arXiv subjects

Paola Caselli

Publications and source records attributed to Paola Caselli.

At least 181 records · Page 10Linked to original sources

arXiv2019

On Simulating the Proton-Irradiation of O$_2$ and H$_2$O Ices Using Astrochemical-type Models, with Implications for Bulk Reactivity

Many astrochemical models today explicitly consider the species that comprise the bulk of interstellar dust grain ice-mantles separately from those in the top few monolayers. Bombardment of these ices by ionizing radiation - whether in the form of cosmic rays, stellar winds, or radionuclide emission - represents an astrochemically viable means of driving a rich chemistry even in the bulk of the ice-mantle, now supported by a large body of work in laboratory astrophysics. In this study, using an existing rate equation-based astrochemical code modified to include a method of considering radiation chemistry recently developed by us, we attempted to simulate two such studies in which (a) pure O$_2$ ice at 5 K and, (b) pure H$_2$O ice at 16 K and 77 K, were bombarded by keV H$^+$ ions. Our aims are twofold: (1) to test the capability of our newly developed method to replicate the results of ice-irradiation experiments, and (2) to determine in such a well-constrained system how bulk chemistry is best handled using the same gas-grain codes that are used to model the interstellar medium (ISM). We find that our modified astrochemical model is able to reproduce both the abundance of O$_3$ in the 5 K pure O$_2$ ice, as well as both the abundance of H$_2$O$_2$ in the 16 K water ice and the previously noted decrease of hydrogen peroxide at higher temperatures. However, these results require the assumption that radicals and other reactive species produced via radiolysis react quickly and non-diffusively with neighbors in the ice.

astro-ph.GA↗

arXiv2019

Dust temperature and time-dependent effects in the chemistry of photodissociation regions

When studying the chemistry of PDRs, time dependence becomes important as visual extinction increases, since certain chemical timescales are comparable to the cloud lifetime. Dust temperature is also a key factor, since it significantly influences gas temperature and mobility on dust grains, determining the chemistry occurring on grain surfaces. We present a study of the dust temperature impact and time effects on the chemistry of different PDRs, using an updated version of the Meijerink PDR code and combining it with the time-dependent code Nahoon. We find the largest temperature effects in the inner regions of high $G$$_{\mathrm{0}}$ PDRs, where high dust temperatures favour the formation of simple oxygen-bearing molecules (especially that of O$_2$), while the formation of complex organic molecules is much more efficient at low dust temperatures. We also find that time-dependent effects strongly depend on the PDR type, since long timescales promote the destruction of oxygen-bearing molecules in the inner parts of low $G$$_{\mathrm{0}}$ PDRs, while favouring their formation and that of carbon-bearing molecules in high $G$$_{\mathrm{0}}$ PDRs. From the chemical evolution, we also conclude that, in dense PDRs, CO$_2$ is a late-forming ice compared to water ice, and confirm a layered ice structure on dust grains, with H$_2$O in lower layers than CO$_2$. Regarding steady state, the PDR edge reaches chemical equilibrium at early times ($\lesssim$10$^5$ yr). This time is even shorter ($<$10$^4$ yr) for high $G$$_{\mathrm{0}}$ PDRs. By contrast, inner regions reach equilibrium much later, especially low $G$$_{\mathrm{0}}$ PDRs, where steady state is reached at $\sim$10$^6$-10$^7$ yr.

astro-ph.GA↗

arXiv2019

Widespread Molecular Outflows in the Infrared Dark Cloud G28.37+0.07: Indications of Orthogonal Outflow-Filament Alignment

We present ALMA CO(2-1) observations toward a massive infrared dark cloud G28.37+0.07. The ALMA data reveal numerous molecular (CO) outflows with a wide range of sizes throughout the cloud. Sixty-two 1.3 mm continuum cores were identified to be driving molecular outflows. We have determined the position angle in the plane-of-sky of 120 CO outflow lobes and studied their distribution. We find that the distribution of the plane-of-sky outflow position angles peaks at about 100 degree, corresponding to a concentration of outflows with an approximately east-west direction. For most outflows, we have been able to estimate the plane-of-sky angle between the outflow axis and the filament that harbors the protostar that powers the outflow. Statistical tests strongly indicate that the distribution of outflow-filament orientations is consistent with most outflow axes being mostly orthogonal to their parent filament in 3D. Such alignment may result from filament fragmentation or continuous mass transportation from filament to the embedded protostellar core. The latter is suggested by recent numerical studies with moderately strong magnetic fields.

astro-ph.GA↗

arXiv2019

The Green Bank Ammonia Survey: A Virial Analysis of Gould Belt Clouds in Data Release 1

We perform a virial analysis of starless dense cores in three nearby star-forming regions : L1688 in Ophiuchus, NGC 1333 in Perseus, and B18 in Taurus. Our analysis takes advantage of comprehensive kinematic information for the dense gas in all of these regions made publicly available through the Green Bank Ammonia Survey Data Release 1, which used to estimate internal support against collapse. We combine this information with ancillary data used to estimate other important properties of the cores, including continuum data from the James Clerk Maxwell Telescope Gould Belt Survey for core identification, core masses, and core sizes. Additionally, we used \textit{Planck} and \textit{Herschel}-based column density maps for external cloud weight pressure, and Five College Radio Astronomy Observatory $^{13}$CO observations for external turbulent pressure. Our self-consistent analysis suggests that many dense cores in all three star-forming regions are not bound by gravity alone, but rather require additional pressure confinement to remain bound. Unlike a recent, similar study in Orion~A, we find that turbulent pressure represents a significant portion of the external pressure budget. Our broad conclusion emphasizing the importance of pressure confinement in dense core evolution, however, agrees with earlier work.

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arXiv2019

Modeling Sulfur Depletion in Interstellar Clouds

The elemental depletion of interstellar sulfur from the gas phase has been a recurring challenge for astrochemical models. Observations show that sulfur remains relatively non-depleted with respect to its cosmic value throughout the diffuse and translucent stages of an interstellar molecular cloud, but its gas-phase constituents cannot account for this cosmic value towards higher-density environments. We have attempted to address this issue by modeling the evolution of an interstellar cloud from its pristine state as a diffuse atomic cloud to a molecular environment of much higher density, using a gas/grain astrochem. code and an enhanced sulfur reaction network. A common gas/grain reaction network has been systematically updated and greatly extended based on previous lit. and models, with a focus on the grain chemistry and processes. A simple model was used to benchmark the resulting network updates, and the results of the model were compared to typical astronomical observations sourced from the literature. Our new gas/grain model is able to reproduce the elemental depletion of sulfur, whereby sulfur can be depleted from the gas-phase by two orders of magnitude, and this process may occur under dark cloud conditions if the cloud has a chemical age of at least 1 Myrs. The resulting mix of sulfur-bearing species on the grain ranges across all the most common chemical elements (H/C/N/O), not dissimilar to the molecules observed in cometary environments. Notably, this mixture is not dominated simply by H2S, unlike all other current astrochem. models. Despite our relatively simple physical model, most of the known gas-phase S-bearing molecular abundances are accurately reproduced under dense conditions, however they are not expected to be the primary molecular sinks of sulfur. Our model predicts that most of the missing sulfur is in the form of organo-sulfur species trapped on grains.

astro-ph.GA↗

arXiv2019

The central 1000 AU of a pre-stellar core revealed with ALMA. I. 1.3 mm continuum observations

Stars like our Sun form in self-gravitating dense and cold structures within interstellar clouds, called pre-stellar cores. Although much is known about the physical structure of dense clouds just before and soon after the switch-on of a protostar, the central few thousand astronomical units (au) of pre-stellar cores are unexplored. It is within these central regions that stellar systems assemble and fragmentation may take place, with the consequent formation of binaries and multiple systems. We present ALMA Band 6 observations (ACA and 12m array) of the dust continuum emission of the 8 Msun pre-stellar core L1544, with angular resolution of 2'' x 1.6'' (linear resolution 270 au x 216 au). Within the primary beam, a compact region of 0.1 Msun, which we call a "kernel", has been unveiled. The kernel is elongated, with a central flat zone with radius Rker ~ 10'' (~ 1400 au). The average number density within Rker is ~1 x 10^6 cm^{-3}, with possible local density enhancements. The region within Rker appears to have fragmented, but detailed analysis shows that similar substructure can be reproduced by synthetic interferometric observations of a smooth centrally concentrated dense core with a similar central flat zone. The presence of a smooth kernel within a dense core is in agreement with non-ideal magneto-hydro-dynamical simulations of a contracting cloud core with a peak number density of 1 x 10^7 cm^{-3}. Dense cores with lower central densities are completely filtered out when simulated 12m-array observations are carried out. These observations demonstrate that the kernel of dynamically evolved dense cores can be investigated at high angular resolution with ALMA.

astro-ph.SR↗

arXiv2018

Multicomponent kinematics in a massive filamentary IRDC

To probe the initial conditions for high-mass star and cluster formation, we investigate the properties of dense filaments within the infrared dark cloud G035.39-00.33 (IRDC G035.39) in a combined Very Large Array (VLA) and the Green Bank Telescope (GBT) mosaic tracing the NH3 (1,1) and (2,2) emission down to 0.08 pc scales. Using agglomerative hierarchical clustering on multiple line-of-sight velocity component fitting results, we identify seven extended velocity-coherent components in our data, likely representing spatially coherent physical structures, some exhibiting complex gas motions. The velocity gradient magnitude distribution peaks at its mode of 0.35 km/s/pc and has a long tail extending into higher values of 1.5 - 2 km/s/pc, and is generally consistent with those found toward the same cloud in other molecular tracers and with the values found towards nearby low-mass dense cloud cores at the same scales. Contrary to observational and theoretical expectations, we find the non-thermal ammonia line widths to be systematically narrower (by about 20%) than those of N2H+ (1-0) line transition observed with similar resolution. If the observed ordered velocity gradients represent the core envelope solid-body rotation, we estimate the specific angular momentum to be about 2 x 10^21 cm^2/s, similar to the low-mass star-forming cores. Together with the previous finding of subsonic motions in G035.39, our results demonstrate high levels of similarity between kinematics of a high-mass star-forming IRDC and the low-mass star formation regime.

astro-ph.GA↗

arXiv2018

Dust charge distribution in the interstellar medium

We investigate the equilibrium charge distribution of dust grains in the interstellar medium (ISM). Our treatment accounts for collisional charging by electrons and ions, photoelectric charging due to a background interstellar radiation field, the collection of suprathermal cosmic ray electrons and photoelectric emission due to a cosmic ray induced ultraviolet radiation field within dense molecular clouds. We find that the charge equilibrium assumption is valid throughout the multi-phase ISM conditions investigated here, and should remain valid for simulations with resolutions down to AU scales. The charge distribution of dust grains is size, composition, and ISM environment dependent: local radiation field strength, $G$, temperature, $T$, and electron number density, $n_{\mathrm{e}}$. The charge distribution is tightly correlated with the `charging parameter', $G\sqrt{T}/n_{\mathrm{e}}$. In the molecular medium, both carbonaceous and silicate grains have predominantly negative or neutral charges with narrow distributions. In the cold neutral medium, carbonaceous and silicate grains vary from negative and narrow distributions, to predominantly positive and wide distributions depending on the magnitude of the charging parameter. In the warm neutral medium, grains of all sizes are positively charged with wide distributions. We derive revised parametric expressions that can be used to recover the charge distribution function of carbonaceous and silicate grains from 3.5 Å to 0.25 $μ$m as a function of the size, composition and ambient ISM parameters. Finally, we find that the parametric equations can be used in environments other than Solar neighborhood conditions, recovering the charge distribution function of dust grains in photon dominated regions.

astro-ph.GA↗

arXiv2018

Dense gas kinematics and a narrow filament in the Orion A OMC1 region using NH3

We present combined observations of the NH3 (J,K) = (1,1) and (2,2) inversion transitions towards OMC1 in Orion A obtained by the Karl G. Jansky Very Large Array (VLA) and the 100 m Robert C. Byrd Green Bank Telescope (GBT). With an angular resolution of 6" (0.01 pc), these observations reveal with unprecedented detail the complex filamentary structure extending north of the active Orion BN/KL region in a field covering 6' x 7'. We find a 0.012 pc wide filament within OMC1, with an aspect ratio of ~37:1, that was missed in previous studies. Its orientation is directly compared to the relative orientation of the magnetic field from the James Clerk Maxwell Telescope BISTRO survey in Orion A. We find a small deviation of ~11 deg between the mean orientation of the filament and the magnetic field, suggesting that they are almost parallel to one another. The filament's column density is estimated to be 2-3 orders of magnitude larger than the filaments studied with Herschel and is possibly self-gravitating given the low values of turbulence found. We further produce maps of the gas kinematics by forward modeling the hyperfine structure of the NH3 (J,K) = (1,1) and (2,2) lines. The resulting distribution of velocity dispersions peaks at ~0.5 km/s, close to the subsonic regime of the gas. This value is about 0.2 km/s smaller than previously measured in single-dish observations of the same region, suggesting that higher angular and spectral resolution observations will identify even lower velocity dispersions that might reach the subsonic turbulence regime in dense gas filaments.

astro-ph.GA↗

arXiv2018

Gas phase detection and rotational spectroscopy of ethynethiol, HCCSH

We report the gas-phase detection and spectroscopic characterization of ethynethiol ($\mathrm{HCCSH}$), a metastable isomer of thioketene ($\mathrm{H_2C_2S}$) using a combination of Fourier-transform microwave and submillimeter-wave spectroscopies. Several $a$-type transitions of the normal species were initially detected below 40 GHz using a supersonic expansion-electrical discharge source, and subsequent measurement of higher-frequency, $b$-type lines using double resonance provided accurate predictions in the submillimeter region. With these, searches using a millimeter-wave absorption spectrometer equipped with a radio frequency discharge source were conducted in the range 280 - 660 GHz, ultimately yielding nearly 100 transitions up to $^rR_0(36)$ and $^rQ_0(68)$. From the combined data set, all three rotational constants and centrifugal distortion terms up to the sextic order were determined to high accuracy, providing a reliable set of frequency predictions to the lower end of the THz band. Isotopic substitution has enabled both a determination of the molecular structure of $\mathrm{HCCSH}$ and, by inference, its formation pathway in our nozzle discharge source via the bimolecular radical-radical recombination reaction $\mathrm{SH + C_2H}$, which is calculated to be highly exothermic (-477 kJ/mol) using the HEAT345(Q) thermochemical scheme.

astro-ph.GA↗

arXiv2018

Perspectives on Astrophysics Based on Atomic, Molecular, and Optical (AMO) Techniques

About two generations ago, a large part of AMO science was dominated by experimental high energy collision studies and perturbative theoretical methods. Since then, AMO science has undergone a transition and is now dominated by quantum, ultracold, and ultrafast studies. But in the process, the field has passed over the complexity that lies between these two extremes. Most of the Universe resides in this intermediate region. We put forward that the next frontier for AMO science is to explore the AMO complexity that describes most of the Cosmos.

physics.atom-ph↗

arXiv2018

HSCO$^+$ and DSCO$^+$: a multi-technique approach in the laboratory for the spectroscopy of interstellar ions

Protonated molecular species have been proven to be abundant in the interstellar gas. This class of molecules is also pivotal for the determination of important physical parameters for the ISM evolution (e.g. gas ionisation fraction) or as tracers of non-polar, hence not directly observable, species. The identification of these molecular species through radioastronomical observations is directly linked to a precise laboratory spectral characterisation. The goal of the present work is to extend the laboratory measurements of the pure rotational spectrum of the ground electronic state of protonated carbonyl sulfide (HSCO$^+$) and its deuterium substituted isotopomer (DSCO$^+$). At the same time, we show how implementing different laboratory techniques allows the determination of different spectroscopical properties of asymmetric-top protonated species. Three different high-resolution experiments were involved to detected for the first time the $b-$type rotational spectrum of HSCO$^+$, and to extend, well into the sub-millimeter region, the $a-$type spectrum of the same molecular species and DSCO$^+$. The electronic ground-state of both ions have been investigated in the 273-405 GHz frequency range, allowing the detection of 60 and 50 new rotational transitions for HSCO$^+$ and DSCO$^+$, respectively. The combination of our new measurements with the three rotational transitions previously observed in the microwave region permits the rest frequencies of the astronomically most relevant transitions to be predicted to better than 100 kHz for both HSCO$^+$ and DSCO$^+$ up to 500 GHz, equivalent to better than 60 m/s in terms of equivalent radial velocity. The present work illustrates the importance of using different laboratory techniques to spectroscopically characterise a protonated species at high frequency, and how a similar approach can be adopted when dealing with reactive species.

astro-ph.GA↗

arXiv2018

Science with an ngVLA: Deuteration in starless and prestellar cores

In dense starless and protostellar cores, the relative abundance of deuterated species to their non-deuterated counterparts can become orders of magnitude greater than in the local interstellar medium. This enhancement proceeds through multiple pathways in the gas phase and on dust grains, where the chemistry is strongly dependent on the physical conditions. In this Chapter, we discuss how sensitive, high resolution observations with the ngVLA of emission from deuterated molecules will trace both the dense gas structure and kinematics on the compact physical scales required to track the gravitational collapse of star-forming cores and the subsequent formation of young protostars and circumstellar accretion regions. Simultaneously, such observations will play a critical role in tracing the chemical history throughout the various phases of star and planet formation. Many low-J transitions of key deuterated species, along with their undeuterated counterparts, lie within the 60-110 GHz frequency window, the lower end of which is largely unavailable with current facilities and instrumentation. The combination of sensitivity and angular resolution provided only by the ngVLA will enable unparalleled detailed studies of the physics and chemistry of the earliest stages of star formation.

astro-ph.IM↗

arXiv2018

Science with an ngVLA: Star-forming Filaments and Cores on a Galactic Scale

Continuum observations of molecular clouds have revealed a surprising amount of substructure in the form of filaments of a few pc length and cores of ~0.1 pc diameter. Understanding the evolution of these substructures towards star formation requires the kinematic and dynamical insights provided uniquely by sensitive line observations at high angular and spectral resolution. In this short paper, we describe how an ngVLA can probe effectively the dynamics of filaments and cores in nearby star-forming molecular clouds using the NH3 rotation-inversion transitions at 24 GHz. Such emission has been proven to trace well the high column density environments of star-forming cores and filaments but higher-resolution observations are needed to reveal important details of how dense gas is flowing within and onto these substructures. In particular, we describe how 150 x 18-m antennas with a maximum baseline of 1 km can be used to map sensitively NH3 emission across high column density locations in clouds in roughly an order of magnitude less time than with the current Jansky VLA.

astro-ph.SR↗

arXiv2018

Production of atomic hydrogen by cosmic rays in dark clouds

The presence of small amounts of atomic hydrogen, detected as absorption dips in the 21 cm line spectrum, is a well-known characteristic of dark clouds. The abundance of hydrogen atoms measured in the densest regions of molecular clouds can be only explained by the dissociation of H$_2$ due to cosmic rays. We want to assess the role of Galactic cosmic rays in the formation of atomic hydrogen, by using recent developments in the characterisation of the low-energy spectra of cosmic rays and advances in the modelling of their propagation in molecular clouds. We model the attenuation of the interstellar cosmic rays entering a cloud and compute the dissociation rate of molecular hydrogen due to collisions with cosmic-ray protons and electrons as well as fast hydrogen atoms. We compare our results with the available observations. The cosmic-ray dissociation rate is entirely determined by secondary electrons produced in primary ionisation collisions. These secondary particles constitute the only source of atomic hydrogen at column densities above $\sim10^{21}$ cm$^{-2}$. We also find that the dissociation rate decreases with column density, while the ratio between the dissociation and ionisation rates varies between about 0.6 and 0.7. From comparison with observations we conclude that a relatively flat spectrum of interstellar cosmic-ray protons, as the one suggested by the most recent Voyager 1 data, can only provide a lower bound for the observed atomic hydrogen fraction. An enhanced spectrum of low-energy protons is needed to explain most of the observations. Our findings show that a careful description of molecular hydrogen dissociation by cosmic rays can explain the abundance of atomic hydrogen in dark clouds. An accurate characterisation of this process at high densities is crucial for understanding the chemical evolution of star-forming regions.

astro-ph.GA↗

arXiv2018

The first detections of the key prebiotic molecule PO in star-forming regions

Phosphorus is a crucial element in prebiotic chemistry, especially the P$-$O bond, which is key for the formation of the backbone of the deoxyribonucleic acid. So far, PO had only been detected towards the envelope of evolved stars, and never towards star-forming regions. We report the first detection of PO towards two massive star-forming regions, W51 e1/e2 and W3(OH), using data from the IRAM 30m telescope. PN has also been detected towards the two regions. The abundance ratio PO/PN is 1.8 and 3 for W51 and W3(OH), respectively. Our chemical model indicates that the two molecules are chemically related and are formed via gas-phase ion-molecule and neutral-neutral reactions during the cold collapse. The molecules freeze out onto grains at the end of the collapse and desorb during the warm-up phase once the temperature reaches $\sim$35 K. The observed molecular abundances of 10$^{-10}$ are predicted by the model if a relatively high initial abundance of phosphorus, 5$\times$10$^{-9}$, is assumed.

astro-ph.GA↗

arXiv2018

Zooming in to Massive Star Birth

We present high resolution (0.2", 1000 AU) 1.3 mm ALMA observations of massive infrared dark cloud clump, G028.37+00.07-C1, thought to harbor the early stages of massive star formation. Using $\rm N_2D^+$(3-2) we resolve the previously identified C1-S core, separating the bulk of its emission from two nearby protostellar sources. C1-S is thus identified as a massive ($\sim50\:M_\odot$), compact ($\sim0.1\:$pc diameter) starless core, e.g., with no signs of outflow activity. Being highly deuterated, this is a promising candidate for a pre-stellar core on the verge of collapse. An analysis of its dynamical state indicates a sub-virial velocity dispersion compared to a trans-Alfvénic turbulent core model. However, virial equilibrium could be achieved with sub-Alfvénic conditions involving $\sim2\:$mG magnetic field strengths.

astro-ph.GA↗

arXiv2018

Magnetic field in a young circumbinary disk

We use polarization observations of a circumbinary disk to investigate how the polarization properties change at distinct frequency bands. Our goal is to discern the main mechanism responsible for the polarization through comparison between our observations and model predictions. We used ALMA to perform full polarization observations at 97.5 GHz, 233 GHz and 343.5 GHz. The target is the Class I object BHB07-11, which is the youngest object in the Barnard 59 protocluster. Complementary VLA observations at 34.5 GHz revealed a binary system within the disk. We detect an extended and structured polarization pattern remarkably consistent among all three bands. The distribution of polarized intensity resembles a horseshoe shape with polarization angles following this morphology. From the spectral index between bands 3 and 7, we derive a dust opacity index $β\sim 1$ consistent with maximum grain sizes larger than expected to produce self-scattering polarization in each band. The polarization morphology do not match predictions from self-scattering. On the other hand, marginal correspondence is seen between our maps and predictions from radiation field assuming the brightest binary component as main radiation source. Molecular line data from BHB07-11 indicates disk rotation. We produced synthetic polarization maps from a rotating magnetized disk model assuming combined poloidal and toroidal magnetic field components. The magnetic field vectors (i. e., the polarization vectors rotated by 90$\degr$) are better represented by a model with poloidal magnetic field strength about 3 times the toroidal one. The similarity of our polarization patterns among the three bands provides a strong evidence against self-scattering and radiation fields. On the other hand, our data are reasonably well reproduced by a model of disk with toroidal magnetic field components slightly smaller than poloidal ones.

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