High Angular Resolution, Sensitive CS J=2-1 and J=3-2 Imaging of the Protostar L1551 NE: Evidence for Outflow-Triggered Star Formation ?

High angular resolution and sensitive aperture synthesis observations of CS ($J=2-1$) and CS ($J=3-2$) emissions toward L1551 NE, the second brightest protostar in the Taurus Molecular Cloud, made with the Nobeyama Millimeter Array are presented. L1551 NE is categorized as a class 0 object deeply embedded in the red-shifted outflow lobe of L1551 IRS 5. Previous studies of the L1551 NE region in CS emission revealed the presence of shell-like components open toward L1551 IRS 5, which seem to trace low-velocity shocks in the swept-up shell driven by the outflow from L1551 IRS 5. In this study, significant CS emission around L1551 NE was detected at the eastern tip of the swept-up shell from $V_{\rm{lsr}}$ = 5.3 km s$^{-1}$ to 10.1 km s$^{-1}$, and the total mass of the dense gas is estimated to be 0.18 $\pm$ 0.02 $M_\odot$. Additionally, the following new structures were successfully revealed: a compact disklike component with a size of $\approx$ 1000 AU just at L1551 NE, an arc-shaped structure around L1551 NE, open toward L1551 NE, with a size of $\sim 5000$ AU, i.e., a bow shock, and a distinct velocity gradient of the dense gas, i.e., deceleration along the outflow axis of L1551 IRS 5. These features suggest that the CS emission traces the post-shocked region where the dense gas associated with L1551 NE and the swept-up shell of the outflow from L1551 IRS 5 interact. Since the age of L1551 NE is comparable to the timescale of the interaction, it is plausible that the formation of L1551 NE was induced by the outflow impact. The compact structure of L1551 NE with a tiny envelope was also revealed, suggesting that the outer envelope of L1551 NE has been blown off by the outflow from L1551 IRS 5.Comment: 29 pages, 12 figures, Accepted for Publication in the Astrophysical Journa

Aperture synthesis observations of low-mass protostars in the Taurus Molecular Cloud: Formation Processes of Protoplanetary Disks in Protostellar Envelopes

Formation process of a protoplanetary disk around a low-mass YSO is one of the most interesting issues in modern astronomy, which is deeply related to the origin of the solar system. Although previous survey observations of low-mass YSOs have revealed evolution from protostar phase to T Tauri star phase, such as dissipations of envelope gases, dust concentrations from envelopes to circumstellar disks, and expansion of accretion disks. However, the formation process of a protoplanetary disk, how and when the protoplanetary disk has formed, is not still understood. In this thesis, I examine the structure and evolution of circumstellar envelopes and disks around protostars to reveal the formation process of a protoplanetary disk in a protostellar envelope through aperture synthesis observations of low-mass YSOs in the Taurus Molecular Cloud (TMC).In the chapter I, since protoplanetary disks have large diversity of disk masses, radii and angular momenta, we tried to reveal one of the causes of the diversity: the outflow-triggered star formation in low-mass star-forming regions. We performed CS J=2-1 and J=3-2 observations of the class 0 protostar L1551 NE to investigate the interaction feature between L1551 NE and the outflow of the nearby protostar L1551 IRS 5, since L1551 NE is deeply embedded in the outflow of L1551 IRS 5. Significant CS emission around L1551 NE was detected at the eastern tip of the swept-up shell of the outflow of L1551 IRS 5, and the following new structures were successfully revealed: a compact disklike component with a size of 1000 AU just at L1551 NE, an arc-shaped structure around L1551 NE, open toward L1551 NE, with a size of 5000 AU, i.e., a bow shock, and a distinct velocity gradient of the dense gas, i.e., deceleration along the outflow axis of L1551 IRS 5. Since the age of L1551 NE is comparable to the timescale of the interaction, it is plausible that the formation of L1551 NE was induced by the outflow impact. This is a first case of the outflow-triggered star formation in low-mass star forming regions. In the following chapters II and III, we presented the results of aperture synthesis observations of 13CO (J=1-0) and (J=2-1) line emissions toward the binary protostar L1551 IRS 5 and the single protostar HL Tau. 13CO (J=1-0) observations of L1551 IRS 5 have revealed the centrally condensed envelope. The envelope shows infall and rotation motion toward the central sources. The infall velocity of the envelope is consistent to the free-fall velocity around a central mass of 0.5 M_\odot, whereas the rotational velocity has a radial dependence of r^-1, suggesting the specific angular momentum of the gases has conversed during the contraction of the envelope. Furthermore, the 13CO (J=2-1) observations of L1551 IRS 5 show a disklike structure in the central part of the envelope. A distinct velocity gradient is detected along the major axis of the structure, whereas no prominent gradient is detected along the minor axis, suggesting a purely rotating disk. The disk radius estimated to be 500 AU. This radius is significantly larger than the centrifugal radius derived from the local specific angular momentum of the envelope of L1551 IRS 5, although a Keplerian disk is theoretically thought to form with the centrifugal radius in the envelope. Neither the gravitational interaction between the disk and the binary sources nor the turbulent viscosity in the disk seems a plausible mechanism to make such a large rotating disk. 13CO (J=1-0) observations of HL Tau have revealed the extended infalling envelope. A velocity gradient of the emission clearly aligns the minor axis of the envelope, whereas no prominent gradient is detected along the major axis of the envelope, suggesting that the envelope around HL Tau is almost infalling. Even in 13CO (J=2-1) imaging, a rotational disklike component could not be found, suggesting that a Keplerian rotating disk around HL Tau seems to be as small as 100 AU and is likely to be comparable to the typical centrifugal radius of the low mass YSOs in the TMC. In the chapter IV, we presented the 13CO(J=1-0) observations of the protostar Haro 6-5B. Although Haro 6-5B is considered as a protostar, only a compact disklike structure which exhibits rotation motion is detected with a tiny extended envelope, suggesting that most of the envelope has already dissipated. Since the rotation radius of Haro 6-5B is about 400 AU which is comparable to that of L1551 IRS 5, Haro 6-5B might be a binary protostars.In the chapter V, we examine the physical properties of low-mass YSOs, both our samples and the data of previous studies to reveal a formation mechanism of a protoplanetary disk. In the case studies of a binary protostar L1551 IRS 5 and a single protostar HL Tau as describe in the chapter II and III, the rotation disk around L1551 IRS 5 is clearly revealed, whereas that around HL Tau is not detected. Furthermore, the envelope around L1551 IRS 5 shows rotation motion with infall motion, whereas that around HL Tau shows almost only rotating, suggesting that the large amount of local specific angular momenta of envelopes might form a binary system and a large rotating disk. A comparison of the local specific angular momenta between single stars and binary systems, however, shows no obvious difference. These results suggest that differences between binary system and single star formations are quite subtle distinctions of physical properties of protostellar cores and envelopes

Plasmonic Color Filters for CMOS Image Sensor Applications

We report on the optical properties of plasmonic hole arrays as they apply to requirements for plasmonic color filters designed for state-of-the-art Si CMOS image sensors. The hole arrays are composed of hexagonally packed subwavelength sized holes on a 150 nm Al film designed to operate at the primary colors of red, green, and blue. Hole array plasmonic filters show peak transmission in the 40–50% range for large (>5 × 5 μm^2) size filters and maintain their filtering function for pixel sizes as small as 1 × 1 μm^2, albeit at a cost in transmission efficiency. Hole array filters are found to robust with respect to spatial crosstalk between pixel within our detection limit and preserve their filtering function in arrays containing random defects. Analysis of hole array filter transmittance and crosstalk suggests that nearest neighbor hole–hole interactions rather than long-range interactions play the dominant role in the transmission properties of plasmonic hole array filters. We verify this via a simple nearest neighbor model that correctly predicts the hole array transmission efficiency as a function of the number of holes

Color Imaging <i>via</i> Nearest Neighbor Hole Coupling in Plasmonic Color Filters Integrated onto a Complementary Metal-Oxide Semiconductor Image Sensor

State-of-the-art CMOS imagers are composed of very small pixels, so it is critical for plasmonic imaging to understand the optical response of finite-size hole arrays and their coupling efficiency to CMOS image sensor pixels. Here, we demonstrate that the transmission spectra of finite-size hole arrays can be accurately described by only accounting for up to the second nearest-neighbor scattering-absorption interactions of hole pairs, thus making hole arrays appealing for close-packed color filters for imaging applications. Using this model, we find that the peak transmission efficiency of a square-shaped hole array with a triangular lattice reaches ∼90% that of an infinite array at an extent of ∼6 × 6 μm², the smallest size array showing near-infinite array transmission properties. Finally, we experimentally validate our findings by investigating the transmission and imaging characteristics of a 360 × 320 pixel plasmonic color filter array composed of 5.6 × 5.6 μm² RGB color filters integrated onto a commercial black and white 1/2.8 in. CMOS image sensor, demonstrating full-color high resolution plasmonic imaging. Our results show good color fidelity with a 6-color-averaged color difference metric (Δ<i>E</i>) in the range of 16.6–19.3, after white balancing and color-matrix correcting raw images taken with f-numbers ranging from 1.8 to 16. The integrated peak filter transmission efficiencies are measured to be in the 50% range, with a FWHM of 200 nm for all three RGB filters, in good agreement with the spectral response of isolated unmounted color filters