Mass Brown Dwarfs Research Articles

THE SOLAR NEIGHBORHOOD. XXXIV. A SEARCH FOR PLANETS ORBITING NEARBY M DWARFS USING ASTROMETRY

Astrometric measurements are presented for seven nearby stars with previously detected planets: six M dwarfs (GJ 317, GJ 667C, GJ 581, GJ 849, GJ 876, and GJ 1214) and one K dwarf (BD $-$10 3166). Measurements are also presented for six additional nearby M dwarfs without known planets, but which are more favorable to astrometric detections of low mass companions, as well as three binary systems for which we provide astrometric orbit solutions. Observations have baselines of three to thirteen years, and were made as part of the RECONS long-term astrometry and photometry program at the CTIO/SMARTS 0.9m telescope. We provide trigonometric parallaxes and proper motions for all 16 systems, and perform an extensive analysis of the astrometric residuals to determine the minimum detectable companion mass for the 12 M dwarfs not having close stellar secondaries. For the six M dwarfs with known planets, we are not sensitive to planets, but can rule out the presence of all but the least massive brown dwarfs at periods of 2 - 12 years. For the six more astrometrically favorable M dwarfs, we conclude that none have brown dwarf companions, and are sensitive to companions with masses as low as 1 $M_{Jup}$ for periods longer than two years. In particular, we conclude that Proxima Centauri has no Jovian companions at orbital periods of 2 - 12 years. These results complement previously published M dwarf planet occurrence rates by providing astrometrically determined upper mass limits on potential super-Jupiter companions at orbits of two years and longer. As part of a continuing survey, these results are consistent with the paucity of super-Jupiter and brown dwarf companions we find among the over 250 red dwarfs within 25 pc observed longer than five years in our astrometric program.

A STATISTICAL ANALYSIS OF SEEDS AND OTHER HIGH-CONTRAST EXOPLANET SURVEYS: MASSIVE PLANETS OR LOW-MASS BROWN DWARFS?

We conduct a statistical analysis of a combined sample of direct imaging data, totalling nearly 250 stars. The stars cover a wide range of ages and spectral types, and include five detections ($\kappa$ And b, two $\sim$60 M$_{\rm J}$ brown dwarf companions in the Pleiades, PZ Tel B, and CD$-$35 2722B). For some analyses we add a currently unpublished set of SEEDS observations, including the detections GJ 504b and GJ 758B. We conduct a uniform, Bayesian analysis of all stellar ages using both membership in a kinematic moving group and activity/rotation age indicators. We then present a new statistical method for computing the likelihood of a substellar distribution function. By performing most of the integrals analytically, we achieve an enormous speedup over brute-force Monte Carlo. We use this method to place upper limits on the maximum semimajor axis of the distribution function derived from radial-velocity planets, finding model-dependent values of $\sim$30--100 AU. Finally, we model the entire substellar sample, from massive brown dwarfs to a theoretically motivated cutoff at $\sim$5 M$_{\rm Jup}$, with a single power law distribution. We find that $p(M, a) \propto M^{-0.65\pm0.60} a^{-0.85\pm0.39}$ (1$\sigma$ errors) provides an adequate fit to our data, with 1.0--3.1\% (68\% confidence) of stars hosting 5--70 $M_{\rm Jup}$ companions between 10 and 100 AU. This suggests that many of the directly imaged exoplanets known, including most (if not all) of the low-mass companions in our sample, formed by fragmentation in a cloud or disk, and represent the low-mass tail of the brown dwarfs.

Astrometric follow-up observations of directly imaged sub-stellar companions to young stars and brown dwarfs★

The formation of massive planetary or brown dwarf companions at large projected separations from their host star is not yet well understood. In order to put constraints on formation scenarios we search for signatures in the orbit dynamics of the systems. We are specifically interested in the eccentricities and inclinations since those parameters might tell us about the dynamic history of the systems and where to look for additional low-mass sub-stellar companions. For this purpose we utilized VLT/NACO to take several well calibrated high resolution images of 6 target systems and analyze them together with available literature data points of those systems as well as Hubble Space Telescope archival data. We used a statistical Least-Squares Monte-Carlo approach to constrain the orbit elements of all systems that showed significant differential motion of the primary star and companion. We show for the first time that the GQ Lup system shows significant change in both separation and position angle. Our analysis yields best fitting orbits for this system, which are eccentric (e between 0.21 and 0.69), but can not rule out circular orbits at high inclinations. Given our astrometry we discuss formation scenarios of the GQ Lup system. In addition, we detected an even fainter new companion candidate to GQ Lup, which is most likely a background object. We also updated the orbit constraints of the PZ Tel system, confirming that the companion is on a highly eccentric orbit with e > 0.62. Finally we show with a high significance, that there is no orbital motion observed in the cases of the DH Tau, HD 203030 and 1RXS J160929.1-210524 systems and give the most precise relative astrometric measurement of the UScoCTIO 108 system to date.

Spectrum of hot methane in astronomical objects using a comprehensive computed line list.

Hot methane spectra are important in environments ranging from flames to the atmospheres of cool stars and exoplanets. A new spectroscopic line list, 10to10, for (12)CH4 containing almost 10 billion transitions is presented. This comprehensive line list covers a broad spectroscopic range and is applicable for temperatures up to 1,500 K. Previous methane data are incomplete, leading to underestimated opacities at short wavelengths and elevated temperatures. Use of 10to10 in models of the bright T4.5 brown dwarf 2MASS 0559-14 leads to significantly better agreement with observations and in studies of the hot Jupiter exoplanet HD 189733b leads to up to a 20-fold increase in methane abundance. It is demonstrated that proper inclusion of the huge increase in hot transitions which are important at elevated temperatures is crucial for accurate characterizations of atmospheres of brown dwarfs and exoplanets, especially when observed in the near-infrared.

DISTRIBUTION OF CCS AND HC3N IN L1147, AN EARLY PHASE DARK CLOUD

We used the Nobeyama 45 m radio telescope to reveal spatial distributions of CCS and HC{sub 3}N in L1147, one of the carbon-chain producing region (CCPR) candidates, where carbon-chain molecules are dominant rather than NH{sub 3}. We found that three cores (two CCS cores and one HC{sub 3}N core), which are away from a very low luminosity object (a source that may turn into a sub-stellar mass brown dwarf), exist along the NE-SW filament traced by the 850 μm dust continuum. The column densities of CCS are 3-7 × 10{sup 12} cm{sup –2} and those of HC{sub 3}N are 2-6 × 10{sup 12} cm{sup –2}, respectively, much lower than those previously reported toward other CCPRs. We also found that two CCS peaks are displaced from the peaks of HC{sub 3}N. In order to interpret such interleaved distributions, we conducted chemical reaction network simulations and found that slightly different gas densities could lead to large variation of the CCS-to-HC{sub 3}N ratio in the early phase of dark cloud evolution. Such a chemical 'variation' may be seen in other CCPRs. Finally, we were able to confirm that the L1147 filament can be regarded as a CCPR.

Multi-object and long-slit spectroscopy of very low mass brown dwarfs in the Orion Nebular Cluster

Abstract We present the results of an H- and K-band multi-object and long-slit spectroscopic survey of substellar mass candidates in the outer regions of the Orion Nebula Cluster. The spectra were obtained using MOIRCS on the 8.2 m Subaru telescope and ISLE on the 1.88 m telescope of Okayama Astrophysical Observatory. Eight out of twelve spectra show strong water absorption and we confirm that their effective temperatures are ≤ 3000 K (spectral type ≥ M6) from a chi-square fit to synthetic spectra. We plot our sources on an HR diagram overlaid with theoretical isochrones of low-mass objects and identify three new young brown dwarf candidates. One of the three new candidates is a cool object near the brown dwarf and planetary mass boundary. Based on our observations and those of previous studies, we determine the stellar (0.08 < M/M⊙ < 1) to substellar (0.03 < M/M⊙ < 0.08) mass number ratio in the outer regions of the Orion Nebular Cluster to be 3.5 ± 0.8. In combination with the number ratio reported for the central region ($3.3^{+0.8} _{-0.7}$), this result suggests the number ratio does not simply change with the distance from the center of the Orion Nebular Cluster.

HOW EMPTY ARE DISK GAPS OPENED BY GIANT PLANETS?

Gap clearing by giant planets has been proposed to explain the optically thin cavities observed in many protoplanetary disks. How much material remains in the gap determines not only how detectable young planets are in their birth environments, but also how strong corotation torques are, which impacts how planets can survive fast orbital migration. We determine numerically how the average surface density inside the gap, sigma_gap, depends on planet-to-star mass ratio q, Shakura-Sunyaev viscosity parameter alpha, and disk height-to-radius aspect ratio h/r. Our results are derived from our new GPU-accelerated Lagrangian hydrodynamical code PEnGUIn, and are verified by independent simulations with ZEUS90. For Jupiter-like planets, we find sigma_gap \propto q^-2.2 alpha^1.4 (h/r)^6.6, and for near brown dwarf masses, sigma_gap \propto q^-1 alpha^1.3 (h/r)^6.1. Surface density contrasts inside and outside gaps can be as large as 10^4, even when the planet does not accrete. We derive a simple analytic scaling, sigma_gap \propto q^-2 alpha^1 (h/r)^5, that compares reasonably well to empirical results, especially at low Neptune-like masses, and use discrepancies to highlight areas for progress.

MICROLENSING DISCOVERY OF A TIGHT, LOW-MASS-RATIO PLANETARY-MASS OBJECT AROUND AN OLD FIELD BROWN DWARF

Observations of accretion disks around young brown dwarfs have led to the speculation that they may form planetary systems similar to normal stars. While there have been several detections of planetary-mass objects around brown dwarfs (2MASS 1207-3932 and 2MASS 0441-2301), these companions have relatively large mass ratios and projected separations, suggesting that they formed in a manner analogous to stellar binaries. We present the discovery of a planetary-mass object orbiting a field brown dwarf via gravitational microlensing, OGLE-2012-BLG-0358Lb. The system is a low secondary/primary mass ratio (0.080 +- 0.001), relatively tightly-separated (~0.87 AU) binary composed of a planetary-mass object with 1.9 +- 0.2 Jupiter masses orbiting a brown dwarf with a mass 0.022 M_Sun. The relatively small mass ratio and separation suggest that the companion may have formed in a protoplanetary disk around the brown dwarf host, in a manner analogous to planets.

THE GEMINI/NICI PLANET-FINDING CAMPAIGN: THE FREQUENCY OF PLANETS AROUND YOUNG MOVING GROUP STARS

We report results of a direct imaging survey for giant planets around 80 members of the Beta Pic, TW Hya, Tucana-Horologium, AB Dor, and Hercules-Lyra moving groups, observed as part of the Gemini NICI Planet-Finding Campaign. For this sample, we obtained median contrasts of \Delta H=13.9 mag at 1" in combined CH4 narrowband ADI+SDI mode and median contrasts of \Delta H=15.1 mag at 2" in H-band ADI mode. We found numerous (>70) candidate companions in our survey images. Some of these candidates were rejected as common-proper motion companions using archival data; we reobserved with NICI all other candidates that lay within 400 AU of the star and were not in dense stellar fields. The vast majority of candidate companions were confirmed as background objects from archival observations and/or dedicated NICI campaign followup. Four co-moving companions of brown dwarf or stellar mass were discovered in this moving group sample: PZ Tel B (36+-6 MJup, 16.4+-1.0 AU, Biller et al. 2010), CD -35 2722B (31+-8 MJup, 67+-4 AU, Wahhaj et al. 2011), HD 12894B (0.46+-0.08 MSun, 15.7+-1.0 AU), and BD+07 1919C (0.20+-0.03 MSun, 12.5+-1.4 AU). From a Bayesian analysis of the achieved H band ADI and ASDI contrasts, using power-law models of planet distributions and hot-start evolutionary models, we restrict the frequency of 1--20 MJup companions at semi-major axes from 10--150 AU to <18% at a 95.4% confidence level using DUSTY models and to <6% at a 95.4% using COND models.

Towards precise ages and masses of Free Floating Planetary Mass Brown Dwarfs

Measurement of the substellar initial mass function (IMF) in very young clusters is hampered by the possibility of the age spread of cluster members. This is particularly serious for candidate planetary mass objects (PMOs), which have a very similar location to older and more massive brown dwarfs on the Hertzsprung-Russell Diagram (HRD). This degeneracy can be lifted by the measurement of gravity-sensitive spectral features. To this end we have obtained medium resolution (R~5000) Near-infrared Integral Field Spectrometer (NIFS) K band spectra of a sample of late M- / early L-type dwarfs. The sample comprises old field dwarfs and very young brown dwarfs in the Taurus association and in the Sigma Orionis cluster. We demonstrate a positive correlation between the strengths of the 2.21micron NaI doublet and the objects' ages. We demonstrate a further correlation between these objects' ages and the shape of their K band spectra. We have quantified this correlation in the form of a new index, the H2(K) index. This index appears to be more gravity-sensitive than the NaI doublet and has the advantage that it can be computed for spectra where gravity-sensitive spectral lines are unresolved, while it is also more sensitive to surface gravity at very young ages (<10 Myr) than the triangular H band peak. Both correlations differentiate young objects from field dwarfs, while the H2(K) index can distinguish, at least statistically, populations of ~1 Myr objects from populations of ~10 Myr objects. We applied the H2(K) index to NIFS data for one Orion nebula cluster (ONC) PMO and to previously published low resolution spectra for several other ONC PMOs where the 2.21micron NaI doublet was unresolved and concluded that the average age of the PMOs is ~1 Myr.

RESOLVED NEAR-INFRARED SPECTROSCOPY OF WISE J104915.57–531906.1AB: A FLUX-REVERSAL BINARY AT THE L DWARF/T DWARF TRANSITION

We report resolved near-infrared spectroscopy and photometry of the recently identified brown dwarf binary WISE J104915.57-531906.1AB, located 2.02+/-0.15 pc from the Sun. Low-resolution spectral data from Magellan/FIRE and IRTF/SpeX reveal strong H2O and CO absorption features in the spectra of both components, with the secondary also exhibiting weak CH4 absorption at 1.6 micron and 2.2 micron. Spectral indices and comparison to low-resolution spectral standards indicate component types of L7.5 and T0.5, the former consistent with the optical classification of the primary. Relative photometry reveals a flux reversal between the J- and K-bands, with the T dwarf component being brighter in the 0.95--1.3 micron range. As with other L/T transition binaries, this reversal likely reflects significant depletion of condensate opacity across the transition, a behavior that may be enhanced in WISE J1049-5319AB if the unusual red color of its L dwarf component is indicative of thick clouds. On the other hand, differing cloud properties may have modified the evolutionary paths of these two components, and we propose a scenario in which the cooler secondary could be the more massive of the two components. Fortunately, the proximity, brightness and small separation (3.12+/-0.25 AU) of this system make it amenable to astrometric and radial velocity orbit measurement during its estimated 25 yr orbit, providing a rare opportunity for the direct determination of individual brown dwarf masses and a unique benchmark for studying cloud evolution across the L dwarf/T dwarf transition.

Spin-orbit alignment in the very low mass binary regime

Studies of solar-type binaries have found coplanarity between the equatorial and orbital planes of systems with $<$40 AU separation. By comparison, the alignment of the equatorial and orbital axes in the substellar regime, and the associated implications for formation theory, are relatively poorly constrained. Here we present the discovery of the rotation period of 3.32 $\pm$ 0.15 hours from 2MASS J0746+20A - the primary component of a tight (2.7 AU) ultracool dwarf binary system (L0+L1.5). The newly discovered period, together with the established period via radio observations of the other component, and the well constrained orbital parameters and rotational velocity measurements, allow us to infer alignment of the equatorial planes of both components with the orbital plane of the system to within 10 degrees. This result suggests that solar-type binary formation mechanisms may extend down into the brown dwarf mass range, and we consider a number of formation theories that may be applicable in this case. This is the first such observational result in the very low mass binary regime. In addition, the detected period of 3.32 $\pm$ 0.15 hours implies that the reported radio period of 2.07 $\pm$ 0.002 hours is associated with the secondary star, not the primary, as was previously claimed. This in turn refutes the claimed radius of 0.78 $\pm$ 0.1 $R_{J}$ for 2MASS J0746+20A, which we demonstrate to be 0.99 $\pm$ 0.03 $R_{J}$.

MICROLENSING DISCOVERY OF A POPULATION OF VERY TIGHT, VERY LOW MASS BINARY BROWN DWARFS

Although many models have been proposed, the physical mechanisms responsible for the formation of low-mass brown dwarfs are poorly understood. The multiplicity properties and minimum mass of the brown-dwarf mass function provide critical empirical diagnostics of these mechanisms. We present the discovery via gravitational microlensing of two very low-mass, very tight binary systems. These binaries have directly and precisely measured total system masses of 0.025 Msun and 0.034 Msun, and projected separations of 0.31 AU and 0.19 AU, making them the lowest-mass and tightest field brown-dwarf binaries known. The discovery of a population of such binaries indicates that brown dwarf binaries can robustly form at least down to masses of ~0.02 Msun. Future microlensing surveys will measure a mass-selected sample of brown-dwarf binary systems, which can then be directly compared to similar samples of stellar binaries.

FIRE: A Facility Class Near-Infrared Echelle Spectrometer for the Magellan Telescopes

ABSTRACTWe describe the design, construction, and commissioning of FIRE, a 0.82–2.51 μ m echelle spectrograph for the 6.5 m Magellan Baade telescope. FIRE may be operated in two modes. Its primary mode is a prism cross-dispersed echelle, which delivers R = 6000 spectra for an 0.6″ slit, with continuous wavelength coverage over the full instrument bandpass in a single setup. Alternatively, the echelle grating may be replaced with a flat mirror to obtain high-throughput R = 400 longslit spectra through the prisms alone—again with full Y/J/H/K coverage. This contribution outlines the details of the optical design and execution, mechanical and thermal design, detector systems, and data analysis software. We also present performance metrics from commissioning observations. These have established that the instrument is achieving its design goals, particularly with regard to throughput, as is required for observations of faint, high-redshift QSOs and the lowest mass brown dwarfs.

CARMA INTERFEROMETRIC OBSERVATIONS OF 2MASS J044427+2512: THE FIRST SPATIALLY RESOLVED OBSERVATIONS OF THERMAL EMISSION OF A BROWN DWARF DISK

We present CARMA 1.3 mm continuum data of the disk surrounding the young brown dwarf 2MASS J044427+2512 in the Taurus molecular cloud. The high angular resolution of the CARMA observations (0.16 arcsec) allows us to spatially resolve for the first time the thermal emission from dust around a brown dwarf. We analyze the interferometric visibilities and constrain the disk outer radius adopting disk models with power-law radial profiles of the dust surface density. In the case of a power-law index equal to or lower than 1, we obtain a disk radius in the range of about 15 - 30 AU, while larger disks are inferred for steeper radial profiles. By combining this information on the disk spatial extent with the sub-mm spectral index of this source we find conclusive evidence for mm-sized grains, or larger, in this brown dwarf disk. We discuss the implications of our results on the models of dust evolution in proto-planetary disks and brown dwarf formation.

NLTT 5306: the shortest period detached white dwarf+brown dwarf binary

We have spectroscopically confirmed a brown dwarf mass companion to the hydrogen atmosphere white dwarf NLTT5306. The white dwarf's atmospheric parameters were measured using Sloan Digital Sky Survey and X-Shooter spectroscopy as T_eff=7756+/-35K and log(g)=7.68+/-0.08, giving a mass for the primary of M_WD=0.44+/-0.04 M_sun, at a distance of 71+/-4 pc with a cooling age of 710+/-50 Myr. The existence of the brown dwarf secondary was confirmed through the near-infrared arm of the X-Shooter data and a spectral type of dL4-dL7 was estimated using standard spectral indices. Combined radial velocity measurements from the Sloan Digital Sky Survey, X-Shooter and the Hobby-Eberly Telescope's High Resolution Spectrograph of the white dwarf gives a minimum mass of 56+/-3 M_jup for the secondary, confirming the substellar nature. The period of the binary was measured as 101.88+/-0.02 mins using both the radial velocity data and i'-band variability detected with the INT. This variability indicates 'day' side heating of the brown dwarf companion. We also observe H{\alpha} emission in our higher resolution data in phase with the white dwarf radial velocity, indicating this system is in a low level of accretion, most likely via a stellar wind. This system represents the shortest period white dwarf + brown dwarf binary and the secondary has survived a stage of common envelope evolution, much like its longer period counterpart, WD0137-349. Both systems likely represent bona-fide progenitors of cataclysmic variables with a low mass white dwarf and a brown dwarf donor.

Binary mass ratios: system mass not primary mass

Abstract Binary properties are usually expressed (for good observational reasons) as a function of primary mass. It has been found that the distribution of companion masses – the mass ratio distribution – is different for different primary masses. We argue that system mass is the more fundamental physical parameter to use. We show that if system masses are drawn from a log-normal mass function, then the different observed mass ratio distributions as a function of primary mass, from M-dwarfs to A-stars, are all consistent with a universal, flat, system mass ratio distribution. We also show that the brown dwarf mass ratio distribution is not drawn from the same flat distribution, suggesting that the process which decides upon mass ratios is very different in brown dwarfs and stars.

SPIRAL ARMS IN THE ASYMMETRICALLY ILLUMINATED DISK OF MWC 758 AND CONSTRAINTS ON GIANT PLANETS

We present the first near-IR scattered light detection of the transitional disk associated with the Herbig Ae star MWC 758 using data obtained as part of the Strategic Exploration of Exoplanets and Disks with Subaru, and 1.1 micron HST/NICMOS data. While sub-millimeter studies suggested there is a dust-depleted cavity with r=0.35, we find scattered light as close as 0.1 (20-28 AU) from the star, with no visible cavity at H, K', or Ks. We find two small-scaled spiral structures which asymmetrically shadow the outer disk. We model one of the spirals using spiral density wave theory, and derive a disk aspect ratio of h ~ 0.18, indicating a dynamically warm disk. If the spiral pattern is excited by a perturber, we estimate its mass to be 5+3,-4 Mj, in the range where planet filtration models predict accretion continuing onto the star. Using a combination of non-redundant aperture masking data at L' and angular differential imaging with Locally Optimized Combination of Images at K' and Ks, we exclude stellar or massive brown dwarf companions within 300 mas of the Herbig Ae star, and all but planetary mass companions exterior to 0.5. We reach 5-sigma contrasts limiting companions to planetary masses, 3-4 MJ at 1.0 and 2 MJ at 1.55 using the COND models. Collectively, these data strengthen the case for MWC 758 already being a young planetary system.

HerschelPACS and SPIRE observations of TW Hydrae association brown dwarf disks

We present Herschel SPIRE observations for the TW Hydrae association (TWA) brown dwarf discs SSSPM J1102-3431 (SS1102) and 2MASSW J1207334-393254 (2M1207). Both discs are undetected in the SPIRE 200-500mu bands. We have also analyzed the archival PACS data and find no detection for either source in the 160mu band. Based on radiative transfer modeling, we estimate an upper limit to the disc mass for both sources of 0.1 M_Jup. The lack of detection in the SPIRE bands could be due to a paucity of millimeter sized dust grains in the 2M1207 and SS1102 discs. We also report a non-detection for the brown dwarf 2MASS J1139511-315921 (2M1139) in the PACS 70 and 160mu bands. We have argued for the presence of a warm debris disc around 2M1139, based on an excess emission observed at 24mu. The mid-infrared colors for 2M1139 are similar to the transition discs in the Taurus and Ophuichus regions. A comparison of the brown dwarf disc masses over a ~1-10 Myr age interval suggests a decline in the disc mass with the age of the system.

ON THE SURVIVAL OF BROWN DWARFS AND PLANETS ENGULFED BY THEIR GIANT HOST STAR

The recent discovery of two Earth-mass planets in close orbits around an evolved star has raised questions as to whether substellar companions can survive encounters with their host stars. We consider whether these companions could have been stripped of significant amounts of mass during the phase when they orbited through the dense inner envelopes of the giant. We apply the criterion derived by Murray et al. for disruption of gravitationally bound objects by ram pressure, to determine whether mass loss may have played a role in the histories of these and other recently discovered low-mass companions to evolved stars. We find that the brown dwarf and Jovian mass objects circling WD 0137-349, SDSS J08205+0008, and HIP 13044 are most unlikely to have lost significant mass during the common envelope phase. However, the Earth-mass planets found around KIC 05807616 could well be the remnant of one or two Jovian mass planets that lost extensive mass during the common envelope phase.