Halting Planet Migration in the Evacuated Centers of Protoplanetary Disks

Precise Doppler searches for extrasolar planets find a surfeit of planets with orbital periods of 3-4 days, and no planets with orbital periods less than 3 days. The circumstellar distance, R_0, where small grains in a protoplanetary disk reach sublimation temperature (~1500 K) corresponds to a period of ~6 days. Interior to R_0, turbulent accretion due to magneto-rotational instability may evacuate the disk center. We suggest that planets with orbital periods of 3-4 days are so common because migrating planets halt once this evacuated region contains the sites of their exterior 2:1 Lindblad resonances.Comment: 9 pages, 1 figure, to appear in ApJ letter

Population-scale organization of cerebellar granule neuron signaling during a visuomotor behavior.

Granule cells at the input layer of the cerebellum comprise over half the neurons in the human brain and are thought to be critical for learning. However, little is known about granule neuron signaling at the population scale during behavior. We used calcium imaging in awake zebrafish during optokinetic behavior to record transgenically identified granule neurons throughout a cerebellar population. A significant fraction of the population was responsive at any given time. In contrast to core precerebellar populations, granule neuron responses were relatively heterogeneous, with variation in the degree of rectification and the balance of positive versus negative changes in activity. Functional correlations were strongest for nearby cells, with weak spatial gradients in the degree of rectification and the average sign of response. These data open a new window upon cerebellar function and suggest granule layer signals represent elementary building blocks under-represented in core sensorimotor pathways, thereby enabling the construction of novel patterns of activity for learning

Pandangan atau Tanggapan Akhir Peserta Mata Kuliah Pendidikan Pancasila terhadap Pendidikan Pancasila di UNPAR

Pancasila adalah philosophische grondslag, weltanschauung bangsa Indonesia. Nilai-nilai Pancasila sudah hidup jauh sebelum dirumuskan para pendiri. Maka tak berlebihan bila dikatakan bahwa Pancasila merupakan inti jiwa bangsa Indonesia. Pancasila bisa juga dikatakan sebagai kerangka pandang orang Indonesia melihat dirinya. Perjalanan sejarah bangsa seharusnya menjadikan Pancasila semakin terbuka untuk dibicarakan. Pancasila yang sudah dirumuskan itu harus terus ditafsirkan dalam konteks Indonesia yang berubah. Dengan demikian, ia menjadi semakin hidup dan kaya makna karena menjadi milik seluruh komponen bangsa.Kesan yang muncul di awal perkuliahan berhadapan dengan perkuliahan Pendidikan Pancasila adalah negatif. Mayoritas mahasiswa menganggap dan memandang mata kuliah Pendidikan Pancasila sekadar menjadi ideologi yang melayani kehendak penguasa. Model pendidikan yang indoktrinatif merupakan salah satu cara yang kerap digunakan untuk menanamkan nilai-nilai Pancasila sehingga cenderung represif. Karena itu, kecurigaan bahwa Pendidikan Pancasila merupakan sebuah upaya manipulasi dalam kerangka tafsir kelompok yang berkuasa. Sehingga menurut mayoritas mahasiswa Pancasila sebagai falsafah hidup bangsa ini tidak lagi memiliki daya atau kekuatan mempengaruhi sehingga mempelajarinya pun sekedar sebuah tuntutan teknis-akademis. Universitas Katolik Parahyangan (UNPAR) sebagai salah satu lembaga pendidikan yang masih setia pada sejarah Pendidikan Pancasila di Perguruan Tinggi merasa selalu terpanggil untuk mencoba merumuskan ulang Pancasila. UNPAR sebagai sebuah universitas yang mendasari diri pada spirit multi-kultur dan muliti-religi menjadikan Pancasila sebagai falsafah. Hal ini pula yang membuat Mata Kuliah Pendidikan Nilai Pancasila tetap dipertahankan di kampus ini. Penelitian ini mencoba menggali dan menganalisis sejauh mana mahasiswa UNPAR menyadari spirit ini.Salah satu komponen penting dinamika pembelajaran mata kuliah ini adalah mahasiswa. Maka penelitian ini mencoba menggali pendapat akhir peserta mata kuliah. Pandangan atau pendapat ini berhubungan dengan pengalaman mereka mengikuti perkuliahan Pendidikan Pancasila selama satu semester. Dalam penelitian ini ada tiga pendapat yang coba digali diantara mahasiswa peserta mata kuliah. Ketiga pendapat itu berkaitan dengan pertama, pendapat akhir tentang materi; kedua, pendapat akhir terhadap metode yang digunakan; ketiga, pendapat akhir terhadap dosen atau pengajar mata kuliah Pendidikan Pancasila.Pendapat akhir mahasiswa ini yang kemudian ditangkap dan dianalisis sehingga akhirnya pendapat akhir dari peserta didik ini memperkaya pengembangan mata kuliah ini. Sehingga sebagai falsafah bangsa Pancasila tetap hidup dalam hati, pikiran dan tindakan manusia Indonesia pada umumnya dan civitas academica UNPAR pada khususnya

Discovery of Reflection Nebulosity Around Five Vega-like Stars

Coronagraphic optical observations of six Vega-like stars reveal reflection nebulosities, five of which were previously unknown. The nebulosities illuminated by HD 4881, HD 23362, HD 23680, HD 26676, and HD 49662 resemble that of the Pleiades, indicating an interstellar origin for dust grains. The reflection nebulosity around HD 123160 has a double-arm morphology, but no disk-like feature is seen as close as 2.5 arcsec from the star in K-band adaptive optics data. We demonstrate that uniform density dust clouds surrounding HD 23362, HD 23680 and HD 123160 can account for the observed 12-100 micron spectral energy distributions. For HD 4881, HD 26676, and HD 49662 an additional emission source, such as from a circumstellar disk or non-equilibrium grain heating, is required to fit the 12-25 micron data. These results indicate that in some cases, particularly for Vega-like stars located beyond the Local Bubble (>100 pc), the dust responsible for excess thermal emission may originate from the interstellar medium rather than from a planetary debris system.Comment: The Astrophysical Journal, in press for March, 2002 (32 pages, 13 figures

A Single Circumbinary Disk in the HD 98800 Quadruple System

We present sub-arcsecond thermal infrared imaging of HD 98800, a young quadruple system composed of a pair of low-mass spectroscopic binaries separated by 0.8'' (38 AU), each with a K-dwarf primary. Images at wavelengths ranging from 5 to 24.5 microns show unequivocally that the optically fainter binary, HD 98800B, is the sole source of a comparatively large infrared excess upon which a silicate emission feature is superposed. The excess is detected only at wavelengths of 7.9 microns and longer, peaks at 25 microns, and has a best-fit black-body temperature of 150 K, indicating that most of the dust lies at distances greater than the orbital separation of the spectroscopic binary. We estimate the radial extent of the dust with a disk model that approximates radiation from the spectroscopic binary as a single source of equivalent luminosity. Given the data, the most-likely values of disk properties in the ranges considered are R_in = 5.0 +/- 2.5 AU, DeltaR = 13+/-8 AU, lambda_0 = 2(+4/-1.5) microns, gamma = 0+/-2.5, and sigma_total = 16+/-3 AU^2, where R_in is the inner radius, DeltaR is the radial extent of the disk, lambda_0 is the effective grain size, gamma is the radial power-law exponent of the optical depth, tau, and sigma_total is the total cross-section of the grains. The range of implied disk masses is 0.001--0.1 times that of the moon. These results show that, for a wide range of possible disk properties, a circumbinary disk is far more likely than a narrow ring.Comment: 11 page Latex manuscript with 3 postscript figures. Accepted for publication in Astrophysical Journal Letters. Postscript version of complete paper also available at http://www.hep.upenn.edu/PORG/web/papers/koerner00a.p

A Resolved Molecular Gas Disk around the Nearby A Star 49 Ceti

The A star 49 Ceti, at a distance of 61 pc, is unusual in retaining a substantial quantity of molecular gas while exhibiting dust properties similar to those of a debris disk. We present resolved observations of the disk around 49 Ceti from the Submillimeter Array in the J=2-1 rotational transition of CO with a resolution of 1.0x1.2 arcsec. The observed emission reveals an extended rotating structure viewed approximately edge-on and clear of detectable CO emission out to a distance of ~90 AU from the star. No 1.3 millimeter continuum emission is detected at a 3-sigma sensitivity of 2.1 mJy/beam. Models of disk structure and chemistry indicate that the inner disk is devoid of molecular gas, while the outer gas disk between 40 and 200 AU from the star is dominated by photochemistry from stellar and interstellar radiation. We determine parameters for a model that reproduces the basic features of the spatially resolved CO J=2-1 emission, the spectral energy distribution, and the unresolved CO J=3-2 spectrum. We investigate variations in disk chemistry and observable properties for a range of structural parameters. 49 Ceti appears to be a rare example of a system in a late stage of transition between a gas-rich protoplanetary disk and a tenuous, virtually gas-free debris disk.Comment: 11 pages, 6 figures, accepted for publication in Ap

Influence of gas flow rate on liquid distribution in trickle-beds using perforated plates as liquid distributors

Two wire mesh tomography devices and a liquid collector were used to study the influence of the gas flow rate on liquid distribution when fluids distribution on top of the reactor is ensured by a perforated plate. In opposition to most of the studies realized by other authors, conditions in which the gas has a negative impact in liquid distribution were evidenced. Indeed, the obtained results show that the influence of gas flow rate depends on the quality of the initial distribution, as the gas forces the liquid to "respect" the distribution imposed at the top of the reactor. Finally, a comparison between the two measuring techniques shows the limitations of the liquid collector and the improper conclusions to which its use could lead

A double-lined spectroscopic orbit for the young star HD 34700

We report high-resolution spectroscopic observations of the young star HD 34700, which confirm it to be a double-lined spectroscopic binary. We derive an accurate orbital solution with a period of 23.4877 +/- 0.0013 days and an eccentricity of e = 0.2501 +/- 0.0068. The stars are found to be of similar mass (M2/M1 = 0.987 +/- 0.014) and luminosity. We derive also the effective temperatures (5900 K and 5800 K) and projected rotational velocities (28 km/s and 22 km/s) of the components. These values of v sin i are much higher than expected for main-sequence stars of similar spectral type (G0), and are not due to tidal synchronization. We discuss also the indicators of youth available for the object. Although there is considerable evidence that the system is young --strong infrared excess, X-ray emission, Li I 6708 absorption (0.17 Angstroms equivalent width), H alpha emission (0.6 Angstroms), rapid rotation-- the precise age cannot yet be established because the distance is unknown.Comment: 17 pages, including 2 figures and 2 tables. Accepted for publication in AJ, to appear in February 200

The Quantum Transverse Field Ising Model on an Infinite Tree from Matrix Product States

We give a generalization to an infinite tree geometry of Vidal's infinite time-evolving block decimation (iTEBD) algorithm for simulating an infinite line of quantum spins. We numerically investigate the quantum Ising model in a transverse field on the Bethe lattice using the Matrix Product State ansatz. We observe a second order phase transition, with certain key differences from the transverse field Ising model on an infinite spin chain. We also investigate a transverse field Ising model with a specific longitudinal field. When the transverse field is turned off, this model has a highly degenerate ground state as opposed to the pure Ising model whose ground state is only doubly degenerate.Comment: 28 pages, 23 figures, PDFlate