262 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
Three planets around HD 27894. A close-in pair with a 2:1 period ratio and an eccentric Jovian planet at 5.4 AU
Aims. Our new program with HARPS aims to detect mean motion resonant
planetary systems around stars which were previously reported to have a single
bona fide planet, often based only on sparse radial velocity data. Methods.
Archival and new HARPS radial velocities for the K2V star HD 27894 were
combined and fitted with a three-planet self-consistent dynamical model. The
best-fit orbit was tested for long-term stability. Results. We find clear
evidence that HD 27894 is hosting at least three massive planets. In addition
to the already known Jovian planet with a period 18 days
we discover a Saturn-mass planet with 36 days, likely in
a 2:1 mean motion resonance with the first planet, and a cold massive planet
( 5.3 ) with a period 5170
days on a moderately eccentric orbit ( = 0.39). Conclusions. HD
27894 is hosting a massive, eccentric giant planet orbiting around a tightly
packed inner pair of massive planets likely involved in an asymmetric 2:1 mean
motion resonance. HD 27894 may be an important milestone for probing planetary
formation and evolution scenarios.Comment: 4 pages, 2 tables, 3 figures. Accepted for publication in A&A Letters
to the Edito
The planet search programme at the ESO CES and HARPS. IV. The search for Jupiter analogues around solar-like stars
In 1992 we began a precision radial velocity (RV) survey for planets around
solar-like stars with the Coude Echelle Spectrograph and the Long Camera (CES
LC) at the 1.4 m telescope in La Silla (Chile). We have continued the survey
with the upgraded CES Very Long Camera (VLC) and HARPS, both at the 3.6 m
telescope, until 2007. The observations for 31 stars cover a time span of up to
15 years and the RV precision permit a search for Jupiter analogues. We perform
a joint analysis for variability, trends, periodicities, and Keplerian orbits
and compute detection limits. Moreover, the HARPS RVs are analysed for
correlations with activity indicators (CaII H&K and CCF shape). We achieve a
long-term RV precision of 15 m/s (CES+LC, 1992-1998), 9 m/s (CES+VLC,
1999-2006), and 2.8 m/s (HARPS, 2003-2009, including archive data), resp. This
enables us to confirm the known planets around Iota Hor, HR 506, and HR 3259. A
steady RV trend for Eps Ind A can be explained by a planetary companion. On the
other hand, we find previously reported trends to be smaller for Beta Hyi and
not present for Alp Men. The candidate planet Eps Eri b was not detected
despite our better precision. Also the planet announced for HR 4523 cannot be
confirmed. Long-term trends in several of our stars are compatible with known
stellar companions. We provide a spectroscopic orbital solution for the binary
HR 2400 and refined solutions for the planets around HR 506 and Iota Hor. For
some other stars the variations could be attributed to stellar activity. The
occurrence of two Jupiter-mass planets in our sample is in line with the
estimate of 10% for the frequency of giant planets with periods smaller than 10
yr around solar-like stars. We have not detected a Jupiter analogue, while the
detections limits for circular orbits indicate at 5 AU a sensitivity for
minimum mass of at least 1 M_Jup (2 M_Jup) for 13% (61%) of the stars.Comment: 63 pages, 24 figures (+33 online figures), 13 Tables, accepted for
publication in A&A (2012-11-13
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
Detection of differential rotation in psi Cap with profile analysis
We report detection of differential rotation on the F5 dwarf psi Cap using
line profile analysis. The Fourier transform of both FeI lambda 5775 and SiI
lambda 5772 are used to obtain a projected rotational velocity of v sini =
(42+-1)km/s. Modelling of the Fourier transformed profiles shows that the
combined effects of equatorial velocity, inclination and differential rotation
dominate the line profile while limb darkening and turbulence velocities have
only minor effects. Rigid rotation is shown to be inconsistent with the
measured profiles. Modelling the line profiles analogous to solar differential
rotation we find a differential rotation parameter of alpha = 0.15+-0.1
(15+-10%) comparable to the solar case. To our knowledge this is the first
successful measurement of differential rotation through line profile analysis.Comment: 4 pages, 4 figures, accepted for publication in A&A Letter
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