Precise Stellar Radial Velocity Measurements Research Articles

Companions to Kepler giant stars: A long-period eccentric sub-stellar companion to KIC 3526061 and a stellar companion to HD 187878

Context. Our knowledge of populations and the occurrence of planets orbiting evolved intermediate-mass stars is still incomplete. In 2010 we started a planet search programme among 95 giant stars observed by the Kepler mission to increase the sample of giant stars with planets and with reliable estimates of stellar masses and radii. Aims. We present the two systems from our planet search programme whose companions we were able to characterise: KIC 3526061 and HD 187878. Methods. We used precise stellar radial velocity measurements taken with four different echelle spectrographs to derive an orbital solution. We used Gaia astrometric measurements to obtain the inclination of the HD 187878 system and Kepler photometric observations to estimate the stellar mass and radius. Results. We report the discovery of a sub-stellar companion and a stellar companion around two intermediate-mass red giant branch stars. KIC 3526061 b is most likely a brown dwarf with a minimum mass of 18.15 ± 0.44 MJupiter in a long-period eccentric orbit, with orbital period 3552−135+158 d and orbital eccentricity e = 0.85 ± 0.01. It is the most evolved system found having a sub-stellar companion with such a high eccentricity and wide separation. HD 187878 B has a minimum mass of 78.4 ± 2.0 MJupiter. Combining the spectroscopic orbital parameters with the astrometric proper motion anomaly, we derived an orbital inclination i = 9.8−0.6+0.4 deg, which corresponds to the companion’s mass in the stellar regime of 0.51−0.02+0.04 M⊙. Conclusions. A sub-stellar companion of KIC 3526061 extends the sample of known red giant branch stars with sub-stellar companions on very eccentric wide orbits, and might provide a probe of the dynamical evolution of such systems over time.

The Radial Velocity Variability of the K-giant γ Draconis: Stellar Variability Masquerading as a Planet

Abstract We present precise stellar radial velocity (RV) measurements of γ Dra taken from 2003 to 2017. The data from 2003 to 2011 show coherent, long-lived variations with a period of 702 days. These variations are consistent with the presence of a planetary companion having m sin i = 10.7 M Jup whose orbital properties are typical for giant planets found around evolved stars. An analysis of the Hipparcos photometry, Ca ii S-index measurements, and measurements of the spectral line shapes during this time show no variations with the RV of the planet, which seems to “confirm” the presence of the planet. However, RV measurements taken from 2011–2017 seem to refute this. From 2011–2013, the RV variations virtually disappear, only to return in 2014 but with a noticeable phase shift. The total RV variations are consistent either with amplitude variations on timescales of ≈10.6 year, or the beating effect between two periods of 666 and 801 days. It seems unlikely that both these signals stem from a two-planet system. A simple dynamical analysis indicates that there is only a 1%–2% chance that the two-planet system is stable. Rather, we suggest that this multi-periodic behavior may represent a new form of stellar variability, possibly related to oscillatory convective modes. If such intrinsic stellar variability is common around K giant stars and is attributed to planetary companions, then the planet occurrence rate among these stars may be significantly lower than thought.

The discovery of a planetary candidate around the evolved low-massKeplergiant star HD 175370

We report on the discovery of a planetary companion candidate with a minimum mass M sin i = 4.6 ± 1.0 MJupiter orbiting the K2 III giant star HD 175370 (KIC 007940959). This star was a target in our programme to search for planets around a sample of 95 giant stars observed with Kepler. This detection was made possible using precise stellar radial velocity measurements of HD 175370 taken over five years and four months using the coudé echelle spectrograph of the 2-m Alfred Jensch Telescope and the fibre-fed echelle spectrograph High Efficiency and Resolution Mercator Echelle Spectrograph of the 1.2-m Mercator Telescope. Our radial velocity measurements reveal a periodic (349.5 ± 4.5 d) variation with a semi-amplitude K = 133 ± 25 m s− 1, superimposed on a long-term trend. A low-mass stellar companion with an orbital period of ∼88 yr in a highly eccentric orbit and a planet in a Keplerian orbit with an eccentricity e = 0.22 are the most plausible explanation of the radial velocity variations. However, we cannot exclude the existence of stellar envelope pulsations as a cause for the low-amplitude radial velocity variations and only future continued monitoring of this system may answer this uncertainty. From Kepler photometry, we find that HD 175370 is most likely a low-mass red giant branch or asymptotic giant branch star.

Long-lived, long-period radial velocity variations in Aldebaran: A planetary companion and stellar activity

We investigate the nature of the long-period radial velocity variations in Alpha Tau first reported over 20 years ago. We analyzed precise stellar radial velocity measurements for Alpha Tau spanning over 30 years. An examination of the Halpha and Ca II 8662 spectral lines, and Hipparcos photometry was also done to help discern the nature of the long-period radial velocity variations. Our radial velocity data show that the long-period, low amplitude radial velocity variations are long-lived and coherent. Furthermore, Halpha equivalent width measurements and Hipparcos photometry show no significant variations with this period. Another investigation of this star established that there was no variability in the spectral line shapes with the radial velocity period. An orbital solution results in a period of P = 628.96 +/- 0.90 d, eccentricity, e = 0.10 +/- 0.05, and a radial velocity amplitude, K = 142.1 +/- 7.2 m/s. Evolutionary tracks yield a stellar mass of 1.13 +/- 0.11 M_sun, which corresponds to a minimum companion mass of 6.47 +/- 0.53 M_Jup with an orbital semi-major axis of a = 1.46 +/- 0.27 AU. After removing the orbital motion of the companion, an additional period of ~ 520 d is found in the radial velocity data, but only in some time spans. A similar period is found in the variations in the equivalent width of Halpha and Ca II. Variations at one-third of this period are also found in the spectral line bisector measurements. The 520 d period is interpreted as the rotation modulation by stellar surface structure. Its presence, however, may not be long-lived, and it only appears in epochs of the radial velocity data separated by $\sim$ 10 years. This might be due to an activity cycle. The data presented here provide further evidence of a planetary companion to Alpha Tau, as well as activity-related radial velocity variations.

The mass of the planet-hosting giant starβGeminorum determined from its p-mode oscillation spectrum

We use precise radial velocity measurements and photometric data to derive the frequency spacing of the p-mode oscillation spectrum of the planet-hosting star Beta Gem. This spacing along with the interferometric radius for this star is used to derive an accurate stellar mass. A long time series of over 60 hours of precise stellar radial velocity measurements of Beta Gem were taken with an iodine absorption cell and the echelle spectrograph mounted on the 2m Alfred Jensch Telescope. Complementary photometric data for this star were also taken with the MOST microsatellite spanning 3.6 d. A Fourier analysis of the radial velocity data reveals the presence of up to 17 significant pulsation modes in the frequency interval 10-250 micro-Hz. Most of these fall on a grid of equally-spaced frequencies having a separation of 7.14 +/- 0.12 micro-Hz. An analysis of 3.6 days of high precision photometry taken with the MOST space telescope shows the presence of up to 16 modes, six of which are consistent with modes found in the spectral (radial velocity) data. This frequency spacing is consistent with high overtone radial pulsations; however, until the pulsation modes are identified we cannot be sure if some of these are nonradial modes or even mixed modes. The radial velocity frequency spacing along with angular diameter measurements of Beta Gem via interferometry results in a stellar mass of M = 1.91 +/- 0.09 solar masses. This value confirms the intermediate mass of the star determined using stellar evolutionary tracks. Beta Gem is confirmed to be an intermediate mass star. Stellar pulsations in giant stars along with interferometric radius measurements can provide accurate determinations of the stellar mass of planet hosting giant stars. These can also be used to calibrate stellar evolutionary tracks.

Spectroscopic properties of cool Ursa Major group members

CONTEXT: Most current radial velocity planet search programs have concentrated on stars of one solar mass. Our knowledge on the frequency of giant planets and brown dwarf companions to more massive stars is thus rather limited. In the case of solar-like stars, the frequency of short-period brown dwarf companions and very massive planets seems to be low. AIMS: Here we present evidence for a substellar companion to 30 Ari B, an F-star of 1.16 $\pm$ 0.04 $\rm M_\odot$ that is a member of a hierarchical triple system. METHODS: The companion was detected by means of precise radial velocity measurements using the 2-m Alfred-Jensch telescope and its 'echelle spectrograph. An iodine absorption cell provided the wavelength reference for precise stellar radial velocity measurements. RESULTS: We analyzed our radial velocity measurements and derived an orbit to the companion with period, P= 335.1+/-2.5 days, eccentricity e = 0.289+/-0.092, and mass function f(m) = (6.1+/-1.7)*10E-7 Modot. CONCLUSIONS: We conclude that the radial velocity variations of 30 Ari B are due to a companion with $m$ sin $i$ of $9.88\pm0.94$ $\rm M_{Jup}$ that is either a massive planet or a brown dwarf. {The object thus belongs to the rare class of massive planets and brown dwarfs orbiting main- sequence stars.

Planetary companion candidates around the K giant stars 42 Draconis and HD 139 357

For the past 3 years we have been monitoring 62 K giant stars using precise stellar radial velocity (RV) measurements with the 2m Alfred Jensch telescope of the Th\"uringer Landessternwarte Tautenburg (TLS). To probe the dependence of planet formation on stellar mass by finding planets around intermediate-mass giant stars. We present high accuracy RV measurements of the K1.5 III star 42 Dra and the K4 III star HD 139357. The wavelength reference for the RV measurements was provided by an iodine absorption cell placed in the optical path of the spectrograph. Our measurements reveal that the time series of the radial velocity of 42 Dra shows a periodic variation of 479.1 days. HD 139357 shows periodic RV variations of 1125.7 days. Keplerian motion is the most likely explanation for the observed radial velocity variations for these stars. Thus the K giant stars 42 Dra and HD 139357 host extrasolar planets.

Stellar oscillations in planet-hosting giant stars

Recently a number of giant extrasolar planets have been discovered around giant stars. These discoveries are important because many of these giant stars have intermediate masses in the range 1.2-3 M⊙. Early-type main sequence stars of this mass range have been avoided by radial velocity planet search surveys due the difficulty of getting the requisite radial velocity precision needed for planet discoveries. Thus, giant stars can tell us about planet formation for stars more massive than the sun. However, the determination of stellar masses for giant stars is difficult due to the fact that evolutionary tracks for stars covering a wide range of masses converge to the same region of the H-R diagram. We report here on stellar oscillations in three planet-hosting giant stars: HD 13189, β Gem, and ι Dra. Precise stellar radial velocity measurements for these stars show variations whose periods and amplitudes are consistent with solar-like p-mode oscillations. The implied stellar masses for these objects based on the characteristics of the stellar oscillations are consistent with the predictions of stellar isochrones. An investigation of stellar oscillations in planet hosting giant stars offers us the possibility of getting an independent determination of the stellar mass for these objects which is of crucial importance for extrasolar planet studies.

The Discovery of Stellar Oscillations in the Planet-hosting Giant Star β Geminorum

We present the results of a long time series of precise stellar radial velocity measurements of the planet hosting K giant star β Geminorum. A total of 20 hr of observations spanning three nights were obtained, and the radial velocity variations show the presence of solar-like stellar oscillations. Our period analysis yields six significant pulsation modes that have frequencies in the range of 30-150 μHz. The dominant mode is at a frequency of 86.9 μHz and has an amplitude of 5.3 m s-1. These values are consistent with stellar oscillations for a giant star with a stellar mass of ≈2 M☉. This stellar mass implies a companion minimum mass of 2.6 MJup. β Gem is the first planet-hosting giant star in which multiperiodic stellar oscillations have been detected. The study of stellar oscillations in planet-hosting giant stars may provide an independent and more accurate determination of the stellar mass.

Discovery of a planet around the K giant star 4 Ursae Majoris

For the past 3 years we have been monitoring a sample of 62 K giant stars using precise stellar radial velocity measurements with high accuracy taken at the Thuringer Landessternwarte Tautenburg. We search for sub-stellar companions to giant stars and try to understand the nature of the diverse radial velocity variations exhibited by K giant stars. We present precise stellar radial velocity measurements of the K1III giant star 4 UMa (HD 73108). These were obtained using the coude echelle spectrograph of 2-m Alfred Jensch Telescope. The wavelength reference for the radial velocity measurements was provided by an iodine absorption cell. Our measurements reveal that the radial velocity of 4 UMa exhibits a periodic variation of 269.3 days with a semiamplitude K = 216.8 m s -1 . A Keplerian orbit with an eccentricity, e = 0.43 ± 0.02 is the most reasonable explanation for the radial velocity variations. The orbit yields a mass function, f(m) = (2.05 ± 0.24) x 10 -7 M ⊙ . From our high resolution spectra we calculate a metallicity of -0.25 ± 0.05 and derive a stellar mass of 1.23 M ⊙ ± 0.15 for the host star. The K giant star 4 UMa hosts a substellar companion with minimum mass M sin i = 7.1 ± 1.6 M Jupiter .

Stellar Oscillations in Giant Stars

Walker et al. (1989) were the first to establish, using precise stellar radial velocity measurements, that K giant stars were a new class of variables. The variability of some of these stars can be quite complex showing several periods ranging from several hours to several hundreds of days. The long-period variations result from sub-stellar companions or rotational modulation, but the short-period variability certainly arises from stellar oscillations. We present recent of precise stellar radial velocity measurements (σ ≈ 6m s −1 ) of two oscillating K giant stars: β Oph and γ Dra. These stars show oscillation periods of hours (β Oph) and days (γ Dra). These periods are consistent with solar-like stellar oscillations, given the stellar properties of the star. Radial modes are a prime candidate for the type of oscillations in these stars.

Confirmation of the planet hypothesis for the long-period radial velocity variations ofβGeminorum

Aims. Our aim is to confirm the nature of the long period radial velocity measurements for β Gem first found by Hatzes & Cochran (1993). Methods. We present precise stellar radial velocity measurements for the K giant star β Gem spanning over 25 years. An examination of the Ca II K emission, spectral line shapes from high resolution data (R = 210 000), and Hipparcos photometry was also made to discern the true nature of the long period radial velocity variations. Results. The radial velocity data show that the long period, low amplitude radial velocity variations found by Hatzes & Cochran (1993) are long-lived and coherent. Furthermore, the Ca II K emission, spectral line bisectors, and Hipparcos photometry show no significant variations of these quantities with the radial velocity period. An orbital solution assuming a stellar mass of 1.7 M ⊙ yields a period, P = 589.6 days, a minimum mass of 2.3 M Jupiter , and a semi-major axis, a = 1.6 AU. The orbit is nearly circular (e = 0.02). Conclusions. The data presented here confirm the planetary companion hypothesis suggested by Hatzes & Cochran (1993). β Gem is one of six intermediate mass stars known to host a sub-stellar companion and suggests that planet-formation around stars much more massive than the sun may common.

Radial velocity variations in pulsating Ap stars – III. The discovery of 16.21-min oscillations in β CrB

We present the analysis of 3 h of a rapid time series of precise stellar radial velocity (RV) measurements (a = 4.5 m s -1 ) of the cool Ap star β CrB. The integrated RV measurements spanning the wavelength interval 5000-6000 A show significant variations (false alarm probability = 10 -5 ) with a period of 16.21 min (v = 1028.17 μHz) and an amplitude of 3.54 ± 0.56 m s -1 The RV measured over a much narrower wavelength interval reveals one spectral feature at λ6272.0 A pulsating with the same 16.21-min period and an amplitude of 138 ± 23 m s -1 . These observations establish β CrB to be a low-amplitude rapidly oscillating Ap star.

Precise radial velocity measurements of G and K giants

We present the first results of our precise radial velocity ( RV ) measurements of G and K giants. A number of stars from our list of 80 targets have been observed for 14 months using the fibre-fed echelle spectrograph FEROS at the 1.52 m ESO telescope in La Silla, Chile. This sample increases the number of giants surveyed with precise stellar radial velocity measurements at least by a factor of 10. During this period we are able to estimate the long-term accuracy of our measurement as better than 11 ${\rm m\,s}^{-1}$. We use the simultaneous Th-Ar calibration and cross-correlation technique to compute the radial velocity by applying a numerical template for K-type stars. Standard deviation σ of mean radial velocity variations between 3 ${\rm m\,s}^{-1}$ and 4 k${\rm m\,s}^{-1}$ with timescales between several days and years are measured for 21 of G and K giants which are presented in this paper. Fifteen stars show definite variability above 3 σ of our measurement uncertainties. Two stars with RV variations above 800 ${\rm m\,s}^{-1}$ are tentatively identified as new binaries. Although definitive trends between RV variations and stellar evolutionary status cannot yet be established, all the luminous cool giants of our sample seem to have significant radial velocity variations, while those stars in the giant's clump region can be either variable or constant.

The Radial Velocity and Spectral Line Bisector Variability of Polaris

We present precise stellar radial velocity measurements for the Cepheid-type star Polaris taken in 1992–1993. The peak-to-peak amplitude of the pulsations was 1.555 ± 0.056 km s-1 in epoch 1992.4 and 1.517 ± 0.047 km s-1 in epoch 1993.2. These amplitudes are comparable to the one measured by Dinshaw et al. in 1987.75 and recent measurements by Kamper &Fernie for 1995–1997. Consequently, these do not support the continued drop in pulsational amplitude first extrapolated by Dinshaw et al. A periodogram analysis for our data yields a pulsational period of 39726769 ± 000011, slightly higher than the value found by Dinshaw et al. and consistent with the value derived from the Kamper &Fernie data. The pulsational period of Polaris may thus be increasing. Residual radial velocity measurements after removal of the component due to radial pulsations show the presence of a 40 day period with a 2K amplitude of about 400 m s-1, first reported by Dinshaw et al. Spectral line bisectors of the Sc II λ5526 and Mg I λ5528 lines also show variations with the 40 day period, strongly indicating that this period is indeed real. The presence of line bisector variations excludes a low-mass companion object as a cause of the residual radial velocity variations. Two models are considered for the residual radial velocity and bisector span variations: starspots and nonradial pulsations. Although both can reproduce the amplitude and shape of the residual radial velocity variations, the spot models considered were unable to reproduce the overall shape and amplitude of the bisector span variations. The best-fit spot model was provided by a single spot with a temperature 500 K cooler than the photosphere with a macroturbulent velocity of zero in the spot. If surface features do exist on Polaris and are responsible for the residual radial variations for this star, then possibly they have a temperature structure and distribution more complicated than the simple models considered here. The best fit to all the observed variations was provided by a nonradial m = 4 mode, although these were not entirely satisfactory having a phase shift of about 0.25 between the observed and predicted bisector variations. Possible explanations for this shift include temperature effects, a source function originating in a dynamic stellar atmosphere, or a different pulsation mode than the one that was considered. Although the exact pulsational mode is yet to be identified, we believe that the residual radial velocity variations are due to a long-period pulsation mode. Supporting evidence comes from the fact that three different residual variations with periods near 40 days have now been seen in Polaris and it is unlikely that these variations are due to rotational modulation by surface features on a star that is differentially rotating.

Long-period, low-amplitude radial velocity variations in the K giant star Herculis: rotation, substellar companion or non-radial pulsations?

Precise stellar radial velocity (RV) measurements (σ≈ 20 m s-1 ) spanning more than 2 yr are presented for the K giant star π Herculis. These show variability with a period of 613 ± 57 d and an amplitude of 150 ± 12 m s-1 . Radial pulsations can be excluded as a mechanism for this variability because the observed period is more than an order of magnitude greater than the expected period of the fundamental radial mode. Tenable hypotheses for the RV variability include rotational modulation by surface structure, non-radial pulsations, or a substellar companion of mass at least 27 MJupiter in orbit 3 au from the star. An upper limit of 1400 d to the rotational period for π Herculis is determined using published values of the angular diameter and distance to π Herculis as well as an estimate of the projected rotational velocity. Rotational modulation is thus a viable mechanism for the RV variability. The RV variations were also fitted using two models: (i) a stellar surface covered with cool spots and (ii) non-radial pulsations. Both models could adequately reproduce the RV curve although the spot model predicts photometric variations of Δ V ∼ 0.1 mag. As a result of the long period, non-radial pulsations may be g-mode or r-mode oscillations, which implies that most of the atmospheric motion of the star is in the horizontal direction. If these modes are indeed present then the corresponding photometric amplitude for non-radial pulsations may be quite small. The radial velocity variations were also measured using subsections of the spectral region. Weak spectral lines yielded an RV amplitude of ≈ 140 m s-1 whereas the stronger lines yielded an RV amplitude of ≈ 220 m s-1 with a phase shift of about 60 ° with respect to the weaker lines. This seems to support the pulsation hypothesis as the cause of the RV variability, although an analysis of more lines is needed to confirm this. Also, a period of 90.3 d with an amplitude 50 m s-1 is found in the residual RV measurements after subtracting the 613-d component. The presence of two periods also argues in favour of non-radial pulsations, although at the present time one cannot exclude low-mass companions or surface structure as a cause for at least one of the observed periods. Photometric measurements as well as detailed analysis of the changes in the spectral line shapes using high-resolution data may be required to distinguish between the companion object and non-radial pulsation hypotheses.

Radial Velocity Variations of the Rapidly Oscillating Ap Star 33 Lib

AbstractWe present precise stellar radial velocity (RV) measurements of the rapidly oscillating Ap (roAp) star 33 Lib taken in rapid succession over a 3-hr time span. A Fourier analysis of the data clearly shows the 8.2 min. pulsation period found previously by photometric investigations and gives a peak-to-peak (2K) amplitude of about 80 ms−1. We find, like in other roAp stars we have studied, that the RV amplitude depends on the spectral region used for measuring the pulsational RV amplitude and is as high as 57 ± 4.7 ms−1 in the region 5411–5500 Å and as low as 7 ± 3 ms−1 in the 5877–5976 Å region. An analysis of individual spectral lines show considerable scatter in the RV amplitude, ranging as high as 320 ms−1 and as low as 7 m−1. There is an overall trend of increasing RV amplitude with decreasing line strength. We also found that spectral lines due to nickel have a higher mean RV amplitude than chromium lines. We believe that the line strength variations result from the vertical atmospheric structure of the pulsations and that the elemental differences are related to the inhomogeneous distribution of elements known to occur on Ap stars. Precise stellar radial velocity studies of roAp stars may be a powerful tool for studying both the spatial (surface) and vertical structure to the pulsational velocity field.