Differential Radial Velocities Research Articles

The Van Hoof Effect over the Blazhko Cycle

We provide the first detection of the Van Hoof effect over a complete Blazhko cycle in the variations of H β, neutral, and ionized metallic lines, measurable on 2782 high-resolution line profiles obtained during the course of a 41 day continuous and unprecedented survey of RR ab star, RR Lyr. We detect phase-lag variations in the velocity–velocity diagrams during the Blazhko period. The Van Hoof effect is maximum at the Blazhko modulation maximum and minimum at the Blazhko modulation minimum. Both turbulent velocities and equivalent widths are correlated with the Van Hoof and Blazhko effects. We investigate the differential motion between the upper and lower photosphere and between the photosphere and high atmosphere in the RR Lyr star. The differential radial velocity and differential acceleration curves show bulges and dents, respectively, during the inward moving atmosphere, but at the pulsation amplitude maximum, the differential acceleration curve is double peaked with a high differential radial-velocity gradient. Correlations between differential radial velocity, separating distance, and differential acceleration are monotonous during the Blazhko cycle, but the double-peak structure, left peak and right peak, is strongly Blazhko antiphased and phased, respectively. We explain the physical mechanisms driving the modulation of these quantities by the ShPM1, ShPM, ShPM2, and ShPM3 receding shocks, and the ShH+He outwarding main shock. The 3D interplay between the Van Hoof, Blazhko, and hypersonic–hydrodynamical effects provides a powerful tool for stellar pulsation and evolution challenges.

Color Difference Makes a Difference: Four Planet Candidates around τ Ceti

Abstract The removal of noise typically correlated in time and wavelength is one of the main challenges for using the radial-velocity (RV) method to detect Earth analogues. We analyze τ Ceti RV data and find robust evidence for wavelength-dependent noise. We find that this noise can be modeled by a combination of moving average models and the so-called “differential radial velocities.” We apply this noise model to various RV data sets for τ Ceti, and find four periodic signals at 20.0, 49.3, 160, and 642 days, which we interpret as planets. We identify two new signals with orbital periods of 20.0 and 49.3 days while the other two previously suspected signals around 160 and 600 days are quantified to a higher precision. The 20.0 days candidate is independently detected in Keck data. All planets detected in this work have minimum masses less than with the two long-period ones located around the inner and outer edges of the habitable zone, respectively. We find that the instrumental noise gives rise to a precision limit of the High Accuracy Radial Velocity Planet Searcher (HARPS) around 0.2 m s−1. We also find correlation between the HARPS data and the central moments of the spectral line profile at around 0.5 m s−1 level, although these central moments may contain both noise and signals. The signals detected in this work have semi-amplitudes as low as 0.3 m s−1, demonstrating the ability of the RV technique to detect relatively weak signals.

Evidence for at least three planet candidates orbiting HD 20794

We explore the feasibility of detecting Earth analogs around Sun-like stars using the radial velocity method by investigating one of the largest radial velocities datasets for the one of the most stable radial-velocity stars HD20794. We proceed by disentangling the Keplerian signals from correlated noise and activity-induced variability. We diagnose the noise using the differences between radial velocities measured at different wavelength ranges, so-called "differential radial velocities". We apply this method to the radial velocities measured by HARPS, and identify four signals at 18, 89, 147 and 330 d. The two signals at periods of 18 and 89 d are previously reported and are better quantified in this work. The signal at a period of about 147 d is reported for the first time, and corresponds to a super-Earth with a minimum mass of 4.59 Earth mass located 0.51 AU from HD20794. We also find a significant signal at a period of about 330 d corresponding to a super-Earth or Neptune in the habitable zone. Since this signal is close to the annual sampling period and significant periodogram power in some noise proxies are found close to this signal, further observations and analyses are required to confirm it. The analyses of the eccentricity and consistency of signals provide weak evidence for the existence of the previously reported 43 d signal and a new signal at a period of about 11.9 d with a semi amplitude of 0.4 m/s. We find that the detection of a number of signals with radial velocity variations around 0.5\,m/s likely caused by low mass planet candidates demonstrates the important role of noise modeling in searching for Earth analogs.

The binarity of Galactic dwarf stars along with effective temperature and metallicity

Abstract The fraction of binary stars fb is one of most valuable tools to probe the star formation and evolution of multiple systems in the Galaxy. We focus on the relationship between fb and stellar metallicity [Fe/H] by employing the differential radial velocity (DRV) method and the large sample observed by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). Main-sequence stars from A- to K-type in the third data release of LAMOST are selected to estimate fb. Contributions to a profile of DRV from the radial velocity (RV) error of single stars σRV and the orbital motion of binary stars are evaluated from the DRV profile. We employ 365 911 stars with randomly repeating spectral observations to present a detailed analysis of fb and σRV in the two-dimensional space of Teff and [Fe/H]. The A-type stars are more likely to be companions in binary star systems than other stars. Furthermore, the reverse correlation between fb and [Fe/H] can be shown statistically, which suggests that fb is a joint function of Teff and [Fe/H]. At the same time, σRV of the sample are fitted for different Teff and [Fe/H]. Metal-rich cold stars in our sample have the best RV measurement.

Tachoastrometry: astrometry with radial velocities

Spectra of composite systems (e.g., spectroscopic binaries) contain spatial information that can be retrieved by measuring the radial velocities (i.e., Doppler shifts) of the components in four observations with the slit rotated by 90 degrees in the sky. By using basic concepts of slit spectroscopy we show that the geometry of composite systems can be reliably retrieved by measuring only radial velocity differences taken with different slit angles. The spatial resolution is determined by the precision with which differential radial velocities can be measured. We use the UVES spectrograph at the VLT to observe the known spectroscopic binary star HD 188088 (HIP 97944), which has a maximum expected separation of 23 milli-arcseconds. We measure an astrometric signal in radial velocity of 276 \ms, which corresponds to a separation between the two components at the time of the observations of 18 $\pm2$ milli-arcseconds. The stars were aligned east-west. We describe a simple optical device to simultaneously record pairs of spectra rotated by 180 degrees, thus reducing systematic effects. We compute and provide the function expressing the shift of the centroid of a seeing-limited image in the presence of a narrow slit.The proposed technique is simple to use and our test shows that it is amenable for deriving astrometry with milli-arcsecond accuracy or better, beyond the diffraction limit of the telescope. The technique can be further improved by using simple devices to simultaneously record the spectra with 180 degrees angles.With tachoastrometry, radial velocities and astrometric positions can be measured simultaneously for many double line system binaries in an easy way. The method is not limited to binary stars, but can be applied to any astrophysical configuration in which spectral lines are generated by separate (non-rotational symmetric) regions.

The VLT-FLAMES Tarantula Survey

We present spectral classifications for 438 B-type stars observed as part of the VLT-FLAMES Tarantula Survey (VFTS) in the 30 Doradus region of the Large Magellanic Cloud. Radial velocities are provided for 307 apparently single stars, and for 99 targets with radial-velocity variations which are consistent with them being spectroscopic binaries. We investigate the spatial distribution of the radial velocities across the 30 Dor region, and use the results to identify candidate runaway stars. Excluding potential runaways and members of two older clusters in the survey region (SL 639 and Hodge 301), we determine a systemic velocity for 30 Dor of 271.6 +/- 12.2 km/s from 273 presumed single stars. Employing a 3-sigma criterion we identify nine candidate runaway stars (2.9% of the single stars with radial-velocity estimates). The projected rotational velocities of the candidate runaways appear to be significantly different to those of the full B-type sample, with a strong preference for either large (>345 km/s) or small (<65 km/s) rotational velocities. Of the candidate runaways, VFTS 358 (classified B0.5: V) has the largest differential radial velocity (-106.9 +/- 16.2 km/s), and a preliminary atmospheric analysis finds a significantly enriched nitrogen abundance of 12+log(N/H) > ~8.5. Combined with a large rotational velocity (vsini = 345 +/- 22 km/s), this is suggestive of past binary interaction for this star.

BLUE STRAGGLER STAR POPULATIONS IN GLOBULAR CLUSTERS. I. DYNAMICAL PROPERTIES OF BLUE STRAGGLER STARS IN NGC 3201, NGC 6218, AND ω CENTAURI

We present the first dynamical study of Blue Straggler Stars (BSSs) in three Galactic globular clusters, NGC\,3201, NGC\,5139 ($\omega$Cen), and NGC\,6218, based on medium-resolution spectroscopy (R 10000) obtained with IMACS. Our BSS candidate selection technique uses HST/ACS and ESO/WFI photometric data out to $>\!4.5\,r_c$. We use radial velocity measurements to discard non-members and achieve a success rate of $\sim93\%$, which yields a sample of 116 confirmed BSSs. Using the penalized pixel fitting method (pPXF) we measure the $v\sin(i)$ values of the sample BSSs and find their distribution functions peaked at slow velocities with a long tail towards fast velocities in each globular cluster. We find that the BSSs in NGC\,3201 and NGC\,6218 which show $v\sin(i)\!>\!50$ km s$^{-1}$ are all found in the central cluster regions, inside a projected $2\,r_c$, of their parent clusters. We find a similar result in $\omega$Cen for BSSs with $v\sin(i)\!>\!70$ km s$^{-1}$ which are all, except for two, concentrated inside $2\,r_c$. In all globular clusters we find rapidly rotating BSSs that have relatively high differential radial velocities which likely put them on hyperbolic orbits, suggestive of strong dynamical interactions in the past. We estimate that all the observed rapidly rotating BSSs are likely to form in their central cluster regions no longer than $\sim\!300$ Myr ago. Using dereddened $V\!-\!I$ colors of our photometric selection we show that blue BSSs in $\omega$Cen with $(V-I)_0$<0.25 mag show a significantly increased $v\sin(i)$ dispersion compared with their red counterparts and all other BSSs in our sample, therefore strongly implying that fast rotating BSSs in $\omega$Cen are preferentially bluer, i.e. more massive. This may indicate that this particular blue BSS population was formed in an unique formation event and/or through an unique mechanism.

THIRTY NEW LOW-MASS SPECTROSCOPIC BINARIES

As part of our search for young M dwarfs within 25 pc, we acquired high-resolution spectra of 185 low-mass stars compiled by the NStars project that have strong X-ray emission. By cross-correlating these spectra with radial velocity standard stars, we are sensitive to finding multi-lined spectroscopic binaries. We find a low-mass spectroscopic binary fraction of 16% consisting of 27 SB2s, 2 SB3s, and 1 SB4, increasing the number of known low-mass spectroscopic binaries (SBs) by 50% and proving that strong X-ray emission is an extremely efficient way to find M-dwarf SBs. WASP photometry of 23 of these systems revealed two low-mass eclipsing binaries (EBs), bringing the count of known M-dwarf EBs to 15. BD−22 5866, the ESB4, was fully described in 2008 by Shkolnik et al. and CCDM J04404+3127 B consists of two mid-M stars orbiting each other every 2.048 days. WASP also provided rotation periods for 12 systems, and in the cases where the synchronization time scales are short, we used Prot to determine the true orbital parameters. For those with no Prot, we used differential radial velocities to set upper limits on orbital periods and semimajor axes. More than half of our sample has near-equal-mass components (q > 0.8). This is expected since our sample is biased toward tight orbits where saturated X-ray emission is due to tidal spin-up rather than stellar youth. Increasing the samples of M-dwarf SBs and EBs is extremely valuable in setting constraints on current theories of stellar multiplicity and evolution scenarios for low-mass multiple systems.

Line bisector analysis of stars with companions in the SARG survey at the TNG

We present an analysis of how bisectors of spectral lines vary, for a few stars observed during the high-accuracy radial-velocity planet survey ongoing at the Telescopio Nazionale Galileo (TNG) using the Galileo High Resolution Spectrograph (Spettrografo Alta Risoluzione Galileo, SARG), and discuss their relation with differential radial velocities. The iodine cell lines employed in the radial velocity measurements were used to improve the wavelength calibration and then removed before bisector analysis. The line bisectors were then computed from average absorption profiles obtained by cross-correlation of the stellar spectra with a mask made from suitable lines of a solar catalog. Bisector velocity spans were determined and the run of bisector velocity span against radial velocity was studied to search for correlations between line asymmetries and radial velocity variations. We present an analysis of spectra of HD 216122B that show a slight contamination likely to be due to a stellar companion, and an analysis of spectra of HD 76036A, a case where the line bisectors are useful for improving the RV measurements. These systems are part of a survey sample being observed with adaptive optics (AdOpt at the TNG since 2006) in an attempt to visually resolve stellar companions.

An Inexpensive Field-Widened Monolithic Michelson Interferometer for Precision Radial Velocity Measurements

ABSTRACTWe have constructed a thermally compensated field-widened monolithic Michelson interferometer that can be used with a medium-resolution spectrograph to measure precise Doppler radial velocities of stars. Our prototype monolithic fixed-delay interferometer is constructed with off-the-shelf components and assembled using a hydrolysis bonding technique. We installed and tested this interferometer in the Exoplanet Tracker (ET) instrument at the Kitt Peak 2.1 m telescope, an instrument built to demonstrate the principles of dispersed fixed-delay interferometry. An iodine cell allows the interferometer drift to be accurately calibrated, relaxing the stability requirements on the interferometer itself. When using our monolithic interferometer, the ET instrument has no moving parts (except the iodine cell), greatly simplifying its operation. We demonstrate differential radial velocity precision of a few m s-1 on well known radial velocity standards and planet bearing stars when using this interferometer. Such monolithic interferometers will make it possible to build relatively inexpensive instruments that are easy to operate and capable of precision radial velocity measurements. A larger multiobject version of the Exoplanet Tracker will be used to conduct a large scale survey for planetary systems as part of the Sloan Digital Sky Survey III (SDSS III). Variants of the techniques and principles discussed in this paper can be directly applied to build large monolithic interferometers for such applications, enabling the construction of instruments capable of efficiently observing many stars simultaneously at high velocity precision.

Anm sin i= 24M⊕Planetary Companion to the Nearby M Dwarf GJ 176

We report the detection of a planetary companion with a minimum mass of -->m sin i = 0.0771 MJup = 24.5 M⊕ to the nearby ( -->d = 9.4 pc) M2.5 V star GJ 176. The star was observed as part of our M dwarf planet search at the Hobby-Eberly Telescope (HET). The detection is based on 5 years of high-precision differential radial velocity (RV) measurements using the High-Resolution Spectrograph (HRS). The orbital period of the planet is 10.24 days. GJ 176 thus joins the small (but increasing) sample of M dwarfs hosting short-period planets with minimum masses in the Neptune-mass range. Low-mass planets could be relatively common around M dwarfs, and the current detections might represent the tip of a rocky planet population.

Improving Earth-like planets' detection with an ELT: the differential radial velocity experiment

Context. Direct detection of Earth-like planets around a nearby star is extremely difficult for ground-based telescopes equipped with the eXtreme Adaptative Optics system. Aims. We propose to combine high-precision radial velocity with an ELT to detect extrasolar planets with a feasible coronagraph. Methods. In this context, we investigated the detectability of Earth-like extrasolar planets with an ELT of 42 m equipped with the spectro-coronagraphic instrument in an eXtrem Adaptive Optics system. Those stellar residuals left uncorrected by the XAO wavefront correction (Strehl ratio >85%) system need to be calibrated. We propose calibrating them by the radial velocity measurement of the planet in a multiple spectrum correlation process. Results. This method allows detection of Terrestrial planets with an ELT but requires a long exposure time (≈200 h) Conclusions. The technique overcomes the main coronagraphic limitation due to the pinned and the low variable speckles and opens a way to detect directly exoplanets with large ground-based telescopes equipped with an efficient XAO system.

Differential Radial Velocities and Stellar Parameters of Nearby Young Stars

ABSTRACTRadial velocity searches for substellar‐mass companions have focused primarily on stars older than 1 Gyr. Increased levels of stellar activity in young stars hinders the detection of solar system analogs, and therefore until recently there has been a prejudice against inclusion of young stars in radial velocity surveys. Adaptive optics surveys of young stars have given us insight into the multiplicity of young stars, but only for massive, distant companions. Understanding the limit of the radial velocity technique, restricted to high‐mass, close‐orbiting planets and brown dwarfs, we began a survey of young stars of various ages. While the number of stars needed to carry out full analysis of the problems of planetary and brown dwarf population and evolution is large, the beginning of such a sample is included here. We report on 61 young stars ranging in age from the β Pictoris association (∼12 Myr) to the Ursa Major association (∼300 Myr). This initial search resulted in no stars showing evidence of companions larger than ∼1MJup–2MJup in short‐period orbits at the 3 σ level. We also present derived stellar parameters, as most have unpublished values. The chemical homogeneity of a cluster, and presumably of an association, may help to constrain true membership, so we present [Fe/H] abundances for the stars in our sample.

Line bisectors and radial velocity jitter from SARG spectra

We present an analysis of spectral line bisector variations for a few stars observed in the SARG high precision radial velocity planet survey, and discuss their relationship with differential radial velocities. The spectra we consider are the same used for determining radial velocities. The iodine cell lines employed in the measurement of radial velocities were removed before bisector analysis. The line bisectors were then computed from average absorption profiles obtained by cross correlation of the stellar spectra with a mask made from suitable lines of a solar catalog. Bisector velocity spans were then determined: errors in these quantities compare well with theoretical expectations based on resolution, S/N and line shape. The plot of bisector velocity span against radial velocity was studied to search for correlations between line asymmetries and radial velocity variations. A correlation was seen for HD 166435 due to stellar activity, and for HD 8071B due to spectral contamination by the companion. No correlation was seen for 51 Peg and rho CrB, stars hosting planets. We conclude that this technique may be useful to separate radial velocity variations due to barycenter motion from spurious signals in spectra acquired with the iodine cell.

Differential Rotation of the Active G5 V Star κ1 Ceti: Photometry from the MOST Satellite11Based on data obtained with the MOST satellite, a Canadian Space Agency mission jointly operated by Dynacon, Inc., the University of Toronto Institute of Aerospace Studies, and the University of British Columbia.

ABSTRACTAbout 30.5 days of nearly uninterrupted broadband photometry of the solar‐type star κ1 Ceti, obtained with the MOST (Microvariability and Oscillations of Stars) satellite, shows evidence for two large starspots with different rotation periods of 8.9 and ∼9.3 days (ΔΩ/Ω≃4%). Ground‐based measurements of Ca ii H and K emission in 2002 and 2003 reveal variations in chromospheric activity with a period of about 9.3 days. The data were obtained during the MOST commissioning phase. When the data are combined with historical observations, they indicate that the 9.3 day spot has been stable in its period for over 30 yr. The photometry, with a sampling rate of approximately once per minute, was also used to search for acoustic (p‐mode) oscillations in the star. We detect no clear evidence for p‐modes in the κ1 Ceti photometry, with a noise level around 7–9 μmag at frequencies in the range of 0.5–4 mHz (3 σ detection limit of 21–27 μmag). There were no flares or planetary transits during 30.5 days of MOST monitoring with light amplitudes greater than 2 mmag (durations >200 minutes) and 3 mmag (2–200 minute durations). While this rules out any close‐in planets with Jupiter diameters ≥0.5 and orbital inclinations close to 90°, the scatter in differential radial velocities permit a close giant planet in a more highly inclined orbit.

HD 137510: An Oasis in the Brown Dwarf Desert

Since the beginning of precise Doppler surveys, which have had stunning success in detecting extrasolar planetary companions, one surprising enigma has emerged: the relative paucity of spectroscopic binaries where the secondary mass lies in between the stellar and planetary mass regime. This gap in the mass function for close-in (a < 3-4 AU) companions to solar-type stars is generally referred to as the "brown dwarf desert." Here we report the detection of a companion to HD 137510 (G0 IV), with a minimum mass of 26MJ, moving in an eccentric orbit (e = 0.4) with a period of 798 days and an orbital semimajor axis of 1.85 AU. The detection is based on precise differential radial velocity data obtained by the McDonald Observatory and Thüringer Landessternwarte Tautenburg planet search programs.

Ultra-High-Precision Velocity Measurements of Oscillations in Centauri A

We have made differential radial velocity measurements of the star alpha Cen A using two spectrographs, UVES and UCLES, both with iodine absorption cells for wavelength referencing. Stellar oscillations are clearly visible in the time series. After removing jumps and slow trends in the data, we show that the precision of the velocity measurements per minute of observing time is 0.42 m/s for UVES and 1.0 m/s for UCLES, while the noise level in the Fourier spectrum of the combined data is 1.9 cm/s. As such, these observations represent the most precise velocities ever measured on any star apart from the Sun.

The planet search program at the ESO Coudé Echelle spectrometer

We present the complete results of the planet search program carried out at the ESO Coudé Echelle Spectrometer (CES) on La Silla, using the Long Camera from Nov. 1992 to April 1998. The CES survey has monitored 37 late-type (F8V – M5V) stars in the southern hemisphere for variations in their differential radial velocities (RV) in order to detect Doppler reflex motions caused by planetary companions. This led to the discovery of the first extrasolar planet in an Earth-like orbit around the young (ZAMS) and active G0V star ι Horologii (Kürster et al. [CITE]). Here we present the RV results for all survey stars and perform a statistical examination of the whole data-set. Each star is tested for RV variability, RV trends (linear and non-linear) and significant periodic signals. β Hyi and ϵ Ind are identified as long-term, low-amplitude RV variables. Furthermore, for 30 CES survey stars we determine quantitative upper mass-limits for giant planets based on our long-term RV results. We find that the CES Long Camera survey would have detected short-period (“51 Peg-type”) planets around all 30 stars but no planets with at orbital separations larger than 2 AU. Finally, we demonstrate that the CES planet search can be continued without applying velocity corrections to the RV results coming from the currently installed Very Long Camera at the CES.

The planet search program at the ESO Coudé Echelle spectrometer

In this article we present the results of the planet search program carried out at the ESO Coude Echelle Spectrometer (CES) for components A and B of the α Centauri system. Since November 1992 we have obtained high precision differential radial velocities (RV s) for both stars. We demonstrate that, after subtraction of the binary orbital motion, no additional residual RV signal is present within our detection limits, which could have been attributed to giant planets orbiting either of the two stars. We performed numerical simulations to determine the detection threshold of the CES survey for planets around both stars. In combination with known dynamical limitations for stable planetary orbits (Wiegert & Holman [CITE]) we can now set strong constraints on the existence of Jupiter-mass-type planets in the α Centauri system.

Stellar Rotation and Precise Radial Velocities

AbstractThis paper focuses on the limitations of establishing rotation rates (to be thought of as differential radial velocities across the surface of the star) and how rotation infringes on our ability to measure precise positions of spectral lines and hence spectroscopic radial velocities.