X. W. Zheng

DISTANCE AND KINEMATICS OF THE RED HYPERGIANT VY CMa: VERY LONG BASELINE ARRAY AND VERY LARGE ARRAY ASTROMETRY

We report astrometric results of phase-referencing very long baseline interferometry observations of 43 GHz SiO maser emission toward the red hypergiant VY Canis Majoris (VY CMa) using the Very Long Baseline Array (VLBA). We measured a trigonometric parallax of 0.83 +/- 0.08 mas, corresponding to a distance of 1.20(-0.10)(+0.13) kpc. Compared to previous studies, the spatial distribution of SiO masers has changed dramatically, while its total extent remains similar. The internal motions of the maser spots are up to 1.4 mas yr(-1), corresponding to 8 km s(-1), and show a tendency for expansion. After modeling the expansion of maser spots, we derived an absolute proper motion for the central star of mu(x) =-2.8 +/- 0.2 and mu(y) = 2.6 +/- 0.2 mas yr(-1) eastward and northward, respectively. Based on the maser distribution from the VLBA observations, and the relative position between the radio photosphere and the SiO maser emission at 43 GHz from the complementary Very Large Array observations, we estimate the absolute position of VY CMa at mean epoch 2006.53 to be alpha(J2000) = 07(h)22(m)58.(s)3259 +/- 0.(s)0007, delta(J2000) =-25 degrees 4603.063 +/- 0.010. The position and proper motion of VY CMa from the VLBA observations differ significantly with values measured by the Hipparcos satellite. These discrepancies are most likely associated with inhomogeneities and dust scattering the optical light in the circumstellar envelope. The absolute proper motion measured with VLBA suggests that VY CMa may be drifting out of the giant molecular cloud to the east of it.

The distance and size of the red hypergiant NML Cygni from VLBA and VLA astrometry

Context. The red hypergiant NML Cyg has been assumed to be at part of the Cyg OB2 association, although its distance has never been measured directly. A reliable distance is crucial to study the properties of this prominent star. For example, its luminosity, and hence its position on the H-R diagram, is critical information to determine its evolutionary status. In addition, a detection of the radio photosphere would be complementary to other methods of determining the stellar size.

Statistical properties of 6.7 GHz methanol maser sources

We present a statistical analysis of 482 6.7 GHz methanol maser sources from the available literature, on their maser emission and the characteristics of their associated infrared sources. On the color-color diagram, more than 70% of the objects fall within a very small region (0.57 ≤ [25-12] ≤ 1.30 and 1.30 ≤ [60-12] ≤ 2.50). This suggests that 6.7 GHz methanol maser emission occurs only within a very short evolutionary phase during the earliest stage of star formation. The velocity ranges of the masers belong to two main groups: one from 1 to 10 km s−1, and one from about 11 to 20 km s−1. These velocity ranges indicate that the masers are probably associated with both disks and outflows. The correlations between the maser and infrared flux densities, and between the maser and infrared luminosities, suggest that far-infrared radiation is a possible pumping mechanism for the masers which most probably originate from some outer molecular envelopes or disks.

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde - II. The Large Magellanic Cloud

Context. The kinetic temperature of molecular clouds is a fundamental physical parameter affecting star formation and the initial mass function. The Large Magellanic Cloud (LMC) is the closest star-forming galaxy with a low metallicity and provides an ideal laboratory for studying star formation in such an environment. Aims. The classical dense molecular gas thermometer NH 3 is seldom available in a low-metallicity environment because of photoionization and a lack of nitrogen atoms. Our goal is to directly measure the gas kinetic temperature with formaldehyde toward six star-forming regions in the LMC. Methods. Three rotational transitions ( J K A K C = 3 03 –2 02 , 3 22 –2 21 , and 3 21 –2 20 ) of para-H 2 CO near 218 GHz were observed with the Atacama Pathfinder EXperiment (APEX) 12 m telescope toward six star-forming regions in the LMC. These data are complemented by C 18 O 2–1 spectra. Results. Using non-local thermal equilibrium modeling with RADEX, we derive the gas kinetic temperature and spatial density, using as constraints the measured para-H 2 CO 3 21 –2 20 /3 03 –2 02 and para-H 2 CO 3 03 –2 02 /C 18 O 2–1 ratios. Excluding the quiescent cloud N159S, where only one para-H 2 CO line could be detected, the gas kinetic temperatures derived from the preferred para-H 2 CO 3 21 –2 20 /3 03 –2 02 line ratios range from 35 to 63 K with an average of 47 ± 5 K (errors are unweighted standard deviations of the mean). Spatial densities of the gas derived from the para-H 2 CO 3 03 –2 02 /C 18 O 2–1 line ratios yield 0.4–2.9 × 10 5 cm -3 with an average of 1.5 ± 0.4 × 10 5 cm -3 . Temperatures derived from the para-H 2 CO line ratio are similar to those obtained with the same method from Galactic star-forming regions and agree with results derived from CO in the dense regions ( n (H 2 ) > 10 3 cm -3 ) of the LMC. A comparison of kinetic temperatures derived from para-H 2 CO with those from the dust also shows good agreement. This suggests that the dust and para-H 2 CO are well mixed in the studied star-forming regions. A comparison of kinetic temperatures derived from para-H 2 CO 3 21 –2 20 /3 03 –2 02 and NH 3 (2,u20092)/(1,u20091) shows a drastic difference, however. In the star-forming region N159W, the gas temperature derived from the NH 3 (2,u20092)/(1,u20091) line ratio is ~16 K (, ApJ, 710, 105), which is only half the temperature derived from para-H 2 CO and the dust. Furthermore, ammonia shows a very low abundance in a 30′′ beam. Apparently, ammonia only survives in the most shielded pockets of dense gas that are not yet irradiated by UV photons, while formaldehyde, less affected by photodissociation, is more widespread and also samples regions that are more exposed to the radiation of young massive stars. A correlation between the gas kinetic temperatures derived from para-H 2 CO and infrared luminosity, represented by the 250 μ m flux, suggests that the kinetic temperatures traced by para-H 2 CO are correlated with the ongoing massive star formation in the LMC.

Kinetic temperature of massive star forming molecular clumps measured with formaldehyde

For a general understanding of the physics involved in the star formation process, measurements of physical parameters such as temperature and density are indispensable. The chemical and physical properties of dense clumps of molecular clouds are strongly affected by the kinetic temperature. Therefore, this parameter is essential for a better understanding of the interstellar medium. Formaldehyde, a molecule which traces the entire dense molecular gas, appears to be the most reliable tracer to directly measure the gas kinetic temperature.We aim to determine the kinetic temperature with spectral lines from formaldehyde and to compare the results with those obtained from ammonia lines for a large number of massive clumps.Three 218 GHz transitions (JKAKC=303-202, 322-221, and 321-220) of para-H2CO were observed with the 15m James Clerk Maxwell Telescope (JCMT) toward 30 massive clumps of the Galactic disk at various stages of high-mass star formation. Using the RADEX non-LTE model, we derive the gas kinetic temperature modeling the measured para-H2CO 322-221/303-202and 321-220/303-202 ratios. The gas kinetic temperatures derived from the para-H2CO (321-220/303-202) line ratios range from 30 to 61 K with an average of 46 K. A comparison of kinetic temperature derived from para-H2CO, NH3, and the dust emission indicates that in many cases para-H2CO traces a similar kinetic temperature to the NH3 (2,2)/(1,1) transitions and the dust associated with the HII regions. Distinctly higher temperatures are probed by para-H2CO in the clumps associated with outflows/shocks. Kinetic temperatures obtained from para-H2CO trace turbulence to a higher degree than NH3 (2,2)/(1,1) in the massive clumps. The non-thermal velocity dispersions of para-H2CO lines are positively correlated with the gas kinetic temperature. The massive clumps are significantly influenced by supersonic non-thermal motions.

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde - III. The Orion molecular cloud 1

We mapped the kinetic temperature structure of the Orion molecular cloud 1 with para-H2CO(303-202, 322-221, and 321-220) using the APEX 12m telescope. This is compared with the temperatures derived from the ratio of the NH3(2,2)/(1,1) inversion lines and the dust emission. Using the RADEX non-LTE model, we derive the gas kinetic temperature modeling the measured averaged line ratios of para-H2CO 322-221/303-202 and 321-220/303-202. The gas kinetic temperatures derived from the para-H2CO line ratios are warm, ranging from 30 to >200 K with an average of 62 K at a spatial density of 10

ATLASGAL-selected massive clumps in the inner Galaxy: VI. Kinetic temperature and spatial density measured with formaldehyde

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Techniques for Accurate Parallax Measurements for 6.7 GHz Methanol Masers

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Trigonometric Parallax of RCW 122

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A kinematical study of the NGC 7538 IRS 1 region

. These temperatures are higher than those obtained from NH3(2,2)/(1,1) and CH3CCH(6-5) in the OMC-1 region. The gas kinetic temperatures derived from para-H2CO agree with those obtained from warm dust components measured in the mid infrared (MIR), which indicates that the para-H2CO(3-2) ratios trace dense and warm gas. The cold dust components measured in the far infrared (FIR) are consistent with those measured with NH3(2,2)/(1,1) and the CH3CCH(6-5) line series. With dust at MIR wavelengths and para-H2CO(3-2) on one side and dust at FIR wavelengths, NH3(2,2)/(1,1), and CH3CCH(6-5) on the other, dust and gas temperatures appear to be equivalent in the dense gas of the OMC-1 region, but provide a bimodal distribution, one more directly related to star formation than the other. The non-thermal velocity dispersions of para-H2CO are positively correlated with the gas kinetic temperatures in regions of strong non-thermal motion (Mach number >2.5) of the OMC-1, implying that the higher temperature traced by para-H2CO is related to turbulence on a 0.06 pc scale. Combining the temperature measurements with para-H2CO and NH3(2,2)/(1,1) line ratios, we find direct evidence for the dense gas along the northern part of the OMC-1 10 km s