We used the HARPS-North high resolution spectrograph (ℛ = 115 000) at Telescopio Nazionale Galileo (TNG) to observe one transit of the highly irradiated planet MASCARA-2b/KELT-20b. Using only one ...transit observation, we are able to clearly resolve the spectral features of the atomic sodium (Na I) doublet and the Hα line in its atmosphere, which are corroborated with the transmission calculated from their respective transmission light curves (TLC). In particular, we resolve two spectral features centered on the Na I doublet position with an averaged absorption depth of 0.17 ± 0.03% for a 0.75 Å bandwidth with line contrasts of 0.44 ± 0.11% (D2) and 0.37 ± 0.08% (D1). The Na I TLC have also been computed, showing a large Rossiter-McLaughlin (RM) effect, which has a 0.20 ± 0.05% Na I transit absorption for a 0.75 Å passband that is consistent with the absorption depth value measured from the final transmission spectrum. We observe a second feature centered on the Hα line with 0.6 ± 0.1% contrast and an absorption depth of 0.59 ± 0.08% for a 0.75 Å passband that has consistent absorptions in its TLC, which corresponds to an effective radius of Rλ/RP = 1.20 ± 0.04. While the signal-to-noise ratio (S/N) of the final transmission spectrum is not sufficient to adjust different temperature profiles to the lines, we find that higher temperatures than the equilibrium (Teq = 2260 ± 50 K) are needed to explain the lines contrast. Particularly, we find that the Na I lines core require a temperature of T = 4210 ± 180 K and that Hα requires a temperature of T = 4330 ± 520 K. MASCARA-2b, like other planets orbiting A-type stars, receives a large amount of UV energy from its host star. This energy excites the atomic hydrogen and produces Hα absorption, leading to the expansion and abrasion of the atmosphere. The study of other Balmer lines in the transmission spectrum would allow the determination of the atmospheric temperature profile and the calculation of the lifetime of the atmosphere with escape rate measurements. In the case of MASCARA-2b, residual features are observed in the Hβ and Hγ lines, but they are not statistically significant. More transit observations are needed to confirm our findings in Na I and Hα and to build up enough S/N to explore the presence of Hβ and Hγ planetary absorptions.
Abstract
We report the first asteroseismic results obtained with the Hertzsprung Stellar Observations Network Group Telescope from an extensive high-precision radial-velocity observing campaign of ...the subgiant
μ
Herculis. The data set was collected during 215 nights in 2014 and 2015. We detected a total of 49 oscillation modes with
l
values from zero to three, including some
l
= 1 mixed modes. Based on the rotational splitting observed in
l
= 1 modes, we determine a rotational period of 52 days and a stellar inclination angle of 63°. The parameters obtained through modeling of the observed oscillation frequencies agree very well with independent observations and imply a stellar mass between 1.11 and 1.15
M
⊙
and an age of
Gyr. Furthermore, the high-quality data allowed us to determine the acoustic depths of the He
ii
ionization layer and the base of the convection zone.
In order to understand stellar evolution, it is crucial to efficiently determine stellar surface rotation periods. Indeed, while they are of great importance in stellar models, angular momentum ...transport processes inside stars are still poorly understood today. Surface rotation, which is linked to the age of the star, is one of the constraints needed to improve the way those processes are modelled. Statistics of the surface rotation periods for a large sample of stars of different spectral types are thus necessary. An efficient tool to automatically determine reliable rotation periods is needed when dealing with large samples of stellar photometric datasets. The objective of this work is to develop such a tool. For this purpose, machine learning classifiers constitute relevant bases to build our new methodology. Random forest learning abilities are exploited to automate the extraction of rotation periods in
Kepler
light curves. Rotation periods and complementary parameters are obtained via three different methods: a wavelet analysis, the autocorrelation function of the light curve, and the composite spectrum. We trained three different classifiers: one to detect if rotational modulations are present in the light curve, one to flag close binary or classical pulsators candidates that can bias our rotation period determination, and finally one classifier to provide the final rotation period. We tested our machine learning pipeline on 23 431 stars of the
Kepler
K and M dwarf reference rotation catalogue for which 60% of the stars have been visually inspected. For the sample of 21 707 stars where all the input parameters are provided to the algorithm, 94.2% of them are correctly classified (as rotating or not). Among the stars that have a rotation period in the reference catalogue, the machine learning provides a period that agrees within 10% of the reference value for 95.3% of the stars. Moreover, the yield of correct rotation periods is raised to 99.5% after visually inspecting 25.2% of the stars. Over the two main analysis steps, rotation classification and period selection, the pipeline yields a global agreement with the reference values of 92.1% and 96.9% before and after visual inspection. Random forest classifiers are efficient tools to determine reliable rotation periods in large samples of stars. The methodology presented here could be easily adapted to extract surface rotation periods for stars with different spectral types or observed by other instruments such as K2, TESS or by PLATO in the near future.
Context. Despite important advances in space asteroseismology during the last decade, the early phases of evolution of stars with masses above ~15 M⊙ (including the O stars and their evolved ...descendants, the B supergiants) have been only vaguely explored up to now. This is due to the lack of adequate observations for a proper characterization of the complex spectroscopic and photometric variability occurring in these stars. Aim. Our goal is to detect, analyze, and interpret variability in the early-B-type supergiant HD 2905 (κ Cas, B1 Ia) using long-term, ground-based, high-resolution spectroscopy. Methods. We gather a total of 1141 high-resolution spectra covering some 2900 days with three different high-performance spectrographs attached to 1–2.6m telescopes at the Canary Islands observatories. We complement these observations with the hipparcos light curve, which includes 160 data points obtained during a time span of ~1200 days. We investigate spectroscopic variability of up to 12 diagnostic lines by using the zero and first moments of the line profiles. We perform a frequency analysis of both the spectroscopic and photometric dataset using Scargle periodograms. We obtain single snapshot and time-dependent information about the stellar parameters and abundances by means of the FASTWIND stellar atmosphere code. Results. HD 2905 is a spectroscopic variable with peak-to-peak amplitudes in the zero and first moments of the photospheric lines of up to 15% and 30 km s−1, respectively. The amplitude of the line-profile variability is correlated with the line formation depth in the photosphere and wind. All investigated lines present complex temporal behavior indicative of multi-periodic variability with timescales of a few days to several weeks. No short-period (hourly) variations are detected. The Scargle periodograms of the hipparcos light curve and the first moment of purely photospheric lines reveal a low-frequency amplitude excess and a clear dominant frequency at ~0.37 d−1. In the spectroscopy, several additional frequencies are present in the range 0.1–0.4 d−1. These may be associated with heat-driven gravity modes, convectively driven gravity waves, or sub-surface convective motions. Additional frequencies are detected below 0.1 d−1. In the particular case of Hα, these are produced by rotational modulation of a non-spherically symmetric stellar wind. Conclusions. Combined long-term uninterrupted space photometry with high-precision spectroscopy is the best strategy to unravel the complex low-frequency photospheric and wind variability of B supergiants. Three-dimensional (3D) simulations of waves and of convective motions in the sub-surface layers can shed light on a unique interpretation of the variability.
The resonant scattering solar spectrophotometer ‘Mark-I’, designed and build at the University of Birmingham (UK) and located at the Observatorio del Teide (Spain), has been continuously in operation ...for the past 38 years. During this period of time, it has provided high-precision measurements of the radial velocity of the Sun as a star, which has enabled the study of the small velocity fluctuations produced by the solar oscillations and the characterization of their spectrum. So far, it has been one of the pioneer experiments in the field of helioseismology and contributed to the development of that area. Moreover, because of its high-sensitivity and long-term instrumental stability, it also provides an accurate determination (to within a few parts in 103) of the absolute daily velocity offset, which contains the so-called solar gravitational red-shift. In this paper, results of the analysis of the measurements of this parameter over the whole period 1976–2013 are presented. The result of this series of measurements is 600.4 ± 0.8 m s−1 with an amplitude variation of ±5 m s−1, which is in anticorrelation with the phase of the solar activity cycle. The 5 per cent difference found with respect to the value predicted by the equivalence principle is probably due to the asymmetry of the solar spectral line used.
Gravity modes (
g
modes), mixed gravito-acoustic modes (mixed modes), and gravito-inertial modes (
gi
modes) possess unmatched properties as probes for stars with radiative interiors. The structural ...and dynamical constraints that they are able to provide cannot be accessed by other means. While they provide precious insights into the internal dynamics of evolved stars as well as massive and intermediate-mass stars, their non-detection in main sequence (MS) solar-type stars make them a crucial missing piece in our understanding of angular momentum transport in radiative zones and stellar rotational evolution. In this work, we aim to apply certain analysis tools originally developed for helioseismology in order to look for
g
-mode signatures in MS solar-type stars. We select a sample of the 34 most promising MS solar-type stars with
Kepler
four-year long photometric time series. All these stars are well-characterised late F-type stars with thin convective envelopes, fast convective flows, and stochastically excited acoustic modes (
p
modes). For each star, we compute the background noise level of the Fourier power spectrum to identify significant peaks at low frequency. After successfully detecting individual peaks in 12 targets, we further analyse four of them and observe distinct patterns of surrounding peaks with a low probability of being noise artifacts. Comparisons with the predictions from reference models suggest that these patterns are compatible with the presence of non-asymptotic low-order pure
g
modes, pure
p
modes, and mixed modes. Given their sensitivity to both the convective core interface stratification and the coupling between
p
- and
g
-mode resonant cavities, such modes are able to provide strong constraints on the structure and evolutionary states of the related targets. Considering the granulation and activity background of the stars in our sample, we subsequently compute the corresponding mode velocity necessary to trigger a detectable luminosity fluctuation. We use it to estimate the surface velocity, ⟨
v
r
⟩, of the candidate modes we have detected. In this case, we find ⟨
v
r
⟩∼10 cm s
−1
. These results could be extremely useful for characterising the deep interior of MS solar-type stars, as the upcoming PLATO mission will considerably expand the size of the available working sample.
ABSTRACT
Here, we report an ensemble study of 214 A- and F-type stars observed by Kepler, exhibiting the so-called hump and spike periodic signal, explained by Rossby modes (r modes) – the hump – and ...magnetic stellar spots or overstable convective (OsC) modes – the spike, respectively. We determine the power confined in the non-resolved hump features and find additional gravity-mode (g-mode) humps always occurring at higher frequencies than the spike. Furthermore, we derive projected rotational velocities from FIES, SONG, and HERMES spectra for 28 stars and the stellar inclination angle for 89 stars. We find a strong correlation between the spike amplitude and the power in the r and g modes, which suggests that both types of oscillations are mechanically excited by either stellar spots or OsC modes. Our analysis suggests that stars with a higher power in m = 1 r-mode humps are more likely to also exhibit humps at higher azimuthal orders (m = 2, 3, or 4). Interestingly, all stars that show g-mode humps are hotter and more luminous than the observed red edge of the δ Scuti instability strip, suggesting that either magnetic fields or convection in the outer layers could play an important role.
Aims. The nearby metal-poor giant HD 122563 is an important astrophysical laboratory in which to test stellar atmospheric and interior physics. It is also a benchmark star for which to calibrate ...methods to apply to large scale surveys. Recently it has been remeasured using various methodologies given the new high precision instruments at our disposal. However, inconsistencies in the observations and models have been found. Methods. In order to better characterise this star using complementary techniques we have been measuring its radial velocities since 2016 using the Hertzsprung telescope (SONG network node) in order to detect oscillations. Results. In this work we report the first detections of sun-like oscillations in this star, and to our knowledge, a detection in the most metal-poor giant to date. We applied the classical seismic scaling relation to derive a new surface gravity for HD 122563 of log gν = 1.39 ± 0.01. Reasonable constraints on the mass imposed by its PopII giant classification then yields a radius of 30.8 ± 1.0 ℛ⊙. By coupling this new radius with recent interferometric measurements we infer a distance to the star of 306 ± 9 pc. This result places it further away than was previously thought and is inconsistent with the HIPPARCOS parallax. Independent data from the Gaia mission corroborate the distance hypothesis (dGDR2 = 290 ± 5 pc), and thus the updated fundamental parameters. Conclusions. We confirm the validity of the classical seismic scaling relation for surface gravity in metal-poor and evolved star regimes. The remaining discrepancy of 0.04 dex between log gGDR2 (= 1.43 ± 0.03) reduces to 0.02 dex by applying corrections to the scaling relations based on the mean molecular weight and adiabatic exponent. The new constraints on the Hertzsprung–Russell diagram (L⋆ν = 381 ± 26 ℒ⊙) significantly reduce the disagreement between the stellar parameters and evolution models, however, a discrepancy of the order of 150 K still exists. Fine-tuned stellar evolution calculations show that this discrepancy can be reconciled by changing the mixing-length parameter by an amount (−0.35) that is in agreement with predictions from recent 3D simulations and empirical results. Asteroseismic measurements are continuing, and analysis of the full frequency data complemented by a distance estimate promises to bring important constraints on our understanding of this star and of the accurate calibration of the seismic scaling relations in this regime.
Stellar evolution models are most uncertain for evolved massive stars. Asteroseismology based on high-precision uninterrupted space photometry has become a new way to test the outcome of stellar ...evolution theory and was recently applied to a multitude of stars, but not yet to massive evolved supergiants.Our aim is to detect, analyse and interpret the photospheric and wind variability of the O9.5 Iab star HD 188209 from Kepler space photometry and long-term high-resolution spectroscopy. We used Kepler scattered-light photometry obtained by the nominal mission during 1460 d to deduce the photometric variability of this O-type supergiant. In addition, we assembled and analysed high-resolution high signal-to-noise spectroscopy taken with four spectrographs during some 1800 d to interpret the temporal spectroscopic variability of the star. The variability of this blue supergiant derived from the scattered-light space photometry is in full in agreement with the one found in the ground-based spectroscopy. We find significant low-frequency variability that is consistently detected in all spectral lines of HD 188209. The photospheric variability propagates into the wind, where it has similar frequencies but slightly higher amplitudes. The morphology of the frequency spectra derived from the long-term photometry and spectroscopy points towards a spectrum of travelling waves with frequency values in the range expected for an evolved O-type star. Convectively-driven internal gravity waves excited in the stellar interior offer the most plausible explanation of the detected variability.
The GOLF instrument on board SoHO has been in operation for almost 25 years, but the ageing of the instrument has now strongly affected its performance, especially in the low-frequency pressure-mode ...(
p
-mode) region. At the end of the SoHO mission, the ground-based network BiSON will remain the only facility able to perform Sun-integrated helioseismic observations. Therefore, we want to assess the helioseismic performances of an échelle spectrograph such as SONG. The high precision of such an instrument and the quality of the data acquired for asteroseismic purposes call for an evaluation of the instrument’s ability to perform global radial-velocity measurements of the solar disk. Data acquired during the Solar-SONG 2018 observation campaign at the Teide Observatory are used to study mid- and low-frequency
p
modes. A Solar-SONG time series of 30 days in duration is reduced with a combination of the traditional IDL
iSONG
pipeline and a new
Python
pipeline described in this paper. A mode fitting method built around a Bayesian approach is then performed on the Solar-SONG and contemporaneous GOLF, BiSON, and HMI data. For this contemporaneous time series, Solar-SONG is able to characterise p modes at a lower frequency than BiSON or GOLF (1750 μHz versus 1946 and 2157 μHz, respectively), while for HMI it is possible to characterise a mode at 1686 μHz. The decrease in GOLF sensitivity is then evaluated through the evolution of its low-frequency
p
-mode characterisation abilities over the years: a set of 30-day-long GOLF time series, considered at the same period of the year from 1996 to 2017, is analysed. We show that it is more difficult to accurately characterise p modes in the range 1680 to 2160 μHz when considering the most recent time series. By comparing the global power level of different frequency regions, we also observe that the Solar-SONG noise level in the 1000 to 1500 μHz region is lower than for any GOLF subseries considered in this work. While the global
p
-mode power-level ratio is larger for GOLF during the first years of the mission, this ratio decreases over the years and is bested by Solar-SONG for every time series after 2000. All these observations strongly suggest that efforts should be made towards deploying more Solar-SONG nodes in order to acquire longer time series with better duty cycles.
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