Astronomy Tea Talk at Caltech

Mondays, Cahill 312

Tea:4 pm, Talk: 4:05 pm

Next talk is listed here, for a complete schedule please check upcoming talks page

Obliquity-Driven Sculpting of Exoplanetary Systems

Sarah Millholland (Carnegie)

Octy 7, 2019

The tilt of a planet’s spin axis off its orbital axis (“obliquity”) is a basic physical characteristic that plays a central role in determining the planet’s atmospheric circulation, energy redistribution, and climate. Moreover, by strongly enhancing the tidal dissipation rate, planetary obliquities can shape not only the physical features of exoplanets, but also their orbital architectures. Large planetary obliquities can be generated by secular spin-orbit resonances. These occur when the rate of a planet’s spin axis precession matches the rate of its orbital precession. I will present evidence that close-in, compact, Kepler-type systems frequently experience this mechanism – a simple but fascinating result of their intrinsic physical and orbital properties. I will highlight the consequences of this obliquity-driven sculpting by linking it to several strange features of the observed planet population, including the mysterious overabundance of planets wide of mean-motion resonances and the remarkably rapid orbital inspiral of the highly inflated hot Jupiter, WASP-12b.

The Southern Stellar Stream Spectroscopic Survey

Ting Li (Carnegie)

Sepy 30, 2019

In this talk, I will present an ongoing spectroscopic program, the Southern Stellar Stream Spectroscopic Survey (S5), which maps newly discovered stellar streams with the fiber-fed AAOmega spectrograph on the Anglo-Australian Telescope. S5 is the first systematic program pursuing a complete census of known streams in the Southern Hemisphere, providing a uniquely powerful sample for understanding the building blocks of the Milky Way's stellar halo, the progenitors and formation of stellar streams, the mass and shape of the Milky Way's halo, and ultimately the nature of dark matter. In additional to observing stars in the stellar streams, we use spare fibers to conduct two auxiliary programs, a Milky Way halo survey as well as a low redshift galaxy survey. In particular, we serendipitously discovered one of the fastest hyper-velocity stars in the Galaxy with the Galactocentric velocity of >1700 km/s and found it ejected by the supermassive black hole from the Galactic Center ~5 Myr ago.

Diffuse Galaxies As a Probe for Dark Matter

Shany Danieli (Yale)

Sepy 23, 2019

Low mass galaxies provide an essential testing ground for theoretical predictions of cosmology. They dominate the counts in the Local Group and have high mass-to-light ratios, making them ideal for studying dark matter on small scales. Recent advances in telescope instrumentation have opened a new window into the population of such low surface brightness galaxies. In this talk, I will present recent results from the Dragonfly Telescope, which has identified large numbers of low surface brightness galaxies beyond the Local Group and discuss its contribution and potential in extending our ability to test LCDM on small scales. I will discuss the recently identified population of ultra-diffuse galaxies (UDGs) that holds the promise of new constraints on low mass galaxies dynamics, as their spatial extent and often significant globular cluster populations provide probes on spatial scales where dark matter should dominate the kinematics. I will also discuss the dynamics of two UDGs that seems to lack most, if not all, of their dark matter. I will finish by presenting our strategy for finding low surface brightness galaxies as part of the recently completed Dragonfly Wide Field Survey, covering 330 sq. deg., in the GAMA and Stripe 82 fields.

Kuiper Belt Occultations and other fast phenomena with W-FAST

Guy Nir (Weizmman Institute)

Sepy 16, 2019

The Weizmann Fast Astronomical Survey Telescope (W-FAST), now in commissioning in Israel, is a wide field fast imaging telescope that is built to detect sub-second phenomena such as occultations of stars by Solar System objects. W-FAST will be running at 25Hz, with a f.o.v of 7 deg^2, producing high cadence lightcurves for thousands of bright objects in the field, while at the same time producing deeper stacked images for observing variability on timescales of seconds to minutes. Beyond the known science objectives, we expect to find new phenomena as we explore the parameter space of very fast and very wide photometry.

Cosmic compass - First tomography of an outer 3D sub-Gauss field via atomic alignment

Heshou Zhang (DESY ; Uni. Potsdam)

Sepy 9, 2019

Sub-Gauss magnetic fields are crucial for many physical processes but hard to measure. We report the first observational results that measure 3D topology as well as the strength of a sub-milliGauss magnetic field simultaneously beyond our solar system (arXiv:1903.08675). This is the first observational proof in line with the magnetic tracer ground state alignment (GSA) proposed a decade ago. Our analysis reveals the first sub-AU scale magnetic field on 89 Her, that is 1.3kpc away, has a 3D orientation aligned to the stellar outflow axis and a strength 70 ~ 150 microGauss, thus substantially improving the accuracy by five orders of magnitude compared to the previous 10G upper limit set by non-detection of Zeeman effect.

GSA has shown great potentials, including abundant observables, the possibility of multi- waveband observation from UV to submillimeter fine-structure lines, as well as a large field

strength coverage (from sub-Gauss at the atmosphere of evolving star all the way down to weaker than nanoGauss InterGalactic medium). We anticipate our observation to start a new era of magnetic field mapping in the universe, where much more information and higher precision can be achieved with current observational facility from our Galaxy to the early universe if accounting for GSA.

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