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Register a new userDynamical masses for the nearest brown dwarf binary: Epsilon Indi Ba, Bb
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We present preliminary astrometric results for the closest known brown dwarf binary to the Sun: Epsilon Indi Ba, Bb at a distance of 3.626 pc. Via ongoing monitoring of the relative separation of the two brown dwarfs (spectral types T1 and T6) with the VLT NACO near-IR adaptive optics system since June 2004, we obtain a model-independent dynamical total mass for the system of 121 MJup, some 60% larger than the one obtained by McCaughrean et al. (2004), implying that the system may be as old as 5 Gyr. We have also been monitoring the absolute astrometric motions of the system using the VLT FORS2 optical imager since August 2005 to determine the individual masses. We predict a periastron passage in early 2010, by which time the system mass will be constrained to < 1 MJup and we will be able to determine the individual masses accurately in a dynamical, model-independent manner.
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We have carried out high angular resolution near-infrared imaging and low-resolution (R~1000) spectroscopy of the nearest known brown dwarf, Eps Indi B, using the ESO VLT NAOS/CONICA adaptive optics system. We find it to be a close binary (as also noted by Volk et al. 2003) with an angular separation of 0.732 arcsec, corresponding to 2.65AU at the 3.626pc distance of the Eps Indi system. In our discovery paper (Scholz et al. 2003), we concluded that Eps Indi B was a ~50Mjup T2.5 dwarf: our revised finding is that the two system components (Eps Indi Ba and Eps Indi Bb) have spectral types of T1 and T6, respectively, and estimated masses of 47 and 28Mjup, respectively, assuming an age of 1.3Gyr. Errors in the masses are +/-10 and +/-7Mjup, respectively, dominated by the uncertainty in the age determination (0.8-2Gyr range). This uniquely well-characterised T dwarf binary system should prove important in the study of low-mass, cool brown dwarfs. The two components are bright and relatively well-resolved: Eps Indi B is the only T dwarf binary in which spectra have been obtained for both components. They have a well-established distance and age. Finally, their orbital motion can be measured on a fairly short timescale (nominal orbital period 15 yrs), permitting an accurate determination of the true total system mass, helping to calibrate brown dwarf evolutionary models.
We present the results of 14 nights of textit{I}-band photometric monitoring of the nearby brown dwarf binary, $epsilon$ Indi Ba,Bb. Observations were acquired over 2 months, and total close to 42 hours of coverage at a typically high cadence of 1.4 minutes. At a separation of just $0.7$, we do not resolve the individual components, and so effectively treat the binary as if it were a single object. However, $epsilon$ Indi Ba (spectral type T1) is the brightest known T-type brown dwarf, and is expected to dominate the photometric signal. We typically find no strong variability associated with the target during each individual night of observing, but see significant changes in mean brightness - by as much as $0.10$ magnitudes - over the 2 months of the campaign. This strong variation is apparent on a timescale of at least 2 days. We detect no clear periodic signature, which suggests we may be observing the T1 brown dwarf almost pole-on, and the days-long variability in mean brightness is caused by changes in the large-scale structure of the cloud coverage. Dynamic clouds will very likely produce lightning, and complementary high cadence textit{V}-band and Htextit{$alpha$} images were acquired to search for the emission signatures associated with stochastic strikes. We report no positive detections for the target in either of these passbands.
We have identified a new early T dwarf only 3.6pc from the Sun, as a common proper motion companion (separation 1459AU) to the K5V star Epsilon Indi (HD209100). As such, Epsilon Indi B is one of the highest proper motion sources outside the solar system (~4.7 arcsec/yr), part of one of the twenty nearest stellar systems, and the nearest brown dwarf to the Sun. Optical photometry obtained from the SuperCOSMOS Sky Survey was combined with approximate infrared photometry from the 2MASS Quicklook survey data release, yielding colours for the source typical of early T dwarfs. Follow up infrared spectroscopy using the ESO NTT and SOFI confirmed its spectral type to be T2.5+/-0.5. With Ks=11.2, Epsilon Indi B is 1.7 magnitudes brighter than any previously known T dwarf and 4 magnitudes brighter than the typical object in its class, making it highly amenable to detailed study. Also, as a companion to a bright nearby star, it has a precisely known distance (3.626pc) and relatively well-known age (0.8-2Gyr), allowing us to estimate its luminosity as logL/Lsun=-4.67, its effective temperature as 1260K, and its mass as ~40-60Mjup. Epsilon Indi B represents an important addition to the census of the Solar neighbourhood and, equally importantly, a new benchmark object in our understanding of substellar objects.
The aim of the project is to characterise both components of the nearest brown dwarf sytem to the Sun, WISE J104915.57-531906.1 (=Luhman16AB) at optical and near-infrared wavelengths. We obtained high signal-to-noise intermediate-resolution (R~6000-11000) optical (600-1000 nm) and near-infrared (1000-2480nm) spectra of each component of Luhman16AB, the closest brown dwarf binary to the Sun, with the X-Shooter instrument on the Very Large Telescope. We classify the primary and secondary of the Luhman16 system as L6-L7.5 and T0+/-1, respectively, in agreement with previous measurements published in the literature. We present measurements of the lithium pseudo-equivalent widths, which appears of similar strength on both components (8.2+/-1.0 Angstroms and 8.4+/-1.5 Angstroms for the L and T components, respectively). The presence of lithium (Lithium 7) in both components imply masses below 0.06 Msun while comparison with models suggests lower limits of 0.04 Msun. The detection of lithium in the T component is the first of its kind. Similarly, we assess the strength of other alkali lines (e.g. pseudo-equivalent widths of 6-7 Angstroms for RbI and 4-7 Angstroms for CsI) present in the optical and near-infrared regions and compare with estimates for L and T dwarfs. We also derive effective temperatures and luminosities of each component of the binary: -4.66+/-0.08 dex and 1305(+180)(-135) for the L dwarf and -4.68+/-0.13 dex and 1320(+185)(-135) for the T dwarf, respectively. Using our radial velocity determinations, the binary does not appear to belong to any of the well-known moving group. Our preliminary theoretical analysis of the optical and J-band spectra indicates that the L- and T-type spectra can be reproduced with a single temperature and gravity but different relative chemical abundances which impact strongly the spectral energy distribution of L/T transition objects.
We present comprehensive orbital analyses and dynamical masses for the substellar companions Gl~229~B, Gl~758~B, HD~13724~B, HD~19467~B, HD~33632~Ab, and HD~72946~B. Our dynamical fits incorporate radial velocities, relative astrometry, and most importantly calibrated Hipparcos-Gaia EDR3 accelerations. For HD~33632~A and HD~72946 we perform three-body fits that account for their outer stellar companions. We present new relative astrometry of Gl~229~B with Keck/NIRC2, extending its observed baseline to 25 years. We obtain a $<$1% mass measurement of $71.4 pm 0.6,M_{rm Jup}$ for the first T dwarf Gl~229~B and a 1.2% mass measurement of its host star ($0.579 pm 0.007,M_{odot}$) that agrees with the high-mass-end of the M dwarf mass-luminosity relation. We perform a homogeneous analysis of the host stars ages and use them, along with the companions measured masses and luminosities, to test substellar evolutionary models. Gl~229~B is the most discrepant, as models predict that an object this massive cannot cool to such a low luminosity within a Hubble time, implying that it may be an unresolved binary. The other companions are generally consistent with models, except for HD~13724~B that has a host-star activity age 3.8$sigma$ older than its substellar cooling age. Examining our results in context with other mass-age-luminosity benchmarks, we find no trend with spectral type but instead note that younger or lower-mass brown dwarfs are over-luminous compared to models, while older or higher-mass brown dwarfs are under-luminous. The presented mass measurements for some companions are so precise that the stellar host ages, not the masses, limit the analysis.
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