arXiv subjects

Shantanu Basu

Publications and source records attributed to Shantanu Basu.

At least 55 records · Page 3Linked to original sources

arXiv2019

Probing the cold magnetized Universe with SPICA-POL (B-BOP)

SPICA, the cryogenic infrared space telescope recently pre-selected for a `Phase A' concept study as one of the three remaining candidates for ESA's fifth medium class (M5) mission, is foreseen to include a far-infrared polarimetric imager (SPICA-POL, now called B-BOP), which would offer a unique opportunity to resolve major issues in our understanding of the nearby, cold magnetized Universe. This paper presents an overview of the main science drivers for B-BOP, including high dynamic range polarimetric imaging of the cold interstellar medium (ISM) in both our Milky Way and nearby galaxies. Thanks to a cooled telescope, B-BOP will deliver wide-field 100-350 micron images of linearly polarized dust emission in Stokes Q and U with a resolution, signal-to-noise ratio, and both intensity and spatial dynamic ranges comparable to those achieved by Herschel images of the cold ISM in total intensity (Stokes I). The B-BOP 200 micron images will also have a factor ~30 higher resolution than Planck polarization data. This will make B-BOP a unique tool for characterizing the statistical properties of the magnetized interstellar medium and probing the role of magnetic fields in the formation and evolution of the interstellar web of dusty molecular filaments giving birth to most stars in our Galaxy. B-BOP will also be a powerful instrument for studying the magnetism of nearby galaxies and testing galactic dynamo models, constraining the physics of dust grain alignment, informing the problem of the interaction of cosmic rays with molecular clouds, tracing magnetic fields in the inner layers of protoplanetary disks, and monitoring accretion bursts in embedded protostars.

astro-ph.GA↗

arXiv2019

The First Two Thousand Years of Star Formation

Starting from a prestellar core with a size of $1.2\times10^4$ AU, we calculate the evolution of a gravitationally collapsing core until $\sim2000$ yr after protostar formation using a three-dimensional resistive magnetohydrodynamic simulation, in which the protostar is resolved with a spatial resolution of $5.6\times10^{-3}$ AU. Following protostar formation, a rotationally supported disk is formed. Although the disk size is as small as $\sim2-4$ AU, it remains present until the end of the simulation. Since the magnetic field dissipates and the angular momentum is then not effectively transferred by magnetic effects, the disk surface density gradually increases and spiral arms develop due to gravitational instability. The disk angular momentum is then transferred mainly by gravitational torques, which induce an episodic mass accretion onto the central protostar. The episodic accretion causes a highly time-variable mass ejection (the high-velocity jet) near the disk inner edge, where the magnetic field is well coupled with the neutral gas. As the mass of the central protostar increases, the jet velocity gradually increases and exceeds $\sim100$ km s$^{-1}$. The jet opening angle widens with time at its base, while the jet keeps a very good collimation on the large scale. In addition, a low-velocity outflow is driven from the disk outer edge. A cavity-like structure, a bow shock and several knots, all of which are usually observed in star-forming regions, are produced in the outflowing region.

astro-ph.SR↗

arXiv2019

Magnetic Field Structure of Dense Cores using Spectroscopic Methods

We develop a new ''core field structure'' (CFS) model to predict the magnetic field strength and magnetic field fluctuation profile of dense cores using gas kinematics. We use spatially resolved observations of the nonthermal velocity dispersion from the Green Bank Ammonia survey along with column density maps from SCUBA-2 to estimate the magnetic field strength across seven dense cores located in the L1688 region of Ophiuchus. The CFS model predicts the profile of the relative field fluctuation, which is related to the observable dispersion in direction of the polarization vectors. Within the context of our model we find that all the cores have a transcritical mass-to-flux ratio.

astro-ph.GA↗

arXiv2018

Magnetic Field of a Compact Spherical Star under f(R,T) Gravity

We present the interior solutions of distributions of magnetised fluid inside a sphere in $f(R,T)$ gravity. The magnetised sphere is embedded in an exterior Reissner-Nordström metric. We assume that all physical quantities are in static equilibrium. The perfect fluid matter is studied under a particular form of the Lagrangian density $f(R,T)$. The magnetic field profile in modified gravity is calculated. Observational data of neutron stars are used to plot suitable models of magnetised compact objects. We reveal the effect of $f(R,T)$ gravity on the magnetic field profile, with application to neutron stars, especially highly magnetized neutron stars found in X-ray pulsar systems. Finally the effective potential $V_{\rm eff}$ and innermost stable circular orbits, arising out of motion of a test particle of negligible mass influenced by attraction or repulsion from the massive center, are discussed.

physics.gen-ph↗

arXiv2018

Magnetic Field Structure in Spheroidal Star-Forming Clouds

A model of magnetic field structure is presented to help test the prevalence of flux freezing in star-forming clouds of various shapes, orientations, and degrees of central concentration, and to estimate their magnetic field strength. The model is based on weak-field flux freezing in centrally condensed Plummer spheres and spheroids of oblate and prolate shape. For a spheroid of given density contrast, aspect ratio, and inclination, the model estimates the local field strength and direction, and the global field pattern of hourglass shape. Comparisons with a polarization simulation indicate typical angle agreement within 1 - 10 degrees. Scalable analytic expressions are given to match observed polarization patterns, and to provide inputs to radiative transfer codes for more accurate predictions. The model may apply to polarization observations of dense cores, elongated filamentary clouds, and magnetized circumstellar disks.

astro-ph.GA↗

arXiv2018

The emergence of the galactic stellar mass function from a non-universal IMF in clusters

We investigate how a single generation galactic mass function (SGMF) depends on the existence of variations in the initial stellar mass functions (IMF) of stellar clusters. We show that cluster-to-cluster variations of the IMF lead to a multicomponent SGMF where each component in a given mass range can be described by a distinct power-law function. We also show that a dispersion of $\approx 0.3$ M$_{\odot}$ in the characteristic mass of the IMF, as observed for young Galactic clusters, leads to a low mass slope of the SGMF that matches the observed Galactic stellar mass function even when the IMFs in the low mass end of individual clusters are much steeper.

astro-ph.GA↗

arXiv2018

A Dual Power Law Distribution for the Stellar Initial Mass Function

We introduce a new dual power law (DPL) probability distribution function for the mass distribution of stellar and substellar objects at birth, otherwise known as the initial mass function (IMF). The model contains both deterministic and stochastic elements, and provides a unified framework within which to view the formation of brown dwarfs and stars resulting from an accretion process that starts from extremely low mass seeds. It does not depend upon a top down scenario of collapsing (Jeans) masses or an initial lognormal or otherwise IMF-like distribution of seed masses. Like the modified lognormal power law (MLP) distribution, the DPL distribution has a power law at the high mass end, as a result of exponential growth of mass coupled with equally likely stopping of accretion at any time interval. Unlike the MLP, a power law decay also appears at the low mass end of the IMF. This feature is closely connected to the accretion stopping probability rising from an initially low value up to a high value. This might be associated with physical effects of ejections sometimes (i.e., rarely) stopping accretion at early times followed by outflow driven accretion stopping at later times, with the transition happening at a critical time (therefore mass). Comparing the DPL to empirical data, the critical mass is close to the substellar mass limit, suggesting that the onset of nuclear fusion plays an important role in the subsequent accretion history of a young stellar object.

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arXiv2017

From Molecular Clouds to the IMF: Spatial and Temporal Effects

We review star formation in molecular clouds and describe why magnetic fields may be important and how they can influence filamentary structure and the column density probability distribution function (PDF). We also comment on the origin of the stellar and substellar initial mass function (IMF), which may require explanations beyond a simple Jeans length argument in turbulent molecular clouds. A mathematical model of the modified lognormal power-law (MLP) distribution function provides a framework within which to connect accretion processes with the IMF.

astro-ph.GA↗

arXiv2017

The Effect of Magnetic Fields and Ambipolar Diffusion on the Column Density Probability Distribution Function in Molecular Clouds

Simulations generally show that non-self-gravitating clouds have a lognormal column density ($Σ$) probability distribution function (PDF), while self-gravitating clouds with active star formation develop a distinct power-law tail at high column density. Although the growth of the power law can be attributed to gravitational contraction leading to the formation of condensed cores, it is often debated if an observed lognormal shape is a direct consequence of supersonic turbulence alone, or even if it is really observed in molecular clouds. In this paper we run three-dimensional magnetohydrodynamic simulations including ambipolar diffusion with different initial conditions to see the effect of strong magnetic fields and nonlinear initial velocity perturbations on the evolution of the column density PDFs. Our simulations show that column density PDFs of clouds with supercritical mass-to-flux ratio, with either linear perturbations or nonlinear turbulence, quickly develop a power-law tail such that $dN/d \log Σ\propto Σ^{-α}$ with index $α\simeq 2$. Interestingly, clouds with subcritical mass-to-flux ratio also proceed directly to a power-law PDF, but with a much steeper index $α\simeq 4$. This is a result of gravitationally-driven ambipolar diffusion. However, for nonlinear perturbations with a turbulent spectrum ($v_{k}^{2} \propto k^{-4}$), the column density PDFs of subcritical clouds do retain a lognormal shape for a major part of the cloud evolution, and only develop a distinct power-law tail with index $α\simeq 2$ at greater column density when supercritical pockets are formed.

astro-ph.GA↗

arXiv2017

Magnetic Ribbons: A Minimum Hypothesis Model for Filaments

We develop a magnetic ribbon model for molecular cloud filaments. These result from turbulent compression in a molecular cloud in which the background magnetic field sets a preferred direction. We use our model to calculate a synthetic observed relation between apparent width in projection versus observed column density. The relationship is relatively flat, in rough agreement with the observations, and unlike the simple expectation based on a Jeans length argument.

astro-ph.GA↗

arXiv2017

Perspectives on Low-Mass Star Formation

I review some recent work on low-mass star formation, with an emphasis on theory, basic principles, and unresolved questions. Star formation is both a gravitational fragmentation problem as well as an accretion problem. Molecular cloud structure can be understood as a fragmentation process driven by the interplay of turbulence, magnetic fields, and gravity (acting on either a dynamical or ambipolar-diffusion time scale). This results in a natural way to understand filamentary structure as magnetic ribbons that have an apparent width that scales differently than the Jeans length. Recent work also shows that stellar mass accretion through a disk is episodic. We show through numerical simulations that bursts of FU Ori type may be clustered, since the clump that accretes to the center is tidally sheared apart in its last stage of infall. Finally, we utilize a simplified model of stellar mass accretion and accretion termination to derive an analytic form for the initial mass function that has a lognormal-like body and a power-law tail. This scenario is consistent with an expectation of a larger number of substellar objects than may have been previously detected.

astro-ph.SR↗

arXiv2017

Ionisation in Turbulent Magnetic Molecular Clouds I. Effect on Density and Mass-to-Flux Ratio Structures

Previous studies show that the physical structures and kinematics of a region depend significantly on the ionisation fraction. In this paper, we extend our previous studies of the effect of ionisation fractions on star formation to clouds that include both non-ideal magnetohydrodynamics and turbulence. We aim to quantify the importance of a treatment of the ionisation fraction in turbulent magnetised media and investigate the effect of turbulence on shaping the clouds and filaments before star formation sets in. In particular, we investigate how the structure, mass and width of filamentary structures depend on the amount of turbulence in ionised media and the initial mass-to-flux ratio. We compare the resulting density and mass-to-flux ratio structures both qualitatively and quantitatively via filament and core masses and filament fitting techniques (Gaussian and Plummer profiles.) We find that even with almost no turbulence, filamentary structure still exists. Comparison of simulations show that for turbulent Mach numbers above 2, there is little structural difference between the Step-Like (SL) and Cosmic Ray only (CR-only) ionisation models, while below this threshold the ionisation structure significantly affects the formation of filaments. Analysis of the mass within cores and filaments show decrease in mass as the degree of turbulence increases. Finally, observed filaments within the Taurus L1495/B213 complex are best reproduced by models with supercritical mass-to-flux ratios and/or at least mildly supersonic turbulence, however, our models show that sterile fibres observed within Taurus may occur in highly ionised, subcritical environments. Based on this, we suggest that regions with fertile fibres likely indicate a trans- or supercritical mass-to-flux ratio within the region while sterile fibres are likely subcritical and transient.

astro-ph.SR↗

arXiv2016

A Magnetic Ribbon Model for Star-Forming Filaments

We develop a magnetic ribbon model for molecular cloud filaments. These result from turbulent compression in a molecular cloud in which the background magnetic field sets a preferred direction. We argue that this is a natural model for filaments and is based on the interplay between turbulence, strong magnetic fields, and gravitationally-driven ambipolar diffusion, rather than pure gravity and thermal pressure. An analytic model for the formation of magnetic ribbons that is based on numerical simulations is used to derive a lateral width of a magnetic ribbon. This differs from the thickness along the magnetic field direction, which is essentially the Jeans scale. We use our model to calculate a synthetic observed relation between apparent width in projection versus observed column density. The relationship is relatively flat, similar to observations, and unlike the simple expectation based on a Jeans length argument.

astro-ph.GA↗

arXiv2016

The Role of Angular Momentum Transport in Establishing the Accretion Rate--Protostellar Mass Correlation

We model the mass accretion rate $\dot{M}$ to stellar mass $M_*$ correlation that has been inferred from observations of intermediate to upper mass T Tauri stars---that is $\dot{M} \propto M_*^{1.3 \pm 0.3}$. We explain this correlation within the framework of quiescent disk evolution, in which accretion is driven largely by gravitational torques acting in the bulk of the mass and volume of the disk. Stresses within the disk arise from the action of gravitationally driven torques parameterized in our 1D model in terms of Toomre's $Q$ criterion. We do not model the hot inner sub-AU scale region of the disk that is likely stable according to this criterion, and appeal to other mechanisms to remove or redistribute angular momentum and allow accretion onto the star. Our model has the advantage of agreeing with large-scale angle-averaged values from more complex nonaxisymmetric calculations. The model disk transitions from an early phase (dominated by initial conditions inherited from the burst mode of accretion) into a later self-similar mode characterized by a steeper temporal decline in $\dot{M}$. The models effectively reproduce the spread in mass accretion rates that have been observed for protostellar objects of $0.2\,\mbox{M}_\odot \le M_* \le 3.0\,\mbox{M}_\odot$, such as those found in the $ρ$ Ophiuchus and Taurus star forming regions. We then compare realistically sampled populations of young stellar objects produced by our model to their observational counterparts. We find these populations to be statistically coincident, which we argue is evidence for the role of gravitational torques in the late time evolution of quiescent protostellar disks.

astro-ph.SR↗

arXiv2016

Kompaneets Model Fitting of the Orion-Eridanus Superbubble II: Thinking Outside of Barnard's Loop

The Orion star-forming region is the nearest active high-mass star-forming region and has created a large superbubble, the Orion-Eridanus superbubble. Recent work by Ochsendorf et al. has extended the accepted boundary of the superbubble. We fit Kompaneets models of superbubbles expanding in exponential atmospheres to the new, larger shape of the Orion-Eridanus superbubble. We find that this larger morphology of the superbubble is consistent with the evolution of the superbubble being primarily controlled by expansion into the exponential Galactic disk ISM if the superbubble is oriented with the Eridanus side farther from the Sun than the Orion side. Unlike previous Kompaneets model fits that required abnormally small scale heights for the Galactic disk (<40 pc), we find morphologically consistent models with scale heights of 80 pc, similar to that expected for the Galactic disk.

astro-ph.GA↗

arXiv2016

Analytic Models of Brown Dwarfs and The Substellar Mass Limit

We present the current status of the analytic theory of brown dwarf evolution and the lower mass limit of the hydrogen burning main sequence stars. In the spirit of a simplified analytic theory we also introduce some modifications to the existing models. We give an exact expression for the pressure of an ideal non-relativistic Fermi gas at a finite temperature, therefore allowing for non-zero values of the degeneracy parameter ($ψ= \frac{kT}{μ_{F}}$, where $μ_{F}$ is the Fermi energy). We review the derivation of surface luminosity using an entropy matching condition and the first-order phase transition between the molecular hydrogen in the outer envelope and the partially-ionized hydrogen in the inner region. We also discuss the results of modern simulations of the plasma phase transition, which illustrate the uncertainties in determining its critical temperature. Based on the existing models and with some simple modification we find the maximum mass for a brown dwarf to be in the range $0.064M_\odot-0.087M_\odot$. An analytic formula for the luminosity evolution allows us to estimate the time period of the non-steady state (i.e., non-main sequence) nuclear burning for substellar objects. Standard models also predict that stars that are just above the substellar mass limit can reach an extremely low luminosity main sequence after at least a few million years of evolution, and sometimes much longer. We estimate that $\simeq 11 \%$ of stars take longer than $10^7$ yr to reach the main-sequence, and $\simeq 5 \%$ of stars take longer than $10^8$ yr.

astro-ph.SR↗

arXiv2015

Molecular Cloud Fragmentation and Core Collapse

I review some steps in the conversion of molecular cloud gas into stars and planets, with an emphasis in this presentation on the early stage molecular cloud fragmentation that leads to elongated filaments/ribbons. Magnetic fields can play a crucial role in all stages and need to be invoked particularly for early stage fragmentation as well as in late core collapse where it may control disk formation. I also review some elements of hydrodynamic modeling of disk evolution.

astro-ph.GA↗

arXiv2015

Variable protostellar accretion with episodic bursts

We present the latest development of the disk gravitational instability and fragmentation model, originally introduced by us to explain episodic accretion bursts in the early stages of star formation. Using our numerical hydrodynamics model with improved disk thermal balance and star-disk interaction, we computed the evolution of protostellar disks formed from the gravitational collapse of prestellar cores. In agreement with our previous studies, we find that cores of higher initial mass and angular momentum produce disks that are more favorable to gravitational instability and fragmentation, while a higher background irradiation and magnetic fields moderate the disk tendency to fragment. The protostellar accretion in our models is time-variable, thanks to the nonlinear interaction between different spiral modes in the gravitationally unstable disk, and can undergo episodic bursts when fragments migrate onto the star owing to the gravitational interaction with other fragments or spiral arms. Most bursts occur in the partly embedded Class I phase, with a smaller fraction taking place in the deeply embedded Class 0 phase and a few possible bursts in the optically visible Class II phase. The average burst duration and mean luminosity are found to be in good agreement with those inferred from observations of FU-Orionis-type eruptions. The model predicts the existence of two types of bursts: the isolated ones, showing well-defined luminosity peaks separated with prolonged periods (~ 10^4 yr) of quiescent accretion, and clustered ones, demonstrating several bursts occurring one after another during just a few hundred years. Finally, we estimate that 40\%--70\% of the star-forming cores can display bursts after forming a star-disk system.

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