Herschel imaging of 61 Vir: implications for the prevalence of debris in low-mass planetary systems

Abstract

This paper describes Herschel observations of the nearby (8.5 pc) G5V multi‐exoplanet host star 61 Vir at 70, 100, 160, 250, 350 and 500 m carried out as part of the DEBRIS survey. These observations reveal emission that is significantly extended out to a distance of >15 arcsec with a morphology that can be fitted by a nearly edge‐on (77° inclination) radially broad (from 30 au out to at least 100 au) debris disc of fractional luminosity 2.7 × 10−5, with two additional (presumably unrelated) sources nearby that become more prominent at longer wavelengths. Chance alignment with a background object seen at 1.4 GHz provides potential for confusion, however, the star’s 1.4 arcsec yr−1 proper motion allows archival Spitzer 70 m images to confirm that what we are interpreting as disc emission really is circumstellar. Although the exact shape of the disc’s inner edge is not well constrained, the region inside 30 au must be significantly depleted in planetesimals. This is readily explained if there are additional planets outside those already known (i.e. in the 0.5–30 au region), but is also consistent with collisional erosion. We also find tentative evidence that the presence of detectable debris around nearby stars correlates with the presence of the lowest mass planets that are detectable in current radial velocity surveys. Out of an unbiased sample of the nearest 60 G stars, 11 are known to have planets, of which six (including 61 Vir) have planets that are all less massive than Saturn, and four of these have evidence for debris. The debris towards one of these planet hosts (HD 20794) is reported here for the first time. This fraction (4/6) is higher than that expected for nearby field stars (15 per cent), and implies that systems that form low‐mass planets are also able to retain bright debris discs. We suggest that this correlation could arise because such planetary systems are dynamically stable and include regions that are populated with planetesimals in the formation process where the planetesimals can remain unperturbed over Gyr time‐scales.

A DEBRIS disk around the planet hosting M-star GJ 581 spatially resolved withHerschel

“…Hence, using these three stars as a sample, the outcome is one disk for one low mass planet system (1/1) and no disk for two high mass planet systems (0/2). Although this is small number statistics, we note that it is suggestive that the correlation between low-mass planets and debris disks recently found for G-stars by Wyatt et al (2012) also applies to M-stars. It is also intriguing that the only debris disk confidently detected in our current analysis surrounds the one star in the sample that hosts low-mass planets.…”

“…This is about three times the black body equilibrium temperature for the dust around this low luminosity M3-type star. Values of f T larger than unity have also been found for the debris disks around the G-type star 61 Vir (Wyatt et al 2012) and several A-type stars (Booth et al 2012). This is akin to disks resolved in scattered light which tend to be more extended than their sizes estimated from blackbody SED (Rodriguez & Zuckerman 2012).…”

“…For selected targets, complementary Herschel observations at 70, 250, 350, 500 μm were also conducted. The first results of this program have already shown that these observations can detect disks down to much fainter levels than previously achieved, and moreover can spatially resolve debris disks at far-IR wavelengths (Matthews et al 2010;Churcher et al 2011;Kennedy et al 2012a,b;Wyatt et al 2012;Booth et al 2012;Broekhoven-Fiene et al 2012).…”

“…Recently, Wyatt et al (2012) have found evidence of the prevalence of debris disks in low-mass planetary systems (also Moro-Martín et al, in prep. ) and suggest that this correlation could arise because such planetary systems are dynamically stable over Gyr timescales.…”

The Gemini NICI Planet-Finding Campaign: The Frequency of Giant Planets around Young B and A Stars

“…Bryden et al (2009) found no significant correlation between presence of RV planets (mainly giant planets) and debris disks. However, studies of hosts of lower-mass RV planets (less than the mass of Saturn) by Wyatt et al (2012) and hosts of Kepler planet candidates by Lawler & Gladman (2012) suggest that stars hosting lower-mass planets are more likely to also host debris disks. High-mass stars are intrinsically brighter than later-type stars, so a faint planet is much harder to directly detect around an A star than an M star.…”