The Kolkata Paise Restaurant Problem and Resource Utilization

We study the dynamics of the "Kolkata Paise Restaurant problem". The problem is the following: In each period, N agents have to choose between N restaurants. Agents have a common ranking of the restaurants. Restaurants can only serve one customer. When more than one customer arrives at the same restaurant, one customer is chosen at random and is served; the others do not get the service. We first introduce the one-shot versions of the Kolkata Paise Restaurant problem which we call one-shot KPR games. We then study the dynamics of the Kolkata Paise Restaurant problem (which is a repeated game version of any given one shot KPR game) for large N. For statistical analysis, we explore the long time steady state behavior. In many such models with myopic agents we get under-utilization of resources, that is, we get a lower aggregate payoff compared to the social optimum. We study a number of myopic strategies, focusing on the average occupation fraction of restaurants.Comment: revtex4, 8 pages, 3 figs, accepted in Physica

Phase transitions in crowd dynamics of resource allocation

We define and study a class of resources allocation processes where $gN$ agents, by repeatedly visiting $N$ resources, try to converge to optimal configuration where each resource is occupied by at most one agent. The process exhibits a phase transition, as the density $g$ of agents grows, from an absorbing to an active phase. In the latter, even if the number of resources is in principle enough for all agents ($g<1$), the system never settles to a frozen configuration. We recast these processes in terms of zero-range interacting particles, studying analytically the mean field dynamics and investigating numerically the phase transition in finite dimensions. We find a good agreement with the critical exponents of the stochastic fixed-energy sandpile. The lack of coordination in the active phase also leads to a non-trivial faster-is-slower effect.Comment: 7 pages, 7 fig

Direction and symmetry transition of the vector order parameter in topological superconductors CuxBi2Se3

Topological superconductors have attracted wide-spreading interests for the bright application perspectives to quantum computing. Cu0.3Bi2Se3 is a rare bulk topological superconductor with an odd-parity wave function, but the details of the vector order parameter d and its pinning mechanism are still unclear. Here, we succeed in growing CuxBi2Se3 single crystals with unprecedented high doping levels. For samples with x = 0.28, 0.36 and 0.37 with similar carrier density as evidenced by the Knight shift, the in-plane upper critical field Hc2 shows a two-fold symmetry. However, the angle at which the Hc2 becomes minimal is different by 90° among them, which indicates that the d-vector direction is different for each crystal likely due to a different local environment. The carrier density for x = 0.46 and 0.54 increases substantially compared to x ≤ 0.37. Surprisingly, the in-plane Hc2 anisotropy disappears, indicating that the gap symmetry undergoes a transition from nematic to isotropic (possibly chiral) as carrier increases

Five-Year Optical and Near Infrared Observations of the Extremely Slow Nova V1280 Scorpii

We present optical ($B$, $V$, $R_{\rm c}$, $I_{\rm c}$ and $y$) and near infrared ($J$, $H$ and $K_{\rm s}$) photometric and spectroscopic observations of a classical nova V1280 Scorpii for five years from 2007 to 2011. Our photometric observations show a declining event in optical bands shortly after the maximum light which continues $\sim$ 250 days. The event is most probably caused by a dust formation. The event is accompanied by a short ($\sim$ 30 days) re-brightening episode ($\sim$ 2.5 mag in $V$), which suggests a re-ignition of the surface nuclear burning. After 2008, the $y$ band observations show a very long plateau at around $y$ = 10.5 for more than 1000 days until April 2011 ($\sim$ 1500 days after the maximum light). The nova had taken a very long time ($\sim$ 50 months) before entering the nebular phase (clear detection of both [\ion{O}{iii}] 4959 and 5007) and is still continuing to generate the wind caused by H-burning. The finding suggests that V1280 Sco is going through the historically slowest evolution. The interval from the maximum light (2007 February 16) to the beginning of the nebular phase is longer than any previously known slow novae: V723 Cas (18 months), RR Pic (10 months), or HR Del (8 months). It suggests that the mass of a white dwarf in the V1280 Sco system might be 0.6 M_\mathrm{\sun} or smaller. The distance, based on our measurements of the expansion velocity combined with the directly measured size of the dust shell, is estimated to be 1.1 $\pm$ 0.5 kpc.Comment: 17 pages, 14 figures, accepted for publication in A&

Discovery of the Coldest Imaged Companion of a Sun-Like Star

We present the discovery of a brown dwarf or possible planet at a projected separation of 1.9" = 29 AU around the star GJ 758, placing it between the separations at which substellar companions are expected to form by core accretion (~5 AU) or direct gravitational collapse (typically >100 AU). The object was detected by direct imaging of its thermal glow with Subaru/HiCIAO. At 10-40 times the mass of Jupiter and a temperature of 550-640 K, GJ 758 B constitutes one of the few known T-type companions, and the coldest ever to be imaged in thermal light around a Sun-like star. Its orbit is likely eccentric and of a size comparable to Pluto's orbit, possibly as a result of gravitational scattering or outward migration. A candidate second companion is detected at 1.2" at one epoch.Comment: 5 pages, 3 figures, 2 tables. Accepted for publication in ApJ Letter

AKARI observations of ice absorption bands towards edge-on YSOs

To investigate the composition and evolution of circumstellar ice around low-mass YSOs, we observed ice absorption bands in the near infrared (NIR) towards eight YSOs ranging from class 0 to class II, among which seven are associated with edge-on disks. We performed slit-less spectroscopic observations using the grism mode of the Infrared Camera (IRC) on board AKARI, which enables us to obtain full NIR spectra from 2.5 $\mu$m to 5 $\mu$m. The spectra were fitted with polynomial baselines to derive the absorption spectra. The molecular absorption bands were then fitted with the laboratory database of ice absorption bands, considering the instrumental line profile and the spectral resolution of the grism dispersion element. Towards the class 0-I sources (L1527, IRC-L1041-2, and IRAS04302), absorption bands of H$_2$O, CO$_2$, CO, and XCN are clearly detected. Column density ratios of CO$_2$ ice and CO ice relative to H$_2$O ice are 21-28% and 13-46%, respectively. If XCN is OCN$^-$, its column density is as high as 2-6% relative to H$_2$O ice. The HDO ice feature at 4.1 $\mu$m is tentatively detected towards the class 0-I sources and HV Tau. Non-detections of the CH-stretching mode features around 3.5 $\mu$m provide upper limits to the CH$_3$OH abundance of 26% (L1527) and 42% (IRAS04302) relative to H$_2$O. We tentatively detect OCS ice absorption towards IRC-L1041-2. Towards class 0-I sources, the detected features should mostly originate in the cold envelope, while CO gas and OCN$^-$ could originate in the region close to the protostar, where there are warm temperatures and UV radiation. We detect H$_2$O ice band towards ASR41 and 2MASSJ1628137-243139, which are edge-on class II disks. We also detect H$_2$O ice and CO$_2$ ice towards HV Tau, HK Tau, and UY Aur, and tentatively detect CO gas features towards HK Tau and UY Aur.Comment: Accepted to A&

SEEDS direct imaging of the RV-detected companion to V450 Andromedae, and characterization of the system

We report the direct imaging detection of a low-mass companion to a young, moderately active star V450 And, that was previously identified with the radial velocity method. The companion was found in high-contrast images obtained with the Subaru Telescope equipped with the HiCIAO camera and AO188 adaptive optics system. From the public ELODIE and SOPHIE archives we extracted available high-resolution spectra and radial velocity (RV) measurements, along with RVs from the Lick planet search program. We combined our multi-epoch astrometry with these archival, partially unpublished RVs, and found that the companion is a low-mass star, not a brown dwarf, as previously suggested. We found the best-fitting dynamical masses to be $m_1=1.141_{-0.091}^{+0.037}$ and $m_2=0.279^{+0.023}_{-0.020}$ M$_\odot$. We also performed spectral analysis of the SOPHIE spectra with the iSpec code. The Hipparcos time-series photometry shows a periodicity of $P=5.743$ d, which is also seen in SOPHIE spectra as an RV modulation of the star A. We interpret it as being caused by spots on the stellar surface, and the star to be rotating with the given period. From the rotation and level of activity, we found that the system is $380^{+220}_{-100}$ Myr old, consistent with an isochrone analysis ($220^{+2120}_{-90}$ Myr). This work may serve as a test case for future studies of low-mass stars, brown dwarfs and exoplanets by combination of RV and direct imaging data.Comment: 15 pages, 9 figures, 7 tables, to appear in Ap

The Structure of Pre-transitional Protoplanetary Disks I: Radiative Transfer Modeling of the Disk+Cavity in the PDS 70 system

Through detailed radiative transfer modeling, we present a disk+cavity model to simultaneously explain both the SED and Subaru H-band polarized light imaging for the pre-transitional protoplanetary disk PDS 70. Particularly, we are able to match not only the radial dependence, but also the absolute scale, of the surface brightness of the scattered light. Our disk model has a cavity 65 AU in radius, which is heavily depleted of sub-micron-sized dust grains, and a small residual inner disk which produces a weak but still optically thick NIR excess in the SED. To explain the contrast of the cavity edge in the Subaru image, a factor of ~1000 depletion for the sub-micron-sized dust inside the cavity is required. The total dust mass of the disk may be on the order of 1e-4 M_sun, only weakly constrained due to the lack of long wavelength observations and the uncertainties in the dust model. The scale height of the sub-micron-sized dust is ~6 AU at the cavity edge, and the cavity wall is optically thick in the vertical direction at H-band. PDS 70 is not a member of the class of (pre-)transitional disks identified by Dong et al. (2012), whose members only show evidence of the cavity in the millimeter-sized dust but not the sub-micron-sized dust in resolved images. The two classes of (pre-)transitional disks may form through different mechanisms, or they may just be at different evolution stages in the disk clearing process.Comment: 28 pages (single column), 7 figures, 1 table, ApJ accepte