arXiv subjects

J. Stutzki

Publications and source records attributed to J. Stutzki.

At least 109 records · Page 6Linked to original sources

arXiv2010

HIFI observations of warm gas in DR21: Shock versus radiative heating

The molecular gas in the DR21 massive star formation region is known to be affected by the strong UV field from the central star cluster and by a fast outflow creating a bright shock. The relative contribution of both heating mechanisms is the matter of a long debate. By better sampling the excitation ladder of various tracers we provide a quantitative distinction between the different heating mechanisms. HIFI observations of mid-J transitions of CO and HCO+ isotopes allow us to bridge the gap in excitation energies between observations from the ground, characterizing the cooler gas, and existing ISO LWS spectra, constraining the properties of the hot gas. Comparing the detailed line profiles allows to identify the physical structure of the different components. In spite of the known shock-excitation of H2 and the clearly visible strong outflow, we find that the emission of all lines up to > 2 THz can be explained by purely radiative heating of the material. However, the new Herschel/HIFI observations reveal two types of excitation conditions. We find hot and dense clumps close to the central cluster, probably dynamically affected by the outflow, and a more widespread distribution of cooler, but nevertheless dense, molecular clumps.

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arXiv2010

PACS and SPIRE photometer maps of M33: First results of the Herschel M33 extended survey (HERM33ES)

Within the framework of the HERM33ES key project, we are studying the star forming interstellar medium in the nearby, metal-poor spiral galaxy M33, exploiting the high resolution and sensitivity of Herschel. We use PACS and SPIRE maps at 100, 160, 250, 350, and 500 micron wavelength, to study the variation of the spectral energy distributions (SEDs) with galacto-centric distance. Detailed SED modeling is performed using azimuthally averaged fluxes in elliptical rings of 2 kpc width, out to 8 kpc galacto-centric distance. Simple isothermal and two-component grey body models, with fixed dust emissivity index, are fitted to the SEDs between 24 and 500 micron using also MIPS/Spitzer data, to derive first estimates of the dust physical conditions. The far-infrared and submillimeter maps reveal the branched, knotted spiral structure of M33. An underlying diffuse disk is seen in all SPIRE maps (250-500 micron). Two component fits to the SEDs agree better than isothermal models with the observed, total and radially averaged flux densities. The two component model, with beta fixed at 1.5, best fits the global and the radial SEDs. The cold dust component clearly dominates; the relative mass of the warm component is less than 0.3% for all the fits. The temperature of the warm component is not well constrained and is found to be about 60K plus/minus 10K. The temperature of the cold component drops significantly from about 24K in the inner 2 kpc radius to 13K beyond 6 kpc radial distance, for the best fitting model. The gas-to-dust ratio for beta=1.5, averaged over the galaxy, is higher than the solar value by a factor of 1.5 and is roughly in agreement with the subsolar metallicity of M33.

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arXiv2009

The molecular environment of the massive star forming region NGC 2024: Multi CO transition analysis

NGC 2024, a sites of massive star formation, have complex internal structures caused by cal heating by young stars, outflows, and stellar winds. These complex cloud structures lead to intricate emission line shapes. The goal of this paper is to show that the complex line shapes of 12 CO lines in NGC 2024 can be explained consistently with a model, whose temperature and velocity structure are based on the well-established scenario of a PDR and the Blister model. We present velocity-resolved spectra of seven CO lines ranging from J=3 to J=13, and we combined these data with CO high-frequency data from the ISO satellite. We find that the bulk of the molecular cloud associated with NGC 2024 consists of warm (75 K) and dense (9e5 cm-3) gas. An additional hot (~ 300 K) component, located at the interface of the HII region and the molecular cloud, is needed to explain the emission of the high-J CO lines. Deep absorption notches indicate that very cold material (20 K) exists in front of the warm material, too. A temperature and column density structure consistent with those predicted by PDR models, combined with the velocity structure of a Blister model, appropriately describes the observed emission line profiles of this massive star forming region. This case study of NGC 2024 shows that, with physical insights into these complex regions and careful modeling, multi-line observations of CO can be used to derive detailed physical conditions in massive star forming regions.

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arXiv2009

A complete 12CO 2--1 map of M51 with HERA: II. Total gas surface densities and gravitational stability

To date the onset of large-scale star formation in galaxies and its link to gravitational stability of the galactic disk have not been fully understood. The nearby face-on spiral galaxy M51 is an ideal target for studying this subject. This paper combines CO, dust, HI, and stellar maps of M51 and its companion galaxy to study the H2/HI transition, the gas-to-dust ratios, and the stability of the disk against gravitational collapse. We combine maps of the molecular gas using 12CO 2--1 map HERA/IRAM-30m data and HI VLA data to study the total gas surface density and the phase transition of atomic to molecular gas. The total gas surface density is compared to the dust surface density from 850 micron SCUBA data. Taking into account the velocity dispersions of the molecular and atomic gas, and the stellar surface densities derived from the 2MASS K-band survey, we derive the total Toomre Q parameter of the disk. The gas surface density in the spiral arms is approximately 2-3 higher compared to that of the interarm regions. The ratio of molecular to atomic surface density shows a nearly power-law dependence on the hydrostatic pressure P_hydro. The gas surface density distribution in M51 shows an underlying exponential distribution with a scale length of h_gas=7.6 kpc representing 55% of the total gas mass, comparable to the properties of the exponential dust disk. In contrast to the velocity widths observed in HI, the CO velocity dispersion shows enhanced line widths in the spiral arms compared to the interarm regions. The contribution of the stellar component in the Toomre Q-parameter analysis is significant and lowers the combined Q-parameter Q_tot by up to 70% towards the threshold for gravitational instability. The value of Q_tot varies from 1.5-3 in radial averages. A map of Q_tot shows values around 1 on the spiral arms.

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arXiv2008

First 450-micron dust continuum mapping of the massive star-forming region NGC 3576 with the P-ArTeMiS bolometer camera

Context: The earliest phases of massive star formation are currently much debated. Aims. In an effort to make progress, we took a census of Class0-like protostellar dense cores in the NGC 3576 region, one of the nearest and most luminous embedded sites of high-mass star formation in the Galaxy. Methods: We used the P-ArTeMiS bolometer camera on the APEX telescope to produce the first 450-micron dust continuum map of the filamentary dense clump associated with NGC 3576. Results: Combining our 450-micron observations with existing data at other wavelengths, we have identified seven massive protostellar sources along the NGC 3576 filament and placed them in the M_env - L_bol evolutionary diagram for protostars. Conclusions: Comparison with theoretical evolutionary tracks suggests that these seven protostellar sources will evolve into massive stars with masses M* ~ 15-50 Msun. Four sources are classified as candidate high-mass Class 0 objects, two sources as massive Class I objects, and one source appears to be at an intermediate stage.

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arXiv2008

The N2D+/N2H+ ratio as an evolutionary tracer of Class 0 protostars

Deuterated ions are abundant in cold (T=10 K), dense (n=10^5 cm^-3) regions, in which CO is frozen out onto dust grains. In such environments, the deuterium fractionation of such ions can exceed the elemental abundance ratio of D/H by a factor of 10^4. In this paper we use the deuterium fractionation to investigate the evolutionary state of Class 0 protostars. In a sample of 20 protostellar objects, we found a clear correlation between the N2D+/N2H+ ratio and evolutionary tracers. As expected, the coolest, i.e. the youngest, objects show the largest deuterium fractionation. Furthermore, we find that sources with a high N2D+/N2H+ ratio show clear indication for infall.

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arXiv2008

The Photon Dominated Region in the IC 348 molecular cloud

In this paper we discuss the physical conditions of clumpy nature in the IC 348 molecular cloud. We combine new observations of fully sampled maps in [C I] at 492 GHz and 12CO 4--3, taken with the KOSMA 3 m telescope at about 1' resolution, with FCRAO data of 12CO 1--0, 13CO 1--0 and far-infrared continuum data observed by HIRES/IRAS. To derive the physical parameters of the region we analyze the three different line ratios. A first rough estimate of abundance is obtained from an LTE analysis. To understand the [C I] and CO emission from the PDRs in IC 348, we use a clumpy PDR model. With an ensemble of identical clumps, we constrain the total mass from the observed absolute intensities. Then we apply a more realistic clump distribution model with a power law index of 1.8 for clump-mass spectrum and a power law index of 2.3 for mass-size relation. We provide detailed fits to observations at seven representative positions in the cloud, revealing clump densities between 4 10$^{4}$ cm$^{-3}$ and 4 10$^{5}$ cm$^{-3}$ and C/CO column density ratios between 0.02 and 0.26. The derived FUV flux from the model fit is consistent with the field calculated from FIR continuum data, varying between 2 and 100 Draine units across the cloud. We find that both an ensemble of identical clumps and an ensemble with a power law clump mass distribution produce line intensities which are in good agreement (within a factor ~ 2) with the observed intensities. The models confirm the anti-correlation between the C/CO abundance ratio and the hydrogen column density found in many regions.

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arXiv2008

A clumpy-cloud PDR model of the global far-infrared line emission of the Milky Way

The fractal structure of the interstellar medium suggests that the interaction of UV radiation with the ISM as described in the context of photon-dominated regions (PDR) dominates most of the physical and chemical conditions, and hence the far-infrared and submm emission from the ISM in the Milky Way. We investigate to what extent the Galactic FIR line emission of the important species CO, C, C+, and O, as observed by the Cosmic Background Explorer (COBE) satellite can be modeled in the framework of a clumpy, UV-penetrated cloud scenario. The far-infrared line emission of the Milky Way is modeled as the emission from an ensemble of clumps with a power law clump mass spectrum and mass-size relation with power-law indices consistent with the observed ISM structure. The individual clump line intensities are calculated using the KOSMA-tau PDR-model for spherical clumps. The model parameters for the cylindrically symmetric Galactic distribution of the mass density and volume filling factor are determined by the observed radial distributions. A constant FUV intensity, in which the clumps are embedded, is assumed. We show that this scenario can explain, without any further assumptions and within a factor of about 2, the absolute FIR-line intensities and their distribution with Galactic longitude as observed by COBE.

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arXiv2008

Structure analysis of interstellar clouds: II. Applying the Delta-variance method to interstellar turbulence

The Delta-variance analysis is an efficient tool for measuring the structural scaling behaviour of interstellar turbulence in astronomical maps. In paper I we proposed essential improvements to the Delta-variance analysis. In this paper we apply the improved Delta-variance analysis to i) a hydrodynamic turbulence simulation with prominent density and velocity structures, ii) an observed intensity map of rho Oph with irregular boundaries and variable uncertainties of the different data points, and iii) a map of the turbulent velocity structure in the Polaris Flare affected by the intensity dependence on the centroid velocity determination. The tests confirm the extended capabilities of the improved Delta-variance analysis. Prominent spatial scales were accurately identified and artifacts from a variable reliability of the data were removed. The analysis of the hydrodynamic simulations showed that the injection of a turbulent velocity structure creates the most prominent density structures are produced on a scale somewhat below the injection scale. The new analysis of a rho Oph continuum map reveals an intermediate stage in the molecular cloud evolution showing both signatures of the typical molecular cloud scaling behaviour and the formation of condensed cores. When analysing the velocity structure of the Polaris Flare we show that a universal power law connects scales from 0.03 pc to 3 pc. However, a plateau in the Delta-variance spectrum around 5 pc indicates that the visible large-scale velocity gradient is not converted directly into a turbulent cascade.

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arXiv2008

Structure analysis of interstellar clouds: I. Improving the Delta-variance method

The Delta-variance analysis, has proven to be an efficient and accurate method of characterising the power spectrum of interstellar turbulence. The implementation presently in use, however, has several shortcomings. We propose and test an improved Delta-variance algorithm for two-dimensional data sets, which is applicable to maps with variable error bars and which can be quickly computed in Fourier space. We calibrate the spatial resolution of the Delta-variance spectra. The new Delta-variance algorithm is based on an appropriate filtering of the data in Fourier space. It allows us to distinguish the influence of variable noise from the actual small-scale structure in the maps and it helps for dealing with the boundary problem in non-periodic and/or irregularly bounded maps. We try several wavelets and test their spatial sensitivity using artificial maps with well known structure sizes. It turns out that different wavelets show different strengths with respect to detecting characteristic structures and spectral indices, i.e. different aspects of map structures. As a reasonable universal compromise for the optimum Delta-variance filter, we propose the Mexican-hat filter with a ratio between the diameters of the core and the annulus of 1.5.

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arXiv2008

Submillimeter Line Emission from LMC N159W: a Dense, Clumpy PDR in a Low Metallicity Environment

Star formation at earlier cosmological times takes place in an interstellar medium with low metallicity. The Large Magellanic Cloud (LMC) is ideally suited to study star formation in such an environment. The physical and chemical state of the ISM in a star forming environment can be constrained by observations of submm and FIR spectral lines of the main carbon carrying species, CO, CI and CII, which originate in the surface layers of molecular clouds illuminated by the UV radiation of the newly formed, young stars. We present high-angular resolution sub-millimeter observations in the N159W region in the LMC obtained with the NANTEN2 telescope of the 12CO J = 4-3, J = 7-6, and 13CO J = 4-3 rotational and [CI] 3P1-3P0 and 3P2-3P1 fine-structure transitions. The 13CO J =4-3 and [CI] 3P2-3P1 transitions are detected for the first time in the LMC. We derive the physical and chemical properties of the low-metallicity molecular gas using an escape probability code and a self-consistent solution of the chemistry and thermal balance of the gas in the framework of a clumpy cloud PDR model. The separate excitation analysis of the submm CO lines and the carbon fine structure lines shows that the emitting gas in the N159W region has temperatures of about 80 K and densities of about 10^4 cm^-3. The estimated C to CO abundance ratio close to unity is substantially higher than in dense massive star-forming regions in the Milky Way. The analysis of all observed lines together, including the [CII] line intensity reported in the literature, in the context of a clumpy cloud PDR model constrains the UV intensity to about χ~220 and an average density of the clump ensemble of about 10^5 cm^-3, thus confirming the presence of high density material in the LMC N159W region.

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arXiv2007

12CO 4-3 and [CI] 1-0 at the centers of NGC 4945 and Circinus

Studying molecular gas in the central regions of the star burst galaxies NGC4945 and Circinus enables us to characterize the physical conditions and compare them to previous local and high-z studies. We estimate temperature, molecular density and column densities of CO and atomic carbon. Using model predictions we give a range of estimated CO/C abundance ratios. Using the new NANTEN2 4m sub-millimeter telescope in Pampa La Bola, Chile, we observed for the first time CO 4-3 and [CI] 3P1-3 P0 at the centers of both galaxies at linear scale of 682 pc and 732 pc respectively. We compute the cooling curves of 12CO and 13CO using radiative transfer models and estimate the physical conditions of CO and [CI]. The centers of NGC4945 and Circinus are very [CI] bright objects, exhibiting [CI] 3P1 - 3 P0 luminosities of 91 and 67Kkms-1kpc2, respectively. The [CI] 3P1-3 P0/CO 4-3 ratio of integrated intensities are large at 1.2 in NGC4945 and 2.8 in Circinus. Combining previous CO J= 1-0, 2-1 and 3-2 and 13CO J= 1-0, 2-1 studies with our new observations, the radiative transfer calculations give a range of densities, n(H2) = 10^3-3*104^cm-3, and a wide range of kinetic temperatures, Tkin = 20 - 100K, depending on the density. Future CO J= 7-6 and [CI] 2-1 observations will be important to resolve the ambiguity in the physical conditions and confirm the model predictions.

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arXiv2007

Clumpy photon-dominated regions in Carina. I. [CI] and mid-J CO lines in two 4'x4' fields

The Carina region is an excellent astrophysical laboratory for studying the feedback mechanisms of newly born, very massive stars within their natal giant molecular clouds (GMCs) at only 2.35 kpc distance. We use a clumpy PDR model to analyse the observed intensities of atomic carbon and CO and to derive the excitation conditions of the gas. The NANTEN2-4m submillimeter telescope was used to map the [CI] 3P1-3P0, 3P2-3P1 and CO 4-3, 7-6 lines in two 4'x4' regions of Carina where molecular material interfaces with radiation from the massive star clusters. One region is the northern molecular cloud near the compact OB cluster Tr14, and the second region is in the molecular cloud south of etaCar and Tr16. These data were combined with 13CO SEST spectra, HIRES/IRAS 60um and 100um maps of the FIR continuum, and maps of 8um IRAC/Spitzer and MSX emission. We used the HIRES far-infrared dust data to create a map of the FUV field heating the gas. The northern region shows an FUV field of a few 1000 in Draine units while the field of the southern region is about a factor 10 weaker. We constructed models consisting of an ensemble of small spherically symmetric PDR clumps within the 38" beam (0.43pc), which follow canonical power-law mass and mass-size distributions. We find that an average local clump density of 2x10**5 cm-3 is needed to reproduce the observed line emission at two selected interface positions. Stationary, clumpy PDR models reproduce the observed cooling lines of atomic carbon and CO at two positions in the Carina Nebula.

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arXiv2007

Gravitational stability in the disk of M51

Star formation laws, like i.e. the Schmidt law relating star formation rate and total gas density, have been studied in several spiral galaxies but the underlying physics are not yet well understood. M51, as a nearby face-on, grand design spiral galaxy studied in many line transitions, is an ideal target to study the connection between physical conditions of the gas and star formation activity. In this contribution we combine molecular, atomic, total gas and stellar surface densities and study the gravitational stability of the gas (Schuster et al.2007, Hitschfeld et al. in prep.). From our IRAM-30m 12 CO2-1 map and complementary HI-, Radio Continuum- and ACS-HST B-band-data we derive maps of the total gas density and the stellar surface density to study the gravitational stability of the gas via the Toomre Q parameter.

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arXiv2007

SCUBA Mapping of Outer Galaxy Protostellar Candidates

We aim to study dust properties of massive star forming regions in the outer Galaxy, in a direction opposite to the Galactic center. We present observations of six outer Galaxy point sources IRAS 01045+6505, 01420+6401, 05271+3059, 05345+3556, 20222+3541 and 20406+4555, taken with the Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT) at 450 and 850 micron. Single temperature greybody models are fitted to the Spectral Energy Distribution of the detected sub-mm cores to derive dust temperature, dust emissivity index and optical depth at 250 micron. The observed radial intensity profiles of the sub-mm cores were fitted with power laws to derive the indices describing the density distribution. At a resolution of 15" all six IRAS point sources show multiple emission peaks. Only four out of fourteen detected sub-mm cores show associated mid-infrared emission. For the sub-mm cores we derive dust temperatures of 32+-5 K and dust emissivity indices between 0.9 and 2.5. The density profiles of the sub-mm cores can be fitted by a single power law distribution with indices -1.5+-0.3, with most cores showing an index of -1.5. This is consistent with most observations of massive star forming regions and supports predictions of models of star formation which consider non-thermal support against gravitational collapse.

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arXiv2007

A PDR-Code Comparison Study

We present a comparison between independent computer codes, modeling the physics and chemistry of interstellar photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical structure of the model clouds, and to converge the output of different codes to a common solution. A number of benchmark models have been created, covering low and high gas densities and far ultraviolet intensities. The benchmark models were computed in two ways: one set assuming constant temperatures, thus testing the consistency of the chemical network and photo-processes, and a second set determining the temperature selfconsistently. We investigated the impact of PDR geometry and agreed on the comparison of results from spherical and plane-parallel PDR models. We identified a number of key processes governing the chemical network which have been treated differently in the various codes, and defined a proper common treatment. We established a comprehensive set of reference models for ongoing and future PDR model bench-marking and were able to increase the agreement in model predictions for all benchmark models significantly.

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arXiv2006

The cooling of atomic and molecular gas in DR21

We present an overview of a high-mass star formation region through the major (sub-)mm, and far-infrared cooling lines to gain insight into the physical conditions and the energy budget of the molecular cloud. We used the KOSMA 3m telescope to map the core ($10'\times 14'$) of the Galactic star forming region DR 21/DR 21 (OH) in the Cygnus X region in the two fine structure lines of atomic carbon CI and four mid-$J$ transitions of CO and $^{13}$CO, and CS $J=7\TO6$. These observations have been combined with FCRAO $J=1\TO0$ observations of $^{13}$CO and C$^{18}$O. Five positions, including DR21, DR21 (OH), and DR21 FIR1, were observed with the ISO/LWS grating spectrometer in the \OI 63 and 145 $μ$m lines, the \CII 158 $μ$m line, and four high-$J$ CO lines. We discuss the intensities and line ratios at these positions and apply Local Thermal Equilibrium (LTE) and non-LTE analysis methods in order to derive physical parameters such as masses, densities and temperatures. The CO line emission has been modeled up to J=20. From non-LTE modeling of the low- to high-$J$ CO lines we identify two gas components, a cold one at temperatures of T$_\RM{kin}\sim 30-40$ K, and one with T$_\RM{kin}\sim 80-150$ K at a local clump density of about n(H$_2$)$\sim 10^4-10^6$ cm$^{-3}$. While the cold quiescent component is massive containing typically more than 94 % of the mass, the warm, dense, and turbulent gas is dominated by mid- and high-$J$ CO line emission and its large line widths. The medium must be clumpy with a volume-filling of a few percent. The CO lines are found to be important for the cooling of the cold molecular gas, e.g. at DR21 (OH). Near the outflow of the UV-heated source DR21, the gas cooling is dominated by line emission of atomic oxygen and of CO.

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arXiv2006

First observations with CONDOR, a 1.5 THz heterodyne receiver

The THz atmospheric windows centered at roughly 1.3 and 1.5~THz, contain numerous spectral lines of astronomical importance, including three high-J CO lines, the N+ line at 205 microns, and the ground transition of para-H2D+. The CO lines are tracers of hot (several 100K), dense gas; N+ is a cooling line of diffuse, ionized gas; the H2D+ line is a non-depleting tracer of cold (~20K), dense gas. As the THz lines benefit the study of diverse phenomena (from high-mass star-forming regions to the WIM to cold prestellar cores), we have built the CO N+ Deuterium Observations Receiver (CONDOR) to further explore the THz windows by ground-based observations. CONDOR was designed to be used at the Atacama Pathfinder EXperiment (APEX) and Stratospheric Observatory For Infrared Astronomy (SOFIA). CONDOR was installed at the APEX telescope and test observations were made to characterize the instrument. The combination of CONDOR on APEX successfully detected THz radiation from astronomical sources. CONDOR operated with typical Trec=1600K and spectral Allan variance times of 30s. CONDOR's first light observations of CO 13-12 emission from the hot core Orion FIR4 (= OMC1 South) revealed a narrow line with T(MB) = 210K and delta(V)=5.4km/s. A search for N+ emission from the ionization front of the Orion Bar resulted in a non-detection. The successful deployment of CONDOR at APEX demonstrates the potential for making observations at THz frequencies from ground-based facilities.

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