arXiv subjects

Sun Kwok

Publications and source records attributed to Sun Kwok.

53 records · Page 3Linked to original sources

arXiv2010

Three-Dimensional Structure of the Central Region of NGC7027: A Quest for Trails of High-Velocity Jets

We report on the results of a radio interferometric observation of NGC7027 in the CO J=2-1 and 13CO J=2-1 lines. The results are analyzed with morpho-kinematic models developed from the software tool Shape. Our goal is to reveal the morpho-kinematic properties of the central region of the nebula, and to explore the nature of unseen high-velocity jets that may have created the characteristic structure of the central region consisting of molecular and ionized components. A simple ellipsoidal shell model explains the intensity distribution around the systemic velocity, but the high velocity features deviate from the ellipsoidal model. Through the Shape automatic reconstruction model, we found a possible trail of a jet only in one direction, but no other possible holes were created by the passage of a jet.

astro-ph.SR↗

arXiv2009

Morphological Structures of Planetary Nebulae

Since various structural components of planetary nebulae manifest themselves differently, a combination of optical, infrared, submm, and radio techniques is needed to derive a complete picture of planetary nebulae. The effects of projection can also make the derivation of the true 3-D structure difficult. Using a number of examples, we show that bipolar and multipolar nebulae are much more common than usually inferred from morphological classifications of apparent structures of planetary nebulae. We put forward a new hypothesis that the bipolar and multipolar lobes of PN are not regions of high-density ejected matter, but the result of ionization and illumination. The visible bright regions are in fact volume of low densities (cleared by high-velocity outflows) where the UV photons are being channelled through. We suggest that multipolar nebulae with similar lobe sizes are not caused by simultaneous ejection of matter in several directions, but by leakage of UV photons into those directions.

astro-ph.SR↗

arXiv2009

Planetary Nebulae Detected in the Spitzer Space Telescope GLIMPSE II Legacy Survey

We report the result of a search for the infrared counterparts of 37 planetary nebulae (PNs) and PN candidates in the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire II (GLIMPSE II) survey. The photometry and images of these PNs at 3.6, 4.5, 5.8, 8.0, and 24mum, taken through the Infrared Array Camera (IRAC) and the Multiband Imaging Photometer for Spitzer (MIPS), are presented. Most of these nebulae are very red and compact in the IRAC bands, and are found to be bright and extended in the 24mum band. The infrared morphology of these objects are compared with Ha images of the Macquarie-AAO-Strasbourg (MASH) and MASH II PNs. The implications for morphological difference in different wavelengths are discussed. The IRAC data allow us to differentiate between PNs and HII regions and be able to reject non-PNs from the optical catalogue (e.g., PNG 352.1-00.0). Spectral energy distributions (SEDs) are constructed by combing the IRAC and MIPS data with existing near-, mid-, and far-IR photometry measurements. The anomalous colors of some objects allow us to infer the presence of aromatic emission bands. These multi-wavelength data provide useful insights into the nature of different nebular components contributing to the infrared emission of PNs.

astro-ph.SR↗

arXiv2009

A molecular line survey of the extreme carbon star CRL 3068 at millimeter wavelengths

We present the results of a molecular line survey of the extreme carbon star CRL 3068. The observations were carried out with the Arizona Radio Observatory (ARO) 12m telescope and the Heinrich Hertz Submillimeter Telescope (SMT) at the 2mm and 1.3mm atmospheric windows. The observations cover the frequency bands from 130--162 GHz and 219.5--267.5 GHz. The typical sensitivities achieved are T_R<15 mK and T_R<7 mK for the ARO 12m and SMT, respectively. Seventy two individual emission features belonging to 23 molecular species and isotopologues were detected. Only three faint lines remain unidentified. The species c-C3H, CH3CN, SiC2, and the isotopologues, C17O and C18O, HC15N, HN13C, C33S, C34S, 13CS, 29SiS, and 30SiS are detected in this object for the first time. Rotational diagram analysis is carried out to determine the column densities and excitation temperatures. The isotopic ratios of the elements C, N, O, S, and Si have also been estimated. The results are consistent with stellar CNO processing and suggest that CRL 3068 is more carbon rich than IRC+10216 and CIT 6. It is also shown that the chemical composition in CRL 3068 is somewhat different from that in IRC+10216 with a more extensive synthesis of cyclic and long-chain molecules in CRL 3068. The results will provide valuable clues for better understanding circumstellar chemistry.

astro-ph.SR↗

arXiv2009

Probing the mass loss history of the yellow hypergiant IRC+10420

We have used the sub-millimeter array to image the molecular envelope around IRC+10420. Our observations reveal a large and clumpy expanding envelope around the star. The molecular envelope shows a clear asymmetry in $^{12}$CO J=2--1 emission in the South-West direction. The elongation of the envelope is found even more pronounced in the emission of $^{13}$CO J=2--1 and SO J$_{\rm K}$=6$_5$--5$_4$. A small positional velocity gradient across velocity channels is seen in these lines, suggesting the presence of a weak bipolar outflow in the envelope of IRC+10420. In the higher resolution $^{12}$CO J=2--1 map, we find that the envelope has two components: (1) an inner shell (shell I) located between radius of about 1"-2"; (2) an outer shell (shell II) located between 3" to 6" in radius. These shells represent two previous mass-loss episodes from IRC+10420. We attempt to derive in self-consistent manner the physical conditions inside the envelope by modelling the dust properties, and the heating and cooling of molecular gas. We estimate a mass loss rate of $\sim$9 10$^{-4}$ M$_\odot$ yr$^{-1}$ for shell I and 7 10$^{-4}$ M$_\odot$ yr$^{-1}$ for shell II. The gas temperature is found to be unusually high in IRC+10420 in comparison with other oxygen-rich envelopes. The elevated gas temperature is mainly due to higher heating rate, which results from the large luminosity of the central s tar. We also derive an isotopic ratio $^{12}$C/$^{13}$C = 6.

astro-ph.SR↗

arXiv2009

A Spitzer Study of 21 and 30 Micron Emission in Several Galactic Carbon-rich Proto-Planetary Nebulae

We have carried out mid-infrared spectroscopy of seven Galactic proto-planetary nebulae (PPNs) using the Spitzer Space Telescope. They were observed from 10-36 microns at relatively high spectral resolution, R~600. The sample was chosen because they all gave some evidence in the visible of a carbon-rich chemistry. All seven of the sources show the broad, unidentified 21 micron emission feature; three of them are new detections (IRAS 06530-0213, 07430+1115, and 19477+2401) and the others are observed at higher S/N than in previous spectra. These have the same shape and central wavelength (20.1 microns) as found in the ISO spectra of the brighter PPNs. The 30 micron feature was seen in all seven objects. However, it is not resolved into two separate features (26 and 33 microns) as was claimed on the basis of ISO spectra, which presumably suffered from the noisy detector bands in this region. All showed the infrared aromatic bands (AIB) at 11.3, 12.4, and 13.3 microns. Five of these also appear to have the C2H2 molecular band at 13.7 microns, one in absorption and four in emission. This is extremely rare, with only one other evolved star, IRC+10216, in which C2H2 emission has been observed. Four also possessed a broad, unidentified emission feature at 15.8 microns that may possibly be related to the 21 micron feature. Model fits were made to the spectral energy distributions for these PPNs to determine properties of the detached circumstellar envelopes. The 21 micron feature has been seen in all Galactic carbon-rich PPNs observed, and thus its carrier appears to be a common component of the outflow around these objects.

astro-ph.SR↗

arXiv2008

Morpho-Kinematic Properties of the 21-Micron Source IRAS 07134+1005

We report the results of a Submillimeter Array (SMA) interferometric observation of 21-micron source IRAS 07134+1005 in the CO J=3-2 line. In order to determine the morpho-kinematic properties of the molecular envelope of the object, we constructed a model using the Shape software to model the observed CO map. We find that the molecular gas component of the envelopes can be interpreted as a geometrically thick expanding torus with an expanding velocity of 8 km/s. The inner and outer radii of the torus determined by fitting Shape models are 1.2" and 3.0", respectively. The inner radius is consistent with the previous values determined by radiative transfer modeling of the spectral energy distribution and mid-infrared imaging of the dust component. The radii and expansion velocity of the torus suggest that the central star has left the asymptotic giant branch about 1140-1710 years ago, and that the duration of the equatorial enhanced mass loss is about 2560-3130 years. From the absence of an observed jet, we suggest that the formation of a bipolar outflow may lack behind in time from the creation of the equatorial torus.

astro-ph↗

arXiv2008

A Molecular Line Survey of the Highly Evolved Carbon Star CIT 6

We present a spectral line survey of the C-rich envelope CIT 6 in the 2mm and 1.3mm bands carried out with the Arizona Radio Observatory (ARO) 12m telescope and the Heinrich Hertz Submillimeter Telescope (SMT). The observations cover the frequency ranges of 131--160 GHz, 219--244 GHz, and 252--268 GHz with typical sensitivity limit of T_R<10 mK. A total of 74 individual emission features are detected, of which 69 are identified to arise from 21 molecular species and isotopologues, with 5 faint lines remaining unidentified. Two new molecules (C4H and CH3CN) and seven new isotopologues (C17O, 29SiC2, 29SiO, 30SiO, 13CS, C33S, and CS) are detected in this object for the first time. The column densities, excitation temperatures, and fractional abundances of the detected molecules are determined using rotation diagram analysis. Comparison of the spectra of CIT 6 to that of IRC+10216 suggests that the spectral properties of CIT 6 are generally consistent with those of IRC+10216. For most of the molecular species, the intensity ratios of the lines detected in the two objects are in good agreement with each other. Nevertheless, there is evidence suggesting enhanced emission from CN and HC3N and depleted emission from HCN, SiS, and C4H in CIT 6. Based on their far-IR spectra, we find that CIT 6 probably has a lower dust-to-molecular gas ratio than IRC+10216. To investigate the chemical evolution of evolved stars, we compare the molecular abundances in the AGB envelopes CIT 6 and IRC+10216 and those in the bright proto-planetary nebula CRL 618. The implication on the circumstellar chemistry is discussed.

astro-ph↗

arXiv2008

A spectral line survey of NGC 7027 at millimeter wavelengths

We report on a recent spectral line survey of the planetary nebula (PN) NGC 7027 using the Arizona Radio Observatory (ARO) 12m telescope and the Heinrich Hertz Submillimeter Telescope (SMT) at millimeter wavelengths. The spectra covering the frequency ranges 71--111GHz, 157--161GHz, and 218--267GHz were obtained with a typical sensitivity of rms<8mK. A total of 67 spectral lines are detected, 21 of which are identified with 8 molecular species, 32 with recombination lines from hydrogen and helium, and 14 remains unidentified. As the widths of emission lines from CO, other neutral molecules, molecular ions, as well as recombination of H+ and He+ are found to be different with each other, the line strengths and profiles are used to investigate the physical conditions and chemical processes of the neutral envelope of NGC 7027. The column densities and fractional abundances relative to H2 of the observed molecular species are calculated and compared with predictions from chemical models. We found evidence for overabundance of N2H+ and underabundance of CS and HNC in NGC 7027, suggesting that X-ray emission and shock wave may play an important role in the chemistry of the hot molecular envelope of the young PN.

astro-ph↗

arXiv2007

Submillimeter Array Observation of the Proto-Planetary Nebula CRL 618 in the CO J=6-5 Line

We report on the results of a Submillimeter Array interferometric observation of the proto-planetary nebula CRL 618 in the 12CO J=6-5 line. With the new capability of SMA enabling us to use two receivers at a time, we also observed simultaneously in the 12CO J=2-1 and 13CO J=2-1 lines. The 12CO J=6-5 and 13CO J=2-1 lines were first interferometrically observed toward CRL 618. The flux of the high velocity component of the 12CO J=6-5 line is almost fully recovered, while roughly 80% of the flux of the low velocity component is resolved out. The low recovery rate suggests that the emission region of the low velocity component of the 12CO J=6-5 line is largely extended. Continuum emission is detected both at 230 and 690 GHz. The flux of the 690 GHz continuum emission seems to be partially resolved out, suggesting dust emission partly contaminates the 690 GHz continuum flux. The cavity structure, which has been confirmed in a previous observation in the 12CO J=2-1 line, is not clearly detected in the 12CO J=6-5 line, and only the south wall of the cavity is detected. This result suggests that the physical condition of the molecular envelope of CRL 618 is not exactly axial symmetric.

astro-ph↗

arXiv2006

Diffraction-limited 3 um Spectroscopy of IRAS 04296+3429 and IRAS 05341+0852: Spatial Extent of Hydrocarbon Dust Emission and Dust Evolutionary Sequence

We present 3 um spectroscopy of the carbon-rich proto-planetary nebulae IRAS 04296+3429 and IRAS 05341+0852 conducted with the adaptive optics system at the Subaru Telescope. We utilize the nearly diffraction-limited spectroscopy to probe the spatial extent of the hydrocarbon dust emitting zone. We find a hydrocarbon emission core extending up to 100--160 mas from the center of IRAS 04296+3429, corresponding to a physical diameter of 400--640 AU, assuming a distance of 4 kpc. On the other hand, we find that IRAS 05341+0852 is not spatially resolved with this instrumentation. The physical extent of these proto-planetary nebulae, along with the reanalyzed data of IRAS 22272+5435 published previously, suggests a correlation between the physical extent of the hydrocarbon dust emission and the spectral evolution of the aliphatic to aromatic features in these post-AGB stars. These measurements represent the first direct test of the proposed chemical synthesis route of carbonaceous dust in the circumstellar environment of evolved stars.

astro-ph↗

arXiv2006

Submillimetre observations of comets with Odin: 2001-2005

The Odin satellite, launched in Feb. 2001, is equipped with a 1.1-m submillimetre telescope. Odin was used to observe the 557 GHz line of water with high spectral resolution in 12 comets between 2001 and 2005. Line shapes and spatial mapping provide information on the anisotropy of the outgassing and constraints on water excitation, enabling accurate measurements of the water production rate. Five comets were regularly observed over periods of more than one month to monitor the variation of their water outgassing rate with heliocentric distance. Observing campaigns have been generally coordinated with ground-based observations of molecular lines at Nançay, CSO or IRAM 30-m telescopes to obtain molecular abundances relative to water. Thanks to Odin's frequency coverage, it was also possible to detect the H_2^18O 548GHz line, first in comet 153P/Ikeya-Zhang in April 2002 (Lecacheux et al., 2003) and then in comets C/2002 T7 (LINEAR), C/2001 Q4 (NEAT) and C/2004 Q2 (Machholz). The ^16O/^18O isotopic ratio (\approx450) is consistent with the terrestrial value. Ammonia has been searched for in three comets through its J_K = 1_0-0_0 line at 572 GHz and was tentatively detected in C/2001 Q4 and C/2002 T7. The derived abundances of NH_3 relative to water are 0.5 % and 0.3 %, respectively, similar to values obtained in other comets with different techniques.

astro-ph↗

arXiv2006

Radio observations of comet 9P/Tempel 1 before and after Deep Impact

Comet 9P/Tempel 1 was the target of a multi-wavelength worldwide investigation in 2005. The NASA Deep Impact mission reached the comet on 4.24 July 2005, delivering a 370 kg impactor which hit the comet at 10.3 km/s. Following this impact, a cloud of gas and dust was excavated from the comet nucleus. The comet was observed in 2005 prior to and after the impact, at 18-cm wavelength with the Nançay radio telescope, in the millimetre range with the IRAM and CSO radio telescopes, and at 557 GHz with the Odin satellite. Post-impact observations at IRAM and CSO did not reveal a significant change of the outgassing rates and relative abundances, with the exception of CH_3OH which may have been more abundant by up to one order of magnitude in the ejecta. Most other variations are linked to the intrinsic variability of the comet. The Odin satellite monitored nearly continuously the H_2O line at 557 GHz during the 38 hours following the impact on the 4th of July, in addition to weekly monitoring. Once the periodic variations related to the nucleus rotation are removed, a small increase of outgassing related to the impact is present, which corresponds to the release of \approx 5000+/-2000 tons of water. Two other bursts of activity, also observed at other wavelengths, were seen on 23 June and 7 July; they correspond to even larger releases of gas.

astro-ph↗

arXiv2006

An Infrared Imaging Study of the Bipolar Proto-Planetary Nebula IRAS 16594-4656

High-resolution mid-infrared images have been obtained in N-band and Q-band for the proto-planetary nebula IRAS 16594-4656. A bright equatorial torus and a pair of bipolar lobes can clearly be seen in the infrared images. The torus appears thinner at the center than at the edges, suggesting that it is viewed nearly edge-on. The infrared lobes correspond to the brightest lobes of the reflection nebula seen in the Hubble Space Telescope (HST) optical image, but with no sign of the point-symmetric structure seen in the visible image. The lobe structure shows a close correspondence with a molecular hydrogen map obtained with HST, suggesting that the dust emission in the lobes traces the distribution of the shocked gas. The shape of the bipolar lobes shows clearly that the fast outflow is still confined by the remnant circumstellar envelope of the progenitor asymptotic giant branch (AGB) star. However, the non-detection of the dust outside of the lobes suggests that the temperature of the dust in the AGB envelope is too low for it to be detected at 20 microns.

astro-ph↗

arXiv2005

Polarization of L Dwarfs by Dust Scattering

The degree of polarization in L dwarfs of spectral types L0 to L6 resulting from dust scattering in a rotation-induced oblate photosphere is calculated. Assuming that forsterite is the main condensate, the atmospheric dust distribution is derived for different spectral types based on a chemical equilibrium model. The degree of polarization at optical is then calculated using a single scattering model. The expected linear polarization at optical is found to peak at around spectral type L1. For a fixed rotational velocity, the degree of polarization decreases from hotter to cooler objects. However, with the increase in mean grain size, the degree of linear polarization reduces significantly. We fit the recently observed linear polarimetric data of L dwarfs and find that single dust scattering model coupled with the chemical equilibrium models of condensates is consistent with the observational results.

astro-ph↗

arXiv2003

Radio Cores of Bipolar Nebulae: Search for Collimated Winds

We present results of our search for collimated ionized winds in bipolar nebulae using the Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA). Our search is motivated by the discovery of an ionized jet in the bipolar nebula M 2-9 (Lim & Kwok 2003) that may be responsible for sculpting the nebula's mirror-symmetric structure. To determine if such jets are a common feature of bipolar nebulae, we searched for optically-thick radio cores - a characteristic signature of ionized jets - in 11 northern nebulae with the VLA at 1.3 cm and 0.7 cm, and in 5 southern nebulae with the ATCA at 6 cm and 3.6 cm. Two northern objects, 19W32 and M 1-91, and two southern objects, He2-84 and and Mz 3, exhibit a compact radio core with a rising spectrum consistent with an ionized jet. Th 2-B exhibits a steeply falling spectrum characteristic of nonthermal radio emission. Here we present a preliminary analysis of these five radio cores and discuss the implications of our results.

astro-ph↗

arXiv2003

High-Resolution Near-Infrared Imaging and Polarimetry of Four Proto-Planetary Nebulae

High-resolution near-infrared HST NICMOS (F160W, F222M) images and polarization (2 um) observations were made of four bipolar proto-planetary nebulae (PPNs): IRAS 17150-3224, IRAS 17441-2411, IRAS 17245-3951, and IRAS 16594-4656. The first three of these are viewed nearly edge-on, and for the first time the central stars in them are seen. Color maps reveal a reddened torus between the bipolar lobes in the edge-on cases, with bluer lobes. The polarization values are high, with maximum values ranging from 40 to 80%. The polarization patterns are basically centrosymmetric, with some deviations in the low polarization equatorial regions. For IRAS 17150-3224, circumstellar arcs are seen at 1.6 um, along with a newly-discovered loop in the equatorial region. Bright caps are seen at the end of the lobes, indicating that they are not open-ended. A distinct point-symmetric pattern is seen in the strengths of the polarization vectors, especially in IRAS 17150-3224. HST NICMOS observations provide a valuable complement to the WFPC2 visible images in deriving the basic structure of bipolar PPNs.

astro-ph↗