arXiv subjects

R. P. Butler

Publications and source records attributed to R. P. Butler.

At least 73 records · Page 4Linked to original sources

arXiv2011

The Frequency of Low-Mass Exoplanets. III. Toward eta-Earth at Short Periods

Determining the occurrence rate of terrestrial-mass planets (m_p < 10M_earth) is a critically important step on the path towards determining the frequency of Earth-like planets (eta-Earth), and hence the uniqueness of our Solar system. Current radial-velocity surveys, achieving precisions of 1 m/s, are able to detect terrestrial-mass planets and provide meaningful estimates of their occurrence rate. We present an analysis of 67 solar-type stars from the Anglo-Australian Planet Search specifically targeted for very high-precision observations. When corrected for incompleteness, we find that the planet occurrence rate increases sharply with decreasing planetary mass. Our results are consistent with those from other surveys: in periods shorter than 50 days, we find that 1.5% of stars host a giant (m_p > 100M_earth) planet, and that 18.5% of stars host a planet with m_p < 10M_earth. The preponderance of low-mass planets in short-period orbits is in conflict with formation simulations in which the majority of terrestrial-mass planets reside at larger orbital distances. This work gives a hint as to the size of eta-Earth, but to make meaningful predictions on the frequency of terrestrial planets in longer, potentially habitable orbits, low-mass terrestrial planet searches at periods of 100-200 days must be made an urgent priority for ground-based Doppler planet searches in the years ahead.

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arXiv2010

On the Frequency of Jupiter Analogs

The Anglo-Australian Planet Search has now accumulated 12 years of radial-velocity data with long-term instrumental precision better than 3 m/s. In this paper, we expand on earlier simulation work, to probe the frequency of near-circular, long-period gas-giant planets residing at orbital distances of 3-6 AU -- the so-called "Jupiter analogs." We present the first comprehensive analysis of the frequency of these objects based on radial-velocity data. We find that 3.3% of stars in our sample host Jupiter analogs; detailed, star-by-star simulations show that no more than 37% of stars host a giant planet between 3-6 AU.

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arXiv2010

The Frequency of Low-Mass Exoplanets. II. The `Period Valley'

Radial-velocity planet search campaigns are now beginning to detect low-mass "Super-Earth" planets, with minimum masses M sin i < 10 M_earth. Using two independently-developed methods, we have derived detection limits from nearly four years of the highest-precision data on 24 bright, stable stars from the Anglo-Australian Planet Search. Both methods are more conservative than a human analysing an individual observed data set, as is demonstrated by the fact that both techniques would detect the radial velocity signals announced as exoplanets for the 61 Vir system in 50% of trials. There are modest differences between the methods which can be recognised as arising from particular criteria that they adopt. What both processes deliver is a quantitative selection process such that one can use them to draw quantitative conclusions about planetary frequency and orbital parameter distribution from a given data set. Averaging over all 24 stars, in the period range P<300 days and the eccentricity range 0.0 7.1 m/s. For the best stars in the sample, we are able to detect or exclude planets with K>3 m/s, corresponding to minimum masses of 8 M_earth (P=5 days) or 17 M_earth (P=50 days). Our results indicate that the observed "period valley," a lack of giant planets (M>100 M_earth) with periods between 10-100 days, is indeed real. However, for planets in the mass range 10-100 M_earth, our results suggest that the deficit of such planets may be a result of selection effects.

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arXiv2010

HAT-P-17b,c: A Transiting, Eccentric, Hot Saturn and a Long-period, Cold Jupiter

We report the discovery of HAT-P-17b,c, a multi-planet system with an inner transiting planet in a short-period, eccentric orbit and an outer planet in a 4.8 yr, nearly circular orbit. The inner planet, HAT-P-17b, transits the bright V = 10.54 early K dwarf star GSC 2717-00417, with an orbital period P = 10.338523 +/- 0.000009 d, orbital eccentricity e = 0.346 +/- 0.007, transit epoch T_c = 2454801.16945 +/- 0.00020, and transit duration 0.1691 +/- 0.0009 d. HAT-P-17b has a mass of 0.530 +/- 0.018 M_J and radius of 1.010 +/- 0.029 R_J yielding a mean density of 0.64 +/- 0.05 g cm^-3. This planet has a relatively low equilibrium temperature in the range 780-927 K, making it an attractive target for follow-up spectroscopic studies. The outer planet, HAT-P-17c, has a significantly longer orbital period P_2 = 1797^+58_-89 d and a minimum mass m_2 sin i_2 = 1.4^+1.1_-0.4 M_J. The orbital inclination of HAT-P-17c is unknown as transits have not been observed and may not be present. The host star has a mass of 0.86 +/- 0.04 M_Sun, radius of 0.84 +/- 0.02, effective temperature 5246 +/- 80 K, and metallicity [Fe/H] = 0.00 +/- 0.08. HAT-P-17 is the second multi-planet system detected from ground-based transit surveys.

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arXiv2010

NACO-SDI imaging of known companion host stars from the AAPS and Keck planet search surveys

Direct imaging of brown dwarfs as companions to solar-type stars can provide a wealth of well-constrained data to "benchmark" the physics of such objects, since quantities like metallicity and age can be determined from their well-studied primaries. We present results from an adaptive optics imaging program on stars drawn from the Anglo-Australian and Keck Planet Search projects, with the aim of directly imaging known cool companions. Simulations have modeled the expected contrast ratios and separations of known companions using estimates of orbital parameters available from current radial-velocity data and then a selection of the best case objects were followed-up with high contrast imaging to attempt to directly image these companions. These simulations suggest that only a very small number of radial-velocity detected exoplanets with consistent velocity fits and age estimates could potentially be directly imaged using the VLT's Simultaneous Differential Imaging system and only under favorable conditions. We also present detectability confidence limits from the radial-velocity data sets and show how these can be used to gain a better understanding of these systems when combined with the imaging data. For HD32778 and HD91204 the detectabilities help little in constraining the companion and hence almost all our knowledge is drawn from the SDI images. Therefore, we can say that these stars do not host cool methane objects, out to on-sky separations of ~2'', with contrasts less than 10-11 magnitudes. However, for HD25874, HD120780 and HD145825, the contrasts and detectabilities can rule out a number of possible solutions, particularly at low angular separations, and for the best case, down to strong methane masses of 40MJ at 1'' separation. The contrast curves constructed for these five stars show 5 sigma contrasts (Delta F1) of ~9.2-11.5 magnitudes at separations of >/=0.6'', which correspond to contrasts of ~9.7-12.0 magnitudes for companions of mid-T spectral type. Such limits allow us to reach down to 40MJ around fairly old field dwarfs that typically constitute high precision radial-velocity programs. Finally, the analysis performed here can serve as a template for future projects that will employ extreme-AO systems to directly image planets already indirectly discovered by the radial-velocity method.

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arXiv2010

A multi-site campaign to measure solar-like oscillations in Procyon. II. Mode frequencies

We have analyzed data from a multi-site campaign to observe oscillations in the F5 star Procyon. The data consist of high-precision velocities that we obtained over more than three weeks with eleven telescopes. A new method for adjusting the data weights allows us to suppress the sidelobes in the power spectrum. Stacking the power spectrum in a so-called echelle diagram reveals two clear ridges that we identify with even and odd values of the angular degree (l=0 and 2, and l=1 and 3, respectively). We interpret a strong, narrow peak at 446 muHz that lies close to the l=1 ridge as a mode with mixed character. We show that the frequencies of the ridge centroids and their separations are useful diagnostics for asteroseismology. In particular, variations in the large separation appear to indicate a glitch in the sound-speed profile at an acoustic depth of about 1000 s. We list frequencies for 55 modes extracted from the data spanning 20 radial orders, a range comparable to the best solar data, which will provide valuable constraints for theoretical models. A preliminary comparison with published models shows that the offset between observed and calculated frequencies for the radial modes is very different for Procyon than for the Sun and other cool stars. We find the mean lifetime of the modes in Procyon to be 1.29 +0.55/-0.49 days, which is significantly shorter than the 2-4 days seen in the Sun.

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arXiv2010

Accurate fundamental parameters for 23 bright solar-type stars

We combine results from interferometry, asteroseismology and spectroscopy to determine accurate fundamental parameters of 23 bright solar-type stars, from spectral type F5 to K2 and luminosity classes III to V. For some stars we can use direct techniques to determine the mass, radius, luminosity and effective temperature, and we compare with indirect methods that rely on photometric calibrations or spectroscopic analyses. We use the asteroseismic information available in the literature to infer an indirect mass with an accuracy of 4-15 percent. From indirect methods we determine luminosity and radius to 3 percent. For Teff we find a slight offset of -40+-20 K between the spectroscopic method and the direct method, meaning the spectroscopic temperatures are too high. From the spectroscopic analysis we determine the detailed chemical composition for 13 elements, including Li, C and O. We find no significant offset between the spectroscopic surface gravity and the value from combining asteroseismology with radius estimates. From the spectroscopy we also determine vsini and we present a new calibration of macro- and microturbulence. From the comparison between the results from the direct and spectroscopic methods we claim that we can determine Teff, log g, and [Fe/H] with absolute accuracies of 80 K, 0.08 dex, and 0.07 dex. The indirect methods are important to obtain reliable estimates of the fundamental parameters of relatively faint stars when interferometry cannot be used. Our study is the first to compare direct and indirect methods for a large sample of stars, and we conclude that indirect methods are valid, although slight corrections may be needed.

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arXiv2009

HAT-P-11b: A Super-Neptune Planet Transiting a Bright K Star in the Kepler Field

We report on the discovery of HAT-P-11b, the smallest radius transiting extrasolar planet (TEP) discovered from the ground, and the first hot Neptune discovered to date by transit searches. HAT-P-11b orbits the bright (V=9.587) and metal rich ([Fe=H] = +0.31 +/- 0.05) K4 dwarf star GSC 03561-02092 with P = 4.8878162 +/- 0.0000071 days and produces a transit signal with depth of 4.2 mmag. We present a global analysis of the available photometric and radial-velocity data that result in stellar and planetary parameters, with simultaneous treatment of systematic variations. The planet, like its near-twin GJ 436b, is somewhat larger than Neptune (17Mearth, 3.8Rearth) both in mass Mp = 0.081 +/- 0.009 MJ (25.8 +/- 2.9 Mearth) and radius Rp = 0.422 +/- 0.014 RJ (4.73 +/- 0.16 Rearth). HAT-P-11b orbits in an eccentric orbit with e = 0.198 +/- 0.046 and omega = 355.2 +/- 17.3, causing a reflex motion of its parent star with amplitude 11.6 +/- 1.2 m/s, a challenging detection due to the high level of chromospheric activity of the parent star. Our ephemeris for the transit events is Tc = 2454605.89132 +/- 0.00032 (BJD), with duration 0.0957 +/- 0.0012 d, and secondary eclipse epoch of 2454608.96 +/- 0.15 d (BJD). The basic stellar parameters of the host star are M* = 0.809+0.020-0.027 Msun, R* = 0.752 +/- 0.021 Rsun and Teff = 4780 +/- 50 K. Importantly, HAT-P-11 will lie on one of the detectors of the forthcoming Kepler mission. We discuss an interesting constraint on the eccentricity of the system by the transit light curve and stellar parameters. We also present a blend analysis, that for the first time treats the case of a blended transiting hot Jupiter mimicing a transiting hot Neptune, and proves that HAT-P-11b is not such a blend.

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arXiv2009

HAT-P-12b: A Low-Density Sub-Saturn Mass Planet Transiting a Metal-Poor K Dwarf

We report on the discovery of HAT-P-12b, a transiting extrasolar planet orbiting the moderately bright V=12.8 K4 dwarf GSC 03033-00706, with a period P = 3.2130598 +- 0.0000021 d, transit epoch Tc = 2454419.19556 +- 0.00020 (BJD) and transit duration 0.0974 +- 0.0006 d. The host star has a mass of 0.73 +- 0.02 Msun, radius of 0.70 +- ^0.02_0.01 Rsun, effective temperature 4650 +- 60 K and metallicity [Fe/H] = -0.29 +- 0.05. We find a slight correlation between the observed spectral line bisector spans and the radial velocity, so we consider, and rule out, various blend configurations including a blend with a background eclipsing binary, and hierarchical triple systems where the eclipsing body is a star or a planet. We conclude that a model consisting of a single star with a transiting planet best fits the observations, and show that a likely explanation for the apparent correlation is contamination from scattered moonlight. Based on this model, the planetary companion has a mass of 0.211 +- 0.012 MJup, and a radius of 0.959 +- ^0.029_0.021 RJup yielding a mean density of 0.295 +- 0.025 g cm^-3. Comparing these observations with recent theoretical models we find that HAT-P-12b is consistent with a ~ 1-4.5 Gyr, mildly irradiated, H/He dominated planet with a core mass Mc <~ 10 Mearth. HAT-P-12b is thus the least massive H/He dominated gas giant planet found to date. This record was previously held by Saturn.

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arXiv2009

The Frequency of Low-mass Exoplanets

We report first results from the Anglo-Australian Telescope Rocky Planet Search - an intensive, high-precision Doppler planet search targeting low-mass exoplanets in contiguous 48 night observing blocks. On this run we targeted 24 bright, nearby and intrinsically stable Sun-like stars selected from the Anglo-Australian Planet Search's main sample. These observations have already detected one low-mass planet reported elsewhere (HD16417b), and here we reconfirm the detection of HD4308b. Further, we have Monte-Carlo simulated the data from this run on a star-by-star basis to produce robust detection constraints. These simulations demonstrate clear differences in the exoplanet detectability functions from star to star due to differences in sampling, data quality and intrinsic stellar stability. They reinforce the importance of star-by-star simulation when interpreting the data from Doppler planet searches. The simulations indicate that for some of our target stars we are sensitive to close-orbiting planets as small as a few Earth masses. The two low-mass planets present in our 24 star sample indicate that the exoplanet minimum mass function at low masses is likely to be a flat alpha ~ -1 (for dN/dM proportional to M^alpha) and that between 15+/-10% (at alpha=-0.3) and 48+/-34% (at alpha=-1.3) of stars host planets with orbital periods of less than 16 days and minimum masses greater than 3 Mearth.

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arXiv2009

Independent confirmation and refined parameters of the hot Jupiter XO-5b

We present HATNet observations of XO-5b, confirming its planetary nature based on evidence beyond that described in the announcement of Burke et al. (2008), namely, the lack of significant correlation between spectral bisector variations and orbital phase. In addition, using extensive spectroscopic measurements spanning multiple seasons, we investigate the relatively large scatter in the spectral line bisectors. We also examine possible blended stellar configurations (hierarchical triples, chance alignments) that can mimic the planet signals, and we are able to show that none are consistent with the sum of all the data. The analysis of the S activity index shows no significant stellar activity. Our results for the planet parameters are consistent with values in Burke et al. (2008), and we refine both the stellar and planetary parameters using our data. XO-5b orbits a slightly evolved, late G type star with mass M_s = 0.88 +/- 0.03, radius R_s = 1.08 +/- 0.04, and metallicity close to solar. The planetary mass and radius are M_p = 1.059 +/- 0.028 M_Jup and R_p = 1.109 +/- 0.050 R_Jup, respectively, corresponding to a mean density of 0.96 -0.11 +0.14 g/cm^3. The ephemeris for the orbit is P = 4.187757 +/- 0.000011, E= 2454552.67168 +/- 0.00029 (BJD) with transit duration of 0.1307 +/- 0.0013 d. By measuring four individual transit centers, we found no signs for transit timing variations. The planet XO-5b is notable for its anomalously high Safronov number, and has a high surface gravity when compared to other transiting exoplanets with similar period.

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arXiv2009

The Jupiter Twin HD 154345b

We announce the discovery of a twin of Jupiter orbiting the slightly metal-poor ([Fe/H] = -0.1) nearby (d = 18 pc) G8 dwarf HD 154345. This planet has a minimum mass of 0.95 Jupiter masses and a 9.2 year, circular orbit with radius 4.2 AU. There is currently little or no evidence for other planets in the system, but smaller or exterior planets cannot yet be ruled out. We also detect a ~ 9-year activity cycle in this star photometrically and in chromospheric emission. We rule out activity cycles as the source of the radial velocity variations by comparison with other cycling late-G dwarfs.

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arXiv2008

Selection Functions in Doppler Planet Searches

We present a preliminary analysis of the sensitivity of Anglo-Australian Planet Search data to the orbital parameters of extrasolar planets. To do so, we have developed new tools for the automatic analysis of large-scale simulations of Doppler velocity planet search data. One of these tools is the 2-Dimensional Keplerian Lomb-Scargle periodogram, that enables the straightforward detection of exoplanets with high eccentricities (something the standard Lomb-Scargle periodogram routinely fails to do). We used this technique to re-determine the orbital parameters of HD20782b, with one of the highest known exoplanet eccentricities (e=0.97+/-0.01). We also derive a set of detection criteria that do not depend on the distribution functions of fitted Keplerian orbital parameters (which we show are non-Gaussian with pronounced, extended wings). Using these tools, we examine the selection functions in orbital period, eccentricity and planet mass of Anglo-Australian Planet Search data for three planets with large-scale Monte Carlo-like simulations. We find that the detectability of exoplanets declines at high eccentricities. However, we also find that exoplanet detectability is a strong function of epoch-to-epoch data quality, number of observations, and period sampling. This strongly suggests that simple parametrisations of the detectability of exoplanets based on "whole-of-survey" metrics may not be accurate. We have derived empirical relationships between the uncertainty estimates for orbital parameters that are derived from least-squares Keplerian fits to our simulations, and the true 99% limits for the errors in those parameters, which are larger than equivalent Gaussian limits by factors of 5-10. (abridged)

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arXiv2008

HAT-P-10b: A light and moderately hot Jupiter transiting a K dwarf

We report on the discovery of HAT-P-10b, the lowest mass (0.46 +/- 0.03 MJ) transiting extrasolar planet (TEP) discovered to date by transit searches. HAT-P-10b orbits the moderately bright V=11.89 K dwarf GSC 02340-01714, with a period P = 3.7224690 +/- 0.0000067 d, transit epoch Tc = 2454729.90631 +/- 0.00030 (BJD) and duration 0.1100 +/- 0.0015 d. HAT-P-10b has a radius of 1.05 +(0.05)-(0.03) RJ yielding a mean density of 0.498+/-0.064 g cm^-3 . Comparing these observations with recent theoretical models we find that HAT-P-10 is consistent with a ~4.5 Gyr, coreless, pure hydrogen and helium gas giant planet. With an equilibrium temperature of Teq = 1030 +(26)-(19)K, HAT-P-10b is one of the coldest TEPs. Curiously, its Safronov number Theta = 0.047 +/- 0.003 falls close to the dividing line between the two suggested TEP populations.

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arXiv2008

Beyond the Iron Peak: r- and s-process Elemental Abundances in Stars with Planets

We present elemental abundances of 118 stars (28 of which are known extrasolar planetary host stars) observed as part of the Anglo-Australian Planet Search. Abundances of O, Mg, Cr, Y, Zr, Ba, Nd and Eu (along with previously published abundances for C and Si) are presented. This study is one of the first to specifically examine planetary host stars for the heavy elements produced by neutron capture reactions. We find that the host stars are chemically different to both the standard solar abundance and non-host stars in all elements studied, with enrichments over non-host stars ranging from 0.06 dex (for O) to 0.11 dex (for Cr and Y). Such abundance trends are in agreement with other previous studies of field stars and lead us to conclude that the chemical anomalies observed in planetary host stars are the result of normal galactic chemical evolution processes. Based on this observation, we conclude that the observed chemical traits of planetary host stars are primordial in origin, coming from the original nebula and not from a ``pollution'' process occurring during or after formation and that planet formation occurs naturally with the evolution of stellar material.

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arXiv2008

HAT-P-7b: An Extremely Hot Massive Planet Transiting a Bright Star in the Kepler Field

We report on the latest discovery of the HATNet project; a very hot giant planet orbiting a bright (V = 10.5) star with a small semi-major axis of a = 0.0377 +/- 0.0005 AU. Ephemeris for the system is P = 2.2047299 +/- 0.0000040 days, mid-transit time E = 2,453,790.2593 +/- 0.0010 (BJD). Based on the available spectroscopic data on the host star and photometry of the system, the planet has a mass of Mp = 1.78+/-^{0.08}_{0.05} MJup and radius of Rp = 1.36+/-^{0.20}_{0.09} RJup. The parent star is a slightly evolved F6 star with M = 1.47+/-^{0.08}_{-0.05} Msun,R = 1.84+/-^{0.23}_{0.11} Rsun, Teff = 6350 +/- 80 K, and metallicity [Fe/H] = +0.26 +/- 0.08. The relatively hot and large host star, combined with the close orbit of the planet, yield a very high planetary irradiance of (4.71+/-^{1.44}_{0.05}) 10^9 erg cm^{-2}s^{-1}, which places the planet near the top of the pM class of irradiated planets as defined by Fortney et al. (2007). If as predicted by Fortney et al. (2007) the planet re-radiates its absorbed energy before distributing it to the night side, the day-side temperature should be about (2730+/-^{150}_{100}) K. Because the host star is quite bright, measurement of the secondary eclipse should be feasible for ground-based telescopes, providing a good opportunity to compare the predictions of current hot Jupiter atmospheric models with the observations. Moreover, the host star falls in the field of the upcoming Kepler mission; hence extensive space-borne follow-up, including not only primary transit and secondary eclipse observations but also asteroseismology, will be possible.

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arXiv2008

HAT-P-6b: A Hot Jupiter transiting a bright F star

In the ongoing HATNet survey we have detected a giant planet, with radius 1.33 +/- 0.06 RJup and mass 1.06 +/- 0.12 MJup, transiting the bright (V = 10.5) star GSC 03239-00992. The planet is in a circular orbit with period 3.852985 +/- 0.000005 days and mid-transit epoch 2,454,035.67575 +/- 0.00028 (HJD). The parent star is a late F star with mass 1.29 +/- 0.06 Msun, radius 1.46 +/- 0.06 Rsun, Teff ~ 6570 +/- 80 K, [Fe=H] = -0.13 +/- 0.08 and age ~ 2.3+/-^{0.5}_{0.7}Gy. With this radius and mass, HAT-P-6b has somewhat larger radius than theoretically expected. We describe the observations and their analysis to determine physical properties of the HAT-P-6 system, and briefly discuss some implications of this finding.

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arXiv2007

Oscillations in Procyon A: First results from a multi-site campaign

Procyon A is a bright F5IV star in a binary system. Although the distance, mass and angular diameter of this star are all known with high precision, the exact evolutionary state is still unclear. Evolutionary tracks with different ages and different mass fractions of hydrogen in the core pass, within the errors, through the observed position of Procyon A in the Hertzsprung-Russell diagram. For more than 15 years several different groups have studied the solar-like oscillations in Procyon A to determine its evolutionary state. Although several studies independently detected power excess in the periodogram, there is no agreement on the actual oscillation frequencies yet. This is probably due to either insufficient high-quality data (i.e., aliasing) or due to intrinsic properties of the star (i.e., short mode lifetimes). Now a spectroscopic multi-site campaign using 10 telescopes world-wide (minimizing aliasing effects) with a total time span of nearly 4 weeks (increase the frequency resolution) is performed to identify frequencies in this star and finally determine its properties and evolutionary state.

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