Radial Velocity Oscillations Research Articles

The first detection of an exoplanet that opened a new field in planetology

Periodic Radial Velocity oscillations of the star 51 Pegasi measured at OHP (Observatoire de Haute-Provence) in 1995 revealed a planet orbiting around it. This first detection of an exoplanet had been expected for a long time, since everyone was convinced that most of the stars were surrounded by planets. This success, leading to the Nobel Prize of physics in 2019, is the result of the evolution of technics for the measurement of radial velocities of stars during half a century at OHP.

MCMCI: A code to fully characterise an exoplanetary system

Context. Useful information can be retrieved by analysing the transit light curve of a planet-hosting star or induced radial velocity oscillations. However, inferring the physical parameters of the planet, such as mass, size, and semi-major axis, requires preliminary knowledge of some parameters of the host star, especially its mass or radius, which are generally inferred through theoretical evolutionary models. Aims. We seek to present and test a whole algorithm devoted to the complete characterisation of an exoplanetary system thanks to the global analysis of photometric or radial velocity time series combined with observational stellar parameters derived either from spectroscopy or photometry. Methods. We developed an integrated tool called MCMCI. This tool combines the Markov chain Monte Carlo (MCMC) approach of analysing photometric or radial velocity time series with a proper interpolation within stellar evolutionary isochrones and tracks, known as isochrone placement, to be performed at each chain step, to retrieve stellar theoretical parameters such as age, mass, and radius. Results. We tested the MCMCI on the HD 219134 multi-planetary system hosting two transiting rocky super Earths and on WASP-4, which hosts a bloated hot Jupiter. Even considering different input approaches, a final convergence was reached within the code, we found good agreement with the results already stated in the literature and we obtained more precise output parameters, especially concerning planetary masses. Conclusions. The MCMCI tool offers the opportunity to perform an integrated analysis of an exoplanetary system without splitting it into the preliminary stellar characterisation through theoretical models. Rather this approach favours a close interaction between light curve analysis and isochrones, so that the parameters recovered at each step of the MCMC enter as inputs for purposes of isochrone placement.

Sectoral r modes and periodic radial velocity variations of Sun-like stars

Context. Radial velocity (RV) measurements are used to search for planets orbiting late-type main-sequence stars and to confirm the transiting planets. Aims. The most advanced spectrometers are now approaching a precision of ~10 cm s−1, which implies the need to identify and correct for all possible sources of RV oscillations intrinsic to the star down to this level and possibly beyond. The recent discovery of global-scale equatorial Rossby waves in the Sun, also called r modes, prompted us to investigate their possible signature in stellar RV measurements. These r modes are toroidal modes of oscillation whose restoring force is the Coriolis force; they propagate in the retrograde direction in a frame that co-rotates with the star. The solar r modes with azimuthal orders 3 ≤ m ≲ 15 were identified unambiguously because of their dispersion relation and their long e-folding lifetimes of hundreds of days. Methods. In this paper, we simulate the RV oscillations produced by sectoral r modes with 2 ≤ m ≤ 5 by assuming a stellar rotation period of 25.54 days and a maximum amplitude of the surface velocity of each mode of 2 m s−1. This amplitude is representative of the solar measurements except for the m = 2 mode, which has not yet been observed on the Sun. Results. Sectoral r modes with azimuthal orders m = 2 and 3 would produce RV oscillations with amplitudes of 76.4 and 19.6 cm s−1 and periods of 19.16 and 10.22 days, respectively, for a star with an inclination of the rotation axis to the line of sight i = 60°. Therefore, they may produce rather sharp peaks in the Fourier spectrum of the radial velocity time series that could lead to spurious planetary detections. Conclusions. Sectoral r modes may represent a source of confusion in the case of slowly rotating inactive stars that are preferential targets for RV planet search. The main limitation of the present investigation is the lack of observational constraints on the amplitude of the m = 2 mode on the Sun.

SPECTROSCOPIC AND PHOTOMETRIC VARIABILITY IN THE A0 SUPERGIANT HR 1040

A time-series analysis of spectroscopic and photometric observables of the A0 Ia supergiant HR 1040 has been performed, including equivalent widths, radial velocities, and Stromgren photometric indices. The data, obtained from 1993 through 2007, include 152 spectroscopic observations from the Ritter Observatory 1 m telescope and 269 Stromgren photometric observations from the Four College Automated Photoelectric Telescope. Typical of late B- and early A-type supergiants, HR 1040 has a highly variable Hα profile. The star was found to have an intermittent active phase marked by correlation between the Hα absorption equivalent width and blue-edge radial velocity and by photospheric connections observed in correlations to equivalent width, second moment and radial velocity in Si ii λλ6347, 6371. High-velocity absorption (HVA) events were observed only during this active phase. HVA events in the wind were preceded by photospheric activity, including Si ii radial velocity oscillations 19–42 days prior to onset of an HVA event and correlated increases in Si ii Wλ and second moment from 13 to 23 days before the start of the HVA event. While increases in various line equivalent widths in the wind prior to HVA events have been reported in the past in other stars, our finding of precursors in enhanced radial velocity variations in the wind and at the photosphere is a new result.

Discovery of new rapidly oscillating Ap pulsators in the UVES survey of cool magnetic Ap stars★

We have carried out a survey of short-period pulsations among a sample of carefully chosen cool Ap stars using time-resolved observations with the UVES spectrometer at the ESO 8-m VLT telescope. Here we report the discovery of pulsations with amplitudes 50-100 m/s and periods 7-12 min in HD132205, HD148593 and HD151860. These objects are therefore established as new rapidly oscillating Ap (roAp) stars. In addition, we independently confirm the presence of pulsations in HD69013, HD96237 and HD143487 and detect, for the first time, radial velocity oscillations in two previously known photometric roAp stars HD119027 and HD185256. At the same time, no pulsation variability is found for HD5823, HD178892 and HD185204. All of the newly discovered roAp stars were previously classified as non-pulsating based on the low-precision ground-based photometric surveys. This shows that such observations cannot be used to reliably distinguish between pulsating and non-pulsating stars and that all cool Ap stars may harbor p-mode pulsations of different amplitudes.

VARIABILITY IN OPTICAL SPECTRA OF ϵ ORIONIS

We present the results of a time-series analysis of 130 echelle spectra of epsilon Ori (B0 Ia), acquired over seven observing seasons between 1998 and 2006 at Ritter Observatory. The equivalent widths of Halpha (net) and He I 5876 were measured and radial velocities were obtained from the central absorption of He I 5876. Temporal variance spectra (TVS) revealed significant wind variability in both Halpha and He I 5876. The He I TVS have a double-peaked profile consistent with radial velocity oscillations. A periodicity search was carried out on the equivalent width and radial velocity data, as well as on wavelength-binned spectra. This analysis has revealed several periods in the variability with time scales of 2-7 d. Many of these periods exhibit sinusoidal modulation in the associated phase diagrams. Several of these periods were present in both Halpha and He I, indicating a possible connection between the wind and the photosphere. Due to the harmonic nature of these periods, stellar pulsations may be the origin of some of the observed variability. Periods on the order of the rotational period were also detected in the He I line in the 98-99 season and in both lines during the 04-05 season. These periods may indicate rotational modulation due to structure in the wind.

Some features of the radial-velocity variations of lines of different intensity in the spectrum of HD 93521. Variability of the stellar wind

CCD spectra taken with the PFES echelle spectrograph of the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences are used to perform a detailed study of the variability of the profiles of Hell, Hβ, and Hα lines in the spectrum of HD 93521. The pattern and nature of the variability of the Hell lines are similar to those of weak HeI lines and are due to nonradial pulsations. The period and amplitude of the radial-velocity variations are the same for the blue and red halves of the absorption profile but their phases are opposite. The behavior of the variations of Hβ and Hα hydrogen lines relative to their mean profiles is the same as that of strong HeI line and is due to nonradial pulsations. The period and phase of the radial-velocity oscillations are the same for the blue and red halves of the absorption profile but their amplitude are different. The behavior of the radial-velocity variations of the absorption and emission components of the Hα line indicates that the latter also are caused by nonradial pulsations. All this is indicative of the complex structure of the stellar wind in the region of its origin. The behavior of variability and wind kinematics differ in different directions and for different regions of the atmosphere and/or envelope.

The HARPS search for southern extra-solar planets

We report the discovery of a planetary companion orbiting the F8V star HD 212301. This is the first very hot discovered by a radial velocity survey. The detection was made possible by the HARPS spectrograph installed at the ESO 3.6 m telescope in Cerro La Silla, Chile. The data were carefully analyzed to exclude activity induced effects on the measured radial velocities. Although no transit has been detected, radial velocity oscillations are best explained with a planet of mass m 2 sin i = 0.45 M Jupiter orbiting the star at 0.036 AU with a period of 2.246 days.

Pulsational line profile variation of the roAp star HR 3831

We report the first comprehensive investigation of the line profile variation caused by non-radial pulsation in an oscillating magnetic chemically peculiar star. Spectrum variation of the well-known roAp star HR3831 was detected using very high-resolution high signal-to-noise spectroscopic time-series observations and was followed through the whole rotation cycle of the star. We confirmed the outstanding diversity of pulsational behaviour of different lines in the HR 3831 spectrum and attributed this phenomenon to an interplay between extreme vertical chemical inhomogeneity of the HR 3831 atmosphere and a running pulsation wave, that is propagating towards the upper photospheric layers with increasing amplitude. The rapid profile variation of the Nd III 6145 A line, which shows the maximum pulsational disturbances in the studied wavelength region, is characterised by measuring changes of its equivalent width and the first three moments. Each of these observables exhibits almost purely sinusoidal variation through the pulsation cycle, with the amplitude and phase clearly modulated by the stellar rotation. In contrast to previous photometric pulsational measurements of roAp stars, our spectroscopic observational material admits straightforward astrophysical interpretation and, hence, opens new and exciting possibilities for direct and accurate analysis of the roAp pulsations. We demonstrate that rotational modulation of the radial velocity oscillations cannot be fully explained by an oblique axisymmetric dipole (l = 1, m = 0) mode, implied by the classical oblique pulsator model of roAp stars. Pulsational variation of the higher moments, in particular the line width, reveals substantial contribution of the high-order (l = 3) spherical harmonics that appear due to distortion of pulsations in HR3831 by the global magnetic field. We interpreted observations with a novel numerical model of the pulsational variation and rotational modulation of the line profile moments in roAp stars. Comparison of the observed and computed radial velocity and line width variation was used to establish parameters of the oblique pulsator model of HR 3831. Furthermore, definite detection of pulsational variation in lines of light and iron-peak elements allows the first 3-D mapping of pulsations in a non-radially oscillating star.

Influence of a sunspot penumbra on the oscillatory regime of neighboring regions

The spectra of radial-velocity oscillations in the immediate vicinity of a sunspot beyond the observable boundary of the penumbra have been analyzed. The radial velocities were derived using a differential method. The oscillation spectra at two heights (lower photosphere and chromosphere) were compared with reference spectra for the middle penumbra and an unperturbed region obtained using the same method. The oscillatory regime characteristic of the sunspot penumbra extends more than 15″ beyond its observable boundary. It is proposed that deep photospheric layers in the region surrounding the penumbra have physical conditions similar to those observed inside its outer boundary.

Physical conditions and velocity fields in coronal holes. i. radial velocity oscillations in and out of coronal holes

Abstract Power spectra of radial velocity in and out coronal holes were studied. Thirteen of spectra with temporal resolution 17–20 s were used for this investigation. The duration of each set varied from 1.5–2 h. We analysed two spectral lines: Si I 10827A (photosphere) and He I 10830A (chromosphere). The following conclusions have been draw. (1) All power spectra in the frequency range 0.001–0.05 Hz have confident peaks in the range 0.002–0.010 Hz. (2) The power of oscillations (the sum of squares of amplitudes in a power spectrum) in the frequency range 0.02–0.010 Hz increases from the photosphere to higher levels of the chromosphere. (3) The power of oscillations in the chromosphere is higher inside coronal holes than out side. (4) There are four peaks in the chromosphere power spectrum of a coronal hole and two in the spectrum of a quiet region. Peak frequencies for coronal holes differ from those for quiet regions. We did not find confident peaks in photosphere power spectra of coronal holes. There ...

Radial velocity observations reveal multimode oscillations in Gamma Equulei

Radial velocity oscillations were observed in the rapidly oscillating Ap star Gamma Equulei. Two peaks in the power spectrum are clearly significant, at v = 1366 and 1427 micro-Hz, in contrast to the single peak at 1339 micro-Hz from an earlier observation reported by Kurtz (1983). These observations suggest that several modes of Gamma Equ with different (l, n) are excited at various times, with a fundamental spacing of roughly 58 micro-Hz. The ratio of velocity to brightness oscillations is inferred to be of order 30 km/s/mag or greater, similar to the approximately 80 km/s/mag observed for the much lower amplitude solar p-mode oscillations.

A Search for Radial Velocity Oscillations in Procyon

I describe here a new technique for precision radial velocity measurements, using an iodine absorption cell in front of the slit of a high resolution spectrograph. A CCD in a continuous readout mode records a timeseries of stellar spectra with a duty cycle of unity. In January 1987, observations were obtained on two clear nights with bad seeing. No oscillations were detected in the data, but the technique shows promise for significantly better results under better conditions.

Further study of a jet stream‐generated Kelvin‐Helmholtz instability

High‐resolution VHF Doppler radar measurements during a jet stream passage reveal radial velocity oscillations produced by a Kelvin‐Helmholtz (KH) instability. A detailed investigation of the velocity data, using digital filters and least squares methods, shows that the growth rate of the KH oscillations decreases linearly with increasing time so that a growth and a decay stage can be defined. During the occurrence of the KHI, the radar signal power is characterized by periodic bursts up to 20 dB above the mean power level. The bursts have a period equal to that of the KH oscillation and are restricted to heights around the critical level at which the KH instability is generated. Model computations indicate that the power bursts are produced by static instabilities which are due to KH‐induced superadiabatic lapse rates. During the growth stage, the vertical profile of the mean wind exhibits characteristic temporal changes that can probably be ascribed to the Reynolds stress of the KH instability. There is some evidence that the observed KH oscillations are parts of a sequence of KH waves initiated by long‐period gravity waves.