176 research outputs found
Strong HI Lyman- variations from the 11 Gyr-old host star Kepler-444: a planetary origin ?
Kepler-444 provides a unique opportunity to probe the atmospheric composition
and evolution of a compact system of exoplanets smaller than the Earth. Five
planets transit this bright K star at close orbital distances, but they are too
small for their putative lower atmosphere to be probed at optical/infrared
wavelengths. We used the Space Telescope Imaging Spectrograph instrument
onboard the Hubble Space Telescope to search for the signature of the planet's
upper atmospheres at six independent epochs in the Ly- line. We detect
significant flux variations during the transits of both Kepler-444e and f
(~20%), and also at a time when none of the known planets was transiting
(~40%). Variability in the transition region and corona of the host star might
be the source of these variations. Yet, their amplitude over short time scales
(~2-3 hours) is surprisingly strong for this old (11.2+-1.0Gyr) and apparently
quiet main-sequence star. Alternatively, we show that the in-transits
variations could be explained by absorption from neutral hydrogen exospheres
trailing the two outer planets (Kepler-444e and f). They would have to contain
substantial amounts of water to replenish such hydrogen exospheres, which would
reveal them as the first confirmed ocean-planets. The out-of-transit
variations, however, would require the presence of a yet-undetected Kepler-444g
at larger orbital distance, casting doubt on the planetary origin scenario.
Using HARPS-N observations in the sodium doublet, we derived the properties of
two Interstellar Medium clouds along the line-of-sight toward Kepler-444. This
allowed us to reconstruct the stellar Ly- line profile and to estimate
the XUV irradiation from the star, which would still allow for a moderate mass
loss from the outer planets after 11.2Gyr. Follow-up of the system at XUV
wavelengths will be required to assess this tantalizing possibility.Comment: Accepted for publication in A&A Name of the system added to the title
in most recent versio
The HARPS search for southern extra-solar planets XLI. A dozen planets around the M dwarfs GJ 3138, GJ 3323, GJ 273, GJ 628, and GJ 3293
Context. Low mass stars are currently the best targets for searches for rocky
planets in the habitable zone of their host star. Over the last 13 years,
precise radial velocities measured with the HARPS spectrograph have identified
over a dozen super-Earths and Earth-mass planets (msin i<10Mearth ) around M
dwarfs, with a well understood selection function. This well defined sample
informs on their frequency of occurrence and on the distribution of their
orbital parameters, and therefore already constrains our understanding of
planetary formation. The subset of these low-mass planets that were found
within the habitable zone of their host star also provide prized targets for
future atmospheric biomarkers searches. Aims. We are working to extend this
planetary sample to lower masses and longer periods through dense and long-term
monitoring of the radial velocity of a small M dwarf sample. Methods. We
obtained large numbers of HARPS spectra for the M dwarfs GJ 3138, GJ 3323, GJ
273, GJ 628 and GJ 3293, from which we derived radial velocities (RVs) and
spectroscopic activity indicators. We searched them for variabilities,
periodicities, Keplerian modulations and correlations, and attribute the
radial-velocity variations to combinations of planetary companions and stellar
activity. Results. We detect 12 planets, of which 9 are new with masses ranging
from 1.17 to 10.5 Mearth . Those planets have relatively short orbital periods
(P<40 d), except two of them with periods of 217.6 and 257.8 days. Among these
systems, GJ 273 harbor two planets with masses close to the one of the Earth.
With a distance of 3.8 parsec only, GJ 273 is the second nearest known
planetary system - after Proxima Centauri - with a planet orbiting the
circumstellar habitable zone.Comment: 19 pages, 24 figures. Astronomy and Astrophysics in pres
Characterisation of stellar activity of M dwarfs. I. Long-timescale variability in a large sample and detection of new cycles
M dwarfs are active stars that exhibit variability in chromospheric emission
and photometry at short and long timescales, including long cycles that are
related to dynamo processes. This activity also impacts the search for
exoplanets because it affects the radial velocities. We analysed a large sample
of 177 M dwarfs observed with HARPS (2003-2020) in order to characterise the
long-term variability of these stars. We compared the variability obtained in
three chromospheric activity indices (Ca II H & K, the Na D doublet, and
Halpha) and with ASAS photometry. We focused on the detailed analysis of the
chromospheric emission based on linear, quadratic, and sinusoidal models. We
used various tools to estimate the significance of the variability and to
quantify the improvement brought by the models. In addition, we analysed
complementary photometric time series for the most variable stars to be able to
provide a broader view of the long-term variability in M dwarfs. We find that
most stars are significantly variable, even the quietest stars. Most stars in
our sample (75%) exhibit a long-term variability, which manifests itself mostly
through linear or quadratic variability, although the true behaviour may be
more complex. We found significant variability with estimated timescales for 24
stars, and estimated the lower limit for a possible cycle period for an
additional 9 stars that were not previously published. We found evidence of
complex variability because more than one long-term timescale may be present
for at least 12 stars, together with significant differences between the
behaviour of the three activity indices. This complexity may also be the source
of the discrepancies observed between previous publications. We conclude that
long-term variability is present for all spectral types and activity level in M
dwarfs, without a significant trend with spectral type or mean activity level.Comment: article accepted in Astronomy and Astrophysics, February 2023, 31
page
The HARPS search for southern extra-solar planets XXXV. Planetary systems and stellar activity of the M dwarfs GJ 3293, GJ 3341, and GJ 3543
Context. Planetary companions of a fixed mass induce larger amplitude reflex
motions around lower-mass stars, which helps make M dwarfs excellent targets
for extra-solar planet searches. State of the art velocimeters with 1m/s
stability can detect very low-mass planets out to the habitable zone of these
stars. Low-mass, small, planets are abundant around M dwarfs, and most known
potentially habitable planets orbit one of these cool stars.
Aims. Our M-dwarf radial velocity monitoring with HARPS on the ESO 3.6m
telescope at La Silla observatory makes a major contribution to this sample.
Methods. We present here dense radial velocity (RV) time series for three M
dwarfs observed over years: GJ 3293 (0.42M), GJ 3341
(0.47M), and GJ 3543 (0.45M). We extract those RVs through
minimum matching of each spectrum against a high S/N ratio stack of
all observed spectra for the same star. We then vet potential orbital signals
against several stellar activity indicators, to disentangle the Keplerian
variations induced by planets from the spurious signals which result from
rotational modulation of stellar surface inhomogeneities and from activity
cycles.
Results. Two Neptune-mass planets - and
- orbit GJ 3293 with periods d and
d, possibly together with a super-Earth -
- with period . A super-Earth
- - orbits GJ 3341 with . The RV
variations of GJ 3543, on the other hand, reflect its stellar activity rather
than planetary signals.Comment: Accepted for publication in A&A, 19 pages, 12 figures, 7 table
Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit
The bright M dwarf K2-18 at 34 pc is known to host a transiting
super-Earth-sized planet orbiting within the star's habitable zone; K2-18b.
Given the superlative nature of this system for studying an exoplanetary
atmosphere receiving similar levels of insolation as the Earth, we aim to
characterize the planet's mass which is required to interpret atmospheric
properties and infer the planet's bulk composition. We obtain precision radial
velocity measurements with the HARPS spectrograph and couple those measurements
with the K2 photometry to jointly model the observed radial velocity variation
with planetary signals and a radial velocity jitter model based on Gaussian
process regression. We measure the mass of K2-18b to be
M with a bulk density of g/cm which may correspond
to a predominantly rocky planet with a significant gaseous envelope or an ocean
planet with a water mass fraction %. We also find strong evidence
for a second, warm super-Earth K2-18c at days with a semi-major axis
2.4 times smaller than the transiting K2-18b. After re-analyzing the available
light curves of K2-18 we conclude that K2-18c is not detected in transit and
therefore likely has an orbit that is non-coplanar with K2-18b. A suite of
dynamical integrations with varying simulated orbital eccentricities of the two
planets are used to further constrain each planet's eccentricity posterior from
which we measure and at 99% confidence. The discovery
of the inner planet K2-18c further emphasizes the prevalence of multi-planet
systems around M dwarfs. The characterization of the density of K2-18b reveals
that the planet likely has a thick gaseous envelope which along with its
proximity to the Solar system makes the K2-18 planetary system an interesting
target for the atmospheric study of an exoplanet receiving Earth-like
insolation.Comment: 13 pages, 8 figures including 4 interactive figures best viewed in
Adobe Acrobat. Submitted to Astronomy & Astrophysics. Comments welcom
A super-Earth orbiting the nearby M-dwarf GJ 536
We report the discovery of a super-Earth orbiting the star GJ 536 based on
the analysis of the radial-velocity time series from the HARPS and HARPS-N
spectrographs. GJ 536 b is a planet with a minimum mass M sin of 5.36 +-
0.69 Me with an orbital period of 8.7076 +- 0.0025 days at a distance of
0.066610(13) AU, and an orbit that is consistent with circular. The host star
is the moderately quiet M1 V star GJ 536, located at 10 pc from the Sun. We
find the presence of a second signal at 43 days that we relate to stellar
rotation after analysing the time series of Ca II H&K and H alpha spectroscopic
indicators and photometric data from the ASAS archive. We find no evidence
linking the short period signal to any activity proxy. We also tentatively
derived a stellar magnetic cycle of less than 3 years.Comment: 14 pages, 14 figures, 5 tables, Accepted in A&
Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS) I. Detection of hot neutral sodium at high altitudes on WASP-49b
High-resolution optical spectroscopy during the transit of HD 189733b, a
prototypical hot Jupiter, allowed the resolution of the Na I D sodium lines in
the planet, giving access to the extreme conditions of the planet upper
atmosphere. We have undertaken HEARTS, a spectroscopic survey of exoplanet
upper atmospheres, to perform a comparative study of hot gas giants and
determine how stellar irradiation affect them. Here, we report on the first
HEARTS observations of the hot Saturn-mass planet WASP-49b. We observed the
planet with the HARPS high-resolution spectrograph at ESO 3.6m telescope. We
collected 126 spectra of WASP-49, covering three transits of WASP-49b. We
analyzed and modeled the planet transit spectrum, while paying particular
attention to the treatment of potentially spurious signals of stellar origin.
We spectrally resolve the Na I D lines in the planet atmosphere and show that
these signatures are unlikely to arise from stellar contamination. The large
contrasts of (D) and (D) require the
presence of hot neutral sodium ( K) at high altitudes
(1.5 planet radius or 45,000 km). From estimating the cloudiness
index of WASP-49b, we determine its atmosphere to be cloud free at the
altitudes probed by the sodium lines. WASP-49b is close to the border of the
evaporation desert and exhibits an enhanced thermospheric signature with
respect to a farther-away planet such as HD 189733b.Comment: Accepted for publication in A&A. 14 page
Atmospheric characterization of Proxima b by coupling the Sphere high-contrast imager to the Espresso spectrograph
Context. The temperate Earth-mass planet Proxima b is the closest exoplanet
to Earth and represents what may be our best ever opportunity to search for
life outside the Solar System. Aims. We aim at directly detecting Proxima b and
characterizing its atmosphere by spatially resolving the planet and obtaining
high-resolution reflected-light spectra. Methods. We propose to develop a
coupling interface between the SPHERE high-contrast imager and the new ESPRESSO
spectrograph, both installed at ESO VLT. The angular separation of 37 mas
between Proxima b and its host star requires the use of visible wavelengths to
spatially resolve the planet on a 8.2-m telescope. At an estimated
planet-to-star contrast of ~10^-7 in reflected light, Proxima b is extremely
challenging to detect with SPHERE alone. However, the combination of a
~10^3-10^4 contrast enhancement from SPHERE to the high spectral resolution of
ESPRESSO can reveal the planetary spectral features and disentangle them from
the stellar ones. Results. We find that significant but realistic upgrades to
SPHERE and ESPRESSO would enable a 5-sigma detection of the planet and yield a
measurement of its true mass and albedo in 20-40 nights of telescope time,
assuming an Earth-like atmospheric composition. Moreover, it will be possible
to probe the O2 bands at 627, 686 and 760 nm, the water vapour band at 717 nm,
and the methane band at 715 nm. In particular, a 3.6-sigma detection of O2
could be made in about 60 nights of telescope time. Those would need to be
spread over 3 years considering optimal observability conditions for the
planet. Conclusions. The very existence of Proxima b and the SPHERE-ESPRESSO
synergy represent a unique opportunity to detect biosignatures on an exoplanet
in the near future. It is also a crucial pathfinder experiment for the
development of Extremely Large Telescopes and their instruments (abridged).Comment: 16 pages, 7 figures, revised version accepted to A&
Deriving High-Precision Radial Velocities
This chapter describes briefly the key aspects behind the derivation of
precise radial velocities. I start by defining radial velocity precision in the
context of astrophysics in general and exoplanet searches in particular. Next I
discuss the different basic elements that constitute a spectrograph, and how
these elements and overall technical choices impact on the derived radial
velocity precision. Then I go on to discuss the different wavelength
calibration and radial velocity calculation techniques, and how these are
intimately related to the spectrograph's properties. I conclude by presenting
some interesting examples of planets detected through radial velocity, and some
of the new-generation instruments that will push the precision limit further.Comment: Lecture presented at the IVth Azores International Advanced School in
Space Sciences on "Asteroseismology and Exoplanets: Listening to the Stars
and Searching for New Worlds" (arXiv:1709.00645), which took place in Horta,
Azores Islands, Portugal in July 201
The SOPHIE search for northern extrasolar planets: VI. Three new hot Jupiters in multi-planet extrasolar systems
We present high-precision radial-velocity measurements of three solar-type
stars: HD 13908, HD 159243, and HIP 91258. The observations were made with the
SOPHIE spectrograph at the 1.93-m telescope of Observatoire de Haute-Provence
(France). They show that these three bright stars host exoplanetary systems
composed of at least two companions. HD 13908 b is a planet with a minimum mass
of 0.865+-0.035 Mjup, on a circular orbit with a period of 19.382+-0.006 days.
There is an outer massive companion in the system with a period of 931+-17
days, e = 0.12+-0.02, and a minimum mass of 5.13+-0.25 Mjup. The star HD
159243, also has two detected companions with respective masses, periods, and
eccentricities of Mp = 1.13+-0.05 and 1.9+-0.13 Mjup, = 12.620+-0.004 and
248.4+-4.9 days, and e = 0.02+-0.02 and 0.075+-0.05. Finally, the star HIP
91258 has a planetary companion with a minimum mass of 1.068+-0.038 Mjup, an
orbital period of 5.0505+-0.0015 days, and a quadratic trend indicating an
outer planetary or stellar companion that is as yet uncharacterized. The
planet-hosting stars HD 13908, HD 159243, and HIP 91258 are main-sequence stars
of spectral types F8V, G0V, and G5V, respectively, with moderate activity
levels. HIP 91258 is slightly over-metallic, while the two other stars have
solar-like metallicity. The three systems are discussed in the frame of
formation and dynamical evolution models of systems composed of several giant
planets.Comment: accepted in A&
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