arXiv subjects

G. Schneider

Publications and source records attributed to G. Schneider.

At least 55 records · Page 3Linked to original sources

arXiv2006

HST NICMOS Imaging of the Planetary-mass Companion to the Young Brown Dwarf 2MASS J1207334-393254

Multi-band (0.9 to 1.6 um) images of the TW Hydrae Association (TWA) brown dwarf, 2MASS J1207334-393254 (also known as 2M1207), and its candidate planetary mass companion (2M1207b) were obtained on 2004 Aug 28 and 2005 Apr 26 with HST/NICMOS. The images from these two epochs unequivocally confirm the two objects as a common proper motion pair (16.0 sigma confidence). A new measurement of the proper motion of 2M1207 implies a distance to the system of 59+-7 pc and a projected separation of 46+-5 AU. The NICMOS and previously published VLT photometry of 2M1207b, extending overall from 0.9 to 3.8 um, are fully consistent with an object of a few Jupiter masses at the canonical age of a TWA member (~8 Myr) based on evolutionary models of young giant planets. These observations provide information on the physical nature of 2M1207b and unambiguously establish that the first direct image of a planetary mass companion in orbit around a self-luminous body, other than our Sun, has been secured.

astro-ph↗

arXiv2006

Polarimetry with NICMOS

NICMOS cameras 1 and 2 each carry a set of three polarizing elements to provide high sensitivity observations of linearly polarized light. The polarizers are bandpass limited and provide diffraction-limited imaging in camera 1 at 0.8 - 1.3um, and in camera 2 at 1.9-2.1um. The NICMOS design specified the intra-camera primary axis angles of the polarizers to be differentially offset by 120 degree, and with identical polarizing efficiency and transmittance. While this ideal concept was not strictly achieved, accurate polarimetry in both cameras, over their full (11" and ~19.2" square) fields of view was enabled through ground and on-orbit calibration of the as-built and HST-integrated systems. The Cycle 7 & 7N calibration program enabled and demonstrated excellent imaging polarimetric performance with uncertainties in measured polarization fractions <=1%. After the installation of the NICMOS Cooling System (NCS), the polarimetric calibration was re-established in Cycle 11, resulting in systemic performance comparable to (or better than) Cycle 7 & 7N. The NCS era NICMOS performance inspired the development of an earlier conceived, but non-implemented, observing mode combining high contrast coronagraphic imaging and polarimetry in camera 2. We successfully executed a program to calibrate and commission the "Coronagraphic Polarimetry'' mode in NICMOS in Cycle 13, and the mode was made available for GO use in Cycle 14. We discuss the data reduction and calibration of direct and coronagraphic NICMOS polarimetry. Importantly, NICMOS coronagraphic polarimetry provides unique access to polarized light near bright targets over a range of spatial scales intermediate between direct polarimetry and ground-based (coronagraphic) polarimetry using adaptive optics.

astro-ph↗

arXiv2005

The Effect of the Transit of Venus on ACRIM's Total Solar Irradiance Measurements: Implications for Transit Studies of Extrasolar Planets

We used the 8 June 2004 transit of Venus (ToV) as a surrogate to test observing methods, strategies and techniques that are being contemplated for future space missions to detect and characterize extrasolar terrestrial planets (ETPs) as they transit their host stars, notably NASA's Kepler mission planned for 2008. As an analog to "Kepler-like" photometric transit observations, we obtained (spatially unresolved) radiometric observations with the ACRIM 3 instrument on ACRIMSAT to follow the effect of the ToV on the total solar irradiance (TSI). Contemporaneous high-resolution broadband imagery with NASA's TRACE spacecraft provided, directly, measures of the stellar (solar) astrophysical noise that can intrinsically limit such transit observations. During the ~ 5.5 h transit, the planet's angular diameter was approximately 1/32 the solar diameter, thus covering ~ 0.1 of the stellar surface. With our ACRIM 3 data, we measure temporal changes in TSI with a 1 sigma per sample (unbinned) uncertainty of approximately 100 mW m^-2 (0.007%). A diminution in TSI of ~ 1.4 W m^-2 (~ 0.1%, closely corresponding to the geometrically occulted area of the photosphere) was measured at mid-transit compared with a mean pre/post transit TSI of ~ 1365.9 W m^-2. These observations serve as a surrogate to future photometric observations of ETPs such as Kepler will deliver. Detailed analysis of the ToV, a rare event within our own solar system, with time-resolved radiometry augmented with high-resolution imagery provides a useful analogue for investigating the detectability and characterization of ETPs from observations that are anticipated in the near future.

astro-ph↗

arXiv2005

An Infrared Coronagraphic Survey for Substellar Companions

We have used the F160W filter (1.4-1.8 um) and the coronagraph on the Near-InfraRed Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope (HST) to survey 45 single stars with a median age of 0.15 Gyr, an average distance of 30 pc, and an average H-magnitude of 7 mag. For the median age we were capable of detecting a 30 M_Jup companion at separations between 15 and 200 AU. A 5 M_Jup object could have been detected at 30 AU around 36% of our primaries. For several of our targets that were less than 30 Myr old, the lower mass limit was as low as a Jupiter mass, well into the high mass planet region. Results of the entire survey include the proper motion verification of five low-mass stellar companions, two brown dwarfs (HR7329B and TWA5B) and one possible brown dwarf binary (Gl 577B/C).

astro-ph↗

arXiv2004

The Core of NGC 6240 from Keck Adaptive Optics and HST NICMOS Observations

We present results of near infrared imaging of the disk-galaxy-merger NGC 6240 using adaptive optics on the Keck II Telescope and reprocessed archival data from NICMOS on the Hubble Space Telescope. Both the North and South nuclei of NGC 6240 are clearly elongated, with considerable sub-structure within each nucleus. In K' band there are at least two point-sources within the North nucleus; we tentatively identify the south-western point-source within the North nucleus as the position of one of the two AGNs. Within the South nucleus, the northern sub-nucleus is more highly reddened. Based upon the nuclear separation measured at 5 GHz, we suggest that the AGN in the South nucleus is still enshrouded in dust at K' band, and is located slightly to the north of the brightest point in K' band. Within the South nucleus there is strong H2 1-0 S(1) line emission from the northern sub-nucleus, contrary to the conclusions of previous seeing-limited observations. Narrowband H2 emission-line images show that a streamer or ribbon of excited molecular hydrogen connects the North and South nuclei. We suggest that this linear feature corresponds to a bridge of gas connecting the two nuclei, as seen in computer simulations of mergers. Many point-like regions are seen around the two nuclei. These are most prominent at 1.1 microns with NICMOS, and in K'-band with Keck adaptive optics. We suggest that these point-sources represent young star clusters formed in the course of the merger.

astro-ph↗

arXiv2004

High Spatial Resolution HST/NICMOS Observations of Markarian 231

Observations of Markarian 231 at 1.1 microns taken with NICMOS on the Hubble Space Telescope are described. The brightness of the object in the near infrared and the inherent short-term stability of the NICMOS optical and instrumental system enables application of special observational and analysis techniques that effectively increase high spatial resolution. By these means, we set an upper limit on the size of the core of the AGN nucleus at 8 mas corresponding to a radial projected distance from the center of MArkarian 231 of ~ 3pc.

astro-ph↗

arXiv2003

Itinerant electron metamagnetism in LaCo$_9$Si$_4$

The strongly exchange enhanced Pauli paramagnet LaCo$_9$Si$_4$ is found to exhibit an itinerant metamagnetic phase transition with indications for metamagnetic quantum criticality. Our investigation comprises magnetic, specific heat, and NMR measurements as well as ab-initio electronic structure calculations. The critical field is about 3.5 T for $H||c$ and 6 T for $H\bot c$, which is the lowest value ever found for rare earth intermetallic compounds. In the ferromagnetic state there appears a moment of about 0.2 $μ_B$/Co at the $16k$ Co-sites, but sigificantly smaller moments at the 4d and $16l$ Co-sites.

cond-mat.str-el↗

arXiv2003

Space Studies of the Black-Drop Effect at a Mercury Transit

Transits of Mercury and Venus across the face of the Sun are rare. The 20th century had 15 transits of Mercury and the 21st century will have 14, the two most recent occuring on 15 November 1999 and 7 May 2003. We report on our observations and analyses of a black-drop effect at the 1999 and 2003 transits of Mercury seen in high spatial resolution optical imaging with NASA's Transition Region and Coronal Explorer (TRACE) spacecraft. We have separated the primary contributors to this effect, solar limb darkening and broadening due to the instrumental point spread function, for the 1999 event. The observations are important for understanding historical observations of transits of Venus, which in the 18th and 19th centuries were basic for the determination of the scale of the solar system. Our observations are in preparation for the 8 June 2004 transit of Venus, the first to occur since 1882. Only five transits of Venus have ever been seen -- in 1639, 1761, 1769, 1874, and 1882. These events occur in pairs, whose members are separated by 8 years, with an interval between pairs of 105 or 122 years. Nobody alive has ever seen a transit of Venus.

astro-ph↗

arXiv2003

Experimental probing of the anisotropy of the empty p states near the Fermi level in MgB2

We have studied the Boron K-edge in the superconductor MgB2 by electron energy loss spectroscopy (EELS) and experimentally resolved the empty p states at the Fermi level that have previously been observed within an energy window of 0.8eV by soft x-ray absorption spectroscopy. Using angular resolved EELS, we find that these states at the immediate edge onset have pxy character in agreement with predictions from first-principle electronic structure calculations.

cond-mat.supr-con↗

arXiv2001

Infrared Views of the TW Hya Disk

The face-on disk around TW Hya is imaged in scattered light at wavelengths of 1.1 and 1.6 micron using the coronagraph in the Near Infrared Camera and Multi Object Spectrometer aboard the Hubble Space Telescope. Stellar light scattered from the optically thick dust disk is seen from 20-230 AU. The surface brightness declines as a power law of r^(-2.6+/-0.1) between 45 and 150 AU. The scattering profile indicates that the disk is flared, not geometrically flat. The disk, while spatially unresolved in thermal radiation at wavelengths of 12 and 18 micron in observations from the W. M. Keck Observatory, shows amorphous and crystalline silicate emission in its spectrum. A disk with silicate grains of a ~1 micron in size in its surface layers can explain the shape of the mid-infrared spectrum. Much larger grains in the disk interior are necessary to fit the millimeter-wave spectral energy distribution, and hence grain growth from an original interstellar size population may have occurred.

astro-ph↗

arXiv2001

Finding Brown Dwarf Companions with HST/NICMOS

We present the results of a HST/NICMOS coronagraphic survey for the direct detection of substellar companions within the young TW Hydrae and Tucana Associations. At the distance of these associations, the lower mass limit of detection, based on models, is well into the high mass planet region for separations > 30 AU. Results presented here include spectra and proper motion verification of two brown dwarf companions, TWA 5B and HR 7329B, located 100 AU and 180 AU from their primaries, respectively. We also present a possible exo-solar giant planet candidate located 125 AU from TWA 6. These few examples demonstrate that the young associations remain fertile ground for discovery and environmental study of planetary systems.

astro-ph↗

arXiv2001

Dust to Dust: Evidence for Planet Formation?

We discuss the properties of several circumstellar debris disk systems imaged with the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer in a survey of young stars with known far-IR excesses. These dusty disks around young ($\sim$ 5--8 Myr) unembedded stars exhibit morphological anisotropies and other characteristics which are suggestive of recent or on-going planet formation. We consider evidence for the evolution of populations of collisionally produced disk grains in light of the significant presence of remnant primordial gas in the optically thick disk of the classical T-Tauri star TW Hya; the dust-dominated and Kuiper-belt like circumstellar ring about the young main-sequence star HR 4796A; and a possible ``intermediate'' case, the complex disk around the Herbig AeBe star HD 141569A. Only a small number of debris disks have been imaged thus far in scattered light. Yet all show structures that may be indicative of dust reprocessing, possibly as a result of planet formation, and speak to the contemporary competing scenarios of disk/planet evolution and formation time scales.

astro-ph↗

arXiv2001

High-resolution Near-Infrared Images and Models of the Circumstellar Disk in HH 30

We present Hubble Space Telescope (HST) Near-Infrared Camera and Multi-object Spectrometer (NICMOS) observations of the reflection nebulosity associated with the T Tauri star HH 30. The images show the scattered light pattern characteristic of a highly inclined, optically thick disk with a prominent dustlane whose width decreases with increasing wavelength. The reflected nebulosity exhibits a lateral asymmetry in the upper lobe on the opposite side to that reported in previously published Wide Field Planetary Camera 2 (WFPC2) images. The radiation transfer model which most closely reproduces the data has a flared accretion disk with dust grains larger than standard interstellar medium grains by a factor of approximately 2.1. A single hotspot on the stellar surface provides the necessary asymmetry to fit the images and is consistent with previous modeling of the light curve and images. Photometric analysis results in an estimated extinction of Av>~80; however, since the photometry measures only scattered light rather than direct stellar flux, this a lower limit. The radiative transfer models require an extinction of Av = 7,900.

astro-ph↗

arXiv2001

Compressibility and Electronic Structure of MgB2 up to 8 GPa

The lattice parameters of MgB2 up to pressures of 8 GPa were determined using high-resolution x-ray powder diffraction in a diamond anvil cell. The bulk modulus, B0, was determined to be 151 +-5 GPa. Both experimental and first-principles calculations indicate nearly isotropic mechanical behavior under pressure. This small anisotropy is in contrast to the 2 dimensional nature of the boron pi states. The pressure dependence of the density of states at the Fermi level and a reasonable value for the average phonon frequency account within the context of BCS theory for the reduction of Tc under pressure.

cond-mat.mtrl-sci↗

arXiv2000

NICMOS Coronagraphic Observations of 55 Cancri

We present new near-infrared (1.1-micron) observations of the circumstellar environment of the planet-bearing star 55 Cancri. With these Hubble Space Telescope images we are unable to confirm the observation of bright scattered radiation at longer NIR wavelengths previously reported by Trilling & Brown (1998), and Trilling, Brown & Rivkin (2000). NICMOS coronagraphic images with detection sensitivities to ~ 100 microJy arcsec$^{-2}$ at 1.1 microns in the region 28 - 60 AU from the star fail to reveal any significant excess flux in point- spread-function (PSF) subtracted images taken in two Hubble Space Telescope orbits. These new observations place flux densities in the 19-28 AU zone at a factor of ten or more below the reported ground-based observations. Applying a suite of a dozen well-matched coronagraphic reference PSFs, including one obtained in the same orbits as the observations of 55 Cnc, yielded consistently null results in detecting a disk. We also searched for, and failed to find, a suggested flux-excess anisotropy in the ratio of ~1.7:1 in the circumstellar background along and orthogonal to the plane of the putative disk. We suggest that, if such a disk does exist, then the total 1.1-micron spectral flux density in an annular zone 28 - 42 AU from the star must be no more than ~0.4mJy, at least ten times smaller than suggested by Trilling and Brown, upon which their very large estimate for the total dust mass (0.4~$M_{\earth}$) was based. Based on the far infrared and submillimeter flux of this system and observations of scattered light and thermal emission from other debris disks, we also expect the intensity of the scattered light to be at least an order of magnitude below our upper limits.

astro-ph↗

arXiv2000

Substellar Companions to Nearby Stars from NICMOS Surveys

A coronagraphic imaging survey of 65 nearby stars was conducted using the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope by the Instrument Definition Team. Using these guaranteed time observations we searched for very low mass stellar and substellar (extra-solar giant planet and brown dwarf) companions to young main sequence and late M dwarf stars. Nineteen additional stars, similarly observed in our circumstellar disk program, were also examined for evidence of companions. We discuss the large depth and dynamic range of these surveys which have given rise to several putative and confirmed detections. Using follow-up spectrographic observations obtained with the Space Telescope Imaging Spectrograph (STIS) we have begun to characterize the physical nature of the objects found in the companion search program.

astro-ph↗

arXiv2000

NICMOS Imaging of Candidate Extra-Solar Planetary Systems

The connection between the formation and growth of planetesimals into larger bodies and the circumstellar debris disks around young and nascent stars from which they arise has been the subject of continual speculation, discussion and inquiry, since the original scenario for such evolution was first expanded upon by Laplace (1796) following the general proposition by Kant (1755). NICMOS coronagraphy has enabled both the direct imaging of such circumstellar disks seen in scattered light about more mature stars (other than b Pictoris) exhibiting morphologically resolved spatial features, and the detection of substellar companions (brown dwarfs and extra-solar giant planets) with masses extending into the Jovian domain for recently formed objects (< a few x 107 years). Exploiting this capability, approximately 18% of the NICMOS IDTs observing time on HST was dedicated to addressing fundamental questions relating to the origins of extra-solar systems while exploring the Environments of Nearby Stars under the umbrella of our EONS project.

astro-ph↗

arXiv2000

A Candidate Substellar Companion to HR 7329

We present the discovery of a candidate substellar companion from a survey of nearby, young stars with the NICMOS coronagraph on the Hubble Space Telescope. The H ~ 12 mag object was discovered approximately 4" from the young A0V star HR 7329. Using follow-up spectroscopy from STIS, we derive a spectral type between M7V and M8V with an effective temperature of ~ 2600 K. We estimate that the probability of a chance alignment with a foreground dwarf star of this nature is ~ 10^(-8) and therefore suggest the object (HR 7329B) is physically associated with HR 7329 with a projected separation of 200 AU. Current brown dwarf cooling models indicate a mass of less than 50 Jupiter masses for HR 7329B based on age estimates of < 30 Myr for HR7329A.

astro-ph↗