arXiv subjects

Ben Zuckerman

Publications and source records attributed to Ben Zuckerman.

33 records · Page 2Linked to original sources

arXiv2016

The International Deep Planet Survey II: The frequency of directly imaged giant exoplanets with stellar mass

Radial velocity and transit methods are effective for the study of short orbital period exoplanets but they hardly probe objects at large separations for which direct imaging can be used. We carried out the international deep planet survey of 292 young nearby stars to search for giant exoplanets and determine their frequency. We developed a pipeline for a uniform processing of all the data that we have recorded with NIRC2/Keck II, NIRI/Gemini North, NICI/Gemini South, and NACO/VLT for 14 years. The pipeline first applies cosmetic corrections and then reduces the speckle intensity to enhance the contrast in the images. The main result of the international deep planet survey is the discovery of the HR 8799 exoplanets. We also detected 59 visual multiple systems including 16 new binary stars and 2 new triple stellar systems, as well as 2,279 point-like sources. We used Monte Carlo simulations and the Bayesian theorem to determine that 1.05[+2.80-0.70]% of stars harbor at least one giant planet between 0.5 and 14M_J and between 20 and 300 AU. This result is obtained assuming uniform distributions of planet masses and semi-major axes. If we consider power law distributions as measured for close-in planets instead, the derived frequency is 2.30[+5.95-1.55]%, recalling the strong impact of assumptions on Monte Carlo output distributions. We also find no evidence that the derived frequency depends on the mass of the hosting star, whereas it does for close-in planets. The international deep planet survey provides a database of confirmed background sources that may be useful for other exoplanet direct imaging surveys. It also puts new constraints on the number of stars with at least one giant planet reducing by a factor of two the frequencies derived by almost all previous works.

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arXiv2015

The bulk composition of exo-planets

Priorities in exo-planet research are rapidly moving from finding planets to characterizing their physical properties. Of key importance is their chemical composition, which feeds back into our understanding of planet formation. For the foreseeable future, far-ultraviolet spectroscopy of white dwarfs accreting planetary debris remains the only way to directly and accurately measure the bulk abundances of exo-planetary bodies. The exploitation of this method is limited by the sensitivity of HST, and significant progress will require a large-aperture space telescope with a high-throughput ultraviolet spectrograph.

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arXiv2013

Young Stars Near Earth: The Octans-Near Association and Castor Moving Group

All cataloged stellar moving groups and associations with ages <100 Myr and within 100 pc of Earth have Galactic space motions (UVW) situated in a "good box" with dimensions ~20 km/s on a side. Torres et al. defined the Octans Association as a group of 15 stars with age "20 Myr?" and located ~140 pc from Earth, but with average V space velocity -3.6 km/s that is well outside of the good box. We present a list of 14 Hipparcos star systems within 100 pc of Earth that we call "Octans-Near"; these systems have UVW similar to those of the much more distant Octans Association. The Octans-Near stars have apparent ages between about 30 and 100 Myr and their relationship to the Octans Association stars is unclear. Six additional star systems have UVW similar to those of Octans-Near stars and likely ages <200 Myr. These six systems include the late-type binary star EQ Peg -- 6.2 pc from Earth with likely age <100 Myr and thus likely to be the nearest known pre-main sequence star system. The UVW of stars in a previously proposed ~200 Myr old Castor moving group are not too dissimilar from the UVW of Octans-Near stars. However, stars in the Castor group -- if it exists at all -- are mostly substantially older than 200 Myr and thus generally can readily be distinguished from the much younger Octans-Near stars.

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arXiv2013

The nearby, young, isolated, dusty star HD 166191

We report an in-depth study of the F8-type star HD 166191, identified in an ongoing survey for stars exhibiting infrared emission above their expected photospheres in the Wide-field Infrared Survey Explorer all-sky catalog. The fractional IR luminosity measured from 3.5 to 70 $μ$m is exceptionally high (L$_{IR}$/L$_{bol}$ $\sim$10%). Near-diffraction limited imaging observations with the T-ReCS Si filter set on the Gemini South telescope and adaptive optics imaging with the NIRC2 Lp filter on the Keck II telescope confirmed that the excess emission coincides with the star. Si-band images show a strong solid-state emission feature at $\sim$10 $μ$m. Theoretical evolutionary isochrones and optical spectroscopic observations indicate a stellar age in the range 10-100 Myr. The large dust mass seen in HD 166191's terrestrial planet zone is indicative of a recent collision between planetary embryos or massive ongoing collisional grinding associated with planet building.

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arXiv2013

Fomalhaut b: Independent Analysis of the Hubble Space Telescope Public Archive Data

The nature and even the existence of a putative planet-mass companion ("Fomalhaut b") to Fomalhaut has been debated since 2008. In the present paper we reanalyze the multi-epoch ACS/STIS/WFC3 Hubble Space Telescope (HST) optical/near infrared images on which the discovery and some other claims were based. We confirm that the HST images do reveal an object in orbit around Fomalhaut but the detailed results from our analysis differ in some ways from previous discussions. In particular, we do not confirm flux variability over a two-year interval at 0.6 microns wavelength and we detect Fomalhaut b for the first time at the short wavelength of 0.43 microns. We find that the HST image of Fomalhaut b at $0.8\,μ$m may be extended beyond the PSF. We cannot determine from our astrometry if Fomalhaut b will cross or not the dust ring. The optical through mid-infrared spectral energy distribution (SED) of Fomalhaut b cannot be explained as due to direct or scattered radiation from a massive planet. We consider two models to explain the SED: (1) a large circumplanetary disk around an unseen planet and (2) the aftermath of a collision during the past 50-150 years of two Kuiper Belt-like objects of radii 50 km.

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arXiv2012

Identifying Nearby, Young, Late-type Stars by Means of Their Circumstellar Disks

It has recently been shown that a significant fraction of late-type members of nearby, very young associations (age <10 Myr) display excess emission at mid-IR wavelengths indicative of dusty circumstellar disks. We demonstrate that the detection of mid-IR excess emission can be utilized to identify new nearby, young, late-type stars including two definite new members ("TWA 33" and "TWA 34") of the TW Hydrae Association. Both new TWA members display mid-IR excess emission in the Wide-field Infrared Survey Explorer (WISE) catalog and they show proper motion and youthful spectroscopic characteristics -- namely Hαemission, strong lithium absorption, and low surface gravity features consistent with known TWA members. We also detect mid-IR excess -- the first unambiguous evidence of a dusty circumstellar disk -- around a previously identified UV-bright, young, accreting star (2M1337) that is a likely member of the Lower-Centaurus Crux region of the Scorpius Centaurus Complex.

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arXiv2012

New Members of the Scorpius Centaurus Complex and Ages of its sub-regions

We have spectroscopically identified about 100 G-, K- and M-type members of the Scorpius Centaurus complex. To deduce the age of these young stars we compare their Li $λ$6708 absorption line strengths against those of stars in the TW Hydrae association and $β$ Pictoris moving group. These line strengths indicate that Sco-Cen stars are younger than $β$ Pic stars whose ages of ~12 Myr have previously been derived from a kinematic traceback analysis. Our derived age,~10Myr, for stars in the LCC and UCL subgroups of ScoCen is younger than previously published ages based on the moving cluster method and upper main sequence fitting. The discrepant ages are likely due to an incorrect (or lack of) cross-calibration between model-dependent and model-independent age-dating methods.

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arXiv2011

Hunting the Coolest Dwarfs: Methods and Early Results

We present the methods and first results of a survey of nearby high proper motion main sequence stars to probe for cool companions with the Gemini camera at Lick Observatory. This survey uses a sample of old (age > 2 Gyr) stars as targets to probe for companions down to temperatures of 500 K. Multi-epoch observations allow us to discriminate comoving companions from background objects. So far, our survey successfully re-discovers the wide T8.5 companion to GJ 1263 and discovers a companion to the nearby M0V star GJ 660.1. The companion to GJ 660.1 (GJ 660.1B) is ~4 magnitudes fainter than its host star in the J-band and is located at a projected separation of ~120AU. Known trigonometric parallax and 2MASS magnitudes for the GJ 660.1 system indicate a spectral type for the companion of M9 +/- 2.

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arXiv2011

The Ultra Cool Brown Dwarf Companion of WD 0806-661: Age, Mass, and Formation Mechanism

We have combined multi-epoch images from the Infrared Side Port Imager on the CTIO 4-meter telescope to derive a 3-sigma limit of J=21.7 for the ultra cool brown dwarf companion to WD 0806-661 (GJ 3483). We find that J-[4.5]>4.95, redder than any other brown dwarf known to date. With theoretical evolutionary models and ages 1.5-2.7 Gyr, we estimate the brown dwarf companion to have mass <10-13 Jupiter masses and temperature <400 K, providing evidence that this is among the coolest brown dwarfs currently known. The range of masses for this object is consistent with that anticipated from Jeans-mass fragmentation and we present this as the likely formation mechanism. However, we find that substellar companions of similar mass (~7-17 Jupiter masses) are distributed over a wide range of semi-major axes, which suggests that giant planet and low-mass brown dwarf formation overlap in this mass range.

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arXiv2010

Resolving debris discs in the far-infrared: early highlights from the DEBRIS survey

We present results from the earliest observations of DEBRIS, a Herschel Key Programme to conduct a volume- and flux-limited survey for debris discs in A-type through M-type stars. PACS images (from chop/nod or scan-mode observations) at 100 and 160 micron are presented toward two A-type stars and one F-type star: beta Leo, beta UMa and eta Corvi. All three stars are known disc hosts. Herschel spatially resolves the dust emission around all three stars (marginally, in the case of beta UMa), providing new information about discs as close as 11 pc with sizes comparable to that of the Solar System. We have combined these data with existing flux density measurements of the discs to refine the SEDs and derive estimates of the fractional luminosities, temperatures and radii of the discs.

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arXiv2008

The Minimum Jeans Mass, Brown Dwarf Companion IMF, and Predictions for Detection of Y-type Dwarfs

Cool L- and T-type objects were discovered first as companions to stars in 1988 and 1995, respectively. A certain example of the yet cooler Y-type spectral class (Teff <~ 500K?) has not been seen. Recent infrared imaging observations of stars and brown dwarfs indicate that substellar companions with large semi-major axes and with masses less than the brown dwarf/giant planet dividing line (~13.5 Mj) are rare. Theoretical considerations of Jeans mass fragmentation of molecular clouds are consistent with this minimum mass cutoff and also with the semi-major axis (hundreds of AU) characteristic of the lowest mass imaged companions. As a consequence, Y-class companions with large semi-major axes should be scarce around stars <2Gyr old, and also around substellar primaries of all ages. By focusing on brown dwarf companions to young stellar primaries, it is possible to derive a first estimate of the brown dwarf IMF over the entire range of brown dwarf masses (13 Mj to 79 Mj) -- the number of companion brown dwarfs is proportional to mass to the -1.2+-0.2 power.

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arXiv2007

The Wide Brown Dwarf Binary Oph 1622-2405 and Discovery of A Wide, Low Mass Binary in Ophiuchus (Oph 1623-2402): A New Class of Young Evaporating Wide Binaries?

We imaged five objects near the star forming clouds of Ophiuchus with the Keck Laser Guide Star AO system. We resolved Allers et al. (2006)'s #11 (Oph 16222-2405) and #16 (Oph 16233-2402) into binary systems. The #11 object is resolved into a 243 AU binary, the widest known for a very low mass (VLM) binary. The binary nature of #11 was discovered first by Allers (2005) and independently here during which we obtained the first spatially resolved R~2000 near-infrared (J & K) spectra, mid-IR photometry, and orbital motion estimates. We estimate for 11A and 11B gravities (log(g)>3.75), ages (5+/-2 Myr), luminosities (log(L/Lsun)=-2.77+/-0.10 and -2.96+/-0.10), and temperatures (Teff=2375+/-175 and 2175+/-175 K). We find self-consistent DUSTY evolutionary model (Chabrier et al. 2000) masses of 17+4-5 MJup and 14+6-5 MJup, for 11A and 11B respectively. Our masses are higher than those previously reported (13-15 MJup and 7-8 MJup) by Jayawardhana & Ivanov (2006b). Hence, we find the system is unlikely a ``planetary mass binary'', (in agreement with Luhman et al. 2007) but it has the second lowest mass and lowest binding energy of any known binary. Oph #11 and Oph #16 belong to a newly recognized population of wide (>100 AU), young (<10 Myr), roughly equal mass, VLM stellar and brown dwarf binaries. We deduce that ~6+/-3% of young (<10 Myr) VLM objects are in such wide systems. However, only 0.3+/-0.1% of old field VLM objects are found in such wide systems. Thus, young, wide, VLM binary populations may be evaporating, due to stellar encounters in their natal clusters, leading to a field population depleted in wide VLM systems.

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arXiv2002

A Keck Adaptive Optics Search for Young Extrasolar Planets

Adaptive optics, large primary mirrors, and careful selection of target stars are the keys to ground-based imaging of extrasolar planets. Our near-IR survey is capable of identifying exoplanets of 1-10 M${_J}$ within 100 AU of young (t$<$60 Myr), nearby (d$<$60 pc) stars. For very young and proximate targets such as GJ 803 (12 Myr, 10 pc) we are able to detect exoplanets with parameters approaching those of planets our solar system (1 M${_J}$ at 19 AU, 2 M${_J}$ at 9 AU; 5 M${_J}$ at 5 AU). We have thus far imaged over 100 stars with Keck AO. Here we report on our progress, discuss specific observing strategies, and present detailed sensitivity limits.

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arXiv2001

Keck adaptive optics observations of TW Hydrae Association members

Adaptive optics (AO) on 8-10 m telescopes is an enormously powerful tool for studying young nearby stars. It is especially useful for searching for companions. Using AO on the 10-m W.M. Keck II telescope we have measured the position of the brown dwarf companion to TWA5 and resolved the primary into an 0.055 arcsecond double. Over the next several years follow-up astrometry should permit an accurate determination of the masses of these young stars. We have also re-observed the candidate extrasolar planet TWA6B, but measurements of its motion relative to TWA6A are inconclusive. We are carrying out a search for new planetary or brown dwarf companions to TWA stars and, if current giant planet models are correct, are currently capable of detecting a 1 Jupiter-mass companion at ~1 arcsecond and a 5 Jupiter-mass companion at ~0.5 arcsecon around a typical TWA member.

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arXiv2000

Extent of Excess Far Infrared Emission around Luminosity Class III Stars

With the Infrared Space Observatory, we conducted 3x3-pixel imaging photometry of twelve luminosity class III stars, which were previously presumed to have dust particles around them, at far infrared wavelengths (60 and 90 um). Eleven out of twelve targets show a peak of excess (above photosphere) far infrared emission at the location of the star, implying that the dust particles are truly associated with stars. To estimate the size of the excess emission source, the flux ratio of center to boundary pixels of the 3x3 array was examined. The radius of the dust emission is found to be ~3000 to ~10000 AU for a thin shell distribution, and ~5000 to ~25000 AU for a uniform distribution. We consider three models for the origin of the dust: disintegration of comets, sporadic dust ejection from the star, and emission from nearby interstellar cirrus. The data seem to rule out the first model (as far as the "Kuiper--belt" like particles are assumed to be large blackbody grains), but do not enable us to choose between the other two models.

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