Improved radial velocity orbit of the young binary brown dwarf candidate ChaHa8

The very young brown dwarf candidate ChaHa8 was recently discovered to have a close (~1AU) companion by means of radial velocity monitoring. We present additional radial velocity data obtained with UVES/VLT between 2007 and 2010, which significantly improve the orbit of the system. The combined data set spans ten years of radial velocity monitoring for ChaHa8. A Kepler fit to the data yields an orbital period of 5.2 yrs, an eccentricity of 0.59, and a radial velocity semi-amplitude of 2.4 km/s. A companion mass M2sini (which is a lower limit due to the unknown orbital inclination) of 25 Mjup and of 31 Mjup is derived when using model-dependent mass estimates for the primary of 0.07 and 0.10 Msun, resp. Assuming random orientation of orbits in space, we find a very high probability that the companion of Chaha8 is of substellar nature: With a greater than 87% probability, the companion mass is between 30 and 69 Mjup and the mass ratio < 0.7. The absence of any evidence of the companion in the cross-correlation function together with the size of the radial velocity amplitude also indicate a mass ratio of at most 0.7, and likely smaller. Furthermore, the new data exclude the possibility that the companion has a mass in the planetary regime (<13 Mjup). We show that the companion contributes significantly to the total luminosity of the system: model-dependent estimates provide a minimum luminosity ratio L2/L1 of 0.2. ChaHa8 is the 4th known spectroscopic brown dwarf or very low-mass stellar binary with determined orbital parameters, and the 2nd known very young one. With an age of only ~3 Myr it is of particular interest to very low-mass formation and evolution theories. In contrast to most other spectroscopic binaries, it has a relatively long period and it might be possible to determine the astrometric orbit of the primary and, thus, the orbital inclination.Comment: Accepted for publication in A&A main Journal, minor changes (language editing

Discovery of an outflow of the very low-mass star ISO 143

We discover that the very young very low-mass star ISO143 (M5) is driving an outflow based on spectro-astrometry of forbidden [SII] emission lines at 6716A and 6731A observed in UVES/VLT spectra. This adds another object to the handful of brown dwarfs and very low-mass stars (M5-M8) for which an outflow has been confirmed and which show that the T Tauri phase continues at very low masses. We find the outflow of ISO143 to be intrinsically asymmetric and the accretion disk to not obscure the outflow, as only the red outflow component is visible in the [SII] lines. ISO143 is only the third T Tauri object showing a stronger red outflow component in spectro-astrometry, after RW Aur (G5) and ISO217 (M6.25). We show here that including ISO143 two out of seven outflows confirmed in the very low-mass regime (M5-M8) are intrinsically asymmetric. We measure a spatial extension of the outflow in [SII] of up to 200-300 mas (about 30-50 AU) and velocities of up to 50-70 km/s. We furthermore detect line emission of ISO143 in CaII (8498), OI (8446), HeI (7065), and weakly in [FeII] (7155). Based on a line profile analysis and decomposition we demonstrate that (i) the CaII emission can be attributed to chromospheric activity, a variable wind, and the magnetospheric infall zone, (ii) the OI emission mainly to accretion-related processes but also a wind, and (iii) the HeI emission to chromospheric or coronal activity. We estimate a mass outflow rate of ISO143 of ~10^{-10} Msol/yr and a mass accretion rate in the range of ~10^{-8} to ~10^{-9} Msol/yer. These values are consistent with those of other brown dwarfs and very low-mass stars. The derived Mout/Macc ratio of 1-20% is not supporting previous findings of this number to be very large (>40%) for very low-mass objects.Comment: Accepted for publication at A&A; 9 pages, 5 figures. Minor changes due to language editin

Improved kinematics for brown dwarfs and very low-mass stars in ChaI and a discussion of brown dwarf formation

We present a precise kinematic study of very young brown dwarfs (BDs) in the ChaI cloud based on radial velocities (RVs) measured with UVES / VLT. This is compared to the kinematics of T Tauri stars (TTS) in the same field, based on both own measurements and on RVs from the literature. More UVES spectra were taken compared with a former paper (Joergens & Guenther 2001), and the reduction of the spectra was improved, while studying the literature for RVs of T Tauri stars in ChaI led to a cleaned and enlarged sample of T Tauri stars. The result is an improved empirical RV distribution of BDs as well as of TTS in ChaI. We found that nine BDs/VLMSs (M6-M8) in ChaI have a RV dispersion of 0.9 km/s measured in terms of a standard deviation. This is consistent with the dispersion measured earlier in terms of fwhm of 2.1 km/s. The studied sample of 25 TTS (G2-M5) has a dispersion of 1.3 km/s (standard deviation). The RV dispersion of the BDs is consistent within the errors with that of TTS, which is in line with the finding of no mass dependence in some theoretical models of the ejection-scenario for the formation of brown dwarfs. In contrast to current N-body simulations, we did not find a high-velocity tail for the BDs RVs. We found hints suggesting different kinematics for binaries compared to predominantly single objects in ChaI. The global RV dispersion for ChaI members (1.24 km/s) is significantly lower than for Taurus members (2.0 km/s), despite higher stellar density in ChaI showing that a fundamental increase in velocity dispersion with stellar density of the star-forming region is not established observationally. The RVs of BDs observed in ChaI are less dispersed than predicted by existing models for the ejection-scenario.Comment: accepted for publication in A&A, 9 pages, 4 figures; minor changes due to language editin

Binary frequency of very young brown dwarfs at separations smaller than 3 AU

Searches for companions of brown dwarfs by direct imaging mainly probe orbital separations > 3-10 AU. On the other hand, previous radial velocity surveys of brown dwarfs are mainly sensitive to separations smaller than 0.6 AU. It has been speculated that the peak of the separation distribution of brown dwarf binaries lies right in the unprobed range. This work extends high-precision radial velocity surveys of brown dwarfs for the first time out to 3 AU. Based on more than six years UVES/VLT spectroscopy the binary frequency of brown dwarfs and (very) low-mass stars (M4.25-M8) in ChaI was determined: 18% for the whole sample and 10% for the subsample of ten brown dwarfs and VLMS (M < 0.1 Msun). Two spectroscopic binaries were confirmed, the brown dwarf candidate ChaHa8 (previously discovered by Joergens & Mueller) and the low-mass star CHXR74. Since their orbital separations appear to be 1 AU or greater, the binary frequency at < 1 AU might be less than 10%. Now for the first time companion searches of (young) brown dwarfs cover the whole orbital separation range, and the following observational constraints for models of brown dwarf formation can be derived: (i) the frequency of brown dwarf and very low-mass stellar binaries at 3 AU; i.e. direct imaging surveys do not miss a significant fraction of brown dwarf binaries; (ii) the overall binary frequency of brown dwarfs and very low-mass stars is 10-30 %; (iii) the decline in the separation distribution of brown dwarfs towards smaller separations seems to occur between 1 and 3 AU; (iv) the observed continuous decrease in the binary frequency from the stellar to the substellar regime is confirmed at < 3 AU providing further evidence of a continuous formation mechanism from low-mass stars to brown dwarfs.Comment: 17 pages, 14 figures, Accepted by A&A, minor language editin

A homogeneous analysis of disks around brown dwarfs

We re-analyzed the Herschel/PACS data of a sample of 55 brown dwarfs (BDs) and very low mass stars with spectral types ranging from M5.5 to L0. We investigated the dependence of disk structure on the mass of the central object in the substellar regime based on a homogeneous analysis of Herschel data from flux density measurements to spectral energy distribution (SED) modeling. A systematic comparison between the derived disk properties and those of sun-like stars shows that the disk flaring of BDs and very low mass stars is generally smaller than that of their higher mass counterparts, the disk mass is orders of magnitude lower than the typical value found in T Tauri stars, and the disk scale heights are comparable in both sun-like stars and BDs. We further divided our sample into an early-type brown dwarf (ETBD) group and a late-type brown dwarf (LTBD) group by using spectral type (=M8) as the border criterion. We systematically compared the modeling results from Bayesian analysis between these two groups, and found the trends of flaring index as a function of spectral type also present in the substellar regime. The spectral type independence of the scale height is also seen between high-mass and very low-mass BDs. However, both the ETBD and LTBD groups feature a similar median disk mass of 10^{-5}Msun and no clear trend is visible in the distribution, probably due to the uncertainty in translating the far-IR photometry into disk mass, the detection bias and the age difference among the sample. Unlike previous studies, our analysis is completely homogeneous in Herschel/PACS data reduction and modeling with a statistically significant sample. Therefore, we present evidence of stellar-mass-dependent disk structure down to the substellar mass regime, which is important for planet formation models. (Abridged Version)Comment: Accepted for publication in A&

Orbit of the young very low-mass spectroscopic binary CHXR 74

The pre-main sequence star CHXR74 (M4.25) in ChaI was detected a few years ago to be a very low-mass spectroscopic binary. Determination of its mass would provide a valuable dynamical mass measurement at young ages in the poorly constrained mass regime of <0.3 Msun. We carried out follow-up radial velocity (RV) monitoring with UVES/VLT between 2008 and 2011 and high-resolution adaptive optic assisted imaging with NACO/VLT in 2008 with the aim to constrain the binary orbit. We present an orbital solution of the system based on the combined RV data set which spans more than 11 years of UVES monitoring for CHXR74. The best-fit Kepler model has an orbital period of 13.1 yrs, zero eccentricity, and a RV semi-amplitude of 2.2 km/s. A companion mass M2sini of 0.08 Msun is derived by using a model-dependent mass estimate for the primary of 0.24 Msun. The binary separation for i=90deg is 3.8 AU (23 mas). Complementary NACO images of CHXR74 were taken with the aim to directly resolve the binary. While there are marginal signs of an extended PSF, we have no convincing companion detected to CHXR74 in these images. From the non-detection of the companion together with a prediction of the binary separation at the time of the NACO observations, we derive an upper limit for the K-band brightness ratio of 0.5. This allows us to estimate an upper limit of the companion mass of 0.14 Msun by applying evolutionary models. Thus, we have confirmed that CHXR74 is a very low-mass spectroscopic binary and constrained the secondary mass to lie within the range of about 0.08 and 0.14 Msun. We predict an astrometric signal of the primary between 0.2 and 0.4 mas when taking into account the luminosity of the companion. The GAIA astrometric mission might well be able to solve the astrometric orbit of the primary and in combination with the presented RV data to determine an absolute companion mass.Comment: accepted for publication in A\&A, minor changes (language editing

16-20 Jupiter mass RV companion orbiting the brown dwarf candidate ChaHa8

We report the discovery of a 16-20 Jupiter mass radial velocity companion around the very young (~3 Myr) brown dwarf candidate ChaHa8 (M5.75-M6.5). Based on high-resolution echelle spectra of ChaHa8 taken between 2000 and 2007 with UVES at the VLT, a companion was detected through RV variability with a semi-amplitude of 1.6 km/s. A Kepler fit to the data yields an orbital period of the companion of 1590 days and an eccentricity of e=0.49. A companion minimum mass M2sini between 16 and 20 Jupiter masses is derived when using model-dependent mass estimates for the primary. The mass ratio q= M2/M1 might be as small as 0.2 and, with a probability of 87%, it is less than 0.4. ChaHa8 harbors most certainly the lowest mass companion detected so far in a close (~ 1 AU) orbit around a brown dwarf or very low-mass star. From the uncertainty in the orbit solution, it cannot completely be ruled out that the companion has a mass in the planetary regime. Its discovery is in any case an important step towards RV planet detections around BDs. Further, ChaHa8 is the fourth known spectroscopic brown dwarf or very low-mass binary system with an RV orbit solution and the second known very young one.Comment: 8 pages, 1 figure, ApJ Letter in pres