147 research outputs found
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
Spin Gaps in High Temperature Superconductors
The phenomenology and theory of spin gap effects in high temperature
superconductors is summarized. It is argued that the spin gap behavior can only
be explained by a model of charge 0 spin 1/2 fermions which become paired into
singlets and that there are both theoretical and experimental reasons for
believing that the pairing is greatly enhanced in the bilayer structure of the
system.
This article will appear in the Proceedings of the Stanford Conference on
Spectroscopies in Novel Superconductors. To obtain postscript files containing
the figures send mail to [email protected]: 9 pages, revtex. To obtain figures contact [email protected]
Superconducting Order Parameter Symmetry in Multi-layer Cuprates
We discuss the allowed order parameter symmetries in multi-layer cuprates and
their physical consequences using highly non-specific forms of the inter- and
intra-plane interactions. Within this framework, the bi-layer case is discussed
in detail with particular attention paid to the role of small orthorhombic
distortions as would derive from the chains in YBCO or superlattice effects in
BSCCO. In the orthorhombic bi-layer case the (s,-s) state is of special
interest, since for a wide range of parameters this state exhibits pi phase
shifts in corner Josephson junction experiments. In addition, its transition
temperature is found to be insensitive to non-magnetic inter-plane disorder, as
would be present at the rare earth site in YBCO, for example. Of particular
interest, also, are the role of van Hove singularities which are seen to
stabilize states with d_{x^2 - y^2}-like symmetry, (as well as nodeless
s-states) and to elongate the gap functions along the four van Hove points,
thereby leading to a substantial region of gaplessness. We find that d_{x^2 -
y^2}-like states are general solutions for repulsive interactions; they possess
the fewest number of nodes and therefore the highest transition temperatures.
In this way, they should not be specifically associated with a spin fluctuation
driven pairing mechanism.Comment: REVTeX documentstyle, 34 pages, 10 figures include
Near-IR and optical radial velocities of the active M-dwarf star Gl 388 (AD Leo) with SPIRou at CFHT and SOPHIE at OHP
Context: The search for extrasolar planets around the nearest M-dwarfs is a
crucial step towards identifying the nearest Earth-like planets. One of the
main challenges in this search is that M-dwarfs can be magnetically active and
stellar activity can produce radial velocity (RV) signals that could mimic
those of a planet.
Aims: We aim to investigate whether the 2.2 day period observed in optical
RVs of the nearby active M-dwarf star Gl 388 (AD Leo) is due to stellar
activity or to a planet which co-rotates with the star as suggested in the
past.
Methods: We obtained quasi-simultaneous optical RVs of Gl 388 from 2019 to
2021 with SOPHIE (R75k) at the OHP in France, and near-IR RV and Stokes V
measurements with SPIRou at the CFHT (R70k).
Results: The SOPHIE RV time-series displays a periodic signal with
2.230.01 days period and 23.60.5 m/s amplitude, which is consistent
with previous HARPS observations obtained in 2005-2006. The SPIRou RV
time-series is flat at 5 m/s rms and displays no periodic signals. RV signals
of amplitude higher than 5.3 m/s at a period of 2.23 days can be excluded with
a confidence level higher than 99%. Using the modulation of the longitudinal
magnetic field (Bl) measured with SPIRou, we derive a stellar rotation period
of 2.23050.0016 days.
Conclusions: SPIRou RV measurements provide solid evidence that the periodic
variability of the optical RVs of Gl 388 is due to stellar activity rather than
to a co-rotating planet. The magnetic activity nature of the optical RV signal
is further confirmed by the modulation of Bl with the same period. The SPIRou
campaign on Gl 388 demonstrates the power of near-IR RV to confirm or infirm
planet candidates discovered in the optical around active stars. SPIRou
observations reiterate how effective spectropolarimetry is at determining the
stellar rotation period.Comment: 25 pages, 23 figures, Accepted by Astronomy and Astrophysic
A pair of tess planets spanning the radius valley around the nearby mid-m dwarf ltt 3780
We present the confirmation of two new planets transiting the nearby mid-M
dwarf LTT 3780 (TIC 36724087, TOI-732, , , =0.374 R, =0.401 M, d=22 pc). The two planet candidates are identified in a single TESS sector and are validated with reconnaissance spectroscopy, ground-based photometric follow-up, and high-resolution imaging. With measured orbital periods of days, days and sizes R, R, the two planets span the radius valley in period-radius space around low mass stars thus making the system a laboratory to test competing theories of the emergence of the radius valley in that stellar mass regime. By combining 63 precise radial-velocity measurements from HARPS and HARPS-N, we measure planet masses
of M and M, which indicates that LTT 3780b has a bulk composition consistent with being Earth-like, while LTT 3780c likely hosts an extended H/He envelope.
We show that the recovered planetary masses are consistent with predictions from both photoevaporation and from core-powered mass loss models. The brightness and small size of LTT 3780, along with the measured planetary parameters, render LTT 3780b and c as accessible targets for atmospheric characterization of planets within the same planetary system and spanning the radius valley
Limb proportions show developmental plasticity in response to embryo movement
Animals have evolved limb proportions adapted to different environments, but it is not yet clear to what extent these proportions are directly influenced by the environment during prenatal development. The developing skeleton experiences mechanical loading resulting from embryo movement. We tested the hypothesis that environmentally-induced changes in prenatal movement influence embryonic limb growth to alter proportions. We show that incubation temperature influences motility and limb bone growth in West African Dwarf crocodiles, producing altered limb proportions which may, influence post-hatching performance. Pharmacological immobilisation of embryonic chickens revealed that altered motility, independent of temperature, may underpin this growth regulation. Use of the chick also allowed us to merge histological, immunochemical and cell proliferation labelling studies to evaluate changes in growth plate organisation, and unbiased array profiling to identify specific cellular and transcriptional targets of embryo movement. This disclosed that movement alters limb proportions and regulates chondrocyte proliferation in only specific growth plates. This selective targeting is related to intrinsic mTOR (mechanistic target of rapamycin) pathway activity in individual growth plates. Our findings provide new insights into how environmental factors can be integrated to influence cellular activity in growing bones and ultimately gross limb morphology, to generate phenotypic variation during prenatal development
Accuracy of Urea Breath Test Performed Immediately After Emergency Endoscopy in Peptic Ulcer Bleeding
Erratum to: 36th International Symposium on Intensive Care and Emergency Medicine
[This corrects the article DOI: 10.1186/s13054-016-1208-6.]
A hot terrestrial planet orbiting the bright M dwarf L 168-9 unveiled by TESS
We report the detection of a transiting super-Earth-sized planet (R = 1.39 ± 0.09 R⊕ ) in a 1.4-day orbit around L 168-9 (TOI-134), a bright M1V dwarf (V = 11, K = 7.1) located at 25.15 ± 0.02 pc. The host star was observed in the first sector of the Transiting Exoplanet Survey Satellite (TESS) mission. For confirmation and planet mass measurement purposes, this was followed up with ground-based photometry, seeing-limited and high-resolution imaging, and precise radial velocity (PRV) observations using the HARPS and Magellan/PFS spectrographs. By combining the TESS data and PRV observations, we find the mass of L 168-9 b to be 4.60 ± 0.56 M⊕ and thus the bulk density to be 1.74-0.33+0.44 times higher than that of the Earth. The orbital eccentricity is smaller than 0.21 (95% confidence). This planet is a level one candidate for the TESS mission's scientific objective of measuring the masses of 50 small planets, and it is one of the most observationally accessible terrestrial planets for future atmospheric characterization
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