HADES RV programme with HARPS-N at TNG Suárez Mascareño, A.; Rebolo, R.; González Hernández, J. I. ...
Astronomy and astrophysics (Berlin),
04/2018, Letnik:
612
Journal Article
Recenzirano
Odprti dostop
We aim to investigate the presence of signatures of magnetic cycles and rotation on a sample of 71 early M-dwarfs from the HADES RV programme using high-resolution time-series spectroscopy of the Ca ...II H&K and H
α
chromospheric activity indicators, the radial velocity series, the parameters of the cross correlation function and the
V
-band photometry. We used mainly HARPS-N spectra, acquired over 4 yr, and add HARPS spectra from the public ESO database and ASAS photometry light-curves as support data, extending the baseline of the observations of some stars up to 12 yr. We provide log
10
(
R
′
HK
) measurements for all the stars in the sample, cycle length measurements for 13 stars, rotation periods for 33 stars and we are able to measure the semi-amplitude of the radial velocity signal induced by rotation in 16 stars. We complement our work with previous results and confirm and refine the previously reported relationships between the mean level of chromospheric emission, measured by the log
10
(
R
′
HK
), with the rotation period, and with the measured semi-amplitude of the activity induced radial velocity signal for early M-dwarfs. We searched for a possible relation between the measured rotation periods and the lengths of the magnetic cycle, finding a weak correlation between both quantities. Using previous
v
sin
i
measurements we estimated the inclinations of the star’s poles to the line of sight for all the stars in the sample, and estimate the range of masses of the planets GJ 3998 b and c (2.5–4.9 and 6.3–12.5
M
⊕
), GJ 625 b (2.82
M
⊕
), GJ 3942 b (7.1–10.0
M
⊕
) and GJ 15A b (3.1–3.3
M
⊕
), assuming their orbits are coplanar with the stellar rotation.
Context. Low-mass stars have been recognised as promising targets in the search for rocky, small planets with the potential of supporting life. As a consequence, Doppler search programmes using ...high-resolution spectrographs like HARPS or HARPS-N are providing huge quantities of optical spectra of M dwarfs. However, determining the stellar parameters of M dwarfs using optical spectra has proven to be challenging. Aims. We aim to calibrate empirical relationships to determine accurate stellar parameters for early-M dwarfs (spectral types M0-M4.5) using the same spectra as those that are used for radial velocity determinations, without the necessity of acquiring IR spectra or relying on atmospheric models and/or photometric calibrations. Methods. Our methodology consists of using ratios of pseudo-equivalent widths of spectral features as a temperature diagnostic, a technique frequently used in solar-type stars. Stars with effective temperatures obtained from interferometric estimates of their radii are used as calibrators. Empirical calibrations for the spectral type are also provided. Combinations of features and ratios of features are used to derive calibrations for the stellar metallicity. Our methods are then applied to a large sample of M dwarfs that are currently being observed in the framework of the HARPS GTO search for extrasolar planets. The derived temperatures and metallicities are used together with photometric estimates of mass, radius, and surface gravity to calibrate empirical relationships for these parameters. Results. A long list of spectral features in the optical spectra of early-M dwarfs was identified. This list shows that the pseudo-equivalent width of roughly 43% of the features is strongly anticorrelated with the effective temperature. The correlation with the stellar metallicity is weaker. A total of 112 temperature sensitive ratios were identified and calibrated over the range 3100−3950 K, providing effective temperatures with typical uncertainties of about 70 K. Eighty-two ratios of pseudo-equivalent widths of features were calibrated to derive spectral types within 0.5 subtypes for stars with spectral types between K7V and M4.5V. We calibrated 696 combinations of the pseudo-equivalent widths of individual features and temperature-sensitive ratios for the stellar metallicity over a metallicity range from −0.54 to +0.24 dex, with estimated uncertainties in the range of 0.07−0.10 dex. We provide our own empirical calibrations for stellar mass, radius, and surface gravity. These parameters depend on the stellar metallicity. For a given effective temperature, lower metallicities predict lower masses and radii as well as higher gravities.
Context.
The 20 Myr old star V1298 Tau hosts at least four planets. Since its discovery, this system has been a target of intensive photometric and spectroscopic monitoring. To date, the ...characterisation of its architecture and planets’ fundamental properties has been very challenging.
Aims.
The determination of the orbital ephemeris of the outermost planet V1298 Tau e remains an open question. Only two transits have been detected so far by
Kepler
/K2 and TESS, allowing for a grid of reference periods to be tested with new observations, without excluding the possibility of transit timing variations. Observing a third transit would allow for better constraints to be set on the orbital period and would also help in determining an accurate radius for V1298 Tau e because the previous transits showed different depths.
Methods.
We observed V1298 Tau with the CHaracterising ExOPlanet Satellite (CHEOPS) to search for a third transit of planet e within observing windows selected to test three of the shortest predicted orbital periods. We also collected ground-based observations to verify the result found with CHEOPS. We reanalysed
Kepler
/K2 and TESS light curves to test how the results derived from these data are affected by alternative photometric extraction and detrending methods.
Results.
We report the CHEOPS detection of a transit-like signal that could be attributed to V1298 Tau e. If so, that result would imply that the orbital period calculated from fitting a linear ephemeris to the three available transits is close to ~45 days. Results from the ground-based follow-up marginally support this possibility. We found that i) the transit observed by CHEOPS has a longer duration compared to that of the transits observed by
Kepler
/K2 and TESS; and ii) the transit observed by TESS is >30% deeper than that of
Kepler
/K2 and CHEOPS, and it is also deeper than the measurement previously reported in the literature, according to our reanalysis.
Conclusions.
If the new transit detected by CHEOPS is found to be due to V1298 Tau e, this would imply that the planet experiences TTVs of a few hours, as deduced from three transits, as well as orbital precession, which would explain the longer duration of the transit compared to the
Kepler
/K2 and TESS signals. Another and a priori less likely possibility is that the newly detected transit belongs to a fifth planet with a longer orbital period than that of V1298 Tau e. Planning further photometric follow-up to search for additional transits is indeed necessary to solve the conundrum, as well as to pin down the radius of V1298 Tau e.
Context. The general theory of relativity predicts the redshift of spectral lines in the solar photosphere as a consequence of the gravitational potential of the Sun. This effect can be measured from ...a solar disk-integrated flux spectrum of the Sun’s reflected light on Solar System bodies. Aims. The laser frequency comb (LFC) calibration system attached to the HARPS spectrograph offers the possibility of performing an accurate measurement of the solar gravitational redshift (GRS) by observing the Moon or other Solar System bodies. Here, we analyse the line shift observed in Fe absorption lines from five high-quality HARPS-LFC spectra of the Moon. Methods. We selected an initial sample of 326 photospheric Fe lines in the spectral range between 476–585 nm and measured their line positions and equivalent widths (EWs). Accurate line shifts were derived from the wavelength position of the core of the lines compared with the laboratory wavelengths of Fe lines. We also used a CO5BOLD 3D hydrodynamical model atmosphere of the Sun to compute 3D synthetic line profiles of a subsample of about 200 spectral Fe lines centred at their laboratory wavelengths. We fit the observed relatively weak spectral Fe lines (with EW< 180 mÅ) with the 3D synthetic profiles. Results. Convective motions in the solar photosphere do not affect the line cores of Fe lines stronger than about ∼150 mÅ. In our sample, only 15 Fe I lines have EWs in the range 150< EW(mÅ) < 550, providing a measurement of the solar GRS at 639 ± 14 m s−1, which is consistent with the expected theoretical value on Earth of ∼633.1 m s−1. A final sample of about 97 weak Fe lines with EW < 180 mÅ allows us to derive a mean global line shift of 638 ± 6 m s−1, which is in agreement with the theoretical solar GRS. Conclusions. These are the most accurate measurements of the solar GRS obtained thus far. Ultrastable spectrographs calibrated with the LFC over a larger spectral range, such as HARPS or ESPRESSO, together with a further improvement on the laboratory wavelengths, could provide a more robust measurement of the solar GRS and further testing of 3D hydrodynamical models.
ABSTRACT
This paper reports on the detailed characterization of the K2-111 planetary system with K2, WASP, and ASAS-SN photometry, as well as high-resolution spectroscopic data from HARPS-N and ...ESPRESSO. The host, K2-111, is confirmed to be a mildly evolved (log g = 4.17), iron-poor (Fe/H = −0.46), but alpha-enhanced (α/Fe=0.27), chromospherically quiet, very old thick disc G2 star. A global fit, performed by using PyORBIT, shows that the transiting planet, K2-111 b, orbits with a period Pb = 5.3518 ± 0.0004 d and has a planet radius of $1.82^{+0.11}_{-0.09}$ R⊕ and a mass of $5.29^{+0.76}_{-0.77}$ M⊕, resulting in a bulk density slightly lower than that of the Earth. The stellar chemical composition and the planet properties are consistent with K2-111 b being a terrestrial planet with an iron core mass fraction lower than the Earth. We announce the existence of a second signal in the radial velocity data that we attribute to a non-transiting planet, K2-111 c, with an orbital period of 15.6785 ± 0.0064 d, orbiting in near-3:1 mean motion resonance with the transiting planet, and a minimum planet mass of 11.3 ± 1.1 M⊕. Both planet signals are independently detected in the HARPS-N and ESPRESSO data when fitted separately. There are potentially more planets in this resonant system, but more well-sampled data are required to confirm their presence and physical parameters.
Context.
Small planets located at the lower mode of the bimodal radius distribution are generally assumed to be composed of iron and silicates in a proportion similar to that of the Earth. However, ...recent discoveries are revealing a new group of low-density planets that are inconsistent with that description.
Aims.
We intend to confirm and characterize the TESS planet candidate TOI-244.01, which orbits the bright (
K
= 7.97 mag), nearby (
d
= 22 pc), and early-type (M2.5 V) M-dwarf star GJ 1018 with an orbital period of 7.4 days.
Methods.
We used Markov chain Monte Carlo methods to model 57 precise radial velocity measurements acquired by the ESPRESSO spectrograph together with TESS photometry and complementary HARPS data. Our model includes a planetary component and Gaussian processes aimed at modeling the correlated stellar and instrumental noise.
Results.
We find TOI-244 b to be a super-Earth with a radius of
R
p
= 1.52 ± 0.12
R
⊕
and a mass of
M
p
= 2.68 ± 0.30
M
⊕
. These values correspond to a density of
ρ
= 4.2 ± 1.1 g cm
−3
, which is below what would be expected for an Earth-like composition. We find that atmospheric loss processes may have been efficient to remove a potential primordial hydrogen envelope, but high mean molecular weight volatiles such as water could have been retained. Our internal structure modeling suggests that TOI-244 b has a 479
−96
+128
km thick hydrosphere over a 1.17 ± 0.09
R
⊕
solid structure composed of a Fe-rich core and a silicate-dominated mantle compatible with that of the Earth. On a population level, we find two tentative trends in the density-metallicity and density-insolation parameter space for the low-density super-Earths, which may hint at their composition.
Conclusions.
With a 8% precision in radius and 12% precision in mass, TOI-244 b is among the most precisely characterized super-Earths, which, together with the likely presence of an extended hydrosphere, makes it a key target for atmospheric observations.
HADES RV Programme with HARPS-N at TNG González-Álvarez, E.; Micela, G.; Maldonado, J. ...
Astronomy and astrophysics (Berlin),
04/2019, Letnik:
624
Journal Article
Recenzirano
Odprti dostop
Aims.
We extend the relationship between X-ray luminosity (
L
x
) and rotation period (
P
rot
) found for main-sequence FGK stars, and test whether it also holds for early M dwarfs, especially in the ...non-saturated regime (
L
x
∝
P
rot
−2
) which corresponds to slow rotators.
Methods.
We use the luminosity coronal activity indicator (
L
x
) of a sample of 78 early M dwarfs with masses in the range from 0.3 to 0.75
M
⊙
from the HArps-N red Dwarf Exoplanet Survey (HADES) radial velocity (RV) programme collected from ROSAT and
XMM-Newton
. The determination of the rotation periods (
P
rot
) was done by analysing time series of high-resolution spectroscopy of the Ca
II
H & K and H
α
activity indicators. Our sample principally covers the slow rotation regime with rotation periods from 15 to 60 days.
Results.
Our work extends to the low mass regime the observed trend for more massive stars showing a continuous shift of the
L
x
∕
L
bol
versus
P
rot
power law towards longer rotation period values, and includes a more accurate way to determine the value of the rotation period at which the saturation occurs (
P
sat
) for M dwarf stars.
Conclusions.
We conclude that the relations between coronal activity and stellar rotation for FGK stars also hold for early M dwarfs in the non-saturated regime, indicating that the rotation period is sufficient to determine the ratio
L
x
∕
L
bol
.
Abstract
We present the detection of three super-Earths transiting the cool star LP415-17, monitored by K2 mission in its 13th campaign. High-resolution spectra obtained with High Accuracy Radial ...velocity Planet Searcher-North/Telescopio Nazionale Galileo (HARPS-N/TNG) showed that the star is a mid-late K dwarf. Using spectral synthesis models, we infer its effective temperature, surface gravity, and metallicity, and subsequently determined from evolutionary models a stellar radius of 0.58 R⊙. The planets have radii of 1.8, 2.6, and 1.9 R⊕ and orbital periods of 6.34, 13.85, and 40.72 d. High-resolution images discard any significant contamination by an intervening star in the line of sight. The orbit of the furthest planet has radius of 0.18 au, close to the inner edge of the habitable zone. The system is suitable to improve our understanding of formation and dynamical evolution of super-Earth systems in the rocky–gaseous threshold, their atmospheres, internal structure, composition, and interactions with host stars.
Aims. We aim at constraining evolutionary models at low mass and young ages by identifying interesting transiting system members of the nearest OB association to the Sun, Upper Scorpius (USco), which ...has been targeted by the Kepler mission. Methods. We produced light curves for M-dwarf members of the USco region that has been surveyed during the second campaign of the Kepler K2 mission. We identified by eye a transiting system, USco J161630.68−251220.1 (=EPIC 203710387) with a combined spectral type of M5.25, whose photometric, astrometric, and spectroscopic properties makes it a member of USco. We conducted an extensive photometric and spectroscopic follow-up of this transiting system with a suite of telescopes and instruments to characterise the properties of each component of the system. Results. We calculated a transit duration of about 2.42 h that occurs every 2.88 days with a slight difference in transit depth and phase between the two components. We estimated a mass ratio of 0.922 ± 0.015 from the semi-amplitudes of the radial velocity curves for each component. We derived masses of 0.091 ± 0.005M⊙ and 0.084 ± 0.004M⊙, radii of 0.388 ± 0.008R⊙ and 0.380 ± 0.008R⊙, luminosities of log (L/L⊙) = −2.020-0.121+0.099 dex and −2.032-0.121+0.099 dex, and effective temperatures of 2901-172+199 K and 2908-172+199 K for the primary and secondary, respectively. Conclusions. We present a complete photometric and radial velocity characterisation of the least massive double-line eclipsing binary system in the young USco association with two components close to the stellar/substellar limit. This system falls in a gap between the least massive eclipsing binaries in the low-mass and substellar regimes at young ages and represents an important addition to constraining evolutionary models at young ages.
Context.
The ESPRESSO spectrograph is a new powerful tool developed to detect and characterize extrasolar planets. Its design allows an unprecedented radial velocity precision (down to a few tens of ...cm s
−1
) and long-term thermomechanical stability.
Aims.
We present the first stand-alone detection of an extrasolar planet by blind radial velocity search using ESPRESSO; our aim is to show the power of the instrument in characterizing planetary signals at different periodicities in long observing time spans.
Methods.
We used 41 ESPRESSO measurements of HD 22496 obtained within a time span of 895 days with a median photon noise of 18 cm s
−1
. A radial velocity analysis was performed to test the presence of planets in the system and to account for the stellar activity of this K5-K7 main-sequence star. For benchmarking and comparison, we attempted the detection with 43 archive HARPS measurements and in this work we compare the results yielded by the two datasets. We also used four TESS sectors to search for transits.
Results.
We find radial velocity variations compatible with a close-in planet with an orbital period of
P
= 5.09071 ± 0.00026 days when simultaneously accounting for the effects of stellar activity on longer timescales (
P
rot
= 34.99
−0.53
+0.58
days). We characterize the physical and orbital properties of the planet and find a minimum mass of 5.57
−0.68
+0.73
M
⊕
, right in the dichotomic regime between rocky and gaseous planets. Although not transiting according to TESS data, if aligned with the stellar spin axis, the absolute mass of the planet must be below 16
M
⊕
. We find no significant evidence for additional signals in the data with semi-amplitudes above 56 cm s
−1
at 95% confidence.
Conclusions.
With a modest set of radial velocity measurements, ESPRESSO is capable of detecting and characterizing low-mass planets and constraining the presence of planets in the habitable zone of K dwarfs down to the rocky-mass regime.
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