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Register a new userDiffractive Microlensing: A New Probe of the Local Universe
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Diffraction is important when nearby substellar objects gravitationally lens distant stars. If the wavelength of the observation is comparable to the Schwarzschild radius of lensing object, diffraction leaves an observable imprint on the lensing signature. The SKA may have sufficient sensitivity to detect the typical sources, giant stars in the bulge. The diffractive signatures in a lensing event break the degeneracies between the mass of the lens, its distance and proper motion.
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We propose the existence of ultracompact minihalos as a new type of massive compact halo object (MACHO) and suggest an observational test to discover them. These new MACHOs are a powerful probe into the nature of dark matter and physics in the high energy Universe. Non-Gaussian energy-density fluctuations produced at phase transitions (e.g., QCD) or by features in the inflaton potential can trigger primordial black hole (PBH) formation if their amplitudes are delta > 30%. We show that a PBH accumulates over time a sufficiently massive and compact minihalo to be able to modify or dominate its microlensing magnification light curve. Perturbations of amplitude 0.03% < delta < 30% are too small to form PBHs, but can nonetheless seed the growth of ultracompact minihalos. Thus, the likelihood of ultracompact minihalos as MACHOs is greater than that of PBHs. In addition, depending on their mass, they may be sites of formation of the first PopIII stars. Ultracompact minihalos and PBHs produce a microlensing light curve that can be distinguished from that of a point-like object if high-quality photometric data are taken for a sufficiently long time after the peak of the magnification event. This enables them to be detected below the stellar-lensing background toward both the Magellanic Clouds and the Galactic bulge.
Microlensings events are predicted for the light coming from cosmological sources. In addition to the microlensing due to gravitation lensing, microlensing produced also by refraction of light due to either ionized, or not, gas clouds can be considered. A detailed prediction is here given assuming that the ray of light coming from the distant source traverses a gas cloud with a Kings density profile for various possible environments. We conclude that the additional deviation due to relativistic refraction is in most cases negligible compared to the gravitational deviation. Deviation due to refraction can anyway become an interesting analysis tool for future facility with great resolving power and the effects can be singled out with dedicated surveys.
The Wide Field Infrared Survey Telescope (WFIRST) will monitor $sim 2$ deg$^2$ toward the Galactic bulge in a wide ($sim 1-2~mu$m) W149 filter at 15-minute cadence with exposure times of $sim$50s for 6 seasons of 72 days each, for a total $sim$41,000 exposures taken over $sim$432 days, spread over the 5-year prime mission. This will be one of the deepest exposures of the sky ever taken, reaching a photon-noise photometric precision of 0.01 mag per exposure and collecting a total of $sim 10^9$ photons over the course of the survey for a W149$_{rm AB}sim 21$ star. Of order $4 times 10^7$ stars will be monitored with W149$_{rm AB}$<21, and 10$^8$ stars with W145$_{rm AB}$<23. The WFIRST microlensing survey will detect $sim$54,000 microlensing events, of which roughly 1% ($sim$500) will be due to isolated black holes, and $sim$3% ($sim$1600) will be due to isolated neutron stars. It will be sensitive to (effectively) isolated compact objects with masses as low as the mass of Pluto, thereby enabling a measurement of the compact object mass function over 10 orders of magnitude. Assuming photon-noise limited precision, it will detect $sim 10^5$ transiting planets with sizes as small as $sim 2~R_oplus$, perform asteroseismology of $sim 10^6$ giant stars, measure the proper motions to $sim 0.3%$ and parallaxes to $sim 10%$ for the $sim 6 times 10^6$ disk and bulge stars in the survey area, and directly detect $sim 5 times 10^3$ Trans-Neptunian objects (TNOs) with diameters down to $sim 10$ km, as well as detect $sim 10^3$ occulations of stars by TNOs during the survey. All of this science will completely serendipitous, i.e., it will not require modifications of the WFIRST optimal microlensing survey design. Allowing for some minor deviation from the optimal design, such as monitoring the Galactic center, would enable an even broader range of transformational science.
Recent studies suggest the existence of very massive stars (VMS) up to 300 solar masses in the local Universe. As this finding may represent a paradigm shift for the canonical stellar upper-mass limit of 150 solar masses, it is timely to evaluate the physics specific to VMS, which is currently missing. For this reason, we decided to construct a book entailing both a discussion of the accuracy of VMS masses (Martins), as well as the physics of VMS formation (Krumholz), mass loss (Vink), instabilities (Owocki), evolution (Hirschi), and fate (theory -- Woosley & Heger; observations -- Smith).
We present htof, an open-source tool for interpreting and fitting the intermediate astrometric data (IAD) from both the 1997 and 2007 reductions of Hipparcos, the scanning-law of Gaia, and future missions such as the Nancy Grace Roman Space Telescope (NGRST). htof solves for the astrometric parameters of any system for any arbitrary combination of absolute astrometric missions. In preparation for later Gaia data releases, htof supports arbitrarily high-order astrometric solutions (e.g. five-, seven-, nine-parameter fits). Using htof, we find that the IAD of 6617 sources in Hipparcos 2007 might have been affected by a data corruption issue. htof integrates an ad-hoc correction that reconciles the IAD of these sources with their published catalog solutions. We developed htof to study masses and orbital parameters of sub-stellar companions, and we outline its implementation in one orbit fitting code (orvara, https://github.com/t-brandt/orvara). We use htof to predict a range of hypothetical additional planets in the $beta$~Pic system, which could be detected by coupling NGRST astrometry with Gaia and Hipparcos. htof is pip installable and available at https://github.com/gmbrandt/htof .
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