arXiv subjects

Paul T. P. Ho

Publications and source records attributed to Paul T. P. Ho.

At least 73 records · Page 4Linked to original sources

arXiv2017

Spiral Arms, Infall, and Misalignment of the Circumbinary Disk from the Circumstellar Disks in the Protostellar Binary System L1551 NE

We report the ALMA Cycle 2 observations of the Class I binary protostellar system L1551 NE in the 0.9-mm continuum, C18O (3-2), 13CO (3-2), SO (7_8-6_7), and the CS (7-6) emission. At 0.18" (= 25 AU) resolution, ~4-times higher than that of our Cycle 0 observations, the circumbinary disk as seen in the 0.9-mm emission is shown to be comprised of a northern and a southern spiral arm, with the southern arm connecting to the circumstellar disk around Source B. The western parts of the spiral arms are brighter than the eastern parts, suggesting the presence of an m=1 spiral mode. In the C18O emission, the infall gas motions in the inter-arm regions and the outward gas motions in the arms are identified. These observed features are well reproduced with our numerical simulations, where gravitational torques from the binary system impart angular momenta to the spiral-arm regions and extract angular momenta from the inter-arm regions. Chemical differentiation of the circumbinary disk is seen in the four molecular species. Our Cycle 2 observations have also resolved the circumstellar disks around the individual protostars, and the beam-deconvolved sizes are 0.29" X 0.19" (= 40 X 26 AU) (P.A. = 144 deg) and 0.26" X 0.20" (= 36 X 27 AU) (P.A. = 147 deg) for Sources A and B, respectively. The position and inclination angles of these circumstellar disks are misaligned with that of the circumbinary disk. The C18O emission traces the Keplerian rotation of the misaligned disk around Source A.

astro-ph.GA↗

arXiv2016

The Greenland Telescope: Antenna Retrofit Status and Future Plans

Since the ALMA North America Prototype Antenna was awarded to the Smithsonian Astrophysical Observatory (SAO), SAO and the Academia Sinica Institute of Astronomy & Astrophysics (ASIAA) are working jointly to relocate the antenna to Greenland. This paper shows the status of the antenna retrofit and the work carried out after the recommissioning and subsequent disassembly of the antenna at the VLA has taken place. The next coming months will see the start of the antenna reassembly at Thule Air Base. These activities are expected to last until the fall of 2017 when commissioning should take place. In parallel, design, fabrication and testing of the last components are taking place in Taiwan.

astro-ph.IM↗

arXiv2016

3.5-Year Monitoring of 225 GHz Opacity at the Summit of Greenland

We present the 3.5-yr monitoring results of 225 GHz opacity at the summit of the Greenland ice sheet (Greenland Summit Camp) at an altitude of 3200 m using a tipping radiometer. We chose this site as our submillimeter telescope (Greenland Telescope; GLT) site, because its location offers favorable baselines to existing submillimeter telescopes for global-scale VLBI. The site shows a clear seasonal variation with the average opacity lower by a factor of two during winter. For the winter quartiles of 25% and 50%, the Greenland site is about 10%-30% worse than the ALMA or the South Pole sites. Estimated atmospheric transmission spectra in winter season are similar to the ALMA site at lower frequencies (<450 GHz), which are transparent enough to perform astronomical observations almost all of the winter time with opacities <0.5, but 10%-25% higher opacities at higher frequencies (>450 GHz) than those at the ALMA site. This is due to the lower altitude of the Greenland site. Nevertheless, half of the winter time at the Greenland site can be used for astronomical observations at frequencies between 450 GHz and 1000 GHz with opacities <1.2, and 10% of the time show >10% transmittance in the THz (1035 GHz, 1350 GHz, and 1500 GHz) windows. One major advantage of the Greenland site in winter is that there is no diurnal variation due to the polar night condition, and therefore the durations of low-opacity conditions are significantly longer than at the ALMA site. Opacities lower than 0.05 or 0.04 can continue for more than 100 hours. Such long stable opacity conditions do not occur as often even at the South Pole; it happens only for the opacity lower than 0.05. Since the opacity variation is directly related to the sky temperature (background) variation, the Greenland site is suitable for astronomical observations that need unusually stable sky background.

astro-ph.IM↗

arXiv2016

AMiBA: Cluster Sunyaev-Zel'dovich Effect Observations with the Expanded 13-Element Array

The Yuan-Tseh Lee Array for Microwave Background Anisotropy (AMiBA) is a co-planar interferometer array operating at a wavelength of 3mm to measure the Sunyaev-Zeldovich effect (SZE) of galaxy clusters. In the first phase of operation -- with a compact 7-element array with 0.6m antennas (AMiBA-7) -- we observed six clusters at angular scales from 5\arcmin to 23\arcmin. Here, we describe the expansion of AMiBA to a 13-element array with 1.2m antennas (AMiBA-13), its subsequent commissioning, and our cluster SZE observing program. The most important changes compared to AMiBA-7 are (1) array re-configuration with baselines ranging from 1.4m to 4.8m covering angular scales from 2\arcmin to 11.5\arcmin, (2) thirteen new lightweight carbon-fiber-reinforced plastic (CFRP) 1.2m reflectors, and (3) additional correlators and six new receivers. From the AMiBA-13 SZE observing program, we present here maps of a subset of twelve clusters. In highlights, we combine AMiBA-7 and AMiBA-13 observations of Abell 1689 and perform a joint fitting assuming a generalized NFW pressure profile. Our cylindrically integrated Compton-y values for this cluster are consistent with the BIMA/OVRA, SZA, and Planck results. We report the first targeted SZE detection towards the optically selected galaxy cluster RCS J1447+0828, and we demonstrate the ability of AMiBA SZE data to serve as a proxy for the total cluster mass. Finally, we show that our AMiBA-SZE derived cluster masses are consistent with recent lensing mass measurements in the literature.

astro-ph.CO↗

arXiv2016

The Fossil Nuclear Outflow in the Central 30 pc of the Galactic Center

We report a new 1-pc (30") resolution CS($J=2-1$) line map of the central 30 pc of the Galactic Center (GC), made with the Nobeyama 45m telescope. We revisit our previous study of the extraplanar feature called polar arc (PA), which is a molecular cloud located above SgrA* with a velocity gradient perpendicular to the Galactic plane. We find that the PA can be traced back to the Galactic disk. This provides clues of the launching point of the PA , roughly $6\times10^{6}$ years ago. Implications of the dynamical time scale of the PA might be related to the Galactic Center Lobe (GCL) at parsec scale. Our results suggest that in the central 30 pc of the GC, the feedback from past explosions could alter the orbital path of the molecular gas down to the central tenth of parsec. In the follow-up work of our new CS($J=2-1$) map, we also find that near the systemic velocity, the molecular gas shows an extraplanar hourglass-shaped feature (HG-feature) with a size of $\sim$13 pc. The latitude-velocity diagrams show that the eastern edge of the HG-feature is associated with an expanding bubble B1, $\sim$7 pc away from SgrA*. The dynamical time scale of this bubble is $\sim3\times10^{5}$ years. This bubble is interacting with the 50 km s$^{-1}$ cloud. Part of the molecular gas from the 50 km s$^{-1}$ cloud was swept away by the bubble to $b=-0.2deg$. The western edge of the HG-feature seems to be the molecular gas entrained from the 20 km s$^{-1}$ cloud towards the north of the Galactic disk. Our results suggest a fossil explosion in the central 30 pc of the GC a few 10$^{5}$ years ago.

astro-ph.GA↗

arXiv2016

Linearly Polarized Millimeter and Submillimeter Continuum Emission of Sgr A* Constrained by ALMA

Our aim is to characterize the polarized continuum emission properties including intensity, polarization position angle, and polarization percentage of Sgr A* at $\sim$100 (3.0 mm), $\sim$230 (1.3 mm), $\sim$345 (0.87 mm), $\sim$500 (0.6 mm), and $\sim$700 GHz (0.43 mm). We report continuum emission properties of Sgr A* at the above frequency bands, based on the Atacama Large Millimeter Array (ALMA) observations. We measured flux densities of Sgr A* from ALMA single pointing and mosaic observations. We performed sinusoidal fittings to the observed (XX-YY)/I intensity ratios, to derive the polarization position angles and polarization percentages. We successfully detect polarized continuum emission from all observed frequency bands. We observed lower Stokes I intensity at $\sim$700 GHz than that at $\sim$500 GHz, which suggests that emission at $\gtrsim$500 GHz is from optically thin part of a synchrotron emission spectrum. Both the Stokes I intensity and the polarization position angle at our highest observing frequency of $\sim$700 GHz, may be varying with time. However, we do not yet detect variation in the polarization percentage at $>$500 GHz. The polarization percentage at $\sim$700 GHz is likely lower than that at $\sim$500 GHz. By comparing the $\sim$500 GHz and $\sim$700 GHz observations with the observations at lower frequency bands, we suggest that the intrinsic polarization position angle of Sgr A* is varying with time. This paper also reports the measurable polarization properties from the observed calibration quasars. The future simultaneous multi-frequency polarization observations are required for clarifying the time and frequency variation of polarization position angle and polarization percentage.

astro-ph.HE↗

arXiv2016

Persistent Asymmetric Structure of Sagittarius A* on Event Horizon Scales

The Galactic Center black hole Sagittarius A* (Sgr A*) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Potentially much richer source information is contained in the phases. We report on 1.3 mm phase information on Sgr A* obtained with the EHT on a total of 13 observing nights over 4 years. Closure phases, the sum of visibility phases along a closed triangle of interferometer baselines, are used because they are robust against phase corruptions introduced by instrumentation and the rapidly variable atmosphere. The median closure phase on a triangle including telescopes in California, Hawaii, and Arizona is nonzero. This result conclusively demonstrates that the millimeter emission is asymmetric on scales of a few Schwarzschild radii and can be used to break 180-degree rotational ambiguities inherent from amplitude data alone. The stability of the sign of the closure phase over most observing nights indicates persistent asymmetry in the image of Sgr A* that is not obscured by refraction due to interstellar electrons along the line of sight.

astro-ph.GA↗

arXiv2015

Resolved Magnetic-Field Structure and Variability Near the Event Horizon of Sagittarius A*

Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizon-scale magnetic-field structure. We report interferometric observations at 1.3-millimeter wavelength that spatially resolve the linearly polarized emission from the Galactic Center supermassive black hole, Sagittarius A*. We have found evidence for partially ordered fields near the event horizon, on scales of ~6 Schwarzschild radii, and we have detected and localized the intra-hour variability associated with these fields.

astro-ph.HE↗

arXiv2015

Local Magnetic Field Role in Star Formation

We highlight distinct and systematic observational features of magnetic field morphologies in polarized submm dust continuum. We illustrate this with specific examples and show statistical trends from a sample of 50 star-forming regions.

astro-ph.GA↗

arXiv2015

First-generation Science Cases for Ground-based Terahertz Telescopes

Ground-based observations at terahertz (THz) frequencies are a newly explorable area of astronomy for the next ten years. We discuss science cases for a first-generation 10-m class THz telescope, focusing on the Greenland Telescope as an example of such a facility. We propose science cases and provide quantitative estimates for each case. The largest advantage of ground-based THz telescopes is their higher angular resolution (~ 4 arcsec for a 10-m dish), as compared to space or airborne THz telescopes. Thus, high-resolution mapping is an important scientific argument. In particular, we can isolate zones of interest for Galactic and extragalactic star-forming regions. The THz windows are suitable for observations of high-excitation CO lines and [N II] 205 um lines, which are scientifically relevant tracers of star formation and stellar feedback. Those lines are the brightest lines in the THz windows, so that they are suitable for the initiation of ground-based THz observations. THz polarization of star-forming regions can also be explored since it traces the dust population contributing to the THz spectral peak. For survey-type observations, we focus on ``sub-THz'' extragalactic surveys, whose uniqueness is to detect galaxies at redshifts z ~ 1--2, where the dust emission per comoving volume is the largest in the history of the Universe. Finally we explore possibilities of flexible time scheduling, which enables us to monitor active galactic nuclei, and to target gamma-ray burst afterglows. For these objects, THz and submillimeter wavelength ranges have not yet been explored.

astro-ph.GA↗

arXiv2015

The connecting molecular ridge in the Galactic center

We report new observations of multiple transitions of the CS molecular lines in the SgrA region of the Galactic center, at an angular resolution of 40" (=1.5 pc). The objective of this paper is to study the polar arc, which is a molecular ridge near the SgrA region, with apparent non-coplanar motions, and a velocity gradient perpendicular to the Galactic plane. With our high resolution dense-gas maps, we search for the base and the origin of the polar arc, which is expected to be embedded in the Galactic disk. We find that the polar arc is connected to a continuous structure from one of the disk ring/arm in both the spatial and velocity domains. This structure near SgrA* has high CS(J=4-3)/CS(J=2-1) ratios >1. That this structure has eluded detection in previous observations, is likely due to the combination of high excitation and low surface brightness temperature. We call this new structure the connecting ridge. We discuss the possible mechanism to form this structure and to lift the gas above the Galactic plane.

astro-ph.GA↗

arXiv2015

Prime Focus Spectrograph for the Subaru telescope: massively multiplexed optical and near-infrared fiber spectrograph

The Prime Focus Spectrograph (PFS) is an optical/near-infrared multifiber spectrograph with 2394 science fibers distributed across a 1.3-deg diameter field of view at the Subaru 8.2-m telescope. The wide wavelength coverage from 0.38 μm to 1.26 μm, with a resolving power of 3000, simultaneously strengthens its ability to target three main survey programs: cosmology, galactic archaeology and galaxy/AGN evolution. A medium resolution mode with a resolving power of 5000 for 0.71 μm to 0.89 μm will also be available by simply exchanging dispersers. We highlight some of the technological aspects of the design. To transform the telescope focal ratio, a broad-band coated microlens is glued to each fiber tip. A higher transmission fiber is selected for the longest part of the cable system, optimizing overall throughput; a fiber with low focal ratio degradation is selected for the fiber-positioner and fiber-slit components, minimizing the effects of fiber movements and fiber bending. Fiber positioning will be performed by a positioner consisting of two stages of piezo-electric rotary motors. The positions of these motors are measured by taking an image of artificially back-illuminated fibers with the metrology camera located in the Cassegrain container; the fibers are placed in the proper location by iteratively measuring and then adjusting the positions of the motors. Target light reaches one of the four identical fast-Schmidt spectrograph modules, each with three arms. The PFS project has passed several project-wide design reviews and is now in the construction phase.

astro-ph.IM↗

arXiv2015

230 GHz VLBI observations of M87: event-horizon-scale structure at the enhanced very-high-energy $\rm γ$-ray state in 2012

We report on 230 GHz (1.3 mm) VLBI observations of M87 with the Event Horizon Telescope using antennas on Mauna Kea in Hawaii, Mt. Graham in Arizona and Cedar Flat in California. For the first time, we have acquired 230 GHz VLBI interferometric phase information on M87 through measurement of closure phase on the triangle of long baselines. Most of the measured closure phases are consistent with 0$^{\circ}$ as expected by physically-motivated models for 230 GHz structure such as jet models and accretion disk models. The brightness temperature of the event-horizon-scale structure is $\sim 1 \times 10^{10}$ K derived from the compact flux density of $\sim 1$ Jy and the angular size of $\sim 40 $ $\rm μ$as $\sim$ 5.5 $R_{\rm s}$, which is broadly consistent with the peak brightness of the radio cores at 1-86 GHz located within $\sim 10^2$ $R_{\rm s}$. Our observations occurred in the middle of an enhancement in very-high-energy (VHE) $\rm γ$-ray flux, presumably originating in the vicinity of the central black hole. Our measurements, combined with results of multi-wavelength observations, favor a scenario in which the VHE region has an extended size of $\sim$20-60 $R_{\rm s}$.

astro-ph.HE↗

arXiv2015

A new era of sub-millimeter GRB afterglow follow-ups with the Greenland Telescope

A planned rapid submillimeter (submm) Gamma Ray Burst (GRBs) follow-up observations conducted using the Greenland Telescope (GLT) is presented. The GLT is a 12-m submm telescope to be located at the top of the Greenland ice sheet, where the high-altitude and dry weather porvides excellent conditions for observations at submm wavelengths. With its combination of wavelength window and rapid responding system, the GLT will explore new insights on GRBs. Summarizing the current achievements of submm GRB follow-ups, we identify the following three scientific goals regarding GRBs: (1) systematic detection of bright submm emissions originating from reverse shock (RS) in the early afterglow phase, (2) characterization of forward shock and RS emissions by capturing their peak flux and frequencies and performing continuous monitoring, and (3) detections of GRBs as a result of the explosion of first-generation stars result of GRBs at a high redshift through systematic rapid follow ups. The light curves and spectra calculated by available theoretical models clearly show that the GLT could play a crucial role in these studies.

astro-ph.HE↗

arXiv2015

Jet Motion, Internal Working Surfaces, and Nested Shells in the Protostellar System HH 212

HH 212 is a nearby (400 pc) highly collimated protostellar jet powered by a Class 0 source in Orion. We have mapped the inner 80" (~ 0.16 pc) of the jet in SiO (J=8-7) and CO (J=3-2) simultaneously at ~ 0.5 resolution with the Atacama Millimeter/Submillimeter Array at unprecedented sensitivity. The jet consists of a chain of knots, bow shocks, and sinuous structures in between. As compared to that seen in our previous observations with the Submillimeter Array, it appears to be more continuous, especially in the northern part. Some of the knots are now seen associated with small bow shocks, with their bow wings curving back to the jet axis, as seen in pulsed jet simulations. Two of them are reasonably resolved, showing kinematics consistent with sideways ejection, possibly tracing the internal working surfaces formed by a temporal variation in the jet velocity. In addition, nested shells are seen in CO around the jet axis connecting to the knots and bow shocks, driven by them. The proper motion of the jet is estimated to be ~ 115+-50 km/s, comparing to our previous observations. The jet has a small semi-periodical wiggle, with a period of ~ 93 yrs. The amplitude of the wiggle first increases with the distance from the central source and then stays roughly constant. One possible origin of the wiggle could be the kink instability in a magnetized jet.

astro-ph.GA↗

arXiv2014

Observations of Infalling and Rotational Motions on a 1,000-AU Scale around 17 Class 0 and 0/I Protostars: Hints of Disk Growth and Magnetic Braking?

We perform imaging and analyses of SMA 1.3 mm continuum, C18O (2-1) and 12CO (2-1) line data of 17 Class 0 and 0/I protostars to study their gas kinematics on a 1,000-AU scale. Continuum and C18O (2-1) emission are detected toward all the sample sources and show central primary components with sizes of ~600-1,500 AU associated with protostars. The velocity gradients in C18O (2-1) have wide ranges of orientations from parallel to perpendicular to the outflows, with magnitudes from ~1 to ~530 km/s/pc. We construct a simple kinematic model to reproduce the observed velocity gradients, estimate the infalling and rotational velocities, and infer the disk radii and the protostellar masses. The inferred disk radii range from <5 AU to >500 AU with estimated protostellar masses from <0.1 Msun to >1 Msun. Our results hint that both large and small disks are possibly present around Class 0 protostars, which could be a sign of disk growth at the Class 0 stage. In addition, the directions of the overall velocity gradients in 7 out of the 17 sources are close to perpendicular to their outflow axes, which is a signature of significant rotational motions. From our model fitting, the specific angular momenta in these sources are estimated to be >2E-4 km/s*pc, suggesting that magnetic braking is unlikely efficient on a 1,000-AU scale in these Class 0 and 0/I sources. In a sub-sample with observed magnetic field orientations, we find no source with large specific angular momenta together with closely aligned magnetic field and outflow axes. This possibly hints that the magnetic field, if originally aligned with the rotational axis, can play a role in removing angular momentum from infalling material at the Class 0 stage. We discuss our results in comparison with theoretical models of collapsing dense cores with and without magnetic fields in the context of disk formation.

astro-ph.SR↗

arXiv2014

The Importance of the Magnetic Field from an SMA-CSO-Combined Sample of Star-Forming Regions

Submillimeter dust polarization measurements of a sample of 50 star-forming regions, observed with the SMA and the CSO covering pc-scale clouds to mpc-scale cores, are analyzed in order to quantify the magnetic field importance. The magnetic field misalignment $δ$ -- the local angle between magnetic field and dust emission gradient -- is found to be a prime observable, revealing distinct distributions for sources where the magnetic field is preferentially aligned with or perpendicular to the source minor axis. Source-averaged misalignment angles $\langle|δ|\rangle$ fall into systematically different ranges, reflecting the different source-magnetic field configurations. Possible bimodal $\langle|δ|\rangle$-distributions are found for the separate SMA and CSO samples. Combining both samples broadens the distribution with a wide maximum peak at small $\langle|δ|\rangle$-values. Assuming the 50 sources to be representative, the prevailing source-magnetic field configuration is one that statistically prefers small magnetic field misalignments $|δ|$. When interpreting $|δ|$ together with an MHD force equation, as developed in the framework of the polarization-intensity gradient method, a sample-based scaling fits the magnetic field tension-to-gravity force ratio $\langleΣ_B\rangle$ versus $\langle|δ|\rangle$ with $\langleΣ_B\rangle = 0.116 \cdot \exp(0.047\cdot \langle|δ|\rangle)\pm 0.20$ (mean error), providing a way to estimate the relative importance of the magnetic field, only based on measurable field misalignments $|δ|$. The force ratio $Σ_B$ discriminates systems that are collapsible on average ($\langle Σ_B\rangle <1$) from other molecular clouds where the magnetic field still provides enough resistance against gravitational collapse ($\langle Σ_B\rangle >1$) (abridged).

astro-ph.GA↗

arXiv2014

Magnetic field structure in the Flattened Envelope and Jet in the young protostellar system HH 211

HH 211 is a young Class 0 protostellar system, with a flattened envelope, a possible rotating disk, and a collimated jet. We have mapped it with the Submillimeter Array in 341.6 GHz continuum and SiO J=8-7 at ~ 0.6 resolution. The continuum traces the thermal dust emission in the flattened envelope and the possible disk. Linear polarization is detected in the continuum in the flattened envelope. The field lines implied from the polarization have different orientations, but they are not incompatible with current gravitational collapse models, which predict different orientation depending on the region/distance. Also, we might have detected for the first time polarized SiO line emission in the jet due to the Goldreich-Kylafis effect. Observations at higher sensitivity are needed to determine the field morphology in the jet.

astro-ph.GA↗