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Shantanu Basu

Publications and source records attributed to Shantanu Basu.

At least 109 records · Page 6Linked to original sources
arXiv2005

The Origin of Episodic Accretion Bursts in the Early Stages of Star Formation

We study numerically the evolution of rotating cloud cores, from the collapse of a magnetically supercritical core to the formation of a protostar and the development of a protostellar disk during the main accretion phase. We find that the disk quickly becomes unstable to the development of a spiral structure similar to that observed recently in AB Aurigae. A continuous infall of matter from the protostellar envelope makes the protostellar disk unstable, leading to spiral arms and the formation of dense protostellar/protoplanetary clumps within them. The growing strength of spiral arms and ensuing redistribution of mass and angular momentum creates a strong centrifugal disbalance in the disk and triggers bursts of mass accretion during which the dense protostellar/protoplanetary clumps fall onto the central protostar. These episodes of clump infall may manifest themselves as episodes of vigorous accretion rate (\ge 10^{-4} M_sun/yr) as is observed in FU Orionis variables. Between these accretion bursts, the protostar is characterized by a low accretion rate (< 10^{-6} M_sun/yr). During the phase of episodic accretion, the mass of the protostellar disk remains less than or comparable to the mass of the protostar.

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arXiv2005

The effect of non-isothermality on the gravitational collapse of spherical clouds and the evolution of protostellar accretion

We investigate the role of non-isothermality in gravitational collapse and protostellar accretion by explicitly including the effects of molecular radiative cooling, gas-dust energy transfer, and cosmic ray heating in models of spherical hydrodynamic collapse. Isothermal models have previously shown an initial decline in the mass accretion rate \dot{M}, due to a gradient of infall speed that develops in the prestellar phase. Our results show that: (1) in the idealized limit of optically thin cooling, a positive temperature gradient is present in the prestellar phase which effectively cancels out the effect of the velocity gradient, producing a near constant \dot{M} in the early accretion phase; (2) in the more realistic case including cooling saturation at higher densities, \dot{M} may initially be either weakly increasing or weakly decreasing with time, for low (T_d ~ 6 K) and high dust temperature (T_d ~ 10 K) cases, respectively. Hence, our results show that the initial decline in \dot{M} seen in isothermal models is definitely not enhanced by non-isothermal effects, and is often suppressed by them. In all our models, \dot{M} does eventually decline rapidly due to the finite mass condition on our cores and a resulting inward propagating rarefaction wave. Thus, any explanation for a rapid decline of $\dot{M}$ in the accretion phase likely needs to appeal to the global molecular cloud structure and possible envelope support, which results in a finite mass reservoir for cores.

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arXiv2005

The effect of a finite mass reservoir on the collapse of spherical isothermal clouds and the evolution of protostellar accretion

Motivated by recent observations which detect an outer boundary for starless cores, and evidence for time-dependent mass accretion in the Class 0 and Class I protostellar phases, we reexamine the case of spherical isothermal collapse in the case of a finite mass reservoir. The presence of a core boundary results in the generation of an inward propagating rarefaction wave. This steepens the gas density profile from r^{-2} to r^{-3} or steeper. After a protostar forms, the mass accretion rate \dot{M} evolves through three distinct phases: (1) an early phase of decline in \dot{M}, which is a non-self-similar effect due to spatially nonuniform infall in the prestellar phase; (2) for large cores, an intermediate phase of near-constant \dot{M} from the infall of the outer part of the self-similar density profile; (3) a late phase of rapid decline in \dot{M} when accretion occurs from the region affected by the inward propagating rarefaction wave. Our model clouds of small to intermediate size make a direct transition from phase (1) to phase (3) above. Both the first and second phase are characterized by a temporally increasing bolometric luminosity L_bol, while L_bol is decreasing in the third (final) phase. We identify the period of temporally increasing L_bol with the Class 0 phase, and the later period of terminal accretion and decreasing L_bol with the Class I phase. The peak in L_bol corresponds to the evolutionary time when 50% \pm 10% of the cloud mass has been accreted by the protostar. This is in agreement with the classification scheme proposed by Andre et al. (1993). We show how our results can be used to explain tracks of envelope mass M_env versus L_bol for protostars in Taurus and Ophiuchus. We also develop an analytic formalism which reproduces the protostellar accretion rate.

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arXiv2004

Numerical simulations of expanding supershells in dwarf irregular galaxies. II. Formation of giant HI rings

We perform numerical hydrodynamic modeling of various physical processes that can form an HI ring as is observed in Holmberg I. Three energetic mechanisms are considered: multiple supernova explosions (SNe), a hypernova explosion associated with a gamma ray burst (GRB), and the vertical impact of a high velocity cloud (HVC). The total released energy has an upper limit of 10^54 ergs. We find that multiple SNe are in general more effective in producing shells that break out of the disk than a hypernova explosion of the same total energy. As a consequence, multiple SNe form rings with a high ring-to-center contrast K<100 in the HI column density, whereas single hypernova explosions form rings with K<10. Only multiple SNe can reproduce both the size (diameter \~1.7 kpc) and the ring-to-center contrast (K ~ 15-20) of the HI ring in Hoolmberg I. High velocity clouds create HI rings that are much smaller in size (< 0.8 kpc) and contrast (K < 4.5) than seen in Holmberg I. We construct model position-velocity (pV) diagrams and find that they can be used to distinguish among different HI ring formation mechanisms. The observed pV-diagrams of Holmberg I are best reproduced by multiple SNe. We conclude that the giant HI ring in Holmberg I is most probably formed by multiple SNe. We also find that the appearance of the SNe-driven shell in the integrated HI image depends on the inclination angle of the galaxy. In nearly face-on galaxies, the integrated HI image shows a ring of roughly constant HI column density surrounding a deep central depression, whereas in considerably inclined galaxies (i > 45 deg) the HI image is characterized by two kidney-shaped density enhancements and a mild central depression.

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arXiv2004

A minimum hypothesis explanation for an IMF with a lognormal body and power law tail

We present a minimum hypothesis model for an IMF that resembles a lognormal distribution at low masses but has a distinct power-law tail. Even if the central limit theorem ensures a lognormal distribution of condensation masses at birth, a power-law tail in the distribution arises due to accretion from the ambient cloud, coupled with a non-uniform (exponential) distribution of accretion times.

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arXiv2004

Magnetic Fields in Molecular Cloud Cores

Observations of magnetic field strengths imply that molecular cloud fragments are individually close to being in a magnetically critical state, even though both magnetic field and column density measurements range over two orders of magnitude. The turbulent pressure also approximately balances the self-gravitational pressure. These results together mean that the one-dimensional velocity dispersion $\sigv$ is proportional to the mean \Alf speed of a cloud $\va$. Global models of MHD turbulence in a molecular cloud show that this correlation is naturally satisfied for a range of different driving strengths of the turbulence. For example, an increase of turbulent driving causes a cloud expansion which also increases $\va$. Clouds are in a time averaged balance but exhibit large oscillatory motions, particularly in their outer rarefied regions. We also discuss models of gravitational fragmentation in a sheet-like region in which turbulence has already dissipated, including the effects of magnetic fields and ion-neutral friction. Clouds with near-critical mass-to-flux ratios lead to subsonic infall within cores, consistent with some recent observations of motions in starless cores. Conversely, significantly supercritical clouds are expected to produce extended supersonic infall.

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arXiv2004

A Mushroom-shaped Structure from the Impact of a Cloud with the Galactic Disk

We propose that the mushroom-shaped structure of the Galactic worm GW 123.4--1.5 is created by a cloud collision with the Galactic gas disk. A hydrodynamic simulation shows that a mushroom-shaped structure is created after the cloud crosses the Galactic midplane. The lifetime of the mushroom-shaped structure is of order the dynamical time scale of the disk, \sim 10^7 years. We find that the velocities across the cap of the mushroom-shaped structure in the simulation are consistent with the observed values. The simulation also predicts a structure on the opposite side of the Galactic plane which is created by the Kelvin-Helmholtz instability after the cloud passes through the disk.

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arXiv2004

Formation and Collapse of Nonaxisymmetric Protostellar Cores in Planar Magnetic Molecular Clouds

We extend our earlier work on ambipolar diffusion induced formation of protostellar cores in isothermal sheet-like magnetic interstellar clouds, by studying nonaxisymmetric collapse for the physically interesting regime of magnetically critical and supercritical model clouds ($\mui \geq 1$, where $\mui$ is the initial mass-to-magnetic flux ratio in units of the critical value for gravitational collapse). Cores that form in model simulations are effectively triaxial, with shapes that are typically closer to being oblate, rather than prolate. Infall velocities in the critical model ($\mui = 1$) are subsonic; in contrast, a supercritical model ($\mui = 2$) has extended supersonic infall that may be excluded by observations. For the magnetically critical model, ambipolar diffusion forms cores that are supercritical ($\muc > 1$) and embedded within subcritical envelopes ($\muenv < 1$). Cores in our models have density profiles that eventually merge into a near-uniform background, which is suggestive of observed properties of cloud cores.

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arXiv2003

On the Power-Law Tail in the Mass Function of Protostellar Condensations and Stars

We explore the idea that the power-law tail in the mass function of protostellar condensations and stars arises from the accretion of ambient cloud material on to a condensation, coupled with a nonuniform (exponential) distribution of accretion lifetimes. This model allows for the generation of power-law distributions in all star-forming regions, even if condensations start with a lognormal mass distribution, as may be expected from the central limit theorem, and supported by some recent numerical simulations of turbulent molecular clouds. For a condensation mass $m$ with growth rate $dm/dt \propto m$, an analytic three-parameter probability density function is derived; it resembles a lognormal at low mass and has a pure power-law high-mass tail. An approximate power-law tail is also expected for other growth laws, and we calculate the distribution for the plausible case $dm/dt \propto m^{2/3}$. Furthermore, any single time snapshot of the masses of condensations that are still accreting (and are of varying ages) also yields a distribution with a power-law tail similar to that of the IMF.

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arXiv2001

The Intrinsic Shapes of Molecular Cloud Fragments over a Range of Length Scales

We decipher intrinsic three-dimensional shape distributions of molecular clouds, cloud cores, Bok globules, and condensations using recently compiled catalogues of observed axis ratios for these objects mapped in carbon monoxide, ammonia, through optical selection, or in continuum dust emission. We apply statistical techniques to compare assumed intrinsic axis ratio distributions with observed projected axis ratio distributions. Intrinsically triaxial shapes produce projected distributions which agree with observations. Molecular clouds mapped in $^{12}$CO are intrinsically triaxial but more nearly prolate than oblate, while the smaller cloud cores, Bok globules, and condensations are also intrinsically triaxial but more nearly oblate than prolate.

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arXiv2001

Constraints on Stirring and Dissipation of MHD Turbulence in Molecular Clouds

We discuss constraints on the rates of stirring and dissipation of MHD turbulence in molecular clouds. Recent MHD simulations suggest that turbulence in clouds decays rapidly, thus providing a significant source of energy input, particularly if driven at small scales by, for example, bipolar outflows. We quantify the heating rates by combining the linewidth-size relations, which describe global cloud properties, with numerically determined dissipation rates. We argue that, if cloud turbulence is driven on small internal scales, the $^{12}$CO flux (enhanced by emission from weakly supersonic shocks) will be much larger than observed; this, in turn, would imply excitation temperatures significantly above observed values. We reach two conclusions: (1) small-scale driving by bipolar outflows cannot possibly account for cloud support and yield long-lived clouds, unless the published MHD dissipation rates are seriously overestimated; (2) driving on large scales (comparable to the cloud size) is much more viable from an energetic standpoint, and if the actual net dissipation rate is only slightly lower than what current MHD simulations estimate, then the observationally inferred lifetimes and apparent virial equilibrium of molecular clouds can be explained.

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arXiv2001

Intrinsic Shapes of Molecular Cloud Cores

We conduct an analysis of the shapes of molecular cloud cores using recently compiled catalogs of observed axis ratios of individual cores mapped in ammonia or through optical selection. We apply both analytical and statistical techniques to deproject the observed axis ratios in order to determine the true distribution of cloud core shapes. We find that neither pure oblate nor pure prolate cores can account for the observed distribution of core shapes. Intrinsically triaxial cores produce distributions which agree with observations. The best-fit triaxial distribution contains cores which are more nearly oblate than prolate.

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arXiv2000

The Razor's Edge: Magnetic Fields and Their Fundamental Role in Star Formation and Observations of Protostellar Coress

We review theoretical models of the early stages of star formation, in which gravitational collapse is strongly regulated by magnetic fields and the associated process of ambipolar diffusion. We discuss results of numerical simulations and analytical studies of core formation and collapse, which can be directly tested against observation. We also focus on recent data which are relevant to this theory of star formation, such as: observations of extended infall in protostellar cores, estimates of evolutionary timescales ~ 1 Myr for cores, measured mass-to-flux ratios of cores, and the relative alignment of polarization vectors with apparent cloud elongation. It is shown that in all of these areas, the data lie within the observationally allowable range of parameter space. Other areas of interest (protostellar accretion rates and the presence of core edges) and issues that remain unresolved or under study (the role of non-thermal motions and cluster formation) are also discussed. Moreover, we highlight some differences between our model predictions and those of highly turbulent star formation models, and discuss how these differences can be distinguished observationally.

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arXiv2000

On the Timescale for the Formation of Protostellar Cores in Magnetic Interstellar Clouds

We revisit the problem of the formation of dense protostellar cores due to ambipolar diffusion within magnetically supported molecular clouds, and derive an analytical expression for the core formation timescale. The resulting expression is similar to the canonical expression = t_{ff}^2/t_{ni} ~ 10 t_{ni} (where t_{ff} is the free-fall time and t_{ni} is the neutral-ion collision time), except that it is multiplied by a numerical factor C(μ_{c0}), where μ_{c0} is the initial central mass-to-flux ratio normalized to the critical value for gravitational collapse. C(μ_{c0}) is typically ~ 1 in highly subcritical clouds (μ_{c0} << 1), although certain conditions allow C(μ_{c0}) >> 1. For clouds that are not highly subcritical, C(μ_{c0}) can be much less than unity, with C(μ_{c0}) --> 0 for μ_{c0} --> 1, significantly reducing the time required to form a supercritical core. This, along with recent observations of clouds with mass-to-flux ratios close to the critical value, may reconcile the results of ambipolar diffusion models with statistical analyses of cores and YSO's which suggest an evolutionary timescale \~ 1 Myr for objects of mean density ~ 10^4 cm^{-3}. We compare our analytical relation to the results of numerical simulations, and also discuss the effects of dust grains on the core formation timescale.

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arXiv2000

Magnetic Fields and the Triaxiality of Molecular Cloud Cores

We make the hypothesis that molecular cloud fragments are triaxial bodies with a large scale magnetic field oriented along the short axis. While consistent with theoretical expectations, this idea is supported by magnetic field strength data, which show strong evidence for flattening along the direction of the mean magnetic field. It is also consistent with early submillimeter polarization data, which show that the projected direction of the magnetic field is often slightly misaligned with the projected minor axis of a molecular cloud core, i.e., the offset angle $Ψ$ is nonzero. We calculate distributions of $Ψ$ for various triaxial bodies, when viewed from a random set of viewing angles. The highest viewing probability always corresponds to $Ψ=0^{\circ}$, but there is a finite probability of viewing all nonzero $Ψ$, including even $Ψ=90^{\circ}$; the average offset typically falls in the range $10^{\circ}-30^{\circ}$ for triaxial bodies most likely to satisfy observational and theoretical constraints.

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arXiv2000

Galactic Worm 123.4-1.5: A Mushroom-shaped HI Cloud

The Dominion Radio Astrophysical Observatory's Synthesis Telescope provides the highest resolution data (1 arcmin and 0.82 km/s) to date of an HI worm candidate. Observed as part of the Canadian Galactic Plane Survey, mushroom-shaped GW 123.4-1.5 extends only a few hundred parsecs, contains ~10^5 solar masses of neutral hydrogen, and appears unrelated to a conventional shell or chimney structure. Our preliminary Zeus-2d models use a single off-plane explosion with a modest (~10^{51} ergs) energy input. These generic simulations generate, interior to an expanding outer blast wave, a buoyant cloud whose structure resembles the morphology of the observed feature. Unlike typical model superbubbles, the stem can be narrow because its width is not governed by the pressure behind the blast wave nor the disk scale height. Using this type of approach it should be possible to more accurately model the thin stem and other details of GW 123.4-1.5 in the future.

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arXiv2000

The HI shell G132.6-0.7-25.3: A Supernova Remnant or an Old Wind-Blown Bubble?

Data from the Canadian Galactic Plane Survey reveal an abundance of HI shells and arcs in the disk of our galaxy. While their shape is suggestive of stellar winds or supernovae influence, very few of these structures have been examined in detail thus far. A fine example is an HI shell in the outer Galaxy with no continuum counterpart discovered in the survey's pilot project. Its size and kinematics suggest that it was created by the winds of a single late-type O star which has since evolved off the main sequence or by a supernova explosion. A B1 Ia star at the centre of the shell, in projection, is a possible candidate for energy source if the shell is assumed to be wind-blown. The shell's shape implies a surprisingly small scale height of less than about 30 pc for the surrounding gas if the elongation is due to evolution in a density gradient.

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