Using all-sky maps obtained from the Cosmic Background Explorer/Diffuse Infrared Background Experiment (DIRBE) at 3.5 and 4.9 mu m, we present a reanalysis of diffuse sky emissions such as zodiacal ...light (ZL), diffuse Galactic light (DGL), integrated starlight (ISL), and isotropic residual emission including the extragalactic background light (EBL). Our new analysis, which includes an improved estimate of ISL using the Wide-field Infrared Survey Explorer data, enabled us to find the DGL signal in a direct linear correlation between diffuse near-infrared and 100 mu m emission at high Galactic latitudes (b> 35degrees). At 3.5 mu m, the high-latitude DGL result is comparable to the low-latitude value derived from the previous DIRBE analysis. In comparison with models of the DGL spectrum assuming a size distribution of dust grains composed of amorphous silicate, graphite, and polycyclic aromatic hydrocarbon (PAH), the measured DGL values at 3.5 and 4.9 mu m constrain the mass fraction of PAH particles in the total dust species to be more than ~2%. This was consistent with the results of Spitzer/IRAC toward the lower Galactic latitude regions. The derived residual emission of 8.9 + or - 3.4 nWm super(-2) sr super(-1) at 3.5 mu m is marginally consistent with the level of integrated galaxy light and the EBL constraints from the gamma -ray observations. The residual emission at 4.9 mu m is not significantly detected due to the large uncertainty in the ZL subtraction, the same as in previous studies. Combined with our reanalysis of the DIRBE data at 1.25 and 2.2 mu m, the residual emission in the near-infrared exhibits the Rayleigh-Jeans spectrum.
We present the design and the mechanical and thermal performances of a prototype rotational mechanism using a superconducting magnetic bearing (SMB) for a space compatible polarization modulator. The ...rotational mechanism consists of an SMB with an optical encoder and a three-grip mechanism that holds a levitating rotor until a high-temperature superconducting array (YBCO) cools down below its critical temperature. After the successful operation of a grip mechanism, the rotor magnet levitates at 10-16 K, and we conduct spin-down measurements. We estimate the heat dissipation from the rotor rotation and an optical encoder. From the mechanical and thermal performances of the prototype rotation mechanism, we did not find the potential no-go results from this SMB technology for use in a future space mission. The development of this rotational mechanism is targeting for use of a polarization modulator for a space mission to probe the comic inflation by measuring the cosmic microwave background polarization.
The disk around the Herbig Ae star HD 169142 was imaged and resolved at 18.8 and 24.5 mu m using Subaru/COMICS. We interpret the observations using a two-dimensional radiative transfer model and find ...evidence for the presence of a large gap. The mid-infrared images trace dust that is emitted at the onset of a strong rise in the spectral energy distribution (SED) at 20 mu m, and are therefore very sensitive to the location and characteristics of the inner wall of the outer disk and its dust. We determine the location of the wall to be 23 super(+3) sub(-5) AU from the star. An extra component of hot dust must exist close to the star. We find that a hydrostatic optically thick inner disk does not produce enough flux in the near-infrared, and an optically thin, geometrically thick component is our solution to fit the SED. Considering the recent findings of gaps and holes in a number of Herbig Ae/Be group I disks, we suggest that such disk structures may be common in group I sources. Classification as group I should be considered a strong case for classification as a transitional disk, though improved imaging surveys are needed to support this speculation.
We imaged circumstellar disks around 22 Herbig Ae/Be stars at 25 mu m using Subaru/COMICS and Gemini/T-ReCS. Our sample consists of an equal number of objects from each of the two categories defined ...by Meeus et al.; 11 group I (flaring disk) and II (flat disk) sources. We find that group I sources tend to show more extended emission than group II sources. Previous studies have shown that the continuous disk is difficult to resolve with 8 m class telescopes in the Q band due to the strong emission from the unresolved innermost region of the disk. This indicates that the resolved Q-band sources require a hole or gap in the disk material distribution to suppress the contribution from the innermost region of the disk. As many group I sources are resolved at 25 mu m, we suggest that many, but not all, group I Herbig Ae/Be disks have a hole or gap and are (pre-)transitional disks. On the other hand, the unresolved nature of many group II sources at 25 mu m supports the idea that group II disks have a continuous flat disk geometry. It has been inferred that group I disks may evolve into group II through the settling of dust grains into the mid-plane of the protoplanetary disk. However, considering the growing evidence for the presence of a hole or gap in the disk of group I sources, such an evolutionary scenario is unlikely. The difference between groups I and II may reflect different evolutionary pathways of protoplanetary disks.
•We introduced a magnetically coupled gear mechanism to continuously rotate a levitating rotor that is supported by a superconducting magnetic bearing.•We conducted the experiments to measure the ...static torque at the room temperature and also to measure the dynamical property of the levitating rotor that is driven by the magnetically coupled gear at the operating temperature at 10 K in the cryostat with the pressure below 10−6 Torr.•We did not observe any slip when the drive gear drives the rotor. We set a limit of the rotational frequency variation to be less than 1 sigma variation of 0.019 Hz at 1 Hz rotational frequency at the operation temperature of 10 K.
We present the design and mechanical performances of a magnetically coupled gear mechanism to drive a levitating rotor magnet of a superconducting magnetic bearing (SMB). The SMB consists of a ring-shaped high-temperature superconducting array (YBCO) and a ring-shaped permanent magnet. This rotational system is designed to operate below 10 K, and thus the design philosophy is to minimize any potential source of heat dissipation. While an SMB provides only a functionality of namely a bearing, it requires a mechanism to drive a rotational motion. We introduce a simple implementation of a magnetically coupled gears between a stator and a rotor. This enables to achieve enough torque to drive a levitating rotor without slip at the rotation frequency of about 1 Hz below 10 K. The rotational variation between the rotor and the drive gear is synchronised within σ=0.019 Hz. The development of this mechanism is a part of the program to develop a testbed in order to evaluate a prototype half-wave plate based polarization modulator for future space missions. The successful development allows this modulator to be a candidate for an instrument to probe the cosmic inflation by measuring the cosmic microwave background polarization.
Context. Little is known about the properties of the warm (Tdust ≳ 150 K) debris disk material located close to the central star, which has a more direct link to the formation of terrestrial planets ...than does the low-temperature debris dust that has been detected to date. Aims. To discover new warm debris disk candidates that show large 18 μm excess and estimate the fraction of stars with excess based on the AKARI/IRC Mid-Infrared All-Sky Survey data. Methods. We searched for point sources detected in the AKARI/IRC All-Sky Survey, which show a positional match with A-M dwarf stars in the Tycho-2 Spectral Type Catalogue and exhibit excess emission at 18 μm compared to what is expected from the KS magnitude in the 2MASS catalogue. Results. We find 24 warm debris candidates including 8 new candidates among A-K stars. The apparent debris disk frequency is estimated to be 2.8 ± 0.6%. We also find that A stars and solar-type FGK stars have different characteristics of the inner component of the identified debris disk candidates. While debris disks around A stars are cooler and consistent with steady-state evolutionary model of debris disks, those around FGK stars tend to be warmer and cannot be explained by the steady-state model.
We report the estimation of the heat dissipation on a levitating rotor over superconducting magnetic bearing operating below 10 K. The continuously rotating mechanism is one of key devices to support ...the rotation of a sapphire half wave plate (HWP) in a polarization modulator of a LiteBIRD satellite. Due to the system requirement, the HWP must be kept at a cryogenic temperature while it is spinning. In order to minimize the frictional energy loss, we employ a superconducting magnetic bearing (SMB) and AC synchronous motor, which enables a contactless rotational mechanism. While we can minimize the frictional heat loss, there exists an energy loss due to the magnetic friction. As a result, it is essential to build a thermal model an estimation of heat dissipation to this contactless rotor is important to predict how much the HWP temperature rises during its rotation. For an estimation of heat dissipation, we conduct an experiment in order to establish the thermal simulation model equivalent to the flight model in size. Each thermal contact conductance between the rotor and the cryogenic rotor holder is also estimated through the experiment data. From the data, we only can know the difference in the rotor temperature before and after the rotor rotation. We monitor the transient temperature profile of grippers after the rotor is gripped by them. The rotational time is related to the total heat dissipation on the rotor because the heat dissipation is attributed to two kinds of energy losses: a magnetic hysteresis and induced eddy currents on metal parts of the rotor. Finally, we make a comparison between the thermal model and the experimental result and estimate the allowable heat dissipation to keep the HWP temperature lower than 20K.
Context. Massive young stellar objects (MYSO) are surrounded by massive dusty envelopes, whose physical structure and geometry are determined by the star formation process. Aims. Our principal aim is ...to establish the density structure of MYSO envelopes on scales of ~1000 AU. This constitutes an increase of a factor ~10 in angular resolution compared to similar studies performed in the (sub)mm. Methods. We have obtained diffraction-limited (0.6´´) 24.5 μm images (field of view of $40\arcsec\times30\arcsec$) of 14 well-known massive star formation regions with the COMICS instrument mounted on the 8.2 m Subaru telescope. We construct azimuthally averaged intensity profiles of the resolved MYSO envelopes and build spectral energy distributions (SEDs) from archival data and the COMICS 24.5 μm flux density. The SEDs range from near-infrared to millimeter wavelengths. Self-consistent 1-D radiative transfer models described by a density dependence of the form $n(r) \propto r^{-p}$ are used to simultaneously compare the intensity profiles and SEDs to model predictions. Results. The images reveal the presence of discrete MYSO sources which are resolved on arcsecond scales, and, to first-order, the observed emission is circular on the sky. For many sources, the spherical models are capable of satisfactorily reproducing the 24.5 μm intensity profile, the 24.5 μm flux density, the 9.7 μm silicate absorption feature, and the submm emission. They are described by density distributions with $p =1.0\pm0.25$. Such distributions are shallower than those found on larger scales probed with single-dish (sub)mm studies. Other sources have density laws that are shallower/steeper than $p=1.0$ and there is evidence that these are viewed near edge-on or near face-on respectively. In these cases spherical models fail to provide good fits to the data. The images also reveal a diffuse component tracing somewhat larger scale structures, particularly visible in the regions S 140, AFGL 2136, IRAS 20126+4104, Mon R2, and Cep A. Conclusions. We find a flattening of the MYSO density law going from scales probed with single-dish submm observations down to scales of ~1000 AU probed with the observations presented here. We propose that this may be evidence of rotational support of the envelope. This finding will be explored further in a future paper using 2-D axisymmetric radiative transfer models.
We report a thermal analysis of a polarization modulator unit (PMU) for use in a space-borne cosmic microwave background (CMB) project. A measurement of the CMB polarization allows us to probe the ...physics of early universe, and that is the best method to test the cosmic inflation experimentally. One of the key instruments for this science is to use a halfwave plate (HWP) based polarization modulator. The HWP is required to rotate continuously at about 1 Hz below 10 K to minimize its own thermal emission to a detector system. The rotating HWP system at the cryogenic environment can be realized by using a superconducting magnetic bearing (SMB) without significant heat dissipation by mechanical friction. While the SMB achieves the smooth rotation due to the contactless bearing, an estimation of a levitating HWP temperature becomes a challenge. We manufactured a one-eighth scale prototype model of PMU and built a thermal model. We verified our thermal model with the experimental data. We forecasted the projected thermal performance of PMU for a full-scale model based on the thermal model. From this analysis, we discuss the design requirement toward constructing the full-scale model for use in a space environment such as a future CMB satellite mission, LiteBIRD.
Context. The first Japanese infrared astronomical satellite, AKARI, has completed an All-Sky Survey at mid- to far-infrared wavelengths with higher spatial resolutions and sensitivities than the ...previous survey with Infrared Astronomical Satellite (IRAS). Aims. We search for new T Tauri star (TTS) candidates with the mid-infrared (MIR) part of the AKARI All-Sky Survey at 9 and 18 μm wavelengths. Methods. We used the point source catalogue (PSC) obtained by the Infrared Camera (IRC) on board AKARI. We combined the 2MASS PSC and the 3rd version of the USNO CCD Astrograph Catalogue (UCAC) with the AKARI IRC-PSC, and surveyed 517 known TTSs over a 1800-square-degree part of the Taurus-Auriga region to develop criteria to extract TTSs. We considered asymptotic giant branch (AGB) stars, post-AGB stars, planetary nebulae (PNe), and galaxies, which have similar MIR colours, to separate TTSs from these sources. We finally searched for new TTS candidates from AKARI IRC-PSC in the same Taurus-Auriga region. Results. Of the 517 known TTSs, we detected 133 sources with AKARI: 46 sources were not detected by IRAS. Based on the colour-colour and colour-magnitude diagrams made from the AKARI, 2MASS, and UCAC surveys, we propose the criteria to extract TTS candidates from the AKARI All-Sky data, and 68/133 AKARI detected TTSs have passed these criteria. On the basis of our criteria, we selected 176/14725 AKARI sources as TTS candidates that are located around the Taurus-Auriga region. Comparing these sources with SIMBAD, we found that 148 are previously identified sources including 115 young stellar objects (YSOs), and 28 unidentified sources. Conclusions. Based on SIMBAD identifications, we infer the TTS-identification probability using our criteria to be ~75%. We find 28 TTS candidates, of which we expect ~21 to be confirmed once follow-up observations can be obtained. Although the probability of ~75% is not so high, it is affected by the completeness of the SIMBAD database, and we can search for TTSs over the whole sky, and all star-forming regions.
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