scholarly journals Ionization: a possible explanation for the difference of mean disk sizes in star-forming regions

2020 ◽  
Vol 639 ◽  
pp. A86
Author(s):  
M. Kuffmeier ◽  
B. Zhao ◽  
P. Caselli
Keyword(s):  
Cosmic Ray ◽  
Ionization Rate ◽  
Ohmic Dissipation ◽  
Star Forming ◽  
Star Forming Regions ◽  
The Difference ◽  
Protostellar Collapse ◽  
Different Levels ◽  
Collapse Phase ◽  
Ionization Rates

Context. Surveys of protoplanetary disks in star-forming regions of similar age revealed significant variations in average disk mass in some regions. For instance, disks in the Orion Nebular Cluster (ONC) and Corona Australis (CrA) are on average smaller than disks observed in Lupus, Taurus, Chamaeleon I, or Ophiuchus. Aims. In contrast to previous models that studied the truncation of disks at a late stage of their evolution, we investigate whether disks may already be born with systematically smaller disk sizes in more massive star-forming regions as a consequence of higher ionization rates. Methods. Assuming various cosmic-ray ionization rates, we computed the resistivities for ambipolar diffusion and Ohmic dissipation with a chemical network, and performed 2D nonideal magnetohydrodynamical protostellar collapse simulations. Results. A higher ionization rate leads to stronger magnetic braking, and hence to the formation of smaller disks. Accounting for recent findings that protostars act as forges of cosmic rays and considering only mild attenuation during the collapse phase, we show that a high average cosmic-ray ionization rate in star-forming regions such as the ONC or CrA can explain the detection of smaller disks in these regions. Conclusions. Our results show that on average, a higher ionization rate leads to the formation of smaller disks. Smaller disks in regions of similar age can therefore be the consequence of different levels of ionization, and may not exclusively be caused by disk truncation through external photoevaporation. We strongly encourage observations that allow measuring the cosmic-ray ionization degrees in different star-forming regions to test our hypothesis.

scholarly journals Hydride spectroscopy of the diffuse interstellar medium: new clues on the gas fraction in molecular form and cosmic ray ionization rate in relation to H 3 +

2012 ◽  
Vol 370 (1978) ◽  
pp. 5174-5185 ◽  
Author(s):  
M. Gerin ◽  
F. Levrier ◽  
E. Falgarone ◽  
B. Godard ◽  
P. Hennebelle ◽  
...  
Keyword(s):  
Molecular Form ◽  
Chemical Properties ◽  
Cosmic Ray ◽  
Building Blocks ◽  
Ionization Rate ◽  
Spectral Lines ◽  
Interstellar Gas ◽  
Interstellar Molecules ◽  
Star Forming ◽  
Star Forming Regions

The Herschel-guaranteed time key programme PRobing InterStellar Molecules with Absorption line Studies (PRISMAS) 1 is providing a survey of the interstellar hydrides containing the elements C, O, N, F and Cl. As the building blocks of interstellar molecules, hydrides provide key information on their formation pathways. They can also be used as tracers of important physical and chemical properties of the interstellar gas that are difficult to measure otherwise. This paper presents an analysis of two sight-lines investigated by the PRISMAS project, towards the star-forming regions W49N and W51. By combining the information extracted from the detected spectral lines, we present an analysis of the physical properties of the diffuse interstellar gas, including the electron abundance, the fraction of gas in molecular form, and constraints on the cosmic ray ionization rate and the gas density.

scholarly journals A new proxy to estimate the cosmic ray ionization rate in dense cores

2020 ◽  
Vol 495 (1) ◽  
pp. L7-L11
Author(s):  
S Bovino ◽  
S Ferrada-Chamorro ◽  
A Lupi ◽  
D R G Schleicher ◽  
P Caselli
Keyword(s):  
Spatial Scales ◽  
Cosmic Ray ◽  
Ionization Rate ◽  
Fundamental Parameter ◽  
Star Forming ◽  
Star Forming Regions ◽  
Dense Cores ◽  
Statistical Relevance ◽  
Theoretical Predictions ◽  
Diffuse Clouds

ABSTRACT Cosmic rays are a global source of ionization, and the ionization fraction represents a fundamental parameter in the interstellar medium. Ions couple to magnetic fields, and affect the chemistry and the dynamics of star-forming regions as well as planetary atmospheres. However, the cosmic ray ionization rate represents one of the bottlenecks for astrochemical models, and its determination is one of the most puzzling problems in astrophysics. While for diffuse clouds reasonable values have been provided from ${\mathrm{ H}_3}^+$ observations, for dense clouds, due to the lack of rotational transitions, this is not possible, and estimates are strongly biased by the employed model. We present here an analytical expression, obtained from first principles, to estimate the cosmic ray ionization rate from observational quantities. The theoretical predictions are validated with high-resolution 3D numerical simulations and applied to the well-known core L1544; we obtained an estimate of ζ2 ∼ 2–3 × 10−17 s−1. Our results and the analytical formulae provided represent the first model-independent robust tool to probe the cosmic ray ionization rate in the densest part of star-forming regions (on spatial scales of R ≤ 0.05 pc). An error analysis is presented to give statistical relevance to our study.

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 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 Chemistry in the Envelopes around Massive Young Stars

2002 ◽  
Vol 12 ◽  
pp. 146-148
Author(s):  
Ewine F. van Dishoeck ◽  
Floris F.S. van der Tak
Keyword(s):  
Young Star ◽  
Young Stars ◽  
High Mass ◽  
Mass Star ◽  
Star Forming ◽  
Star Forming Regions ◽  
Chemical Studies ◽  
H Ii Region ◽  
Infrared Wavelengths ◽  
Collapse Phase

AbstractRecent chemical studies of high-mass star-forming regions at submillimeter and infrared wavelengths reveal large variations in the abundances depending on evolutionary state. Such variations can be explained by freezing out of molecules onto grains in the cold collapse phase, followed by evaporation and high-temperature chemical reactions when the young star heats the envelope. Thus, the chemical composition can be a powerful diagnostic tool. A detailed study of a set of infrared-bright massive young stars reveals systematic increases in the gas/solid ratios and abundances of evaporated molecules with temperature. This ‘global heating’ plausibly results from the gradual dispersion of the envelopes. We argue that these objects form the earliest phase of massive star formation, before the ‘hot core’ and ultracompact H II region phase.

Performance of VERA in 10 micro-arcsecond astrometry

2020 ◽  
Vol 72 (4) ◽  
Author(s):  
Takumi Nagayama ◽  
Hideyuki Kobayashi ◽  
Tomoya Hirota ◽  
Mareki Honma ◽  
Takaaki Jike ◽  
...  
Keyword(s):  
Monitoring Program ◽  
High Elevation ◽  
Reference Source ◽  
Star Forming ◽  
Star Forming Regions ◽  
Long Baseline ◽  
Residual Contribution ◽  
Open Issue ◽  
The Difference ◽  
High Elevations

Abstract Very Long Baseline Interferometry (VLBI) astrometry using the phase-referencing technique remains an open issue for the quantitative characterization of the observing conditions to achieve a feasible parallax precision of 10 micro-arcseconds (μas). To address this issue, we evaluated the astrometric performance of the VLBI Exploration of Radio Astrometry (VERA) through the parallax measurements of five distant star-forming regions under good observing conditions of close separations (${0{^{\circ}_{.}}5}$–${1{_{.}^{\circ}}3}$) and high elevations (≥50°). Their parallaxes measured 89–200 μas, corresponding to distances of 5–11 kpc with an error of 11–20 μas. Furthermore, we investigated the contributions to the position error budget and concluded that the tropospheric residual contribution is the dominant error source. We also confirmed that the astrometric error propagation strongly depends on the term $\Delta \sec Z$, which stands for the difference between $\sec Z$ of the target and its reference source, where Z is the zenith angle during the observations. We found that for a source pair with a $\Delta \sec Z$ less than 0.01 (for example, a set of a close separation of $\le {{0{^{\circ}_{.}}5}}$ and a high elevation of ≥50°), we can achieve the parallax precision of 10 μas using a typical monitoring program comprising 10 observing epochs over a span of two years.

scholarly journals A study of interstellar aldehydes and enols as tracers of a cosmic ray-driven nonequilibrium synthesis of complex organic molecules

2016 ◽  
Vol 113 (28) ◽  
pp. 7727-7732 ◽  
Author(s):  
Matthew J. Abplanalp ◽  
Samer Gozem ◽  
Anna I. Krylov ◽  
Christopher N. Shingledecker ◽  
Eric Herbst ◽  
...  
Keyword(s):  
Organic Molecules ◽  
Cosmic Ray ◽  
Formation Mechanisms ◽  
Molecular Precursors ◽  
Star Forming ◽  
Star Forming Regions ◽  
Interstellar Ices ◽  
A Chain ◽  
Nonequilibrium Chemistry ◽  
Complex Organic

Complex organic molecules such as sugars and amides are ubiquitous in star- and planet-forming regions, but their formation mechanisms have remained largely elusive until now. Here we show in a combined experimental, computational, and astrochemical modeling study that interstellar aldehydes and enols like acetaldehyde (CH3CHO) and vinyl alcohol (C2H3OH) act as key tracers of a cosmic-ray-driven nonequilibrium chemistry leading to complex organics even deep within low-temperature interstellar ices at 10 K. Our findings challenge conventional wisdom and define a hitherto poorly characterized reaction class forming complex organic molecules inside interstellar ices before their sublimation in star-forming regions such as SgrB2(N). These processes are of vital importance in initiating a chain of chemical reactions leading eventually to the molecular precursors of biorelevant molecules as planets form in their interstellar nurseries.

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.

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.

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