Melanie Freed

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

AN ADAPTIVE OPTICS SURVEY OF M6.0-M7.5 STARS: DISCOVERY OF THREE VERY LOW MASS BINARY SYSTEMS INCLUDING TWO PROBABLE HYADES MEMBERS

A survey of 30 nearby M6.0–M7.5 dwarfs with Ks 12 and 0>29 (3–10 AU) with similar mass ratios (q > 0:8, DKs 20 AU), very low mass (Mtot 3 AU. This binary frequency is less than the � 32% measured among early M dwarfs over the same separation range. Two of the low-mass binaries are probable Hyades open cluster members based on proper motions, cluster membership probabilities, radial velocities, and near-IR photometry. LP 415-20 has the distinction of being the tightest (3.6 AU) multiple system ever spatially resolved in the cluster, and the companions of LP 415-20 and LP 475-855 are among the least massive objects ever resolved in the Hyades, with estimated masses of 0:081 þ0:009 � 0:010 and 0:082 þ0:009 � 0:009 M� . Subject headings: binaries: general — instrumentation: adaptive optics — open clusters and associations: individual (Hyades) — stars: individual (LP 415-20, LP 475-855, 2MASSW J1750129+442404) — stars: low-mass, brown dwarfs

MMT/AO 5 μm Imaging Constraints on the Existence of Giant Planets Orbiting Fomalhaut at ~13-40 AU

A candidate3MJup extrasolar planet was recently imaged by Kalas et al. usingHubble Space Telescope/Advanced Camera for Surveys and Keck II at 12. �� 7 (96 AU) separation from the nearby (d = 7.7 pc) young (∼200 Myr) A2V star Fomalhaut. Here, we report results from M-band (4.8 μm) imaging of Fomalhaut on 2006 December 5 using the Clio IR imager on the 6.5 m MMT with the adaptive secondary mirror. Our images are sensitive to giant planets at orbital radii comparable to the outer solar system (∼10–40 AU). Comparing our 5σ M-band photometric limits to theoretical evolutionary tracks for substellar objects, our results rule out the existence of planets with masses >2 MJup from ∼13 to 40 AU and objects >13 MJup from ∼ 8t o 40 AU.

Guiding on the edge (V~19): results from an AO survey of very low mass stars searching for extremely faint companions

Use of a highly sensitive curvature wavefront sensor (WFS) with avalanche photodiode (APD) detectors at the Gemini North telescope has allowed direct AO guiding on very low mass stars (V=18-20, I=14-16) producing 0.1” images in K’. This resolution (which is near to the 0.07” diffraction limit) has enabled us to conduct ground-based searches for substellar companions (<0.075 M) to within ~3 AU of their primary stars. We have been able to use the faint science targets themselves to close the AO loop. The key to this capability is the zero readout noise, photon-counting APDs used in the curvature WFS. The amplification of incident photons from low luminosity sources allows sufficient signal at the subapertures to reconstruct the aberrated wavefronts with relatively little reconstruction error. This is advantageous compared to typical Shack-Hartmann WFSs which require greater signal from their guide stars to compensate for their CCDs’ readout noise. The technique presented here is currently the only working one from the ground studying such cool and faint targets. Additionally, we report the discovery of 3 new binary stellar systems from a survey of 30 low mass stars (~0.095 M, spectral type M6.0-M7.5, V=18-20) at separations between 0.12-0.29” (3.5-8.3 AU) using this technique.

Klio: A 5‐micron Camera for the Detection of Giant Exoplanets

We plan to take advantage of the unprecedented combination of low thermal background and high resolution provided by the 6.5m MMT’s adaptive secondary mirror, to target the 3–5 micron atmospheric window where giant planets are expected to be anomalously bright. We are in the process of building a 3–5 micron coronograph that is predicted to be sensitive to planets as close as 0.4 arcsec to the parent star. We expect to be able to detect giant planets down to 5 times Jupiter’s mass for a 1 Gyr old system at 10 pc. We plan to carry out a survey which is complementary to the radial velocity detections of planets and constructed to characterize the prevalence and distribution of giant planets around nearby, Sun‐like stars.

MEDI: An instrument for direct-detection of massive extrasolar planets

We have developed an instrument, MEDI (Massive Exoplanet Differential Imager), that takes advantage of a unique method of starlight rejection, simultaneous differential imaging, in order to directly image massive planets around nearby stars. Using this technique we expect to achieve suppression of starlight to the photon-noise limit, which means that increased exposure time will translate into higher sensitivities. This is in contrast to past sequential and two-color simultaneous studies that reach a sensitivity floor due to speckle-noise limitations. MEDI is currently installed in ARIES, the infrared camera that will be commissioned at the newly refurbished 6.5 MMT in January 2003, with the world’s first adaptive secondary. This should allow us to take Nyquist sampled, diffraction-limited images in the near-IR. The adaptive secondary will also give us unprecedented throughput while minimizing the thermal background and providing a smooth PSF. Based on lab results, we expect to be able to detect objects 106 times fainter than their primaries at 0.5” separations in 2 hours, limited only by photon noise. This suggests that we will be sensitive to objects with masses as small as ~5 MJupiter at separations of greater than ~5 AU for G2 V stars that are ~300 Myr old and within about 10 pc. Therefore, we will probe a unique search space compared with current radial velocity methods, which are so far restricted to close-in (<6 AU) orbits.