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Register a new userDiscovery of a Very Nearby Brown Dwarf to the Sun: A Methane Rich Brown Dwarf Companion to the Low Mass Star SCR 1845-6357
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We present VLT/NACO SDI images of the very nearby star SCR 1845-6357 (hereafter SCR 1845). SCR 1845 is a recently discovered (Hambly et al. 2004) M8.5 star just 3.85 pc from the sun (Henry et al. 2006). Using the capabilities of the unique SDI device, we discovered a substellar companion to SCR 1845 at a separation of 4.5 AU (1.170+-0.003 on the sky) and fainter by 3.57+-0.057 mag in the 1.575 um SDI filter. This substellar companion has an H magnitude of 13.16+0.31-0.26 (absolute H magnitude of 15.30+0.31-0.26), making it likely the brightest mid-T dwarf known. The unique Simultaneous Differential Imager (SDI) consists of 3 narrowband filters placed around the 1.6 um methane absorption feature characteristic of T-dwarfs (Teff < 1200 K). The flux of the substellar companion drops by a factor of 2.7+-0.1 between the SDI F1(1.575 um) filter and the SDI F3(1.625 um) filter, consistent with strong methane absorption in a substellar companion. We estimate a spectral type of T5.5+-1 for the companion based on the strength of this methane break. The chances that this object is a background T dwarf are vanishing small -- and there is no isolated background T-dwarf in this part of the sky according to 2MASS. Thus, it is a bound companion, hereafter SCR 1845-6357B. For an age range of 100 Myr - 10 Gyr and spectral type range of T4.5-T6.5, we find a mass range of 9 - 65 MJup for SCR 1845B from the Baraffe et al. 2003 COND models. SCR 1845AB is the 24th closest stellar system to the Sun (at 3.85 pc); the only brown dwarf system closer to the Sun is Eps Indi Ba-Bb (at 3.626 pc). In addition, this is the first T-dwarf companion discovered around a low mass star.
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This paper was accidentally submitted twice to astro-ph. This version has been withdrawn -- please see astro-ph/0601440 instead.
The recently discovered star SCR 1845-6357 is the first late M/T dwarf binary discovered. SCR 1845 is a particular object due to its tight orbit (currently around 4 AU) and its proximity to the Sun (3.85 pc). We present spatially resolved VLT/NACO images and low resolution spectra of SCR 1845 in the J, H and K near-infrared bands. Since the T dwarf companion, SCR 1845B, is so close to the primary SCR 1845A, orbital motion is evident even within a year. Following the orbital motion, the binarys mass can be measured accurately within a decade, making SCR 1845B a key T-dwarf mass-luminosity calibrator. The NIR spectra allow for accurate determination of spectral type and also for rough estimates of the objects physical parameters. The spectral type of SCR 1845B is determined by direct comparison of the flux calibrated JHK spectra with T dwarf standard template spectra and also by NIR spectral indices obtained from synthetic photometry. Constrained values for surface gravity, effective temperature and metallicity are derived by comparison with model spectra. Our data prove that SCR 1845B is a brown dwarf of spectral type T6 that is co-moving with and therefore gravitationally bound to the M8.5 primary. Fitting the NIR spectrum of SCR 1845B to model spectra yields an effective temperature of about 950K and a surface gravity log(g)=5.1 (cgs) assuming solar metallicity. Mass and age of SCR 1845B are in the range 40 to 50 Jupiter masses and 1.8 to 3.1 Gyr.
We report the discovery of a bright, brown dwarf companion to the star HIP 64892, imaged with VLT/SPHERE during the SHINE exoplanet survey. The host is a B9.5V member of the Lower-Centaurus-Crux subgroup of the Scorpius Centaurus OB association. The measured angular separation of the companion ($1.2705pm0.0023$) corresponds to a projected distance of $159pm12$ AU. We observed the target with the dual-band imaging and long-slit spectroscopy modes of the IRDIS imager to obtain its SED and astrometry. In addition, we reprocessed archival NACO L-band data, from which we also recover the companion. Its SED is consistent with a young (<30 Myr), low surface gravity object with a spectral type of M9$_{gamma}pm1$. From comparison with the BT-Settl atmospheric models we estimate an effective temperature of $T_{textrm{eff}}=2600 pm 100$ K, and comparison of the companion photometry to the COND evolutionary models yields a mass of $sim29-37$ M$_{text{J}}$ at the estimated age of $16^{+15}_{-7}$ Myr for the system. HIP 64892 is a rare example of an extreme-mass ratio system ($qsim0.01$) and will be useful for testing models relating to the formation and evolution of such low-mass objects.
We report the discovery of an L dwarf companion to the A3V star beta{} Circini. VVV J151721.49-585131.5, or beta{} Cir B, was identified in a proper motion and parallax catalogue of the Vista Variables in the V{i}a L{a}ctea survey as having near infrared luminosity and colour indicative of an early L dwarf, and a proper motion and parallax consistent with that of beta{} Cir. The projected separation of $sim$3.6 corresponds to $6656$ au, which is unusually wide. The most recent published estimate of the age of the primary combined with our own estimate based on newer isochrones yields an age of $370-500$ Myr. The system therefore serves as a useful benchmark at an age greater than that of the Pleiades brown dwarfs and most other young L dwarf benchmarks. We have obtained a medium resolution echelle spectrum of the companion which indicates a spectral type of L1.0$pm$0.5 and lacks the typical signatures of low surface gravity seen in younger brown dwarfs. This suggests that signs of low surface gravity disappear from the spectra of early L dwarfs by an age of $sim370-500$ Myr, as expected from theoretical isochrones. The mass of beta{} Cir B is estimated from the BHAC15 isochrones as $0.056pm0.007$ M$_{odot}$.
We present the direct imaging discovery of a substellar companion to the nearby Sun-like star, HD 33632 Aa, at a projected separation of $sim$ 20 au, obtained with SCExAO/CHARIS integral field spectroscopy complemented by Keck/NIRC2 thermal infrared imaging. The companion, HD 33632 Ab, induces a 10.5$sigma$ astrometric acceleration on the star as detected with the $Gaia$ and $Hipparcos$ satellites. SCExAO/CHARIS $JHK$ (1.1--2.4 $mu$m) spectra and Keck/NIRC2 $L_{rm p}$ (3.78 $mu$m) photometry are best matched by a field L/T transition object: an older, higher gravity, and less dusty counterpart to HR 8799 cde. Combining our astrometry with $Gaia/Hipparcos$ data and archival Lick Observatory radial-velocities, we measure a dynamical mass of 46.4 $pm$ 8 $M_{rm J}$ and an eccentricity of $e$ $<$0.46 at 95% confidence. HD 33632 Abs mass and mass ratio (4.0% $pm$ 0.7%) are comparable to the low-mass brown dwarf GJ 758 B and intermediate between the more massive brown dwarf HD 19467 B and the (near-)planet mass companions to HR 2562 and GJ 504. Using $Gaia$ to select for direct imaging observations with the newest extreme adaptive optics systems can reveal substellar or even planet-mass companions on solar system-like scales at an increased frequency compared to blind surveys.
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