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Amir Siraj

Publications and source records attributed to Amir Siraj.

44 records · Page 3Linked to original sources
arXiv2020

Observational Signatures of Sub-Relativistic Meteors

It is currently unknown whether solid particles larger than dust from supernova ejecta rain down on Earth at high speeds. We develop a hydrodynamic and radiative model to explore the detectability of $\gtrsim 1 \mathrm{\; mm}$ sub-relativistic meteors. We find that a large fraction of the meteor energy during its passage through the Earth's upper atmosphere powers the adiabatic expansion of a hot plasma cylinder, giving rise to acoustic shocks detectable by infrasound microphones. Additionally, a global network of several hundred all-sky optical cameras with a time resolution of $\lesssim 10^{-4} \mathrm{\;s}$ would be capable of detecting $\gtrsim 1 \mathrm{\; mm}$ sub-relativistic meteors.

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arXiv2019

Exporting Terrestrial Life Out of the Solar System with Gravitational Slingshots of Earthgrazing Bodies

Exporting terrestrial life out of the Solar System requires a process that both embeds microbes in boulders and ejects those boulders out of the Solar System. We explore the possibility that Earthgrazing long-period comets and interstellar objects could export life from Earth by collecting microbes from the atmosphere and receiving a gravitational slingshot effect from the Earth. We estimate the total number of exportation events over the lifetime of the Earth to be $\sim 1 - 10$ for long-period comets and $\sim 1 - 50$ for interstellar objects. If life existed above an altitude of 100 km, then the number is dramatically increased up to $\sim 10^5$ exportation events over Earth's lifetime.

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arXiv2019

An Argument for a Kilometer-Scale Nucleus of C/2019 Q4

C/2019 Q4 (Borisov), discovered by Gennady Borisov on August 30, 2019, is tentatively an interstellar comet. We show that a kilometer-scale nucleus would provide consistency with both `Oumuamua and CNEOS-2014-01-08, resulting in a single power-law distribution with an equal amount of mass per logarithmic bin of interstellar objects.

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arXiv2019

Halo Meteors

The stellar halo contains some of the oldest stars in the Milky Way galaxy and in the universe. The detections of `Oumuamua, CNEOS 2014-01-08, and interstellar dust serve to calibrate the production rate of interstellar objects. We study the feasibility of a search for interstellar meteors with origins in the stellar halo. We find the mean heliocentric impact speed for halo meteors to be $\sim 270 \mathrm{\;km\;s^{-1}}$, and the standard deviation is $\sim 90 \mathrm{\;km\;s^{-1}}$, making the population kinematically distinct from all other meteors, which are an order-of-magnitude slower. We explore the expected abundance of halo meteors, finding that a network of all-sky cameras covering all land on Earth can take spectra and determine the orbits of a few hundred halo meteors larger than a few mm per year. The compositions of halo meteors would provide information on the characteristics of planetary system formation for the oldest stars. In addition, one could place tight constraints on baryonic dark matter objects of low masses.

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arXiv2019

Probing Extrasolar Planetary Systems with Interstellar Meteors

The first interstellar object, `Oumuamua, was discovered in the Solar System by Pan-STARRS in 2017, allowing for a calibration of the impact rate of interstellar meteors of its size $\sim 100\;$m. The discovery of CNEOS 2014-01-08 allowed for a calibration of the impact rate of interstellar meteors of its size $\sim 1\;$m. Analysis of interstellar dust grains have allowed for calibrations of the impact rate of smaller interstellar meteors down to the size $\sim 10^{-8}\;$m. We analyze the size distribution of interstellar meteors, finding that for smooth power-law fits of the form $N(r)\propto r^{-q}$, the possible values of $q$ are in the range $3.41 \pm 0.17$. We then consider the possibility of analyzing interstellar meteors to learn about their parent planetary systems. We propose a strategy for determining the orbits and chemical compositions of interstellar meteors, using a network of $\sim 600$ all-sky camera systems to track and conduct remote spectroscopy on meteors larger than $\sim 5$cm once every few years. It should also be possible to retrieve meteorites from the impact sites, providing the first samples of materials from other planetary systems.

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arXiv2019

Identifying Interstellar Objects Trapped in the Solar System through Their Orbital Parameters

The first interstellar object, `Oumuamua, was discovered in the Solar System by Pan-STARRS in 2017, allowing for a calibration of the abundance of interstellar objects of its size and an estimation of the subset of objects trapped by the Jupiter-Sun system. Photographing or visiting these trapped objects would allow for learning about the conditions in other planetary systems, saving the need to send interstellar probes. Here, we explore the orbital properties of captured interstellar objects in the Solar System using dynamical simulations of the Jupiter-Sun system and initial conditions drawn from the distribution of relative velocities of stars in the Solar neighborhood. We compare the resulting distributions of orbital elements to those of the most similar population of known asteroids, namely Centaurs, to search for a parameter space in which interstellar objects should dominate and therefore be identifiable solely by their orbits. We find that there should be thousands of `Oumuamua-size interstellar objects identifiable by Centaur-like orbits at high inclinations, assuming a number density of `Oumuamua-size interstellar objects of $\sim 10^{15} \; \mathrm{pc^{-3}}$. We note eight known objects that may be of interstellar origin. Finally, we estimate that LSST will be able to detect several hundreds of these objects.

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arXiv2018

Infrared Lightcurves of Near Earth Objects

We present lightcurves and derive periods and amplitudes for a subset of 38 near earth objects (NEOs) observed at 4.5 microns with the IRAC camera on the the Spitzer Space Telescope, many of them having no previously reported rotation periods. This subset was chosen from about 1800 IRAC NEO observations as having obvious periodicity and significant amplitude. For objects where the period observed did not sample the full rotational period, we derived lower limits to these parameters based on sinusoidal fits. Lightcurve durations ranged from 42 to 544 minutes, with derived periods from 16 to 400 minutes. We discuss the effects of lightcurve variations on the thermal modeling used to derive diameters and albedos from Spitzer photometry. We find that both diameters and albedos derived from the lightcurve maxima and minima agree with our previously published results, even for extreme objects, showing the conservative nature of the thermal model uncertainties. We also evaluate the NEO rotation rates, sizes, and their cohesive strengths.

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