arXiv subjects

J. Harju

Publications and source records attributed to J. Harju.

At least 19 recordsLinked to original sources

arXiv2026

Corona Australis 151: an extremely young protostar

Context. Prestellar cores are the birthplace of stars and planetary systems, but they are short-lived objects, since the initial stages of dense core evolution, collapse, and the formation of a protostellar seed are fast. In an effort to build a catalogue of bona-fide prestellar cores in the Solar neighbourhood, we used APEX observations to identify dynamically evolved cores among dense cores observed with Herschel. One of them, Corona Australis 151, stood out because of its centrally peaked structure, with densities above 1e7cm-3 in the central 500-1000 au, and high deuteration levels (N2D+/N2H+~0.5), suggestive of an evolved, prestellar stage. Aims. Corona Australis 151 appears to be an evolved prestellar core, but the presence of broad wings in some of the detected lines and a tentative 70micron detection with Herschel partially challenges its prestellar stage. We intend to assess its evolutionary stage. Methods. We analyse new ALMA data of the continuum emission and of several line tracers (including typical outflow tracers) at a resolution of~150-200au, tracing the envelope. Results. We unveil the presence of a compact and young outflow (projected dynamical age:~500yr), traced by SiO, CO, and H2CO emission. The continuum emission traces an envelope structure of size~1000 au and mass 0.33M_sun(assuming Tdust=20 K). Conclusions. Corona Australis 151 is an extremely young protostar, possibly one of the youngest known in the Solar neighbourhood, and hence a new laboratory to study the chemical and dynamical evolution at the dawn of star formation.

astro-ph.GA↗

arXiv2025

Hunting pre-stellar cores with APEX: overview

[Abridged] $Context.$ Pre-stellar cores are centrally concentrated starless cores on the verge of star formation and they represent the initial conditions for star and planet formation. Pre-stellar cores host an active organic chemistry and isotopic fractionation, kept stored into thick icy mantles, which can be inherited by the future protoplanetary disks and planetesimals. So far, only a few have been studied in detail, with special attention being paid to L1544 in the Taurus Molecular Cloud. $Aims.$ The aim is to identify nearby ($<$200 pc) pre-stellar cores in an unbiased way, to build a sample that can then be studied in detail. $Methods.$ We first used the Herschel Gould Belt Survey archival data, selecting all those starless cores with central H$_2$ number densities higher than or equal to 3$\times$10$^5$ cm$^{-3}$, the density of L1544 within the Herschel beam. The selected 40 (out of 1746) cores have then been observed in N$_2$H$^+$(3-2) and N$_2$D$^+$(4-3) using the APEX antenna. $Results.$ A total of 17 bona-fide (i.e., with a deuterium fraction larger than 10%) pre-stellar cores have been identified. Other 16 objects can also be considered pre-stellar, as they are dynamically evolved starless cores, but their deuterium fraction is relatively low ($<$10%). The remaining 7 objects have been found associated with very young stellar objects. $Conclusions.$ Dust continuum emission, together with spectroscopic observations of N$_2$H$^+$(3-2) and N$_2$D$^+$(4-3), is a powerful tool to identify pre-stellar cores in molecular clouds. Detailed modeling of the physical structure of the objects is now required for reconstructing the chemical composition as a function of radius. This work has provided a statistically significant sample of 33 pre-stellar cores, a crucial step in the understanding of the process of star and planet formation.

astro-ph.GA↗

arXiv2025

Hunting pre-stellar cores with APEX: Corona Australis 151, the densest pre-stellar core or the youngest protostar?

Context. Pre-stellar cores are the birthplaces of Sun-like stars and represent the initial conditions for the assembly of protoplanetary systems. Due to their short lifespans, they are rare. In recent efforts to increase the number of such sources identified in the Solar neighbourhood, we have selected a sample of 40 starless cores from the publicly available core catalogs of the Herschel Gould Belt survey. In this work, we focus on one of the sources that stands out for its high central density: Corona Australis 151. Aims. We use molecular lines that trace dense gas (n>=10^6 cm-3) to confirm the exceptionally high density of this object, to study its physical structure, and to understand its evolutionary stage. Methods. We detected the N2H+ 3-2 and 5-4 transitions, and the N2D+ 3-2, 4-3, and 6-5 lines with the APEX telescope. We use the Herschel continuum data to infer a spherically symmetric model of the core's density and temperature. This is used as input to perform non-local-thermodynamic-equilibrium radiative transfer to fit the observed five lines. Results. Our analysis confirms that this core is characterised by very high densities (a few x 10^7 cm-3 at the centre) and cold temperatures. We infer a high deuteration level of N2D+/N2H+=0.50, indicative of an advanced evolutionary stage. In the large bandwidth covered by the APEX data, we detect several other deuterated species, including CHD2OH, D2CO, and ND3. We also detect multiple sulphurated species that present broader lines with signs of high-velocity wings. Conclusions. The observation of high-velocity wings and the fact that the linewidths of N2H+ and N2D+ become larger with increasing frequency can be interpreted either as an indication of supersonic infall motions developing in the central parts of a very evolved pre-stellar core or as the signature of outflows from a very low luminosity object (VeLLO). *SHORTENED*

astro-ph.SR↗

arXiv2024

Hunting pre-stellar cores with APEX: IRAS16293E (Oph464)

Pre-stellar cores are the first steps in the process of star and planet formation. However, the dynamical and chemical evolution of pre-stellar cores is still not well understood. We aim at estimating the central density of the pre-stellar core IRAS16293E and at carrying out an inventory of molecular species towards the density peak of the core. We observed high-$J$ rotational transitions of N$_2$H$^+$ and N$_2$D$^+$, and several other molecular lines towards the dust emission peak using the Atacama Pathfinder EXperiment (APEX) telescope, and derived the density and temperature profiles of the core using far-infrared surface brightness maps from $Herschel$. The N$_2$H$^+$ and N$_2$D$^+$ lines were analysed by non-LTE radiative transfer modelling. Our best-fit core model consists in a static inner region, embedded in an infalling envelope with an inner radius of approximately 3000 au (21" at 141 pc). The observed high-J lines of N$_2$H$^+$ and N$_2$D$^+$ (with critical densities greater than 10$^6$ cm$^{-3}$) turn out to be very sensitive to depletion; the present single-dish observations are best explained with no depletion of N$_2$H$^+$ and N$_2$D$^+$ in the inner core. The N$_2$D$^+$/N$_2$H$^+$ ratio that best reproduces our observations is 0.44, one of the largest observed to date in pre-stellar cores. Additionally, half of the molecules that we observed are deuterated isotopologues, confirming the high-level of deuteration towards this source. Non-LTE radiative transfer modelling of N$_2$H$^+$ and N$_2$D$^+$ lines proved to be an excellent diagnostic of the chemical structure and dynamics of a pre-stellar core. Probing the physical conditions immediately before the protostellar collapse is a necessary reference for theoretical studies and simulations with the aim of understanding the earliest stages of star and planet formation and the time scale of this process.

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arXiv2019

Modeling deuterium chemistry in starless cores: full scrambling versus proton hop

We constructed two new models for deuterium and spin-state chemistry for the purpose of modeling the low-temperature environment prevailing in starless and pre-stellar cores. The fundamental difference between the two models is in the treatment of ion-molecule proton-donation reactions of the form $\rm XH^+ + Y \longrightarrow X + YH^+$, which are allowed to proceed either via full scrambling or via direct proton hop, i.e., disregarding proton exchange. The choice of the reaction mechanism affects both deuterium and spin-state chemistry, and in this work our main interest is on the effect on deuterated ammonia. We applied the new models to the starless core H-MM1, where several deuterated forms of ammonia have been observed. Our investigation slightly favors the proton hop mechanism over full scrambling because the ammonia D/H ratios are better fit by the former model, although neither model can reproduce the observed $\rm NH_2D$ ortho-to-para ratio of 3 (the models predict a value of $\sim$2). Extending the proton hop scenario to hydrogen atom abstraction reactions yields a good agreement for the spin-state abundance ratios, but greatly overestimates the deuterium fractions of ammonia. However, one can find a reasonably good agreement with the observations with this model by increasing the cosmic-ray ionization rate over the commonly-adopted value of $\sim$$10^{-17}\,\rm s^{-1}$. We also find that the deuterium fractions of several other species, such as $\rm H_2CO$, $\rm H_2O$, and $\rm CH_3$, are sensitive to the adopted proton-donation reaction mechanism. Whether the full scrambling or proton hop mechanism dominates may be dependent on the reacting system, and new laboratory and theoretical studies for various reacting systems are needed to constrain chemical models.

astro-ph.GA↗

arXiv2019

Dust opacity variations in the pre-stellar core L1544

Context: The study of dust emission at millimeter wavelengths is important to shed light on the dust properties and physical structure of pre-stellar cores, the initial conditions in the process of star and planet formation. Aims: Using two new continuum facilities, AzTEC at the LMT and MUSTANG-2 at the GBO, we aim to detect changes in the optical properties of dust grains as a function of radius for the well-known pre-stellar core L1544. Methods: We determine the emission profiles at 1.1 and 3.3 mm and examine whether they can be reproduced in terms of the current best physical models for L1544. We also make use of various tools to determine the radial distributions of the density, temperature, and the dust opacity in a self-consistent manner. Results: We find that our observations cannot be reproduced without invoking opacity variations. With the new data, new temperature and density profiles, as well as opacity variations across the core, have been derived. The opacity changes are consistent with the expected variations between uncoagulated bare grains, toward the outer regions of the core, and grains with thick ice mantles, toward the core center. A simple analytical grain growth model predicts the presence of grains of ~3-4 um within the central 2000 au for the new density profile.

astro-ph.GA↗

arXiv2017

Species-to-species rate coefficients for the $\rm H_3^+ + H_2$ reacting system

Aims. We study whether rotational excitation makes a difference to the abundances of the $\rm H_3^+$ isotopologs, including spin states, in physical conditions corresponding to starless cores and protostellar envelopes. Methods. We developed a new rate coefficient set for the $\rm H_3^+$ isotopologs, allowing for rotational excitation, using the state-to-state rate coefficients from Hugo et al. These new so-called species-to-species rate coefficients are compared with previously-used ground state-to-species rate coefficients. Results. The species-to-species and ground state-to-species model results differ at high density and toward increasing temperatures ($T > 10$ K). The species-to-species model predicts a lower $\rm H_3^+$ deuteration degree at high density owing to an increase of the rate coefficients of endothermic reactions that decrease deuteration. At 20 K the ground state-to-species model overestimates the abundance of $\rm H_2D^+$ by a factor of about two while the abundance of $\rm D_3^+$ can differ by an order of magnitude between the models. Spin-state abundance ratios are also affected, and the new model better reproduces recent observations of ortho and para $\rm H_2D^+$ and $\rm D_2H^+$. The applicability regime of the new rate coefficients depends on the critical densities of the various rotational transitions. Conclusions. The difference in the abundances of the $\rm H_3^+$ isotopologs predicted by the two models is negligible at 10 K but excited states are very important in studies of deuteration at higher temperatures, for example in protostellar envelopes. The species-to-species rate coefficients provide a more realistic approach to the chemistry of the $\rm H_3^+$ isotopologs than the ground state-to-species rate coefficients do, and so the former should be adopted in chemical models describing the chemistry of the $\rm H_3^+ + H_2$ reacting system.

astro-ph.GA↗

arXiv2017

Search for grain growth towards the center of L1544

In dense and cold molecular clouds dust grains are surrounded by thick icy mantles. It is however not clear if dust growth and coagulation take place before the switch-on of a protostar. This is an important issue, as the presence of large grains may affect the chemical structure of dense cloud cores, including the dynamically important ionization fraction, and the future evolution of solids in protoplanetary disks. To study this further, we focus on L1544, one of the most centrally concentrated pre-stellar cores on the verge of star formation, and with a well-known physical structure. We observed L1544 at 1.2 and 2 mm using NIKA, a new receiver at the IRAM 30 m telescope, and we used data from the Herschel Space Observatory archive. We find no evidence of grain growth towards the center of L1544 at the available angular resolution. Therefore, we conclude that single dish observations do not allow us to investigate grain growth toward the pre-stellar core L1544 and high sensitivity interferometer observations are needed. We predict that dust grains can grow to 200 $μ$m in size toward the central ~300 au of L1544. This will imply a dust opacity change by a factor of ~2.5 at 1.2 mm, which can be detected using the Atacama Large Millimeter and submillimeter Array (ALMA) at different wavelengths and with an angular resolution of 2".

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

On the stability of non-isothermal Bonnor-Ebert spheres. II. The effect of gas temperature on the stability

Aims. We investigate the stability of non-isothermal Bonnor-Ebert spheres with a model that includes a self-consistent calculation of the gas temperature. This way we can discard the assumption of equality between the dust and gas temperatures, and study the stability as the gas temperature changes with chemical evolution of the gas. Methods. We use a gas-grain chemical model including a time-dependent treatment of depletion onto grain surfaces, which strongly influences the gas temperature as the main coolant, CO, depletes from the gas. Dust and gas temperatures are solved with radiative transfer. For comparison with previous work, we assume that the cores are deeply embedded in a larger external structure, corresponding to visual extinction $A_{\rm V}^{\rm ext}=10$ mag. Results. We find that the critical non-dimensional radius $ξ_1$ derived here is similar to our previous work where we assumed $T_{\rm dust}=T_{\rm gas}$; the $ξ_1$ values lie below the isothermal critical value $ξ_0\sim6.45$, but the difference is less than 10%. Chemical evolution does not affect notably the stability condition of low-mass cores (<0.75 $M_\odot$). For higher masses the decrease of cooling owing to CO depletion causes substantial temporal changes in the temperature and density profiles of the cores. In the mass range 1-2 $M_\odot$ , $ξ_1$ decreases with chemical evolution, whereas above 3 $M_\odot$ , $ξ_1$ instead increases. We also find that decreasing $A_{\rm V}^{\rm ext}$ increases the gas temperature especially when the gas is chemically old, causing $ξ_1$ to increase with respect to models with higher $A_{\rm V}^{\rm ext}$. The derived $ξ_1$ values are close to $ξ_0$. The density contrast between the core center and edge varies between 8 to 16 depending on core mass and the chemical age of the gas, compared to the constant value $\sim$ 14.1 for the isothermal BES.

astro-ph.GA↗

arXiv2015

Spin-state chemistry of deuterated ammonia

Aims. We aim to develop a chemical model that contains a consistent description of spin-state chemistry in reactions involving chemical species with multiple deuterons. We apply the model to the specific case of deuterated ammonia, to derive values for the various spin-state ratios. Methods. We apply symmetry rules in the complete scrambling assumption to calculate branching ratio tables for reactions between chemical species that include multiple protons and/or deuterons. Reaction sets for both gas-phase and grain-surface chemistry are generated using an automated routine that forms all possible spin-state variants of any given reaction with up to six H/D atoms. Single-point and modified Bonnor-Ebert models are used to study the density and temperature dependence of ammonia and its isotopologs, and the associated spin-state ratios. Results. We find that the spin-state ratios of the ammonia isotopologs are, at late times, very different from their statistical values. The ratios are rather insensitive to variations in the density, but present strong temperature dependence. We derive high peak values ($\sim$ 0.1) for the deuterium fraction in ammonia, in agreement with previous (gas-phase) models. The deuterium fractionation is strongest at high density, corresponding to a high degree of depletion, and also presents temperature dependence. We find that in the temperature range 5 to 20 K, the deuterium fractionation peaks at $\sim$ 15 K while most of the ortho/para (and meta/para for $\rm ND_3$) ratios present a minimum at 10 K (ortho/para $\rm NH_2D$ has instead a maximum at this temperature). Conclusions. Owing to the density and temperature dependence found in the abundances and spin-state ratios of ammonia and its isotopologs, it is evident that observations of ammonia and its deuterated forms can provide important constraints on the physical structure of molecular clouds.

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arXiv2015

The physical state of selected cold clumps

Context. he study of prestellar cores is essential to understanding the initial stages of star formation. With $Herschel$ more cold clumps have been detected than ever before. For this study we have selected 21 cold clumps from 20 $Herschel$ fields observed as a follow-up on original $Planck$ detections. We have observed these clumps in $^{13}$CO (1-0), C$^{18}$O (1-0), and N$_2$H$^+$ (1-0) lines. Aims. Our aim is to find out if these cold clumps are prestellar. We have examined to what extent independent analysis of the dust and the molecular lines lead to similar conclusions about the masses of these objects. Methods. We calculate the clump masses and densities from the dust continuum and molecular line observations and compare these to each other and to the virial and Bonnor-Ebert masses calculated for each clump. Finally we examine two of the fields with radiative transfer models to estimate CO abundances. Results. When excitation temperatures could be estimated, the column densities derived from molecular line observations were comparable to those from dust continuum data. The median column density estimates are 4.2$\times 10^{21}$cm$^{-2}$ and 5.5$\times 10^{21}$cm$^{-2}$ for the line and dust emission data, respectively. The calculated abundances, column densities, volume densities, and masses all have large uncertainties and one must be careful when drawing conclusions. Abundance of $^{13}$CO was found in modeling the two clumps in the field G131.65$+$9.75 to be close to the usual value of 10$^{-6}$. The abundance ratio of $^{13}$CO and C$^{18}$O was $\sim$10. Molecular abundances could only be estimated with modeling, relying on dust column density data. Conclusions. The results indicate that most cold clumps, even those with dust color temperatures close to 11 K, are not necessarily prestellar.

astro-ph.GA↗

arXiv2015

Benchmarking spin-state chemistry in starless core models

Aims. We aim to present simulated chemical abundance profiles for a variety of important species, with special attention given to spin-state chemistry, in order to provide reference results against which present and future models can be compared. Methods. We employ gas-phase and gas-grain models to investigate chemical abundances in physical conditions corresponding to starless cores. To this end, we have developed new chemical reaction sets for both gas-phase and grain-surface chemistry, including the deuterated forms of species with up to six atoms and the spin-state chemistry of light ions and of the species involved in the ammonia and water formation networks. The physical model is kept simple in order to facilitate straightforward benchmarking of other models against the results of this paper. Results. We find that the ortho/para ratios of ammonia and water are similar in both gas-phase and gas-grain models, at late times in particular, implying that the ratios are determined by gas-phase processes. We derive late-time ortho/para ratios of ~0.5 and ~1.6 for ammonia and water, respectively. We find that including or excluding deuterium in the calculations has little effect on the abundances of non-deuterated species and on the ortho/para ratios of ammonia and water, especially in gas-phase models where deuteration is naturally hindered owing to the presence of abundant heavy elements. Although we study a rather narrow temperature range (10-20 K), we find strong temperature dependence in, e.g., deuteration and nitrogen chemistry. For example, the depletion timescale of ammonia is significantly reduced when the temperature is increased from 10 to 20 K; this is because the increase in temperature translates into increased accretion rates, while the very high binding energy of ammonia prevents it from being desorbed at 20 K.

astro-ph.GA↗

arXiv2013

High-resolution ammonia mapping of the very young protostellar core Chamaeleon-MMS1

Cha-MMS1 was mapped in the NH_3(1,1) line and the 1.2 cm continuum using the Australia Telescope Compact Array, ATCA. The angular resolution of the ATCA observations is 7'' (~ 1000 AU), and the velocity resolution is 50 m s^{-1}. The core was also mapped with the 64-m Parkes telesope in the NH_3(1,1) and (2,2) lines. Observations from Herschel Space Observatory and Spitzer Space telescope were used to help interpretation. A compact high column density core with a steep velocity gradient is detected in ammonia, with a fractional ammonia abundance compatible with determinations towards other dense cores. The direction of the velocity gradient agrees with previous single-dish observations, and the overall velocity distribution can be interpreted as rotation. The rotation axis goes through the position of a compact far-infrared source detected by Spitzer and Herschel. The specific angular momentum of the core is typical for protostellar envelopes. A string of 1.2 cm continuum sources is tentatively detected near the rotation axis. The ammonia spectra suggest the presence of warm embedded gas in its vicinity. An hourglass-shaped structure is seen in ammonia at the cloud's average LSR velocity, also aligned with the rotation axis. Although this structure resembles a pair of outflow lobes the ammonia spectra show no indications of shocked gas. The observed ammonia structure mainly delineates the inner envelope around the central source. The velocity gradient is likely to originate in the angular momentum of the contracting core, although influence of the outflow from the neighbouring young star IRS4 is possibly visible on one side of the core. The tentative continuum detection and the indications of a warm background component near the rotation axis suggest that the core contains a deeply embedded outflow which may have been missed in previous single-dish CO surveys owing to beam dilution.

astro-ph.GA↗

arXiv2013

HD depletion in starless cores

Aims: We aim to investigate the abundances of light deuterium-bearing species such as HD, H2D+ and D2H+ in a gas-grain chemical model including an extensive description of deuterium and spin state chemistry, in physical conditions appropriate to the very centers of starless cores. Methods: We combine a gas-grain chemical model with radiative transfer calculations to simulate density and temperature structure in starless cores. The chemical model includes deuterated forms of species with up to 4 atoms and the spin states of the light species H2, H2+ and H3+ and their deuterated forms. Results: We find that HD eventually depletes from the gas phase because deuterium is efficiently incorporated to grain-surface HDO, resulting in inefficient HD production on grains. HD depletion has consequences not only on the abundances of e.g. H2D+ and D2H+, whose production depends on the abundance of HD, but also on the spin state abundance ratios of the various light species, when compared with the complete depletion model where heavy elements do not influence the chemistry. Conclusions: While the eventual HD depletion leads to the disappearance of light deuterium-bearing species from the gas phase in a relatively short timescale at high density, we find that at late stages of core evolution the abundances of H2D+ and D2H+ increase toward the core edge and the disributions become extended. The HD depletion timescale increases if less oxygen is initially present in the gas phase, owing to chemical interaction between the gas and the dust predecing the starless core phase. Our results are greatly affected if H2 is allowed to tunnel on grain surfaces, and therefore more experimental data not only on tunneling but also on the O + H2 surface reaction in particular is needed.

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arXiv2013

Chemical and Physical Conditions in Molecular Cloud Core DC 000.4-19.5 (SL42) in Corona Australis

Chemical reactions in starless molecular clouds are heavily dependent on interactions between gas phase material and solid phase dust and ices. We have observed the abundance and distribution of molecular gases in the cold, starless core DC 000.4-19.5 (SL42) in Corona Australis using data from the Swedish ESO Submillimeter Telescope. We present column density maps determined from measurements of C18O(J=2-1,1-0) and N2H+(J=1-0) emission features. Herschel data of the same region allow a direct comparison to the dust component of the cloud core and provide evidence for gas phase depletion of CO at the highest extinctions. The dust color emperature in the core calculated from Herschel maps ranges from roughly 10.7 to 14.0 K. This range agrees with the previous determinations from Infrared Space Observatory and Planck observations. The column density profile of the core can be fitted with a Plummer-like density distribution approaching n(r) ~ r^{-2} at large distances. The core structure deviates clearly from a critical Bonnor-Ebert sphere. Instead, the core appears to be gravitationally bound and to lack thermal and turbulent support against the pressure of the surrounding low-density material: it may therefore be in the process of slow contraction. We test two chemical models and find that a steady-state depletion model agrees with the observed C18O column density profile and the observed N(C18O) versus AV relationship.

astro-ph.GA↗

arXiv2012

Reliability of NH3 as the temperature probe of cold cloud cores

The temperature is a central parameter affecting the chemical and physical properties of dense cores of interstellar clouds and their evolution to star formation. The chemistry and the dust properties are temperature dependent and the interpretation of observation requires the knowledge of the temperature and its variations. Measurement of the gas kinetic temperature is possible with molecular line spectroscopy, the ammonia molecule, NH3, being the most commonly used tracer. We want to determine the accuracy of the temperature estimates derived from ammonia spectra. The normal interpretation of NH3 observations assumes that all the hyperfine line components are tracing the same gas volume. In the case of temperature gradients they may be sensitive to different layers and cause errors in the optical depth and gas temperature estimates. We examine a series of spherical cloud models, 1.0 and 0.5 M_Sun Bonnor-Ebert spheres, with different radial temperature profiles. We calculate synthetic NH3 spectra and compare the derived column densities and temperatures to the true values. For high signal-to-noise observations, the estimated gas kinetic temperatures are within ~0.3 K of the real mass averaged temperature and the column densities are correct to within ~10%. When the S/N ratio of the (2,2) spectrum decreases below 10, the temperature errors are of the order of 1K but without a significant bias. When the density of the models is increased by a factor of a few, the results begin to show significant bias because of the saturation of the (1,1) main group. The ammonia spectra are found to be a reliable tracer of the mass averaged gas temperature. Because the radial temperature profiles of the cores are not well constrained, the central temperature could still differ from this value. If the cores are optically very thick, there are no guarantees of the accuracy.

astro-ph.GA↗

arXiv2011

On the stability of non-isothermal Bonnor-Ebert spheres

Aims: We aim to derive a stability condition for non-isothermal Bonnor-Ebert spheres and compare the physical properties of critical non-isothermal and isothermal gas spheres. These configurations can serve as models for prestellar cores before gravitational collapse. Methods: A stability condition for non-isothermal spheres is derived by constructing an expression for the derivative of boundary pressure with respect to core volume. The temperature distribution is determined by means of radiative transfer calculations. Based on the stability analysis, we derive the physical parameters of critical cores for the mass range 0.1 - 5.0 M_sun. In addition, the properties of roughly Jupiter-mass cores are briefly examined. Results: At the low-mass end the critical non-isothermal sphere has lower central density and a slightly larger physical radius than the corresponding isothermal sphere (i.e. one with the same mass and average temperature). The temperature decrease towards the core centre becomes steeper towards smaller masses as the central density becomes higher. The slope depends on the adopted dust model. We find that the critical dimensionless radius increases above the isothermal value xi_0 = 6.45 for very low-mass cores (< 0.2 M_sun). However, in the mass-range studied here the changes are within 5% from the isothermal value. Conclusions: The density structures of non-isothermal and isothermal Bonnor-Ebert spheres for a given mass are fairly similar. However, the present models predict clear differences in the average temperatures for the same physical radius. Especially for low-mass cores, the temperature gradient probably has implications on the chemistry and the observed line emission. We also find that hydrostatic Jupiter-mass cores with radii less than 100 AU would have very high boundary pressures compared with typical pressures in the interstellar space.

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