dwarf star with a mass of ML=0.29+0.33−0.16 M⊙ located at DL=6.7+1.1−1.2 kpc away from the Earth and the companion's mass is mP=0.47+0.54−0.26 MJup. The projected planet-host separation is a⊥=1.6+0.4−0.3 AU. Because the lens-source relative proper motion is relatively high, future high resolution images would detect the lens host star and determine the lens properties uniquely. This system is likely a Saturn-mass exoplanet around an M-dwarf and such systems are commonly detected by gravitational microlensing. This adds an another example of a possible pileup of sub-Jupiters (0.2 less than mP/MJup less than 1) in contrast to a lack of Jupiters (∼1−2 MJup) around M-dwarfs, supporting the prediction by core accretion models that Jupiter-mass or more massive planets are unlikely to form around M-dwarfs.

EPIC 205117205b: a Neptune Class Exoplanet Orbiting a pre Main Sequence Star

Zodiacal Exoplanets in Time (ZEIT) III: A Neptune-sized planet orbiting a pre-main-sequence star in the Upper Scorpius OB Association

Authors:

Mann et al

Abstract:

We confirm and characterize a close-in (Porb = 5.425 days), super-Neptune sized (5.04+0.34−0.37 Earth radii) planet transiting EPIC 205117205 (2MASS J16101473-1919095), a late-type (M3) pre-main sequence (≃11 Myr-old) star in the Upper Scorpius subgroup of the Scorpius-Centaurus OB association. The host star has the kinematics of a member of the Upper Scorpius OB association, and its spectrum contains lithium absorption, an unambiguous sign of youth (less than 20 Myr) in late-type dwarfs. We combine photometry from K2 and the ground-based MEarth project to refine the planet's properties and constrain the average stellar density. We determine EPIC 205117205's bolometric flux and effective temperature from moderate resolution spectra. By utilizing isochrones that include the effects of magnetic fields, we derive a precise (6-7%) radius and mass for the host star, and a stellar age consistent with the established value for Upper Scorpius. Follow-up high-resolution imaging and Doppler spectroscopy confirm that the transiting object is not a stellar companion or a background eclipsing binary blended with the target. The shape of the transit, the constancy of the transit depth and periodicity over 1.5 years, and the independence with wavelength rules out stellar variability, or a dust cloud or debris disk partially occulting the star as the source of the signal; we conclude it must instead be planetary in origin. The existence of EPIC 205117205b suggests close-in planets can form in situ or migrate within ≃10 Myr, e.g., via interactions with a disk, and that long-timescale dynamical migration such as by Lidov-Kozai or planet-planet scattering is not responsible for all short-period planets.

How Difficult is it to Detect Habitable Planets Around M Dwarfs?

The Impact of Stellar Rotation on the Detectability of Habitable Planets Around M Dwarfs

Authors:

Newton et al

Abstract:

Stellar activity and rotation frustrate the detection of exoplanets through the radial velocity technique. This effect is particularly of concern for M dwarfs, which can remain magnetically active for billions of years. We compile rotation periods for late-type stars and for the M dwarf planet-host sample in order to investigate the rotation periods of older field stars across the main sequence. We show that for stars with masses between 0.25 and 0.5 solar masses (M4V to M1V), the stellar rotation period typical of field stars coincides with the orbital periods of planets in the habitable zone. This will pose a fundamental challenge to the discovery and characterization of potentially habitable planets around early M dwarfs. Due to the longer rotation periods reached by mid M dwarfs and the shorter orbital period at which the planetary habitable zone is found, stars with masses between 0.1 and 0.25 solar masses (M6V to M4V) offer better opportunities for the detection of habitable planets via radial velocities.

Intrinsic Lyman Alpha and Extreme Ultraviolet Spectra of K and M Dwarfs with Exoplanets

The MUSCLES Treasury Survey II: Intrinsic Lyman Alpha and Extreme Ultraviolet Spectra of K and M Dwarfs with Exoplanets

Authors:

Youngblood et al

Abstract:

The ultraviolet (UV) spectral energy distributions of low-mass (K- and M-type) stars play a critical role in the heating and chemistry of exoplanet atmospheres, but are not observationally well-constrained. Direct observations of the intrinsic flux of the Lyman alpha line (the dominant source of UV photons from low-mass stars) are challenging, as interstellar HI absorbs the entire line core for even the closest stars. To address the existing gap in empirical constraints on the UV flux of K and M dwarfs, the MUSCLES HST Treasury Survey has obtained UV observations of 11 nearby M and K dwarfs hosting exoplanets. This paper presents the Lyman alpha and extreme-UV spectral reconstructions for the MUSCLES targets. Most targets are optically inactive, but all exhibit significant UV activity. We use a Markov Chain Monte Carlo technique to correct the observed Lyman alpha profiles for interstellar absorption, and we employ empirical relations to compute the extreme-UV spectral energy distribution from the intrinsic Lyman alpha flux in ~100 {\AA} bins from 100-1170 {\AA}. The reconstructed Lyman alpha profiles have 300 km/s broad cores, while greater than 1% of the total intrinsic Lyman alpha flux is measured in extended wings between 300 km/s to 1200 km/s. The Lyman alpha surface flux positively correlates with the MgII surface flux and negatively correlates with the stellar rotation period. Stars with larger Lyman alpha surface flux also tend to have larger surface flux in ions formed at higher temperatures, but these correlations remain statistically insignificant in our sample of 11 stars. We also present HI column density measurements for 10 new sightlines through the local interstellar medium.

M Dwarf and Brown Dwarf Stars in the TW Hya Association: Stellar X-rays and Disk Dissipation

M Stars in the TW Hya Association: Stellar X-rays and Disk Dissipation

Authors:

Kastner et al

Abstract:

To investigate the potential connection between the intense X-ray emission from young, low-mass stars and the lifetimes of their circumstellar, planet-forming disks, we have compiled the X-ray luminosities (LX) of M stars in the ∼8 Myr-old TW Hya Association (TWA) for which X-ray data are presently available. Our investigation includes analysis of archival Chandra data for the TWA binary systems TWA 8, 9, and 13. Although our study suffers from poor statistics for stars later than M3, we find a trend of decreasing LX/Lbol with decreasing Teff for TWA M stars wherein the earliest-type (M0--M2) stars cluster near log(LX/Lbol)≈−3.0 and then log(LX/Lbol) decreases, and its distribution broadens, for types M4 and later. The fraction of TWA stars that display evidence for residual primordial disk material also sharply increases in this same (mid-M) spectral type regime. This apparent anticorrelation between the relative X-ray luminosities of low-mass TWA stars and the longevities of their circumstellar disks suggests that primordial disks orbiting early-type M stars in the TWA have dispersed rapidly as a consequence of their persistent large X-ray fluxes. Conversely, the disks orbiting the very lowest-mass pre-MS stars and pre-MS brown dwarfs in the Association may have survived because their X-ray luminosities and, hence, disk photoevaporation rates are very low to begin with, and then further decline relatively early in their pre-MS evolution.

The Size and Shape of the Milky Way Disk and Halo from M-type Brown Dwarfs in the BoRG Survey

The Size and Shape of the Milky Way Disk and Halo from M-type Brown Dwarfs in the BoRG Survey

Authors:

van Vledder et al

Abstract:

We have identified 274 M-type Brown Dwarfs in the Hubble Space Telescope's Wide Field Camera 3 (WFC3) pure parallel fields from the Brightest of Reionizing Galaxies (BoRG) survey for high redshift galaxies. These are near-infrared observations with multiple lines-of-sight out of our Milky Way. Using these observed M-type Brown Dwarfs we fitted a Galactic disk and halo model with a Markov chain Monte Carlo (MCMC) analysis. This model worked best with the scale length of the disk fixed at h = 2.6 kpc. For the scale height of the disk, we found z0=0.29+0.02−0.019 kpc and for the central number density ρ0=0.29+0.20−0.13 \#/pc3. For the halo we derived a flattening parameter κ = 0.45±0.04 and a power-law index p = 2.4±0.07. We found the fraction of M-type brown dwarfs in the local density that belong to the halo to be fh = 0.0075+0.0025−0.0019. We found no correlation between subtype of M-dwarf and any model parameters.

The total number of M-type Brown Dwarfs in the disk and halo was determined to be 58.2+9.81−6.70×109. We found an upper limit for the fraction of M-type Brown Dwarfs in the halo of 7+5−4\%. The upper limit for the total Galactic Disk mass in M-dwarfs is 4.34+0.73−0.5×109 M⊙, assuming all M-type Brown Dwarfs have a mass of 80MJ.

OGLE-2014-BLG-0257L: A Brown Dwarf in Binary With a low Mass M Dwarf

OGLE-2014-BLG-0257L: A Microlensing Brown Dwarf Orbiting a Low-mass M Dwarf

Authors:

Han et al

Abstract:

In this paper, we report the discovery of a binary composed of a brown dwarf and a low-mass M dwarf from the observation of the microlensing event OGLE-2014-BLG-0257. Resolution of the very short-lasting caustic crossing combined with the detection of subtle continuous deviation in the lensing light curve induced by the Earth's orbital motion enable us to precisely measure both the Einstein radius \theta_E and the lens parallax pi_E, which are the two quantities needed to unambiguously determine the mass and distance to the lens. It is found that the companion is a substellar brown dwarf with a mass 0.036 +/- 0.005 Msun (37.7 +/- 5.2\ M_J) and it is orbiting an M dwarf with a mass 0.19 +/- 0.02 Msun. The binary is located at a distance 1.25 +/- 0.13 kpc toward the Galactic bulge and the projected separation between the binary components is 0.61 +/- 0.07 AU. The separation scaled by the mass of the host is 3.2 AU/Msun. Under the assumption that separations scale with masses, then, the discovered brown dwarf is located in the zone of the brown dwarf desert. With the increasing sample of brown dwarfs existing in various environments, microlensing will provide a powerful probe of brown dwarfs in the Galaxy.

"Hot" Brown Dwarfs may not be Possible Around Stars Larger Than M Dwarfs

Can brown dwarfs survive on close orbits around convective stars?

Authors:

Damiani et al

Abstract:

Brown dwarfs straddle the mass range transition from planetary to stellar objects. There is a relative paucity of brown dwarfs companions around FGKM stars compared to exoplanets for orbital periods less than a few years, but most of the short-period brown dwarf companions fully characterised by transits and radial velocities are found around F-type stars. We examine the hypothesis that brown dwarf companions could not survive on close orbit around stars with important convective envelopes because the tides and angular momentum loss through magnetic breaking should lead to a rapid orbital decay and quick engulfment of the companion. We use a classical Skumanich-type braking law, and constant time-lag tidal theory to assess the characteristic timescale for orbital decay for the brown dwarf mass range as a function of the host properties. We find that F-type stars may host massive companions for a significantly longer time than G-type stars for a given orbital period, which may explain the paucity of G-type hosts for brown dwarfs with orbital period less than 5 days. On the other hand, we show that the small radius of early M-type stars contributes to orbital decay timescales that are only half those of F-type stars, despite their more efficient tidal dissipation and magnetic braking. For fully convective later type M-dwarfs, orbital decay timescales could be orders of magnitude greater than for F-type stars. For orbital periods greater than 10 days, brown dwarf occurrence should largely be unaffected by tidal decay, whatever the mass of the host. On closer orbital periods, the rapid engulfment of massive companions could explain the lack of G and K-type hosts in the sample of known systems with transiting brown dwarfs. However, the paucity of M-type hosts can not be an effect of tidal decay alone, but may be the result of a selection effect in the sample and/or the formation mechanism.