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

Modeling of Closure Phase Measurements with AMBER/VLTI - Towards Characterization of Exoplanetary Atmospheres

Differential phase observations with a near-IR interferometer offer a way to obtain spectra of extrasolar planets. The method makes use of the wavelength dependence of the interferometer phase of the planet/star system, which depends both on the interferometer geometry and on the brightness ratio between the planet and the star. The differential phase is strongly affected by instrumental and atmospheric dispersion effects. Difficulties in calibrating these effects might prevent the application of the differential phase method to systems with a very high contrast, such as extrasolar planets. A promising alternative is the use of spectrally resolved closure phases, which are immune to many of the systematic and random errors affecting the single-baseline phases. We have modeled the response of the AMBER instrument at the VLTI to realistic models of known extrasolar planetary systems, taking into account their theoretical spectra as well as the geometry of the VLTI. We present a strategy to determine the geometry of the planetary system and the spectrum of the extrasolar planet from closure phase observations in two steps. We show that there is a close relation between the nulls in the closure phase and the nulls in the corresponding single-baseline phases: every second null of a single-baseline phase is also a null in the closure phase. In particular, the nulls in the closure phase do not depend on the spectrum but only on the geometry. Therefore the geometry of the system can be determined by measuring the nulls in the closure phase, and braking the remaining ambiguity due to the unknown system orientation by means of observations at different hour angles. Based on the known geometry, the planet spectrum can then be directly synthesized from the closure phases.Comment: replaced version with corrected Fig.5; 9 pages, 6 figures, Proceeding of the SPIE conference, Glasgow, 2004, Proc. SPIE 5491, in pres

Where lies the peak of the brown dwarf binary separation distribution ?

Searches for companions of brown dwarfs by direct imaging probe mainly 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 if the peak of the separation distribution of brown dwarf binaries lies right in the unprobed range. Very recent work for the first time extends high-precision radial velocity surveys of brown dwarfs out to 3 AU (Joergens 2008, A&A). 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: it is 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, these are the brown dwarf candidate ChaHa8, 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 of the separation distribution of brown dwarfs towards smaller separations seem to occur between 1 and 3 AU; (iv) the observed continuous decrease of the binary frequency from the stellar to the substellar regime is confirmed at < 3 AU providing further evidence for a continuous formation mechanism from low-mass stars to brown dwarfs.Comment: Proceedings article of the conference 'Cool Stars 15' held July 2008 in St. Andrew

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

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

The Formation of Brown Dwarfs

In the presented work, a population of twelve very young bona fide and candidate brown dwarfs in the ChaI star forming cloud (ChaHa1-12) was studied observationally in terms of their kinematic properties, the occurrence of multiple systems among them as well as their rotational characteristics. Based on high-resolution spectra taken for nine out of the twelve objects with UVES at the VLT, radial and rotational velocities have been measured with high accuracy. A kinematic study of the sample showed that their radial velocity dispersion is relatively small (2.2 kms). It is significantly smaller than the radial velocity dispersion of the T~Tauri stars in the field (3.6 kms) and slightly larger than that one of the surrounding molecular gas (1.2 kms). This result indicates that the studied brown dwarfs are not ejected during their formation with velocities large than ~2 kms as proposed in recent formation scenarios. The brown dwarfs may have larger 3D velocities. However, brown dwarfs ejected during the early accretion phase in directions with a significant fraction perpendicular to the line-of-sight, would have flown out of the field a long time ago. By means of time-resolved UVES spectra, a radial velocity survey for close companions to the targets was conducted. In addition, a direct imaging survey for wide companions was carried out with the WFPC camera on board the HST, with FORS at the VLT as well as with SofI at the NTT. With these two complementary search methods, a wide range of possible companion separations has been covered. For brown dwarf companions (> 13 M_Jup) to the targets, separations < 3 AU and between 50 and 1000 AU were covered. With more restricted separations (< 0.1 AU and 300-1000 AU) the surveys were sensitive also to companion masses down to 1 M_Jup. HST images of ChaHa2 hint at a binary system comprised of two approximately equal-mass companions with a separation of ~30 AU. No further indications for companions have been found in the images. Furthermore, the radial velocities of the targets turned out to be rather constant setting upper limits for the mass Msini of possible companions to 0.1 M_Jup to 2 M_Jup These findings hint at a rather low (smaller or equal 10%) multiplicity fraction of the studied brown dwarfs. Furthermore, a photometric monitoring campaign of the targets yielded the determination of rotational periods for ChaHa2, 3 and 6 in the range of 2.2 to 3.4 days. These are the first rotational periods for very young brown dwarfs and among the first for brown dwarfs at all. They are complemented by measurements of rotational velocities vsini from UVES spectra. The observations show that brown dwarfs at an age of 1-5 Myr display surface spots like T Tauri stars and are moderately fast rotators in contrast to rapidly rotating old brown dwarfs consistent with them being in an early contracting stage. A comparison with rotational periods from the literature indicates that most of the acceleration of brown dwarfs takes place in the first 30 million years or less of their lifetime.In der vorliegenden Arbeit wurden zwölf sehr junge Braune Zwerge und Kandidaten fuer Braune Zwerge in der ChaI Sternentstehungswolke (ChaHa1-12) im Hinblick auf ihre kinematischen und Rotationseigenschaften, sowie auf das Vorkommen von Mehrfachsystemen hin untersucht. Hochaufgelöste Spektren aufgenommen mit UVES am VLT von neun der zwölf Objekte, erlaubten die Messung von Radial- und Rotationsgeschwindigkeiten mit hoher Genauigkeit. Eine kinematische Studie des Samples zeigte, dass ihre Radialgeschwindigkeitsdispersion relativ gering ist (2.2 kms). Sie ist deutlich geringer als die von T Tauri Sternen im gleichen Feld (3.6 kms) und etwas höher als die des sie umgebenden Gases (1.2 kms). Dieses Ergebnis deutet darauf hin, dass die untersuchten Braunen Zwerge während ihrer Entstehung nicht herausgeschleudert wurden, jedenfalls nicht mit Geschwindigkeiten grösser als ~2 kms, wie kürzlich vorgeschlagen wurde. Sie könnten eine grössere 3D Geschwindigkeit haben. Allerdings hätten Braune Zwerge, die in ihrer frühen Akkretionsphase in Richtungen mit einem signifikanten Anteil senkrecht zur Sehline geschleudert wurden, das Feld schon lange verlassen. Mittels zeitaufgelöster UVES Spektren wurde eine Radialgeschwindigkeitssuche nach engen Begleitern durchgeführt. Zusätzlich wurde mittels hochaufgelösten Bildern nach Begleitern in weiten Orbits gesucht, basierend auf Aufnahmen mit der WFPC Kamera an Bord des HST, mit FORS am VLT sowie mit SofI am NTT. Mit diesen beiden, sich ergänzenden Suchprogrammen konnte ein grosser Bereich möglicher Begleiterabstände erfasst werden. Für Braune Zwerg Begleiter (>13 M_Jup) wurde der Bereich < 3 AU und 50-1000 AU abgedeckt. In einem stärker eingeschränkten Bereich (< 0.1 AU und 300-1000 AU) sind die Surveys in der Lage Begleitermassen bis zu 1 M_Jup zu detektieren. HST Bilder von ChaHa2 deuten auf ein Doppelsystem mit zwei etwa gleichschweren Komponenten im Abstand von ~30 AU hin. Es wurden keine weiteren Hinweise auf Begleiter in den Bildern gefunden. Die Radialgeschwindigkeiten der untersuchten Objekte sind zudem relativ konstant und setzen obere Grenzen für die Masse Msini möglicher Begleiter von 0.1 M_Jup bis 2 M_Jup. Diese Ergebnisse deuten auf eine eher kleine Rate von Mehrfachsystemen (kleiner gleich 10%) unter den untersuchten Braunen Zwerge hin. Desweiteren wurden basierend auf photometrischen Beobachtungen Rotationsperioden für ChaHa2, 3 und 6 im Bereich von 2.2 bis 3.4 Tagen bestimmt. Dies sind die ersten Rotationsperioden für sehr junge Braune Zwerge und eine der ersten für Braune Zwerge überhaupt. Sie werden ergänzt durch die Messung von Rotationsgeschwindigkeiten vsini in UVES Spektren. Die Beobachtungen zeigen, dass Braune Zwerge in einem Alter von 1-5 Myr Flecken auf ihrer Oberfläche haben, ähnlich wie T Tauri Sterne und dass sie mit mittleren Geschwindigkeiten rotieren im Gegensatz zu schnell rotierenden alten Braunen Zwergen. Ein Vergleich mit bisher veröffentlichten Rotationsperioden von älteren Braunen Zwergen, weist darauf hin, dass ein Grossteil der Beschleunigung Brauner Zwerge in ihren ersten 30 Millionen Lebensjahren stattfindet

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