A high density portion of the Orion Molecular Cloud 1 (OMC-1) contains the prominent, warm Kleinmann-Low (KL) nebula that is internally powered by an energetic event plus a farther region in which ...intermediate to high mass stars are forming. Its outside is affected by ultraviolet radiation from the neighboring Orion Nebula Cluster and forms the archetypical photon-dominated region (PDR) with the prominent bar feature. Its nearness makes the OMC-1 core region a touchstone for research on the dense molecular interstellar medium and PDRs. Using the Atacama Pathfinder Experiment telescope (APEX), we have imaged the line emission from the multiple transitions of several carbon monoxide (CO) isotopologues over the OMC-1 core region. Our observations employed the 2x7 pixel submillimeter CHAMP+ array to produce maps (~ 300 arcsec x 350 arcsec) of 12CO, 13CO, and C18O from mid-J transitions (J=6-5 to 8-7). We also obtained the 13CO and C18O J=3-2 images toward this region. The 12CO line emission shows a well-defined structure which is shaped and excited by a variety of phenomena, including the energetic photons from hot, massive stars in the nearby Orion Nebula's central Trapezium cluster, active high- and intermediate-mass star formation, and a past energetic event that excites the KL nebula. Radiative transfer modeling of the various isotopologic CO lines implies typical H2 densities in the OMC-1 core region of ~10^4-10^6 cm^-3 and generally elevated temperatures (~ 50-250 K). We estimate a warm gas mass in the OMC-1 core region of 86-285 solar masses.
Abstract
HD 50138 is one of the brightest Be stars and is located at a distance of ∼380 pc with strong infrared excess. The star was observed in O
i
63
μ
m and C
ii
158
μ
m with high velocity ...resolution with upGREAT on SOFIA. The velocity-resolved O
i
emission provides evidence for a large gas disk, ∼760 au in size, around HD 50138. Whereas previous interferometric observations gave strong evidence for a hot gas and dust disk in Keplerian rotation, our observations are the first to provide unambiguous evidence for a large warm disk around the star.
Herschel
/PACS observations showed that the C
ii
emission is extended, therefore the C
ii
emission most likely originates from an ionized gas shell created by a past outflow event. We confirm the isolated nature of HD 50138. It is far from any star-forming region and has low proper motion. Neither is there any sign of a remnant cloud from which it could have formed. The extended disk around the star appears to be carbon-poor. It shows OH and O
i
emission, but no CO. The CO abundance appears to be at least an order of magnitude lower than that of OH. Furthermore,
13
CO is enriched by more than a factor of five, confirming that the star is not a Herbig Be star. Finally, we note that our high-spectral-resolution O
i
and C
ii
observations provide a very accurate heliocentric velocity of the star, 40.8 ± 0.2 km s
−1
.
Context. We present the technology and first scientific results of a new generation of very flexible and sensitive spectrometers, well-suited for the needs of spectral-line radio and (sub)millimeter ...astronomy: Fast Fourier Transform Spectrometers (FFTS), which are in operation at the Atacama Pathfinder EXperiment (APEX) telescope. Aims. The FFTS for APEX is a novel high-resolution $2\times 1$ GHz bandwidth digital spectrometer backend. Due to its high frequency resolution, and the demonstrated capability of operating at high altitude, the FFTS became the facility spectrometer for spectral line observations at APEX. Methods. The FFTS is based on one of the currently most powerful digitizer/analyzer boards available from Acqiris, Switzerland. The board incorporates two 1 Gsamples/s analog-to-digital converters (ADCs) with 8-bit resolution which feed an on-board complex field programmable gate array (FPGA) chip. The enormous processing power by today's FPGAs allow for a complete real-time FFT signal processing pipeline to decompose a 1 GHz band into 16 384 spectral channels in just one chip. Results. Since May 2005 an MPIfR FFTS has been extensively used in all regular spectroscopic science observations. The performance at APEX was demonstrated to be very reliable and as good as measured in the first laboratory tests which finally led to the request to provide a second, facility type FFTS for APEX. The unit was delivered and commissioned in March this year. Conclusions. Using a commercially available digitizer board, it was possible to develop a complete FFTS in only a few months. Successful observations at APEX demonstrate that this new generation of FPGA-based spectrometers easily matching and superseding the performance of older technology spectrometers and can built up much more easily. Furthermore, the by now available class of new high-speed ADCs and the continuous increase of FPGA processing power makes it very likely that FFTS can be pushed to broader bandwidth and even more spectral channels in the near future.
Context. High-mass stars are formed within massive molecular clumps, where a large number of stars form close together. The evolution of the clumps with different masses and luminosities is mainly ...regulated by their high-mass stellar content and the formation of such objects is still not well understood. Aims. In this work, we characterise the mid-J CO emission in a statistical sample of 99 clumps (TOP100) selected from the ATLASGAL survey that are representative of the Galactic proto-cluster population. Methods. High-spatial resolution APEX-CHAMP+ maps of the CO (6–5) and CO (7–6) transitions were obtained and combined with additional single-pointing APEX-FLASH+ spectra of the CO (4–3) line. The data were convolved to a common angular resolution of 13.′′4. We analysed the line profiles by fitting the spectra with up to three Gaussian components, classified as narrow or broad, and computed CO line luminosities for each transition. Additionally, we defined a distance-limited sample of 72 sources within 5 kpc to check the robustness of our analysis against beam dilution effects. We have studied the correlations of the line luminosities and profiles for the three CO transitions with the clump properties and investigate if and how they change as a function of the evolution. Results. All sources were detected above 3-σ in all three CO transitions and most of the sources exhibit broad CO emission likely associated with molecular outflows. We find that the extension of the mid-J CO emission is correlated with the size of the dust emission traced by the Herschel-PACS 70 μm maps. The CO line luminosity (LCO) is correlated with the luminosity and mass of the clumps. However, it does not correlate with the luminosity-to-mass ratio. Conclusions. The dependency of the CO luminosity with the properties of the clumps is steeper for higher-J transitions. Our data seem to exclude that this trend is biased by self-absorption features in the CO emission, but rather suggest that different J transitions arise from different regions of the inner envelope. Moreover, high-mass clumps show similar trends in CO luminosity as lower mass clumps, but are systematically offset towards larger values, suggesting that higher column density and (or) temperature (of unresolved) CO emitters are found inside high-mass clumps.
Context. The disc around the Herbig Ae/Be star HD 100546 is one of the most extensively studied discs in the southern sky. Although there is a wealth of information about its dust content and ...composition, not much is known about its gas and large-scale kinematics. Many recent results have stressed the importance of studying both the gas and dust in discs. 12CO is an excellent gas tracer in the submillimetre, and the intensity ratio between lines originating from low and high rotational levels probes the gas temperature. Emerging submillimetre facilities in the Southern hemisphere allow us to characterise the gas and dust content in objects like HD 100546 better. Aims. Our aim is to establish whether the disc is gas-rich and to study the disc temperature and kinematics. Methods. We detected and studied the molecular gas in the disc at spatial resolution from 7$\farcs$7 to 18$\farcs$9 using the Atacama Pathfinder Experiment telescope (This publication is based on data acquired with the Atacama Pathfinder Experiment (APEX). APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.). We observed the lines 12CO J = 7–6, J = 6–5, J = 3–2, 13CO J = 3–2 and C I 3P2–3P1, as diagnostics of disc temperature, size, chemistry, and kinematics. We use parametric disc models that reproduce the low-J 12CO emission from Herbig Ae stars and we vary the basic disc parameters – temperature, mass, and size. With the help of a molecular excitation and radiative transfer code we fit the observed spectral line profiles. Results. Our observations are consistent with more than 10-3 $M_{\odot}$ of molecular gas in a disc of ≈400 AU radius in Keplerian rotation around a 2.5 $M_{\odot}$ star, seen at an inclination of 50°. The detected 12CO lines are dominated by gas at 30–70 K. Not detecting the C I line indicates excess ultraviolet emission above that of a B9 type model stellar atmosphere. Asymmetry in the 12CO line emission suggests that one side of the outer disc is colder by 10–20 K than the other. A plausible scenario is a warped geometry in the inner disc casting a partial shaddow on the outer disc. We exclude pointing offsets, foreground cloud absorption, and asymmetry in the disc extent as possible causes of the observed line asymmetry. Conclusions. Efficient heating of the outer disc by the star HD 100546 ensures that low- and high-J 12CO lines are dominated by the outermost disc regions, indicating a 400 AU radius. The 12CO J = 6–5 line arises from a disc layer higher than disc midplane, and warmer by 15–20 K than the layer emitting the J = 3–2 line. The existing models of discs around Herbig Ae stars, asuming a B9.5 type model stellar atmosphere, overproduce the CI 3P2–3P1 line intensity from HD 100546 by an order of magnitude.
We report the detection of the ground state N,J = 1,3/2 $\rightarrow$ 1,1/2 doublet of the methylidyne radical CH at ~532 GHz and ~536 GHz with the Herschel/HIFI instrument along the sight-line to ...the massive star-forming regions G10.6–0.4 (W31C), W49N, and W51. While the molecular cores associated with these massive star-forming regions show emission lines, clouds in the diffuse interstellar medium are detected in absorption against the strong submillimeter background. The combination of hyperfine structure with emission and absorption results in complex profiles, with overlap of the different hyperfine components. The opacities of most of the CH absorption features are linearly correlated with those of CCH, CN, and HCO+ in the same velocity intervals. In specific narrow velocity intervals, the opacities of CN and HCO+ deviate from the mean trends, giving rise to more opaque absorption features. We propose that CCH can be used as another tracer of the molecular gas in the absence of better tracers, with CCH/ H2 ~ 3.2±1.1×10-8. The observed CN/CH, CCH/CH abundance ratios suggest that the bulk of the diffuse matter along the lines of sight has gas densities nH = n(H) + 2n(H2) ranging between 100 and 1000 cm-3.
With the GREAT receiver at the Stratospheric Observatory for Infrared Astronomy (SOFIA), nine massive molecular clumps have been observed in the ammonia 32+−22− line at 1.8 THz in a search for ...signatures of infall. The sources were selected from the ATLASGAL submillimeter dust continuum survey of our Galaxy. Clumps with high masses covering a range of evolutionary stages based on their infrared properties were chosen. The ammonia line was detected in all sources, leading to five new detections and one confirmation of a previous detection of redshifted absorption in front of their strong THz continuum as a probe of infall in the clumps. These detections include two clumps embedded in infrared dark clouds. The measured velocity shifts of the absorptions compared to optically thin C17O (3–2) emission are 0.3–2.8 km s-1, corresponding to fractions of 3% to 30% of the free-fall velocities of the clumps. The ammonia infall signature is compared with complementary data of different transitions of HCN, HNC, CS, and HCO+, which are often used to probe infall because of their blue-skewed line profiles. The best agreement with the ammonia results is found for the HCO+ (4–3) transitions, but the latter is still strongly blended with emission from associated outflows. This outflow signature is far less prominent in the THz ammonia lines, which confirms it as a powerful probe of infall in molecular clumps. Infall rates in the range from 0.3 to 16 × 10-3 M⊙/yr were derived with a tentative correlation with the virial parameters of the clumps. The new observations show that infall on clump scales is ubiquitous through a wide range of evolutionary stages, from L/M covering about ten to several hundreds.
Context. Protostars of intermediate-mass provide a bridge between theories of low- and high-mass star formation. Molecular outflows emerging from such sources can be used to determine the influence ...of fragmentation and multiplicity on protostellar evolution through the apparent correlation of outflow forces of intermediate-mass protostars with the total luminosity instead of the individual luminosity. Aims. The aim of this paper is to derive outflow forces from outflows of six intermediate-mass protostellar regions and validate the apparent correlation between total luminosity and outflow force seen in earlier work, as well as remove uncertainties caused by different methodologies. Methods. By comparing CO 6–5 observations obtained with APEX with non-LTE radiative transfer model predictions, the optical depths, temperatures and densities of the gas of the molecular outflows are derived. Outflow forces, dynamical timescales, and kinetic luminosities are subsequently calculated. Results. Outflow parameters, including the forces, were derived for all sources. Temperatures in excess of 50 K were found for all flows, in line with recent low-mass results. However, comparison with other studies could not corroborate conclusions from earlier work on intermediate-mass protostars which hypothesized that fragmentation enhances outflow forces in clustered intermediate-mass star formation. Any enhancement in comparison with the classical relation between outflow force and luminosity can be attributed to the use of a higher excitation line and improvement in methods. They are in line with results from low-mass protostars using similar techniques. Conclusions. The role of fragmentation on outflows is an important ingredient to understand clustered star formation and the link between low- and high-mass star formation. However, detailed information on spatial scales of a few 100 AU, covering all individual members is needed to make the necessary progress.
Context. Observations of dense molecular gas lie at the basis of our understanding of the density and temperature structure of protostellar envelopes and molecular outflows. The Atacama Pathfinder ...EXperiment (APEX) opens up the study of southern (Dec $< -35^{\circ}$) protostars. Aims. We aim to characterize the properties of the protostellar envelope, molecular outflow and surrounding cloud, through observations of high excitation molecular lines within a sample of 16 southern sources presumed to be embedded YSOs, including the most luminous Class I objects in Corona Australis and Chamaeleon. Methods. Observations of submillimeter lines of CO, HCO+ and their isotopologues, both single spectra and small maps (up to $80''\times 80''$), were taken with the FLASH and APEX-2a instruments mounted on APEX to trace the gas around the sources. The HARP-B instrument on the JCMT was used to map IRAS 15398-3359 in these lines. HCO+ mapping probes the presence of dense centrally condensed gas, a characteristic of protostellar envelopes. The rare isotopologues C18O and H13CO+ are also included to determine the optical depth, column density, and source velocity. The combination of multiple CO transitions, such as 3–2, 4–3 and 7–6, allows to constrain outflow properties, in particular the temperature. Archival submillimeter continuum data are used to determine envelope masses. Results. Eleven of the sixteen sources have associated warm and/or dense (≥ 106 cm-3) quiescent gas characteristic of protostellar envelopes, or an associated outflow. Using the strength and degree of concentration of the HCO+ 4–3 and CO 4–3 lines as a diagnostic, five sources classified as Class I based on their spectral energy distributions are found not to be embedded YSOs. The C18O 3–2 lines show that for none of the sources, foreground cloud layers are present. Strong molecular outflows are found around six sources, with outflow forces an order of magnitude higher than for previously studied Class I sources of similar luminosity. Conclusions. This study provides a starting point for future ALMA and Herschel surveys by identifying truly embedded southern YSOs and determining their larger scale envelope and outflow characteristics.
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![Velocity-resolved [O i] 63 ...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0004-637X_1538-4357/small "Velocity-resolved [O i] 63 μm Emission in the HD 50138 Circumstellar Disk")
