The Mt John University Observatory search for Earth-mass planets in the habitable zone of α Centauri - PDF

International Journal of Astrobiology, Page 1 of 8 doi: /s Cambridge University Press 2014 The Mt John University Observatory search for Earth-mass planets in the habitable zone of

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International Journal of Astrobiology, Page 1 of 8 doi: /s Cambridge University Press 2014 The Mt John University Observatory search for Earth-mass planets in the habitable zone of α Centauri Michael Endl 1, Christoph Bergmann 2, John Hearnshaw 2, Stuart I. Barnes 1,2, Robert A. Wittenmyer 3, David Ramm 2, Pam Kilmartin 2, Fraser Gunn 2 and Erik Brogt 4 1 McDonald Observatory, The University of Texas at Austin, Austin TX 78712, USA 2 Department of Physics & Astronomy, The University of Canterbury, Christchurch 8041, New Zealand 3 Department of Astrophysics and Optics, School of Physics, University of New South Wales, Sydney, Australia 4 Academic Development Group, The University of Canterbury, Christchurch 8041, New Zealand Abstract: The holy grail in planet hunting is the detection of an Earth-analogue: a planet with similar mass as the Earth and an orbit inside the habitable zone. If we can find such an Earth-analogue around one of the stars in the immediate solar neighbourhood, we could potentially even study it in such great detail to address the question of its potential habitability. Several groups have focused their planet detection efforts on the nearest stars. Our team is currently performing an intensive observing campaign on the α Centauri system using the High Efficiency and Resolution Canterbury University Large Échelle Spectrograph (HERCULES)at the 1 m McLellan telescope at Mt John University Observatory in New Zealand. The goal of our project is to obtain such a large number of radial velocity (RV) measurements with sufficiently high temporal sampling to become sensitive to signals of Earth-mass planets in the habitable zones of the two stars in this binary system. Over the past few years, we have collected more than spectra for both stars combined. These data are currently processed by an advanced version of our RV reduction pipeline, which eliminates the effect of spectral cross-contamination. Here we present simulations of the expected detection sensitivity to low-mass planets in the habitable zone by the HERCULES programme for various noise levels. We also discuss our expected sensitivity to the purported Earth-mass planet in a 3.24-day orbit announced by Dumusque et al. (2012). Received 7 July 2013, accepted 14 March 2014 Key words: individual (α Cen A, α Cen B) techniques, planetary system stars, radial velocities. Introduction The search for a true Earth-analogue planet is one of the boldest scientific and intellectual endeavours ever undertaken by humankind. If such a planet can be found orbiting a nearby Sun-like star, it will constitute an ideal target for extensive follow-up studies from the ground and with future space missions. These follow-up studies can include a detailed characterization of the planetary system and, ultimately, a search for bio-signatures in the atmosphere of an Earth-like planet in the habitable zone. For the far future, many decades to centuries from now, one can even imagine that the first interstellar probe will be launched to travel to one of the systems, where we found evidence for a nearby Earth twin. The discovery of such a planet will have an unprecedented cultural as well as scientific impact. NASA s Kepler mission (Borucki et al. 2010) has been extremely successful in finding small, possibly rocky planets orbiting stars in the Kepler search field. Some of them even reside within the circumstellar habitable zones: e.g., Kepler- 22b (Borucki et al. 2012) and two planets in the Kepler-62 system (Borucki et al. 2013). One of the most significant results from Kepler is the planet occurrence rates, which show that small radius planets are quite frequent and outnumber the giant planets to a large extent (e.g., Howard et al. 2012; Dressing & Charbonneau 2013; Fressin et al. 2013; Petigura et al. 2013). We are tempted to extrapolate from these high Kepler planet frequencies to the immediate solar neighbourhood and conclude that many nearby stars, possibly even the nearest star to the Sun, are orbited by one or more Earth-like planets. The precision of stellar radial velocity (RV) measurements has steadily improved from a modest 15 m s 1 (Campbell & Walker 1979), more than three decades ago, to a routine 3ms 1 (Butler et al. 1996), and in the best case, with the highly stabilized HARPS spectrograph (Mayor et al. 2003), even 1ms 1 or better. The discovery space of the RV method was, therefore, extended from the giant planet domain down to Neptunes and super-earths (with minimum masses between 2 and 10 M ). In terms of RV precision, we are still more than an order of magnitude from the 0.09 m s 1 RV amplitude of an Earth at 1 AU orbiting a G-type star. Future projects aim for RV precision of 0.1 m s 1, but they are still years away from being operational. ESPRESSO (Pepe et al. 2014) is currently 2 Michael Endl et al. Table 1. Precise radial velocity surveys that target the α Cen system Site Spectrograph/telescope Reference/Project website La Silla HARPS/ESO 3.6 m Pepe et al.(2011), Dumusque et al. (2012) CTIO CHIRON/SMARTS 1.5 m MJUO HERCULES/1 m McLellan under construction for the ESO Very Large Telescope and G-CLEF (Szentgyorgyi et al. 2012) has been selected as a firstlight instrument for the Giant Magellan Telescope (GMT). However, there is an alternative to extreme precision: with a large enough number of measurements, even signals with amplitudes orders of magnitude below the individual measurement uncertainties can be detected with high significance. Instead of waiting for the new instruments to be deployed, several groups have started ambitious RV programmes that observe a small sample of suitable stars in the solar neighbourhood with high temporal cadence. Owing to the extreme observational effort, these searches have to be dedicated to a few systems rather than to include as many targets as possible. The need of RV searches to focus on nearby bright stars has an attractive side-effect: the targets are all very close to the Sun, unlike the Kepler targets or microlensing systems, which are at typical distances of several hundreds or thousands of parsecs. The HARPS team has focused on ten Solar-type stars and reported very low-mass planets around HD 20794, HD and HD (Pepe et al. 2011). In order to properly perform such an RV search for low-mass planets, it is important to pay careful attention to RV signals that are intrinsic to the star and that can have larger amplitude than a planetary signal. Special attention has been given to our closest neighbour in space, α Centauri, and several groups (see Table 1) have chosen this star system as the prime target of their planet detection efforts. Recently, Dumusque et al. (2012) presented the case for the presence of a low-mass planet in a 3.2-day orbit around α Cen B using 459 highly precise RV measurements with HARPS obtained over a time span of 4 years. Tuomi et al. (2013) discussed the possible existence of a five-planet system around τ Ceti, another very close Sun-like star for which no planets have yet been reported. These important results need to be confirmed by independent data and analysis. Indeed, Hatzes (2013) re-analysed the HARPS RV results for α Cen B using a different approach to filter out the stellar activity signals than that of Dumusque et al. and cast serious doubt on the reality, or at least on the planetary nature of the 3.2-day signal. In this paper, we describe our α Cen programme with the High Efficiency and Resolution Canterbury University Large Échelle Spectrograph (HERCULES) at the McLellan 1 m telescope at Mt John University Observatory (MJUO) in New Zealand. The α Centauri system The α Cen system is our closest neighbour in space (d =1.347 pc) and thus constitutes a target of fundamental importance for any exoplanet search programme. It is so close that a future spacecraft travelling at 0.1 c reaches the system within 50 years. The α Centauri binary consists of a G2V primary (HR 5459, HD , V = 0.01) and a K1V secondary (HR 5460, HD , V =1.33) moving in an eccentric (e=0.518) orbit with a semi-major axis of a =23AU and a period of almost 80 years (Heintz 1982; Pourbaix et al. 1999). The possible third member of the system, Proxima Cen (M5V, V=11.05), is located at a much larger separation of &12000 AU. It is not clear yet whether Proxima is indeed gravitationally bound to the inner binary (Wertheimer & Laughlin 2006). Stringent upper mass limits for planets in the habitable zone of Proxima Cen have been presented by Endl & Kürster (2008) and Zechmeister et al. (2009). The star α Cen A is also very similar to our Sun. A mass ratio of the binary (m B /m A =0.75±0.09) was first presented by Murdoch et al. (1993). A recent study of the atmospheric parameters and abundances of the two stars was presented by Porto de Mello et al. (2008). Both stars are almost twice as metal-rich as the Sun (England 1980; Furenlid & Meylan 1990 and Porto de Mello et al. 2008). The age of the system has been estimated to be between 5.6 and 6.5 Gyr (e.g., Eggenberger et al. 2004). The masses of the two stars have been determined with high accuracy by Pourbaix et al. (2002): M A =1.105 ±0.007 M and M B =0.934±0.006 M. The α Cen system was included in the original 1992 target sample of the RV planet search at the ESO Coudé Echelle Spectrometer (CES) (Kürster et al. 1994). In Endl et al. (2001) we presented our CES RV data and a full analysis of the system including mass upper limits for planets. Based on the first five and a half years of CES RV measurements, we were able to set stringent constraints on the presence of any gas giant planets in the systems. We basically excluded the presence of any giant planet with a minimum mass greater than 3.5 M Jup. Moreover, we combined the RV-derived mass limits with the limits on stable orbits around each star imposed by the presence of the other star from Wiegert & Holman (1997). They found that stable regions for planets orbiting in the binary plane (corresponding to a viewing angle i = ) extend to roughly 3 AU around each star, for prograde orbits, and to 4 AU, for retrograde orbits. If the planets orbit in the binary plane, the RV measured m sini values are close to the actual mass. The formation of terrestrial planets in the α Cen system was studied by Quintana et al. (2002, 2007), Barbieri et al. (2002), Quintana & Lissauer (2006) and Guedes et al. (2008). These studies suggest that the formation of several Earth-mass planets at separations 2 AU was possible, despite the perturbing influence of the stellar companion. The MJUO search for Earth-mass planets in the habitable zone of α Centauri 3 Guedes et al. simulated the RV detectability of these planets using synthetic RV data sets. They demonstrated that a 1.7 M planet with a period of 1.2 years can be clearly detected after 3 years of observations, if the noise distribution is sufficiently close to Gaussian (see Fig. 5 in their paper). They used a noise level of 3 m s 1 and nearly 10 5 synthetic RV points. Thebault et al. (2008, 2009), however, argue that these Earth-like planets might not have formed around α Cen B as the mutual velocities of planetesimals were too high to allow accretion and continued growth to proto-planets beyond the planetesimal phase. These authors also state that the situation would have been more benign for the formation of terrestrial planets if the binary was initially wider (by about 15 AU) or had a lower eccentricity than currently. On the other hand, Xie et al. (2010) conclude that the formation of terrestrial planets around α Cen B was possible, even in the current binary configuration. The MJUO α Centauri programme The University of Canterbury in Christchurch, New Zealand, owns and operates the MJUO at the northern end of the Mackenzie Basin in the South Island. We selected MJUO as the site for this project, as α Cen, at lower culmination, is still 15 above the horizon. While other observatories are limited to observe α Cen for 9 10 months, we are able to obtain RV data for 12 months a year. We are in a unique position that allows us optimal sampling of periods close to 1 year. Observing strategy Terrestrial planets orbiting inside the classical habitable zone around F, G and K-type stars only induce reflex velocities of &0.1 m s 1 on their host stars. This is still well below the current state-of-the-art level of measurement precision. But the sensitivity to low-amplitude signals is not only a function of measurement precision, but also of data quantity. With a large enough number of measurements, even signals with amplitudes below the individual measurement uncertainties can be detected with high significance. Cochran & Hatzes (1996) presented analytical expressions for the detection sensitivity of an RV survey of a given precision. Their analytical relationship between noise and the total number of independent measurements (N ) demonstrates that at least N=100 is necessary to detect a signal with an amplitude equal to the noise. If the errors are sufficiently close to white noise, this trend continues towards lower amplitudes and with the appropriate high value of N even signals with amplitudes much lower than the noise can be detected. From this we see that, to find Earth-mass planets with minimum masses of M with habitable zone periods, N has to be of the order of several , depending on the measurement uncertainties. This is the strategy that we adopted for the Mt John programme: to obtain a very large number of RV measurements at a modest 2 3ms 1 precision level. The HERCULES spectrograph The HERCULES (Hearnshaw et al. 2002) has been in operation at MJUO since It is fibre fed from the 1 m McLellan telescope, and is enclosed inside a vacuum tank (P 0.005 atm). The spectrograph uses an R2 échelle grating in combination with a 200 mm collimated beam size. Cross-dispersion uses a BK7 prism in double pass. A large format CCD with pixels each 15 μm in size allows complete wavelength coverage from 370 to 850 nm. A 4.5 arcsecond fibre feeding a 2.25 arcsecond entrance slit gives a resolving power of R =70000 with high throughput (& 12% including telescope, fibres and CCD quantum efficiency). The spectrograph is located in a temperaturestabilized room (T&20 ±0.05 C) and has no moving parts apart from focus, which remains unchanged. An exposure meter is used to ensure precise control of exposure lengths and signal-to-noise (S/N) ratios. Because it was known that when observing bright stars the RV precision of HERCULES was limited by the effects of small guiding and centring errors, HERCULES was also equipped with an iodine cell, which superimposes a dense reference spectrum on the stellar spectrum and also allows us to reconstruct the shape of the spectrograph s instrumental profile at the time of observation. All RV results shown here were obtained with the iodine cell. HERCULES RV results for α Cen From 2007 to 2009 we collected RV data over seven observing runs and 28 nights, with typical exposure times for component A ranging from 30 to 45 s and for B from 90 to 120 s. The RV results have a long-term RV scatter of m s 1 after subtracting the large trend due to the binary motion (see Fig. 1). About 4% of the data were rejected as outliers by a simple σ-clipping routine, removing all poor S/N-ratio spectra. In April 2009, we obtained more than 500 individual spectra for α Cen B over the course of five nights. The weighted means of the nights have a very small RMS scatter of 0.5 m s 1 (see Fig. 2). These first RV results demonstrate that we achieve a single shot RV precision of *2.5 m s 1, sufficient to carry out this programme. However, since 2010 the angular separation of the two stars has shrunk from &7 to 5 arcseconds. With a mean seeing at Mt John of 2 arcsecond (with 95% of the time better than 3 arcsecond) we have an increasing level of cross-contamination in our α Cen spectra. The nominal 4.5 arcsecond fibre of HERCULES is simply too large for isolated observations of the two components of α Cen, as discussed in Wittenmyer et al. (2012). To minimize this effect we placed a pinhole at the focal plane of the telescope followed by a set of lenses to relay the light passing through the pinhole into the fibre. Apertures of 3 to 2 arcsecond can be used to reduce the size of the fibre on the sky. This pinhole relay system was installed in February 2011, and reduced, but did not remove the spectral cross-contamination. Figure 3 shows the level of contamination in our data for α Cen B (the effect for component B is, of course, stronger than for component A) as measured with a line index R. 4 Michael Endl et al. Velocity (m/s) Velocity (m/s) Residuals (m/s) Alpha Cen A: RMS = 3.02 m/s (n = 1245) Residuals (m/s) Alpha Cen B: RMS = 2.57 m/s (n = 719) Fig. 1. The HERCULES +I 2 RV data for α Cen A & B. The top panels show the data with the strong slope due to the binary orbit (solid line) and the lower panel displays the residuals. The residual scatter after removal of the slope is m s 1. The data also show a larger level of RV variability for A than for B (which is expected from their spectral types and level of magnetic activity). drv (m/s) RMS = 0.46 m/s over 5 nights compute the RV of the main target independent of the level of contamination from the second star. A detailed description of this new version of AUSTRAL is the topic of a paper that is currently in preparation (Bergmann et al. in preparation). We use this new data modelling algorithm now to reprocess all of our data for the α Cen system. Detectability of low-mass planets with the HERCULES programme 10 Weighted mean of all velocities Fig. 2. Five consecutive nights of HERCULES RV measurements for α Cen B from our 2009 April run. The 500 individual measurements have a total scatter of 2.71 m s 1 and show very little night to night variations. The weighted mean values of these five nights have a standard deviation of only 0.5 m s 1. The quantity R is defined as the ratio of the two line indices calculated as the flux in the line core as compared to the surrounding continuum for the Hα and Na D lines, respectively. As the shapes of these two strong lines are different for the A and B we can estimate the amount of contamination from the other star. The pinhole relay system also contains the iodine cell on a moving stage to insert it into the collimated beam between the Cassegrain focus and the fibre entrance. It can be quickly removed for flat field spectra and thorium-argon spectra, but inserted for the stellar spectra. We pursue now an additional approach (Bergmann et al. 2012): to include and account for the spectrum of the contaminating star in our iodine cell data modelling pipeline AUSTRAL (Endl et al. 2000). First test results for synthetic spectra with different amounts of contamination and S/N levels are shown in Fig. 4. These initial test results show that with a sufficiently high S/N level ( 300) we are able to We performed simulations to determine the expected detection efficiency of our programme, once the effect of spectral contamination is removed. We use the actual times of our current observations of α Cen B taken over 5 years and created artificial RV data sets with various different levels of white noise. The noise in our data is currently dominated by the effect of cross-contamination, which is highly systematic in nature. This is a critical issue, as the strategy to detect lowamplitude signals with a large number of measurements is only successful if the dominant noise in the data is random (white noise). Our goal is therefore to correct for the contamination and get as close as possible to white noise for the final noise structure in our RV data. The detection simulations will therefore inform our expected sensitivity to low-mass planets once the systematic noise from the cross-contamination is removed. Sensitivity to the purported 3.24-day planet signal As a f
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