Self-regulated gravitational accretion in protostellar discs

We present a numerical model for the evolution of a protostellar disc that has formed self-consistently from the collapse of a molecular cloud core. The global evolution of the disc is followed for several million years after its formation. The capture of a wide range of spatial and temporal scales is made possible by use of the thin-disc approximation. We focus on the role of gravitational torques in transporting mass inward and angular momentum outward during different evolutionary phases of a protostellar disc with disc-to-star mass ratio of order 0.1. In the early phase, when the infall of matter from the surrounding envelope is substantial, mass is transported inward by the gravitational torques from spiral arms that are a manifestation of the envelope-induced gravitational instability in the disc. In the late phase, when the gas reservoir of the envelope is depleted, the distinct spiral structure is replaced by ongoing irregular nonaxisymmetric density perturbations. The amplitude of these density perturbations decreases with time, though this process is moderated by swing amplification aided by the existence of the disc's sharp outer edge. Our global modelling of the protostellar disc reveals that there is typically a residual nonzero gravitational torque from these density perturbations, i.e. their effects do not exactly cancel out in each region. In particular, the net gravitational torque in the inner disc tends to be negative during first several million years of the evolution, while the outer disc has a net positive gravitational torque. Our global model of a self-consistently formed disc shows that it is also self-regulated in the late phase, so that it is near the Toomre stability limit, with a near-uniform Toomre parameter Q\approx 1.5-2.0. (Abstract abridged).Comment: 9 pages, 9 figures, accepted for publication in MNRA

Mass accretion rates in self-regulated disks of T Tauri stars

We have studied numerically the evolution of protostellar disks around intermediate and upper mass T Tauri stars (0.25 M_sun < M_st < 3.0 M_sun) that have formed self-consistently from the collapse of molecular cloud cores. In the T Tauri phase, disks settle into a self-regulated state, with low-amplitude nonaxisymmetric density perturbations persisting for at least several million years. Our main finding is that the global effect of gravitational torques due to these perturbations is to produce disk accretion rates that are of the correct magnitude to explain observed accretion onto T Tauri stars. Our models yield a correlation between accretion rate M_dot and stellar mass M_st that has a best fit M_dot \propto M_st^{1.7}, in good agreement with recent observations. We also predict a near-linear correlation between the disk accretion rate and the disk mass.Comment: Accepted for publication in ApJ Letter

The Democratic Prospects of Digital Urban Futures: Lessons from India's Smart Cities Mission

This paper explains the potential implications of digital interventions for social accountability through the Smart Cities Mission in India. The SCM represents India’s transition to a new political economy based on rapid urbanisation and wide scale application of digital technology to reform public service delivery while simultaneously creating new markets for urban transformation. Within this wider context, the paper considers the future of democratic practices in urban governance. We argue that while citizen-led accountability practices were trialed by civil society organisations since 1990s, the SCM presented a unique opportunity and challenge to institutionalise these tools within the framework of multi scalar governance – between central, state and local level institutions and between communities, private vendors and public bodies. Zooming into 4 smart city projects – Indore, Kakinada, Panaji and Ranchi we explain how each city engaged with citizens groups, communities and civil society and what their experiences tell us about the prospects and challenges of democratising digital urban futures

My City (In)visible: Colombo

This photo essay was funded by the British Academy funded Virtual Sandpit project called My City(In)visible. https://www.thebritishacademy.ac.uk/funding/virtual-sandpits/virtual-sandpits-follow-on-funding/ The essay curates photos collected by citizens of Colombo Slave Island and neighbouring communities on their everyday engagement with infrastructural and economic transformations in post war Colombo

Elite Discourse Coalitions and the Governance of 'Smart Spaces':Politics, Power and Privilege in India's Smart Cities Mission

This article reveals the elite discourse coalition which frames the priorities and institutional arrangements of the Smart Cities Mission in India. After locating the SCM within India's neoliberal urban policy trajectory and the new elite coalition it encompasses I describe three separate elite discourses: Modi's technocratic nationalism IT and consulting firms' technological utopia and middle class urban citizenship. Together the discourses have: shifted the moral sphere of urban policy from inclusive rights to exclusive privilege, privatised governance instead of local democracy and turned digital technocracy into a simplified solution for structural problems of urban inequality. While the elite discourses contest each other on certain areas which could lead to delays and stops in Smart City projects, together they have left the imprint of an elite led governance arrangement for urban planning and development in India

Line asymmetry of solar p-modes: Reversal of asymmetry in intensity power spectra

The sense of line asymmetry of solar p-modes in the intensity power spectra is observed to be opposite of that seen in the velocity power spectra. Theoretical calculations provide a good understanding and fit to the observed velocity power spectra whereas the reverse sense of asymmetry in the intensity power spectrum has been poorly understood. We show that when turbulent eddies arrive at the top of the convection zone they give rise to an observable intensity fluctuation which is correlated with the oscillation they generate, thereby affecting the shape of the line in the p-mode power spectra and reversing the sense of asymmetry (this point was recognized by Nigam et al. and Roxburgh & Vorontsov). The addition of the correlated noise displaces the frequencies of peaks in the power spectrum. Depending on the amplitude of the noise source the shift in the position of the peak can be substantially larger than the frequency shift in the velocity power spectra. In neither case are the peak frequencies precisely equal to the eigenfrequencies of p-modes. We suggest two observations which can provide a test of the model discussed here.Comment: Revised version. To appear in Ap