90 research outputs found
Physical and Chemical Properties of Molecular Gas in Star Forming Regions
In this thesis I study the physical and chemical properties of molecular gas in star forming regions. This work takes advantage of the newly developed CONDOR receiver, which allows spectrally resolved observations of emission lines at frequencies of about 1.4 THz. Thus the thesis contains observational data obtained with CONDOR as well as observations motivated by the science one can do with receivers at terahertz frequencies are largely included. In the massive star forming region NGC 2024, seven emission lines of 12CO and 13CO have been observed, including 12CO J=13-12 (f=1.497 THz) observations with CONDOR. These high-J CO data reveal a thin layer of hot (~300 K) and dense (~10e6 cm-3) molecular gas, which has not been detected with any other tracer before. This newly discovered component is located at the interface between an HII region and the molecular cloud. Furthermore, the narrow line width indicates that this layer is heated by radiation rather than by shocks. On the background of well established physical scenarios, such as the ''Blister Model'' and the PDR scenario, I developed a model of NGC 2024, using radiative transfer computations. This model explains both, the spatial distribution of density and temperature along the line of sight and the velocity structure of the source. The complex profiles of the observed 12CO and 13CO lines, which include emission by multiple velocity components as well as self-absorption by foreground material, are reproduced by this model remarkably well. Furthermore, the integrated intensities of five additional lines (J=14 to J=19) can also be explained. My study of low mass protostars was motivated by the H2D+ line observable with CONDOR, and shows the variation of chemical abundances along the star forming process. Next to its cosmological importance, deuterium plays also a significant role in the chemistry in the early phases of star formation. In such objects extremely high abundances of deuterated molecules have been measured. These abundances are mainly determined by the freeze out of CO onto dust grains, and the implication of the lowered CO abundance on gas phase chemistry. A consequence of the freeze out of CO at temperatures below 20 K, is that N2H+ is very abundant in the inner parts of prestellar cores, except for some possible depletion of NH itself in the coldest spots. In objects, which harbor already a protostar, the CO depleted zone forms a shell around the warm core. The more evolved such an embedded protostar is, i.e., the more it heats up the surrounding material, the smaller gets the CO depleted shell. The lack of CO in the gas phase leads not only to an enhanced abundance of N2H+, but also deuterium bearing molecules, such as H2D+, N2D+ and NH2D, are formed in a significant amount. As I will show in this work, the abundance of these deuterated species, and especially the abundance ratio of the deuterated to the non-deuterated form of molecule can be used as a tracer for the evolutionary stage of young protostellar cores. A high N2D+/N2H+ ratio (>0.15) indicates, that in such a core the collapse, which forms a star, has just begun. This ratio decreases as the protostar evolves further. I conducted follow up observations of NH2D and NH3 in the same objects. These observations give a similar trend for the NH2D/NH3 ration. However, other ratios, e.g., DCO+/HCO+ do not show such an enhancement, and stay at a level of a few percent throughout the protostellar evolution. The differences in the deuterium fractionation during the stellar evolution help us to understand the physical and chemical processes taking place. The lines, now observable at high spectral resolution with CONDOR, provide new possibilities to study the physical processes in the interstellar medium. The observations of highly excited CO enable us for the first time to investigate the hot, molecular gas in the ISM specifically. Therefore the temperature and density of this material can be determined with much higher accuracy than previously. In addition, the shapes of the high-J CO lines draw conclusions on the heating mechanisms of the gas. H2D+ observations give us insights into the physics of the extremely cold part of the ISM, where standard tracers, like CO, are frozen out onto dust grains. As I will show in this work, observation at THz frequencies allow us to study the ISM under extreme physical conditions, which have been very difficult to investigate so far
Hydrogen Fluoride in High-Mass Star-forming Regions
Hydrogen fluoride has been established to be an excellent tracer of molecular
hydrogen in diffuse clouds. In denser environments, however, the HF abundance
has been shown to be approximately two orders of magnitude lower. We present
Herschel/HIFI observations of HF J=1-0 toward two high-mass star formation
sites, NGC6334 I and AFGL 2591. In NGC6334 I the HF line is seen in absorption
in foreground clouds and the source itself, while in AFGL 2591 HF is partially
in emission. We find an HF abundance with respect to H2 of 1.5e-8 in the
diffuse foreground clouds, whereas in the denser parts of NGC6334 I, we derive
a lower limit on the HF abundance of 5e-10. Lower HF abundances in dense clouds
are most likely caused by freeze out of HF molecules onto dust grains in
high-density gas. In AFGL 2591, the view of the hot core is obstructed by
absorption in the massive outflow, in which HF is also very abundant 3.6e-8)
due to the desorption by sputtering. These observations provide further
evidence that the chemistry of interstellar fluorine is controlled by freeze
out onto gas grains.Comment: accepted in Ap
Hot, metastable hydronium ion in the Galactic center: Formation pumping in X-ray irradiated gas?
With a 3.5-meter diameter telescope passively cooled to ∼80 K and a science payload comprising two direct detection cameras/medium resolution imaging spectrometers, PACS and SPIRE, and a very high spectral resolution heterodyne spectrometer, HIFI, the Herschel Space Observatory is providing extraordinary observational opportunities in the 55-670 µm spectral range. HIFI has opened for the first time to high-resolution spectroscopy the submillimeter band that includes the fundamental rotational transitions of interstellar hydrides, the basic building blocks of astrochemistry. We discuss a recent HIFI discovery of metastable rotational transitions of the hydronium ion (protonated water, H 3 O + ), with rotational level energies up to 1200 K above the ground state, in absorption towards Sagittarius B2(N) in the Galactic center. Hydronium is an important molecular ion in the oxygen chemical network. Earlier HIFI observations have indicated a general deficiency of H 3 O + in the diffuse gas in the Galactic disk. The presence of hot H 3 O + towards Sagittarius B2(N) thus appears to be related to the unique physical conditions and the widespread presence of abundant H + 3 in the Central Molecular Zone. The high rotational temperature characterizing the population of the metastable levels may be indicative of H 3 O + formation pumping in molecular gas irradiated by X-rays emitted by the Galactic center black hole
CASIMIR: a high resolution far-IR/submm spectrometer for airborne astronomy
CASIMIR, the Caltech Airborne Submillimeter Interstellar Medium Investigations Receiver, is a far-infrared and submillimeter heterodyne spectrometer, being developed for the Stratospheric Observatory For Infrared Astronomy, SOFIA. CASIMIR will use newly developed superconducting-insulating-superconducting (SIS) mixers. Combined with the 2.5 m mirror of SOFIA, these detectors will allow observations with high sensitivity to be made in the frequency range from 500 GHz up to 1.4 THz. Initially, at least 5 frequency bands in this range are planned, each with a 4-8 GHz IF passband. Up to 4 frequency bands will be available on each flight and bands may be swapped readily between flights. The local oscillators for all bands are synthesized and tuner-less, using solid state multipliers. CASIMIR also uses a novel, commercial, field-programmable gate array (FPGA) based, fast Fourier transform spectrometer, with extremely high resolution, 22000 (268 kHz at 6 GHz), yielding a system resolution > 10^6. CASIMIR is extremely well suited to observe the warm, ≈ 100K, interstellar medium, particularly hydrides and water lines, in both galactic and extragalactic sources. We present an overview of the instrument, its capabilities and systems. We also describe recent progress in development of the local oscillators and present our first astronomical observations obtained with the new type of spectrometer
Herschel observations of EXtraordinary Sources: Analysis of the full Herschel/HIFI molecular line survey of Sagittarius B2(N)
A sensitive broadband molecular line survey of the Sagittarius B2(N)
star-forming region has been obtained with the HIFI instrument on the Herschel
Space Observatory, offering the first high-spectral resolution look at this
well-studied source in a wavelength region largely inaccessible from the ground
(625-157 um). From the roughly 8,000 spectral features in the survey, a total
of 72 isotopologues arising from 44 different molecules have been identified,
ranging from light hydrides to complex organics, and arising from a variety of
environments from cold and diffuse to hot and dense gas. We present an LTE
model to the spectral signatures of each molecule, constraining the source
sizes for hot core species with complementary SMA interferometric observations,
and assuming that molecules with related functional group composition are
cospatial. For each molecule, a single model is given to fit all of the
emission and absorption features of that species across the entire 480-1910 GHz
spectral range, accounting for multiple temperature and velocity components
when needed to describe the spectrum. As with other HIFI surveys toward massive
star forming regions, methanol is found to contribute more integrated line
intensity to the spectrum than any other species. We discuss the molecular
abundances derived for the hot core, where the local thermodynamic equilibrium
approximation is generally found to describe the spectrum well, in comparison
to abundances derived for the same molecules in the Orion KL region from a
similar HIFI survey.Comment: Accepted to ApJ. 64 pages, 14 figures. Truncated abstrac
Broadband analysis techniques for Herschel/HIFI spectral surveys of chemically rich star-forming regions
The Heterodyne Instrument for the Far Infrared (HIFI) aboard the Herschel
Space Observatory has acquired high-resolution broadband molecular spectra of
star-forming regions in a wavelength range that is mostly inaccessible from
ground-based astronomical observatories. These spectral surveys provide new
insight into the chemical composition and physical properties of molecular
clouds. In this manuscript, we present initial results from the HIFI spectral
survey of the Sagittarius B2(N) molecular cloud, which contains spectral
features assigned to at least 40 different molecules in a range of physical
environments. While extensive line blending is observed due to the chemical
complexity of this region, reliable molecular line identifications can be made,
down to the noise floor, due to the large number of transitions detected for
each species in the 1.2 THz survey bandwidth. This allows for the extraction of
new weakly emitting species from the line forest. These HIFI surveys will be an
invaluable archival resource for future investigations into interstellar
chemistry.Comment: 14 pages, 2 figures; accepted to the Journal of Molecular
Spectroscop
Reversal of infall in SgrB2(M) revealed by Herschel/HIFI observations of HCN lines at THz frequencies
To investigate the accretion and feedback processes in massive star
formation, we analyze the shapes of emission lines from hot molecular cores,
whose asymmetries trace infall and expansion motions. The high-mass star
forming region SgrB2(M) was observed with Herschel/HIFI (HEXOS key project) in
various lines of HCN and its isotopologues, complemented by APEX data. The
observations are compared to spherically symmetric, centrally heated models
with density power-law gradient and different velocity fields (infall or
infall+expansion), using the radiative transfer code RATRAN. The HCN line
profiles are asymmetric, with the emission peak shifting from blue to red with
increasing J and decreasing line opacity (HCN to HCN). This is most
evident in the HCN 12--11 line at 1062 GHz. These line shapes are reproduced by
a model whose velocity field changes from infall in the outer part to expansion
in the inner part. The qualitative reproduction of the HCN lines suggests that
infall dominates in the colder, outer regions, but expansion dominates in the
warmer, inner regions. We are thus witnessing the onset of feedback in massive
star formation, starting to reverse the infall and finally disrupting the whole
molecular cloud. To obtain our result, the THz lines uniquely covered by HIFI
were critically important.Comment: A&A, HIFI special issue, accepte
Herschel Observations of Extraordinary Sources: Analysis of the HIFI 1.2 THz Wide Spectral Survey toward Orion KL. I. Methods
We present a comprehensive analysis of a broadband spectral line survey of the Orion Kleinmann-Low nebula (Orion KL), one of the most chemically rich regions in the Galaxy, using the HIFI instrument on board the Herschel Space Observatory. This survey spans a frequency range from 480 to 1907 GHz at a resolution of 1.1 MHz. These observations thus encompass the largest spectral coverage ever obtained toward this high-mass star-forming region in the submillimeter with high spectral resolution and include frequencies >1 THz, where the Earth's atmosphere prevents observations from the ground. In all, we detect emission from 39 molecules (79 isotopologues). Combining this data set with ground-based millimeter spectroscopy obtained with the IRAM 30 m telescope, we model the molecular emission from the millimeter to the far-IR using the XCLASS program, which assumes local thermodynamic equilibrium (LTE). Several molecules are also modeled with the MADEX non-LTE code. Because of the wide frequency coverage, our models are constrained by transitions over an unprecedented range in excitation energy. A reduced χ^2 analysis indicates that models for most species reproduce the observed emission well. In particular, most complex organics are well fit by LTE implying gas densities are high (>10^6 cm^(–3)) and excitation temperatures and column densities are well constrained. Molecular abundances are computed using H_2 column densities also derived from the HIFI survey. The distribution of rotation temperatures, T_(rot), for molecules detected toward the hot core is significantly wider than the compact ridge, plateau, and extended ridge T_(rot) distributions, indicating the hot core has the most complex thermal structure
Herschel observations of deuterated water towards Sgr B2(M)
Observations of HDO are an important complement for studies of water, because
they give strong constraints on the formation processes -- grain surfaces
versus energetic process in the gas phase, e.g. in shocks. The HIFI
observations of multiple transitions of HDO in Sgr~B2(M) presented here allow
the determination of the HDO abundance throughout the envelope, which has not
been possible before with ground-based observations only. The abundance
structure has been modeled with the spherical Monte Carlo radiative transfer
code RATRAN, which also takes radiative pumping by continuum emission from dust
into account. The modeling reveals that the abundance of HDO rises steeply with
temperature from a low abundance () in the outer envelope
at temperatures below 100~K through a medium abundance () in
the inner envelope/outer core, at temperatures between 100 and 200~K, and
finally a high abundance () at temperatures above 200~K in
the hot core.Comment: A&A HIFI special issue, accepte
Herschel observations of extra-ordinary sources: Detecting spiral arm clouds by CH absorption lines
We have observed CH absorption lines ()
against the continuum source Sgr~B2(M) using the \textit{Herschel}/HIFI
instrument. With the high spectral resolution and wide velocity coverage
provided by HIFI, 31 CH absorption features with different radial velocities
and line widths are detected and identified. The narrower line width and lower
column density clouds show `spiral arm' cloud characteristics, while the
absorption component with the broadest line width and highest column density
corresponds to the gas from the Sgr~B2 envelope. The observations show that
each `spiral arm' harbors multiple velocity components, indicating that the
clouds are not uniform and that they have internal structure. This
line-of-sight through almost the entire Galaxy offers unique possibilities to
study the basic chemistry of simple molecules in diffuse clouds, as a variety
of different cloud classes are sampled simultaneously. We find that the linear
relationship between CH and H column densities found at lower by UV
observations does not continue into the range of higher visual extinction.
There, the curve flattens, which probably means that CH is depleted in the
denser cores of these clouds.Comment: Accepted for publication in A&A, HIFI Special Issu
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