arXiv subjects

F. Daniel

Publications and source records attributed to F. Daniel.

At least 19 recordsLinked to original sources

arXiv2026

Magnetohydrodynamic instabilities in stellar radiative regions. I. Linear study of shear-driven instabilities

This paper is the first in a series investigating magnetohydrodynamic instabilities that may contribute to angular-momentum transport and magnetic-field evolution in stellar radiative zones. We focus on shear-driven instabilities, specifically the Goldreich-Schubert-Fricke (GSF) instability and the magnetorotational instability (MRI), which are expected to play key roles in the internal dynamics of radiative regions. We carried out a local linear stability analysis using a numerical approach that extends beyond classical limiting cases and includes stabilizing effects such as stratification and magnetic tension, allowing the exploration of realistic flow regimes. These results were validated through a global mode analysis in a Taylor-Couette configuration. We recovered the standard MRI and azimuthal MRI stability criteria and quantified the effects of stratification, magnetic tension, and diffusion on their growth. In strongly sheared regimes, we derived a new criterion for the magnetised GSF (MGSF) instability and clarified the transition from SMRI to MGSF as stratification and magnetic effects narrow the unstable domain. We also provided approximate growth-time formulae that identify the dominant instability under given stellar conditions and can be implemented in 1D stellar evolution codes. Global Taylor-Couette calculations validate the local WKB analysis. Applied to subgiants and young red giants, our results show that shear-driven instabilities can grow rapidly for magnetic fields below 100 kG. Strong axial fields (100 kG) confined to the hydrogen-burning shell suppress instabilities unless the shear is sufficiently distant. These results support incorporating our criteria and growth estimates into stellar evolution models to assess the efficiency of shear-driven transport.

astro-ph.SR↗

arXiv2017

Collisional excitation of NH3 by atomic and molecular hydrogen

We report extensive theoretical calculations on the rotation-inversion excitation of interstellar ammonia (NH3) due to collisions with atomic and molecular hydrogen (both para- and ortho-H2). Close-coupling calculations are performed for total energies in the range 1-2000 cm-1 and rotational cross sections are obtained for all transitions among the lowest 17 and 34 rotation-inversion levels of ortho- and para-NH3, respectively. Rate coefficients are deduced for kinetic temperatures up to 200 K. Propensity rules for the three colliding partners are discussed and we also compare the new results to previous calculations for the spherically symmetrical He and para-H2 projectiles. Significant differences are found between the different sets of calculations. Finally, we test the impact of the new rate coefficients on the calibration of the ammonia thermometer. We find that the calibration curve is only weakly sensitive to the colliding partner and we confirm that the ammonia thermometer is robust.

physics.chem-ph↗

arXiv2016

N$_2$H$^+$ and N$^{15}$NH$^+$ towards the prestellar core 16293E in L1689N

Understanding the processes that could lead to enrichment of molecules in $^{15}$N atoms is of particular interest in order to shed light on the relatively large variations observed in the $^{14}$N/$^{15}$N ratio in various solar system environments. Currently, the sample of molecular clouds where $^{14}$N/$^{15}$N ratios have been measured is small and has to be enlarged in order to allow statistically significant studies. In particular, the N$_2$H$^+$ molecule currently shows the largest spread of $^{14}$N/$^{15}$N ratios in high-mass star forming regions. However, the $^{14}$N/$^{15}$N ratio in N$_2$H$^+$ was obtained in only two low-mass star forming regions (L1544 and B1b). The current work extends this sample to a third dark cloud. We targeted the 16293E prestellar core, where the N$^{15}$NH$^+$ $J$=1-0 line was detected. Using a model previously developed for the physical structure of the source, we solved the molecular excitation with a non-local radiative transfer code. For that purpose, we computed specific collisional rate coefficients for the N$^{15}$NH$^+$-H$_2$ collisional system. As a first step of the analysis, the N$_2$H$^+$ abundance profile was constrained by reproducing the N$_2$H$^+$ $J$=1-0 and 3-2 maps. A scaling factor was then applied to this profile to match the N$^{15}$NH$^+$ $J$=1-0 spectrum. We derive a column density ratio N$_2$H$^+$ / N$^{15}$NH$^+$ = $330^{+170}_{-100}$. The current estimate $\sim$330 agrees with the value typical of the elemental isotopic ratio in the local ISM. It is however lower than in some other cores, where values as high as 1300 have been reported.

astro-ph.GA↗

arXiv2016

Collisional excitation of doubly and triply deuterated ammonia ND$_2$H and ND$_3$ by H$_2$

The availability of collisional rate coefficients is a prerequisite for an accurate interpretation of astrophysical observations, since the observed media often harbour densities where molecules are populated under non--LTE conditions. In the current study, we present calculations of rate coefficients suitable to describe the various spin isomers of multiply deuterated ammonia, namely the ND$_2$H and ND$_3$ isotopologues. These calculations are based on the most accurate NH$_3$--H$_2$ potential energy surface available, which has been modified to describe the geometrical changes induced by the nuclear substitutions. The dynamical calculations are performed within the close--coupling formalism and are carried out in order to provide rate coefficients up to a temperature of $T$ = 50K. For the various isotopologues/symmetries, we provide rate coefficients for the energy levels below $\sim$ 100 cm$^{-1}$. Subsequently, these new rate coefficients are used in astrophysical models aimed at reproducing the NH$_2$D, ND$_2$H and ND$_3$ observations previously reported towards the prestellar cores B1b and 16293E. We thus update the estimates of the corresponding column densities and find a reasonable agreement with the previous models. In particular, the ortho--to--para ratios of NH$_2$D and NHD$_2$ are found to be consistent with the statistical ratios.

astro-ph.GA↗

arXiv2016

The NH$_2$D hyperfine structure revealed by astrophysical observations

The 1$_{11}$-1$_{01}$ lines of ortho and para--NH$_2$D (o/p-NH$_2$D), respectively at 86 and 110 GHz, are commonly observed to provide constraints on the deuterium fractionation in the interstellar medium. In cold regions, the hyperfine structure due to the nitrogen ($^{14}$N) nucleus is resolved. To date, this splitting is the only one which is taken into account in the NH$_2$D column density estimates. We investigate how the inclusion of the hyperfine splitting caused by the deuterium (D) nucleus affects the analysis of the rotational lines of NH$_2$D. We present 30m IRAM observations of the above mentioned lines, as well as APEX o/p-NH$_2$D observations of the 1$_{01}$-0$_{00}$ lines at 333 GHz. The hyperfine spectra are first analyzed with a line list that only includes the hyperfine splitting due to the $^{14}$N nucleus. We find inconsistencies between the line widths of the 1$_{01}$-0$_{00}$ and 1$_{11}$-1$_{01}$ lines, the latter being larger by a factor of $\sim$1.6$\pm0.3$. Such a large difference is unexpected given the two sets of lines are likely to originate from the same region. We next employ a newly computed line list for the o/p-NH$_2$D transitions, where the hyperfine structure induced by both nitrogen and deuterium nuclei is included. With this new line list, the analysis of the previous spectra leads to linewidths which are compatible. Neglecting the hyperfine structure owing to D leads to overestimate the linewidths of the o/p-NH$_2$D lines at 3 mm. The error for a cold molecular core is about 50\%. This error propagates directly to the column density estimate. It is therefore recommended to take into account the hyperfine splittings caused by both the $^{14}$N and D nuclei in any analysis relying on these lines.

astro-ph.GA↗

arXiv2015

Stratified NH and ND emission in the prestellar core 16293E in L1689N

High degrees of deuterium fractionation are commonly found in cold prestellar cores and in the envelopes around young protostars. As it brings strong constraints to chemical models, deuterium chemistry is often used to infer core history or molecule formation pathways. Whereas a large number of observations is available regarding interstellar deuterated stable molecules, relatively little is known about the deuteration of hydride radicals, as their fundamental rotational transitions are at high frequencies where the atmosphere is mostly opaque. Nitrogen hydride radicals are important species in nitrogen chemistry, as they are thought to be related to ammonia formation. Observations have shown that ammonia is strongly deuterated, with [NH2D]/[NH3] ~ 10%. Models predict similarly high [ND]/[NH] ratios, but so far only one observational determination of this ratio is available, towards the envelope of the protostar IRAS16293-2422. In order to test model predictions, we aim here at determining [ND]/[NH] in a dense, starless core. We observed NH and ND in 16293E with the HIFI spectrometer on board the Herschel Space Observatory as part of the CHESS guaranteed time key programme, and derived the abundances of these two species using a non-LTE non-local radiative transfer model. Both NH and ND are detected in the source, with ND in emission and NH in absorption against the continuum arising from the cold dust emission. Our model shows however that the ND emission and the NH absorption originate from different layers in the cloud, as further evidenced by their different velocities. In the central region of the core, we can set a lower limit to the [ND]/[NH] ratio of ~2%. This estimate is consistent with recent pure gas-phase models of nitrogen chemistry

astro-ph.GA↗

arXiv2014

Collisional excitation of water by hydrogen atoms

We present quantum dynamical calculations that describe the rotational excitation of H$_2$O due to collisions with H atoms. We used a recent, high accuracy potential energy surface, and solved the collisional dynamics with the close-coupling formalism, for total energies up to 12 000 cm$^{-1}$. From these calculations, we obtained collisional rate coefficients for the first 45 energy levels of both ortho- and para-H$_2$O and for temperatures in the range T = 5-1500 K. These rate coefficients are subsequently compared to the values previously published for the H$_2$O / He and H$_2$O / H$_2$ collisional systems. It is shown that no simple relation exists between the three systems and that specific calculations are thus mandatory.

astro-ph.GA↗

arXiv2014

Discovery of Time Variation of the Intensity of Molecular Lines in IRC+10216 in The Submillimeter and Far Infrared Domains

We report on the discovery of strong intensity variations in the high rotational lines of abundant molecular species towards the archetypical circumstellar envelope of IRC+10216. The observations have been carried out with the HIFI instrument on board \textit{Herschel}\thanks{\textit{Herschel} is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA} and with the IRAM\thanks{This work was based on observations carried out with the IRAM 30-meter telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain)} 30-m telescope. They cover several observing periods spreading over 3 years. The line intensity variations for molecules produced in the external layers of the envelope most probably result from time variations in the infrared pumping rates. We analyze the main implications this discovery has on the interpretation of molecular line emission in the envelopes of Mira-type stars. Radiative transfer calculations have to take into account both the time variability of infrared pumping and the possible variation of the dust and gas temperatures with stellar phase in order to reproduce the observation of molecular lines at different epochs. The effect of gas temperature variations with stellar phase could be particularly important for lines produced in the innermost regions of the envelope. Each layer of the circumstellar envelope sees the stellar light radiation with a different lag time (phase). Our results show that this effect must be included in the models. The sub-mm and FIR lines of AGB stars cannot anymore be considered as safe intensity calibrators.

astro-ph.SR↗

arXiv2014

Collisional excitation of singly deuterated ammonia NH$_2$D by H$_2$

The availability of collisional rate coefficients with H$_2$ is a pre-requisite for interpretation of observations of molecules whose energy levels are populated under non local thermodynamical equilibrium conditions. In the current study, we present collisional rate coefficients for the NH$_2$D / para--H$_2$($J_2 = 0,2$) collisional system, for energy levels up to $J_\tau = 7_7$ ($E_u$$\sim$735 K) and for gas temperatures in the range $T = 5-300$K. The cross sections are obtained using the essentially exact close--coupling (CC) formalism at low energy and at the highest energies, we used the coupled--states (CS) approximation. For the energy levels up to $J_\tau = 4_2$ ($E_u$$\sim$215 K), the cross sections obtained through the CS formalism are scaled according to a few CC reference points. These reference points are subsequently used to estimate the accuracy of the rate coefficients for higher levels, which is mainly limited by the use of the CS formalism. Considering the current potential energy surface, the rate coefficients are thus expected to be accurate to within 5\% for the levels below $J_\tau = 4_2$, while we estimate an accuracy of 30\% for higher levels.

astro-ph.GA↗

arXiv2013

Unveiling the dust nucleation zone of IRC+10216 with ALMA

We report the detection in IRC+10216 of lines of HNC $J$=3-2 pertaining to 9 excited vibrational states with energies up to $\sim$5300 K. The spectrum, observed with ALMA, also shows a surprising large number of narrow, unidentified lines that arise in the vicinity of the star. The HNC data are interpreted through a 1D--spherical non--local radiative transfer model, coupled to a chemical model that includes chemistry at thermochemical equilibrium for the innermost regions and reaction kinetics for the external envelope. Although unresolved by the present early ALMA data, the radius inferred for the emitting region is $\sim$0.06'' (i.e., $\simeq$ 3 stellar radii), similar to the size of the dusty clumps reported by IR studies of the innermost region ($r <$ 0.3''). The derived abundance of HNC relative to H$_2$ is $10^{-8} <$ $\chi$(HNC) $< 10^{-6}$, and drops quickly where the gas density decreases and the gas chemistry is dominated by reaction kinetics. Merging HNC data with that of molecular species present throughout the inner envelope, such as vibrationally excited HCN, SiS, CS, or SiO, should allow us to characterize the physical and chemical conditions in the dust formation zone.

astro-ph.GA↗

arXiv2013

Nitrogen isotopic ratios in Barnard 1: a consistent study of the N2H+, NH3, CN, HCN and HNC isotopologues

The 15N isotopologue abundance ratio measured today in different bodies of the solar system is thought to be connected to 15N-fractionation effects that would have occured in the protosolar nebula. The present study aims at putting constraints on the degree of 15N-fractionation that occurs during the prestellar phase, through observations of D, 13C and 15N-substituted isotopologues towards B1b. Both molecules from the nitrogen hydride family, i.e. N2H+ and NH3, and from the nitrile family, i.e. HCN, HNC and CN, are considered in the analysis. As a first step, we model the continuum emission in order to derive the physical structure of the cloud, i.e. gas temperature and H2 density. These parameters are subsequently used as an input in a non-local radiative transfer model to infer the radial abundances profiles of the various molecules. Our modeling shows that all the molecules are affected by depletion onto dust grains, in the region that encompasses the B1-bS and B1-bN cores. While high levels of deuterium fractionation are derived, we conclude that no fractionation occurs in the case of the nitrogen chemistry. Independently of the chemical family, the molecular abundances are consistent with 14N/15N~300, a value representative of the elemental atomic abundances of the parental gas. The inefficiency of the 15N-fractionation effects in the B1b region can be linked to the relatively high gas temperature ~17K which is representative of the innermost part of the cloud. Since this region shows signs of depletion onto dust grains, we can not exclude the possibility that the molecules were previously enriched in 15N, earlier in the B1b history, and that such an enrichment could have been incorporated into the ice mantles. It is thus necessary to repeat this kind of study in colder sources to test such a possibility.

astro-ph.GA↗

arXiv2013

Non--local radiative transfer in strongly inverted masers

Maser transitions are commonly observed in media exhibiting a large range of densities and temperatures. They can be used to obtain information on the dynamics and physical conditions of the observed regions. In order to obtain reliable constraints on the physical conditions prevailing in the masing regions, it is necessary to model the excitation mechanisms of the energy levels of the observed molecules. We present a numerical method that enables us to obtain self-consistent solutions for both the statistical equilibrium and radiative transfer equations. Using the standard maser theory, the method of Short Characteristics is extended to obtain the solution of the integro-differential radiative transfer equation, appropriate to the case of intense masing lines. We have applied our method to the maser lines of the H2O molecule and we compare with the results obtained with a less accurate approach. In the regime of large maser opacities we find large differences in the intensity of the maser lines that could be as high as several orders of magnitude. The comparison between the two methods shows, however, that the effect on the thermal lines is modest. Finally, the effect introduced by rate coefficients on the prediction of H2O masing lines and opacities is discussed, making use of various sets of rate coefficients involving He, o-H2 and p-H2. We find that the masing nature of a line is not affected by the selected collisional rates. However, from one set to the other the modelled line opacities and intensities can vary by up to a factor ~2 and ~10 respectively.

astro-ph.GA↗

arXiv2012

On the influence of collisional rate coefficients on the water vapour excitation

Water is a key molecule in many astrophysical studies. Its high dipole moment makes this molecule to be subthermally populated under the typical conditions of most astrophysical objects. This motivated the calculation of various sets of collisional rate coefficients (CRC) for H$_2$O (with He or H$_2$) which are necessary to model its rotational excitation and line emission. We performed accurate non--local non--LTE radiative transfer calculations using different sets of CRC in order to predict the line intensities from transitions that involve the lowest energy levels of H$_2$O (E $<$ 900 K). The results obtained from the different CRC sets are then compared using line intensity ratio statistics. For the whole range of physical conditions considered in this work, we obtain that the intensities based on the quantum and QCT CRC are in good agreement. However, at relatively low H$_2$ volume density ($n$(H$_2$) $<$ 10$^7$ cm$^{-3}$) and low water abundance ($\chi$(H$_2$O) $<$ 10$^{-6}$), these physical conditions being relevant to describe most molecular clouds, we find differences in the predicted line intensities of up to a factor of $\sim$ 3 for the bulk of the lines. Most of the recent studies interpreting early Herschel Space Observatory spectra used the QCT CRC. Our results show that although the global conclusions from those studies will not be drastically changed, each case has to be considered individually, since depending on the physical conditions, the use of the QCT CRC may lead to a mis--estimate of the water vapour abundance of up to a factor of $\sim$ 3.

astro-ph.GA↗

arXiv2012

Herschel/HIFI observation of highly excited rotational lines of HNC toward IRC +10 216

We report the detection in emission of various highly excited rotational transitions of HNC (J = 6-5 through J =12-11) toward the carbon-star envelope IRC +10 216 using the HIFI instrument on-board the Herschel Space Observatory. Observations of the J = 1-0 and J = 3-2 lines of HNC with the IRAM 30-m telescope are also presented. The lines observed with HIFI have upper level energies corresponding to temperatures between 90 and 340 degrees Kelvin, and trace a warm and smaller circumstellar region than that seen in the interferometric maps of the J = 1-0 transition, whose emission extends up to a radius of 20". After a detailed chemical and radiative transfer modeling, we find that the presence of HNC in the circumstellar envelope of IRC +10 216 is consistent with formation from the precursor ion HCNH+, which in turn is produced through several proton transfer reactions which are triggered by the cosmic-ray ionization. We also find that the radiative pumping through 21 um photons to the first excited state of the bending mode v2 plays a crucial role to populate the high-J HNC levels involved in the transitions observed with HIFI. Emission in these high-J rotational transitions of HNC is expected to be strong in regions which are warm and dense and/or have an intense infrared flux at wavelengths around 21 um.

astro-ph.GA↗

arXiv2012

Molecular abundances in the inner layers of IRC +10216

Observations towards IRC +10216 of CS, SiO, SiS, NaCl, KCl, AlCl, AlF, and NaCN have been carried out with the IRAM 30-m telescope in the 80-357.5 GHz frequency range. A large number of rotational transitions covering a wide range of energy levels, including highly excited vibrational states, are detected in emission and serve to trace different regions of the envelope. Radiative transfer calculations based on the LVG formalism have been performed to derive molecular abundances from the innermost out to the outer layers. The excitation calculations include infrared pumping to excited vibrational states and inelastic collisions, for which up-to-date rate coefficients for rotational and, in some cases, ro-vibrational transitions are used. We find that in the inner layers CS, SiO, and SiS have abundances relative to H$_2$ of 4e-6, 1.8e-7, and 3e-6, respectively, and that CS and SiS have significant lower abundances in the outer envelope, which implies that they actively contribute to the formation of dust. Moreover, in the inner layers, the amount of sulfur and silicon in gas phase molecules is only 27 % for S and 5.6 % for Si, implying that these elements have already condensed onto grains, most likely in the form of MgS and SiC. Metal-bearing molecules lock up a relatively small fraction of metals, although our results indicate that NaCl, KCl, AlCl, AlF, and NaCN, despite their refractory character, are not significantly depleted in the cold outer layers. In these regions a few percent of the metals Na, K, and Al survive in the gas phase, either in atomic or molecular form, and are therefore available to participate in the gas phase chemistry in the outer envelope.

astro-ph.GA↗

arXiv2012

On the physical structure of IRC+10216. Ground-based and Herschel observations of CO and CCH

The C-rich AGB star IRC+10216 undergoes strong mass loss, and quasi-periodic density enhancements in the circumstellar matter have been reported. CO is ubiquitous in the CSE, while CCH emission comes from a spatially confined shell. With the IRAM 30m telescope and Herschel/HIFI, we recently detected unexpectedly strong emission from the CCH N=4-3, 6-5, 7-6, 8-7, and 9-8 transitions, challenging the available chemical and physical models. We aim to constrain the physical properties of IRC+10216's CSE, including the effect of episodic mass loss on the observed emission. In particular, we aim to determine the excitation region and conditions of CCH and to reconcile these with interferometric maps of the N=1-0 transition. Via radiative-transfer modelling, we provide a physical description of the CSE, constrained by the SED and a sample of 20 high-resolution and 29 low-resolution CO lines. We further present detailed radiative-transfer analysis of CCH. Assuming a distance of 150pc, the SED is modelled with a stellar luminosity of 11300Lsun and a dust-mass-loss rate of 4.0\times10^{-8}Msun/yr. Based on the analysis of 20 high resolution CO observations, an average gas-mass-loss rate for the last 1000yrs of 1.5\times10^{-5}Msun/yr is derived. This gives a gas-to-dust-mass ratio of 375, typical for an AGB star. The gas kinetic temperature throughout the CSE is described by 3 powerlaws: it goes as r^{-0.58} for r<9R*, as r^{-0.40} for 9<=r<=65R*, and as r^{-1.20} for r>65R*. This model successfully describes all 49 CO lines. We show the effect of wind-density enhancements on the CCH-abundance profile, and the good agreement of the model with the CCH N=1-0 transition and with the lines observed with the 30m telescope and HIFI. We report on the importance of radiative pumping to the vibrationally excited levels of CCH and the significant effect this has on the excitation of all levels of the CCH-molecule.

astro-ph.SR↗

arXiv2011

HIFI detection of HF in the carbon star envelope IRC +10216

We report the detection of emission in the J=1-0 rotational transition of hydrogen fluoride (HF), together with observations of the J=1-0 to J=3-2 rotational lines of H35Cl and H37Cl, towards the envelope of the carbon star IRC +10216. High-sensitivity, high-spectral resolution observations have been carried out with the HIFI instrument on board Herschel, allowing us to resolve the line profiles and providing insights into the spatial distribution of the emission. Our interpretation of the observations, with the use of radiative transfer calculations, indicates that both HF and HCl are formed in the inner regions of the envelope close to the AGB star. Thermochemical equilibrium calculations predict HF and HCl to be the major reservoirs of fluorine and chlorine in the atmospheres of AGB stars. The abundances relative to H2 derived for HF and HCl, 8e-9 and 1e-7 respectively, are substantially lower than those predicted by thermochemical equilibrium, indicating that F and Cl are likely affected by significant depletion onto dust grains, although some chlorine may be in the form of atomic Cl. The H35Cl/H37Cl abundance ratio is 3.3 +/- 0.3. The low abundance derived for HF in IRC +10216 makes it likely that the fluorine abundance is not enhanced over the solar value by nucleosynthesis in the AGB star, although this conclusion may not be robust because the HF abundance we derive is a lower limit to the elemental abundance of F. These observations suggest that both HF and HCl should be detectable through low J rotational transitions in other evolved stars.

astro-ph.GA↗

arXiv2011

Warm water vapour in the sooty outflow from a luminous carbon star

In 2001, the discovery of circumstellar water vapour around the ageing carbon star IRC+10216 was announced. This detection challenged the current understanding of chemistry in old stars, since water vapour was predicted to be absent in carbon-rich stars. Several explanations for the occurrence of water vapour were postulated, including the vaporization of icy bodies (comets or dwarf planets) in orbit around the star, grain surface reactions, and photochemistry in the outer circumstellar envelope. However, the only water line detected so far from one carbon-rich evolved star can not discriminate, by itself, between the different mechanisms proposed. Here we report on the detection by the Herschel satellite of dozens of water vapour lines in the far-infrared and sub-millimetre spectrum of IRC+10216, including some high-excitation lines with energies corresponding to ~1000 K. The emission of these high-excitation water lines can only be explained if water vapour is present in the warm inner region of the envelope. A plausible explanation for the formation of warm water vapour appears to be the penetration of ultraviolet (UV) photons deep into a clumpy circumstellar envelope. This mechanism triggers also the formation of other molecules such as ammonia, whose observed abundances are much higher than hitherto predicted.

astro-ph.SR↗