scholarly journals Complex cyanides as chemical clocks in hot cores

2018 ◽  
Vol 616 ◽  
pp. A67 ◽  
Author(s):  
V. Allen ◽  
F. F. S. van der Tak ◽  
C. Walsh
Keyword(s):  
Cosmic Ray ◽  
Ionization Rate ◽  
High Mass ◽  
Small Scale ◽  
Mass Star ◽  
Chemical Difference ◽  
Gas Density ◽  
Warm Up ◽  
Dust Temperature ◽  
Ionization Rates

Context. In the high-mass star-forming region G35.20−0.74N, small scale (~800 AU) chemical segregation has been observed in which complex organic molecules containing the CN group are located in a small location (toward continuum peak B3) within an apparently coherently rotating structure. Aims. We aim to determine the physical origin of the large abundance difference (~4 orders of magnitude) in complex cyanides within G35.20−0.74 B, and we explore variations in age, gas/dust temperature, and gas density. Methods. We performed gas-grain astrochemical modeling experiments with exponentially increasing (coupled) gas and dust temperature rising from 10 to 500 K at constant H2 densities of 107 cm−3, 108 cm−3, and 109 cm−3. We tested the effect of varying the initial ice composition, cosmic-ray ionization rate (1.3 × 10−17 s−1, 1 × 10−16 s−1, and 6 × 10−16 s−1), warm-up time (over 50, 200, and 1000 kyr), and initial (10, 15, and 25 K) and final temperatures (300 and 500 K). Results. Varying the initial ice compositions within the observed and expected ranges does not noticeably affect the modeled abundances indicating that the chemical make-up of hot cores is determined in the warm-up stage. Complex cyanides vinyl and ethyl cyanide (CH2CHCN and C2H5CN, respectively) cannot be produced in abundances (vs. H2) greater than 5 ×10−10 for CH2CHCN and 2 ×10−10 for C2H5CN with a fast warm-up time (52 kyr), while the lower limit for the observed abundance of C2H5CN toward source B3 is 3.4 ×10−10. Complex cyanide abundances are reduced at higher initial temperatures and increased at higher cosmic-ray ionization rates. Reaction-diffusion competition is necessary to reproduce observed abundances of oxygen-bearing species in our model. Conclusions. Within the context of this model, reproducing the observed abundances toward G35.20−0.74 Core B3 requires a fast warm-up at a high cosmic-ray ionization rate (~1 × 10−16 s−1) at a high gas density (>109 cm−3). The abundances observed at the other positions in G35.20-0.74N also require a fast warm-up but allow lower gas densities (~108 cm−3) and cosmic-ray ionization rates (~1 × 10−17 s−1). In general, we find that the abundance of ethyl cyanide in particular is maximized in models with a low initial temperature, a high cosmic-ray ionization rate, a long warm-up time (>200 kyr), and a lower gas density (tested down to 107 cm−3). G35.20−0.74 source B3 only needs to be ~2000 years older than B1/B2 for the observed chemical difference to be present, which maintains the possibility that G35.20−0.74 B contains a Keplerian disk.

Using H 3 + and H 2 D + as probes of star-forming regions

2006 ◽  
Vol 364 (1848) ◽  
pp. 3101-3106 ◽  
Author(s):  
Floris F.S van der Tak
Keyword(s):  
Cosmic Ray ◽  
Ionization Rate ◽  
High Mass ◽  
Mass Star ◽  
Future Prospects ◽  
Star Forming ◽  
Star Forming Regions ◽  
Chemical Conditions ◽  
Physical And Chemical ◽  
Kinematic Properties

The and H 2 D + ions are important probes of the physical and chemical conditions in regions of the interstellar medium where new stars are forming. This paper reviews how observations of these species and of heavier ions such as HCO + and H 3 O + can be used to derive chemical and kinematic properties of nearby pre-stellar cores and the cosmic ray ionization rate towards more distant regions of high-mass star formation. Future prospects in the field are outlined at the end.

scholarly journals Hot Molecular Cores and High Mass Star Formation

2004 ◽  
Vol 221 ◽  
pp. 59-66 ◽  
Author(s):  
F.F.S. Van Der Tak
Keyword(s):  
Star Formation ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Chemical Processes ◽  
High Mass ◽  
Mass Star ◽  
Low Mass Stars ◽  
Low Mass

This review covers hot cores in the context of high-mass star formation. After giving an overview of chemical processes and diversity during high-mass star formation, it reviews the ‘warm envelope’ phase which probably precedes the formation of hot cores. Some recent determinations of the cosmic-ray ionization rate are discussed, as well as recent evidence for hot cores around low-mass stars. Routes for future hot core research are outlined.

scholarly journals Survey of ortho-H2D+ in high-mass star-forming regions

2020 ◽  
Vol 644 ◽  
pp. A34
Author(s):  
G. Sabatini ◽  
S. Bovino ◽  
A. Giannetti ◽  
F. Wyrowski ◽  
M. A. Órdenes ◽  
...  
Keyword(s):  
Star Formation ◽  
Formation Process ◽  
Strong Dependence ◽  
Cosmic Ray ◽  
High Mass ◽  
Clear Correlation ◽  
Mass Star ◽  
Large Sample ◽  
Star Forming ◽  
Star Forming Regions

Context. Deuteration has been suggested to be a reliable chemical clock of star-forming regions due to its strong dependence on density and temperature changes during cloud contraction. In particular, the H3+ isotopologues (e.g. ortho-H2D+) seem to act as good proxies of the evolutionary stages of the star formation process. While this has been widely explored in low-mass star-forming regions, in the high-mass counterparts only a few studies have been pursued, and the reliability of deuteration as a chemical clock remains inconclusive. Aims. We present a large sample of o-H2D+ observations in high-mass star-forming regions and discuss possible empirical correlations with relevant physical quantities to assess its role as a chronometer of star-forming regions through different evolutionary stages. Methods. APEX observations of the ground-state transition of o-H2D+ were analysed in a large sample of high-mass clumps selected from the ATLASGAL survey at different evolutionary stages. Column densities and beam-averaged abundances of o-H2D+ with respect to H2, X(o-H2D+), were obtained by modelling the spectra under the assumption of local thermodynamic equilibrium. Results. We detect 16 sources in o-H2D+ and find clear correlations between X(o-H2D+) and the clump bolometric luminosity and the dust temperature, while only a mild correlation is found with the CO-depletion factor. In addition, we see a clear correlation with the luminosity-to-mass ratio, which is known to trace the evolution of the star formation process. This would indicate that the deuterated forms of H3+ are more abundant in the very early stages of the star formation process and that deuteration is influenced by the time evolution of the clumps. In this respect, our findings would suggest that the X(o-H2D+) abundance is mainly affected by the thermal changes rather than density changes in the gas. We have employed these findings together with observations of H13CO+, DCO+, and C17O to provide an estimate of the cosmic-ray ionisation rate in a sub-sample of eight clumps based on recent analytical work. Conclusions. Our study presents the largest sample of o-H2D+ in star-forming regions to date. The results confirm that the deuteration process is strongly affected by temperature and suggests that o-H2D+ can be considered a reliable chemical clock during the star formation processes, as proved by its strong temporal dependence.

scholarly journals On the Cosmic Ray-Induced Ionization Rate in Molecular Clouds

2011 ◽  
Vol 2011 ◽  
pp. 1-10 ◽  
Author(s):  
Ararat G. Yeghikyan
Keyword(s):  
Molecular Clouds ◽  
Column Density ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Electronic Interaction ◽  
Absorbing Medium ◽  
Be Star ◽  
Good Agreement ◽  
Ionization Rates

The transformation of the energy dependence of the cosmic ray proton flux in the keV to GeV region is investigated theoretically when penetrating inside molecular clouds ( mag). The computations suggest that energy losses of the cosmic ray particles by interaction with the matter of the molecular cloud are principally caused by the inelastic (electronic) interaction potential; the transformed energy distribution of energetic protons is determined mainly by the column density of the absorbing medium. A cutoff of the cosmic ray spectrum inside clouds by their magnetic fields is also phenomenologically taken into account. This procedure allows a determination of environment-dependent ionization rates of molecular clouds. The theoretically predicted ionization rates are in good agreement with those derived from astronomical observations of absorption lines in the spectrum of the cloud connected with the Herbig Be star LkH 101.

scholarly journals Probing the initial conditions of high-mass star formation

2019 ◽  
Vol 627 ◽  
pp. A85 ◽  
Author(s):  
Chuan-Peng Zhang ◽  
Timea Csengeri ◽  
Friedrich Wyrowski ◽  
Guang-Xing Li ◽  
Thushara Pillai ◽  
...  
Keyword(s):  
Star Formation ◽  
Multiple Scales ◽  
Initial Conditions ◽  
Dust Emission ◽  
Radio Continuum ◽  
High Mass ◽  
Small Scale ◽  
High Angular Resolution ◽  
H Ii Regions ◽  
Mass Star

Context. Fragmentation and feedback are two important processes during the early phases of star formation. Aims. Massive clumps tend to fragment into clusters of cores and condensations, some of which form high-mass stars. In this work, we study the structure of massive clumps at different scales, analyze the fragmentation process, and investigate the possibility that star formation is triggered by nearby H ii regions. Methods. We present a high angular resolution study of a sample of massive proto-cluster clumps G18.17, G18.21, G23.97N, G23.98, G23.44, G23.97S, G25.38, and G25.71. Combining infrared data at 4.5, 8.0, 24, and 70 μm, we use a few arcsecond resolution, radiometer and millimeter inteferometric data taken at 1.3 cm, 3.5 mm, 1.3 mm, and 870 μm to study their fragmentation and evolution. Our sample is unique in the sense that all the clumps have neighboring H ii regions. Taking advantage of that, we tested triggered star formation using a novel method where we study the alignment of the center of mass traced by dust emission at multiple scales. Results. The eight massive clumps, identified based on single-dish observations, have masses ranging from 228 to 2279 M⊙ within an effective radius of Reff ~ 0.5 pc. We detect compact structures towards six out of the eight clumps. The brightest compact structures within infrared bright clumps are typically associated with embedded compact radio continuum sources. The smaller scale structures of Reff ~ 0.02 pc observed within each clump are mostly gravitationally bound and massive enough to form at least a B3-B0 type star. Many condensations have masses larger than 8 M⊙ at a small scale of Reff ~ 0.02 pc. We find that the two infrared quiet clumps with the lowest mass and lowest surface density with <300 M⊙ do not host any compact sources, calling into question their ability to form high-mass stars. Although the clumps are mostly infrared quiet, the dynamical movements are active at clump scale (~1 pc). Conclusions. We studied the spatial distribution of the gas conditions detected at different scales. For some sources we find hints of external triggering, whereas for others we find no significant pattern that indicates triggering is dynamically unimportant. This probably indicates that the different clumps go through different evolutionary paths. In this respect, studies with larger samples are highly desired.

scholarly journals The distribution of cosmic-ray ionization rates in diffuse molecular clouds as probed by H 3 +

2012 ◽  
Vol 370 (1978) ◽  
pp. 5142-5150 ◽  
Author(s):  
Nick Indriolo
Keyword(s):  
Molecular Clouds ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Line Of Sight ◽  
Interstellar Clouds ◽  
Upper Limits ◽  
The Galaxy ◽  
Cosmic Ray Flux ◽  
Made In ◽  
Ionization Rates

Owing to its simple chemistry, H is widely regarded as the most reliable tracer of the cosmic-ray ionization rate in diffuse interstellar clouds. At present, H observations have been made in over 50 sight lines that probe the diffuse interstellar medium (ISM) throughout the Galaxy. This small survey presents the opportunity to investigate the distribution of cosmic-ray ionization rates in the ISM, as well as any correlations between the ionization rate and line-of-sight properties. Some of the highest inferred ionization rates are about 25 times larger than the lowest upper limits, suggesting variations in the underlying low-energy cosmic-ray flux across the Galaxy. Most likely, such variations are caused predominantly by the distance between an observed cloud and the nearest site of particle acceleration.

scholarly journals On determination of the cosmic ray flux using molecular hydrogen absorption lines

2003 ◽  
pp. 15-20
Author(s):  
Milan Cirkovic ◽  
I. Damjanov
Keyword(s):  
Molecular Hydrogen ◽  
Hydrogen Absorption ◽  
High Redshift ◽  
Cosmic Ray ◽  
Absorption Lines ◽  
High Mass ◽  
Proton Density ◽  
Mass Star ◽  
Cosmic Ray Flux

We outline a procedure for estimating the cosmic ray flux at remote locations where molecular hydrogen absorption lines have been detected. The method relies on several assumptions whose validity in the local Galactic ISM has been independently verified, so it might be useful for much less accessible objects, especially damped Ly? absorption systems. Since most of low-energy cosmic rays in the Galactic environment are thought to originate in supernovae remnants, the link to the rate of high-mass star formation could in principle, be established. We applied the method to a particular case of high redshift damped Ly? absorption system towards 0528?250 and obtained an estimate of proton density and some useful constraints.

scholarly journals The EDIBLES survey

2019 ◽  
Vol 622 ◽  
pp. A31 ◽  
Author(s):  
Xavier L. Bacalla ◽  
Harold Linnartz ◽  
Nick L. J. Cox ◽  
Jan Cami ◽  
Evelyne Roueff ◽  
...  
Keyword(s):  
Band System ◽  
Cosmic Ray ◽  
Far Infrared ◽  
Ionization Rate ◽  
Electronic Band ◽  
Interstellar Clouds ◽  
Near Ultraviolet ◽  
Diffuse Interstellar Bands ◽  
Range Of Values ◽  
Ionization Rates

We report cosmic ray ionization rates toward ten reddened stars studied within the framework of the EDIBLES (ESO Diffuse Interstellar Bands Large Exploration Survey) program, using the VLT-UVES. For each sightline, between two and ten individual rotational lines of OH+ have been detected in its (0,0) and (1,0) A3Π − X3Σ− electronic band system. This allows constraining of OH+ column densities toward different objects. Results are also presented for 28 additional sightlines for which only one or rather weak signals are found. An analysis of these data makes it possible to derive the primary cosmic ray ionization rate ζp in the targeted diffuse interstellar clouds. For the ten selected targets, we obtain a range of values for ζp equal to (3.9–16.4) × 10−16 s−1. These values are higher than the numbers derived in previous detections of interstellar OH+ in the far-infrared/submillimeter-wave regions and in other near-ultraviolet studies. This difference is a result of using new OH+ oscillator strength values and a more complete picture of all relevant OH+ formation and destruction routes (including the effect of proton recombinations on PAHs), and the relatively high N(OH+) seen toward those ten targets.

scholarly journals Photodissociation region diagnostics across galactic environments

2021 ◽  
Vol 502 (2) ◽  
pp. 2701-2732
Author(s):  
Thomas G Bisbas ◽  
Jonathan C Tan ◽  
Kei E I Tanaka
Keyword(s):  
Three Dimensional ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Energy Distributions ◽  
Line Energy ◽  
Conversion Factors ◽  
Line Intensities ◽  
Wide Range ◽  
Photodissociation Region ◽  
Ionization Rates

ABSTRACT We present three-dimensional astrochemical simulations and synthetic observations of magnetized, turbulent, self-gravitating molecular clouds. We explore various galactic interstellar medium environments, including cosmic ray ionization rates in the range of ζCR = 10−17–$10^{-14}\, {\rm s}^{-1}$, far-UV intensities in the range of G0 = 1–103 and metallicities in the range of Z = 0.1–$2\, {\rm Z}_{\odot }$. The simulations also probe a range of densities and levels of turbulence, including cases where the gas has undergone recent compression due to cloud–cloud collisions. We examine: (i) the column densities of carbon species across the cycle of C ii, C i, and CO, along with O i, in relation to the H i-to-H2 transition; (ii) the velocity-integrated emission of [C ii] 158 μm, [13C ii] 158 μm, [C i] 609 μm and 370 μm, [O i] 63 μm and 146 μm, and of the first ten 12CO rotational transitions; (iii) the corresponding Spectral Line Energy Distributions; (iv) the usage of [C ii] and [O i] 63 μm to describe the dynamical state of the clouds; (v) the behaviour of the most commonly used ratios between transitions of CO and [C i]; and (vi) the conversion factors for using CO and C i as H2-gas tracers. We find that enhanced cosmic ray energy densities enhance all aforementioned line intensities. At low metallicities, the emission of [C ii] is well connected with the H2 column, making it a promising new H2 tracer in metal-poor environments. The conversion factors of XCO and XC i depend on metallicity and the cosmic ray ionization rate, but not on FUV intensity. In the era of ALMA, SOFIA, and the forthcoming CCAT-prime telescope, our results can be used to understand better the behaviour of systems in a wide range of galactic and extragalactic environments.

Revealing magnetic fields towards massive protostars: a multi-scale approach using masers and dust

2018 ◽  
Vol 14 (A30) ◽  
pp. 111-112
Author(s):  
Daria Dall’Olio ◽  
W. H. T. Vlemmings ◽  
M. V. Persson
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Fields ◽  
Spatial Scales ◽  
High Mass ◽  
Small Scale ◽  
Evolutionary Stage ◽  
Mass Star ◽  
Filamentary Structure ◽  
The Magnetic Field

AbstractMagnetic fields play a significant role during star formation processes, hindering the fragmentation and the collapse of the parental cloud, and affecting the accretion mechanisms and feedback phenomena. However, several questions still need to be addressed to clarify the importance of magnetic fields at the onset of high-mass star formation, such as how strong they are and at what evolutionary stage and spatial scales their action becomes relevant. Furthermore, the magnetic field parameters are still poorly constrained especially at small scales, i.e. few astronomical units from the central object, where the accretion disc and the base of the outflow are located. Thus we need to probe magnetic fields at different scales, at different evolutionary steps and possibly with different tracers. We show that the magnetic field morphology around high-mass protostars can be successfully traced at different scales by observing maser and dust polarised emission. A confirmation that they are effective tools is indeed provided by our recent results from 6.7 GHz MERLIN observations of the massive protostar IRAS 18089-1732, where we find that the small-scale magnetic field probed by methanol masers is consistent with the large-scale magnetic field probed by dust (Dall’Olio et al. 2017 A&A 607, A111). Moreover we present results obtained from our ALMA Band 7 polarisation observations of G9.62+0.20, which is a massive star-forming region with a sequence of cores at different evolutionary stages (Dall’Olio et al. submitted to A&A). In this region we resolve several protostellar cores embedded in a bright and dusty filamentary structure. The magnetic field morphology and strength in different cores is related to the evolutionary sequence of the star formation process which is occurring across the filament.

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