Person: Loeb, Abraham
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Loeb
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Loeb, Abraham
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Publication Absence of a Thick Atmosphere on the Terrestrial Exoplanet LHS 3844b(Springer Science and Business Media LLC, 2019-08-19) Kreidberg, Laura; Hu, Renyu; Schaefer, Laura; Deming, Drake; Stevenson, Kevin B.; Dittmann, Jason; Vanderburg, Andrew; Berardo, David; Guo, Xueying; Stassun, Keivan; Crossfield, Ian; Charbonneau, David; Loeb, Abraham; Ricker, George; Seager, Sara; Vanderspek, Roland; Koll, Daniel; Morley, Caroline; Latham, DavidThe majority of terrestrial planets in the Galaxy orbit small stars with radii less than 60% that of the Sun1,2. Theoretical models predict that these planets are more vulnerable to at- mospheric escape and collapse than their counterparts orbiting Sun-like stars3–5. To deter- mine whether a thick atmosphere has survived, one approach is to search for signatures of atmospheric heat redistribution in a planet’s thermal phase curve6–9. This technique was previously applied to the super-Earth 55 Cancri e, which showed an offset hot spot indicative of atmosphere heat circulation10. Here we report a phase curve measurement for the exo- planet LHS 3844b, a 1.3 R⊕ world in an 11-hour orbit around a small, nearby star. This is the first such measurement for a planet smaller than 1.6 Re, the size marking the transition from rocky to gaseous worlds11. The phase variation is symmetric and has a large amplitude, implying a dayside brightness temperature of 1040±40 K and a nightside temperature con- sistent with zero K (1σ confidence). The data are best fit by a bare rock model with a low Bond albedo (< 0.2 at 2σ confidence), or a tenuous atmosphere with surface pressure below 0.1 bar. These results support theoretical predictions that hot terrestrial planets orbiting small stars may not retain substantial atmospheres.Publication Jet-Launching Structure Resolved Near the Supermassive Black Hole in M87(American Association for the Advancement of Science (AAAS), 2012) Doeleman, Sheperd; Fish, V. L.; Schenck, D. E.; Beaudoin, C.; Blundell, Raymond; Bower, G. C.; Broderick, Alithia Carol; Chamberlin, R.; Freund, R.; Friberg, P.; Gurwell, Mark; Ho, Po-Yi; Honma, M.; Inoue, M.; Krichbaum, T. P.; Lamb, Justin; Loeb, Abraham; Lonsdale, C.; Marrone, D. P.; Moran, James; Oyama, T.; Plambeck, R.; Primiani, Rurik; Rogers, A. E. E.; Smythe, D. L.; SooHoo, J.; Strittmatter, P.; Tilanus, R. P. J.; Titus, M.; Weintroub, Jonathan; Wright, Bennett Bennett; Young, K. H.; Ziurys, L. M.Approximately 10% of active galactic nuclei exhibit relativistic jets, which are powered by accretion of matter onto super massive black holes. While the measured width profiles of such jets on large scales agree with theories of magnetic collimation, predicted structure on accretion disk scales at the jet launch point has not been detected. We report radio interferometry observations at 1.3mm wavelength of the elliptical galaxy M87 that spatially resolve the base of the jet in this source. The derived size of 5.5 +/- 0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, suggesting that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole.Publication Cosmological Recombination of Lithium and Its Effect on the Microwave Background Anisotropies(IOP Publishing, 2002) Stancil, Phillip C.; Loeb, Abraham; Zaldarriaga, Matias; Dalgarno, Alexander; Lepp, StephenThe cosmological recombination history of lithium, produced during big bang nucleosynthesis, is presented using updated chemistry and cosmological parameters consistent with recent cosmic microwave background (CMB) measurements. For the popular set of cosmological parameters, about a fifth of the lithium ions recombine into neutral atoms by a redshift z ~ 400. The neutral lithium atoms scatter resonantly the CMB at 6708 Å and distort its intensity and polarization anisotropies at observed wavelengths around ~300 μm, as originally suggested by Loeb. The modified anisotropies resulting from the lithium recombination history are calculated for a variety of cosmological models and found to result primarily in a suppression of the power spectrum amplitude. Significant modification of the power spectrum occurs for models that assume a large primordial abundance of lithium. While detection of the lithium signal might prove difficult, it offers the possibility of inferring the lithium primordial abundance and is the only probe proposed to date of the large-scale structure of the universe for z ~ 500-100.Publication Delay Spectrum with Phase-Tracking Arrays: Extracting the HI power spectrum from the Epoch of Reionization(American Astronomical Society, 2016) Paul, Sourabh; Sethi, Shiv K.; Morales, Miguel F.; Dwarkanath, K. S.; Shankar, N. Udaya; Subrahmanyan, Ravi; Barry, N.; Beardsley, A. P.; Bowman, Judd D.; Briggs, F.; Carroll, P.; de Oliveira-Costa, A.; Dillon, Joshua S.; Ewall-Wice, A.; Feng, L.; Greenhill, Lincoln; Gaensler, B. M.; Hazelton, B. J.; Hewitt, J. N.; Hurley-Walker, N.; Jacobs, D. J.; Kim, Han-Seek; Kittiwisit, P.; Lenc, E.; Line, J.; Loeb, Abraham; McKinley, B.; Mitchell, D. A.; Neben, A. R.; Offringa, A. R.; Pindor, B.; Pober, J. C.; Procopio, P.; Riding, J.; Sullivan, I. S.; Tegmark, M.; Thyagarajan, Nithyanandan; Tingay, S. J.; Trott, C. M.; Wayth, R. B.; Webster, R. L.; Wyithe, J. S. B.; Cappallo, Roger; Johnston-Hollitt, M.; Kaplan, D. L.; Lonsdale, C. J.; McWhirter, S. R.; Morgan, E.; Oberoi, D.; Ord, S. M.; Prabu, T.; Srivani, K. S.; Williams, A.; Williams, C. L.The Detection of redshifted 21 cm emission from the epoch of reionization (EoR) is a challenging task owing to strong foregrounds that dominate the signal. In this paper, we propose a general method, based on the delay spectrum approach, to extract HI power spectra that is applicable to tracking observations using an imaging radio interferometer (Delay Spectrum with Imaging Arrays (DSIA)). Our method is based on modelling the HI signal taking into account the impact of wide field effects such as the w-term which are then used as appropriate weights in cross-correlating the measured visibilities. Our method is applicable to any radio interferometer that tracks a phase center and could be utilized for arrays such as MWA, LOFAR, GMRT, PAPER and HERA. In the literature the delay spectrum approach has been implemented for near-redundant baselines using drift scan observations. In this paper we explore the scheme for non-redundant tracking arrays, and this is the first application of delay spectrum methodology to such data to extract the HI signal. We analyze 3 hours of MWA tracking data on the EoR1 field. We present both 2-dimensional (k∥,k⊥) and 1-dimensional (k) power spectra from the analysis. Our results are in agreement with the findings of other pipelines developed to analyse the MWA EoR data.Publication Interferometric Measurement of Acceleration at Relativistic Speeds(American Astronomical Society, 2017) Christian, Pierre; Loeb, AbrahamWe show that an interferometer moving at a relativistic speed relative to a point source of light offers a sensitive probe of acceleration. Such an accelerometer contains no moving parts, and is thus more robust than conventional "mass-on-a-spring" accelerometers. In an interstellar mission to Alpha-Centauri, such an accelerometer could be used to measure the masses of planets around other stars as well as the mass distribution of the Milky Way Galaxy.Publication Prospects for Characterizing the Atmosphere of Proxima Centauri B(American Astronomical Society, 2016) Kreidberg, Laura; Loeb, AbrahamThe newly detected Earth-mass planet in the habitable zone of Proxima Centauri could potentially host life - if it has an atmosphere that supports surface liquid water. We show that thermal phase curve observations with the James Webb Space Telescope (JWST) from 5-12 microns can be used to test the existence of such an atmosphere. We predict the thermal variation for a bare rock versus a planet with 35% heat redistribution to the nightside and show that a JWST phase curve measurement can distinguish between these cases at 4σ confidence, assuming photon-limited precision. We also consider the case of an Earth-like atmosphere, and find that the ozone 9.8 micron band could be detected with longer integration times (a few months). We conclude that JWST observations have the potential to put the first constraints on the possibility of life around the nearest star to the Solar System.Publication A Novel Approach for Identifying Host Galaxies of Nearby FRBs(Royal Astronomical Society, 2017) Rane, Akshaya; Loeb, AbrahamWe report on a search for host galaxies of a subset of Rotating Radio Transients (RRATs) that possess a dispersion measure (DM) near or above the maximum Galactic value in their direction. These RRATs could have an extragalactic origin and therefore be Fast Radio Bursts (FRBs). The sizes of related galaxies on the sky at such short distances are comparable to the beam size of a single-dish telescope (for example, the 7.0′ radius of the Parkes beam). Hence the association, if found, could be more definitive as compared to finding host galaxies for more distant FRBs. We did not find any host galaxy associated with six RRATs near the maximum Galactic DM. This result is consistent with the fact that the probability of finding an FRB host galaxy within this volume is also very small. We propose that future follow-up observations of such RRATs be carried out in searching for local host galaxies as well as the sources of FRBs.Publication First Season MWA EoR Power Spectrum Results at Redshift 7(American Astronomical Society, 2016) Beardsley, A.; Hazelton, B.; Sullivan, I.; Carroll, P.; Barry, N.; Rahimi, M.; Pindor, B.; Trott, C.; Line, J.; Jacobs, Daniel; Morales, M.; Pober, J.; Bernardi, G.; Bowman, Judd; Busch, M.; Briggs, F.; Cappallo, R.; Corey, B.; de Oliveira-Costa, A.; Dillon, Joshua; Emrich, D.; Ewall-Wice, A.; Feng, L.; Gaensler, B.; Goeke, R.; Greenhill, L.; Hewitt, J.; Hurley-Walker, N.; Johnston-Hollitt, M.; Kaplan, D.; Kasper, Justin; Kim, Hanna; Kratzenberg, E.; Lenc, E.; Loeb, Abraham; Lonsdale, C.; Lynch, M.; McKinley, B.; McWhirter, S.; Mitchell, D.; Morgan, E.; Neben, A.; Thyagarajan, Nithyanandan; Oberoi, D.; Offringa, A.; Ord, S.; Paul, S.; Prabu, T.; Procopio, P.; Riding, J.; Rogers, A.; Roshi, A.; Shankar, N.; Sethi, Shiv; Srivani, K.; Subrahmanyan, R.; Tegmark, M.; Tingay, S.; al, etThe Murchison Widefield Array (MWA) has collected hundreds of hours of Epoch of Reionization (EoR) data and now faces the challenge of overcoming foreground and systematic contamination to reduce the data to a cosmological measurement. We introduce several novel analysis techniques such as cable reflection calibration, hyper-resolution gridding kernels, diffuse foreground model subtraction, and quality control methods. Each change to the analysis pipeline is tested against a two dimensional power spectrum figure of merit to demonstrate improvement. We incorporate the new techniques into a deep integration of 32 hours of MWA data. This data set is used to place a systematic-limited upper limit on the cosmological power spectrum of Δ2≤2.7×104 mK2 at k=0.27 h~Mpc−1 and z=7.1, consistent with other published limits, and a modest improvement (factor of 1.4) over previous MWA results. From this deep analysis we have identified a list of improvements to be made to our EoR data analysis strategies. These improvements will be implemented in the future and detailed in upcoming publications.Publication The Interaction of Relativistic Spacecrafts with the Interstellar Medium(American Astronomical Society, 2017) Hoang, Thiem; Lazarian, A.; Burkhart, Blakesley; Loeb, AbrahamThe Breakthrough Starshot initiative aims to launch a gram-scale spacecraft to a speed of v∼0.2c, capable of reaching the nearest star system, α Centauri, in about 20 years. However, a critical challenge for the initiative is the damage to the spacecraft by interstellar gas and dust during the journey. In this paper, we quantify the interaction of a relativistic spacecraft with gas and dust in the interstellar medium. For gas bombardment, we find that damage by track formation due to heavy elements is an important effect. We find that gas bombardment can potentially damage the surface of the spacecraft to a depth of ∼0.1 mm for quartz material after traversing a gas column of NH∼2×1018cm−2 along the path to α Centauri, whereas the effect is much weaker for graphite material. The effect of dust bombardment erodes the spacecraft surface and produces numerous craters due to explosive evaporation of surface atoms. For a spacecraft speed v=0.2c, we find that dust bombardment can erode a surface layer of ∼0.5 mm thickness after the spacecraft has swept a column density of NH∼3×1017cm−2, assuming the standard gas-to-dust ratio of the interstellar medium. Dust bombardment also damages the spacecraft surface by modifying the material structure through melting. We calculate the equilibrium surface temperature due to collisional heating by gas atoms as well as the temperature profile as a function of depth into the spacecraft. Our quantitative results suggest methods for damage control, and we highlight possibilities for shielding strategies and protection of the spacecraft.Publication Lessons from Mayan AstronomyLoeb, AbrahamThe Mayan culture collected exquisite astronomical data for over a millennium. However, it failed to come up with the breakthrough ideas of modern astronomy because the data was analyzed within a mythological culture of astrology that rested upon false but mathematically sophisticated theories about the Universe. Have we learned the necessary lessons to prevent our current scientific culture from resembling Mayan Astronomy? Clearly, data collection by itself is not a guarantee for good science as commonly assumed by funding agencies. A vibrant scientific culture should cultivate multiple approaches to analyzing existing data and to collecting new data.