INDRANI DAS

Das, Shang, & Krasnopolsky, ApJ, 2025
The enigma of star formation in galaxies continues to captivate astronomers around the world, with a fundamental question remaining: how do self-gravity, magnetic fields, and thermal pressure play a role in protostar formation? Our work reveals a constraint on the degree of non-isothermality in the choice of Equation of states (EOSs), that says a polytropic monoatomic index of Γ no stiffer than 4/3, complemented by the magnetized virial theorem, ensures sufficient cooling in allowing a protostar to form and grow continuously through the magnetized gravitational collapse of interstellar molecular gas clouds. The global MHD collapse models with a Γ softer than or equal to 4/3, the infall dynamics qualitatively follow the behaviour of the isothermal case. However, for the model collapse with a Γ harder than 4/3, causes a reduction in the mass accretion rate into the central (point-mass like) protostar, thus delaying the collapse. At the scales of protostar’s surface, which is likely several solar radii, the heating from the accumulated material within may begin to significantly influence the EOS by transitioning to the regime of nonisothermality. However, this regime of nonisothermality may remain confined to within the protostar’s surface only, similar to the case of a chromosphere, and certainly does not impede the mass infall during collapse.

2. Evolution of Protoplanetary Disks and Prospects of Planetesimal Formation

Das, Vorobyov, & Basu, ApJ, 2025
In this work, we investigated We investigate the occurrence of accretion bursts, dust accumulation, and the prospects for planetesimal formation in a gravitationally unstable magnetized protoplanetary disk (PPD) with globally suppressed but episodically triggered magnetorotational instability (MRI), particularly in young intermediate-mass stars (YIMSs) but with a comparison to its low-mass cousins. Massive gas concentrations and dust rings form within the inner disk region owing to the radially varying efficiency of mass transport by gravitational instability. These rings are initially susceptible to streaming instability (SI). The ensuing MRI bursts destroy the dust rings, making planetesimal formation via SI problematic. In the later evolution phase, when the burst activity starts to diminish, SI becomes inefficient because of growing dust drift velocity and a more extended inner dead zone, both acting to reduce the dust concentration below the threshold for the SI to develop. Low-mass objects appear to be less affected by these adverse effects. Our results suggest that disks around YIMSs may be challenging environments for planetesimal formation via SI. This may explain the dearth of planets around stars with M_★ > 3M_⊙.

3. A semi-analytical model for the temporal evolution of the episodic disc-to-star accretion rate during star formation

Das & Basu, MNRAS, 2022
In this work, we developed a semi-analytic formalism (refer to Figure 6 for the numerical prescription) for episodic accretion from disk to star during star formation. Our model provides a self-consistent evolution of the mass accretion rate by joining the spherical envelope accretion (dominant at the earlier stage) with the disk accretion (important at the later stage) and accounts for the (gravitational instability driven) episodic accretion bursts at times. Our self-developed model can explain the basic features of the hydrodynamic simulations of episodic accretion (e.g., Vorobyov & Basu 2005, 2006, 2007). We find that the histogram of total luminosities obtained from our semi-analytic model provides a good match to the observed distribution of bolometric luminosities of YSOs, whereas neither a constant nor a time-dependent but smoothly increasing or decreasing mass accretion rate is able to do so.

4. Variation of the Core Lifetime and Fragmentation Scale in Molecular Clouds as an Indication of Ambipolar Diffusion

Das, Basu, & André, A&A Letters, 2021
Fragmentation in large molecular clouds probed by SMA, ALMA, VLA, JCMT, and Herschel has been of great interest in recent years. In this work, we apply the magnetically (nonideal MHD) modulated threshold for fragmentation scale (length scale, time scale, and mass) to fit the data of the evolutionary time/lifetime and fragmentation mass of prestellar cores identified with Herschel Space Observatory in the Aquila cloud as well as the number of enclosed cores formed in a parent clump measured in Perseus cloud complex with the Submillimeter Array (SMA). By varying a single parameter, the normalized mass-to-flux ratio (lying in the range 1 ≤ μ ≤ 2), over the range of observationally measured densities, we fit the range of prestellar core lifetimes that varies from 0.1 to a few Myr. Moreover, we found the estimated lifetime is few times the free-fall toward the low density regime and essentially merges with the free-fall time toward the higher end of density. This cannot be explained in a purely hydrodynamic scenario.

5. Linear Stability Analysis of a Magnetic Rotating Disk with Ohmic Dissipation and Ambipolar Diffusion

Das & Basu, ApJ, 2021
We perform a linear analysis of the stability of isothermal, rotating, magnetic, self-gravitating sheets that are weakly ionized. We include a self-consistent treatment of thermal pressure, gravitational, rotational, and magnetic (pressure and tension) forces together with two nonideal magnetohydrodynamic (MHD) effects: Ohmic dissipation and ambipolar diffusion. Our results show that there is always a preferred lengthscale and associated minimum timescale for gravitational instability. The addition of rotation leads to a generalized Toomre criterion (that includes a magnetic dependence) and modified lengthscales and timescales for collapse. We show that both the nonideal MHD effects qualitatively behave in a similar way but have quantitative differences. We apply our results to protostellar disk properties in the early embedded phase and find that the preferred scale of instability can significantly exceed the thermal (Jeans) scale and the peak preferred fragmentation mass is likely to be ~10–90 Jupiter masses.