Correlations Between the Stellar, Planetary, and Debris Components of Exoplanet Systems Observed by Herschel⋆
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A&A 565, A15 (2014) Astronomy DOI: 10.1051/0004-6361/201323058 & c ESO 2014 Astrophysics Correlations between the stellar, planetary, and debris components of exoplanet systems observed by Herschel J. P. Marshall1,2, A. Moro-Martín3,4, C. Eiroa1, G. Kennedy5,A.Mora6, B. Sibthorpe7, J.-F. Lestrade8, J. Maldonado1,9, J. Sanz-Forcada10,M.C.Wyatt5,B.Matthews11,12,J.Horner2,13,14, B. Montesinos10,G.Bryden15, C. del Burgo16,J.S.Greaves17,R.J.Ivison18,19, G. Meeus1, G. Olofsson20, G. L. Pilbratt21, and G. J. White22,23 (Affiliations can be found after the references) Received 15 November 2013 / Accepted 6 March 2014 ABSTRACT Context. Stars form surrounded by gas- and dust-rich protoplanetary discs. Generally, these discs dissipate over a few (3–10) Myr, leaving a faint tenuous debris disc composed of second-generation dust produced by the attrition of larger bodies formed in the protoplanetary disc. Giant planets detected in radial velocity and transit surveys of main-sequence stars also form within the protoplanetary disc, whilst super-Earths now detectable may form once the gas has dissipated. Our own solar system, with its eight planets and two debris belts, is a prime example of an end state of this process. Aims. The Herschel DEBRIS, DUNES, and GT programmes observed 37 exoplanet host stars within 25 pc at 70, 100, and 160 μm with the sensitiv- ity to detect far-infrared excess emission at flux density levels only an order of magnitude greater than that of the solar system’s Edgeworth-Kuiper belt. Here we present an analysis of that sample, using it to more accurately determine the (possible) level of dust emission from these exoplanet host stars and thereafter determine the links between the various components of these exoplanetary systems through statistical analysis. Methods. We have fitted the flux densities measured from recent Herschel observations with a simple two parameter (Td, LIR/L) black-body model (or to the 3σ upper limits at 100 μm). From this uniform approach we calculated the fractional luminosity, radial extent and dust temper- ature. We then plotted the calculated dust luminosity or upper limits against the stellar properties, e.g. effective temperature, metallicity, and age, and identified correlations between these parameters. Results. A total of eleven debris discs are identified around the 37 stars in the sample. An incidence of ten cool debris discs around the Sun-like exoplanet host stars (29 ± 9%) is consistent with the detection rate found by DUNES (20.2 ± 2.0%). For the debris disc systems, the dust temper- −6 −4 atures range from 20 to 80 K, and fractional luminosities (LIR/L) between 2.4 ×10 and 4.1 ×10 . In the case of non-detections, we calculated typical 3σ upper limits to the dust fractional luminosities of a few ×10−6. Conclusions. We recover the previously identified correlation between stellar metallicity and hot-Jupiter planets in our data set. We find a correla- tion between the increased presence of dust, lower planet masses, and lower stellar metallicities. This confirms the recently identified correlation between cold debris discs and low-mass planets in the context of planet formation by core accretion. Key words. infrared: stars – infrared: planetary systems – circumstellar matter – planet-disk interactions 1. Introduction as arising from two distinct belts at different stellocentric radii (Chen et al. 2009; Morales et al. 2011). The cool discs are Circumstellar debris discs around main-sequence stars are com- more commonly seen and analogous to the Edgeworth-Kuiper posed of second-generation dust produced by the attrition of belt (EKB) in our own solar system (Greaves & Wyatt 2010; larger bodies (Backman & Paresce 1993), which are remnants of Vitense et al. 2012). The EKB’s existence has been inferred primordial protoplanetary discs (Hernández et al. 2007). Debris from the detection of over a thousand trans-Neptunian objects2, discs can be detected and analysed based on their thermal in- through ground-based surveys and in situ dust measurement frared emission from the constituent dust particles. Around +2.8 from Voyager 1 and 2 (Gurnett et al. 1997) and New Horizons 16.4−2.9% of main-sequence Sun-like stars have evidence of (Poppe et al. 2010; Han et al. 2011), although direct observa- circumstellar dust emission at 70 μm with Spitzer (Trilling 1 tion of the dust emission from the EKB is confounded by the et al. 2008). From observations of FGK stars by the Herschel bright foreground thermal emission from the zodiacal dust in ± DUNES survey an incidence of 20.2 2.0% was measured the inner solar system (Backman et al. 1995). The less com- (Eiroa et al. 2013), whereas the DEBRIS survey measures an monly seen warm debris disc asteroid-belt analogues, which are ± incidence of 16.5 2.5% (Sibthorpe et al., in prep.). more difficult to observe around other stars due to the larger flux Many circumstellar discs around Sun-like stars are seen to density contribution from the stellar photosphere at mid-infrared have two temperature components, which has been interpreted wavelengths compared with the dust excess, have been detected around ∼2% of Sun-like stars (3/7 FGK stars with 24 μm excess − Tables 2 4 are available in electronic form at and Tdust > 100 K from a sample of 184, Trilling et al. 2008). http://www.aanda.org 1 Herschel is an ESA space observatory with science instruments pro- vided by European-led Principal Investigator consortia and with impor- 2 1258 as of 29th October 2013, see: tant participation from NASA. http://www.minorplanetcenter.net/iau/lists/TNOs.html Article published by EDP Sciences A15, page 1 of 14 A&A 565, A15 (2014) Exoplanets3 around Sun-like stars have been identified established between the presence of debris discs and the pres- through radial velocity (e.g. Marcy & Butler 2000; Tinney ence of planets (Greaves et al. 2006; Moro-Martín et al. 2007; et al. 2001; Mayor & Queloz 2012)ortransitsurveyse.g. Bryden et al. 2009; Kóspál et al. 2009), and larger scale direct CoRoT (Auvergne et al. 2009), WASP (Pollacco et al. 2006), imaging surveys have found little evidence of massive planets HAT (Bakos et al. 2002)andKepler (Borucki et al. 2011). See around debris disc host stars (Wahhaj et al. 2013; Janson et al. Perryman (2011) for a summary of exoplanet detection tech- 2013). However, new analysis by Maldonado et al. (2012) identi- niques. The majority of all exoplanet searches have taken place fied a trend between the presence of a debris disc and cool Jupiter at optical wavelengths, with a sample focus on mature, Sun-like exoplanet around a star, whilst Wyattetal.(2012) identified a stars as the most suitable candidates for the radial velocity de- possible trend between cool dust and low-mass planet host stars. tection technique. A stars are avoided as their atmospheres lack Direct measurement of the spatial distribution of debris in the narrow lines necessary for radial velocity detections through other stellar systems will reveal whether our own EKB is com- accurate Doppler measurements, but these stars are prime can- mon or unusual. The EKB and its interaction with the outer plan- didates for direct imaging surveys, resulting in the detection ets is thought to have played a significant role in the develop- of several exoplanet systems around debris disc host stars, e.g. ment of life on Earth, having supplied a significant fraction of Fomalhaut (Kalas et al. 2008), HR 8799 (Marois et al. 2008), the impactors thought to have been involved in the Late Heavy β Pic (Lagrange et al. 2010) and HD 95086 (Rameau et al. 2013). Bombardment (Gomes et al. 2005), dictated the development of M stars have likewise been avoided because they exhibit high the terrestrial planets (Walsh et al. 2011), and continues to pro- levels of stellar variability and because of their emission peaks vide bodies for the short-period comet population through in- in the near-infrared, which render them noisy and faint, although teraction with Jupiter (Horner & Jones 2009). It may also have great efforts have been made to overcome these problems be- been involved in the hydration of Earth (Morbidelli et al. 2000), cause of their sheer number and potential to yield low-mass a topic that is still widely debated (Raymond et al. 2004, 2006, planets through either transit or radial velocity detections (e.g. 2007; O’Brien et al. 2006; Horner et al. 2009; Bond et al. 2010; Reiners et al. 2010; Rodler et al. 2011; Giacobbe et al. 2012; Horner & Jones 2010; Izidoro et al. 2013). As such, the nature of Anglada-Escudé & Tuomi 2012). This has led to unavoidable exo-EKBs may play a vital role in the determination of the hab- bias in the types of stars around which exoplanets have been itability of their host systems (e.g. Horner & Jones 2010), and studied. Comparative analysis or aggregation of results from ra- it is therefore vital that we are able to judge whether the solar dial velocity surveys is further complicated by both the differ- system’s architecture and EKB are the norm, or unusual. ences in sensitivity and the variable baseline of observations for The Herschel (Pilbratt et al. 2010) guaranteed time (GT) the stellar samples, although broad conclusions, for instance on debris disc programme and the open time key programmes the absence of Jupiter analogues around most nearby stars, may Disc Emission from Bias-free Reconnaissance Infrared Survey be drawn (Cumming et al. 1999; Fischer et al. 2014). Recent (DEBRIS4; Matthews et al. 2010) and DUst around NEarby results from an analysis of microlensing surveys suggest that Stars (DUNES5; Eiroa et al.