High-cadence spectroscopy of M-dwarfs – II. Searching for stellar pulsations with HARPS

Abstract

Stellar oscillations appear all across the Hertzsprung-Russell diagram. Recent theoretical studies support their existence also in the atmospheres of M dwarfs. These studies predict for them short periodicities ranging from 20 min to 3 h. Our Cool Tiny Beats (CTB) programme aims at finding these oscillations for the very first time. With this goal, CTB explores the short time domain of M dwarfs using radial velocity data from the HARPS-ESO and HARPS-N high-precision spectrographs. Here we present the results for the two most long-term stable targets observed to date with CTB, GJ 588 and GJ 699 (i.e. Barnard's star). In the first part of this work we detail the correction of several instrumental effects. These corrections are specially relevant when searching for sub-night signals. Results show no significant signals in the range where M dwarfs pulsations were predicted. However, we estimate that stellar pulsations with amplitudes larger than ∼ 0.5 m s −1 can be detected with a 90% completeness with our observations. This result, along with the excess of power regions detected in the periodograms, open the possibility of non-resolved very low amplitude pulsation signals. Next generation more precise instrumentation would be required to detect such oscillations. However, the possibility of detecting pulsating M-dwarf stars with larger amplitudes is feasible due to the short size of the analysed sample. This motivates the need for completeness of the CTB survey.

Activity and telluric contamination in HARPS observations of Alpha Centauri B

“…High cadence observations in 2013 were excluded from this analysis as most measurements from the spectra (like RV and spectral indices) correlate closely with airmass. We expect that a combination of two factors are the cause: light contamination from α Cen A (Bergmann et al 2015) and point-spread function variations over a night (Berdiñas et al 2016(Berdiñas et al , 2017. These observations should be analysed separately.…”

The Quest for Pulsating M Dwarf Stars

“…One of the reasons is the extremely high time-cadence needed to sample the 20 min to 3 h pulsating range, coupled to the high competition and cost of observing nights at medium class telescopes. Berdiñas et al (2017) derived that a pulsating signal with a 0.5 m/s RV amplitude could be detected with a 90% confidence with four continuous nights of observations. Unfortunately, these amount of nights are not easily awarded for exploratory programs.…”

“…Despite the volume of radial velocity (RV) data of M dwarfs in the archives of these kind of instruments, such as HARPS (Bonfils et al, 2013), that do consistent searches for exoplanets around several types of stars, these archive RV time-series have long time baselines with a low time cadence, typically a few RV measurements per month, which are not useful to search for rapid pulsations in the time domain from around 20 min to 3 h, expected for main sequence M dwarfs. Berdiñas et al (2017) carried out the first search for M dwarfs pulsations using high-cadence, high-precision time-series spectroscopy obtained with HARPS, for which they derive an amplitude detection limit of ∼0.5 m s −1 to achieve a confidence level of 90% for the detection of M dwarfs pulsations. They present results for the two most long-term stable stars in their sample, for which they found no signals with false alarm probability (FAP) below 1% in the expected range of pulsations.…”

A super-Earth on a close-in orbit around the M1V star GJ 740. A HADES and CARMENES collaboration

“…The high resolution of R ∼ 115 000 provided by this fiber-fed echelle spectrograph along with its spectral range from 3830 to 6900 Å are ideal for high-precision RV searches. We obtained a total of 129 HARPS-N spectra over a time-span of 6 yr. Every spectrum was taken using an exposure time of 900 s to average out the shorttime periodic oscillations of the star (Dumusque et al 2011), although this phenomenon has not yet been detected in M-dwarfs (Berdiñas et al 2017). The average signal-to-noise ratio (S/N) achieved at 5500 Å per pixel was 110, enough to ensure a good exposure level of the blue part of the spectra, which contains the Ca II H&K lines that are especially weak for M-type stars (Giampapa et al 1989;Lovis et al 2011).…”