Correlations Between the Stellar, Planetary and Debris Components Of
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Astronomy & Astrophysics manuscript no. dunes˙debris˙exoplanets˙v18 c ESO 2018 September 3, 2018 Correlations between the stellar, planetary and debris components of exoplanet systems observed by Herschel Jonathan P. Marshall1,2,⋆, A. Moro-Mart´ın3,4, 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 —; accepted — 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 3-σ upper limits at 100 µm). From this uniform approach we calculate the fractional luminosity, radial extent, dust temperature and disc mass. We then plotted the calculated dust luminosity or upper limits against the stellar properties, e.g. effective temperature, metallicity, 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 3-σ 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 corre- lation 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, stars: circumstellar matter, stars: planetary systems, planet-disc interactions 1. Introduction more commonly seen and analogous to the Edgeworth-Kuiper belt (EKB) in our own Solar system (Greaves & Wyatt 2010; Circumstellar debris discs around main sequence stars are com- Vitense et al. 2012). The EKB’s existence has been inferred posed of second generation dust produced by the attrition of from the detection of over a thousand Trans-Neptunian Objects2 larger bodies (Backman & Paresce 1993) which are remnants of , through ground-based surveys and in-situ dust measurement primordial protoplanetary discs (Hern´andez et al. 2007). Debris from Voyager 1 and 2 (Gurnett et al. 1997) and New Horizons discs can be detected and analyzed based on their excess infrared (Poppe et al. 2010; Han et al. 2011), although direct observation +2.8 emission from the constituent dust particles. Around 16.4−2.9 % of the dust emission from the EKB is confounded by the bright of main sequence Sun-like stars have evidence of circumstel- foreground thermal emission from the zodiacal dust in the inner arXiv:1403.6186v1 [astro-ph.EP] 24 Mar 2014 lar dust emission at 70 µm with Spitzer (Trilling et al. 2008). Solar system (Backman et al. 1995). The less commonly seen 1 From observations of FGK stars by the Herschel DUNES sur- warm debris disc Asteroid-belt analogues, which are more diffi- vey an incidence of 20.2 ± 2.0 % was measured (Eiroa et al. cult to observe around other stars due to the larger flux density 2013), whereas the DEBRIS survey measure an incidence of contribution from the stellar photosphere at mid-infrared wave- 16.5 ± 2.5 % (Sibthorpe et al. in prep). lengths compared to the dust excess, have been detected around Many circumstellar discs around Sun-like stars are seen ∼ 2 % of Sun-like stars (3/7 FGK stars with 24 µm excess and to have two temperature components, which has been inter- Tdust > 100 K from a sample of 184, Trilling et al. 2008). preted as arising from two distinct belts at different stellocentric Exoplanets3 around Sun-like stars have been identified radii (Chen et al. 2009; Morales et al. 2011). The cool discs are through radial velocity (e.g. Marcy & Butler 2000; Tinney et al. ⋆ email: [email protected] 2 1258 as of 29th October 2013, see: 1 Herschel is an ESA space observatory with science instruments http://www.minorplanetcenter.net/iau/lists/TNOs.html provided by European-led Principal Investigator consortia and with im- 3 Databases of exoplanet properties are maintained at portant participation from NASA. http://exoplanet.eu and http://exoplanets.org 1 J.P. Marshall, A. Moro-Mart´ın, et al.: Herschel observations of exoplanet host stars 2001; Mayor & Queloz 2012) or transit surveys e.g. CoRoT 2013). On the contrary, Maldonado et al. (2012) identify a trend (Auvergneet al. 2009), WASP (Pollacco et al. 2006), HAT between the presence of a debris disc and cool Jupiter exo- (Bakoset al. 2002) and Kepler (Boruckiet al. 2011). See planet around a star whilst Wyatt et al. (2012) identified a possi- Perryman (2011) for a summary of exoplanet detection tech- ble trend between cool dust and low mass planet host stars. niques. The majority of all exoplanet searches have taken place Direct measurement of the spatial distribution of debris in at optical wavelengths, with a sample focus on mature, Sun- other stellar systems will reveal whether our own EKB is com- like stars as the most suitable candidates for the radial ve- mon or unusual. The EKB and its interaction with the outer plan- locity detection technique. A stars are avoided as their atmo- ets is thought to have played a significant role in the develop- spheres lack the narrow lines necessary for radial velocity de- ment of life on Earth, having supplied a significant fraction of tections through accurate doppler measurements, however these the impactors thought to have been involved in the Late Heavy stars are prime candidates for direct imaging surveys, result- Bombardment (Gomes et al. 2005), and continues to provide ing in the detection of several exoplanet systems around de- bodies for the short period comet population through interaction bris disc host stars, e.g. Fomalhaut (Kalas et al. 2008), HR 8799 with Jupiter (Horner & Jones 2009). It may also have been in- (Marois et al. 2008), β Pic (Lagrange et al. 2010) and HD 95086 volved in the hydration of the Earth, a topic which is still widely (Rameau et al. 2013). M stars have likewise been avoided be- debated (O’Brien et al. 2006; Horner et al. 2009; Bond et al. cause they exhibit high levels of stellar variability and their emis- 2010; Horner & Jones 2010; Izidoro et al. 2013). As such, the sion peaks in the near-infrared, rendering them noisy and faint, nature of exo-EKBs may play a vital role in the determination although great efforts have been made to overcome these is- of the habitability of their host systems (e.g. Horner & Jones sues due to their sheer number and potential to yield low mass 2010), and it is therefore vital that we are able to judge whether planets through either transit or radial velocity detections (e.g. the Solar system’s architecture and EKB are the norm, or un- Reiners et al. 2010; Rodler et al. 2011; Giacobbeet al. 2012; usual. Anglada-Escud´e& Tuomi 2012). This has led to unavoidable The Herschel (Pilbratt et al. 2010) Guaranteed Time (GT) bias in the types of stars around which exoplanets are known debris disc programme and the Open Time Key Programmes to exist. Comparative analysis or aggregation of results from DEBRIS4 (Disc Emission from Bias-free Reconnaissance radial velocity surveys is further complicated due to both the Infrared Survey; Matthews et al. 2010) and DUNES5 (DUst differences in sensitivity and the variable baseline of observa- around NEarby Stars; Eiroa et al. 2010, 2013) have observed tions for the stellar samples, although broad conclusions on e.g. nearby Sun-like stars using the PACS (Photodetector Array the absence of Jupiter analogues around most nearby stars may Camera and Spectrometer, Poglitsch et al. 2010) instrument at be drawn (Cumming et al. 1999; Fischer et al. 2014). Recent far-infrared wavelengths looking for excess emission due to the results from an analysis of microlensing surveys suggest that presence of circumstellar dust discs analogous to the Solar sys- almost all stars may have one or more exoplanets, with low tem’s EKB (Vitense et al.