Magnetic field tomography in two clouds towards Ursa Major using HI fibers

The atomic interstellar medium (ISM) is observed to be full of linear structures that are referred to as "fibers". Fibers exhibit similar properties to linear structures found in molecular clouds known as striations. Suggestive of a similar formation mechanism, both striations and fibers appear to be ordered, quasi-periodic, and well-aligned with the magnetic field. The prevailing formation mechanism for striations involves the excitation of fast magnetosonic waves. Based on this theoretical model, and through a combination of velocity centroids and column density maps, Tritsis et al. (2018) developed a method for estimating the plane-of-sky (POS) magnetic field from molecular cloud striations. We apply this method in two H\textsc{I} clouds with fibers along the same line-of-sight (LOS) towards the ultra-high-energy cosmic-ray (UHECR) hotspot, at the boundaries of Ursa Major. For the cloud located closer to Earth, where Zeeman observations from the literature were also available, we find general agreement in the distributions of the LOS and POS components of the magnetic field. We find relatively large values for the total magnetic field (ranging from $\sim$$\rm{10}$ to $\sim$$\rm{20} ~\rm{\mu G}$) and an average projection angle with respect to the LOS of $\sim$ 50$^\circ$. For the cloud located further away, we also find a large value for the POS component of the magnetic field of $15^{+8}_{-3}~\rm{\mu G}$. We discuss the potential of our new magnetic-field tomography method for large-scale application. We consider the implications of our findings for the accuracy of current reconstructions of the Galactic magnetic field and on the propagation of UHECR through the ISM.Comment: 11 pages, 8 figures, published in Ap

A new method for probing magnetic field strengths from striations in the interstellar medium

Recent studies of the diffuse parts of molecular clouds have revealed the presence of parallel, ordered low-density filaments termed striations. Flows along magnetic field lines, Kelvin-Helmholtz instabilities and hydromagnetic waves are amongst the various formation mechanisms proposed. Through a synergy of observational, numerical and theoretical analysis, previous studies singled out the hydromagnetic waves model as the only one that can account for the observed properties of striations. Based on the predictions of that model, we develop here a method for measuring the temporal evolution of striations through a combination of molecular and dust continuum observations. Our method allows us to not only probe temporal variations in molecular clouds but also estimate the strength of both the ordered and fluctuating components of the magnetic field projected on the plane-of-the-sky. We benchmark our new method against chemical and radiative transfer effects through two-dimensional magnetohydrodynamic simulations coupled with non-equilibrium chemical modelling and non-local thermodynamic equilibrium line radiative transfer. We find good agreement between theoretical predictions, simulations and observations of striations in the Taurus molecular cloud. We find a value of $\rm{27 \pm 7} ~\rm{\mu G}$ for the plane-of-sky magnetic field, in agreement with previous estimates via the Davis-Chandrasekhar-Fermi method, and a ratio of fluctuating to ordered component of the magnetic field of $\sim$ 10\%.Comment: 12 pages, 14 figures, Accepted for publication in MNRA

A multilevel implementation of the Goldreich-Kylafis effect into the radiative transfer code PyRaTE

Among all the available observational techniques for studying magnetic fields in the dense cold phase of the interstellar medium, linear polarization of spectral lines, referred to in the literature as the Goldreich-Kylafis effect (Goldreich & Kylafis 1981; hereafter "GK effect"), remains one of the most underutilized methods. In this study, we implement the GK effect into the multilevel, non-local thermodynamic equilibrium radiative transfer code PyRaTE. Different modes of polarized radiation are treated individually with separate optical depths computed for each polarization direction. We benchmark our implementation against analytical results and provide tests for various limiting cases. In agreement with previous theoretical results, we find that in the multilevel case the amount of fractional polarization decreases when compared to the two-level approximation, but this result is subject to the relative importance between radiative and collisional processes. Finally, we post-process an axially symmetric, non-ideal magnetohydrodynamic chemo-dynamical simulation of a collapsing prestellar core and provide theoretical predictions regarding the shape (as a function of velocity) of the polarization fraction of CO during the early stages in the evolution of molecular clouds. The code is freely available to download.Comment: 13 pages and 11 figures. Submitted to A&A. Comments welcom

Can we observe the ion-neutral drift velocity in prestellar cores?

Given the low ionization fraction of molecular clouds, ambipolar diffusion is thought to be an integral process in star formation. However, chemical and radiative-transfer effects, observational challenges, and the fact that the ion-neutral drift velocity is inherently very small render a definite detection of ambipolar diffusion extremely non-trivial. Here, we study the ion-neutral drift velocity in a suite of chemodynamical, non-ideal magnetohydrodynamic (MHD), two-dimensional axisymmetric simulations of prestellar cores where we alter the temperature, cosmic-ray ionization rate, visual extinction, mass-to-flux ratio, and chemical evolution. Subsequently, we perform a number of non-local thermodynamic equilibrium (non-LTE) radiative-transfer calculations considering various idealized and non-idealized scenarios in order to assess which factor (chemistry, radiative transfer and/or observational difficulties) is the most challenging to overcome in our efforts to detect the ion-neutral drift velocity. We find that temperature has a significant effect in the amplitude of the drift velocity with the coldest modelled cores (T = 6 K) exhibiting drift velocities comparable to the sound speed. Against expectations, we find that in idealized scenarios (where two species are perfectly chemically co-evolving) the drift velocity ``survives" radiative-transfer effects and can in principle be observed. However, we find that observational challenges and chemical effects can significantly hinder our view of the ion-neutral drift velocity. Finally, we propose that $\rm{HCN}$ and $\rm{HCNH^+}$, being chemically co-evolving, could be used in future observational studies aiming to measure the ion-neutral drift velocity.Comment: 14 pages, 11 figures. Accepted for publication in MNRA

Magnetic Seismology of Interstellar Gas Clouds: Unveiling a Hidden Dimension

Simulation outputs for filament and sheet like geometry and the setup scripts for FLASH code<br

Multiscale study of the structure of molecular clouds: connecting theory and observations

This thesis is a multilateral and multiscale study of the structure of molecular clouds and the importance of magnetic fields in shaping them. First, we concentrate in the translucent parts of molecular clouds where elongated, quasi-periodic, magnetic-field aligned structures, termed "striations'', have been recently discovered. We perform a series of numerical experiments and we find that striations are formed due to compressible fast magnetosonic waves. Using the properties of these magnetohydrodynamic waves revealed by the presence of striations in an isolated molecular cloud we reconstruct its 3D shape through a normal-mode analysis. Turning our attention to the smallest, densest parts of molecular clouds we study the relation between the magnetic field and gas density in contracting prestellar cores. We find that previous studies severely underestimated the observational uncertainties in gas density. By properly accounting for these uncertainties and performing an independent analysis of the projected shapes of cores, we show that the data are in agreement with the predictions of the ambipolar diffusion theory of star formation. Driven by the need for accurate density estimates we have also performed numerical simulations of collapsing prestellar cores coupled with non-equilibrium chemical modelling and we have developed a method for probing the 3-dimensional shapes of cores using two-dimensional molecular column density maps. Finally, in order to directly compare our numerical models to observations we have developed a state-of-the-art, non local thermodynamic equilibrium (non-LTE) line radiative transfer code.Η παρούσα διατριβή είναι μία πολύπλευρη και σε πολλές διαφορετικές χωρικές κλίμακες, μελέτη των δομών των μοριακών νεφών και της σημασίας του μαγνητικού πεδίου στο σχηματισμό τους. Αρχικά μελετούμε τη δημιουργία νεοανακαλυφθέντων, νηματοειδών, ημιπεριοδικών δομών, ονόματι "striations", οι οποίες απατούνται στις περιοχές των μοριακών νεφών με χαμηλή πυκνότητα και είναι παράλληλες με το μαγνητικό πεδίο. Μέσω δισδιάστατων και τρισδιάστατων προσομοιώσεων μαγνητοϋδροδυναμικής, εξετάσαμε όλα τα πιθανά ενδεχόμενα για τη δημιουργία των "striations" και βρήκαμε ότι η μόνη δόκιμη εξήγηση για το σχηματισμό τους, προκύπτει μέσω της διέγερσης γρήγορων μαγνητοακουστικών κυμάτων. Στη συνέχεια, αξιοποιώντας το αποτελεσμά μας και δια μέσου της πρώτης στα χρονικά ανακάλυψης μας μιας περιοχής συντονισμού κυμάτων και των αρμονικών της συχνοτήτων σε ένα μοριακό σύννεφο, αναπαράγουμε την τρισδιάστατη δομή του.Κατόπιν, στρέφουμε την προσοχή μας στην δυναμική εξέλιξη των πιο πυκνών σημείων των μοριακών νεφών και τη σχέση ανάμεσα στο μαγνητικό πεδίο και στην πυκνότητα του αερίου σε καταρρέοντες προαστρικούς πυρήνες. Βρίσκουμε ότι οι παρατηρησιακές αβεβαιότητες στην πυκνότητα του αερίου είχαν υποτίμηθεί σε προηγούμενες μελέτες. Λαμβάνουμε υπόψη τις αβεβαιότητες σε αυτές τις μετρήσεις και αναλύοντας ανεξάρτητα τις δισδιάστατες προβολές των προαστρικών πυρήνων, βρίσκουμε ότι τα τα δεδομένα συμφωνούν με τις προβλέψεις της θεωρίας της αμφιπολικής διάχυσης. Με αφορμή την ανάγκη για ακριβείς εκτιμήσεις της πυκνότητας, προσομοιώνουμε επίσης την κατάρρευσης προαστρικών πυρήνων σε συνδυασμό με μοντελοποίηση της χημείας που τους συνοδεύει και αναπτύσουμε μία μέθοδο ανέυρεσης των τρισδιάστατων σχημάτων τους, χρησιμοποιώντας χάρτες πυκνότητας στήλης συγκεκριμένων μορίων. Τέλος, για να μπορούμε να συγκρίνουμε άμεσα τα αριθμητικά μας μοντέλα με τις παρατηρήσεις, αναπτύσσουμε έναν σύγχρονο κώδικα μεταφοράς διάδοσης της ακτινοβολίας μοριακών και ατομικών γραμμών σε συνθήκες μη-τοπικής θερμοδυναμικής ισορροπίας

Magnetic field-gas density relation and observational implications revisited

We revisit the relation between magnetic-field strength (B) and gas density (ρ) for contracting interstellar clouds and fragments (or, cores), which is central in observationally determining the dynamical importance of magnetic fields in cloud evolution and star formation. Recently, it has been claimed that a relation B ∝ ρ2/3 is statistically preferred over B ∝ ρ1/2 in molecular clouds, when magnetic-field detections and non-detections from Zeeman observations are combined. This finding has unique observational implications on cloud and core geometry: the relation B ∝ ρ2/3 can only be realized under spherical contraction. However, no indication of spherical geometry can be found for the objects used in the original statistical analysis of the B-ρ relation. We trace the origin of the inconsistency to simplifying assumptions in the statistical model used to arrive at the B ∝ ρ2/3 conclusion and to an underestimate of observational uncertainties in the determination of cloud and core densities. We show that, when these restrictive assumptions are relaxed, B ∝ ρ1/2 is the preferred relation for the (self-gravitating) molecular-cloud data, as theoretically predicted four decades ago

CO enhancement by magnetohydrodynamic waves. Striations in the Polaris Flare

Context. The formation of molecular gas in interstellar clouds is a slow process, but can be enhanced by gas compression. Magneto-hydrodynamic (MHD) waves can create compressed quasi-periodic linear structures, referred to as striations. Striations are observed at the column densities at which the transition from atomic to molecular gas takes place. Aims: We explore the role of MHD waves in the CO chemistry in regions with striations within molecular clouds. Methods: We targeted a region with striations in the Polaris Flare cloud. We conducted a CO J = 2−1 survey in order to probe the molecular gas properties. We used archival starlight polarization data and dust emission maps in order to probe the magnetic field properties and compare against the CO morphological and kinematic properties. We assessed the interaction of compressible MHD wave modes with CO chemistry by comparing their characteristic timescales. Results: The estimated magnetic field is 38-76 \ub5G. In the CO integrated intensity map, we observe a dominant quasiperiodic intensity structure that tends to be parallel to the magnetic field orientation and has a wavelength of approximately one parsec. The periodicity axis is ~17\ub0 off from the mean magnetic field orientation and is also observed in the dust intensity map. The contrast in the CO integrated intensity map is ~2.4 times higher than the contrast of the column density map, indicating that CO formation is enhanced locally. We suggest that a dominant slow magnetosonic mode with an estimated period of 2.1-3.4 Myr and a propagation speed of 0.30-0.45 km s−1 is likely to have enhanced the formation of CO, hence created the observed periodic pattern. We also suggest that within uncertainties, a fast magnetosonic mode with a period of 0.48 Myr and a velocity of 2.0 km s−1 could have played some role in increasing the CO abundance. Conclusions: Quasiperiodic CO structures observed in striation regions may be the imprint of MHD wave modes. The Alfv\ue9nic speed sets the dynamical timescales of the compressible MHD modes and determines which wave modes are involved in the CO chemistry