Colloquia: Spring 2021
NOTE: colloquia with dates in red are not at the regular times; dates in blue indicate talks broadcast from Hilo
One Minute Colloquium
Wed Mar 3
Host: Eric Baxter
Wed Mar 10
Wed Mar 17
Wed Mar 24
Wed Mar 31
Thomas de Jaeger
Host: Benjamin Shappee
Wed Apr 7
Host: Dan Huber
Wed Apr 14
Wed Apr 21
Host: Xudong Sun
Wed Apr 28
Host: Dan Huber
What can Astronomy Teach us about Cosmology?
University of Hawaii at Manoa
Our best-fitting theory of the universe for over 20 years is that the structure we see around us formed due to the gravitational interactions of visible matter with cold dark matter, and that the expansion of the universe as a whole is governed by a mysterious space filling fluid called dark energy. Together, cold dark matter and dark energy are thought to constitute over 95% of the universe’s constituents. As we have entered the era of data-driven, precision cosmology, cracks in this paradigm have begun to emerge. Tensions between different cosmological data sets have worsened, dark matter has yet to be detected, and we are still no closer to understanding the underlying nature of dark energy. This has led theorists to speculate on alternative scenarios, such as dark matter having very weak interactions with visible matter, or that dark energy is due to a breakdown of the laws of gravity on cosmological scales. These alternative scenarios open up the possibility of testing cosmology using astrophysical objects since dark matter could be produced inside stars, and changing the laws of gravity changes how stars work. In this talk, I will tell you about this exciting program of testing cosmology using astrophysical objects by showing how they can help to answer two important questions: can dark matter destroy stars? and is gravitation universal?
Construction of a spectrograph and quasar target selection for the Dark Energy project, DESI
University of Hawaii at Manoa
|The accelerating expansion of the universe is one of the main topics of modern cosmology. It may stem from a new component, so-called dark energy, which would make up 70% of the energy content of the universe. To study its nature through its equation of state, one can measure a standard ruler given by baryonic acoustic oscillations (BAO) at various redshifts or for different slices of the universe. The BAO signal was confirmed in 2012 by the BOSS project (SDSS-III) and then by the eBOSS project (SDSS-IV), both with galaxies and Ly-alpha absorbers revealed in quasar spectra. During my thesis, I was involved in the preparation of the next generation of BAO experiments, the new Dark Energy Spectroscopic Instrument (DESI) program. This project will perform a 3D survey of several tens of millions of galaxies and quasars with the 4-meter Mayall telescope located at Kitt Peak in Arizona. To this end, the Mayall Telescope has been upgraded with a reconfigurable focal plane made up of 5000 robotically controlled fiber tips that collect light from the observed objects. The light is transmitted down to 10 spectrograph modules which are able to simultaneously measure the spectra of the 5000 objects. Despite these unique capabilities, it is first necessary to select the targets to be pointed using data coming from extensive imaging surveys of the sky probed in several photometric bands. Among these targets, quasars are very bright and distant. They are ideal candidates for mapping the structures of the universe at high redshift. In this talk, I will introduce the science conducted by DESI using BAO measurement. I will then describe the general architecture of the instrument and the activities I participated in for the construction of the spectrograph and the alignment of the CCD sensors. I will also discuss the algorithms that I have developed for the selection of quasar candidates based on their photometry properties by making use of machine learning tools.|
Evidence that 1I/2017 U1 (`Oumuamua) was composed of molecular hydrogen ice
University of Chicago
|`Oumuamua (I1 2017) was the first macroscopic (l~100 m) body observed to traverse the inner solar system on an unbound hyperbolic orbit. Its light curve displayed strong periodic variation, and it showed no hint of a coma or emission from molecular outgassing. Astrometric measurements indicate that ‘Oumuamua experienced non-gravitational acceleration on its outbound trajectory, but energy balance arguments indicate this acceleration is inconsistent with a water ice sublimation-driven jet of the type exhibited by solar system comets. We show that all of `Oumaumua’s observed properties can be explained if it contained a significant fraction of molecular hydrogen (H_2) ice. H_2 sublimation at a rate proportional to the incident solar flux generates a surface-covering jet that reproduces the observed acceleration. Mass wasting from sublimation leads to monotonic increase in the body axis ratio, explaining `Oumuamua’s shape. Back-tracing `Oumuamua’s trajectory through the Solar System permits calculation of its mass and aspect ratio prior to encountering the Sun. We show that H_2-rich bodies plausibly form in the coldest dense cores of Giant Molecular Clouds, where number densities are of order n~10^5, and temperatures approach the T=3 K background. Post-formation exposure to galactic cosmic rays implies a tau ~ 100 Myr age, explaining the kinematics of `Oumuamua’s inbound trajectory.|
Type II supernova cosmology: past and future
Thomas de Jaeger
University of Hawaii at Manoa
|Measuring accurate extragalactic distances is one of the most challenging tasks in Astronomy but remains one of the best observational probes to understand the Universe’s content. For more than two decades, Type Ia supernovae have been used as standard candles to measure extragalactic distances with an accuracy of 5-6%. In 1998, SNe Ia led to the measurement of the Universe expansion history and revealed the surprising accelerated growth rate of the Universe driven by an unknown effect attributed to dark energy. However, even if SN Ia cosmology is one of the most mature methods, the nature of dark energy remains unknown. The precise value of the Universe expansion rate H0 is also vigorously debated and a 4-5 sigma discrepancy is seen between measurements from SNe Ia calibrated using Cepheid stars and the baryon acoustic oscillations calibrated using the cosmic microwave background radiation. Since truth will likely emerge from the combination of different independent approaches, it is essential to develop as many methods as possible.
Among the new techniques found in the literature, one of the most interesting for deriving accurate distances and measuring cosmological parameters is the use of Type II supernovae. Even though detecting SNe II at high redshift is challenging owing to their relatively low luminosity, their use as cosmic distance indicators is motivated by the fact that their progenitors and environments are better understood than those of SNe Ia. The SN II family displays a large range of peak luminosities; but as for SNe Ia, their extrinsic differences can be calibrated.
In this talk, I will present in detail the Standard Candle Method which is the most used method. I will show our ability to extend the current SN II Hubble diagram beyond z = 0.3 and derive the cosmological parameters. Using Cepheid distances, I will also present the most precise Hubble constant measurement from Type II supernovae, showing that they can play a role in the Hubble tension. As the coming era of large photometric wide-field surveys will increase the detection rate of supernovae by orders of magnitude, which will prohibit spectroscopic follow-up in the vast majority of cases, I will also present a new method to standardize SNe II based on photometric parameters alone.
The Planet We Could Not Imagine
|A fundamental aspect of understanding the limits of habitable environments and detectable signatures is the study of where the boundaries of such environments can occur, and the conditions under which a planet is rendered into a hostile environment. In our solar system, Venus is the most Earth-like planet, yet at some point in planetary history there was a bifurcation between the two: Earth has been continually habitable since the end-Hadean, whereas Venus became uninhabitable. Indeed, Venus is the type-planet for a world that has transitioned from habitable and Earth-like conditions, through the inner edge of the Habitable Zone (HZ); thus it provides a natural laboratory to study the evolution of habitability. In this talk I will describe the gaps in our knowledge regarding Venus within the context of how these gaps are impacting our ability to model exoplanet atmospheres and interiors. I will discuss various factors that relate to a possible habitable past of Venus, including orbital evolution. I will outline exoplanet target selection for testing the conditions of runaway greenhouse and present examples of potential Venus analogs. Finally, I will summarize the primary exoplanet science questions that would be addressed by a return surface mission to Venus.|
Identifying ancient asteroid families: prospecting the original planetesimal population
|Asteroid families are the remnant fragments of asteroids broken apart by collisions. There are only a few known Main Belt (MB) asteroid families with ages greater than 2 Ga (Broz et al., 2013; Spoto et al., 2015). Estimates based on the family producing collision rate suggest that the lack of >2 Ga-old families may be due to a selection bias in classic techniques used to identify families. Family fragments disperse in their orbital elements, semi-major axis, a, eccentricity, e, and inclination, i, due to secular resonances and the non-gravitational Yarkovsky force. This causes the family fragments to be more difficult to identify with the hierarchical clustering method (HCM), which attempts to find clusters in orbital element space when applied to family fragments’ elements as the fragments age. We have developed a new technique that is insensitive to the spreading of fragments in e and i by searching for V-shaped correlations of family members in a and asteroid diameter, D. A group of asteroids is identified as a collisional family if its boundary in the a vs. 1/D plane has a characteristic V-shape which is due to the size-dependent Yarkovsky effect. The V-shape technique is demonstrated on the known families and families difficult to identify by HCM, and used to discover a 4 Ga-old family linking most dark asteroids in the inner MB not included in any known family (Delbo` et al., 2017). The 4 Ga-old family reveals asteroids with D > 35 km that do not belong to any asteroid family implying that they originally accreted from the protoplanetary disk and support recent theories on the formation of asteroids (Morbidelli et al., 2009).|
The Smallest X-ray Flares: the Keys to Energy Release on the Sun?
University of Minnesota Institute for Astrophysics
|While large, cataclysmic solar flares and eruptions tend to dominate our attention, there is a great deal to be learned from small, faint flares. Indeed, one of the benefits that comes from using the Sun as a case study for astrophysical processes is that relatively small, weakly energetic events can be well observed, not just the large, pathological ones. In the past, determining how energy is released and particles are accelerated in solar flares has proved to be an elusive topic. Understanding this would elucidate how and when flares occur, how they relate to coronal mass ejections, the extent to which they heat the corona, and what connections they have with the rest of the heliosphere. Of particular interest is the energy carried by accelerated electrons, and the mechanisms by which they attain those energies. We attack this by studying how energy release properties scale across flare size. This talk will show recent results from the smallest hard X-ray flares observed by the FOXSI rocket and NuSTAR spacecraft. The talk will also discuss some of the joys and challenges in utilizing space-based telescopes for solar observation that were not designed for the Sun.|
Solar System Chaos and its Utility in Geologic Dating
Richard E. Zeebe
|The motion of the solar system is chaotic, i.e., small differences in initial conditions grow exponentially, with a Lyapunov time for the inner planets of only about 5 Myr. For example, a difference in initial coordinates of 1 mm grows to ca. 1 AU after 163 Myr. Thus, the chaotic nature of the planetary orbits appears to limit accurate prediction of orbital evolution beyond a certain time interval (limit of predictability, ca. 50 Myr). In this presentation, I will provide a brief historic perspective on our understanding of the chaotic behavior of the solar system. Next, I will describe the utility of solar system dynamics (e.g., changes in Earth’s orbital parameters, so-called Milankovic cycles) in geologic dating, a.k.a. astrochronology, which uses astronomical solutions of the planetary orbits to provide highly accurate ages of events in Earth’s history (astronomical time scale). Unfortunately, until recently astronomers and geologists have struggled to extend the astronomical time scale farther back than ca. 50 Myr due to the limit of predictability from solar system chaos. However, I will show how geologic records and the fingerprint of chaos in those records can be used to overcome the limit by using long-term numerical ensemble integrations of the solar system. Furthermore, I will comment on important integration parameters such as the solar quadrupole moment and the number of asteroids included in the simulations. Finally, I will highlight the unique opportunity provided by the current approach to reconstruct the chaotic behavior of the solar system.|