arXiv subjects

Aparna Venkatesan

Publications and source records attributed to Aparna Venkatesan.

40 records · Page 3Linked to original sources

arXiv2001

Observational Signatures of the First Stars

At present, there are several feasible observational probes of the first stars in the universe. Here, we examine the constraints on early stellar activity from the metallicity of the high-redshift Ly-$α$ clouds, from the effects of stellar ionizing photons on reionization and the cosmic microwave background (CMB), and from the implications of gravitational microlensing results for the presence of stellar remnants in galactic halos. We also discuss whether the above signatures are consistent with each other, i.e., if they reflect the same population of stars.

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arXiv2000

The Optical Depth to Reionization as a Probe of Cosmological and Astrophysical Parameters

Current data of high-redshift absorption-line systems imply that hydrogen reionization occurred before redshifts of about 5. Previous works on reionization by the first stars or quasars have shown that such scenarios are described by a large number of cosmological and astrophysical parameters. Here, we adopt a semi-analytic model of stellar reionization in order to quantify how the optical depth to reionization depends on such parameters, and combine this with constraints from the cosmic microwave background (CMB). We find this approach to be particularly useful in alleviating the well-known degeneracy in CMB parameter extraction between the optical depth to reionization and the amplitude of the primordial power spectrum, due to the complementary information from the reionization model. We also examine translating independent limits on astrophysical parameters into those on cosmological parameters, or conversely, how improved determinations of cosmological parameters will constrain astrophysical unknowns.

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arXiv1999

Neutron Stars and Black Holes as MACHOs

We consider the contribution of neutron stars and black holes to the dynamical mass of galactic halos. In particular, we show that if these compact objects were produced by an early generation of stars with initial metallicity less than about $10^{-4} Z_\odot$, they can contribute at most 30--40% of the Galactic halo mass without creating supersolar levels of enrichment. We show that the case for halo neutron stars and black holes cannot be rejected on metal overproduction arguments alone, due to the critical factor of the choice of progenitor metallicity in determining the yields. We show that this scenario satisfies observational constraints, similar to but no more severe than those faced by halo white dwarfs. We also discuss the recent results on halo microlensing, the presence of enriched hot gas in clusters and groups of galaxies, and other observations. If there are halo neutron stars and black holes, they will be detected by microlensing experiments in the future as longer-timescale events.

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arXiv1997

Constraints on the Production of Ultra-High-Energy Cosmic Rays by Isolated Neutron Stars

The energetics, spectrum, and composition of cosmic rays with energies below about $10^{15}$ eV are fairly well explained by models involving supernova shocks. In contrast, no widely accepted theory exists for the origin of ultra-high energy cosmic rays (UHECRs), which have energies above $10^{15}$ eV. Instead of proposing a specific model, here we place strong constraints on any model of UHECRs involving isolated neutron stars (no companions). We consider the total power requirements and show that the only viable power source associated with isolated neutron stars is rotation. Mechanisms based on accretion from the interstellar medium fall short of the necessary power despite the most optimistic assumptions. Power considerations also demonstrate that not enough rotational energy is tapped by a "propeller"-like acceleration of interstellar matter. The most promising source of energy is rotational spindown via magnetic braking. We examine microphysical energy loss processes near magnetized neutron stars and conclude that the most likely site for yielding UHECRs from isolated neutron stars is near or beyond the light cylinder.

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