1801.01120
New parallaxes of galactic Cepheids from spatially scanning the Hubble Space Telescope: Implications for the Hubble constant
Riess, et al
New parallax measurements of 7 long-period (>10 ays) MW Cepheids using astrometry from spatial scanning of WFC3 on HST. Observations were obtained at 6 month intervals over 4 years. The distances are 1.7--3.6 kpc with a mean precision of 45 microarcseconds and a best of 29 microarcseconds (SNR=14). The accuracy of the parallaxes is demonstrated through independent analyses of >100 reference stars. This raises to 10 the number of long-period Cepheids in the hosts of 19 SNeIa. This sample addresses two outstanding systematic uncertainties affecting prior comparisons of MW and extragalactic Cepheids used to calibrate H0: their dissimilarity of ladder gives a ratio (or independent scale for H0) of 1.034±0.036, consistent with no change and inconsistent at the 3.3 sigma level with a ratio of 0.91 needed to match the value predicted by Planck+LCDM. Using these data instead to augment the Riess+2016 measurement of H0 improves the precision to 2.3%, yielding 73.45±1.66 km/s/Mpc, and tension with Planck+LCDM increases to 3.7 sigma. The future combination of Gaia parallaxes and HST spatial scanning photometry of 50 MW Cepheids can support a <1% calibration of H0.
1801.01257
Testing weak equivalence principle with strongly lensed cosmic transients
Yu, Wang
Compare the time delays between lensed images seen in different energy bands, or in gravitational waves (GWs) and their electromagnetic (EM) counterparts in strongly lensed cosmic transients, to robustly test for WEP. If the time delay of cosmic transient can be measured with accuracy about 0.1s, show that the upper limit on the differences of the parameterized post-Newtonian parameter gamma values is Delta gamma <1e-7 with a one-month strong lensing time delay event. This accuracy of WEP can be improved by a a factor of 1e7, if the leasing is a galaxy cluster and the strongly lensed cosmic transients have much shorter duration, such as fast radio bursts.
1801.01474
Galactic effects on habitability
Kaib
The galactic environment has been suspected to influence planetary habitability in many ways. Very metal-poor regions of the Galaxy (those largely devoid of atoms more massive than H and He) are thought to be unable to form habitable planets. Moreover, if such planets do form, the young system is subjected to close stellar passages while it resides in its stellar birth cluster. Various potential hazards remain after clusters disperse. For instance, central galactic regions may present risks to habitability via nearby supernovae, GRBs, and frequent comet showers. In addition, planets residing within very wide binary star systems are affected by the Galaxy, as local gravitational perturbations from the Galaxy can increase the binary's eccentricity until it destabilizes the planets it hosts. Here, review the most recent work on the main galactic influences over planetary habitability. Although there must be some metallicity limit below which rocky planets cannot form, recent exoplanet surveys show that they form around stars with a very large range of metallicities. Once formed, the probability of star clusters destabilizing planetary systems only becomes high for rare, extremely long-lived clusters. Regarding threats to habitability from supernovae, GRBs, and comet showers, many recent studies suggest that their hazards are more limited than originally thought. Finally, denser regions of the Galaxy enhance the threat that very wide binary companions pose to planetary habitability, but the probability that a very wide binary star disrupts habitability will always be substantially below 100% for any environment. While some MW regions must be more hospitable to habitable planets than others, it is difficult to state that habitable planets are confined to any well-defined regions of the Galaxy or that any other particular region of the Galaxy is uninhabitable.
1801.01506
Time Delay lens modeling challenge: I. Experimental Design
Ding, Treu, et al
Following the experience of the past challenge on time delay, where it was shown that time delays can indeed be measured precisely and accurately at the sub-percent level, now present the "Time Delay Lens Modeling Challenge" (TDLMC). The goal of this challenge is to assess the present capability of lens modeling codes and assumptions and test the level of accuracy of inferred cosmological parameters given realistic mock datasets. Invite scientists to model a set of simulated HST observations of 50 mock lens systems . The systems are organized in rungs, with the complexity and realism increasing going up the ladder. The goal of the challenge is to infer H0 for each rung, given the HST images, the time delay, and a stellar velocity dispersion of the deflector, for a fixed background cosmology. The TDLMC challenge will start with the mock data release on 2018 January 8th, with a deadline for blind submission of 2018 August 8th. This first paper gives an overview of the challenge including the data design, and a set of metrics to quantify the modeling performance and challenge details. After the deadline, the results of the challenge will be presented in a companion paper with all challenge participants as co-authors.
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