1,045 research outputs found

    Occurrence Frequency of CO Outflows in Massive Protostellar Candidates

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    We mapped 12 massive protostellar candidates in the CO J=2-1 line, which in combination with Zhang et al. (2005) completes an unbiased survey of outflows for all 48 sources with l>50^o in a sample of 101 massive protostellar candidates. We detected outflows in 10 sources, implying 88% occurrence frequency of outflows for the 48 sources. This supports the conclusion of previous studies that bipolar outflows are an integral component in the formation process of massive stars. The vast majority of the observed outflows are much more massive (>10~Mo) and energetic (>100~Mo~km/s) than outflows from low-mass protostars. They also have large mass outflow rates (>2x10^{-4}~Mo/yr), suggesting large (~1x10^{-4}~Mo/yr) accretion rates sufficient to overcome radiation pressure of the central massive protostars. We compared the frequency distribution of collimation factors of 40 massive outflows including those of this study with that of 36 low-mass outflows from the literature, and found NO significant difference between the two. All these results are consistent with the suggestion that massive stars form through accretion as do low-mass stars but with much higher accretion rates.Comment: 15 pages, 3 figures, Accepted by Ap

    Studies on the effect of heat shock, culture conditions, and packaging conditions on the heat resistance, recovery, and virulence of Listeria monocytogenes in ground pork

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    The heat resistance of two Listeria monocytogenes subspecies was determined, according to various heating conditions. In general, serotype 1 was more heat resistant than Scott A, and both subspecies were more heat resistant if packaged aerobically than anaerobically during heating. Cells exposed to 48°C for 2h prior to heating (heat shock) were more heat resistant than nonheat-shocked controls. Less survivors were detected when the cells were inoculated in pork stored for 3 months compared with fresh pork, suggesting that oxidative components can decrease cell survival during heating. Heating the samples at a slow heating rate (1.3°C/min) resulted in the highest number of survivors to heating at 62°C when compared with samples heated at a fast rate (8.0°C/min). The recovery of heat-injured Listeria during storage at 4-30°C in aerobically-packaged or vacuum-packaged ground pork was determined. Addition of 100-300 ppm of butylated hydroxyanisole (BHA) and 300-700 ppm of BHT (butylated hydroxytoluene) was examined in aerobically packaged ground pork at 7°C and 30°C. Listeria was recovered more rapidly in vacuum-packaging at 4°C, but BHA and BHT did not significantly affect cell growth at 7 and 30°C;The effect of various growth conditions on production of LLO were examined. Serotype 1 produced more LLO than Scott A and, in a fed-batch method, maximum LLO production was detected at pH 5.5-6.5 after 2h stationary phase. The production rate of LLO by heat-shocked cells increased more than 40-fold, in comparison with a two-fold rate increase for controls within 4h of incubation after the heat shock treatment. However, heat shocking inactivated existing LLO, so the levels of LLO were many times higher in controls than in heat-shocked cells after 4h of incubation;An enzyme-linked immunosorbent assay was developed for quantitative analysis of LLO by using 5 mg/ml ASO and a 1:1,000 dilution of antibody conjugated with alkaline phosphatase. The LLO was inactivated easily with heat but the protein structure was stable at temperatures below 80°C for 3 min;These studies can serve the industry to design procedures that will more effectively prevent this organism from surviving during heating and storage in ground pork products

    The HCN/HNC abundance ratio toward different evolutionary phases of massive star formation

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    Using the HCN and HNC J=1--0 line observations, the abundance ratio of HCN/HNC has been estimated for different evolutionary stages of massive star formation: Infrared dark clouds (IRDCs), High-mass protostellar object (HMPOs), and Ultra-compact HII regions (UCHIIs). IRDCs were divided into `quiescent IRDC cores' and `active IRDC cores', depending on star formation activity. The HCN/HNC ratio is known to be higher at active and high temperature regions related to ongoing star formation, compared to cold and quiescent regions. Our observations toward 8 quiescent IRDC cores, 16 active IRDC cores, 23 HMPOs, and 31 UCHIIs show consistent results; the ratio is 0.97~(± 0.10\pm~0.10), 2.65~(± 0.88\pm~0.88), 4.17~(± 1.03\pm~1.03) and 8.96~(± 3.32\pm~3.32) in these respective evolutionary stages, increasing from quiescent IRDC cores to UCHIIs. The change of the HCN/HNC abundance ratio, therefore, seems directly associated with the evolutionary stages of star formation, which have different temperatures. One suggested explanation for this trend is the conversion of HNC to HCN, which occurs effectively at higher temperatures. To test the explanation, we performed a simple chemical model calculation. In order to fit the observed results, the energy barrier of the conversion must be much lower than the value provided by theoretical calculations.Comment: 55 pages, 8 figures, accepted for publication in ApJ

    Infrared Excess and Molecular Gas in Galactic Supershells

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    We have carried out high-resolution observations along one-dimensional cuts through the three Galactic supershells GS 064-01-97, GS 090-28-17, and GS 174+02-64 in the HI 21 cm and CO J=1-0 lines. By comparing the HI data with IRAS data, we have derived the distributions of the I_100 and tau_100 excesses, which are, respectively, the 100 mum intensity and 100 mum optical depth in excess of what would be expected from HI emission. We have found that both the I_100 and tau_100 excesses have good correlations with the CO integrated intensity W_CO in all three supershells. But the I_100 excess appears to underestimate H_2 column density N(H_2) by factors of 1.5-3.8. This factor is the ratio of atomic to molecular infrared emissivities, and we show that it can be roughly determined from the HI and IRAS data. By comparing the tau_100 excess with W_CO, we derive the conversion factor X = N(H_2)/W_CO = 0.26-0.66 in the three supershells. In GS 090-28-17, which is a very diffuse shell, our result suggests that the region with N(H_2) < 3*10^20 cm^-2 does not have observable CO emission, which appears to be consistent with previous results indicating that diffuse molecular gas is not observable in CO. Our results show that the molecular gas has a 60/100 mum color temperature T_d lower than the atomic gas. The low value of T_d might be due either to the low equilibrium temperature or to the lower abundance of small grains, or a combination of both.Comment: Latex file, 13 pages, 7 postscript figures, uses jkas32.sty Accepted for publication in Jour. of Korean Astron. Societ

    Hierarchical Structure of HII Regions - Ultracompact, Compact, and Extended Components

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    We have observed extended emission physically associated with 15 out of 16 UC HII regions with large ratios of single-dish to VLA fluxes. We discuss their implications for the age problem of UC HII regions and the evolution of HII regions.Comment: 2 pages, To appear in Hot Star Workshop III: The Earliest Phases of Massive Star Birth, ASP Conference Series, 2002, ed. P. A. Crowthe

    The inflow signature toward different evolutionary phases of massive star formation

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    We analyzed both HCN J=1-0 and HNC J=1-0 line profiles to study the inflow motions in different evolutionary stages of massive star formation: 54 infrared dark clouds (IRDCs), 69 high-mass protostellar object (HMPOs), and 54 ultra-compact HII regions (UCHIIs). The inflow asymmetry in HCN spectra seems to be prevalent throughout all the three evolutionary phases, with IRDCs showing the largest excess in blue profile. In the case of HNC spectra, the prevalence of blue sources does not appear, excepting for IRDCs. We suggest that this line is not appropriate to trace inflow motion in evolved stages of massive star formation because the abundance of HNC decreases at high temperatures. This result spotlights the importance of considering chemistry in the dynamics study of massive star-forming regions. The fact that the IRDCs show the highest blue excess in both transitions indicates that the most active inflow occurs in the early phase of star formation, i.e., the IRDC phase rather than in the later phases. However, mass is still inflowing onto some UCHIIs. We also found that the absorption dips of the HNC spectra in 6 out of 7 blue sources are red-shifted relative to their systemic velocities. These red-shifted absorption dips may indicate global collapse candidates, although mapping observations with better resolution are needed to examine this feature in more detail.Comment: 36 pages, 7 figures, 5 Tables, accepted for publication in ApJ

    Magnetic fields in multiple bright-rimmed clouds in different directions of H ~II region IC1396 - II

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    Bright-rimmed clouds form on the edges of H II regions affected by the high energy radiation from a central ionizing source. The UV radiation from the ionizing source results in compression and ionization causing either cloud disruption or further star formation. In this work, we present R-band polarization measurements towards four bright-rimmed clouds, IC1396A, BRC 37, BRC 38, and BRC 39, located in the different directions of the H II region, Sh2-131, in order to map magnetic fields (B-fields) in the plane of the sky. These BRCs are illuminated by the O star HD206267 and present a range of projected on sky geometries. This provides an opportunity to understand the magnetized evolution of BRCs. The B-field geometries of the clouds deduced from the polarization data, after correction for foreground ISM contamination, are seen to be connected to the ambient B-fields on the large scale. They seem to play an important role in shaping the cloud IC1396A and BRC 37. BRCs 38 and 39 show a broader and snubber head morphology possibly due to the B-fields being aligned with incoming radiation as explained in the simulations. A good general agreement is noted on comparing our observational results with the simulations supporting the importance of B-fields in BRC evolution. This work is the first step towards systematic mapping the B-fields morphology in multiple BRCs in an expanding H II region, extending the work presented by Soam et al. (2017b).Comment: 13 pages, 8 figures, accepted for publication in MNRA

    Simultaneous Survey of Water and Class I Methanol Masers toward Red MSX Sources

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    We report simultaneous single-dish surveys of 22 GHz H2O and 44 and 95 GHz class I CH3OH masers toward 299 Red MSX Sources in the protostellar stage. The detection rates are 45% at 22 GHz, 28% at 44 GHz, and 23% at 95 GHz. There are 15, 53, and 51 new discoveries at 22, 44, and 95 GHz, respectively. We detect high-velocity (>30 km/s) features in 27 H2O maser sources. The 95 GHz maser emission is detected only in 44 GHz maser sources. The two transitions show strong correlations in the peak velocity, peak flux density, and isotropic maser luminosity, indicating that they are likely generated in the same sites by the same mechanisms. The 44 GHz masers have much narrower distributions than 22 GHz masers in the relative peak velocity and velocity range, while 6.7 GHz class II CH3OH masers have distributions intermediate between the two. The maser luminosity significantly correlates with the parental clump mass, while it correlates well with the bolometric luminosity of the central protostar only when data of the low-mass regime from the literature are added. Comparison with the results of previous maser surveys toward massive star-forming regions suggests that the detection rates of 22 and 44 GHz masers tend to increase as the central objects evolve. This is contrary to the trends found in low- and intermediate-mass star-forming regions. Thus the occurrence of both masers might depend on the surrounding environments as well as on the evolution of the central object.Comment: 65 pages, 18 figures, 6 tables, accepted for publication in ApJ

    Simultaneous observation of water and class I methanol masers toward class II methanol maser sources

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    We present a simultaneous single-dish survey of 22 GHz water maser and 44 GHz and 95 GHz class I methanol masers toward 77 6.7 GHz class II methanol maser sources, which were selected from the Arecibo methanol maser Galactic plane survey (AMGPS) catalog.Water maser emission is detected in 39 (51%) sources, of which 15 are new detections. Methanol maser emission at 44 GHz and 95 GHz is found in 25 (32%) and 19 (25%) sources, of which 21 and 13 sources are newly detected, respectively. We find 4 high-velocity (> 30 km/s) water maser sources, including 3 dominant blue- or redshifted outflows.The 95 GHz masers always appear with the 44 GHz maser emission. They are strongly correlated with 44 GHz masers in velocity, flux density, and luminosity, while they are not correlated with either water or 6.7 GHz class II methanol masers. The average peak flux density ratio of 95 GHz to 44 GHz masers is close to unity, which is two times higher than previous estimates. The flux densities of class I methanol masers are more closely correlated with the associated BGPS core mass than those of water or class II methanol masers. Using the large velocity gradient (LVG) model and assuming unsaturated class I methanol maser emission, we derive the fractional abundance of methanol to be in a range of 4.2*10^-8 to 2.3*10^-6, with a median value of 3.3\pm2.7*10^-7.Comment: 45 pages, 15 figures, 5 tables. Accepted for publication in ApJ

    Galactic HCO+ Absorption toward Compact Extragalactic Radio Sources

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    As part of the search for the "dark molecular gas (DMG)," we report on the results of HCO+^+ J = 1-0 absorption observations toward nine bright extragalactic millimeter wave continuum sources. The extragalactic sources are at high Galactic latitudes (|b| > 10deg\deg) and seen at small extinction (E(B-V) \lesssim 0.1 mag). We have detected the HCO+^+ absorption lines toward two sources, B0838+133 and B2251+158. The absorption toward B2251+158 was previously reported, while the absorption toward B0838+133 is a new detection. We derive hydrogen column densities or their upper limits toward the nine sources from our observations and compare them to those expected from CO line emission and far-infrared dust continuum emission. Toward the seven sources with no HCO+^+ detection, CO emission has not been detected, either. Thus the sight lines are likely to be filled with almost pure atomic gas. Toward the two sources with HCO+^+ detection, CO emission has been also detected. Comparison of the H2_2 column densities from HCO+^+ absorption and CO emission suggests a non-negligible amount of DMG toward B0838+133.Comment: 10 pages, 3 figures; accepted in Astrophysics and Space Science (ASTR-D-18-00258R1