115 research outputs found
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
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
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&
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
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