arXiv subjects

Amir Siraj

Publications and source records attributed to Amir Siraj.

At least 37 records · Page 2Linked to original sources

arXiv2021

Intelligent Responses to Our Technological Signals Will Not Arrive In Fewer Than Three Millennia

What is the chance we start a conversation with another civilization like our own? Our technological society produced signals that could be received by other extraterrestrial civilizations, within a sphere around us with a radius of $\sim 10^2$ light years. Given that, the Copernican principle provides a lower limit on the response time that we should expect from transmitters on Earth-like planets around Sun-like stars. If our civilization lives longer, the expected number of responses could increase. We explore the chance of detecting a response in the future, and show that a response should only be expected to arrive after a few millennia.

physics.pop-ph↗

arXiv2021

Interstellar Objects Outnumber Solar System Objects in the Oort Cloud

Here, we show that the detection of Borisov implies that interstellar objects outnumber Solar system objects in the Oort cloud, whereas the reverse is true near the Sun due to the stronger gravitational focusing of bound objects. This hypothesis can be tested with stellar occultation surveys of the Oort cloud. Furthermore, we demonstrate that $\sim 1 \%$ of carbon and oxygen in the Milky Way Galaxy may be locked in interstellar objects, saturating the heavy element budget of the minimum mass Solar nebula model.

astro-ph.EP↗

arXiv2021

Explaining Neptune's Eccentricity

Early migration damped Neptune's eccentricity. Here, we assume that the damped value was much smaller than the value observed today, and show that the closest flyby of $\sim 0.1 \; \mathrm{M_{\odot}}$ star over $\sim 4.5 \mathrm{\; Gyr}$ in the field, at a distance of $\sim 10^3 \mathrm{\; AU}$ would explain the value of Neptune's eccentricity observed today.

astro-ph.EP↗

arXiv2021

Reply to Desch et al. (2021) on "Breakup of a long-period comet as the origin of the dinosaur extinction"

We reply to criticisms by Desch et al. (2021) regarding our Scientific Reports paper, "Breakup of a long-period comet as the origin of the dinosaur extinction." The background impact rates of main-belt asteroids and long-period comets have been previously dismissed as being too low to explain the Chicxulub impact event. Our work demonstrates that a fraction of long-period comets are tidally disrupted after passing close to the Sun, each producing a collection of smaller fragments that cross the orbit of Earth. This population greatly increases the impact rate of long-period comets capable of producing Chicxulub, making it consistent with the age of the Chicxulub impact crater. Our results are subject to an uncertainty in the number of fragments produced in the breakup event that leaves the conclusion of a cometary origin being more likely than an asteroidal one unchanged.

astro-ph.EP↗

arXiv2021

Eliminating the Remaining Window for Primordial Black Holes as Dark Matter from the Dynamics of the Cold Kuiper Belt

The nature of dark matter (DM) is unknown. One compelling possibility is DM being composed of primordial black holes (PBHs), given the tight limits on some types of elementary particles as DM. There is only one remaining window of masses available for PBHs to constitute the entire DM density, $10^{17} - 10^{23} \mathrm{\; g}$. Here, we show that the kernel population in the cold Kuiper belt rules out this window, arguing in favor of a particle nature for DM.

astro-ph.CO↗

arXiv2021

Breakup of a Long-Period Comet as the Origin of the Dinosaur Extinction

The origin of the Chicxulub impactor, which is attributed as the cause of the K/T mass extinction event, is an unsolved puzzle. The background impact rates of main-belt asteroids and long-period comets have been previously dismissed as being too low to explain the Chicxulub impact event. Here, we show that a fraction of long-period comets are tidally disrupted after passing close to the Sun, each producing a collection of smaller fragments that cross the orbit of Earth. This population could increase the impact rate of long-period comets capable of producing Chicxulub impact events by an order of magnitude. This new rate would be consistent with the age of the Chicxulub impact crater, thereby providing a satisfactory explanation for the origin of the impactor. Our hypothesis explains the composition of the largest confirmed impact crater in Earth's history as well as the largest one within the last million years. It predicts a larger proportion of impactors with carbonaceous chondritic compositions than would be expected from meteorite falls of main-belt asteroids.

astro-ph.EP↗

arXiv2021

The Copernican Principle Rules Out BLC1 as a Technological Radio Signal from the Alpha Centauri System

Without evidence for occupying a special time or location, we should not assume that we inhabit privileged circumstances in the Universe. As a result, within the context of all Earth-like planets orbiting Sun-like stars, the origin of a technological civilization on Earth should be considered a single outcome of a random process. We show that in such a Copernican framework, which is inherently optimistic about the prevalence of life in the Universe, the likelihood of the nearest star system, Alpha Centauri, hosting a radio-transmitting civilization is $\sim 10^{-8}$. This rules out, \textit{a priori}, Breakthrough Listen Candidate 1 (BLC1) as a technological radio signal from the Alpha Centauri system, as such a scenario would violate the Copernican principle by about eight orders of magnitude. We also show that the Copernican principle is consistent with the vast majority of Fast Radio Bursts being natural in origin.

physics.pop-ph↗

arXiv2020

Repeated Impact-Driven Plume Formation On Enceladus Over Megayear Timescales

Water plumes erupting from the `tiger stripe' features on the south pole of Enceladus are thought to connect to a global subsurface ocean. Proposed origins for the initial stress necessary to form the `tiger stripes' include a giant impact, which would require true polar wander to explain the location of the plumes if the impact did not occur at the South Pole, or tensile stresses, which would require volumetric expansion associated with partial freezing of the subsurface ocean. In addition to the requirement of a fine-tuned impact, true polar wander, or partial freezing of the subsurface ocean, a further issue with these hypotheses is that the `tiger stripes' may be short-lived. We show here that impact resurfacing can seal off plumes and mass loss can lead to their compression and closure over $\sim 1 \mathrm{\;Myr}$. Since plumes are observed at present, a mechanism by which new plumes can be generated every $\sim 1 \mathrm{\;Myr}$ and by which such plumes are most likely to form at the south pole is needed. We propose and investigate the possibility that impacts constitute an adequate repeating source for the continual instigation of fractures and plumes. We find that the rate of impacts on Enceladus suggests the formation of $\sim 10^3$ independent plume systems per Gyr, the vast majority on the south pole, and is consistent with the Cassini-derived age of the south pole for a lunar-like bombardment history, our estimates of fracture lifetimes, and with the needed parameters for parallel fracture propagation. The model favors a bombardment history similar to that of Triton over one more similar to that of the Galilean satellites.

astro-ph.EP↗

arXiv2020

The Case for an Early Solar Binary Companion

We show that an equal-mass, temporary binary companion to the Sun in the solar birth cluster at a separation of $\sim 10^3 \; \mathrm{\; AU}$ would have increased the likelihood of forming the observed population of outer Oort cloud objects and of capturing Planet Nine. In particular, the discovery of a captured origin for Planet Nine would favor our binary model by an order of magnitude relative to a lone stellar history. Our model predicts an overabundance of dwarf planets, discoverable by LSST, with similar orbits to Planet Nine, which would result from capture by the stellar binary.

astro-ph.EP↗

arXiv2020

Risks for Life on Proxima b from Sterilizing Impacts

We consider the implications that a debris belt located between Proxima b and Proxima c would pose for the rate of large asteroid impacts that could sterilize Proxima b from life. Future observations by ALMA or JWST could constrain the existence of an asteroid belt in the life-threatening regime. We generalize our rate calculation of sterilizing impacts for habitable planets in systems with an asteroid belt and an outer planet.

astro-ph.EP↗

arXiv2020

Observable Signatures of the Ejection Speed of Interstellar Objects from their Birth Systems

`Oumuamua and Borisov were the first two interstellar objects confirmed in the Solar system. The upcoming commencement of the Vera C. Rubin Observatory's Legacy Survey of Space of Time (LSST) will enhance greatly the discovery rate of interstellar objects. This raises the question, what can be learned from large-number statistics of interstellar objects? Here, we show that discovery statistics provided by LSST will allow low- and high-ejection-speed populations to be distinguished using the velocity dispersion and angular anisotropy of interstellar objects. These findings can be combined with physical characterizations to yield a better understanding of planetary system origin and nature.

astro-ph.EP↗

arXiv2020

Transfer of Life Between Earth and Venus with Planet-Grazing Asteroids

Recently, phosphine was discovered in the atmosphere of Venus as a potential biosignature. This raises the question: if Venusian life exists, could it be related to terrestrial life? Based on the known rate of meteoroid impacts on Earth, we show that at least $\sim 6 \times 10^5$ asteroids have grazed Earth's atmosphere without being significantly heated and later impacted Venus, and a similar number have grazed Venus's atmosphere and later impacted the Earth, both within a period of $\sim 10^5$ years during which microbes could survive in space. Although the abundance of terrestrial life in the upper atmosphere is unknown, these planet-grazing shepherds could have potentially been capable of transferring microbial life between the atmospheres of Earth and Venus. As a result, the origin of possible Venusian life may be fundamentally indistinguishable from that of terrestrial life.

astro-ph.EP↗

arXiv2020

Searching for Black Holes in the Outer Solar System with LSST

Planet Nine has been proposed to potentially be a black hole in the outer solar system. We investigate the accretion flares that would result from impacts of small Oort cloud objects, and find that the upcoming LSST observing program will be able to either rule out or confirm Planet Nine as a black hole within a year. We also find that LSST could rule out or confirm the existence of trapped planet-mass black holes out to the edge of the Oort cloud, indirectly probing the dark matter fraction in subsolar mass black holes and potentially improving upon current limits by orders of magnitude.

astro-ph.HE↗

arXiv2020

Time Series Methods and Ensemble Models to Nowcast Dengue at the State Level in Brazil

Predicting an infectious disease can help reduce its impact by advising public health interventions and personal preventive measures. Novel data streams, such as Internet and social media data, have recently been reported to benefit infectious disease prediction. As a case study of dengue in Brazil, we have combined multiple traditional and non-traditional, heterogeneous data streams (satellite imagery, Internet, weather, and clinical surveillance data) across its 27 states on a weekly basis over seven years. For each state, we nowcast dengue based on several time series models, which vary in complexity and inclusion of exogenous data. The top-performing model varies by state, motivating our consideration of ensemble approaches to automatically combine these models for better outcomes at the state level. Model comparisons suggest that predictions often improve with the addition of exogenous data, although similar performance can be attained by including only one exogenous data stream (either weather data or the novel satellite data) rather than combining all of them. Our results demonstrate that Brazil can be nowcasted at the state level with high accuracy and confidence, inform the utility of each individual data stream, and reveal potential geographic contributors to predictive performance. Our work can be extended to other spatial levels of Brazil, vector-borne diseases, and countries, so that the spread of infectious disease can be more effectively curbed.

stat.AP↗

arXiv2020

Impacts of Dust Grains Accelerated by Supernovae on the Moon

There is evidence that ejecta from nearby supernovae have rained down on Earth in the past. Supernovae can accelerate pre-existing dust grains in the interstellar medium to speeds of $\sim 0.01 \mathrm{\;c}$. We investigate the survival and impact of dust grains from supernovae on the moon, finding that supernova dust grains can form detectable tracks with widths of $\sim 0.01 - 0.07 \mathrm{\; μm}$ and depths of $\sim 0.1 - 0.7 \mathrm{\; mm}$ in lunar rocks. These tracks could potentially shed light on the timings, luminosities, and directions of nearby supernovae.

astro-ph.EP↗

arXiv2020

Possible Transfer of Life by Earth-Grazing Objects to Exoplanetary Systems

Recently, a 30-cm object was discovered to graze the Earth's atmosphere and shift into a Jupiter-crossing orbit. We use the related survey parameters to calibrate the total number of such objects. The number of objects that could have exported terrestrial microbes out of the Solar System is in the range $2 \times 10^{9}$--$3 \times 10^{11}$. We find that $10^{7}$--$10^{9}$ such objects could have been captured by binary star systems over the lifetime of the Solar System. Adopting the fiducial assumption that one polyextremophile colony is picked up by each object, the total number of objects carrying living colonies on them upon capture could be $10$--$10^3$.

astro-ph.EP↗

arXiv2020

A Real-Time Search for Interstellar Impacts on the Moon

The discovery of `Oumuamua and CNEOS 2014-01-08 allowed for a calibration of the impact rate of interstellar objects. We propose a new telescope in lunar orbit to study in real-time interstellar meteoroid impacts and to serve as a laboratory for hypervelocity collisions. We show that a telescope with diameter $D_a \gtrsim 2 \mathrm{\; m}$ would be capable of detecting $\gtrsim 1$ interstellar meteoroid impacts (among hundreds of Solar System meteoroid impacts) per year. For each meteoroid, measurements of the reflected sunlight and shadow, as well as the impact's optical flash and crater, would allow for the determination of the 3D velocity, mass, density, and composition, as well as the radiative efficiency.

astro-ph.EP↗

arXiv2020

Detecting Interstellar Objects Through Stellar Occultations

Stellar occultations have been used to search for Kuiper Belt and Oort Cloud objects. We propose a search for interstellar objects based on the characteristic durations ($\sim 0.1 \mathrm{s}$) of their stellar occultation signals and high inclination relative to the ecliptic plane. An all-sky monitoring program of all $\sim 7 \times 10^6$ stars with $R \lesssim 12.5$ using 1-m telescopes with $0.1 \; \mathrm{s}$ cadences is predicted to discover $\sim 1$ interstellar object per year.

astro-ph.EP↗