Radial Velocity Modulation Research Articles

Precise Fourier parameters of Cepheid radial velocity curves: Towards refining the Hertzsprung progression models

Context. Radial velocity (RV) curves of Classical Cepheids allow precise determination of the resonant periods, which in turn help to constrain fundamental parameters of these stars. The RV curves of Cepheids are also useful for identifying their pulsation modes and for distance determination using the parallax-of-pulsation method. Aims. The primary goal of this paper is to derive precise Fourier parameters of the RV curves for fundamental and first-overtone Galactic Cepheids. Our secondary objectives are then to analyze the progression of the Fourier parameters up to the seventh harmonic, and to propose an identification of the pulsation modes of the stars. Methods. For each star, we carefully selected RV measurements available in the literature that yield the highest precision of Fourier parameters according to the procedure that follows. We performed a Fourier decomposition of the RV curves using the unweighted least-square method and the standard deviation of the fit was used to derive the uncertainty on the Fourier parameters. We corrected for zero-point differences between datasets and RV modulations caused by binary motion. Results. With this study we have more than doubled the number of Cepheids with published RV curve Fourier parameters and with their uncertainty properly estimated. Our sample includes 178 fundamental-mode and 33 first-overtone pulsators, as well as 7 additional Cepheids whose pulsation mode is uncertain or undetermined according to our criteria. For the fundamental-mode Cepheids, the precision of the obtained low-order Fourier phases and amplitudes is about seven times and 25% better, respectively, as compared to the precision achieved in previously published Fourier parameter surveys. With highly accurate RV Fourier phases ϕ21, we are able to firmly identify V495 Cyg as a new first-overtone Cepheid and we confirm the first-overtone nature of several other stars. In particular, α UMi should be firmly classified as a first-overtone pulsator. In three objects (VY Per, AQ Pup, and QZ Nor), we find significant γ-velocity variations, which for the first two objects (and possibly for QZ Nor as well) can be attributed to the spectroscopic binarity of these stars. Finally, the analysis of the fundamental mode Fourier parameters up to seventh order reveals tight progression of Fourier phases for all pulsation periods. Conclusions. We provide new precise Fourier parameters of Cepheid RV curves determined from RV measurements available in the literature together with unpublished data. The pulsation period coverage and the precision obtained, in particular for Fourier phase ϕ21, will be useful for studying the dynamics of Cepheid pulsations with the help of hydrodynamical models. Further RV measurements from modern high-resolution spectroscopic instruments will be important to improve these results.

Unraveling the binary nature of HQ Tau

Context. Both the stellar activity and the accretion processes of young stellar objects can induce variations in their radial velocity (RV). This variation is often modulated on the stellar rotation period and may hide a RV signal from a planetary or even a stellar companion. Aims. The aim of this study is to detect the companion of HQ Tau, the existence of which is suspected based on our previous study of this object. We also aim to derive the orbital elements of the system. Methods. We used multi-variate Gaussian process regression on the RV and the bisector inverse slope of a six-month high-resolution spectroscopic follow-up observation of the system to model the stellar activity. This allowed us to extract the Keplerian RV modulation induced by the suspected companion. Results. Our analysis yields the detection of a ∼50 Mjup brown dwarf companion orbiting HQ Tau with a ∼126 day orbital period. Although this is consistent with the modulation seen on this dataset, it does not fit the measurements from our previous work three years earlier. In order to include these measurements in our analysis, we hypothesise the presence of a third component with orbital elements that are consistent with those of the secondary according to our previous analysis (MB ∼ 48 Mjup, Porb, B ∼ 126 days), and a ∼465 Mjup tertiary with a ∼767 day orbital period. However, the hypothesis of a single companion with MB ∼ 188 Mjup and Porb ∼ 247 days can fit both datasets and cannot be completely excluded at this stage of the analysis. Conclusions. At minima, HQ Tau is a single-lined spectroscopic binary, and several factors indicate that the companion is a brown dwarf and that a third component is responsible for larger RV variation on a longer timescale.

Constraining the Binarity of Black Hole Candidates: A Proof-of-concept Study of Gaia BH1 and Gaia BH2

Nearly a hundred binary black holes (BBHs) have been discovered with gravitational-wave signals emitted at their merging events. Thus, it is quite natural to expect that significantly more abundant BBHs with wider separations remain undetected in the Universe or even in our Galaxy. We consider a possibility that star–BH binary candidates may indeed host an inner BBH instead of a single BH. We present a detailed feasibility study of constraining the binarity of the currently available two targets, Gaia BH1 and Gaia BH2. Specifically, we examine three types of radial velocity (RV) modulations of a tertiary star in star–BBH triple systems; short-term RV modulations induced by the inner BBH, long-term RV modulations induced by the nodal precession, and long-term RV modulations induced by the von Zeipel-Kozai–Lidov oscillations. Direct three-body simulations combined with approximate analytic models reveal that the Gaia BH1 system may exhibit observable signatures of the hidden inner BBH if it exists at all. The methodology that we examine here is quite generic and is expected to be readily applicable to future star–BH binary candidates in a straightforward manner.

Stable accretion in young stars: the cases of EX Lupi and TW Hya

ABSTRACT We examine the long-term spectroscopic and photometric variability of EX Lupi and TW Hya, studying the presence of stable accretion and the role it plays in the observed variability. Analysing the velocity modulations of the emission lines with star-melt, we obtain information on the structure of the accretion columns and the disc–star connection. The emission-line radial velocities reveal that TW Hya, like EX Lupi, has a remarkably stable or slow-varying accretion column footprint, locked to the star for several years. The line-emitting regions are non-polar for both EX Lupi and TW Hya, and species with different energies differ in position. In contrast, the continuum emission as observed in the photometry is very variable and can be modelled by hotspot(s) that change over time in phase, shape, temperature, size, and location with respect to the emission-line region. The continuum emission region may not be limited to the stellar surface, especially during episodes of high accretion. The broad-line emission observed in EX Lupi during episodes of increased accretion reveals a further structure, which can be fitted by non-axisymmetric disc in Keplerian rotation inwards of the corotation radius. Since the radial velocity modulation due to accretion footprints is so stable, we used it to search for further velocity modulations. While no residual modulation (other than caused by stellar rotation) is found in these objects, a similar analysis could help to identify young planets/companions. Therefore, determining whether stable accretion footprints are common among young stars is a key to detect young planets.

New periodograms separating orbital radial velocities and spectral shape variation

We present new periodograms that are effective in distinguishing Doppler shift from spectral shape variability in astronomical spectra. These periodograms, building upon the concept of partial distance correlation, separate the periodic radial velocity modulation induced by orbital motion from that induced by stellar activity. These tools can be used to explore large spectroscopic databases in search of targets in which spectral shape variations obscure the orbital motion; such systems include active planet-hosting stars or binary systems with an intrinsically variable component. We provide a detailed prescription for calculating the periodograms, demonstrate their performance via simulations and real-life case studies, and provide a public Python implementation.

Unveiling the Architecture of a Pulsar–Binary Black Hole Triple System with Pulsar Arrival Time Analysis

Abstract A large number of binary black holes (BBHs) with longer orbital periods are supposed to exist as progenitors of BBH mergers recently discovered with gravitational wave (GW) detectors. In our previous papers, we proposed to search for such BBHs in triple systems through the radial velocity modulation of the tertiary orbiting star. If the tertiary is a pulsar, high-precision and high-cadence observations of its arrival time enable an unambiguous characterization of the pulsar–BBH triples located at several kiloparsecs, which are inaccessible with the radial velocity of stars. The present paper shows that such inner BBHs can be identified through the short-term Rømer delay modulation, on the order of 10 ms for our fiducial case, a triple consisting of a 20 M ⊙ BBH and 1.4 M ⊙ pulsar with P in = 10 days and P out = 100 days. If the relativistic time delays are measured as well, one can determine basically all the orbital parameters of the triple. For instance, this method is applicable to inner BBHs of down to ∼1 hr orbital periods if the orbital period of the tertiary pulsar is around several days. Inner BBHs with ≲1 hr orbital period emit the GW detectable by future space-based GW missions, including LISA, DECIGO, and BBO, and very short inner BBHs with subsecond orbital period can be even probed by the existing ground-based GW detectors. Therefore, our proposed methodology provides a complementary technique to search for inner BBHs in triples, if they exist at all, in the near future.

Persistent Starspot Signals on M Dwarfs: Multiwavelength Doppler Observations with the Habitable-zone Planet Finder and Keck/HIRES

Abstract Young, rapidly rotating M dwarfs exhibit prominent starspots, which create quasiperiodic signals in their photometric and Doppler spectroscopic measurements. The periodic Doppler signals can mimic radial velocity (RV) changes expected from orbiting exoplanets. Exoplanets can be distinguished from activity-induced false positives by the chromaticity and long-term incoherence of starspot signals, but these qualities are poorly constrained for fully convective M stars. Coherent photometric starspot signals on M dwarfs may persist for hundreds of rotations, and the wavelength dependence of starspot RV signals may not be consistent between stars due to differences in their magnetic fields and active regions. We obtained precise multiwavelength RVs of four rapidly rotating M dwarfs (AD Leo, G227-22, GJ 1245B, GJ 3959) using the near-infrared (NIR) Habitable-zone Planet Finder and the optical Keck/HIRES spectrometer. Our RVs are complemented by photometry from Kepler, TESS, and the Las Cumbres Observatory network of telescopes. We found that all four stars exhibit large spot-induced Doppler signals at their rotation periods, and investigated the longevity and optical-to-NIR chromaticity for these signals. The phase curves remain coherent much longer than is typical for Sunlike stars. Their chromaticity varies, and one star (GJ 3959) exhibits optical and NIR RV modulation consistent in both phase and amplitude. In general, though, we find that the NIR amplitudes are lower than their optical counterparts. We conclude that starspot modulation for rapidly rotating M stars frequently remains coherent for hundreds of stellar rotations and gives rise to Doppler signals that, due to this coherence, may be mistaken for exoplanets.

The magnetic field and accretion regime of CI Tau

ABSTRACT This paper exploits spectropolarimetric data of the classical T Tauri star CI Tau collected with ESPaDOnS at the Canada–France–Hawaii Telescope, with the aims of detecting and characterizing the large-scale magnetic field that the star hosts, and of investigating how the star interacts with the inner regions of its accretion disc through this field. Our data unambiguously show that CI Tau has a rotation period of 9.0 d, and that it hosts a strong, mainly poloidal large-scale field. Accretion at the surface of the star concentrates within a bright high-latitude chromospheric region that spatially overlaps with a large dark photospheric spot, in which the radial magnetic field reaches −3.7 kG. With a polar strength of −1.7 kG, the dipole component of the large-scale field is able to evacuate the central regions of the disc up to about 50 per cent of the co-rotation radius (at which the Keplerian orbital period equals the stellar rotation period) throughout our observations, during which the average accretion rate was found to be unusually high. We speculate that the magnetic field of CI Tau is strong enough to sustain most of the time a magnetospheric gap extending to at least 70 per cent of the co-rotation radius, which would explain why the rotation period of CI Tau is as long as 9 d. Our results also imply that the 9 d radial velocity (RV) modulation that CI Tau exhibits is attributable to stellar activity, and thus that the existence of the candidate close-in massive planet CI Tau b to which these RV fluctuations were first attributed needs to be reassessed with new evidence.

Spectroscopic confirmation of the binary nature of the hybrid pulsator KIC 5709664 found with the frequency modulation method

We started a program to search for companions around hybrid $\delta$ Sct/$\gamma$ Dor stars with the frequency modulation method using Kepler data. Our best candidate was KIC 5709664, where we could identify Fourier peaks with sidelobes, suggesting binary orbital motion. We determined the orbital parameters with the phase modulation method and took spectroscopic measurements to confirm unambiguously the binary nature with radial velocities. The spectra show that the object is a double-lined spectroscopic binary, and we determined the orbital solutions from the radial velocity curve fit. The parameters from the phase modulation method and the radial velocity fits are in good agreement. We combined a radial velocity and phase modulation approach to determine the orbital parameters as accurately as possible. We determined that the pulsator is a hybrid $\delta$ Sct/$\gamma$ Dor star in an eccentric binary system with an orbital period of ~95 d and an eccentricity of 0.55. The measured mass ratio is 0.67. We analysed the pulsation content and extracted 38 frequencies with amplitudes greater than 20 $\mu$mag. At low frequencies, we found broad power excesses which are likely attributed to spots on the rotating surface of the lower-mass component. We inferred rotation periods of 0.56 and 2.53 d for the primary and secondary, respectively.

Multi-Wavelength High-Resolution Spectroscopy for Exoplanet Detection: Motivation, Instrumentation and First Results

Exoplanet research has shown an incessant growth since the first claim of a hot giant planet around a solar-like star in the mid-1990s. Today, the new facilities are working to spot the first habitable rocky planets around low-mass stars as a forerunner for the detection of the long-awaited Sun-Earth analog system. All the achievements in this field would not have been possible without the constant development of the technology and of new methods to detect more and more challenging planets. After the consolidation of a top-level instrumentation for high-resolution spectroscopy in the visible wavelength range, a huge effort is now dedicated to reaching the same precision and accuracy in the near-infrared. Actually, observations in this range present several advantages in the search for exoplanets around M dwarfs, known to be the most favorable targets to detect possible habitable planets. They are also characterized by intense stellar activity, which hampers planet detection, but its impact on the radial velocity modulation is mitigated in the infrared. Simultaneous observations in the visible and near-infrared ranges appear to be an even more powerful technique since they provide combined and complementary information, also useful for many other exoplanetary science cases.

Evidence of a substellar companion around a very young T Tauri star

We present results from a Near Infrared multi-epoch spectroscopic campaign to detect a young low-mass companion to a T Tauri star. AS 205A is a late-type dwarf (~K5) of ~1 Msun that belongs to a triple system. Independent photometric surveys discovered that AS 205A has two distinct periods (P1=6.78 and P2=24.78 days) detected in the light curve that persist over several years. Period P1 seems to be linked to the axial-rotation of the star and is caused by the presence of cool surface spots. Period P2 is correlated with the modulation in AS 205A brightness (V) and red color (V-R), consistent with a gravitating object within the accretion disk. We here derive precise Near Infrared radial velocities to investigate the origin of period P2 which is predicted to correspond to a cool source in a Keplerian orbit with a semi-major axis of ~0.17 AU positioned close to the inner disk radius of 0.14 AU. The radial velocity variations of AS 205A were found to have a period of P ~ 24.84 days and a semi-amplitude of 1.529 kms-1. This result closely resembles the P2 period in past photometric observations (P ~ 24.78 days). The analysis of the cross-correlation function bisector has shown no correlation with the radial velocity modulations, strongly suggesting that the period is not controlled by stellar rotation. Additional activity indicators should however be explored in future surveys. Taking this into account we found that the presence of a substellar companion is the explanation that best fits the results. We derived an orbital solution for AS 205A and found evidence of a m2sini 19.25 MJup object in an orbit with moderate eccentricity of e~0.34. If confirmed with future observations, preferably using a multiwavelength survey approach, this companion could provide interesting constraints on brown dwarf and planetary formation models.

SEEDS DIRECT IMAGING OF THE RV-DETECTED COMPANION TO V450 ANDROMEDAE, AND CHARACTERIZATION OF THE SYSTEM

ABSTRACT We report the direct imaging detection of a low-mass companion to a young, moderately active star V450 And, that was previously identified with the radial velocity (RV) method. The companion was found in high-contrast images obtained with the Subaru Telescope equipped with the HiCIAO camera and AO188 adaptive optics system. From the public ELODIE and SOPHIE archives we extracted available high-resolution spectra and RV measurements, along with RVs from the Lick planet search program. We combined our multi-epoch astrometry with these archival, partially unpublished RVs, and found that the companion is a low-mass star, not a brown dwarf, as previously suggested. We found the best-fitting dynamical masses to be and . We also performed spectral analysis of the SOPHIE spectra with the iSpec code. Hipparcos time-series photometry shows a periodicity of P = 5.743 day, which is also seen in the SOPHIE spectra as an RV modulation of the star A. We interpret it as being caused by spots on the stellar surface, and the star to be rotating with the given period. From the rotation and level of activity, we found that the system is Myr old, consistent with an isochrone analysis ( Myr). This work may serve as a test case for future studies of low-mass stars, brown dwarfs, and exoplanets by combination of RV and direct imaging data.

The Sun as a planet-host star: proxies fromSDOimages for HARPS radial-velocity variations

The Sun is the only star whose surface can be directly resolved at high resolution, and therefore constitutes an excellent test case to explore the physical origin of stellar radial-velocity (RV) variability. We present HARPS observations of sunlight scattered off the bright asteroid 4/Vesta, from which we deduced the Sun's activity-driven RV variations. In parallel, the HMI instrument onboard the Solar Dynamics Observatory provided us with simultaneous high spatial resolution magnetograms, Dopplergrams, and continuum images of the Sun in the Fe I 6173A line. We determine the RV modulation arising from the suppression of granular blueshift in magnetised regions and the flux imbalance induced by dark spots and bright faculae. The rms velocity amplitudes of these contributions are 2.40 m/s and 0.41 m/s, respectively, which confirms that the inhibition of convection is the dominant source of activity-induced RV variations at play, in accordance with previous studies. We find the Doppler imbalances of spot and plage regions to be only weakly anticorrelated. Lightcurves can thus only give incomplete predictions of convective blueshift suppression. We must instead seek proxies that track the plage coverage on the visible stellar hemisphere directly. The chromospheric flux index R'_HK derived from the HARPS spectra performs poorly in this respect, possibly because of the differences in limb brightening/darkening in the chromosphere and photosphere. We also find that the activity-driven RV variations of the Sun are strongly correlated with its full-disc magnetic flux density, which may become a useful proxy for activity-related RV noise.

Daily variability of Ceres’ albedo detected by means of radial velocities changes of the reflected sunlight

Abstract Bright features have been recently discovered by Dawn on Ceres, which extend previous photometric and Space Telescope observations. These features should produce distortions of the line profiles of the reflected solar spectrum and therefore an apparent radial velocity variation modulated by the rotation of the dwarf planet. Here we report on two sequences of observations of Ceres performed in the nights of 2015 July 31, August 26 and 27 by means of the high-precision High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph at the 3.6 m La Silla European Southern Observatory (ESO) telescope. The observations revealed a quite complex behaviour which likely combines a radial velocity modulation due to the rotation with an amplitude of ≈±6 m s−1 and an unexpected diurnal effect. The latter changes imply changes in the albedo of Occator's bright features due to the blaze produced by the exposure to solar radiation. The short-term variability of Ceres’ albedo is on time-scales ranging from hours to months and can both be confirmed and followed by means of dedicated radial velocity observations.

A search and modeling of peculiar narrow transient line components in novae spectra

The formation of peculiar transient narrow emission line components observed in the spectra of a few novae is discussed. We aim to constrain the possible physical sources responsible for those unexpected components that present orbital radial velocity modulations, which were first observed in the post-outburst recombination lines of Nova U Sco 2010. A search for candidates showing similar narrow components is presented. Exploratory photoionization simulations indicate that the forming region cannot be restricted to the Roche Lobe of the primary, but could be located around the outer Lagrangian point L3. Further analysis disfavors an origin at the companion star. In addition, we analyze possible correlations between the presence of the narrow components, the basic nova parameters and the spectral classification in the initial permitted line phase.

Radial dependence of line profile variability in seven O9–B0.5 stars

Massive stars show a variety of spectral variability: presence of discrete absorption components in UV P-Cygni profiles, optical line profile variability, X-ray variability, radial velocity modulations. Our goal is to study the spectral variability of single OB stars to better understand the relation between photospheric and wind variability. For that, we rely on high spectral resolution, high signal-to-noise ratio optical spectra collected with the spectrograph NARVAL on the Telescope Bernard Lyot at Pic du Midi. We investigate the variability of twelve spectral lines by means of the Temporal Variance Spectrum (TVS). The selected lines probe the radial structure of the atmosphere, from the photosphere to the outer wind. We also perform a spectroscopic analysis with atmosphere models to derive the stellar and wind properties, and to constrain the formation region of the selected lines. We show that variability is observed in the wind lines of all bright giants and supergiants, on a daily timescale. Lines formed in the photosphere are sometimes variable, sometimes not. The dwarf stars do not show any sign of variability. If variability is observed on a daily timescale, it can also (but not always) be observed on hourly timescales, albeit with lower amplitude. There is a very clear correlation between amplitude of the variability and fraction of the line formed in the wind. Strong anti-correlations between the different part of the temporal variance spectrum are observed. Our results indicate that variability is stronger in lines formed in the wind. A link between photospheric and wind variability is not obvious from our study, since wind variability is observed whatever the level of photospheric variability. Different photospheric lines also show different degrees of variability.

A SEARCH FOR GIANT PLANET COMPANIONS TO T TAURI STARS

We present results from an ongoing multiwavelength radial velocity (RV) survey of the Taurus-Auriga star forming region as part of our effort to identify pre--main sequence giant planet hosts. These 1-3 Myr old T Tauri stars present significant challenges to traditional RV surveys. The presence of strong magnetic fields gives rise to large, cool star spots. These spots introduce significant RV jitter which can mimic the velocity modulation from a planet-mass companion. To distinguish between spot-induced and planet-induced RV modulation, we conduct observations at ~6700 Angstroms and ~2.3 microns and measure the wavelength dependence (if any) in the RV amplitude. CSHELL observations of the known exoplanet host Gl 86 demonstrate our ability to detect not only hot Jupiters in the near infrared but also secular trends from more distant companions. Observations of nine very young stars reveal a typical reduction in RV amplitude at the longer wavelengths by a factor of ~2-3. While we can not confirm the presence of planets in this sample, three targets show different periodicities in the two wavelength regions. This suggests different physical mechanisms underlying the optical and K band variability.

SEVEN NEW BINARIES DISCOVERED IN THEKEPLERLIGHT CURVES THROUGH THE BEER METHOD CONFIRMED BY RADIAL-VELOCITY OBSERVATIONS

We present seven newly discovered non-eclipsing short-period binary systems with low-mass companions, identified by the recently introduced BEER algorithm, applied to the publicly available 138-day photometric light curves obtained by the Kepler mission. The detection is based on the beaming effect (sometimes called Doppler boosting), which increases (decreases) the brightness of any light source approaching (receding from) the observer, enabling a prediction of the stellar Doppler radial-velocity modulation from its precise photometry. The BEER algorithm identifies the BEaming periodic modulation, with a combination of the well known Ellipsoidal and Reflection/heating periodic effects, induced by short-period companions. The seven detections were confirmed by spectroscopic radial-velocity follow-up observations, indicating minimum secondary masses in the range of 0.07-0.4 Msun. The discovered binaries establish for the first time the feasibility of the BEER algorithm as a new detection method for short-period non-eclipsing binaries, with the potential to detect in the near future non-transiting brown dwarfs secondaries, or even massive planets.

C I 5380 Å: A SPECTRAL LINE WITH LOW INFLUENCE FROM STARSPOTS DYNAMICS AND MAGNETIC CYCLES

In the Sun-as-a-star Project, the sun was observed spectroscopically and photometrically for more than 25 years in order to determine variability and luminosity changes. This project detected systematic longterm decrease in the total irradiance as a consequence of the solar magnetic activity cycle (scale of years) and variability on solar activity from a time scale of days-months. The solar magnetic activity cycles could mimic the radial velocity modulation signal of a long-period companion in several spectral lines. This effect is an important limitation for the exoplanet searches programs using the radial velocity technique. The Lomb-Scargle periodogram analysis of the Sun-as-a-star spectroscopic data shows that the photospheric line C I 5380 Å and other 11 lines seems to not show significant influence from the rotational or cromospheric magnetic activity modulation. Thus, our analysis suggest that C I 5380 Å line could be used in programs that require extremely line stability.

STARSPOT-INDUCED OPTICAL AND INFRARED RADIAL VELOCITY VARIABILITY IN T TAURI STAR HUBBLE I 4

We report optical (6150 Ang) and K-band (2.3 micron) radial velocities obtained over two years for the pre-main sequence weak-lined T Tauri star Hubble I 4. We detect periodic and near-sinusoidal radial velocity variations at both wavelengths, with a semi-amplitude of 1395\pm94 m/s in the optical and 365\pm80 m/s in the infrared. The lower velocity amplitude at the longer wavelength, combined with bisector analysis and spot modeling, indicates that there are large, cool spots on the stellar surface that are causing the radial velocity modulation. The radial velocities maintain phase coherence over hundreds of days suggesting that the starspots are long-lived. This is one of the first active stars where the spot-induced velocity modulation has been resolved in the infrared.