Nick Siegler

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

SPITZER OBSERVATIONS OF IC 348: THE DISK POPULATION AT 2-3 MILLION YEARS

We present near- and mid-infrared photometry obtained with the Spitzer Space Telescope of ~300 known members of the IC 348 cluster. We merge this photometry with existing ground-based optical and near-infrared photometry in order to construct optical-infrared spectral energy distributions (SEDs) for all the cluster members and present a complete atlas of these SEDs. We employ these observations to investigate both the frequency and nature of the circumstellar disk population in the cluster. The Spitzer observations span a wavelength range between 3.6 and 24 μm, corresponding to disk radii of ~0.1-5 AU from the central star. The observations are sufficiently sensitive to enable the first detailed measurement of the disk frequency for very low mass stars at the peak of the stellar initial mass function. Using measurements of infrared excess between 3.6 and 8.0 μm, we find the total frequency of disk-bearing stars in the cluster to be 50% ± 6%. However, only 30% ± 4% of the member stars are surrounded by optically thick, primordial disks, while the remaining disk-bearing stars are surrounded by what appear to be optically thin, anemic disks. Both these values are below previous estimates for this cluster. The disk fraction appears to be a function of spectral type and stellar mass. The fraction of stars with optically thick disks ranges from 11% ± 8% for stars earlier than K6 to 47% ± 12% for K6-M2 stars to 28% ± 5% for M2-M6 stars. The disk longevity and thus conditions for planet formation appear to be most favorable for the K6-M2 stars, which are objects of comparable mass to the Sun for the age of this cluster. The optically thick disks around later type (>M4) stars appear to be less flared than the disks around earlier type stars. This may indicate a greater degree of dust settling and a more advanced evolutionary state for the late M disk population. Finally, we find that the presence of an optically thick dust disk is correlated with gaseous accretion, as measured by the strength of Hα emission. A large fraction of stars classified as classical T Tauri stars possess robust, optically thick disks, and very few such stars are found to be diskless. The majority (64%) of stars classified as weak-lined T Tauri stars are found to be diskless. However, a significant fraction (12%) of these stars are found to be surrounded by thick, primordial disks. These results suggest that it is more likely for dust disks to persist in the absence of active gaseous accretion than for active accretion to persist in the absence of dusty disks.

Spitzer 24 μm Observations of Open Cluster IC 2391 and Debris Disk Evolution of FGK Stars

We present 24 � m Spitzer MIPS photometric observations of the � 50 Myr open cluster IC 2391. Thirty-four cluster members ranging in spectral type from B3 to M5 were observed in the central square degree of the cluster. Excesses indicative of debris disks were discovered around one A star, six FGK stars, and possibly one M dwarf. For the cluster members observed to their photospheric limit, we find a debris disk frequency of 10 þ17 � 3 % for B‐A stars and 31 þ13 � 9 % for FGK stars using a 15% relative excess threshold. Relative to a model of decaying excess frequency, thefrequencyofdebrisdisksaroundA-typestarsappearsmarginallylowforthecluster’sagewhilethatof FGKstars appears consistent. Scenarios that may qualitatively explain this result are examined. We conclude that planetesimal activity in the terrestrial region of FGK stars is common in thefirst � 50 Myr and decays on timescales of � 100 Myr. Despite luminosity differences, debris disk evolution does not appear to depend strongly on stellar mass. Subject headingg infrared: stars — open clusters and associations: individual (IC 2391) — planetary systems: protoplanetary disks

Discovery of two very low mass binaries: Final results of an adaptive optics survey of nearby M6.0-M7.5 stars

We present updated results of a high-resolution, magnitude-limited (Ks 0.8, ΔKs 20 AU), very low mass (VLM; Mtot < 0.19 M☉) binary systems are rare. The projected semimajor axis distribution of these systems peak at ~5 AU, and we report a sensitivity-corrected binary fraction of 9 % for stars with primaries of spectral type M6.0-M7.5 with separations 3 AU and mass ratios q 0.6. Within these instrumental sensitivities, these results support the overall trend that both the semimajor axis distribution and binary fraction are a function of the mass of the primary star and decrease with decreasing primary mass. These observations provide important constraints for low-mass binary star formation theories.

Identifying Primordial Substructure in NGC 2264

We present new Spitzer Space Telescope observations of the young cluster NGC 2264. Observations at 24 μm with the Multiband Imaging Photometer have enabled us to identify the most highly embedded and youngest objects in NGC 2264. This Letter reports on one particular region of NGC 2264 where bright 24 μm sources are spatially configured in curious linear structures with quasi-uniform separations. The majority of these sources (~60%) are found to be protostellar in nature, with Class I spectral energy distributions. Comparison of their spatial distribution with submillimeter data from Wolf-Chase et al. and millimeter data from Peretto et al. shows a close correlation between the dust filaments and the linear spatial configurations of the protostars, indicating that star formation is occurring primarily within dense, dusty filaments. Finally, the quasi-uniform separations of the protostars are found to be comparable in magnitude to the expected Jeans length, suggesting thermal fragmentation of the dense filamentary material.

Spitzer 24 μm Survey of Debris Disks in the Pleiades

We performed a 24 μm 2° × 1° survey of the Pleiades cluster, using the MIPS instrument on Spitzer. Fifty-four members ranging in spectral type from B8 to K6 show 24 μm fluxes consistent with bare photospheres. All Be stars show excesses attributed to free-free emission in their gaseous envelopes. Five early-type stars and four solar-type stars show excesses indicative of debris disks. We find a debris disk fraction of 25% for B-A members and 10% for F-K3 ones. These fractions appear intermediate between those for younger clusters and for the older field stars. They indicate a decay with age of the frequency of the dust production events inside the planetary zone, with similar timescales for solar-mass stars as have been found previously for A stars.

24 μm Detections of Circum(sub)stellar Disks in IC 348: Grain Growth and Inner Holes?

We present observations of six late-type members of the young cluster IC 348 detected at 24 μm with the Multiband Imaging Photometer for Spitzer (MIPS). At least four of the objects are probably substellar. Combining these data with ground-based optical and near-infrared photometry and complementary observations with the Infrared Array Camera (IRAC), we have modeled the spectral energy distributions using detailed models of irradiated accretion disks. We are able to fit the observations with models using a range of maximum grain sizes from ISM-type dust to grains as large as 1 mm. Two objects show a lack of excess emission at wavelengths shortward of 5.8-8 μm but significant excess at longer wavelengths, indicative of large optically thin or evacuated inner holes. Our models indicate an inner hole of radius ~0.5-0.9 AU for the brown dwarf L316; this is the first brown dwarf with evidence for an AU-scale inner disk hole. We examine several possible mechanisms for the inner disk clearing in this case, including photoevaporation and planet formation.

Discovery of nine new companions to nearby young M stars with the altair ao system

We present results of a high-resolution, near-infrared survey of 41 nearby, young (300 Myr) M0-M5.0 dwarfs using the Altair natural guide star adaptive optics system at the Gemini North telescope. Twelve of the objects appear to be binaries, seven of which are reported here for the first time. One triple system was discovered. Statistical properties are studied and compared with earlier (F to K) and later (≥M6 very low mass [VLM]) populations. We find that the separation distribution of the binaries in this sample peaks at 13 AU, which is consistent with previous measurements of early M binaries. Hence, early M binaries seem to occur in—on average—tighter systems than G binaries. At the same time they are significantly wider than field VLM binary stars. The distribution of mass ratios q of primary and secondary stars was found to show an intermediate distribution between the strongly q → 1 peaked distribution of field VLM systems and the almost flat distribution of earlier type stars. Consequently, we show evidence for relatively young, early M binaries representing a transition between the well-known earlier star distributions and the recently examined field VLM population characteristics. Despite the fact that this survey was dedicated to the search for faint brown dwarf and planetary mass companions, all planetary mass candidates were background objects. We exclude the existence of physical companions with masses greater than 10 Jupiter masses (MJ) at separations of 40 AU and masses greater than 24MJ for separations 10 AU around 37 of the 41 observed objects.

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

SPITZER AND MAGELLAN OBSERVATIONS OF NGC 2264: A REMARKABLE STAR-FORMING CORE NEAR IRS 2

We analyze Spitzer and Magellan observations of a star-forming core near IRS 2 in the young cluster NGC 2264. The submillimeter source IRAS 12 S1, previously believed to be an intermediate-mass Class 0 object is shown to be a dense collection of embedded, low-mass stars. We argue that this group of stars represents the fragmenting collapse of a dense, turbulent core, using a number of indicators of extreme youth. With reasonable estimates for the velocity dispersion in the group, we estimate a dynamical lifetime of only a few times 104 yr. Spectral energy distributions of stars in the core are consistent with Class I or Class 0 assignments. We present observations of an extensive system of molecular hydrogen emission knots. The luminosity of the objects in the core region are consistent with roughly solar mass protostars.