334 research outputs found
A probable close brown dwarf companion to GJ 1046 (M2.5V)
Context. Brown dwarf companions to stars at separations of a few AU or less
are rare objects, and none have been found so far around early-type M dwarfs
M0V-M5V). With GJ 1046 (M2.5V), a strong candidate for such a system with a
separation of 0.42 AU is presented.
Aims. We aim at constraining the mass of the companion in order to decide
whether it is a brown dwarf or a low-mass star.
Methods. We employed precision RV measurements to determine the orbital
parameters and the minimum companion mass. We then derived an upper limit to
the companion mass from the lack of disturbances of the RV measurements by a
secondary spectrum. An even tighter upper limit is subsequently established by
combining the RV-derived orbital parameters with the recent new version of the
Hipparcos Intermediate Astrometric Data.
Results. For the mass of the companion, we derive m>26.9 MJup from the RV
data. Based on the RV data alone, the probability that the companion exceeds
the stellar mass threshold is just 6.2%. The absence of effects from the
secondary spectrum lets us constrain the companion mass to m <229 MJup. The
combination of RV and Hipparcos data yields a 3sigma upper mass limit to the
companion mass of 112 MJup with a formal optimum value at m=47.2 MJup. From the
combination of RV and astrometric data, the chance probability that the
companion is a star is 2.9%.
Conclusions. We have found a low-mass, close companion to an early-type M
dwarf. While the most likely interpretation of this object is that it is a
brown dwarf, a low-mass stellar companion is not fully excluded.Comment: Accepted by A&A, 6 pages, 3 figures with 4 jpg files (Fig. 3 has two
panels), original version replaced as sidecaption in Figure 1 did not work.
Changes in 2nd replacement: (1) Conclusions: 99.9% --> 99.73% . (2) one
sentence below: the the --> the . (3) References: Joergens, V., M\"uller, A..
2007. ApJL, in press --> ... A., 2007, ApJ 666, L11
Toward detection of terrestrial planets in the habitable zone of our closest neighbor: Proxima Centauri
The precision of radial velocity (RV) measurements to detect indirectly
planetary companions of nearby stars has improved to enable the discovery of
extrasolar planets in the Neptune and Super-Earth mass range. Discoveries of
Earth-like planets by means of ground-based RV programs will help to determine
the parameter Eta_Earth, the frequency of potentially habitable planets around
other stars. In search of low-mass planetary companions we monitored Proxima
Centauri (M5V) as part of our M dwarf program. In the absence of a significant
detection, we use these data to demonstrate the general capability of the RV
method in finding terrestrial planets. For late M dwarfs the classic liquid
surface water habitable zone (HZ) is located close to the star, in which
circumstances the RV method is most effective. We want to demonstrate that late
M dwarfs are ideal targets for the search of terrestrial planets with the RV
technique. We obtained differential RV measurements of Proxima Cen over a time
span of 7 years with the UVES spectrograph at the ESO VLT. We determine upper
limits to the masses of companions in circular orbits by means of numerical
simulations. The RV data of Proxima Cen have a total rms scatter of 3.1 m/s and
a period search does not reveal any significant signals. As a result of our
companion limit calculations, we find that we successfully recover all test
signals with RV amplitudes corresponding to planets with m sin i > 2 - 3
M_Earth residing inside the HZ of Proxima Cen with a statistical significance
of >99%. Over the same period range, we can recover 50% of the test planets
with masses of m sin i > 1.5 - 2.5 M_Earth. Based on our simulations, we
exclude the presence of any planet in a circular orbit with m sin i > 1
M_Neptune at separations of a < 1 AU.Comment: 8 pages, 4 figures, accepted for publication in Astronomy &
Astrophysic
The M dwarf planet search programme at the ESO VLT + UVES. A search for terrestrial planets in the habitable zone of M dwarfs
We present radial velocity (RV) measurements of our sample of 40 M dwarfs
from our planet search programme with VLT+UVES begun in 2000. Although with our
RV precision down to 2 - 2.5 m/s and timebase line of up to 7 years, we are
capable of finding planets of a few Earth masses in the close-in habitable
zones of M dwarfs, there is no detection of a planetary companion. To
demonstrate this we present mass detection limits allowing us to exclude
Jupiter-mass planets up to 1 AU for most of our sample stars. We identified 6 M
dwarfs that host a brown dwarf or low-mass stellar companion. With the
exception of these, all other sample stars show low RV variability with an rms
< 20 m/s. Some high proper motion stars exhibit a linear RV trend consistent
with their secular acceleration. Furthermore, we examine our data sets for a
possible correlation between RVs and stellar activity as seen in variations of
the Halpha line strength. For Barnard's star we found a significant
anticorrelation, but most of the sample stars do not show such a correlation.Comment: 13 pages, 12 figures, 5 tables, accepted by A&
Disentangling 2:1 resonant radial velocity orbits from eccentric ones and a case study for HD 27894
In radial velocity observations, a pair of extrasolar planets near a 2:1
orbital resonance can be misinterpreted as a single eccentric planet, if data
are sparse and measurement precision insufficient to distinguish between these
models. We determine the fraction of alleged single-planet RV detected systems
for which a 2:1 resonant pair of planets is also a viable model and address the
question of how the models can be disentangled. By simulation we quantified the
mismatch arising from applying the wrong model. Model alternatives are
illustrated using the supposed single-planet system HD 27894 for which we also
study the dynamical stability of near-2:1 resonant solutions. From the data
scatter around the fitted single-planet Keplerians, we find that for of
the putative single-planet systems, a 2:1 resonant pair cannot be
excluded as a viable model, since the error due to the wrong model is smaller
than the scatter. For stars -probabilities can be used to reject
the Keplerian models with a confidence of for of the stars and
with for of the stars. For HD 27894 a considerable fit
improvement is obtained when adding a low-mass planet near half the orbital
period of the known Jovian planet. Dynamical analysis demonstrates that this
system is stable when both planets are initially placed on circular orbits. For
fully Keplerian orbits a stable system is only obtained if the eccentricity of
the inner planet is constrained to . A large part of the allegedly RV
detected single-planet systems should be scrutinized in order to determine the
fraction of systems containing near-2:1 resonant pairs of planets. Knowing the
abundance of such systems will allow us to revise the eccentricity distribution
for extrasolar planets and provide direct constraints for planetary system
formation.Comment: 12 pages, 8 figures, one of them composed by two files, accepted by
A&A, citations may appear in a non-standard way (double brackets) due to
reformatting needs. Abstract slightly adjuste
X-ray rotational modulation of a supersaturated star in IC 2391
We present evidence of X-ray rotational modulation on VXR45, a young fast
rotator star, member of IC 2391. It is a dG9 spectral type star whose
rotational period and X-ray luminosity make it a supersaturated star. Our X-ray
observation, made with EPIC/PN on XMM-Newton, covers about two photometric
rotational periods. The detection of rotational modulation implies the presence
of structural inhomogeneities. Possible interpretations are presented and
discussed.Comment: 4 pages with 4 PostScript figures. Accepted for publication in
Astronomy & Astrophysics Letter
TESS exoplanet candidates validated with HARPS archival data. A massive Neptune around GJ143 and two Neptunes around HD23472
We aim at the discovery of new planetary systems by exploiting the transit
light curve results from TESS orbital observatory's Sector 1 and 2 observations
and validating them with precise Doppler measurements obtained from archival
HARPS data. Taking advantage of the reported TESS transit events around GJ143
(TOI 186) and HD23472 (TOI 174) we model their HARPS precise Doppler
measurements and derive orbital parameters for these two systems. For the GJ143
system TESS has reported only a single transit, and thus its period is
unconstrained from photometry. Our RV analysis of GJ143 reveal the full
Keplerian solution of the system, which is consistent with an eccentric planet
with a mass almost twice that of Neptune and a period of =
days. Our estimates of the GJ143 b planet are fully
consistent with the transit timing from TESS. We confirm the two-planet system
around HD23472, which according to our analysis is composed of two Neptune mass
planets in a possible 5:3 MMR.Comment: Submitted to A&A on 10th December 2018, Accepted on 14 January 2019,
Published online on 30 January 201
The planet search program at the ESO Coude Echelle spectrometer. III. The complete Long Camera survey results
We present the complete results of the planet search program carried out at the ESO Coude Echelle Spectrometer (CES) on La Silla, using the Long Camera from Nov. 1992 to April 1998. The CES survey has monitored 37 late-type (F8V - M5V) stars in the southern hemisphere for variations in their differential radial velocities (RV) in order to detect Doppler reflex motions caused by planetary companions. This led to the discovery of the first extrasolar planet in an Earth-like orbit around the young (ZAMS) and active G0V star iota Horologii (Kuerster et al. 2000). Here we present the RV results for all survey stars and perform a statistical examination of the whole data-set. Each star is tested for RV variability, RV trends (linear and non-linear) and significant periodic signals. Beta Hyi and eps Ind are identified as long-term, low-amplitude RV variables. Furthermore, for 30 CES survey stars we determine quantitative upper mass-limits for giant planets based on our long-term RV results. We find that the CES Long Camera survey would have detected short-period (51 Peg-type) planets around all 30 stars but no planets with m sin i < 1 M_Jup at orbital separations larger than 2 AU. Finally, we demonstrate that the CES planet search can be continued without applying velocity corrections to the RV results coming from the currently installed Very Long Camera at the CES
Activity-induced radial velocity jitter in a flaring M dwarf
We investigate the effect of stellar activity and flares on short-term radial
velocity measurements in the mid-M flare star CN Leo. Radial velocity
variations are calculated from 181 UVES spectra obtained during three nights.
We searched for spectral orders that contain very few atmospheric absorption
lines and calibrated them against the telluric A-band from O in the Earth's
atmosphere. One giant flare occurred during our observations, which has a very
strong effect on radial velocity. The apparent radial velocity shift due to the
flare is several hundred m s and clearly correlated with H
emission. Outside the flare, only spectral orders containing the most prominent
emission lines of H, He, and Ca show a correlation to chromospheric activity
together with a radial velocity jitter exceeding a few 10 m s. We
identify a number of spectral orders that are free of strong emission lines and
show no flaring-related radial velocity jitter, although flares occurred as
strong as 0.4 dex in normalized H luminosity. The mean radial velocity
jitter due to moderate flaring is less than 10 m s. Strong flares are
easily recognized directly in the spectra and should be neglected for planet
searches.Comment: accepted by A&
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