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 km emission toward 12 stars. Combined with an earlier study, we find an overall 70 km excess detection rate of 13% c 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 km excesses (fluxes 610% above the stellar photosphere), we confirm a trend found in earlier studies wherein a weak 24 km excess is associated with a strong 70 km 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.
The Hunt for Observable Signatures of Terrestrial Systems survey searches for dust near the habitable zones (HZs) around nearby, bright main-sequence stars. We use nulling interferometry in the N ...band to suppress the bright stellar light and to probe for low levels of HZ dust around the 30 stars observed so far. Our overall detection rate is 18%, including four new detections, among which are the first three around Sun-like stars and the first two around stars without any previously known circumstellar dust. The inferred occurrence rates are comparable for early-type and Sun-like stars, but decrease from % for stars with previously detected cold dust to % for stars without such excess, confirming earlier results at higher sensitivity. For completed observations on individual stars, our sensitivity is five to ten times better than previous results. Assuming a lognormal excess luminosity function, we put upper limits on the median HZ dust level of 13 zodis (95% confidence) for a sample of stars without cold dust and of 26 zodis when focusing on Sun-like stars without cold dust. However, our data suggest that a more complex luminosity function may be more appropriate. For stars without detectable Large Binocular Telescope Interferometer (LBTI) excess, our upper limits are almost reduced by a factor of two, demonstrating the strength of LBTI target vetting for future exo-Earth imaging missions. Our statistics are limited so far, and extending the survey is critical to informing the design of future exo-Earth imaging surveys.
Inference of demographic history from genetic data is a primary goal of population genetics of model and nonmodel organisms. Whole genome-based approaches such as the pairwise/multiple sequentially ...Markovian coalescent methods use genomic data from one to four individuals to infer the demographic history of an entire population, while site frequency spectrum (SFS)-based methods use the distribution of allele frequencies in a sample to reconstruct the same historical events. Although both methods are extensively used in empirical studies and perform well on data simulated under simple models, there have been only limited comparisons of them in more complex and realistic settings. Here we use published demographic models based on data from three human populations (Yoruba, descendants of northwest-Europeans, and Han Chinese) as an empirical test case to study the behavior of both inference procedures. We find that several of the demographic histories inferred by the whole genome-based methods do not predict the genome-wide distribution of heterozygosity, nor do they predict the empirical SFS. However, using simulated data, we also find that the whole genome methods can reconstruct the complex demographic models inferred by SFS-based methods, suggesting that the discordant patterns of genetic variation are not attributable to a lack of statistical power, but may reflect unmodeled complexities in the underlying demography. More generally, our findings indicate that demographic inference from a small number of genomes, routine in genomic studies of nonmodel organisms, should be interpreted cautiously, as these models cannot recapitulate other summaries of the data.
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 mu m. 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 mu m, 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 plus or minus 0.000315, implying significant redistribution of heat from the dayside to the nlghtside. 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.
This paper confronts a simple analytical model for the steady state evolution of debris disks due to collisions with Spitzer observations of dust around main-sequence A stars. It is assumed that ...every star has a planetesimal belt, the initial mass and radius of which are drawn from distributions. In the model disk mass is constant until the largest planetesimals reach collisional equilibrium, whereupon mass falls unk. We find that the detection statistics and trends seen at 24 and 70 mu m can be fitted well by the model. While there is no need to invoke stochastic evolution or delayed stirring to explain the statistics, a moderate rate of stochastic events is not ruled out. Potentially anomalous systems are identified by a high dust luminosity compared with the maximum permissible in the model (HD 3003, HD 38678, HD 115892, HD 172555); their planetesimals may have unusual properties (high strength or low eccentricity), or this dust could be transient. The overall success of our model, which assumes planetesimals in all belts have the same strength, eccentricity, and maximum size, suggests the outcome of planet formation is reasonably uniform. The distribution of planetesimal belt radii, once corrected for detection bias, follows N(r) proportional to r- super(0.8 plus or minus 0.3) for 3-120 AU. Since belt boundaries may be attributed to unseen planets, this provides a unique constraint on A star planetary systems. It is also shown that P-R drag may sculpt the inner edges of A star disks close to the Spitzer detection threshold (HD 2262, HD 19356, HD 106591, HD 115892). This model can be readily applied to the interpretation of future surveys, and predictions for the upcoming SCUBA-2 survey include that 17% of A star disks should be detectable at 850 mu m.
We observed 69 A3-F8 main-sequence binary star systems using the Multiband Imaging Photometer for Spitzer on board the Spitzer Space Telescope. We find emission significantly in excess of predicted ...photospheric flux levels for 9 super(+4-3)% and 40 super(+7-6)% of these systems at 24 and 70 km, respectively. Twenty-two systems total have excess emission, including four systems that show excess emission at both wavelengths. A very large fraction (nearly 60%) of observed binary systems with small (<3 AU) separations have excess thermal emission. We interpret the observed infrared excesses as thermal emission from dust produced by collisions in planetesimal belts. The incidence of debris disks around main-sequence A3-F8 binaries is marginally higher than that for single old AFGK stars. Whatever combination of nature (birth conditions of binary systems) and nurture (interactions between the two stars) drives the evolution of debris disks in binary systems, it is clear that planetesimal formation is not inhibited to any great degree. We model these dust disks through fitting the spectral energy distributions and derive typical dust temperatures in the range 100-200 K and typical fractional luminosities around 10 super(-5), with both parameters similar to other Spitzer-discovered debris disks. Our calculated dust temperatures suggest that about half the excesses we observe are derived from circumbinary planetesimal belts and around one-third of the excesses clearly suggest circumstellar material. Three systems with excesses have dust in dynamically unstable regions, and we discuss possible scenarios for the origin of this short-lived dust.
Genome sequence data are now being routinely obtained from many nonmodel organisms. These data contain a wealth of information about the demographic history of the populations from which they ...originate. Many sophisticated statistical inference procedures have been developed to infer the demographic history of populations from this type of genomic data. In this review, we discuss the different statistical methods available for inference of demography, providing an overview of the underlying theory and logic behind each approach. We also discuss the types of data required and the pros and cons of each method. We then discuss how these methods have been applied to a variety of nonmodel organisms. We conclude by presenting some recommendations for researchers looking to use genomic data to infer demographic history.
Twentieth century industrial whaling pushed several species to the brink of extinction, with fin whales being the most impacted. However, a small, resident population in the Gulf of California was ...not targeted by whaling. Here, we analyzed 50 whole-genomes from the Eastern North Pacific (ENP) and Gulf of California (GOC) fin whale populations to investigate their demographic history and the genomic effects of natural and human-induced bottlenecks. We show that the two populations diverged ~16,000 years ago, after which the ENP population expanded and then suffered a 99% reduction in effective size during the whaling period. In contrast, the GOC population remained small and isolated, receiving less than one migrant per generation. However, this low level of migration has been crucial for maintaining its viability. Our study exposes the severity of whaling, emphasizes the importance of migration, and demonstrates the use of genome-based analyses and simulations to inform conservation strategies.
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 Delta *mm) of 11.8% +/- 2.4%. The success rate for excesses in the short-wavelength band (8.5-12 Delta *mm) is ~1% including sources from other surveys. For stars with no excess at 8.5-12 Delta *mm, the IRS data set 3 Delta *s limits of around 1000 times the level of zodiacal emission present in our solar system, while at 30-34 Delta *mm data 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 Delta *mm) 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 Multiband Imaging Photometer for Spitzer (MIPS) wavelengths. Only three stars in this sample show a MIPS 70 Delta *mm excess with no IRS excess, implying that very cold dust is rare around solar-type stars.
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