Beth A. Biller

Detection of Nine M8.0-L0.5 Binaries: The Very Low Mass Binary Population and Its Implications for Brown Dwarf and Very Low Mass Star Formation

Use of the highly sensitive Hokupa’a/Gemini curvature wave front sensor has allowed direct adaptive optics (AO) guiding on very low mass (VLM) stars with SpT = M8.0–L0.5. A survey of 39 such objects detected nine VLM binaries (seven of which were discovered for the first time to be binaries). Most of these systems are tight (separation 2:4 mag and consist of a VLM star orbited by a much cooler L7–L8 brown dwarf companion. On the basis of this flux-limited (Ks 20 AU) VLM/brown dwarf binaries may be explained if the binary components were given a significant differential velocity kick. Such a velocity kick is predicted by current ‘‘ ejection ’’ theories, where brown dwarfs are formed because they are ejected from their embryonic minicluster and therefore starved of accretion material. We find that a kick from a close triple or quadruple encounter (imparting a differential kick of � 3k m s � 1 between the members of an escaping binary) could reproduce the observed cutoff in the semimajor axis distribution at � 20 AU. However, the estimated binarity (d5%) produced by such ejection scenarios is below the 15% � 7% observed. Similarly, VLM binaries could be the final hardened binaries produced when a minicluster decays. However, the models of Sterzik & Durisen and Durisen, Sterzik, & Pickett also could not produce a VLM binary fraction of 15% and a G star binary fraction of 57%. The observed VLM binary frequency could possibly be produced by cloud core fragmentation. However, our estimate of a fragmentation-produced VLM binary semimajor axis distribution contains a significant fraction of ‘‘ wide ’’ VLM binaries with a > 20 AU in contrast to observation. In summary, more detailed theoretical work will be needed to explain these interesting results that show VLM binaries to be a significantly different population from more massive M & G dwarf binaries. Subject headings: binaries: general — instrumentation: adaptive optics — stars: evolution — stars: formation — stars: low-mass, brown dwarfs

Direct Imaging of a Cold Jovian Exoplanet in Orbit around the Sun-like Star GJ 504

Several exoplanets have recently been imaged at wide separations of >10?AU from their parent stars. These span a limited range of ages ( 0.5?mag), implying thick cloud covers. Furthermore, substantial model uncertainties exist at these young ages due to the unknown initial conditions at formation, which can lead to an order of magnitude of uncertainty in the modeled planet mass. Here, we report the direct-imaging discovery of a Jovian exoplanet around the Sun-like star GJ 504, detected as part of the SEEDS survey. The system is older than all other known directly imaged planets; as a result, its estimated mass remains in the planetary regime independent of uncertainties related to choices of initial conditions in the exoplanet modeling. Using the most common exoplanet cooling model, and given the system age of 160?Myr, GJ 504b has an estimated mass of 4 Jupiter masses, among the lowest of directly imaged planets. Its projected separation of 43.5?AU exceeds the typical outer boundary of ~30?AU predicted for the core accretion mechanism. GJ 504b is also significantly cooler (510 K) and has a bluer color (J ? H = ?0.23?mag) than previously imaged exoplanets, suggesting a largely cloud-free atmosphere accessible to spectroscopic characterization. Thus, it has the potential of providing novel insights into the origins of giant planets as well as their atmospheric properties.

Chandra Spectra of the Soft X-Ray Diffuse Background

We present an exploratory Chandra ACIS-S3 study of the diffuse component of the cosmic X-ray background (CXB) in the 0.3–7 keV band for four directions at high Galactic latitudes, with emphasis on details of the ACIS instrumental background modeling. Observations of the dark Moon are used to model the detector background. A comparison of the Moon data and the data obtained with ACIS stowed outside the focal area showed that the dark Moon does not emit significantly in our band. Point sources down to 3 � 10 � 16 ergs s � 1 cm � 2 in the 0.5–2 keV band are excluded in our two deepest observations. We estimate the contribution of fainter, undetected sources to be less than 20% of the remaining CXB flux in this band in all four pointings. In the 0.3–1 keV band, the diffuse signal varies strongly from field to field and contributes between 55% and 90% of the total CXB signal. It is dominated by emission lines that can be modeled by a kT ¼ 0:1 0:4 keV plasma. In particular, the two fields located away from bright Galactic features show a prominent line blend at E � 580 eV (O vii+O viii) and a possible line feature at E � 300 eV. The two pointings toward the North Polar Spur exhibit a brighter O blend and additional bright lines at 730–830 eV (Fe xvii). We measure the total 1–2 keV flux of 1:0 1:2 � 0:2 ðÞ �10 � 15 ergs s � 1 cm � 2 arcmin � 2 (mostly resolved) and the 2–7 keV flux of 4:0 4:5 � 1:5 ðÞ �10 � 15 ergs s � 1 cm � 2 arcmin � 2 .A tE > 2 keV, the diffuse emission is consistent with zero, to an accuracy limited by the short Moon exposure and systematic uncertainties of the S3 background. Assuming Galactic or local origin of the line emission, we put an upper limit of � 3 � 10 � 15 ergs s � 1 cm � 2 arcmin � 2 on the 0.3–1 keV extragalactic diffuse flux. Subject headings: intergalactic medium — ISM: general — methods: data analysis — X-rays: diffuse background — X-rays: ISM

A dynamical calibration of the mass-luminosity relation at very low stellar masses and young ages

Mass is the most fundamental parameter of a star, yet it is also one of the most difficult to measure directly. In general, astronomers estimate stellar masses by determining the luminosity and using the ‘mass–luminosity’ relationship, but this relationship has never been accurately calibrated for young, low-mass stars and brown dwarfs. Masses for these low-mass objects are therefore constrained only by theoretical models. A new high-contrast adaptive optics camera enabled the discovery of a young (50 million years) companion only 0.156 arcseconds (2.3 au) from the more luminous (> 120 times brighter) star AB Doradus A. Here we report a dynamical determination of the mass of the newly resolved low-mass companion AB Dor C, whose mass is 0.090 ± 0.005 solar masses. Given its measured 1–2-micrometre luminosity, we have found that the standard mass–luminosity relations overestimate the near-infrared luminosity of such objects by about a factor of ∼2.5 at young ages. The young, cool objects hitherto thought to be substellar in mass are therefore about twice as massive, which means that the frequency of brown dwarfs and planetary mass objects in young stellar clusters has been overestimated.

An imaging survey for extrasolar planets around 45 close, young stars with the simultaneous differential imager at the very large telescope and MMT

Wepresent theresultsof asurveyof 45young(P250Myr), close(P50pc) starswiththeSimultaneous Differential Imager (SDI) implemented at the VLT and the MMT for the direct detection of extrasolar planets. As part of the survey, we observed 54 objects, consisting of 45 close, young stars; two more distant ( 2 � which behaved consistently like a real object. From our survey null result,we can rule out (with 93% confidence) a model planet population where N(a) / constant out to a distance of 45 AU.

The Gemini NICI Planet-Finding Campaign : Discovery of a Substellar L Dwarf Companion to the Nearby Young M Dwarf CD-35 2722

We present the discovery of a wide (67 AU) substellar companion to the nearby (21 pc) young solar-metallicity M1 dwarf CD-35 2722, a member of the ~100 Myr AB Doradus association. Two epochs of astrometry from the NICI Planet-Finding Campaign confirm that CD-35 2722 B is physically associated with the primary star. Near-IR spectra indicate a spectral type of L4\pm1 with a moderately low surface gravity, making it one of the coolest young companions found to date. The absorption lines and near-IR continuum shape of CD-35 2722 B agree especially well the dusty field L4.5 dwarf 2MASS J22244381-0158521, while the near-IR colors and absolute magnitudes match those of the 5 Myr old L4 planetary-mass companion, 1RXS J160929.1-210524 b. Overall, CD-35 2722 B appears to be an intermediate-age benchmark for L-dwarfs, with a less peaked H-band continuum than the youngest objects and near-IR absorption lines comparable to field objects. We fit Ames-Dusty model atmospheres to the near-IR spectra and find T=1700-1900 K and log(g) =4.5\pm0.5. The spectra also show that the radial velocities of components A and B agree to within \pm10 km/s, further confirming their physical association. Using the age and bolometric luminosity of CD-35 2722 B, we derive a mass of 31\pm8 Mjup from the Lyon/Dusty evolutionary models. Altogether, young late-M to mid-L type companions appear to be over-luminous for their near-IR spectral type compared to field objects, in contrast to the under-luminosity of young late-L and early-T dwarfs.

A global cloud map of the nearest known brown dwarf

Brown dwarfs—substellar bodies more massive than planets but not massive enough to initiate the sustained hydrogen fusion that powers self-luminous stars—are born hot and slowly cool as they age. As they cool below about 2,300 kelvin, liquid or crystalline particles composed of calcium aluminates, silicates and iron condense into atmospheric ‘dust’, which disappears at still cooler temperatures (around 1,300 kelvin). Models to explain this dust dispersal include both an abrupt sinking of the entire cloud deck into the deep, unobservable atmosphere and breakup of the cloud into scattered patches (as seen on Jupiter and Saturn). However, hitherto observations of brown dwarfs have been limited to globally integrated measurements, which can reveal surface inhomogeneities but cannot unambiguously resolve surface features. Here we report a two-dimensional map of a brown dwarf’s surface that allows identification of large-scale bright and dark features, indicative of patchy clouds. Monitoring suggests that the characteristic timescale for the evolution of global weather patterns is approximately one day.

Stellar companions to exoplanet host stars: Lucky Imaging of transiting planet hosts

Observed properties of stars and planets in binary/multiple star systems provide clues to planet formation and evolution. We extended our survey for visual stellar companions to the hosts of transiting exoplanets by 21 stars, using the Lucky Imaging technique with the two AstraLux instruments: AstraLux Norte at the Calar Alto 2.2-m telescope, and AstraLux Sur at the ESO 3.5-m New Technology Telescope at La Silla. We present observations of two previously unknown binary candidate companions, to the transiting planet host stars HAT-P-8 and WASP-12, and derive photometric and astrometric properties of the companion candidates. The common proper motions of the previously discovered candidate companions with the exoplanet host stars TrES-4 and WASP-2 are confirmed from follow-up observations. A Bayesian statistical analysis of 31 transiting exoplanet host stars observed with AstraLux suggests that the companion star fraction of planet hosts is not significantly different from that of solar-type field stars, but that the binary separation is on average larger for planet host stars.

Constraints on Extrasolar Planet Populations from VLT NACO/SDI and MMT SDI and Direct Adaptive Optics Imaging Surveys: Giant Planets are Rare at Large Separations

We examine the implications for the distribution of extrasolar planets based on the null results from two of the largest direct imaging surveys published to date. Combining the measured contrast curves from 22 of the stars observed with the VLT NACO adaptive optics system by Masciadri and coworkers and 48 of the stars observed with the VLT NACO SDI and MMT SDI devices by Biller and coworkers (for a total of 60 unique stars), we consider what distributions of planet masses and semimajor axes can be ruled out by these data, based on Monte Carlo simulations of planet populations. We can set the following upper limit with 95% confidence: the fraction of stars with planets with semimajor axis between 20 and 100 AU, and mass above 4 MJup, is 20% or less. Also, with a distribution of planet mass of -->dN/dM M?1.16 in the range of 0.5-13 MJup, we can rule out a power-law distribution for semimajor axis ( -->dN/da a?) with index 0 and upper cutoff of 18 AU, and index -0.5 with an upper cutoff of 48 AU. For the distribution suggested by Cumming et al., a power-law of index ?0.61, we can place an upper limit of 75 AU on the semimajor axis distribution. In general, we find that even null results from direct imaging surveys are very powerful in constraining the distributions of giant planets (0.5-13 MJup) at large separations, but more work needs to be done to close the gap between planets that can be detected by direct imaging, and those to which the radial velocity method is sensitive.

A survey for massive giant planets in debris disks with evacuated inner cavities

The commonality of collisionally replenished debris around main-sequence stars suggests that minor bodies are frequent around Sun-like stars.Whether or not debris disks in general are accompanied by planets is yet unknown, but debris disks with large inner cavities—perhaps dynamically cleared—are considered to be prime candidates for hosting large-separation massive giant planets. We present here a high-contrast VLT/NACO angular differential imaging survey for eight such cold debris disks. We investigated the presence of massive giant planets in the range of orbital radii where the inner edge of the dust debris is expected. Our observations are sensitive to planets and brown dwarfs with masses >3-7 Jupiter mass, depending on the age and distance of the target star. Our observations did not identify any planet candidates. We compare the derived planet mass upper limits to the minimum planet mass required to dynamically clear the inner disks.While we cannot exclude that single giant planets are responsible for clearing out the inner debris disks, our observations constrain the parameter space available for such planets. The nondetection of massive planets in these evacuated debris disks further reinforces the notion that the giant planet population is confined to the inner disk (<15 AU).