We present high-resolution near-infrared spectropolarimetric observations using the SPIRou instrument at Canada-France-Hawaii Telescope (CFHT) during a transit of the recently detected young planet ...AU Mic b, with supporting spectroscopic data from iSHELL at NASA InfraRed Telescope Facility. We detect Zeeman signatures in the Stokes
V
profiles and measure a mean longitudinal magnetic field of ¯
B
ℓ
= 46.3 ± 0.7 G. Rotationally modulated magnetic spots likely cause long-term variations of the field with a slope of d
B
ℓ
/d
t
= −108.7 ± 7.7 G d
−1
. We apply the cross-correlation technique to measure line profiles and obtain radial velocities through CCF template matching. We find an empirical linear relationship between radial velocity and
B
ℓ
, which allows us to estimate the radial-velocity induced by stellar activity through rotational modulation of spots for the five hours of continuous monitoring of AU Mic with SPIRou. We model the corrected radial velocities for the classical Rossiter-McLaughlin effect, using MCMC to sample the posterior distribution of the model parameters. This analysis shows that the orbit of AU Mic b is prograde and aligned with the stellar rotation axis with a sky-projected spin-orbit obliquity of
λ
= 0°
−15°
+18°
. The aligned orbit of AU Mic b indicates that it formed in the protoplanetary disk that evolved into the current debris disk around AU Mic.
The naked-eye star 55 Cancri hosts a planetary system with five known planets, including a hot super-Earth (55 Cnc e) extremely close to its star and a farther out giant planet (55 Cnc b), found in ...milder irradiation conditions with respect to other known hot Jupiters. This system raises important questions on the evolution of atmospheres for close-in exoplanets, and the dependence with planetary mass and irradiation. These questions can be addressed by Lyman-α transit observations of the extended hydrogen planetary atmospheres, complemented by contemporaneous measurements of the stellar X-ray flux. In fact, planet “e” has been detected in transit, suggesting the system is seen nearly edge-on. Yet, planet “b” has not been observed in transit so far. Here, we report on Hubble Space Telescope STIS Lyα and Chandra ACIS-S X-ray observations of 55 Cnc. These simultaneous observations cover two transits of 55 Cnc e and two inferior conjunctions of 55 Cnc b. They reveal the star as a bright Lyα target and a variable X-ray source. While no significant signal is detected during the transits of 55 Cnc e, we detect a surprising Lyα absorption of 7.5 ± 1.8% (4.2σ) at inferior conjunctions of 55 Cnc b. The absorption is only detected over the range of Doppler velocities where the stellar radiation repels hydrogen atoms towards the observer. We calculate a false-alarm probability of 4.4%, which takes the a-priori unknown transit parameters into account. This result suggests the possibility that 55 Cnc b has an extended upper H i atmosphere, which undergoes partial transits when the planet grazes the stellar disc. If confirmed, it would show that planets cooler than hot Jupiters can also have extended atmospheres.
Context. The search for extra-solar planets similar to Earth is becoming a reality, but as the level of the measured radial-velocity reaches the sub-m s-1, stellar intrinsic sources of noise capable ...of hiding the signal of these planets from scrutiny become more important. Aims: Other stars are known to have magnetic cycles similar to that of the Sun. The relationship between these activity variations and the observed radial-velocity is still not satisfactorily understood. Following our previous work, which studied the correlation between activity cycles and long-term velocity variations for K dwarfs, we now expand it to the lower end of the main sequence. In this first paper our aim is to assess the long-term activity variations in the low end of the main sequence, having in mind a planetary search perspective. Methods: We used a sample of 30 M0-M5.5 stars from the HARPS M-dwarf planet search program with a median timespan of observations of 5.2 years. We computed chromospheric activity indicators based on the Ca ii H and K, Hα, He i D3, and Na i D1 and D2 lines. All data were binned to average out undesired effects such as rotationally modulated atmospheric inhomogeneities. We searched for long-term variability of each index and determined the correlations between them. Results: While the SCa II, Hα, and Na i indices showed significant variability for a fraction of our stellar sample (39%, 33%, and 37%, respectively), only 10% of our stars presented significant variability in the He i index. We therefore conclude that this index is a poor activity indicator at least for this type of stars. Although the Hα shows good correlation with SCa II for the most active stars, the correlation is lost when the activity level decreases. This result appears to indicate that the Ca ii - Hα correlation is dependent on the activity level of the star. The Na i lines correlate very well with the SCa II index for the stars with low activity levels we used, and are thus a good chromospheric activity proxy for early-M dwarfs. We therefore strongly recommend the use of the Na i activity index because the signal-to-noise ratio in the sodium lines spectral region is always higher than for the calcium lines. 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).
Large-scale magnetic topologies of late M dwarfs Morin, J.; Donati, J.-F.; Petit, P. ...
Monthly notices of the Royal Astronomical Society,
October 2010, Letnik:
407, Številka:
4
Journal Article
Recenzirano
Odprti dostop
We present here the final results of the first spectropolarimetric survey of a small sample of active M dwarfs, aimed at providing observational constraints on dynamo action on both sides of the ...full-convection threshold (spectral type M4). Our two previous studies were focused on early and mid M dwarfs. The present paper examines 11 fully convective late M dwarfs (spectral types M5–M8). Tomographic imaging techniques were applied to time-series of circularly polarized profiles of six stars, in order to infer their large-scale magnetic topologies. For three other stars we could not produce such magnetic maps, because of low variability of the Stokes V signatures, but were able to derive some properties of the magnetic fields. We find two distinct categories of magnetic topologies: on the one hand strong axisymmetric dipolar fields (similar to mid M dwarfs), and on the other hand weak fields generally featuring a significant non-axisymmetric component, and sometimes a significant toroidal one. Comparison with unsigned magnetic fluxes demonstrates that the second category of magnetic fields shows less organization (less energy in the large scales), similarly to partly convective early M dwarfs. Stars in both categories have similar stellar parameters, our data do not evidence a separation between these two categories in the mass–rotation plane. We also report marginal detection of a large-scale magnetic field on the M8 star VB 10 featuring a significant toroidal axisymmetric component, whereas no field is detectable on VB 8 (M7).
The isotopic ratio of nitrogen measured in primitive Solar System bodies shows a broad range of values, the origin of which remains unknown. One key question is whether these isotopic reservoirs of ...nitrogen predate the comet formation stage or are posterior to it. Another central question is elucidating the processes that can produce the observed variations in the 14N/15N isotopic ratio. Disks that orbit pre-main-sequence (T Tauri) stars provide unique opportunities for observing the chemical content of analogs of the protosolar nebula and therefore for building a comprehensive scenario that can explain the origin of nitrogen in the Solar System and in planet-forming disks. With ALMA, it has become possible to measure isotopic ratios of nitrogen-bearing species in such environments. We present spectrally and spatially resolved observations of the hyperfine structure of the 4−3 rotational transition of HCN and its main isotopologs H13CN and HC15N in the disk orbiting the 8 Myr old T Tauri star TW Hya. The sensitivity allows directly measuring the HCN/H13CN and HCN/HC15N abundance ratios with minimal assumptions. Averaged spatially over the disks, the ratios are 86 ± 4 and 223 ± 21, respectively. The latter value is significantly lower than the CN/C15N ratio of 323 ± 30 in this disk and thus provides the first evidence that two isotopic reservoirs of nitrogen are present in a disk at the stage of giant planet and comet formation. Furthermore, we find clear evidence for an increase in the ratio of HCN to HC15N with radius. The ratio in the outer disk, at 45 au, is 339 ± 28, in excellent agreement with direct measurements in the local interstellar medium, and with the bulk nitrogen isotopic ratio predicted from galactic evolution calculations. In the comet formation region at r = 20 au, the ratio is a factor ≈3 lower, 121 ± 11. This radial increase qualitatively agrees with the scenario in which selective photodissociation of N2 is the dominant fractionation process. However, our isotopic ratios and kinetic temperature of the HCN-emitting layers quantitatively disagree with models of nitrogen chemistry in disks.
The combination of high-contrast imaging and high-dispersion spectroscopy, which has successfully been use to detect the atmosphere of a giant planet, is one of the most promising potential probes of ...the atmosphere of Earth-size worlds. The forthcoming generation of extremely large telescopes (ELTs) may obtain sufficient contrast with this technique to detect O2 in the atmosphere of those worlds that orbit low-mass M dwarfs. This is strong motivation to carry out a census of planets around cool stars for which habitable zones can be resolved by ELTs, i.e. for M dwarfs within ~5 parsec. Our HARPS survey has been a major contributor to that sample of nearby planets. Here we report on our radial velocity observations of Ross 128 (Proxima Virginis, GJ447, HIP 57548), an M4 dwarf just 3.4 parsec away from our Sun. This source hosts an exo-Earth with a projected mass m sini = 1.35 M⊕ and an orbital period of 9.9 days. Ross 128 b receives less than 1.5 times as much flux as Earth from the Sun and its equilibrium ranges in temperature between 269 K for an Earth-like albedo and 213 K for a Venus-like albedo. Recent studies place it close to the inner edge of the conventional habitable zone. An 80-day long light curve from K2 campaign C01 demonstrates that Ross 128 b does not transit. Together with the All Sky Automated Survey (ASAS) photometry and spectroscopic activity indices, the K2 photometry shows that Ross 128 rotates slowly and has weak magnetic activity. In a habitability context, this makes survival of its atmosphere against erosion more likely. Ross 128 b is the second closest known exo-Earth, after Proxima Centauri b (1.3 parsec), and the closest temperate planet known around a quiet star. The 15 mas planet-star angular separation at maximum elongation will be resolved by ELTs (>3λ∕D) in the optical bands of O2.
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 ~103–104 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 5σ 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 3.6σ detection of O2 could be made in about 60 nights of telescope time. Those would need to be spread over three 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, in particular the E-ELT and its high-resolution visible and near-IR spectrograph.
Using high-contrast imaging with the SPHERE instrument at the Very Large Telescope (VLT), we report the first images of a cold brown dwarf companion to the exoplanet host star HD 4113A. The brown ...dwarf HD 4113C is part of a complex dynamical system consisting of a giant planet, a stellar host, and a known wide M-dwarf companion. Its separation of 535 ± 3 mas and H-band contrast of 13.35 ± 0.10 mag correspond to a projected separation of 22 AU and an isochronal mass estimate of 36 ± 5 MJ based on COND models. The companion shows strong methane absorption, and through fitting an atmosphere model, we estimate a surface gravity of logg = 5 and an effective temperature of ~500–600 K. A comparison of its spectrum with observed T dwarfs indicates a late-T spectral type, with a T9 object providing the best match. By combining the observed astrometry from the imaging data with 27 years of radial velocities, we use orbital fitting to constrain its orbital and physical parameters, as well as update those of the planet HD 4113A b, discovered by previous radial velocity measurements. The data suggest a dynamical mass of 66−4+5 $66^{+5}_{-4}$ 66−4+5 MJ and moderate eccentricity of 0.44−0.07+0.08 $0.44^{+0.08}_{-0.07}$ 0.44−0.07+0.08 for the brown dwarf. This mass estimate appears to contradict the isochronal estimate and that of objects with similar temperatures, which may be caused by the newly detected object being an unresolved binary brown dwarf system or the presence of an additional object in the system. Through dynamical simulations, we show that the planet may undergo strong Lidov-Kozai cycles, raising the possibility that it formed on a quasi-circular orbit and gained its currently observed high eccentricity (e ~ 0.9) through interactions with the brown dwarf. Follow-up observations combining radial velocities, direct imaging, and Gaia astrometry will be crucial to precisely constrain the dynamical mass of the brown dwarf and allow for an in-depth comparison with evolutionary and atmosphere models.
We present a suite of Atacama Large Millimeter Array (ALMA) interferometric molecular line and continuum images that elucidate, on linear size scales of ∼30-40 au, the chemical structure of the ...nearby, evolved, protoplanetary disk orbiting the close binary system V4046 Sgr. The observations were undertaken in the 1.1-1.4 mm wavelength range (ALMA Bands 6 and 7) with antenna configurations involving maximum baselines of several hundred meters, yielding subarcsecond-resolution images in more than a dozen molecular species and isotopologues. Isotopologues of CO and HCN display centrally peaked morphologies of integrated emission-line intensity, whereas the line emission from complex nitrile group molecules (HC3N, CH3CN), deuterated molecules (DCN, DCO+), hydrocarbons (as traced by C2H), and potential CO ice line tracers (N2H+, and H2CO) appears as a sequence of sharp and diffuse rings. The dimensions and morphologies of HC3N and CH3CN emission are suggestive of photodesorption of organic ices from the surfaces of dust grains, while the sequence of increasing radius of peak intensity represented by DCN (smallest), DCO+, N2H+, and H2CO (largest) is qualitatively consistent with the expected decline of midplane gas temperature with increasing disk radius. Empirical modeling indicates that the sharp-edged C2H emission ring lies at relatively deep disk layers, leaving open the question of the origin of C2H abundance enhancements in evolved disks. This study of the "molecular anatomy" of V4046 Sgr should serve as motivation for additional subarcsecond ALMA molecular line imaging surveys of nearby, evolved protoplanetary disks aimed at addressing major uncertainties in protoplanetary disk physical and chemical structure and molecular production pathways.
Context.
Observations of molecular clouds, prestellar cores, and protoplanetary disks have established that the HNC/HCN ratio may be a potent diagnostic of molecular gas physical conditions. The ...processes that govern the relative abundances of these molecules nevertheless remain poorly understood.
Aims.
We seek to exploit the wide range of UV irradiation strengths within the ∼pc diameter Helix planetary nebula to explore the potential role of UV radiation in driving HNC/HCN.
Methods.
We performed IRAM 30 m and APEX 12 m radio line observations across six positions within the Helix Nebula, making use of radiative transfer and photodissociation modeling codes to interpret the results for line intensities and line ratios in terms of the molecular gas properties.
Results.
We have obtained the first detections of the plasma-embedded Helix molecular knots (globules) in HCN, HNC, HCO
+
, and other trace molecules. Analysis of the HNC/HCN integrated line intensity ratio reveals an increase with radial distance from the Helix central star. In the context of molecular line ratios of other planetary nebulae from the literature, the HNC/HCN ratio appears to be anticorrelated with UV emission over four orders of magnitude in incident flux. Models of the photodissociation regions within the Helix using the RADEX and Meudon codes reveal strong constraints on the column density (1.5–2.5 × 10
12
cm
−2
) of the molecular gas, as well as pressure and temperature. Analysis of the molecular ion HCO
+
across the Helix indicates that X-ray irradiation is likely driving HCO
+
production in the outer regions of planetary nebulae, where photodissociation is limited but cold gas and ionized molecules are abundant.
Conclusions.
Although the observational results clearly indicate that UV irradiation is important in determining the HNC/HCN ratio, our photodissociation region modeling indicates that the UV flux gradient alone cannot reproduce the observed variation in HNC/HCN across the Helix Nebula. Instead, HNC/HCN appears to be dependent on both UV irradiation and gas pressure and density.
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