High-resolution observations of edge-on protoplanetary disks in emission from molecular species that sample different critical densities and formation pathways offer the opportunity to trace the ...vertical chemical and physical structures of protoplanetary disks. Among the problems that can be addressed is the origin and significance of the bright CN emission that is a ubiquitous feature of disks. Based on analysis of subarcsecond-resolution Atacama Large Millimeter Array archival data for the edge-on Flying Saucer disk (2MASS J16281370-2431391), we establish the vertical and radial differentiation of the CN emitting regions of the disk with respect to those of
12
CO and CS, and we model the physical disk conditions from which the CN emission arises. We demonstrate that the
12
CO (2–1), CN (2–1), and CS
J
= 5–4 emitting regions of the disk decrease in scale height above the midplane, such that
12
CO, CN, and CS trace layers of increasing density and decreasing temperature. We find that at radii >100 au from the central star, CN emission predominantly arises from intermediate layers, while in the inner region of the disk CN appears to arise from layers closer to the midplane. We investigate the physical conditions of the disk within the CN emitting regions, as well as the ranges of CN excitation temperature and column density, via RADEX non-local thermodynamic equilibrium (non-LTE) modeling of the three brightest CN hyperfine lines. Near the disk midplane, where we derive densities of
n
H
2
~ 10
7
cm
−3
at relatively low
T
kin
(~12 K), we find that CN is thermalized, while sub-thermal, non-LTE conditions appear to obtain CN emission from higher (intermediate) disk layers. We consider whether and how the particular spatial location and excitation conditions of CN emission from the Flying Saucer can be related to CN production that is governed, radially and vertically, by the degree of irradiation of the flared disk by X-rays and UV photons from the central star.
Context.
HD 13724 is a nearby solar-type star at 43.48 ± 0.06 pc hosting a long-period low-mass brown dwarf detected with the CORALIE echelle spectrograph as part of the historical CORALIE ...radial-velocity search for extra-solar planets. The companion has a minimum mass of 26.77
−2.2
+4.4
M
Jup
and an expected semi-major axis of ~240 mas making it a suitable target for further characterisation with high-contrast imaging, in particular to measure its inclination, mass, and spectrum and thus establish its substellar nature.
Aims.
Using high-contrast imaging with the SPHERE instrument on the Very Large Telescope (VLT), we are able to directly image a brown dwarf companion to HD 13724 and obtain a low-resolution spectrum.
Methods.
We combine the radial-velocity measurements of CORALIE and HARPS taken over two decades and high-contrast imaging from SPHERE to obtain a dynamical mass estimate. From the SPHERE data we obtain a low-resolution spectrum of the companion from
Y
to
J
band, as well as photometric measurements from IRDIS in the
J
,
H
, and
K
bands.
Results.
Using high-contrast imaging with the SPHERE instrument at the VLT, we report the first images of a brown dwarf companion orbiting the host star HD 13724. It has an angular separation of 175.6 ± 4.5 mas and an
H
-band contrast of 10.61 ± 0.16 mag, and using the age estimate of the star to be ~1 Gyr gives an isochronal mass estimate of ~44
M
Jup
. By combining radial-velocity and imaging data we also obtain a dynamical mass of 50.5
−3.5
+3.3
M
Jup
. Through fitting an atmospheric model, we estimate a surface gravity of log
g
= 5.5 and an effective temperature of 1000 K. A comparison of its spectrum with observed T dwarfs estimates a spectral type of T4 or T4.5, with a T4 object providing the best fit.
Owing to their low mass and luminosity, M dwarfs are ideal targets if one hopes to find low-mass planets similar to Earth using the radial velocity (RV) method. However, stellar magnetic cycles could ...add noise or even mimic the RV signal of a long-period companion. We extend our previous study of the correlation between activity cycles and long-term RV variations for K dwarfs to the lower-end of the main sequence. Our objective is to detect any correlations between long-term activity variations and the observed RV of a sample of M dwarfs. We use a sample of 27 M-dwarfs with a median observational timespan of 5.9 years. The cross-correlation function (CCF) with its parameters RV, bisector inverse slope (BIS), full width at half maximum (FWHM), and contrast are computed from the HARPS spectrum. The activity index is derived using the Na i D doublet. These parameters are compared with the activity level of the stars to search for correlations. We detect RV variations up to ~5 m s-1 that we can attribute to activity cycle effects. However, only 36% of the stars with long-term activity variability appear to have had their RV affected by magnetic cycles, on the typical timescale of ~6 years. Therefore, we suggest a careful analysis of activity data when searching for extrasolar planets using long-timespan RV data. Based on observations made with the HARPS instrument on the ESO 3.6-m telescope at La Silla Observatory under programme ID 072.C-0488(E).Tables with the data used for Figs. A.1-A.27 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/541/A9 Appendix A is available in electronic form at www.aanda.org
The GJ 581 planetary system is already known to harbour three planets, including two super-Earth planets that straddle its habitable zone. We report the detection of an additional planet - GJ 581e - ...with a minimum mass of 1.9 $M_\oplus$. With a period of 3.15 days, it is the innermost planet of the system and has a ~5% transit probability. We also correct our previous confusion about the orbital period of GJ 581d (the outermost planet) with a one-year alias, benefitting from an extended time span and many more measurements. The revised period is 66.8 days, and positions the semi-major axis inside the habitable zone of the low mass star. The dynamical stability of the 4-planet system imposes an upper bound on the orbital plane inclination. The planets cannot be more massive than approximately 1.6 times their minimum mass.
Large-scale magnetic topologies of mid M dwarfs Morin, J.; Donati, J.-F.; Petit, P. ...
Monthly notices of the Royal Astronomical Society,
10/2008, Letnik:
390, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We present in this paper, the first results of a spectropolarimetric analysis of a small sample (∼20) of active stars ranging from spectral type M0 to M8, which are either fully convective or possess ...a very small radiative core. This study aims at providing new constraints on dynamo processes in fully convective stars. This paper focuses on five stars of spectral type ∼M4, i.e. with masses close to the full convection threshold (≃0.35 M⊙), and with short rotational periods. Tomographic imaging techniques allow us to reconstruct the surface magnetic topologies from the rotationally modulated time-series of circularly polarized profiles. We find that all stars host mainly axisymmetric large-scale poloidal fields. Three stars were observed at two different epochs separated by ∼1 yr; we find the magnetic topologies to be globally stable on this time-scale. We also provide an accurate estimation of the rotational period of all stars, thus allowing us to start studying how rotation impacts the large-scale magnetic field.
We report improved masses, radii, and densities for four planets in two bright M-dwarf systems, K2-3 and GJ3470, derived from a combination of new radial velocity and transit observations. ...Supplementing K2 photometry with follow-up Spitzer transit observations refined the transit ephemerides of K2-3 b, c, and d by over a factor of 10. We analyze ground-based photometry from the Evryscope and Fairborn Observatory to determine the characteristic stellar activity timescales for our Gaussian Process fit, including the stellar rotation period and activity region decay timescale. The stellar rotation signals for both stars are evident in the radial velocity data and is included in our fit using a Gaussian process trained on the photometry. We find the masses of K2-3 b, K2-3 c, and GJ3470 b to be 6.48 , 2.14 , and 12.58 M⊕, respectively. K2-3 d was not significantly detected and has a 3 upper limit of 2.80 M⊕. These two systems are training cases for future TESS systems; due to the low planet densities ( < 3.7 g cm−3) and bright host stars (K < 9 mag), they are among the best candidates for transmission spectroscopy in order to characterize the atmospheric compositions of small planets.
This Letter reports on the detection of two super-Earth planets in the Gl 581 system, which is already known to harbour a hot Neptune. One of the planets has a mass of 5 M_{\oplus} and resides at the ..."warm" edge of the habitable zone of the star. It is thus the known exoplanet that most resembles our own Earth. The other planet has a 7.7 M_{\oplus} mass and orbits at 0.25 AU from the star, close to the "cold" edge of the habitable zone. These two new light planets around an M3 dwarf further confirm the formerly tentative statistical trend toward (i) many more very low-mass planets being found around M dwarfs than around solar-type stars and (ii) low-mass planets outnumbering Jovian planets around M dwarfs.
ABSTRACT
We present near-infrared spectropolarimetric observations of a sample of 43 weakly to moderately active M dwarfs, carried with SPIRou at the Canada–France–Hawaii Telescope in the framework ...of the SPIRou Legacy Survey from early 2019 to mid-2022. We use the 6700 circularly polarised spectra collected for this sample to investigate the longitudinal magnetic field and its temporal variations for all sample stars, from which we diagnose, through quasi-periodic Gaussian process regression, the periodic modulation and longer-term fluctuations of the longitudinal field. We detect the large-scale field for 40 of our 43 sample stars, and infer a reliable or tentative rotation period for 38 of them, using a Bayesian framework to diagnose the confidence level at which each rotation period is detected. We find rotation periods ranging from 14 to over 60 d for the early-M dwarfs, and from 70 to 200 d for most mid- and late-M dwarfs (potentially up to 430 d for one of them). We also find that the strength of the detected large-scale fields does not decrease with increasing period or Rossby number for the slowly rotating dwarfs of our sample as it does for higher-mass, more active stars, suggesting that these magnetic fields may be generated through a different dynamo regime than those of more rapidly rotating stars. We also show that the large-scale fields of most sample stars evolve on long time-scales, with some of them globally switching sign as stars progress on their putative magnetic cycles.
Aims. Extra-solar planet search programs require high-precision velocity measurements. They need to determine how to differentiate between radial-velocity variations due to Doppler motion and the ...noise induced by stellar activity. Methods. We monitored the active K2V star HD 189 733 and its transiting planetary companion, which has a 2.2-day orbital period. We used the high-resolution spectograph SOPHIE mounted on the 1.93-m telescope at the Observatoire de Haute-Provence to obtain 55 spectra of HD 189 733 over nearly two months. We refined the HD 189 733b orbit parameters and placed limits on both the eccentricity and long-term velocity gradient. After subtracting the orbital motion of the planet, we compared the variability in spectroscopic activity indices with the evolution in the radial-velocity residuals and the shape of spectral lines. Results. The radial velocity, the spectral-line profile, and the activity indices measured in He I (5875.62 Å), Hα (6562.81 Å), and both of the Ca II H&K lines (3968.47 Å and 3933.66 Å, respectively) exhibit a periodicity close to the stellar-rotation period and the correlations between them are consistent with a spotted stellar surface in rotation. We used these correlations to correct for the radial-velocity jitter due to stellar activity. This results in achieving high precision in measuring the orbital parameters, with a semi-amplitude K = 200.56 ± 0.88 m s-1 and a derived planet mass of MP = 1.13 ± 0.03 MJup.
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