S. M. Lawler

FREQUENCY OF DEBRIS DISKS AROUND SOLAR-TYPE STARS: FIRST RESULTS FROM A SPITZER MIPS SURVEY

We have searched for infrared excesses around a well-defined sample of 69 FGK main-sequence field stars. These starswereselectedwithoutregardto theirage,metallicity,oranypreviousdetectionof IRexcess; they have amedian ageof � 4Gyr.Wehavedetected70 � mexcessesaroundsevenstarsatthe3 � confidencelevel.Thisextraemissionis produced by cool material (<100 K) located beyond 10 AU, well outside the ‘‘habitable zones’’ of these systems and consistent with the presence of Kuiper Belt analogs with � 100 times more emitting surface area than in our own planetary system. Only one star, HD 69830, shows excess emission at 24 � m, corresponding to dust with temperaturesk300Klocatedinsideof1AU.WhiledebrisdiskswithLdust/L? � 10 � 3 arerarearoundoldFGKstars,wefind that thediskfrequencyincreasesfrom2% � 2%forLdust/L? � 10 � 4 to12% � 5%forLdust/L? � 10 � 5 .Thistrendin the disk luminosity distribution is consistent with the estimated dust in our solar system being within an order of magnitude greater or less than the typical level around similar nearby stars. Although there is no correlation of IR excesswithmetallicity orspectraltype,there isaweak correlationwithstellarage,withstarsyoungerthanagigayear more likely to have excess emission.

New Debris Disks Around Nearby Main Sequence Stars: Impact on The Direct Detection of Planets

Using the MIPS instrument on Spitzer, we have searched for infrared excesses around a sample of 82 stars, mostly F, G, and K main-sequence field stars, along with a small number of nearby M stars. These stars were selected for their suitability for future observations by a variety of planet-finding techniques. These observations provide information on the asteroidal and cometary material orbiting these stars, data that can be correlated with any planets that may eventually be found. We have found significant excess 70 μm emission toward 12 stars. Combined with an earlier study, we find an overall 70 μm excess detection rate of 13% ± 3% for mature cool stars. Unlike the trend for planets to be found preferentially toward stars with high metallicity, the incidence of debris disks is uncorrelated with metallicity. By newly identifying four of these stars as having weak 24 μm excesses (fluxes ~10% above the stellar photosphere), we confirm a trend found in earlier studies wherein a weak 24 μm excess is associated with a strong 70 μm excess. Interestingly, we find no evidence for debris disks around 23 stars cooler than K1, a result that is bolstered by a lack of excess around any of the 38 K1-M6 stars in two companion surveys. One motivation for this study is the fact that strong zodiacal emission can make it hard or impossible to detect planets directly with future observatories such as the Terrestrial Planet Finder (TPF). The observations reported here exclude a few stars with very high levels of emission, >1000 times the emission of our zodiacal cloud, from direct planet searches. For the remainder of the sample, we set relatively high limits on dust emission from asteroid belt counterparts.

PLANETS AND DEBRIS DISKS: RESULTS FROM A SPITZER/MIPS SEARCH FOR INFRARED EXCESS

Using the MIPS camera on the Spitzer Space Telescope, we have searched for debris disks around 104 stars known from radial velocity studies to have one or more planets. Combining this new data with 42 already published observations of planet-bearing stars, we find that 14 of the 146 systems have IR excess at 24 and/or 70 μm. Only one star, HD 69830, has IR excess exclusively at 24 μm, indicative of warm dust in the inner system analogous to that produced by collisions in the solar systems asteroid belt. For the other 13 stars with IR excess the emission is stronger at 70 μm, consistent with cool dust (<100 K) located beyond 10 AU, well outside of the orbital location of the known planets. Selection effects inhibit detection of faint disks around the planet-bearing stars (e.g., the stars tend to be more distant), resulting in a lower detection rate for IR excess than in a corresponding control sample of nearby stars not known to have planets (9% ± 3% versus 14% ± 3%). Even taking into account the selection bias, we find that the difference between the dust emission around stars with planets and stars without known planets is not statistically significant.

IRS spectra of solar-type stars: A search for asteroid belt analogs

We report the results of a spectroscopic search for debris disks surrounding 41 nearby solar-type stars, including eight planet-bearing stars, using the Infrared Spectrometer (IRS) on the Spitzer Space Telescope. With the accurate relative photometry of the IRS between 7 and 34 μm we are able to look for excesses as small as ~2% of photospheric levels, with particular sensitivity to weak spectral features. For stars with no excess, the 3 σ upper limit in a band at 30-34 μm corresponds to ~75 times the brightness of our zodiacal dust cloud. Comparable limits at 8.5-13 μm correspond to ~1400 times the brightness of our zodiacal dust cloud. These limits correspond to material located within the <1 to ~5 AU region that, in our solar system, originates predominantly from debris associated with the asteroid belt. We find excess emission longward of ~25 μm from five stars, of which four also show excess emission at 70 μm. This emitting dust must be located in a region starting around 5-10 AU. One star has 70 μm emission but no IRS excess. In this case, the emitting region must begin outside 10 AU; this star has a known radial velocity planet. Only two stars of the five show emission shortward of 25 μm, where spectral features reveal the presence of a population of small, hot dust grains emitting in the 7-20 μm band. One of these stars, HD 72905, is quite young (300 Myr), while the other, HD 69830, is older than 2 Gyr. The data presented here strengthen the results of previous studies to show that excesses at 25 μm and shorter are rare: only 1 out of 40 stars older than 1 Gyr or ~2.5% shows an excess. Asteroid belts 10-30 times more massive than our own appear are rare among mature, solar-type stars.

THE MID-INFRARED SPECTRUM OF THE TRANSITING EXOPLANET HD 209458b

We report the spectroscopic detection of mid-infrared emission from the transiting exoplanet HD 209458b. Using archive data taken with the Spitzer IRS instrument, we have determined the spectrum of HD 209458b between 7.46 and 15.25 μm. We have used two independent methods to determine the planet spectrum, one differential in wavelength and one absolute, and find the results are in good agreement. Over much of this spectral range, the planet spectrum is consistent with featureless thermal emission. Between 7.5 and 8.5 μm, we find evidence for an unidentified spectral feature. If this spectral modulation is due to absorption, it implies that the dayside vertical temperature profile of the planetary atmosphere is not entirely isothermal. Using the IRS data, we have determined the broadband eclipse depth to be 0.00315 ± 0.000315, implying significant redistribution of heat from the dayside to the nightside. This work required the development of improved methods for Spitzer IRS data calibration that increase the achievable absolute calibration precision and dynamic range for observations of bright point sources.

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

This paper describes Herschel observations of the nearby (8.5pc) G5V multi-exoplanet host star 61Vir at 70, 100, 160, 250, 350 and 500m carried out as part of the DEBRIS survey. These observations reveal emission that is significantly extended out to a distance of >15arcsec with a morphology that can be fitted by a nearly edge-on (77° inclination) radially broad (from 30au out to at least 100au) 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 1.4GHz provides potential for confusion, however, the stars 1.4arcsecyr -1 proper motion allows archival Spitzer 70m images to confirm that what we are interpreting as disc emission really is circumstellar. Although the exact shape of the discs inner edge is not well constrained, the region inside 30au must be significantly depleted in planetesimals. This is readily explained if there are additional planets outside those already known (i.e. in the 0.5-30au 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 60G stars, 11 are known to have planets, of which six (including 61Vir) 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 (HD20794) is reported here for the first time. This fraction (4/6) is higher than that expected for nearby field stars (15per 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.

EXPLORATIONS BEYOND THE SNOW LINE: SPITZER/IRS SPECTRA OF DEBRIS DISKS AROUND SOLAR-TYPE STARS

We have observed 152 nearby solar-type stars with the Infrared Spectrometer (IRS) on the Spitzer Space Telescope. Including stars that met our criteria but were observed in other surveys, we get an overall success rate for finding excesses in the long wavelength IRS band (30-34 micron) of 11.8% +/- 2.4%. The success rate for excesses in the short wavelength band (8.5-12 micron) is ~1% including sources from other surveys. For stars with no excess at 8.5-12 microns, the IRS data set 3 sigma limits of around 1,000 times the level of zodiacal emission present in our solar system, while at 30-34 microns set limits of around 100 times the level of our solar system. Two stars (HD 40136 and HD 10647) show weak evidence for spectral features; the excess emission in the other systems is featureless. If the emitting material consists of large (10 micron) grains as implied by the lack of spectral features, we find that these grains are typically located at or beyond the snow line, ~1-35 AU from the host stars, with an average distance of 14 +/- 6 AU; however smaller grains could be located at significantly greater distances from the host stars. These distances correspond to dust temperatures in the range ~50-450 K. Several of the disks are well modeled by a single dust temperature, possibly indicative of a ring-like structure. However, a single dust temperature does not match the data for other disks in the sample, implying a distribution of temperatures within these disks. For most stars with excesses, we detect an excess at both IRS and MIPS wavelengths. Only three stars in this sample show a MIPS 70 micron excess with no IRS excess, implying that very cold dust is rare around solar-type stars.

THE OUTER SOLAR SYSTEM ORIGINS SURVEY. I. DESIGN AND FIRST-QUARTER DISCOVERIES

National Research Council of Canada; National Science and Engineering Research Council of Canada; Academia Sinica Postdoctoral Fellowship

THE DEBRIS DISK AROUND γ DORADUS RESOLVED WITH HERSCHEL

We present observations of the debris disk around gamma Doradus, an F1V star, from the Herschel Key Programme DEBRIS (Disc Emission via Bias-free Reconnaissance in the Infrared/Submillimetre). The disk is well-resolved at 70, 100 and 160 micron, resolved along its major axis at 250 micron, detected but not resolved at 350 micron, and confused with a background source at 500 micron. It is one of our best resolved targets and we find it to have a radially broad dust distribution. The modelling of the resolved images cannot distinguish between two configurations: an arrangement of a warm inner ring at several AU (best-fit 4 AU) and a cool outer belt extending from ~55 to 400 AU or an arrangement of two cool, narrow rings at ~70 AU and ~190 AU. This suggests that any configuration between these two is also possible. Both models have a total fractional luminosity of ~10^{-5} and are consistent with the disk being aligned with the stellar equator. The inner edge of either possible configuration suggests that the most likely region to find planets in this system would be within ~55 AU of the star. A transient event is not needed to explain the warm dusts fractional luminosity.

Observational Signatures of a Massive Distant Planet on the Scattering Disk

The orbital element distribution of trans-Neptunian objects (TNOs) with large pericenters has been suggested to be influenced by the presence of an undetected, large planet at >200 AU from the Sun. To find additional observables caused by this scenario, we here present the first detailed emplacement simulation in the presence of a massive ninth planet on the distant Kuiper Belt. We perform 4 Gyr N-body simulations with the currently known Solar System planetary architecture, plus a 10 Earth mass planet with similar orbital parameters to those suggested by Trujillo & Sheppard (2014) or Batygin & Brown (2016), and 10^5 test particles in an initial planetesimal disk. We find that including a distant superearth-mass planet produces a substantially different orbital distribution for the scattering and detached TNOs, raising the pericenters and inclinations of moderate semimajor axis (50<a<500 AU) objects. We test whether this signature is detectable via a simulator with the observational characteristics of four precisely characterized TNO surveys. We find that the qualitatively very distinct Solar System models that include a ninth planet are essentially observationally indistinguishable from an outer Solar System produced solely by the four giant planets. We also find that the mass of the Kuiper Belts current scattering and detached populations is required to be 3-10 times larger in the presence of an additional planet. We do not find any evidence for clustering of orbital angles in our simulated TNO population. Wide-field, deep surveys targeting inclined high-pericenter objects will be required to distinguish between these different scenarios.