Gas Kinetic Temperature Research Articles

Impact of inhomogeneous CMB heating of gas on the HI 21-cm signal during dark ages

Observations of redshifted 21-cm signal from neutral hydrogen (HI) appear to be the most promising probe of the cosmic dark ages. The signal carries information about the thermal state along with density distribution of the intergalactic medium (IGM). The cosmic microwave background radiation (CMBR), through its interaction with charged particles, plays a major role in determining the kinetic and spin temperature of HI gas in the IGM during dark ages. A Spatially fluctuating ionization fraction, which is caused by inhomogeneous recombinations, causes heat transfer from the CMBR to the IGM gas inhomogeneous. We revisit the impact of this inhomogeneous heat transfer on spatial fluctuations in the observed HI 21-cm signal over a large redshift range during dark ages. Our study shows that the effect negatively impacts fluctuations in the HI spin temperature and results in an enhanced HI 21-cm power spectrum. We find that the effect is particularly important during the transition of the gas kinetic temperature being coupled to the CMBR to fully decoupled from it, i.e., in the redshift range $30 \lesssim z \lesssim 300$. It is found that, on the average the HI power spectrum, $P_{T_b}(k, z)$ is enhanced by $\sim 4\%$, $\sim10 \%$ , $\sim 20\%$, and $\sim 30 \%$ at redshifts $60$, $90$, $140$, and $200$ respectively at $k=0.1 \, {\rm Mpc}^{-1}$. The effect becomes even more significant at lower values of $k_{\parallel}^2/k^2$ due to the reduced dominance of the peculiar velocity. It is observed that the power spectrum is enhanced by $\sim 49\%$ and $\sim 93\%$ at redshifts $140$ and $200$ respectively at $k=0.1 \, {\rm Mpc}^{-1}$ for $k_{\parallel}^2/k^2=0$. This enhancement has a weak $k$-mode dependence.

HCN J = 4–3, HNC J = 1–0, H13CN J = 1–0, and HC3N J = 10–9 Maps of the Galactic Center Region. I. Spatially Resolved Measurements of Physical Conditions and Chemical Composition

Abstract This supplement paper presents the maps of HCN J = 4–3, HNC J = 1–0, J = 1–0, and HC3N J = 10–9 for the Galactic central molecular zone (CMZ), which have been obtained using the Atacama Submillimeter Telescope Experiment and Nobeyama Radio Observatory 45 m telescope. Three-dimensional maps (2D in space and 1D in velocity) of the gas kinetic temperature (T kin), hydrogen volume density ( ), and fractional abundances of eight molecules (HCN, HNC, , HCO+, , SiO, CS, and ) have been constructed from our and archival data. We have developed a method with hierarchical Bayesian inference for this analysis, which has successfully suppressed the artificial correlations among the parameters created by systematic errors due to the deficiency in the simple one-zone excitation analysis and the calibration uncertainty. The typical values of T kin and are and , respectively, and the presence of an additional cold, low-density component is also indicated. The distribution of high-temperature regions is poorly correlated with known active star-forming regions, while a few of them coincide with shocked clouds. Principal component analysis has identified two distinct groups in the eight analyzed molecules: one group with large PC1 and PC2 scores, and the other with a large T kin dependence, which could be explained using two regimes of shock chemistry with fast ( ) and slow ( ) velocity shocks, respectively. This supports the idea that the mechanical sputtering of dust grains and the mechanical heating play primary roles in the chemical and thermal processes in CMZ clouds.

Detection of the Galactic warm neutral medium in H i 21-cm absorption

ABSTRACT We report a deep Giant Metrewave Radio Telescope search for Galactic H i 21-cm absorption towards the quasar B0438−436, yielding the detection of wide, weak H i 21-cm absorption, with a velocity-integrated H i 21-cm optical depth of 0.0188 ± 0.0036 km s−1. Comparing this with the H i column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of 3760 ± 365 K, one of the highest measured in the Galaxy. This is consistent with most of the H i along the sightline arising in the stable warm neutral medium. The low-peak H i 21-cm optical depth towards B0438−436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the H i 21-cm absorption and emission spectra. This yields a gas kinetic temperature of $T_{\rm k} \le (4910 \pm \lt t\gt 1900)$ K, and a spin temperature of $T_{\rm s} = (1000 \pm 345)$ K for the gas that gives rise to the H i 21-cm absorption. Our data are consistent with the H i 21-cm absorption arising from either the stable WNM, with $T_{\rm s} \ll T_{\rm k}$, $\rm T_k \approx 5000$ K, and little penetration of the background Lyman-α radiation field into the neutral hydrogen, or the unstable neutral medium, with $T_{\rm s} \approx T_{\rm k} \approx 1000\,{\rm K}$.

ATLASGAL-selected massive clumps in the inner Galaxy

Context. Formaldehyde (H2CO) is a reliable tracer to accurately measure the physical parameters of dense gas in star-forming regions. Aim. We aim to determine directly the kinetic temperature and spatial density with formaldehyde for the ~100 brightest ATLASGAL-selected clumps (the TOP100 sample) at 870 μm representing various evolutionary stages of high-mass star formation. Methods. Ten transitions (J = 3–2 and 4–3) of ortho- and para-H2CO near 211, 218, 225, and 291 GHz were observed with the Atacama Pathfinder EXperiment (APEX) 12 m telescope. Results. Using non-LTE models with RADEX, we derived the gas kinetic temperature and spatial density with the measured para-H2CO 321–220/303–202, 422–321/404–303, and 404–303/303–202 ratios. The gas kinetic temperatures derived from the para-H2CO 321–220/303–202 and 422–321/404–303 line ratios are high, ranging from 43 to >300 K with an unweighted average of 91 ± 4 K. Deduced Tkin values from the J = 3–2 and 4–3 transitions are similar. Spatial densities of the gas derived from the para-H2CO 404–303/303–202 line ratios yield 0.6–8.3 × 106 cm−3 with an unweighted average of 1.5 (±0.1) × 106 cm−3. A comparison of kinetic temperatures derived from para-H2CO, NH3, and dust emission indicates that para-H2CO traces a distinctly higher temperature than the NH3 (2, 2)/(1, 1) transitions and the dust, tracing heated gas more directly associated with the star formation process. The H2CO line widths are found to be correlated with bolometric luminosity and increase with the evolutionary stage of the clumps, which suggests that higher luminosities tend to be associated with a more turbulent molecular medium. It seems that the spatial densities measured with H2CO do not vary significantly with the evolutionary stage of the clumps. However, averaged gas kinetic temperatures derived from H2CO increase with time through the evolution of the clumps. The high temperature of the gas traced by H2CO may be mainly caused by radiation from embedded young massive stars and the interaction of outflows with the ambient medium. For Lbol/Mclump ≳ 10 L⊙/M⊙, we find a rough correlation between gas kinetic temperature and this ratio, which is indicative of the evolutionary stage of the individual clumps. The strong relationship between H2CO line luminosities and clump masses is apparently linear during the late evolutionary stages of the clumps, indicating that LH_2CO does reliably trace the mass of warm dense molecular gas. In our massive clumps H2CO line luminosities are approximately linearly correlated with bolometric luminosities over about four orders of magnitude in Lbol, which suggests that the mass of dense molecular gas traced by the H2CO line luminosity is well correlated with star formation.

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde

We mapped the kinetic temperature structure of the Orion molecular cloud 1 (OMC-1) with para-H2CO (JKaKc = 303–202, 322–221, and 321–220) using the APEX 12 m 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 ± 2 K at a spatial density of 105 cm-3. 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 (n(H2) ≳ 104 cm-3) 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-1 filament heated by radiation from the central Orion nebula.

The Survey of Water and Ammonia in the Galactic Center (SWAG): Molecular Cloud Evolution in the Central Molecular Zone

Abstract The Survey of Water and Ammonia in the Galactic Center (SWAG) covers the Central Molecular Zone (CMZ) of the Milky Way at frequencies between 21.2 and 25.4 GHz obtained at the Australia Telescope Compact Array at ∼0.9 pc spatial and ∼2.0 km s−1 spectral resolution. In this paper, we present data on the inner ∼250 pc (1.°4) between Sgr C and Sgr B2. We focus on the hyperfine structure of the metastable ammonia inversion lines (J, K) = (1, 1)–(6, 6) to derive column density, kinematics, opacity, and kinetic gas temperature. In the CMZ molecular clouds, we find typical line widths of 8–16 km s−1 and extended regions of optically thick (τ > 1) emission. Two components in kinetic temperature are detected at 25–50 K and 60–100 K, both being significantly hotter than the dust temperatures throughout the CMZ. We discuss the physical state of the CMZ gas as traced by ammonia in the context of the orbital model by Kruijssen et al. that interprets the observed distribution as a stream of molecular clouds following an open eccentric orbit. This allows us to statistically investigate the time dependencies of gas temperature, column density, and line width. We find heating rates between ∼50 and ∼100 K Myr−1 along the stream orbit. No strong signs of time dependence are found for column density or line width. These quantities are likely dominated by cloud-to-cloud variations. Our results qualitatively match the predictions of the current model of tidal triggering of cloud collapse, orbital kinematics, and the observation of an evolutionary sequence of increasing star formation activity with orbital phase.

The Green Bank Ammonia Survey: Observations of Hierarchical Dense Gas Structures in Cepheus-L1251

Abstract We use Green Bank Ammonia Survey observations of NH3 (1, 1) and (2, 2) emission with 32″ FWHM resolution from a ∼10 pc2 portion of the Cepheus-L1251 molecular cloud to identify hierarchical dense gas structures. Our dendrogram analysis of the NH3 data results in 22 top-level structures, which reside within 13 lower-level parent structures. The structures are compact and are spatially correlated with the highest H2 column density portions of the cloud. We also compare the ammonia data to a catalog of dense cores identified by higher-resolution (18.″2 FWHM) Herschel Space Observatory observations of dust continuum emission from Cepheus-L1251. Maps of kinetic gas temperature, velocity dispersion, and NH3 column density, derived from detailed modeling of the NH3 data, are used to investigate the stability and chemistry of the ammonia-identified and Herschel-identified structures. We show that the dust and dense gas in the structures have similar temperatures, with median T dust and T K measurements of 11.7 ± 1.1 K and 10.3 ± 2.0 K, respectively. Based on a virial analysis, we find that the ammonia-identified structures are gravitationally dominated, yet may be in or near a state of virial equilibrium. Meanwhile, the majority of the Herschel-identified dense cores appear to be not bound by their own gravity and instead confined by external pressure. CCS (20 − 10) and HC5N emission from the region reveal broader line widths and centroid velocity offsets when compared to the NH3 (1, 1) emission in some cases, likely due to these carbon-based molecules tracing the turbulent outer layers of the dense cores.

Temperature structure and kinematics of the IRDC G035.39–00.33

Aims. Infrared dark clouds represent the earliest stages of high-mass star formation. Detailed observations of their physical conditions on all physical scales are required to improve our understanding of their role in fueling star formation. Methods. We investigate the large-scale structure of the IRDC G035.39-00.33, probing the dense gas with the classical ammonia thermometer. This allows us to put reliable constraints on the temperature of the extended, pc-scale dense gas reservoir and to probe the magnitude of its non-thermal motions. Available far-infrared observations can be used in tandem with the observed ammonia emission to estimate the total gas mass contained in G035.39-00.33. Results. We identify a main velocity component as a prominent filament, manifested as an ammonia emission intensity ridge spanning more than 6 pc, consistent with the previous studies on the Northern part of the cloud. A number of additional line-of-sight components are found, and a large scale, linear velocity gradient of ~0.2 km s$^{-1}$ pc$^{-1}$ is found along the ridge of the IRDC. In contrast to the dust temperature map, an ammonia-derived kinetic temperature map, presented for the entirety of the cloud, reveals local temperature enhancements towards the massive protostellar cores. We show that without properly accounting for the line of sight contamination, the dust temperature is 2-3 K larger than the gas temperature measured with NH$_3$. Conclusions. While both the large scale kinematics and temperature structure are consistent with that of starless dark filaments, the kinetic gas temperature profile on smaller scales is suggestive of tracing the heating mechanism coincident with the locations of massive protostellar cores.

A Herschel/PACS Far-infrared Line Emission Survey of Local Luminous Infrared Galaxies

Abstract We present an analysis of , [O iii]88, [N ii]122, and far-infrared (FIR) fine-structure line observations obtained with Herschel/PACS, for ∼240 local luminous infrared galaxies (LIRGs) in the Great Observatories All-sky LIRG Survey. We find pronounced declines (“deficits”) of line-to-FIR continuum emission for [N ii]122, , and as a function of FIR color and infrared luminosity surface density, . The median electron density of the ionized gas in LIRGs, based on the [N ii]122/[N ii]205 ratio, is = 41 cm−3. We find that the dispersion in the deficit of LIRGs is attributed to a varying fractional contribution of photodissociation regions (PDRs) to the observed emission, f( ) = / , which increases from ∼60% to ∼95% in the warmest LIRGs. The / ratio is tightly correlated with the PDR gas kinetic temperature in sources where is not optically thick or self-absorbed. For each galaxy, we derive the average PDR hydrogen density, , and intensity of the interstellar radiation field, G, in units of and find G/ ratios of ∼0.1–50 cm3, with ULIRGs populating the upper end of the distribution. There is a relation between G/ and , showing a critical break at ≃ 5 × 1010 L ⊙ kpc−2. Below , G/ remains constant, ≃0.32 cm3, and variations in are driven by the number density of star-forming regions within a galaxy, with no change in their PDR properties. Above , G/ increases rapidly with , signaling a departure from the typical PDR conditions found in normal star-forming galaxies toward more intense/harder radiation fields and compact geometries typical of starbursting sources.

The Validity of 21 cm Spin Temperature as a Kinetic Temperature Indicator in Atomic and Molecular Gas

Abstract The gas kinetic temperature (T K ) of various interstellar environments is often inferred from observations that can deduce level populations of atoms, ions, or molecules using spectral line observations; H i 21 cm is perhaps the most widely used, and has a long history. Usually the H i 21 cm line is assumed to be in thermal equilibrium and the populations are given by the Boltzmann distribution. A variety of processes, many involving Lyα, can affect the 21 cm line. Here we show how this is treated in the spectral simulation code Cloudy, and present numerical simulations of environments where this temperature indicator is used, with a detailed treatment of the physical processes that determine level populations within H0. We discuss situations where this temperature indicator traces T K , cases where it fails, as well as the effects of Lyα pumping on the 21 cm spin temperature. We also show that the Lyα excitation temperature rarely traces the gas kinetic temperature.

The Green Bank Ammonia Survey: First Results of NH3 Mapping of the Gould Belt

Abstract We present an overview of the first data release (DR1) and first-look science from the Green Bank Ammonia Survey (GAS). GAS is a Large Program at the Green Bank Telescope to map all Gould Belt star-forming regions with mag visible from the northern hemisphere in emission from NH3 and other key molecular tracers. This first release includes the data for four regions in the Gould Belt clouds: B18 in Taurus, NGC 1333 in Perseus, L1688 in Ophiuchus, and Orion A North in Orion. We compare the NH3 emission to dust continuum emission from Herschel and find that the two tracers correspond closely. We find that NH3 is present in over 60% of the lines of sight with mag in three of the four DR1 regions, in agreement with expectations from previous observations. The sole exception is B18, where NH3 is detected toward ∼40% of the lines of sight with mag. Moreover, we find that the NH3 emission is generally extended beyond the typical 0.1 pc length scales of dense cores. We produce maps of the gas kinematics, temperature, and NH3 column densities through forward modeling of the hyperfine structure of the NH3 (1, 1) and (2, 2) lines. We show that the NH3 velocity dispersion, , and gas kinetic temperature, T K, vary systematically between the regions included in this release, with an increase in both the mean value and the spread of and T K with increasing star formation activity. The data presented in this paper are publicly available (https://dataverse.harvard.edu/dataverse/GAS_DR1).

Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde

The Large Magellanic Cloud (LMC), the closest star forming galaxy with low metallicity, provides an ideal laboratory to study star formation in such an environment. The classical dense molecular gas thermometer NH3 is rarely 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. Three rotational transitions of para-H2CO near 218 GHz were observed with the APEX 12m telescope toward six star forming regions in the LMC. Those data are complemented by C18O 2-1 spectra. Using non-LTE modeling with RADEX, we derive the gas kinetic temperature and spatial density, using as constraints the measured para-H2CO 321-220/303-202 and para-H2CO 303-202/C18O 2-1 ratios. Excluding the quiescent cloud N159S, where only one para-H2CO line could be detected, the gas kinetic temperatures derived from the preferred para-H2CO 321-220/303-202 line ratios range from 35 to 63 K with an average of 47 K. Spatial densities of the gas derived from the paraH2CO 303-202/C18O 2-1 line ratios yield 0.4-2.9x10^5 cm^-3 with an average of 1.5x10^5 cm^-3. Temperatures derived from the para-H2CO 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 of the LMC. A comparison of kinetic temperatures derived from para-H2CO with those from the dust also shows good agreement. This suggests that the dust and para-H2CO are well mixed in the studied star forming regions. A correlation between the gas kinetic temperatures derived from para-H2CO and infrared luminosity, represented by the 250um flux, suggests that the kinetic temperatures traced by para-H2CO 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.

Evolutionary status of dense cores in the NGC 1333 IRAS 4 star-forming region

Protostellar evolution, following the formation of the protostar is becoming reasonably well characterized, but the evolution from a prestellar core to a protostar is not well known, although the first hydrostatic core (FHSC) must be a pivotal step. NGC 1333 IRAS 4C is a potentially very young object, that we directly compare with the nearby Class 0 IRAS 4A and IRAS 4B. Observational constraints are provided by spectral imaging from the JCMT Spectral Legacy Survey (330-373 GHz) and continuum and line observations from CARMA. We present integrated intensity and velocity maps of several species, including CO, H2CO and CH3OH. The velocity of an observed outflow, the degree of CO depletion, the deuteration of DCO+/HCO+ and gas kinetic temperatures are observational signatures that we present. We report differences between the three sources in four aspects: a) the kinetic temperature is much lower towards IRAS 4C, b) the line profiles of the detected species show strong outflow activity towards IRAS 4A and IRAS 4B but not towards IRAS 4C, c) the HCN/HNC is < 1 towards IRAS 4C, which confirms the cold nature of the source, d) the degree of CO depletion and the deuteration are the lowest towards the warmest of the sources, IRAS 4B. IRAS 4C seems to be in a different evolutionary state than the IRAS 4A and IRAS 4B sources. We can probably exclude the FHSC stage due to the relatively low Lsmm/Lbol (~6\%) and we investigate the earliest accretion phase of Class 0 stage and the transition between Class 0 to Class I. Our results do not show a consistent scenario for either case, with the major issue being the absence of outflow activity and the cold nature of IRAS 4C. The number of FHSC candidates in Perseus is ~10 times higher than current models predict, which suggests that the lifespan of these objects is greater than 1000 years, possibly due to an accretion rate lower than 4x10^-5 msun/yr.

Analytical methods for measuring the parameters of interstellar gas using methanol observations

We analyze methanol excitation in the absence of external radiation and consider LTE methods for probing interstellar gas. We show that rotation diagrams correctly estimate the gas kinetic temperature only if they are built from lines with the upper levels located in the same K-ladders, such as the J_0-J_{-1}E lines at 157~GHz, the J_1-J_0E lines at 165~GHz or the J_2-J_1E lines at 25~GHz. The gas density should be no less than 10^7~cm^{-3}. Rotation diagrams built from lines with different K values of the upper levels (2_K-1_K at 96~GHz, 3_K-2_K at 145~GHz, or 5_K-4_K at 241~GHz) significantly underestimate the temperature but allow a density estimation. In addition, the diagrams based on the 2_K-1_K lines make possible methanol column density estimates within a factor of about 2--5. We suggest that rotation diagrams should be used in the following manner. First, one should build two rotation diagrams, one from the lines at 96, 145, or 241~GHz, and another from the lines at 157, 165, or 25~GHz. The former diagram is used to estimate the gas density. If the density is about 10^7~cm^{-3} or higher, the latter diagram reproduces the temperature fairly well. If the density is around 10^6~cm^{-3}, the temperature obtained from the latter diagram should be multiplied by a factor of 1.5--2. If the density is about 10^5~cm^{-3} or lower, then the latter diagram yields a temperature that is lower than the kinetic temperature by a factor of three or larger and should be used only as a lower limit on the kinetic temperature. Errors of methanol column density determined from the integrated intensity of a single line may be larger than an order of magnitude even when the gas temperature is well-known. However, if the J_0-(J-1)_0E lines, as well as the J_1-(J-1)_1A^{+} or A^{-} lines are used, the relative error of the column density proves to be no larger than several units.

Numerical modeling of a slow axial flow CW CO2 laser with simultaneous striking of two discharge tubes

A five temperature model which consists of a system of differential equations is applied to describe the operation of locally made a slow axial flow CW CO2 laser with simultaneous striking of two discharge tubes. The model is solved using Runge-Kutta method. The laser works on the mixture of CO2:N2: He with a ratio of 1:1:4, at a total pressure of 8–20mbar and a discharge current of 120–140mA.According to the experimental measurements of the laser characteristics at certain operational conditions, the “BOLTZOLD” software has been used to determine the electronic excitation rates of CO2 and N2 molecules and electron drift velocity in the discharge. Therefore, the numerical solution of the rate equations for laser levels and energy balance equation of the discharge has been carried out using a written FORTRAN program under these conditions.Consequently, the temporal behaviour of the output power, the photon density of the laser emission, and the laser output power versus the input power are defined under the taken pressures and currents. In addition, the effect of gas kinetic temperature on the output power is studied taking into account the variation of loss factor depending on the operating conditions of the laser.The calculated and measured results of the output power versus the pressure, the current and the input power are in a good agreement under certain values of loss coefficient.In resume, the developed model could be used to predict and evaluate the performance of slow axial flow CW CO2 lasers.

Exocometary gas in the HD 181327 debris ring

An increasing number of observations have shown that gaseous debris discs are not an exception. However, until now, we only knew of cases around A stars. Here we present the first detection of 12CO (2–1) disc emission around an F star, HD 181327, obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) observations at 1.3 mm. The continuum and CO emission are resolved into an axisymmetric disc with ring-like morphology. Using a Markov chain Monte Carlo method coupled with radiative transfer calculations, we study the dust and CO mass distribution. We find the dust is distributed in a ring with a radius of 86.0 ± 0.4 au and a radial width of 23.2 ± 1.0 au. At this frequency, the ring radius is smaller than in the optical, revealing grain size segregation expected due to radiation pressure. We also report on the detection of low-level continuum emission beyond the main ring out to ∼200 au. We model the CO emission in the non-local thermodynamic equilibrium regime and we find that the CO is co-located with the dust, with a total CO gas mass ranging between 1.2 × 10−6 M⊕ and 2.9 × 10−6 M⊕, depending on the gas kinetic temperature and collisional partners densities. The CO densities and location suggest a secondary origin, i.e. released from icy planetesimals in the ring. We derive a CO+CO2 cometary composition that is consistent with Solar system comets. Due to the low gas densities, it is unlikely that the gas is shaping the dust distribution.

THE BOLOCAM GALACTIC PLANE SURVEY. XIV. PHYSICAL PROPERTIES OF MASSIVE STARLESS AND STAR-FORMING CLUMPS

ABSTRACT We sort 4683 molecular clouds between 10° &lt; ℓ &lt; 65° from the Bolocam Galactic Plane Survey based on observational diagnostics of star formation activity: compact 70 μm sources, mid-IR color-selected YSOs, H2O and CH3OH masers, and UCH ii regions. We also present a combined NH3-derived gas kinetic temperature and H2O maser catalog for 1788 clumps from our own GBT 100 m observations and from the literature. We identify a subsample of 2223 (47.5%) starless clump candidates (SCCs), the largest and most robust sample identified from a blind survey to date. Distributions of flux density, flux concentration, solid angle, kinetic temperature, column density, radius, and mass show strong (&gt;1 dex) progressions when sorted by star formation indicator. The median SCC is marginally subvirial (α ∼ 0.7) with &gt;75% of clumps with known distance being gravitationally bound (α &lt; 2). These samples show a statistically significant increase in the median clump mass of ΔM ∼ 170–370 M ⊙ from the starless candidates to clumps associated with protostars. This trend could be due to (i) mass growth of the clumps at M ˙ ∼ 200 – 440 M ⊙ Myr−1 for an average freefall 0.8 Myr timescale, (ii) a systematic factor of two increase in dust opacity from starless to protostellar phases, and/or (iii) a variation in the ratio of starless to protostellar clump lifetime that scales as ∼M −0.4. By comparing to the observed number of CH3OH maser containing clumps, we estimate the phase lifetime of massive (M &gt; 103 M ⊙) starless clumps to be 0.37 ± 0.08 Myr (M/103 M ⊙)−1; the majority (M &lt; 450 M ⊙) have phase lifetimes longer than their average freefall time.

OH 18 cm TRANSITION AS A THERMOMETER FOR MOLECULAR CLOUDS

We have observed the four hyperfine components of the 18 cm OH transition toward the translucent cloud eastward of Heiles Cloud 2 (HCL2E), the cold dark cloud L134N, and the photodissociation region of the $\rho$-Ophiuchi molecular cloud with the Effelsberg 100 m telescope. We have found intensity anomalies amongst the hyperfine components in all three regions. In particular, an absorption feature of the 1612 MHz satellite line against the cosmic microwave background has been detected toward HCL2E and two positions of the $\rho$-Ophiuchi molecular cloud. On the basis of statistical equilibrium calculations, we find that the hyperfine anomalies originate from the non-LTE population of the hyperfine levels, and can be used to determine the kinetic temperature of the gas over a wide range of H$_2$ density (10$^2$ - 10$^7$ cm$^{-3}$). Toward the center of HCL2E, the gas kinetic temperature is determined to be 53$\pm$1 K, and it increases toward the cloud peripheries ($\sim$ 60 K). The ortho-to-para ratio of H$_2$ is determined to be 3.5 $\pm$ 0.9 from the averaged spectrum for the 8 positions. In L134N, a similar increase of the temperature is also seen toward the periphery. In the $\rho$-Ophiuchi molecular cloud, the gas kinetic temperature decreases as a function of the distance from the exciting star HD147889. These results demonstrate a new aspect of the OH 18 cm line as a good thermometer of molecular cloud envelopes. The OH 18 cm line can be used to trace a new class of warm molecular gas surrounding a molecular cloud, which is not well traced by emission of CO and its isotopologues.

A comparison of gas temperatures measured by ultraviolet laser scattering in atmospheric plasma sources

A laser scattering system utilizing an ultraviolet laser with a triple grating spectrometer has been assembled in order to measure gas temperature in atmospheric plasma sources. Such laser scattering interactions offer a non-invasive technique for investigating atmospheric microplasma sources, which have potential applications in remote optical sensing, materials processing, and environmental decontamination. This particular system is unique in that it utilizes a ultraviolet laser line (266 nm), which increases the cross section for Rayleigh and Raman scattering by a factor of 16 in comparison to the more common 532 nm laser operating in the visible range. In this work, the laser scattering system is used to directly compare the rotational gas temperature (Tr) and gas kinetic temperature (Tg) in two different atmospheric plasma sources []: a direct current plasma jet operating on nitrogen and [] a conventional pin–pin glow microdischarge in air. Results show agreement between Tr and Tg both in the low temperature afterglow of the plasma jet (300–700 K) and the hot center of the atmospheric glow (1500–2000 K). These observations lend credence to the common assumption of rotational relaxation in atmospheric plasmas and validate the ultraviolet laser diagnostic for future application in atmospheric microplasma sources.